Magnesium alloy strip produced by a melt-conditioned twin roll casting process

Abstract

Twin roll casting (TRC) offers a promising route for the economic production of Mg sheet, but unfortunately, it produces strip with coarse and non-uniform microstructures and severe centre line segregation. Recently, a novel magnesium strip casting process termed melt conditioned twin roll casting (MC-TRC) was developed that, compared with the conventional TRC process, emphasizes solidification control at the casting stage rather than hot rolling. This was achieved by melt conditioning under intensive forced convection prior to twin roll casting resulting in enhanced heterogeneous nucleation followed by equiaxed growth. In this study the development of TRC and MC-TRC processes and a microstructural comparison of the MC-TRC Mg-alloy strip with that of conventional TRC strip, have been investigated. Emphasis has been focused on the solidification behaviour of the intensively sheared liquid metal, and on the mechanisms for microstructural refinement and compositional uniformity in the MCTRC process. The results of the process development indicate that the MC-TRC process reduces considerably or eliminates defects such as the centre line segregation, voids and cracks at or near the strip surface that are always present in conventional TRC strip. The newly-designed homogenization treatment investigated for TRC and MC-TRC magnesium alloy strips was based on microstructural evolution obtained during heat treatment. The results of the MC-TRC strips showed a much faster recrystallization rate with finer recrystallized grains, which are due to more homogeneous and a finer grain size of the as-cast MC-TRC strips compared with the as-cast TRC strips. During down-stream processing, the effects of MC-TRC process on microstructural evolution of hot-rolled magnesium strips have been understood thoroughly by accurate control of the hot-rolling procedure during each step of strip thickness reduction. This study indicates that the MC-TRC strip requires fewer rolling steps when compared to TRC strip, thus offering reduced processing cost and carbon footprint. Mechanical properties at room temperature of MC-TRC as-cast and rolled sheets are much improved when compared with the conventional TRC as-cast and rolled sheets which can result in a higher quality of final components. The mechanical properties at elevated temperature shows for the first time that the higher elongation and lower yield strength of MC-TRC as-cast strips at a temperature close to its optimised hot-rolling temperature results in better ability for rolling and higher ductility of MC-TRC Mg strip compared with the TRC Mg strip.