Friction stir processing (FSP) and standard hardening T6 heat treatment were applied for local surface hardening of the АК12D aluminum (Al-12.8Si-1.67Cu-1.03Ni-0.84Mg-0.33Mn-0.23Co-0.24Fe) alloy plates obtained by hot-compression at elevated temperature. FSP was carried out by introducing a pin into the bulk of the material, followed by its movement along the surface at a traverse speed of 8 and 16 mm / min, accompanied by rotation of the pin at a speed of 2000 rpm. The effect of FSP parameters and T6 treatment on structure and hardness were investigated. It was established that FSP at 2000 rpm and 8 mm / min speed resulted in the formation of a monolithic and defect-free processing zone. The FSP and T6 treatment led to fragmentation of the primary Si and intermetallic phases and partial dissolution of intermetallic phases in the α-Al solid solution followed by decomposition and the formation of dispersed precipitates. The formation of a quasi-equiaxed finegrained structure was observed after FSP and T6 heat treatment in the stir zone. The measured Brinell hardness demonstrated an average value of 128 HB after treatment versus 103 HB in the initial state. The work reveals the features of the local surface structure refinement allowing to figure out approaches to the design of constructions with enhanced properties.