1998 / Solid Freeform Fabrication Proceedings
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Author E. Schlienger, M. Griffith, M. Oliver, J. A. Romero, J. Smugeresky, Sandia National Labs
Source Solid Freeform Fabrication Proceedings, 1998, pp 205-210
Abstract The direct additive manufacturing of metallic components can present several process challenges. At present, there are several techniques for the accomplishment of this goal, each with its own set of strong points and limitations. At Sandia, Laser Engineered Net Shaping, or LENS, is a process which has been developed for the direct additive manufacturing of fully dense three dimensional parts. In LENS, a ND-YAG laser is focused onto a metallic substrate or onto previously deposited material. The laser melts the metal and a small pool of metal forms. Powder is injected into the pool and a bead forms. By mastering an x-y table to which the part is affixed in a controlled fashion, the bead is pulled and a fully dense metal part is formed. As currently configured, LENS is a 2 1/2 D process. A potential drawback of this technique is that in order for deposition to occur, a substrate must be present. This limitation defines the degree of overhang that can be achieved with this process. Since LENS deposition is based within an orthogonal coordinate system, the degree of overhang that can be achieved seems to be related to surface tension issues. At present, an overhang of about 15 deg is what may be readily achieved. Although similar processes utilizing movable deposition heads are more effective at producing overhangs, there will always be applications where some form of support structure is required to produce the desired geometry. One example of such a geometry might be injection molding tools with conformal cooling passages. In such a case, closing off the top of the cooling channel represents a process challenge. However, there are several ways of accomplishing this task. Cooling passages may be built into a part simply by utilizing the overhang capacity inherent in the system or tubing can be readily incorporated into LENS produced components. Both of theses techniques have applications in the production of many different types of parts. Further, manipulation of the deposition head can provide an even greater overhang capability and further extend the ability of the process to produce enclosed shapes. However, even with these various capabilities, some shapes are most readily produced if some form of underlying support is available. Such a capability is typically achieved by using a sacrificial material. The requirement for such a material is that it must be readily placed, and easily removed from the finished part. Typically, this means that the sacrificial material must not bond to the main structure, or it must be readily removed through some process such as acid etching. Finding a material that is readi1y deposited via LENS, can withstand molten metal deposition, and not bond to such a deposition is a challenge. The removal of support structures via acid also has some complications associated with it, and since LENS is a melting process, acid etching is not expected to produce good interfaces at the boundary of the sacrificial material. These problems have been addressed by using powder of the parent material as the necessary support structure. (Auth abstract) [References: 3]
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