Gas Atomization of Amorphous Aluminum: Part I. Thermal Behavior Calculations
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rocessing, powder experiences rapid solidification, which occurs under nonequilibrium conditions and is completed in a few milliseconds,[9] whereas the heat is extracted at high rates from the liquid mass via the smallest dimension. Recording temperature variations over such a short time and small scale is difficult. In particular, it is not possible to measure directly the temperatures and cooling rates of droplets during their residence in a high-velocity gas during GA. Accordingly, it is customary to estimate the cooling rate by numerical calculation of the droplet thermal histories during GA. Numerical simulation is a powerful tool for solving these types of problems. The modeling results can be used to provide fundamental insight in support of the optimization of GA processing parameters. Inspection of the technical literature reveals some excellent research on numerical simulation of GA[16–19] and spray forming (SF),[20–22] which were principally concerned with relevant recalescence, crystal nucleation, and growth phenomena of conventional crystal alloys during GA or SF. A review of the scientific literature reveals that published numerical studies on the thermal behavior and process optimization of GA for metallic amorphous powders are essentially nonexistent. The fact that MGs generally have a higher melt viscosity than that of conventional alloys, and there is no obvious phase transformation, as well as no crystal nucleation and growth occurring during droplet solidification, suggest that such a study would be useful. The primary technological goal of the work described herein is to provide insight into the thermal behavior of atomized Al MGs and to use this fundamental insight to optimize the processing parameters. This goal will be accomplished via a combination of numerical (part I of this study) and experimental (part II) studies. The scientific objective is to establish a fundamental understanding of the relationship between processing, thermal behavior, and microstructure evolution of amorphous Al powder during GA. In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powder were calculated based on the assumption of Newtonian cooling with forced convection. The rationale for the assumptions used was rationalized in light of available experimental data. The effects of processing parameters, such as gas composition, gas pressure, melt superheat temperature, and gas/melt flow ratios, on the thermal history and cooling rate were numerical simulated and analyzed. The complexities of the system studied required several simplifying assumptions as well as the use of empirical equations that will undoubtedly lead to discrepancies between theory and experimentation. Accordingly, experimental validation was deemed necessary and is reported in part II[23] of this study.
II.
NUMERICAL FORMULATION
During the GA processing, a stream of molten metal is disintegrated into micron-sized droplets by the impact of high-energy gas jets.[9] The liquid breakup process METALLURGICAL A
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