Non-dendritic Structure Research Articles

Semisolid metal processing

Semisolid metal processing is a relatively new method for forming alloys in the semisolid condition to near net shaped products. It relies on the behaviour of semisolid slurries in which the solid exists in the form of spheroidal particles: if left undisturbed such slurries are stiff and may be handled, but flow like liquids on being sheared. Slugs of semisolid alloy may therefore be injected into a die (thixocasting) or shaped between closed dies (thixoforging) to produce components near-to-shape with good surface finish, free from porosity, and possessing fine uniform microstructures, which may be heat treated to give superior mechanical properties. The rheology of non-dendritic alloy slurries and the recent ideas which have been proposed to account for their pseudoplastic and thixotropic behaviour are considered. This is followed by a description of the technologies available for producing non-dendritic structures and the processes for subsequently shaping the semisolid alloy. Finally, some indication is given of the mechanical properties of components produced by the semisolid processing route.

Semisolid metal processing

Semisolid metal processing is a relatively new method for forming alloys in the semisolid condition to near net shaped products. It relies on the behaviour of semisolid slurries in which the solid exists in the form of spheroidal particles: if left undisturbed such slurries are stiff and may be handled, but flow like liquids on being sheared. Slugs of semisolid alloy may therefore be injected into a die (thixocasting) or shaped between closed dies (thixoforging) to produce components near-to-shape with good surface finish, free from porosity, and possessing fine uniform microstructures, which may be heat treated to give superior mechanical properties. The rheology of non-dendritic alloy slurries and the recent ideas which have been proposed to account for their pseudoplastic and thixotropic behaviour are considered. This is followed by a description of the technologies available for producing non-dendritic structures and the processes for subsequently shaping the semisolid alloy. Finally, some indication is given of the mechanical properties of components produced by the semisolid processing route.

Magnesium rheocasting: a study of processing-microstructure interactions

The rheocasting solidification process has been applied in the production of AZ91D magnesium alloy. A batch-type rheocaster has been designed to produce rheocast ingots in an argon-covered, closed chamber system, under different processing conditions. Processing variables which were evaluated include shear rate, isothermal stirring time and volume fraction solid during isothermal stirring. A detailed examination was made of the resultant microstructure using three different techniques to reveal the rheocast structure: etching techniques, reheating at the isothermal stirring temperature and quenching, and scanning microscopy via backscattering techniques. Rheocasting yielded a non-dendritic structure, wherein the primary phase has a rosette-shaped morphology surrounded by a divorced eutectic. The evolution of the structure and the morphological changes which occur during processing are described. The uniformity in the primary particle size, as well as their sphericity, was increased by increasing the stirring time, or the shear rate, or by decreasing the stirring temperature (increasing the volume fraction solid present). Increasing the shear rate gave rise to a more efficient break-up of the dendritic structure. Porosity was present in the samples which was detrimental to the mechanical properties; however, the mechanical properties after extrusion were quite outstanding.

Mechanical properties of zinc-aluminium alloys extruded in the liquid-solid state

Zinc-aluminium alloy bars and wires were successfully extruded in the liquid-solid state with a liquid fraction higher than 15%. Fully dense materials were obtained in this way with pressure lower than 100 MPa. The relationship between flow stress and solid fraction was obtained and the role of the liquid component in the deformation behaviour was clarified. Mechanical properties of the extruded material are presented and discussed with regard to the initial microstructure of the metals before deformation, i.e. the conventional dendritic structure and non-dendritic structure composed of solid spheroids dispersed in the remaining liquid obtained by stir-casting techniques. The microstructure of the extruded products was generally fine and a fibrous structure was easily observed particularly for the stir-cast material where primary solid spheroids were highly deformed. Optimization of these parameters leads to a desirable high-strength material.

Supercooling susceptibility of molten Fe-C alloys with rare earth metal addition.

A new technique has been developed to obtain large supercooling for Fe-C alloys. A mother alloy was prepared by the addition of rare earth metal (REM) of 0.75mass% to molten Fe-C alloy of 2kg. A specimen weighing 0.08kg was taken out o f the mother alloy and charged into an alumina crucible. The procedure of melting and freezing of the specimen was repeated in a Tammann furnace. The surface layer of the solidified ingot was removed after being repeated the procedure of melting and freezing for several times, and then the procedure was further repeated. By this procedure the supercooling of 100K was obtained. Furthermore the non-dendritic structure and uniform concentration of carbon were obtained by supercooling.

Comparison of Structures of Gas Atomized and of Emulsified Highly Undercooled Ni-Sn Alloy Droplets

ABSTRACTA comparison is made of microstructures of droplets of Ni-Sn alloys rapidly solidified (a) by gas atomization and (b) in a glass emulsifying medium. Cooling rate of the gas atomized particles ranged from 103 to 106K/s depending on droplet diameter (20-230μm). In the hypoeutectic alloy studied (Ni-25wt%Sn), most particles showed a dendritic structure. These same particles, melted and resolidified in a glass medium using DTA (Differential Thermal Analysis), showed undercoolings up to 280K: the structures were dendritic at low undercoolings and non-dendritic at undercoolings above about 220K. It is concluded that (1) the gas atomized particles exhibited little or no undercooling before nucleation, (2) the solidification time of the undercooled emulsified droplets is substantially less than that of gas atomized droplets, and (3) the undercooling required to achieve non-dendritic structure depends on sample size.

Effect of solidification conditions on deformation behaviour of semi-solid Sn-Pb alloys

The compressive behaviour of Sn-Pb alloys is studied with materials of conventionally dendritic structure and non-dendritic (rheocast) structure obtained through mechanical stirring during solidification. The alloys are found to deform similarly in the fully solid state but their behaviour becomes very different in the semi-solid range depending on the solidification mode. The holding time in the semi-solid state before compression also affects the mechanical properties: the influence of these two parameters is discussed in terms of the initial structure of the alloy and its evolution. Advantages of using semi-solid materials in metal forming processes are also presented.

Compression of semisolid dendritic and non-dendritic ZnAl alloys at low strain rates

The behaviour of semisolid ZnAl alloys with a dendritic or a non-dendritic structure was studied in compression between two parallel plates. Small cylindrical specimens were deformed at constant strain rates ranging from 0.012 to 0.72 mm mm −1 s −1 and at various temperatures ranging from 350 to 443 °C. Dendritic specimens deformed with severe segregation and their structure broke down. Stir-cast specimens behaved more homogeneously. At 433 °C, their spheroid particle structure was preserved and the deformation strength was lower.

Structural characteristics and extrusion behaviour of Pb–Sn alloys in semisolid state

The extrusion behaviour of semisolid Pb–19Sn alloys has been studied on materials with conventionally solidified dendritic structures and with non-dendritic structures obtained through mechanical stirring during solidification. This stirring performed at a constant temperature above the solidus produces a semisolid slurry with approximately spherical primary particles. The volume fraction, particle size, and interparticle spacing of the primary phase decrease as the stirring temperature increases. Whereas the alloy with the non-dendritic structure (rheocast) can be extruded in the semisolid state under low extrusion forces giving a perfectly continuous rod, the alloy with the dendritic structure requires larger forces and produces a fragmented product. These experiments suggest that extrusion can be performed advantageously in the semisolid state with alloys obtained by mechanical stirring during solidification.