optics and lasers in engineering - unistsf.unist.ac.kr/research/variation_weldseam.pdfcorrelation...

Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at SciVerse ScienceDirect

Optics and Lasers in Engineering

0143-81http://d

n CorrE-m

Pleasgalva

journal homepage: www.elsevier.com/locate/optlaseng

Correlation analysis of the variation of weld seam and tensile strengthin laser welding of galvanized steel

Amit Kumar Sinha a, Duck Young Kim a,n, Darek Ceglarek b

a School of Design and Human Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulsan, Republic of Koreab International Digital Laboratory, WMG, University of Warwick, West Midlands CV4 7AL, United Kingdom

a r t i c l e i n f o

Article history:Received 11 February 2013Received in revised form17 April 2013Accepted 17 April 2013

Keywords:Laser weldingVariation of weld seamTensile strengthPart-to-part gap

66/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.optlaseng.2013.04.012

esponding author. Tel.: +82 52 217 2713; fax:ail address: [email protected] (D.Y. Kim).

e cite this article as: Sinha AK, et alnized steel. Opt Laser Eng (2013), ht

a b s t r a c t

Many advantages of laser welding technology such as high speed and non-contact welding make the useof the technology more attractive in the automotive industry. Many studies have been conducted tosearch the optimal welding condition experimentally that ensure the joining quality of laser welding thatrelies both on welding system configuration and welding parameter specification. Both non-destructiveand destructive techniques, for example, ultrasonic inspection and tensile test are widely used in practicefor estimating the joining quality. Non-destructive techniques are attractive as a rapid quality testingmethod despite relatively low accuracy. In this paper, we examine the relationship between the variationof weld seam and tensile shear strength in the laser welding of galvanized steel in a lap jointconfiguration in order to investigate the potential of the variation of weld seam as a joining qualityestimator. From the experimental analysis, we identify a trend in between maximum tensile shearstrength and the variation of weld seam that clearly supports the fact that laser welded parts havinglarger variation in the weld seam usually have lower tensile strength. The discovered relationship leadsus to conclude that the variation of weld seam can be used as an indirect non-destructive testing methodfor estimating the tensile strength of the welded parts.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In the automotive industry, there has been a growing interest inthe use of laser welding technology for joining new materials tomeet the increasing demand of corrosion resistant, lightweight anddurable auto-body parts. Laser welding has many advantages overconventional joining methods such as high speed and non-contactwelding, deep penetration, low heat input per unit volume, low heataffected zone, effective integration with industrial robots, and thecapability of joining materials by single side access [1–3].

Tensile strength, hardness, fatigue and toughness are the fourmajor mechanical properties that are widely used to judge the qualityof welding. In particular, tensile strength is most popularly used inpractice due to the fact that it includes very important informationabout the breaking point, maximum load, fracture position and thepercentage elongation of weldment. In order to measure the tensilestrength of weldment, it is necessary to carry out the tensile testwhich requires time, cost, and wastage of materials. Therefore theestimation of the tensile strength in a non-destructive way is a verychallenging task for academician and practitioners.

Several studies have been made to scrutinize the effects of thechanges in welding process parameters on the resulting geometry

ll rights reserved.

+82 52 217 2709.

. Correlation analysis of thetp://dx.doi.org/10.1016/j.opt

of weld seam (usually called weld pool or weld bead interchange-ably in the literature). For example, Chen et al. [4] and Furusakoet al. [5] investigated relations between the geometry of weldseam and tensile strength using conventional destructive techni-ques for measuring the geometry of the weld seam and tensilestrength. Interestingly, some researchers [6,7] reported that thewidth of weld seam is correlated with welding quality.

However, little attention has been given to analysing therelationship between the variation in the width of weld seamsand tensile strength in a non-destructive way. Therefore, weconduct laser welding experiments to examine the statisticalcorrelation between them, using 1.8 mm and 1.4 mm thick galva-nized steel parts in a single lap joint configuration. We make part-to-part gaps purposely between the upper and the lower parts toexamine the effects of zinc vapour on the tensile shear strength ofweldment. Two quality characteristics namely, variations in thewidth of the top weld seam and tensile shear strength (hereaftercalled ‘tensile strength’ in short), and three welding parameters:laser power, welding speed, and part-to-part gap are consideredfor the experiment.

2. Related work

Many studies in the domain of laser welding have focused onempirical search of welding process parameters in order to

variation of weld seam and tensile strength in laser welding oflaseng.2013.04.012i

www.elsevier.com/locate/optlaseng

www.elsevier.com/locate/optlaseng

http://dx.doi.org/10.1016/j.optlaseng.2013.04.012



mailto:[email protected]





A.K. Sinha et al. / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎2

maximize welding quality by minimizing the formation of poros-ity, spatter and intermetallic brittle phase [8]. On the basis ofobserving the state (welding defects) around the weld-zone,Kawamoto et al. [9] obtained ranges of laser welding parameterswhich will improve the tensile strength of laser welded aluminumalloy in a lap joint configuration. They concluded that heat inputper unit length and part-to-par gap are the two important factorsfor deciding the tensile strength of laser welding in a lap jointconfiguration. Acherjee et al. [10] proposed a sequentially inte-grated optimization approach for predicting the optimal level ofweld strength and the width of weld seam. Their optimizationapproach is developed based on Taguchi method, response surfacemethodology (RSM), and desirability function analysis.

