Hideki Okada

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.

Refill Friction Stir Spot Joining for aerospace aluminum alloys

Refill Friction Stir Spot Joining (RFSJ) developed by Kawasaki Heavy Industries (KHI) is a derivative technology of friction stir spot welding for joining aerospace aluminum alloys. The aerospace aluminum alloys were previously considered not weldable using conventional fusion welding methods. RFSJ does not consume any filler materials so that no additional weight is introduced to the assembly. As the solid-to-liquid phase transition is not involved in RFSJ in general, there is no lack of fusion or material deterioration caused by liquefaction and solidification. Unlike the conventional friction stir spot welding, RFSJ produces a spot joint with a perfectly flush surface finish without a key/exit hole. KHI has advanced the original friction stir spot welding concept and developed a robotic system that is capable of producing refill friction stir spot joints. The goal of this study is to demonstrate process parameter optimization of RFSJ for baseline aerospace aluminum alloys.