a b s t r a c tThe current trend towards miniaturization has led to the increasing use of stainless steel powders as raw material in the manufacture small parts with complex shapes, e.g. medical devices. This paper focuses on the role of coated and uncoated low carbon stainless steel powders, having d 50 = 7.3 m, normal particle size distribution and a shape factor of 1 on additive processes used in micromanufacturing, e.g. micro powder injection moulding (PIM). Surfaces were treated using a magnetron sputtering deposition apparatus equipped with a high frequency powder vibration and a continuous feed system. The results clearly show that the coated powders assume an "artichoke" morphology, which significantly increases the surface area. This combined with their nanocrystalline character leads to higher flowability than uncoated powders. However, no improvements have been observed concerning the critical powder volume concentration (CPVC) in feedstocks for PIM processing. In consolidation processes, the debinding temperatures can attain values higher than 500 • C. After debinding and sintering, the coating of steel powders can show lower carbon contamination in solid solution than uncoated ones. This result is very important particularly for powder microtechnology of low carbon stainless steel.
The present study aims to prepare feedstocks for MIM (Metal Injection Molding) where the metal powders are 316L stainless steel powders (SS). The master objective is to compare the performance of a biodegradable binder with a commercial one based on polyolefins. Different challenges must be overcome in SS injection molding, as follows: to decrease binder/carbon content in feedstocks; to decrease carbon contamination during debinding and sintering; to avoid the formation of chromium carbide and presence of precipitation-free zones; to avoid the grain growth during sintering and to reduce the feedstock price. The optimization of the feedstocks was performed using a torque rheometry technique. Feedstocks of coated and uncoated SS powders mixed with an agar-based binder were used to produce sound parts. A feedstock constituted by SS powders mixed with a high quality commercial binder was the standard. SS with agar-based on feedstocks can admit solids content similar to those based on the commercial binder (62 vol.%). For similar powder content, the sinters resulting from feedstocks with the agar-based binder shows a lower quantity of solid solution of carbon and chromium carbides, absence of precipitation-free zones than commercial feedstocks and good sinter soundness. Coating powders with nanocrystalline stainless steel contribute to control grain growth during debinding and sintering.
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