Collaborative Research: Formation and Stability of Eutectic Nanostructures in Laser-Irradiated Particle Suspensions

粉末冶金是一种通过将金属粉末熔合在一起来制造复杂零件的技术，在美国具有很大的经济影响力。随着增材制造（AM）技术的普及，这一足迹只会增长。增材制造的一个紧迫问题是，生产的零件通常比锻造或机加工生产的零件具有更差的机械性能。这项合作提案的目标是开发新的方法，在粉末颗粒中创建纳米级物理结构，以提高最终零件的机械性能（强度和导热性）。为了实现这一目标，这里提出的研究将使用激光熔化不同的金属和陶瓷合金，这些合金具有特殊的化学成分，称为共晶，可以产生内部片状结构。当粉末处于快速吸收热量的流体中时，通过激光照射，当颗粒凝固时，可以保留共晶结构。这项工作将粉末颗粒悬浮在液体和固体介质中，这些介质可以除去热量，但这些介质不能发生反应，沸腾或遮挡激光。实验将与建模相结合，以了解热量的去除，并模拟在这些条件下单个球形颗粒的内部结构是如何形成的。这项研究的更广泛影响将是创造用于烧结和增材制造的定制原料的新加工方法，以快速制造具有改进技术性能的复杂部件。这将对航空航天、汽车、医疗产品和国防工业产生重大的直接效益。脉冲激光熔化可以提供非常快的凝固速度，形成纳米级共晶结构。不幸的是，在具有高光吸收的材料中，纳秒脉冲宽度的激光只熔化近表面区域，不能被认为是真正的体加工技术。本研究将利用激光照射粉末形式的共晶合金来克服这一限制。对离散颗粒中共晶凝固的大多数研究都使用了通过雾化形成的熔融液滴，其中从颗粒中向周围气体中提取热量的效率很低。虽然快速共晶凝固仍然发生，但雾化导致内部组织高度不均匀，这对于许多技术性能来说是不希望的。这项研究将使用悬浮在液体或固体介质中的颗粒，这些颗粒可以在脉冲激光熔化后更有效地提取热量。悬浮液的体积将受到限制，从而抑制与熔融颗粒接触的介质的沸腾，从而保持有效的散热。此外，通过控制激光功率和脉冲数，可以部分熔化粒子，使小的固体部分充当现成的成核位置，避免深度过冷。实验将在金属、半导体和金属氧化物合金粉末中检验这些过程。本文将采用相场建模方法对体块和三维颗粒的动态快速凝固过程进行详细的模拟。在火花等离子烧结生产的颗粒材料的纳米级共晶结构的稳定性也将被检查。这项研究的广泛影响将是新的加工方法，可以在烧结和增材制造中使用的原料材料中创建定制的微观结构，从而提高快速制造部件的技术性能。这将对航空航天、汽车、医疗产品和国防工业产生重大的直接效益。

Powder metallurgy, in which complex parts are made by fusing together a metal powder, has a large economic footprint in the USA. This footprint will only grow as additive manufacturing (AM) techniques become more pervasive. A pressing concern with AM is that the parts produced often have worse mechanical properties than parts produced from forging or machining. The goal of this collaborative proposal is to develop new approaches to create nanoscale physical structure within powder particles in order to improve mechanical properties (strength and thermal conductivity) of final parts. To accomplish this, the research proposed here will use lasers to melt different metal and ceramic alloys that have a special chemical composition, known as a eutectic, which produces an internal sheet-like structure. By lasing the powders while they are in a fluid that extracts heat quickly, the eutectic structure can be preserved when the particle solidifies. This work will suspend the powder particles in liquid and solid media that can remove heat, but these media must not react, boil, or obscure the laser. Experiments will be combined with modeling to understand the removal of heat, and to simulate how the internal structure forms within individual spherical particles under these conditions. The Broader Impact of this research will be the new processing approaches that create tailored feedstock materials for use in sintering and additive manufacturing to rapidly manufacture complex pieces with improved technical properties. This will have significant direct benefits for the aerospace, automotive, medical products, and defense industries.  Pulsed laser melting can provide very fast solidification rates to form nanoscale eutectic structures. Unfortunately, in materials with high optical absorption, lasers with nanosecond pulse widths only melt the near-surface region and cannot be considered true bulk processing techniques. This research will circumvent this limitation by using laser irradiation of eutectic alloys in powder form. Most investigations of eutectic solidification in discrete particles have used molten droplets formed by atomization, where heat extraction from the particle into a surrounding gas is inefficient. Although rapid eutectic solidification still occurs, atomization results in highly heterogeneous internal microstructures, which are undesirable with respect to many technical properties. This research will employ particles suspended in liquid or solid media that can extract heat much more efficiently following pulsed laser melting. The suspensions will be volume-restricted such that boiling of the media in contact with the molten particles is suppressed, thereby maintaining effective heat dissipation. In addition, by controlling the laser power and number of pulses, it is possible to partially melt a particle, such that a small solid portion acts as a ready nucleation site, avoiding deep undercooling. The experiments will examine these processes in metallic, semiconducting and metal-oxide alloy powders. The dynamic rapid solidification process in bulk and three-dimensional particles will be simulated in detail using the phase field modeling approach. The stability of the nanoscale eutectic structure during spark plasma sintering of the produced particulate material will also be examined. The Broader Impact of this research will be the new processing approaches that create tailored microstructures in feedstock materials for use in sintering and additive manufacturing that will yield improved technical properties in rapidly-manufactured componetns. This will have significant direct benefits for the aerospace, automotive, medical products, and defense industries.