Investigation, prediction, and optimization of residual stresses induced by cutting parameters during machining of hot forged aluminum alloy

Abstract

Residual stresses generated during machining operations play a decisive role in determining the surface integrity, dimensional stability, fatigue performance, and service life of engineering components. This effect is particularly critical in high-strength, precipitation-hardened aluminum alloys such as AA7075-T6, which are extensively used in aerospace and automotive applications due to their superior strength-to-weight ratio. The thermo-mechanical loads imposed during machining processes may lead to undesirable tensile residual stresses, surface distortion, and premature failure if not properly controlled. Although machining-induced residual stresses have been widely investigated, existing studies predominantly focus on individual machining operations or simplified geometries, and a unified understanding covering multiple machining processes applied to industrially relevant hot-forged components remains limited. This doctoral thesis aims to analyze, predict, and optimize the effects of cutting parameters on residual stress formation in hot-forged and T6 heat-treated AA7075 aluminum alloy components. The research addresses turning, milling, and drilling operations in an integrated manner in order to establish a comprehensive understanding of residual stress evolution under different machining conditions. The thesis combines systematic experimental investigations with statistical analysis, predictive modeling, and energy-based interpretation to provide both fundamental insight and practical guidelines for industrial applications. All experimental studies were conducted under dry cutting conditions using cutting parameters representative of industrial practice. Machining forces and torque were measured using a multi-component dynamometer to characterize the mechanical loading during material removal. Residual stresses induced on the machined surfaces were determined non-destructively by X-ray diffraction (XRD), enabling reliable evaluation of near-surface stress states without altering the material condition. The experimental methodology ensured repeatability and consistency across all machining operations, allowing meaningful comparison between turning, milling, and drilling processes. In the turning study, both face turning and circumferential turning operations were investigated to evaluate the influence of cutting speed, feed rate, and depth of cut on residual stress formation. The results revealed that residual stress evolution is governed by the balance between mechanical deformation and thermal loading. When thermal effects dominated the cutting process, tensile residual stresses developed on the machined surface, whereas mechanically dominated conditions promoted compressive stress states. Among the examined parameters, cutting speed was identified as the most influential factor affecting residual stress magnitude and sign in turning operations. In the milling study, the combined effects of cutting parameters on residual stress and surface roughness were examined using Taguchi and full-factorial experimental designs. The results demonstrated that feed rate was the dominant parameter controlling surface roughness, while cutting speed played a critical role in regulating residual stress magnitude. To enable predictive capability and reduce experimental cost, regression-based modeling approaches were employed. Support Vector Regression (SVR) models provided higher prediction accuracy for residual stress and surface roughness compared to Lasso and Ridge regression models, demonstrating the effectiveness of machine learning techniques for machining optimization with limited experimental datasets. In the drilling study, the formation mechanisms of thrust force, torque, active work, and axial residual stress were systematically analyzed. Residual stresses were evaluated at two distinct locations along the hole depth, namely the hole entrance and the hole exit, in order to capture the effects of cyclic tool engagement and breakthrough mechanics. The results indicated that feed rate was the dominant parameter influencing thrust force, torque, and residual stress formation, while spindle speed primarily affected thermal and frictional conditions in the cutting zone. A pronounced asymmetry in residual stress distribution was observed between the entrance and exit regions, with the hole exit exhibiting enhanced stress relaxation behavior due to breakthrough-induced instability. An explicit energy-based framework was introduced by correlating active work with axial residual stress evolution, providing a physically meaningful interpretation of drilling-induced residual stress formation. Overall, this thesis establishes a unified thermo-mechanical and energy-based framework for interpreting machining-induced residual stresses in hot-forged AA7075-T6 aluminum alloy components across turning, milling, and drilling operations. The findings contribute to a deeper understanding of residual stress formation mechanisms and provide practical guidance for selecting cutting parameters to control residual stress states, enhance surface integrity, and improve component performance in industrial manufacturing applications.