During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

Behavior of metal alloys in the semisolid state

The critical need for high strength, high performance, and cost effective materials has resulted in the use of novel processing techniques for the development of new aluminium alloys for structural applications. The highly attractive combinations of microstructure and mechanical properties achievable through rapid solidification technology (RST) have prompted the study and application of rapid solidification as a means of improving the behaviour of existing alloy systems and also developing novel alloying compositions. The rapid extraction of thermal energy associated with RST permits large deviations from equilibrium, as evidenced by the extension in solid solubility limits, sharp reductions in grain size, and a reduction in the size and number of segregated phases with concomitant development of non-equilibrium phases. In the present paper, the fundamental scientific principles underlying RST are discussed and particular emphasis is given to published studies of composition-processing-microstruc...

Rapid solidification processing with specific application to aluminium alloys

It was first shown in 1960 that rapid quenching of certain alloys from the melt could form completely extended solid solutions, new metastable crystalline phases, and amorphous solid phases. These effects were attributable to the high cooling rates (>or approximately=105 K s-1) from the melt achieved by new techniques of rapidly spreading a few milligrams of liquid alloy into a thin layer in close contact with a good heat conductor (splat cooling). A comprehensive review of developments to data which have resulted from the application of this technique in laboratories throughout the world is presented. It includes an analysis of the range of methods now available for quenching the melt and of present understanding of how they form and quench specimens. The structural features observed as-quenched are then discussed under the divisions of microstructure and extended solid solubility, and of formation of both metastable crystalline and amorphous solid phases. A survey of the response of the as-quenched microstructure to annealing then precedes treatment of the properties and applications of splat-cooled alloys. It is concluded that splat cooling is being established in ever widening fields of study as a tool for controlling structure and properties, and that further applications can be anticipated.

http://iopscience.iop.org/article/10.1088/0034-4885/36/11/002/pdf

Splat cooling and metastable phases

http://arc.nucapt.northwestern.edu/refbase/files/Knipling-2008_1909.pdf

Riei Ichikawa

Papers

Effects of Cooling Rate and Supercooling Degree on Solidified Structures of Al–Mn, Al–Cr and Al–Zr Alloys in Rapid Solidification

Effect of Fe and Si on Age Hardening Properties of Supersaturated Solid Solutions of Al-Zr Alloy by Rapid Solidification

Resistance Spot Welding of Dissimilar Metals-Commercial Pure Titanium to Some Aluminum Alloys :

Apparent Viscosity and Structure in Partially Solidified Al-Cu Alloys

Supercooling in the solidification of Al-Cr, Al-Mn and Al-Zr binary alloys