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Journal of Manufacturing Processes

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Technical Paper

Effects of tilt angle between laser nozzle and substrate on bead morphologyin multi-axis laser cladding

Jingbin Haoa,⁎, Qingdong Menga, Congcong Lia, Zhixiong Lib, Dazhong Wub

a School of Mechatronic Engineering, China University of Mining and Technology, Xuzhou, 221116, ChinabDepartment of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL, 32816, USA

A R T I C L E I N F O

Keywords:Laser claddingBead morphologyTilting postureLaser beam power distributionGravity effect

A B S T R A C T

Laser cladding has been increasingly used for repairing and remanufacturing critical and high-value componentsdue to its unique benefits such as high solidification rates and a small heat-affected zone. In laser cladding, tiltangle between a laser nozzle and a substrate has a significant impact on deposited bead morphology. To ensurethe quality of laser cladding, the effects of tilt angle on bead morphology are investigated in this study. Ananalytical model is introduced to predict bead shapes for three tilting postures. In the first case, a substrateremains horizontal while the nozzle is tilted. All three parameters, including width, height, and peak point offset,will be influenced by the laser beam power distribution. In the second case, a substrate is tilted while the lasernozzle is kept axial to the substrate’s normal, the peak point offset will ascend along with the increasing of thetilt angle (gravity effect). In the third case, the laser nozzle remains vertical while the substrate is tilted, whichleads to variations of cladding width, cladding height, and especially peak point shifting value. These parameterswill be dependent on the integrated effect of gravity and the laser beam power distribution. A set of experimentsis conducted to demonstrate the effectiveness of the proposed model. This study illustrates that the variation ofcladding width and height with the tilt angle can be accurately calculated by the predictive model, and that thepeak point shifting value is roughly smaller than 5% of cladding width when the tilt angle is less than 30°. Thesefindings show that trajectory planning of multi-axis laser cladding can be optimized using an acceptable range oftilt angle between the laser nozzle and substrate.

1. Introduction

Laser cladding, also known as laser metal deposition (LMD), is ajoining process used for adding a cladding material to the surface ofanother base material [1]. A laser cladding process uses a laser beam tofuse enhanced materials by creating a metallurgical bond between thecladding and the base materials, as shown in Fig. 1. In the laser clad-ding process, the cladding material (in either a powder or a wire form)is fed by a laser nozzle onto a melt pool, which is created by the laserbeam [2]. Laser cladding has been increasingly adopted in the repairand remanufacturing of critical and high-value components such asturbine blades, gears, crankshafts, and centrifuge hubs [3–5]. Com-paring to traditional cladding processes such as thermal spray andplasma coating [6], laser cladding offers unique benefits such as highsolidification rates, full metallurgical bond, small heat-affected zone,low porosity, and very low dilution [7,8].

Although laser cladding has many advantages, there are some lim-itations. For example, it is difficult to control the metallurgical quality

of cladding layers, and surface cracks and internal pores can becommon. Moreover, the composition and structure of the cladding layerare not uniform over the entire surface [9,10]. Some of the dominantfactors affecting the quality of laser cladding include laser power,powder feed rate, nozzle tilt angle, and scan speed [11,12]. The mi-crostructure and mechanical properties of cladding layers also varywith the alloy composition and cladding path [13–15]. Therefore, it’simportant to build a robust model of single bead shape to generatereferential boundary conditions for overlapping coating. Furthermore, amulti-axis laser cladding robot is often used to repair large-scale,complex parts or interior components [16,17]. Considering the reach-ability of a nozzle, a tilt angle always exists between the surface normalvector and the laser beam axis. This brings uncertainties to path plan-ning and machining simulation of laser cladding.

By conducting experiments in a facility housing the robot arm androtary platform, this paper focuses on investigating the effects of tiltangle between a laser nozzle and a substrate on bead morphology. Apredictive model was developed to illustrate how a substrate with an

https://doi.org/10.1016/j.jmapro.2019.04.025Received 6 October 2018; Received in revised form 26 February 2019; Accepted 22 April 2019

⁎ Corresponding author.E-mail address: jingbinhao@cumt.edu.cn (J. Hao).

