Development of Materials for Extreme Environment Applications by Laser Powder Bed Fusion

Abstract

As the desire for greener, more efficient energy production creates an urgent push for advances in reactor and turbine designs, rapid innovation must be achieved in the field of extreme environment materials. Recent technological progress in metal additive manufacturing has enabled new techniques in materials design that might prove essential in closing current gaps. In this study, I identify two critical advanced energy systems that may be improved with materials produced by metal additive technology, fusion reactor vacuum vessels and jet engine turbine blades, and explore the practical usage of laser powder bed fusion to improve production of relevant materials.

First, material enhancements are considered in the context of increasing survivability for full-power operation of the ARC tokamak fusion reactor. Based on evaluation of relevant properties for four candidate vacuum vessel materials, Inconel 718 is identified as the most likely selection for construction of an initial ARC pilot plant in the short-term. Maximizing the lifetime of such a vessel will require tailoring properties to increase resistance to neutron effects and, especially, to enhance mechanical properties in the range of 800℃. Both targets are expected to be addressable by the formation of a 718-based metal matrix composite, which is enabled by additive manufacturing. Based on rapid evaluation of various ceramics in Al-based composites, SiC is selected as a promising candidate, so an Inconel 718 composite reinforced with 2 vol% SiC is produced by laser powder bed fusion. Microstructural analysis reveals breakdown of SiC and in-situ formation of silicides and carbides, which result in decreased porosity and grain size. Room temperature mechanical tests show good strengthening over base Inconel 718 with low loss in ductility. Improvement in high temperature ductility is achieved over the unreinforced material, but the effects appear inadequate to merit use over a wrought Inconel. An additional composite is then developed using 2 vol% ZrB2 as the reinforcing material. Microstructural results for this composite follow a similar trend to SiC and verify the capability for reducing porosity. Room temperature mechanical testing shows higher strength and lower ductility than the SiC. However, elongation at failure is found to increase drastically around 800℃, reaching more than 8x that of printed unreinforced Inconel. These results suggest high potential for ARC implementation.

Second, the use of functionally graded printing is discussed as a potential method of establishing improved oxidation resistance for the use of niobium for turbine blade applications. To enable future studies of this, a process flow is developed to establish niobium printability. The viability of high-throughput single-layer testing with fixed-depth wells is first assessed for rapid, material-efficient parameterization. Promising conditions are then transferred to multi-layer printing, and further iteration is conducted to minimize surface agglomeration and allow continuous build-up. Finally, a Gaussian regression code is applied to recommend optimum conditions. The resulting print quality is found to achieve the first scalable printing of niobium powder in a commercial powder bed fusion system, which is necessary to enable the exploration of techniques such as functional grading.