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

增材制造（AM），也称为3D打印，涉及连续添加通过熔化合金粉末或线材形成的新材料薄层，并将其固化到先前的层上以构建3D组件。这个过程直接构建了使用传统技术无法创建的复杂形状的零件。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部件可能遭受可能导致性能退化的微观结构特征，例如孔隙率和外延晶粒生长。孔隙可以由凝固前沿中夹带的气泡形成，导致最终构建中的空隙。当新的晶粒呈现前一层的晶体取向时，发生外延晶粒生长，产生通常不期望的方向依赖性质。我们希望利用磁场作用于热电（TE）电流来控制这些特性。TE效应将两种导电材料交界处的温度变化转化为电流。它们在常见应用中是众所周知的，例如Peltier冷却器、用于废热回收的TE发电机和热电偶。在该提案中，我们的目标是使用热电流和施加的磁场的相互作用，以在AM工艺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，或TEMHD。我们的可行性研究表明，TEMHD可以改变AM组件的微观结构，防止形成微观结构特征，如孔隙率或外延生长。我们将表明，热电效应是AM过程的自然和固有的一部分，由于AM中遇到的巨大的热梯度而形成高电流。我们将施加受控的外部磁场，使这些电流相互作用并产生驱动TEMHD流的洛伦兹力。我们的初步数值预测表明，即使是一个温和的磁场所产生的永磁体是足够的TEMHD占主导地位的熔池流体力学和流量大小是高度敏感的磁场的方向和大小。这种灵敏度将使我们能够调节热量，质量和动量传输，使微观结构的演变，包括外延生长和气体夹带的控制。我们的愿景是揭示TEMHD引入AM的基本机制，并最终开发出一条在工业应用中利用其生产改进和一致的材料性能的组件的途径。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射线照相术将产生用于验证数值模型的数据，并为更广泛的科学界提供基准。数值模型将捕捉熔池中复杂的相互作用，并提供对工作中复杂物理机制的理解。模型的理论预测将指导实验方案，而直接观测将指导数值模型的开发。利用经过验证的数值模型，将对磁场条件沿着关键AM加工条件进行参数化研究，以确定生产提供每种应用所需性能的微结构所需的条件。使用TEMHD设计微结构的能力将在实验方案中得到证明。在整个项目过程中，我们将寻求工业合作伙伴的投入，在后期阶段，我们将举办一个研讨会，以开发将这些技术扩展和实施到下一代AM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.