Extremely fine and uniform microstructure of magnesium AZ91D alloy sheets produced by melt conditioned twin roll casting Zan Bian*, I. Bayandorian and Z. Fan Magnesium AZ91D alloy strips consisting of equiaxed grains with a mean size less than 100 mm were fabricated successfully by using the melt conditioned twin roll casting (MC-TRC) process. A melt conditioning by advanced shear technology (MCAST) process for conditioning liquid metals at temperatures either above or below the alloy liquidus using a high shear twin screw mechanism was combined with the twin roll casting (TRC) process to form an innovative technology, namely, the MC-TRC process for casting Al alloy and Mg alloy strips. During the MC-TRC process, liquid melt with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have a fine and uniform microstructure. It consists of equiaxed grains with a mean size of 60–70 mm, and also displays extremely uniform grain size and composition throughout the entire cross-section. Investigation also shows that the MC-TRC processes are extremely effective to reduce the formation of defects, particularly the formation of the central line segregations. Keywords: Melt conditioned twin roll casting, Magnesium alloy, Microstructure, Grain size, Segregation Introduction It is well known that Mg metal, with a hexagonal close packed structure, has only three slip systems. This makes it difficult to undergo plastic deformation in the solid state Mg alloy to fabricate Mg alloy strips at room temperature. In the conventional process, Mg based alloy strips are fabricated by hot rolling slabs which are prepared by direct chill (DC) casting. As DC cast Mg alloys are coarse grained materials, a sophisticated rolling process has to be carried out carefully to fabricate much thinner strips. These include controlling high rolling temperatures exactly, reheating strips at each step of the rolling process, and utilising precise control systems for rolling and applying low rolling speed. 1–5 These extra steps cause low production efficiency and high cost. To overcome the difficulties associated with the solid state deformation process of Mg alloys, improve production efficiency and reduce cost, a twin roll casting (TRC) technology has been developed to produce Mg alloy strips directly from liquid Mg alloys. 3–8 In the recent years, the TRC technique has been developed extensively for producing both Al and Mg alloy strips with high quality and good mechanical properties, and research has also been made to improve the understanding of the solidification mechanism of the TRC process. 4–7 The conventional TRC process integrates casting and rolling into a single process 8–11 and especially stresses the large deformation in the hot rolling process. The ultimate objective of TRC is to produce a strip with a fine and uniform microstructure and little defects such as segregation, and a good surface finish. However, lots of problems have to be solved before such an ideal TRC strip is achieved. The major problems with the conventional TRC process include coarse columnar grains in the size of nearly 1 mm, many types of severe segregations and defects, only few alloys suitable for strip production, poor surface finish and low casting speed. Most of such problems are associated with the deformation pro- cess. 11–13 The conventional TRC process involves a substantial amount of force to weld the two presolidi- fied shells in the bite of the two rolls. As a consequence of this large deformation, the solute rich liquid is squeezed out from the growing dendrite, resulting in severe central line segregation and other casting defects such as bleedings. It is worth considering that TRC is a Materials Science and Technology mst8415.3d 24/6/08 13:09:53 The Charlesworth Group, Wakefield +44(0)1924 369598 - Rev 7.51n/W (Jan 20 2003) Brunel Centre for Advanced Solidification Technology, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK *Corresponding author, email zan.bian@brunel.ac.uk ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 6 May 2008; accepted 23 May 2008 DOI 10.1179/174328408X326129 Materials Science and Technology 2008 VOL 000 NO 000 1