Improving Microstructure and Mechanical Properties for Large-Diameter 7075 Aluminum Alloy Ingots by a Forced Convection
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ULTRAHIGH strength Al-Zn-Mg-Cu alloys possess excellent mechanical properties and are extensively applied in the aerospace, shipping, and armament industries.[1–3] To meet the increasing demands of the above systems, the large-diameter Al-Zn-Mg-Cu alloy ingots with high strength, high ductility, and high wear resistance are desired urgently.[4,5] However, the solidified microstructure of Al-Zn-Mg-Cu alloy ingots is coarse and non-uniform, and the ingots have inferior formability because of the high element content.[6] Casting crack is easily generated during solidification, especially when casting large-diameter ingots. Casting crack can be removed partially using grain refinement which, meanwhile, boosts both the ductility and strength of the as-cast alloys.[7] The principles of the grain refining casting techniques are as follows: (1) an addition of grain refiners, (2) an increase in cooling rates, and (3) an application of external forces. Before, many experiments on grain refiners have been conducted, and it is certified that the effect of refiners on
MINGFAN QI, YONGLIN KANG, and GUOMING ZHU are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] YANGDE LI and WEIRONG LI are with Dongguan EONTEC Corporation, Ltd., Dongguan 523662, China. Manuscript submitted April 5, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
ultrahigh strength Al-Zn-Mg-Cu alloys is restricted. Also, some rapid solidification methods[4,8,9] have been invented to prepare the bulk alloys with a finer microstructure in the last few decades, however, these routes, need extra and expensive steps for preparing the alloys. It also results in the serious oxidation and contamination, which, in turn, decrease the alloys properties, resulting in lower reliability. The use of external forces, including mechanical stirring, electromagnetic field, and supersonic vibration, can improve surface qualities, refine microstructures, decrease macrosegregations, and inhibit casting cracks of ingots.[9,10] Despite the fact that the techniques of the supersonic vibration[11,12] and various electromagnetic fields such as the low frequency magnetic field,[10,13] annular magnetic field,[14] static magnetic field,[15] high magnetic field,[16] and dual-frequency magnetic field[17] have been developed and applied for a long time, they have, at best, only got the ingots with the diameter of 250 mm. Also, the above techniques also have drawbacks of low production efficiency, hard to stir the bulk melt fully, and complex devices. To solve these issues, a forced convection stirring (FCS) device was developed for the preparation of the large-diameter ingots of Al-Zn-Mg-Cu alloys by Zhou et al.[18] In this work, the ingots of the 7075 alloy with a dimension of 300 mm were prepared by the FCS casting (FCSC) and normal casting (NC) processes. Effects of the FCS on the macrosegregation, microstructure, and mechanical properties of the ingots were studied. The flow behavior, temperature, a
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