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Dierk Raabe Dissertation Topics

Cluster-hardening alloys by intrinsic heat treatment

Commercially available materials are designed and optimised for the conventional processing route (e.g. casting, rolling and annealing) and therefore might not be optimal for LAM or might even not be suitable for LAM at all. The time-temperature profile experienced by a part produced by LAM is very different from the one produced by conventional manufacturing. 

During SLM, for example, a thin powder layer is melted by the laser beam. When the laser beam moves away, the material in the melt pool cools down at very high cooling rates due to rapid heat conduction into the underlying layers and the substrate material. During the deposition of subsequent layers, the consolidated material gets re-heated by the laser beam. It experiences a series of temperature pulse cycles up to temperatures close to the melting point (decaying in intensity with every additional layer). This “intrinsic heat treatment“ is unlike any annealing conducted in conventional processing and can be exploited e.g. for solid-state transformations after deposition.

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Schematic representation of the different temperature profiles experienced by conventionally-produced material and material processed by laser additive manufacturing.

Schematic representation of the different temperature profiles experienced by conventionally-produced material and material processed by laser additive manufacturing.

© Max-Planck-Institut für Eisenforschung GmbH

© Max-Planck-Institut für Eisenforschung GmbH

In this project, we are developing alloys with in-situ strengthening reactions, i.e. alloys that achieve age-hardening by rapid clustering/precipitation of nano-scaled intermetallic phases during the intrinsic heat treatment of LAM. Exploiting the rapid alloy prototyping capabilities of LMD we designed an Fe-Ni-Al maraging steel that responds very well to the intrinsic heat treatment. Number densities up to 1025 NiAl nano precipitates per m3 are formed already during the LMD process, without any further heat treatment.

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Atom probe measurement of the newly developed Fe-Ni-Al steel showing the high number density of NiAl precipitates visualized by drawing an isoconcentration surface at 15 at% Al. Corresponding proximity histogram plotting the chemical composition as a function of the distance to the isoconcentration surface.

Atom probe measurement of the newly developed Fe-Ni-Al steel showing the high number density of NiAl precipitates visualized by drawing an isoconcentration surface at 15 at% Al. Corresponding proximity histogram plotting the chemical composition as a function of the distance to the isoconcentration surface.

© MPIE GmbH

© MPIE GmbH

In Al-Sc alloys fully coherent Al3Sc precipitates in the size range of 5-40nm in diameter are formed by the intrinsic heat treatment during the LMD process. Currently we investigate the influence of powder quality, alloying elements and process parameters on the size and distribution of Al3Sc precipitates, their coarsening behavior during the intrinsic heat treatment as well as microstructure of the LAM produced parts.

Details of the precipitation kinetics during the strongly non-linear intrinsic heat treatment will be studied in a dedicated project. Fundamental understanding of the impact of the intrinsic heat treatment on the final part and its precise control during the manufacturing process allows to better exploit the potential of LAM processes by combining shaping and heat treatment in one processing step. This research is a collaboration of MPIE, Düsseldorf, with the Fraunhofer Institute for Laser Technology in Aachen.

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Microstructural hierarchy of a LAM-produced maraging steel, showing the layer-wise build-up of the sample, the cellular solidification structure together with the corresponding microsegregation, and finally, nano-sized intermetallic precipitates which give the material its strength.

Microstructural hierarchy of a LAM-produced maraging steel, showing the layer-wise build-up of the sample, the cellular solidification structure together with the corresponding microsegregation, and finally, nano-sized intermetallic precipitates which give the material its strength.

© Max-Planck-Institut für Eisenforschung GmbH

© Max-Planck-Institut für Eisenforschung GmbH

Curriculum Vitae

Born on April 18, 1965 in Hilden. Study of music, metallurgy and metal physics; doctorate in metallurgy and metal physics (Dipl.-Ing.) at RWTH Aachen (1992). Staff Scientist and Group Leader at the Institute for Metal Physics, RWTH Aachen (1992-1997), German Habilitation at RWTH Aachen (1997), Visiting Scientist Dpt. for Materials Science and Engineering Carnegie Mellon Univ. Pittsburgh, USA, and US National High Magnetic Field Laboratory, Tallahassee, Florida. Director and Scientific Member at the Max-Planck-Institut für Eisenforschung (since 1999), Professor at RWTH Aachen.

Amongst others Prof. Raabe holds the following offices and prizes: Masing Award of the German Society for Materials Science (DGM) (1996); Heisenberg Scholarship, German Research Foundation (DFG) (1997); Materials Science and Technology Price, Federation of European Materials Societies FEMS (2001); Gottfried Wilhelm Leibniz Award (2004); Lee Hsun Lecture Award of the Chinese Academy of Sciences (2008); DGM Award (Award of the German Society for Materials Science) (2011); Weinberg Lecture Award, University of British Columbia (2011); ERC Advanced Grant (2012); member of the National Academy Chairman of the Board of Governors (Hochschulrat) of RWTH Aachen University (since 2012); member of the National Academy Leopoldina (2014) and Honorary Professor at Katholieke Universiteit Leuven, Belgium.