A study of laser welding modes with varying beam energy levels

1141

A study of laser welding modes with varying beamenergy levelsG Buvanashekaran1, N Siva Shanmugam2∗, K Sankaranarayanasamy2, and R Sabarikanth3

1Welding Research Institute, Bharat Heavy Electricals Limited, Tiruchirappalli, Tamil Nadu, India2Department of Mechanical Engineering, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India3M/s. Optilase, Chennai, Tamil Nadu, India

The manuscript was received on 13 May 2008 and was accepted after revision for publication on 7 October 2008.

DOI: 10.1243/09544062JMES1177

Abstract: The energy of a laser beam is generally calculated based on the laser power andits processing speed. In this work, the laser welding modes such as conduction, conduction–penetration, and keyhole welding of thickness 1.6, 2, and 2.5 mm AISI304 stainless steel sheets,respectively, are studied at different beam energy levels. A series of bead-on-plate trials areconducted using a 500W continuous wave Nd:YAG laser source to study the beam–materialinteraction and the influence of laser power and welding speed on the formation of weld pool.In addition to the experimental study, a three-dimensional finite-element model is developedto analyse the transient heat flow and to predict the formation of the weld pool. The correlationamong the parameters including laser power, welding speed, beam incident angle, and the char-acteristic geometry of weld pool are established. Temperature-dependent thermal properties ofAISI304 stainless steel, the effect of latent heat of fusion, and the convective and radiative aspectsof boundary conditions are considered in the model. The heat input to the developed model isassumed to be a three-dimensional conical Gaussian heat source. Finite-element simulationsare carried out by using finite-element code, SYSWELD, and FORTRAN subroutines availablewithin the code are used to obtain the numerical results. The result of the numerical analysisprovides the shape of the molten pool with different beam energy levels, which is then comparedwith the results obtained through experimentation. It is observed that the results obtained fromfinite-element simulation and the experimental trials are in good agreement.

Keywords: laser welding, beam energy, finite-element analysis, temperature field, stainless steel,bead-on-plate

1 INTRODUCTION

In recent years, rapid progress has taken place inthe application of laser processing. Particularly, laserwelding has become one of the important manufac-turing processes in many industries like automobile,electrical, ship building, etc. The joining of struc-tures with laser welding is becoming more commonin the processing industries because of the narrowheat affected zone (HAZ) and low distortion com-pared to the conventional welding processes. During

∗Corresponding author: Department of Mechanical Engineering,

National Institute of Technology, Tiruchirappalli, Tamil Nadu,

620015, India. email: [email protected]

laser welding, a very high density of supplied power(105−106 W/cm2) brings metal to vaporization, aftera few milliseconds of irradiation. The pressure cre-ated by the intense vaporization tends to dig themolten metal zone, which allows the laser beam topenetrate deeper into the material, making the irra-diated zone a thin hole, known as a keyhole [1]. Themolten front of the keyhole will have higher depthas compared to the width due to the constant spotdiameter even for varying power density. This can leadto a cylindrical-shaped molten pool and heat trans-fer occurs predominantly via radiative and convectivemodes through the vapour and molten material.Whenthe laser power is low (<105 W/cm2), however, the con-duction mode of heat transfer dominates, resulting ina low depth-to-width ratio. Laser beam power, weldingspeed, beam energy, and spot size are the key process

JMES1177 © IMechE 2009 Proc. IMechE Vol. 223 Part C: J. Mechanical Engineering Science

1142 G Buvanashekaran, N Siva Shanmugam, K Sankaranarayanasamy, and R Sabarikanth

parameters adjusted to achieve the desired laser welddimensions, appearance, and heat input. Weld dimen-sions are the first requirement of laser welding, sincestrength and final shapes are often critical measures ofquality.

The laser welding of AISI304 stainless steel is usedin several areas, including electronics, medical instru-ments, home appliances, automotive, and specializedtube industry. The advantage of using laser to weldstainless steels is the low weld distortion produceddue to the low but concentrated heat input and theweld shape [2]. When developing the welding pro-cedure for a given application, the laser parametersmust be characterized and fully specified. This is usu-ally done by trial and error where the knowledge andexperience of engineers plays a major role. As the laserbeam intermittently interacts with the workpiece overa very short span of time, the workpiece is subjected tovery rapid heating and cooling cycles. The weld beadis the product of a number of overlapping spot weldsand every point in the weld experiences a complexseries of thermal cycles during the passage of the laserbeam. This makes the condition quite complicatedfor analytical modelling techniques. The numericalmodel, therefore, is the preferred option, although theanalysis requires a very large number of small timesteps – much smaller than those needed for the contin-uous analysis [3]. The thermal cycles experienced bya weld have a major influence on many of its charac-teristics like the mechanical properties, metallurgicalstructures, distortion, and residual stresses, etc. Whilethe advances in computing technology and numeri-cal techniques, the numerical simulations have begunto take shape as a realistic method for the predictionof laser welding parameters for a specific application.The simulations based on finite-element method arevery useful in predicting the weld pool shape from thethermal simulation at an early stage of product designand welding process development.

