Investigating the Effect of Optimum Welding Parameters on the Microstructural and Mechanical Properties of St37 Steel and 316L Stainless Steel Welded by the Friction Stir Welding Process

In this research, St37 and 316L steel sheets were welded using friction stir welding (FSW) process and effective parameters such as the rotational speed, linear speed of the tool, pin diameter and their appropriate values were studied. The microstructure, hardness, and strength of the different welding regions were investigated. It was observed that in the stir zone (SZ), a mechanical operation takes place which causes grain refinement up to 10-20 times and improves the mechanical properties of the joint. The thermo-mechanically affected zone (TMAZ) is less affected and causes the refinement of the structure. In the heat affected zone (HAZ), no mechanical operation was performed but in some parts, the grain size was larger and more stretched than the grains of the base metal. SEM microscopic images of the weld metal showed alternating onion rings and layers consisting of poor and rich alloying elements due to the non-equilibrium cooling rate of the melt.