Modeling of Microporosity Size Distribution in Aluminum Alloy A356
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ALONGSIDE defects such as oxide films and intermetallic particles, porosity is an important microstructural phenomenon as its presence may impact negatively on the mechanical properties of cast components. Components under cyclic loading are often observed to have surface or internal pores as crack initiators. The porosity distribution, therefore, is a crucial parameter for fatigue life assessment. According to Dabaye et al.[1] and Zhu et al.,[2] the pore size distribution can be divided in two populations. The first population is related to small defects where the number is relatively high but the volume fraction of porosity is low (these defects are typically homogeneously distributed in the casting). The second population is of larger defects: the number of these defects is comparatively low, they make up a significant portion of the porosity volume fraction, and they are typically less homogeneously distributed. The existence of these two defect populations was confirmed by X-ray microtomography (XMT) observations performed on A356 in a previous study.[3] Threedimensional (3-D) XMT experimental observations of the porosity distribution are required as two-dimensional (2-D) metallographic observations can lead to errors in porosity size distribution measurement, as shown by Lashkari et al.[3] and Nicoletto et al.[4] LU YAO, PhD Candidate, STEVE COCKCROFT, Professor, JINDONG ZHU, Research Associate, and CARL REILLY, Postdoctoral Fellow, are with the Department of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] Manuscript submitted March 14, 2011. Article published online September 15, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Although the role of the small pore population on fatigue behavior is still unclear due to the competition with other microstructural features (such as the primary eutectic and intermetallics), it is well recognized that assessment of the large defect population is crucial for accurate fatigue assessment.[2] Many authors observed a reduction of the fatigue life of A356 T6 with increasing pore size.[5,6] These observations were often performed using fatigue life as the observed variable, so that the effect of defect size could be considered for fatigue life assessment. The reduction in fatigue life with increasing pore size is also supported by the research showing crack initiation to be a negligible factor in the fatigue life of A356 T6 under tension fatigue loading.[7,8] The complex morphology of pores and how it impacts fatigue life was the subject of recent work, which has shown that 3-D computations are needed to assess local stresses[4] and that an equivalent homogeneous 3-D ellipsoid can be a reasonable approximation of a complex pore morphology.[9] Therefore, the simulation of the porosity size distribution remains an important issue for the optimization of casting processes where component fatigue life is critical. As Li et al. showed,[9] it is appropriate to model an equivalent pore size rather than
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