MATERIALS FORUM VOLUME 29 - Published 2005 Edited by J.F. Nie and M. Barnett © Institute of Materials Engineering Australasia Ltd 119 APPLICATION OF POWDER METALLURGY FOR THE PRODUCTION OF HIGHLY POROUS FUNCTIONAL PARTS WITH OPEN POROSITY M. Bram, A. Laptev, H.P. Buchkremer, D. Stöver Forschungszentrum Jülich GmbH, Institute for Materials and Processes in Energy Systems (IWV-1: Materials Synthesis and Processing), D-52425 Jülich, Germany ABSTRACT The production of highly porous parts by powder metallurgy using suitable spacer materials is a well established technique for titanium and stainless steel. Porosities up to 80 Vol.% are directly related to the ratio of the spacer material. In addition, pore sizes up to 2 mm can be well adjusted by fractionating the spacer particles. Up to now, net shape fabrication of functional parts was limited by the fact that during machining of sintered parts the surface pores tend to be closed by plastic deformation of the porous framework. Therefore, machining in the green state followed by decomposition of the additive and subsequent sintering was found to be suitable to overcome this drawback. Each step of the production route is demonstrated on the prototype of an acetabular cup provided as biomedical implant in hip replacement. 1. INTRODUCTION The application of suitable place holder materials (e.g. ammoniumhydrogencarbonate (NH 4 )HCO 3 ) enables the powder metallurgical production of highly porous materials. To produce this material, a homogeneous mixture of the metal powder and the place holder material is prerequisite. The porosity and pore size can be well adjusted by the amount of the place holder material and by fractionizing it. After compaction of the mixture the place holder is removed by decomposition in air. The subsequent sintering step leads to the formation of sintering necks and is responsible for the stability of the remaining network. The production route was intensively studied at the Forschungszentrum Juelich and was successfully applied for titanium, stainless steel, Ni-based superalloys, molybdenum and copper 1,2 . The application of this technique for the production of net shape parts was hindered by the limited machinability of the porous structure after sintering. The pores tend to be closed by the plastic deformation of the metal struts 3 . Machining of porous metal is also coupled with high tool wear. The infiltration of smearing and grinding media in the porous structure deteriorates the potential especially for biomedical applications. To overcome these problems the production route was modified. Shaping is done on pressed compacts before decomposition of the place holder material. The stability of the compacts was found to be sufficient to machine them in the unsintered state (“green” state) by conventional methods like drilling, turning, sawing or milling. The tool wear was reduced to a minimum and the application of smearing liquids was not necessary. The main advantage of machining in the green state is the retention of the open porosity, which is prerequisite for applications, where a functional porosity is required (e.g. in biomedical implants). The place holder method is attractive for the production of highly porous titanium implants 4 . Titanium is a well known implant material due to its biocompatibility 5 . The open porosity enhances the structural and functional bonding between implant surface and human bone 4,6,7 . Fractionizing the place holder enables a well adjustment of the pore size to the range of 350 – 500 μm which is discussed to be preferred for bone ingrowth 7 . The application potential was demonstrated on the prototype of a hemispherical cup provided for hip replacement. The feasibility study was supplemented by analyzing the impurity content considering the ISO-Norm for biomedical implants 8 and by investigation of the compression strength depending on the porosity. 2. EXPERIMENTAL PROCEDURES The feasibility study was done on the prototype of a hemispherical cup provided as implant for hip replacement. The outer diameter was 50.0 mm, the inner diameter 41.2 mm. Titanium powder with an irregular shape produced by GfE, Germany was used as starting material. The particle size was d 10 = 24.0 μm, d 50 = 48.3 μm and d 90 = 85.0 μm. Table 1 summarizes the content of oxygen, carbon and nitrogen in the starting powders. Enhanced amounts of these impurities are critical due to the formation of titanium oxides and carbides deteriorating the mechanical properties of the sintered parts. The starting powder fulfilled the requirements of ISO 5832-2 for titanium grade 4. Irregular shaped ammoniumhydrogen- carbonate (NH 4 )HCO 3 was preferred as place holder because it decomposes in air at temperatures above 60°C to ammonia, carbon dioxide and water. In the present investigation the decomposition was done at 150°C in air to remove all reactants in the gaseous state. To achieve a well defined pore size the place holder material was fractionized to 350 – 500 μm by sieving. Pores of this size are discussed to be