Extension of the power factor model for aluminium

Júlio Cristiano Rato Rafael Coroado

Thesis to obtain the Master of Science Degree in

Mechanical Engineering

Supervisors: Prof. Eurico Gonçalves Assunção

Prof.ª Maria Luísa Coutinho Gomes de Almeida

Examination Committee

Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista

Supervisor: Prof. Eurico Gonçalves Assunção

Members of the Committee: Dr.ª Sónia Andreia Martins Meco

Prof.ª Rosa Maria Mendes Miranda

November 2015

“The purpose shapes the plan. The plan shapes the action. The action reaches the results. The

results bring success.”

John C. Maxwell (Garden City, 1947)

Falling Forward

I

Agradecimentos

Em primeiro lugar, gostaria de agradecer à Professora Luísa Coutinho pela oportunidade única que me

proporcionou para realizar este trabalho e também por todo o apoio científico, disponibilidade e

orientação prestados ao longo deste desafio.

Ao Dr. Eurico Assunção, que também me acompanhou durante toda esta etapa, expresso os meus

sinceros agradecimentos pela oportunidade dada bem como as sugestões propostas e know how que

trouxe a este trabalho.

Quero expressar o meu apreço à Sónia Meco e ao Gonçalo Pardal da Universidade de Cranfield pelo

acompanhamento prestado dentro e fora da vida académica. Um muito obrigado por todos os

ensinamentos teóricos e práticos, ajuda a preparar os testes e discussões sobre este projecto.

O meu profundo agradecimento ao Dr. Wojciech Suder pelas críticas construtivas e essenciais ao

desenvolvimento e preparação das experiências e na análise de resultados.

À Universidade de Cranfield e a todo o centro engenharia de soldadura e processamento laser,

agradeço a hospitalidade com que me receberam e que fez de Cranfield uma segunda casa. Gostaria

de agradecer em particular ao Professor Stewart Williams e ao Dr. Supriyo Ganguly pela oportunidade

de trabalhar no centro de investigação com o acesso a todos os equipamentos indispensáveis ao

desenvolvimento deste projecto.

Aos meus amigos e colegas de universidade, um especial obrigado pelo apoio e amizade durante estes

anos que me ajudaram a levar esta missão a bom porto.

À Marina dos Santos, pela paciência, incentivo, dedicação e por estar sempre presente, muito obrigado.

Agradeço-lhe também os bons momentos que me proporcionou e que foram essenciais nestes anos

que cresceu comigo.

Por último, mas não menos importante, um obrigado do fundo do coração a toda a minha família que

me formou como pessoa, que me apoiou incondicionalmente em todas as decisões e que me ajudou a

alcançar sempre o sucesso em qualquer desafio. Pai, mãe, irmãos e avós, a vocês dedico este trabalho.

II

III

Resumo

A profundidade de penetração é um dos mais importantes parâmetros que o utilizador deseja controlar

em aplicações de soldadura a laser. A criação de um modelo necessita da utilização dos parâmetros

fundamentais de interação material-laser em qualquer diâmetro do feixe de laser, o que permite a

aplicação de um conjunto de parâmetros em diferentes sistemas laser. Este “modelo de fator de

potência” foi inicialmente desenvolvido, testado e validado para aço macio e posteriormente para titânio,

apoiando o utilizador na seleção do correto sistema de parâmetros, possibilitando escolher uma

soldadura adequada para uma determinada aplicação em termos de profundidade de penetração e

largura do cordão. Esta tese tem por objetivo estender este modelo para alumínio através de um estudo

dos parâmetros básicos de interação material-laser em termos de densidade de potência, tempo de

interação e ponto específico de energia e a sua correlação com os perfis de cordão de soldadura.

Através da combinação de diferentes potências, velocidades de soldadura e diâmetros do feixe de

laser, vários ensaios foram realizados analisando a relação da profundidade de penetração com o fator

de potência e tempo de interação. A independência entre a profundidade de penetração e o diâmetro

do feixe de laser, para valores de fator de potência e tempos de interação constantes, foi observada e

isto comprova a funcionalidade do modelo em alumínio.

Palavras-chave: modelo de fator de potência, tempo de interação, profundidade de penetração,

soldadura a laser, diâmetro do feixe de laser, largura de soldadura.

IV

V

Abstract

The depth of penetration is one of the most important laser parameters that the user wishes to control

in laser welding applications. Creating a model requires the use of the fundamental laser material

interaction parameters for any beam diameter, which allows the application of a set of parameters for

different laser systems. The "power factor model" has been initially developed, tested and validated for

mild steel and posteriorly for titanium, for supporting the user on the selection of the correct system

parameters, enabling to choose a suitable weld for a given application in terms of depth of penetration

and weld width. This thesis is intended to extend this model to aluminium through a study of the basic

laser material interaction parameters in terms of power density, interaction time, specific point energy,

power factor and their correlation with the weld bead profiles. By combining different powers, travel

speeds and beam diameters, several trials were conducted analysing the relation of penetration depth

with the power factor and interaction time. The independence between the depth of penetration and

beam diameter, for constant values of power factor and interaction time, was observed and this proves

the model functionality for aluminium.

Keywords: power factor model, interaction time, depth of penetration, laser welding, laser beam

diameter, weld width.

VI

VII

Contents

List of Figures ......................................................................................................................................... IX

List of tables ......................................................................................................................................... XIII

Abbreviations .........................................................................................................................................XV

1. Introduction .......................................................................................................................................... 1

1.1 Background .................................................................................................................................... 1

1.2 Thesis Objectives .......................................................................................................................... 3

1.3 Thesis structure ............................................................................................................................. 4

2. Literature review .................................................................................................................................. 5

2.1 Historical Note ............................................................................................................................... 5

2.2 Fibre Lasers ................................................................................................................................... 7

2.3 Measuring methods of laser beam properties ............................................................................. 10

2.3.1 Laser output power measurement ........................................................................................ 11

2.3.2 Laser beam diameter measurement ..................................................................................... 11

2.4 Operational regimes in laser welding .......................................................................................... 12

2.5 Controlling depth of penetration in laser welding ........................................................................ 14

2.5.1 Plasma plume effect ............................................................................................................. 15

2.5.2 Effect of pressure .................................................................................................................. 16

2.5.3 Absorption effect ................................................................................................................... 17

2.6 Different types of laser ................................................................................................................. 18

2.7 Fundamental Laser-material interaction parameters in laser welding ......................................... 20

2.8 Laser welding of aluminium ......................................................................................................... 23

2.9 Power Factor ............................................................................................................................... 25

2.9.1 Effect of the system parameters on the weld bead geometry .............................................. 25

2.9.2 Effect of the beam diameter on the weld bead geometry ..................................................... 26

2.9.3 Depth of penetration ............................................................................................................. 27

2.9.4 Sensitivity analysis ................................................................................................................ 28

2.9.5 Power factor – application model .......................................................................................... 29

2.9.6 Limitation of the power factor ................................................................................................ 30

2.10 Summary ................................................................................................................................... 32

3. Experimental approach ..................................................................................................................... 33

3.1 Material ........................................................................................................................................ 33

VIII

3.2 Laser system ............................................................................................................................... 34

3.3 Motion and clamping system ....................................................................................................... 35

3.4 Samples preparation ................................................................................................................... 38

3.4.1 Before welding ...................................................................................................................... 38

3.4.2 Macrograph preparation ........................................................................................................ 38

3.5 Experimental procedure .............................................................................................................. 39

3.5.1 Preliminary experiments ....................................................................................................... 39

3.5.2 Extension of the power factor model for aluminium .............................................................. 40

3.6 Methodology ................................................................................................................................ 41

4. Results presentation and discussion................................................................................................. 42

4.1 Preliminary experiments .............................................................................................................. 42

4.1.1 Effect of the power density on the weld ................................................................................ 42

4.1.2 Effect of graphite on the weld geometry ............................................................................... 44

4.1.3 Effect of the shielding gas on the weld geometry ................................................................. 45

4.1.4 Effect of shielding gas flow rate on the weld geometry ........................................................ 46

4.2 Extension of the power factor model for aluminium .................................................................... 48

4.2.1 Development of the power factor model for one beam diameter.......................................... 48

4.2.2 Effect of interaction time and power factor on the depth of penetration for different beam

diameters ....................................................................................................................................... 50

5. Conclusions and future developments .............................................................................................. 62

5.1 Conclusions ................................................................................................................................. 62

5.2 Future developments ................................................................................................................... 63

6. References ........................................................................................................................................ 64

7. Appendix ........................................................................................................................................... 70

Appendix A – Material properties. ..................................................................................................... 70

Appendix B - Transversal cuts for a power factor of 4.9 MW/m. ....................................................... 71

IX

List of Figures

Figure 1: Schematic representation of the power factor model selection parameters. ........................... 2

Figure 2: Laser welding of two plates in a butt joint configuration [10]. ................................................... 5

Figure 3: Estimated laser operating costs [11]. ....................................................................................... 6

Figure 4: Schematic representation of a double-clad fibre laser [15]. ..................................................... 7

Figure 5: Schematic presentation of a fibre laser architecture [16]. ........................................................ 8

Figure 6: Beam divergence. .................................................................................................................... 9

Figure 7: Definition of M2 [45]. ................................................................................................................ 9

Figure 8: Effect of BPP on beam diameter and power density [44]. ..................................................... 10

Figure 9: Schematic representation of calorimetric power meter [19]. .................................................. 11

Figure 10: Experimental setup (top view). (a) The BS, SD, RD, and TS stand for the beam splitter, signal

detector, reference detector and translational stage, respectively. (b) Illustration of the transmitted

energy S1 as a function of x and its derivative [22]. .............................................................................. 12

Figure 11: Schematic representation of the welding modes: conduction (left) and “keyhole” (right) [27].

............................................................................................................................................................... 13

Figure 12: Visible keyhole area for 1, 4 and 7 m/min feed rates on one 5 mm stainless steel plate. The

dashed lines show the trend of the keyhole opening area [31]. ............................................................ 15

Figure 13: The plume inclination and the radiance change with rising laser power: 1.5 kW (left) and 3.5

kW (right) [31]. ....................................................................................................................................... 15

Figure 14: Dynamic pressure p, closing pressure ps and keyhole pressure pk for p = 0.1 and 1 bar in

dependence of keyhole diameter [32]. .................................................................................................. 16

Figure 15: Relation between depth of penetration with ambient pressure as a function of travel speed

for two different materials: 304 stainless steel (left) and 5052 aluminium (right) [33]. .......................... 17

Figure 16: Relationship between keyhole depths and laser absorptions during Type 304 stainless and

A5052 aluminium welding at 10 kW laser power with tightly focused spot diameter of 200 µm [34]. ... 18

Figure 17: Dependence of absorption on incidence angle for different polarizations [37]. ................... 18

Figure 18: Penetration depth versus power density for an interaction time of 10 ms (left) and 20 ms

(right). Beam diameter of 0.95 mm for the CW welds and a beam diameter of 0.9 mm for the PW welds

[43]. ........................................................................................................................................................ 19

Figure 19: Effect of parameters on interaction with material [44]. ......................................................... 21

Figure 20: Penetration versus laser power for constant travel speeds in cm/min (left) and Penetration

versus welding speed for constant power levels (right) [3]. .................................................................. 22

Figure 21: Weld beads produced in A5083 plate with 10 kW fibre laser beam of 200 mm spot diameter

at various welding speeds [52]. ............................................................................................................. 24

Figure 22: Macrographs of bead-on-plate welds for 0.78 mm beam diameter, combinations of

parameters required for 5 mm depth of penetration a) PL = 2 kW, v = 0.3 m min-1; b) PL = 5 kW, v= 2 m

min-1; c) PL = 8 kW, v = 5 m min-1 [44]. .................................................................................................. 25

X

Figure 23: Macrographs of bead-on-plate welds for 0.38 mm beam diameter, combinations of

parameters required for 6 mm depth of penetration a) PL = 2 kW, v = 0.3 m min-1; b) PL = 5 kW, v = 2 m

min-1; c) PL = 8 kW, v = 5 m min-1 [44]. .................................................................................................. 26

Figure 24: Effect of beam diameter on depth of penetration at different interaction times and a constant

power factor of 11 MW m-1 [44]. ............................................................................................................ 27

Figure 25: Required power factor for depths of penetration of 8 mm, 6 mm and 4 mm as a function of

interaction time (range of beam diameters 0.38 mm to 0.78 mm) [44]. ................................................ 27

Figure 26: Depth of penetration as a function of interaction time at 10 MW m-1 power factor, for two beam

diameters of 0.5 mm and 0.78 mm [44]. ................................................................................................ 28

Figure 27: Macrographs achieved at a constant power factor of 10 MW m-1 and various interaction times

with two beam diameters of 0.5 mm and 0.78 mm [44]. ....................................................................... 29

Figure 28: Parameter selection chart of the power factor model [44]. .................................................. 30

Figure 29: Effect of beam diameter at constant power factor of 10 MW/m for two beam diameters 0.5

mm and 0.78 mm at two extreme cases of interaction time: 6 ms and 100 ms [44]. ............................ 31

Figure 30: Schematics of the measures of the plates used and direction of the welding. .................... 33

Figure 31: Optical head tilted relative to the vertical axis. ..................................................................... 34

Figure 32: Experimental set-up. ............................................................................................................ 35

