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Procedia CIRP 11 (2013) 139 144

Available online at www.sciencedirect.com

2212-8271 2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of the International Scientific Committee of the 2nd International Through-life

Engineering Services Conference and the Programme Chair Ashutosh Tiwari

doi:10.1016/j.procir.2013.07.039

ScienceDirect

2ndInternational Through-life Engineering Services Conference

Friction stir welding of Al alloys: analysis of processing parameters

affecting mechanical behavior

Pasquale Cavaliere*

Department of Innovation Engineering, University of Salento, Lecce 73100, Italy

* Corresponding author. Tel.: +39 0832 297357; fax: +39 0832 297357.E-mail address:pasquale.cavaliere@unisalento.it

Abstract

Friction Stir Welding is a well known solid state joining technology. Many processing conditions and materials properties affect the

microstructural evolution and mechanical behavior of the produced joints. The main parameters involved in the welding process have been

studied and the results presented in the present paper. The fatigue life and crack behavior of several aluminum alloys FSW joints have been

presented. The analysis was conducted through employing a multi-objective optimization tool capable of correlating all the material propertiesand processing parameters to the final mechanical performances of the welds.

2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of the International Scientific Committee of the "2nd

International Through-life Engineering

Services Conference" and the Programme Chair Ashutosh Tiwari.

Keywords: FSW; Fatigue properties; crack behavior; optimization.

1.IntroductionThousands of papers have been published in the last 20

years on Friction Stir Welding (FSW). The solid state joining

technology is well known; a complete description of

processing, physical and mechanical behavior can be found in

[1, 2]. In the recent literature, such technology is becoming

very promising in repairing technology especially in the case

of Al aeronautical structures [3-5]. Actually, due to safety and

legal constrictions and to maintenance procedures, the

analyses and the control of welding parameters in order to

obtain high resistance, uniform microstructure and fatigue

performances become fundamental in the case of such

technology. Many papers are presented in the literature on

microstructural, physical and mechanical behavior of friction

stir welded Al-alloys, only few papers focus on the effect of

processing parameters to obtain joints good efficiency in terms

of tensile and fatigue properties. In [6] the authors show the

results of an optimization study of tool geometry in order to

improve the tensile properties of FSW joints. In [7, 8] the

authors focus the attention on the effect of tool rotation speed,

advancing speed and tool geometry on fatigue strength of

5083 alloy. The effect of processing parameters on residual

stresses affecting crack initiation and propagation and fatiguelife in 2XXX alloys is described in [9-18]. A review on fatigue

properties of FSW joints is described in [19]. Fatigue crack

behavior of 7XXX alloys is shown in [20-23]. Fatigue

behavior of 6XXX alloys is described as a function of

processing parameters in [24-29]. The paper is aimed to give a

contribution to the understanding of all the main variables

affecting solid state joining performances and quality. The

contribution can be useful also in the prospective of further

FSW international standardization to weld and repair. The

employed multi-objective optimization software is

modeFRONTIER (ESTECO), through which a set of input

parameters, governing the FSW process, was defined. It was

possible to evaluate the weight of the processing parameters

with respect to the final performances and the correlationsbetween different sets of parameters [30, 31].

2013 The Authors. Published by Elsevier B.V.

Selection and peer-review under responsibility of the International Scientific Committee of the 2nd International Through-life

Engineering Services Conference and the Programme Chair Ashutosh Tiwari

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140 Pasquale Cavaliere / Procedia CIRP 11 (2013) 139 144

2.Experimental procedureThe materials under investigation were different heat and

non heat treatable aluminium alloys from 2XXX, 3XXX,

5XXX, 6XXX and 7XXX series.

Sheets with different thicknesses were friction stir welded

by employing various welding advancing and rotating speeds

of the tools, such tools were all threaded ones and they werecharacterized by different height and shoulder diameter. The

welds were performed by employing different tools

inclinations. The machine used for the production of the joints

was instrumented with a Kistler three channel load cell in

order to record both forces along the tool axis, hereon denoted

as FZ, and along the welding direction, hereon denoted as FX,

for all the produced welds. Acquisition scan rate changed as a

function of the rotating speed in order to record two time

samples per tool revolution, in all the examined conditions.

