Engineering Fellowships for Growth: Solidification Processing of Alloys for Sustainable Manufacturing

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We rely on metallic objects every day, from bicycles and bridges to the solder joints in our electronics. In each case, a key step in manufacturing is the solidification of liquid alloy, and it is through controlling solidification that we can control grain structure and defects. Solidification is at the heart of current challenges facing the UK: steel and aluminium production contributes more than 10% to global industrial CO2 emissions, and new solder technologies are required to enable the manufacturing of smaller, more powerful portable electronics. In all these industries, advances will involve controlling the solidification microstructure and controlling solidification defects.Key to the development of grain structure in solder joints and structural castings are the earliest stages of solidification when the number-density of grains is determined by the number density of nucleation events. The project will use new microscopy techniques which combine focussing an ion beam to micro-machine into the centre of crystals and find nucleant particles with electron diffraction to understand how the particles catalyse nucleation. With this information, new ways to control nucleation will be explored.After nucleation, the semi-solid grain structure goes on to significantly affect the formation of defects in castings and solder joints. Part of tackling this challenge is to develop a deeper understanding of how and why casting defects form. It is known that the origin of semi-solid cracking is the stresses and strains that develop during solidification but, to understand the details, we need to observe and measure how numerous solidifying crystals respond to loads during solidification. Metals and alloys are opaque to visible light and their inner structure is therefore hidden from our eyes. By pouring liquid alloy, we can see that they have a low viscosity and that the viscosity increases considerably as alloys solidify, but we cannot see or measure what structural changes are causing these changing flow properties. X-rays can be transmitted through metals, offering the potential to observe the development of microstructure, but it is only in the last decade that X ray sources have become available with sufficient flux and coherence to allow real-time imaging of crystal growth in alloys. This was an enormous step forward as it became possible to test solidification theories developed in 'post-mortem' studies using real metallic samples.This project will extend these synchrotron techniques to observe and measure the solidification of intermetallic grains in solder joints, and to study how deformation of the semi-solid grain structure leads to casting defect formation. We aim to observe and measure for the first time where intermetallics nucleate in solder joints and how they grow during solder reactions. This will give us insights that we can use to engineer solder joint microstructures and tackle the final frontiers in the transition to Pb-free soldering such as a replacement for high-Pb solder for use at T>180C.Similar techniques will be applied to imaging the formation of inter-columnar cracking in experiments analogous to the continuous casting of steel, a process used to produce more than one billion tonnes of steel annually. An exciting aspect of this part of the research is that much about semi-solid alloy deformation is unknown: How is force transmitted from crystal to crystal? What happens when two crystals are pushed into one another? Do they bend? Do they fragment? Do they behave as rigid bodies? Why do strain instabilities develop? Where do cracks begin and how fast do they grow? These questions can only be fully answered with in-situ observations of deformation at the scale of the microstructure. We have begun to address these questions in pilot studies and now we aim to expand this to crack movement in the mush.