Inconel is a nickel-based alloy with poor machinability
[33]. Low thermal conductivity of inconel alloy (about 11 W/m °C) caused higher cutting temperature and high tool wear rate
[1] and additional effort was required to reduce the cutting temperature. Wang and Rajurkar reported that machining Inconel 718 with LN
2 cooling was able to delay the tool wear and improve the surface roughness of workpiece surface
[1]. Similar results were reported by Pusavec et al.
[34]. In addition to tool wear and surface roughness, a thicker compressive zone is noticed beneath the machining surface and smaller grain size is observed after cryogenic machining with liquid nitrogen. Beside this, the sub-surface layer under cryogenic machining is harder and thinner than sub-surface layer under dry or Minimum Quantity Lubrication (MQL) machining. In other words, cryogenic machining is able to improve the surface roughness, and increase the hardness of the machined workpiece. The majority of previous cryogenic machining works supplied cryogen at a constant supply pressure. To study the effect of supply pressure and the corresponding flowrate of the cryogen, Klocke et al. conducted cryogenic machining tests at supply pressures of 7–30 MPa on Inconel 718 and then measured maximum flank wear of the cutting inserts
[35]. They reported that the higher the supply pressure and flowrate of cryogen, the lower the maximum flank wear at a cutting speed of 500 m/min. However, they obtained a negative result at cutting speed of 60 m/min, where higher supply pressure and flowrate produced higher maximum flank wear. Earlier researchers used cryogen only in their cryogenic turning experiments, but recently hybrid cooling which combined cryogen with cutting fluids has been proposed. Bagherzadeh and Budak used four different cooling strategies in turning titanium alloy Ti6Al4V and Inconel 718
[36]. They introduced a new method, CMOL, where CO
2 and vegetable oil were mixed in the form of frozen oil particles, before reaching the tool–work interface. The new method, CMOL, is better than cryogenic turning with carbon dioxide cooling only, in terms of tool wear and surface finish. Yildirim compared cryogenic, nanofluids, and hybrid cooling in turning Inconel 625
[37]. Six cooling techniques such as dry, pure MQL, nMQL, LN
2, and their hybrid were used in his work. Three types of nanofluids based on Al
2O
3, hBN, and hBN + Al
2O
3 were included in the vegetable cutting fluid in 0.5 vol% and 1.0 vol%. Hybrid cooling (cryogenic liquid nitrogen and 0.5 vol% hBN) outperformed cryogenic cooling only and vegetable oil with nanofluids only in terms of reducing cutting temperature, prolonging tool life and improving surface roughness. Tantalum has low thermal conductivity, low specific heat, high shear strength, high work-hardening capacity, and gummy consistency, which cause it to be difficult to machine. In order to study the roles of cryogenic machining in machining tantalum, Wang and Rajurkar conducted a preliminary study on cryogenic machining of tantalum. They reported that liquid nitrogen was able to reduce the cutting temperature and extend the tool life
[1]. In their further research in cryogenic machining of tantalum with liquid nitrogen
[22], they found that cryogenic machining reduced tool wear by 70%, improved surface roughness by 200% and reduced cutting forces by 60%.