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Art to Part: Rapid Manufacturing in Aluminum
12-26-02
Producing functional parts directly from a 3-D computer file, commonly referred to as rapid prototyping (RP) or rapid manufacturing (RM) (along with an alphabet soup of other acronyms), has become a commercially accepted technology for plastics, ceramics, and various non-aluminum metallic materials. The direct formation of aluminum parts has been more challenging, yet promising processing approaches are beginning to emerge. How soon they will have an impact on the way aluminum parts are made is still a question mark.
The first and probably most familiar technology used for making parts directly from a CAD drawing of a part was the stereolithography apparatus, or SLA for short. This method uses a laser to interact with a photo-sensitive polymer that causes it to solidify at the point of interaction. The key development in SLA was the creation of the stereolithography or .stl file that is used in the translation from a solid CAD model into a file that represents the same model in 2-D slices. This has allowed the now-familiar yellow plastic parts to be used quite regularly as design models, patterns for casting molds, and general show-and-tell articles.
Out of these beginnings has sprung a large industry and development effort directed toward further exploitation of the general "art to part" concept. Another result has been the establishment of a bewildering array of acronyms for various processes and approaches, which can make navigating the field a bit difficult. First, a brief review on some of the key acronyms. Generally, the field can be roughly divided into rapid prototyping (RP) and rapid manufacturing (RM). RP is generally used to make models, often out of polymer-based materials, for concept visualization, and is usually not applicable for the production of net shape products from metals. (These processes, however, can be used in the process of producing molds for investment casting.) Other labels for processes used in the RP process include solid ground curing (SGC), fused deposition modeling (FDM), selective laser sintering (SLS), and laminated object manufacturing (LOM).
Rapid manufacturing, or RM, technologies are those that reduce the production time in manufacturing by elimination of process steps through the combined use of computer-aided design (CAD) and computer-aided manufacturing (CAM). RM technologies have the potential to produce functional near net shape components directly. It is this area that will be the emphasis of this article, with specific reference to progress being made on the rapid manufacturing of aluminum-based parts. At this point it is important to note that RP and RM in aluminum can be viewed broadly as including processes such as casting and high speed machining, in which net or near-net shape parts are produced in a single step quite regularly and economically. For the discussion here, however, the focus will be on a group of processes termed (yes, another acronym) solid freeform fabrication (SFF). SFF processes are additive processes that produce freestanding objects without the use of a mold, mandrel, or other tooling.
SFF processes for aluminum alloys, as for other metal systems, generally employ some form of powder or particulate material in order to produce the precise features of the net shape parts. With aluminum, the presence of the tenacious oxide layer on the surface of powders can be a significant barrier to utilizing the process approaches that are employed for other metallic systems. Thus the focus of development work in producing aluminum parts via SFF processes has been to overcome the oxide to enable the good bonding required to achieve structural properties.
Many SFF technologies incorporate deposition or fusion of metal powder through heat supplied by a laser, and have been successful in producing parts from metals such as titanium, stainless steel, and nickel-based alloys. One specific approach, developed originally at Sandia National Laboratory, is called the laser engineered net shaping (LENSÔ) process. In this process, a high power laser is focused onto a substrate to create a molten puddle, and then metal powder is injected into the puddle. The substrate is moved relative to the laser beam in a controlled fashion to deposit thin metallic lines of a finite width and height. These lines are deposited side-by-side in the desired regions to create the pattern for each layer. In this fashion, each layer is built up line-by-line while the entire object evolves layer-by-layer. Key to success in this process is effective coupling of the metal with the laser, and recent developments involving additions of specific constituents to aluminum to increase absorption have allowed one company, Optomec Inc., to produce thin-walled aluminum cylinders by the LENSÔ process. The company is quoted as saying they have "developed process conditions for aluminum component fabrication and repair". Further information is available at the Optomec, Inc. web site at www.optomec.com.
An alternative approach to producing net shape parts from powders is to consolidate
them without melting. The challenge here is to produce adequate particle-to-particle
bonding, which usually requires some liquid phase. One process technology that
is under development is called cold gas dynamic spraying (CGDS), both in the U.S.
by a consortium involving Sandia National Laboratory and industry (www.sandia.gov),
as well as by the University of Liverpool in the United Kingdom (www.liv.ac.uk).
In this process, powder entrained in a high velocity gas stream impacts the substrate
surface, the oxide layer is broken, and consolidation occurs through forge bonding.
Because this process is carried out at relatively low temperatures in the range
of 150-300oC, the metal powder remains solid and further oxidation is avoided.
Deposition rates of 10-30 kg/hour are expected, compared to 0.5-2.0 kg/hour for
other processes. The work at the University of Liverpool has focused on aluminum
as the test material since it's difficult to process by laser methods. Efforts
there will develop an understanding of the mechanics of the deposition process
as well as an evaluation of the deposited materials in comparison to standard
cast and wrought forms. Industrial partners in the University of Liverpool work
are BAE Systems and DERA, and a manufacturer's group is also being set up.
Yet another method for producing net shape parts from aluminum powders is called dynamic magnetic compression (DMC), which utilizes high intensity magnetic fields to provide the energy necessary to rapidly consolidate aluminum powders. By loading the aluminum powder into an aluminum tube and then discharging a capacitor into an electromagnetic coil, the powder is formed into the shape of an inner die. The process is only applicable to cylindrical parts with inner features such as holes, splines, and gear teeth. Further information is available from the web site of the company that offers this technology. Magnepress Products LLC, at www.magnepress.com.
A final approach to net shape forming, very exploratory in nature, is generically termed droplet-based manufacturing (DBM, of course). An offshoot of the more commercially well-known process of spray forming, this process produces uniform droplet sprays that deposit metal in a controlled fashion to build up a part. Traditional spray forming is not particularly suitable for producing complicated net shape parts because of the wide size distribution of the metal droplets in the spray. The DBM approach instead uses fixed size droplets that enable controlled deposition and solidification conditions to be obtained. A development completed in 1998 at Arizona State University under the Department of Energy's Inventions and Innovations Program funding involved the concept of variable diameter droplet jets that would allow different sized droplets to be used based on the specific outline geometry and desired internal microstructure of the product. While hard to imagine this process ever becoming a large volume method, it is nonetheless interesting and illustrative of the rapid manufacturing process mindset.
Looking at these various methods for rapid manufacturing of solid freeform fabricated parts in aluminum, one might ask why go to all this trouble? Certainly, processes such as high speed machining and the wide variety of metal casting processes that are employed routinely for aluminum will be just as capable of producing the desired part geometries, and probably much more rapidly and inexpensively. That's a very good question. My view is that the potential benefit of these sorts of processes will more likely be realized in the fabrication of parts from aluminum-based alloys and materials that cannot be produced by conventional wrought or casting fabrication processes. Examples in this category could include high temperature aluminum alloys, certain aluminum metal matrix composites, and nanocrystalline aluminum. In addition, the potential for producing engineered parts through functional grading of microstructure is a possibility that will probably be more easily realized via solid freeform fabrication processes.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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