Determining stress in turbocharger impellers due to component machining

Turbocharger impeller wheels are traditionally manufactured using a casting process. However, with the improvement of multi-axial machining technology, machined impeller wheels are becoming popular among turbo machinery manufacturers due to their enhanced durability. Nonetheless, machining a complex impeller shape from a solid billet, results in tool marks being left on the component surface. As presented in this paper, repeatedly running a wheel to 5% beyond the design speed limit can result in fatigue failure initiating from the machining marks.
In this paper, the ‘as machined’ geometry of sample wheels has been determined using both CT scanning and optical surface measurement techniques. The data from these measurements has been used to generate solid CAD models suitable for finite element analysis to simulate the stress distribution of reverse engineered wheels. The maximum principal stress predicted is 15% higher than that obtained from the nominal CAD model. In order to model the measured geometry efficiently, a novel technique has been used to enforce cyclic symmetry on geometry that is not precisely cyclically symmetric.
The work has demonstrated that it is possible to predict the stress raising effect of the machining marks at the design stage. The analysis methodology presented in the paper will enable future integrated optimisation of both the design and manufacture of impeller wheels to ensure that wheels with a specified operating envelope are machined as efficiently as possible.