Laser welding joints are not axisymmetric, and thus the geometryof weld seam affects the strength of laser welded joints [11–13]. Inother words, the fatigue levels, the depth of penetration, the length ofweld seam and the width of weld seam are yet another importantfactor for the prediction of laser welding quality. Stoschka et al. [14]obtained the optimum value of feasible fatigue levels of laser weldingjoints. Lawrence et al. [15] identified that fatigue resistance of laserwelding mainly depend upon the weld seam geometry. They alsoproposed a mathematical model on the basis of weld seam geometryfor predicting the fatigue resistance of welds.

Siva Shanmugam et al. [16] predicted the weld seam geometry(in terms of the depth of penetration, and the length and the widthof weld seam) in laser welding of stainless steel by using a finiteelement method (FEM)-based simulation model. Their simulationmodel is developed based on the calculation of the total heat inputby assuming three dimensional conical Gaussian heat sources. Theyalso considered the effects of latent heat of fusion, the convectiveand radiative aspects of boundary conditions for simulation.

Chen et al. [4] evaluated the quality of full penetration laserwelding of 5A90 Al–Li alloy on the basis of the width of weld seam.They found that the ratio of the width of the bottom (root) weldseam to the width of the top (face) weld seam influences thetensile strength, and further, if this ratio is larger than 0.4 in fullpenetration laser welding then a stable formation of keyhole, lessdefect and higher tensile strength can be expected. They con-cluded that this ratio has a polynomial relationship with thetensile strength, and follows a normal distribution with thepercentage elongation of the laser welded joint. Furusako et al.[5] established an ad hoc model to predict the tensile shearstrength of a laser welded lap joint based on the width and thelength of weld seam. They identified three fracture types of laserwelded lap joints. The first one is the fracture at the base metal;the second is the fracture at the weld seam; and the last is thefracture at the curvature in between the base metal and theweld seam.

Fig. 1. The laser welding system (2.5 axis gantr

Please cite this article as: Sinha AK, et al. Correlation analysis of thegalvanized steel. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.op

Acherjee et al. [17] developed a RSM-based optimization modelto predict the weld strength and the width of weld seam of laserwelding. This model, by selecting an appropriate combination oflaser power and welding speed, determines an energy thresholdwhere the weld strength reaches the maximum. Zhao et al. [18]also utilized the RSM for investigating the effect of changes in laserwelding process parameters (i.e. laser power, welding speed, part-to-part gap and focal position) on the geometry of weld seam (i.e.the depth of penetration, the width of weld seam and the surfaceconcavity) of galvanized steel in a lap joint configuration. Ghosaland Chaki [19] developed an Artificial Neural Networks (ANN)model for predicting the depth of penetration. Sathiya et al. [20]further investigated the relationship between laser welding pro-cess parameters (laser power, welding speed and focal length) andresponse variables (the width of weld seam, the depth of penetra-tion and tensile strength) by using the optimized process para-meters obtained by a Genetic algorithm-based ANN model.

In general, the size of weld seam, particularly, the width ofweld seam decreases with the increasing welding speed becauseheat input per unit length decreases. Benyounis et al. [21] foundthat higher heat input does not guarantee the higher tensilestrength of a laser lap welded joint because higher heat inputusually facilitates the formation of a wider heat affected zone.Neither can welding with relatively lower laser power and higherwelding speed, guarantee a higher tensile strength due to the lackof full penetration.

A number of laser welding experiments have been carried outto study the effects of process parameter change (e.g. weldingspeed, focal length, laser power, and the flow of shielding gas) anddifferent materials on welding quality. Several reports havedetailed the role of the weld seam geometry on the quality ofwelding. In particular, some studies reported that the width ofweld seam is an important property of welded joints. However, wefound that it is necessary to take a close look at the variation in thewidth of weld seam, rather merely than the width itself. In thisregard, we examine the importance of the variation of weld seamfor the indirect estimation of laser welding quality.

3. The experiment

3.1. The laser welding system

The laser welding system (see Fig. 1) used for the experiment isa gantry-based automated welding system that delivers a laserbeam from IPG YLS-2000-CT fiber laser source with a maximumoutput discharge of 2 kW in the TEM01 mode of laser radiation.A laser beam of 35 mm diameter was focused by a parabolic mirror

y robot, maximum power of 2 kW).

variation of weld seam and tensile strength in laser welding oftlaseng.2013.04.012i




A.K. Sinha et al. / Optics and Lasers in Engineering ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3

with a focal length of 168 mm. Table 1 lists the laser technicalparameters used in the experiment. The intensity distribution ofthe focused beam was measured using PRIME’s FM-35 beamanalyser; the focused spot size was approximately 600 mm dia-meter which contained 86% of the total power passed. The laserwelding head is mounted directly on a 3-axis gantry system thatconsists of belt-drive linear modules, travel limit/home positionsensors, and servo motor system.

3.2. The experimental materials

Laser welding experiments are conducted on the lap joint of twodifferent galvanized steels: SGARC440 (lower part) and SGAFC590DP(upper part). The dimensions (length�width� thickness) of thelower and the upper parts are 130 mm�30 mm�1.8 mm and130mm�30 mm�1.4 mm, respectively. The amount of Zinc coat-ing on the lower and the upper parts are 45.5 g/m2 and 45.4 g/m2,respectively. Chemical compositions and mechanical properties ofthe tested materials are listed in Tables 2 and 3, respectively.

