Surface Integrity in Sustainable Drilling of AZ31BMg Alloy Using Cryogenic Cooling 

In the machining literature, in-process cryogenic cooling during machining has been credited with reduced cutting temperatures leading to longer tool life and enhanced productivity and, ultimately, contributing to more sustainable manufacturing processes. However, not until recently, that it was suggested that in-process cryogenic cooling contributes to enhanced integrity of machined surfaces. Such enhanced surface integrity properties include: smoother surfaces, refinement of grain size, and increase in the closely related surface hardness. In this paper, we study the effect of liquid nitrogen (LN) cryogenic cooling and cutting parameters (mainly feed) on the surface hardness of drilled holes in magnesium alloy AZ31b. Drilling experiments of in-process cryogenic cooling were run where thrust force and torque were measured. Surface micro-hardness (HV) values and the grain structure at the surface of the drilled holes were also measured. Furthermore, a robust FEM model for simulating this cryogenically cooled drilling operation was developed. Numerical output such as strains, strain rates, and temperature was used to predict the Zener-Hollomon parameter (Z-parameter) values at the surface of the holes. Using a relation that relates the Z-parameter to grain size, average values of the latter were estimated. Surface hardness (HV) values were also estimated from a Hall-Petch like relation. Cryogenic cooling was found to have a pronounced effect in increasing surface hardness, presumably, due to the accompanying refinement  in grain size at the surface. Experimental measurements of grain size and hardness values compare favorably with the numerical results of the FEM model. Such findings suggest that in-process application of cryogenic cooling results in improved surface hardness of drilled holes as compared with those drilled while dry.