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This present work consists of developing and testing a model for the prediction of precipitation kinetics and strengthening in Al-Cu-Mg alloys with composition in the . + S phase field. The model is applied to a range of conditions including isothermal and non-isothermal treatments. The non-isothermal treatments include controlled slow heating and cooling cycles and rapid heating and cooling cycles as experienced during fusion welding. In these Al-Cu-Mg alloys the Cu:Mg ratio is close to 1 and the Cu:Mg co-clusters and the S phase precipitates are the dominant strengthening phases. The model consists of two integrated modules, one for the prediction of the microstructural evolution of the Cu:Mg co-clusters and the S phase precipitates and the other for the prediction of yield strength or hardness. The modelling of precipitation kinetics of S phase is based on the Kampmann and Wagner (KW) numerical model. The major predictions of the microstructural model are the volume fraction and average radius of the S phase precipitates and the volume fraction of the Cu:Mg co-clusters evolving during the isothermal and non-isothermal treatments. The modelling of the thermal profile representing fusion welding is based on the Rosenthal’s thin plate solution for two dimensional heat flow. In the strength model the total critical resolved shear stress (CRSS) of the grains is evaluated by including contributions from the precipitates, solid solution, dislocations and the aluminium matrix. The strengthening due to the Cu:Mg co-clusters is based on the modulus strengthening mechanism and the strengthening due to the S phase precipitates is modelled using the Orowan looping mechanism. The predicted CRSS is then converted to yield strength and hardness data in order to compare with the experimental results. The testing of the model predictions is carried out by experimental data on 2024 T351 aluminium alloys. Some of the experimental data has been taken from other published works. The model is tested not only by the strength and hardness data but also by heat flow measured by the calorimetry experiments and the S phase average size measured from the transmission electron microscopy (TEM) micrographs. The predictions of the model correspond well with the experimental results for all the three models (one isothermal and two non-isothermal).

This thesis describes the processing and characterisation of Al-70Si alloys manufactured by gas atomised spray forming at Sandvik-Osprey (Neath, UK) and Oxford University using a newly commissioned spray forming pilot-plant facility. Spray formed Al-70Si (CE7) provides an attractive balance of thermophysical properties making it suitable for thermal management applications. Microstructural characterisation of CE7 was conducted using optical microscopy, image analysis, electron probe micro analysis (EPMA) and electron backscatter diffraction (EBSD). Microscopy revealed an interpenetrating network microstructure consisting of fine, randomly oriented polycrystalline primary Si interpenetrated by large, α-Al grains devoid of eutectic Si. Mechanical testing and thermal cycling simulated a service environment and revealed for the first time crack initiation, growth and blunting mechanisms, the effect of intermetallic phases on the bulk mechanical properties, and anisotropy effects resulting from macrosegregation of Al during solidification. A relationship between the inter-phase interface length and the fracture toughness has been proposed and methods of interface length refinement have been investigated, including chill casting and spray forming. Spray formed CE7 modified with separate additions of B, P, P+Ce and Sr have been microstructurally and mechanically characterised and compared with binary CE7. While alloy additions were effective in refining primary and eutectic Si in chill cast alloys, spray formed alloys showed little change in interface length. Particle injection of Si-Al powder was effective in refining the scale of the spray formed microstructure, and improving mechanical properties. The deleterious effect of intermetallic phases on bulk mechanical properties has been demonstrated and highlighted the importance of melt cleanliness and materials control during manufacturing.