Shape distortion in liquid-phase-sintered tungsten heavy alloys
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I.
INTRODUCTION
LIQUID phase sintering is a net-shaping technology applicable to many high performance alloys. The wetting liquid provides a capillary force that pulls the solid grains together and induces rearrangement. In addition, when the solid is soluble in the liquid, the liquid gives rapid mass transport at the sintering temperature.[1] This results in solution-reprecipitation and improved grain packing by grain shape accommodation. Hence, the presence of a liquid phase during sintering assists densification. Despite its advantages, liquid phase sintering is only applicable to selected compositions. A major problem is compact slumping when excess liquid is present.[2] Slumping or distortion during liquid phase sintering is defined as a nonisotropic shape change that accompanies densification. Usually, to maintain structural rigidity, the solid content must be high.[3] So far, distortion has been analyzed from a kinetic standpoint.[4,5,6] This research investigates the effect of microstructure on distortion during liquid phase sintering. Distortion is quantified and related to grain size, dihedral angle, solid volume fraction, contiguity, and connectivity. Based on experiments, a model is derived that correlates the dihedral angle with the critical solid content required to preserve structural rigidity. Background Compact distortion during sintering is a long-standing problem in powder metallurgy. Lenel et al.[7] observed nonuniform shrinkage in sintered copper compacts, which was attributed to the presence of gravity. Recently, distortion in hard metals during solid-state sintering has been numeri-
ANISH UPADHYAYA, Director of Materials Development, and RANDALL M. GERMAN, Brush Chair Professor in Materials, are with the P/M Lab, Engineering Science and Mechanics Department, The Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted July 29, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
cally modeled using finite element techniques.[8,9] These models include the green density gradients in the as-pressed compact. However, they do not include the effect of gravity and microstructure upon distortion. Compared to the solid-state sintering, the problem of compact slumping is further aggravated during liquid phase sintering. Usually, the constituent liquid and solid phases have differing densities; therefore, the denser constituent tends to settle, causing solid-liquid segregation and distortion. Detailed investigation of slumping in liquid-phase-sintered tungsten heavy alloy (WHA) compacts was performed by Kipphut et al.[10] Their experimental results underscore the role of gravity and density difference in governing settling and distortion. Mani and German[4] investigated the kinetic aspect of gravity-induced distortion in liquid-phase-sintered WHAs. They proposed a macroscopic model that predicts the role of various processing variables, such as compact mass, temperature, sample shape, green density, and composition, on distortion. Raman and German[5] modeled the gravity role
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