Introduction:
The rheocasting process entails the application of semi-solid slurry in metal casting. The process has numerous benefits that are directly connected to solid fractioning during the duration of casting. The process is also known as the semi-liquid casting because it involves the usage of the slurry in the semi-liquid or semi-solid state. The process can be said to consist of two stages, slurry making and direct slurry casting. The method was first developed by R. Shibata for commercial purposes at Hitachi Co. Ltd (Beeley & Smart, 2004).
Rheocasting process description:
The process involves preparation of high quality non-dendritic slurry through the application of a rotating table close to a casting machine without the need for a stirring process. The slurry, a molten lowly superheated semi-liquid, is poured into metallic containers. The cooling of the melt is allowed at a preferred semi-solid temperature that involves a uniform distribution of temperature in the melt. After pouring the melt, the top most part, as well as the bottom part of the metallic vessel, is shielded with insulators made of the ceramic material, a bad conductor of heat. This is aimed at preventing the top and the bottom parts of the vessel from overcooling in order to attain uniform cooling. The slurry is allowed to cool to preferred temperature with a steady cooling rate. At the last phase of the cooling process, induction heating is introduced to regulate the distribution of temperature throughout the vessel. Then, the slurry is transferred into the shot container of the casting machine at high pressure via metallic vessels. It is then ejected by turning it in an upside-down direction. There are two types of rheocasting processes i.e. the new rheocasting process, and the semi-solid dies casting process.
The new rheocasting process
This kind of the rheocasting process applies the carousel from the vessels on a circular rotating table. In this process, the aluminum in the molten liquid that is slightly above the liquidus temperature is poured into the carousel container. The achievement of the process can be attained once the superheat of the molten liquid is kept low. During the pouring stage of this process, there is the formation of many solid nuclei. The nuclei are the one responsible for the creation of the globular microstructure (Vinarcik, 2003). The slug in the semi-liquid state is then transferred into the shot cavity within the vertical cold cavity die-casting instrument.
Semi-liquid rheocasting
The Massachusetts Institute of Technology initially developed this process and it applies dendrite fragmentation as well as the stirring methods for making the slurry. The Institute revealed that once the solid nuclei are available in enough quantities in the molten state, the melt is cooled slightly below its liquid temperature; advanced cooling makes the nuclei quickly spheroidze as well as grow with a morphology that is spherical in shape.
In this process, a robot is used to place a coated ceramic vessel into a holding furnace wholly of aluminum melt maintained at numerous degrees above the aluminum’s liquidus temperature as well as take it to the station of the SSR (Groover, 2010). Then a cooled graphite rod that is rotating is placed into the molten metal. The molten aluminum is then cooled quickly for a short duration of time, within the interval of two to five seconds. Just a small fraction of the solid is developed within the stirring stage and once the stirring stick is taken away, extra cooling must occur to ensure that the semi-liquid state of the aluminum is cooled to a fraction of solid ranging between twenty-five to seventy-five percent. While the molten aluminum is cooling, the particles made begin to grow and form solid globular particles that spread all over the liquid. On achieving the expected solid fraction, the liquid alloy is poured from the crucibles into the shot cavity of the die-casting machine; the injection into the die happens here.
Material used in the process
One on the material that is mostly used in this process is the aluminum alloy. This material is used because of its mechanical properties that are necessary for the process. The aluminum alloy possesses the properties that exhibit the desired output (Polmear, 2000). Another material that is employed in the rheocasting process is strontium and refined eutectic silicon. Magnesium is used in the process as a strengthening element
Advantages and disadvantages of the process
The primary benefit of the process is that there is the production of a semi-liquid feed material directly from the liquid slurry in the area of the casting machine. This permits the usage of the traditional ingot material, enhancing the elimination of the premium that is connected to the billet of the thixocasting process. The second advantage is that it promotes the recycling of the runner as well as the biscuits straight into the stream of the casting (Ashby, & Jones, 2012). This results in the reduction of production costs of the rheocasting process. The third advantage of the method is that it is possible to do metal casting for an extensive range of solid fractions. This is mainly caused by the benefits of using the semi-liquid as well as a non-dendritic alloy that relies on the quantity of the solid during the duration of the casting. It is an advantage because an increase in the fractions of solid causes a reduction in the shrinkage of the product, as well as a reduced amount of the latent heat associated with the rheocasting process.
The prospective disadvantage of the process is related to the lack of consistency in the product produced. This can be caused by unequal distribution of the cooling temperature as well as non-uniformity in the alloy that was used. It has an application that the limited its commercial production. The rheocasting process requires significant investment in facilities and materials, and this makes the process economically feasible only when applied in large scale. Small scale production can be very expensive because of the resources and material used during the process. The consistency problem in the rheocasting process is due to the usage of a dosing that is single shot liquid based (Kaufman & Rooy, 2007). During the single-shot liquid process, a shot of molten metal is poured to give out each casting. This makes it hard to uphold the level desired for the quality as well as a pure product during the pouring of the five pounds of the molten liquid as compared to the pouring of more than fifty thousand pounds. The result of this is unclear that makes thixocasting more reliable. There is an increase in the fraction of the solid cause the semi-solid of the casting to start to deviate from the traditional process of die-casting. The high fraction of solid that is related to the rheocasting process produces a short cycle time. This makes the changes that are required to be expensive for the die-casting instrument to control the viscous material during the process.
Conclusion
The rheocasting process entails the application of the slurry in semi-solid state and its main advantage is production of a semi-liquid feed material directly from the liquid slurry in the area of the casting machine. The main disadvantage is lack of consistency in the product produced.
References
Kaufman, J. G., & Rooy, E. L. (2007). Aluminum alloy castings: Properties, processes, and applications. Schaumburg, IL: American Foundry Soc.
Ashby, M. F., & Jones, D. R. H. (2012). Engineering materials: 1. Amsterdam: Boston.
Polmear, I. J. (2000). Light alloys: Metallurgy of the light metals. Oxford: Butterworth- Heinemann.
Beeley, P. R., & Smart, R. F. (2004). Investment casting. London: Institute of Materials.
Groover, M. P. (2010). Fundamentals of modern manufacturing: Materials, processes, and systems. Hoboken, NJ: J. Wiley & Sons.
Vinarcik, E. J. (2003). High integrity die casting processes. New York: Wiley.
