Nov_Dec_AMP_Digital

6 A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 3D PRINTING NOT A CURE-ALL I read with interest the article on “Addi- tive Manufacturing for Aerospace Appli- cations—Part II” (September 2017) by Sam Froes, Rod Boyer, and Bhaskar Dut- ta. 3D printing of components and pat- terns will positively change every aspect of our lives. We must be cautious in our application of 3D printing because as in- triguing as it is, a specific material or pro- cess application may or may not be the best fit. Direct metal printing is the hot- test new application of 3D printing and is rightfully drawing significant attention from the engineering and manufacturing communities, but all material processes have limitations. For the second time in as many months, I have seen articles discussing direct metal printing and claiming that printed metal parts “reduce energy use by up to 50%.” Having seen no editori- al reply to those claims, I thought it ap- propriate to respond. All metal components start as a casting. Metal casting is the most di- rect, and lowest energy, line of man- ufacturing to take metal ores from the ground and convert them into a final product. Refined ores are made into in- gots, which are remelted, alloyed, and cast to near net shape. Adding a step where the metal is remelted, atomized, and then welded back together with a 50-micron laser or electron beam (EB) is much more energy intensive than cast- ing it. The article claims that “half the cost of the final (cast) part is associat- ed with the machining cost” and im- plies that the final finishing/machin- ing cost for direct metal printed parts is less. That is just not the case. The sup- port structures required for direct metal printed parts are every bit as substan- tial as the gating/runner system for a casting. (The cutoff casting gating sys- tem is subsequently recycled as low-en- ergy chargematerial in themanufacture of subsequent castings.) Investment casting polymeric patterns are rou- tinely produced by 3D printing and in- vestment shells can now be 3D print- ed without tooling or cores. 3D-printed patterns and/or shells eliminate the need for draft in design and allow all of the advantages claimed by direct metal printed parts. The cleaning, machining, heat treating, HIP, and finishing require- ments for both direct metal printed parts and investment cast parts (and the energy embodied in them) are simi- lar. Currently, direct metal printed parts are more expensive than investment castings, a strong implication that they embody more energy in their manufac- ture than investment castings. FEEDBACK structure and subsequent finishing operations, but one cannot remove the energy from meticulously weld- ing together metal powders with lasers and EB. New casting technologies are developing that use fully recyclable, 3D-printed molds with no tooling, fea- turing all of the design freedom of di- rect metal printing and requiring much less energy. Direct metal printing di- rectly competes with the metal cast- ing process. Both processes will benefit from advances in 3D printing technolo- gies and from competition with the oth- er. The engineering and manufacturing community must scrutinize new mate- rial/process combinations to seek the best solutions for their applications. No one material/process combination is the best at everything and, like it or not, this tends to shake out in a proper cost analysis. John R. (Chip) Keough, PE, FASM AUTHORS’ RESPONSE: 3D PRINTING ARTICLE W e, the authors of the article on additive manufacturing (AM), appreciate the interest taken in the article by Mr. Keough, and compli- ment him on his insightful remarks. In particular, we find his comments on the fabrication of investment casting shells by 3D printing very interesting. Casting or forging is certainly more energy efficient for larger batches of parts, but when the quantity is low or for a one-off prototype part, casting and forging require a lot of energy when the die manufacturing and energy consumption for die material is considered. In addition, for parts with a high buy-to-fly ratio, casting/forging of- ten yields a lot of scrap material and re- quires much more machining time than AM parts. Every pound of material waste and additional machining time is also as- sociatedwith energy consumption. There BALD bracket for USAF joint strike fighter built using AM electron beammelting technology. Courtesy of Oak Ridge National Laboratory. Investment casting currently em- bodies less energy than direct metal printed parts. (The difference can vary significantly by material/process com- bination, component configuration, and product volume.) Much of the energy embodied in investment castings re- sults from the amount of resources re- quired to produce the ceramic shells and cores. As 3D printing of ceramic shells is mainstreamed, the energy in the investment casting process will be reduced. The challenge for direct metal printed parts is to minimize the support

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