Effect of Laser Shock Peening on the Passivation Behavior of Subtractively and Additively Manufactured Ti-6Al-4V...

The effects of laser shock peening (LSP) on subtractively manufactured (SM) and additively manufactured (AM) Ti-6Al-4V alloys in pH 2 buffer solution were investigated. LSP increased the surface roughness from 0.25 ± 0.05 μm to 0.6 ± 0.1 μm, raised Vickers hardness by 12-16%, and introduced compressive residual stresses of 400-950 MPa. Microstructural analysis indicated that LSP promoted β-phase formation and grain refinement in SM alloys, while reducing the α'-phase fraction in AM alloys. Electrochemical testing revealed that all LSP-treated specimens exhibited active-passive transitions, unlike the stable passive response of unpeened samples. The corrosion rate (i_corr) decreased from approximately 5 × 10^-6 to 1 × 10^-6 A·cm^-2 after LSP. During 24 h potentiostatic polarization at 1.3 V_SCE, the passive current density stabilized at 10^-8-10^-7 A·cm^-2, with LSP AM specimens exhibiting values approximately twice those of their unpeened counterparts. Mott-Schottky analysis confirmed that the donor density (N_D) in the SM alloy changed negligibly after LSP, indicating a stable passive alloy. In contrast, the N_D for the AM alloy increased from 1 × 10¹⁹ to 3 × 10¹⁹ cm^-3, suggesting an oxygen-vacancy-rich, less stable passive film. Overall, LSP reduces the corrosion rate primarily through the introduction of compressive residual stress but may impair the long-term passive-film stability of AM Ti-6Al-4V owing to defect generation. In contrast, the SM alloy maintains passive-film stability under identical treatment conditions.