Refill friction stir spot welding of dissimilar materials consisting of A6061 and hot dip zinc-coated steel sheets

Abstract

Friction stir welding (FSW) and friction stir spot welding (FSSW) are being paid much attention in automotive and aerospace industries. Especially, in automotive industry, the light weight body composed of multi materials such as aluminum alloys and high strength steel is strongly desired to satisfy the demand for fuel consumption saving, crash safety and reduce the cost for the car. However, conventional resistance spot welding (RSW) forms intermetallic compounds (IMC) that occasionally result in the brittle joint in the weld interface between aluminum alloys and steel. On the other hand, mechanical fastening such as rivet or self-piercing rivet can join dissimilar material, but it needs additional parts which must increase cost and weight. FSSW is expected to inhibit the formation of IMC because it is solid state joining technique. In addition, Refill FSSW, which is newly developed keyhole-less FSSW process, can bring benefits such as high strength and low scatter joints. Because of those reason, Refill FSSW is expected to contribute to produce aluminum-steel hybrid car body. In this study, configuration of 6000 series aluminum alloy and hot dip zinc-coated steel was attempted with Refill FSSW. Sound joint in terms of its appearance and cross sectional examination was obtained. In the microstructure examination, IMC layer in some micro mater thickness was not observed. Hardness distribution showed its reduction in stir zone and outside of stir zone and minimum hardness on aluminum sheet. In lap shear test, Refill FSSW exceeded JIS standard value for RSW. The fracture modes in lap shear tensile test were the shear fracture and the pull out fracture and the fracture modes seemed to relate to shoulder plunging depth, and every specimen was fractured in aluminum sheet.