Journal of Manufacturing Processes xxx (xxxx) xxx–xxx

1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

Please cite this article as: Jingbin Hao, et al., Journal of Manufacturing Processes, https://doi.org/10.1016/j.jmapro.2019.04.025

inclined angle (gravity effect) impacts the variation tendency of beadshape parameters (width w, height h, and peak shifting value Δ). Inaddition, a laser nozzle tilt angle (beam power distribution) also has asignificant impact on bead shapes. The experimental results demon-strate the accuracy of the predictive model, and show that the claddingbead and overlapping surface quality will not be influenced within acertain range of tilt angle, which exists between orientations of the lasernozzle and substrate. Our research will provide some theoretical basisfor laser nozzle posture optimization in trajectory planning of multi-axis laser cladding.

The remainder of the paper is organized as follows: Section 2 re-views the related work on modelling and prediction of bead shape inthe laser cladding process. Section 3 introduces four postures betweenthe laser nozzle and the substrate, and presents a mathematical pre-dictive model of bead shapes for three tilting postures. Section 4 pre-sents an experimental setup of the laser remanufacturing system andexperimental data acquired from the materials microscope. Section 5discusses experimental results, demonstrates the accuracy of the pre-dictive model, and analyses the reason for the predictive model de-viation of peak point shifting. Section 6 provides conclusions that in-clude a discussion of research contributions and future work.

2. Related work

Han and Liou [18] developed a mathematical model and relatednumerical methods to simulate the process of laser material interactionfor different laser beam modes. This model incorporated several phy-sical phenomena, including melting, solidification, vaporization, andevolution. Fathi et al. [19] presents a mathematical model of laserpowder deposition to predict temperature field, and studied the effectof laser beam and bead height on the melt pool depth and dilution.Akkas et al. [20] studied the relation between bead shape geometry andlaser deposition parameters by applying the method of Artificial NeuralNetwork (ANN) and neuro-fuzzy system. The predicted and resultingbead geometry values were within the 95th percentile accuracy. Saqibet al. [21] used design of experiment (DOE) and response surfacemethod (RSM) to design an experiment, which confirmed the impact offive parameters (laser power, scan speed, and powder flow rate et al.)on the bead height, width, and penetration. Goodarzi et al. [22] appliedthe DOE approach and observed that laser power and powder feed ratewere the main factors determining melted area. Furthermore, they es-tablished a relationship between main laser cladding process variablesand substrate melt shape. Based on experimental results of two metalcomponents, Barroi et al. [23] illustrated the influence of the laserpower of a diode laser in the range of 500W to 1000W on the shapes ofsingle tracks in scanner-based laser wire cladding, and discusses thisinfluence in respect to the heat dissipation in the substrate material.Ding et al. [24] built up a bead model prediction method, and thenintroduced the medial axis transformation algorithm for path planning.Urbanic et al. [25] conducted single-track experiments through the

DOE method, then set up a general predictive model of single beadshape by analysis of variance and generalized reduced gradient.Moreover, they identified the bead shape through data clustering.

There are also multiple analytical and numerical models for thelaser deposition process via applying finite element approach withelement death-and-birth function in commercial software. Toyserkaniet al. [26] introduced a 3-D transient finite element model of lasercladding by powder injection to investigate the effects of laser pulseshaping on the process, which is simulated for different laser pulsefrequencies and energies. Qian et al. [27] established a finite elementmodel to predict the temperature history of direct laser fabricatedTi–6Al–4 V thin wall samples, and analyzed the effects of laser powerand the effect of location within a sample on its temperature history.Wang et al. [28] developed a 3-D finite element model to predict thetemperature distribution and phase transformation in deposited stain-less steel 410during the Laser Engineered Net Shaping rapid fabricationprocess. Peyre et al. [29] presented a finite element model to predict theshapes of manufactured structures and thermal loadings induced by thedirect metal deposition process. Their thermal model can provide anadequate representation of temperatures near the melt-pool, and canreproduce with the thermal cycles and melt-pool dimensions with highaccuracy. Fallah et al. [30] developed a transient finite element ap-proach to simulate the temporal evolution of the melt-pool morphologyand dimensions during laser powder deposition without assuming anyof the geometrical characteristics. Michalrie [31] used finite elementtechniques for modelling metal deposition heat transfer analyses ofadditive manufacturing. He also proposed a new hybrid quiet inactivemetal deposition method to accelerate computer run times. Liu et al.[32] used finite element techniques to simulate multi-layer coating onthe cast iron, and investigated the microstructural evolution with em-phasis on the variation of the bonding zone. Although the finite elementmethod is widely used in modelling deposition processes through se-quential solutions, the process is interdependent, which needs a fullycoupled solution.

