Dependence of Microstructure Evolution, Texture, and Mechanical Behavior of A Mg Alloy on Thermo-Mechanical Input during Friction Stir Processing

In this thesis, the relationship among friction stir processing (FSP) parameters, microstructure evolution, texture development, and mechanical hehavior of AZ31B Mg alloy was investigated.

First of all, in order to reveal the correlation among the deformation conditions, dynamic recrystallization (DRX) mechanisms, and microstructure evolution in the Mg alloy, hot compression tests at a wide range of Zener-Hollomon parameter (Z) values were conducted. Through optical microscopic examination, it was found out that above a critical Z value, twinning influences the DRX process resulting in a more effective grain refinement, which is manifested in a significant change in the slope of the Z-drec relationship, where drec is the recrystallized grain size. Moreover, EBSD examination revealed that the twinning also contributed to a distinct change in the recrystallization texture. Compression tests were performed along both through-thickness and in-rolling-plane directions of the plate to study the orientation dependency of twinning activities and its influence on the DRX process. X-ray line profile analysis (XLPA) provides further insights by highlighting the differences in the dislocation density/types, subgrain sizes, and twin densities during the DRX processes operating with or without the twinning.

Secondly, the constitutive behaviour study was applied to the investigation of microstructure evolution during FSP. By varying the key FSP parameters systematically, i.e. rotation and travel rates of the tool, a series of FSP specimens were prepared with a wide range of thermo-mechanical inputs in terms of Z. The resulting tensile behavior in the stir zone (SZ) showed a dramatic change as a function of Z, caused by a systematic change in the texture within SZ measured by neutron diffraction.

A three-dimensional transient model was developed to investigate the detailed deformation history including the temperature and strain rate profiles and material flow pattern during FSP of the Mg alloy. Such deformation history can be combined with the constitutive study from the compression tests in order to analyze the developments of micro-texture and DRX grains during FSP, which will, in turn, dominate the mechanical properties.

Based on the studies above, new fundamental understandings were gained on the governing mechanisms for the deformation and recrystallization processes during FSP and the influence of thermo-mechanical input during FSP on ductility enhancement in the Mg alloy.