8/6/2019 CFDandFEMAnalysisofLaserWeldShapeanditsCharacteristic

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CFD and FEM Analysis of Laser Weld Shape and its Characteristic

M Sundar, R Eghlio and L LiLaser Processing Research Centre, School of Mechanical, Aerospace and Civil Engineering

1. IntroductionLaser butt welding simulation has been performed by many researchers in the past and in all previous simulations, the

urface of the weld bead is assumed to be perfectly flat which is a crude assumption. In this work a coupled thermo-structural

nalysis was carried out with an objective to predict the effect of laser parameters on change in surface topology of the weld

ead and also its subsequent effect on thermal and structural results. The numerical simulation results agree well with the

xperimental results conducted on a mild steel sheet using a 1 kW fibre laser.

2. FormulationThe 3D CFD analysis was performed incorporating Navier-

Stokes mass, energy and momentum equations. Heat input

is modelled as a Gaussian volumetric heat source and heat

loss is due to convection and radiation in the surfaces. The

weld surface topological changes are primarily due to the

fluid flow in the weld pool, which is driven by combined

surface tension and buoyancy force. The FEM

4. FEM Results

Speed = 75 mm/s

Speed = 100 mm/s

Speed = 125 mm/s

0.0E+00

5.0E+07

1.0E+08

1.5E+08

2.0E+08

2.5E+08

3.0E+08

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

Stress(Pa)

Distance (mm)

Sp ee d = 75 mm/s Sp ee d = 10 0 m m/ s S pe ed = 1 25 mm /s

geometry and the temperature history predicted by the CFDanalysis. To understand the effect of bead geometry on the

tensile strength characteristics of the welds, a non-linear

finite element analysis incorporating multilinear isotropic

hardening was performed.

3. CFD Results

6. DiscussionAt low weld speeds, the molten pool on the surface flows

outwards causing a depression in the weld pool centre and

at high speeds, the molten pool flows inward causing a

humped weld and at some particular welding speed there

Figure 1: Different weld bead top surface profiles for 600 W laser powerand speed: a) 75 mm/s, b) 100 mm/s, c) 125 mm/s

(a) (b) (c)

Fi ure 2: Com arison of ex erimental (left side) and simulated (ri ht side) weld(a) (b) (c)

Figure 4: Comparison of residual stress for difference welding conditions

a) along the cross section b) across the weld bead surfaces

(a) (b)

Figure 5: Comparison of experimental and simulated tensile test resultsfor a speed of : a) 75 mm/s, b) 100 mm/s, c) 125 mm/s

(a) (b) (c)

5. Tensile Test Results

Weld line

Failure zone

-

.22E-4

-.1

71E-4

-.1

22E-4

-.7

34E-5

-.1

85E-7

7. ConclusionsThe CFD simulation effectively predicts and paves way to control the weld bead surface geometry. CFD modelling has

shown the main reason for the different weld bead surface geometry formation as the Marangoni effect with flipping

surface tension gradient signs as the melt pool temperature changes. In FEM analysis the net-shape weld shows smooth andminimal stress distribution also, it shows better tensile test performance largely due to the lack of stress concentrators at the

weld zones which is also the case in experimentation.

gradients. The FEM results show relatively smaller and

smoother residual stresses for net shape welding shape,

possibly due to reduced heat input to the material. In the

tensile test simulation the net shaped weld, shows high

distortion away from the welding zone because of its flat

surface geometry shifting and spreading the stress

concentration to places away from the weld zones.

bead cross section profiles for a speed of: a)75 mm/s, b)100 mm/s, c)125 mm/s

Figure 3: Comparison of top surface velocity vector for 600 W laser powerand speed: a) 75 mm/s, b) 100 mm/s, c) 125 mm/s

(a) (b) (c)