Microstructure and Mechanical Properties of Dissimilar Friction Stir Welding of High-Strength Martensitic Steel with Q235 Steel Plates

In this study, friction stir welding (FSW) technique was applied to dissimilar butt joints between a 1500 MPa grade high-strength martensitic steel and Q235 steel plate. Sound dissimilar joints could be successfully obtained under different rotation rates (300, 500, 700 rpm). The microstructure, interfacial characteristics and mechanical properties of the dissimilar FSW joints were systemically investigated. It was revealed that the bonded interface changed from “S” shape to onion-ring pattern when the rotation rates increased from 300 to 700 rpm. Along the interface, elements such as Cr, Mn and Ni were observed to diffuse from the martensitic side to the Q235 side. The thickness of the transitional layer varied from 1.0 μm at 300 rpm to 8.0 μm at 700 rpm. Obvious grain refinement of ferrite was formed in the stir zone at 300 rpm, with an average grain size of 5.0 μm. Furthermore, hardness reduction of about 150 HV in the heat-affected zone of martensite side occurred due to the tempering of martensite. In tensile tests, all the global tensile specimen of the joints fractured at the base metal of the Q235 side, while the miniatures tensile specimens of the stir zone fractured within the Q235 area, situated away from the bonded interface. In addition, the miniature tensile specimens exhibited the highest tensile strength of 547 MPa for the joint processed at 300 rpm.