A number of simple analytical and numerical mod-els have been developed to determine the transienttemperature field of the laser welding process. Theelementary welding heat source models were basedmostly on Rosenthal’s solutions [4]. He proposed amathematical model for a moving heat source underthe assumptions of quasi-stationary state and con-centrated point heating in three-dimensional analysis.Sabbaghzadeh et al. [5] developed two numericalmodels in finite difference and finite element, to com-pute thermal phenomena during pulsed laser weld-ing. They found that the temperature contours anddepth of penetration were strongly dependent on thepulse parameters of the laser beam. Spina et al. [6]investigated the efficiency of numerical simulationsto predict the thermo-mechanical fields induced bylaser welding of aluminium alloy sheets. The depthof penetration, nugget size, and bead width of laser

spot welds on AISI304 stainless steel were studied byChang and Na [2] using the finite-element methodand artificial neural network. They also investigatedthe shape of the weld pool by placing the speci-men with and without air gap. Tsai and Asibu [7]developed a dynamic model of the conduction modelaser welding process based on two-dimensional heatflow, assuming a tear-drop shape of the weld poolfor process control. Trivedi et al. [8] proposed athree-dimensional transient conduction heat transfer-based coupled thermo-mechanical analysis of laserspot welding process. They considered a volumetricheat source, to accommodate enhanced heat transferin weld pool at higher temperature owing to con-vective flow and greater amount of absorption ofthe laser energy. Frewin et al. [9] presented a three-dimensional finite-element model of the heat flowduring pulsed laser beam welding and also they cal-culated transient temperature profiles, dimensionsof fusion, and HAZs by incorporating temperaturedependent thermo-physical properties and experi-mentally measured beam profiles. Tsirkas et al. [3]performed the simulation of laser welding processusing a non-linear heat transfer analysis, based ona keyhole formation model and a coupled tran-sient thermo-mechanical analysis. The analysis takesinto account metallurgical transformations using thetemperature-dependent material properties and thecontinuous cooling transformation diagram. Monemand Batahgy [10] evaluated the effect of laser weldingparameters such as the laser power, welding speed,defocusing distance, and the type of shielding gas onfusion zone shape and final solidification structureof different austenitic stainless steels with differentthicknesses. There are many research papers [2–10]which deal with the shape and size of the weld poolof laser beam welds with reference to different laserinput parameters.

This paper presents a transient temperature fieldin a bead-on-plate made on a 1.6 mm AISI304 stain-less steel sheet by laser welding, using FE transientthermal analysis. The heat source model is assumedto be a three-dimensional conical Gaussian. Therequired FORTRAN subroutines [11] are developedin order to define a moving distributed heat sourcefor the simulations carried out using finite-elementcode SYSWELD. The depth of penetration and beadwidth of bead-on-plate welds are measured exper-imentally from weld specimens are compared withthe results obtained from simulation for validation.However, the effect of beam energy on three modesof laser welding namely conduction, conduction–penetration, and keyhole welding have not beenadequately reported in the available literature. Thecombination of low beam power (450W) and lowprocessing speed (1000 mm/min) leads to high beamenergy level producing a keyhole mode of welding and

Proc. IMechE Vol. 223 Part C: J. Mechanical Engineering Science JMES1177 © IMechE 2009

Laser welding modes with varying beam energy levels 1143

the effect of various beam energy levels on the weldpool geometry has to be analysed in detail.

2 GOVERNING EQUATION AND BOUNDARYCONDITION

To develop the mathematical formulation for laserwelding process the geometry of substrate as illus-trated in Fig. 1 is considered. The figure shows a typicalweld seam for a bead-on-plate configuration and thetwo-dimensional transverse section analysed in thepresent work. The laser beam is assumed to follow athree-dimensional conical Gaussian distribution withthe focused spot on the workpiece surface.

2.1 Governing equation for heat transfer analysis

In the heat transfer analysis, the transient temperaturefield T of the laser welded sheet is a function of timet and the spatial coordinates (x, y, z) and is determinedby the non-linear heat transfer equation

ρCp

[∂T∂t

+ (−vw)∂T∂y

]= k

(∂2T∂x2

+ ∂2T∂y2

+ ∂2T∂z2

)

+ Qv(x, y, z) (1)

where Qv is the volumetric heat source term whichvaries with beam power and welding speed. k, Cp,ρ, and vw are the thermal conductivity, specific heat,density of the sheet material, and velocity of the work-piece, respectively. Figure 1 shows the schematic rep-resentation of the region considered for mathematicalformulation.

2.2 Boundary conditions

Figure 2 illustrates the thermal boundary conditionson a typical transverse section A-B-C-D (refer Fig. 1).

Fig. 1 Schematic representation of the region consid-ered for mathematical formulation

Fig. 2 Schematic representation of thermal boundaryconditions

The focused laser beam is irradiated on the top surfacewithin a circular zone defined by the position of centreof the laser beam and its effective radius. The rest ofthe steel substrate surface facing air is subjected to theconvection and radiative heat losses [8].

The initial condition is

T (x, 0, z, t) = T0(x, y, z) (2)

The essential boundary condition for the transientanalysis is

T (x, y, z, 0) = T0(x, y, z) (3)

on the boundary S1. This condition prescribes nodaltemperatures at the inlet surface S1.

The natural boundary condition can be defined by

kn∂T∂n

− q + h(T − T0) + σε(T 4 − T 40 ) = 0 (4)

on the boundary S2, where n is the direction nor-mal to surface and kn is the thermal conductivitynormal to surface (W/mK); h and ε are the surfaceheat transfer coefficient (25W/m2K) and emissivity,respectively; σ is Stefan–Boltzmann constant (5.67 ×10−8 W/m2K4), and T0 is the ambient temperature (K).The first, third, and fourth terms on the left-hand sideof equation (2), respectively, refer to conductive, con-vective, and radiative heat loss from the surface (S2)of the substrate. The term ‘q’ stands for the imposedheat flux on to the surface due to an external heatsource [9].