All the sheets were machined before tests in order to eliminate

surface inhomogeneities. Tensile tests were performed in

order to evaluate the mechanical properties obtained in the

different welding conditions. The Residual Stresses (RS) were

also calculated in longitudinal direction respect to the loadingone, by employing the sin2method. The RS were measured

in longitudinal direction, being the one affecting the crack tip

stress field. The tensile tests were carried out at room

temperature using a MTS 810 testing machine with initial

strain rate of 10-3 s-1. Specimens were sectioned in the

perpendicular direction to the weld line by employing an

electrical discharge machine (EDM), the tensile specimens

measured 12 mm width, 80 mm length for a gauge length of

25 mm. Endurance fatigue tests were performed by a resonant

electromechanical testing machine under constant loading

control, with sine wave loading (TESTRONICTM 25 25

kN, produced by RUMUL, SUI) in high cycles regimes. The

cyclic fatigue tests were conducted under axial stress

amplitude control mode with different loading ratios. The

fatigue specimens gauge dimensions measured 12.5-mm

width, 50-mm length and tests were performed up to failure.

The fatigue crack propagation experiments were performed by

employing single edge (1 mm) notched specimens obtained on

the advancing side. The FSW, in fact, does not produce a

symmetric deformation respect to the center line of the

advancing tool. Due to such inhomogeneities, when a clock

wise direction rotation is employed, the less resistant zone

results the one on the advancing side of the tool. Due to

materials, thicknesses and tool differences they were taken

some choices to build the database. The base materials

indication come through the hardness. The effect of the tool

dimension is indicated into the database through the shoulder

diameter/pin height ratio. Revolutionary pitch effect is

indicated into the database. A broader discussion is necessary

for the indication of the residual stresses, they were measured

in five different points, for each sheet the distance between the

measurement points were equal to the sheet thickness; after

the calculation it was calculated the area of the plot residual

stress-distance from the weld center (on the advancing side of

the tool); the value of such area, for each experimental

condition was used as the indication of the residual stresses

effect into the database. The effect of processing parameters

on the tensile mechanical properties was underlined through

the ration between the FSW properties and base materials ones

for yield stress, UTS and elongation (%). Fatigue properties

were evaluated through the ratio between the stress at 104, 105,

106and 107cycles of the FSW and base materials ones (%).

The crack behavior was evaluated through the ratio between

the K values at 6E-3, 7E-4, E-5and 2E-6mm/cycle of the FSW

and base materials ones (%). In such a way it was possible tohave uniform values to be employed together into the

database. Now it is possible to indicate as inputs: Base

material properties, thickness, tool geometry, tool rotation

speed, tool advancing speed, revolutionary pitch, tool

inclination, Fz welding force (in stationary conditions), fatigue

load ratio; and as outputs: residual stresses, tensile properties,

fatigue and crack propagation properties.

The experimental design consists of 136 input and output

conditions obtained from experimental data.

3.Results and discussionMany different results should be put in evidence

concerning the relationships between the different processing

parameters. The main welding parameters, which can be

controlled in the FSW process for a given tool, are the

rotational speed of the tool, the traverse speed of the tool, the

axial force of the tool shoulder on the workpiece, the angle of

contact between the tool and the workpiece, tool geometry and

revolutionary pitch. Vertical force was inserted into the

database because it is strongly larger with respect to the other

two and, in particular, it is generally recognized as the force

limiting the welding machine design and the main force

influencing tool wear or fracture particularly during the plunge

stage. Processing parameters in FSW affect temperature

profile/heat input, defects, microstructure and residual

stresses. Generally, the welding force decrease as increasing

the revolutionary pitch, the welding force increases as

decreasing the tool inclination angle.

Fig. 1. Welding force in Z direction as a function of the base material

hardness and of the revolutionary pitch.

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The welding force decreases with increasing base material

hardness up to a minimum around 140 HV, then the welding

force increases with increasing the base material hardness

(Figure 1). Finally it was observed that the welding force

increases with increasing the rate tool shoulder/pin height.

Such factor is fundamental in analyzing the effect of welding

conditions on consequent mechanical properties. Many factors

are involved, first of all variation of the pin design did have aneffect on the welding force which directly opposes translation

of the tool; during the translational stage the forging pressure

exerted by the shoulder generates the vertical force.

Normally the severe plastic deformation of material during

FSW decreases for thicker sheets. In this way all the heating

and material mixing is affected by the dimensions.

Fig. 2. Yield strength as a function of the welding force and of the

revolutionary pitch.

In particular, the transition zone between TMAZ and HAZis strongly influenced by the geometric conditions (thickness,

tool geometry), this is well known as the weaker section of

FSW joints governing tensile and fatigue behavior, for this

reason, in the present study, a large emphasis was put on such

a factor.

Fig. 3. UTS as a function of the ratio tool shoulder/pin eight and of the

welding force.

