X-Ray Microtomographic Characterization of Porosity in Aluminum Alloy A356
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TRODUCTION
ALUMINUM alloys are an attractive option for automotive components due to their high strength-toweight ratio. The trend is to a greater use of cast components to reduce manufacturing costs. However, castings produced from aluminum alloys are prone to the usual casting related defects including macro- and microporosity, oxide inclusion, film entrainment, and hot and cold cracking. Microporosity, the subject of this study, forms as a result of the combined effect of the partitioning of hydrogen to the liquid during solidification and the restriction of liquid feeding in the late stages of solidification. The percentage and size of the microporosity increase with increasing initial hydrogen content[1–3] and the availability of nucleation sites, such as oxide inclusions.[1,4] The alloy composition and the solidification conditions are also known to be a factor.[1,4] In structural applications involving fatigue, the presence of microporosity can limit the life of the component if present in areas subject to high tensile loading. Mechanistically, both large individual pores and clusters of pores can act as crack initiation sites, thereby eliminating or greatly reducing the crack initiation stage in fatigue failure.[5] Over the last decade, increasing attention has been focused on studying and modeling microporosity formation during the solidification of aluminum alloy castings. The studies have helped to understand the factors that OMID LASHKARI, Postdoctoral Fellow, LU YAO, PhD Candidate, and DAAN MAIJER, Associate Professor, are with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC, V6T 1Z4. Contact e-mail: omid.lashkari@ ubc.ca STEVE COCKCROFT, Senior Associate Dean, is with the Faculty of Applied Science, The University of British Columbia Manuscript submitted September 29, 2008. Article published online February 14, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
influence the formation of microporosity, and the models have reached a level of sophistication where they are useful to foundry engineers.[6–10] The prediction of hydrogen porosity requires quantification of many physical phenomena, including the evolution of the thermal field, the solid fraction, and the pressure field within the mushy zone, particularly at high fractions solid. It also requires an understanding of the partitioning of hydrogen between the solid and the liquid, the solubility limit of hydrogen in liquid aluminum, nucleation processes with respect to hydrogen pore formation, and transport processes with respect to pore growth.[7] Despite the increased use of these models, there still remains a limited amount of good quantitative data that can be used for the purposes of validation and on-going model development. The aim of the present study is to quantify the effect of hydrogen content and the solidification conditions (cooling rate and temperature gradient) on the volume fraction and size distribution of microporosity in directionally solidified aluminum alloy A356. Characterization via both conventi
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