Investigation of microstructure and wear resistance of AZ31B/SiO2/graphite hybrid surface composite produced by friction stir processing (FSP)

Magnesium hybrid surface composites are widely used in automotive and aerospace industries because they have low weight, high specific strength, and suitable wear properties. In this research, a mixture of silica and graphite nanoparticles was used to produce hybrid surface composites on AZ31B magnesium alloy by the use of friction stir processing (FSP). The effect of FSP passes on microstructure, microhardness, and wear resistance of composites was investigated. According to microstructural investigations, after 4 passes, the particles dispersed well and properly prevented grain growth so that the mean grain size of AZ31B–SiO2–graphite composite decreased about 80% compared with as-received AZ31B and above 50% compared with FSPed as-received AZ31B. Furthermore, in comparison with as-received AZ31B, microhardness of the composite increased more than 17% and its wear resistance increased more than 33% after 4 passes. The results indicated that grain size decrease plays a more significant role in composite hardness than dispersion hardening and other strengthening mechanisms. Furthermore, by increasing FSP passes, grain size variance and hardness variance decreased in the stir zone, which indicates that particles are dispersed and the microstructure becomes homogenized.