laser welding of low carbon steel and thermal stress analysis
DESCRIPTION
TRANSCRIPT
ARTICLE IN PRESS
Optics & Laser Technology 42 (2010) 760–768
Contents lists available at ScienceDirect
Optics & Laser Technology
0030-39
doi:10.1
n Corr
E-m
journal homepage: www.elsevier.com/locate/optlastec
Laser welding of low carbon steel and thermal stress analysis
B.S. Yilbas n, A.F.M. Arif, B.J. Abdul Aleem
Mechanical Engineering Department, KFUPM, Saudi Arabia
a r t i c l e i n f o
Article history:
Received 13 November 2008
Received in revised form
17 November 2009
Accepted 26 November 2009Available online 21 December 2009
Keywords:
Laser welding
Temperature
Finite element model
92/$ - see front matter & 2009 Elsevier Ltd. A
016/j.optlastec.2009.11.024
esponding author. Tel.: +966 3 860 4481; fax
ail address: [email protected] (B.S. Yilba
a b s t r a c t
Laser welding of mild steel sheets is carried out under nitrogen assisting gas ambient. Temperature and
stress fields are computed in the welding region through the finite element method. The residual stress
developed in the welding region is measured using the XRD technique and the results are compared
with the predictions. Optical microscopy and the SEM are used for the metallurgical examination of the
welding sites. It is found that von Mises stress attains high values in the cooling cycle after the
solidification of the molten regions. The residual stress predicted agreed well with the XRD results.
& 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Laser is widely used as a thermal source for industrialapplications. This is because of the local treatment, preciseoperation, and short processing time. One of the importantindustrial applications of laser processing is the laser welding,which offers considerable advantages over the conventionalwelding methods. High intensity laser beam melts and partiallyevaporates the welded material during the process. Attainment ofhigh temperature gradient during the heating and cooling periodsresults in the development of high thermal stresses in the weldingzone. Once the cooling period ends, the residual stress in thewelding zone is resulted. This, in turn, influences the mechanicalperformances of the resulting welds. Consequently, investigationinto thermal stress development and residual stress formation inthe weld zone becomes essential.
Considerable research studies were carried out to examine thelaser welding process. An extensive review on laser welding andrelated process was carried out by Mackwood and Crafer [1]. Theypresented the applications laser welding processes under differ-ent welding categories such as laser spot welding, laser buttwelding, etc. Nd:YAG laser repair welding of tool steels andmicrostructural changes in the welded region were examined byVedani[2]. They indicated that heat affected zone was narrow andcarbides dissolve during the heating phase of the welding process.Li et al. [3] studied the laser forming process using the finiteelement method. They showed that thermal expansion and
ll rights reserved.
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s).
contraction took place during the laser processing, which in turnallowed the thermo-mechanical forming of complex shapes. Laserwelding and heating analysis were carried out by Papadakis andTobias [4]. They examined the lap and fillet seam welds andmeasured the distortion during the processing. Moreover, theexperimental findings were compared with the model predic-tions. A high speed laser pulse welding of metallic substrates wasexamined by Holtz et al. [5]. They demonstrated that animprovement in the structural integrity of the weld site wasresulted for high speed pulsed contour welding than the seamwelding process. The transient effects on the formation of laserproduced weld pool were studied by Ehlen et al. [6]. Theyaccommodated the Marangoni effect using a semi-empiricalmodel for the temperature dependent surface tension gradient.The thermal stress developed during the lap welding of thermo-plastic films was examined by Coelho et al. [7]. They showed thatthe influence of thermal stresses and expansion and contractionforces played an important role on the achievement of strongwelds; in which case, tensile strength improved 80% as comparedto the original thermo-plastic substrate. The high power laserwelding of construction steel was investigated by Engstron et al.[8]. They indicated that laser welding offered new designopportunities, which improved the manufacturing process byshortening lead and production times as well as reducing theamount of materials used. The metallurgical and residual stressevaluation of CO2 laser welded super-austenitic stainless steelwas carried out by Zambon et al. [9]. They showed that theresidual stress was tensile and close to the yielding strength of thesubstrate material in the longitudinal direction in the weld beadwhile the stresses were compressive in the transverse direction inthe base material. Laser welding of steel and residual stress
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30 mm
Laser Beam
xy
z
Laser Irradiated Spot
x = 0y = 0z = 0
U
Nitrogen
Weld section
40 mm
2 mm
Fig. 1. View of laser melting process.
