Surface Densification PM Process
Traverse gear rolling increases the dimensional precision and performance of conventionally produced Powder Metal gears through the controlled densification of critical surfaces.
In order to selectively densify only the critical high stress region of sintered gears, this technique can be added to the well-known conventional powder metal manufacturing methods. Figure 1 illustrates the processing sequence that allows the manufacturing of sintered gears with improved precision as well as high loaded carrying capacity.
Figure 1: Processing sequence to manufacture highly loaded sintered gears
The process starts with the conventional powder manufacture, followed by compaction and sintering of a gear preform and the subsequent transverse rolling and optional heat-treatment of the densified gear. Many conventional prealloyed, admixed or partially alloyed metal powders can be used for this process.
However - similar to other P/M processes – specific formulations may perform better than others. New material developments are being investigated in order to provide materials with optimum performance. Depending on the strength requirements of the gear, different sintered (core) densities can be provided. Low strength requirements can be fulfilled by part densities around 7,1 g/cm3, while more highly loaded applications may require core densities greater than 7,3 g/cm3 with full density in the critical stress regions at the tooth flank and at the tooth root region.
After preform production, the surface layer can be densified by the transverse rolling operation. Depending on the core density and the amount of local stock – or overmeasure - on the P/M preform, different density gradients will result. Essentially fully dense material can be achieved at the critical near-surface highly stressed region. As the distance from the surface increases, the density will reduce as a function of the parameters mentioned above (Figure 2).
Figure 2: Typical hardness and density gradients after rolling
High dimensional precision requirements can be fulfilled by running the process with a CNC-controlled machine with rotational and transversal movement of the rolling tools.
As dictated by the product application, heat-treatment of the surface densified gears may be required after rolling. If carburizing is required, the process chosen must consider the specific carburizing behavior of a surface densified gear. Depending on the gear quality requirements, hard finishing may be necessary after heat treatment.