Tribological Conditions Using CO2 as Volatile Lubricant in Dry Metal Forming

Deep drawing is the most important manufacturing process in order to produce sheet metal parts in mass production. In this study, volatile carbon dioxide (CO2) is used as lubricant in order to replace mineral oil-based lubricants. CO2 is injected into the void between the forming tool and the metal part through laser drilled microholes in the forming tool and acts there as a highly efficient lubricant. In order to investigate the tribological conditions during the drawing process, a new test bench having optical access to the CO2 lubricant is designed, built and used in this study. Additionally, the CO2 flow through the microholes is modeled using CFD simulation. Both the experimental and the theoretical study reveal a critical influence of the geometry of the microholes on the phase of the CO2 in the forming tool and on the friction during the drawing process. Thereby, these investigations confirm that the CO2 is in solid and/or gaseous state dependent on the surface pressure and the geometry of the microholes. Consequently, these results support the concept of microstructure-designed forming tools allowing dry metal forming for locally adjusted and environmentally friendly friction conditions.