Metal Additive Manufacturing Conference 2018

Maraging steels show outstanding mechanical properties regarding strength and toughness caused by a martensitic microstructure that is strengthened by a high number density of intermetallic precipitates upon ageing heat treatment. Laser Metal Deposition (LMD; DED) is a nozzle-based Laser Additive Manufacturing (LAM) process that allows to produce custom-made parts directly from a CAD model and metallic powders. Material produced by LMD exhibits a unique thermal history: Initially the material is cooled rapidly from the liquid state in the meltpool. Subsequently, the material experiences a cyclic reheating, the so-called intrinsic heat treatment (IHT), as neighboring tracks and further layers are deposited during the LAM process. The present study aims at exploiting this intrinsic heat treatment to produce Maraging steel parts that are already in-situ precipitation hardened during the manufacturing process, avoiding an ageing heat treatment after the LMD process. For this purpose, we atomized an Fe19Ni (at%) master alloy powder. We produced two compositionally graded specimen with varying Al and Ti concentrations ranging from 3at% to 20at% by varying the relative feed rates of the master alloy and pure element powders in the LMD process. This allowed fast and efficient screening of different alloy compositions. We could show that indeed the IHT can be used to in-situ harden the Fe-Ni-Al [1] as well as the Fe-Ni-Ti Maraging steel system. We evaluated mechanical properties such as hardness and related them to the precipitate number density, spatial distribution and chemistry as found from Atom Probe Tomography (APT) and High Energy X-ray Diffraction (HEXRD) experiments. At low Al concentrations in the Fe-Ni-Al system, the solute atoms Ni and Al were distributed randomly. In contrast, at higher Al concentrations, pronounced clustering of Ni and Al occurred. We found exceptionally high number densities of up to 10^25 NiAl nano-precipitates per m^3 at 10at% Al in the as-produced samples. Hardness of the material showed a steep increase from 300HV (at 0% Al) to 530HV associated with the high number density of those NiAl precipitates. In the Fe-Ni-Ti system, depending on processing parameters, either η-phase (Ni3Ti) or Laves phase (Fe,Ni)2Ti precipitates formed upon IHT. The η-phase exhibited a bimodal size distribution consisting of nanometer sized spherical precipitates and a dense network of rod-shaped precipitates. The Laves phase showed rod shaped as well as roughly spherical precipitates in the size range of 100nm. In a graded sample exhibiting Laves phase, we observed a steep increase in hardness from 300HV at 0% Ti up to 700HV at 15at% Ti by precipitation of 20vol% Laves phase. [1] P. Kürnsteiner, M. B. Wilms, A. Weisheit, P. Barriobero-Vila, E. A. Jägle, D. Raabe: Massive nanoprecipitation in an Fe-19Ni-xAl maraging steel triggered by the intrinsic heat treatment during laser metal deposition. Acta Materialia 129, 52 (2017)