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PhD

Sarah George

This thesis investigates the feasibility of utilising the AA7075 composition to produce high strength castings via the semi-solid metal (SSM) casting process.

The advantages of the SSM process is the development of a globular primary grain structure and lower porosity than conventional high pressure die-casting practice. However, segregation during non-equilibrium solidification places high demands on post-solidification solution heat treatment in order to derive optimum strength during subsequent ageing treatments.

Research has been aimed at characterising and understanding the solute distribution in SSM castings and at determining the impact that solute segregation has on the homogenisation practice and homogenisation efficiency. To this end, the as-cast structures have been analysed using microscopy, X-ray analysis (EDS) and differential scanning calorimetry (DSC). The detailed grain structure has also been mapped using electron backscattered diffraction (EBSD) in the SEM which has revealed unusual misorientation structures within the primary grains.

The latter observations are employed to trace the solidification path during the semi-solid conditioning step and to explain the factors leading to the high solute segregation levels in the SSM castings. Based on the observations from the DSC analysis and the detection of incipient melting, homogenisation practices have been performed to promote reduction in solute segregation. The outcome of these treatments have been assessed in terms of both composition distribution and ageing behaviour.