Minimization of Macrosegregation in DC Cast Ingots Through Jet Processing
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INTRODUCTION
MORE than two-thirds of the aluminum produced in the United States is first cast into ingots using the Direct-Chill (DC) process,[1] prior to being transformed into sheet, plate, extrusions, or foil. The process, devised in its current version in the 1940’s has subsisted thanks to its simplicity, robustness, and wide range of applicability. In spite of its industrial importance, metallurgical defects persist and must be addressed before the process may continue to evolve. One longstanding quality issue for large castings of aluminum alloys is macrosegregation. Defined as compositional variation visible across the dimension of the casting, this macroscopic defect dictates variations of properties throughout the entire finished product.[2,3] Compositional variations are perhaps one of the earliest recorded metallurgical defects but their origin and prediction continue to be an active area of research. In spite of a large volume of results, no reliable solution has been proposed to mitigate the deleterious effects of macrosegregation at production-level quantities for DC cast slabs. II.
BACKGROUND
The fundamental cause behind macrosegregation is the relative movement of liquid and solid phases during solidification.[4,5] This relative movement implies that the micro-scale partitioning of solute between liquid and solid phases (microsegregation) translates into large-scale
SAMUEL R. WAGSTAFF, Graduate Student, and ANTOINE ALLANORE, T.B. King Assistant Professor of Metallurgy, are with the Department of Materials Science & Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139. Contact e-mail: [email protected] Manuscript submitted March 29, 2016. Article published online June 14, 2016. 3132—VOLUME 47B, OCTOBER 2016
differences in chemical composition (macrosegregation). The movement can be driven by a number of factors, whose magnitude depends not only on casting practice, but also alloy composition, and shape of the transition region (sump).[4,6] In the present work, the focus is primarily on the transport and preferential sedimentation of free-moving grains, as justified below. Commercially cast aluminum alloys tend to solidify as equiaxed grains due to the addition of exogenous nucleation sites (grain refiner). In the slurry region (defined as the region between the liquidus and coherency isotherms[4]) these grains are mobile and can travel short or long distances depending on convection conditions. The preferential sedimentation of these grains at the bottom of the sump is often considered as the origin of centerline segregation in DC cast ingots.[6–8] Grain sedimentation was one of the first mechanisms proposed for segregation,[9] due to the observation of a duplex structure, characterized by a mixture of coarse and fine cell dendritic grains, near the ingot centerline.[10–13] As each position in the sump has a distinct solidification pattern, thus a unique microstructure; the observation of several distinct microstructures suggests differing origins and thermal histori
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