5.07 - Routes to Spheroidal Starting Material for Semisolid Metal Processing

Abstract

Spheroidal or globular primary phase microstructure is a standard requirement for semisolid metal (SSM) processing. This type of microstructure improves material formability during processing and allows the production of dense near-net-shape components with detailed geometries. There are many techniques that have been used in order to attain a spheroidal microstructure within the laboratory and industrial settings since the initial discovery of the improved formability provided. The unique forming advantages produced from this type of microstructure are found for both ferrous and nonferrous materials. Initial laboratory routes for producing SSM feedstock were based on mechanical stirring methods. In the early 1990s, SSM forming was taken on to a greater extent by industry where the main method of feedstock production was electromagnetic stirring. Relatively few large-scale aluminum suppliers provided this feedstock to cast houses, which in turn formed this into final component geometries. This feedstock was therefore of higher cost than standard aluminum billets and therefore presented a cost barrier to the use of the SSM processing route. From the mid-1990s onward, new thermal methods to produce the spheroidal microstructure were developed within which the melt is rapidly solidified and held for a short time in the semisolid state before forming. These techniques allowed for a multitude of small nuclei to be quickly formed and held at a semisolid temperature during which the primary phase spheroidized before injection of the material into the mold. Two main advantages of this technique were that these processes had become single-step processes and that the technique could be implemented within the cast houses directly rather than requiring purchase of more expensive feedstock and subsequent reheating. This provided a significant cost reduction for the process and allowed its more widespread use within engineering products. Many research groups have also developed combinations of techniques for the production of SSM feedstock. Although these provide their own individual benefits and drawbacks, these techniques are largely laboratory based due to their higher associated costs. This chapter provides an overview of the techniques that have been used for SSM feedstock production and provides discussion on the important parameters that need to be well controlled in order to provide the required spheroidal microstructures.