In laser welding the energy is absorbed by the basemetal and converted into heat. The process of laserdeep penetration welding (high depth-to-width ratio)



is characterized by the existence of plasma and a key-hole. The plasma is formed by ionized metal vapourand moves along with the molten pool as the weld-ing progress, which leads to the wider molten poolat the top surface of the weld. At the same time, thelaser energy is transferred to the keyhole wall by directabsorption of the base metal. During the computa-tion, it is considered that the heat source comprisesa plane heat source on the top surface and a conicalheat source along the depth (z) direction [12].

The three-dimensional conical heat source (Fig. 3)is a volumetric heat source that considers the heatintensity distribution along the workpiece thickness.

As shown in Fig. 3, the heat intensity depositedregion is maximum at the top surface of workpieceand is minimum at the bottom surface of workpiece.Along the thickness of the workpiece, the regionswere the diameter of the heat density distribution lin-early decreases [13]. At any plane perpendicular to thez-axis, the heat intensity distribution (Gaussian form)may be written as

Qv(r, z) = Q0 exp(−3r2

r20

)(5)

where Q0 is the maximum heat intensity, r0 is the radiusof the heat source at z (distribution parameter), and ris the radial coordinate of interior point.

The height of the conical heat source is H = ze − zi,the z-coordinates of the top and bottom surfaces are ze

and zi, respectively, and the radii at the top and bottomsurfaces are re and ri, respectively.

r and r0 are given as

r = (x2 + y2)1/2, r0(z) = re − (re − ri)(ze − z)

(ze − zi)(6)

The heat flux distribution in the keyhole volume fol-lows a conical Gaussian distribution, which can be

Fig. 3 Schematic of three-dimensional conical heatsource

Table 1 Chemical composition of SS304 sheet metalversus the weight percentage

C Cr Fe Mn Ni P S Si

0.055 18.28 66.34 1.00 8.48 0.029 0.005 0.6

expressed as

Qv(r, z) = 2Pπr2

0 He1−(r/r0)2

(1 − z

H

)(7)

where r0 is the initial radius at the top of the keyhole,H is the keyhole depth, r is the current radius (i.e. thedistance from the cone axis), z is the vertical axis, andP is the absorbed laser power [3].

The Gaussian heat distribution parameter is themain factor to adjust the heat input distribution on thedomain of the weld pool. Additionally, convection andradiation are applied on the domain (S2). Therefore,the heat input on the domain is expressed as

Qv(r, z) = 2Pπr2

0 He1−(r/r0)2

(1 − z

H

)− h(T − T0)

− σε(T 4 − T 40 ) (8)

3 MATERIAL PROPERTIES

The material used for this investigation is commercialAISI304 stainless steel sheet of thickness 1.6 mm. Thechemical composition of this material under annealedcondition is given in Table 1. It has many desir-able characteristics such as low thermal conductivity,high resistance to corrosion and high stability at ele-vated temperature. Thermo-physical properties of thismaterial are assumed to be isotropic and homoge-neous and are taken (Figs 4(a) to (d)) according toSabbaghzadeh et al. [5]. The latent heat of fusion Lf

is 247 kJ/kg, to be released or absorbed over the tem-perature range from solidus temperature TS = 1400 ◦Cto liquidus temperature TL = 1500 ◦C and the latentheat of vaporization Lv is 7600 kJ/kg [2].

4 FINITE-ELEMENT ANALYSIS

To model the temperature field induced by the laserirradiation in steel substrate more accurately, finite-element method (FEM) is adopted. FEM providesmore flexibility while modelling and has the capabilityto obtain full field numerical solution [13–20].

The non-linear finite-element code, SYSWELD isemployed to solve the transient thermal analysis in thisstudy. Figure 5 depicts the schematic representationof overall geometry and boundary conditions of thesolution domain. In the analysis the phase transfor-mation is also considered since the peak temperature



Fig. 4 Variation of thermo-physical material properties of AISI304 SS [5]

Fig. 5 Schematic representation of physical model

reaches the boiling point of the steel substrate [12].The convection and radiation loads are simulated assurface loads on the element-free faces, specifying thefilm coefficient, ambient temperature, and emissiv-ity of the material. Temperature-dependent materialproperties of AISI304 stainless steel are specified toconduct the non-linear transient thermal analysis. Thewhole solution domain is discretized into uniformeight-node hexahedrons consisting of 14 927 nodesand 18 362 elements as shown in Fig. 6. The accuracyof the finite-element method depends upon the den-sity of the mesh used in the analysis. The temperaturearound the laser beam is higher than the boiling pointof the material, and it drops sharply in regions awayfrom the molten pool. Therefore, in order to obtain thecorrect temperature field in the laser irradiation region



Fig. 6 Finite-element model of laser welding

it is necessary to have a more dense mesh close to theweld area while in regions located away from weld areaa more coarse mesh is used. This will help in obtaininga good solution and accuracy within a short time.

The following points are taken into account whiledeveloping the finite-element model:

(a) the workpiece initial temperature is 30 ◦C;(b) thermal properties of the material such as conduc-

tivity, specific heat, and density are temperaturedependent;

(c) the convection and radiation loads are considered;(d) the latent heat of fusion and vaporization for the

steel substrate is considered;

Fig. 7 Heat source fitting (a) in the weld line and (b)offset from the weld line

Fig. 8 Orientation of laser beam with respect to vertical(z) axis

(e) it is also assumed that the laser energy gets com-pletely transferred to the base metal by directabsorption.

