Master sintering curve concepts as applied to the sintering of molybdenum
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I. INTRODUCTION
MATERIALS suited for high-temperature applications usually have high melting temperatures, making them difficult to manufacture by methods such as casting. Powder metallurgy techniques offer alternative processing routes that avoid melting, yet deliver net-shape products. A prime example of such a material is molybdenum.[1] Molybdenum has a melting temperature of 2622 °C, yet after consolidation, the powder can be sintered at temperatures near 2000 °C. Indeed, considerable sintering is evident at much lower temperatures. In materials that require such high sintering temperatures, it is attractive to design sintering cycles that minimize the energy consumption while attaining a target density. Also, minimizing the sintering time will reduce grain growth, giving better sintered strength and other properties.[2,3] The master sintering curve is a model based on diffusion theories that links the work of sintering to the relative density at any point in the sintering cycle.[4] The work of sintering needed to obtain a target density can be generated by various combinations of heating rate, hold time, and isothermal temperatures. Thus, once mapped for density vs the work of sintering, the master sintering curve enables process optimization. Sintering behavior is typically influenced by parameters such as initial density o, particle size D, and grain size G. Variations between powder vendors and powder lots lead to noticeable changes in the sintering response. In this study, sintering of six different molybdenum powders from two different vendors is evaluated through master sintering curves constructed for each powder. Comparisons between these DEBORAH C. BLAINE, Materials Research Engineer, formerly with the Center for Innovative Sintered Products, Pennsylvania State University, is with Bleistahl Productions GmbH & Co., Wetter D-58300, Germany. Contact e-mail: [email protected] JOHN D. GUROSIK, Process Engineer, formerly with the Center for Innovative Sintered Products, Pennsylvania State University, is with GKN Sinter Metals, Emporium, PA 15857. SEONG JIN PARK, Research Professor, and RANDALL M. GERMAN, CAVS Professor and Director, formerly with the Center for Innovative Sintered Products, Pennsylvania State University, are with the Center for Advanced Vehicular Systems, Mississippi State University, Mississippi State, MS 39762-5405. DONALD F. HEANEY, Director Process Development, is with the Center for Innovative Sintered Products, Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted July 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
master sintering curves provide insight into the changes in sintering work due to changes in powders as well as green density and presintering treatments. We introduce a new method that normalizes the relative density data to eliminate green density effects on the master sintering curve. This provides a versatile tool for designing optimal sintering cycles. II. MASTER SINTERING CURVE THEORY Early research identified that diffusion pla
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