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.
C.G. Kang, S.K. Kim and S.Y. Lee
M.C. Flemings and R. A. Martinez, "Principles of Microstructural Formation in Semi-Solid Metal Processing", Solid State Phenomena, Vols. 116-117, pp. 1-8, 2006