As mentioned above, although there is has been substantial researchinto bead shape modelling and prediction, a premise of most of thesestudies is that the laser nozzle is vertical and the substrate is horizontal(i.e. laser beam axis coincides with surface normal vector). Ramiro et al.[33] studied the cladding efficiency of different LMD nozzles by using ahybrid multi-process machine. They found that the continuous coaxialnozzle of 1mm is the best option for coating with the laser headworking vertically in terms of efficiency, productivity and mass de-position rate without loss hardness and quality of the coating. Liu et al.[34] investigated the relationship between the process parameters andgeometrical characteristics of cladding layer, which is deposited byhigh power diode laser with rectangle beam spot. They adopted a cir-cular arc to describe the geometry profile of the cross-section of thesingle track cladding. Paul et al. [35] focused on laser manufacturing invertical surface configuration and indicated that gravity flow broughtpeak point shifting of the deposited track. They developed an analyticalmodel incorporating gravity, which was validated by experiment. Zhuet al. [36] investigated the influences of the substrate-inclined angle onthe section size of cladding layers, and analyzed the forces applied tothe molten pool. They changed the substrate-inclined angles from 0° to150° with perpendicular laser nozzle. Wang et al. [37] proposed amethod of trajectory planning of laser cladding remanufacturing onNURBS (non-uniform rational B-splines) surface. This method kept thelaser gun head and the cladding surface normal vector on the same line.However, this study did not consider laser beam inclining situationsand relative orientation of the laser nozzle and the substrate.

3. Analytical model of bead morphology

As previously studied, laser beam moves with a known heat profile(Gaussian source) to melt and solidify a layer on a substrate. When thelaser beam moves on the substrate surface, the melt pool is being

Fig. 1. Schematic diagram of the laser cladding process.

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formed in the irradiated area. The boundary of melt pool can be ap-proximately described as two semi-ellipses dimensioned by L1, L2(Fig. 2). Meanwhile, the powder streams melt into beads over the meltpool, and then solidify into the cladding layer after the laser beammoves away.

Generally, the laser nozzle remains perpendicular to the substrate inthe process of laser cladding (the vertical posture, as shown inFig. 3(a)). The maximum height of bead shape is written by:

=hη f

ρ d vmaxc

L 1 (1)

where η f ρ d v, , , ,c L 1 respectively represent powder catchment effi-ciency, powder feed rate, density of liquid metal, diameter of laserbeam, and laser travel speed.

It is assumed that the upper clad height value satisfies the parabolicequation in the x–z plane, the track height at each point (x, y) can bededuced as:

⎜ ⎟⎜ ⎟= ⎛⎝

− ++

⎞⎠

⎛⎝

− ⎞⎠

h x y h x LL L

xw y

( , ) 1 ( )( )

1( )max

22

1 22

2

2 (2)

The melt pool width at section x is obtained as:

= ⎡

⎣⎢ − + − ∙ ⎤

⎦⎥w x w sign x x

wsign x( ) ( ) 1

( /2)( )0

2

02

(3)

where w0 refers to the cladding width at section x , and

= ≥≤{sign x x

x( ) 1 , 00 , 0 .

