Novel approaches to grain refinement of magnesium alloys

Ali, Yahia (2017). Novel approaches to grain refinement of magnesium alloys PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2017.877

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Author Ali, Yahia
Thesis Title Novel approaches to grain refinement of magnesium alloys
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2017.877
Publication date 2017-07-14
Thesis type PhD Thesis
Supervisor Mingxing Zhang
Dong Qiu
Total pages 130
Language eng
Subjects 0912 Materials Engineering
Formatted abstract
Grain refinement of cast metals through inoculation has been one of the most favourable approaches in the industry due to its convenience, low cost and reproducible good results in obtaining fine equiaxed as-cast microstructures. Grain refinement is also considered one of the most effective approaches to simultaneously improve castability, strength, ductility and formability of metals. Since the 1990’s, Mg has been one of the most important structural materials, particularly in the automotive industry, due to its abundance in the earth’s crust, lightness and good castability. However, as-cast Mg is usually associated with low strength, ductility and creep resistance. In the last couple of decades, grain refinement of Mg alloys has been an active research topic, because achieving finer grains is found to be increasing both strength and ductility of the alloys. So far, it is believed that grain refinement by inoculation can be achieved through restricting the grains growth by controlling the constitutional undercooling during solidification or through increasing the nucleation rate in the melt or both. However, a number of important details in the grain refinement mechanism are still unknown and there has not been a unique model found in the literature that can fully satisfy all experimental findings. In addition, most of the developed grain refiners are not as effective as zirconium which is considered the most effective grain refiner for Mg so far; but it does not work with Mg-Al alloys which form the majority of commercially used Mg alloys.

In order to design a new grain refiner, growth restriction factors (Q-values) of a number of solutes that have not been found in the literature were calculated in binary Mg alloys. Following that, calculations using the edge-to-edge crystallographic matching (E2EM) model predicted FCC-CaO as an effective nucleant for the HCP-Mg. Casting experiments were then undertaken through addition of various amounts of CaO into the Mg melts. As-cast optical micrographs showed dramatic reduction in grains size with CaO particles addition. At 0.3 wt.% CaO addition level, columnar to equiaxed (CTE) transition of Mg alloys was firstly observed. Further CaO addition led to further grain refinement. Through SEM analysis CaO particles were spotted at the centres of the grains, which indicated that CaO particles can successfully nucleate Mg grains. In addition, Ca solute was introduced into the melt through decomposition of some CaO particles, which further improved the grain refinement efficiency. Addition of CaO into Mg-Ca and Mg-Zn alloys led to even more grain refinement; but grain coarsening was obtained in Mg-Al-CaO alloys. The effect of CaO on the mechanical properties of Mg alloys was then studied through preparing two groups of samples (Mg-Ca and Mg-CaO based alloys). Tensile and hardness results showed that both sample groups were strengthened. However, the grain refinement strengthening component was higher in Mg-CaO base alloys.

Fast cooling is another common approach to produce fine as-cast grains. But, a comprehensive study of the effect of cooling rate on the grain refinement of Mg alloys is lacking. To investigate the effect of cooling rate on the microstructure of as-cast Mg alloys, a V-shaped copper mould was designed that produces different cooling rates along its height. Pure Mg, Mg-Al, Mg-Zn, Mg-Zr and Mg-CaO base alloys were then cast using this mould and the as-cast microstructure of the alloys was then investigated. Unlike what was normally expected, results showed that higher cooling rate does not always lead to grain refinement. Mg-1.0wt.%Zr, Mg-1.4wt.%CaO and Mg-3.0wt.%Al alloys suffered from grain coarsening at the tip of the mould, where the highest cooling rate was achieved. It was considered that at high cooling rate, due to the high thermal gradient, the constitutional undercooled zone formed ahead of the solid/liquid interface is smaller. As a result, most of this zone might lie within the nucleation free zone proposed by the interdependence theory. Thus, in this situation grain growth might be favourable over nucleation of new grains resulting in grain coarsening.

Due to the high commercial importance of Mg-Al based alloys, the effects of various tertiary elements addition into Mg-Al based alloys were studied in terms of microstructure and strengthening mechanisms. Silicon, copper, titanium, manganese, calcium and tin solutes were added separately into Mg-3.0wt.%Al and Mg-9.0wt.%Al based alloys in various concentrations. Grain size and hardness were measured for the as-cast alloys. Results showed that the grain refining significance of Sn, Ti or Mn containing alloys is very low. Thus, further solution treatment and aging experiments were designed and performed on Cu, Ca and Si containing alloys. Hardness and microstructure results of the as-solution treated and aged alloys suggested that in Mg-3.0wt.%Al based alloy, only the Ca solute showed a precipitation hardening effect. On the other hand, in Mg-9.0wt.%Al based alloys, Mg17Al12 intermetallic compounds still played a major role in precipitation hardening.

In summary, the investigations presented in this PhD thesis have contributed to understanding the mechanism of grain refinement in Mg alloys. Based on the results, it is concluded that in order to design a successful grain refiner for Mg alloys many parameters need to be considered. In addition, the current work highlights the major role played by cooling rate in controlling the grains size. Finally, it comprehensively discusses the effects of various tertiary solutes on Mg-Al based alloys.
Keyword Grain refinement
Magnesium alloys
Light metals
Grain coarsening
Cooling rate
V-shaped mould
mechanical properties
Edge-to-edge matching model

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Created: Wed, 12 Jul 2017, 12:07:47 EST by Yahia Ali on behalf of Learning and Research Services (UQ Library)