Role of Grain Refinement in the Hot Tearing of Cast Al-Cu Alloy
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tearing is a common and severe defect encountered in alloy castings and perhaps the pivotal issue defining an alloy’s castability. Once it occurs, the casting has to be repaired or scrapped, resulting in significant productivity losses. Hot tearing has been a focus topic in metallurgy since publications in foundry metallurgy began. Over the years, many theories and models have been proposed and developed, but unfortunately many of the tests that have been proposed are qualitative in nature. Furthermore, many of the predictive models are not satisfactory as they lack quantitative data. The need exists for a reliable, robust, and quantitative test to evaluate and characterize hot tearing susceptibility of cast alloys. The team at MPI and CANMET created a joint research project to study hot tearing. An apparatus with an instrumented constrained rod mold was developed. The system is designed to quantitatively evaluate and investigate hot tearing behavior by measuring the real time contraction force developed during solidification and cooling of the casting. Details of the experimental setup and the methodology for conducting hot tearing test are presented elsewhere.[1] Quantitative information obtained from the measurement helped reveal details of hot tear formation and also shed light on the operative SHIMIN LI, Post-Doctoral Associate, and DIRAN APELIAN, Alcoa-Howmet Professor of Engineering, are with Metal Processing Institute, WPI, Worcester, MA, 01609. Contact e-mail: dapelian@ wpi.edu KUMAR SADAYAPPAN, Senior Research Scientist, is with the CANMET—Materials Technology Laboratory, Hamilton, ON L8P 0A5, Canada The following copyright notice pertains only to K. Sadayappan: Printed by permission of Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2013. Manuscript submitted December 20, 2010. Article published online February 22, 2013. 614—VOLUME 44B, JUNE 2013
mechanism. The effects of key processing factors on hot tear formation were studied using this system; the effects of grain refinement on hot tearing are presented in this paper. A. Effects of Grain Size and Morphology on Hot Tearing: A Review A number of studies have been conducted to investigate the effects of grain refinement or grain morphology and size on hot tearing.[2–17] However, the results are not consistent and the literature is confusing. Matsuda et al.[15] studied the effect of different elements on hot tearing susceptibility of an Al-2 pct Zn-2 to 3 pct Mg weld alloy. They found that the results were related to the element’s grain refining efficiency. An alloy with large columnar grains exhibited severe hot tearing tendency with larger cracks and reached the maximum value for hot tearing rating. On the other hand, when grains became equiaxed and smaller in size, the propensity for hot tearing was lowered with reduced crack length; this trend was independent of the grain refining element and amount added. Among 13 elements evaluated, the most favorable ones were Ti+B, Ti, and Zr, and the detrimental element was Cu. Ea
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