Thermodynamics-based Interpretation of White Layer Formation in Metal Cutting

In metal cutting, the formation of white layers in the machined surface is a common phenomenon and refers to severe grain refinements, which appear white in etched micrographs. Due to the resulting modifications of the mechanical properties in the workpiece surface, understanding the initiation and evolution of white layer formation is of great interest for the industry. In order to save time as well as costs during tool and process design, the industry therefore demands valid predictive models of white layer formation. This requires the theoretical understanding of the underlying metal-physical mechanisms governing these structural material modifications. The relevant mechanisms include dynamic recrystallization (DRX), shearing and possibly phase transformations. Recently it was shown that DRX is predictable by thermodynamic potentials such as the Helmholtz free energy, which describe white layer formation as chains of spontaneous, irreversible state changes. In this paper, we provide a new interpretation of the energy transformations governing these thermodynamic state changes in the machined surface. The individual impacts of dissipation, heat conduction, mechanical work as well as the associated flow and production of entropy on the initiation and kinetics of DRX during cutting are discussed. The interpretation provides new fundamental insight into white layer formation by the example of orthogonal cutting quenched/tempered AISI 4140 using cemented carbide tools.