Enhancing fatigue performance and microstructural integrity of additively manufactured Ti-6Al-4 V alloy via laser peening without coating (LPwC)

This study comprehensively investigates the effects of coating-free laser shock peening (LPwC) on surface characteristics, microstructural integrity, and fatigue performance of Ti-6Al-4 V alloy parts fabricated by laser powder bed fusion (L-PBF), with a comparative evaluation against sandblasted (SB) and polished (P) surface conditions. LPwC treatments were applied at two different laser power densities (4 GW/cm2 and 8 GW/cm2) without any protective coating, enabling direct interaction with the as-built surfaces. LPwC treatment effectively eliminated tensile residual stresses and introduced compressive residual stresses up to-850 MPa, extending approximately 0.5 mm beneath the surface. This stress profile, combined with a 12-20 % increase in surface hardness and the closure of surface-adjacent pores (up to 400-600 mu m depth), contributed to improved micro-structural integrity. Although LPwC slightly increased surface roughness, it promoted subsurface work-hardening and densification, which more significantly influenced fatigue resistance. As a result, fatigue crack initiation was delayed and propagation was suppressed, leading to fatigue strength improvements of 36 % and 75 % at power densities of 4 GW/cm2 and 8 GW/cm2, respectively. These outcomes highlight LPwC's ability to overcome common AM surface limitations, offering a robust and machine-independent solution for enhancing the mechanical reliability of Ti-6Al-4 V parts.