Sand casting is a method used to make ordered-cell metal foams. Aluminum ordered-cell foam is lightweight, moves heat, and absorbs impact. Binder-jet 3D-printed sand castings could create a hybrid method to manufacture metal foam.
Excerpt from the April 2013 Machine Design article by Lindsey Frick.
Metal foams, also called cellular materials, are metallic bodies with interspersed voids called cells. They have a reputation for high strength, low density, and absorbing impact. This combination works well for military vehicles where lighter parts can save money and the degree of impact cushioning can be the difference between life or death.
Traditionally made metal foam is stochastic, meaning the foam has irregular cells that are spaced unevenly. If the cell structure in metal foam is arranged and regular it is called ordered-cell foam. Stochastic materials reduce weight as do ordered-cell metal foams. However, ordered-cell foam has superior strength, stiffness, energy absorption, and it moves heat.
Goodfellow’s aluminum foam is made by sand casting. The foam has regular, evenly spaced, 10-mm open pore cells.
While there are several methods of making ordered-cell foam, one company is seeing consistent results by making it through the process of sand casting. The problem is that current methods of making sand-casting patterns are from the age of subtractive machining processes where part features must be relatively simple. Now, researchers are testing additive manufacturing to make complex sand castings which could create a new breed of ordered-cell metal foams.
Mixing old and new techniques
The researchers are from the Design, Research, & Education for Additive Manufacturing Systems (DREAMS) Laboratory at Virginia Polytechnic Institute and State University, and the Virginia Tech Foundry Institute for Research and Education (VT FIRE). And what they’re studying is mesoscopic topology. Basically, this is the geometric arrangement of the solid phases and voids ranging from 0.1 to 10 mm within a material or product.
Originally the team considered direct-metal additive manufacturing methods to make ordered-cell metal foam. Methods include selective laser melting, electron-beam melting, and direct-metal laser sintering. While these methods can fabricate parts with cellular geometries, unfortunately each option posed such limitations as a lack of compatible working materials, the need for support structures which are difficult to remove, and cost. At the end of the day, the team decided these techniques are incapable of fabricating metal foam for large-scale applications.
Goodfellow uses a vertical sand-casting process to make reproducible metal foam with evenly spaced pores.
In addition, the team looked into tried-and-true methods of making ordered-cell foam. Common methods include stamping or crimping thin sheets of metal into a corrugated shape and joining them to create periodic structures, joining and bonding slotted metal sheets, extrusion and electrodischarge machining, and weaving and brazing metal filaments to form a periodic textile. Each method creates repeatable part quality, but they limit the macrostructure of parts to planar geometries and constrain designers to use a specific homogeneous mesostructure throughout a part.
Sand casting of foams is a relatively new method dating to 10 years ago when a French research organization, Centre Technique des Industries de la Fonderie, wanted to develop a foam that could be manufactured using foundry methods. The development led to a patent in 2008. Today, a company called Goodfellow Corp., Coraopolis, Pa., has released an ordered-cell metal foam made using the technique found in the patent. Goodfellow says it has the first commercially available metal foam made by sand casting. This is also said to be the first foam with a defined, reproducible, regular structure.
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