Effects of Multi-Pass Friction Stir Processing on Microstructures and Mechanical Properties of the 1060Al/Q235 Composite Plate

Steel cuttings, holes and cracks always exist at the interfaces in the composite plate. Multi-pass friction stir processing (M-FSP) is proposed in this research to optimize the interface microstructure and the interface connection for the 1060Al/Q235 composite plate. Results show that the microstructures of 1060Al after M-FSP are fine and uniform owing to the strong stirring effect and recrystallization. Micro-defects formed by the welding can be repaired by the M-FSP. However, tunnel defects can also be formed in the matrix of aluminum by M-FSP, especially when the passes are one and two. The melting block and the melting lump in the composite plates are easy to become the source of crack. The shear strengths and the bending properties for the 1060Al/Q235 composite plate after M-FSP are the best when the passes are three, with the tool rotation speed of 1200 rpm and the forward speed of 60 mm/min. The optimized interfaces for the composite plate after M-FSP are mainly by the metallurgical bondings, with a certain thickness and discontinuous mechanical connections. Therefore, the crack extension stress is the largest and the mechanical properties are the best.