Microstructure and mechanical properties in dissimilar friction stir welding of 316 stainless steel to 4140 steel

In the current study, 316 stainless steel and 4140 steel sheets were successfully joined using friction stir butt welding (FSBW). The tool's rotational speed and the linear welding speed were assumed to be 1400-1700 rpm and 30-50 mm/min, respectively. The weld microstructures were examined by X-ray diffraction (XRD), optical microscopy, and scanning electron microscopy (SEM). In this welding, three zones with different structures relative to the base metals, including thermomechanical zones for each base metal and stir zone, were formed. Observations indicated that in the microstructure of 4140 steel containing martensite, the martensite blades vanished, and the grains extended in the thermomechanical zone. Furthermore, in the 316 stainless steel containing austenite, the austenite grain size was shrunk due to the dynamic recrystallization in the thermomechanical zone. Also, the microhardness test results showed that the stir zones have higher hardness than base metals in all specimens because of the dynamic recrystallization and fine-grained structure. Moreover, the 1550 rpm - 40 mm/min welded specimen in the stir zone has the highest yield strength of 350 MPa in the tensile test, which was higher than the yield strength of 316 stainless steel.