a b s t r a c tHigh power ultrasonic spot welding (USW) is a low heat input solid-state joining process that may offer a solution for welding difficult dissimilar-material couples, like magnesium (Mg) to aluminium (Al) for automotive body applications. However, the high strain rate dynamic deformation in USW has been claimed to accelerate inter-diffusion rates in dissimilar joints. The interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy. For the optimum welding condition of 600 J (0.4 s) the reaction layer thickness was already $ 5 mm thick. Intermetallic reaction centres were found to nucleate within microwelds at the interface at very short welding times and spread and grow rapidly to form a continuous layer, composed of two sub-layers of Al 12 Mg 17 and Al 3 Mg 2 . Interface liquation was also found for longer welding times at temperatures below the recognised lowest eutectic reaction temperature in the Al-Mg binary system. Modelling has been used to show that the solid state reaction kinetics were over twice the rate expected from parabolic growth predictions made using rate constants obtained under static test conditions. The reasons for this discrepancy and the depressed melting reaction are discussed.

Characterization of microstructures containing small grains or low-angle grain boundaries by electron backscattered diffraction (EBSD) is limited by the spatial and angular resolution limits of the technique. It was found that the best effective spatial resolution (60 nm) for aluminium alloys in a tungsten-filament scanning electron microscope (SEM) was obtained for an intermediate probe current which provided a compromise between pattern quality and specimen interaction volume. The same specimens and EBSD equipment when used with a field-emission gun SEM showed an improvement in spatial resolution by a factor of 2-3. For characterizing low-angle boundary microstructures, the precision of determining relative orientations is a limiting factor. It was found that the orientation noise was directly related to the probe current and this was interpreted in terms of the effect of probe current on the quality of the diffraction patterns.

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A method of accurately measuring misorientations by electron backscatter diffraction (EBSD), which is an extension of that proposed by Wilkinson and based on the comparison of diffraction patterns, is described. The method has been applied to linescans, and found to improve the angular resolution by a factor of more than 30. The consequent improvement in determining misorientation axes is also analysed. Small changes of orientation very close to some low-angle boundaries were investigated and found to be artefacts of the analysis. Measurements of the area from which diffraction patterns are generated show this to be much larger than the effective spatial resolution of EBSD, and it is concluded that this may be a limiting factor in the use of EBSD for microstructural characterization.