Distortion Analysis of BS L168 Aeronautical Aluminum Alloy Thin-Walled Parts during High-Speed Milling
摘要
In the modern aviation industry, the use of thin-walled monolithic integrated parts made of aluminum (Al) alloys has significantly increased due to their high strength-to-weight ratio (175-205 MPa·cm3/g) which reduces the overall weight of the aircraft, shortens assembly build cycle times, lowers fuel consumption and improved performance. However, when slender thin-walled components with varying contours are machined from BS L168 Al alloy bars—widely used in combat aircraft structures—distortions ranging from 3 to 12 mm are frequently observed along the wall thickness. These deformations result from the components’ inherent lack of stiffness, exacerbated by machining-induced cutting forces, elevated temperatures, and internal stress redistribution. Such distortion compromises surface finish and often misaligns features critical to subsequent operations. Consequently, post-machining rework is commonly required and parts exceeding allowable tolerances may be classified as non-conforming and rejected. This research addresses these challenges through High-speed milling (HSM) and presents a scientifically optimized model based on Grey Relational Analysis (GRA) to manage multiple response variables—including cutting force components, surface roughness, cutting temperature and chip morphology—under both dry and wet machining conditions for BS L168 Al alloy. To analyze distortion during HSM, equivalent sections of aircraft thin-walled monolithic components with varying thicknesses (1-4 mm) and complex contours were milled using optimized cutting parameters: a cutting speed of 10,000 RPM, a feed rate (f) of 0.35 mm/tooth and a depth of cut (DOC) of 1 mm, yielding a material removal rate of 112,000 mm3/min. The machining strategy incorporated enhanced clamping, increased rigidity, wax filling in pockets for added firmness and intermittent stress-relieving operations, after rough machining to minimize distortion. Under wet machining conditions, main cutting forces at 1 mm wall thickness were reduced by 34.33% (136.89 to 89.89 N), and surface roughness improved by 50.78% (5.477 to 2.696 µm). Residual tensile stresses, measured using x-ray diffraction, decreased by 40.9% (from 83 to 49 MPa). Additionally, tool wear and chip morphology analyses conducted via Scanning Electron Microscopy (SEM) revealed a notable reduction in built-up layer (BUL) formation—from 4.01 mm (dry) to 2.85 mm (wet) in 4 mm walls and from 1.40 to 0.89 mm in 1 mm walls. Results revealed that HSM under wet conditions, combined with optimized parameters and enhanced techniques, reduced distortion by 30.56% to 48.93% across thin-wall thicknesses compared to dry machining, demonstrating its efficacy in improving dimensional accuracy and structural performance.