In the local reference frame of finite-element codeSYSWELD, it is possible to define the (refer Figs 7 (a) to(b)) offset of the heat source centre (x0, y0, z0) and theangle of the laser beam relative to z-axis (refer Fig. 8)and oriented by the y-axis (ay ). Based on the orienta-tion of y-axis (beam angle), the source intensity Qv iscalculated and applied into the finite-element modeland moving load is implemented by using a FORTRANsubroutine [3, 18].

5 MODES OF LASER WELDING

The laser beam strikes the surface of a base metalonly a fraction of the light energy is absorbed at thesurface and converted to heat while the remaining isradiated from the surface. The absorbed light energyis used to melt the base metal, resulting in three dis-tinct modes of laser welding. These are conductionmode, conduction–penetration mode, and keyholemode welding [1]. In conduction mode welds, heatis conducted from the incident surface into the bulkof the metal mainly by thermal conduction and con-vection in the molten pool. The bead depth-to-width(d/w) ratio of conduction mode welds is typicallybelow 0.5. At higher power densities (105−106 W/cm2),the metal surface is heated above the vaporizationtemperature of the metal which results in keyholemode weld. A low power laser operating at larger focallength (spot diameter >0.6 mm) cannot form a stablekeyhole. The welding regime is somewhere betweenconduction mode welding and keyhole mode weldingnamed as conduction–penetration mode welding.

6 EXPERIMENTAL INVESTIGATION

Bead-on-plate welds are made on AISI304 stainlesssteel sheets of 1.6, 2, and 2.5 mm thickness using a500W GSI Lumonics JK500 Nd:YAG CW laser weld-ing system (Fig. 9) available at M/s. Optilase, Chennai.Specimens with machined surfaces are prepared withdimensions of 60 × 100 mm and are held firmly usingfixture to prevent distortion. Weld powers are varied



Fig. 9 Nd:YAG laser welding system

in 50W increments, beginning with 50W and end-ing at the power of 450W. During the experimentaltrials, industrial pure Argon gas with a flowrate of10 l/min is used as shielding gas. The laser head istitled 5◦ from normal to avoid back reflection dam-ages the laser delivery optics. For each run, the beamfocal length is 120 mm, the beam radius focused onthe surface of the samples is taken to be 0.3 mm, andthe beam radius at the end of the lens is estimated tobe 3 mm. Prior to welding, the test samples are sandedwith #200 sand papers and cleaned with acetone. Tostudy the effects of welding parameters on the weldgeometry, the tested coupons are sectioned perpen-dicular to the welding direction for metallographicexaminations. The sectioned samples are mountedin resin matrix, sanded using sand papers, and pol-ished using alumina (Al2O3, 0.05 μ) colloidal solution(Fig. 10). Samples are cleaned before being etched witha solution of ferric chloride + water. Then they areobserved under an image analyser to determine theweld geometry, i.e bead width and depth. A numberof trials for three different sets of welding parame-ters (beam power, welding speed, and welding speed)are performed. The values of the welding parametersused and the number of experiments conducted foreach set are shown in Table 2. The welding parametersare chosen based on the expertise available at WeldingResearch Institute, BHEL, Trichy, where laser weldingwas successfully used for many industrial applications

like welding of fins, batteries, dental clips, etc. and tofind the impact of beam energy on different modes oflaser welding.

7 RESULTS AND DISCUSSION

Finite-element simulations are carried out usingfinite-element code SYSWELD for each welding condi-tion listed in Table 2 to determine the weld geometry,i.e. bead width and depth of penetration taking theappropriate material properties for AISI 304 SS. Allthe weld dimensions are predicted by the melting andvaporization isotherms of the base metal using finite-element simulation. Since the AISI304 stainless steelmelts and vaporizes at about 1500 ◦C and 2467 ◦C,respectively [14], the zone covered above the meltingpoint of the material gives the corresponding moltenpool shape.

The simulation and the experimental results are tab-ulated in Table 3 (located in Appendix 1) for 1.6 mmthick sheet; it indicates that the laser weld, depth ofpenetration d and bead width w as calculated by thesimulation to the corresponding experimental valuesgives the standard error percentage of 6.3 and 1.4per cent, respectively. Figure 11 presents the tempera-ture distribution contours at three different positions(start, middle, and end) during the laser irradiation

Table 2 Laser welding parameters

Fig. 10 Mounted specimen for macro studies



for one of the cases, where the beam power is 450W,welding speed is 1000 mm/min, and the beam angle is5◦, from which the high temperature gradients in thevicinity of the weld line close to the laser source maybe clearly observed.

An elliptical shape of the molten pool arises aroundthe high energy heat source on the surface of theworkpiece as observed from Fig. 11. The shape andsize of the molten pool vary depending on the beampower and welding speed. Figure 12 shows the com-puted thermal cycles at nine locations on the spec-imen surface in the middle of the laser beam. Itis seen from Fig. 12 that the upper surface tem-perature of welding line (x = 0 mm) is the highestaround 2150 ◦C. Further, the distance away fromthe weld line is around 600 ◦C (x = 0.45 mm), whichindicates the characteristics of laser welding having

narrow HAZ. With the increment of the distance fromthe weld line, the velocity of the rising temperaturedecreases.

