Starch Consolidation as a New Process for Manufacturing Powder Metallurgy High-Speed Steels
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DURING the last decades, new routes for the manufacturing of high-speed steel (HSS) have been developed, and, among them, metal injection molding (MIM) has established itself as an important alternative.[1,2,3] One of the main advantages of the MIM is the freedom offered regarding component shape and size. However, the use of large amounts of polymer (35 to 40 vol pct) causes large stresses during debinding and limits the possible component size. With the aim of overcoming this disadvantage, as well as developing an environmentally friendly powder shaping route, ‘‘starch consolidation’’ has appeared recently. The potential advantages of this direct consolidation method are the preservation of a high degree of homogeneity achieved in the slurry, as well as a good control of the shrinkage during drying and sintering and, therefore, of the ultimate dimensions of the components.[4] Starch consolidation has been succesfully used in the shaping of ceramic bodies.[5–8] This technique allows the manufacturing of porous ceramics,[9] Al2O3, Si3N4, cordierite, and thermally insulated materials[10] as well as bioceramics.[4] In the field of metal powders, some studies using HSS have already been carried out.[11–15] A. Starch Consolidation The new forming method presented is this work uses the ability of the starch to swell in contact with water
F.J. VELASCO and J.M. TORRALBA are with the Department of Material Science, Universidad Carlos III, E-28911 Madrid, Legane´s, Spain. Contact e-mail: [email protected] P. ROMANO is with the Microstructure Physics and Metal Forming, Max Planck Institute for Iron Research, D-40237 Du¨sseldorf, Germany. Manuscript submitted May 24, 2006 METALLURGICAL AND MATERIALS TRANSACTIONS A
and at a certain temperature, which depends mainly on the type of starch. All starches consist of mixtures of two types of polysaccharide: a linear one, amylose, and a highly branched one, amylopectin. Amylose gives the starch its gelling property in aqueous suspensions. The glucose units that build up the polymeric chains in starch expose a large number of hydroxyl groups and, therefore, give a strong hydrophilic character to starch granules.[9] When an aqueous suspension of powder and starch is heated up to 60 to 80 C, the starch particles swell due to the water uptake (Figure 1).[16] The amount of water available for flowing gradually decreases, causing the particles to stick together and consolidate into a rigid body.[6] Gas-atomized HSS powders normally consist of spherical and smooth particles that promote a high particle concentration and form a liquid phase during sintering that helps densification.[17] To avoid settling of the metal powders during forming, thickening of the water phase is required. Because any conventional thickener will absorb a certain amount of water during the process in direct competition with the starch, the amount of thickener should be kept as low as possible. The presence of small spherical particles with a wide size distribution will improve the packing and therefore will help in redu
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