Effect of low-frequency electromagnetic casting on the castability, microstructure, and tensile properties of direct-chi
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9/27/04
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Effect of Low-Frequency Electromagnetic Casting on the Castability, Microstructure, and Tensile Properties of Direct-Chill Cast Al-Zn-Mg-Cu Alloy JIE DONG, JIANZHONG CUI, FUXIAO YU, CHUNYAN BAN, and ZHIHAO ZHAO Microstructure and grain boundary segregation of the direct-chill (DC) cast Al-Zn-Mg-Cu (7A60) alloy, with and without low-frequency electromagnetic field, were investigated. The surface quality, hot-tearing tendency, and tensile properties of the ingots manufactured by DC and low-frequency electromagnetic field casting (LFEC) were compared. The results show that LFEC significantly improves the surface quality and reduces the hot-tearing tendency of DC ingots. It is possible to generate a fine, uniform, and equiaxed microstructure with LFEC. Under optimum conditions, the average grain size varies from 30 m near the surface to 45 m at the center of the LFEC ingots. Decreasing electromagnetic frequency or increasing intensity significantly refines microstructure, suppresses grain boundary segregation, and increases as-cast fracture strength and elongation. In the range of frequency and ampere turns employed in the experiments, the optimum frequency is found to be 15 to 25 Hz and the number of ampere turns to be 18,000 to 20,000 At.
I. INTRODUCTION
ALUMINUM alloys with superhigh strength and toughness are very important materials to the military and aerospace industries.[1–4] Traditional high-strength aluminum alloys such as 7055 and 7093 cannot meet the requirement of the rapid development of the aforementioned industries; thus, a series of superhigh-strength and high-alloying Al-Zn-Mg-Cu alloys was designed and studied recently.[3–9] However, it was proved that with normal solidification processes, due to nonequilibrium solidification, a great amount of intermetallic compounds and eutectics of Al, Zn, Mg, and Cu segregate at the grain boundaries, which leads to a great reduction of the strength and toughness after heat treatment and mechanical deformation.[6,7] In order to reduce the grain boundary segregation, rapid solidification methods are often adopted. For instance, Kusui et al.[7] and Wei et al.[8] have obtained superhigh strength and supertough Al-Zn-Mg-Cu alloys using powder metallurgy and spray forming processes, respectively. However, the rapid solidification methods require complicated processing or special expensive devices. A simple and efficient way, low-frequency electromagnetic casting[10] (LFEC), was developed in our laboratory with the frequencies lower than that in the casting, refining, and electromagnetic process (CREM), which was put forward by Vives[11,12] to refine structures and improve the surface quality of direct-chill (DC) ingots. The aims of the present investigation are to study whether LEFC can significantly refine the microstructure, and decrease grain boundary segregation and hot-tearing tendency, while JIE DONG Postdoctoral Candidate, and ZHIHAO ZHAO, Postgraduate Student, JIANZHONG CUI, Professor, FUXIAO YU, Vice-Professor, and CHUNYAN BAN,
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