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

粉末冶金是一种将金属粉末熔化在一起来制造复杂零件的方法，在美国有很大的经济足迹。随着添加制造(AM)技术变得更加普遍，这种足迹只会增长。AM的一个紧迫问题是，生产的零件的机械性能往往比锻造或机械加工生产的零件差。该合作项目的目标是开发新的方法，在粉末颗粒内创建纳米级的物理结构，以改善最终部件的机械性能(强度和导热系数)。为了实现这一目标，这项研究将使用激光熔化不同的金属和陶瓷合金，这些合金具有一种特殊的化学成分，即所谓的共晶，它会在内部产生片状结构。当粉末在一种快速吸热的流体中时，通过激光照射，当颗粒凝固时，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，但这些介质不得反应、沸腾或遮挡激光。实验将与建模相结合，以了解热量的释放，并模拟在这些条件下单个球形颗粒内部结构是如何形成的。这项研究的成功将使一类新的定制材料能够用于烧结和添加剂制造，以快速制造具有更好技术性能的复杂零件。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好处。此外，PI在他们的研究中招募代表不足的少数族裔学生的历史很好，这一努力将继续下去。脉冲激光熔化和在某些流体中冷却可以提供非常快的凝固速度，形成纳米级的共晶结构。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了近表面区域，不能被认为是真正的整体加工技术。这项研究将通过激光照射粉末形式的共晶合金来绕过这一限制。大多数关于离散颗粒中共晶凝固的研究都使用雾化形成的熔滴，这种熔滴将热量从颗粒中提取到周围的气体中是效率低下的。虽然快速共晶凝固仍然发生，但雾化会导致高度不均匀的内部组织，这在许多技术性能方面是不可取的。这项研究将使用悬浮在液体或固体介质中的颗粒，这些颗粒在脉冲激光熔化后可以更有效地提取热量。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量，如果需要，可以部分熔化颗粒。这些实验将研究金属、半导体和金属氧化物合金粉末中的这些过程。采用相场模拟的方法，对块体和三维颗粒的动态快速凝固过程进行了详细的模拟。这项研究还将检查产生的颗粒材料在放电等离子烧结过程中纳米级共晶结构的稳定性。这项研究的更广泛影响包括新的加工方法，这些方法在烧结和添加剂制造中使用的原料中创建定制的微结构，从而在快速制造的部件中产生更好的机械和电气性能。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好处。

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 project is to develop new approaches to create a nanoscale physical structure within powder particles in order to improve mechanical properties (strength and thermal conductivity) of final parts. To accomplish this, this research 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 indivdual spherical particles under these conditions. Success in this research would enable a new class of tailored materials to be used 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. In addition, the PIs have a good history of recruiting underrepresented minority students in their research, and this effort will continue.Pulsed laser melting and cooling in certain fluids 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 if desired. 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 Impacts of this research include the new processing approaches that create tailored microstructures in feedstock materials for use in sintering and additive manufacturing that will yield improved mechanical and electrical properties in rapidly-manufactured components. This will have significant direct benefits for the aerospace, automotive, medical products, and defense industries.