7.1 Effect of laser power and welding speed

Laser power is varied in the range between 50 and450W in steps of 50W as shown in Table 2. Weldingspeed, spot diameter, and angle of the beam are keptconstant at 1500 mm/min, 0.6 mm, and 5◦, respec-tively. The effect of the laser power on the depth ofpenetration is illustrated in Fig. 13. It is observedthat the depth of penetration increases from 0.04 to0.57 mm with increasing laser power from 50 to 450W.It also indicates that by increasing the laser powerthe depth of penetration tends to decrease to lower

Fig. 11 Temperature distribution during the welding process at three different time periods(a) 0.48 s (start), (b) 2.1 s (middle), and (c) 3.54 s (end)



Fig. 12 Thermal cycles at nine locations on the topsurface during the welding process

Depth of Penetration of laser weld at 1500 mm/min. process speed

y = -8E-11x4 + 7E-08x3 - 2E-05x2 + 0.0016x - 0.0037R2 = 0.9976

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

Beam Power, Watts

Dep

th o

f Pen

etra

tion,

mm

Exp-DOP FEM-DOP

0 100 200 300 400 500

Fig. 13 Effect of beam power on the depth of weld pool(welding speed = 1500 mm/min)

value when the laser power equals 50W and furtherstarts to rise up when laser power equals 450W. Theresults shown in Fig. 14 indicate that an increase inlaser power to 450W leads to noticeable increase inbead width from 0.33 to 0.69 mm. The molten pool cor-responding to laser power operating between 50 and

Bead Width of Laser weld at 1500 mm/min. process speed

y = 0.0011x + 0.2453R2 = 0.9889

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 100 200 300 400 500Beam power, Watts

Bea

d W

idth

, mm

Exp-BW FEM-BW

Fig. 14 Effect of welding speed on the bead width(welding speed = 1500 mm/min)

300W are nearly semi-circular (d/w < 0.5), which is atypical conduction mode welding. When the power isincreased to 350W, the conduction mode of weldingwill get transformed to conduction and penetrationmode of welding with the d/w ratio of 0.695. As thepower is further increased to 400 and 450W, the depthof penetration increases substantially in comparisonwith bead width resulting in keyhole mode weldingwith the d/w of 0.806 and 0.826, respectively. Fig-ures 15 and 16 shows the macrographs of AISI304 SSsheet bead on plate weld made using laser powers of250 and 450W, respectively. The weld bead geometriesexplain two different modes of laser welding: (a) con-duction mode welding with depth–width ratio closeto 0.308; (b) keyhole mode welding with depth–widthratio of 0.826. It is evident from the micrographs thatno weld cracks or porosity are found in any of thebead on plate welds, this may possibly be due to thegood crack resistance of the base metal and the weld-ing conditions provided. Similar behaviour is observedfor a welding speed of 1000 mm/min (Figs 17 and 18)with varying laser power from 50 to 450W. The depthof penetration and bead width is higher in compar-ison with the welding speed of 1500 mm/min for allthe beam powers. It is noticed from Figs 19 and 20that the welding speed is decreased from 1500 to 1000



Fig. 15 Comparison of (a) experimental and (b) finite-element simulation for laser power of 250Wand welding speed of 1500 mm/min

Fig. 16 Comparison of (a) experimental and (b) finite-element simulation for laser power of 450W,welding speed of 1500 mm/min, and beam angle of 5◦

mm/min at 450W laser power, the fusion line becomealmost parallel to the laser beam axis, resulting inan inverted thumb-like shape of the molten zoneand a higher weld depth-to-width ratio of 0.902 isachieved. The data points shown in Figs 13 and 17 arefitted with fourth-order polynomial equation and itshows a good fit with R2 value of 0.9976 and 0.9985,respectively. Figures 14 and 18 shows a plot betweenbeam power and bead width, where the data pointsare fitted by linear equation with R2 value of 0.9869and 0.9932. The dependence of d/w ratio on weld-ing speed is confirmed at different laser powers bymeans of conducting 15 experimental trials. A lowerwelding speed results in a considerable increase inthe molten pool shape and consequently an increasein d/w ratio leading to acceptable weld profile. Thisis due to the interaction rate of laser beam on basemetal at low speed which is higher when comparedto high-processing speed. Because of low processingspeed, the heat input (beam power/welding speed)increases leading to a greater volume of the base metalbeing melted, consequently the depth and width of themolten zone increases.

Figure 21(a) shows the connection between the dis-tance at the centre of the weld and the distribution

of temperature for one of the cases, i.e. laser power450W, welding speed 100 mm/min, and beam angle 5◦.It can be seen from Fig. 21(a) that the temperaturegoes down rapidly with the increase in distance to thecentre of the weld. As the distance is of about 2 mmfrom the weld centre, the temperature falls to 150 ◦C(high cooling rate). Figure 21(b) shows the tempera-ture distribution along the z-direction (depth). Fromthese two plots, bead width and depth of penetra-tion can be calculated and it is found as 0.84 and0.75 mm, respectively. The experimentally obtainedweld pool shapes shown in Figs 15, 16, 19, and 20to confirm the numerical calculations and it empha-sis that the finite-element simulations are the bestsuited method for welding analysis. A good correla-tion between the finite-element simulation and theexperimental results is observed with a standard errorpercentage of 1.4 and 6.3 for bead width and depth ofpenetration respectively for all the cases considered inthis investigation.

