Thursday, 30 April 2020

Speaker: Dr. Eric A. Jägle

Bundeswehr University, Munich | Max Planck Institute for Iron Research, Düsseldorf

Controlling phase transformations during laser AM - the intrinsic heat treatment effect and beyond

 Jägle
© Max-Planck-Institut
für Eisenforschung GmbH

Abstract

By now it is well established that the complex time-temperature profile during laser-based additive manufacturing can have an unexpected and significant impact on solid-state diffusion and phase transformations. In this context, Intrinsic Heat Treatment (IHT) means that the cyclic re-heating by the deposition of additional tracks and layers can trigger e.g. precipitation reactions. This is the case in Fe-Ni-Al maraging steels produced by DED. However, depending on alloy composition and processing conditions, additional boundary conditions need to be met for the IHT to be effective. In Fe-Ni-Ti maraging steels, for example, the IHT can be so strong that the material remains in the austenitic state during processing by DED and no precipitation at all occurs. In Al- Sc-alloys, on the other hand, the most significant factor controlling precipitation is not re-heating, but rather the cooling rate immediately after deposition in DED. This not only controls precipitation, core-shell formation and precipitate coarsening in the solid state, but also primary precipitation from the liquid. In these materials, the precipitation progress as a function of deposition height and hence the hardness gradient in the final part can be the opposite to the one observed in maraging steels by IHT. In L-PBF, on the other hand, the re-heating effect of IHT is of limited effectiveness due to the much faster scan speeds and lower laser powers as compared to DED. Here, base plate pre-heating is the most significant factor controlling solid-state transformations and introducing gradients into the alloys. In this talk, I will attempt to summarize our findings from Al- and Fe-based alloys produced by DED and L-PBF. Precipitation analysis is performed by APT and HEXRD, and precipitation kinetics is captured by KWN-type modelling. I will discuss how process parameters need to be selected in order to put the IHT effect to good use and to achieve desired hardness gradients in AM-produced parts.


Biography

Dr. Jägle studied materials science at the University of Stuttgart, Germany, receiving a Dipl.- Ing. degree with distinction in 2006. In 2006/2007 he spent on year at the University of Cambridge, UK. In the M.Phil. course in Materials Modelling, he worked with H.K.D.H. Bhadeshia on simulating the origin of banding in hot-rolled steel. Afterwards, he returned to Stuttgart for his Ph.D. at the Max-Planck-Institut für Metallforschung (MPI for Metals Research) under the supervision of Prof. E. J. Mittemeijer. His work focused on the mesoscopic simulation of microstructure development during phase transformations, in particular during recrystallization. After receiving his Ph.D. in 2011 with distinction, he moved to the Max-Planck-Institut für Eisenforschung (MPI for Iron Research) in Düsseldorf, Germany. There, he worked as postdoctoral researcher in the department of Prof. D. Raabe on Atom Probe Tomography analysis of electrical steels, precipitation transformations and mechanical alloying. In 2015 he became leader of a newly-formed group in the same department working on alloys for Additive Manufacturing. The group focuses on various aspects of alloys used in AM such as particle reinforcement, in-process strengthening reactions, hot cracking behaviour, residual stress and in-process metal-gas reactions. The investigated materials include steels, Niand Al- based alloys and composites. In 2020, he moved to the Institute of Materials Science of the Bundeswehr University Munich as full professor, continuing his work on materials for additive manufacturing.


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Documents compiled by Camille Guévenoux.