In-situ formation of Al₃Ni nano particles in synthesis of Al 7075 alloy by friction stir processing with Ni powder addition

Bulk aluminum 7075 alloy strengthened by nano-sized Al-Ni intermetallic particles is synthesized by friction stir processing (FSP). Al 7075 alloy powders mixed with 10 wt% Ni nano powders were used as feedstock. The extensive thermal-mechanical actions by FSP accelerate the reaction and inter-diffusion between Al 7075 powder particles and Ni nano powder particles, resulting in the formation of Al₃Ni nano particles, in-situ during FSP, confirmed by scanning electron microscopy and X-ray diffraction. The presence of the Al₃Ni nano particles in the Al 7075 matrix increases the micro hardness by 19.0 % and 14.6 %, respectively, under as-processed and T6 aged conditions, compared to the one synthesized by FSP from 100 % 7075 alloy powder. The hardness increment agrees with the estimation from the Orowan strengthening effect of the Al₃Ni reinforcement. Furthermore, Al 7075 alloy with in-situ formed Al₃Ni shows a tendency of promoting T-Al₂Mg₃Zn₃ and inhibiting η-MgZn₂ precipitation during post synthesis heat treatment between 50 and 500 °C. This work suggests the potential of FSP as an effective process to bulk synthesize Al alloys with in-situ formation of Al₃Ni nano particles to further enhance the properties.