Novel Cooling Rate Correlations in Molten Metal Gas Atomization
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MECHANICAL properties of metal alloys strongly depend on their microstructure, which can be adjusted with cooling conditions during solidification.[1] The rate of heat extraction during the transition from a liquid state to a solid material determines the morphology of the solidified microstructure. Any solidification process with cooling rates ranging from 103 to 106 K s1 is considered rapid solidification.[2] Achieving these cooling rates requires a large surface-to-volume ratio of the alloy, leading to a high heat extraction rate. A high cooling rate provides materials with small grain sizes, good chemical homogeneity, and high solid solubility. It also offers advantages over slow solidification processes such as the absence of crystallization as observed in metallic glasses.[3,4] Gas atomization processes often meet these requirements. Free-fall atomization (FFA) and close-coupled atomization (CCA) are established atomization techniques that produce fine, spherical
N. CIFTCI is with the Leibniz Institute for Materials Engineering IWT, Badgasteiner Straße 3, 28359 Bremen, Germany. N. ELLENDT, G. COULTHARD, E. SOARES BARRETO, L. MA¨DLER, and V. UHLENWINKEL are with the Faculty of Production Engineering, University of Bremen, Badgasteiner Straße 1, 28359 Bremen, Germany and also with the Leibniz Institute for Materials Engineering IWT. Contact e-mail: [email protected] Manuscript submitted August 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
metal powders.[5] These powders can be applied in sintering processing,[6,7] additive manufacturing,[8–10] and MIM technology.[11,12] The process design of atomization facilities requires thermal models to adjust the solidification behavior of the melt droplets and thus, the final microstructure of the atomized particles. Several thermal models[13–15] and experimental investigations[16,17] have been proposed to estimate the cooling rate during melt atomization. Table I summarizes cooling rate correlations with their corresponding validity conditions. All referenced studies show a strong correlation between the cooling rate and the melt droplet size. The cooling rate follows a power function with respect to droplet diameter dp: CR ðK s1 Þ ¼ a0 dp ðlmÞa1 ;
½1
where a0 is a pre-exponential factor describing the material and gas properties and a1 is an additional exponent. However, these cooling rate calculations in the spray cone during molten metal atomization are limited as the actual cooling conditions under which highly undercooled melt droplets solidify are difficult to measure experimentally or predict theoretically. Containerless methods such as drop tubes or electromagnetic/electrostatic levitation are commonly used techniques to determine the solidification behavior of melt droplets, both close to and far away from thermodynamic equilibrium.[18] However, these methods cannot be directly linked to gas atomization, as for instance,
METALLURGICAL AND MATERIALS TRANSACTIONS B
Pd83Si17 and Pd82Si18 Pd84.5Si15.5
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