An experimental and numerical study of the effect of cryogenic cooling on the surface integrity of drilled holes in AZ31B Mg alloy

In-process cryogenic cooling has been credited with more precise products and enhanced integrity of machined surfaces. Enhanced surface integrity properties include smoother and more precise surfaces, grain refinement, and increase in surface hardness. This paper reports on the effect of liquid nitrogen (LN) cryogenic cooling and cutting parameters on surface hardness of drilled holes in magnesium alloy AZ31B. Drilling experiments of in-process cryogenic cooling were conducted while measuring thrust force and torque. Surface microhardness (HV) values and grain structure at drilled hole surfaces were measured. Finite element analysis (FEM) and model for simulating this cryogenically cooled drilling operation was developed. Numerical output of the state variables strain, strain rate, and temperature were used to predict Zener–Hollomon parameter (Z-parameter) values at the surfaces of the holes. Using an expression that relates Z-parameter to grain size, average values of the latter were estimated. Surface hardness values were also estimated from a Hall–Petch-like relation. Cryogenic cooling was found to have a pronounced effect in increasing surface hardness. Experimental measurements of grain size and hardness values compare favorably with the numerical results of the FEM model. In-process application of cryogenic cooling resulted in improved surface hardness of drilled holes as compared with those drilled while dry.