Study on the Cooling and Lubrication Mechanism and Properties of Soybean Oil-Based MQL and Different Cryogenic Media in Titanium Alloy Processing

The machining of Ti-6Al-4V thin-walled parts is characterized by high cutting temperatures, significant force fluctuations, and complex thermomechanical coupling. Cryogenic Minimum Quantity Lubrication Technology (CMQL) uses bio-lubricant as the lubrication carrier, combined with the cooling characteristics of cryogenic temperature medium, showing good cooling and lubrication performance and environmental friendliness. However, the cooling and lubrication mechanism of different cryogenic media in synergy with bio-lubricants is still unclear. This paper establishes convective heat transfer coefficient and penetration models for cryogenic media in the cutting zone, based on the jet core theory and the continuum medium assumption. The model results show that cryogenic air has a higher heat transfer coefficient, while cryogenic CO2 exhibits a better penetration ability in the cutting zone. Further milling experiments show that compared with cryogenic air, the average temperature rise, average cutting force and surface roughness of workpiece surface with cryogenic CO2 as cryogenic medium are reduced by 23.6%, 32.8%, and 11.8%, respectively. It is considered that excellent permeability is the key to realize efficient cooling and lubrication in the cutting zone by Cryogenic CO2 Minimum Quantity Lubrication Technology. This study provides a theoretical basis and technical reference for efficient precision machining of titanium alloy thin-walled parts.