7.2 Effect of laser linear energy

As discussed earlier, the results indicate that: (a)increasing the beam power increases the depth of



Depth of Penetration of Laser weldat 1000 mm/min. process speed

y = 1E-10x4 - 1E-07x3 + 6E-05x2 - 0.006x + 0.2364R2 = 0.9985

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

Beam power, Watts

Dep

th o

f Pen

etra

tion,

mm

Exp-DOP FEM-DOP

0 100 200 300 400 500

Fig. 17 Effect of beam power on the depth of weld pool(welding speed = 1000 mm/min)

penetration and bead width; (b) increasing the weld-ing speed decreases the depth of penetration and beadwidth. The characteristics of laser welding are lowheat input, high energy density, and depth–width ratio.Even though the laser is operated at low power witha low processing speed the energy delivered is highcompared to the cases when a higher power and pro-cessing speed of laser is being used. This investigation

Bead Width of Laser weldat 1000mm/min. process speed

y = 0.0012x + 0.2951R2 = 0.9932

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

0 100 200 300 400 500Beam power, Watts

Bea

d W

idth

, mm

Exp-BW FEM-BW

Fig. 18 Effect of welding speed on the bead width(welding speed = 1000 mm/min)

emphasizes the same. Fig. 22 shows an example of aplot between linear energy and d/w ratio. A compar-ison of data shows that the weld dimensions changesignificantly as the laser power is kept constant at250W and welding speed is increased from 1000 to1500 mm/min. In the first case (i.e. laser power 250Wand welding speed 1000 mm/min), the d/w ratio is 0.7,giving keyhole mode of welding, and the latter gives

Fig. 19 Comparison of (a) experimental and (b) finite-element simulation for laser power of 400Wand welding speed of 1500 mm/min



Fig. 20 Comparison of (a) experimental and (b) finite-element simulation for laser power of 400W,welding speed of 1000 mm/min, and beam angle of 5◦

Fig. 21 The distribution of temperature in the (a) x-direction and (b) z-direction

conduction mode of welding (d/w ratio 0.308). Thisis because the linear energy produced by low powerand processing speed, respectively, which is higher(15 J/mm) as compared to the trials when the samelaser power is maintained and the processing speed isincreased (10 J/mm).

It is inferred from Fig. 22 that the weld dimensions(d/w ratio) do not change significantly as the linearenergy is increased from 2 to 6 J/mm. In contrast, anincrease in linear energy to 27 J/mm shows a markedincrease in d/w ratio from 0.239 (at 8 J/mm) to 0.902 (at27 J/mm). When the linear energy is below 6 J/mm, theshape of the molten pool is nearly semi-circular whichis typical conduction mode welding. As the linearenergy is increased above 15 J/mm, drastic improve-ment in the d/w ratio is absorbed leading to keyholemode welding. This is because at the higher beamenergy levels (<15 J/mm) a large volume of metalvapour is produced with sufficient pressure.The devel-oped pressure in turn is used to hold the liquid layeragainst the edges of the keyhole, leading to formation

of a stable keyhole mode welding. When the linearenergy is operated between 6 and 15 J/mm, the result-ing weld region is somewhere between conductionmode welding and keyhole welding termed conduc-tion and penetration mode welding. The data pointsshown in Fig. 22 are fitted with sixth-order polyno-mial equations and it shows a good fit with R2 valueof 0.9969. Also it is observed that there is an excel-lent correlation between the results of simulation andexperiment with a standard error percentage of 8.4.

7.3 Effect of laser beam power density

Figure 23 shows the effect of laser beam power densityon weld beam geometry. It is noted that at low powerdensity (103−104 W/cm2) the d/w ratio is around 0.4to produce a conduction mode weld. This is becauseat low irradiance, the weld puddle depression does notoccur and the laser input energy creates a shallow weldpool. When the power density of the beam is between104 and 105 W/cm2, the resulting d/w is around 0.4–0.7



6 + 2E-05x5 - 0.0006x4 + 0.0092x3 - 0.0659x2 + 0.2155x - 0.087R2 = 0.9969

Fig. 22 Effect of linear energy on the weld poolgeometry

leading to penetration mode weld. In this mode, thelaser energy delivered into the weld puddle is higherthan the energy lost to conduction into the rest of thesolid metal.

Fig. 23 Effect of laser beam power density on the weldpool geometry

It is also noted that at higher power density(>105 W/cm2) the d/w ratio greater than 0.8 leadsto a stable keyhole mode weld. At this mode, themetal surface is heated well above the vaporiza-tion temperature of the metal and a stable keyholemode weld is produced. Similarly, the data pointsshown in Fig. 23 are fitted with sixth-order polyno-mial equations and it show a good fit with R2 value of0.9974.

Fig. 24 Effect of the thickness of the workpiece on the temperature distribution (a) top surfaceand (b) depth direction



Fig. 25 Temperature distribution during the welding process at two different workpiece thickness(a) 1.6 and (b) 2.0 mm

Fig. 26 Temperature distribution during the welding process at three different beam incident angle(a) 5◦, (b) 10◦, and (c) 15◦



7.4 Effect of workpiece thickness

Figures 24(a) and (b) show results for the effects ofworkpiece thickness on temperature distribution forlaser power of 450W, welding speed of 1000 mm/min,and beam angle of 5◦. It is seen that temperaturedecreases as the thickness of the workpiece increases.It is also noted that the HAZ in the x- and z-directionsdecreases as the workpiece thickness increases (referFigs 24(a) and (b)). Particularly, the HAZ in z-direction(0.7 mm) is higher when compared to x-direction(0.5 mm) for the 1.6 mm thick sheet. Both the peaktemperature and the temperature gradient decrease asthe workpiece thickness increases; this is because theheat transfer rate is higher in high thickness workpiececompared to low thickness workpiece (refer Fig. 25).The relationship between the peak temperature valuesand the temperature gradient is non-linear in nature.

