The effect of friction stir processing treatment (FSP) on the microstructure and physical and mechanical properties of an aluminum alloy of the Al-Fe-Mg-Si system (AD31T alloy) has been studied. In particular, the influence of various processing modes on the formation of structural and bulk defects in the stir zone (SZ) was studied. The optimal technological parameters that ensure the formation of a tool with a relatively homogeneous defect-free structure for a given tool geometry is a tool rotation speed of 1120 rpm at a tool linear speed of 200 mm/min. It is shown that an increase in the linear speed of the tool in the range of 50–200 mm/min leads to a decrease of the heat-affected zone (HAZ), a smaller size of volume defects in the SZ, and more uniform deformation. The microstructure analysis data is confirmed by the microhardness and electrical conductivity measurement values. It is shown that the decrease in the electrical conductivity in the SZ does not exceed 2 % of the electrical conductivity of the base material.