Build parts by printing

A new 3D printing process is under development at the departments of Mechanical Engineering, Materials Science and Engineering, and Chemical Engineering at the Massachusetts Institute of Technology (MIT) in Cambridge, MA, USA. Its developers hope that it will allow prototype parts and tooling to be produced directly from a CAD model.

The 3D printing process works by building parts in layers (Figure 1). First, a thin distribution of powder is spread over the surface of a powder bed. From a computer model of the desired part, a slicing algorithm computes information for the layer. Using a technology similar to ink-jet printing, a binder material joins particles where the object is to be formed. A piston then lowers so that the next powder layer can be spread and selectively joined. This layer-by-layer process repeats until the part is completed. Following heat treatment, the unbound powder is removed, leaving the fabricated part.

3D printing can create parts of any geometry, including undercuts, overhangs, and internal volumes. It can form any material that can be obtained as a powder. And because different materials can be dispensed by different printheads, the technique can exercise control over local material composition. The proper placement of droplets can be used to create surfaces of controlled texture and to control the internal microstructure of the printed part.

CAD-Casting is a term used to connote a casting process where the mould is fabricated directly from a computer model with no intervening steps, resulting in a streamlining of the casting process.

Metal parts in a range of materials including stainless steel, tungsten and tungsten carbide can be created from metal powder with the three dimensional process. Printed parts are post-processed using techniques borrowed from metal injection moulding to produce metal parts with greater than 92% of theoretical density. These parts may be infiltrated with low melting point alloys to produce fully dense parts. The three dimensional process is adaptable to a variety of materials systems, allowing the production of metallic and ceramic parts with novel compositions.

The process has been used to prepare dense alumina components by spreading submicron alumina powders and printing a latex binder. The green parts are then isostatically pressed and sintered to increase the density of the component. The polymeric binder is removed by thermal decomposition. Figure 2 shows a ceramic shell fabricated by three dimensional printing and the metal casting of a hollow turbine blade made from an identical sample.

The technique can create composite materials as well. Ceramic moulds can be 3D printed, filled with particulate matter, and then pressure infiltrated with a molten material. Silicon carbide reinforced aluminium alloys can be produced directly by 3D Printing a complex SiC preform and infiltrating it with aluminium, allowing localised control of toughness.

The MIT effort is funded by the 3D Printing Consortium whose members believe that the technique can substantially reduce time to market for new products.

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