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

我们每天都依赖于金属物体，从自行车和桥梁到电子产品中的焊点。在每种情况下，制造的关键步骤是液态合金的凝固，并且通过控制凝固，我们可以控制晶粒结构和缺陷。固化是英国目前面临的挑战的核心：钢铁和铝生产对全球工业二氧化碳排放的贡献超过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，技术进步将涉及控制凝固微观结构和控制凝固缺陷。焊点和结构铸件中晶粒结构发展的关键是凝固的最早阶段，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，结合联合收割机聚焦离子束，以微加工进入晶体中心，并通过电子衍射找到成核剂颗粒，以了解颗粒如何催化成核。有了这些信息，我们将探索控制形核的新方法。形核后，半固态晶粒结构继续显著影响铸件和焊点中缺陷的形成。应对这一挑战的一部分是更深入地了解铸造缺陷的形成方式和原因。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大量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，我们可以看到它们具有低粘度，并且粘度随着合金凝固而显著增加，但我们无法看到或测量是什么结构变化导致这些变化的流动特性。X射线可以透过金属，提供了观察微观结构发展的潜力，但直到最近十年，X射线源才具有足够的通量和相干性，可以对合金中的晶体生长进行实时成像。这是一个巨大的进步，因为可以使用真实的金属样品来检验“事后”研究中发展起来的凝固理论。该项目将把这些同步加速器技术扩展到观察和测量焊点中金属间化合物晶粒的凝固，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成核的位置以及它们在焊料反应过程中如何生长。这将为我们提供一些见解，我们可以利用这些见解来设计焊点微观结构，并解决向无铅焊接过渡的最终前沿问题，例如在T> 180 C下使用的高铅焊料的替代品。类似的技术将被应用于在类似于钢的连续铸造的实验中对柱间裂纹的形成进行成像，该过程每年生产超过10亿吨钢。这部分研究的一个令人兴奋的方面是，关于半固态合金变形的许多知识是未知的：力是如何从晶体传递到晶体的？当两块水晶相互挤压时会发生什么？它们会弯曲吗？它们会碎裂吗？它们是否表现为刚体？为什么会产生应变不稳定性？裂缝从哪里开始开始，它们生长的速度有多快？这些问题只能通过在微观结构尺度上对变形的原位观察来完全回答。我们已经开始在试点研究中解决这些问题，现在我们的目标是扩大这一点，以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.