Gradient microstructures and mechanical properties of Ti–6Al–4V alloy induced by laser surface quenching

Laser surface quenching (LSQ) was employed to fabricate gradient microstructures in a Ti–6Al–4V alloy. The influence of the LSQ parameters on the surface morphology, the depth of the LSQ layer, gradient microstructure, and microhardness were investigated. The results showed that as the laser energy density increases, the surface roughness and thickness of the heat-affected zone (HAZ) increase. From the internal matrix to the surface, the microstructure in the HAZ changes from the equiaxed structure to the mixt structure, martensitic structure, Widmanstätten structure, and then the oxide layer. The size of the β grains gradually decreases as the distance from the surface increases. The different microstructures and elemental distributions in the HAZ result in different microhardness values, which gradually decrease from the surface to the matrix. The laser energy density of 8.0 J/mm² is recommended to obtain a HAZ with a thickness of 1200 μm and a peak microhardness of 393 ± 7.3 H V without surface remelting. The LSQ process may help to increase the longevity of Ti–6Al–4V alloy implants by hardening the surface.