In-situ observation of microcrack initiation and damage nucleation modes on the HAZ of laser-welded DP1180 joint

In-situ SEM can intuitively observe the changes in microstructure and microcrack initiation during the deformation process of the specimen. In this paper, the microstructural evolution, mechanical properties, and damage nucleation modes of DP1180 laser welded joints were systematically discussed by the transmission electron microscope (TEM), focused ion beam (FIB), and in-situ tensile test. The precipitation of carbides and disappearance of dislocations tangles deteriorated the mechanical properties of tempered martensite in the sub-critical heat affected zone (SCHAZ), which caused its microhardness slightly decreased (∼27 HV) and also called the softened zone. The joints fractured in the SCHAZ during the tensile and Erichsen cupping tests and microcracks initiated from the tempered martensite, while the microcrack initiation of base material (BM) occurred at the ferrite/martensite interface. The dominant damage nucleation mode of the joint was tempered martensite cracking, and that of BM was ferrite/martensite interface decohesion during tests.