The impact of sintered density upon the microstructural and residual stress development in an ultrasonic pulsed waterjet peened Al-alloy/AlN composite

Abstract

A powder metallurgy processed, aluminum-based metal matrix composite (Al-MMC) was sintered to either 95.4 % or 98.8 % of theoretical density, and subsequently surface treated via ultrasonic pulsed waterjet (UPWJ) peening. During peening, the UPWJ traverse speed was varied while the stand-off distance, frequency, and pressure were maintained constant. The microstructures, residual porosity, surface topography, phase composition, and residual stress were subsequently evaluated as a function of the peening condition. The results indicated that the less dense Al-MMC variant was slightly more receptive to plastic deformation at slower traverse speeds. The surface roughness decreased with increasing traverse speed for both sinter conditions. Regardless of the density, the UPWJ peening process was effective in instilling residual compressive stresses on the surface of the Al-MMC, which initially was in residual tension after sintering. The concentration of sub-surface pores was found to be reduced after UPWJ peening for both sintered densities. This phenomenon is linked to the development of residual compressive stresses which penetrated about 380 μm into the UPWJ peened material.