Improving superficial microstructure and properties of the laser-processed ultrathin kerf in Ti-6Al-4V alloy by water-jet guiding

Abstract

In the present research, the gas-assisted laser (GAL) and water-jet guided laser (WGL) processing technologies were applied to machine the ultrathin kerf in the wrought Ti-6Al-4V alloy. The microstructure, microhardness, and wear properties of the superficial layer were investigated. The results reveal that the GAL processing could machine the kerf with a high depth-to-width ratio of 12–15, but the increased processing times enhance the depth little. Due to the oxygen entrainment and relatively low heat and mass transferring efficiency, the assisted gas promotes the formation of a scaled recast layer containing β-Ti phase and oxides, which increases the roughness to 20 μm. The WGL processed kerf has a low depth-to-width ratio with a value of 1.9–2.5 and the depth could be increased by increasing the WGL processing times. With the assistance of the water jet, the remelted debris and heat could be eliminated immediately, which restrains the formation of the recast layer and heat-affected zone. The ultrathin oxide outer layer with hundreds of nanometers and ultrafine α-Ti grain inner layer are formed on the surface, which decreases the roughness to 12 μm. Compared with the as-received Ti-6Al-4V alloy, the microhardness of GAL processed kerf surface is increased to 382.8 HV accompanied by residual tensile stress, while the microhardness of WGL processed kerf surface is increased to 481.6 HV accompanying with residual compressive stress. In addition, the GAL processing increases the wear rate at room temperature but decreases the wear rate at high temperatures. Comparatively, the WGL processing decreases the wear rate at room and high temperatures, simultaneously. Such wear behaviors could be ascribed to their different superficial microstructures and phase constituents.