3.3. Part-to-part gap

Strict assembly tolerance between the upper and the lowerparts to be joined needs to be guaranteed during the joiningoperations of laser welding. In general, the gap can be controlledwithin 10% of the thickness of the upper part upon which the laserbeam is incident [22].

This tight part-to-part gap control is even more critical in laserlap welding of galvanized steel since we have to additionally allowfor the minimum gap between the parts, so that the vaporized zincproduced by the welding heat can be exhausted through the gap. If

Table 1Laser technical parameters.

Parameters Value

Maximum laser power 2 kWLaser mode TEM01

Focus length 168 mmOperation mode CWSwitching ON/OFF time 80 msEmission wavelength 1070 nmOutput power Instability 2.0%BPPn(1/e2) at the output of fiber 2.0 mmnmrad

Table 2Chemical composition (wt%) of the test materials.

Tested material C (%) Si (%) Mn (%) P (%) S (%)

SGARC440 (1.8 mm thickness) 0.12 0.5 1.01 0.021 0.004SGAFC590DP (1.4 mm thickness) 0.09 0.26 1.79 0.03 0.003

Table 3Mechanical properties of the test materials.

Tested material Tensile test

Yield strength(N/m2)

Max-tensile strength(N/m2)

Elongation(%)

SGARC440 (1.8 mmthickness) 327.5 451.1 38

SGAFC590DP (1.4 mmthickness) 413.8 625.7 28

Please cite this article as: Sinha AK, et al. Correlation analysis of thegalvanized steel. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.opt

the gap of galvanized steel sheets is too small, it may then createweld defects such as porosity, spatter, intermetallic brittle phaseand discontinuity formed by zinc vapour entrapment in thewelding joints [23,24]. These types of weld defects occur usuallybecause the boiling point of Zn (906 1C) is lower than the meltingpoint of Fe (1538 1C) [25].

On the other hand, if the gap is too large, it is generally difficultto obtain good bonding at the interface between sheets becauselaser welding does not induce any forces on the parts to be joined,as does resistance spot welding [6]. According to Akhter [26], theeffective gap through which zinc vapour can escape from the weldpool should be in between 0.1 to 0.2 mm with respect to thethickness of the upper and the lower parts as well as the thicknessof zinc coating. Particularly, when we apply clamping force to thejoint, we should then increase the gap in a certain percentage [27].

Akhter et al. [28] proposed a mathematical relationshipbetween part-to-part gap and sheet thickness, given by:

gt¼ AvtZn

t3=2

where

g: Part-to-part gapt: Sheet thicknessA: Empirical constantv: Welding speedtZn: Thickness of Zinc coating on each part at the interface.

They presented that if the values of g/t is approximately inbetween 0.2 and 0.3, then we can expect good quality of laserwelding of galvanized steel.

Several techniques have been developed for part-to-part gapcontrol in laser welding of galvanized steel in lap joint configura-tion. Shims (e.g. metal gauge type) are inserted in between theupper and the lower parts to create a fine gap that provides arequired clearance for zinc vapour [29]. Pre-stamped projectiontechnique creates V-shaped tabs in the lower part which act as gasventing channels that allow zinc vapour to escape during laserwelding process [30].

The laser dimpling technique is a practical method to create arequired part-to-part gap in between the upper and the lowerparts by humping effects [31]. The main advantage of thistechnique is that dimples can be produced with the same lasersystem used for the laser welding [32].

‘Fill the part-to-part gap with a porous metal’ is yet anothertechnique to maintain part-to-part gap where a porous powermetal will provide room for zinc vapour to pass through. It isunfortunately difficult to implement this technique in a realproduction environment due to its long processing time.

‘Pre-drilling vent hole along the welding line’ allows zincvapour to escape through the weld zone without causingexpulsion of molten metal [1]. A time consuming preprocess,pre-drilling is necessary by nature.

‘Prior zinc removal from the weld area on a part joint’ is also aninnovative technique to control part-to-part gap in laser weldingof galvanized steel in lap joint configuration [33]. In order toprovide corrosion protection, the zinc removed zone could befurther coated with nickel, which has a higher vaporizationtemperature as compare to the fusion temperature of steel.

2.5 kW pulse CO2 laser welding of galvanized steel in lap jointconfiguration without a tight part-to-part gap control withvisually sound welding has been proposed first by [34]. Kennedyet al. [35] also claimed successful laser welding of galvanized steelwithout a tight part-to-part gap control by using pulsed Nd:YAGlaser. Tzeng et al. [36] reported successful lap welding of galva-nized steel with porosity/spatter free welds by using a 400 W






pulsed Nd:YAG laser. They suggested that by careful control ofpulse energy, pulse duration, peak power density, mean power,and welding speed, zinc gas can be reduced in the pulsed modeand effectively exhausted by stabilized keyholes. However,the limitation of low laser power causes slow welding speed of

Table 4Part-to-part gap control techniques in laser welding of galvanized steel.

Industrialsolutions

Description Advantages

Shiminsertion

Insert shims in between sheets Intuitive and easy toclearance

Pre-stampedprojection

A preprocessing creates V-shaped tabs in thelower part which act as a gas venting channelsthat allow zinc vapour to escape during laserwelding process

Useful for hem jointsconfiguration)

Laserdimplingtechnique

In this technique a preprocessing is carried outin which the laser beam generates dimples tomaintain a part-to-part gap.

