Design of microstructure and degradation behavior of oxide-particle modified Fe-based alloys processed by selective electron beam melting

• Applicants: Professor Dr.-Ing. Guido Grundmeier; Professor Dr.-Ing. Hans Jürgen Maier; Professor Dr.-Ing. Thomas Niendorf
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes ; Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 413259151

Funding period one focused on the processability of pure iron as well as powders modified by CeO2 and Fe2O3 via electron powder bed fusion (E-PBF, also PBF-EB/M). The microstructure and the mechanical and corrosive properties were intended to be influenced by minimal additions of the oxide particles. Quasi-static and cyclic mechanical investigations showed that this is robustly possible and that, due to an unexpectedly high damage tolerance, fatigue strengths significantly superior to those of hot-rolled iron can be achieved. Despite a process-related high defect density present in these conditions, these effects were particularly evident in the CeO2 modifications. Therefore, an effective increase in hardness as a result of these oxides is assumed. Complementary investigations also showed that a local acidic environment in pores can impede the formation of surface layers, leading in turn to higher corrosion rates of the material. Since positive effects were observed as a result of the CeO2 particles and since the material also offers a high damage tolerance to defects, selectively introduced pores can thus be used as a further degree of freedom to adjust the corrosion rate. Consequently, in funding period two E-PBF process routes must be established to allow for robust processing of iron powders with even higher CeO2 contents. In order to promote immediate process stability, the electrical conductivity of the powder will be considered. When dense material can be processed reproducibly, samples will be manufactured with porosity specifically tailored in terms of shape, size and distribution. This represents a further degree of freedom for adjusting the material properties, and is only possible by additive manufacturing processes. Furthermore, on the material side the very high damage tolerance is required. Computed tomography investigations during loading can be used to expand and analyse pores that otherwise are too small for detection. The corrosive processes in the pores are to be investigated by means of a specifically designed setup. Complementing to high-cycle fatigue and corrosive loading, low-cycle fatigue and fracture mechanics investigations will be conducted to characterize crack advance starting from the pores. Finally, a fatigue life model will be established to assess and predict crack propagation as a function of material constants, the crack length and a corrosion factor. Based on the model elaborated, a prediction of the behavior of other materials under highly complex loading scenarios shall be possible as well, so that additively manufactured, bioresorbable implants can finally be individually tailored for the intended application.

• DFG Programme: Research Grants