The compaction of carbonyl nickel powders, nickel-based mixtures, and iron, aluminum, and magnesium powders in cylindrical molds is experimentally studied, the billet height being continuously recorded in a testing machine. The theory of plasticity for powder bodies is used to analyze the strain hardening of the matrix forming a powder body in pressing. It is established that shear yield stress and strain hardening of the matrix determine the shape of the compaction curve and the final density of the powder body.
INTRODUCTIONThe pressing of a plastic powder body hardens the material of its particles in addition to its compaction. Unlike a solid body, whose hardening is easy to determine from results of mechanical tests (stress-strain curves), the strain hardening of the powder matrix after pressing can only be calculated using the theory of plasticity for porous bodies [1]. The shape of the compaction curve is determined by the yield stress of the particulate material and by its change due to strain hardening during deformation. Discrete values of the relative density of the compact obtained from large increments of compaction pressure do not give a complete picture of the strain hardening of the powder matrix [2,3]. In addition, discrete data on density are scattered, which complicates the analysis as well. It is incorrect to compare hardening data on porous and cast materials either since the deformation of particles in powder materials is much more inhomogeneous than that of cast materials [4]. There is an indirect method to determine the strain hardening using the microhardness of particulate material [5]. The x-ray method used to assess the dislocation density and compare data with those for deformed compact materials with the same dislocation density may give the averaged value of hardening in powder pressing [6].To obtain complete information on the compaction of a powder body and on the hardening of its particulate material, a compaction curve needs to be continuously plotted. For this purpose, a mechanical testing machine may be used. In this connection, this paper examines how powders of carbonyl nickel and nickel-based mixtures, reduced iron, aluminum, and magnesium are compacted in cylindrical molds; pressing force and height of a powder briquette in a mechanical testing machine being continuously recorded.
EXPERIMENTAL MATERIALS AND PROCEDUREPowders of reduced iron, PA-2 aluminum, magnesium, and PNK-1L6 carbonyl nickel and its mixtures with a nickel alloy powder containing 4 wt.% Si and 3 wt.% B (Tables 1 and 2) were used in the experiment.We used two molds whose diameters were measured to 10 -3 mm and were 11.299 mm (one-piece mold) and 15.057 mm (dismountable triclinic mold). The pressing conditions corresponded to the standard compaction testing procedure [7]. The mold walls that contacted with the powder were lubricated with suspension of zinc stearate in acetone (100 g per 1000 ml). Preliminary pressing to 40 MPa and then double-action pressing were conducted. The cross-bar of a universal mechan...