Figure 33: Clamping system. ................................................................................................................. 36

Figure 34: Shielding device and its position dimensions. ...................................................................... 36

Figure 35: Metallic foam used inside the shielding gas nozzle. ............................................................ 37

Figure 36: Robot control console. .......................................................................................................... 37

Figure 37: Experimental set-up for laser characterization. .................................................................... 37

Figure 38: Intensity distribution and beam profile of the fibre laser for 0.75 mm beam diameter. ........ 38

Figure 39: Positions where the welds were cross-sectioned................................................................. 39

Figure 40: Effect of power density and interaction time on the depth of penetration. ........................... 43

Figure 41: Macrographs for ti of 30 ms and qp of (a) 0.81 MW/cm2 and (b) 0.40 MW/cm2. .................. 43

Figure 42: Effect of graphite on the weld without shielding gas. ........................................................... 44

Figure 43: Macrographs without shielding gas for ti of 15 ms and power density of 0.81MW/cm2: (a) with

graphite and (b) without graphite. .......................................................................................................... 44

Figure 44: Effect of the shielding gas on the weld. ................................................................................ 45

Figure 45: Macrograph without graphite and no shield gas for ti of 30 ms. .......................................... 46

Figure 46: Macrograph without graphite and with shielding gas (20 l/min) for ti of 30 ms. ................... 46

Figure 47: Influence of the shielding gas flow rate on the weld............................................................. 47

Figure 48: Macrographs at 0.81 MW/cm2 power density (shielding gas flow rate: (a) 10 l/min; (b) 20 l/min;

(c) 30 l/min). ........................................................................................................................................... 47

Figure 49: Power factor for depths of penetration between 2 mm and 7 mm as function of interaction

time for a beam diameter of 0.61 mm. .................................................................................................. 49

Figure 50: Power factor for depths of penetration of 2 mm, 4 mm and 6 mm as function of interaction

time for a beam diameter of 0.61 mm. .................................................................................................. 49

XI

Figure 51: Effect of the beam diameter on the depth of penetration at different interaction times and a

constant power factor of 4.9 MW/m. ...................................................................................................... 51

Figure 52: Effect of the beam diameter on the depth of penetration at different interaction times and a

constant power factor of 6.6 MW/m. ...................................................................................................... 51

Figure 53: Effect of the beam diameter on the depth of penetration at different interaction times and a

constant power factor of 8.2 MW/m. ...................................................................................................... 52

Figure 54: Deviation of the depth of penetration for different beam diameters at different interaction times

and a constant power factor of 8.2 MW/m. ............................................................................................ 52

Figure 55: Longitudinal cuts for a power factor of 8.2 MW/m: (a) ti = 9.8 ms, d = 0.75 mm; (b) ti = 7.3 ms,

d = 0.49 mm). ........................................................................................................................................ 53

Figure 56: Transversal cuts for a power factor of 8.2 MW/m: (a) ti = 9.8 ms, d = 0.75 mm; (b) ti = 7.3 ms,

d = 0.49 mm. .......................................................................................................................................... 53

Figure 57: Power factor for depths of penetration of 2 mm, 4 mm and 6 mm as function of interaction

time for beam diameters of 0.49 mm, 0.61 mm and 0.75 mm. ............................................................. 54

Figure 58: Deviation of the depth of penetration for mild steel using different beam diameters. .......... 55

Figure 59: Macrographs of bead-on-plate for aluminium welds for 0.75 mm beam diameter, combinations

of parameters required for 4 mm depth of penetration a) ti = 29 ms, PF = 4.9 MW/m; b) ti = 14.7 ms, PF

= 6.6 MW/m; c) ti = 7.3 ms, PF = 8.2 MW/m. ........................................................................................ 56

Figure 60: Macrographs of bead-on-plate aluminium welds for 0.61 mm beam diameter, combinations

of parameters required for 6 mm depth of penetration a) ti = 29 ms, PF = 4.9 MW/m; b) ti = 14.7 ms, PF

= 6.6 MW/m; c) ti = 9.8 ms, PF =9.8 MW/m. .......................................................................................... 56

Figure 61: Comparison of the trend lines of the power factor between aluminium, titanium and mild steel

for a depth of penetration of 4 mm. ....................................................................................................... 58

Figure 62: Comparison of the trend lines of the power factor model between aluminium and mild steel

for a depth of penetration of 6 mm. ....................................................................................................... 58

Figure 63: Effect of power density and specific point energy on the depth of penetration for different

materials. ............................................................................................................................................... 60

Figure 64: Dependence of depth of penetration with the power density and specific point energy for ti =

7.3 ms using different beam diameters. ................................................................................................ 60

XII

XIII

List of tables

Table 1: Main characteristics of laser welding [2].................................................................................... 6

Table 2: Advantages and disadvantages of keyhole and conduction laser welding [29]........................ 14

Table 3: Parameters used to characterize laser welding [45….............................................................. 20

Table 4: Variation of fundamental laser interaction parameters with beam diameter at 10 MW/m power

factor of and three different interaction times of 100 ms, 15 ms and 6 ms [45]….................................. 31

Table 5: Chemical composition of aluminium 5083. .............................................................................. 33

Table 6: Beam properties of different optical set-ups. ........................................................................... 34

Table 7: Laser system parameters. ....................................................................................................... 39

Table 8: Calculated parameters. ........................................................................................................... 39

Table 9: Fundamental material interaction parameters. ........................................................................ 40

Table 10: Laser system parameters. ..................................................................................................... 40

Table 11: Calculated parameter. ........................................................................................................... 40

Table 12: Fundamental material interaction parameters. ...................................................................... 41

XIV

XV

Abbreviations

As cross sectional area of the laser spot on the surface

α thermal diffusivity

BPP beam parameter product

BS beam splitter

CW continuous wave

CO carbon monoxide

CO2 carbon dioxide

dwaist diameter of the laser beam at the laser source

df diameter of laser the beam at the focal point

df keyhole diameter

d diameter of beam on the surface of workpiece

ESP specific point energy

Ed energy density

Eut energy per unit thickness

F focal length

HAZ heat affected zone

HI heat input

k thermal conductivity

λ wavelength

M2 definition of beam diameter

Nd:YAG neodymium dopped yttrium aluminium garnet

p dynamic pressure

p∞ ambient pressure

Pe Peclet number

PD depth of penetration

PF power factor

pk pressure inside the keyhole

PL Laser power

Pr recoil pressure

ps closing pressure

PW pulsed wave

qp power density

RD reference detector

S heat function

SD signal detector

ti interaction time

TS translational stage

XVI

Θ divergence angle

v travel speed

X normalised power input

Y normalised speed-weld width

1

1. Introduction

1.1 Background Joining processes and procedures have had rapid development with new applications and new

materials. The heat source for laser welding may be a carbon dioxide gas (CO2) laser, YAG laser, diode

laser (LD), LD pumped solid laser, fibre laser and disc laser. Recently there has been a rapid increase

in the use of the fibre laser, which has the flexibility of the laser being conducted by fibre and can be

miniaturized with high beam quality and high efficiency, as a welding heat source with increased power

and high power density [1]. The new multi-kilowatt fibre lasers have other advantages, including: robust

setup for mobile applications, small beam focus diameter; high productivity and low distortion.

High power fibre lasers can be used for deep penetration welding in a diversity of materials due to their

capability to be narrowly focused, and the fibre delivery system provides the necessary flexibility on the

positioning of the beam. Due to its non-contact character, laser technology has a great potential which

is revolutionising manufacturing in micro-joining, marking, hardening, brazing, deep penetration welding

and cutting.

The solution-integration between the machining and laser companies and the well-developed worldwide

laser centres network, led to new developments in an industrial laser scenario. With an increasing

availability of new laser sources and a steady decrease of the prices of the workstations, there was a

high demand for lasers resulting in an increase in volume production [62]. Laser welding is a suitable

joining process for the construction of aluminium structures, such as bridges, buildings and transport

industry, which includes automobiles, planes, trains and ships. Laser welding aluminium is a promising

process because of the high laser beam energy density and a consequent high ratio penetration/width,

when compared to other fusion welding processes. It also has a very precise control resulting in high

thermal gradients and thus very small heat affected zones [48, 51, 62]. However, due to its high

reflectivity and superior thermal conductivity, aluminium is a challenge in laser processing [34].

The versatility of lasers is demonstrated by the application on different materials with a flexibility in terms

of the energy delivered to the work piece. Different combinations of power and travel speed give different

power density, interaction time and specific point energy, which are responsible for the depth of

penetration. Depending on the processing conditions, there are two different operational regimes;

conduction mode and keyhole mode. The last one is an unstable process, used for deep penetration

depth and a relatively small heat affected zone, whilst for conduction laser welding no vaporization takes

place, making this a very stable process that allows control of the heat delivered to the work piece.

2

Laser keyhole welding or cutting requires intense vaporisation of metal which is achieved by a very small

beam diameter which provides a high power density. However, other applications, such as surface

treatment or brazing, require a larger beam diameter either to process a larger surface or to have lower

power density and avoid vaporisation [1].

Figure 1: Schematic representation of the power factor model selection parameters.

Achieving a certain weld profile is a very hard task due to the large number of variables in the welding

process. Many parameters can affect the depth of penetration such as beam diameter, laser power and

travel speed, however each combination result in a different weld profile, which may affect the

mechanical properties of the joint. Based on requirements from the laser user, it is possible to achieve

a desired depth of penetration and weld width in any laser system.

The power factor model, developed for keyhole mode, gives the optimal laser parameters settings for

laser power and travel speed for a given beam diameter, as shown in Figure 1. It was previously

developed for mild steel and titanium at Cranfield University [44]. Nevertheless it is necessary to prove

the model functionality for other materials, such as aluminium.

The extension of the power factor for aluminium is the purpose of this thesis. It is intended to create a

system of welding parameters which gives the user the correct values of laser power and travel speed

according the user specifications in terms of depth of penetration and weld width. The depth of

penetration will be controlled by power density and specific point energy whilst the weld width will be

controlled by the interaction time. The model would enable to achieve the same weld geometry using

3

different laser systems. Comparing various materials is also necessary to analyse the model influence

on them.

1.2 Thesis Objectives

This project, developed at Cranfield University, is focused on extending a previous developed model for

a new material. The influence of the beam diameter, travel speed and laser power were researched.

The motivation behind this MSc thesis was the possibility of add to the knowledge to an already

established model, proving its functionality for aluminium.

Since the laser processing is more difficult for aluminium than for mild steel, the extension of the power

factor model to aluminium was a very challenging goal, being also necessary prevent welding defects

such as porosity, keeping always a good weld quality. The main focus is to find parameters which allow

a given depth of penetration to be independent of the beam diameter. This was achieved through

several steps leading to the model functionality to the new material:

• Understanding the influence of graphite, shielding gas type and flow rate on the depth of

penetration and on the weld quality;

• Find the fundamental laser material interaction parameters which gives only a keyhole mode;

• Understanding how the laser welding parameters of power density, interaction time and specific

point energy change the weld geometry;

• Determine the power factor versus interaction time trend lines for three different depths of

penetration in aluminium;

• Validate the model achieving the same depth of penetration for different beam diameters,

keeping constant the power factor and the interaction time;

• Compare the power factor model results of different materials and analyse how they react to

the same power factor and interaction time values;

4

1.3 Thesis structure

The thesis is structured in 5 chapters. Its format is as follows:

Chapter 2 begins by reviewing the current state of the art and literature of laser welding and its variants.

Some of the current knowledge of keyhole laser welding mode and the different parameters involved in

process are shown and discussed in this chapter. The current knowledge of the power factor model,

which is behind the motivation of this thesis, is also reviewed.

Chapter 3 consists in the experimental approach. The equipment, the main characteristics of the material

used the correct procedure to prepare the samples during the experiments and the methodology of all

experiments of this project is explained in detail. The experimental set-up for laser beam characterization

with its correspondent software is also shown.

Chapter 4 shows the main results obtained during preliminary experiments to achieve the best

parameters to use on the next chapter. The effects of graphite, shielding gas type and flow rate are

analysed. It also looks into the parameter selection criteria and the issue of data transferability between

different laser systems using a power factor model. The results are compared and discussed amongst

different materials.

The thesis is closed-up in chapter 5, which summarizes the overall conclusions from this work, followed

by suggestions for expanding this research in the future.

5

2. Literature review

2.1 Historical Note

Light Amplification by Stimulated Emission of Radiation (Laser) is, as the name refers, a process that

transforms light radiation into heat which is dependent of the interaction of the laser with the material.

In 1917, studies conducted by Albert Einstein lead to the construction of the first laser. Although, only in

1960 the first laser was built, its inventor was Theodore Maimann, in the Hughes research laboratories

in California. The first laser was a solid ruby laser that used a mercury vapour fluorescent lamp.

Since industrial lasers were first introduced nearly 40 years ago, revolutionary developments in laser

welding technology have enabled new and innovative joining applications. In the early years, lasers

were used primarily for exotic applications; however advancements in the maximum power capacity and

the beam quality of lasers made deep-penetration keyhole welding possible and expanded the

applications. Pulsed Nd: YAG lasers that followed in the early 1990s provided means of welding highly

reflective materials such as aluminum and copper. Lamp-pumped Nd: YAG lasers reached the

multikilowatt level toward the end of the 1990s and made their entrance into high-production welding

and cutting applications. With the advancements, the use of lasers by the industry was increased by

fiber lasers, disk lasers and high power diode lasers [1, 2].

