Metal Additive Manufacturing Conference 2019

Maraging steels show outstanding mechanical properties especially regarding strength and toughness. They exhibit a big advantage over e.g. conventional C-containing tool steels regarding their precessability in Laser Additive Manufacturing (LAM). Parts produced by Laser Metal Deposition (LMD; DED) possess a unique thermal history: after a rapid quenching from the liquid state, the material experiences a cyclic reheating - the so-called intrinsic heat treatment (IHT). Our aim is to design new steels that are tailor-made for LAM processes exploiting the IHT. We want to produce Maraging steel parts that are already in-situ precipitation hardened during the manufacturing process, avoiding an ageing heat treatment after the LAM process. For this purpose, we apply rapid alloy prototyping to find alloy compositions that respond well to the IHT. We use LMD, a nozzle-based, blown-powder AM-process as well as the powder bed process Selective Laser Metling (SLM). We could show shown that the IHT can be used to in-situ harden Fe-Ni-Al [1] as well as Fe-Ni-Ti Maraging steel parts printed by LMD. Mechanical properties such as hardness and tensile strength were evaluated and related to the precipitate density, size and chemistry determined by Atom Probe Tomography (APT) and High Energy X-ray Diffraction (HEXRD). Exceptionally high number densities of up to 10^25 NiAl nano-precipitates per m^3 were found in the as-produced Fe-Ni-Al samples. The hardness of the material showed a steep increase of over 200 HV associated with the high number density of those NiAl precipitates. In the Fe-Ni-Ti system, a dense network of rod shaped η-phase (Ni3Ti) precipitates together with fine, nanometer-sized spherical precipitates are formed upon IHT. An increase in hardness of 300 HV could be achived by precipitation of the η-phase. Precipitation of NiAl and η-phase can occur only in a martensite matrix. In the Fe-Ni-Ti system we study in detail how martensite start (Ms) temperature, cooling rate as well as IHT strength influence in-situ precipitation. We were able to show how only specific thermal histories lead to precipitation of η-phase and thus how the microstructure can be controlled locally, without changing the alloy composition. In SLM, the base plate heating is another factor influencing the thermal history. Only if the sample temperature, which is determined by both the base plate heating and the laser heat input, drops below Ms temperature the IHT can trigger precipitation. On Fe-Ni-Al samples we used HEXRD and APT together with hardness measurments to study the in-situ precipitation hardening at different base plate heating temperatures as well as different energy densities (i.e. different strenghts of the IHT). We demonstrated that the IHT during LAM can be used to trigger phase transformations already during the AM process. This effect should be taken into account for future alloy design for AM. [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)