The structure and mechanical properties of the AK12D (Al–Si–Cu–Ni–Mg) aluminum alloy subjected to friction stir processing

Abstract

The application of friction stir processing (FSP) to modify the structure of the Al–Si alloys, in particular the fragmentation of large silicon particles, can lead to an increase in the level of mechanical properties. This work is aimed to study features of local surface hardening of AK12D aluminum alloy (Al–Si–Cu–Ni–Mg system) during FSP and subsequent T6 hardening heat treatment. The authors investigated the influence of FSP and subsequent heat treatment parameters on the structure, microhardness, and hardness of the AK12D alloy. FSP was carried out at speeds of processing tool rotation and traverse of 2000 rpm and 8, 16 mm/min, respectively. The subsequent hardening T6 heat treatment was carried out according to the standard regime for the AK12D alloy. The paper shows that the FSP mode at a rotation speed of 2000 rpm and a traverse speed of 8 mm/min contributed to the formation of a monolithic and defect-free treatment zone. The study revealed that the formed microstructure is heterogeneous due to the influence of various thermomechanical effects. The most intense structural changes occurred in the stir zone. Friction stir processing and subsequent heat treatment led to fragmentation and partial dissolution of intermetallide particles in the α-Al solid solution followed by its decomposition and formation of secondary hardening phases. Moreover, the FSP and T6 heat treatment led to the formation of quasi-equiaxed fine-grained structure. The AK12D alloy microhardness after treatment under the study varied nonmonotonically and depended on the structure in different zones. At the same time, the Brinell hardness values after FSP and subsequent T6 heat treatment increased compared to the initial heat-treated state.