Improvement of microstructure and properties in twin-roll casting 7075 sheet by lower casting speed and compound field

摘要

Well-developed dendrites and severe macro and micro segregations in 7075 sheet produced by horizontal twin roll casting (TRC) deteriorates the hot-workability and properties of the sheet, which makes an obstacle for the successful use of this technology. In this paper, lower casting speed and a pulsed electric-magnetostatic compound field are used to refine microstructure and abate segregation in TRC 7075 sheet. The dendrite arm space decreases from 20 pm to 8-13 mu m and the micro-segregation degree of Mg, Zn and Cu decreases when casting speed decreases from 1.5 m/min to 0.75 m/min. The center macro-segregation belt disappears in the 0.75 m/min sheet. The as-cast structure and the dendritic segregation in the 0.75 m/min sheet are further refined and abated respectively by the compound field. The secondary dendrite arm size decreases to 5-8 mu m in the field sheet. The 0.75 m/min sheet casted with the field shows better mechanical properties after homogenization and hot rolling. The optimization mechanism of lower casting speed and the field was discussed with the aid of classical solidification theory and electromagnetism. (C) 2017 Published by Elsevier Inc.