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Abstract: The concept of semisolid processing was explored to generate alloys by mixing coarse particulate precursors with different chemistries. The experiments with several magnesium alloys revealed that the control of chemistry and the proportion of precursors, as well as the solid to liquid ratio during their partial melting, allowed the selective partition of alloying elements between the solid and liquid phases, thus designing unique solidification microstructures. [Copyright &y& Elsevier]

Abstract: Analytical electron microscopy, supported by microchemical and microdiffraction techniques, was applied to assess the microstructural changes accompanying semisolid molding of a creep-resistant magnesium alloy with nominal chemistry of Mg–5%Al–2%Sr. In an ingot precursor, the Sr containing precipitates exhibited a morphology of Al4Sr platelets within eutectic colonies and Al–Mg–Sr films, both residing at grain/cell boundaries, while sub-boundaries were occupied by fine Mg17Sr2 plates. Trace quantities of Mg17Al12 were present as plate-shaped continuous precipitates within selected grain/cell interiors. Semisolid processing of mechanically comminuted ingots generated thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al4Sr phase, surrounding the secondary α-Mg. Traces of Mg17Al12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn–Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains. The possible implications of semisolid processing on the creep behavior of magnesium alloys are discussed. [Copyright &y& Elsevier]

Abstract: A novel application of the injection molding technique for net-shape forming of Mg–9Al–1Zn and Mg–6Al alloys, starting from a particulate precursor, was investigated. It was revealed that tight control of the alloy’s temperature within a narrow range around the liquidus level generated structures with high integrity and diminished some disadvantages inherent for conventional casting which utilizes superheated melts. The homogeneous matrix consisted of fine near-equiaxed α-Mg grains/cells surrounded by mostly discontinuous precipitates of Mg17Al12 intermetallics. The primary solid phase was either completely absent or present in negligible amounts, not exceeding 5%, with a morphology ranging from globules to degenerated rosettes. The structural integrity and fine microstructure improved mechanical properties which exhibited a superior combination of strength and ductility. The role of Mg17Al12 phase as a factor controlling ductility in Mg–Al and Mg–Al–Zn alloys is considered. [Copyright &y& Elsevier]

Abstract: A prototype system was used to process the semisolid Mg–9% Al–1% Zn alloy with a content of 75–85% of the unmelted phase. The influence of operating parameters on mold filling characteristics, integrity and microstructure of the component with a weight of 0.5kg, varying geometry and wall thickness, was examined. The mold filling time was found to be the primary factor controlling the process. Although the turbulent flow created porosity at intermediate stages of mold filling, highly dense structures were obtained unless the injection velocity was high enough to preserve the semisolid state until the final densification. A laminar flow of the slurry, observed for lower injection velocities, resulted in a low porosity at all stages of the mold filling. The final microstructure was dominated by near globular primary α-Mg particles with a size of 40μm which, for thicker walls, evolved into equiaxed grains. For high injection velocities and large distances from the gate, an inhomogeneity in distribution of α-Mg was detected. The eutectics of the secondary α-Mg and Mg17Al12 with a volume fraction of 15–25%, existed as thin films at particle boundaries and islands at triple junctions. Some suggestions for the processing of magnesium alloys at ultra high solid contents are defined. [Copyright &y& Elsevier]

Abstract: A number of Mg–Al–Zn alloys with thixotropic microstructures were created by the semisolid mixing of AZ91D and AM60B mechanically comminuted precursors in the Thixomolding® system. The microstructure formation was analyzed along with the role of structural constituents in controlling strength, ductility and the fracture behaviour of the created alloy. It was found that the inhomogeneity in the partition of alloying elements intensified with a reduction in the processing temperature and the liquid fraction was highly influenced by the alloy with the lower melting range. Tensile strength showed a strong correlation with corresponding elongations and was predominantly controlled by the solid particles’ content in the microstructure, with negligible influence derived from changes in the alloy’s chemistry. Although elongation was affected by both the solid content and the alloy’s chemical composition, a larger role was still exerted by the former. The contribution of individual precursors to the tensile properties of the created alloy depended on the processing temperature. While near to complete melting, both of them contributed almost equally; with a temperature reduction, the deviation from the rule of mixtures enlarged, and properties were increasingly influenced by the precursor with the lower melting range. [Copyright &y& Elsevier]

Techniques based on a rapid solidification concept were explored for manufacturing particulates of magnesium alloys with the potential use as feedstock for Thixomolding®. Basic features of granulation methods are outlined along with dimensional, morphological, microstructural and chemical characterizations of particulates with a composition of Mg–9% Al–1% Zn. In spite of the known tendency of Mg and Zn to evaporate, the globule chemistry was within the alloy’s specification. The microstructure comprised of dendritic morphologies of α-Mg and Mg17Al12 phases where the content of the latter was reduced compared to the conventionally cast state. It is suggested that the microchemical and phase segregations which accompanied the morphology of fine equiaxed dendrites are the key factors which control the transformation of particulates during subsequent heating in a solid state and their melting behaviour. [Copyright &y& Elsevier]

Microscopical techniques were used to provide the microstructural details and identify mechanisms governing phase and morphological transformations during the heating of Mg–8%Al–2%Zn pellets in solid and semisolid states. It was found that an as-extruded matrix of equiaxed α-Mg grains with a twinning substructure, was thermally unstable and experienced complete recrystallization after reheating to 200 °C. The precipitates of the Mg17Al12 phase and augmented concentrations of alloying elements within migrating grain boundaries and triple junctions played a key role in transformation of the equiaxed grains into thixotropic structures during partial melting. A direct link exists between sizes of equiaxed grains in the solid state and unmelted particles in the semisolid slurry. Although the morphology of primary solid particles did not change during the melting progress, the rate of particles’ coarsening at high temperatures and their internal microstructure in subsequently solidified alloy were influenced by the solid–liquid ratio. The importance of these findings for semi-solid injection molding practice is emphasized. [Copyright &y& Elsevier]