Investigation of mechanical properties and microstructures of aluminum-fly ash composite processed by friction stirring

An aluminum-fly ash (ALFA) composite comprising aluminum alloy ADC6 and 5 wt% fly ash was produced by the stir casting method, and multipath friction stir processing (MP-FSP) was then applied to the composite. The microstructure of the processed area and the fracture surfaces of a tensile test specimen were investigated to determine the distribution of the fly ash in the aluminum matrix, the grain refinement, and the failure mechanisms. The experimental results showed that the grain of the composite was significantly refined by MP-FSP, with the original fly ash particle size of 53–106 μm reduced to 10 μm or less. Compared to the ALFA composite without stir processing, the tensile strength and elongation parallel to the direction of stirring were respectively increased from 143 MPa to 227 MPa and from 1.19% to 7.18%; whereas the same properties perpendicular to the stirring direction were respectively increased from 143 MPa to 226 MPa and from 1.19% to 3.12%. The presence of many rosette patterns and dimples on the fracture surface of the specimen clearly indicated that plastic deformation occurred in the composite after MP-FSP, resulting in recrystallization refinement and hence significant improvement of the mechanical properties.