Developing magnesium alloys with a combination of strength and ductility based on friction stir-based technologies

Abstract

Magnesium (Mg) and its alloys are the lightest structural metallic materials. The density of Mg and its alloys is approximately two-thirds of the density of aluminium (Al) alloys and nearly four to five times lighter than steel. Due to the low density and high strength-to-weight ratio, Mg alloys have been used in transportation applications where mass reduction is essential, such as automobiles, aeronautics, and astronautics. The reduction of structural weight can improve the efficiency of fuel usage and produce less emission. However, the high energy cost of Mg production weakens the benefit of less emission by using light-weight Mg alloy structures. Moreover, because of the hexagonal close packed (HCP) crystal structure, Mg alloys often face severe strength-ductility trade-off. As a result, producing Mg alloys with a combination of strength and ductility with lower energy consumption is essential for the development of Mg industry.
Friction stir-based technologies are considered promising for the production of Mg alloys with a combination of strength and ductility. As variants developed from friction stir welding (FSW), friction stir based technologies share similar features of the microstructure evolution such as grain refinement, large second phase particle fragmentation, pore closure, etc. The work involved three variants, including stationary shoulder friction stir channelling (SS-FSC, Coreflow®), friction stir processing (FSP), and additive friction stir deposition (AFSD). SS-FSC and FSP were used in combination with spark plasma sintering (SPS) to recycle waste Mg-4Y-3RE (WE43C) scraps, turning them into wires and disks, respectively. AFSD was used to achieve the additive manufacturing (AM) of WE43C.
The mechanical stirring and generated friction heat during the processing, and thus dynamic recrystallisation (DRX), especially the continuous dynamic recrystallisation (CDRX) actively participated in the grain evolution. DRX occurred during the fabrication produced refined grains with an average grain size ~1-10 µm in the final products. Besides, the presence of fine thermal stable oxide particles either originated from the oxide film grown during the SPS or in-situ formed during the processing can further assist in controlling the grain size. Due to the mechanical stirring and elevated temperature during the processing, pre-existing second phase particles can be fragmented and partly re-dissolve into the matrix. Pre-existing oxide films, if there were some in the material, can also be fragmented but not re-dissolved. These particles were further redistributed throughout the material. Oxide nanoparticles were also in-situ formed during AFSD and provided extraordinary thermal stability at elevated temperature by pinning the grain boundary migration. Basal texture developed during the deformation. The orientation of the {0001} planes was strongly dependent on the material flow behaviour, which is mainly parallel to the material flow plane. Pores were detected in the material. Pore shape is closely related to the material flow behaviour as they all exhibited flake-like shapes and the thickness were parallel to the normal direction of the material flow plane.
Refined grain size provided strengthening effects by the volume increment in grain boundaries, contributing to the strength improvement of the material. For WE43C Mg alloys, when subjected to further ageing treatment at 200 °C, plate-like β1 phase precipitated on {101̅0} planes of α-Mg matrix. These precipitates acted as obstacles to the movement of dislocations and thus increased the strength of the fabricated material. In addition, a quicker ageing response around 30-40 h in these materials was observed, at the cost of a reduction in hardness enhancement. Apart from these factors, pores also played an essential role in the mechanical properties, especially the elongation of the material. FSPed material, where the pore size and fraction were higher than the other two samples, exhibited much lower elongation and even partly failure of the material during tensile tests. Refined grains, weakened gradient basal texture produced by friction stir-based technologies provided the combination of strength and ductility, yet if pores were not well controlled, a significant drop in the ductility could occur.
This work explored producing Mg alloy with a good combination of strength and ductility with lower environmental impact using friction stir based technologies. It specifically reveals the effects of processing on the final texture gradient in SS-FSC and the role of oxide formation on the subsequent heat treatment, including the ageing peak shift and strengthening effect, which were not previously investigated in detail.

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Identifiers

ORCID for Xingjian Zhao: orcid.org/0000-0001-5909-3043

ORCID for Dikai Guan: orcid.org/0000-0002-3953-2878

ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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