Figure 2: Laser welding of two plates in a butt joint configuration [10].

6

Table 1: Main characteristics of laser welding [2].

Laser beam welding is a fusion welding process where radiant energy is used to produce the heat

required to melt materials to be joined [1]. A concentrated beam of coherent, monochromatic light is

directed by optical system and focused to a small spot, for higher power density, on the abutting surfaces

of the parts to be joined (Figure 2). Gas shielding is generally used to prevent oxidation of the material

during welding. Different parts and even different metals can be joined in a non-contact process. The

required accessibility to the work piece being from one side only and the opportunity to abandon filling

material are the main advantages of this process. Welding can be performed using either pulsed or

continuous wave mode lasers. In Table 1 it is possible to see some other characteristics of laser welding

and how these characteristics influence the welds joints.

Figure 3: Estimated laser operating costs [11].

7

It is well know that the use of laser welding requires high investment costs but it is wise not forget that

the running costs will also play an important part in the production costs. A comparison of operation

costs for different laser systems it’s given in Figure 3 [11].

Today, laser welding is a full-fledged part of the metalworking industry, routinely producing welds for so

many industries, such as automotive [6, 11], shipbuilding [6], railway [6], aerospace [8, 11], medical

appliances [10] and so many others. The welding of metals was one of the first industrial applications of

lasers. This process is applied in a very broad variety of materials: Magnesium, titanium [12], aluminum

[10, 11], copper [13] and steel [12, 14].

2.2 Fibre Lasers

The first use of fibre lasers dates from 1960s for low power lasers. Currently, multi-kilowatt fibre lasers

have been introduced for materials processing. These new lasers have a lot of advantages: high

efficiency compared to lamp or diode pumped rod lasers; compact design, which simplifies installation;

good beam quality, due to the use of small diameters fibres, and thus small beam focus diameter; and

a robust setup for mobile applications. They also have several benefits for industrial purposes, namely

high power with low beam divergence, flexible beam delivery and low maintenance costs [14].

Figure 4: Schematic representation of a double-clad fibre laser [15].

8

Figure 5: Schematic presentation of a fibre laser architecture [16].

The concept of fibre lasers is based on an active gain medium of a fibre with a core of the fibre doped

with rare earth elements. Most commonly, this is a single-mode fibre laser made of silica. The pump

beam is launched longitudinally along the fibre length and it may be guided by either core itself, as

occurs for single-mode lasers or by an inner cladding around this core (double-clad fibre laser) as shown

in Figure 4 [15]. The outer cladding is made of a polymeric material with low refraction index to minimise

attenuation (Figure 5) [16]. The resulting laser beam is essentially diffraction limited and when fitted with

an integral collimator, produces a parallel beam.

The high power fiber lasers in the market are based on the active fibers with a patented pumping

technique that allows the utilization of broad area multimode diodes rather than diode bars [15].

A high beam quality is crucial when one intends to focus the laser beam to a small size point to achieve

higher power density. It is characterized by the good focus ability and larger depth of field. The quality

of the laser beam is a measure of how the beam can be strongly focused in certain conditions. The most

commonly used tools to access the beam quality are trough the Beam Parameter Product (BPP) and

𝑀2, defined as follows:

2.2

Where 𝑑𝑤𝑎𝑖𝑠𝑡 (Figure 6) is the beam diameter in waist and Θ is the full divergence angle. The 𝑀2 factor,

also referred to as factor beam quality or beam propagation factor, is currently the most common

measure of laser beam quality. According to ISO standard 11146:1999 if is defined as the ratio of BPP

and BPP0 which is the beam parameter product for a diffraction-limited Gaussian beam, with the same

length wave, λ.

9

Figure 6: Beam divergence.

Figure 7: Definition of M2 [45].

The 𝑀2 definition (Figure 7) of the beam quality is usually used for comparison of lasers with different

wavelengths. A beam has limited diffraction when it corresponds to 𝑀2 = 1, and is a Gaussian beam.

This value can be achieved by conventional solid state lasers operating at a low-power single transverse

spatial mode, for fiber lasers based on single-mode fibers, and some low-power laser diodes. However,

some high-power lasers may have very high values of 𝑀2, often above 100 and in some cases very

close to 1000. In conventional solid-state lasers, this high value is often the result of distortions of

thermally induced wave front among gain. In the case of high power semiconductor lasers, low beam

quality results from the operation with a highly multimode waveguide [17].

10

Figure 8: Effect of BPP on beam diameter and power density [44].

The dependency of beam diameter and power density with BPP is shown in Figure 8. A reduction of

BPP by a factor of two results in a decrease of beam diameter by a factor of two, for the same optical

set-up. This corresponds to an increase of power density by a factor of four. Thus lasers with a lower

BPP can be operated at lower powers to achieve the same power density.

Wavelengths also provide information about beam quality as, at shorter wavelengths, the more energetic

photons can be absorbed by a greater bound electron and so the reflectivity falls and the absorption of

the surfaces increases [18].

2.3 Measuring methods of laser beam properties

The average output power, beam diameter and intensity distribution are basic properties that have to be

measured in order to fully characterise the laser beam. Difficulties with measuring properties of laser

sources resulted in several problems, such as changes degradation of the optics. Any kind of

misalignment in the optical path or if the real beam diameter is unknown, the laser system have to be

subjected to a series of parametrical studies before it could be employed into applications. Nowadays

there are many devices to characterize lasers in terms of output power and beam propagation

properties.

11

2.3.1 Laser output power measurement

Various measuring techniques of laser power differ in terms of response time, accuracy and damage

threshold. The most common power meters used for high power continuous wave lasers are

calorimetric-based devices. They measure the temperature of a cooling medium, which attains the heat

from the laser beam absorbed in a highly absorbing chamber [19]. It’s possible to see a schematic

representation of the device in Figure 9.

Figure 9: Schematic representation of calorimetric power meter [19].

2.3.2 Laser beam diameter measurement

There are several methods of acquiring the optical intensity of a particular laser beam. The most popular

among these are the camera based sensors, slit cameras, knife-edge scanners and pinhole scanners

[20].These systems use a mechanical component which scans the beam across the plane perpendicular

to the beam propagation direction and records intensity with respect to the position of this mechanical

component. The possibility of measuring high power lasers beams directly is the main advantage of

these types of devices [20, 21].

12

Figure 10: Experimental setup (top view). (a) The BS, SD, RD, and TS stand for the beam

splitter, signal detector, reference detector and translational stage, respectively. (b) Illustration

of the transmitted energy S1 as a function of x and its derivative [22].

In the knife-edge method the power or intensity is analysed by means of a power meter or a photodiode,

whilst the beam is sliced with a razor. First, as a reference, the total energy is recorded when the entire

beam reaches the detector. Second, the energy is recorded with respect to the position of the razor,

which is continuously cutting-off the beam, until no energy can reach the sensor. Then the normalized

sensor response is plotted as a function of razor position and the error function is fitted to the data points,

as shown in Figure 10. Next, the slit or the razor is rotated by 90º along the propagation direction and

the measurement is carried out in a perpendicular direction.

2.4 Operational regimes in laser welding

The interaction of a laser beam with the workpiece result into three regimes, conduction limited, mixed

and keyhole mode. The key difference between these three modes is the power density and the

interaction time applied to the welding area. Conduction takes place when the intensity of the laser is

not sufficient to rise the material temperature above the vaporisation point. The surface is heated to a

temperature between melting and boiling and the amount of molten metal is determined by the balance

between the energy absorbed and the heat losses. Furthermore, the material properties including,

thermal conductivity, latent heat of melting, density, specific heat capacity, melting and vaporization

temperatures and latent heat of vaporisation determine the upper and lower limits for the conduction

mode. Aluminium and mild steel have a transition mode separating the conduction mode from the

keyhole mode. When the keyhole starts, the threshold power density for both materials is 0.5215

MW/cm2 [28] and when it impinges the material substrate surface starts to heat up. The surface

13

temperature quickly reaches the melting point and a molten layer is formed. Subsequently, vaporization

produces a recoil pressure that acts on the molten layer, forming a thin capillary (the so-called keyhole)

for the laser beam to be efficiently delivered into the material and consequently propagating the melt

front into the metal bulk. Spatter, which is the ejection of melt droplets from the weld pool, is a common

problem observed in this regime. Melt pool ejections in the form of spatter will result in an unsteady

appearance of the weld seam. Irregular weld surface features such as under-fill, under-cut, craters, and

blowouts act as stress raisers which can severely reduce the mechanical properties of a weld [24].

Figure 11: Schematic representation of the welding modes: conduction (left) and “keyhole”

(right) [27].

Keyhole mode laser welding is characterized by deep weld penetration and low energy. Because of

these advantages, many experimental and mathematical modeling studies have been undertaken on

the keyhole mode laser welding process to seek better understanding of the laser material interaction

parameters [25].The two main operational regimes are represented in Figure 11 and their advantages

and disadvantages are shown in Table 2. The mixed mode regime occurs when the power density is

enough to heat the material and just above the vaporization threshold the keyhole is not stable and does

not extend beyond the melt front. Detecting the transition between these two welding modes is a crucial

topic for the quality monitoring of the process. In fact, the weld bead geometry, such as the width and

the penetration depth, are key factors in estimating weld quality. Indeed, insufficient penetration reduces

weld strength resulting in a poor weld quality. Moreover, the instability and eventual collapse of the

keyhole is considered as a possible cause of porosity in laser welding of Al alloys due to the entrapment

of vaporized alloying elements or shielding gas [26].

14

Process Advantages Disadvantages

Keyhole laser welding

High productivity

Deep penetration

welds with a high

aspect ratio

Low distortion

Narrow HAZ

High amount of

spatter

Unstable process

High levels of porosity

Degradation of

mechanical properties

Loss of alloying

elements

Conduction laser welding No porosity, no

cracking and no

undercut on the welds

No spatter

Stable process

Control of the heat

input

Applicable to laser

systems with any

beam quality

Good gap bridging

ability due to the large

beams used

High energy

Slow process

Low productivity

Low coupling

efficiency

Table 2: Advantages and disadvantages of keyhole and conduction laser welding [28].

2.5 Controlling depth of penetration in laser welding

Depth of penetration because it is a key parameter in laser welding. This parameter is determined by

the balance of many aspects: melt flow dynamics [23, 24, 29-31], pressure [32, 33] and absorption

conditions [34-37].

15

2.5.1 Plasma plume effect

An intensive vaporization may be induced by the interaction of a high power density laser beam with the

workpiece. This causes a vapour cloud suspended over the laser-material interaction point, which can

reduce the efficiency of the laser process by reducing the amount of energy absorbed by the material.

Figure 12: Visible keyhole area for 1, 4 and 7 m/min feed rates on one 5 mm stainless steel

plate. The dashed lines show the trend of the keyhole opening area [31].

Figure 13: The plume inclination and the radiance change with rising laser power: 1.5 kW (left)

and 3.5 kW (right) [31].

R. Fabbro et al. published a study where correlations between the plasma plume and the melt pool of

liquid metal are shown [29]. However, the melt pool is just an indicator for a process situation which has

already evolved in the keyhole. It is therefore not possible to prevent process instabilities that occur on

short time scales by detecting the melt pool characteristics. For this purpose, monitoring of the keyhole

is better suited. The results of J. Wang et al. indicate that the periodical oscillation of the vapour plume

can be attributed to the oscillation of the keyhole during high-power fibre laser welding [30]. Tenner et

16

al. studied the dynamic behaviour as well as the correlations between the keyhole and the plasma plume

using two high- speed cameras [31].They found a strong dependency of the size of the keyhole’s visible

top area on the applied processing speed (Figure 12). They also found out that the dynamic behaviour

of the keyhole and the plume is well correlated, especially for high laser power and feed rates. It could

be observed that the trend of both, the keyhole and the plasma plume behaviour, changes when

reaching a threshold laser power. The vapour plume inclines in direction of the melt pool as the

penetration depth increases (Figure 13). B. Brock et al. [23] also observed too that the stability of the

vapour plume position responds to the occurrence of welding defects such as spatter formation or the

formation of holes in the weld seam. These findings indicate that information on the keyhole geometry

can be obtained by the evaluation of the shape of the vapour plume during the welding process. The

identified correlations may allow for the implementation of a closed-loop control system based on

detection of the vapour plume position.

2.5.2 Effect of pressure

During laser keyhole welding a surplus pressure has to act at the keyhole front which drives the molten

material around the keyhole. Depending on the travel speed and keyhole diameter, the pressure can

reach several bar and may, therefore, represent a significant contribution to the pressure balance in the

keyhole. P. Berger et al. concluded that at very small diameters, pk (pressure inside the keyhole) is

dominated by the ps (closing pressure). At large keyhole diameters, the keyhole pressure is determined

by the ambient pressure because ps is of minor importance here. Since, for a given v (travel speed), p

( dynamic pressure) increases with df (keyhole diameter) the influence of flow around the keyhole will

be more pronounced at higher df which means that here already at lower values of v a deviation from

axisymmetric equilibrium conditions will occur. At larger diameters, therefore, a reduction of 𝒑∞ (ambient

pressure) will enhance the effect of p (Figure 14) [32].

