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

粉末冶金，其中复杂的零件是由熔融在一起的金属粉末，在美国有很大的经济足迹。随着增材制造（AM）技术变得越来越普遍，这种足迹只会增加。AM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，在粉末颗粒内创建纳米级物理结构，以提高最终部件的机械性能（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，产生内部片状结构。当粉末处于快速吸热的流体中时，通过激光照射粉末，当颗粒固化时，共晶结构可以被保留。这项工作将使粉末颗粒悬浮在可以散热的液体和固体介质中，但这些介质不得发生反应，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内部结构的形成。这项研究的更广泛影响将是新的加工方法，这些方法可以创建用于烧结和增材制造的定制原料材料，以快速制造具有改进技术性能的复杂零件。这将为航空航天、汽车、医疗产品和国防工业带来巨大的直接利益。 脉冲激光熔化可以提供非常快的固化速率以形成纳米级共晶结构。不幸的是，在具有高光学吸收的材料中，具有纳秒脉冲宽度的激光仅熔化近表面区域，并且不能被认为是真正的体加工技术。本研究将通过使用粉末形式的共晶合金的激光辐照来规避这一限制。大多数离散颗粒中的共晶凝固的研究使用通过雾化形成的熔融液滴，其中从颗粒到周围气体中的热提取是低效的。虽然快速共晶凝固仍然发生，雾化导致高度不均匀的内部微观结构，这是不可取的，就许多技术性能。这项研究将采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。悬浮液将是体积受限的，使得与熔融颗粒接触的介质的沸腾被抑制，从而保持有效的散热。此外，通过控制激光功率和脉冲数，可以部分地熔化颗粒，使得小的固体部分充当准备好的成核位点，避免深度过冷。实验将研究这些过程中的金属，半导体和金属氧化物合金粉末。采用相场模拟方法，详细模拟了块状和三维颗粒的动态快速凝固过程。还将检查所产生的颗粒材料在放电等离子体烧结期间的纳米级共晶结构的稳定性。这项研究的更广泛影响将是新的加工方法，这些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，从而提高快速制造组件的技术性能。这将为航空航天、汽车、医疗产品和国防工业带来巨大的直接利益。

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.