The Microstructures And Properties Research Of Friction Stir Spot Welding Of DP600Galvanized Steel To6061Al Alloy Sheets Without Keyhole

Along with the increasing lack of energy resources, the lightweight technique has aroused wider concern. Alumnium/steel structure component, which can reduce the weight and guarantee excellent strength, will have broad application prospects. However, the joining of aluminum to steel has always been a great challenge because of great differences in the physical and chemical properties. As a solid phase joining technique, friction stir spot welding (FSSW) which exhibits many attractive advantages, such as lower heat input and thinner intermetallic compounds (IMCs), will be one of the most important and potential techniques joining alumnium to steel.In this paper, a new process of friction stir spot welding without keyhole was carried out for spot joining DP600galvanized steel with the thickness of1mm and6061Al alloy with the thickness of3mm. The beautiful flat joints without keyhole and burr were achieved. The influences of main parameters on the shear resistance of joints were studied by the orthogonal test method. The results show that the influence factors from main to secondary were tool rotation rate, pin length and preheating time. The shear force of joints reached above10KN with the optimum technologic parameters, namely the rotation speed of1200r/min, preheating time of6s, pin length of3.2mm.Microstructures were analysed by the scanning electron microscopy (SEM). A welded joint consisted of the stir zone and diffusion zone. In the stir zone, some steel nails and hook-shaped distribution were formed in the aluminium substrate. Meanwhile, annular lamellar onion ring microstructures composed of different IMCs, which existed at the interface between aluminium and steel. Only a layer of uniform IMC FeAl3with average thickness about5μm was produced at the bonding interface of diffusion zone. Both mechanical joining and metallurgical bonding were formed in the FSSW joints, which made the joint have excellent mechanical property.Two different fracture modes were observed. One fractured at the whole aluminum/steel lap interface, and the other fractured in the diffusion zone after some plastic deformation and left a big hole on the top steel sheet. By comparison, the greater bonding strength can be attained in the second fracture mode. The brittle fracture microstructure consisted of river-shaped cleavage step and some FeAl3existed on the edges of these steps. So it can be inferred that the joint fractured at the interface between Al substrate and IMC layer in the diffusion zone. Besides, microvoids and cracks in the IMC layer can contribute to the forming and extension of the fracture.The hardness transverse distribution on the top steel and bottom aluminum sheets present the letter "W" shape and the maximum value appeared in the stir zone. For the longitudinal distribution of the hardness, the hardness of the aluminum/steel interface reached up to800HV, which was much higher than that of two base metals. It can be concluded that some hard IMCs were formed at the interface.