Enhanced Hot Workability and Post-Hot Deformation Microstructure of the As-Cast Al-Zn-Cu-Mg Alloy Fabricated by Use of a
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INTRODUCTION
ELECTROMAGNETIC force can be used in the solidification process to improve surface quality, refine grains, and reduce macro-segregation.[1–6] There are two electromagnetic casting methods. One is high-frequency electromagnetic casting (HFEMC) using high-frequency magnetic fields of tens of kHz,[7–10] and the other is low-frequency electromagnetic casting (LFEMC) using S.Y. PARK is with the Advanced Fusion Process R&D Group, Korea Institute of Industrial Technology, Yeonsu-gu, Songdo-dong 7-47, Incheon 406-840, Korea, and also with Department of Materials Science and Engineering, Hongik University, Mapo-gu, Sangsu-dong 72-1, Seoul 121-791, Korea. W.J. KIM is with Department of Materials Science and Engineering, Hongik University. Contact e-mail: kimwj@ wow.hongik.ac.kr Manuscript submitted October 5, 2016. Article published online May 3, 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A
low-frequency magnetic fields of 10 to 200 Hz.[11–13] In the former method, the meniscus is stable so that the quality of the initial solidified shell can be improved by suppressing the formation of oscillation marks on the surfaces of solidifying ingots, leading to production of high surface quality ingots. In the latter method, there is a major difficulty in stabilizing the meniscus, but it has the advantage of not requiring special molds and power devices. In HFEMC, a high-frequency alternating current is used to generate an alternating, high magnetic field in the molten metal. The electromagnetic forces, exerted by the interaction of heavy eddy current in the molten metal with the electromagnetic field, enlarge the meniscus curvature of the molten metal in contact with the mold. This improves the inflow of the mold flux and prevents the metal from touching the mold by reducing contact pressure between the shell and the mold.[9,14] As a result, surface quality and sub-surface quality can be improved, VOLUME 48A, JULY 2017—3523
allowing for increase of casting speed. Electromagnetic stirring (EMS), the device for which is often located below the induction coils for EMC, provides enhanced fluid motion and thus promotes fragmentation of dendrite structure and reduction of the temperature gradient in the molten metal, resulting in the formation of fine equiaxed grains that are homogeneously distributed within the entire cross section of the ingot.[15,16] EMS is also effective in homogenizing alloy elements, reducing porosity and macro-segregation, minimizing internal cracks, and promoting a homogeneous distribution in the size of precipitate particles in the matrix.[15,16] The 7075 aluminum alloy, which has a high strength-to-density ratio and high resistance to stress-corrosion cracking, has been widely used for marine, automotive, and aviation applications. The hot compressive properties and processing maps of the extruded or rolled 7075 alloys have been studied by researchers.[17–24] Rajamuthamilselvan and Ramanathan[17] constructed the processing map of the 7075 alloy extruded from the billet prepared using stir casting technique a
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