Semi-Solid Rheocasting Moves into Commercial Production

Process involves agitating molten metal during solidification

In response to automakers’ growing acceptance of safety-critical, structural castings, IdraPrince Inc. set out to provide the technology necessary to produce these castings. In early 2002, the diecasting equipment builder identified a rheocasting process being developed at Massachusetts Institute of Technology as the one with the most potential to overcome the operating limits of existing rheocasting processes. The company secured an exclusive license from MIT to supply the special rheocasting technology, known now as Semi-Solid Rheocasting, or SSR for short, for diecasting of aluminum worldwide.

Following considerable development activity, IdraPrince recently announced that it had supplied a Semi-Solid Rheocasting (SSR) process system to Japan’s Yamaha Motor Co. Mark Los, president of the Holland, MI, firm, stated: “We are pleased with Yamaha’s commitment to implement SSR technology and believe they will take full advantage of the many cost and quality benefits this process offers.”

Rheocasting is a metalcasting process in which molten metal is agitated strongly as it begins to solidify. This creates a globular, highly viscous material that is very fluid even when over 60% of the metal has solidified.

SSR is fundamentally different from other rheocasting processes because the molten alloy is converted to semi-solid slurry with applied rapid cooling and convection. Desirable, spheroidal microstructures are formed quickly and efficiently from molten alloy right at the liquidus. According to IdraPrince, this gives the SSR process several advantages:
- Control of cooling and stirring allows for production of consistent slurry in the foundry environment;
- One shot of metal in process at any one given time (true slurry-on-demand ... much simpler to operate with less downtime and scrap);
- It is retrofittable to existing diecasting machines and furnaces; and,
- SSR slurry can be used at any percentage solid, and many advantages are identified in using solid contents of less than 20%.

SSR makes it possible to modify ordinary aluminum alloy into semi-solid material directly at the diecasting cell. This allows SSR to use conventional diecasting machinery to produce high-integrity, near-net-shape parts with strength and ductility comparable to squeeze- or permanent-mold cast components, but with the faster cycle times.

In the SSR process, molten alloy is held above the liquidus temperature. A rotating, cool rod made of graphite is inserted into the melt, starting solidification. After the proper interval, the rotating rod is removed and the quiescent melt is further cooled to the casting temperature.

With SSR, the diecaster can produce high-integrity, 356-type alloy (AlSi7Mg) castings that were previously difficult or impossible to make with diecasting. These include castings that are T5 and T6 heat-treatable (blister free) and weldable, with excellent mechanical properties [YS > 32.5 ksi (225 MPa), TS > 40 ksi (275 MPa), El% > 10%]. SSR also permits production of near-net shape, thick- and thin-walled castings that require less machining than other casting processes

According to IdraPrince, dwell time with SSR is decreased by 30-60% resulting in a decrease in casting cycle time of 12-25%. It also results in an increase in tool life of at least 50%.

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