Formulation of rod-forming models and their application in spray forming
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I. INTRODUCTION
OBTAINING both high yields and good-quality sprayformed rods is essential in order to achieve a successful commercialization of the spray-forming process.[1] Therefore, the spray-formed rod has to have a good dimensional tolerance as well as a homogeneous microstructure. This requires an accurate control of the spray-forming process, which involves sequential stages of droplet spray (the atomization step) and droplet consolidation (the deposition step) in order to manufacture near-net-shape preforms in a single operation.[2] Extensive studies[3–6] on the atomization step have been carried out to gain an understanding of the droplet dynamics and cooling behavior of the sprayed droplet. Analyses of droplet consolidation during the deposition step[7–11] and formulation of forming models12,13,14] to calculate the rod preform shape have been conducted. Among these, formulation of the forming models to calculate the preform shape is considered to be the most complex and critical step in spray forming. Once the preform shapes are calculated, the thermal history of the preform during spray forming can be predicted.[13,15] Frigaard[12] carried out an analysis of the steady-state and transient-state rod growth. Mathur et al.[13] calculated the rod shape by numerically integrating the change in height of the rod-surface grid points during spray forming with a scanning atomizer. Effects of the scanning mode on rod forming were reported by Muhamad et al.[14] Recently, Seok et al.[16,17] studied the effects of various process parameters on the shape and dimensions of the rod by formulating a HYUN-KWANG SEOK, Research Scientist, JAE-CHUL LEE, Chief Researcher, and HO-IN LEE, Chief Researcher, are with the Division of Metals, Korea Institute of Science and Technology, Seoul, 136-791, Korea. HUI CHOON LEE, Senior Researcher, is with Kangwon Ind. Ltd., KyungBuk, 790-370, Korea. KYU HWAN OH, Associate Professor, and HYUNG YONG RA, Professor, are with the School of Materials, Seoul National University, Seoul, 151-742, Korea. Manuscript submitted December 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
three-dimensional time-dependent model (3-D TDM) using a simple vector calculus method. However, these studies mentioned neither the shape-evolution mechanism nor the optimal initial setting conditions in spray forming. During spray forming, the rod preform changes its shape continually from that of a disc to a rod (transient-state rod growth) and eventually maintains its top surface profile once it has settled down (steady-state rod growth), as can be seen in Figure 1. The transient-state rod growth, which needs subsequent machining such as scalping, has to be minimized. Investigating the rod shape-evolution mechanism will provide the knowledge base to better control the rod shape during spray forming. In this study, three different rod-forming models were formulated to investigate the rod shape-evolution mechanism. The effects of various spray-forming parameters such as initial eccentric distance, substrate withdraw
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