Semi-solid metal casting

Semi-solid metal casting (SSM) is a near net shape variant of die casting.[1] The process is used with non-ferrous metals, such as aluminium, copper,[2] and magnesium. The process combines the advantages of casting and forging. The process is named after the fluid property thixotropy, which is the phenomenon that allows this process to work. Simply, thixotropic fluids shear when the material flows, but thicken when standing.[3] The potential for this type of process was first recognized in the early 1970s.[3] There are four different processes: thixocasting, rheocasting, thixomolding, and SIMA.

SSM is done at a temperature that puts the metal between its liquidus and solidus temperature. Ideally, the metal should be 30 to 65% solid. The metal must have a low viscosity to be usable, and to reach this low viscosity the material needs a globular primary surrounded by the liquid phase.[2] The temperature range possible depends on the material and for aluminum alloys is 5–10 °C, but for narrow melting range copper alloys can be only several tenths of a degree.[4]

Semi-solid casting is typically used for high-end castings. For aluminum alloys typical parts include engine suspension mounts, air manifold sensor harness, engine blocks and oil pump filter housing.[5]

Processes

There are a number of different techniques to produce semi-solid castings. For aluminum alloys the more common processes are thixocasting and rheocasting.

With magnesium alloys, the most common process is molding.[6]

Thixocasting

Thixocasting utilizes a pre-cast billet with a non-dendritic microstructure that is normally produced by vigorously stirring the melt as the bar is being cast. Induction heating is normally used to re-heat the billets to the semi-solid temperature range, and die casting machines are used to inject the semi-solid material into hardened steels dies. Thixocasting is being performed commercially in North America, Europe and Asia. Thixocasting has the ability to produce extremely high quality components due to the product consistency that results from using pre-cast billet that is manufactured under the same ideal continuous processing conditions that are employed to make forging or rolling stock.[7] The main disadvantage is that it is expensive due to the special billets that must be used. Other disadvantages include a limited number of alloys, and scrap cannot be directly reused.[8]

Rheocasting

Unlike thixocasting, which re-heats a billet, rheocasting develops the semi-solid slurry from the molten metal produced in a typical die casting furnace/machine.[7] This is a big advantage over thixocasting because it results in less expensive feedstock, in the form of typical die casting alloys, and allows for direct recycling.[8]

Thixomolding

For magnesium alloys, thixomolding uses a machine similar to injection molding. In a single step process, room temperature magnesium alloy chips are fed into the back end of a heated barrel through a volumetric feeder. The barrel is maintained under an argon atmosphere to prevent oxidation of the magnesium chips. A screw feeder located inside the barrel feeds the magnesium chips forward as they are heated into the semi-solid temperature range. The screw rotation provides the necessary shearing force to generate the globular structure needed for semi-solid casting. Once enough slurry has accumulated, the screw moves forward to inject the slurry into a steel die.[9]

SIMA

In the SIMA method the material is first heated to the SMM temperature. As it nears the solidus temperature the grains recrystallize to form a fine grain structure. After the solidus temperature is passed the grain boundaries melt to form the SSM microstructure. For this method to work the material should be extruded or cold rolled in the half-hard tempered state. This method is limited in size to bar diameters smaller than 37 mm (1.5 in); because of this only smaller parts can be cast.[10]

Advantages

The advantages of semi-solid casting are as follows:[11]

Due to the lower pressures and temperatures required to die cast semi-solid metal the die material does not need to be as exotic. Often graphite or softer stainless steels may be used. Even non-ferrous dies can be used for one time shots. Because of this the process can be applied to rapid prototyping needs and mass production. This also allows for the casting of high melting point metals, such as tool steel and stellite, if a higher temperature die material is used. Other advantages include: easily automated, consistent, production rates are equal to or better than die casting rates, no air entrapment, low shrinkage rates, and a uniform microstructure.[3]

Disadvantages

Because thixotropy (semi-solid state) is a middle state in physical or rheological sense process conditions form a band so narrow even environmental temperature differences have to be considered. Thus production facilities need a high level of technology and operators require similar knownledge and training.

See also

References

Notes

  1. ↑ "Welcome to MyNADCA!". diecasting.org. Retrieved 2015-08-20.
  2. 1 2 Young, p. 1.
  3. 1 2 3 Lowe, Anthony; Ridgway, Keith; Atkinson, Helen (September 1999), "Thixoforming", Materials World, 7 (9): 541–543.
  4. ↑ Vinarcik, Edward J. (2003), High integrity die casting processes, 1, Wiley-IEEE, pp. 91–101, ISBN 978-0-471-20131-1.
  5. ↑ P. Kapranos, Proc. 10th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Aachen, Germany & Liege, Belgium, 2008
  6. ↑ S. LeBeau & R Decker, "Microstructural Design of Thixomolded Magnesium Alloys", Proc. 5th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Golden, Colorado, 1998
  7. 1 2 Stephen P. Midson, Semi-Solid Casting of Aluminum Alloys: An Update, Die Casting Engineer, Sept. 2008
  8. 1 2 John L., Jorstad (September 2006), "Aluminum Future Technology in Die Casting", Die Casting Engineering: 18–25, archived from the original (PDF) on 2010-12-11.
  9. ↑ Stephen P. Midson, Robert K. Kilbert, Stephen E. Le Beau & Raymond Decker, "Guidelines for Producing Magnesium Thixomolded Semi-Solid Components used in Structural Applications", Proc. 8th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Limasol, Cyprus, 2004
  10. ↑ Young, p. 2.
  11. ↑ Stephen P. Midson, NADCA Semi-Solid & Squeeze Casting Conference, Rosemont, Illinois, 1996

Bibliography

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