Rough turning Inconel 750 with supercritical CO2-based minimum quantity lubrication

Abstract

This paper describes preliminary results of replacing water-based (aqueous) flood coolant with supercritical CO2-based minimum quantity lubrication (scCO2 MQL) in an external turning operation on an Inconel 750 combustor housing. Two series of tests were performed: the first series to compare tool wear performance observed with aqueous flood coolant and scCO2 MQL under identical machining conditions, and the second series to investigate tool wear performance with scCO2 MQL at higher metal removal rates (MRR) than the MRR used in production practice with aqueous flood coolant. All tests were performed using roughing cuts on unaged Inconel with coated carbide tooling, and vegetable oil lubricant. As a key enabler, special flank jet tool holders were used to eliminate chip blockage of the lubricant stream.In the first series of tests, tool wear was observed to be consistently lower with scCO2 MQL than with the aqueous flood coolant. In the second series of tests, two process conditions were demonstrated for which MRR increased by 25% and 40%, respectively, with scCO2 MQL compared to aqueous flood coolant at equivalent tool life. Notch wear, the limiting factor for tool life under baseline conditions, was reduced for scCO2 MQL, but crater wear and chip hammering were more pronounced. Overall the results indicate that scCO2 MQL can provide increased tool life or material removal rate compared to aqueous flood coolants when machining Inconel 750 and similar nickel alloys by improving lubricity and changing the dominant wear mechanism from rapid notch wear to gradual crater wear and chip hammering. These tests, which involved extended cuts of over 10min under production conditions, represent an important extension of MQL machining to a hard metal alloy that cannot be machined by conventional MQL methods.