Melt conditioning by intensive shear was used prior to twin-roll casting of AZ31 magnesium alloy strip to promote heterogeneous nucleation and to provide a refined and uniform microstructure without severe macrosegregation. The cast strip was then processed by homogenization, hot rolling, and annealing, and its downstream processing was compared with a similar cast strip produced without melt conditioning. Melt conditioning produced strip with accelerated kinetics of recrystallization during homogenization and improved performance in hot rolling and improved tensile properties. An average tensile elongation of~28 pct was achieved, which is substantially higher than the~9 pct obtained for the strip produced without melt conditioning which is consistent with reported values (~6 pct to 16 pct). The as-cast, homogenized, and hot-rolled microstructures of the strip were characterized. The kinetics of homogenization and hot-rolling process have been discussed in detail.

Magnesium AZ91D alloy strips consisting of equiaxed grains with a mean size less than 100 μm 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 μm, 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.

MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently by BCAST at Brunel University for conditioning liquid metal prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence to the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys.

Melt conditioning by intensive shear was used prior to twin roll casting of an AZ91 magnesium alloy strip to promote heterogeneous nucleation and produce a refined and uniform microstructure without severe macro-segregation. The as-cast strip was then processed by homogenization, hot rolling and annealing and the microstructural behaviour during the downstream processing was examined and compared with the strip of the same alloy produced without melt conditioning. The melt conditioned strip after downstream processing displayed significantly improved mechanical properties with an average tensile elongation of ~16%, compared with ~10% for the strip produced without melt conditioning and the reported values of ~1.5-6.2% in the literature.