Non-destructive measurement of residual stresses in welded aluminium 2024 airframe alloy

Abstract

Welding is being considered as a potential alternative metal joining technique to mechanical fastening, not only to save cost on existing aircraft versions but also to face the challenges in the aerospace industry of the next generation mass air transportation. However, before change of a fabrication process, the residual stress generation due to welding needs accounting for in structural integrity calculations of safety critical components subjected to fatigue loading spectra. The present work is focused on determination of the full 3D residual stress distribution, and its subsequent evolution due to machining, in fusion welded airframe aluminium alloy 2024-T351. Within the overall program two fusion welding processes - Metal Inert Gas (MIG) and Variable Polarity Plasma Arc (VPPA) were studied. A combination of diffraction techniques - neutron, synchrotron X-ray, and laboratory X-ray - were used in the course of the project. The recent advances in neutron and synchrotron X-ray diffraction techniques were utilised to obtain a very detailed three dimensional picture of residual stress distribution with high spatial accuracy. The measurement requirements and the size specificity of different samples determine the appropriate diffraction technique. Material characterisation of grain size distribution and crystallographic texture was performed and used as a vital input to the diffraction experiments. In this dissertation an integrated approach was adopted whereby stress magnitude and distribution in an as-welded sample was determined and then stress evolution and redistribution was compared before and after machining. Two welding processes were studied and compared. The residual stress in fatigue samples used to monitor short crack generation and growth (up to 1 mm) were measured for surface stress distribution and its evolution following testing. Fatigue samples for long crack (over 1 mm) growth were also studied. The effects of welding structural assemblies will not be manifested in coupon samples, and therefore, a structural wing skin and stringer assembly was also measured for residual stress generation. The weld residual stress generation is found to be higher in a MIG weld than in a VPPA weld. A distinct profile and magnitude of stress distribution is observed in both the welds studied. A systematic stress evolution is observed in the fatigue geometry samples for short and long crack growth monitoring. The magnitude of stress reduction due to machining and cutting of fatigue specimens, however, is different for the different weld processes. The wing skin stringer assembly measured also shows a different magnitude and stress distribution pattern. However, the basic footprint of stress distribution was observed to be same in the different weld processes and the samples studied.