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Aluminium foils (99.99% purity) and single crystals (99.999% purity) were charged with hydrogen using a gas plasma method and electrochemical methods, resulting in the introduction of a large amount of hydrogen. X‐ray diffraction measurements indicated that within experimental error there was a zero change in lattice parameter after plasma charging. This result is contradictory to almost all other face‐centred cubic (f.c.c.) materials, which exhibit a lattice expansion when the hydrogen enters the lattice interstitially. It is hypothesized that the hydrogen does not enter the lattice as an interstitial solute, but instead forms an H–vacancy complex at the surface that diffuses into the volume and then clusters to form H2 bubbles. Small‐ and ultra‐small‐angle neutron scattering (SANS, USANS) and small‐angle X‐ray scattering (SAXS) were primarily employed to study the nature and agglomeration of the H–vacancy complexes in the Al–H system. The SAXS results were ambiguous owing to double Bragg scattering, but the SANS and USANS investigation, coupled with results from inelastic neutron scattering, and transmission and scanning electron microscopy, revealed the existence of a large size distribution of hydrogen bubbles on the surface and in the bulk of the Al–H system. The relative change in lattice parameter is calculated from the pressure in a bubble of average volume and is compared with the experimentally determined value.

This paper investigates the potential of mechanical tensioning (MT) to reduce the magnitude of residual stresses in welds and to eliminate buckling distortion. Both friction stir (FSW) and arc welds have been produced from the aluminium alloy AA2024, with different levels of tensile stress applied along the weld line either during or after welding. The resulting welds have been characterised in terms of out of plane distortion, residual stresses and microstructure. Buckling distortion was eliminated by stretching plates to between 35 and 70% of the yield stress of the material during welding. For each set of welding parameters investigated, an optimum tensioning stress has been identified, which eliminates the tensile residual stress peak across the weld zone, along with distortion. This optimum tensioning stress increases in line with the heat input of the welding process. When MT stresses are increased beyond this optimum value, then distortion arises once more and a band of compressive stress is formed across the weld zone.

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