Figure 14: Dynamic pressure p, closing pressure ps and keyhole pressure pk for p = 0.1 and 1

bar in dependence of keyhole diameter [32].

17

Figure 15: Relation between depth of penetration with ambient pressure as a function of travel

speed for two different materials: 304 stainless steel (left) and 5052 aluminium (right) [33].

Abe Y. et al. [33] studied the correlation between penetration depth and the reduction of ambient

pressure as a function of travel speed for two different materials. They concluded that the highest depth

of penetration was evident at low travel speeds and is inversely proportional to the ambient pressure.

As the travel speed exceeded 3 m/min the depth of penetration of the weld produced in vacuum was

similar to that obtained at the atmospheric conditions, as shown in Figure 15.

2.5.3 Absorption effect

A high-power fibre laser can produce deep penetration welds. High power and the corresponding power

density can easily generate a deep keyhole in the molten pool, and the laser beam is expected to be

efficiently absorbed. Y. Kawahito et al. [34] studied the relationships between keyhole depth and laser

absorption between Type 304 stainless steel and A5052 aluminium alloy. Their laser absorptivity as

improved by increasing the keyhole depth. The improvement appears to be dependent on the increase

in multi-reflections of the incident beam inside the keyhole (Figure 16). Fresnel equations describe how

much energy is absorbed when an electromagnetic wave, such as laser beam heats a surface. The

amount of absorbed light is dependent on the wavelength, polarization direction and incidence angle.

A. Kaplan [35] calculated the keyhole geometry based on the Fresnel model and found that the front

wall of the keyhole tended to a specific angle, at which a sufficient amount of energy could be intercepted

to balance the heat losses. This determined the wall formation. When the processing speed was

increased the keyhole angle and absorption changed accordingly to the new conditions (Figure 17). The

amount of laser energy absorbed in a side wall of keyhole increases, but the energy at the bottom of

keyhole decreases with increasing travel speed, hence the depth of penetration reduces [36].

18

Figure 16: Relationship between keyhole depths and laser absorptions during Type 304

stainless and A5052 aluminium welding at 10 kW laser power with tightly focused spot

diameter of 200 µm [34].

Figure 17: Dependence of absorption on incidence angle for different polarizations [37].

2.6 Different types of laser

There are two main types of lasers, continuous wave (CW) and pulsed wave (PW) [38]. Laser welding

technology with continuous wave (CW) mode has such advantages as easy controlling, small

heataffected zone and deformation, high level of automatization and control precision. However, the

defects such as blowhole have frequently been observed in the joints when non full penetration welding

was carried out. That decreases the strength of the welded joint. The deep penetration welding can be

achieved by PW laser at high peak power. Such laser beam can make welding pool intensively flow up

and down when operated at available parameter so that the gas bubbles can be easily driven out from

the welding pool [39]. The normal parameters used in PW laser welding are pulse energy, pulse duration,

beam diameter and average peak power [40]. On the other hand the parameters normally used in CW

laser welding are welding speed, power and beam diameter [41]. P.

19

Fuershbach et al. made a comparison between a pulsed and a continuous wave laser weld without using

any parameter which gives a correlation between pulse duration and welding speed used in the CW

welds, concluding that CW lasers show higher penetration than PW lasers. In this study the effect of

interaction time of the laser beam with the material was neglected, only power density was used for

comparison [42].

Figure 18: Penetration depth versus power density for an interaction time of 10 ms (left) and 20

ms (right). Beam diameter of 0.95 mm for the CW welds and a beam diameter of 0.9 mm for the

PW welds [43].

E. Assunção et al. [43] investigated the fundamental material interaction parameters, power density and

interaction time, and their influence on CW and PW lasers welds. They found a linear relationship

between power density and penetration (Figure 18). For the same power density the PW laser welds

have deeper penetration. However, the difference between the penetration depth in CW and PW is more

evident in the keyhole regime and it is even bigger for shorter interaction times as shown in Figure 18

for 10 and 20 ms of interaction time. This difference in penetration depth results from the added force

on the keyhole produced by the movement of the weld pool which leads to a delay of the onset of the

keyhole and extends the transition mode region. The level of the opposing force will depend on the

welding velocity and the interaction time depends inversely on welding velocity. For shorter interaction

times, the closure force will be higher, which means higher penetration depth difference.

20

2.7 Fundamental Laser-material interaction parameters in laser welding

Depth of penetration and weld width are the most critical parameters in keyhole laser welding which

users need to adjust. Many combinations of travel speed and laser power can be used to reach a

required depth of penetration for a given beam diameter. On the other hand for a given combination of

travel speed and power, if different beam diameters are used, different depths of penetration are

obtained. The laser beam diameter is dependent of the laser properties and the optical system, for this

reason it is expected that different laser systems have different beam diameters [1]. This causes many

problems in transferring parameters between laser systems and in the selection of the optimum laser

parameters. Laser practitioners use a simple approach to choose system parameters, such as laser

output power and travel speed, based on the trial and error method due to different phenomena affecting

depth of penetration. Due to the unique beam diameter, the system parameter approach makes the

process dependent on the particular laser system.

Table 3: Parameters used to characterize laser welding [44].

An improvement of data transferability between laser systems has been attempted by many authors and

most of parameters are summarized in Table 3.

21

Figure 19: Effect of parameters on interaction with material [44].

It’s intended to determine basic parameters which control the laser welding in order to understand their

interactions between the system parameters set on a given welding system and the response in the

workpiece. In Figure 19 the effect of system parameters on the material in laser processing is

demonstrated.

Power density and interaction time can distinguish various laser processes. The interaction of a laser

beam with a workpiece is determined by the size and shape of the heat source, time of irradiation and

the power density. The average power density 𝒒𝒑, for a top-hat energy distribution laser beam, is defined

by the ratio of the laser power 𝑷𝑳 to the area of laser spot on the surface 𝑨𝒔, and it is given by the

Equation 2.3.

While the beam is moving with a constant speed, interaction time defines the time in which a specific

point in the weld centreline is exposed to the laser beam considering a circular laser beam. Due to the

reduction of beam length as we move from the weld centreline, the interaction time may vary across the

beam. Equation 2.4 defines the interaction time 𝒕𝒊 in case of a particular beam with a diameter d, which

travels with a welding speed v.

2.4

Laser processing is also characterized by a third parameter because the product of interaction time by

power density determines the energy density, which is independent of the laser beam. This means that

if the same energy is used with different beam diameters the total energy delivered to the workpiece

22

would be different. This energy delivered through the laser spot is called specific point energy, , and it

is proportional to the product of laser spot area on the material surface, 𝑨𝒔, interaction time 𝒕𝒊 and the

power density 𝒒𝒑, which also corresponds to the product of 𝒕𝒊 and the absorbed laser power 𝑷𝑳. This

definition assumes a constant interaction time across the weld centreline in the transverse direction to

weld centreline and a uniform intensity distribution, which is only true rectangular top-hat or square

beams. Thus it is relevant for relatively small beam diameters. The definition is given by Equation 2.5.

Intensity, specific point energy and interaction time define the conditions of any laser processing system

and in welding, these parameters determine the thermal field and the resultant molten volume, cooling

rate, microstructure changes and other properties.

2.5

Figure 20: Penetration versus laser power for constant travel speeds in cm/min (left) and

Penetration versus welding speed for constant power levels (right) [3].

L. Quintino et al. [3] and K. Hansen et al. [45] studied the relation between some parameters with depth

of penetration for a constant beam diameter. They analyzed the behavior of weld bead penetration as a

function of laser power and welding speed, represented in Figure 20. It’s possible to observe that the

weld depth has almost a linear relationship with the laser power and decreases exponentially with the

welding speed. W. Suder et al. did a similar study, however they compared two beam diameters for the

same experience and concluded that dividing the beam diameter by half resulted in a small increase in

penetration depth. This increases the power density by a factor of four but simultaneously reduces the

interaction time by a factor of two [46]. The result also explains that as the beam moves out of focus the

intensity reduces but the interaction time increases and compensates the large depth of focus seen in

laser welding [47]. However, these parameters cannot be used as an application model, because there

is a unique combination of the laser system parameters for any beam diameter. In laser applications the

beam diameter is a parameter often fixed for a given laser system.

23

2.8 Laser welding of aluminium

Nowadays aluminium is a very common and important material which is used in many different

applications. The biggest market for aluminium is the application in packaging and containers. This

material is easily welded and this fact allows its application in bridges, buildings and transport industry,

which includes automobiles, planes, trains and ships [48].

Aluminium alloys used in manufacturing, which includes thin and thick sheet and extruded rod and wire,

are classified according their alloying elements. Magnesium is the main element added to 5xxx

aluminium series alloys and its percentage ranges between 0.2 and 6.2 wt%, which means that these

alloys are not heat treated and show the higher mechanical strength. These alloys series with a

magnesium percentage above 3 wt% is not recommended to be used in applications above 65ºC due

to their susceptibility to corrosion under tension [49, 50].

Laser welding of aluminium is a very promising process because it permits high productivity, high quality

welds, high travel speeds, high depth of penetration, low distortion, low energy and high process

automation. This process applied to this material has a high potential due to high laser beam energy

density and to a consequent high penetration/width ratio, comparatively with other fusion welding

processes [51].

Y. Kawahito et al. [34] studied the A5052 aluminium alloy, which is problematic in laser processing due

to its high reflectivity and superior thermal conductivity. The authors concluded that whilst the welding

speed increases, the penetration depth and the welding fusion zone decreases and the weld bead

becomes much thinner. S. Katayama et al. [52] investigated the effects of various welding conditions on

penetration and defect formation on several aluminium alloys. They found that aluminium type 1000 has

lower penetration depth comparing to type 5000 and 7000 because it has a greater reflectivity and

thermal conductivity and lower vapour pressure. Type 5000 and 7000 alloys contain magnesium and

zinc, respectively, which have high vapour pressures, and the vapour recoil force increases with the

content of these. The results of X-ray inspection showed porosities in all aluminium alloys, but the largest

number of porosities occurred in type 5000 and 7000, which have a low boiling point.

24

Figure 21: Weld beads produced in A5083 plate with 10 kW fibre laser beam of 200 mm spot

diameter at various welding speeds [52].

In Figure 21 at a speed of 3 m/min, the appearance of the weld bead is relatively stable, the bead width

was reduced to 3.5mm, and a deep penetration of 13 mm was obtained. When the welding speed was

increased to the high speed, a keyhole penetration of at least 6mm in depth was obtained. Even when

the welding speed was increased to 20 m/min, the porosities decreased in size but couldn’t be

eliminated. It was clear that the penetration of the weld bead became deeper as the spot diameter

became smaller and the power density increased. For a constant speed, when the spot diameter was

increased to 360 and 560 µm, the power density decreased, the penetration became shallower and the

bead surface became very rough and irregular. On the other hand, when the spot diameter was

increased to 1500 µm, the penetration became very shallow and the weld bead surface was relatively

smooth.

S. Katayama et al. [52] also tested the influence of the shielding gas flow rate on aluminium. The flow

rate was changed from 15 l/min to 50 l/min, using a bigger gas nozzle. It was achieved a smooth weld

bead surface with a good surface appearance. However the incidence of porosities did not improve

notably but undercut was inhibited.

Due to the high reflectivity of aluminium, E. Assunção [28] coated the surface of the plates with graphite

in order to improve the coupling of the laser beam to the material. It was found that changes occur in

absorptivity with travel speed due to the pre-heating of the material and thus vaporization of the graphite

coating in front of the laser beam. This means that a slower heat source causes more efficient pre-

heating of the material ahead of the beam. This likely destroys the graphite coating, thus reduces the

absorptivity.

25

2.9 Power Factor

The necessity to have a set of parameters, which could be applied on various beam diameters and

therefore different laser systems, motivated some authors to investigate if it is possible to find a

phenomenological model of parameters, which would specify depth and gives a relative value of weld

width of the fusion zone in laser welding, independent of the beam diameter. W. Suder [46] concluded

that for a given interaction time the depth of penetration, in keyhole regime, is proportional to the product

of power density and beam diameter, which is consistent with the results reported by P. Fuershbach et

al. [42]. Power factor model determines the system parameters which should be used in order to achieve

a suitable weld in terms of depth penetration and weld width for a particular application in any laser

system, with different beam diameters. Power factor, 𝑷𝑭, it is defined by the product of power density 𝒒𝒑

and beam diameter d, which also corresponds to the ratio of the laser power P to the beam diameter d

and is given by Equation 2.6.

2.6

2.9.1 Effect of the system parameters on the weld bead geometry

W. Suder investigated the effect of the system parameters (power, travel speed and beam diameter) on

the shape of the laser welds. The experimental results in Figure 22 and Figure 23 show that the weld

width decreases with increasing travel speed at a constant beam diameter despite the fact the laser

power was increased with the travel speed to maintain the same depth of penetration. Paying attention

to the weld defects, for slow speeds and for both beam diameters, a large heat affected zone and severe

porosity are observed.

Figure 22: Macrographs of bead-on-plate welds for 0.78 mm beam diameter, combinations of

parameters required for 5 mm depth of penetration a) PL = 2 kW, v = 0.3 m min-1; b) PL = 5 kW,

v= 2 m min-1; c) PL = 8 kW, v = 5 m min-1 [44].

