Role of CO2 cooling strategies in managing tool wear during the shoulder milling of grade 2 commercially pure titanium

As a result of their high hardness, osseointegration and low chemical reactivity titanium alloys have become an invaluable material species for use in medical applications. Unfortunately, many of the material properties which make titanium alloys desirable biomedical materials, also contribute towards difficulties in tool wear and surface quality during machining. Historically, this has been met via the application of robust metalworking fluid (MWF) strategies, e.g. flood, or high-pressure (HP) soluble oil coolant (emulsion). However, this brings with it its own challenges, not least of which the problem of coolant residue contamination. This study describes efforts to minimise and characterise tool wear during the shoulder milling of grade 2 commercially pure titanium (CP–Ti) by using a novel external supercritical CO2 (scCO2) coolant strategy. Whilst the results imply that the sole use of scCO2 coolant contributes to increased tool wear (relative to emulsion), an optimised strategy of combining scCO2 with minimum quantity lubrication (MQL) is shown to support up to a 245% increase in tool life. Despite this, machinability outcomes are found to be highly contingent upon proper selection of trial process variables such as scCO2 mass flow rate and cutting speed, wherein cutting speeds at or below 115 m/min, and 40 kg/h of scCO2 is generally shown to be the most efficacious parameters for this material/operation. Ultimately, scCO2 + MQL is found to be an effective compromise between the cleanliness of scCO2 and the improved function of emulsion.