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B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768 761
distribution were examined by Olabi et al. [10]. They examinedthe effect of laser parameters on the residual stress developedthrough the statistical analysis. Laser welding and the simulationof temperature field were carried out by Zeng-rong et al. [11].They indicated that the finite element method predictions were inagreement with the experimental findings. Laser welding char-acteristics of cold rolled carbon steel were examined by Shin et al.[12]. They showed that the optimal welding conditions, whichresulted in no defect sites in the vicinity of the welded area, werepossible for a certain combination of laser output power, weldingspeed, and focus setting of focusing lens. The influence ofshielding condition on the weld quality of underwater laserwelding was investigated by Zhang et al. [13]. They showed thatthere existed the relationship between shielding conditions andthe quality of the weld bead. The porosity formation andpenetration in pulsed laser welding were examined by Zhouand Tsai [14]. They indicated that the porosity formation wasstrongly related to the aspect ratio (depth to width) of the keyhole formed during the welding process. The hybrid laser beamwelding and its applications were studied by Mahrle and Beyer[15]. They demonstrated some practical examples of potentialhybrid technologies, which would be used to extend theapplication spectrum of the laser beam welding. Laser micro-welding of copper and aluminum was investigated by Ihor andSchmidt [16]. They showed that an application of suitable fillermaterial helped in avoiding the brittle intermetallic phases at theinterface between copper and the solidified melt in the weldedjoints. The experimental study on welding characteristics of CO2
laser TIG hybrid welding process was carried out by Chen et al.[17]. They indicated that when the current was increased to acritical value, the laser induced key hole disappeared and the arcexpanded, which, in turn, decreased the penetration depth. Themechanical and metallurgical aspects of tailored welded blankswere examined by Bayraktar et al. [18]. They showed that thefracture surfaces tested at low temperatures presented thefracture topography for cleavage in the notched section for steelsamples. However, at high temperatures, the fractured alwaysappeared in the ductile mode in the specimens.
In the present study, laser welding of low carbon steel plates iscarried out. Temperature and thermal stress fields are modeledusing the finite element method. The residual stress developed inthe welding zone is measured using the XRD technique andcompared with the predictions. The metallurgical and morpholo-gical changes in the weld zone are examined using the opticalmicroscopy, the SEM, the EDS and the XRD.
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2. Heating analysis
Since the welding situation is involved with the transientheating during laser scanning, a transient equation for heattransfer needs to be considered. In this case, the transient heattransfer equation in the Cartesian coordinates is
rDE
Dt¼ ðrðkrTÞÞþSo ð1Þ
where D represents the material derivative or substantialderivative D
Dt ¼ u @@x þv @
@y þw @@z þ
@@t
� �, E is the energy gain by the
substrate material and So is the volumetric heat source term and
So ¼ Iodð1�rf Þe�dyeð�x2þ z2=a2Þ
Io is laser peak intensity, d is the absorption depth, rf is the surfacereflectivity, r is the density, Cp is the specific heat capacity, k is thethermal conductivity, a is the Gaussian parameter, and x and z arethe axes while the laser beam scans the surface along the z-axisand the laser beam axis is the y-axis (Fig. (1)).