26

Figure 23: Macrographs of bead-on-plate welds for 0.38 mm beam diameter, combinations of

parameters required for 6 mm depth of penetration a) PL = 2 kW, v = 0.3 m min-1; b) PL = 5 kW, v

= 2 m min-1; c) PL = 8 kW, v = 5 m min-1 [44].

In the experiments, the author also shows that the width of the welds is almost independent of the beam

diameter. However higher laser power was required to achieve the same depth of penetration, as the

beam diameter increased at a given travel speed. The effect of the beam diameter on the weld width is

more significant at higher travel speeds and all welds with a higher beam diameter are significantly wider

for the same travel speed and comparable depths of penetration. It was also shown the same depth of

penetration can be achieved by different beam diameters and travel speeds.

2.9.2 Effect of the beam diameter on the weld bead geometry

A constant depth of penetration at any interaction time is demonstrated in Figure 24 for four different

beam diameters, having a maximum difference of 10% of total depth of penetration. It was concluded

that the power factor and interaction time allow a particular depth of penetration to be reached with any

beam diameter.

27

Figure 24: Effect of beam diameter on depth of penetration at different interaction times and a

constant power factor of 11 MW m-1 [44].

2.9.3 Depth of penetration

It can be seen in Figure 25 that for all beam diameters all data follow the same trend. Figure 25 shows

the curves of the power factor model for different weld depths. A small beam diameter or a high laser

power corresponds to a high power factor. The grater the power factor the shorter the interaction time

that needs to be applied in order to achieve a particular depth of penetration. A suitable power factor

can be selected depending on the interaction time and required depth of penetration.

Figure 25: Required power factor for depths of penetration of 8 mm, 6 mm and 4 mm as a

function of interaction time (range of beam diameters 0.38 mm to 0.78 mm) [44].

28

2.9.4 Sensitivity analysis

The sensitivity analysis of the power factor is necessary to attribute the limits of the model. W. Suder

based on the results showed in Figure 26, concluded that the effect of the beam diameter seems to be

small in the entire range of interaction times. Below 10 ms of interaction time, the difference in depth of

penetration between the beam diameters approximates 25% of the maximum depth. From 10 ms to 100

ms of interaction time this difference doesn’t exceed 10%. The author observed higher depths of

penetration for a larger beam diameter in a range of interaction times above 15 ms. However, higher

depths of penetration are achieved with a smaller beam diameter as the interaction time decreases

below 15 ms. These results are shown on macrographs of the corresponding welds in Figure 27.

Figure 26: Depth of penetration as a function of interaction time at 10 MW m-1 power factor, for

two beam diameters of 0.5 mm and 0.78 mm [44].

29

Figure 27: Macrographs achieved at a constant power factor of 10 MW m-1 and various

interaction times with two beam diameters of 0.5 mm and 0.78 mm [44].

2.9.5 Power factor – application model

The power factor and the interaction time are proportional to the depth of penetration under certain

conditions, and it is almost independent of the beam diameter, which means this model can be applied

to different laser systems with different beam diameters. Based on the interaction time, which is

dependent of the beam diameter and travel speed, a parameter selection model for the laser for the

laser keyhole welding process is outlined in Figure 28, which suggests the beam diameter and the output

power required for the process.

According to the model, the interaction time is selected according to the quality or the weld profile

requirement. Second the travel speed must be selected based on the required productivity. The model

will then suggest the beam diameter based on the selected combination of travel speed and interaction

time. There is only one beam diameter for every particular combination of these two parameters, and if

it is unrealistic, then another travel speed or interaction time has to be selected. The next step is to

determine the power factor which is selected to accommodate a particular material thickness and an

appropriate laser power is selected by the model. However, if it is unrealistic, a longer interaction time

has to be used. An alternative way of using the model is, if the beam diameter is fixed then the interaction

30

time determines the travel speed, or vice-versa, while the depth of penetration, which needs to be

achieved, will be dependent on the power factor.

In this model the user decides and tries to reach the type of weld which is intended for a particular

application. The appropriate combination of interaction time and power factor can be transferred into the

system parameter and achieved on various laser systems with various beam diameters, which is useful

in selecting a laser system.

Figure 28: Parameter selection chart of the power factor model [44].

2.9.6 Limitation of the power factor

Despite a constant power factor and interaction time, there are discrepancies in depth of penetration

between different beam diameters. At short interaction times this difference reached up to 25%, which

means that the power factor model becomes slightly dependent on the beam diameter as the interaction

time decreases below 10 ms. However at very long interaction times this difference is of the order of the

experimental error, it did not exceed 10% of the maximum depth of penetration (Figure 24). The power

factor is still by far less sensitive to the variation of beam diameter than the laser power and travel speed.

The specific point energy and the power density control the depth of penetration in laser welding and it

is linearly proportional to the beam diameter, which explains why the power factor works. The depth of

penetration is maintained constant when the beam diameter is changed by using a trade-off between

the power density and the specific point energy. However, this trade-off led up to 25% difference in

depth of penetration in some conditions.

31

Table 4: Variation of fundamental laser material interaction parameters with beam diameter at

10 MW/m power factor of and three different interaction times of 100 ms, 15 ms and 6 ms [44].

Figure 29: Effect of beam diameter at constant power factor of 10 MW/m for two beam

diameters 0.5 mm and 0.78 mm at two extreme cases of interaction time: 6 ms and 100 ms [44].

Table 4 shows that, at a constant power factor and interaction time, increasing the beam diameter

induces a reduction of power density and an increase of specific point energy.

The trade-off between the power density and the specific point energy provides a constant depth of

penetration as long as at medium range of interaction times. This trade-off has its optimum operating

range, where the increase of specific point energy almost equally compensates for the drop of power

density. The optimum conditions were in a range of interaction times between 10 ms and 50 ms, as

shown in Figure 29, and the welds were similar in terms of weld shape and depth of penetration. For

100 ms of interaction time there is a fluctuation of depth of penetration and weld width caused by the

keyhole instabilities, due to the large melt pool. The problem becomes more significant as the interaction

times decreases below 6 ms and the difference in depth of penetration reached up to 25%. The weld

produced with the larger beam diameter was near the transition between keyhole and conduction regime

and the weld produced with the smaller beam diameter was still in the keyhole regime. As the welding

conditions are close to the threshold energy density for the keyhole regime, the higher the power density

32

the longer the process is in this regime. At this low energy density the trade-off between power density

and specific point energy failed to maintain deep penetration.

For the same power factor and interaction time a larger beam diameter provides more specific point

energy. However if there is not enough energy density this additional specific point energy is utilized for

melting instead of drilling. If the process is far beyond the threshold energy density, the drilling is efficient

and both power density and specific point energy affect the depth almost equally. The power factor

provides the same depth of penetration with different beam diameters on the surface at this range.

However, high specific point energy will provide a similar melting rate as compared to high power density

if the process is near the threshold energy density for keyhole regime, but a higher power density will

lead to deeper welds.

2.10 Summary

The literature review was useful to realize that there are three fundamental parameters to characterise

the laser welding process: interaction time, specific point energy and power density. The depth of

penetration is determined by power density and specific point energy, which is represented by the power

factor, whilst the width of the bead is controlled by interaction time.

The complex character of keyhole in deep penetration laser welding was demonstrated by several

studies which revealed a balance between the heat distribution, pressure and absorption conditions

driven by the multiple reflections of the laser light inside the keyhole cavity. Absorbed energy, defined

by power density and interaction time characterise the response of the material to the imposed laser

energy, which together with material properties determine evaporation rate, pressure, melting rate, fluid

flow and other conditions.

The power factor is a new parameter which combined with interaction time allows the specification of

the depth of penetration and weld width, independent of the laser system, in laser keyhole welding.

This recent discovery brought to mild steel and titanium two important advantages:

• Transfer of the process between different laser systems with different beam diameters;

• Development of process specifications for application requirements;

Since the power factor model is quite useful for mild steel, is needed testing its functionality on other

materials, such as aluminium.

33

3. Experimental approach

In this experimental work the main objective was to determine the working envelop for laser keyhole

welding of aluminium and evaluate the effect of the welding conditions on the bead-on-plate weld seam.

This chapter presents the set-up and the main equipment used for the purpose of this thesis, including

the laser system and the equipment used for the characterization of the laser beam used for laser

welding. The procedure for sample preparation and the chemical composition of the material are also

shown. All parameters used and some details are specified in the methodology sections in each

corresponding chapter.

3.1 Material

All bead-on-plate laser welds were carried out in 5083 aluminium alloy plates already cut with the

following measurements: 250 mm long, 200 mm width and 12 mm thick (6 mm plate thick was used only

for the preliminary experiments), as shown in Figure 30. The chemical composition of the material is

given in Table 5.

Figure 30: Schematics of the measures of the plates used and direction of the welding.

Table 5: Chemical composition of aluminium 5083.

34

3.2 Laser system

An IPG YLR-8000 CW multimode fibre laser with a maximum power of 8 kW was used in all experiments

presented in this thesis. The laser beam was delivered through an optical fibre of 300 μm of diameter

and was collimated with a 125 mm focal length lens. A set of focusing lenses with focal lengths of 200

mm, 250 mm and 300 mm were used to achieve different laser beam diameters. This enabled variation

of the beam diameter, while the same intensity distribution was kept constant. All welding experiments

were carried out at the focal point, i.e. with the laser beam focused on the material surface. The beam

diameter at the focal point for each focusing lenses is given in Table 6.

Table 6: Beam properties of different optical set-ups.

Figure 31: Optical head tilted relative to the vertical axis.

To protect the laser system from laser back reflection, an angle of 10° from the weld centreline was

applied. This is shown in Figure 31.

35

3.3 Motion and clamping system

A six-axis Fanuc M700i B45 robot integrated with a single axis translation stage was used to achieve

the translation of the workpiece relative to the laser beam. The robot end-effector holds the laser

perpendicular to the workpiece and allows the laser head to be moved in 3D work space. The

experimental set-up is shown in Figure 32 and the clamping system is presented in Figure 33.

Figure 32: Experimental set-up.

36

Figure 33: Clamping system.

Figure 34: Shielding device and its position dimensions.

There is a guide laser incorporated in the laser head, Figure 34, which simulates where the welding is

going to take place. In the same figure is also possible to see the shielding gas device and its relative

position in relation to the laser spot incident on the base material. This permits a full protection with a

constant gas flow during the entire length of the weld. Inside the nozzle there were two layers of metallic

foam aimed to create a laminar flow. One of them is presented in Figure 35. The flow rate used in all

experiments was 30 l/min. The welding path is predefined by a program saved in a console connected

to the robot, shown in Figure 36, which allows an easy programming of the initial and the final points.

37

Figure 35: Metallic foam used inside the shielding gas nozzle.

Figure 36: Robot control console.

Properties of the laser beam such as beam diameter, focus position, divergence angle were measured

using a Primes GmbH focus monitor, shown in Figure 37. All the beams exhibited a top-hat intensity

distribution at the focal position. An example of the intensity distribution profile and its cross section is

shown in Figure 38.

Figure 37: Experimental set-up for laser characterization.

Figure 36: Robot control console.

38

Figure 38: Intensity distribution and beam profile of the fibre laser for 0.75 mm beam diameter.

3.4 Samples preparation

3.4.1 Before welding

All plates for bead-on-plate testing were washed with water and soap to remove any contaminants from

the surface and afterwards the plates were ground to remove the oxide layer. The samples were cleaned

with acetone prior to welding to remove any residuals of grease.

3.4.2 Macrograph preparation

All welds were cross-sectioned in two positions, A and B, as shown in Figure 39. Afterwards they were

cut and mounted in plastic moulds using an epoxy resin mixed with a hardener, in order to fit them into

an automatic polisher. The samples were mechanically ground in three steps using different grades of

grinding paper as follows: 240, 1200 and 2500. Next the polishing using 9 μm diamond suspension was

performed, followed by 3 μm diamond compound. In the final step, a 1 μm colloidal silica suspension

was used. Before the microscopic examination, in order to reveal the microstructure and distinguish

various zones within the welds, the samples were etched with Keller´s reagent [61]. The macrographs

were analysed in terms of weld width and depth of penetration and measured using an Carl Zeiss Axio

Vision 4.8 image analysis software.

39

Figure 39: Positions where the welds were cross-sectioned.

3.5 Experimental procedure

3.5.1 Preliminary experiments

In order to reach different depths of penetration and determine the working envelope for laser keyhole

welds in the first set of experiments, the power and the travel speed were varied using always the same

beam diameter. The range of the laser system parameters used is shown in Table 7 (power and travel

speed). Using these values and equations 2.3 to 2.6, it was possible to calculate the fundamental

material interaction parameters.

Table 7: Laser system parameters.

Table 8: Calculated parameter.

40

Table 9: Fundamental material interaction parameters.

3.5.2 Extension of the power factor model for aluminium

The effect of power factor and interaction time on the depth of penetration for one beam diameter (0.61

mm) was investigated in the beginning of the second set of experiments.

For a constant interaction time and power factor, the effect of the beam diameter was investigated in the

second part of the second set of experiments. The same data was used to reach the same depth of

penetration. Thus a constant power factor of 4.9 MW/m, 6.6 MW/m and 8.2 MW/m and different values

of interaction time were used for the three previous power factor values. By using two focusing lenses

with different focusing lengths as described in section 3.2, two new beam diameters were achieved (0.49

mm and 0.75 mm) to test the model. According to equations 2.4 and 2.6, for power factor and interaction

time, respectively, the travel speed and the laser power were adjusted to keep the power factor and

interaction time constant to reach the intended depth of penetration previously with a beam diameter of

0.61 mm.

