ALUMINUM SAND CASTING
Sand Casting is one of the quickest and most cost effective casting methods adopted in the production of metal prototypes and is an excellent solution for low to medium runs of parts that do not require precise shape repeatability, as well as being the only solution for very large objects which cannot be produced with other mass production casting techniques.
Sand is an excellent low cost cast material because it is refractory and chemically inert. Sand casting is also ideal for the production of very complex components requiring sand cores (cold box or shell sand) for the most intricate details and having internal areas with variations in thickness.
The Aluminum Sand Casting Process
Green sand, which is new or regenerated sand mixed with natural or synthetic binders, is the most commonly used material for making aluminum expendable molds. Green sand molds get their name from the fact that they are still moist when the molten metal is poured into them. The process of aluminum sand casting using green sand and the gravity filling method can be summarized as follows:
- a mold is created by placing the mixture of sand, clay and water on a pattern (the replica of the object to cast). Although this process can be done by hand, machinery is normally used in order to achieve better precision of the mold. When the pattern is removed the clay will have a cavity that corresponds to the shape of the pattern
- the sand mold has two or more parts, the upper part is known as the cope while the bottom one is called the drag. Additional parts known as cheeks can also be used. The molds are encased in a two part (or more if cheeks are used) box called a flask for protection. Before the flask is closed, any sand cores needed to manufacture the part details are placed in the mold halves. The gating system is placed inside, and a sprue is formed in order for the molten alloy to be fed into the cast
- the two halves are closed and clamped together and molten metal is then poured into the mold. As the metal starts to cool and some contraction takes place, molten metal is fed in from the risers that were placed in the casting system
- because sand and clay do not absorb heat, the cooling time is a lot longer than that of permanent mold or die casting. Chills (metal plates) can be inserted into the sand mold in order to help provide an equal cooling rate throughout the cast. As a consequence of the slower cooling, there is an appreciable decrease in the mechanical properties of alloys such as Aluminum 319 and 356, magnesium and bronze when compared to those of the same alloys cast with the permanent or die casting methods based on the Secondary Dendrite Arm Spacing (SDAS) value
- after a preset dwell time to allow the metal to solidify, the cast shake out takes place. The heat from the molten metal that is poured into it dries out the moisture making the cast easy to crack open when the metal has cooled
Aluminum sand casting defects to look out for are residual oxide film, inclusion, core erosion, gas holes and shrinkage porosity.
Sand cast aluminum components are widely used in the automotive and transportation industries including aerospace. Parts commonly produced with sand casting include the power-train, supports, suspensions, casings, gears and many others.
Aluminum Sand Casting from the Numerical Simulation Perspective
A simulation model that optimizes the process and layout of sand casting requires a complete fluid-dynamics simulation, including the change of a laminar flow of molten metal to an undesired turbulent flow. The model should also account for the natural air permeability of the mold, allowing gases and steam created by the casting process to easily escape, thanks to the good permeability of sand molds and cores. Some typical process parameters and their corresponding outcomes to be included in the model are:
- the risk of inclusion in the case of a turbulent vortex and molten metal velocity above the suggested 0.5 m/s range
- the temperature changes during the filling process in order to predict the emergence of cold shuts
- possible overheating where the cores touch the metal caused by the low heat absorption of the sand which could result in surface defects such as sinks
- different cooling rates as these can generate a high level of residual stress and significant casting distortion
Mastering the Sand Casting Process through Simulation
Using engineering simulation to optimize the sand casting process for aluminum and light alloys, we can lead foundries to “zero defect” manufacturing resulting in the dimensional conformity of the objects produced. Engineering simulation can help with:
- optimizing the gating system design to avoid filling defects caused by turbulence
- avoiding the presence of shrinkage porosities by acting on the process variables and feeder geometry right at the design stage
- reducing casting trials
- predicting the local microstructure and mechanical properties
- the proper evaluation of casting deformation and residual stress
And all of this can be achieved while shortening the time to market of the final product.
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Innovative integrated technologies for the development of aeronautic components
TMS2010, Seattle, 14-17 February 2010