High speed cryogenic finish machining of Ti-6Al4V with polycrystalline diamond tools

In this work, Ti-6Al4 V alloy was machined using polycrystalline diamond (PCD) tools under three different cooling/lubricating environments: cryogenic cooling (liquid nitrogen (LN₂) at −195.8 °C), hybrid cooling/lubrication (LN₂ and oil based MQL − minimum quantity lubrication) as well as conventional flood cooling (emulsion). The machining parameters were selected to achieve very low kinematic surface roughness (f = 0.01 mm/rev, a_p = 0.1 mm) and three cutting speeds of v_c = 120, 240 and 360 m/min were evaluated. The as-machined surface integrity was compared for all conditions, showing significant improvements in surface and sub-surface properties with both cryogenic and hybrid cooling as compared to flood cooling. Although the PCD tools are not commonly used in high speed machining of Ti alloys because of the problem of thermally induced chemical wear, it was found that with the use of cryogenic cooling, the tribological system changed sufficiently to allow for sustained machining performance, with very low tool-wear (VB_C(cryogenic) < 10 μm after 65 min of cutting at v_c = 240 m/min). Surface roughness values were as low as R_a = 40 nm for cryogenic machining. Interestingly, flood machining did not lead to reduce surface roughness values. However, hybrid cooling/lubrication did yield the lowest cutting forces compared to both flood and cryogenic conditions. All conditions evaluated during this study resulted in cutting forces of less than 15 N, suggesting that cryogenic precision machining with PCD tools may be a suitable alternative to grinding, even for slender and thin-walled components.