Considering the reachability of a nozzle, an angle always existsbetween the surface normal vector and the laser beam axis. In additionto the vertical posture, there are three tilting postures: 1) tilted nozzlerelative to a horizontal substrate, which is abbreviated to Nozzle Tilting(Fig. 3(b)); 2) substrate is tilting with perpendicular laser nozzle, whichis abbreviated to Both Tilting (Fig. 3(c)); 3) vertical nozzle relative to aninclined substrate, which is abbreviated to Substrate Tilting (Fig. 3(d)).

3.1. Tilted nozzle relative to a horizontal substrate

In the case of nozzle tilting, the substrate remains horizontal, whilethe laser nozzle is tilted with a small angle between the laser beam axisand Z-axis direction (Fig. 4 (a)). If the defocusing distance of the laserbeam is kept constant, the contour of the cladding bead will change(Fig. 4 (b)). According to the sine function formula, the laser spotdiameter on the tilting substrate d1 will increase with the increase of thetilt angle θ. The specific derivation is as follows:

− − = − = +d π θ α d sin π α as α π βsosin ( ) ( ),

2 2,1 0

=+

= =+

ε εdcos β

cos β θd d

cos βcos β θ

( /2)( /2 )

, (( /2)

( /2 ))θ θ1 0 0

(4)

The coefficient >ε 1θ . Since d0 and β are fixed, the laser spot dia-meter on the tilting substrate d1 is larger than the normal laser spotdiameter d0. According to the forming rule of laser cladding, the moltenpool becomes larger with the increase of effective spot diameter. If thepowder feeding is sufficient and stable, the cross-sectional width of thecladding bead becomes large after rapid solidification. However, thecross-sectional width is not the same as the laser spot diameter d1.Because the distribution of the laser beam power changes on the sub-strate after the laser beam is tilted, the energy density of the OA sidetends to increase in Fig. 5(a), and the effective spot size is proportionalto Eq. (4). On the other side, the calculated spot size on the substrate isOB, but the effective distribution of laser beam power is the OC area, sothe effective size of the molten pool should be the projection of OC–

∙OC cos θ| | .Therefore, after the laser beam is tilted, the original spot diameter d1

will be divided into two segments to form an effective spot size. Thed /20 segment near the tilt direction will increase, and the increasecoefficient is defined as εθ; the other side of d /20 will decrease, theincrease coefficient is defined as cos θ, and thus the overall effectivespot diameter ′d1 is:

= ∙ + ∙ = = +′ ε ε ε εd d d cosθ d cosθ2 2

, (2

).θ wθ

w10 0

0 (5)

Since the cladding bead width is approximately equal to the effec-tive spot diameter, the relationship between the cladding bead width w1in the case of nozzle tilting and the cladding bead width w0 in thevertical case is = εw ww1 0. In this case, εw is the width variation coef-ficient of cladding beads.

Fig. 2. Schematic of melt pool surface.

Fig. 3. Four postures between a laser nozzle and a substrate.

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Similarly, due to the rotation of Gaussian energy profile, the heightof the cladding bead will have some changes. Taking peak point heighthmax for example, if powder feeding is enough, and though the laserbeam is inclining, the largest powder accumulation point should still belocated where the laser beam axis has the highest power. Since theenergy density at the axis increases by a multiple of εθ, the height ofpowder accumulation increases along the axis of the laser beam. Theincrease influence factor can also be considered as εθ, and the maximumheight of powder accumulation in the final axial direction is ∙εh θmax . Asshown in Fig. 5(b), the cumulative height of powder in the z-axis

direction can be determined. The position of the highest point is rotatedby an angle around the pointO, and its projection in the z-axis direction(height of the cladding bead) ′hmax is:

= ∙ ∙ = ∙ = ∙′ ε ε ε εh h cosθ h set cosθ, ( )θ h θmax max h max (6)

The peak shifting of the cladding bead in the X-direction is Δ1, whichis the projection of the powder accumulation along the axis of the laserbeam on the x-axis, can be calculated as:

= ∙ ∙ = ∙ = ∙ε ε ε εh sinθ h set sinθΔ , ( ).θ θmax max1 Δ Δ (7)

Based on Eqs. (6) and (7), the cross-section of the cladding bead inthe case of nozzle tilting can be obtained by rotating all contour pointsof the cladding bead in the vertical case by the angle θ around the pointO. According to the measurement data of the width and height of thecladding bead, the corresponding three variation coefficients εw, εh, εΔ,are calculated, the cross-section of the single-pass cladding can be ob-tained in the case of nozzle tilting. The two steps of cladding beadchange are as shown in Fig. 5.

