On the mechanism of mushy layer formation during droplet-based processing
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INTRODUCTION
DROPLET-BASED processes, such as thermal spraying and spray deposition, are of interest, not only as a result of their potential for net-shape manufacturing, but also due to their inherent ability to generate nonequilibrium thermal and solidification conditions.[1,2,3] Droplet-based processes are being investigated in an effort to improve the microstructure and physical properties of advanced materials, such as composites and nanostructured coatings.[4–7] These processes offer the opportunity to combine the benefits associated with fine particulate technology (e.g., microstructural refinement, alloy modifications, etc.) with in situ processing. The manufacture of composite structures by droplet-based processes, for example, typically involves the mixing of reinforcement and matrix under highly nonequilibrium conditions, and as a result, these processes offer the opportunity to modify the properties of existing alloy systems and develop novel alloy compositions. In principle, such an approach will inherently avoid the extreme thermal excursions, with concomitant macrosegregation, normally associated with more classical manufacturing processes. Furthermore, droplet-based processes also eliminate the need to handle fine reactive particulates, normally associated with powder metallurgical processes. In this article, the focus is placed on bulk materials produced by spray deposition. An important, but heretofore poorly understood, phenomenon that is associated with spray deposition is the formation of a mushy (i.e., both solid and liquid phases simultaneously coexist) layer in the upper region of the deposited material. Q. XU, Graduate, V.V. GUPTA, Assistant Specialist, and E.J. LAVERNIA, Professor, are with the Department of Chemical and Biochemical Engineering and Materials Science, University of California, Irvine, CA 92697. Manuscript submitted May 11, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
The existence of this layer was originally proposed in an effort to rationalize the absence of prior droplet boundaries, an observation that is well documented for numerous spraydeposited materials.[8,9,10] Subsequently, other investigators have used both experimental[11,12,13] and numerical techniques[14,15,16] to ascertain the presence of mushy layer during deposition. Grant et al.[11] recorded the temperature profile on the upper surface of the deposited Sn-38 wt pct Pb alloy by using an infrared thermal-imaging camera. Bewlay and Cantor[12] and Mathur et al.[13] used thermocouples to monitor the thermal histories of the deposited Sn-38 wt pct Pb and Fe-20 wt pct Mn alloys, respectively. Their results revealed that the upper regions of the deposited materials were in a semiliquid or solid state. However, the information provided by such in situ techniques is limited. For example, it is difficult to determine the formation position of a mushy layer, its thickness, and the solid-liquid interfacial velocity by using currently available experimental techniques. As a consequence, investigators have applied n
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