Hook formation and joint strength in friction stir spot welding of Al alloy and Al-Si-coated hot-press forming steel

In the automotive industry, the demand for ultra-high-strength steel is increasing due to the CO2 emission and safety regulations. Hot-press forming (HPF) steels are a type of boron-alloyed high-strength steel fabricated via hot-press forming, which enables both high strength and elongation. Because HPF is conducted at high temperatures (900-950 degrees C) for a few minutes, its surface is coated with Al-Si or Zn to prevent surface oxidation and decarburizing. However, the coating layer often influences the properties of the welds. In this study, friction stir spot welding (FSSW) is used to weld dissimilar metals, i.e., an Al alloy and Al-Si-coated HPF steel. The effects of Al-Si coating on the mechanical and metallurgical properties of the hook formation on the weld are investigated. The shape of the hook, which is formed during FSSW, changes from bent to straight shape due to the presence of Al-Si and the HPF process. The joint strength of the straight hook-shaped specimens is demonstrated to be lower than that of the bent hook-shaped specimens. This difference in strength is because the hard Fe-Al-Si intermetallic (IMC) layer on the outer surface of the hook disturbs the bending of the hook during the welding. On the outer surface of the hook, a Fe-Al-Si IMC layer of chemical composition similar to that of the coating layer formed during HPF is observed. This formation is different from the inner surface of the hook, wherein a thin Fe-Al IMC layer is reconstructed between aluminum and steel. Thus, the hard Fe-Al-Si intermetallic layer transformed during the HPF process is the primary cause of the straight hook shape.