A soil and magma mechanics approach to understanding defects in cast metals manufacturing

We rely on metallic components every day, from cars and bridges to the solder joints in our electronics. In almost all cases, a step in the manufacture of these components is the solidification of liquid alloy, and it is during solidification that most defects arise. We are all familiar with ice expanding as it solidifies, making ice float on lakes and causing water-filled crevices in rocks and roads to crack open. Most metals do not expand on solidification, but shrink. If this shrinkage is not fed by liquid from elsewhere, a variety of defects can form: the outer surface of the casting can be deformed inwards, pores can grow in the liquid, or cracks can propagate along liquid films between grains, pulling the metal apart. In order to produce metals with fewer defects at a competitive cost, predictive models of defect formation in casting are required. To develop accurate models, we first need a better understanding of the fundamentals of deformation in semi-solid alloys.

It has recently been found that solidifying metals share striking similarities to the soils that support our buildings and the partially-molten rock in the earth. In their semi-solid state, metals are made up of numerous crystals (solid particles) surrounded by liquid. Just as is the case of sand grains, these particles have been shown to move around each other when the material as a whole deforms, and the particles rotate and transmit forces between each other. An exciting aspect of this discovery is that the answers to solving metal-casting defects may not lie in the metallurgy section of the library but in the Civil Engineering and Earth Sciences sections. Indeed, models already exist for the deformation of soils and are widely used in Civil Engineering. However, the analogy between metals and soils has only been proven in small-scale experiments carried out to observe the individual particles in semi-solid metals. In the proposed research, we seek to conduct experiments inspired by soil and rock mechanics that will produce results suitable for testing whether the framework at the heart of soil mechanics theory can describe the deformation of semi-solid alloys.

We aim to fit semi-solid alloy deformation into an over-arching framework for soils, magmas and metals. We will test scientifically whether semi-solid metals meet rules for behaviour specified within Critical State Soil Mechanics theory, developed in the UK in the 1960s. Three main hypotheses must be demonstrated:
1. That the mechanical behaviour depends on the initial packing-density of the crystals: a densely packed material should experience a reduction in packing-density (dilation) when a shearing deformation is applied. The opposite effect (contraction) should be experienced in a loosely packed material.
2. That the peak shear-stress that the material can resist depends on the overall-pressure acting on it.
3. That there is some combination of crystal shape, packing-density and confining pressure where the material can deform without any overall change in packing-density.

To achieve this goal, we will combine experimental approaches from soil, magma and metals research. We will use apparatus developed to study partially-liquid rock (magma) to obtain data on deforming semi-solid aluminium alloys at more than 500C. Next, to ensure the correct microscopic interpretation of the measurements, we will directly observe crystals within a semi-solid alloy as it is being deformed in a small-scale two-dimensional experiment using X-ray imaging in Japan. We will then develop an equivalent particle-scale computer model, based on soil mechanics, of the X-ray experiments to explore the forces acting at crystal-crystal contacts. When combined, the results from the experiments and modelling should enable us to put forward a new idea for the modelling of semi-solid metals.

Academic impact:
The critical state theory of mush mechanics developed here would be useful to modellers of most casting processes and could form a component of future models of defect formation in castings.
The project will draw together the currently distinct fields of soil mechanics, magma mechanics and metal casting through direct engagement with each field, speeding-up academic progress in each of these fields.
A one-day workshop on "deformation of sand, magma and mushy-metal" will increase the international research effort in mush mechanics and build momentum in multi-disciplinary research between the three disciplines.
Academic impact will be maximised through publications and presentations to each of the three fields related to the proposal. Example international peer-reviewed journals include Granular Matter, Acta Materialia, Proceedings of the Royal Society etc.

Economic impact:
Metal manufacturing is of strategic importance to the UK as identified by industrial and government agencies. An improved ability to understand and predict casting defects would impact on major UK exporters (Jaguar-Land Rover, TATA steel, Rolls Royce etc.) and their supply chains, helping them to compete into the future.
The multi-disciplinary approach to metal casting will help us encourage students to specialise in metallurgy and rebuild the research base in an economically important area to the UK.
For broader dissemination to industry, research summaries will be published in industrial trade magazines such as "Foundry Trade Journal" and "Die Casting Engineer".

Societal impact:
Research on the analogy between metal casting, soil behaviour and volcanic eruptions is of general interest and has the potential to attract young people to science and engineering. To maximise this impact, we will target "popular science" presentations at venues such as the Royal Institution and "Friends of Imperial College", and incorporate our research findings into outreach activities at schools.