Ultrasonic pulsed waterjet surface peening of an industrial aluminum-based metal matrix composite

Abstract

Peening is used across a wide range of materials to improve fatigue response. Recently, a new approach to peening has been developed using an ultrasonic-pulsed waterjet (UPWJ). In the present work, UPWJ peening has been conducted on a powder metallurgy derived aluminum-based metal matrix composite (Al-MMC), containing 5 wt% aluminum nitride (AlN) additions. Traverse speed is one of the most important parameters in UPWJ peening, which primarily modulates the mechanistic interaction between the waterjet and the target surface. In the current study, a focus was placed on assessing the effects of traverse speed (which was varied from 150 to 1000 mm/s), while the other primary process parameters, including the pressure, frequency and standoff distance were kept constant. Confocal laser scanning microscopy and scanning electron microscopy were used to evaluate the surface roughness and microstructure of the Al-MMC samples, in both un-peened and UPWJ peened states. The induced residual stress generated by UPWJ was also determined, with profiles obtained by electropolishing to remove micro-layers of material until an attenuation of the residual stresses was realized. It is shown that the Al-MMC was generally responsive to UPWJ peening. The surface roughness increased due to UPWJ processing, though the extent of increase was lower for increasing traverse speed. Mass losses were recorded at UPWJ traverse speeds lower than 500 mm/s due to material removal. Generally, the UPWJ peening process produced significant increases in compressive stresses, with the best combination of low surface roughness and high compressive stress obtained at a traverse speed of 700 mm/s.