Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy

International audienceIn 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

Influence of a 3D realistic crack path in the driving forces for fatigue crack growth under mode I+II loading

International audienceThe aim of this paper is to test the influence of different crack path models, from oversimplification currently used in literature of a straight crack front orthogonal to the specimen sides to a realistic 3D crack path. On the basis of experimental features observed for Ti17 and Ti6242 alloys under multiaxial loading, a sensitivity analysis is proposed to address the impact of realistic 3D crack path on SIF and FCGR assessment

Characterization of damage in a cast aluminum alloy during cyclic loading test at high temperature by x-ray tomography

International audienceThe aim of this work is to clarify the role of microstructure heterogeneity in the initiation and growth of cracks in a lost foam cast A319 for low cycle fatigue (LCF) loading condition. Because 2D analysis fail to establish the scenario of damage events, this study is based on 3D in situ analysis using synchrotron X-ray tomography during LCF test performed at 200°C. If X-ray tomography is now an established technique to study damage development in 3D, its application for relatively high temperature cyclic tests has never been done so far. Initial 3D observations of damage show that the cracks that lead to fracture initiate near large pores and propagate into the hard particles 3D network of the eutectic regions

In situ 3D characterization of high temperature fatigue damage mechanisms in a cast aluminum alloy using synchrotron X-ray tomography

International audienceFatigue tests were performed at 250 °C on a cast AlSi7Cu3Mg aluminum alloy and monitored with Synchrotron in situ X-ray tomography in order to understand the micro-mechanisms of crack initiation and propagation. The analysis of the 3D images reveals that internal shrinkage pores are responsible for the main crack initiation. Crack propagation is mainly due to the complex and highly interconnected network of hard particles of the eutectic regions