Carbon emissions, techno-economic and machinability assessments to achieve sustainability in drilling Ti6Al4V ELI for medical industry applications

In the last decade, cryogenic machining has made a significant impact as a sustainable machining technique, demonstrating machinability improvements in terms of tool life, surface quality of machine parts, low environmental effects and reduced cost of overall machining operation. A near-to-dry machining technique, Electrostatic Minimum Quantity Lubrication (EMQL) has also emerged as a sustainable machining technique, which is an environmentally-friendly alternative to the flood machining process. Although these coolant/lubrication strategies have been studied extensively, there is no direct comparison of both these machining techniques in the aspects of ecology, economy and machining performances. In this novel study, cryogenic liquid carbon dioxide (LCO2) and EMQL techniques are used for high-speed drilling of Ti6Al4V ELI (extra low interstitial) grade, and the sustainability and machinability assessment of these techniques are evaluated which can help medical implant manufacturing industries to implement these techniques as an economic and sustainable replacement of the conventional cooling/lubrication techniques. Ti6Al4V ELI is widely used in the medical industry for Hip and Knee joint endoprosthesis, bone screws, collar bone and dental implants. The sustainability aspects were evaluated in terms of carbon emission and cost analysis of the processes. It was found that the EMQL cutting condition exhibited 27.69% higher carbon emission and 67.60% higher cost of machining operation compared to cryogenic LCO2 cutting condition. Moreover, the tool life experienced when machining using cryogenic LCO2 cutting condition was at least 4 times higher than tool life found when using the EMQL cutting condition. The surface roughness of the workpiece material was higher (56.40%) when using the EMQL coolant. These results were attributed to adhesion wear and chip clogging phenomena which were found to be the main factors for reduced tool life and higher surface roughness in EMQL cutting condition. Therefore, from this study, it can be concluded that cryogenic LCO2 coolant performs significantly better compared to EMQL coolant, providing improved machinability of the titanium workpiece and superior sustainability of the cutting operation.