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I.

INTRODUCTION

WROUGHT magnesium alloys in sheet form are of interest for applications in the automotive and electronic industries because of their low density, outstanding electromagnetic shielding capability, and excellent heat dissipation.[1,2] However, their production is inefficient and expensive because of low ductility. Grain refinement in magnesium alloys enhances strength and improves ductility by promoting the operation of nonbasal slip systems[3,4] and restricting twinning.[5–7] Several procedures have been developed to produce fine-grain structures in magnesium alloys, including powder metallurgy,[8] rapid solidification,[9] and severe plastic deformation,[10–12] but only small quantities of material can be obtained from these process routes. Although thermomechanical processing through repetitive hot/ warm rolling/extrusion of ingot slabs combined with heat treatment can produce well-refined microstructures,[13–15] it is extremely costly and has an adverse effect on the anisotropy and ductility as strong basal texture develops.[14,16,17] Twin-roll casting can produce a magnesium alloy strip directly from the melt with a thickness less than 6 mm, eliminating the need for the use of a breakdown mill and most of the passes in the finishing mill that are required in conventional processing.[18] Twin-roll casting offers both significant cost savings and the potential to I. BAYANDORIAN, Research Fellow, Y. HUANG, Lecturer, and Z. FAN, Professor, are with the EPSRC Centre for Innovative Manufacturing in Liquid Metal Engineering and BCAST, Brunel University, Uxbridge, Middlesex UB8 3PH, United Kingdom. Contact e-mail: [email protected] S. PAWAR, Research Student, X. ZHOU, Reader, and G.E. THOMPSON, Professor, are with the Corrosion and Protection Centre, School of Materials, University of Manchester, Manchester M13 9PL, United Kingdom. Manuscript submitted September 28, 2010. Article published online November 19, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

improve strip quality by refining grain size based on the higher cooling rate compared with the conventional ingot casting. Another benefit of producing magnesium alloy strip by this route is the possibility of providing a weak or even random texture if the process is controlled to require only a minimum of plastic deformation. Many reports showed that the solidification rate achieved by twin-roll casting can improve alloy properties by homogenizing microstructure, refining grain size, and reducing segregation, etc.[19–21] However, the quality of magnesium alloy strip produced by the existing twin roll casting techniques is still limited by the formation of columnar dendrite grains and centerline segregation.[22–24] Optimizing the process and developing alloys that better suit the process can help resolve these problems in conjunction with effective grain refinement at the solidification stage. The addition of a grain refiner works well for most alloy casting processes. However, there are potential problems with inoculation of grain refiners in twin-roll casting suc