SOLIDIFICATION PROCESSING is one of the oldest manufacturing processes as it is the principal component of metal casting processing. While solidification science evolved from the need to better understand and further develop casting processes, in 2004 solidification science is at the base of many new developments that fall out of the realm of traditional metal casting. Solidification is, strictly speaking, the transformation of liquid matter into solid matter. The microstructure that results from solidification may be the final one, in which case it directly affects the mechanical properties of the product. In other cases, heat treatment or other processes may be used after solidification to further modify the solidification microstructure. However, the outcome of this additional processing will be greatly affected by the solidification microstructure.

Fundamentals of Solidification

The microstructure formation, during casting, of alloys being immiscible in the liquid state such as copperlead or aluminium-lead has gained renewed scientific and technical interest during the last fifteen years. Especially, a new experimental tool, research under reduced gravity conditions, was able to cast new, unexpected results and theories into the discussion on the nature of the complex process of microstructure evolution in such alloys. Prior to the first experiments performed at reduced levels of gravity acceleration, it was generally agreed that the process of phase separation during cooling through the miscibility gap is dictated solely by gravity-induced effects such as natural convection and sedimentation. Fundamental and applied research in space and in earth laboratories could show that there are other mechanisms operating concurrently and under suitable conditions with equal strength. In addition applied research was able to utilize the often unexpected results from space experimentation to develop new casting processes which allow one to produce microstructures on earth suitable for bearings in automotive applications. Therefore this article describes the extensive progress that has been made during the last decade and also the fundamentals of immiscibles. In addition it will be shown that the combination of classical laboratory research, research under reduced gravity conditions and a newly developed computational modelling technique seems to be just becoming available to solve the problems of decomposition, spatial phase separation and microstructure evolution during cooling of an alloy through the liquid miscibility gap.

Liquid immiscible alloys

Most models that describe the interaction of an insoluble particle with an advancing solid-liquid interface are based on the assumption of steady state. However, as demonstrated by experimental work, the process does not reach steady state until the particle is pushed for a while by the interface. In this work, a dynamic mathematical model was developed. The dynamic model demonstrates that this interaction is essentially non-steady state and that steady state eventually occurs only when solidification is conducted at sub-critical velocities. The model was tested for three systems: aluminum-zirconia particles, succinonitrilepolystyrene particles, and biphenyl-glass particles. The calculated values for critical velocity of the pushing/engulfment transition were in same range with the experimental ones.

An analytical model for the interaction between an insoluble particle and an advancing solid/liquid interface

Multiphase solidification in multicomponent alloys is pertinent to many commercial materials and industrial processes, while also raising challenging questions from a fundamental point of view. Within the past few years, research activities dedicated to multiphase solidification of ternary and multicomponent alloys experienced considerable amplification. This paper gives an overview of our present understanding in this field and the experimental techniques and theoretical methods research relies on. We start with an introduction to thermodynamic databases and computations and emphasize the importance of thermophysical property data. Then, we address pattern formation during coupled growth in ternary alloys and cover microstructure evolution during successive steps of phase formation in solidifying multicomponent alloys. Subsequently, we review advances made in phase field modeling of multiphase solidification in binary and multicomponent alloys, including various approaches to crystal nucleation and growth. Concluding, we address open questions and outline future prospects on the basis of a close interaction among scientists investigating the thermodynamic, thermophysical and microstructural properties of these alloys.

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Multiphase solidification in multicomponent alloys

Settling of silicon carbide particles in cast metal matrix composites

Cu-Pb and Bi-Ga monotectic alloys of nominal hypermonotectic compositions were directionally solidified under various furnace translation rates, temperature gradients, and gravity levels. Gravity was varied by solidifying the alloys under ground conditions and in the furnace aboard NASA KC-135 aircraft, flying on parabolic trajectories. High translation rates, high gradients, high gravity levels, and higher density and lower thermal conductivity of the L2 phase favored the formation of fiber composite structure, while the opposite conditions resulted in structures consisting of L2 droplets in α matrix. A modified particle engulfment theory as originally enunciated by Ulhmannet al. is proposed to explain these observations.

Directional solidification of Cu-Pb and Bi-Ga monotectic alloys under normal gravity and during parabolic flight

A method of processing steel strip such that the steel strip can be temper rolled at the increased speeds and better surface texture control of prior art dry lubricants, yet the steel strip can be temper rolled with less frequent replacement of temper mill working rolls. Further, the resulting steel strip has increased stretchability and can be temper rolled to achieve sufficient reduction of YPE at lower working roll pressures and/or lower strip tension, previously only effective when the steel strip was lubricated with a wet lubricant film. Enhanced corrosion resistance is another advantage of this process.

Ajay Kumar Singh

Papers

Directional solidification of Cu-Pb and Bi-Ga monotectic alloys under normal gravity and during parabolic flight

Method of lubricating steel strip for cold rolling, particularly temper rolling