Fatigue Analysis of an Aluminium Radial Impeller

The mechanical properties of the precipitation hardened aluminium alloy AL2618-T851 have been determined to obtain data for the optimization at design of the back-disc of an impeller in a turbocharger. Hardness testing was done at room temperature, while fatigue testing has been carried out at 200ºC and tensile testing at both temperatures. Light microscopy and scanning electron microscopy have been used to characterize the microstructure while Energy Dispersive Spectroscopy (EDS) gave information on the distribution of the alloying elements in the alloy. Normalized stress, strain and temperature data for the alloy are presented in this thesis. Data from LCF and HCF experiments at 200ºC have been used to generate S – N curves. Corresponding curves for other temperatures were obtained by scaling. Fractographic studies were made using SEM and light microscopy in order to determine the nature of fracture in the samples studied. It was observed that both Rm and Re decreased approximately 35% when the temperature was increased from room temperature to 200C. It was also observed that the high temperature fatigue life was very sensitive to changes in stress levels. In a stress level around 215 MPa, a 3% decrease in stress can improve the impeller’s lifespan by a factor of about 4.5. The study also showed that the deformation before fracture was significant and that the fracture always is ductile at 200ºC, independent of applied stress level.Outgoin