Thermomechanical fatigue of cast aluminium alloys for cylinder head applications–experimental characterization and life prediction

Abstract

TMF loading in diesel cylinder heads made of cast aluminium alloys is expected to steadily become more severe in the future. This increasing loading condition will lead to an early crack initialization and eventually determine component life. Against this background several cast aluminium alloys have been studied with respect to their thermal and mechanical properties. In addition to standardized specimens, near component-shaped samples were tested under thermomechanical fatigue conditions in a testing device which represents near-service conditions, in order to characterize life until a long crack is formed. The sample geometry tries to represent the valve bridge of a diesel cylinder head, which is known to be the most thermomechanically loaded and hence critical part in this component. The results of the TMF tests with this new type of sample were compared with the results obtained on standardized specimens.The experimental results formed a sound basis to improve the thermomechanical life prediction methodology for cast aluminium alloys. Two different life prediction models were used to assess the TMF life of the near-component samples. Furthermore, an aging model for cast aluminium alloys was derived from the experimental results including the pronounced change of the mechanical properties which result from the combined effects of aging and cyclic plastic deformation. The comparison of the experimental and predicted results clearly shows that the new model leads to an improvement of the TMF life prediction.