Different levels of power and travel speed were used, as shown on Table 10. Using these values is

possible to compute the parameters in Table 11 and Table 12.

Table 10: Laser system parameters.

Table 11: Calculated parameter.

41

Table 12: Fundamental material interaction parameters.

3.6 Methodology

This section includes the methodology used for the investigation of the influence of some parameters

on the depth of penetration, weld width and quality of the weld seam obtained during the first set of

experiments. The second set uses the best parameters of the first results to extend the power factor

model for aluminium.

In the first case a 0.61 mm beam diameter was used with a focal point incident on the surface of the

base material, using focusing lens of 250 mm focal length. The parameters were chosen aiming to

ensure only the keyhole regime, to avoid changes in the laser coupling, which occurs in conduction

mode. The starting welding parameters were based on the power factor model graph for mild steel

(Figure 25, page 27). The welds were made on 6 mm thick 5083 aluminium and afterwards sectioned,

polished and examined under an optical microscope in order to measure the depth of penetration and

to analyse the porosity content.

For the second set of experiments the laser beam diameters used were 0.49 mm and 0.75 mm with a

focal point on the surface of the base material, focused using focusing lens of 200 mm and 300 mm

focal lengths. The parameters were chosen aiming to ensure only keyhole regime. The welds were

made on 12 mm thick 5083 aluminium and afterwards sectioned, polished and examined under an

optical microscope in order to measure the depth of penetration, weld width and welding defects.

42

4. Results presentation and discussion

4.1 Preliminary experiments

The first set of experiments is considered preliminary since it was not directed to validate the power

factor model for aluminium, but to analyse the effect of the graphite, shielding gas type and flow rate on

the weld, which was very important to define the best welding parameters used on the following set of

experiments.

The experiments were divided in five different conditions:

• A – Graphite and no shielding gas;

• B – No graphite and no shielding gas;

• C – No graphite and shielding gas (10 l/min);

• D - No graphite and shielding gas (20 l/min);

• E - No graphite and shielding gas (30 l/min);

The experimental error bars presented in Figures 40, 42 and 44 are the difference of the depth of

penetration observed between side A and side B of the cross-sectional cuts for the same welding

parameters.

4.1.1 Effect of the power density on the weld

The effect of power density (for a constant beam diameter of 0.61 mm) on depth of penetration using

graphite and no shielding gas for different interaction times is shown in Figure 40. The depth of

penetration increases with increasing interaction time. For a power density of 0.81 MW/cm2, the depth

of penetration is much higher than that for half of this value. For both conditions, using graphite and no

shielding gas, the porosity level is high as shown in Figure 41.

43

Figure 40: Effect of power density and interaction time on the depth of penetration.

Figure 41: Macrographs for ti of 30 ms and qp of (a) 0.81 MW/cm2 and (b) 0.40 MW/cm2.

It was shown in Figure 40 that the depth of penetration is much higher for a high power density value.

However, this difference is quite sharp, which means that there are different welding modes. According

to the results of J. Sanchez-Amaya for laser welding of aluminium under conduction mode regime [64]

while the interaction time increases for a low and constant value of power density, the increasing of the

depth of penetration is very low. On the other hand, the high increasing of the depth of penetration while

the interaction time increases for a high and constant value of power density, happens for a keyhole

regime [31].

This means that for 0.40 MW/cm2 the regime is not intended to use on the power factor model, while

0.81 MW/cm2 is the only one intended to start the next set of experiments, which also allows higher

depths of penetration (Figure 41 - a).

(a) (b)

44

4.1.2 Effect of graphite on the weld geometry

In Figure 42, keeping power density constant and equal to 0.81 MW/cm2 and without using shielding

gas, is shown almost the same depth of penetration using or not graphite. This depth of penetration

increases with increasing interaction time as shown before. Macrographs in Figure 43 show not only the

previous conclusions but also that the graphite increases the porosity content in the weld.

Figure 42: Effect of graphite on the weld without shielding gas.

Figure 43: Macrographs without shielding gas for ti of 15 ms and power density of 0.81MW/cm2:

(a) with graphite and (b) without graphite.

Without shielding gas and with a constant power density, the depth of penetration is similar when

graphite was applied on the base material before welding than without graphite (Figure 42). In order to

help the coupling of the CW laser beam to the material, graphite was used to coat the material’s surface

(a) (b)

45

[28]. For a CW laser, the CO presence influences the pressure balance at keyhole walls, which result in

lower keyhole wall temperatures [55]. In turn, this lower temperature causes a higher keyhole

penetration [41, 54]. It was found that there is no benefit to coat the aluminium plates with graphite to

increase the coupling and thus, increase the weld depth. The reasons are the difficulty to keep a constant

thickness of graphite on the surface of all plates in an industrial environment this would cause a dirty

process, the porosity content is higher in graphite presence (Figure 43) and the depth of penetration

was almost the same with and without graphite. For this reason, the next set of experiments were done

without graphite.

4.1.3 Effect of the shielding gas on the weld geometry

The use of the shielding gas does not influence the depth of penetration, for a constant power density

(0.81 MW/cm2) and without using graphite, as shown in Figure 44. The depth of penetration also

increases with the interaction time as before. Analysing Figure 45 and Figure 46 it is possible to

conclude that the shielding gas causes a tremendous difference on the quality without shielding gas of

the weld seam. The surface of the weld is much rougher and it has much more porosity than the welds

with shielding, as shown in Figure 45.

Figure 44: Effect of the shielding gas on the weld.

46

Figure 45: Macrograph without graphite and no shield gas for ti of 30 ms.

Figure 46: Macrograph without graphite and with shielding gas (20 l/min) for ti of 30 ms.

In laser welding, the shielding gas is commonly used to stabilize the welding process, to improve welded

joints features and to protect the welded seam against oxidization. Metals at high temperature are highly

reactive and tend to form oxides that lead to defects which decrease the welding seam quality when no

shielding gas is used [63]. Pure argon was used as shielding gas because of its good effect in terms of

seam surface, splashes and blowholes, despite of the high cost [55]. It was shown in Figure 44 that for

the same power density, the depth of penetration was not changed by the shielding gas. However,

analysing the macrograph in Figure 45 the porosity content is high and it is quite rougher, revealing the

effect of the oxides. On the other hand, in Figure 46 the macrograph for the same welding conditions

has no porosity and no undercut, which means that the shielding gas has a powerful effect in terms of

weld seam quality. These conclusions led to the use of shielding gas on all the next set of experiments.

4.1.4 Effect of shielding gas flow rate on the weld geometry

In Figure 47 for different shielding gas flow rates, the depth of penetration and the weld width are almost

the same, for constant laser parameters. Analysing Figure 48, the macrographs corroborate this

conclusion and also show the same good quality level in terms of porosity. For all flow rates, the weld

seams have the same good superficial shape.

47

Figure 47: Influence of the shielding gas flow rate on the weld.

Figure 48: Macrographs at 0.81 MW/cm2 power density (shielding gas flow rate: (a) 10 l/min; (b)

20 l/min; (c) 30 l/min).

The efficiency of the laser process is dependent on the flow rate of the shielding gas, owing to the more

stable keyhole [54]. Some authors showed that the irregularity of the bead surface and undercut can be

improved by increasing the diameter of the shielding gas nozzle and optimizing the flow rate [52]. It was

shown in Figure 47 that for a constant power density and interaction time, the different flow rates don’t

influence the depth of penetration neither the weld width. These results are shown on the macrographs

in Figure 48. Since the porosity content is similar in all macrographs it was chosen to use a flow rate of

30 l/min to achieve a completely shielded melting pool with the small local shielding device with a laminar

flow in all the next set of experiments.

.

(a)

(b)

(c)

48

4.2 Extension of the power factor model for aluminium

The second set of experiments was done without graphite and with shielding gas with 30 l/min of flow

rate. Many combinations of parameters such as power and travel speed, only for power factor and

interaction time constant, will give the same depths of penetration. However, different combinations of

system parameters will give a specific weld with a particular bead quality, weld width, mechanical

properties and other joint characteristics. It’s investigated in this project if is possible to find a model of

parameters in laser welding, which would specify the depth of penetration of the fusion zone for

aluminium. It would be useful to create a set of parameters that would allow a user to select a specific

weld for different laser systems with different beam diameters. The extension of this model to aluminium

would solve a big issue, because nowadays for any beam diameter there is a unique combination of the

laser material interaction parameters. It aim to determine the power factor versus interaction time trend

lines for different depths of penetration, validating the model for different beam diameters keeping

constant the power factor and the interaction time values. At the end, the model is compared between

aluminium, mild steel and titanium.

The experimental error bars presented in Figures 49 to 53 and in Figure 57 are the difference of the

depth of penetration achieved between the side A and side B of the transversal cuts for the same welding

parameters.

4.2.1 Development of the power factor model for one beam diameter

The combination of parameters used in the laser, as described in the first set of experiments of the last

section, is shown in Figure 49. In the second set of experiments in Figure 50 the data is the same as in

Figure 49 but with more points for only three different depths of penetration. It can be seen that the

power factor and the interaction time are related for a given depth of penetration. For a constant beam

diameter, a high power factor corresponds to a higher laser power which could be a limitation of the

laser system. In order to achieve a specific depth of penetration, the shorter the interaction time the

greater the power factor required and faster processing speed.

49

Figure 49: Power factor for depths of penetration between 2 mm and 7 mm as function of

interaction time for a beam diameter of 0.61 mm.

Figure 50: Power factor for depths of penetration of 2 mm, 4 mm and 6 mm as function of

interaction time for a beam diameter of 0.61 mm.

50

As indicated in the literature review, there is a huge variety of parameters that have to be considered to

achieve a certain weld in a given material. The depth of penetration is the most important user's

requirement and the first thing that needs to be determined in most welding applications, which is

controlled by two parameters, the power density and the specific point energy [47]. However there are

a lot of conditions that occur simultaneously, making this task very hard and complex, such as: melt flow

dynamics [29-31], pressure [32, 33] and absorption conditions [23, 33, 35, 37, 44].Using equations 2.4

and 2.6 is possible to obtain different penetrations changing the system parameters according to the

power factor and interaction time shown in Figure 49. Different depths of penetration were chosen and

the trend lines were completed only for one beam diameter, as shown in Figure 50. It was proven in this

graph that constant penetration depth curves can be generated for aluminium for different combinations

of power factor and interaction time.

4.2.2 Effect of interaction time and power factor on the depth of penetration for

different beam diameters

It was shown in section 4.2.1 that a specific depth of penetration is achieved by a combination of power

factor and interaction time using one beam diameter. To create an application model it’s necessary to

investigate if the depths of penetration shown in Figure 50 (page 49) are achievable by changing the

beam diameter and keeping constant the power factor and the interaction time values.

The effect of the beam diameter on the depth of penetration for an interaction time and a constant power

factor is shown in Figure 51, Figure 52, and Figure 53. For this range of beam diameters, a constant

depth of penetration is demonstrated in Figure 51 and Figure 52 for different interaction times. In Figure

53 there is a deviation on the linear depth of penetration trend lines. The maximum deviation of the two

new beam diameters (0.49 mm and 0.75 mm), for the intended depths of penetration achieved on

section 4.2.1 (3.6 mm, 4.8 mm and 5.7 mm) with a beam diameter of 0.61 mm, is 0.5 mm, which

corresponds to 13% of total depth of penetration, shown in Figure 54.

51

Figure 51: Effect of the beam diameter on the depth of penetration at different interaction times

and a constant power factor of 4.9 MW/m.

Figure 52: Effect of the beam diameter on the depth of penetration at different interaction times

and a constant power factor of 6.6 MW/m.

0

1

2

3

4

5

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Beam Diameter [mm]

PF = 4.9 MW/m

29.0 ms

14.7 ms

7.3 ms

0

1

2

3

4

5

6

7

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Beam Diameter [mm]

PF = 6.6 MW/m

29.0 ms

14.7 ms

7.3 ms

52

Figure 53: Effect of the beam diameter on the depth of penetration at different interaction times

and a constant power factor of 8.2 MW/m.

Figure 54: Deviation of the depth of penetration for different beam diameters at different

interaction times and a constant power factor of 8.2 MW/m.

The macrographs of the longitudinal cuts with biggest deviation shown in Figure 54 are in Figure 55.

Two lines are limiting the maximum and minimum of the depth of penetration, with a difference of 0.76

mm in case (a) and 0.86 mm in case (b). The corresponding transversal cuts are shown in Figure 56,

where it’s possible to see an evident difference on the depth of penetration for the same parameters.

0

1

2

3

4

5

6

7

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Beam Diameter [mm]

PF= 8.2 MW/m

14.7 ms

9.8 ms

ms 7.3

53

Figure 55: Longitudinal cuts for a power factor of 8.2 MW/m: (a) ti = 9.8 ms, d = 0.75 mm; (b) ti =

7.3 ms, d = 0.49 mm).

Figure 56: Transversal cuts for a power factor of 8.2 MW/m: (a) ti = 9.8 ms, d = 0.75 mm; (b) ti =

7.3 ms, d = 0.49 mm.

