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

Cold gas dynamic spraying (“cold spraying”) at low pressure (150 psig) was used to fabricate Al-Al₂O₃ metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained -45 µm Al stock powder admixed with -10 Al₂O₃ in fractions ranging from 0-90 wt%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted on the as-sprayed coatings. The coatings were then friction-stir processed (FSP) using a milling machine and a 12 mm diameter cylindrical tool. Microhardness testing, SEM, and image analysis were then repeated to study the effect that FSP had on the MMC coating hardness. Hardness increased with increasing fraction of Al₂O₃ in the feedstock powder, resulting in a maximum as-sprayed coating hardness of 85 HV when 90 wt% Al₂O₃ is used. After FSP, the hardness of the MMC fabricated from a 90 wt% Al₂O₃ powder blend increased to a maximum of 140 HV. SEM micrographs showed that the as-sprayed MMC coatings contained Al₂O₃ particles that had been trapped between the larger Al particles. FSP succeeded in redistributing the Al₂O₃ particles, decreasing the mean free interparticle distance and increasing the probability of load sharing between the reinforcing particles. It was suggested that this redistribution may be the primary reason for hardness improvement in the MMC coatings.