MATERIALS

The feedstock for the PM production is a compressible metallic powder. The production process consists of the shape forming (pressing) of the powder and a subsequent heat treatment (sintering) below melting temperature. Often, in order to raise the material density and the geometrical accuracy, another process step for redensification or calibration is added. The manufacturing is free of burrs and waste, and at the same time the energy input of the pressing and the sintering is comparatively lower than in metal-cutting manufacturing and in drop forging. By means of powder-metallurgical procedures, components can be generated both out of conventional alloys and out of metal-ceramic and metal-carbide composites. A procedure which allows for providing sinter components with a protection against plagiarism is currently being developed at the Institute of Metal Forming and Metal-Forming Machines (IFUM) of the Leibniz Universität Hannover (LUH). For this purpose, an identification mark, which also allows the storage of coded information, is placed inside the component. This mark is neither visible from the outside nor manipulable. The stored information can be easily read out by means of radiography.

Powder-metallurgical production

In the die-pressing process, the most commonly used and economically most successful shape-forming procedure, the powder is axially compressed between two punches in a die. Depending on the initial density of the fill, the powder is compressed to about half of the original volume. The final density most notably depends on the forming pressures used, which range from 600 to 800MPa for functional components.
However, the powder cannot be completely compressed during pressing, leaving a residual porosity of 5 to 10% in the compact. This has a negative effect on the component's stability. The reason for the remaining porosity is the friction of the powder particles with one another, but friction also occurs between powder and die wall, and this depends on the instantaneous density of the powder and the surface roughness of the die wall.
The powder layers cover the largest distance at the punches during the pressing process. With rising forming pressure, the powder density and the pressure on the die wall increase. When this happens, the powder movement at the wall is slowed down by the friction, and thus density gradients are generated. In addition, the wall friction causes an increase of the powder density close to the rams. The described powder movement is visualized by means of colored powder in Fig. 1.

Relative movement of the powder particles during pressing

Current problems in the production of powder-metallurgical components are distortion due to sintering and crack formation, resulting from an inhomogeneous density allocation in the compact. Differences in density and powder movements can be described using the Finite Element Analysis (FEA). The Drucker-Prager-Cap-Model is used for the material modeling. This model describes the material as an elasto-plastic, compressible continuum, and it can represent hardening of the material as well as decreasing strength. The FE-Software ABAQUS 6.6 is used for the analysis. This program has an implemented version of the Drucker-Prager-Cap-Model. By adding a user-defined subroutine this model was modified in order to formulate it as function of the relative density.
The numerical investigation is concerned with the FEA of a toothed belt disk. This component has several areas each with a different ratio of height to width, and it is thus suitable for the description of wall friction influences on the powder's particle movement.
Fig. 2 shows the displacement of powder particles at the end of the pressing process. In slender areas of the component in particular, the friction makes for notably less powder movement close to the wall than inside the component. The higher the wall friction is, the stronger is the development of a so-called neutral level, in which the powder is compressed the least, during the two-sided pressing.

Numerical investigation of the powder pressing of complex components

The authors would like to thank the German Research Foundation (Deutsche Forschungsgemeinschaft DFG) for the financial support of the research project and the company Comet Schleifscheiben GmbH for the component, shown in Fig.1.

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