A novel process for forming T-section components with low residual stresses in aluminium alloys

Abstract

The overall aim of this study is to develop a process for manufacturing extra-large aviation components with low residual stresses to minimise their subsequent distortion during the final machining process. The research concentrates on characterising and modelling the residual stress distribution at different stages of the manufacturing process, i.e. solution heat treatment (SHT), quenching and the subsequent residual stress relief. In addition, an effective method has been developed for manufacturing T-shaped components with low residual stresses. Compression tests were conducted to determine the temperature dependent mechanical properties of AA 7050 aluminium alloy. Based on the experimental results, the visco-plastic material behaviour was characterised for a range of test temperatures and strain rates. A dislocation-based constitutive model, describing the thermo-mechanical response of AA 7050 Al alloy in a condition corresponding to the manufacturing process, was developed. Values of material constants in the model were calibrated from isothermal compression test results. To quantify the residual stress evolution in the material during the manufacturing process, residual stress measurements were performed using neutron diffraction, X-ray diffraction and contour techniques on quenched and quenched & cold rolled specimens. The experimental data was used to verify finite element simulations of the residual stress mitigation processes, enabling residual stress predictions to be made. To study the effect of cold rolling on laboratory scale T-section components, a roll assembly was designed and manufactured. According to the measurement results, after cold rolling, the compressive stresses due to quenching at the surfaces of material were removed, though tensile residual stresses were induced near the surface. Generally, the stress predictions from the FE models were in close agreement with the measurement results which show that the FE results are reliable. Feasibility studies were performed to mitigate quench-induced residual stresses in extra-large T-section component. An integrated model was built to predict the effectiveness of cold rolling on residual stresses in heat-treated samples. The optimal parameters (incl. rolling deformation ratio and roll diameter) to minimise residual stresses in scaled-down T-section panels were predicted. Based on these simulation results, the integrated numerical models of cold compression and rolling on residual stress distribution in large-sized T-section components were developed. To reduce the residual stress in large-sized heat-treated T-section components, this study showed that, although cold compression without overlap can effectively relax residual stress in the material, for the case of multiple compressions with overlap regions, the residual stresses distribution post-compression are still sufficient to lead to component distortion. The cold rolling technique is a more cost-effective cold working technique to improve the residual stress distribution in T-profile large-sized components, compared with multiple cold compressions technique. For the cases considered, for reducing residual stresses in extra-large T-section components the FE models also indicate that the optimised parameters to relax residual stresses in the component is a deformation ratio of 1.5 % via a set of rolls with 400 mm radius. These results provide a valuable guideline for future industrial production activities.