Non-monotonic evolution of microstructure and fatigue properties of round bar–plate rotary friction welding joints in 304 austenitic stainless steel

In this study, the structure and fatigue properties of a new type asymmetric round bar–plate rotary friction welding (RFW) joints with three kinds of welding time, 0.8 s, 1.2 s and 3.6 s, were investigated. The results show that the RFW processing evolves from the heat stage to the quasi-steady stage with a trade-off of dynamic recrystallization (DRX) and grain boundary sliding (GBS). From 0 s for the base metal (BM) to 3.6 s, the DRX intensity changes from strong to weak relative to the GBS intensity, resulting in the non-monotonic evolution of the microstructure, e.g., the texture, the average grain size and the geometrically necessary dislocation, with welding time. The evolution results in non-monotonic evolution of the fatigue properties: specimens with welding time of 0.8 s, 1.2 s and 3.6 s, have fatigue lives of approximately 150,000, 200,000, and 100,000 cycles, respectively. The microstructure evolves with the welding time through the texture strength, grain size, and dislocation density, affecting the dislocation and slip mobility. This evolution not only directly affects the rates of crack initiation and extension, but also makes the initiation point of fatigue crack change on the fillet, which affects the fatigue life.