Effect of the Intensity of Melt Shearing on the As Cast Structure of Direct Chill Cast 2024 Aluminum Alloy
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IN the direct chill (DC) casting of a wrought aluminum alloy, a fine and uniform equiaxed as cast structure across the ingot is desired.[1–3] Grain refinement can reduce the porosity, decrease the tendency of hot tearing and improve the mechanical properties. However, during the DC casting process, solidification happens from the surface to the center of the ingot under the primary cooling in a mold and the secondary cooling generated by the direct spraying of water, resulting in a deep sump and large temperature gradient within the ingot. These solidification characteristics determine that the ingots prepared by the DC casting have some inherent defects, such as a coarse as cast structure in the center of the ingot and serious macrosegregation.[4,5] These defects will be more serious in the production of large-size aluminum alloy ingots. The main methods to solve these problems are adding grain refiners and applying external fields such as an electromagnetic field, ultrasonic field, or stirring.[6–9] With regard to the addition of a grain refiner, although XUDONG LIU, QINGFENG ZHU, YUBO ZUO, CHENG ZHU, and JIANZHONG CUI are with Key Lab of Electromagnetic Processing of Materials, Northeastern University, Shenyang 110004, P.R. China and also with the College of Materials Science and Engineering, Northeastern University, Shenyang 110004, P.R. China. Contact e-mail: [email protected] ZHIHAO ZHAO is with College of Materials Science and Engineering, Northeastern University. Manuscript submitted May 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the grain structure can be refined by enhancing the heterogeneous nucleation, it cannot eliminate the structure difference caused by the different cooling rates on the cross section of the ingot. For the electromagnetic field, due to the skin effect, its intensity at the center of the ingot is weak, which makes the grain refinement limited. For an ultrasonic field, although it has a good effect on the grain refinement, it is limited by the relatively small action zone. Recently, Fan[10] proposed intensive melt shearing and achieved a good grain refining effect. Compared with other technologies, the intensive melt shearing in the DC casting process has the following characteristics: (1) the melt is sucked from the bottom of the unit by a high speed rotating rotor, and then ejected rapidly through a small hole in the stator, which produces a controllable forced convection in the melt[10,11]; (2) it can provide a relatively high-shear rate which is beneficial to enhance the heterogeneous nucleation by breaking and dispersing the solid particles in the melt[12–14]; (3) the stator–rotor high-shear unit is low in cost and easy to operate. Zuo[15] and Li[16] suggest that the grain refinement caused by intensive melt shearing is achieved because the unit can break the oxide particles and fragmentations and disperse them. During the DC casting process, there are liquid region, slurry region and mushy region above the solidus, and the solid region (ingot) is below the solidus. The active reg
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