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

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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.