Effect of friction stir spot processing on microstructure and mechanical properties of cold-sprayed Al coating on Ti substrate

Abstract

This study aims to investigate the influence of friction stir spot processing (FSSP) on the microstructure, microhardness, and adhesion strength of the cold-sprayed Al coatings on Ti substrates. A sound and dense coating is obtained at 2400 rpm. The pores and inter-particle interfaces completely disappear, and the equiaxed grains with low dislocation density are observed in the stir zone (SZ) after FSSP. The varying grain sizes and dislocation densities are found in different positions of the SZ under different rotation speeds. However, the Taylor factors are almost unchanged before and after FSSP. Compared with the as-deposited coating, the microhardness dramatically increases in the SZ after FSSP. Furthermore, when the dislocation density and crystallographic orientation are similar in different positions of the SZ, the microhardness variations can be interpreted by the Hall-Petch relationship, and the kH and H0 of the Hall–Petch relationship for the SZ of the FSSPed Al coating are obtained. The influence of the dislocation density and crystallographic orientation on the microhardness in the SZ is also discussed. Moreover, the TiAl3 layer generates at the interface between the coating and substrate after FSSP. The continuity of the TiAl3 layer is strongly dependent on the rotation speeds during FSSP, which further affects the coating adhesion strength.