Investigation of the effects of thread pitch and water cooling action on the mechanical strength and microstructure of friction stir processed AZ31

Friction stir process (FSP), as a severe plastic deformation, has a considerable effect on the mechanical and microstructure properties of a material. Recently, in new industrial applications, magnesium alloys are used widely due to their low density and appropriate material properties. One of the widely used alloys in automobile and aerospace industries is, AZ31. In this research, friction stir process was conducted on the AZ31 and the effects of cooling action, thread pitch of the pin, and traverse speed during FSP on material properties such as tensile strength, ductility, hardness, and microstructure were investigated. The FSP process tested pitch levels of the pin thread at one and three millimeters; with and without water cooling action, and at three different traverse speeds of 28, 40, and 58 mm/min. According to the results, the pin with one millimeter pitch thread improved mechanical properties and microstructure uniformity more than the pin with three millimeter pitch thread. So, it can be concluded that thread pitch had a special effect on material properties of tensile strength, micro-hardness, and size and uniformity of grain. But, its effect on ductility was low. Furthermore, cooling action decreased grain size and increased tensile strength, hardness, and ductility of the material.