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

我们每天都依赖金属物体，从自行车和桥梁到电子产品的焊点。在每种情况下，制造的关键步骤都是液态合金的凝固，通过控制凝固，我们可以控制晶粒结构和缺陷。固化是英国当前面临的核心挑战：钢铁和铝生产对全球工业二氧化碳排放量的贡献超过10%，需要新的焊接技术来制造更小，更强大的便携式电子产品。在所有这些行业中，进步将涉及控制凝固组织和控制凝固缺陷。焊点和组织铸件晶粒组织发展的关键是凝固的最初阶段，此时晶粒的数量密度由形核事件的数量密度决定。该项目将使用新的显微镜技术，将离子束聚焦到晶体中心的微型机器上，并通过电子衍射找到成核粒子，以了解粒子如何催化成核。有了这些信息，将探索控制成核的新方法。在形核后，半固态晶粒组织继续显著影响铸件和焊点缺陷的形成。解决这一挑战的一部分是深入了解铸造缺陷是如何以及为什么形成的。众所周知，半固态裂纹的起源是凝固过程中产生的应力和应变，但为了了解细节，我们需要观察和测量凝固过程中无数凝固晶体对载荷的响应。金属和合金对可见光是不透明的，因此它们的内部结构对我们的眼睛是隐藏的。通过浇注液态合金，我们可以看到它们的粘度很低，而且随着合金凝固，粘度会显著增加，但我们无法看到或测量是什么结构变化导致了这些流动特性的变化。X射线可以穿透金属，提供了观察微观结构发展的潜力，但直到最近十年，X射线源才有足够的通量和相干性，可以实时成像合金中的晶体生长。这是一个巨大的进步，因为它可以使用真实的金属样品来测试“死后”研究中发展的凝固理论。本项目将扩展这些同步加速器技术，观察和测量焊点金属间晶粒的凝固，并研究半固态晶粒结构的变形如何导致铸造缺陷的形成。我们的目标是首次观察和测量金属间化合物在焊点中的成核位置以及它们在焊接反应中如何生长。这将为我们提供可用于设计焊点微结构的见解，并解决向无铅焊接过渡的最终前沿问题，例如替换用于T bb0 - 180C的高铅焊料。类似的技术将应用于在类似于连续铸钢的实验中成像柱间裂纹的形成，连续铸钢是每年生产超过10亿吨钢的一种工艺。这部分研究的一个令人兴奋的方面是，半固态合金变形的很多方面是未知的：力是如何从晶体传递到晶体的？当两个晶体相互挤压时会发生什么？它们会弯曲吗？它们会分裂吗？它们是刚体吗？应变不稳定性为什么会发展？裂缝从哪里开始，发展有多快？这些问题只能通过在微观结构尺度上的变形现场观察来完全回答。我们已经开始在试点研究中解决这些问题，现在我们的目标是将其扩展到破解糊状物中的运动。

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.