In the case of a moving heat source along the z-axis with aconstant velocity U, energy gain by the substrate material yields
rDE
Dt¼ r @E
@t�rU
@E
@zð2Þ
or
rDE
Dt¼ r @ðCpTÞ
@t-rU
@ðCpTÞ
@zð3Þ
Combining equations (1) and (3) yields
r @ðCpTÞ
@t¼ rðkrTÞð ÞþrU
@ðCpTÞ
@zþSo ð4Þ
At the free surfaces of the welded workpiece (the irradiatedsurface, Fig. (1)), the convective boundary is assumed and at therear side of the workpiece, the convective and radiative boundaryconditions are considered. Therefore, the corresponding boundarycondition is:
At the irradiated surface
@T
@z¼
h
kðTs�TambÞ ð5Þ
and at the rear side of the surface:
@T
@y¼
h
kðTs�TambÞþ
eskðT4
s �T4ambÞ and
@T
@z¼
h
kðTs�TambÞ ð6Þ
where h is the heat transfer coefficient due to natural convection,and Ts and Tamb are the surface and ambient temperatures,respectively, e is the emissivity (e=0.63 is considered [19]), and sis the Stefan–Boltzmann constant (s=5.67�10�8 W/m2 K). At faraway boundary (at edges of the solution domain) constanttemperature boundary is assumed (T=293 K), i.e.
x¼1; y¼1; z¼1-T ¼ 293K
Initially (prior to laser treatment), the substrate material isassumed to be at constant ambient temperature, i.e. T=Tamb,which is considered as constant (Tamb=293 K).
Equation (4) is solved numerically with the appropriateboundary conditions to predict the temperature field in thesubstrate material. However, to analyze the phase changeproblem, a non-linear transient thermal analysis is performedemploying the enthalpy method. To account for latent heatevolution during phase change, the enthalpy of the material as afunction of temperature is incorporated in the energy equation.
3. Modeling of thermal stresses
For structural response, the finite element formulation is basedon the principle of virtual work. From the principle of virtual work(PVW), a virtual (very small) change in the internal strain energy(dU) must be offset by an identical change in external work due tothe applied loads (dV). Considering the strain energy due tothermal stresses resulting from the constrained motion of a body
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t = 1 s
0
600
1200
1800
2400
3000
0.0E+00DISTANCE (m)
TEM
PER
ATU
RE
(K) y = 0 mm; z = 15 mm
y = 0.8 mm; z = 15 mmy = 1.6 mm; z = 15 mm
t = 2 s
1000
1500
2000
PER
ATU
RE
(K) y = 0 mm; z = 15 mm
y = 0.8 mm; z = 15 mmy = 1.6 mm; z = 15 mm
2.0E-03 4.0E-03 6.0E-03 8.0E-03 1.0E-02
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768762
during a temperature change, PVW yields
fdugTZ
vol½BT �½D�½B�dvfug ¼ fdugT
Zvol½B�T ½D�fethgdv ð7Þ
Noting that the {du}T vector is a set of arbitrary virtualdisplacements common in all of the above terms; the conditionrequired to satisfy above equation reduces to
½K�fug ¼ fFthg
where
½K� ¼R
vol½B�T ½D�½B�dv¼ element stiffness matrix
fFthg ¼R
vol½B�T ½D�fethgdv¼ element thermal load vector
fethg ¼ fagDT ¼ thermal strain vector
fag ¼ vector of coefficients of thermal expansion
In the present study, the effect of mechanical deformation onheat flow has been ignored and the thermo-mechanical phenom-enon of melting is idealized as a sequentially-coupled unidirec-tional problem. For thermal analysis, the given structure ismodeled using the thermal element (SOLID70). SOLID70 has a3D thermal conduction capability (ANSYS code). The element haseight nodes with a single degree of freedom, and temperature, ateach node. The element is applicable to a 3D, steady-state ortransient thermal analysis. Since the model containing theconducting solid element is also to be analyzed structurally, theelement is replaced by an equivalent structural element (such asSOLID45) for the structural analysis. SOLID45 is used for the 3Dmodeling of solid structures. The element is defined by eightnodes having three degrees of freedom at each node: translationsin the nodal x, y, and z directions. The element has plasticity,creep, swelling, stress stiffening, large deflection, and large straincapabilities.
