Distortion Analysis of BS L168 Aeronautical Aluminum Alloy Thin-Walled Parts during High-Speed Milling

Abstract In the modern aviation industry, the use of thin-walled monolithic integrated parts made of aluminum alloys has significantly increased due to their high strength-to-weight ratio which reduces the overall weight of the aircraft, shortens assembly build cycle times, lower fuel consumption and improved performance. When slender, thin-walled components with varying contours are machined from BS L168 aluminum alloy bars commonly used in combat aircraft, distortion ranging from 5 to 12 mm along the wall thickness are often observed prior to lug removal. This is due to high milling forces, increased temperatures at the cutter-component interface zone, plastic deformation, low rigidity and the redistribution of internal stresses. Thin-walled components, which inherently lacking stiffness, are more prone to deformation, adversely affecting surface finish and often preventing parts from being properly aligned for subsequent operations. As a result, post-machining rework is frequently required, or, in some cases, parts are rejected due ti tight tolerance. This research analyses how cutting forces, thermal loads, surface finish, high material removal rates, residual stresses, cutter wear, and chip generation influence the distortion behaviour of thin-walled parts made from BS L168 Al alloy bars during high-speed milling (HSM) operations, aiming to identify factors contributing to distortion in real industrial scenarios and to develop strategies to minimize them, ensuring the production of high-precision parts with optimal structural integrity. Thin-walled, monolithic integrated parts of varying thickness and contour are milled into complex aircraft components using optimized cutting parameters under both dry and wet machining conditions. This is achieved with improved clamping, increased rigidity, the introduction of wax filling into pockets to add firmness to the workpiece, and intermittent stress-relieving operations after rough machining. The initial machining-induced residual stresses (MIRS) were studied using XRD, as high tensile stresses can cause micro-cracks and surface damage, impacting the part's structural integrity. Cutter wear and chip generation are also critical factors influencing stress and deformation in thin walls and were examined using a Scanning Electron Microscope (SEM) to analyse the impact of distortion across the wall thickness. Distortions in wall thickness were quantified with a coordinate measuring machine (CMM). The results indicated that HSM under wet machining of aircraft monolithic thin-walled parts, with optimized cutting parameters, improved clamping, and enhanced processes, had a substantial impact on reducing distortions. This technique led to a 30.56–48.93% reduction in distortion along the wall thickness of thin-walled components compared to dry machining.