Surface Oxidation and Subsurface Deformation in a Laser-Peened Ti-6Al-4V

Laser peening without coating (LPwC) was carried out at multiple passes on Ti-6Al-4V at a power density of 6 GW cm −2 . Tensile residual stress (65 MPa) was observed on the LPwC sample surface, without surface melting and re-solidification. Further, an oxide layer (TiO) had formed on the surface of the LPwC samples, without alpha case formation. Both these results have not been reported in Ti-6Al-4V. Secondary ion mass spectrometry (SIMS) revealed the presence of oxide layer < 10 μm in thickness on LPwC samples. Acid pickling was used as a practical post-processing technique to eliminate the oxide layer, without causing hydrogen embrittlement. Laser peening induced a residual stress of −233 MPa at the surface and maximum residual stress of −552 MPa at a depth of 50 μm after pickling in Ti-6Al-4V samples. Further, high- and low-angle grain boundaries were found to have increased after peening, increasing further the strength of the alloy, as shown by microhardness profiles.