Development of a method for carbide additivation on tool steel powders via a functional polymer binder to enhance the processability, microstructural isotropy, and strength of hard ferrous alloys

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Steels with higher carbon contents, e.g. cold-work and high-speed steels, have proven to be difficult to process by LPBF. High carbon steels show a tendency for cold crack formation during LPBF processing. The influence of the processing parameters as well as the alloying content on the cracking formation mechanisms of high alloyed tool steels processed by LPBF is not yet fully understood. Whereas pre-alloyed powders seem to be limited in the alloying content due to cracking, the alloying content can be enhanced by using differently prepared feedstocks. In order to enhance the range of processable alloys, reinforced alloys have gained focus. Various carbides have been added to base steels, mainly by ball milling, and were processed by LPBF. Ball milling is time consuming and can result in reduced sphericity of the powder. Satelliting is another method for decorating large parent powder particles with fine additives by a bonding agent. Satellited particles prevent a local demixing due to size and density effects of the powder and have been reported to reduce anisotropy and grain size. Whereas Ti, Al, Co alloys have been satellited, corrosion susceptible steels have not been satellited yet. Recently, a number of successful phosphorylation of polysaccharides (as bio-based polymer network) with a different degree of substituents have been published. An increase of the degree of phosphate groups substituents in the polysaccharides structure could enhance anti-oxidant activities (which are related to corrosion inhibition) due to activation of hydrogen atoms present on anomeric carbon of polysaccharides. The main objective of this project is to provide a new and reliable method of carbide additivation to hard ferrous alloys such as tool steels via a functional polymer binder (polysaccharide phosphate) in order to enhance the microstructural isotropy as well as the processability. The modification of the tool steel aims at improving the processability by reducing the tendency to cold and hot cracking, and at a homogeneous isotropic and fine-grained microstructure. Unmolten carbides function as nucleations sites for the isotropic solidification of the alloy during LPBF. Grain size and texture become key performance indicators for the success of the project. The alloys are developed by thermodynamic equilibrium and Scheil-Gulliver solidification calculations. The investigation includes the influence of powder satelliting on rheological and thermophysical powder properties. The processability in LPBF and the microstructure formation is analysed by scale-bridging microstructural characterizations and the advantages of an isotropic microstructure on mechanical properties is characterized by static and dynamic mechanical tests.