Pedal to the metal - light metals workshop report
Ledetta Asfa-Wossen reports on the progress of the UK’s two leading light metals projects and the quest for sustainable metallurgy solutions at the recent Light Metals Workshop held at Brunel University, London, UK.
Applications for light alloys within the transport sector are set to double in the next decade in a race to reach reduced emission targets. However, the cost of materials and the associated manufacturing processes are key inhibitors of weight reduction.
Full metal circulation was top of the agenda at the workshop, as speakers discussed the need to transform metallurgy into a sector that is not only preoccupied with mining and primary metals production but also reuse, recycling and remanufacturing.
Further weight savings can be made by creating stronger, more efficiently designed alloys that combine the best attributes of advanced aluminium and magnesium alloys with composites, laminates, and cheaper steel products in multi-material structures. The workshop discussed ways in which this could be achieved, such as better control of cast microstructures, improving the tolerance of secondary alloys to impurity elements and taking advantage of normally detrimental inclusions, all of which can be achieved through improved liquid metal engineering.
The two EPSRC-funded projects that have been leading the way are the Centre for Innovative Manufacturing Liquid Metal Engineering (LIME) at Brunel University and the Light Alloys Towards Environmentally Sustainable Transport 2nd Generation Solutions for Advanced Metallic Systems at the University of Manchester (LATEST2). Here is a review of some recent LIME projects in metallurgy processing conducted at Brunel University:
1 Melt- conditioned direct chill casting technology
A ﬁne, uniform microstructure is always desirable for aluminum billets produced by direct-chill (DC) casting. This is usually achieved using a chemical grain reﬁner during DC casting, but scientists have demonstrated a physical approach that is chemical-free. Intensive melt shearing is applied to DC casting by submerging a rotor stator-based high shear device in the sump of a conventional DC caster. It has been shown that DC casting with intensive melt shearing can achieve one order of magnitude grain size reduction. Such ﬁne and equiaxed grain structure in the Al billets can enhance heterogeneous nucleation and growth under reduced temperature conditions.
2 Twin roll casting of magnesium alloys
BCAST researchers have devised a melt-conditioned TRC (MC-TRC) process for magnesium alloys that eliminates scalping, hot rolling and the heating cycle to produce magnesium alloy strips for direct stamping into engineering components. The group claims the method can achieve alloys with superior mechanical properties at elevated temperatures and a uniform grain structure.
While LIME has been looking at energy-efficient processes for the metallurgy industry, LATEST2 is heading work into forming, joining and surface engineering technologies for light alloys used in naval, automotive and aerospace structures. A key challenge is that dissimilar material combinations required by industry, for example Al-steel, Mg-Al, metals, metal-polymer laminates and Ti-composites, are very difficult to join by traditional resistance or other fusion welding methods. The aim has therefore been to create new low-energy friction joining processes to facilitate metal combinations and composite to metal joining.
Dr Joseph Robson of University of Manchester explained, ‘Magnesium alloys deform in a very different way. It is difficult to deform due to its HCP structure. We need to better understand the link between microstructure, strength and texture to improve forming performance. Rare earth elements are expensive, so understanding formation of wrought magnesium alloys, is critical to increase formability and reduce anisotropy and asymmetry.’
Another research area has been in the control or suppression of twinning — largely responsible for mechanical asymmetry. A research group has also been looking at the development of corrosion-resistant finishes to tolerate high levels of impurities in recycled alloys. Other projects include corrosion-resistant finishes for more aggressive environments and an alkali-resistant finish for the automotive sector.
LATEST 2: Under the lens
To understand the formability of different light alloys, scientists are assessing the degree of anisotropy in a material by measuring r-values and building forming limit diagrams (FLDs). In the past, these have been measured by measuring the deformation on grids etched on the surface. However, these methods provide limited spatial resolution of deformation during straining, which often provides valuable clues. Using digital image correlation it is possible to make time-resolved, in situ measurements in 2D and 3D. The technique can be used to measure deformation development at different scales to analyse how microstructural features affect local deformation. Two sets of images of the surface of the specimen are recorded at pre-determined time intervals during testing, using two digital high-speed cameras. These images are then calibrated and cross-correlated to give deformation data and their distribution and inhomogeneity.
For more information on other research discussed at the event, visit www.lime.ac.uk/news/LMTWorkshop2013.aspx