Rheological and Thermal Debinding Behaviors in Titanium Powder Injection Molding
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I.
INTRODUCTION
TITANIUM and its alloys have low densities, high specific strengths, excellent corrosion resistance, and high-temperature endurance as well as nonmagnetic properties. However, industrial applications for complex shaped components are limited due to their poor machinability and high manufacturing cost. As a proven net shaping technology, powder injection molding (PIM) offers a unique opportunity for the fabrication of complex shaped, microstructurally uniform, and high-performance Ti components.[1–3] Although the world’s first PIM Ti component was produced in 1990 by Nippon Tungsten,[2,3] systematic research on Ti PIM had not been reported until the late 1990s.[4–10] The keys to successful Ti PIM were found to include (a) careful control over impurity elements, especially oxygen; (b) fairly good moldability of Ti powder; and (c) acceptable Ti powder expenses. The presence of excess oxygen in Ti compacts decreased not only the sintered density, but also ductility and toughness.[3] To reduce the oxygen concentration in sintered titanium, gas-atomized (GA) Ti powder, which contains a low initial oxygen content, is preferred.[3–8,11] The GA Ti powder also possesses good moldability due to its spherical particle shape. However, its use is limited to a SEONG-JIN PARK, Associate Research Professor, is with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759. Contact e-mail: [email protected] YUNXIN WU and GUOSHENG GAI, Professors of Materials, and XIN ZOU, Graduate Student, are with the Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China. DONALD F. HEANEY, Associate Professor, is with the Center for Innovative Sintered Products, Pennsylvania State University, University Park, PA 16802. RANDALL M. GERMAN, Professor and Associate Dean, is with the College of Engineering, San Diego State University, San Diego, CA 92182-1326. Manuscript submitted February 11, 2008. Article published online November 6, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
few applications because of its high cost. Most often, the less costly hydride dehydride (HDH) Ti powder is chosen to be mixed with GA powder. The problem with PIM of HDH Ti powder is associated with its poor moldability resulting from the irregular particle shape as well as with its relatively high oxygen content. To better facilitate the use of HDH Ti powder for PIM, a spheroidizing treatment is developed in this article to improve the powder moldability, and the feedstock mixing and thermal debinding processes are optimized to reduce additional impurity contaminations. Moreover, the rheological and thermogravimetric behaviors of PIM feedstocks prepared with HDH Ti powder, spheroidized HDH Ti powder, and GA Ti powder are compared.
II.
EXPERIMENTAL
A. Powder and Binder Characteristics Three types of Ti powder were used in the present work: hydride-dehydride (HDH) powder, spheroidized HDH powder, and GA powder. The powder characteristics and morphologies are given in Table I and Figu
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