| With the increasing requirement of lighter vehicle body and safety of automobile, ultra-high strength hot stamping steel which one of the most ideal automobile structural parts has been used more and more widely in automobile manufacturing field. Galvanized steel sheet with good corrosion resistance is widely used in the vehicle body, so the resistance spot welding(RSW) between ultra-high strength hot stamping steel and galvanized steel is inevitable in vehicle manufacturing. Because of that the hot stamping steel has high hardness, high resistivity, high thermal conductivity and Al-Si coating on surface, so the welding process of high strength steel is complex, easy to form welding defects and only a narrow welding process scope. Therefore, it has important practice significance to research the macro/microstructure, mechanical properties and the related mechanism of the RSW between ultra-high strength hot stamping steel and galvanized steel.In this paper, the RSW between 1.35 mm hot stamping boron steel 22MnB5 and 1.5mm galvanized steel HSLA350 is researched. In orthogonal experiment, the welding joint with good performance and no splash is obtained under the parameter of I=10 kA,t=12 cycles and F=3.2 k N. The effect of main parameters on the macro characteristics of welded spot was studied by single variable method. With the increase of welding heat, the diameter of nugget is gradually increased, but the diameter of nugget will decrease when the joint occur seriously splashing. In all parameters, the pressure rate of hot stamping boron steel side is lower than that of galvanized steel side.From the analysis of the microstructure characteristics, the welded joint can be divided into three parts: base material(BM), heat affected zone(HAZ) and fusion zone(FZ). From the base matel to nugget, the microstructure of 22MnB5 side could be expressed as M / tempered M + granular carbides / F+M / fine M / lath M, and on the HSLA350 side, F + M / F+ flocculent P+ tempered M / F+ flocculent P+ M / F+M / lath M are observed. Because of the change of microstructure in each area, the phenomenon of hardening and softening occurred in HAZ on the 22 Mn B5 side. In the HAZ, on the HSLA350 side, the microhardness increases gradually along the direction from the BM to the FZ.There are four different failure modes, which are termed as the interfacial failure(IF), the pullout failure from the galvanized steel(PFG), the pullout failure on boron steel and tearing the galvanized steel(PFB-TG) and the pullout failure from the boron steel(PFB), in the tensile shear test. The fracture surface of the four kinds of fracture modes all have dimples with different size and depth.In order to study the formation mechanism of nugget, we have made a comparative analysis on the formation process of nugget of five combinations between the two matels. When the welding time is short, the nugget presents a cup cone shape and shows an obvious nugget offset. In the 22MnB5 side, the nugget diameter is slightly larger than the HSLA350 side, but with the extension of welding time, the difference of the nugge diameter between two base matels becomes smaller. In the five different combinations, the greater difference of base matel’s thickness the greater nugget offest. |
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