An FEM analysis with experimental validation to study the hardness of in-process cryogenically cooled drilled holes in Mg AZ31b

The goal of a number of many recent studies is to assess the potential of cryogenic cooling in improving the sustainability of manufacturing processes. One prime interest is the enhancement of surface integrity properties of newly machined surfaces. In this paper, we study the effect of liquid nitrogen cryogenic cooling on the surface integrity of drilled holes in magnesium AZ31b using an indexable drill. Utilized are both experimental techniques and numerical (FEM) simulations. Specifically, liquid nitrogen at cryogenic temperatures was pumped through the drill's built-in through spindle coolant holes. HV micro-hardness measurements were performed on the newly machined holes surfaces. Furthermore, the process was modelled in FEM via an appropriate convective cooling approximation superimposed on the drilling process. Outputs of the numerical model such as strains, strain rates, and temperatures were used to predict the grain size at the surface of the holes and, consequently, hardness. Hardness values (determined from both experiments and FEM analyses) with different feed rates for dry drilled holes were compared against those from cryogenically cooled holes with the latter being found to have higher hardness values when compared to non-cooled ones.