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The effects of the main welding variables, apart form tool design, on the microstructure and properties of the nugget zone in friction stir welds have been investigated for a typical high strength Al alloy (AA7010). It has been found that there is an optimum rotational speed, for a given travel speed, that gives the highest strength and ductility for the nugget zone. As the travel speed is increased, it is necessary to increase the spindle speed to maintain this condition, although the optimum condition does not directly relate to a constant ratio of rotationalto travel speed. This condition coincides with a heat input, for a given travel speed, that gives the maximum level of solute in solution in the nugget and the lowest density of coarse second phase particles. For low travel speeds, this is limited at excessive heat inputs by reprecipitation occurringduring the weld cooling cycles, whereas for high travel speeds incipient melting within the nugget appears to be an important factor.

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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The corrosion behaviour of a dissimilar metal friction stir weld between AA2024-T351 and AA7010-T7651 was investigated. Gel visualisation and immersion tests were used to examine the effect of galvanic coupling between the two components. A microelectrochemical cell with a 400 mm diameter glass tip was used to measure the anodic and cathodic reactivities at different positions in the weld region as well as in the base alloys. It was found that the net anodic attack was in the 7010 alloy with the highest susceptibility in the nugget region. The nugget region of 2024 was protected by its high net cathodic reactivity owing to the precipitation of S phase particles.

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