Effect of Powder Recycling in Electron Beam Melting on the Surface Chemistry of Alloy 718 Powder
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ALLOY 718 (also known as Inconel 718 or IN718) is a well-known age-hardenable Ni-Fe-base superalloy. Owing to its high Nb content, Alloy 718 is primarily strengthened by the disk-shaped gamma double prime phase (Ni3Nb) which maximizes its strength at temperatures up to around 650 C. As Alloy 718 also contains Al and Ti, the cuboidal or spherically shaped, matrix-coherent gamma prime (Ni3(Al, Ti)) also contributes to the precipitation strengthening. Alloy 718 exhibits excellent mechanical properties, corrosion resistance and oxidation resistance, at a wide range of temperatures. Besides being extensively used for gas turbine disks and compressor blades in aircraft engines, it finds service in a wide range of industrial application areas such as rocket motors, nuclear reactors, as well as in the oil industry.[1–3]
HANS GRUBER, MIKAEL HENRIKSSON, EDUARD HRYHA, and LARS NYBORG are with the Department of Industrial and Materials Science, Chalmers University of Technology, Gothenburg, Ra¨nnv. 2A, 412 96 Gothenburg, Sweden. Contact e-mail: [email protected] Manuscript submitted February 17, 2019. Article published online July 2, 2019 4410—VOLUME 50A, SEPTEMBER 2019
To date, Alloy 718 is predominately produced using the ingot-metallurgy route, including double melting (VIM + VAR/ESR) or triple melting (VIM + VAR + ESR) followed by homogenization, forging, and finally machining: a sequence associated with high cost and long lead times.[4] Even though Alloy 718 is considered to have a relatively good fabricability, its high strength, poor thermal conductivity, and work-hardening characteristics make both forging and machining challenging steps in the production chain. In addition, high material costs and increasing demands on sustainable production routes further incentivize minimal machining and improved buy-to-fly ratio.[1,5] The foregoing aspects motivate the recent rising interest that electron beam melting (EBM), a member of the powder bed fusion additive manufacturing (AM) family, has seen from both industry, and research community. Being able to produce complex-shaped metal components without tooling and extensive machining, while also reducing the total lead time, EBM has been recognized as a potential candidate for the aerospace industry as a means of obtaining high-performance, light-weight components through structural optimization.[6] However, to fully utilize the potential that AM offers, high reliability, and robustness is required from both material and process. In
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particular, the amount of material imperfections must be held at a minimum to meet the requirements for fatigue and fracture mechanical properties, especially under the hostile conditions at which aerospace engine components operate. Even though additive manufacturing has been subjected to extensive research during the recent years, there is still lack of knowledge in many different aspects. One such area is the effect of the powder feedstock material on the process stability, and/or quality of t
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