Surface integrity and corrosion behaviour in AZ91D under multi-pass water jet peening: A process perspective

A coupled Eulerian-Lagrangian (CEL) finite element model coupled with the Johnson-Cook constitutive law was developed to model single- and multi-pass water jet peening (WJP) of AZ91D Mg alloy. The model captures the interaction between jet and surface, plastic deformation, and development of surface residual stress at the repeated impact areas. Model predictions were compared with X-ray diffraction measurements and confirmed the good agreement with deviations less than 10%, indicating that it is reliable in predicting subsurface stress fields. The simulations indicate that additional peening passes enhance plastic deformation and compressive residual stress buildup to an optimal condition beyond which stress saturation and partial stress relaxation occur as a result of over-peening. This behaviour is explained by the repetitive removal of surface materials and the redistribution of the highly strained layer. The modelling predictions are corroborated by experimental observations, with the increased micro strain and dislocation density of 0.0024 and 1.25 × 10 15 lines/m 2 , respectively, and the crystallite size also decreased from 50 nm on the unpeened surface to 39 nm after three passes, resulting in an improvement in surface microhardness by 46.4%. Surface topography development to a valley-dominant morphology increases surface energy and wettability, decreasing the contact angle to 73.22 ± 3.53°. The corrosion resistance also improved with an increase in polarisation resistance from 45.92 Ω·cm 2 to 104.83 Ω·cm 2 in 3.5 wt% NaCl. These findings indicate that the controlled multi-pass WJP enhances surface integrity through synergistic effects of strain hardening, crystallite refinement, and compressive residual stress, and over-peening causes surface degradation.