Machinery's Handbook, 31st Edition
1560 METAL ADDITIVE MANUFACTURING PROCESSES In one version of this process, the part is lightly sintered, and the remaining volume be- tween particles is filled by infiltration with a lower-melting-point alloy. In such cases, the printed powder is typically stainless steel and the infiltrating material bronze. Average particle size for this method is 0.001–0.002 in. (0.030–0.060 mm). The resulting mate- rial is useful for making functional prototypes, because it has properties comparable to high-strength steel. In another variation, the printed material is sintered to full density. In this case, the materials used may be stainless steel, Inconel, or another high-strength metal. To pro- mote sintering to full density, very small particle size is used, typically 0.0004–0.0006 in. (0.010–0.015 mm). Metals produced by BJ processes may have residual porosity as high as 2 percent. Be- cause these processes do not use a laser, they can be used to produce parts of copper, gold, and other metals that would reflect laser light. In addition, BJ can be used for ceramic ma - terials. Typical print resolutions are 1200 dots per inch, corresponding to a voxel length of 0.0077 in. (0.196 mm). Directed Energy Deposition (DED).—In this metal AM process, the energy source may be a laser, electron beam, or plasma arc. The material source may be metal powder or metal wire. Unlike the powder bed processes described above, DED processes the powder or feed wire into the focal point of the energy source. Typically, the substrate and part are fitted to a 5-axis motion system, while the energy and powder nozzles remain stationary. Powder-Fed DED: In the most common application of directed energy deposition (Fig. 28 ) , metal powder flows through nozzles at high speed into the focal point of a laser. The powder melts and is deposited layer upon layer onto a substrate or a previously built part. As in powder bed fusion processes, liquid metal in this molten pool solidifies very quickly. To prevent oxidation of the molten metal, an inert shield gas (argon or nitrogen) engulfs the metal deposition region. Alternately, the entire build system may be enclosed in a chamber containing inert gas. In one typical system, the 5-axis motion and powder ap- plication systems are enclosed in a chamber measuring 31 by 31 by 24 in. (800 by 800 by 600 mm) in length, width, and height. A major advantage of this process is that it does not involve spreading a layer of powder; therefore, it can be used to apply a material surface to an existing part or substrate. DED processes, as a result, are widely used for cladding and repair of existing parts. Another advantage of DED is the high build rate of up to 2.2 lb (1 kg) per hour. Unlike powder bed processes, powder-fed DED processes have multiple nozzles, each of which can supply a different material. Therefore, material can be deposited with vary- ing composition. For example, if one nozzle applies stainless steel and another supplies
Fig. 28. Directed Energy Deposition with Powder Feed
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