Towards understanding the thermal history of microstructural surface deformation when cutting a next generation powder metallurgy nickel-base superalloy

Despite the ongoing progress in metallurgical characterisation of machined surfaces, knowledge of the thermal conditions under which they originate during the workpiece-flank interaction is still lacking. When cutting advanced superalloys, little is known about temperature evolution in the machined part volume, where workpiece material interacts with the tool flank. In this work, the characteristics of the thermal field and the resulting surface metallurgy induced in a next generation nickel-base superalloy have been studied for cutting scenarios involving different combinations of thermo-mechanical boundary conditions. Analysis of the thermal field evolution in the workpiece subsurface has allowed the heating and cooling rates induced by cutting to be revealed, allowing description of two distinct types of thermal cycle, with a Heating-Peaking-Cooling (H–P–C) and a Heating-Quasi-isothermal Deformation-Cooling (HQC) structure depending on the process aggressiveness. Subsurface thermal history has been found to relate with the severity of the cutting-induced deformation, as it combines information on thermal field magnitude and on the process rates. Furthermore, thermal balance equations have been applied to study the rate of the heat generation in the machined subsurface due to its own plastic deformation while interacting with the tool flank. This has revealed that the highest rate of heat generation induced by plastic deformation occurred in thin surface layers at the beginning of the workpiece-flank contact, which has been associated to the conditions under which white layers (WLs) are generated. Energy balance analysis has furthermore indicated the development of a less severe and less impulsive deformation process at higher subsurface depths, which has been linked to the formation mechanism of material drag (MD) layers. In this way, the thermal history of machined surfaces has been related to their resulting metallurgical integrity, allowing in-depth understanding of the physical conditions developing when cutting next-generation superalloys.