THERMO-MECHANICAL NUMERICAL ANALYSIS OF HOT CRACKING DURING LASER WELDING OF 6XXX ALUMINUM ALLOYS

摘要

The finite element method is applied to study the mechanisms inducing hot cracking during laser welding of 6xxx aluminum alloys. The welding process of a single-sheet plate is modeled by adopting a three dimensional fully coupled thermo-mechanical finite element formulation, where the energy distribution due to a fiber laser heat source and the motion trajectory of the moving laser beam are specified. The temperature dependent material behavior of aluminum alloys is described by a thermoelastic-plastic constitutive law which assumes isotropic hardening, and the plastic strain history is re-initialized during the melting and solidification process. With this numerical model, the critical mechanical and thermal responses in the welded metal sheet are used to investigate potential hot cracking mechanisms. It is found that the heat flux vector field obtained through simulation is consistent with the grain orientation in the weld bead observed experimentally. The large tensile stress in the semi-solid mushy zone near the fusion line is used as an indicator for identification of hot crack initiation, which is shown to be in the direction normal to the welding line. As the crack grows from the fusion line into the weld centerline, due to the high transverse tensile thermal stress developed inside the weld central region, the crack propagation then shifts to the weld line direction. Furthermore, the effects of several manufacturing process parameters on hot cracking susceptibility are investigated. The results reveal that a smaller weld line-to-edge distance would result in a higher tensile stress state near the fusion line in the mushy zone, and thus increase the possibility of crack formation which is consistent with the experimental observations. Preliminary results also suggest that higher laser power or slower welding speed could increase the hot cracking susceptibility.