Surface Integrity and High-Cycle Fatigue Performance of Laser Powder Bed Fusion TC4 Titanium Alloy Treated by Microscale Laser Shock Peening without Coating

This study introduces a novel surface modification technique, micro-scale laser shock peening without coating (μLSPwC), to address three key limitations in laser powder bed fusion (L-PBF) of TC4 titanium alloy components: detrimental tensile residual stress, excessive surface roughness, and insufficient fatigue resistance. Compared to conventional laser shock peening without coating (LSPwC), μLSPwC offers significant improvements in surface integrity and fatigue performance. Both μLSPwC and LSPwC reduce surface roughness (μLSPwC: 25.15 μm, LSPwC: 23.73 μm), eliminate 77–85% of surface spheroidized particles, and induce beneficial compressive residual stress. However, μLSPwC excels in three areas: 1) better control of oxygen content (1.38% increase vs. 6.95% increase), 2) superior surface hardening (435.8 HV vs 407.3 HV), and 3) higher compressive stress (−643.04 MPa vs −173.98 MPa). Rotating bending fatigue tests show an extraordinary life extension of 264 times for μLSPwC-treated specimens (from 10 827 to 2.87 × 10⁶ cycles), compared to a 10.4 times improvement for LSPwC. Fractographic analysis reveals μLSPwC induces sub-surface crack nucleation at ≈200 μm, whereas LSPwC causes surface-initiated failure. These findings highlight μLSPwC as an effective technique for improving both surface quality and fatigue durability in complex L-PBF titanium alloy components.