Processing of Wrought Magnesium Alloys by a Rheoforming Approach

Magnesium (Mg) is the 8th most plentiful element in the world, comprising 2.7% of the earth's crust. Mg alloys are as light as wood, but as strong as Al-alloys. Due to the increasing environmental concerns and tightening government regulations, Mg alloys find extensive applications in the automotive industry for vehicle weight reduction to reduce fuel consumption and CO2 emissions. We have seen a 15% annual growth rate in Mg applications in the automobile industry since 1993, and it is predicted that this growth trend will continue in the first decade of the 21st century. However, Mg as an industry is still in its infancy and very much underdeveloped, especially in the sector of wrought products. Technologies for processing wrought Mg alloys are copied directly from the Al industry with little modifications and have proven to be unsuitable. The current applications of Mg in the automotive industry constitute almost 100% cast components without any significant contribution from the wrought products, which hold the key to most significant weight saving. The main barrier to the penetration of the wrought Mg alloys into the motor vehicle is their poor deformability, low productivity and high cost. Therefore, it is crucial to develop alternative processing technologies to overcome such problems.The proposed project aims to develop the rheoforming technologies, which include direct chill (DC) rheocasting for billets or slabs, rheoextrusion for extruded profiles and twin-roll rheocasting for flat products. In the rheoforming processes, semi-finished wrought Mg products are shaped in the semisolid state, somewhat similar to squeezing toothpaste. In addition, the rheoformed products have a fine and uniform microstructure, and thus can be further processed by the conventional solid deformation techniques, because fine-grained Mg-alloys can deform plastically through alternative deformation mechanisms. The rheoforming technologies in combination with the conventional technologies offer the Mg industry a complete solution to the semi-finished wrought Mg products. The specific research activities include development of the rheoforming process and equipment, demonstration of the rheoforming processes at industrial scale, understanding the fluid flow and solidification behaviour during rheoforming, evaluation of the mechanical properties of the rheoformed Mg alloys and evaluation of the deformability, productivity and production cost of the rheoformed Mg-alloys.The significance of the rheoforming technologies can be understood from the following aspects. Technologically, the rheoforming processes represent a step change in the manufacturing technology for production of lightweight automotive components. Rheoforming offers a complete technological solution to the magnesium industry and provides numerous opportunities for new applications. Scientifically, solidification under intensive forced convection opens a new dimension for solidification research. Control of alloy solidification has been traditionally achieved by chemical means (i.e., through variation of alloy composition). The rheoforming processes have successfully demonstrated that both nucleation and crystal growth can be effectively controlled by application of an intensive shear stress-strain field. This will allow the development of a theoretical framework for solidification under externally applied physical fields, which is anticipated to have profound implications to future solidification research and technological development. Commercially, rheoforming offers to the UK industry competitive edge in the global market in terms of technology advantage, improved product quality and reduced cost. The potential benefit to the UK economy is expected to be huge.