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Impact of melt conditioning and filtration on iron-rich β phase in AlSi9Cu3 and its fatigue lifeThursday (08.10.2020) 10:30 - 10:50
The present study deals with the influence of iron-rich phases on the fatigue life of AlSi9Cu3 alloy and the impact of melt conditioning and filtration on these phases. Iron-rich phases occur due to insufficient scrap separation within the recycling process and decrease the mechanical properties such as ductility, tensile strength and fatigue life of aluminum alloys. The harmful effect of iron-rich phases can be reduced by combining different methods: (i) changing the morphology of brittle β phase (Al4.5FeSi), and (ii) reducing the iron content by sedimentation and subsequent filtration. In the current study, this combined method, called melt conditioning, is compared to conventional casting (reference) with a higher Fe content using the commercial alloy AlSi9Cu3. Both batches are investigated regarding the microstructure and chemical composition of the occurring phases by means of scanning electron microscope (EDX, EBSD), tensile tests, X-ray microtomography (µCT) and ultrasonic fatigue tests. The reference batch revealed a high proportion of β plates, which are responsible for low strength compared to melt conditioned batch under uniaxial tensile stress. The fatigue life of melt conditioned batch is significantly improved compared to the reference state.
X-ray microtomography scans before and after ultrasonic fatigue tests were evaluated by machine learning algorithms (Trainable Weka Segmentation) in order to study the role of different phases regarding fatigue crack propagation. The superposition of the segmented fatigue crack with the initial, undeformed state showed that the fatigue crack propagation is strongly influenced by the occurring phases. While the crack in the melt conditioned batch propagates mainly through the Al matrix, the β phase considerably influences the crack propagation in the reference batch. Detailed knowledge of the phases responsible for crack propagation is an important step in the further development of optimal requirements for alloy conditioning by sedimentation and filtration.