The thermal and structural properties used in the currentsimulations are given in the Table 1. It should be noted that theconditions for the current simulations resemble the actualexperiments carried out in the present study.
0
500TEM
0.0E+00DISTANCE (m)
2.0E-03 4.0E-03 6.0E-03 8.0E-03 1.0E-02
Fig. 2. Temperature distribution along the x-axis at different depths below the
surface and for two time periods.
4. Residual stress measurements
XRD Technique: The measurement relies on the stresses in finegrained polycrystalline structure. The position of the diffractionpeak undergoes shifting as the specimen is tilted by an angle c.The magnitude of the shift is related to the magnitude of theresidual stress. The relationship between the peak shift and the
Table 1Mechanical and thermal properties of mild steel used in the simulations.
Temperature (K) 294 366 477 589
E (GPa) 203 199 191 184
Temperature (K) 273 373 473 573 673
a�10�6 (1/K) 11.2 11.2 12.1 13.0 13.6
k (W/mK) 51.9 50.7 48.2 45.6 41.9
Cp (J/kgK) 486 486 515 548 586
u 0.3
r (kg/m3) 7700
Table 2. Laser welding conditions.
Feed Rate (mm/min) Power (W) Frequency (Hz) Nozzle Gap (mm)
1000 2000 1500 1.5
residual stress (s) is given [20]
s¼ E
ð1þuÞSin2cðdn�doÞ
doð8Þ
where E is the Young’s modulus, n is the Poisson’s ratio, c is thetilt angle, and di are the d spacing measured at each tilt angle. Ifthere are no shear strains present in the specimen, the d spacingchanges linearly with Sin2 c.
644 700 755 811 866
176 167 154 141 124
773 873 973 1073 1273 1473
14.0 14.6 14.8 11.8 13.6 13.6
38.1 33.9 30.1 24.7 26.8 29.7
649 708 770 624 548 548
Nozzle Diameter (mm) Focus setting (mm) N2 Pressure (kPa)
1.5 127 600
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Fig. 3. Three-dimensional view of temperature distribution during welding process for two time periods. Temperature is in K.
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768 763
5. Experimental
The CO2 laser (LC-ALPHAIII) delivering nominal output powerof 2 kW at pulse mode with different frequencies is used toirradiate the workpiece surface. The nominal focal length of thefocusing lens is 127 mm. Nitrogen assisting gas emerging fromthe conical nozzle and co-axially with the laser beam is used. Thewelding conditions are given in Table 2. The workpieceaccommodated is mild steel at 2 mm in thicknesses.
Material characterization of the laser nitrided surfaces iscarried out using the SEM, the XRD and the XPS. The Jeol 6460electron microscopy is used for the SEM examinations and theBroker D8 Advanced having MoKa radiation is used for the XRDanalysis. A typical setting of the XRD was 40 kV and 30 mA.Microphotonics digital microhardness tester (MP-100TC) was
used to obtain microhardness across the weld zone (parallel tothe workpiece surface). The standard test method for Vickersindentation hardness of advanced ceramics (ASTM C1327–99)was adopted. The measurements were repeated three times ateach location.
6. Results and discussions
Laser welding of low carbon steel sheets is carried out.Temperature and stress fields developed during the welding arecomputed using the finite element method. The residual stressdeveloped in the weld region is measured using the XRDtechnique while metallurgical and morphological changes in the
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t = 1 s
1.0E+06
1.0E+07
1.0E+08
1.0E+09
STR
ESS
(Pa)
y = 0 mm; z = 15 mmy = 0.8 mm; z = 15 mmy = 1.6 mm; z = 15 mm
t = 2 s
1.0E+05
1.0E+06
1.0E+07
1.0E+08
1.0E+09
0.0E+00DISTANCE (m)
STR
ESS
(Pa)
y = 0 mm; z = 15 mmy = 0.8 mm; z = 15 mmy = 1.6 mm; z = 15 mm
2.0E-03 1.0E-028.0E-036.0E-034.0E-03
0.0E+00DISTANCE (m)
2.0E-03 1.0E-028.0E-036.0E-034.0E-03
Fig. 4. von Mises stress distribution along the x-axis at different depths below the
surface and for two time periods.
