Neutron and Synchrotron X-ray Residual Stress Mapping of 7XXX Aluminium Alloy Aerospace Welds

Stelmukh, Vadim A. (2004). Neutron and Synchrotron X-ray Residual Stress Mapping of 7XXX Aluminium Alloy Aerospace Welds. PhD thesis The Open University.



It has been long appreciated that residual stresses can exert significant influences on fatigue and fracture behaviour. Diffraction methods offer the engineering community the ability to obtain macro residual stress profiles, which are needed for designing safety critical parts. Because of their penetrability, neutron and synchrotron X-rays can be used to measure the strain tensor in the interior of engineering components. Despite the fact that diffraction techniques are well developed by scientists, it will take some time before engineers can use them as routine tools for obtaining the experimental data which are convertible into stress distributions with anticipated precision.

This work deals with high-strength AI-Zn-Mg-Cu alloys, which are widely used by the aerospace industry and have recently been found to be weldable due to significant progress in the development of new welding processes and automation of existing ones. There are several problems specific to stress measurement in aluminium alloy welds: variation in grain size, preferred orientations and the associated spatial resolution and speed at which the diffraction data can be collected. Based on the findings of preliminary examinations, experimental strategies are proposed to overcome these difficulties for the alloys studied with a view of obtaining usable diffraction data. They include a hybrid neutron/synchrotron diffraction technique designed to optimize the mapping of the full 3D stress tensor for the whole weld cross-section and the use of hard X-rays for effective triaxial strain measurements in highly textured zones of the welds. These and other procedures developed in the present work are also tested by the author on welds fabricated using the most advanced joining technologies: Metal Inert Gas, Variable Polarity Plasma Arc and Friction Stir welding. The role of "elastic stress-free" reference measurements, as a means of intrinsic separation of macrostrains from microstrains, is outlined. It has been shown that accurate macrostrain determination requires the reference measurements to be made under conditions identical to the bulk measurements.

The longitudinal stress distribution in the Metal Inert Gas weld is found to be characterized by the presence of distinct maxima in the regions of the heat affected zones bordering the double-V shaped fusion zone. The through-thickness asymmetry of the stress field is assumed to be responsible for the residual stress relaxation that occurs on the thinning to size of the weld, unambiguously determined in this thesis. The experimental data obtained for the Friction Stir welds with neutron diffraction shows that the welding speed has a significant influence on the magnitude and spatial distribution of the residual stresses: the specimen made at lower welding speed exhibits lower residual stress, which is accompanied by a lowering of the minimum hardness and a widening of the heat affected zone. The effect of heat inputs associated with the above welding processes on the development of residual stresses, and possible stress reducing measures are discussed in the light of the results obtained.

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