Magnesium is a promising material that can substitute for steel and aluminium alloys to achieve weight reduction in automobile, aerospace and allied industries, which is considered to contribute for energy efficiency and eco-friendly. However, one of the major impediment is the limited formability of magnesium.
Researchers from the Department of Materials Science and Engineering and Department of Mechanical and Aerospace Engineering, Monash University, Australia and Automotive Steel Research Institute, China have reported a breakthrough in the development of polycrystalline pure magnesium that can be tailored to be super-formable at room temperature by conventional processes.
The study reveals that polycrystalline pure magnesium becomes super-formable at room temperature after it is extruded ≤ 80 °C, exhibit no work hardening and shows no sign of fracture during compression at room temperature and at a strain rate of 10−3 s-1. In contrast, those extruded at 150 to 400 °C exhibit poor formability at room temperature, high work hardening and show clear signs of fracture when compressed by 20–30% reduction in height (Fig. 1).
Fig. 1 Room temperature compression of specimens extruded at 80 and 400 °C. Photographs in the inset show the specimens before and after compression test. Specimens extruded at 400 °C fractures after ~20% height reduction while those extruded at 80 °C can be compressed from 10 to 1.5 mm without fracture.
The super-formability of polycrystalline pure magnesium specimens extruded at 80 °C was demonstrated by rolling them at room temperature (cold rolling) without any intermediate annealing stage. Continuous reduction in their thickness from 3 to 1 mm fails to display any edge cracking (Fig. 2(a)). The ability of 1 mm-thick sheet to bent through 180° (hemming) without any failure (Fig. 2(b)), suggests its suitability for the fabrication of automotive panels. The 1 mm thick sheet can be further cold rolled to 0.5 mm, and even 0.12 mm strips (96 % reduction in total thickness equivalent to a true strain of 3.2). The 0.12 mm strips are amenable for cutting and shaping in the form of letters “m” and “g” (Fig. 2(a)). Folding twice followed by complete unfolding fails to display any visible cracks (Fig.2 (c)), which refute the conventional belief that magnesium would fracture after heavy cold work or bending.
Fig. 2 Cold rolling of extruded specimens: (a) Photograph of 3 mm thick magnesium plate extruded at 80 °C, and after 67 and 96% cold rolling without any trimming of specimen edges along the rolling direction. The strip cold rolled by 96% was cut and shaped in the form of letters “m” and g”; (b) Photograph of cold-rolled 1 mm thick strip bent by ~180° at room temperature; (c) Photographs showing folding and unfolding of 0.12 mm strip without any visible cracks. (Scale bars in a –c: 20, 3 and 5 mm, respectively)
Specimens extruded at 80 and 400 °C posses strong basal texture and contain predominantly equiaxed grains with an average grain size of ~1.3 and ~82 μm, respectively. For the specimen extruded at 400 °C followed by 20% cold compression or rolling, the average grain size is decreased to 56–61 μm. In contrast, there observed to be very little change in the size and shape of grains for the specimen extruded at 80 °C followed by 50% cold compression or rolling. Microstructural evolution reveals the presence of a large number of deformation twins (Fig. 3(a)) and slip traces (Fig. 3 (b)) for the specimen extruded at 400 °C followed by 20% cold compression or rolling. In contrast, these features are not detected in the specimen extruded at 80 °C.
Fig. 3 Secondary electron micrographs showing: (a) deformation twins (T); and (b) slip traces (S) in the specimen extruded at 400 °C and compressed by 20%.
The results of the study reveals the occurrence of significantly different deformation modes during cold forming of specimens extruded at 80 °C, even though they also possess a strong basal texture. The superformability behaviour is due to the occurrence of dynamic recrystallisation during extrusion of pure magnesium specimens at room temperature. In addition, dynamic recrystallisation is also possible during compression/rolling at room temperature, either to accommodate grain boundary sliding or to act as an independent softening mechanism. The study demonstrates that extruded pure magnesium remains superformable, even after substantial plastic deformation at room temperature. The findings of the study provide a new avenue for the design and development of highly formable magnesium products by conventional thermomechanical processes that are cost-effective, efficient and industrially scalable. This attribute assumes significance for the development of light weight materials in automobile, aerospace and allied industries.
T.S.N. Sankara Narayanan
For more detailed information, the reader may kindly refer: Zeng et al., Super-formable pure magnesium at room temperature, NATURE COMMUNICATIONS | 8: 972 | DOI: 10.1038/s41467-017-01330-9
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