Characterization and Optimization of Microstructure and Performance of 3D Printed Metallic Components

Cecilie Vase Funch

Research output: Book/ReportPh.D. thesis

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Abstract

Metal additive manufacturing (MAM) enables the production of components with a high level of geometrical freedom and complexity. Furthermore, MAM is especially beneficial for production of expensive alloys due to low levels of material waste and for materials that are difficult to machine. The layer-wise nature of the MAM processes produces a unique cyclic thermal history with steep thermal gradients and high growth rates. This produces unconventional microstructures in otherwise conventional alloys resulting in often unconventional or anisotropic properties.

Therefore, this thesis focuses on understanding these unconventional microstructures and properties through advanced characterization techniques. Furthermore, part of the thesis focuses on optimizing the unusual properties through heat and surface treatments. Different properties are optimized based on the requirements of a specific area of application. Four different materials are investigated: austenitic stainless steel, α+β titanium, maraging steel and a nickel-based superalloy.

The hierarchical nature of the as-built microstructure was characterized on a range of different length scales. An additional level of hierarchy has been added to the usual understanding of the microstructures. The cells were found to be grouped together in 2-5 μm domains of a similar orientation with a small misorientation to adjacent domains. The as-built condition exhibited a distinct plastic and elastic mechanical anisotropy with the vertical direction exhibiting early yielding. Generally, the as-built condition had an excellent combination of high strength and good ductility with a low level of cold-work hardening. The thermal stability of this complex hierarchical microstructure was evaluated, which showed a dissolution of the cell structure and melt pool boundaries, transformation of amorphous silicates and stable elongated austenite grains and domains. The effect of using edge parameters to improve the surface finish during MAM was investigated on austenitic stainless steel. The use of edge parameters was found to induce a change in grain shape (by recrystallization or abnormal grain growth) at the surface during high temperature solution nitriding. The as-built condition contains a significant amount of nitrogen in solid solution, which needs to be considered during heat treatment. While vacuum austenitization caused a denitriding of the surface, an active austenitization kept the nitrogen content of the surface stable. High temperature solution nitriding increased the nitrogen content significantly in the surface. The high temperature treatment reduced the strength of the material through dissolution of the cell structure. The mechanical anisotropy is also significantly reduced. The hardness of the surface was increased by low temperature surface hardening and the formation of nitrogen and/or carbon expanded austenite. As-built Ti-6Al-4V exhibits a fine hierarchical martensitic microstructure. While this microstructure has an extremely high yield and ultimate tensile strength, it exhibits poor ductility. The mechanical properties were found to be sensitive to build orientation with the vertical direction having a higher strength. Also found a clear impact of surface finish was found, i.e. improved ductility was obtained by machining of the surface. Furthermore, the pick-up of interstitials (O and N) was demonstrated to be sensitive to contamination in the build chamber as well as the build volume and thermal history. The mechanical properties were improved through targeted heat treatment, producing a bi-lamellar microstructure, which improved the ductility by more than 250 %, while maintaining a high strength. The bi-lamellar heat treatment was also shown to be compatible with different surface hardening treatments, in order to combat the poor wear resistance of titanium. Conducting the surface treatment below β-transus showed remnants of the as-built martensite, while the treatment above β-transus fully reset the microstructure independently of the initial condition.

Maraging steel is a promising MAM material due to the as-built soft bcc martensite that limits crack formation. Surface hardening of the MAM maraging steel was conducted by nitriding and the nitriding response was evaluated in the solution treated condition and aged condition. The aged condition produced a shallower case and surface hardness because the nitride forming elements were already bound in strengthening precipitates. These intermetallics need to be converted to the more stable Ti and Mo based nitrides. Solution treated and aged maraging steel possesses little to no corrosion resistance, so the effect of adding chromium to the surface by chromizing was investigated. The chromizing treatment produced a hard, brittle case of σ-phase supported by a layer of austenite on top of the bulk martensite. The chromizing treatments significantly reduced the corrosion rate of the maraging steel. The corrosion rate was further reduced by subsequent decomposition heat treatment. The nitriding response of a nickel-based superalloy was investigated for both a solution treated and aged specimen.

The nitriding mechanism depended on the nitriding temperature: a low temperature produced a case of expanded austenite, while a higher temperature was dominated by chromium nitride and iron/nickel nitride formation. An intermediate nitriding temperature provided a deeper hardened case than the higher temperature likely due to development of N2 gas on the surface effectively lowering the available nitrogen for diffusion and potentially initial expanded austenite formation.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages348
ISBN (Print)978-87-7475-682-8
Publication statusPublished - 2022

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