Philipp Kürnsteiner

Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing

Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure—in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. Here, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition—i.e., the intrinsic heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing.

Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale

Abstract Ti and its alloys have a high affinity for hydrogen and are typical hydride formers. Ti-hydride are brittle phases which probably cause premature failure of Ti-alloys. Here, we used atom probe tomography and electron microscopy to investigate the hydrogen distribution in a set of specimens of commercially pure Ti, model and commercial Ti-alloys. Although likely partly introduced during specimen preparation with the focused-ion beam, we show formation of Ti-hydrides along α grain boundaries and α/β phase boundaries in commercial pure Ti and α+β binary model alloys. No hydrides are observed in the α phase in alloys with Al addition or quenched-in Mo supersaturation.

In-process Precipitation During Laser Additive Manufacturing Investigated by Atom Probe Tomography

Laser Metal Deposition (LMD) is a specific Laser Additive Manufacturing (LAM) process in which a focused laser beam creates a melt pool in the component’s surface. Metallic powder is then injected through a nozzle into the melt pool. As neighboring tracks and subsequent layers are deposited during the LMD process, already consolidated material experiences a cyclic re-heating. This intrinsic heat treatment (IHT) can be used to trigger the precipitation reaction in precipitation hardening alloys [1]. We used the rapid alloy prototyping capabilities of the LMD process to efficiently screen different alloy compositions by producing compositionally graded samples. In a simple model maraging steel containing 19at% Ni, we varied the Al concentration from 0 to 25at% to identify an alloy composition that responds well to the IHT of the LAM process to produce an in-process precipitation strengthened maraging steel. Due to its sub-nanometer resolution, Atom Probe Tomography (APT) provides compositional information at high-resolution and is therefore ideally suited for analyzing small clusters and precipitates. Additionally, we used High Energy Synchrotron X-Ray Diffraction (HEXRD) to provide crystallographic information with high sensitivity.