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

我们每天都依赖于金属部件，从汽车、桥梁到电子产品中的焊点。在几乎所有的情况下，制造这些部件的一个步骤是液态合金的凝固，而大多数缺陷正是在凝固过程中产生的。我们都熟悉冰在凝固时膨胀，使冰漂浮在湖面上，并导致岩石和道路上充满水的裂缝破裂。大多数金属在凝固时不会膨胀，而是会收缩。如果这种收缩不是来自其他地方的液体提供的，可能会形成各种缺陷：铸件外表面可能向内变形，气孔可能在液体中生长，或者裂纹可能会沿着颗粒之间的液膜扩展，从而将金属拉开。为了以具有竞争力的成本生产缺陷较少的金属，需要建立铸造缺陷形成的预测模型。为了开发准确的模型，我们首先需要更好地了解半固态合金中变形的基本原理。最近发现，凝固的金属与支撑我们建筑物的土壤和地球上部分熔融的岩石有惊人的相似之处。在半固态状态下，金属由许多晶体(固体颗粒)组成，周围环绕着液体。就像沙粒的情况一样，当材料作为一个整体变形时，这些颗粒被证明相互围绕着移动，这些颗粒相互旋转并传递力。这一发现的一个令人兴奋的方面是，解决金属铸造缺陷的答案可能不在图书馆的冶金部分，而在土木工程和地球科学部分。事实上，土的变形模型已经存在，并在土木工程中得到广泛应用。然而，金属和土壤之间的类比只有在观察半固态金属中的单个颗粒的小规模实验中才得到证实。在拟议的研究中，我们寻求进行受土壤和岩石力学启发的实验，这些实验将产生适合于测试土力学理论核心框架是否能够描述半固态合金变形的结果。我们的目标是将半固态合金变形适合于土壤、岩浆和金属的综合框架。我们将科学地测试半固态金属是否符合英国在20世纪60年代发展起来的临界状态土壤力学理论中规定的行为规则。必须证明三个主要假设：1.力学行为取决于晶体的初始堆积密度：当施加剪切变形时，密集堆积的材料应该经历堆积密度(膨胀)的减小。相反的效果(收缩)应该在松散包装的材料中经历。材料所能承受的峰值剪应力取决于作用在其上的总压力。在晶体形状、堆积密度和围压的某种组合中，材料可以在不改变堆积密度的情况下变形。为了实现这一目标，我们将结合土壤、岩浆和金属研究的实验方法。我们将使用为研究部分液态岩石(岩浆)而开发的仪器来获得在500℃以上变形半固态铝合金的数据。接下来，为了确保对测量结果的正确微观解释，我们将使用日本的X射线成像技术，在小型二维实验中直接观察半固态合金中的晶体。然后，我们将开发一个基于土力学的等效粒子尺度计算机模型，用于探索作用在晶体-晶体接触处的力。将实验和模拟的结果结合起来，应该能使我们对半固态金属的模拟提出一个新的想法。

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.