Appendix I: Advanced Machine and Process Architecture for Refractories via Laser Powder Bed Fusion Additive Manufacturing

Refractory metals and alloys, e.g., tungsten and its alloys, have exceptionally intrinsic properties, including ultra-high melting temperature, low thermal expansion, high thermal conductivity, low sensitivity to hydrogen and low neutron yield. These properties make them top candidates for a wide range of high-temperature applications. Enabled by additive manufacturing (AM), the design innovation, and ability to integrate systems, coupled with the potential to circumvent time-consuming powder metallurgy and subtractive machining processes, will promote disruptive technologies such as fusion-powered spacecraft and advanced thermal management systems to advance NASA’s space exploration initiatives. Laser powder bed fusion additive manufacturing (LPBFAM) process is one of the preferred AM technologies to produce tungsten parts with fine feature resolution. To date, approximately 98.5% of theoretical density has been achieved in LPBFAM for tungsten. However, microcracks are consistently observed as a result of the high ductile-to-brittle transition temperature and the high residual stress during fabrication. In this project, the University of South Carolina (UofSC) proposes to explore an ultrasound-assisted machine and process architecture based on LPBFAM to increase the printability of tungsten. This project will integrate unique capabilities of a customized open-architecture LPBFAM machine, established multiscale computational models, and advanced ultrasound technologies at UofSC to: 1) Pioneer a customized LPBFAM process that incorporates in-situ ultrasound processing with applications of advanced laser scanning strategies; and 2) Initiate an integrated numerical solution that optimizes local thermal history and global residual stress for defect-free microstructure of tungsten. The impact of ultrasound on rapid solidification and the defect formation will be investigated via printing of single tracks and cubic samples. Thin wall structures will be demonstrated at the end of this project to identify potential challenges and pathways to further improve the current technology. This study will pioneer an architecture of in-situ ultrasound processing in LPBFAM. Continuing the current research, a comprehensive process-structure-property relationship can be investigated towards manufacturing large and complex geometries with fine features for refractory metals and alloys. The system and methodology developed here are expected to be applicable to other refractories and other difficult-to-process alloys in AM, such as high-strength Al alloys and Ni-based superalloys. The successful development will not only enable a novel method to increase the printability of materials, but it will also induce potential fine/fully equiaxed grains for superior homogeneous mechanical properties. This will further extend the applications of AM and materials selection for NASA’s diverse applications.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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