Abstract
Magnesium alloys show promise to be materials for lightweighting of automotive and aerospace vehicles improving fuel efficiencies and vehicle performance. A majority of magnesium alloy components are produced using casting where susceptibility to forming inclusions and coarse grain sizes could result. Development of effective inclusion removal techniques and better understanding of grain refinement of magnesium alloys could help in improving their mechanical properties to advance them to more structurally demanding applications. This research aimed to develop an environmentally friendly alternative to the grain refinement and inclusion removal capabilities of carbon based hexachloroethane as it releases dioxins, chlorine gas and corrodes foundry equipment. A secondary aim was to pioneer in-situ neutron diffraction to examine the solidification of magnesium alloys. The research involved preparing tensile samples of AZ91E magnesium alloy using permanent mould casting. Inclusion removal was conducted by using filtration and argon gas bubbling. Castings grain refined using hexachloroethane (0.25, 0.50 and 0.75 wt.%) were compared against ex-situ aluminum-silicon carbide and in-situ aluminum-carbon based grain refiners combined with filtration and argon gas bubbling. Further, in-situ neutron diffraction was utilized for phase analysis and fraction solid determination of magnesium-zinc and magnesium-aluminum alloys. There was a significant improvement in yield strength, ultimate tensile strength and elongation with filtration plus argon bubbling, carbon inoculation or both filtration plus argon bubbling and carbon inoculation. The results indicated that the mechanism of the observed ~20% reduction in grain sizes with carbon inoculation (hexachloroethane, ex-situ aluminum-silicon carbide and in-situ aluminum-carbon) was explained through duplex nucleation of Mn-Al and Al-Mg-C-O (likely Al2MgC2) phases. Finally, in-situ neutron diffraction was used to follow the formation of Mg17Al12 eutectic phase in a magnesium-9 wt.% aluminum alloy. For the magnesium-zinc alloys, in-situ neutron diffraction enabled characterization of the effects of zirconium to the fraction solid growth of (1010), (0002) and (1011) α-Mg planes. The societal and environmental impact of this research is significant. There is a clear demonstration of alternatives to the universally used hexachloroethane grain refiner promoting harmful emissions. Improved mechanical properties resulting from new grain refinement and iv inclusion filtration are a major advancement in promoting weight reduction, improved castability and decreased environmental impact for automotive and aerospace industries.
Highlights
Lightweighting of automotive and aerospace vehicles is a crucial strategy to improve fuel efficiencies, reduce exhaust emissions, improve performance and reduce impact on the environment
The results indicated that the mechanism of the observed ~20% reduction in grain sizes with carbon inoculation was explained through duplex nucleation of Mn-Al and Al-Mg-C-O phases
[172] Zr cores are nearly pure Zr and are typically circular or elliptical found within grains or at grain boundaries [170] 1Nucleant size of Mg for homogeneous nucleation from Equation 11, r* is ~1.3 μm [16] or d* is ~2.6 μm Improvements in Zr refinement can be made by introducing more appropriately sized Zr particles that are able to act as potent substrates [4] and better understanding of Zr cores [116] to maximize Zr recovery and reduce refining costs
Summary
Lightweighting of automotive and aerospace vehicles is a crucial strategy to improve fuel efficiencies, reduce exhaust emissions, improve performance and reduce impact on the environment. An economical technique for producing Mg components is casting such as permanent mould or sand casting. These casting processes are effective for producing large structural automotive components such as wheel rims and engine blocks. Mg alloys have potential to be used in more structurally demanding applications at room and elevated temperatures. This could expand their application from small components such as seat frames and steering wheels to more structurally demanding applications such as engine blocks, wheel rims and transmission cases. The drawbacks of Mg alloys can be countered by grain refinement which imparts improvements in strength without hindering ductility [2,3] and inclusion removal techniques such as filtration and inert gas bubbling
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