Fabrication of Aluminum-Alumina Metal Matrix Composite Via Low-Pressure Cold Spraying Followed by Friction Stir Processing

Cold gas dynamic spraying at low pressure (150 psig) was used to deposit 200 – 800 µm thick aluminum-alumina (Al-Al₂O₃) metal matrix composite coatings on 6061 Al alloy. The coating powder consisted of -45µm Al stock powder admixed with -10 µm Al₂O₃ powder in fractions ranging from 0 – 90 wt.%. Deposition parameters such as cold spray nozzle speed, stand-off distance, and powder feed rates were modified in order to optimize the content of alumina in the final coating and the coating hardness. The as-sprayed coatings were friction stir processed in order to redistribute the reinforcing Al₂O₃ particles and modify the microstructure, resulting in an increase in hardness. Optical and scanning electron microscopy (SEM) imaging confirmed the redistribution and content of the Al₂O₃ particles in the coating. It was found that increasing the content of the Al₂O₃ powder in the feed-stock powder increased the corresponding percentage in the coating, resulting in an overall hardness increase by up to 90%.  After friction stir processing, the hardness of the coating with the highest Al₂O₃ content increased by an additional 55%. These results show that a significant increase in coating hardness is possible by combining these two processes.