Influence of Melt Feeding Scheme and Casting Parameters During Direct-Chill Casting on Microstructure of an AA7050 Bille
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T-CHILL (DC) casting is the major technology for producing aluminum ingots and billets for further deformation processing. Usually, a fine, equiaxed grain structure and a uniform grain structure distribution across billet (ingot) are desired in aluminum casting for improving yield strength, fracture toughness, ductility, and other required properties by minimizing shrinkage, hot tearing, and segregation.[1,2] The main casting process parameters in this technology, such as casting speed, casting temperature, melt flow feeding system, and cooling water flow conditions, are crucial to the evolution of microstructures during solidification, as well as for the formation of defects. Although invention and commercial use of DC casting could be dated back to the 1930s, and a number of studies were dedicated to the effects of process parameters on structure, relatively little is known about structure under different melt flow feeding schemes. Generally, higher casting speed and lower casting temperature result in a finer microstructure,[3–5] while cooling water-flow rate has relatively minor effect on grain structure.[6,7] The distribution of structure parameters in the horizontal section of a DC casting billet (along the L. ZHANG, Ph.D. Researcher, T. SUBROTO, Ph.D. Student, and L. KATGERMAN, Professor, are with Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: Liang.Zhang@ tudelft.nl D.G. ESKIN, Professor, is with BCAST, Brunel University, Uxbridge, UB8 3PH, U.K. A. MIROUX, Research Fellow, is with Materials Innovation Institute, Mekelweg 2, 2628 CD Delft, The Netherlands. Manuscript submitted December 16, 2011. Article published online August 21, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
radius or diameter) is of great importance since it determines the homogeneity of technological properties during downstream processing. The distribution of grain structure depends to a great extent on the profile of a sump (sump depth and shape), temperature distribution and the flow pattern in liquid and transition regions, which are difficult to observe directly in the real DC casting processing.[8] The lack of these data restricts our understandings on the evolution of microstructures during DC casting. Nowadays, various numerical and analytical simulations allow us to visualize and analyze the solidification patterns during DC casting. To fully understand what really happens during solidification and how the microstructure is formed in DC casting, both experimental and numerical analysis are essential. The numerical simulation complements the experiments study and becomes a tool of research. Before we move on, let us specify the terms which are related to structure formation during DC casting. Solidification front is the macroscopically continuous and microscopically discontinuous front of the growing solid phase, can be also named coherency isotherm.[8] The condition of coherency can be defined as the moment when solid grains begin to interact with each
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