| Welding is a complex process that contains the mass transfer, heat transfer, metallurgy and mechanics. There’s on doubt that some welding parameters played a decisive effect on the quality and stability of the composite joints such as laser power, welding speed, defocusing amount, etc. Thin panels steel of car body is thin and has a low self-restraint. So it has a large deformation when welding. And purely theoretical study is difficult to solve of the practical problems of production. A large number of tests extremely increase the cost. Therefore, this study uses numerical simulation technology to build a mathematical model of laser welding. Welding deformation and stress sheet of thin plate structure can be predicted by calculating the temperature field, strain and stress field of thin plate laser welding. The bearing capacity of the structure can be improved by improve the welding process, reducing welding stress and deformation. It has a special meaning to solve sheet metal welding process problem and discover the evolution laws of the temperature field, thermal stress, thermal deformation and the residual stress and deformation after welding of the sheet butt welding components. It has a very important theoretical and practical significance to the automation of welding processes by applying it to production practices.Firstly, the finite element model of thin plate laser welding is established by the nonlinear finite element software MSC.Marc. At the same time, the temperature-dependent material properties of the thermal physical parameters are determined. The boundary conditions of finite element numerical simulation are sated. The laser weld puddle was in "nail head" shape because of the effect of the bath surface and "keyhole" of laser welding process. However, it can not be described accurately by one species heat model. In order to simulate the "nail head" heat source accurately, this paper uses Gaussian surface source combined with bodv heat source to simulate the laser heat source. Secondary development function of MSC.Marc is used to the body heat of the combination heat source. The subroutines write in language FORTRAN. Then by adjusting the surface and body heat resource distribution coefficient, the combination of heat resource is made in a good tune "nail head" shape and in good agreement with laser welding heat.Secondly, the paper focuses on the analysis of finite element numerical simulation on different sheets of laser power and laser welding under different welding speeds.(1) Set the welding speed of30mm/s, the laser power is set to1400W,1600W,1800W,2000W. Then carry out numerical simulations separately. The thickness direction of the maximum deformation results are0.076mm,0.0844mm,0.1018mm,0.1242mm. The maximum equivalent residual stresses are381.5MPa,384MPa,387.3MPa,390.7MPa. By analyzing the results, welding laser power as well as welding deformation and residual stress are positively correlated.(2) Set the laser power of2000W, the welding speed is set to20mm/s,30mm/s,40mm/s,50mm/s. Through numerical simulations, the maximum amounts of deformation of the plate thickness direction obtained are0.1262mm,0.1242mm,0.1227mm,0.1215mm. The maximum equivalent residual stresses are392.5MPa,390.7MPa,388.9MPa,387.7MPa. By analyzing the results, welding speed as well as welding deformation and residual stress are negatively correlated. Compared to welding speed, the effect of changing of laser power is more pronounced in deformation and residual stress.Finally, this paper analyzes the impact of pre-stretching method to welding deformation and stress. Then go on numerical simulation of laser welding by applying the pre-tensile stress of0.2times,0.5times,0.8times to the material in the flat ends of the yield. Through the simulation results, the conclusions are as follows:(1) When the material yield strength pre-tensile stress are0.2times,0.5times,0.8times, the maximum deformation are-0.1116mm,0.1426mm,0.3201mm respectively. And the deformation in the form of maximum is gradually changed from concave to raised. This shows that the tensile stress has become a major pre-impact factor to deformation after welding at this time and its effect on welding deformation is remarkable.(2) With the pre-tension stress of yield strength of the material increases from0to0.2times,0.5times,0.8times, the flat longitudinal residual stresses after welding are also reduced by the389MPa turn to328.3MPa,236.3MPa,132.7MPa. The longitudinal residual stresses are reduced by15.60%,39.25%,65.89%. The transverse residual stresses are reduced by6.21%,18.44%,36.59%. Followed by the equivalent residual stresses are reduced by6.15%,18.05%,35.59%. This indicates that pre-applied tensile stress can effectively influence the welding deformation as well as controlling residual stress after welding. Especially for controlling longitudinal residual stress, the effect is more obvious.Through numerical simulation method, laser welding sheet under different welding parameters as well as different pre-stretch stress can be analyzed. It has great practical significance to provide some predictions, guidance and basis for the welding practice.
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