3.2. Nozzle perpendicular to an inclined substrate

According to the effects of gravity, surface tension, and viscousforces, the melted materials will shift along the slope during the lasermelt pool solidifying time in the case of both tilting. Fig. 6 shows theschematics of bead shape variation from start time ( =t 0) to end time( =t d v/0 ) of melt solidification. In the vertical surface ( = °θ 90 ), liquidmetal will fall by the resultant force of gravity, while resisted by surfacetension and viscous forces. Based on the research of Paul et al., [30], the

Fig. 4. Schematic of bead shape changes in the case of nozzle tilting.

Fig. 5. Predictive model of cladding bead in the case of nozzle tilting.

Fig. 6. Schematic of bead shape changes in the case of both tilting.

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expression of the relationship between gravity and viscous forces,combined with temperature surface tension, can be expressed as:

=∙

−u U zz ρ g

γ D z( )2LL slop

TS

2

1 (8)

where uL refers to kinematic viscosity,U z( ) refers to fluid flow velocity,z refers to the height of the cladding bead, ρL refers to density of liquidmetal, gslop refers to component of gravity in z-axis direction, and γ1 andDTS refer to surface tension and temperature on the top boundary of themolten deposit.

The peak shifting of cladding bead Δ2 can be expressed as thefunction of fluid flow velocity U z( ) concerned with cladding beadheight z and solidifying time ( =t d v/0 ).

= −dvu

z ρ gsinθ γ D zΔ4

( 2 )L

L TS20 2

1 (9)

Based on Eq. (9), the local shifting value of cladding bead border( =z 0) is zero, which means that the overall shifting of cladding beadwill not happen in the case of both tilting. Additionally, the solidifyingtime is reduced under the condition of higher scan speed, and the peakshifting of cladding bead will shrink. If all the relevant parameters inEq. (9) can be obtained, the top boundary (i.e. shifting bead shape) canbe approximately obtained.

3.3. Vertical nozzle relative to an inclined substrate

For most of the Cartesian coordinate robots, the laser nozzle usuallyhas vertical downward posture. There may be the situation of a verticalnozzle with substrate tilting in practical cladding process. Since thissituation is a combination of the above two situations, the claddingbead shape of substrate tilting can be determined by the combinedinfluence of nozzle tilting and both tilting.

As shown in Fig. 7 (a), at time =t 0, the cladding bead width in-creases in the case of nozzle tilting. All contour points of the claddingbead rotate by an angle θ to the upside direction of the tilting substrate.The parameters of cross-section ( ′ ′d h, , Δmax1 1) can be calculated by Eqs.(5)–(7). When it comes to =t d v/0 , the melting metal will slide to thelow side of the substrate due to the effect of gravity (Fig. 7 (b)). Thepeak shifting of cladding bead Δ2 will be calculated by Eq. (9). Sincethe laser nozzle is vertically downwards in posture, the tilt directions ofthe laser nozzle and the substrate are opposite, therefore the final peakshifting value is obtained as = −Δ Δ Δ3 2 1. In certain special condi-tions, the tilt directions of the laser nozzle and the substrate may be thesame, and the final peak shifting value would be obtained by

= +Δ Δ Δ3 2 1.

4. Experimental setup

In this study, a laser cladding machine (Zhongke Dagang Laser Co.,Ltd.) is used to conduct a set of experiments (As shown in Fig. 8). Thesystem uses a coaxial nozzle to feed metal powders to the melt pool. Themotion control system of the machine contains a KUKA's 6-DOF roboticarm and a 2-DOF rotating platform. The machine uses an IPG highpower laser (YLS-4000 model) with a rated output power of 4KW, and amodulation frequency (Max) of 5KHZ. The experiment uses a DPSF-2type powder feeder, which adopts coaxial powder feeding. The nozzlehas a diameter of 1.5mm. The carrier gas feeds the powder (BeijingAirlines Group 625), and the powder feeding gas is argon gas.

