A. Sullivan

Microstructural Modelling for Friction Stir Welding of Aluminium Alloys

Predictive models have been developed to calculate the hardness profile and precipitate distribution after friction stir welding of aerospace aluminium alloys. A coupled modelling approach has been used, linking a thermal model to predict the thermal cycle to microstructure and property models. Two levels of modelling have been considered. The first is a semi-empirical approach that predicts hardness profiles. This simple model is demonstrated to correctly reproduce measured profiles in an example weld. The second model provides a detailed prediction of the precipitate evolution. Particle nucleation, growth, dissolution, and coarsening are all accounted for. Model predictions have been compared with experiments for the case of friction stir welded AA7449 thick plate.

Microstructural Evolution in AA7449 Plate Subject to Friction Stir Welding and Post Weld Heat Treatment

The effect of friction stir welding (FSW) and post weld heat treatment (PWHT) on the second phase particle distribution and cross weld hardness profile in AA7449 plate has been investigated. The alloy was received in an underaged condition, welded, then PWHT to give an overaged condition (in the parent material) . The effect of this complex treatment on the precipitate distribution in the weld and parent plate has been investigated over a range of length scales using small angle X-ray scattering (SAXS), TEM and FEGSEM. It is shown that the PWHT does not improve the hardness in the heat affected zone (HAZ), which is the location of the strength minimum after welding, but it does reduce the difference between the hardness in the HAZ and the nugget and parent hardness. The reduction in nugget strength after PWHT is particularly marked and is due to replacement of fine GP zones formed on post weld natural ageing by coarse overaged precipitates.

Process model for strength of age hardenable aluminium alloy welds

Abstract An existing process model for hardness prediction in age hardenable aluminium alloy welds is presented and analysed. One of the key criticisms of this model is that its derivation assumes softening is due to precipitate dissolution alone. The influence of precipitate coarsening has been determined by developing an equivalent model for softening owing to coarsening. It is shown that the experimentally derived master curves that form the basis of the model are capable of representing softening by a mixture of precipitate coarsening and dissolution. Methods to predict post-weld natural aging are discussed, and a new method is presented based on direct prediction of the Guinier–Preston zone fraction. The model has been applied to friction stir welding. Model predictions agree well with measured hardness profiles, and the sensitivity of the predictions to temperature is discussed.

Process Modelling of Friction Stir Welding for Aerospace Aluminium Alloys

A process model for the prediction of post weld hardness has been applied to Friction Stir Welding (FSW) of 2xxx, 6xxx and 7xxx aerospace aluminium alloys. The model has been used to predict hardness maps for welds in both thin sheet and thick plate material. The different weld configurations and tool geometries have been simulated using a proven thermal model to predict the temperature profile during welding. The thermal data have then been used as inputs to the hardness prediction model. This model is calibrated using isothermal softening data for each alloy and postweld natural ageing is also accounted for. A direct comparison of the model performance with measured hardness has been performed, and the predicted and measured profiles agree well, despite the fact that the model ignores the effect of deformation during FSW on hardness. The model is used to predict the effect of tool geometry (in particular the shoulder/pin ratio) on the hardness throughout the weld zone.

Modelling precipitate evolution during friction stir welding of aerospace aluminium alloys

Two models to predict the microstructural evolution and post-weld properties of friction stir welds in aerospace aluminium alloys are presented. The first model is a develop- ment of an existing semi-empirical method for the prediction of hardness profiles after welding, calibrated using isothermal hardness data. Post-weld natural ageing is accounted for, and a new method that predicts natural ageing kinetics is introduced. Once calibrated, the model is shown to accurately predict weld hardness profiles. However, this model does not explicitly predict the microstructure and therefore cannot readily be extended to model other properties. It can also only be applied to alloys welded in peak or overaged conditions. The second model aims to explicitly predict the heterogeneous precipitate distributions obtained after welding for any initial condition. It is based on classical kinetic theory and the numerical framework of Kampmann and Wagner. Multiple nucleation sites and multiple phases are accounted for. This model provides detailed microstructural information required for prediction of complex properties.