镁合金电阻点焊接头的拉剪断裂

Tensile shear failure of resistance spot welded magnesium alloy joints

采用体视显微镜、激光共聚焦扫描显微镜(CLSM)和扫描电镜(SEM)等设备研究了镁合金电阻点焊接头的断裂模式和断口形貌。结果表明,在拉剪试验中,接头断裂模式主要有两种,即结合面断裂和纽扣断裂。熔核内的结晶裂纹对结合面断裂接头性能影响较大,而对纽扣断裂接头性能影响不大。对于纽扣断裂,裂纹在熔核的胞状树枝晶区萌生,而后沿着熔核周围的胞状树枝晶区迅速扩展,并沿板厚方向依次通过热影响区和母材。结合面断裂断口表面具有金属光泽,且较为平整。而纽扣断裂断口形貌在不同位置上具有不同的特点,这些特点与不同位置所受的应力状态有关。

Failure modes and fracture appearance of resistance spot welded magnesium alloy joints were investigated with stereoscopic microscope, confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM). The results show that two failure modes were often observed during tensile shear test, which are interfacial failure and button pullout failure, respectively. Effect of solidification cracking in weld nugget on mechanical properties of joints is bad for interracial failure, while it can be ignored for button pullout failure. For button pullout failure, the crack initiates at cellular dendrite structure, which first propagates along cellular dendrite structure of the nugget circumference, and then pass through heat affected zone (HAZ) and base metal in sequence along the direction of sheet thickness. Fracture surface displayed metallic luster and relatively flat for interracial failure. Different fracture surface appearances could be observed at different position for button pullout failure, which is related to stress distribution at different position.