Evolution of residual stress and its microstructural mechanisms in 7075-T6 aluminum alloy subjected to a combination surface modification technique

In this study, a combination surface modification technique integrating high-speed cutting and solid projectile entrained water jet (HSC-SPEWJ) was employed to strengthen the surface of a 7075-T6 aluminum alloy. By combining analyses of surface morphology, microhardness, residual stress, transmission electron microscopy (TEM), and numerical simulation, the evolution of surface properties and the underlying microstructural transformation mechanisms during the combined modification process were systematically investigated. The results indicate that, after samples after HSC machining, the sample surface was mainly characterized by machining feed marks accompanied by localized plastic deformation. Following the introduction of SPEWJ surface modification, the surface morphology was dominated by uniformly distributed plowing grooves, while localized impact pits and irregular damage features were significantly reduced. As the jet pressure increases from 5 MPa to 11 MPa, the surface residual compressive stress increases from −176.57 MPa to −300.85 MPa, and the depth of the compressive residual stress layer increases from 247.87 μ m to 375.39 μ m. In contrast, both microhardness and residual compressive stress show a negative correlation with the jet standoff distance. Compared with single HSC and SPEWJ treatments, the HSC-SPEWJ combined modification significantly enhances the surface strengthening effect of 7075-T6 aluminum alloy, increasing the surface microhardness to 188.46–207.22 HV, corresponding to improvements of approximately 10.37%–21.36% and 4.13%–14.49% relative to single HSC and single SPEWJ, respectively. Finite element simulations indicate that the water jet impact induces stress superposition within the cutting-induced plastic layer and promotes stress propagation toward deeper regions. TEM analyses reveal that a high density of dislocations with a non-uniform distribution is formed during the samples after HSC machining stage, whereas SPEWJ surface modification facilitates dislocation multiplication and rearrangement, leading to a more homogeneous plastic deformation behavior.