Machining of titanium alloys using phosphonium-based halogen-free ionic liquid as lubricant additives

Purpose Nowadays, ionic liquids (ILs) are used as lubricant additives because of their ability to improve tribological characteristics. However, majority of the ILs contains halogen-based anions. They are extremely sensitive to moisture and produce detrimental halides and halogen acids when reacted with moisture and water. These halides and halogen acids are harmful to the health and environment of the operator’s. This paper aims to study four different lubricants including two halogen-based ILs blended in canola oil and two phosphonium-based halogen-free ILs blended in canola oil and in pure canola oil. Design/methodology/approach Viscosity and contact angle were measured by using rheometer and contact angle goniometer, respectively. Machining experiments were performed using turning centre with four different lubricants at two different cutting speeds and temperatures, and the machining forces, tool morphology and roughness of the machined surface were analysed. Findings The results showed that the inclusion of 1% phosphonium-based halogen-free ILs blended in canola oil increased the viscosity by 44.8% and reduced the cutting and thrust force by 21.7% and 26.8%, respectively, compared to pure canola oil lubricant. Microscopic analysis of tool showed lower adhesive and abrasive wear when machined with phosphonium-based halogen-free ILs blended in canola oil. The workpiece surface roughness reduced by 32% for phosphonium-based halogen-free ILs blended in canola oil compared with pure canola oil. Originality/value Halogen-free ILs are possible green lubricants, as they do not contain harmful elements such as F, S, B and Cl like halogen-based ILs. To the best of the authors’ knowledge, this is presumably the first work which shows machining performance of halogen-free ILs as lubricant additives.