By taking a look to the outputs, it can be underlined that

Yield strength of the joints increases as increasing the welding

force at intermediate values of the revolutionary pitch (Figure

2). At higher values of revolutionary pitch it is generally

observed a strong decrease in mechanical properties, such

condition is indicative of a decrease of material stirring and

sufficient heating to avoid joints defects. At too low values of

revolutionary pitch the excess of heating leads to highmaterial tearing and consequently to high microstructural

inhomogeneities in the joint.

Ductility increases as increasing the welding force from

low to intermediate values of the revolutionary pitch, and

increases as increasing the tool inclination angle. UTS

decreases as increasing the ratio tool shoulder/pin eight up to

a minimum, then it starts to increase as increasing the ratio;

UTS increases as increasing the welding force (Figure 3). The

difference between tensile strength of the joints and of the

base metal is mainly governed by the heating generated

during welding, such heating is governed by the welding

conditions and by the geometry of the tool, such aspect is very

critical for precipitation hardening alloys.

Fig. 4. Residual stresses as a function of the base material hardness and of the

ratio tool shoulder/pin eight.

Heating and cooling is also strongly related to the ductility

of the joints, in fact, if softening acts on the weld line necking

is anticipate and, as a consequence, such aspect strongly

reduces elongation of the joints. It can be underlined that such

aspect can be monitored through the analysis of softening

effect on elongation. In the present study such factor was

monitored through the calculation of the reduction in hardness

with respect to the base material. The main softening effect is

observed in the 7XXX and Li-2XXX alloys, the alloys that

are less effected from softening are the 3XXX and 5XXX

series materials. In such alloys, improvement in tensile and

fatigue properties is expected for those welding conditions

inducing higher levels of heat input if compared with

precipitation hardening alloys. In heat treatable alloys, in

fact, excess in heating can lead to overageing and then to

dissolution of hardening precipitates and/or particles

coarsening.

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The microstructural evolution acting during FSW are

accompanied by significant residual stress changes. Both

tensile and compressive residual stresses exist within the

weld. Residual stresses increases as increasing the base

material hardness up to a maximum and then they start to

decrease as increasing the base material hardness; the same

behavior is observed as a function of the ratio tool

shoulder/pin eight (Figure 4). Residual stresses increase asincreasing the revolutionary pitch and they increase as

increasing the welding force up to a maximum, then residual

stresses decrease as increasing the welding force (Figure 5).

Fig. 5. Residual stresses as a function of the revolutionary pitch and of the

welding force.

By comparing different fatigue behavior it is convenient to

indicate the fatigue strength in terms of stress at a fixed

reference value of number of cycles to failure, the first

reference value was taken at 107cycles to failure.

Fig. 6. Fatigue limit as a function of the revolutionary pitch and of the base

material hardness.

The fatigue limit increases as increasing the revolutionary

pitch up to a maximum and then it starts to decrease as

increasing the revolutionary pitch; the same behavior has been

observed as a function of the base material hardness (Figure

6). Fatigue limit increases for residual stresses going from a

compressive to a tensile state up to a maximum and then it

decreases as increasing tensile residual stresses;

contemporary, fatigue limit increases as increasing the

welding force up to a maximum and then decreases as

increasing the vertical force (Figure 7). In previous studies the

compressive aspect of residual stresses has been explained as

due to very complex thermal and rigid clamping used duringFSW process.

Fig. 7. Fatigue limit as a function of the residual stresses and of the welding

force.

For fixed geometry conditions and revolutionary pitch, in

fact, high Fz forces lead to an excessive flash in the weld (also

governed by the tool inclination), while low vertical forces

produce excessive tearing of the material. In such condition it

is observed a strong drop of fatigue properties mainly

observed in heat treatable alloys.

Fig. 8. Fatigue limit as a function of the tool rotation speed and of the ratio

tool shoulder/pin eight.

Too small tools, in fact, does not lead to an optimal mixing

and stirring of material, while too big tools produce an

excessive heating with a strong softening effect and with large

microstructural inhomogeneities. As a general behavior, the

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fatigue efficiency decreases as increasing the revolutionary

pitch. Heat treatable alloys resulted much more sensitive to

welding conditions variation if compared to non age

hardenable materials. The fatigue limit decreases as

increasing the tool rotation speed for intermediate values of

the ratio tool shoulder/pin eight (Figure 8). So for each set of

processing conditions there is an optimal set of pin and

shoulder diameter that should be employed to obtain good

fatigue performances.

