Compilation of a thermodynamics based process signature for the formation of residual surface stresses in metal cutting

Abstract

The abrasive wear behaviour and fatigue life of many components in mechanical engineering depend on their surface stress states. Compressive surface stresses hinder ductile or brittle crack formation as well as crack propagation into the bulk material and therefore increase the lifetime of dynamically or abrasively loaded parts. The thermo-mechanical loads acting on the workpiece surface during metal cutting determine the stress state after processing. Under the high strain rates of cutting, the underlying mechanisms include the accumulation of stress fields around dislocations resulting from plastic deformation as well as thermal expansion and shrinking phenomena associated with the dissipation of mechanical energy. Until now, the underlying thermodynamics of residual stress formation in metal cutting are hardly understood quantitatively, which explains the current dominance of empirical-iterative design procedures of cutting processes regarding residual stresses.In this work, the derivation and experimental validation of a thermodynamics based finite element model for the energy transformations during residual stress formation are presented. For the first time, the residual surface stress state is correlated with the mechanical and dissipative thermal energies, which are transformed during processing. It is shown how each residual stress component relates to these energy transformations. These findings are applied to formulate characteristic process signatures, which may be used to describe the formation of residual stresses in other manufacturing technologies as well.The proposed work includes orthogonal cutting tests on quenched and tempered AISI 4140, subsequent determination of the residual stress states using a diffractometric measurement technique, the analytical description of the energy transformations during residual stress formation as well as its implementation into a finite element process model.