Three-Dimensional CFD Simulation Coupled with Thermal Contraction in Direct-Chill Casting of A390 Aluminum Alloy Hollow
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TRODUCTION
THE direct-chill (DC) casting process is being used widely to produce extrusion billets and rolling slabs of aluminum alloys due to its relative simplicity and robust nature.[1] When pipes are to be produced via mandrel extrusion, the extrusion billets are preferred to be in hollow shape for the purpose of ease of operation and high yield. A schematic of the DC casting process for hollow billet is shown in Figure 1. This process is similar to the DC casting process for solid billets except that it contains a water-cooled inner mold (often made of copper). The molten metal is horizontally fed from the delivery system into the cavity surrounded by the hot top, core hot top, mold, inner mold (Cu core), and starting block. After the molten metal is cooled by the mold or Cu core, the initial solid shells would form at the outer and inner walls of hollow billet. As soon as the solid shells are strong enough to embay the interior molten metal, the starting block is translated downward together with the solid shell. As the starting block exits the bottom of mold, the outer wall of hollow billet is cooled rapidly by a direct flow of water. However, a macroscopic air gap forms once the inner wall of hollow KESHENG ZUO, Lecturer, and QINGZHANG CHEN, Professor, are with the School of Automotive Engineering, Changshu Institute of Technology, Changshu 215500, China. Contact e-mail: [email protected] HAITAO ZHANG and KE QIN, Associate Professors, and JIANZHONG CUI, Professor, are with the Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China. Manuscript submitted June 21, 2016. Article published online November 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
billet is separated from the outer wall of Cu core. The heat dissipation at the inner wall is dominated by the surrounding air. This process terminates when the hollow billet with a desired height is obtained. The hollow billet can be then sectioned, homogenized, and hot extruded to produce pipes. Except for the adjustments of conventional casting parameters, such as casting speed, casting temperature, and cooling water flow rate, the formability of hollow billet largely depends upon the design of taper angle of the Cu core (h). An appropriate core taper angle should be designed to ensure the success of DC casting process for hollow billet. Because the diameters of both the outer and the inner walls decrease as the solid shells shrink during the DC casting process. If the core taper angle is smaller than the certain value, the shrinking inner solid wall of hollow billet would directly contact with the outer wall of the core. The excessive contact pressure propels the core to be drawn into the hollow billet and leads to the occurrence of ‘‘hanging’’ problem. However, when the core taper angle is too large, which would enlarge the air gap width between the core and hollow billet, leading to partial remelting of the inner wall and the formation of liquid metal exudation, the extreme case is
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