Dimples can be produsystem used for the l

Fill the part-to-part gap

with aporouspowdermetal

The porous powder metal allows zinc vapour toescape without disturbing molten metal

No need to remove thafter welding

Pre-drillingvent holetechniques

Pre-drilling vent holes along the welding lineallow zinc vapour to escape without causingexpulsion of the molten metal

No need to provide p

Prior zincremoval

techniques

Remove zinc coating from welding zone andthen coat the treated zone with nickel in orderto provide corrosion protection

Nickel coating not onresistance but also revapour because nicketemperature as comptemperature of steel

Low powerpulsed laserwelding

By careful control of pulse energy, pulseduration, peak power density, mean power, andwelding speed, zinc gas can be reduced in thepulse mode and effectively exhausted throughstabilized keyholes

Literature survey reveYAG pulsed laser prowelds

Altered jointgeometrytechniques

Altered joint geometry offers channels betweenthe metal parts to exhaust zinc vapour

This technique is verydimensional variationgap is high

Dual beamhybrid

technique

The first beam creates a slot as an effect ofpreheating and the second beam performsactual welding process

The first beam facilitazinc that will prevent

Addition ofoxygen as ashielding gas

to argon

Addition of a small amount of oxygen (2–5%) asa shielding gas to argon facilitates the zinc toreact with oxygen and reduce the effect ofvaporized zinc

No need to provide p

Verticalpositioningof metalparts

Metal parts are positioned and moved verticallyduring welding while the laser beam is staticand applied to parts horizontally

Vertical position of mvapour to escape thro

Synchronousrolling

techniques

The additional roller generates pressurebetween part-to-part gap as well as createsfavorable conditions for rapid heat transfer fromthe upper part

Decrease the formatiocompound like zinc o

Fig. 2. Weld joint configuration (material


less than 2.4 mm/s, which is hard to maintain in an industrialenvironment.

Bilge et al. [37] claimed that if the metal parts are positionedand moved vertically during welding while the laser beam is staticand applied to parts horizontally, then continuous wave CO2 laser

Disadvantages Reference

control the required Needs additional work and tool for shiminsertion

[29]

(special case of lap joint Needs preprocessing [30]

ced with the same laseraser welding

Needs two-step process: (i) pre-processing by which dimples willgenerate and (ii) actual welding

[31,32]

e porous powder metal Difficult to implement in a realproduction environment

[33]

art-to-part gap Time consuming and expensive process [33]

ly provides good corrosionmoves the problem of zincl has a higher vaporizationare to the fusion

Preprocessing is necessary [34]

als that both CO2 and Nd:vide porosity/spatter free

The limitation of low laser power causesslow welding speed of less than 2.4 mm/s, which is hard to maintain in anindustrial environment

[37]

useful when thein between part-to-part

Intentionally, we need to create alteredjoint geometry in the form of eitherconcave or convex on the top surface ofthe metal part

[39]

tes vaporization of theweld defects

Needs additional complex equipment [40], [41],

art-to-part gap The flow rate of shielding gas must beoptimized otherwise plasma willdissipates and appears as oxides porosityon the surface of the weldment

[42]

etal parts allow zincugh the weld zone

Due to difficult positioning andmovement of parts this idea has beenrarely applied in industry

[38]

n of brittle intermetallicxide

Needs additional roller during welding [43,44]

compositions are presented in wt%).






welding on a lap joint produces better results. However, precisepositioning and movement of parts are required which are not soeasy in practice.

The altered joint geometry techniques, which offers controlledchannels between the metal parts to exhaust zinc gas, was used by[38] in laser welding of galvanized sheet steels. In general, alteredjoint geometry is usually created in the form of either concave orconvex on the top surface of metal part. This technique is veryuseful when the dimensional variation in between part-to-partgap is high.

Dual beam hybrid technique (called also as bi-focal hybrid orshaped beam techniques) [39,40] that will give rise to preheatingeffects to eliminate zinc, is another most utilized technique inindustry [39]. This technique however needs additional complexequipment.

The addition of a small amount of oxygen (2–5%) as a shieldinggas in the argon facilitates the zinc to react with oxygen andreduces its explosive effect [41]. However, the flow rate ofshielding gas must be optimized otherwise plasma will dissipateand appear as oxides porosity on the surface of weldment Table 4.

In this experiment, we have intentionally created part-to-partgap by inserting a conventional metal thickness gauge (thickness:

Fig. 4. Schematic diagram for formation of weld pool during laser welding of zinc

Fig. 3. Part-to-part gap created by metal thickness gauges.


0.1 mm, 0.2 mm, 0.3 mm) in between the upper and the lowerparts to be joined as shown in Fig. 2 and Fig. 3.

3.4. Weld pool geometry

Fig. 4 illustrates the formation of weld pool during laserwelding of zinc-coated steel sheets with a part-to-part gap. Thethickness of the upper part (t1) and the lower part (t2) are 1.4 mmand 1.8 mm, respectively. The details of weld pool geometry areillustrated in Fig. 5. In the literature, weld pool geometry (weldseam geometry or weld bead geometry) includes the width of thetop (face) weld seam (W1), the width of the bottom (root) weldseam (W2), the depth of penetration (D), up sunk height on the topweld seam (L1), up protruded height on the top weld seam (L2),down sunk height on the bottom weld seam (L3), down protrudedheight on the bottom weld seam (L4), and aspect ratio (D/W1)[44,45]. There have been many research efforts to utilize the weldpool geometry for measuring the welding quality indirectly, forexample, Mistry [46] found that the tensile strength of laser lapweld joints is proportional to the undercut depth of the top part,given by 0.15� (t1+part-to-part gap).