In the experiment, Fe-base powders are deposited on a substratemade of Q235 steel. The carbon steel was selected as the substrate witha dimension of 100mm x 50mm x 10mm. The nominal chemicalcomposition of Fe-base powder is listed in Table 1. The substrate (steelplate) is clapped on the rotating platform, while the laser nozzle isequipped on the multi-axis robot arm. Therefore, both types of tiltangles were easily adjusted.

Because the experiment is designed to investigate how tilt anglebetween laser nozzle and substrate affects bead morphology, it is notnecessary to consider all the parameters of the laser cladding process.Table 2 shows the design of the experiments. The experiments areconducted using different tilt angles and powder feed rates, whilekeeping the other parameters constant (laser power is 1900W and scanspeed is 5mm/min). To ensure the stability of the experiments, thedefocus amount of the laser beam must be kept constant. In addition, itis necessary to clad multiple tracks for each parameter set, so we havedone the same experiment on three substrates (1#, 2#, and 3#). Auniform track is then chosen to prepare samples. By EDM cutting,mounting, polishing, and etching, the cross-sectional images of the

Fig. 7. Schematic of bead shape changes in the case of substrate tilting.

Fig. 8. Multi-axis laser cladding robot platform.

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cladding beads are obtained with the upright materials microscope(LEICA DM4 M). These images are then imported into AutoCAD tomeasure and compare bead shapes. Finally, the parameters of beadshape (w h, , Δ) are obtained according to measuring date.

5. Results and discussions

Fig. 9 shows the cross-section of a single track in the vertical case.The cladding bead is substantially axisymmetric in this traditional case,and the centreline of the width passes through the peak point. However,when the relative posture of the laser beam and the substrate is notperpendicular, the most obvious change is the peak point offset. Asshown in Fig. 10, the cross-section of cladding bead has a significantshifting Δ3 between the peak point and the width center line in the caseof substrate tilting.

Based on the sampling-drawing-measuring method, three para-meters of bead shape (w h, , Δ) are qualitatively obtained, and then theproposed predictive model is verified and analyzed based on the mea-sured contour parameters. Fig. 11 shows the cross-section of several

cladding tracks from the experimental results.

5.1. Titled nozzle relative to a horizontal substrate

In the first case, the impact of laser beam power and powder feedrate on cladding width and height is first analyzed. While other para-meters are kept constant, cladding height is gradually increasing withpowder feed rate increasing as long as the laser beam power is suffi-cient. Concurrently with this process, cladding width is being reduced(Fig. 12).

The impact of the nozzle tilt angle on cladding width and height isanalyzed. It is apparent that the trend of cladding width w closelyfollows the tilt angle θ, as seen in Fig. 13. When θ is going up, w is alsoincreasing. The reason for this phenomenon is that when laser nozzletilts, the actual spot size (i.e. diameters of upper melt pool) on thesubstrate becomes bigger. Consequently, the area where the powder ismelted and re-solidified has been enlarged, so the width of the claddingbead increases. The predictive and experimental values are compared inFig. 13. To avoid initial error, the common cladding situation ( =θ 0)was directly chosen as the basic data to verify the prediction of Eq. (5).

Along with nozzle tilting, the distribution of powder accumulatedpoints has also been influenced, but not significantly. As shown inFig. 14, the bead height h will slightly increase while the peak point willdeviate towards the direction of the laser beam tilt. The predictive andexperimental values of the cladding height are compared at three tiltangles ranging from 10°to 30°, as illustrated in Fig. 14(b)-(d). Thepredictive value is close to the experimental value at each tilt angle,which indicates that the theoretical calculation error of cladding heightin Eq. (6) is small.

