Comparative Study of Friction Stir Welding and Underwater Friction Stir Welding on Magnesium ZE41 Alloy

5-mm-thick ZE41 magnesium alloy plates were subjected to normal friction stir welding (in air) and under water friction stir welding (UWFSW). A comparative study was made to investigate the process parameter and resultant properties of microstructure and tensile behaviour. The friction stir welding (FSW) was carried out at two different speeds 660 and 1220 rpm in order to obtain the influence of rotation speed on the performance of underwater joints. And the tool transverse speed and the tool tilt angle were fixed at 40 mm/min and 2.5°, respectively. From this investigation, it can be found that the UWFSW joint made using the higher tool rotational speed of 1220 rpm exhibited good tensile properties when compared to FSW joints without any defects like tunnel-type defects or hot cracking. The higher cooling rate in the UWFSW caused reduced thermomechanically affected zone (TMAZ) in the weld. The joint welded at UWFSW made the grain size fine when compared to the grain size of joints at the air weld condition. It is also noted that the hardness was increased in the UWFSW specimen due to the fine grains present in the stir zone. For air weld, the fracture occurred in the weld region and for UWFSW the fracture moved away from the stir zone.