Post-processing of material extrusion additively manufactured 17-4 PH stainless steel via aging and multi-step shot peening: surface integrity and wear response

This study investigates the influence of aging, single and double shot peening treatments on 17-4 PH stainless steel fabricated via material extrusion additive manufacturing (MEX). The focus is on improving surface integrities—specifically mitigating surface imperfections inherent to MEX parts, such as printing-related strip patterns and sintering-induced roughness—and enhancing linear reciprocating wear performance using a WC-Co counterpart. The results show that both single and double shot peening treatments significantly reduce surface imperfections, leading to a notable decrease in the coefficient of friction (COF). Aging also contributes to COF reduction. Surface grain refinement is observed after shot peening, with nanocrystalline structures (grain size <100 nm) forming in the as-sintered specimen subjected to double shot peening, while other specimens exhibit larger grains or earlier stages of grain refinement. Aging enhances overall hardness, while the single and double shot peening further increases surface hardness, which gradually diminishes at depths beyond 80 μm from the surface. In addition, the wear rate decreases progressively with the application of aging and subsequent shot peening treatments. These improvements are attributed to a combination of surface grain refinement, the formation of a work-hardened layer, compressive residual stress from shot peening, and precipitation hardening from aging. As a result, the dominant wear mechanisms shift from oxidation-induced delamination and low cycle fatigue—most severe in untreated, as-sintered specimen—to predominantly abrasive wear in aged and peened condition. Overall, the synergistic effects of aging and double shot peening markedly enhance the wear resistance and surface integrity of MEX-fabricated 17-4 PH stainless steel components.