Modeling the Deposition Dynamics of a Twin-Atomizer Spray Forming System
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nce its invention in the 1970s, spray forming has developed steadily to become a niche casting technology for highly alloyed metallic alloys, including hypereutectic Al-Si alloys, high-speed steels, Ni superalloys, etc.[1,2] It has become a standard industrial practice to use a meltatomization system consisting of two scanning atomizers (a twin atomizer) for the spray forming of Al, Cu, and steel preforms in a large-volume production. Two droplet sprays can spread the metal droplets uniformly onto a large surface area and achieve a production rate that competes with powder-batch processes or semicontinuous billet casting.[3] The ability to tailor the operating parameters of each atomizer separately provides an extra degree of freedom to control the mass and coupled heat distribution input at the growing billet surface[4] that, in turn, link to the final shape, thermal, and microstructural aspects such as grain size and porosity.[5–7] However, it is a significant technical challenge to integrate a twinatomizer system with a large-capacity vacuum induction melting (VIM) furnace for Ni superalloys. The high cost of the Ni feedstock requires that experimental trials to optimize products should be minimized, and numerical modeling for process optimization should be used wherever possible. However, most of the previous simulations[7–11] have concerned a single atomizer, and G. ZHANG, Professor, Z. LI, Research Engineer, and Y. ZHANG, Senior Research Engineer, are with the National Key Laboratory of Advanced High Temperature Structural Materials, Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. J. MI, Research Fellow, and P.S. GRANT, Professor, are with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Contact e-mail: [email protected] Manuscript submitted October 3, 2009. Article published online January 6, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
the dynamics of atomizer scanning and its relative movement to the rotation of the deposition surface (termed ‘‘substrate’’ hereafter) were averaged generally over one or a few substrate revolutions, so the spray was treated as a continuous, non-temporally changing mass flux. This approach has shown to be an acceptable approximation for single spray arrangements,[7–11] but in the case of two sprays, possibly with atomizers running under different conditions, time-averaging may obscure the dynamic interplay between the two sprays and their interaction with the rotating substrate. This article presents a numerical study of the spray deposition dynamics of the twin-atomizer system recently established at the Beijing Institute of Aeronautical Materials (BIAM) China—the first of its kind—by integrating a VIM furnace of 45 L and a twin-atomizer system shown in Figure 1(a). Compared with most previous numerical simulations, this study is focused on the scan dynamics of the twin-atomizer, the resulting mass distribution because of change in the twin-atomizer scan frequency and the substrate rotation frequency, as wel
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