New Process Requirements for Additive Powders for Microplasma Powder Deposition

The distribution of doping elements and impurities between the external surfaces and internal volumes of typical fine particles in samples of gas-atomized commercial additive powders of hightemperature creep-resistant (Inconel 939, ZhS32) and high-temperature oxidation-resistant (Inconel 625, Hastelloy C22) nickel superalloys was examined employing energy-dispersive X-ray analysis (EDX). Significant concentration gradients were observed between the surfaces and internal volumes of powder particles for doping elements reaching 4–5 wt.%: Re, Mo Ta, and Nb in the high-temperature creep-resistant alloys and Al, Nb, Co, Fe, V, and Mn in the Inconel 625 hightemperature oxidation-resistant alloy. Besides doping elements, concentration gradients of O, N, S, P, and Si impurities were found in the near-surface layers of the additive powders. The EDX findings and data from the reduction–extraction method were used to calculate the amounts of oxygen and nitrogen in the internal volumes and the near-surface layer of typical fine powder particles and the thickness of this layer corresponding to the increased content of impurities. The surface layer of typical fine particles was shown to increase the total weight-average content of impurities in the samples of commercial additive powders: oxygen up to 2.5 times and nitrogen up to 1.8 times. To assess the influence of impurity amounts of oxygen <0.16 wt.% and nitrogen <0.13 wt.% on the welding process properties of the atomized additive powders, additional samples of hightemperature oxidation-resistant (ChS40) and high-temperature creep-resistant (ZhS6U, ZhS32, Renè 80) nickel superalloys were tested to ascertain their suitability for microplasma powder deposition at a welding current of up to 15 A. It was found that the suitability of the additive powder for lowamperage deposition was mainly determined by the limited oxygen impurity content: weight-average content up to 0.025 wt.% and content in the 1–3 μm thick near-surface layer of a typical fine particle up to 0.1 wt.%.