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The gear rolling densification of Powder Metal (PM) gears leads to better mechanical properties due to the selectively closed porosity of some hundreds of a micrometer under the surface of the tooth. The optimization of this process is critical in order to reduce the overall design time of the process and to increase the quality of the rolled gear. One way to optimize the process, tool design and gear with stock is to conduct Finite Element (FE) simulations with a plasticity material model for porous metals. The simulations should be able to predict the resulted geometry and densification, after that the opposite procedure can be conducted and by using simulations as a design tool to predict the process parameters. In this paper, FE simulations are being performed for the gear rolling densification process. The aim is to investigate how such simulations can be used to improve the quality of the rolled gear. Moreover, to identify how more advanced plasticity material models such as the anisotropic model of Ponte-Castaneda Kailasam and coworkers can help and increase the accuracy of the calculations, compared to more commonly used models such as the one suggested by Gurson-TveergardNeedleman (GTN). Furthermore, the results from the simulations in densification and involute profile are also correlated with experimental results to validate the accuracy of the simulations. Finally, the accuracy of the simulations in densification and involute profile will define if the target of optimizing the gear rolling densification process through FE simulations is realistic.Keywords : Gears, Powder etal m , FEM, Plasticity, Densification, Gear olling, r Simulations IntroductionPowder Metallurgy (PM) is an established technology for manufacturing components in many applications. The reason is the high productivity of highly complex parts with good mechanical properties and tolerances. At the same time it is an environmentally friendly process. The stages for manufacturing a component from powder metal are three, first iron powder is mixed with carbon and other alloying elements that are chosen depended on the application. The mixed powder is then filled into a mold and pressed by the tool. At this stage the pressed component possesses a low strength due to the fact that the grains are stacked together from plasticization during compaction. The final process is the sintering process, the components undergo a heat treatment under certain atmosphere conditions and in a temperature bellow the melting temperature of the alloy in order to bond by means of atomic diffusion between adjacent powder particles and attain the required strength. The product that is taken after sintering is a ferrous porous steel component with lower mechanical properties than the full dense part due to the presence of porosity.One of the component that is increasingly becoming very interesting to the PM industry are highly loaded gears. Highly loaded PM gears can be produced by pressing sintering and then treated with post sintering operations which ...

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INTRODUCTIONSimulations are continuously becoming more and more important to predict the behaviour of materials, components and structures. By applying advanced material modelling as a supplement to experiments, it is often possible to shorten lead times during development considerably. One example is surface densification of sintered gears through gear rolling, see for instance [1][2][3]. By using finite element simulations it is possible to simulate the process and optimize for instance the tool geometry or the material stock on the work pieces.However, the accuracy of the simulations will depend on how well the material model predicts the material behaviour. Classic metal plasticity models typically rely on von Mises plasticity, and results in zero volumetric strain. This is clearly not correct for densification of PM steels, and special plasticity models for porous materials should be used instead.One model that is often used is the Gurson plasticity model [4], that is also found in some commercial finite element software such as Abaqus [5]. The Gurson yield criterion combines the von Mises stress with the hydrostatic stress, while accounting for different porosity levels, into a model suitable for porous materials. Fig. 1 illustrates the yield surfaces for different porosity levels, note that for zero porosity (f=0) the model is identical to classic von Mises plasticity.The purpose of this paper is first to investigate how the necessary input to the Gurson model can be calibrated from tensile tests on PM materials. Next, to simulate surface densification, indentation tests are made where a steel ball is pressed into the surface of a PM material. The experimental indentation tests are then compared to calculated ones, testing the application of the Gurson model.