Semi-solid Processing of Alloys: Principles, Thermodynamic Selection Criteria, Applicability

Abstract

Semi-solid processing (SSP) of alloys makes use of the thixotropic properties of two-phase mixtures of globulites and liquid. These materials whose viscosity decreases under shear deformation and increases again once the shear action ceases, can be handled almost like solids in the absence of shear stress and they can flow like viscous liquids when deformation forces are applied. SSP requires a wide melting interval of the alloy, a lowtemperature sensitivity of the phase fractions in the slurry and a tendency to form a fine-grained globulitic microstructure.The numeric simulation of solidification has proven useful for the selection of alloys to be processed in the semi-solid state. To obtain results applicable in practice, the methods to be used depend on the alloy systems, reaching from equilibrium calculations and Scheil-Gulliver estimations to diffusion simulations. All simulations depend on reliable thermodynamic and kinetic databases. Instead of using only the slope of the fraction liquid curve at 50% liquid as selection criterion, we suggest to use the thixo ranges ΔT40–60 for thixocasting and ΔT20–40 for thixoforging. Using these selection criteria we investigated the suitability of three Al alloys and three steels including the influence of compositional variations.So far, the industrial large-scale production of parts by SSP has been confined to aluminium alloys, although extensive development efforts are being made to apply the process to magnesium, to nickel alloys and to steels.