Abstract
The detrimental effect of nitrogen and oxygen when it comes to the precipitation of the strengthening γ’’ and γ’ phases in Alloy 718 is well-known from traditional manufacturing. Hence, the influence of the two processing atmospheres, namely argon and nitrogen, during the laser powder bed fusion (L-PBF) of Alloy 718 parts was studied. Regardless of the gas type, considerable losses of both oxygen of about 150 ppm O2 (≈30%) and nitrogen on the level of around 400 ppm N2 (≈25%) were measured in comparison to the feedstock powder. The utilization of nitrogen as processing atmosphere led to a slightly higher nitrogen content in the as-built material—about 50 ppm—compared to the argon atmosphere. The presence of the stable nitrides and Al-rich oxides observed in the as-built material was related to the transfer of these inclusions from the nitrogen atomized powder feedstock to the components. This was confirmed by dedicated analysis of the powder feedstock and supported by thermodynamic and kinetic calculations. Rapid cooling rates were held responsible for the limited nitrogen pick-up. Oxide dissociation during laser–powder interaction, metal vaporization followed by oxidation and spatter generation, and their removal by processing atmosphere are the factors describing an important oxygen loss during L-PBF. In addition, the reduction of the oxygen level in the process atmosphere from 500 to 50 ppm resulted in the reduction in the oxygen level in as-built component by about 5%.
Highlights
Additive manufacturing (AM) of Alloy 718, a precipitation-strengthened nickel–iron superalloy, has gained a lot of attention since it offers the possibility to produce complex components with high-temperature mechanical and chemical resistance
Gas atomized Alloy 718 powder provided by Höganäs AB (Sweden), with particle sizes in the range of 15 to 45 μm was employed as the feedstock material to build cubes of dimensions 10 × 10 × 15 mm3 with the standard parameters developed by the machine manufacturer with a 40 μm layer thickness
This highlights interesting differences between these two materials when exposed to the laser powder bed fusion (L-PBF) conditions, which is assumed to be connected to the lower solubility of nitrogen in the austenitic γ matrix in case of Alloy 718 compared to iron matrix in case of 316L stainless steel [21,22]
Summary
Additive manufacturing (AM) of Alloy 718, a precipitation-strengthened nickel–iron superalloy, has gained a lot of attention since it offers the possibility to produce complex components with high-temperature mechanical and chemical resistance. Alloy 718 has been preferably produced following the traditional ingot–metallurgy processing route for superalloys, including vacuum induction melting and electroslag refining and/or vacuum arc melting [2], followed by homogenization, forging, and machining to final dimensions. It is known that Alloy 718 is a difficult-to-machine material because of its high strength at elevated temperatures, its low thermal conductivity, high ductility, and critical work hardening [3]. Many challenges are to be addressed to make L-PBF an economically viable manufacturing route for Alloy 718
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