Rheocasting helps in the production of high-volume, high-integrity cast components.

Semi-solid casting technology has not completely caught on in the metalcasting industry because of limitations in process control. But if the technology is correctly harnessed, semi-solid casting can widen the range of cast components produced on diecasting machines, open up new markets dominated by slower cycle time processes used to make high-integrity castings and extend tooling life.

IdraPrince, Inc., Holland, Mich., has recently developed a rheocasting machine that allows the production of castings on conventional diecasting machines that were previously only produced by permanent mold or squeeze casting. Known as SSR (Semi-Solid Rheocasting), the process is based upon technology licensed exclusively from the Massachusetts Institute of Technology and is currently undergoing commercial trials. The process allows for the high-volume production of high-integrity, near-net-shape parts with the strength and ductility of squeeze and permanent mold cast components.

The basis of the SSR process is the use of a rotating, cool, graphite rod that rapidly removes heat to begin solidification in a shot of molten alloy. The rapid application of controlled cooling and convection creates enough solid nuclei within the melt that the metal is ready for semi-solid casting within seconds. Stirring and cooling is unnecessary after solidification begins because it has no impact on the final microstructure. By using the cooling rod, operators have greater control over the process.

During the process, molten metal is delivered from the furnace to the rheocasting machine in a ladle. The molten metal is then stirred and cooled by the chilled rod for 10 sec. The exact length of the stir time changes depending on the temperature of the furnace and rod. The casting machine houses four rods, but only one is used per cycle (the rest are air-cooled for subsequent cycles). The graphite rod temperature remains below 350-400C (662-752F) throughout the cycle to limit oxidation. Once the molten metal is properly cooled to the semi-solid state, it is immediately transferred to the diecasting machine.

The machine also incorporates only one stirring cup, so if it must be taken down, there is no back up. The machine cuts down on dendritic skin forming on the wails of the cup because the rod also acts as the source of agitation.

Trials of the system have focused on castings ranging from 5-15 lb with a variety of alloys, including 356 and 380 aluminum. Through the trials, it has been shown that semi-solid rheocasting offers numerous advantages over other processes, including:

* rapid cooling from within the melt, which forms the ideal microstructure suitable for semi-solid casting;

* no dendritic skin is formed on the outside of the slurry;

* no need for special grain refiners or other consumables;

* effects from variations in metal holding temperature, cycle time delays, shot sleeve temperature, etc. are negated using the controlled cooling of the rod;

* the slurry is produced outside of the diecasting machine, which allows for tighter process control and improvement of overall casting cycle time;

* the slurry is at a low fraction solid when it enters the cold chamber (< 0.20) so it can be handled like a liquid and does not require major changes to a diecasting machine.

Compared with diecasting, dwell time was reduced by at least 25% and mechanical properties were comparable to other semi-solid and squeeze casting processes. The machine is small (the footprint is 4 ft x 3ft) and can be retrofitted to existing horizontal or vertical diecasting machines.

Select No. 101 at www.moderncasting.com/info