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Experiments have been carried out to ÔfreezeÕ the friction stir welding process by stopping the tool and immediately quenching the work piece in an Al-2195 plate welded under typical conditions. Sectioning through the ÔfrozenÕ weld keyhole with the tool in place has allowed the microstructure development, leading to the formation of the ultrafine grained nugget material, to be directly observed as fresh material encounters the deformation field surrounding the rotating pin. The grain refinement process is shown to be driven by grain subdivision at the colder periphery of the tools deformation zone, and the geometric effects of strain, which together reduce the overall high angle boundary spacing with increasing deformation. However, it also involves thermally activated high angle grain boundary migration, which increases as the temperature rises towards the tool. The higher temperature latter stages of the refinement process are closely reminiscent of geometric dynamic recrystallisation seen in high strain hot torsion experiments. The nugget grain structure has also been found to become more equiaxed and coarsens slightly, due to static annealing in the thermal wake of the tool.

a b s t r a c tResistance spot welding (RSW) is difficult to apply to aluminium automotive alloys. High power ultrasonic spot welding (HP-USW) is a new alternative method which is extremely efficient, using ∼2% of the energy of RSW. However, to date there have been few studies of the mechanisms of bond formation and the material interactions that take place with this process. Here, we report on a detailed investigation where we have used X-ray tomography, high resolution SEM, and EBSD, and dissimilar alloy welds, to track the interface position and characterise the stages of weld formation, and microstructure evolution, as a function of welding energy. Under optimum conditions high quality welds are produced, showing few defects. Welding proceeds by the development and spread of microwelds, until extensive plastic deformation occurs within the weld zone, where the temperature reaches ∼380 • C. The origin of the weld interface 'flow features' characteristic of HP-USW are discussed.

a b s t r a c tAn Mg-7Sn-1Al-1Zn alloy known to have excellent extrudability and superior strength was subjected to artificial cooling during indirect extrusion by directly spraying water onto the extruded rod at the die exit. The results obtained revealed that this artificial cooling dramatically reduces the temperature of the deformation zone during extrusion, thereby creating a finer grain size, an intensified texture and a greater amount of precipitates when compared to extrusion without artificial cooling. The yield and tensile strength of the extruded alloy is also significantly improved, which is attributed to the effects of grain refinement in combination with an enhanced texture and precipitate hardening.

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