This is especially the case for helical gears with a helix angle above 30 degrees. Additionally, although PM technology offers lower cost components, there are currently still some limitations in static and fatigue properties of sintered steel materials which preclude the use of conventionally produced PM parts for these applications.
DensiForm® Helical Powder Metal Gears for Automotive Transmissions
A major step forward in automotive helical gear production by Powder Metallurgy was presented in a paper at PM2012 World Congress in Yokohama, by Dr Salvator Nigarura, Director of Research and Development at PMG Corporation and colleagues in Columbus, IN, USA.
Dr Nigarura presented information on a new and powder metal compatible surface densification technology called DensiForm®, which he stated has been adapted to produce complex, highly loaded helical transmission gears having superior dimensional accuracy and uniform surface densification of the gear teeth from the root to the active flank surfaces. Dr Nigarura stated that the DensiForm® process has the added benefit of substantially increasing the core density of the sintered helical gears whilst performing sizing and surface densification, bringing the levels of mechanical and fatigue properties in the gears up to the level required for high torque transmission applications.
A development manual transmission helical gear (Fig.1) having 39 teeth and a helix angle of 33 degrees was produced by compacting a Mo prealloyed steel powder (0.85 wt% Mo) mixed with 0.2 wt% graphite and pressing lubricant. The gears were compacted to a green density of 7.2 g/cm3, followed by sintering at conventional temperature (1150°C/2100°F) on a belt furnace. Dr Nigarura stated that the DensiForm® process is performed in the same Powder Metallurgy press used for powder compaction with the added benefit of substantially increasing the core density to 7.35 g/cm3 whilst performing sizing and surface densification. This new process is significantly different from the surface rolling process for PM gears which is performed in a separate circular die force controlled rolling machine.
Fig.2 shows the layout of tools used for the surface densification of the development helical gear shown in Fig.1. Given the helical nature of the external form of the part, the upper (outer and inner) and lower outer punches follow a helical motion that is in keeping with the geometry of the gear (the punches rotate together as they advance axially). The lower inner punch remains stationary, as do the DensiForm® tools and die. This action is different from that seen at powder compaction where the die floats.
Dr Nigarura said that simulation of the DensiForm® process, especially for helical gears, is capable of predicting with high level of precision the amount of densification and the uniformity of the densified layer. The important benefit of simulation is the design of DensiForm® tools and of the blank geometry to achieve the level of densification needed. Furthermore, simulation of the DensiForm® process allows for accurate predictions of final part geometry and dimensions as well as density levels following surface densification.
The results of the simulation analysis given by Dr Nigarura are presented in Figs.3 and 4. Planar sections at two axial locations are shown in Fig.3 with accompanying density maps at those locations. The extent of densification is shown in terms of relative density, which is the ratio of the current density to the theoretical density of fully dense steel. A density of 7.83 g/cm3 (with corresponding relative density of 0.996) is seen at a depth of 0.4 mm from the surface at location 1. Similarly, at location 2, a density of 7.8 gm/cm3 is reported at a depth of 0.4 mm on the left flank. Adequate depths of densification are seen to be achieved on the gear flanks, while substantial densification depths are achieved in the root area.
The surface and core densified gears were subsequently heat treated in two groups. The first group was atmospheric carburized using endothermic gas and then quenched in oil. The second group was vacuum carburized using low pressure acetylene (less than 20 mbar) and then quenched using high pressure helium gas (20 bar) to minimize distortions.
Dr Nigarura found that whilst heat treatment introduces distortion, vacuum carburization with high pressure gas quenching reduces significantly the level of distortions. A high gear quality, AGMA A-4 for profile error and A-6 for helix error is obtained after high pressure gas quenching. In terms of properties Dr Nigarura reported a Young’s Modulus of 150,000 MPa and Poisson’s Ratio of 0.27 at the sintered core density >7.35 g/cm3. Further work is being done to characterize DensiForm® produced sintered steel gears in a real transmission and results will be presented in a future paper.
 From paper: ‘DensiForm® Helical Gears for Automotive Transmissions’ by S. Nigarura, R. Parameswaran, M. Bird, M. Scott, G. Rau. Presented at the PM2012 World PM Congress, Yokohama, October 15-18, and published in the Congress Proceedings by JPMA/JSP&PM, Japan
PM2012 World Congress Proceedings
Papers presented at the PM2012 World Congress will be published by the JPMA/JSP&PM in the Congress Proceedings, available in early 2013. Further information will be posted on the conference website.