Effect of induction preheating on microstructure and mechanical properties of friction stir welded dissimilar material joints of Inconel 718 and SS316L

Using an external heat source during friction stir welding (FSW) of high melting point materials is a promising approach to enhance the material flow and reduce the axial load, improving the tool life. In this study, conventional and induction heating-assisted friction stir welding (I-FSW) in joining dissimilar Inconel 718, and stainless steel (SS316L) alloy using the WC-10%Co tool was experimentally investigated. The welding was performed at a constant rotational speed of 300 rpm with welding speeds of 70 and 140 mm/min and 300 degrees C preheating temperature in the assisted FSW. The results show that the application of friction stir welding resulted in highly refined grains with deformed carbide particles in the stir zone of both materials, which increased mechanical properties. An increase in temperature during the I-FSW process led to a slight increase in the size of the particles and high atomic diffusion of Cr and Mo elements at the interface of the SS316L and Inconel 718 joint, which showed enhancement of the weld strength. In addition, highly dragged SS316L material into the advancing side of the Inconel 718 was observed, which was directly influenced by the preheating temperature and welding speed. The mechanical properties of the joint with preheating significantly improved at a high welding speed (i.e., at 140 mm/min). The tensile strength of the I-FSW joint at 140 mm/min was 683 MPa which was 99.8% of the base SS316L. Dynamic recrystallization resulted in grain refinement, which raised the hardness value. The results also show that preheating reduces downward axial force, enhancing the tool life.