Tailored Microstructures via Thermoelectric-Magnetohydrodynamics for Additive Manufacturing (TEAM)

增材制造（AM），也称为3D打印，涉及连续添加由熔化合金粉末或电线形成的薄层新材料，并将其固化到先前的层上，以构建3D组件。这个过程直接构建复杂形状的部件不可能创建使用传统技术。此外，增材制造有望提高能源和材料效率。潜在的应用是深远的，包括生物医学、能源和航空航天。然而，AM组件可能会受到微观结构特征的影响，这些特征可能导致性能退化，例如孔隙率和外延晶粒生长。孔隙度可以由气泡夹带在凝固前沿形成，导致最终构建的空洞。外延晶粒生长发生在新晶粒采用前一层的晶体取向时，产生通常不希望的方向依赖特性。我们希望利用作用于热电（TE）电流的磁场来控制这些特征。TE效应将两种导电材料连接处的温度变化转化为电流。它们在常见应用中众所周知，例如Peltier冷却器，用于废热回收的TE发电机和热电偶。在这个提议中，我们的目标是利用热电电流和外加磁场的相互作用，在增材制造过程中形成材料的金属熔池中产生流体流动。这种相互作用被称为热电磁流体动力学，简称TEMHD。我们的可行性研究表明，TEMHD可以改变增材制造部件的微观结构，防止气孔或外延生长等微观结构特征的形成。我们将展示热电效应是AM过程中自然和固有的一部分，由于AM中遇到的巨大热梯度而形成高电流。我们将施加可控的外部磁场，使这些电流相互作用并产生驱动TEMHD流动的洛伦兹力。我们的初步数值预测表明，即使是由永磁体产生的中等磁场也足以使TEMHD主导熔池流体动力学，并且流动大小对磁场的方向和大小高度敏感。这种灵敏度将使我们能够调节热量、质量和动量输运，从而控制微观结构的演变，包括外延生长和气体夹带。我们的愿景是揭示TEMHD引入增材制造的基本机制，然后最终开发出一种途径，在工业应用中利用它来生产改进的、一致的部件材料性能。为了实现这些目标，研究人员将采用最先进的实验和数值模拟技术。该过程的高速原位同步加速器x射线摄影将为数值模型的验证产生数据，并为更广泛的科学界提供基准。数值模型将捕捉到熔池中复杂的相互作用，并提供对工作中的复杂物理机制的理解。该模型的理论预测将指导实验计划，而直接观测将指导数值模型的发展。通过验证的数值模型，将对磁场条件以及关键的AM加工条件进行参数化研究，以确定生产微结构所需的条件，从而提供每种应用所需的性能。使用TEMHD设计微结构的能力将在实验程序中进行演示。在整个项目中，我们将寻求我们的工业合作伙伴的投入，在后期阶段，我们将举行一个研讨会，以开发扩展和实施这些技术到下一代增材制造机器的转化途径。

Additive Manufacturing (AM), also termed 3D printing, involves successively adding thin layers of new material formed by melting alloy powders or wires and solidifying them onto prior layers to construct 3D components. This process directly builds intricately shaped parts impossible to create using traditional techniques. Further, AM promises to be both more energy and materials efficient. Potential applications are far reaching, including biomedical, energy and aerospace. However, AM components can suffer from microstructural features that may lead to degraded properties, such as porosity and epitaxial grain growth. Porosity can form from gas bubbles entrained in the solidification front, leading to voids in the final built. Epitaxial grain growth occurs when new grains take on the crystal orientation of the previous layer, producing often undesirable direction dependent properties. We hope to control these features using magnetic fields acting on Thermoelectric (TE) currents.TE effects translate temperature variations at the junction of two conductive materials into electric current. They are well known in common applications such as Peltier coolers, TE generators for waste heat recovery and in thermocouples. In this proposal we aim use the interaction of thermoelectric currents and applied magnetic fields to generate fluid flow in the molten pool of metal that forms material in the AM process. This interaction is called Thermoelectric Magnetohydrodynamics, or TEMHD. Our feasibility studies indicate that TEMHD can transform the microstructure in AM components, preventing the formation of microstructural features such as porosity or epitaxial growth. We will show that thermoelectric effects are a natural and inherent part of AM processes, with high currents forming due to the huge thermal gradients encountered in AM. We will apply controlled external magnetic fields, causing these currents to interact and generate a Lorentz force that drives TEMHD flow. Our preliminary numerical predictions show that even a moderate magnetic field generated by permanent magnets is sufficient for TEMHD to dominate the melt pool hydrodynamics and that the flow magnitude is highly sensitive to the orientation and magnitude of the magnetic field. This sensitivity will enable us to modulate the heat, mass and momentum transport, enabling control of microstructural evolution, including epitaxial growth and gas entrainment. Our vision is to reveal the fundamental mechanisms that TEMHD introduces to AM and to then ultimately develop a pathway to exploit it in industrial applications producing improved and consistent material properties of components.To achieve these goals the investigators will employ state-of-the-art experimental and numerical modelling techniques. High speed in situ synchrotron X-ray radiography of the process will generate data for validation of the numerical model and provide benchmarks for the wider scientific community. The numerical model will capture the complex interactions in the melt pool and provide understanding of the complex physical mechanisms at work. Theoretical predictions from the model will guide the experimental programme, while direct observations will guide the numerical model development. With a validated numerical model, a parametric study of the magnetic field conditions along with key AM processing conditions will be conducted to determine conditions required to produce microstructures that give the properties required for each application. The ability to use TEMHD to design the microstructures will be demonstrated in the experimental programme. Throughout the project we will seek input from our industrial partners, and during the latter stages we will hold a workshop to develop translational pathways for scaling and implementing these techniques to the next generation of AM machines.