FSP pass number and cooling effects on the microstructure and properties of AZ31

In this study, the effects of passes number and cooling during friction stir processing (FSP) on the microstructure and properties of AZ31 magnesium alloy has been investigated. The process was carried out at tool rotational speed, transverse speed and tool tilt angle of 1100 rpm, 52 mm/min, and 3°, respectively. Cooling was performed by water. The microstructure of FSPed samples was explored by optical microscopy, atomic force microscope (AFM), and scanning electron microscope (SEM). Microhardness and tensile tests were used to characterize the mechanical properties of the samples. In addition, wear and corrosion resistance of processed samples were evaluated. Microstructural investigations showed that the cooling process during FSP led to finer and more homogenized grains because of the suppression of grain growth and as a result, hardness and tensile strength were improved. The best properties obtained after 4 passes by an increase in hardness and tension strength equal to 57 and 21%, respectively. In addition, wear characteristics of this specimen was considerably improved and electrochemical tests showed better corrosion resistance due to the decreasing grain size.