Characterization and correlation of microstructure and hardness of Ti–6Al–4V sheet surface-treated by pulsed laser

Abstract A hot-rolled Ti–6Al–4V (Ti-64) sheet was surface-treated by pulsed laser at two different powers (100 and 200 W), with microstructural features and hardness before and after the laser surface treatment (LST) systematically investigated. Results show that after the LST at both powers there are two modification zones with distinct microstructural characteristics: melted zone (MZ) near the laser beam center, completely composed of fine martensitic plates with dense {10–11} nanotwins inside them; heat-affected zone (HAZ) far away from the laser beam center, comprised of mixed structures of short-rod β particles, martensitic plates and untransformed bulk α grains. Hardness measurements reveal that the hardness of the Ti-64 sheet can be markedly increased (especially in the MZ) after the LST. In-depth analyses suggest that the hardness increase in the MZ can be ascribed to combined contributions from grain refinement, presence of nanotwins and solid solution of alloying elements, while only the structural refinement by fine plate structures contributes to hardening in the HAZ. Comparisons between both the LSTed specimens reveal that increasing the laser power from 100 W to 200 W can effectively enlarge the laser-modified zones (both the MZ and the HAZ) and simultaneously refine plate structures, leading to further hardness increase.