Tribological Effect of Lubricated Liquid Carbon Dioxide on TiAl6V4 and AISI1045 under Extreme Contact Conditions

Abstract

One of the trends in machining is to reduce the consumption of conventional cutting fluids via the use of alternative cooling-lubrication techniques such as Minimum Quantity Lubrication (MQL) or even move to dry cutting. High friction during machining leads to heat generation, which adversely affects the tool life, workpiece surface quality and energy consumption. Reducing the cooling potential with such options can thus have detrimental effects, especially when difficult-to-machine materials are considered, pushing the end-users to limit the cutting conditions. Cryogenic machining based on the injection of liquid nitrogen (LN) or liquid carbon dioxide (LCO2) emerged over the last decade as a promising sustainable assistance in machining. If the cooling capability is definitely enhanced with both, the lubrication capability has not really been proved and is expected to be highly limited. A recent approach to improve both cooling and lubrication is the use of LCO2 in combination with oil, delivered via minimum quantity lubrication (MQL). However, the actual potential of this technique has not really been investigated from a tribological point of view. The proposed work thus aims at assessing the tribological performance of lubricated liquid carbon dioxide when machining TiAl6V4 and AISI1045 with carbide tools. A specially designed open tribometer has been used to characterize the friction coefficient under different sliding velocities. A novel single-channel MQL-delivery system using lubricated LCO2 has especially been applied to perform experiments under LCO2+MQL conditions but also cryogenic LCO2 and dry conditions for a comparison purpose. Whereas a strong influence could be observed on the AISI1045 with a reduction in terms of friction coefficient when applying LCO2+MQL, neither LCO2 nor LCO2+MQL were able to decrease the friction coefficient on TiAl6V4 compared to dry over all the considered sliding velocities, opening the possibility of further research to understand this specific behaviour.