Abstract
The present study was carried out to evaluate the microstructure and mechanical properties of the friction stir welding (FSW) of dissimilar alloys of low-carbon (SPHC) and high-carbon (SK85) steels. For this work, FSW was performed at two tool rotation speed of 400 rpm and 800 rpm, and traveling speed was fixed at 200 mm/min. Phase transformation of the microstructures in the joints was investigated by scanning electron microscopy imaging. Vickers hardness and tensile test were used to evaluate the mechanical properties of the joints. In the base metal (BM), SPHC and SK85 each has single ferrite phase and globular cementite phase + ferrite phase. After welding, fine ferrite phase and martensite phase were observed in the joint due to phase transformation and dynamic recrystalization. Volume fractions of martensite phase were different according to tool rotation speed. The area fraction of the martensite phase at the higher tool rotation speed was less than that at the lower tool rotation speed. The lower area fraction at the higher tool rotation speed was attributed to the enhanced diffusion of carbon atoms during FSW. Hardness distribution also showed that hardness at higher tool rotation speed was less than that at lower tool rotation speed and it is related to different volume fractions of the martensite phase. After the tensile test, the test specimen was fractured at the SPHC BM region and showed the same yield strength (YS) and ultimate tensile strength (UTS) of SPHC BM.
