Numerical Simulation of Thermal–Mechanical Coupling in Laser–Metal Inert Gas Hybrid Welding Considering Grain Heterogeneity and Study of Phase Field Method during Solidification Process

Considering the microstructure grain heterogeneity of welded material can effectively reveal the mechanism during welding and improve welding quality. In this paper, a random microcrystalline model during laser-MIG hybrid welding was established based on the Voronoi method. The grain heterogeneity coefficient was determined, and the grain types were divided by nanoindentation experiments. The material properties were randomly assigned to Voronoi cells with a certain probability by writing a Python script program to introduce the grain heterogeneity structure. A moving heat source model of laser-MIG hybrid welding was established by programming a Fortran subroutine to couple Gaussian cone heat source and double ellipsoid heat source. The influence of the angle for the MIG welding gun on the heat input to weld pool was considered in the modeling, and the double ellipsoid heat source model was modified. Finally, the dendrite growth process of pure material was established by the phase field method, and the effects of anisotropy and flow velocity on dendrite growth were considered. The calculation shows that, compared with the conventional finite element model, considering the grain heterogeneity, the temperature field and stress field during laser-MIG hybrid welding show different changes. Among them, the temperature field difference is not significant, but the stress field shows an obvious uneven distribution. The stress at the adjacent grain boundary within the model abruptly changes, and the greater the difference in mechanical properties between grains, the more significant the mutation phenomenon. The phase field results reveal that the dendrite morphology is obviously asymmetrical when considering the flow velocity during welding solidification. This study provides an effective method to reveal the micro-evolution mechanism during laser-MIG hybrid welding and provides a reliable theoretical basis for improving the quality of hybrid welding and optimizing the hybrid welding process.