Rotation speed controls microstructure evolution and joint strength in pre holed pinless friction stir rivet welding of aluminum to steel

Traditional riveting is widely used in aircraft assembly, but there are initial defects such as rivet loosening. In this study, an improved friction stir rivet welding (FSRW) process was developed to connect AA6061-T6 aluminum (Al) alloy and 304 stainless steel plates. In order to clarify the influence of the rotation speed of the stirring head, the joint temperature, surface forming, microstructure, failure mode and mechanical properties were studied experimentally. Experimental results indicated that increasing the rotational speed from 1500 rpm to 3500 rpm led to a rise in interfacial peak temperature from 440 °C to 650 °C, which significantly influenced the interfacial oxidation and microstructural evolution. With the increase of heat input, the oxidation degree of the joint surface gradually increases, and the microstructure also changes. The area of rivet-steel plate fusion zone increases first and then decreases with the increase of rotational speed, and reaches the highest at 2500 rpm. In addition, the thickness of IMC layer at the steel-Al interface increases continuously, and a hook structure is formed near the rivet. The mechanical interlocking induced by the hook structure, combined with the metallurgical bonding evidenced by the interfacial IMC layer, synergistically contributed to the joint strength. The average peak load of the joint reaches a maximum value of 8311 N at a speed of 2500 rpm. The failure mode of the joint is shear-plug fracture, and the joint presents a brittle-ductile mixed fracture mechanism.