Determination of Strain during Hot Tearing by Image Correlation
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HOT tearing refers to cracks that can occur within the mushy zone at high fraction solid during cooling from the liquid to solid state in near-net-shape and ingot castings. Both ferrous and nonferrous alloys may be affected and there is some evidence to suggest that alloy chemistries with long freezing ranges are more susceptible.[1,2] It is believed that tearing occurs due to thermally and/or mechanically induced stresses acting on semisolid material with limited ductility.[3] For example, complex casting geometries may result in constraints that can place regions of the casting in tension.[4] Due to the nature of this defect, the economic impact is often significant and can result in an immediate productivity loss.[2,5] It is therefore important for industry to be able to better predict the susceptibility of various alloys, casting geometries, and/or process conditions to hot tearing. The formation of macroscopic cracks within semisolid alloy castings has been the subject of metallurgical research since the 1940s, and they have been referred to as hot cracks, hot tears, solidification cracks, etc. and are a major cause of defects in aluminum alloys.[6] Published work in the field of hot tearing has usually presented an experimental method to assess the probability or susceptibility of particular alloys to hot tearing, in conjunction with a theory or model to explain the phenomenon observed. The majority of the experimental methods for J.B. MITCHELL, Ph.D. Candidate, Graduate Student, and S.L. COCKCROFT, Professor, Department Head, are with the Department of Materials Engineering, The University of British Columbia, V6T 1Z4, Vancouver, BC, Canada. Contact e-mail: [email protected] D. VIANO, Ph.D. Candidate, Graduate Student, and D. STJOHN, Professor, are with the CAST Cooperative Research Centre and School of Engineering, University of Queensland, Brisbane, QLD 4072, Australia. C. DAVIDSON, Project Leader, Mechanical Metallurgy & Modelling, is with the CAST Cooperative Research Centre, University of Queensland, and CSIRO Manufacturing & Materials Tech, Kenmore, QLD 4069, Australia. Manuscript submitted October 17, 2006. Article published online September 13, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
determining hot tearing susceptibility aim to induce the defect by means of constraining the solidifying casting (thus producing stresses that act on the semisolid material) and then quantifying the severity of tearing. Experimental designs have generally been of the ring-type casting,[7–10] the restrained bar-type casting,[3–5,11–15] or variations thereof. The ring casting techniques work on the principle of a central core that acts to resist thermal contraction during solidification and cooling, thereby causing tensile stresses to develop in the semisolid. If these tensile stresses result in the accumulation of strain in excess of the ductility, then a crack forms. The major drawback of this method is that it is difficult to extract quantitative data and therefore interpret the results. The cracks are often distri
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