Coupling between inherent and machining-induced residual stresses in aluminum components

Abstract

This work investigates the coupling of inherent residual stress (IRS) and machining-induced residual stress (MIRS) on the final-state of residual stress (FRS) and distortion when high-speed machining (HSM) high aspect-ratio aluminum components. Motivation for this work stems from the simplifications in related numerical investigations that give rise to two limitations: First, the mapping of incompatible, incomplete, and/or spatially scarce IRS profiles generates unrealistic distortions and incorrect stress fields during static equilibration. Second, the simulation of machining via element deletion, inactive elements approach, or Boolean subtraction (removal) of material either ignores thermal and MIRS effects, or implements them based on simplified analytical/empirical models. Such practices therefore prevent a thorough understanding of how IRS and MIRS are coupled. Accordingly, two wrought aluminum 7050 blocks having different IRS profiles (based on stress relief) are considered in this work. An iterative stress reconstruction algorithm is implemented to numerically model a spatially-complete and fully-compatible IRS field in each aluminum block using limited data from slitting measurements documented in the literature. A 2D orthogonal cutting model is used to validate the material and damage models employed, as well as to elucidate the influences of IRS and MIRS on FRS. A 3D end milling model, which adopts the validated material and damage definitions, is then applied with different tool paths to reveal the coupled effects of IRS and MIRS on the distortion when HSM a C-channel featuring high aspect-ratio walls. The results reveal that the interaction between IRS and MIRS is nonlinear in nature, thus contrasting assumptions allowing for their superposition, as are widely reported and adopted in the literature. The results also show that the nonlinear coupling between IRS and MIRS varies according to both the component and location of stress within the machined part. Moreover, the findings reveal that the final part distortion is significantly influenced by the nonlinear coupling, as well as the specific machine tool path implemented.