Coupled thermal-mechanical-contact analysis of hot cracking in laser welded lap joints

Laser welding offers numerous advantages and has been widely adopted in the automotive industry. However, its application in light-weight vehicle manufacturing is still limited due to hot cracking that commonly occurs in Al-Mg-Si alloys. An in-depth understanding of the hot cracking mechanism is of great importance to the optimal design of welding processes. To this end, a 3-D coupled thermomechanical finite element model is developed to investigate the laser welding of lap joints made of 5xxx and 6xxx Al sheets. The temperature-dependent material behavior of aluminum alloys is described by a thermo-elastic-plastic constitutive law, the plastic history variable of which is re-initialized during material recrystallization processes. Moreover, a material state variable representing the irreversible welding process is introduced into the model to properly describe the thermal and mechanical contact behaviors at the interface between two alloy sheets. Using the simulated thermal field, the weld mushy zone susceptible to crack initiation during solidification is identified. Accordingly, critical mechanical state and field variables are analyzed to investigate the potential cracking mechanisms. It is found that the tensile interfacial stress is a key factor for the initiation of hot cracks close to the free edge of the welding specimen. The influence of weld line-to-edge distance on the cracking susceptibility is numerically evaluated and compared with experimental observations. The numerical simulation reveals that increasing the weld line-to-edge distance can reduce the cracking susceptibility, which is consistent with the experimental observation.