Simulations are becoming very important in the development of products and production processes. Lately, computing power availability and upgraded algorithms have made simulation tools more accessible to R&D engineers. FE (Finite Elements) is a method that is used for instance to predict the stress levels and the elastic-plastic deformation of a component during certain loading conditions. In this study, FE simulations are used as a tool to gain an understanding if the plastic behaviour of a PM gear during the rolling densification process can be achieved with a good level of accuracy.
Progress of the PM manufacturing in gear technology
Figure 1. Helical gear pressed in Höganäs ab facilities
Powder Metallurgy is an established technology for manufacturing components for many applications. The reason is the high productivity to lower cost ratios that produces parts with good mechanical properties, at the same time it is an environmentally friendly process. PM gears are produced since many years and for different applications especially in the automotive industry. Recently, many improvements have been attained in compacting robust helical gears with the PM technology. This was demonstrated for instance through the Powder Metal Gearbox Initiative (https://www.youtube.com/watch?v=pnR10rTk4BA) and the compaction of the fourth gear for the M32 gearbox in Höganäs ab prototyping centre (PoP centre) (https://www.youtube.com/watch?v=iIVhUxVhjCg).
PM manufacturing process
Figure 2 Schematic of the PM manufacturing process
The stages for manufacturing a component from PM are three.
- 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 die and pressed by the tool to the desirable shape. At this stage, the pressed component possesses a low strength because the grains are bonded together from cold welding during compaction.
- The final process is the sintering, the components undergo a heat treatment under certain atmosphere conditions and in temperatures below 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 resulting component is porous with sufficiently good mechanical properties for many industrial and automotive applications. In addition, secondary operations can improve the mechanical properties of the PM components. Accordingly, for gears, the rolling densification is a purely mechanical process that improves the fatigue properties and increase the life expectancy of the component.
Gear rolling densification
Gear rolling densification is a post sintering process in which the porosity is closed at the surface and in some depth under the surface of the gear. The rolling process improves the mechanical properties such as the contact strength. Moreover if a good geometric quality is
obtained by rolling, no extra process such as grinding is needed which will reduce the manufacturing cost. The process starts by placing the gear with added stock material between two tools. The tools during the process have two degrees of freedom, the rotational and the translational in the horizontal direction. The gear with the added stock material can only rotate. The tools rotate and press at the same time the gear until the optimum involute profile is formed and a good surface quality is achieved. Figure 3a shows a gear rolling machine from Profiroll Technologies GmbH and how the gear is placed between the tools. Figure 3b shows the kinematics of the process schematically.
Figure 3 a. Gear rolling machine from Profiroll Technologies GmbH. b. Schematic of the gear rolling densification process
The process, the geometry of the tool and the added stock to the gear are the important parameters that can be modified in order to achieve an optimal result. Until recently, these parameters were identified through experiments, which are costly and time consuming.
FE simulations of the PM gear rolling densification
The main question that is addressed in this research is how the FE method can improve the PM gear rolling process. Improvement by means of reduced development time for the design of the tool and gear with stock distribution, coupled with the necessary input process parameters. Even more, the ability to use simulation tools for optimization reasons can lead to solutions that have not being tested and may produce enhanced results. To accomplish this, a good accuracy between experimental and simulation results should be realized that can justify the attempt to optimize the process.
Therefore, advanced plasticity material modelling should be used. Furthermore, advancements in contact algorithms have led to solutions that are more robust and consequently realistic simulation conditions can be applied. Some work has been done in simulations by considering the tool as a rigid body and the pores as initially spherical. In this study, FE simulations are conducted with the pores to be considered initially as randomly distributed ellipsoids, the material model to be initially anisotropic and the tools as flexible bodies.
Vasilis Angelopoulos, M.Sc., Development engineer
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Höganäs AB | PM-Components | SE-263 83 Höganäs | Sweden
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