Semi-Solid Casting of Magnesium and Aluminum Alloys via the CRP (Continuous Rheo-conversion Process) 2006-01-0509
Semi-solid processing (SSM) has many advantages in that the alloy is cast at lower temperatures (i.e., in the two-phase region) giving rise to reduced die wear, as well as giving rise to novel microstructures. The resultant SSM processed castings are dendrite-free and do not contain hot tears; rather, the SSM structure is globular, and the liquid phase surrounding the globules acts as a “lubricant” during processing. Moreover, the flow of the slurry into the die cavity is more laminar than turbulent, since the starting metal is in the mushy region. This concept of SSM processing was realized by the development of a continuous process titled: CRP - Continuous Rheo-conversion Process. In this process, one allows the incipient solidification of alloy melt(s) under the combined effects of forced convection and rapid cooling rates. In the CRP, two liquids held at particular level of superheat, are passively mixed within a reactor. The reactor provides heat extraction, copious nucleation, and forced convection during the initial stages of solidification, leading to dendrite-free structures. The process also works well with only one melt source; thus having two liquid melts is not a requirement, but the latter does give one the flexibility to custom design alloys via the CRP.
In this study, a magnesium alloy (AZ91D) and an aluminum alloy (206) were processed using the CRP under various processing conditions (different melt superheats, fraction solids, etc), and optimal process windows for the alloys were established. The results show that one can consistently produce ideal semi-solid structures with small, spheroidized primary particles free of entrapped liquid. These results will be reviewed and discussed within the context of automotive applications wherein weight reduction is the target.