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 ind...

Semisolid metal processing

The microstructure of alloys with a near-equiaxed microstructure, produced by spray casting, magnetohydrodynamic (MHD) casting and the stress induced, melt activated (SIMA) process, as it evolves within short times in the semisolid state, is examined by rapid quenching and isothermal soaking experiments. Quenching experiments reveal the morphology and distribution of solid phase at high and medium volume fractions of liquid. At medium liquid content, the microstructure of spray-cast and SIMA alloys consists of discrete equiaxed grains uniformly dispersed in the liquid phase, while the corresponding microstructure of MHD-cast alloys exhibits extensive agglomerates consisting of incompletely spheroidized grains. The connectivity of solid phase and the formation of a solid skeleton in the semisolid state are discussed in terms of grain misorientation. Isothermal soaking experiments investigated grain growth and degree of spheroidization as a function of soaking time and liquid content in the semisolid state. Results demonstrated that MHD-cast microstructures are less equiaxed compared with SIMA and spray-cast alloys even after 5 min of soaking in the semisolid state. It is also shown that the grain growth rate is smaller in spray-cast alloys than in SIMA alloys. The role of coalescence and the effects of alloying elements are also discussed.

Evolution of near-equiaxed microstructure in the semisolid state

A synchrotron X-ray radiography study of dendrite fragmentation induced by a pulsed electromagnetic field in an Al–15Cu alloy

Processing of Discontinuously-Reinforced Metal Matrix Composites by Rapid Solidification

Aluminum becomes the most popular nonferrous metal and is widely used in many fields such as packaging, building transportation and electrical materials due to its rich resource, light weight, good mechanical properties, suitable corrosion resistance and excellent electrical conductivity. Grain refinement, which is obtained by changing the size of grain structure by different techniques, is a preferred method to improve simultaneously the strength and plasticity of metallic materials. Therefore, grain refining of aluminum is regarded as a key technique in aluminum processing industry. Up to now, there have been a number of techniques for aluminum grain refining. All the techniques can be classified as four categories as follows: grain refining by vibration and stirring during solidification, rapid solidification, the addition of grain refiner and severe plastic deformation. Each of them has its own merits and demerits as well as applicable conditions, and there are still some arguments in the understanding of the mechanisms of these techniques. In this article, the research progresses and challenges encountered in the present techniques and the future research issues and directions are summarized.

Ho-In Lee

Papers

Microstructure of stir-cast metals