In the powder shaping process, friction is often considered as one of the main limiting factors since it can be the cause of bad green density distributions as well as ejection defects. Therefore, knowledge of this is important for better controlling the compaction stage. This paper makes an attempt to bring out the different process parameters (density, normal stress, sliding velocity, temperature, and displacement) which influence the friction between a standard iron powder and a tool in tungsten carbide. With this view, a sliding piece device has been built and is described. Several tests have been performed; from the results emerge a decrease of the friction coefficient with increasing density or normal stress, conversely the velocity increases the friction while temperature has PM75554.10.1 no significant role. In this work, particular attention 1 Sliding piece device: 1=optical transducer, 2= is given to the amplitude of the displacement showing transducer, 3=disc, 4=parallelipipedic slab, 5=force that it can highly affect friction and wear of green transducer, 6=hydraulic press, 7=cylindrical die, 8= parts. An attempt is made at explaining the observed press phenomena and results are correlated with an analytical expression. PM/0952 the process parameters on friction. Using an iron powder, authors4,9 have observed that the friction coefficient Dr Doremus and Dr Toussaint are at Institut decreased when increasing normal stress while Mallender National Polytechnique de Grenoble, Laboratoire et al. are the only authors who have measured an increase Sols Solides Structures, Domaine Universitaire, of this coefficient with density. The second type of BP 53, 38041 Grenoble cedex 9, France and Dr Pavier equipment allows one to study the role of the different is with Ferroxdure, Aimants permanents, 41, rue parameters independently, such as the density of the Pierre Brossollette,
pression is not yet available in the literature and conventional models which have been developed to describe the This paper investigates die compression of tungsten compressibility of powder never refer to WC-Co mixtures. carbide-cobalt powder mixtures, which is an import-In this context, the present study is an attempt to obtain a ant step in the processing route of cemented carbides.better characterisation and understanding of the pressing Simple action compression of miscellaneous mixtures of these mixtures. It focuses on die pressing, which is the has been investigated with an industrial type press most classical pressing technique. allowing measurement of the force applied on both Because of the friction between the powder and the die, upper and lower punches. In addition, an original the state of stress within the powder is not homogeneous apparatus was used to measure the average radial during die pressing. Thus, the interpretation of data drawn stress applied upon the die, and the frictional shearing from this test requires a mechanical analysis. A simple force at the powder/die interface was determined model, which has already been used by Mosbah et al.1 for through a powder/wall friction apparatus. A mechaninvestigating the compressibility of iron powder, is proposed ical analysis of the data obtained for various tungsten for this purpose. This analysis, which is supposed to provide carbide-cobalt powder mixtures provided the intrinthe intrinsic compressibility law of the material, is validated sic stress v. density compressibility relation as well by comparing results of tests performed with several as the radial stress transmission coefficient and the devices, as well as data obtained with compacts of different powder/die friction coefficient. In this framework, the geometries. effect of several material and process parameters on With the help of this model, the influence of material the densification is discussed.PM/0825 parameters (cobalt content, carbide grain size) and process parameters (granulation, compression rate) on the com-At the time the work was carried out Dr Roure pressibility of WC-Co systems is investigated and the was with the Centre Europe ´en de Recherches en mechanisms responsible for this behaviour are discussed. Me ´tallurgie des Poudres,
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