The low boiling point of the alloying elements, high thermal conductivity and low viscosity are some

reasons that make the keyhole laser welding of aluminium an unstable process causing porosity and

(a)

(b)

(a)

(b)

54

blow holes [28]. The magnesium vaporisation is the most import responsible for the keyhole fluctuations

mechanism, affecting the properties of the weld. Tensile strength and ductility is also degraded by the

magnesium losses [60]. This Keyhole instability is responsible for different depths of penetration over

the same weld. The waviness shown in Figure 55 causes a big difference on the measured depth of

penetration which was shown in Figure 53, which means that the place where the two cross sections

were cut for the same weld can greatly influence the results. Thus, the depth of penetration deviation in

Figure 54 could be lower and be inside the experimental error (<10 %), as happens for mild steel [44].

The undercut and bead irregularity is shown not only in Figure 55 but also in Figure 56. They are also

caused by the shielding device and the non-optimized flow rate. The porosity could also be markedly

improved by increasing the forward incident angle of laser [52].

Based on the represented data in Figure 51, Figure 52 and Figure 53 the graph in Figure 50 was

completed with two more beam diameters for 2 mm, 4 mm and 6 mm depths of penetration. Even though

different beam diameters were used, the curves for different depths of penetration follow the same trend.

Figure 57: Power factor for depths of penetration of 2 mm, 4 mm and 6 mm as function of

interaction time for beam diameters of 0.49 mm, 0.61 mm and 0.75 mm.

It is possible to see in Figure 57 that using different beam diameters, the same depth of penetration was

achieved in Figure 50 for just one beam diameter, adjusting the travel speed and the output power for

new beams. The trend lines are very evident and with a similar curvature as for mild steel in Figure 25

(page 27).

For a constant power factor and interaction time, the independent character of the depth of penetration

from the beam diameter was shown in Figure 51 to Figure 53 and in Figure 57. This conclusion is very

55

important because for different laser systems with different beam diameters, the same conditions are

achievable. In Figure 51 and Figure 52 the linear trend lines of the depth of penetration are evident, for

the same power factor and interaction time. However, there is a deviation on these trend lines in Figure

53 resultant from the instability of the keyhole, which gives different depths of penetration despite the

same power factor and interaction time. This deviation, as shown in Figure 54 is 13%, which is not high

and is close to the results shown in Figure 58 for the same experiments for mild steel in Figure 24 (page

27). The results did not exceed 8%, in the order of the experimental error. Taking into account the

proximity of the deviations for the different materials, and for aluminium it only exceed 8% in two points,

is possible to say that the power factor model is not only valid for mild steel but also for aluminium.

Figure 58: Deviation of the depth of penetration for mild steel using different beam diameters.

The changes of the aluminium weld profile with the processing parameters are shown for in Figure 59

and in Figure 60. All the macrographs show that the weld width decreases with increasing power factor

and decreasing interaction time, keeping constant the beam diameter. In Figure 59 the weld widths

correspondent to the macros a), b) and c) are: 4.6 mm, 3.4 mm and 2.5 mm, respectively. In Figure 60

the weld widths correspond to the macros a), b) and c) are: 3.5 mm, 3.4 mm, and 2.3 mm, respectively.

There is no porosity and the undercut is shown in all macrographs.

56

Figure 59: Macrographs of bead-on-plate for aluminium welds for 0.75 mm beam diameter,

combinations of parameters required for 4 mm depth of penetration a) ti = 29 ms, PF = 4.9

MW/m; b) ti = 14.7 ms, PF = 6.6 MW/m; c) ti = 7.3 ms, PF = 8.2 MW/m.

Figure 60: Macrographs of bead-on-plate aluminium welds for 0.61 mm beam diameter,

combinations of parameters required for 6 mm depth of penetration a) ti = 29 ms, PF = 4.9

MW/m; b) ti = 14.7 ms, PF = 6.6 MW/m; c) ti = 9.8 ms, PF =9.8 MW/m.

The shape of the weld-bead is determined by the laser welding parameters, due to its combination

control the heat input. Good weld quality is controlled by a combination of the output power, welding

speed, focal position, shielding gas and position accuracy which should be correctly selected.

Nevertheless the final quality of the welded joints is also affected by the size of the plasma plume and

its contamination by the surrounding atmosphere. For both the aluminium and mild steel [44], the weld

width, is important to the fit-up tolerance, residual stresses and distortions, and decreases with

decreasing the interaction time, keeping constant the beam diameter.

For aluminium, the porosity content is apparently reduced on the cross-sections. However the heat

affected zone is not visible and this does not happen for mild steel. A visible heat affected zone is present

in all macrographs of the latter, especially for high interaction time [44], where the porosity content is

higher as well. In terms of undercut, for aluminium it is visible in all macrographs, while for mild steel it

is more apparent for a small beam diameter combined with high speed, such as in Figure 23 c) in the

literature review.

57

Observing the common trend lines for aluminium, titanium and mild steel in Figure 61, there is an offset

on these lines. The trend lines of aluminium and mild steel cross, at 20 ms and a power factor of 6

MW/m. Different conditions on the experimental set-ups used in both materials could explain this once

for mild steel a flow rate between 10 l/min and 15 l/min was used with a different shielding gas device

[44]. The flow rate and the nozzle position of the shielding device relatively to the keyhole deflect the

plume from the interaction zone [23, 56], changing the density of plasma inside the keyhole and

consequently the laser absorption. Different shielding gas velocities can also interact with the melting

pool and reduce the keyhole in some conditions, influencing the depth of penetration [29], which could

have happened for these fundamental laser material interaction parameters. However this crossover

point is no visible in Figure 62 for a 6 mm depth of penetration, the trend line of aluminium is always

lower than mild steel for the range of interaction times studied.

Titanium was also investigated in Cranfield University’s Internal Report due to its applications for

aerospace, nuclear and automotive. It has a high strength-to-weight ratio, fracture toughness, corrosion

resistance, good fatigue behaviour and high temperature properties desired by industry [57, 58].

Because of the precision and rapid processing capability, laser welding was used for this material [58]

and the power factor model was also successfully tested, having a similar trend line as aluminium and

mild steel in Figure 61. This material requires less power factor and a higher interaction time, to reach

the same depth of penetration. A different shielding gas device with a different set-up was used,

changing the formation of excited metal vapour above the keyhole and also causing significantly

changes in the hydrodynamic behaviour of the weld pool [58, 60], which may have influenced the results.

It is known that there is a changing the of melting efficiency with laser power and travel speed and it is

also dependent on the thermophysical properties of the base material, such as thermal diffusivity and

the latent heat of melting [4]. The last one is similar for all the tested materials, as shown in Appendix

A. However the thermal diffusivity for titanium is an order of magnitude lower than aluminium and mild

steel, which means that the losses are lower as well and the melting efficiency would be higher. Thus

less energy is required for titanium, which explains why ts trend line is under that of the aluminium and

mild steel. According E. Assunção [28] the thermal properties of the materials change when changing

from a solid state into a liquid state. The rate of increase of the temperature is not controlled simply by

the material thermal conductivity but a relation between the material properties (thermal diffusivity,

specific heat capacity, thermal conductivity, latent heat of vaporisation, latent heat of melting and

density). As shown in Appendix A, all of these properties are similar between mild steel and titanium but

some of them are quite different from aluminium, as the latent heat of vaporisation, which is an order of

magnitude higher than titanium and mild steel. Some authors have also concluded that the material

thermal properties in keyhole mode have low influence on the depth of penetration, once it was very

similar for aluminium and mild steel [28]. However these results were achieved for just one value of

power density, keeping constants the beam diameter and the interaction time.

58

Figure 61: Comparison of the trend lines of the power factor between aluminium, titanium and

mild steel for a depth of penetration of 4 mm.

Figure 62: Comparison of the trend lines of the power factor model between aluminium and

mild steel for a depth of penetration of 6 mm.

According W. Suder's experiments [44], for the same conditions (beam diameter, power density and

interaction time) aluminium reached higher depth of penetration comparing to mild steel. However, for

the same power density achieved with a smaller beam diameter than the former, the specific point

energy (ESP) was reduced, exhibiting a shallowest depth of penetration for aluminium. The explanation

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160 180

Interaction Time [ms]

Depth of Penetration = 6mm

Aluminium

Mild Steel

59

for this is because the high thermal conductivity of aluminium became dominant for lower ESP. Aluminium

alloy can dissipate much greater portion of heat by conduction, thus the threshold energy density

(product of power density and interaction time) required for the keyhole formation in aluminium is higher

than in steel. This resulted in lower depth of penetration at low energy level in aluminium, than compared

to mild steel. Alternatively, for a higher energy level, where the threshold energy density for keyhole was

exceeded, the depth of penetration achieved in aluminium was higher than that for steel. This indicates

that above the threshold energy density for the keyhole, conduction losses have secondary effects [44].

However, for the same power density, somewhere in the middle of the range of the ESP, the depth of

penetration should be the same for mild steel and aluminium (as previously mentioned by E. Assunção

[28]). This is shown in Figure 63. This figure is based on data presented in Figure 61, showing how

power density (qp) and ESP interact. It’s possible to achieve the same depth of penetration on aluminium

and mild steel only on the crossover point of the trend lines with the same depth of penetration for

aluminium and for mild steel only on the crossover point of the trend lines with the same ESP (70 J) and

qp (1.4 MW/cm2). This crossover point is an equilibrium state for both materials, which means that the

welding parameters compensate the different thermal properties of the materials. Thus the thermal

conductivity of aluminium is only dominant before the crossover point if qp equal to 1.4 MW/cm2. To

reach the same depth of penetration for the same ESP, the qp needs to be increased for both materials,

being higher for mild steel than to aluminium. From this crossover point onwards the threshold energy

density for aluminium is exceeded if qp is equal to 1.4 MW/cm2. To reach the same depth of penetration,

keeping ESP constant, the power density needs to be decreased for both materials, being higher for

aluminium than for mild steel. This explains the trend lines in Figure 61 for mild steel and aluminium and

it’s the proof that the power factor model works for different materials, being possible to reach the same

depth of penetration with different beam diameters. All the materials reach the same depth of penetration

of 4 mm with different values of qp for the same ESP because this is between 3.5 mm to 4.4 mm.

60

Figure 63: Effect of power density and specific point energy on the depth of penetration for

different materials.

Figure 64: Dependence of depth of penetration with the power density and specific point

energy for ti = 7.3 ms using different beam diameters.

61

It’s shown in Figure 64 that for the same interaction time and using different beam diameters, the depth

of penetration is controlled by the power density and the specific point energy. For higher values of

beam diameter, less power density and more specific point energy is needed to achieve the same depth

of penetration. For a constant value of power density, the depth of penetration increases steadily with

increasing beam diameter, while for a constant value of specific point energy, the depth of penetration

increases with decreasing beam diameter. This is valid for mild steel as well [44].

62

5. Conclusions and future developments

5.1 Conclusions

During this study of the extension of the power factor for aluminium, the knowledge development of this

model has been obtained for a new material. The following conclusions derived from the work presented

in this thesis are:

• The graphite coating caused a slight difference on the depth of penetration for the range of

parameters studied, but does not justify its use on the experiments;

• The shielding gas is essential for a good weld seam quality. However different flow rates don’t

influence the depth of penetration;

• Power density, interaction time and specific point energy characterise the laser welding process.

The weld width is controlled by interaction time while the depth of penetration is determined by

the power density and specific point energy, which combined give the power factor;

• Aluminium is a new material where the power factor model can be applied. It is possible to get

a certain depth of penetration through the combination of different values of power factor and

interaction time to different beam diameters. This brings important advantages, such as:

o Satisfaction of the laser user requirements for a certain weld for a particular application,

introducing on the system the correct values of laser power and travel speed given by

the model;

o System parameters transfer between different laser systems with different beam

diameters;

• From the equilibrium point backwards, the heat losses caused by the high thermal conductivity

of aluminium are controlled by increasing the power density, being possible to reach the same

depth of penetration for the aluminium and mild steel using different beam diameters

• From the equilibrium point onwards, the threshold energy density for keyhole is not exceeded

by decreasing the power density, being possible to reach the same depth of penetration for the

aluminium and mild steel using different beam diameters.

• The process instability causes the waviness of the keyhole along the weld giving different depths

of penetration along the weld seem for the same welding parameters;

• It is possible to reach the same depth of penetration with different values of power factor and

interaction time for different beam diameters;

63

5.2 Future developments

• The power factor model was tested for three different materials. However a full study on a wider

range of output powers and beam diameters, using the same experimental conditions, would

be required to enhance the viability of the model;

• It would be helpful to find the influence of each thermal properties of the material on the power

factor model to get a better explanation for the off sets of the trend lines;

• More experimental points would be necessary for titanium to have a better comparison with the

other materials;

• A software development which provides welding parameters based on the user requirements,

such as depth of penetration, material and weld width would be the next step to make this model

easier to use;

• The power factor model could be used to control, during the welding process, the depth of

penetration, weld width and welding defects.

64

6. References

1. Ion, J.C., Laser processing of engineering materials. Oxford UK: Elsivier, 2005.

2. Steen, W. M., Mazunder, J., Laser Material Processing. 3rd ed, Springer, 2003.

3. Quintino, L., et al., “Welding with high power fiber lasers - A preliminary study”, Materials and

Design, vol. 28, no. 4, pp. 1231-1237, 2007.

