Functionally graded materials (FGMs) represent a type of material with spatially varying functional properties in one or more directions, which stem from internal gradients of chemical composition, phases, pores and/or microstructure. The optimized functional response of a particular FGM is triggered by a locally-tailored structure-property relationship, which is a result of the multi-scale material response, often encompassing several length scales and hierarchical levels. There is a need to apply advanced characterization approaches operating at the macro-, micro and nano-scale in order to understand the correlation between process parameters used to fabricate FGMs, their internal structural, chemical and morphological gradients and the spatially varying functional response.
The methodological focus of this thesis lies on the experimental characterization of near surface and bulk properties of metallic FGMs using newly introduced high-energy synchrotron cross-sectional synchrotron X-ray micro-diffraction (CSmicroXRD), which was complemented by micro- and nanohardness testing, optical characterization, electron microscopy and chemical analysis. The thesis presents three cases for the application of correlative cross-sectional analytics to characterize mechanically and thermo-chemically induced near-surface gradients, and further three cases of laser-based powder bed-fused samples. Correlative characterization is used to assess spatial variations of structural properties, residual stresses, grain morphology, phase and chemical gradients and local mechanical properties. The obtained results are correlated with the applied process parameters.
The second focus is to apply the developed methodology, enabling the characterization of multi-metal hybrid structures produced by liquid-dispersed metal powder bed fusion. The experimental work concentrates on the analysis of the in plane and out of plane variation of structural and mechanical properties with special attention on understanding the residual stress build-up, and on attributes and the role of interfaces between metallic alloys.
In particular, the following topics are covered in the thesis and in the attached four publications:
- Near-surface properties of a low-pressure carburized case hardening steel 18CrNiMo7-6, a plasma-nitrided hot work tool steel W300 and a shot-peened high-strength steel 300M are characterized at the high-energy materials science beamline P07 of the storage ring PETRAIII at Deutsches Elekronensynchrotron (DESY) in Hamburg using CSmicroXRD. The evaluated residual stresses distributions are correlated with gradients of hardness, phases, crystallite sizes, chemical gradients and/or microstructural features as well as with the applied process conditions.
- Rod-like samples from stainless steel S316L are manufactured by powder bed fusion using three different hatch strategies. The samples are investigated by optical microscopy, CSmicroXRD and using tensile tests in order to understand the correlation between different crystallographic textures induced by the particular hatch strategies and the observed differences in the deformation behaviour. The study presents a strategy to design functionally graded materials with dedicated mechanical properties by laser-based powder bed fusion.
- Liquid-dispersed metal powder bed fusion is used for the production of a multi-metal hybrid structure consisting of stainless steel S316L and the nickel-based superalloy IN625 with an alloy variation along the build direction. The process allows to fabricate different sub-regions and morphologically sharp interfaces between the two alloys, which are characterized with nanoscale spatial resolution. The periodic occurrence of S316L and IN625 with different crystallographic textures results in a complex distribution of residual stresses along the sample’s build direction. Moreover, the formation of spherical CrMOx precipitates enriched with Si and Nb during the build process indicates the possibility for a hierarchical design of FGMs by reactive additive manufacturing. This study is the first step in confirming that liquid-dispersed metal powder bed fusion is an effective tool that allows to fabricate unique hierarchical microstructures consisting of two alloys.
- Based on results revealed in the application case above, another S316L-IN625 FGM with an inter-, as well as an intralayer variation of the two metal alloys is designed and analysed on its cross-section. CSmicroXRD mapping is used to assess the distribution of phases, texture and residual stresses on the entire cross-section of the sample. The results reveal the sharpness of in-plane and out-of-plane-oriented interfaces between the alloys and allow to assess the possibility to design truly biomimetic structures with alternating metallic alloys, which can be used to alter crack propagation behaviour in multi-metal materials prepared by laser-based powder bed fusion.