Effect of friction stir back extrusion rotational speed on microstructure, mechanical properties, and corrosion behaviour of AZ91-Ca alloy

In this study, the effect of severe plastic deformation via friction stir back extrusion on microstructure, mechanical properties, and corrosion behavior of AZ91-Ca alloy were investigated. The results show that due to the high temperature during the process at a rotation speed of 1200 rpm, all the β-Mg17Al12, and Al2Ca phases are dissolved, and next to the manganese-rich compounds, re-precipitation of the β-Mg17Al12 and Al2Ca phases is observed. Also, increasing the rotation speed from 800 to 1200 rpm increases the accumulated strain from 0.186 to 0.351. With decreasing rotational speed from 1200 to 800 rpm, the texture parameter of the basal and prismatic planes is reduced, and on the contrary, the texture parameter of the pyramidal planes is increased. The ultimate tensile strength value increases from 332.14±8.71 MPa to 375.13±7.45 MPa by decreasing the rotational speed from 1200 to 800 rpm. The ultimate tensile strength, yield strength, elongation percent, and corrosion resistance in the sample processed at 800 rpm increased by 78, 49, and 32 %, respectively, when compared to the as-cast base metal.