The function of the compressor wheel in a turbocharger is to compress the air drawn by the turbine. During this process the wheel reaches temperatures up to 230°C. Aluminium alloys have been developed to withstand both the elevated temperatures and the cyclic loading that the wheel experiences during its working life. The 2618-T6 aluminium alloy is used due to its good combination of price, mechanical properties and suitability at temperatures up to 300°C. However, the manufacturer found during inspections of catastrophic failed wheels a considerable contribution of "intergranular" fracture (fracture along grain boundaries). This mode of failure could be the result of a complex combination of factors, such as a creep mechanism operating at high temperature, and/or the effect of microstructural features such as second phase particles, quenched induced intergranular and transgranular preCipitates, and precipitate free-zones located along grain boundaries. A quantitative metallography analysis was made within the compressor wheel net-shape to understand the influence of the process on the grain size and the distribution of second phase particles. It was found that the grain size is smaller in the sections where the wheel tends to fail, and that the percentage of second phase particles bigger than 5~lm is less than 10% in the same zones. Fatigue tests at both room and elevated temperature (230°C) were carried out to understand the fatigue behaviour of the material under such conditions. It was found that "intergranular" failure does not depend on temperature. A new methodology was developed during this research to quantify the percentage of "intergranular" failure (flat areas) on the fracture surfaces. The methodology is a combination of stereo-photogrammetry and image analysis. Stereo-pairs were randomly taken along the crack paths and were analysed using the MeX software. Results showed a higher contribution of flat areas at room temperature than at 230°C. The mechanisms which could promote this "intergranular" fracture were discussed by means of extensive literature review and observations during this research. The effect of these areas on the fatigue life was explored using a model based on a modification of the Paris law. The main idea of this model was to consider the material as the mixture of two phases associated with the two modes of failure observed on the fracture surfaces. The properties of a softer material were selected to simulate the "intergranular" (flat areas) behaviour. The model gave a good prediction of both the fatigue lives measured during this research and the fatigue lives provided by Cummins Turbo Technologies. Results from the model showed that the fatigue life is overestimated by a factor 10 if the presence of the flat areas is not accounted for.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:500233 |
| Date | January 2008 |
| Creators | Zamarripa, Adrian Salas |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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