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768764
weld zone are examined using the optical microscopy, the SEMand the EDS.
Fig. (2) shows temperature variation along the x-axis at threedifferent locations in the y-axis while Fig. (3) shows 3D view oftemperature distribution. The z-axis location is 15 mm from thewelding starting point (Fig. (1)) in Fig. (2). It should be noted thatthe time t=1 s corresponds to the laser beam spot center at thelocation x=0, y=0, and z=15 mm. Temperature decays sharply inthe surface region and as the depth below the surface increasestemperature decay becomes sharp and further increase in thedepth results in gradual decay of temperature. In the region,where the sharp delay occurs, temperature gradient attains highvalues. This is more pronounced at some depth below the surface.Temperature in the surface region exceeds the meltingtemperature of the substrate material. Although temperature atthe melting–solid interface remains the same – due to theinfluence of latent heat of fusion on the energy transfer in thevicinity of melting–solid interface – temperature gradient inthe molten region becomes different in the solid phase. Moreover,the decay rate of temperature as well as its magnitude changes atdifferent y-axis locations in the melting zone. The y-axis locationrepresents different depths below the surface inside the substratematerial. The difference in temperature in the molten zone isattributed to the heat conduction in this region. In the case oft=2 s heating duration, the laser beam scans over this region andenergy transfer from the surface through convection and
conduction reduces temperature significantly in this region. Theheat conduction becomes the sole mechanism after this time inthis region. In addition, temperature profiles along the depthbelow the surface becomes almost self similar.
Fig. (4) shows von Mises stress along the x-axis at tree depthsbelow the surface for the same z-axis location as shown in Fig. (1)for two heating periods while Fig. (5) shows 3D view of von Misesstress in the substrate material. von Mises stress attains lowvalues in the surface region because of the melting; in which case,molten metal is free to expand in this region. von Mises stressincreases sharply as the depth below the surface increases alongthe y-axis. Moreover, it remains high as the distance increasesfurther. This sharp rise of von Mises stress is related to the sharpincrease in temperature gradient at some depth below thesurface. The variation of von Mises stress along the x-axis issimilar at different y-axis locations, since the difference in thestress magnitude is negligibly small. This situation is associatedwith the similar behavior of temperature distribution at differenty-axis locations along the x-axis. In the case of time period t=2 s,von Mises stress rises with increase in distance along the x-axis.This is true for all the y-axis locations. This increase is attributedto the strain developed at early periods when the laser peakintensity scans over the z-axis locations corresponding to thefigure. Therefore, thermal stress developed during hightemperature heating contributes to the stress field developedduring the cooling period for t=2 s. Consequently, von Misesstress for this time period does not follow exactly the temperaturedistribution as shown in Fig. (2). Moreover, the maximum stresslevel along the x-axis is below the elastic limit of the substratematerial.
Fig. (6) shows temperature distribution along the z-axis (laserscanning direction) at three different y-axis locations and forthree time periods. It should be noted that the x-axis location isx=0 (Fig. (1)). The location of peak temperature changes along thez-axis for different periods, which is because of the location oflaser beam intensity along the scanning direction (z-axis). In thiscase, the laser peak intensity is at z=1.5�10�2 m for t=1 s,z=2.25�10�2 m for t=1.5 s, and z=3�10�2 m for t=2 s. Thesmall irregular behavior is observed in temperature profile atabout 1600 K. This is because of the phase change process(melting), which suppresses the rate of rise of temperature inthe solid due to the latent heat of fusion. However, the sharpincrease in temperature occurs at a location where the laser beamintensity is the maximum. This is due to the absorption of thelaser beam energy by the substrate material, which enhancessignificantly the internal energy gain in this region. Therefore,temperature increases sharply in the molten material. Thissituation is true for different y-axis locations, provided thattemperature gradient around the peak temperature reduces as they-axis location increases below the surface. The decrease in timederivative of temperature (qT/qt) is because of less energy transferthrough conduction from the surface in this region where thelaser energy absorption is significantly high. The rise and delay oftemperature around the peak temperature is not the same. Thissuggests that temperature remains high at the surfaces during theheating period when the laser scans the workpiece surface.However, once the welding is over, temperature delay is sharp inthe cooling cycle. This is true for all the y-axis locations and for allperiods in the cooling cycle.