It is known that the width of weld seam is usually correlatedwith welding speed. In general, W1 is greater than W2 althoughthe difference between these two values depends on weldingspeed and incident laser intensity [6]. Kimara et al. [47] reportedthat the difference between W1 and W2 decreases with weldingspeed at the constant laser power. Cline and Anthony [48]developed a simulation method to approximate a set of weld poolgeometry curves that provide a good estimation of penetrationdepth versus laser power and welding speed. Akhter et al. [49]developed a mathematical model to show that the powerabsorbed by the laser welded plate (i.e. amount of heat content)is correlated with the width of weld seam and the depth ofpenetration.

According to Zhang [7] the width of weld seam and the depthof penetration are the most important factors that determine thewelding quality. They have shown that the width of the bottomweld seam is the direct physical parameter reflecting the depth ofpenetration. However, in practice, it is not easy to inspect thewidth of the bottom weld seam directly, and thus, they estimatedthe width of the bottom weld seam according to the shape of thetop weld seam [7]. For this reason, this paper also investigates the

coated steel plate in a lap joint configuration in existence of part-to-part gap.





Table 5Full factorial design with three welding process parameters each at three-levels.

Code unit Experimental factor

Laser power (W) Welding speed (mm/min) Gap (mm)

−1 1600 1200 0.10 1800 1500 0.21 2000 1800 0.3


variation in the width of the top weld seam to estimate the tensilestrength of the weld joint.

3.5. Experimental design

The major welding parameters that affect the quality of laserwelding of galvanized steel in the lap joint configuration aresummarized in Fig. 6. It has been reported that several weldingparameters such as laser power, welding speed, part-to-part gap,clamp pressure, and focused position have the major effect on thewidth of weld seam during laser lap welding [17,45]. In general,laser power is proportional to the width of weld seam. In otherwords, the width of weld seam is increased as we increase thelaser power up to a certain limit. Welding speed, on the otherhand, is inversely proportional to both the width of weld seam andthe width of heat affected zone [45]. It is well-known that thepart-to-part gap for laser lap welding must be controlled within acertain threshold [48] otherwise the two parts would not bejoined correctly. This threshold value of part-to-part gap iscorrelated with the width of weld seam and the thickness ofparts. Part-to-part gap has a negative effect on the depth ofpenetration but it is proportional to the width of weld seam [50].

Welding speed has a negative effect on tensile shear strengthbut both laser power and clamp pressure have a little positiveeffect on the tensile shear strength [17,51]. In the laser welding of

Fig. 6. Cause and effect diagram for laser welding

Fig. 5. Weld poo


zinc coated steel, shielding gas also play an important role fordeciding the weld quality [52]. Chen et al. [53] observed uniformhardness as well as higher tensile strength throughout weldseams, when they use Nitrogen gas (N2) as shielding gas. Weldingspeed is the most important factor for residual stress which arisesin the heat affected zone while focus position is not so relevantwith residual stress [54]. The tensile strength of laser welded lapjoints depends mainly on the width of weld seam, the depth ofpenetration and the weld seam length [5].

According to the literature survey, laser power, welding speedand part-to-part gap are the most important welding processparameters for the weld pool geometry, (particularly, the width ofweld seam) and the resulting mechanical properties (particularly,tensile strength) of laser-welded lap joints [54]. Therefore, in this

of galvanized steel in a lap joint configuration.

l geometry.






experiment, these three factors are involved at three levels each inthe mechanical response of tensile strength, namely, three factorsthree levels full factorial design.

In order to find the initial values of welding process para-meters, trial weld runs (approximately 200 experiments) wereperformed a priori by changing one of the process parameters at atime. Absence of clear welding defect, visual soundness of the topand the bottom seams are the criteria of specifying the initialranges of the welding process parameters. Table 5 shows thewelding process parameters considered in the experiment, theircoded and actual values.

Table 6The laser welding experimental data.

No Runorder Laserpower(W)

Weldingspeed(mm/min)

Gap(mm)

Maximumtensilestrength(MPa)

Topweld seamwidth (μm)

mean variation

1 3 1800 1200 0.1 161.30 1528.00 3.482 11 1800 1200 0.3 101.77 930.64 7.3243 6 1600 1200 0.1 161.50 1550.80 2.4514 5 1800 1800 0.3 124.00 934.14 3.4445 13 1800 1200 0.2 156.30 1721.40 0.5916 17 1800 1800 0.2 148.00 1358.39 2.7317 12 2000 1800 0.2 162.00 1480.17 0.5228 7 2000 1800 0.3 114.70 762.57 6.0279 1 1600 1800 0.2 123.33 731.93 6.511

10 23 2000 1500 0.1 165.69 1438.16 2.1311 18 1600 1500 0.1 134.51 1339.16 1.25712 20 1355.35 1.991

3.6. Variation of weld seam

The procedure to estimate the variation of weld seam is asfollows: we first take a series of magnified pictures of a top weldseam in section wise by Olympus LEXT OLS 3100 confocal laserscanning microscope at the total magnification of 160� . Since thelength of a top weld seam is approximately 20 mm and themaximum field of view of the microscope is 2560 μm�2560 μm,we create panorama images of top weld seams, computed fromabout eight weld seam patch images each, as illustrated in Fig. 7.We then import the top weld seam image into CATIA V5 to extractthe two boundary curves of the top weld seam efficiently. Finally,we measure the variation in the width of the top weld seamby means of the deviation analysis between the two extractedcurves.

