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Completed TRL 2 (started at 2, targeting 4)
TThis project addresses the NASA Technology area TA12 supporting advanced manufacturing, structures, and materials. To reduce costs associated with the fabrication of the Space Launch System (SLS), various additive manufacturing (AM) processes are being explored. While processes, such as selected laser melting (SLM) can print monolithic components, other components on the RS-25 liquid rocket engine (LRE) require a combination of Cu for conductivity and a higher strength-lower density superalloy material, for structural stability. The application of directed energy deposition (DED) processes can be used to print these bimetallic components. This approach greatly reduces the time and hence cost by eliminating multiple processing steps, such as brazing, to fabricate the multi component heritage hardware. Although use of blown powder DED was recently used to fabricate an RS-25 augmented spark igniter (ASI) that was hot fire tested in July 2017, little is known about the reliability and stability of the resulting interface formed as the metals are directly deposited on one another. This proposed effort would quantify the material properties of the bimetallic interface and also evaluate its stability at elevated temperatures. Specimens proposed for this study have been fabricated by 3 different vendors using 2 different DED processes. By quantifying the bimetallic joint, the overall technical readiness level (TRL) of an additively manufactured RS-25 LRE can be increased.
One of NASA Marshall Space Flight Center's (MSFC) current thrust areas revolves around the use of additive manufacturing (AM) methods to reduce costs associated with production and assembly of launch vehicles and in-space propulsion. A specific need involves AM fabrication of the various components for the RS-25 LRE, intended to power the first stage of the SLS. While many AM processes can build monolithic components, several components in a regeneratively cooled LRE require a copper (Cu) core for improved thermal conductivity reinforced with an outer structural shell of a higher strength-lower density superalloy material. Currently, heritage combustion chambers are fabricated using multiple processing steps to machine and braze or weld the multiple pieces into the final assembly. Fabrication changes have considered shrink brazing an outer machined Inconel structural jacket to a Cu liner.
Use of a free-form, or directed energy deposition (DED) AM process would greatly reduce the time and cost while increasing the reliability with the ability to directly deposit a structural jacket onto a Cu liner. DED AM was recently demonstrated in the fabrication of a RS-25 augmented spark igniter (ASI) and also subscale combustion chamber. During the development of the DED process for the ASI, specimens from the bimetallic joint were qualitatively verified to have a completely bonded interface. Although the ASI recently underwent successful hot fire testing in July 2017 at the NASA-MSFC, there is currently no quantified analysis to impart confidence in the reliability and stability of the bimetallic interface. Therefore, this study proposes to evaluate the material properties of the interface between Inconel 625 and GrCop84, as well as its stability at elevated temperatures. Through quantifiably analyzing the bimetallic joint, the overall technical readiness level (TRL) of an AM RS-25 LRE can be increased.
Feed stocks for AM processes utilize either metal powder or solid wire. Metallic powder is used in both selected laser melting (SLM) powder bed and DED blown powder processes. In addition to the use of blown powder for the DED AM process, wire fed can also be used. Both DED processes are built on readily scalable CNC machine or robotic platforms. In addition, mounting a DED AM system onto a CNC platform, allows both additive and subtractive processes to be used together further reducing manufacturing time.
The NASA has invested heavily in SLM powder bed processes, which use a laser to melt the nominal 45 μm diameter powder, to build parts layer by layer. SLM AM processing has produced monolithic components including: RS-25 Pogo Z-baffle, subscale chamber liners, subscale injectors and single elements. While SLM powder bed techniques are limited to monolithic builds, such as the GrCop84 combustion chamber liner, forming the outer Inconel structural jacket requires a DED AM process. This bimetallic joint is common to other LRE components such as the augmented spark igniter (ASI).
DED AM technology has been demonstrated in the fabrication of the ASI igniter and structural Inconel 625 jacket onto a SLM fabricated GrCop84 liner. Various processes have been demonstrated including: blown powder DED additive/subtractive deposition by DMG-Mori, blown powder DED/DMD by DM3D, and wire fed DED by Keystone Synergistic, all under NASA contracts. UAH was actively involved with these companies in development of their processes providing metallurgical support. Since this previous study only provided visual confirmation and basic metallography of the bimetallic joint, advancing the TRL of this process requires the interfacial properties between the Inconel 625 and GrCop84 to be quantified.
The main Technical Challenge in reaching the goal of reliably using DED AM for the LRE components on the SLS lies in quantifying the mechanical properties of the various DED AM processes including not only the monolithic properties but also the bimetallic interfaces. Obtaining a database of material properties to support engineering design allowables for AM materials is complicated due to the lack of standards and specifications to guide both heat treatments and number of specimens. Although efforts are being undertaken to develop these standards and specifications for monolithic materials, the usage of AM bimetallic components is ongoing in parallel. Testing methods, such as proposed in this effort, must be demonstrated to provide guidance for the quantification of the bimetallic components.
To overcome this current technical issue, this proposal seeks funds to evaluate the post build processing, resulting mechanical properties and stability of the bimetallic interface of DED specimens of Inconel 625 to GrCop84. While heat treatments have been designed for the monolithic materials based on experience with heritage hardware, the treatment of bimetallic builds will require new approaches as the Cu alloys cannot withstand the temperatures the Inconel is exposed to during its traditional heat treatments. To evaluate the long term stability of the interface, specimens will also be subjected to temperatures simulating those experienced during typical hot fire tests of the RS-25. The temperatures and times will be selected based on analysis efforts of the Engine Components Development and Technology Branch (contact: Paul Gradl).
The approach of this proposed study will be to quantify the properties of the bimetallic interface produced using 3 different DED processes. Specimens from each vendor build will be separated into 3 sets. One set will remain in the as-built state for baseline comparison. Two of the 3 sets will be subjected to 2 different heat treatments: (1) homogenization of the AM deposited metal and (2) simulated hot fire conditions. The homogenization treatment will consider suitable times and temperatures for the Inconel 625 that are compatible with the GrCop84 liners. Selection of the time and temperature will be selected based on prior NASA-MSFC studies in addition to consultation with the NASA engineers. All sample interfaces will be recorded via optical microscopy (OM), followed by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction to document the elemental distribution and phases present, respectively.
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