Fig. (7) shows von Mises stress along the z-axis for three y-axislocations and three time periods. It should be noted that thelocations and time periods are the same as Fig. (6). von Misesstress attains high values immediately after temperature reachesits peak value (Fig. (6)). This is associated with the temperaturegradient, which is higher after the peak temperature. It should benoted that material in the liquid phase, where temperature
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Fig. 5. Three-dimensional view of von Mises stress distribution during welding process for two time periods. Stress is in Pa.
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768 765
reaches its peak, expands freely and stress level becomes verylow. However, upon decaying from the peak temperature, thesolidification takes place rapidly and conduction cooling from theheated region results in attainment of high temperature gradient.This causes a sharp increase in von Mises stress in this region. vonMises stress level rises sharply and decays gradually for all timeperiods and the y-axis locations, despite the fact that temperaturedecays sharply after its peak value. Consequently, straindeveloped during the heating period plays an important role onthe stress formation after the laser beam passes over theconcerned location along the z-axis (laser scanning direction).The maximum value of von Mises stress attains almost 195 MPa,which is less than the elastic limit of the substrate material. Thisis true for all the y-axis locations and time periods. Moreover, the
decay in von Mises stress is observed before rising to itsmaximum. This may be associated with the initial solidificationand temperature variation immediately after the solidification, i.e.temperature change is small in the region of solidificationresulting in attainment of low temperature gradient whilelowering von Mises stress in this region.
Fig. (8) shows optical photographs of welding surface anddifferent regions in the weld cross-section while Fig. (9) showsthe SEM micrographs of the weld cross-section. The heat affectedzone (HAZ) is evident from the SEM micrograph and the heataffected zone extends both sides of the weld section almost at asame size. This indicates the plane symmetry of the heatingduring the actual welding process, which is also adapted in thesimulations. The grain size coarsening is evident in the HAZ due to
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x = 0 mm and y = 0 mm
0
600
1200
1800
2400
3000
0.0E+00DISTANCE (m)
TEM
PER
ATU
RE
(K)
t = 1 s
t = 2 s
x = 0 mm and y = 0.8 mm
0
600
1200
1800
2400
3000
0.0E+00DISTANCE (m)
TEM
PER
ATU
RE
(K)
t = 1 s
t = 2 s
x = 0 mm and y = 1.6 mm
0
600
1200
1800
2400
3000
0.0E+00DISTANCE (m)
TEM
PER
ATU
RE
(K)
t = 1 s
t = 2 s
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
Fig. 6. Temperature distribution along the z-axis at different depths below the
surface and for two time periods.
x = 0 mm and y = 0 mm
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
0.0E+00DISTANCE (m)
STR
ESS
(Pa)
t = 1 st = 2 s
x = 0 mm and y = 0.8 mm
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
STR
ESS
(Pa)
t = 1 st = 2 s
x = 0 mm and y = 1.6 mm
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
STR
ESS
(Pa)
t = 1 st = 2 s
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
0.0E+00DISTANCE (m)
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
0.0E+00DISTANCE (m)
6.0E-03 1.2E-02 1.8E-02 2.4E-02 3.0E-02
Fig. 7. von Mises stress distribution along the z-axis at different depths below the
surface and for two time periods.
