Mechanical and metallurgical properties of friction stir welded austenitic steel pipes in position control mode

Abstract

The current work involves friction stir welding of austenitic steel pipes under position control mode. AISI 316L pipes of thickness 6.75 mm has been joined by considering parameters viz. feed rate (mm/min), spindle speed (rpm), tool depth (mm). The welding was conducted at same rotational speed of 400 rpm but with two different welding speeds of 50 and 100 mm/min and with fixed plunge depth of 6 mm. The microstructural study has been done through Optical microscopy (OM) and Field Emission Scanning Electron Microscope (FE-SEM). Hardness of the weld zone is found to increase significantly compared to the base metal which is due to grain refinement. Tensile tests show that the weldments have a higher yield strength compared to the base metal. However, the ultimate tensile strength decreases due to the appearance of defects viz. microcracks. These defects may be primarily due to the lack of heat generation. The joints display an efficiency of almost 87%. The present study also provides an idea about the optimal range of vertical force which may yield good quality weldments. With this range of vertical force, welding is carried out under force control mode and found to achieve higher efficiency. Moreover, the present experiments provide idea about the process parameters and how they impact the characteristics of the weldments.