Microstructural and mechanical property investigation of machined surface layer in high-speed milling of Ti-6Al-4V alloy

The machining-induced material’s microstructure evolution, which can be determined from the thermo-mechanical loading history, dramatically influences the end products’ service performances. The present research investigates the microstructural and mechanical property changes of the machined surface layer of Ti-6Al-4V alloy by utilizing some advanced characterization techniques. First, high-speed milling experiments of Ti-6Al-4V alloy under various cutting parameters were carried out. The effect of machining parameters on cutting forces and workpiece temperature was investigated. Secondly, based on the cutting forces and workpiece temperature, the machined surface layer’s microstructural and phase composition changes were observed using optical and scanning electron microscopy and X-ray diffraction analysis, respectively. The experimental results indicate that the machined surface and sub-surface exhibit a transformed layer (white layer) and a localized plastic deformation, respectively. The intensity of the transformed layer increases with both the cutting speed and feed rate. In contrast, the plastic deformation shows a slight variation among cutting speeds but increases gradually with the feed rates. The phase transformation analysis reveals that the conversion of β-Ti into α-Ti is not time dependent and can coincide. Finally, the nano-indentation technique was employed to measure the hardness from the machined surface down to the bulk region. The improved machined surface hardness is mainly attributed to grain refinement, deformation twinning, and the material phase proportions. The current research can provide a better understanding of the machined surface integrity in high-speed milling of Ti-6Al-4V alloy.