Maraging steels belong to a class of Advanced High Strength Steels (AHSS) that combine ultra-high strength with good toughness. These superior mechanical properties are caused by a martensitic microstructure that is hardened by a high number density of intermetallic precipitates upon aging heat treatment. Laser Metal Deposition (LMD) is a Laser Additive Manufacturing (LAM) process that allows to produce small, custom-made parts directly from a CAD model and metallic powders. The metallic powder is injected into the melt pool created by the laser beam in the substrate through a nozzle by means of a carrier gas.
As neighboring tracks and subsequent layers are deposited during the LMD process, the material experiences a cyclic reheating. The present study aims at the investigation of the precipitation reaction that occurs during this intrinsic heat treatment and at designing a maraging steel that is in-process precipitation strengthened. For this purpose, an Fe19Ni (at%) master alloy powder was produced. Different Al contents ranging from 3at% to 20at% were obtained by Rapid Alloy Prototyping, an approach that can easily be implemented by LMD. Thus, graded samples in which the Al content was increased from layer to layer were produced. They allow a fast and efficient investigation of different alloy compositions.
Mechanical properties such as hardness were evaluated and related to the precipitation density, distribution and chemistry as found from APT and TEM experiments. In the as-produced state, a completely homogenous Al and Ni distribution with no clusters present was found at an Al content of 3at% despite the cyclic reheating. Already at an Al content of 6at%, a high precipitate density of 5*10^24 was present, which even increased with increasing Al content. Using size-distribution based models, the influence of the intrinsic heat treatment on precipitation progress was modeled to guide future optimization of the LMD parameters for in-process precipitation strengthening.
Additionally to the nanometer sized precipitates, microstructural features such as grain boundary segregation and micrometer sized regions of retained austenite were analyzed by XRD, SEM, TEM and APT.
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|Präsentation||Figure 1||APT measurement showing the high precipitation density of nanometer sized Ni-Al precipitates represented by an isoconcentration surface at 5at% Al.||434 KB||Download|