Centerline Depletion in Direct-Chill Cast Aluminum Alloys: The Avalanche Effect and Its Consequence for Turbulent Jet Ca
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I.
INTRODUCTION
DIRECT-CHILL (DC) casting is one of the major current processing routes for producing large-scale castings before subsequent mechanical deformation, e.g., rolling or extruding. In spite of many years of research into the development and advancement of this technology, the fundamental defects remain consistent: hot and cold cracks, inclusions, a rough or uneven surface, and macrosegregation. This final defect has received our attention in the recent years and in particular, centerline segregation is addressed in this paper. The mechanisms driving macrosegregation are generally known and their review is available elsewhere.[1,2] The typical location of interest for macrosegregation in DC cast ingots is the centerline region, where up to a 15 pct difference from the nominal alloy composition can be observed.[3,4] This difference in composition is ultimately deemed responsible for physical property variations in rolled plate products.[5,6] Two mechanisms are traditionally proposed for centerline segregation: shrinkage-induced flow and sedimenting (or floating) grains. In a previous study,[4] we put forth the later mechanism as dominant for centerline depletion in DC cast aluminum ingots. We subsequently demonstrated that centerline depletion could be minimized by the introduction of a turbulent jet, which impinges on the base of the molten pool and causes the resuspension of sedimented grains.[6] We have also previously reported evidences that the degree of centerline depletion varies as a function of cast length within the ingot itself for standard practices.[4] Numerous investigators have concluded that the depth of the solidification interface (sump) impacts the degree of macrosegregation caused 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 May 7, 2016. Article published online July 25, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
by shrinkage-induced flow.[2] Herein, in the context of the sedimenting grain hypothesis, we propose to apply the basics of avalanche dynamics and evaluate its possible role in DC casting. In particular, we postulate that the sump depth not only impacts shrinkage-induced flow, but also the volume of sedimenting grains found at the ingot center. We therefore first propose an analytical model describing the role of the inclination of the sump on grain accumulation (stacking) in traditional DC casting. We then compare the prediction with prior experimental reports. Secondly, we present new experimental results obtained with a turbulent jet designed to resuspend grains.
II.
THEORY AND MODEL
The solidification path of a given aluminum alloy is marked by several distinct events occurring in the sump. The onset of nucleation begins approximately at the liquidus temperature. The young grains are mobile and free to move independen
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