Principles of microstructural formation in semi-solid metal processing

The essential first step in semi-solid forming is to obtain a high grain density during the initial stages of solidification of an alloy melt. This is usually done by a combination of cooling and convection. If the grain density is too low, the resulting structure is one of conventional coarse dendrites, unsuitable for semi-solid forming. At higher grain densities, fine dendrites grow that can be coarsened to a spheroidal structure in a short enough time to be of practical interest for semi-solid forming. If the initial grain density is still higher, the grains grow in a fully spheroidal manner. Thus there are two distinctly different paths to formation of the desired structure: 1) coarsening of fine dendrites, and 2) direct spheroidal growth. We are beginning to understand quantitatively the conditions necessary to form spheroids in these two different ways.