Processing of Wrought Magnesium Alloys by a Rheoforming Approach

镁（Mg）是世界上第八丰富的元素，占地壳的2.7%。镁合金和木材一样轻，但和铝合金一样坚固。由于环境问题的日益严重和政府法规的日益严格，镁合金在汽车工业中获得了广泛的应用，用于减轻车辆重量以减少燃料消耗和CO2排放。自1993年以来，镁在汽车工业中的应用每年以15%的速度增长，据预测，这种增长趋势将在21世纪世纪的前十年继续下去。然而，镁作为一个行业仍处于起步阶段，非常不发达，特别是在锻造产品领域。加工变形镁合金的技术是直接从铝工业复制的，几乎没有修改，已经证明是不合适的。目前镁在汽车工业中的应用几乎是100%的铸造部件，而锻造产品没有任何显着的贡献，这是最显着减轻重量的关键。变形镁合金在汽车上应用的主要障碍是变形性能差、生产率低和成本高。本项目旨在开发流变成形技术，包括用于坯料或板坯的直接激冷（DC）流变铸造、用于挤压型材的流变挤压和用于平板产品的双辊流变铸造。在流变成形工艺中，半成品锻造镁产品在半固体状态下成形，有点类似于挤压牙膏。此外，流变成形的产品具有精细和均匀的微观结构，因此可以通过常规的固体变形技术进一步加工，因为细晶粒镁合金可以通过替代的变形机制塑性变形。流变成形技术与传统技术相结合，为镁工业提供了一个完整的半成品锻造镁产品的解决方案。具体的研究活动包括流变成形工艺和设备的开发，工业规模的流变成形工艺的演示，了解流变成形过程中的流体流动和凝固行为，流变成形镁合金的机械性能评估和变形性能评估，流变成形镁合金的生产率和生产成本。流变成形技术的意义可以从以下几个方面来理解。从技术上讲，流变成形工艺代表了轻型汽车零部件制造技术的一个阶段性变化。流变成形为镁工业提供了一个完整的技术解决方案，并为新的应用提供了许多机会。在科学上，强对流条件下的凝固为凝固研究开辟了一个新的维度。合金凝固的控制传统上是通过化学手段（即，通过改变合金成分）。流变成形过程已经成功地证明，通过施加强烈的剪切应力-应变场，可以有效地控制成核和晶体生长。这将允许一个理论框架的发展，凝固下施加的物理场，预计将有深远的影响，未来的凝固研究和技术发展。在商业上，流变成形为英国工业在全球市场上提供了技术优势，提高了产品质量和降低了成本。预计英国经济的潜在利益将是巨大的。

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.