Effects of Aluminum Plate Residual Stress on Machined-part Distortion

Dimensional tolerance requirements for high-speed-machined aluminum products continue to tighten due to strong demand for automated assembly of complex monolithic aluminum parts in aerospace and other industries. Understanding the contribution of inherent residual stress in wrought aluminum 7050-T7451 plate, a common alloy in aircraft manufacture, in the distortion of high-aspect-ratio machined monolithic parts is critical but remains problematic. The difficulty stems from the alloy’s low magnitude of residual stress, distributed over relatively large geometries. The numerous prior studies aimed at investigating residual stress effects on machined part distortion, however, suffer from inadequate characterization of the inherent stress field within the wrought material—because of low fidelity issues due to slitting methods of residual stress measurement, confounding effects from machined-layer removal methods, or because of small number of measurements when using neutron diffraction (ND). In this work, inherent residual stress is measured using ND at over 860 locations throughout the volume of a 90.5 mm thick 7050-T7454 aluminum plate having dimensions 399 mm in the longitudinal (rolling) direction and 335 mm in the transverse direction. Unlike prior studies, the ND residual stress field is reconstructed using an iterative stress reconstruction algorithm to ensure a fully compatible and equilibrated 3D field prior to examining its effect on the distortion of a high- aspect-ratio monolithic part. Validation of the equilibrated stress field is accomplished by comparison of corresponding aggregate fields generated by both experimental and simulated slitting techniques. To isolate and study the potential contribution of residual stress on part distortion, an element deletion technique to simulate material removal is performed to avoid confounding with any machining-induced effects. The findings reveal that the inherent residual stress is not negligible, and alone is sufficient to distort a high-aspect-ratio part beyond tolerances necessary to meet current aerospace industry manufacturing requirements (>0.75 mm distortion over 400 mm span). Moreover, the work reveals that a residual stress field developed only from slitting data, per the literature, underrepresents both residual stress and part distortion. The results show that parts created from different locations within the plate thickness can lead to reversed distortion patterns due to the corresponding residual stress induced effects. The research gives insight into fixturing and shimming to compensate for distortion as well as provides an algorithm to further address distortion of the finished part by applying weights to in accordance with industry practices.