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

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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.