Fundamental Aspects

Abstract

The original semisolid forming process, developed at MIT in 1972, involved stirring an alloy during solidification to produce a slurry of spheroidal primary particles in a liquid matrix, which was then injected directly into a die to produce a solid component. This was termed “rheocasting.” Subsequently, it was found more convenient to solidify the slurry completely during the continuous casting of an electromag netically stirred strand, which was then cut into slugs for partial remelting back into semisolid billets on demand. These could be loaded into a diecasting machine in this state for injection into the die. This alternative process route is called “thixoforming,” and until recently, it was the preferred industrial process. For this reason, the microstructure developed during the reheating and melting for thixo forming will be considered first. Experimentally, it has been found that the most effective fraction solid f _s for thixoforming, lies between 0.5 and 0.6. Below this range, the semisolid slug becomes too soft to support its own weight and sags during remelting; above this range, it is too stiff to flow readily and fill the die. However, the slurry technologies used in rheocasting typically operate at lower fraction solids and rely on the ability to pour the semisolid alloy much like a liquid (see Chap. 4). It has been observed in practice that the fraction solid is in fact a critical factor for effective thixoforming, and therefore, both good temperature control and lack of sensitivity of f _s to small temperature variations of the alloy are essential to efficient manufacturing.