Hot-Tear Susceptibility of Aluminum Wrought Alloys and the Effect of Grain Refining
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earing is a defect that involves the formation of a macroscopic tear during the solidification of a casting. Early studies on hot tearing have been carried out mainly on casting alloys, with emphasis on wrought alloys emerging only in the last 15 years.[1–7] Hot tearing is seen in direct-chill (DC) casting of wrought alloys as a result of induced stresses and strains in the shell zone of ingots and billets.[2,4,5] The objective of this study was to investigate the hot-tearing susceptibility of aluminum wrought alloys. II.
considered the role of the liquid film in the semisolid metal during solidification.[15,16,17] In 1981, Hunt and Durans[18] pioneered the analysis of hot tearing as two different events of hot-tear nucleation and growth, and found that it was not possible to initiate a tear in a clean material in the absence of a nucleus (inclusion). Grain boundaries in the hot spots were seen as likely sites for tear initiation.[19] Spittle et al.[20] related tear growth to grain size and shape (equiaxed vs columnar). In more recent years, many studies were conducted on mathematical modeling of hot tearing, which brought the focus on the development and use of hot-tear criteria. These have been recently reviewed by Suyitno et al.[21]
THEORIES OF HOT TEARING
Many theories have been developed since the 1930s to explain hot tearing.[8–27] The early theories were based on stress and strain,[12,13,14] and the subsequent theories S. LIN, formerly M. Eng Student, Faculte´ d’Inge´nierie, University of Quebec at Chicoutimi, Chicoutimi, PQ, Canada, is Program Manager, L&P Incorporated, Schukra North America, Lakeshore Ontario, ON, Canada N8N 4Y3. C. ALIRAVCI, formerly Research Associate, Faculte´ d’Inge´nierie, University of Quebec at Chicoutimi, is Postdoctoral Fellow, CRCT, Ecole Polytechnique, Universite´ de Montre´al, Montreal, PQ, Canada H3C 3A7. M.O. PEKGULERYUZ, formerly Professor and Alcan Chairholder, Faculte´ d’Inge´nierie, University of Quebec at Chicoutimi, is Associate Professor, McGill University, Montreal, PQ, Canada H3A 2B2. Contact e-mail: mihriban. [email protected] Manuscript submitted October 27, 2005. Article published online May 10, 2007. 1056—VOLUME 38A, MAY 2007
A. Hot-Tearing Theories Based on Stress and Strain Early theories considered hot tearing to be a result of stress buildup in the solidified metal, which arises principally because of volume contraction, b, associated with the liquid-to-solid phase change and which can be made worse by thermal contraction in the solid and by the constraints of the mold. Vero[12] in 1936 proposed the brittleness theory, which described hot tearing occurring as a result of stresses set up in the interlocked coherent semisolid network of dendrites during solidification. Pumphrey[13] introduced the concept of the brittle temperature range (BTR) between the coherency temperature (temperature above the solidus where the dendrites interlock producing a coherent network) and the effective solidus temperature. He related the METALLURGICAL AND MATERIALS TRANSACTIONS A
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