Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy |
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| Affiliation: | 1. Ecole Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Qc H3C 3A7, Canada;2. INSA-Lyon, MATEIS CNRS UMR 5510, F-69621 Villeurbanne, France;3. Centre des Matériaux, Mines ParisTech, CNRS UMR 7633, BP 87, 91 003 Evry Cedex, France;1. Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstraße 11, 79108 Freiburg, Germany;2. Offenburg University of Applied Sciences, Badstraße 24, 77652 Offenburg, Germany;1. School of Materials Science and Engineering, Tongji University, Shanghai Key Laboratory for R&D and Application of Metallic Functional Materials, Shanghai 201804, China;2. Gen. Motors Co., Materials Technology, Global Powertrain Engineering, Pontiac, MI 48340, USA;1. PPRIME institute, ISAE-ENSMA, UPR CNRS 3346, Material Engineering Department, 1 Avenue Clément Ader, Téléport 2 – BP 40109, F86961 FUTUROSCOPE CHASSENEUIL Cedex, France;2. Laboratoire de Mécanique de Sousse, Ecole Nationale d’Ingénieurs de Sousse, Bp 264 erriadh, 4023 Sousse, Tunisia;3. School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, B38g Sackville St. Building, Manchester M13 9PL, UK;4. Dept. of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada |
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| Abstract: | In order to observe the influence of strontium (Sr) modification and hot isostatic pressing (HIP) on an aluminum–silicon cast alloy A357 (AlSi7Mg0.6), the microstructure and the high cycle fatigue behavior of three batches of materials produced by investment casting (IC) were studied. The parts were produced by an advanced IC proprietary process. The main process innovation is to increase the solidification and cooling rate by immersing the mold in cool liquid. Its advantage is to produce finer microstructures. Microstructural characterization showed a dendrite arm spacing (DAS) refinement of 40% when compared with the same part produced by conventional investment casting. Fatigue tests were conducted on hourglass specimens heat treated to T6, under a stress ratio of R = 0.1 and a frequency of 25 Hz. One batch of material was unmodified but two batches were modified with 0.007% and 0.013% Sr addition, from which one batch was submitted to HIP after casting. Results reported in S–N diagrams show that the addition of Sr and the HIP process improve the 106 cycles fatigue strength by 9% and 34% respectively. Scanning electron microscopy (SEM) observation of the fracture surfaces showed a variety of crack initiation mechanisms. In the unmodified alloy, decohesion between the coarse Si particles and the aluminum matrix was mostly observed. On the other hand, in the modified but non HIP-ed alloy, cracks initiated from pores. When the same alloy was subjected to HIP, a competition between crystallographic crack initiations (at persistent slip bands) and decohesion/failure of intermetallic phases was observed. When compared to fatigue strength reported for components produced by permanent mold casting, the studied material are more resistant to fatigue even in the unmodified and non HIP-ed states. |
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| Keywords: | Aluminum alloy A357 Investment casting Microstructure High cycle fatigue Crack initiation |
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