Laser surface processing of Ti6Al4V alloy precoated with hard particles

Laser gas-assisting processing of preprepared Ti6Al4V alloy is carried out and resulting surface topology, wetting state, metallurgical changes, and hardness are examined. A thin carbon film accommodating TiC and B4C particles is developed over the surface in the preparation cycle. Laser surface treatment consisting of surface ablation and melting is performed at constant laser scanning speeds and high-pressure nitrogen gas jet. Laser treatment results in surface texture topology with hierarchically distributed micro/nanopillars having 5.6 mu m average surface roughness (Ra). The texture of the surface demonstrates a hydrophobic state with a contact angle of about 109 degrees +/- 3 degrees and hysteresis of 37 degrees +/- 4 degrees. The treated surface free energy yields similar to 132 mJ/m(2), which is greater than that of TiC (similar to 120 mJ/m(2)). The high spreading rate of the molten alloy wets the surface of the hard particles while minimizing nano/mesopores around the particles in the treated layer. The presence of carbon film and TiC particles forms (Ti, Al)N, (Ti, Al)CxN1-x, TiN, TiNx, Ti2N, TiCxN1-x compounds on the surface, which contributes to microhardness enhancement in the treated zone. Surface microhardness of 1670 +/- 50 HV is achieved, which remains higher than that of TiN coated, plasma nitrided, and laser nitrided surfaces.