A simple model for recrystallisation and grain growth in the nugget of friction stir welds has been developed on the basis that the dominant recrystallisation mechanism is geometric dynamic recrystallisation. In combination with a process model capable of predicting the strain, strain rate and temperature history of any region within the weld, the current model can predict the grain size after friction stir welding. The model has been tested against experimental data in the literature for a friction stir welding in AA2524 and found to give a reasonable prediction of the final nugget grain size without arbitrary fitting. The model has then been used to predict the influence of dispersoid particles and the effect of cooling rate behind the tool on the grain size evolution in the nugget.

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The material constitutive behaviour and microstructure of Aluminium alloys 6082 and 7449 were studied with the Gleeble hot compression test. The novel aspect of the work is that the testing was done at high strain-rates and at temperatures within 5 K of the solidus. The results indicated that the strength was maintained up to near solidus temperatures, with no dramatic strength reduction being observed. There was however, a distinct change in the slope observed with the 7449 results around 720 K. The experimental results were then fit to the Zener-Holloman equation, which describes the relationship between the material flow stress, temperature and strain-rates. The material microstructure of the hot compression test samples was analysed, and the averaged grain size was calculated to compare with friction stir weld nuggets. This will be used to infer the processing conditions that exist in the dynamically recrystallized weld nugget. Finally, a simple model was used to understand how processing conditions affected the deformation behaviour.

Tungsten heavy alloys with compositions ranging from 35 to 93 wt pct tungsten were liquidphase sintered at 1500°C under microgravity conditions for isothermal hold times ranging from 1 to 600 minutes. The solid-volume fraction, grain size, grain size distribution, connectivity, and contiguity of the sintered microstructures were quantitatively measured. From these data, graingrowth-rate constants are determined for solid-volume fractions ranging from 0.048 to 0.858 and are compared to the predictions of several grain-coarsening models. The measured grain size distributions are shown to be self-similar and are fit to a Weibull distribution. Threedimensional (3-D) grain size distributions from several coarsening models are transformed into grain size distributions for two-dimensional (2-D) cross sections, for comparison with the experimental data. Chi-squared tests and G-tests show that a coalescence model for grain growth fits the experimental observations better than solution-reprecipitation models, even for dilute tungsten heavy alloys.

Friction stir welding is conceptually simple but metallurgically highly complex due to thecombination of severe deformation and high temperature. This is particularly true in the case ofprecipitation strengthened alloys, such as high strength aerospace aluminium alloys, where theheat and deformation of FSW lead to profound changes in both grain structure and precipitatedistribution that ultimately determine weld performance.