Modeling the Spray Forming of H13 Steel Tooling
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INTRODUCTION
THE conventional method for making molds and dies starts with forged plate, bar, etc. and involves a series of precision machining, benching, and heattreatment steps. The process is both costly and time consuming. Spray forming provides an alternative approach for making molds and dies.[1,2] The approach involves depositing atomized metal droplets onto a predesigned pattern. As the deposit builds up, the shape and surface texture of the pattern are replicated precisely and completely, as a result of the good filling characteristics and high cooling rate of micron-sized droplets upon deposition. An additional advantage is simplified heat treatment. Compared with the heattreatment operations in conventional mold and die fabrication techniques,[3] annealing, austenitization, and quenching can be avoided due to the formation of a homogeneous and macrosegregation-free microstructure that is characteristic of as-spray-formed material.[4] Rapid solidification during spray forming suppresses carbide precipitation and growth, allowing tool steels to be artificially aged directly from the as-spray-formed condition to obtain the final mold/die products. As a consequence, production costs, energy use, and lead time are significantly reduced. YAOJUN LIN, formerly with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, is R&D Engineer, Department of Technology, Praxair Electronics, Orangeburg, NY 10962. Contact e-mail: yaojun8l@yahoo. com YIZHANG ZHOU, Associate Researcher, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. KEVIN M. MCHUGH, Senior Advisory Scientist, is with the Industrial and Material Technologies Department, Idaho National Laboratory, Idaho Falls, ID 83415. Manuscript submitted August 28, 2006. Article published online June 26, 2007. 1632—VOLUME 38A, JULY 2007
A critical issue when spray forming molds and dies is the removal of porosity in the region near the pattern (substrate). Oftentimes, there is high porosity in this region that results from insufficient liquid fraction in the spray and high cooling rate by a relatively cold substrate.[5] Therefore, a preheated substrate may be beneficial. However, preheating a substrate is an added expense and may not be practical. Moreover, a preheated substrate will decrease the cooling rate of the as-spray-formed metal, which can degrade properties and possibly eliminate the ability to age the metal by failing to suppress carbide precipitation. Thus, conditions that avoid preheating the substrate while maintaining a balance between removing porosity and suppressing carbide precipitation are optimal. Determination of the appropriate conditions via experimentation is time and cost intensive. In contrast, a modeling framework can be effectively implemented to determine the optimal spray conditions, once the desired microstructure is established. The present study seeks to implement an existing numerical framework to a
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