1800 1800 0.1 141.2413 22 1600 1200 0.2 140.60 1352.12 1.93714 4 1800 1500 0.1 141.73 1335.01 1.88315 27 1600 1800 0.3 34.43 555.48 12.24316 25 1800 1500 0.3 137.90 1047.93 4.4417 15 1600 1500 0.2 145.40 1051.72 4.35918 9 1600 1800 0.1 134.48 1319.14 2.45119 2 2000 1500 0.3 159.95 931.07 5.15920 24 1800 1500 0.2 156.19 1702.29 1.65221 26 1600 1200 0.3 90.90 817.32 8.07522 21 2000 1200 0.2 181.00 1641.91 2.33223 16 2000 1800 0.1 153.02 1338.06 1.72824 19 2000 1200 0.1 170.10 1625.75 2.9625 8 2000 1500 0.2 158.90 951.07 4.72926 14 1600 1500 0.3 57.52 649.90 11.24927 10 2000 1200 0.3 163.00 950.38 6.511

4. Analysis of experiment

The result of laser welding experiments is summarized inTable 6. Maximum tensile strengths are measured by the INSTRON5582 universal testing machine and the mean and the variation ofthe width of each weld seam, listed in the last two columns, areestimated by the procedure described in the previous section.

Fig. 8 shows panorama images of all the 27 specimens. Thewidth of weld seam and their variation are listed with correspond-ing process variables: laser power, welding speed, and part-to-part gap.

Fig. 7. Definition of the width of the top weld seam


The analysis of variance (ANOVA) for the laser welding experi-ment, by considering maximum tensile strength as a responsevariable is summarized in Table 7. We conclude from the P-valuesthat laser power, welding speed, part-to-part gap, and the inter-action between laser power and gap have statistically significanteffects on the maximum tensile strength at the 0.05 level ofsignificance.

Statistical correlation analyses are carried out to clarify therelation between weld seam and welding quality. First, as shown

by using the two extracted boundary curves.





Fig. 8. Variations of top weld seams.

Table 7ANOVA table for thelaser welding experiment.

Source Sum of squares DF Mean square F Prob4F

Laser power (x1) 9280 2 4,640 21.7 0.0006Welding speed (x2) 2,086.3 2 1,043.14 4.88 0.0412Gap (x3) 10,896.5 2 54,448.26 25.47 0.0003Laser power (x1)�welding speed (x2) 924.8 4 231.2 1.08 0.4267Laser power (x1)� gap (x3) 4,227.6 4 1,056.9 4.94 0.0265Welding speed (x2)� gap(x3) 524 4 130.99 0.61 0.6656Error 1711 8 213.87

Total 29,650.1 26


Please cite this article as: Sinha AK, et al. Correlation analysis of the variation of weld seam and tensile strength in laser welding ofgalvanized steel. Opt Laser Eng (2013), http://dx.doi.org/10.1016/j.optlaseng.2013.04.012i





in Fig. 9, the following regression model describes the relationbetween the average width of the top weld seam ‘w’ andmaximum tensile strength ‘T’:

T ¼ 86:248 lnðwÞ−469:56; R2 ¼ 0:6472

Fig. 9. A positive correlation between the log-transformed average width of the topweld seam and maximum tensile strength.

Fig. 10. A negative correlation between the variance of the width of the top weldseam and maximum tensile strength.

Fig. 11. Top weld seams: (a) and (b) have lower variations with higher maximum tensstrength.


The coefficient of determination R2 shows that 64.72% of thevariation of maximum tensile strength is accounted for by theabove regression model with the transformed average width of thetop weld seam (log-transformation). The computed P-value of‘4.2478E-7’ indicates that the investigated positive correlationbetween T and ln(w) is statistically acceptable at the 0.05 levelof significance.

In the same way, the observed relation between the variance ofthe top weld seam width ‘v’ and maximum tensile strength ‘T’ isdescribed by the following regression model (see Fig. 10):

T ¼−0:8087� v2 þ 158:63; R2 ¼ 0:7867

The coefficient of determination R2 and the negative coefficientof ‘v2’ explain a strong negative correlation between T and v2. Thecomputed P-value of ‘7.2109E-10’ indicates that the investigatednegative correlation is statistically acceptable at the 0.05 level ofsignificance.

In order to visually verify the discovered trends in the weldseam, we select the specimen 5, 22, 2 and 17 as shown in Fig. 11;the first two have small variation in the width of top weld seamwith high tensile strength, while the last two have large variationwith low tensile strength.

5. Conclusion

We have investigated the correlation between (i) the shapecomplexity of the top weld seam, that is, the variation in the widthof the weld seam, and (ii) welding quality, for the case of laser lapwelding of galvanized steel. Laser power, welding speed, part-to-part gap and their interactions were taken as the experimentalfactors, and tensile tests have been made to determine the weldingquality of each specimen.