Actually, the main differences between the crack behavior

of the FSWed joints and the base material is observed at low

K, such difference tends to decrease as increasing K.

Normally, it is accepted that such behavior is due to the aspect

that the crack grow into a material with different

microstructural and mechanical properties, at the beginning

into the HAZ and into the TMAZ which are characterized by

lower stiffness with respect to the parent material; when the

crack starts to grow into the recrystallized nugget the situation

strongly varies. The nugget is, in fact, strongly recrystallized

and it is characterized by finer grains; normally the finer

grains favor the increase of crack growth but, in the case of

FSW nuggets, such situation is inversed leading to a similar

behavior between the crack growth in the nugget and in the

parent material. Generally, K calculated at a value of da/dN

of 6E-3mm/cycle decreases as increasing residual stresses up

to a minimum and then it increases as increasing residual

stresses; it decreases as increasing the welding force (Figure

9); compressive residual stresses lead to a decrease in crack

growth rate so it is much more difficult to observe a crack

initiation in this zone, tensile residual stresses lead to an

increase in crack growth rate. The worst crack behavior was

observed in those joints with less compressive state close to

the crack tip and characterized by a strong increase of residual

stresses toward high tensile state. In addition the coupled

effect of microstructure changes and increase of residual

stresses toward tensile state lead to an acceleration of crack

growth with respect to the parent material behavior.

Fig. 9. K as a function of the residual stresses and of the welding force.

For those specimens tested at different load ratios it can

be underlined that the effect of residual stresses decreases as

increasing R, in fact the increase of maximum load level leads

the residual stress to be a negligible portion of the total stress

acting on the material. K increases as increasing base

material hardness, contemporary it increases as increasing the

revolutionary pitch up to a maximum, then it decreases as

increasing the revolutionary pitch. K has a sinusoidal

behavior as a function of low values of the ratio tool

shoulder/pin eight and it decreases as increasing the tool

rotation speed (Figure 10).

By focusing on the FSW joints and on the parent material

behavior it can be underlined how, crack initiation is strongly

governed by microstructural features and residual stresses; in

the Paris region, all the joints assume a general behavior

which is closer to the parent material one; the cracks

accelerate with a rate proportional to the increase in residual

stresses and to the difference in microstructure depending on

processing parameters governing heating and stirring of the

material. In particular excess of heating, at constant mixing

conditions, leads to strong softening of HAZ and TMAZ with

local enhanced mechanical properties differences with the

nugget zone (in which the rate of dynamic recrystallization is

mainly governed by material mixing then by revolutionary

pitch and tool inclination). In addition, as underlined by

different authors, such microstructural features decrease crack

closure contribution with consequent increase in crack growth

rate. Such behavior is normally characterized by a transition

from ductile to brittle fracture appearance on the fracture

surfaces; such behavior is favored, in the heat treatable

alloys, when heating conditions lead to precipitate coarsening

due to overageing effects.

Fig. 10. K as a function of the rotation speed and of the ratio tool

shoulder/pin eight.

It is also important, in the present analysis, to employ the so

called scatter matrix that allows to immediately recognize

how much the different variables are correlated between them,

actually the parameters are strongly correlated if the

corresponding value in the table are distant from zero in a

range between -1 and 1, if the value is 1 the parameters are

directly correlated, while if the value is -1 the parameters are

inversely correlated. From such matrix it is also possible to

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observe the different weight of all the parameters, the more

the value differs from 0 the more it influence the

corresponding variable. By taking a look at the influence of

the processing parameters on the mechanical properties of the

joints it should be underlined that, tensile properties are

strongly dependent on the ratio tool shoulder/pin eight and on

tool inclination with an inverse proportionality, and on the

tool rotation speed.

The welding force is mainly dependent on the base

material hardness, on the ratio tool shoulder/pin eight and on

the tool inclination. Residual stresses show the same behavior.

The fatigue limit is strongly dependent on welding force, on

residual stresses (both with direct proportionality), on the tool

rotation speed and on the ratio tool shoulder/pin eight, on the

base material hardness and on the tool inclination. The K

value at 6E-3mm/cycle is dependent on the material hardness,

tool geometry, inclination and rotating speed and it is directly

related to welding force and residual stresses.

4.ConclusionsIn the present study the effect of processing parameters on

tensile, fatigue and crack behavior of several aluminum alloys

is described. The experimental data were employed to build a

database capable of developing a model useful for predicting

mechanical performances of FSW joints. It was underlined the

different weight of processing parameters on final

performances of the welds. The quality of the model was

evaluated through the potential error calculation of each

output monitored during the analysis.

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