7.5 Effect of beam incident angle

Figure 26 shows the effects of beam incident angleon the temperature profiles for the laser power of450W and welding speed of 1000 mm/min. It is notedthat there is a slight variation in the peak tempera-ture value as the analysis performed between lowerbeam incident angle (5◦) and higher beam incidentangle (15◦). The peak temperature values attained atlow beam incident angle and high beam incidentangle are approximately 2417 and 2371 ◦C, respectively(Figs 26(a) to (c)). These deviations are mainly due tothe decreasing heat flux of the surface load for increas-ing beam angle and to the application of an inclinedheat generation rate that covers a greater region inx-direction. Furthermore, the convection and radia-tion losses at the element’s surfaces have a greatereffect on the inclined cases, because the temperatureisotherms get wider in the x-direction and lie in agreater area compared with those in the lower beamincident angle. Also, there is some reduction in depthof penetration and noticeable increase in bead widthwith increase in beam angle.

8 CONCLUSION

In this study, a three-dimensional finite-elementmodel is developed to predict the laser welding modeswith varying beam energy levels through tempera-ture fields for AISI304 stainless steel sheet. When theresults are compared with the experimental values, itis observed that the finite-element simulation usingfinite-element code SYSWELD is used for calculat-ing the laser weld bead geometry. According to thesimulation and experimental results, the followingconclusions are drawn.

1. There are three basic modes of laser welding that aredetermined by the laser power and welding speed.

2. The result shows that the laser power is directlyproportional to the depth of penetration and thewelding speed is inversely proportional to the depthof penetration.

3. Below 6 J/mm linear energy, the weld profile isalmost semi-circular leading to conduction modewelding.

4. It is observed from the results that conduction andpenetration mode welding will occur when the lin-ear energy is operated in the range between 6 and15 J/mm.

5. The transition from conduction to keyhole modewelding is achieved with 400W beam power anda welding speed of 1000 mm/min (linear energy>15 J/mm).

6. The results show that at low power density(103−104 W/cm2) the d/w ratio is around 0.4 result-ing in conduction mode weld, the power densityof the beam is between 104 and 105 W/cm2, pen-etration mode weld is achieved, at higher powerdensity (>105 W/cm2) the d/w ratio is greater than0.8 leading to stable keyhole mode weld.

7. It is noticed from the investigation that the tem-perature decreases as the thickness of the work-piece increases and further, the HAZ in the x- andz-directions decreases as the workpiece thicknessincreases.

8. There is a slight variation in the peak tempera-ture value as the analysis performed between lowerbeam incident angle (5◦) and higher beam incidentangle (15◦).

9. A very good agreement between the finite-elementsimulation and the experiment has been achievedfor bead width and depth of penetration with a stan-dard error percentage of 1.4 and 6.3, respectively.

ACKNOWLEDGEMENTS

The authors thank the Head, Welding ResearchInstitute and Management, Bharat Heavy ElectricalsLimited, Tiruchirappalli and Optilase, Chennai forextending the facilities to carryout this research workand allowing to present the results in this paper. Theauthors also thank NITT for providing computationfacilities (SYSWELD).

REFERENCES

1 Reddy, J. F. and Farson, D. F. LIA handbook of lasermaterials processing, 1st edition, 2001 (Laser Institute ofAmerica, Magnolia Publishing, USA).

2 Chang, W. S. and Na, S. J. Prediction of laser spot weldshape by numerical analysis and neural network. Metall.Mater. Trans. B, 2001, 32B, 723–731.

3 Tsirkas,S. A.,Papanikos,P.,and Kermanidis,Th. Numer-ical simulation of the laser welding process in butt-jointspecimens. J. Mater. Process. Technol., 2003, 134, 59–69.



4 Rosenthal, D. The theory of moving source of heat andit’s application to metal treatment. Trans. ASME, 1946,68, 849–866.

5 Sabbaghzadeh, J., Azizi, M., and Torkamany, M. J.Numerical and experimental investigation of seam weld-ing with a pulsed laser. J. Opt. Laser Technol., 2008, 40,289–296.

6 Spina, R., Tricarico, L., Basile, G., and Sibillano, T.Thermo-mechanical modeling of laser welding ofAA5083 sheets. J. Mater. Process. Technol., 2007,

7 Tsai, F. R. and Asibu, E. K. Modeling of conduction modelaser welding process for feedback control. J. Manuf. Sci.Technol., 2000, 122, 420–428.

8 Trivedi, A., Bag, S., and De, A. Three-dimensional tran-sient heat conduction and thermomechanical analysisfor laser spot welding using adaptive heat source. Sci.Technol. Weld. Joining, 2007, 12(1), 24–31.

9 Frewin, M. R. and Scott, D. A. Finite element model ofpulsed laser welding. Weld. J., 1999, 78, 15s–22s.

10 Monem,A. and Batahgy,E. Effect of laser welding param-eters on fusion zone shape and solidification struc-ture of austenitic stainless steels. Mater. Lett., 1997, 32,155–163.

11 ESI-Group. SYSWELD reference manual, 2007.12 Du, H., Hu, L., Liu, J., and Hu, X. A study on the metal

flow in full penetration laser beam welding for titaniumalloy. Comput. Mater. Sci., 2004, 29, 419–427.