We observed that if the heat input per unit length is notsufficient due to low laser power, large part-to-part gap or highwelding speed, weld seam is not formed sufficiently from the initialmelting zone which is usually created nearby at the mating plane ofthe two metal parts to be joined, outwardly onto the top surface ofthe upper part. Consequently, unstable top weld seam is oftencreated, so that the variation in the width of the top weld seam isrelatively large. We found that laser welded parts having largervariation of the top weld seam usually have lower tensile strength,and vice versa. This result leads us to conclude that the variation of

ile strength, while (c) and (d) have higher variations with lower maximum tensile






weld seams can be used as an indirect non-destructive testingmethod for estimating the tensile strength of the welded parts.

Further experiments will be conducted to verify the resultsin terms of hardness, toughness, surface roughness and fatigue.Furthermore, in this paper, we manually extracted two boundarycurves of the top weld seam to calculate its variation. It is howevernecessary to develop a more programmed way of measuring thevariation in order to evaluate the quality of welded partsefficiently.

Acknowledgments

The research reported in this paper is supported by KoreaInstitute for Advancement in Technology (EUFP-M0000224) andthe European Commission (FP7 Project 285051).

References

[1] Chen W, Ackerson P, Molian P. CO2 laser welding of galvanized steel sheetsusing vent holes. Mater Des 2009;30:245–51.

[2] Ribolla A, Damoulis GL, Batalha GF. The use of Nd: YAG laser weld for largescale volume assembly of automotive body in white. J Mater Process Technol2005;164:1120–7.

[3] Rizzi D, Sibillano T, Pietro Calabrese P, Ancona A, Mario Lugarà P. Spectro-scopic, energetic and metallographic investigations of the laser lap welding ofAISI 304 using the response surface methodology. Opt Lasers Eng 2011;49:892–8.

[4] Chen L, XuW, Gong S. The effect of weld size on joint mechanical properties of5A90 Al–Li alloy. Adv Mater Res 2011;146–147:1831–8.

[5] Furusako S, Miyazaki Y, Hashimoto K, Kobayashi J. Establishment of a modelpredicting tensile shear strength and fracture portion of laser-welded lapjoints. In: First international symposium on high-power laser macroproces-sing. SPIE proceedings 2003; 4831. p. 197–202.

[6] Duley WW. Laser welding. Wiley International Publication; 1999.[7] Zhang YM. Real-time weld process monitoring. Woodhead Publishing; 2008.[8] Tzeng YF. Effects of operating parameters on surface quality for the pulsed

laser welding of zinc-coated steel. J Mater Process Technol 2000;100:163–70.[9] Kawamoto K, Yoshimitsu S, Noguchi H, Yoshimura T. Laser welding conditions

to improve tensile strength and fatigue strength of a lap joint in aluminumalloys. J Test Eval 2007;35:416–23.

[10] Acherjee B, Kuar AS, Mitra S, Misra D. A sequentially integrated multi-criteriaoptimization approach applied to laser transmission weld quality enhance-ment—a case study. Int J Adv Manuf Technol 2012:1–10 Doi: 10.100/s00170-012-4203-3.

[11] Miyazaki Y, Furusako S. Tensile shear strength of laser welded lap joints. ;28–34 Nippon steel technical report.

[12] Sathiya P, Abdul Jaleel MY, Katherasan D, Shanmugarajan B. Optimization oflaser butt welding parameters with multiple performance characteristics. OptLaser Technol 2011;43:660–73.

[13] Remes H, Varsta P. Statistics of weld geometry for laser-hybrid welded jointsand its application within notch stress approach. Weld. World 2010;54:189–207.

[14] Stoschka M, Thaler M, Huemer H, Eichlseder W. Contribution to the fatigueassessment of laser welded joints. Procedia Eng 2011;10:1789–94.

[15] Lawrence F, Ho N, Mazumdar PK. Predicting the fatigue resistance of welds.Annu Rev Mater Sci 1981;11:401–25.

[16] Siva Shanmugam N, Buvanashekaran G, Sankaranarayanasamy K. Somestudies on weld bead geometries for laser spot welding process using finiteelement analysis. Mater Des 2012;34:412–26.

[17] Acherjee B, Misra D, Bose D, Venkadeshwaran K. Prediction of weld strengthand seam width for laser transmission welding of thermoplastic usingresponse surface methodology. Opt Laser Technol 2009;41:956–67.

[18] Zhao Y, Zhang Y, Hu W, Lai X. Optimization of laser welding thin-gagegalvanized steel via response surface methodology. Opt Lasers Eng 2012;50:1267–73.

[19] Ghosal S, Chaki S. Estimation and optimization of depth of penetrationin hybrid CO2 LASER-MIG welding using ANN-optimization hybrid model.Int J Adv Manuf Technol 2010;47:1149–57.

[20] Sathiya P, Panneerselvam K, Abdul Jaleel MY. Optimization of laser weldingprocess parameters for super austenitic stainless steel using artificial neuralnetworks and genetic algorithm. Mater Des 2012;36:490–8.

[21] Benyounis K, Olabi A, Hashmi M. Multi-response optimization of CO2 laser-welding process of austenitic stainless steel. Opt Laser Technol 2008;40:76–87.


[22] Havrilla D. Photonics applied: materials processing-successful laser weldingdemands optimized laser joint design. Laser Focus World 2012;48:43–8 Sep.

[23] Akhter R, Steen W. The gap model for welding zinc-coated steel sheet. In:International conference on laser system applications in industry. Torino,Italy; 1990. p. 219–236.

[24] Tzeng YF. Process characterisation of pulsed Nd: YAG laser seam welding.Int J Adv Manuf Technol 2000;16:10–8.

