Formation of Hot Tear Under Controlled Solidification Conditions
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elongs to a group of high-strength aluminum alloys. Thanks to its superior mechanical properties, it is widely used in aerospace industry.[1] The main production route of this alloy includes direct-chill (DC) casting.[2] In such a process, a just-solidified billet shell is directly cooled with water. Consequently, a severe thermomechanical condition is created inside the billet, involving high temperature gradients, inhomogeneous contraction, and tensile thermal stresses. All these make the billet prone to cracking. From previous research, AA7050 is known as a ‘‘hard-to-cast’’ alloy
TUNGKY SUBROTO, Ph.D. Student, and ALEXIS MIROUX, Senior Researcher, are with the Materials innovation institute (M2i), Mekelweg 2, Delft 2628 CD, The Netherlands and also with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, Delft 2628 CD, The Netherlands. Contact e-mail: [email protected]; [email protected] LIONEL BOUFFIER, Ph.D. Student, CHARLES JOSSEROND, Engineer, LUC SALVO, Associate Professor, and MICHEL SUE´RY, Professor, are with the Grenoble INP, SIMaP/GPM2, UMR CNRS 5266, UJF, BP46, 38402, Saint-Martin d’He`res Cedex, France. DMITRY G. ESKIN, Professor, is with the BCAST, Brunel University, Uxbridge, Middlesex UB8 3PH, U.K. LAURENS KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted October 3, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
because of its susceptibility to both hot and cold cracking.[3] Hot tears or cracks form above the solidus while cold cracks form below the solidus. Cold-cracking occurrence is seemingly random and difficult to predict. In addition, the propagation of such cracks is usually catastrophic. Therefore, not only does it reduce the yield of DC casting production and increases the production cost but this type of cracking can also induce severe damage to personnel and equipment in the vicinity of the casting area.[4,5] Progress in predicting the cold-cracking occurrence was recently made using a fracture mechanics approach.[6,7] The developed criterion for the crack states that solidification defects that survive until the ingot is fully solid could become the initiation point of the cold crack if they exceed a critical crack size. It was also suggested that underdeveloped or microscopic hot tear could be such an initiation point of the catastrophic cold crack. However, the validation of this assumption requires the presence of a defect of known size before the cold-cracking testing. The development of an experimental technique that would allow creating a hot tear under controlled conditions is the subject of the current article. Recent experimental advances give the possibility to perform high-resolution and in situ observation of hot tearing.[8–10] However, most of these studies are directed toward the development of hot tearing up to complete
failure. Our main focus is on the contrary, on undeveloped hot tears that remain within the billet and could act as a
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