Development of surface composite based on Al-Cu system by friction stir processing: Evaluation of microstructure, formation mechanism and wear behavior

In this study, friction stir processing (FSP) was used to fabricate in-situ surface composite based on Al-Cu system on 1060 Al substrate. The effects of the number of FSP passes on phase compositions, microstructure and wear behavior of the resulting surface composites were investigated. The XRD analysis showed that with increasing the number of FSP passes, only Al2Cu phase was formed, irrespective of FSP passes, and its amount increased. This was in a good agreement with the SEM and TEM results, which showed that during multi-pass FSP, there were two types of in-situ formed particles, including fine Al2Cu particles and relatively coarse core-shell structured Cu-Al2Cu particles, until most of core-shell structured Cu-Al2Cu particles were converted into small Al2Cu particles. Moreover, increasing FSP passes, with the opposite travelling direction of FSP tool between the consecutive passes, increased the area of the stir zone, decreased the particle size, improved the particle dispersion homogeneity, and favored the grain refinement. The formation mechanism of in-situ Al2Cu/Al surface composite produced by multi-pass FSP was also clarified based on the thermodynamic and kinetic standpoints as well as the process characteristics of FSP. The FSPed AMC layers exhibited significantly improved hardness and wear resistance as compared with as-received Al (similar to 24 HV), which were both improved with increasing FSP passes. The maximum microhardness was achieved in the 5-pass FSPed AMC layer, reaching a level as high as similar to 75 HV. Particularly, the wear mechanism was transformed from adhesive wear in the as-received Al to the combination of abrasive and delamination wear in 5-pass FSPed AMC layer.