Variation of Initial Bulk Residual Stresses in Aluminum Alloy 7050-T7451 and Its Effect on Distortion of Thin-Walled Structural Parts

Milled thin-walled monolithic aluminum structural parts are widely used in the aerospace industry due to their appropriate properties such as a high overall strength-to-weight ratio. The semifinished products made of aluminum alloy 7050 undergo a heat treatment to gain this increased strength and hardness: Typically, three steps including solution heat treatment, quenching, and age hardening are carried out. This also leads to high initial bulk residual stresses (IBRS) within the part in the range of +/- 200 MPa. In rolled aluminum plate, plastic stretch (T7451 heat treatment designation) provides very effective relief of residual stress, decreasing IBRS to a level of +/- 20 MPa. In large parts with low bending stiffness, even that low level of IBRS can cause appreciable distortion. Besides the IBRS, the machining-induced residual stresses (MIRS) contribute to the distortion. This study investigates how IBRS in different stress relieved 7050-T7451 semifinished products vary and how this variation affects the distortion of milled thin-walled monolithic structural parts. A linear elastic finite element distortion prediction model, which considers the IBRS as well as the MIRS as input, was used to analyze the effect of varying IBRS on the distortion for different part sizes and geometries. The model was validated by machining of those parts and measuring their distortion. IBRS were measured via slitting technique and MIRS via incremental hole-drilling.