The Use of Ashes in Surface Metal Matrix Composites Produced by Friction Stir Processing

The article presents an assessment of the possibility of using ashes from the combustion of coal and biomass as a reinforcing phase in metal matrix composites. The composite surface layer was produced by means of the FSP (friction stir processing) method, using an original hole solution with a shifted working zone of the pin. The composite matrix was AA7075 alloy. The obtained composite was subjected to microscopic observations, tribological tests, and hardness measurements. The microscopic examinations revealed favorable changes in the microstructure, in particular, strong refinement of the grains, uniform distribution of the reinforcing phase, and good bonding of the particles of this phase with the matrix material. Changes in the microstructure resulted in a significant increase in the hardness (from 36 to 41% depending on the type of reinforcing phase) and wear resistance (from 24.1 to 32.9%), despite partial dissolution of the intermetallic phases. It was found that the effect of strengthening the matrix and the uniformity of the distribution of the reinforcing phase depend on the physicochemical properties of the used powders, especially on the shape and size of the particles. The research shows that the use of ashes as a reinforcing phase in composites is fully justified.