Residual Stress Prediction in Machined Workpiece Surfaces

The increasing drive for productivity improvements and consistent part dimensionality in aerospace structures has lead to the advent of machined monolithic parts. High speed machining technology employed affords the manufacture of thin-walled parts from single billets of material, resulting in the removal of approximately 85% of the initial workpiece material. With thin-walled monolithic parts come the increased propensity for workpiece distortion and few arresting mechanisms for crack propagation, largely due to potentially unfavorable residual stress states. These imposed states of stress can be a result of the machining conditions used (e.g., feeds, speeds and cutter geometry). A general method is presented to model the residual stress state induced by metal cutting operations which takes into account workpiece thermo-mechanical properties, cutter geometry and process parameters. In this paper the model is specifically applied to Al7050. Results indicate the magnitude and sign of the state of stress is shown to have no intuitive correlation to machining process parameters such as speed and chip load. Similar results are shown for stress-induced bending moments, a potential strong contributor to part distortion. In addition, the machining-affected layer is shown to be on the order of 1mm, easily on the same length scale as the wall thickness of aerospace structures.