Numerical Simulation Of Stress Field During Selective Laser Melting Of Mg-Y-Sm-Zn-Zr

Selective Laser Melting(SLM)has been considered as a promising additive manufacturing technology for the fabrication of lightweight structural parts of Mg-alloys.However,due to the large temperature gradient of the material affected by the laser heat source in the actual production process,it leads to the large residual stress remaining inside the part after forming which makes it deformation and failure.The unstable quality of formed parts hinders the application and development of this technology.Numerical simulation technology can provide in-depth analysis of the SLM process with low cost and efficiency,and has become an important means to study the evolution of thermal stress.In this study,the Mg-3.4Y-3.6Sm-2.6Zn-0.8Zr alloy is used as the object of a 3D finite element numerical model based on thermal-structural sequential coupling.The material nonlinear thermal physical parameters and the anisotropic thermal conductivity enhancement factor are considered to reduce the accuracy error arising from neglecting the melt pool convection.The thermo-mechanical evolution behavior of single-layer scanning and block forming was analyzed.The influence of process parameters(laser power 50-80 W and scanning speed 0.2-0.5 m/s),single-layer scanning methods(continuous,island and orthogonal scanning)and multi-layer scanning strategies(0°,37°,67°and 90°interlayer rotational scanning)on the thermal-stress field was investigated.Accordingly,the SLM process strategies were selected and optimized,and the simulation results were verified by experiments.The results show that the peak temperature of the melt pool and the liquid phase retention time decrease with the decrease of laser power and the increase of scanning speed.The effect of different scanning methods on the thermal behavior is obvious.The instantaneous peak temperature at orthogonal scanning between 4×4 island(817.20℃)is 15.82%lower than that of the long-side reciprocating scanning.There is a remelting effect between different layers in multilayer forming.With the increase of the interlayer angle,the peak temperature decreased and then increased,and the lowest relative peak temperature(1133.35℃)was obtained by 67°rotating scan.With the decrease of laser power and the increase of scanning speed,theσ_x andσ_ystress keep increasing.The equivalent stress shows a trend of decreasing and then increasing with the increase of laser power and scanning speed.The most uniform stress field distribution and low stress values were observed with the laser power P=60 W and scanning speed v=0.3 m/s.Island scanning reduces the high stress areas in the deposited layer and the residual stress within the island decreases with increasing number of island number.Orthogonal scanning can improve the problem of stress concentration.The interlayer rotation angle reduces the overall stress level of the deposited layer and acts to reduce the residual stress difference in the X and Y directions.The orthogonal scanning between 4×4 island and 37°rotating scan both obtained the lowest average equivalent stress,S11 stress and S22 stress,and the most uniform stress distribution.It is concluded that these two scanning methods are the optimal scanning strategies.Based on the numerical simulation,the SLMed samples of the rare-earth Mg-alloys were characterized by experimental means.The errors of experimental melt pool size and model prediction were obtained as 7.26%and 9.95%,and the error value of melt path morphologyθwas within 2.24%.The residual stress values are in general agreement with the experimental results,thus verifying the reliability of the model at multiple scales.