Friction stir processing of AZ31 magnesium alloy

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Magnesium alloys are known to be one of the lightest structural alloys and hence they are of interest to the potential applications in aerospace and automobile industry. However, widescale commercial usage of magnesium alloys is limited given its low formability at room temperature. Conventional processing techniques fail to alter the formability and other mechanical properties significantly. A severe plastic deformation processing technique called friction stir processing (FSP) developed on the principles of friction stir welding (FSW) is touted to significantly change the microstructure, texture and hence the mechanical properties of magnesium alloys. This thesis is primarily based on investigating the mechanisms of microstructure and texture evolution during FSP of a commercial AZ31 magnesium alloy. The work presented in the thesis comprise of four projects, all under the broad theme of investigating the microstructure and texture evolution in AZ31 alloy: (i) Microstructural origin of friction stir processed zone in a magnesium alloy, (ii) Microstructural evolution during multipass friction stir processing of a magnesium alloy, (iii) Friction stir processing under different thermal history, and (iv) Study of grain structure evolution during annealing of a twin roll cast AZ31 Mg alloy. In (i) microstructural and texture evolution at the edge regions of a friction stir processed (FSP) magnesium alloy AZ31 was studied which was aimed at explaining the mechanism of the microstructural evolution during FSP. A model of microstructural development through grain boundary sliding of the ultra-fine grains was proposed based on the texture and microstructural observations in the concerned regions. Further, in (ii) the microstructure and texture developments and also prediction of texture using visco-plasticself-consistent (VPSC) modelling of commercial magnesium alloy AZ31 being friction stir processed through multi-pass and multi-directional (unidirectional, reverse and transverse tool movements) was studied. Later, in (iii) the effect of various coolant mediums viz. air, liquid nitrogen and water on the microstructural and thermal profiles in the friction stir processed zone was investigated. Experimental observations, on the process zone dimensions and temperature profiles, showed significant differences: which highlighted the effect of the thermal history. A numerical simulation was made using multi-physics finite element method. A three-dimensional steady-state coupled laminar fluid flow and heat transfer model was developed, which could capture the experimentally observed process zone dimensions and temperature profiles. Lastly, in (iv) evolution of microstructure under static annealing at 300°C and 500°C for different times of twin-roll-cast (TRC) magnesium alloy AZ31 was outlined. Grain path envelope analysis was used for quantitatively analyzing the microstructural evolution. Twin-rolled structure had bimodal grain size which was preserved after annealing at 300°C. However, annealing at 500°C led to a unimodal grain size. A deformation induced recrystallization recovery (DIRR) model was proposed which could explain the observed microstructural features. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.