Quantitative Assessment of Deformation-Induced Damage in a Semisolid Aluminum Alloy via X-ray Microtomography
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INTRODUCTION AND BACKGROUND
THE control of defects during the processing of metals is critical to the production of high quality products. In the context of strain-induced defects, damage initiates as vacancies that coalesce and subsequently grow, resulting in macroscopic flaws. One example of this process is the formation of internal voids when semisolid material is strained. In direct chill casting of aluminum ingots and billets, the thermal stresses are sufficient to induce localized damage leading to product rejection, even though the metal is not externally constrained during solidification. This phenomenon, often termed hot tearing or hot cracking, is an important defect in a range of processes from shape casting[1] to welding,[2] but is poorly understood. Over the years, a number of researchers have experimentally investigated hot tearing. Pellini[3] was the first author to demonstrate that hot tears form in the semisolid. Feurer[4] examined the influence of alloy composition and solidification conditions on hot tearing, and proposed that this defect was a result of the inability of liquid to feed solidification shrinkage. Warrington and McCartney[5] examined the effect of grain refining on hot tearing, and found that hot tears formed easily in columnar and equiaxed-globular grain structures, but not in equiaxed-dendritic structures. Using experimental data from ring castings, Guven A.B. PHILLION, formerly Doctoral Candidate, Department of Materials Engineering, University of British Columbia, is Postdoctoral Fellow, Computational Materials Laboratory, Institute of Materials, Ecole Polytechnique Fe´de´rale de Lausanne, Lausanne, CH 1015, Switzerland. Contact e-mail: andre.phillion@epfl.ch P.D. LEE, Professor, is with Department of Materials, Imperial College London, Prince Consort Road, London, SW7 2BP, United Kingdom. E. MAIRE, Research Associate, is with Universite´ de Lyon, INSALyon, Villeurbanne 69621, France. S.L. COCKCROFT, Professor and Department Head, is with Department of Materials Engineering, University of British Columbia, Vancouver, Canada V6T 1Z4. Manuscript submitted January 21, 2008. Article published online July 15, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
and Hunt[6] showed that hot tears initiate in a thin film of liquid between two grains. While these studies have proved insightful in developing processes that are less susceptible to hot tearing, they lack insight into the underlying mechanisms controlling hot tear formation. To examine these underlying mechanisms, a few authors have designed experiments that allow for observation of semisolid crack initiation. Pellini[3] used X-ray radiography to make the first observations of hot tear formation in aluminum-copper alloys. Fredriksson and Lehtinen[7] performed hot tensile tests inside a scanning electron microscope, showing that hot cracks occur if the alloy contains a eutectic liquid with good ability to wet the solid grain boundaries. Davidson et al.[8] recorded the formation of hot tears in an aluminum-copper alloy during solidification
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