Thixocasting needs special feedstock
However, it also has several disadvantages, such as the need for special feedstock with near spherical globular microstructure. To cast such special billets of limited sizes for thixocasting one has to pay a higher-than-normal premium. The elimination of this additional special feedstock with near spherical globular microstructure in new semisolid casting machine would lead to both cost and time savings without adverse impact on our environment.
Compared with conventional casting technologies, thixocasting has a lower forming temperature, significantly longer die life, high part precision, efficient production processes, and comprehensive mechanical properties. In comparison with hot forging technologies, thixocasting has quite a low yield strength, high fluidity, low forming load and low surface roughness.
In the thixocasting process, a complex geometric product can be obtained through one-step forming. Experimental results indicate that defects such as micro porosity, micro shrinkage, dendritic solidification in liquid pool, hot tearing in liquid phase, micro segregation at grain boundaries, and liquid segregation in the sharp corner of the die were observed. To avoid these casting defects, the thixocasting process parameters (liquid fraction, semisolid temperature, injection speed, injection pressure and die temperature) must be tightly controlled.
Rheocasting: thixocast properties but from fluidized melt
Rheocasting produces output with thixocast properties, by developing semisolid slurry from the molten metal produced in a typical die casting furnace, rather than by re-heating billets. The resulting feedstock (in the form of typical die casting alloys) is less expensive and directly recyclable.
Fine globular microstructure with SIMA
The stress induced melt activation (SIMA) method is suitable for industrial applications because it is relatively inexpensive and offers an easy way to obtain the non-dendritic feedstock by just heating the highly stressed feedstock to the semisolid state. The SIMA cycle starts from the conventionally cast ingot. The feedstock is hot-worked material, for example, rolled, forged or extruded in the solid state above recrystallization temperature and then cooled; such feedstock develops fine globular microstructure after being heated up to the semisolid state.