As shown in Fig. 15 (a), the experimental value of peak pointshifting increases as the tilt angle θ increases. However, there is a largedeviation between the experimental value and the predictive value inEq. (7). In Fig. 15 (b), when the powder feeding rate is 15 g/min, theprediction error is small. However, when the powder feeding rate in-creases, the deviation between the experimental value and the pre-dictive value also increases (as shown in Fig. 15 (c) and (d)). It ispossible that surface tension and interfacial viscous shearing force havea great influence on the final forming of the cladding bead. Thesefactors should be considered in subsequent research to obtain accuratetheoretical prediction results.

5.2. Nozzle perpendicular to an inclined substrate

In the second case, the impact of the effect of gravity (i.e. increasingtilt angle of a substrate) on cladding width, cladding height, and peakpoint shifting is analyzed. Fig. 16 shows that cladding width and heightremain approximately constant except for shift value Δ2. Although the

Table 1Chemical compositions (wt%) of the powder.

elements

C Cr Si Fe Mo Ni Mn

JG-2 0.05-0.09 16.0-19.0 0.5-1.1 Bal 2.0-3.0 22-30 0.5-1.0

Table 2Design of experiments.

Set Tilt angle Powder Feed Rate(degree°) (g/min)

1 0 152 0 203 0 254 10 155 10 206 10 257 20 158 20 209 20 2510 30 1511 30 2012 30 2513 40 1514 40 2015 40 25

Fig. 9. Single-track clad of nozzle tilting.

Fig. 10. Single-track clad of substrate tilting.

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Fig. 11. Cross-section of clad beads.

Fig. 12. Effect of powder feed rate on cladding width and height.

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Fig. 13. Predictive and experimental values of cladding width in the case of nozzle tilting.

Fig. 14. Predictive and experimental values of cladding height in the case of nozzle tilting.

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peak point is most affected by gravity during the solidification process,it will shift along the direction parallel to the substrate, and its finalpowder accumulation in the Z-direction will not change. It is indicatedthat solidifying time is a critical factor for the level at which tilt angle is

able to impact the melt pool displacement, similarly to the time d v/0 inEq. (9).

To demonstrate this assertion, the impact factor of the powder feedrate is substituted by the scan speed. The scan speed was set at three

Fig. 15. Predictive and experimental values of peak point shifting in the case of nozzle tilting.

Fig. 16. Experimental result of bead shape parameters in the case of both tilting.

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levels, with a high level of 6mm/s and a low level of 4mm/s. With thesame tilt angle, the peak point shifting will gradually decrease with theincrease of scan speed (Fig. 16 (d)), which is consistent with the pre-dictive value in Eq. (9). The reason for this is that the component ofgravity in the direction of the substrate slope is rising from zero to g dueto the increase of tilt angle (generally less than 90°). The flow velocityof melt pool increases as well. The peak point shifting value Δ2 is alsorelated to the cladding height. With the same tilt angle, the peak pointshifting value will increase with the increase of the cladding height.

5.3. Vertical nozzle relative to an inclined substrate

The third case could be regarded as a superposition of the previoustwo cases. Firstly, when the laser spot is irradiated to the substrate(t= 0), the contour parameters of cladding bead will change similarlyto the case of nozzle tilting. Then, during the solidifying time ( =t d v/0 ),the shape of the cladding bead will be affected by gravity, similarly tothe case of both tilting.

According to previous experimental verification, the height andwidth of the cladding bead will be influenced only in the case of nozzletilting. The peak point shifting will be influenced in both cases, how-ever. As shown in Fig. 17, the width of the cladding bead increases withthe increase of the tilt angle, which is the same as the result of nozzletilting. Therefore, the theoretical value also adopts the calculation re-sult of Eq. (5) in the case of laser tilting. The comparison between thepredictive value and the experimental value of the cladding height isshown in Fig. 17(b)-(d). The theoretical calculation error of claddingwidth in Eq. (5) is small.

Similarly, the height of the cladding bead increases with the in-crease of the tilt angle, which is the same as the result of nozzle tilting.Therefore, the theoretical value also adopts the calculation result of Eq.(6). The comparison between the predictive value and the experimental

value of the cladding height is shown in Fig. 18, which indicates thatthe prediction results are reliable.