13 Wu, C. S., Wang, H. G., and Zhang, Y. M. A new heatsource model for keyhole plasma arc welding in FEManalysis of the temperature profile. Weld. J., 2006, 85(12),284s–291s.

14 Tsoukantas, G. and Chryssolouris, G. Theoretical andexperimental analysis of the remote welding processon thin, lap-joined AISI304 sheets. Int. J. Adv. Manuf.Technol., 2008, 35(9–10), 880–894.

15 Moraitis, G. A. and Labeas, G. N. Residual stress anddistortion calculation of laser beam welding for alu-minum lap joints. J. Mater. Process. Technol., 2008, 198,260–269.

16 GuoMing, H., Jian, Z., and JianQang, L. Dynamic sim-ulation of the temperature field of stainless steel laserwelding. J. Mater. Des., 2007, 28, 240–245.

17 Xu, B. Q., Shen, Z. H., Lu, J., Ni, X. W., and Zhang,S. Y. Numerical simulation of laser-induced transienttemperature field in film-substrate system by finite ele-ment method. Int. J. Heat Mass Transf., 2003, 46, 4963–4968.

18 Balasubramanian, K. R., Siva Shanmugam, N.,Buvanashekaran, G., and Sankaranarayanasamy, K.Numerical and experimental investigation of laser beamwelding of AISI 304 stainless steel sheet. Adv. Prod. Eng.Manage. J., 2008, 3(2), 93–105.

19 Siva Shanmugam, N., Buvanashekaran, G., Sankara-narayanasamy, K., and Arunachalam, S. Modellingof temperature distribution for high densityheat source used in material joining process. InProceedings of the 24th International Manufactur-ing Conference, Waterford Institute of Technology,Waterford, Ireland, 29–31 August 2007, pp. 1061–1070.

20 Ji, Z. and Wu, S. FEM simulation of the temperature fieldduring the laser forming of sheet metal. J. Mater. Process.Technol., 1998, 74, 89–95. A

PP

EN

DIX

1

Tab

le3

Aco

mp

aris

on

ofc

alcu

late

dd

epth

ofp

enet

rati

on

dan

db

ead

wid

thw

asa

fun

ctio

no

flas

erp

ower

,wel

din

gsp

eed

,an

dli

nea

ren

ergy

for

1.6

mm

thic

ksh

eet

Lin

ear

Bea

mW

eld

ing

Wel

din

gE

xper

imen

tal

Exp

erim

enta

l(F

EM

)b

ead

(FE

M)

dep

tho

fE

rro

r%

Err

or

%F

EM

Err

or

ener

gyp

ower

spee

dsp

eed

bea

dd

epth

of

wid

thp

enet

rati

on

bea

dd

epth

of

Exp

erim

enta

ld

/w%

d/w

Sl.n

o(J

/mm

)(W

)(m

m/m

in)

(mm

/s)

wid

th(m

m)

(mm

)p

enet

rati

on

(mm

)(m

m)

wid

thp

enet

rati

on

d/w

rati

ora

tio

rati

o

12

5015

0025

.000

0.33

00.

040

0.31

00.

045

0.06

1−0

.125

0.12

10.

145

−0.1

982

350

1000

16.6

670.

370

0.05

00.

340

0.05

80.

081

−0.1

600.

135

0.17

1−0

.262

34

100

1500

25.0

000.

390

0.06

00.

360

0.06

50.

077

−0.0

830.

154

0.18

1−0

.174

46

100

1000

16.6

670.

450

0.07

00.

420

0.08

00.

067

−0.1

430.

156

0.19

0−0

.224

58

200

1500

25.0

000.

460

0.11

00.

440

0.12

00.

043

−0.0

910.

239

0.27

3−0

.140

69

150

1000

16.6

670.

500

0.15

00.

470

0.15

00.

060

0.00

00.

300

0.31

9−0

.064

710

250

1500

25.0

000.

520

0.16

00.

490

0.17

00.

058

−0.0

630.

308

0.34

7−0

.128

812

300

1500

25.0

000.

580

0.28

80.

540

0.30

00.

069

−0.0

420.

497

0.55

6−0

.119

912

200

1000

16.6

670.

580

0.30

00.

570

0.32

00.

017

−0.0

670.

517

0.56

1−0

.085

1014

350

1500

25.0

000.

590

0.41

00.

630

0.42

0−0

.068

−0.0

240.

695

0.66

70.

041

1115

250

1000

16.6

670.

600

0.42

00.

610

0.44

0−0

.017

−0.0

480.

700

0.72

1−0

.030

1216

400

1500

25.0

000.

620

0.50

00.

680

0.51

0−0

.097

−0.0

200.

806

0.75

00.

070

1318

450

1500

25.0

000.

690

0.57

00.

720

0.60

0−0

.043

−0.0

530.

826

0.83

3−0

.009

1424

400

1000

16.6

670.

730

0.66

00.

780

0.67

0−0

.068

−0.0

150.

904

0.85

90.

050

1527

450

1000

16.6

670.

820

0.74

00.

840

0.75

0−0

.024

−0.0

140.

902

0.89

30.

011

Tota

l0.

215

−0.9

46−1

.262

Err

or

0.01

4−0

.063

−0.0

84


A study of laser welding modes with varying beam energy levels

Documents

Transcript of A study of laser welding modes with varying beam energy levels