Machinery's Handbook, 31st Edition
METAL ADDITIVE MANUFACTURING PROCESSES
1561
Fig. 29. Directed Energy Deposition with Wire Feed Inconel nickel-based superalloy, a part could be built with a relatively low-cost stainless steel base that transitions to the higher cost Inconel surface where greater heat resistance is required. A major disadvantage of this process is rough surface finish. In addition, because the powder is blown onto the surface, not all of the powder becomes melted, and material losses can range from 20 to 60 percent. Wire-Fed DED: Another approach to DED involves feeding a metal wire from a spool into the focal point of an electron beam (Fig. 29). Energy from an electron beam continu- ously melts wire fed from a coil into the beam focal point. The motion of the base plate beneath the beam and wire feed builds up the part as directed by the CAD file. In this case, the process must be carried out in a vacuum chamber, because any gas in the environment would interfere with the electron beam. Wire-fed electron beam DED pro- cesses are capable of very high build rates, up to 25 lb (11.3 kg) per hour, and do not result in any direct metal loss. But since such processes essentially involve weld metal build-up, the finish is rough; machining the entire surface is needed to reach desired dimensions and surface finish. Hybrid Processes: Both wire-fed and powder-fed approaches to DED provide the basis for hybrid manufacturing, or combined additive/subtractive manufacturing. The basic form of the part to be produced is made by one of the DED processes, and the finished dimensions and surface finish are achieved by material removal, usually cutting. This se - quence of operations can be carried out in one machine with one setup to ensure accuracy. The metal addition device (powder deposition or wire deposition) is attached to the CNC tool disk or carriage and is used to add metal where it is needed, while the usual cutting tools are then used to carry out the subsequent machining and finishing operations. One way to use such hybrid systems is to fix a conventionally manufactured (by forging or casting) base shape in the hybrid machine, and then add features, such as nozzles or fins, that would be impossible or difficult to form in the conventional process. Then, the overall part can be finish-machined in the same setup. Hybrid additive/subtractive manufacturing offers a convenient approach for machine shops to enter into additive manufacturing. Some hybrid equipment providers offer ret- rofit systems—including powder feed, laser, and software in one package—to convert a standard 5-axis CNC system into a hybrid additive/subtractive manufacturing cell. Comparing AM with Conventional Processes.—AM technology is just a few decades old, but great progress has been made in applying these processes to production of tai- lored, one-of-a-kind or limited-production parts. The majority of current applications involve specialized materials used in aerospace and medical fields. However, serial (mass) production of other manufactured items is economically feasible in some cases— increasingly so as rapid development of AM processes, materials, and applications continues.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online