Effect of liquid nitrogen cooling on surface integrity in cryogenic milling of Ti-6Al-4 V titanium alloy

Owing to poor thermal conductivity, heat dissipation, and high chemical reactivity toward most of the tool materials, temperature elevation in the machining of titanium alloy leads to poor surface quality. Based on analyzing the variation laws of the milling forces under cryogenic cooling, the present investigation concerns the surface integrity (surface roughness, micro-hardness, microstructures, and residual stresses) in cryogenic milling of Ti-6Al-4 V alloy under the application of liquid nitrogen (LN2) as a cooling mode. Findings have indicated a dramatic increase in milling forces, and decreasing surface roughness was observed under variation of jet temperature (20~−196 °C). Besides an increase in cutting speed from 60 to 120 m/min, a linear increase in cutting forces, surface roughness, micro-hardness, and residual compressive stress was observed. The minimum micro-hardness decreased at cutting speed of 90 m/min and up to 30 μm in depth. A holistic comparison between obtained results under cryogenic milling and previously studied results under dry milling at same cutting conditions depicted higher micro-hardness and higher compressive residual stress under cryogenic LN2 on the machined surface. However, the residual stress under LN2 cooling conditions tends to decrease relatively slower compared to dry milling. Also, there are no significant differences in grain refinement and twisting under dry and cryogenic LN2 machining. The research work proves the effectiveness of cryogenic milling in improving the surface integrity of the Ti-6Al-4 V alloy.