Non-equilibrium globular microstructure suitable for semisolid casting of light metal alloys by rapid slug cooling technology (RSCT)

As the semisolid processing of metals continues to gain importance and acceptance not only within research centers but also in industry, new methods and technologies are being developed to improve the quality of the semisolid cast parts and increase the productivity and reliability of the process. Within the so called thixocasting processes pre-material slugs with a globular microstructure serve as feedstock for the semisolid casting process. Studies have shown however, that for many alloys the main parameter for the production of suitable pre-material billets with a globular microstructure is the cooling rate. Therefore, the new rapid slug cooling technology (RSCT)-process has been developed, where the alloy is quenched in a specially designed cooling device. With this process, a fine and homogeneous non-equilibrium microstructure of globular α-phase dendrites was achieved. Due to micro-segregations within the globular grains melting proceed from outside towards the grain center. Thus, it is possible to produce a semisolid slurry with a well defined fraction of solid using technical Mg alloys with very low contents of eutectics like the AZ31(MgAl3Zn1), AZ61 (MgAl6Zn1) and alloys with rare earths. These alloys were processed in thixocasting to test the effectiveness of the RSCT with very promising results.