4. C. A., Walsh, “LASER WELDING - Literature Review”, Materials Science and Metallurgy

Department, University of Cambridge, England, July 2002.

5. S. Tsukamoto, Laser Welding. Welding International, 2010.

6. M. Graudenz, et al., “Applications of laser welding in automotive industry”. Handbook of Laser

Welding Technologies, pp. 555-574, 2013.

7. Wang, H., “Applications of laser welding in the railway industry”. Handbook of Laser Welding

Technologies”, pp. 575-595, 2013.

8. J. Blackburn, “Laser welding of metals for aerospace and other applications”. Welding and joining

of Aerospace Materials, pp. 75-108, 2012.

9. Jones, I., Transmission laser welding strategies for medical plastics. Joining and Assembly of

Medical Materials and Devices, pp. 344-369, 2013.

10. L. Quintino, et al., “Laser Welding of Structural Aluminium”, Springer-Verlag Berlin Heidelberg, vol.

8, pp. 33-57, 2012.

11. W.V Vaidya, et al., “Structure-property investigations on a laser beam welded dissimilar joint of

aluminium AA6056 and titanium Ti6A14 V for aeronautical applications Part I: local gradients in

microstructure, hardness and strength”. Materialwiss. Werkstofftech. vol. 40, no. 10, pp. 623-633,

2009.

12. G. Sierra, P. Peyre, F. Deschaux-Beaume, D. Stuart, and G. Fras, “Steel to aluminium key-hole

laser welding,” Mater. Sci. Eng. A, vol. 447, no. 1–2, pp. 197–208, 2007.

13. Y. Zhang, G. Chen, H. Wei, and J. Zhang, “A novel ‘sandwich’ method for observation of the

keyhole in deep penetration laser welding,” Opt. Lasers Eng., vol. 46, no. 2, pp. 133–139, 2008.

65

14. Thomy, C., et al., “Application of high power fibre lasers in laser and MIG welding of steel and

aluminium”. In: Junek L, editor. Proceedings of the IIW conference on benefits of new methods and

trends in welding to economy, productivity and quality, Prague, Czech Republic; pp. 88-98, 10-15

July, 2005.

15. A new generation of lasers for industrial: fibre lasers. Available from: <http://www.zugo.com.sg>.

16. B. Shiner, “High power fiber lasers impact material processing”. Industrial Laser Solutions, pp. 911,

February 2003.

17. M. Melo, et al., “Lasers de fibra óptica de alta potência: conceito, limitações e aplicações”.

Gazeta de física. Artigo geral nº22, no. 34, pp 22-28, 2011.

18. G. Gahan and B. Shiner. New High-Power Fiber Laser, pp. 29-31, 2004.

19. R. K. Soni, V. K. Mandloie, M. B. Pote, and a. K. Nath, “Spinning cone water film power meter for

high-power CO2 lasers,” Opt. Laser Technol., vol. 39, no. 1, pp. 196–201, 2007.

20. Handbook of Optical and Laser Scanning ed. G.F. Marshall. New York: Marcel Dekker, INC.

2004.

21. Fundamentals of beam profiling and measurement, Materials of CVI Melles Griot available at:

www.cvimellesgriot.com. 2011.

22. Y. Qin, T. Nakajima, H. Zen, X. Wang, T. Kii, and H. Ohgaki, “Characterization of non-Gaussian

mid-infrared free-electron laser beams by the knife-edge method,” Infrared Phys. Technol., vol.

66, pp. 146–151, 2014.

23. C. Brock, F. Tenner, F. Klämpfl, R. Hohenstein, and M. Schmidt, “Detection of Weld Defects by

High Speed Imaging of the Vapor Plume,” Phys. Procedia, vol. 41, pp. 539–543, 2013.

24. M. J. Zhang, G. Y. Chen, Y. Zhou, S. C. Li, and H. Deng, “Observation of spatter formation

mechanisms in high-power fiber laser welding of thick plate,” Appl. Surf. Sci., vol. 280, pp. 868–

875, 2013.

25. Rai R, Roy GG, Debroy T. A computationally efficient model of convective heat transfer and

solidification characteristics during keyhole mode laser welding. J Appl Phys. 2007;101(5)

66

26. Sibillano, T., et al., “Optical detection of conduction/keyhole mode transition in laser welding”.

Journal of Materials Processing Technology, vol. 191, pp. 364–367, 2007.

27. D70 operator’s manual (Disk Laser HLD 4002 manual), Trumpf, 2005

28. Assunção, Eurico; “Investigation of conduction to keyhole mode transition”, PhD Thesis, Cranfield

University, Scholl of Applied Sciences, 2012.

29. R. Fabbro, S. Slimani, I. Doudet, F. Coste, and F. Briand, “Experimental study of the dynamical

coupling between the induced vapour plume and the melt pool for Nd–Yag CW laser welding,” J.

Phys. D. Appl. Phys., vol. 39, no. 2, pp. 394–400, 2006. .

30. J. Wang, C. Wang, X. Meng, X. Hu, Y. Yu, and S. Yu, “Study on the periodic oscillation of

plasma/vapour induced during high power fibre laser penetration welding,” Opt. Laser Technol.,

vol. 44, no. 1, pp. 67–70, Feb. 2012.

31. F. Tenner, C. Brock, F. Klämpfl, and M. Schmidt, “Analysis of the correlation between plasma plume

and keyhole behavior in laser metal welding for the modeling of the keyhole geometry,” Opt. Lasers

Eng., vol. 64, pp. 32–41, Jan. 2015.

32. P. Berger and H. Hügel, “Fluid dynamic effects in keyhole welding an attempt to characterize

different regimes,” Phys. Procedia, vol. 41, pp. 216–224, 2013.

33. Abe, Y., et al., Deep penetration welding with high power laser under vacuum, in Proceedings of

29th International Congress on Applications of Lasers and Electro-Optics, ICALEO: Anaheim, CA.

pp. 648-653, 2010.

34. Y. Kawahito, N. Matsumoto, Y. Abe, and S. Katayama, “Relationship of laser absorption to keyhole

behavior in high power fiber laser welding of stainless steel and aluminum alloy,” J. Mater. Process.

Technol., vol. 211, no. 10, pp. 1563–1568, Oct. 2011.

35. A. F. H. Kaplan, “Fresnel absorption of 1μm- and 10μm-laser beams at the keyhole wall during

laser beam welding: Comparison between smooth and wavy surfaces,” Appl. Surf. Sci., vol. 258,

no. 8, pp. 3354–3363, Feb. 2012.

36. R. Fabbro, “Dynamic approach of the keyhole and melt pool behavior for deep penetration NdYag

laser welding,” AIP Conf. Proc., vol. 1047, no. 2008, pp. 18–24, 2008.

67

37. R. Weber, A. Michalowski, M. Abdou-Ahmed, V. Onuseit, V. Rominger, M. Kraus, and T. Graf,

“Effects of radial and tangential polarization in laser material processing,” Phys. Procedia, vol. 12,

no. PART 1, pp. 21–30, 2011.

38. J.F. Ready, D.F. Farson, LIA handbook of laser materials processing, vol. xxv, Laser Institute of

America, Orlando, pp. 715, Fl. 2001.

39. S. Yan et al., “CW/PW dual-beam YAG laser welding of steel/aluminum alloy sheets”. Opt Lasers

Eng., vol. 48, pp. 732-736, 2010.

40. B. Upadhyaya, et al., “A highly efficient 5 kW peak power Nd:YAG laser with time-shared fiber optic

beam delivery”. Optics & Laser Technology, vol. 40, pp. 337-342, 2008.

41. K. Benyounis et al., “Effect of laser welding parameters on the heat input and weld-bead profile”.

Journal of Materials Processing Technology, pp 164-165, 2005.

42. P.W. Fuerschbach, G.R. Eisler, “Effect of laser spot weld energy and duration on melting and

absorption”. Sci. Technol. Weld Joining, pp. 241–246, 2002.

43. E. Assunção and S. Williams, “Comparison of continuous wave and pulsed wave laser welding

effects”. Optics and Lasers in Engineering., vol. 51, pp. 674–680, 2013.

44. Suder, Wojciech; “Study of fundamental parameters in hybrid laser welding”, PhD Thesis, Cranfield

University, Scholl of Applied Sciences, 2012.

45. K. Hansen et al., “Beam shaping to control of weldpool size in width and depth”. Physics Procedia,

Vol.56, pp. 467 – 476, 2014.

46. Williams, Stewart, and Wojciech Suder. "Use of fundamental laser material interaction parameters

in laser welding." CLEO: 2011-Laser Science to Photonic Applications. 2011.

47. Suder, W., et al., Absolute spot size effect on penetration depth in laser welding. Proceedings of

the Fifth international WLT-Conference on Lasers in Manufacturing 2009 Munich, June 2009.

48. Web-Page – www.world-aluminium.org – The International Aluminium Institute, Actualized © 2015

Microsoft Corporation.

49. R. Leal; Soldadura de Ligas de Alumínio Pelos Processos de Fricção Linear e Laser de Díodos,

Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 2004.

68

50. ASM Handbook; Properties and Selection: Nonferrous Alloys and Special-Purpose Materials;

American Society for Metals (ASM); Vol. 2, 2004.

51. L. Quintino et al., “Soldadura por laser de Nd:YAG, em Liga de Alumínio”, Tecnologia & Qualidade,

Instituto de Soldadura e Qualidade, nº 46, Outubro/Dezembro, 2003.

52. S. Katayama, H. Nagayama, M. Mizutani, and Y. Kawahito, “Fibre laser welding of aluminium alloy,”

Weld. Int., vol. 23, no. 10, pp. 744–752, 2009.

53. B. Ribic, S. Tsukamoto, R. Rai, and T. DebRoy, “Role of surface-active elements during keyhole-

mode laser welding,” J. Phys. D. Appl. Phys., vol. 44, no. 48, p. 485203, 2011.

54. S. Kelly et al., "Examination of the process efficiency during hybrid laser-arc welding". Materials

Science and Technology, no. 3, pp. 69-76, 2005.

55. Trumpf User’s manual guide

56. C. Gerritsen and C. Olivier, “Optimisation of plasma/plume control for high power Nd:YAG laser

welding of 15mm thickness C-Mn steels”, in ASM Proceedings of the International Conference:

Trends in Welding Research. Pheonix, Arizona. pp. 465-470, 2002.

57. G. Casalino, M. Mortello, and S. L. Campanelli, “Ytterbium fiber laser welding of Ti6Al4V alloy,” J.

Manuf. Process., Jul. 2015.

58. E. Akman, A. Demir, T. Canel, and T. Sınmazçelik, “Laser welding of Ti6Al4V titanium alloys,” J.

Mater. Process. Technol., vol. 209, no. 8, pp. 3705–3713, Apr. 2009.

59. J. Blackburn, C. Hilton and L. Li, " Nd:YAG laser welding of titanium alloys using a directed gas

jet", Journal of Laser Applications, vol.22., no.2, pp. 71-78, 2010.

60. J. Ion, "Laser beam welding of wrought aluminium alloys", Science and Technology of Welding and

Joining, Vol.5, no.5, pp. 265-276, 2000.

61. Web-page - www.metprep.co.uk - Metprep Limited; Coventry CV4 8AW; United Kingdom.

62. Milton S. F. Lima ; Rudimar Riva ; Aline C. de Oliveira and Gabriel R. Siqueira "Laser beam welding

aerospace aluminum using fiber lasers", Proc. SPIE 7131, XVII International Symposium on Gas

Flow, Chemical Lasers, and High-Power Lasers, 713128, April 21. 2009.

69

63. Silva, João; "Laser Welding of Aluminium Rings", MSc Thesis, Instituto Superior Técnico, 2011.

64. J. M. Sánchez-Amaya, T. Delgado, L. González-Rovira, and F. J. Botana, “Laser welding of

aluminium alloys 5083 and 6082 under conduction regime,” Appl. Surf. Sci., vol. 255, no. 23, pp.

9512–9521, Sep. 2009.

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7. Appendix

Appendix A – Material properties.

Property Symbol Unit Material

Mild Steel Titanium Aluminium

Melting temp. Tm K 1803 1630 843

Vaporisation temp. Tv K 3133 3260 2650

Density ρ kg m-3 7800 4506 2800

Specific heat of

solid phase

cps J kg-1 K-1 480 526 850

Latent heat of

melting

Hm J kg-1 2.7 x 105 4.19 x 105 3.5 x 105

Latent heat of

vaporisation

Hv J kg-1 6.1 x 106 9.83 x 106 1.19 x 107

Thermal

conductivity

average

k W m-1 K-1 39 6.7 135

Thermal diffusivity

average α m2 s-1 1.04 x 10-5 8.85 x 10-6 5.67 x 10-5

Viscosity η m Pas 8 4.42 1.3

Vaporisation

constant B0 kg m-1 s-2 3.9 x 1012 3.9 x 1012 2.05 x 1012

71

Appendix B - Transversal cuts for a power factor of 4.9 MW/m.

d = 0.61 mm; ti = 29 ms d = 0.75 mm; ti = 29 ms

d = 0.61 mm; ti = 14.6 ms d = 0.75 mm; ti = 14.6 ms

d = 0.61 mm; ti = 7.3 ms d = 0.75 mm; ti = 7.3 ms