The Hub’s research activities are divided into platform research that focuses on fundamental science and user-led research driven by our industrial partners.
Our platform research aims: (1) to mitigate the harmful effects of inclusions and impurity elements through positive use of inclusions to enhance heterogeneous nucleation; and (2) to effectively control crystal structure, morphology, size and distribution of the IMCs to minimise their harmful effects on the performance of Al-alloys.
The specific objectives of the platform research are: To understand atomic ordering at the liquid/inclusion interface at temperatures above the liquidus; To understand the mechanisms of heterogeneous nucleation of both primary α-Al and IMCs; To understand adsorption at the liquid/inclusion interface and its effect on heterogeneous nucleation; To understand the competition for nucleation between different inclusions during solidification; To understand the formation dynamics of IMCs during solidification of recycled alloys; To develop practical approaches to control the chemical and physical nature of IMCs.
Heterogeneous nucleation is a complex phenomenon, which is closely linked with the atomic activity at the liquid/substrate interface. Over the last 5 years the EPSRC Centre – LiME has made substantial progress in understanding heterogeneous nucleation at the atomic level using a unique combination of MD (Molecular Dynamics) simulation, novel sample preparation and high resolution electron microscopy.
Iron (Fe) in secondary Al-alloys is the most harmful impurity/tramp element due to its detrimental effect on ductility, fracture toughness, impact energy, fatigue resistance and corrosion resistance. A careful analysis of the existing data in the literature and our recent experimental results suggests that for effective control of the size, morphology and distribution of IMCs we need to focus on their nucleation rather than their growth in the traditional approach. In the Hub we will concentrate our research effort on understanding of both nucleation and growth characteristics of IMCs through a combination of modelling and new experimental approaches.
Fe in secondary Mg-alloys is also the most harmful impurity particularly because the corrosion rate of Mg-alloys is closely related to the content of Fe, as well as Ni and Cu. In addition, we have recently found that intensive melt shearing is effective for dispersing MgO, which can refine both IMCs and the a-Mg , and therefore mitigate the harmful effects of both inclusions and impurities.
Phase-field modelling is one of the most successful approaches to have emerged over the last 25 years for the simulation of microstructural evolution, including during solidification. Research on phasefield modeling in the Hub will focus on the development of models for the simulation of facetted IMCs with realistic solidification morphologies within the framework of coupled thermo-solutal solidification models specifically designed for non-equilibrium solidification.
Grain refinement requires not only potent particles for activating heterogeneous nucleation, but also a large population of particles with adequate number density, suitable size and correct size distribution to deliver effective grain refinement . In practice, other than the deliberately added grain refining particles (exogenous), there are often a number of naturally occurring particles (endogenous) present in the alloy melt. Naturally, there exists a competition for heterogeneous nucleation among such co-existing particles, and the final solidified grain size will be determined by such competition. In the Hub we will study the competition between different particles (both endogenous and exogenous) to define the basic principles for grain refinement.
Real-time radiographic and tomographic studies of solidification have become routine in recent years using 3rd generation synchrotron X-ray sources, all these studies have, however, used model binary alloys, deriving contrast from the preferential partitioning of one X-ray absorbing element (usually Cu) to the liquid. It is therefore not possible to study directly the behaviour of multiple elements, which is required to understand the formation of IMCs in real alloys. Together with colleagues at the Diamond Light Source, we have recently demonstrated for the first time the use of a combined fluorescence spectroscopy and radiographic technique to study re-melting and solidification of an Al-Cu-Ag-Zr-Mo alloy. We will also explore simultaneous use of diffraction imaging alongside fluorescence and radiography to identify the crystal structure in situ, which will be particularly useful for the study of IMCs.
Grand Challenge Research Activities
By working closely with our industrial collaborators, comprising automotive OEMs and their supply chains, we have identified jointly the following high level future needs for automotive materials and their processing technologies to enable vehicle weight reduction and hence reduced CO2 emission:
• Stronger: we need recycling-friendly automotive alloys with higher stiffness, higher strength and elevated operating temperature so that a reduced amount of material can be used for the same functions.
• Lighter: a combination of recycling-friendly high performance light alloys and sustainable advanced casting technologies for ultra-large and ultra-thin walled castings for hybrid structures and feature integration.
• Faster: development of multi-scale modelling and simulation tools, and databases, and their integration towards virtual component development.
• Lower cost: resource efficiency achieved through integration of recycling-friendly high performance materials, sustainable manufacturing technology and virtual component development.
Based on our long term vision for full metal circulation and in consideration of the above industrial needs, closed-loop recycling of light metals has been identified as our grand challenge research theme. We have identified the following generic approaches to address the grand challenge:
• Underpinning: To establish new nucleation centred solidification science to underpin closed-loop recycling.
• Enabling: To develop alloys tolerant to both inclusions and impurities to enable closed-loop recycling.
• Facilitating: To develop sustainable metal processing technologies to facilitate closed-loop recycling.
All our grand challenge research activities (GCRAs) have been co-created with our industrial partners. With the exception of two GCRAs that work on melt treatment processes generic to all solidification processing technologies and relevant to all industrial partners, our GCRAs are specific to industrial partners’ needs.