Introduction

THE PRODUCTION OF P/M PARTS generally requires admixing of various constituents into metal powders in order to promote part compaction and ejection from the die and obtain adequate strength through diffusion of these elements during the sintering process. Solid lubricants are generally used in a range of 0.5 to 1.5%, depending on the part density and size, to allow part ejection and maintain good surface finish. In the case of iron and steel powders, constituents such as graphite, copper, nickel, or ferroalloys are used to increase strength through diffusion during the sintering process. Finally, other elements such as manganese sulfide or boron nitride may also be admixed to improve machinability of P/M parts. These elements must be uniformly mixed to ensure part-to-part consistency during the compaction and sintering operations. Also, because of differences in particle shape, size, and density of these various powders and low specific gravity, fine particles are susceptible to demixing and dusting, which are a major cause of variations in chemical composition of mixes and hence of the dimensional change and mechanical properties of P/M parts.

Two blending techniques are generally used in the P/M industry to prepare press-ready mixes. The first technique consists of dry mixing the various constituents to get a homogeneous mixture. The second technique, binder treatment blending, makes use of binding agents to reduce segregation and dusting during powder handling.

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