Cryogenic Machining of Ti-6Al-4V

Licentiate thesis, 2018

The use of cryogenic coolants such as liquid nitrogen or liquid carbon dioxide has emerged as a sustainable alternative to conventional cutting fluids during machining operations, with the potential to increase tool life, leading to productivity gains, as well as to improve the surface integrity of the machined components. In addition, the machined surfaces are left residue-free after the process and there are no used cutting fluids to dispose of. This work aims to increase the understanding of the effects of cryogenic cooling when machining titanium alloy Ti‑6Al‑4V.

Turning experiments on Ti-6Al-4V revealed that using the same setup (same tool holder and nozzle configuration, comparable coolant pressure) gives very similar results in terms of surface integrity when using cryogenic cooling with liquid nitrogen compared to flood-cooling with emulsion. The choice of coolant did not have a significant impact on surface roughness, microstructure in the near-surface layer or residual stresses. The residual stresses were compressive in all cases, but increased tool wear shifted the residual stress depth profiles towards more compressive stresses. Due to the relatively small nozzle size used, however, the flow of cryogenic coolant was insufficient to match the tool life obtained with emulsion cooling.

In face milling of Ti-6Al-4V with liquid carbon dioxide and emulsion, it was shown that notch wear and chipping were the main wear mechanisms determining the tool life, rather than abrasive wear, i.e. flank wear. Comb cracks were formed in both cooling conditions. However, cryogenic cooling with liquid carbon dioxide greatly delayed the lateral propagation of the comb cracks, thereby delaying the chipping of the cutting edge. Tool life was shown to improve with higher flow rates of coolant, the effect being stronger when using liquid carbon dioxide compared to flood-cooling with emulsion. Moreover, the difference in terms of tool life between cryogenic cooling and flood-cooling with emulsion decreased at higher cutting data.