Abstract:

The microstructure evolution and deformation behavior of resistance spot-welded joints of the two layer plates of 1500HS hot-formed steel sheets are studied in this paper. Through metallographic analysis， heat input distribution map， and alloy material property map， the microstructural changes at various positions relative to the weld nugget are analyzed. As the distance from the center area of the weld nugget increases， the microstructure of the welded joint can be divided into columnar crystal martensite， coarse-grained martensite， fine-grained martensite， ferrite-martensite dual phase microstructure and tempered martensite microstructure. Combined with Vickers hardness analysis， the differences in hardness under different organizational characteristics are clarified. The results show that the hardness decreases significantly in the ferrite martensite dual phase structure and tempered martensite region， which are the weak areas of the welding joints. Based on the experimental results of fusion size， maximum failure load， fracture surface macro-morphology， initial fracture location， and Vickers hardness of different plate thickness combinations， the influence of plate thickness and plate strength on the fracture mode， initial fracture location， and maximum failure load of the spot welded joints is explained.

Key words: resistance spot welding, welding joint failure, fracture mode, microstructural evolution