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768766
the slow cooling rates in this region. However, some fine grainsare observed in the region away from the weld interface. Thegrain refining is formed because of high cooling rates due to hightemperature gradient. In general, the fine grains consist of circularferrite and fine pearlite structures (Fig. (9)), in which case, thelight region is ferrite while dark region is pearlite. Moreover, thenucleation of pearlite results in partial formation of cementitelamellae structure. The pearlitic modulus grows from the nucleialong the austenite boundaries and the scattered colonies ofalternating ferrite and cementite lamellae are formed within thepearlitic structure. In the weld core, the austenite phase allowinga considerable grain growth is observed. In this case, grainboundary ferrite and ferrite/carbide aggregate appear to be finepearlite. In addition, in the fusion zone, very fine grains, as well aslocally scattered large grains, where crystallization takes place,are evident.
Fig. (10) shows microhardness distribution across the weldingcross-section along the line parallel to the free surface (transverseto the weld section). The hardness decays sharply with increase indistance across the weld. The peak hardness is almost 40% higherthan the base material hardness. It should be noted that theincrease in hardness is one of the indications of formation of finestructures and the residual stress developed during the weldingprocess. The residual stress measured using the XRD technique
and observed from the simulations are 105 MPa and 90 MPa,respectively. It should be noted that the residual stress ismeasured 2.5 mm away from the welding line and at themidway of the welded sample along the welding direction.Moreover, the prediction of the residual stress is compared atthe same location of the measurement. It can be observed thatboth results are in agreement (in the order of 100 MPa), providedthat a small discrepancy occurs, which is because of theassumptions made in the simulations such as uniform structureand mechanical properties are considered.
7. Conclusions
Laser welding of mild steel sheets is carried out and stress fielddeveloped in the welding zone is modeled using the finiteelement method (FEM). The residual stress developed in the weldregion is measured using the XRD technique and compared withthe FEM predictions. The morphological and metallurgicalchanges in the welding region are examined using the opticalmicroscopy and the SEM. Microhardness distribution across thewelding zone is measured. It is found that temperature decay ratein the molten zone is lower than in the solid. This is because of theabsorption and dissipation of the laser energy in the molten zone,
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2 mm
Top view of laser welded section
1 mm Cross-section of laser welded section.
0.2 mm Fusion Zone and Heat Affected Zone Interface.
Fusion Zone
Heat Affected Zone
Base Material
Fig. 8. Optical photographs of welding cross-section.
Fusion Zone
Heat Affected Zone
Partially formedpearlite
Fig. 9. SEM micrographs of welding cross-section.
100
120
140
160
180
200
0.0E+00DISTANCE (m)
HA
RD
NES
S (H
V)
6.0E-04 1.2E-03 1.8E-03 2.4E-03 3.0E-03
Fig. 10. Microhardness across the weld cross-section and parallel to the free
surface.
B.S. Yilbas et al. / Optics & Laser Technology 42 (2010) 760–768 767
which is generated in the surface region. This, in turn, increasesthe internal energy gain in this region. In the solid region,temperature variation along the x-axis becomes similar for all they-axis locations. This results in similar von Mises stress distribu-tion along the x-axis provided that at large y-axis locations (atsome depth below the surface); the strain developed in this regionmodifies this behavior slightly due to temperature increase,which reaches its peak at the z-axis location when the laserbeam intensity is the maximum. Moreover, once the laser beamscans over this region, temperature decays sharply. This causessharp increase in von Mises stress due to the attainment of hightemperature gradient in this region. This situation is true for allthe y-axis locations and time periods considered in this study.However, the maximum stress level is less than the elastic limit ofthe substrate material. The residual stress predicted from the FEMagrees well with the XRD results, which is in the order of100 MPa. The grain coarsening occurs in the HAZ and grainrefining and recrystallization takes place in the fusion zone asobserved from the optical and the SEM micrographs. The closedexamination of welding section reveals that the weld section isfree from cracks and voids.
Acknowledgements
The authors acknowledge the support of King Fahd Universityof Petroleum and Minerals, Dhahran, Saudi Arabia, for the fundedproject, Project #SB080003.
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