Effect of Tool Rotational Speed on Friction Stir Welding of ASTM A516-70 Steel Using W–25%Re Alloy Tool

W–25%Re tool was used to friction stir weld ASTM A516-70 steel. This paper presents the results of studying the effects of rotational speed on tool reaction loads, tool wear, weld defects, and weld microstructure. The measured tool axial forces profile were found to be strongly coupled with the weld surface features. At high values of rotational speed, weld microstructure examinations revealed a wide heat-affected zone, coarse grains, and partial dissolving of ferrite and pearlite. These are attributed to the excessive heat generation at high levels of rotational speeds. Stir zone grain refinement occurred at all levels of rotational speeds under investigation, with the finest grains occurring at lower values of the rotational speeds. W–25%Re tool demonstrated excellent wear resistance at low rotational speed. However, at high values of rotational speeds fluctuating loads and high heat input resulted in excessive wear of the tool. Greater amount of wear took place at the tool shoulder, which experienced 50% reduction in the tool shoulder shank. Moreover, elemental diffusion of tungsten occurred into the weld and was found to increase with the increase in tool rotational speed.