Study on combination process of resistance and laser spot welding for hot stamping boron steel

In order to avoid splash when large amount of heat of resistance spot welding is input for hot stamping high strength steel and meet the strict requirements for assembly under laser spot welding,a new welding process that is combination of resistance spot welding and laser spot welding is proposed. The welding joints were obtained through the combination of resistance and laser spot welding process. The microstructures of various regions for welding joints were analyzed with scanning electron microscopy. The micro-hardness distribution of welding joints was tested by micro-hardness testing. The mechanical properties of welded joints under different welding process were obtained by using universal tensile machine,and the fracture mode and fracture mechanism were analyzed. The results show that the welding area under process of combination of resistance and laser spot welding is composed of resistance welding zone and laser welding zone. The microstructure of laser nugget zone and base metal are lath martensite. The heat affected zone locates outside of laser ring and near the base metal,and the nugget of original resistance spot welding are tempering organization. The hardness of laser nugget zone is the same as that of base metal,and the hardness of softening zone corresponding to the tempering zone decreases to 60% of that of base metal. The softening zone outside of laser ring is the weak region for tensileshear fracture. The obtained welding joints by this combination process have better load carrying capacity and toughness than those by individual resistance spot welding or laser welding.