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Thermo-mechanical processing of advanced magnesium alloys
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
Through process modelling (TPM) is a powerful tool to help understand and optimiseindustrial metal processing routes to obtain the desired microstructure and properties inthe final product. In this study, the development of a TPM to predict key microstructuralparameters in extruded rare-earth (RE) containing wrought magnesium alloy is discussed.Such alloys have a complex microstructure after processing. Additions of RE elementsto extruded magnesium alloys can lead to a weaker texture and the emergence of a new‘RE-texture’ component. Understanding the formation of this component after extrusionis difficult because the final microstructure is often fully dynamically recrystallised.To produce such a model, it has been necessary to improve understanding of the microstructuredevelopment during processing and how these are influenced by process parameters.Laboratory simulations and microstructural analysis have been used to identifythe key relationships between process parameters and microstructural evolution.These results have been used to guide the development of a TPM model that couplesa finite element prediction of temperature, strain, and strain rate with simple microstructuremodels to estimate the fraction of recrystallisation, the grain size and the fraction anddistribution of second phase particles. The model predictions have been compared againstthe experimental data and applications of the model are discussed.To investigate the emergence of the RE-texture, a Mg alloy with high RE content hasbeen cold drawn into wire. This is an axis-symmetric process with a strain path somewhatsimilar to extrusion. Using this method has enabled both the deformation and staticrecrystallisation behaviours to be separated, providing insights into the essential conditionsneeded to form the RE texture in (hot) extrusion.The drawn microstructure was characterised by elongated grains with basal fibre texture.Tensile, compression and double twins were visible but shear bands were not. Duringannealing, new statically recrystallised grains formed at grain and twin boundaries and atparticles, leading to weakening of the drawn texture but no emergence of the RE-textureoccurred, which suggests that shear banding has a critical role in forming the RE texture.