Fig. 19 (a) shows that under the three feeding rates, as the angle ofinclination increases, the shifting value of the peak point is expandingin the direction of the lower slant plate due to the effect of gravity.However, the effect of gravity is opposite to the effect of the laser beamtilting, thus the peak point shifting distance should be obtained by

= −Δ Δ Δ3 2 1. Fig. 19 (b) shows the comparison between the predictivevalue and the experimental value of peak point shifting with the feedingrate of 15 g/min. The deviation between the experimental value and thepredictive value is still obvious. Since the experimental value of is Δ1generally lower than the predictive value (As shown in Fig. 15), it isalso reasonable that the experimental value of Δ3 is generally higherthan the predictive value. In the future, the effects of gravity, surfacetension, and viscous shear force on the formation of the molten layershould be further studied to obtain a more accurate predictive model.

Based on the above experimental analysis, the cladding width,cladding height, and peak point shifting are all influenced by the tiltingangle. If the tilting angle is less than 30°, the increase of cladding widthand height is relatively minor, and the peak point shifting value isroughly smaller than 5% of the cladding width. Considering that thecladding surface is usually multi-tracks overlapping, the minor error ofcladding bead can be eliminated using a reasonable overlap ratio. Thus,the bead shape and overlapping surface quality will not be influencedwithin a certain range of tilt angles, which exist between orientations ofthe laser nozzle and substrate.

6. Conclusion

In this work, the effects of tilt angle between a laser nozzle andsubstrate on bead morphology have been studied. Three tilting pos-tures, including nozzle tilting, both tilting, and substrate titling, have

Fig. 17. Predictive and experimental values of cladding width in the case of substrate tilting.

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been investigated. The analytical model of bead morphology was de-veloped to predict bead shapes under these conditions. Three para-meters that measure the cladding bead, including width w, height h,and peak point offset Δ, were determined using the proposed model. Aset of experiments was conducted. The analytical model was demon-strated using the experimental data. Compared with the experimentalvalues, the predictive values for the width and height of the claddingbead are fairly reliable. The deviation between the predictive and ex-perimental values of cladding width and cladding height is less than3%. It turns out that the predictive values for peak point shifting are notas accurate, and that the deviation between the experimental value andthe predictive value increases along with the powder feeding rate in-creasing. Some other factors, such as surface tension and interfacialviscous shearing force, may have certain influences on the final formingof the cladding bead.

Based on the predictive model and experimental results, this studyillustrates that the variation of cladding width and height with thetilting angle can be accurately calculated by the predictive model, andthat the peak point shifting value is roughly smaller than 5% of clad-ding width when the tilting angle is less than 30°. This minor error canbe eliminated using a reasonable overlap ratio. Within an acceptablerange of tilt angle, the laser nozzle can be non-coaxial with the normalvector of the cladding surface in real time. Therefore, the variationrange of laser nozzle posture provides the possibility for path optimi-zation of the robot arm. This study provides a certain theoretical andpractical basis for the laser nozzle posture optimization in laser clad-ding path planning of a multi-axis robot arm.

In the future, we will develop a predictive model of the laser clad-ding process by taking into account more factors such as surface tensionand shear forces associated with liquid metal forming. In addition, our

Fig. 18. Predictive and experimental values of cladding height in the case of substrate tilting.

Fig. 19. Predictive and experimental values of peak point shifting in the case of substrate tilting.

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future work will be focused on investigating the effect of various pro-cess parameters on the microstructure, fracture toughness, and hard-ness of the components built by laser cladding.

Acknowledgments

This research was partially supported by National Natural ScienceFoundation of China (51305443, 51475459), Natural ScienceFoundation of Jiangsu Province (bk20130184, bk20160258), StateScholarship Fund of the China Scholarship Council (201706425017),Science and Technology Planning Project of Xuzhou City (KC18073),and A Project Funded by Priority Academic Program Development ofJiangsu Higher Education Institutions (PAPD). The authors would alsolike to thank Mr. Jason Benoit for his proofreading of this paper.

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