[25] Mei L, Chen G, Jin X, Zhang Y, Wu Q. Research on laser welding of high-strength galvanized automobile steel sheets. Opt Lasers Eng 2009;47:1117–24.

[26] Akhter R. Laser welding of zinc coated steel. PhD thesis. Imperial CollegeLondon (University of London); 1990.

[27] Gu H, Shulkin B. Remote laser welding of zinc-coating sheet metal componentin a lap configuration utilizing humping effect. Canada: Stronach Centre forInnovation, Magna International Inc; 380–5.

[28] Akhter R, Steen W, Watkins K. Welding zinc-coated steel with a laser and theproperties of the weldment. J Laser Appl 1991;3:9–20.

[29] Rito N, Ohta M, Yamada T, Gotoh J, Kitagawa T. Laser welding method. UnitedStates Patent Number: 4,745,257; 1988.

[30] Petrick FD. Method of making hemmed joints utilizing laser welding. UnitedStates Patent Number: 4,916,248; 1990.

[31] Daimler Chrysler AG. Patent Number: DE 10241593; 2005.[32] Colombo D, Colosimo BM, Previtali B. Comparison of methods for data analysis

in the remote monitoring of remote laser welding. Opt Lasers Eng 2013;51:34–46.

[33] Pennington EJ. Laser welding of galvanized steel. United States PatentNumber: 4,642,446; 1987.

[34] Heyden J, Nilsson K, Magnusson C. Laser welding of zinc coated steel. In: Proc.sixth international lasers in manufacturing; 1989. p. 93–104.

[35] Kennedy SC, Norris IM. Nd-YAG laser welding of bare and galvanised steels,international congress and exposition. Technical paper series 890887; 1989.

[36] Tzeng Y. Pulsed Nd: YAG laser seam welding of zinc-coated steel. Weld J1999;78(7):238–44.

[37] Bilge U, Jurczyszyn CE, Jenuwine WC. Laser welding method. United StatesPatent Number: 4,745,257; 1993.

[38] Delle Piane A, Sartorio F, Cantello M, Ghiringhello G. Method of laser weldingsheet metal protected by low-vaporizing-temperature materials. United StatesPatent Number: 4,682,002; 1987.

[39] Milberg J, Trautmann A. Defect-free joining of zinc-coated steels by bifocalhybrid laser welding. Prod Eng 2009;3:9–15.

[40] Gualini M. Laser welding of zinc coated steel sheets: an old problem with apossible solution. In: 20th international congress on ICALEO 2001: applica-tions of lasers and electro-optics, Jacksonville, USA; 2001. p. 492–501.

[41] Wu Q, Gong J, Chen G, Xu L. Research on laser welding of vehicle body. OptLaser Technol 2008;40:420–6.

[42] Hanicke L, Strandberg Ö. Roof laser welding in series production. Society ofautomotive engineers technical paper. http://dx.doi.org/10.4271/930028;1993.

[43] Ozaki H, Kutsuna M, Nakagawa S, Miyamoto K. Laser roll welding of dissimilarmetal joint of zinc coated steel and aluminum alloy. J Laser Appl 2010;22:1–6.

[44] Huang Q, Hagstrom J, Skoog H, Kullberg G. Effect of CO2 laser parametervariations on sheet metal welding. Int J Joining Mater 1991;3:79–88.

[45] Benyounis K, Olabi A, Hashmi M. Effect of laser welding parameters on theheat input and weld-bead profile. J Mater Process Technol 2005;164:978–85.

[46] Mistry A. Basic vechile design rules and guidance. Jaguar Land Rover: AdvanceManufacturing Technology; 2012.

[47] Kimara S, Sugiyama S, Mizutame M. Welding properties with high powerpulsed CO2 laser. Changing Front Laser Mater Process 1986:89–96.

[48] Cline H, Anthony TR. Heat treating and melting material with a scanning laseror electron beam. J Appl Phys 1977;48:3895–900.

[49] Akhter R, Davis M, Dowden J, Kapadia P, Ley M, Steen W. A method forcalculating the fused zone profile of laser keyhole welds. J Phys D: Appl Phys1989;22:23–8.

[50] Olabi A, Casalino G, Benyounis K, Rotondo A. Minimisation of the residualstress in the heat affected zone by means of numerical methods. Mater Des2007;28:2295–302.

[51] Anawa E, Olabi A. Optimization of tensile strength of ferritic/austenitic laser-welded components. Opt Lasers Eng 2008;46:571–7.

[52] Sibillano T, Ancona A, Berardi V, Schingaro E, Basile G, Mario Lugarà P. A studyof the shielding gas influence on the laser beam welding of AA5083aluminium alloys by in-process spectroscopic investigation. Opt Lasers Eng2006;44:1039–51.

[53] Chen HC, Pinkerton AJ, Li L, Liu Z, Mistry AT. Gap-free fibre laser welding ofZn-coated steel on Al alloy for light-weight automotive applications. MaterDes 2011;32:495–504.

[54] Khan M, Romoli L, Fiaschi M, Dini G, Sarri F. Experimental design approach tothe process parameter optimization for laser welding of martensitic stainlesssteels in a constrained overlap configuration. Opt Laser Technol 2011;43:158–72.


http://refhub.elsevier.com/S0143-8166(13)00133-4/sbref1










































































dx.doi.org/doi:10.4271/930028





































optics and lasers in engineering - unistsf.unist.ac.kr/research/variation_weldseam.pdfcorrelation...

Documents