Characterization of bi-metallic joints formed by different processes

Development of the Space Launch System (SLS) vehicle at the NASA- Marshall Space Flight Center (MSFC) supports the Artemis Mission to the moon, Mars and beyond [1]. To sustain these missions, there is a need for fabrication of landers as well as reusable, reliable materials and processes for extraterrestrial fabrication and repair. Thus, the materials and fabrication processes must be suitable for a multitude of components as well as environments. Terrestrially, to support the SLS vehicle fabrication, upgrades are underway to improve the performance and reliability of the RS-25 liquid rocket engine (LRE) [2]. While this provides an opportunity to insert new materials and manufacturing processes, such as additive manufacturing (AM) into the RS-25 production upgrade, it is urgent to develop material property databases to ensure designers have relevant information for designing robust and reliable engine components.

A regeneratively cooled, RS-25 combustion chamber consists of a Copper (Cu) liner joined to a structural jacket [3, 4]. Heritage hardware initially used stainless steels (Fe based) whose coefficient of thermal expansion (CTE) closely matched that of the Cu liner [5]. Eventually, the Fe base material was replaced by higher strength nickel (Ni) based superalloys such as Inconel 625. While Inconel 625 provided improved strength, there was a trade off in increased CTE mismatch that affected the fatigue life of the combustion chamber. More recently, the NASA developed an advanced Ni based superalloy called NASA HR-1 that decreased the CTE mismatch [6]. The current evaluation of manufacturing methods, such as AM, provides an opportunity to evaluate improved materials to retain the high strength of the Ni based superalloy while minimizing the CTE mismatch to improve reliability.

Of the various AM processes being evaluated for the RS-25 LRE, direct energy deposition (DED) processes offer the best approach toward fabrication of bi-metallic combinations without size restrictions [7]. Improved reliability can be obtained if the resulting bi-metallic interface has adequate strength and microstructural stability at elevated temperatures during hot fire operation. This requires quantification of the properties of the AM materials in parallel with the development of AM processes for LREs. Thus quantifying the material properties from AM processing also must include those of the resulting interface between different families of materials encountered in regeneratively cooled LREs.

Prior research has observed significant differences in the interface of samples obtained using different materials, processes and vendors [8-12]. Since there are no current standards or specifications for production of this hardware, the vendors currently apply their best practices, which can widely vary. This means that there is no consistency in the feedstock supplier or equipment settings, factors known to affect the melt pool formation and mixing between materials. Within the liquid metal of the melt pool, Marangoni currents influence the intermixing as influenced by atmospheric contaminations, injection velocity, heat source power density, and elemental composition, resulting in vastly different properties at the interface [13-15]. Since DED is of interest for the bi-metallic structures, characterization of two processes involving blown powder are proposed: blown powder deposition versus cold spray. Thus understanding how variations in DED processing affect the overall properties of the bi-metallic joint will contribute toward increasing the overall technical readiness level (TRL) of an AM RS-25 LRE. This is of relevant interest to both NASA and the commercial space companies [16-19]. Using the unique capabilities at UAH, this proposal will leverage mini-samples from actual hardware [12] in collaboration with NASA and commercial space to characterize and disseminate data.

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