Materials Science Forum Vols. 546-549

Abstract: In view of the increasing needs for efficient usage of natural resources and environmental protection in our modern society, weight reduction in transportation such as cars, trains or aircrafts is of fundamental interest. In order to solve this major issue, improved concepts are necessary which also emphasize the usage of light weight materials in construction. Especially magnesium and its alloys as the lightest available constructional metals have a major potential in this regard. Since magnesium cast components have found their application, interest is now spreading towards wrought alloys for use as structural components. However, the use of wrought magnesium alloys in the transportation industry is still limited at present. In this paper we give an overview on the present state of the art as well as on specific requirements for the processing of wrought magnesium alloys. We will show the technical potential in terms of improved economic aspects for wrought magnesium and discuss research topics such as process-specific alloy design.

Abstract: Expanding world economic prosperity and probable peaking of conventional petroleum production in the coming decades requires efforts to increase the efficiency of, and the development of alternatives to, petroleum-based fuels used in automotive transportation. North America has been aggressively pursuing both approaches for over ten years. Mainly as a result of lower prices due to global sourcing, magnesium has recently emerged as a serious candidate for lightweighting, and thus increasing the fuel efficiency of, automotive transportation. Automotive vehicles produced in North America currently use more Mg than vehicles produced elsewhere in the world, but the amounts per vehicle are very small in comparison to other materials such as steel, aluminum and plastics. The reasons, besides price, are primarily a less-developed state of technology for Mg in automotive transportation applications and lack of familiarity by the vehicle manufacturers with the material. This paper reviews some publicly-known, recent, present and future North American research and development activities in Mg for automotive applications.

Abstract: In order for magnesium to continue to grow, all forms of magnesium scrap need to be recycled – for both economic and environmental reasons. This paper extensively reviews the techniques for the refining and recycling of magnesium. Removal of inter-metallic elements and inclusions through gas fluxing, filtration and other methods are summarized. Flux refining technology is the old but most common method to purify magnesium metal. Fluxless refining technologies are under development, such as salt furnace technology, inert gas and filter refining, vacuum distillation refining, and hydrometallurgical methods. Magnesium scraps recycling is discussed, techniques to protecting Mg (or Mg alloy) melt with protective atmosphere and salt flux are briefly introduced.

Abstract: This paper reviews the latest research and development status of processing technologies for wrought magnesium alloys including extrusion, forging, (twin-roll) thin strip casting, rolling, stamping, and superplastic forming. Existing problems and the development trends of these processing technologies are analyzed and discussed. Up to the present, the R&D on the processing technologies of wrought magnesium alloys has made great global progress. Nevertheless there is a long way to achieve large-scale industrialization and application of wrought magnesium alloys.

Abstract: The CAST Cooperative Research Centre was established in 1993 as a joint venture between industry, research and government partners to carry out research and development in the area of light metals. Over the thirteen years since its formation CAST has developed a world class research program and, in addition to the significant application of outcomes by our industry partners, much of our research is being commercialised. In addition, CAST has developed a significant education and training program, a best practice technology transfer program for small and medium enterprises, consulting and design activities and established an alliance with Advanced Magnesium Technologies as a provider of their research and development needs. The development of CAST has been achieved by cooperation between all our partners who represent most of the light metals research groups and a significant proportion of light metals industries in Australia. The close involvement of the industry partners in all aspects of the operation of CAST has ensured we have a program of activities focused on delivering benefits to Australia’s light metals industry. This paper presents an overview of the magnesium research undertaken by CAST and other research groups such as the Centre of Excellence in Design of Light Metals and the CSIRO including their Light Metals Flagship program. Some highlights are new magnesium powertrain alloys, an alloy suitable for decorative and domestic applications called AM-lite, and new CSIRO casting technologies T-Mag and a twin roll strip casting process.

Abstract: In Mg-Al-Zn and Mg-Al-Mn alloys containing 2.0~6.0mass%Al and 0~1.5mass%Zn, grain refinement in the as-rolled (F) specimens containing large amount of Al and Zn are achieved by both dynamic recrystallization and dynamic precipitation during hot rolling and leads to high strength and high ductility at room temperature. At high temperatures, the tensile strength of the investigated alloys is almost the same, while the elongation of the F-specimens increases with increasing Al and Zn contents, leading to 150% in Mg-4.5%Al-1.5%Zn alloy. High Al and Zn contents alloys significantly accumulate large working strain in grain interiors, and involve large amounts of high angle grain boundaries and fine spherical precipitates, which can become the nucleation sites for recrystallization. Therefore, dynamic recrystallization in such alloys occurs at small strain region during tensile test. This dynamic recrystallization causes reduction of flow stress and large elongation by grain boundary sliding at high temperatures. Furthermore, .fine recrystallized grains contributes to deformation in normal direction, resulting in isotropic deformation behavior. Authors attempt to improve proof stress and its anisotropic property of Mg-Al-Zn wrought alloys by grain size and precipitates controls utilizing dynamic recrystallization and dynamic precipitation during hot extrusion. In the alloy specimens extruded at lower temperatures increasing Al and Zn contents enhance dynamic recrystallization and dynamic precipitation, resulting in grain refinement and large amount of Mg17Al12 precipitates. As a result, the extruded Mg-9%Al-1%Zn alloy specimen shows high tensile strength of 370MPa, 0.2% tensile proof stress of 240MPa and moderate elongation of 20%, which are almost same as standard values of tensile properties of T5-treated 6N01 Al extruded alloy. Furthermore, a ratio of compressive proof stress to tensile proof stress of the as-extruded specimen improves up to a higher ratio of 0.9 than that of Mg-3%Al-1%Zn alloy specimen with no precipitation, 0.5, due to prevention of tensile twin, which easily occurs during compressive deformation even under a low applied stress perpendicular to the extrusion direction, by dynamic precipitation of Mg17Al12 phase.

Abstract: Mechanical properties of AZ91 cast alloy depend strongly on the morphology (size and distribution) of the second (β-Mg17Al12) phase. It was observed that low ductility of AZ91 alloy was attributed to the brittle nature of the β phase particles at which microcracks initiated. These microcracks then coalesced contributing to the fracture of alloy. Quantitative study on microcracking progress revealed that cast samples with coarse microstructures fractured at low strain due to the non-uniform distribution of bulk blocky β particles at interdendrite region. These fracture surfaces exhibited clear cleavage mode. Fine cast microstructure presented quasicleavage fracture mode with clear dimple and tear ridges. The partial melting (and resolidification) heat treatment improved tensile properties, which was in disagreement with the available data from literature.

Abstract: The development of new Mg-Sn based alloys that have shown improved corrosion resistance needs further investigations to develop standard alloys for industrial use. In the present study, the influence of heat treatment was investigated by examining the creep resistance of Mg-Sn and Mg-Sn-Ca alloys at 85 MPa under 135 °C identify the best conditions to improve the mechanical properties of such alloys. Additionally, the changes in microstructure of these alloys were studied and analysed by light microscopy, X-ray diffraction and scanning electron microscopy. It was found that the heat treatments can affect the microstructure of the binary alloy while no apparent change in the microstructure was found in the Mg-Sn-Ca alloys, indicating that the second phase CaMgSn is thermal stable in this alloy. Based on the obtained results, the relationship between the microstructure and creep behaviour are discussed.