Ultrasonic Peening-Water Jet combined Surface Modification Mechanism and Surface Integrity Study of 7075 Aluminum Alloy

This study employs an orthogonal experimental framework to investigate the effects of ultrasonic peening and solid projectile with water jet (UIT-SPEWJ) as a hybrid surface enhancement method on the surface integrity of 7075-T6 aluminum alloy. The research delves into the impact of various processing parameters on the alloy’s surface characteristics, encompassing surface finish, microhardness, residual stresses, and microstructural alterations. Our findings reveal a variation in surface roughness post-treatment, ranging from 0.852 to 2.411 μm; When the jet pressure was 25 MPa, the jet target distance was 7.5 mm, and the ultrasonic amplitude was 5 μm (referred to as UIT-SPEWJ-6), the surface roughness was the lowest at 0.852 μm. In contrast, when the jet pressure was 20 MPa, the jet target distance was 12.5 mm, and the ultrasonic amplitude was 20 μm (referred to as UIT-SPEWJ-4), the surface roughness was the highest at 2.411 μm. Surface textures displayed notable features such as crater-like indentations, adhered debris, and microporosity. The microhardness values on the treated surfaces were measured between 180 and 240 HV, marking an enhancement of 2.2% to 36.3% over the untreated samples. Among the specimens, UIT-SPEWJ-6 exhibited the deepest hardening effect with a layer reaching 240 μm, whereas UIT-SPEWJ-4 showed the shallowest at approximately 210 μm. Residual compressive stresses were observed to lie between 277 and 530 MPa, which denotes a substantial increase of 394.6% to 846.4% over the baseline stress of 56 MPa in untreated samples. Morphologically, UIT-SPEWJ-4 displayed continuous precipitate-free zones (PFZs) measuring 11 to 23 nm and abundant precipitates within 42 to 61 nm accompanied by pronounced dislocation activity. In contrast, UIT-SPEWJ-6 revealed discontinuous PFZs ranging from 7 to 16 nm, smaller precipitates between 22 and 36 nm, refined grain structures, and a higher dislocation density.