EXPERIMENTAL EVALUATION OF FRICTION COEFFICIENT AND LIQUID NITROGEN LUBRICATION EFFECT IN CRYOGENIC MACHINING

Abstract

Cryogenic machining is considered an environmentally safe alternative to conventional machining where cutting fluid is used. However, the improved machinability in cryogenic machining has been attributed to the cooling effect of liquid nitrogen (LN2) in past research. Our recent studies indicate that LN2 may possess a lubrication effect in machining, as evidenced by a reduction of tool wear, the apparent coefficients of friction calculated by a mathematical model, and the secondary deformation in chip metallurgy. However, there is a need for a direct proof of LN2's lubrication effect before it can be claimed to be a lubricant in cryogenic machining. This paper presents the methodology and data from an experiment that measures the normal force and friction force directly in an altered machining setup. This procedure simulates the pure frictional behavior of the tool-chip interface in cryogenic cutting. The results show that LN2 cooled condition has a significantly lower coefficient of friction than dry conditions in all cases. The data also shows that the friction is lower for Ti-6Al-4V but mixed results obtained for mild steel AISI 1018 when LN2 is applied properly as in the experiment setup as compared to traditional emulsion flooding. Based on the unique pattern of the friction behavior in the sliding tests, possible lubrication mechanisms using liquid nitrogen are proposed.