Microstructure and mechanical properties of Al/graphite- zirconium oxide hybrid composite fabricated by friction stir processing

Aluminum matrix surface composites (AMSCs) reinforced by various content of zirconium dioxide (ZrO2) nanoparticles and graphite (Gr) microparticles were fabricated by four passes of friction stir processing (FSP). Processing of composites reinforced by Gr particles did not lead to the sound and well-mixed composites due to a decline in the maximum experienced temperature during the process resulting from the low friction coefficient of graphite particles. Combination of graphite and zirconia particles resulted in a more homogenous dispersion of particles in the stir zone (SZ) due to a higher frictional heat generation and a lower tendency for agglomeration of nanoparticles as compared to relevant mono-composites. The occurrence of dynamic recovery (DRV) and dy-namic recrystallization (DRX) along with particle stimulated nucleation (PSN) and pinning effect of the particles led to fine and equiaxed grains in the SZ as compared to coarse and elongated grains of the base metal. X-ray diffraction (XRD) analysis demonstrated formation of the Al3Zr intermetallic compounds in Al/ZrO2 nano -composites because of in-situ reactions. Tensile tests and microhardness measurements showed that the ultimate tensile stress (UTS), yield stress (YS), and microhardness of composites gradually improved by an increase in the weight percent of the secondary phases. The UTS of Al/9 wt% ZrO2, Al/2 wt% Gr, and Al/3 wt% (Gr + ZrO2) with a hybrid ratio of (Gr/ZrO2 = 2) increased-42%,-53%, and-32% as compared to the base metal, respectively. Elongation of composites, especially in graphite-reinforced composites, decreased after FSP. The hybrid composite with a hybrid powder ratio of Gr/ZrO2 = 1 showed the lowest fluctuation of microhardness values because of a uniform particle distribution in the SZ. Hybrid composites with higher Gr content (Gr/ZrO2 = 2) showed greater UTS and microhardness values and lower elongation as compared to other hybrid composites.