UBC Theses and Dissertations
Microstructure-property models for heat treatment of A356 aluminum alloy Colley, Leo John
The evolution of microstructure and mechanical properties during heat treatment of an industrially-cast A356 aluminum alloy was studied in an extensive experimental investigation. The temperature ranges of interest were; solution treatment at 500-560°C, natural ageing at room temperature, and artificial ageing at 150-200°C. The changes in dendritic composition and eutectic morphology due to solution treatment were quantified by microprobe and image analysis for a wide range of processing conditions. Subsequently, a microstructure model for solution treatment was constructed using sub-models for; i) the dissolution of Mg₂Si particles, ii) the fragmentation of eutectic fibres, and iii) the coarsening of the fragmented eutectic. For the ageing investigations, characterisation of mechanical properties was done by hardness and tensile testing, and the kinetics of precipitation was determined by an isothermal calorimetry technique. A model to predict the evolution of yield strength during artificial ageing was developed based on established physical theories. A yield strength model for natural ageing was also proposed using data from isothermal calorimetry tests performed close to room temperature. Two model Al-Si-Mg alloys were investigated in order to extend both ageing models to include the effects of; i) alloy chemistry, ii) incomplete solution treatment and iii) natural ageing prior to artificial ageing. The validity of the models was verified using independent experimental measurements and literature data, and they were subsequently used as a tool to identify potential optimisation strategies for industrial heat treatment processes. The linkages between the models revealed details of processing challenges arising from the interdependence of the heat treatment stages, such as reduced strengthening during ageing due to incomplete solution treatment, and delayed strengthening during artificial ageing as a result of prior natural ageing.
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