The role of melt pool behavior in free-jet melt spinning
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25/3/04
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The Role of Melt Pool Behavior in Free-Jet Melt Spinning R.E. NAPOLITANO and H. MECO The influence of melt pool behavior on the competition between the nucleation of crystalline solidification products and glass formation is examined for an Fe-Si-B alloy. High-speed imaging of the melt pool, analysis of ribbon microstructure, and measurement of ribbon geometry and surface character all indicate upper and lower limits for melt spinning (MS) rates for which fully amorphous ribbons can be achieved. Comparison of the relevant time scales reveals that surface-controlled melt pool oscillation may be the dominant factor governing the onset of unsteady thermal conditions accompanied by varying amounts of crystalline nucleation observed near the lower limit. At high rates, the influence of these oscillations is minimal due to very short melt pool residence times. However, microstructural evidence suggests that the entrapment of gas pockets at the wheel-metal interface may play a critical role in establishing the upper rate limit. An observed transition in wheel-side surface character with an increasing MS rate supports this contention.
I. INTRODUCTION
THE techniques of rapid solidification continue to be of central interest because of their utility in producing materials with microstructures and properties unattainable through other methods. Accordingly, the literature contains numerous reports of ultra-refined microstructures,[1] growth mode transitions,[2] anomalous solubility,[3] and the presence of metastable phases,[4] all achieved through these methods. More generally, the conditions that prevail during a process such as melt spinning (MS) offer an opportunity to study various mechanisms of alloy solidification at the high-rate extreme. Of course, the careful experimental study of solidification dynamics is impeded by a generally poor understanding of the local conditions that exist within the melt pool. Such difficulties highlight the demand for modeling tools capable of handling the complex problems of heat transfer, fluid flow, and interfacial behavior that dominate the MS process. Indeed, as our ability to predict the local thermal conditions improves, the kinetics of the operative nucleation and growth processes can be better quantified. Because of the interdependence between local conditions and microstructural dynamics, the investigation of nucleation and growth during MS not only serves to enhance our understanding of solidification dynamics, but may also provide a means to understand better the evolution of local conditions within the melt pool. It is in this iterative sense that the current observations and interpretations of melt spun structures are offered. The Fe-Si-B system is employed for the experimental work. There is considerable interest in the melt spun microstructures in these alloys because of the magnetic properties that may be exhibited after rapid solidification.5,6 The rapid solidification technique of MS arises from the general method known as splat-quenching, in
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