Synchrotron Tomographic Characterization of Damage Evolution During Aluminum Alloy Solidification
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THE processing of metal via casting offers substantial economic and design advantages compared to many thermo-mechanical processing routes. However, the control of solidification defects, especially porosity and hot tears, remains a major challenge limiting production yield and quality. The presence of porosity can degrade the final mechanical properties of the as-cast components, especially fatigue life.[1,2] Hot tearing is the term applied to large void formation when an alloy is still in the semi-solid or ‘‘mushy’’ state. The formation of hot tears in the semi-solid state usually results in part rejection during production. These damage voids are assumed to be caused by insufficient liquid feeding into the spaces left by the volume reduction upon solidification. It has been shown that hot tears are localized at grain boundaries and can nucleate on pores, oxides, and liquid films.[3–5] Historically, a number of experimental and numerical investigations have been performed to understand how alloying and processing affect hot tearing susceptibility.[5–8] However, most of these studies only observed the influence of processing parameters postmortem, i.e., after failure. An alternative to postmortem metallographic characterization is the use of in situ observation, as pioneered in 1966 by Hunt et al.[9] They used transparent analogs and optical microscopy in a thermal
CHEDTHA PUNCREOBUTR, PhD Student, and RICHARD W. HAMILTON, Visiting Scientist, are with the Department of Materials, Imperial College London, Prince Consort Road, London, SW7 2BP, U.K. PETER D. LEE, Professor, and BIAO CAI, PhD Student, are with the School of Materials, The University of Manchester, Oxford Road, Manchester, M1 7HS, U.K. Contact e-mail: [email protected] THOMAS CONNOLLEY, Beamline Scientist, is with the Diamond Light Source Ltd, Harwell Science & Innovation Campus, Didcot, OX11 0DE, U.K. Manuscript submitted September 4, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
gradient stage to study solidification microstructures in situ. The next year, Forsten and Miekk-oja[10] used X-ray imaging (on film) to observe the planar solid–liquid interface in aluminum-gold alloys. In 1972, Miller and Beech[11] produced the first high quality X-ray film images of dendrites forming in Al-30 wt pct Cu. In situ observation of microstructural defects during solidification was shown to also give direct quantitative values for model validation by Lee and Hunt, with their quantitative observations’ porosity using in situ radiography.[12–14] Various techniques for observing hot tearing have also been attempted. Farup et al.[4] used succinonitrileacetone as an analog of a binary metallic alloy, mimicking the solidification conditions, but not the mechanical behavior. Davidson et al.[15] and Mitchell et al.[16] successfully performed in situ optical observations of hot tearing in Al-Cu alloys, but observations were limited to the sample surface. Pellini[17] used X-ray radiography and simultaneous temperature measurement to observe hot tearing in an Al-Cu all
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