Additive Manufacturing of Refractory Metal Nb Alloy C103 for Propulsion Applications

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The primary member of the refractory metals are Nb, Ta, Mo, W, and Re. As one of the most dominate industrial refractory alloy, Nb C103 (89Nb-10Hf-1Ti), this precipitation strengthened niobium alloy is used in aerospace applications in sustained high temperature operating environments, particularly in the propulsion systems where regenerative cooling is not available. NASA Marshall Space Flight Center (MSFC) is involved in developing Additive Manufacturing (AM) methodologies primarily for of propulsion systems technology. AM shows great potential in producing complex and optimized components from an array of materials with potential to decrease cost and lead time when compared to traditional manufacturing. One area of development is the implementation of refractory materials for green propulsion, NTP, etc. Although Nb C103 is machinable by conventional subtractive process, it is expensive to machine them; whereas processes for forming sophisticated shapes, such as investment casting, are associated with extreme high cost and rarely applied have limited this alloy’s applications. In typical propulsion applications, when Nb C103 is used, more than 95% of the bar stock materials were removed to produce final part that is significant waste of expensive resources. Almost 100 percent Nb are imported. Metallic materials additive manufacturing, on the other hand, has taken the manufacturing industry by storm not only because it offers significant savings in cost, materials, and schedule but it also provides the ability to make geometrically complex monolithic parts that are unattainable or extremely difficult to make through conventional manufacturing processes. Among many additive processes the most prominent process is the powder bed laser layer-build additive process – Selective Laser Melting (SLM) also known as Laser Powder Bed Fusion (LPBF). The LPBF process not only can produce fine features that the other additive process such as electron beam powder bed fusion (EBPBF) cannot, but also can produce superior surface finish needing little or no secondary processing. The adaption of additive manufacturing for C103 would open up the alloy’s addressable market, and with AM’s efficiency in use the materials (high buy-to-fly ratio), could increase the competiveness of the C103 in the aerospace landscape and achieve significant cost savings. To harness this potential, this proposed work based on the previously successful initial assessment work by Castheon will further demonstrate the feasibility of additive manufacturing of C103 alloy using LPBF process to further mature processing parameters, shape building capabilities, assess the material integrity, post-process requirements, AM C103 materials properties, and establish design and application criteria to enable the AM production of C103 hardware. The successful demonstration of this work, affordable high performance material manufacturing, will also produces positive impact other refractory metal to be additively manufactured.

Although, Castheon has demonstrated the possibility of producing full dense AM Nb C103 alloy. It is far from mature to say that AM Nb C103 can be made into components for aerospace applications. This work will further demonstrate the feasibility of additive manufacturing of C103 alloy using LPBF process to further mature processing parameters, shape building capabilities, assess the material integrity, post-process requirements, AM C103 materials properties, and establish design and application criteria to enable the AM production of Nb C103 hardware.

The objectives are to optimize C103 SLM process parameters with associated shape build capabilities, assess material integrity, post-process requirements, basic material properties, and establish design criteria to enable the AM production of C103 hardware. ATI will provide C103 powder and conduct powder characterization to include particle size distribution, morphology, density, and chemical analysis. 

A design of experiments (DOE) will be conducted to identify optimized build parameters which include laser power, speed, hatch distance, and layer thickness. Castheon will apply initial build parameters developed using a Concept Laser M2 and interpret such parameter to be applied on MSFC’s EOS M100. MSFC will expand the DOE matrix in line with the finer laser beam diameter of the EOS M100. Continued effort to optimize parameters with metallurgical feedback will be based on the metallurgical specimen blocks. Parameter development effort will focus on minimizing porosity with a lower selection criteria of >99.75% relative density and enhancing AM feature build capability. The results from the development trails will be used to identify an optimized set of parameters as basis for developing AM C-103 design criteria and AM manufacturing cost model.

Detailed DOE matrix for SLM parameter development includes three level - four variables fractional factorial design that is closely coupled with the two level - two variable contour scan parameters matrix. The iterative process initially produces twenty four (24) evaluation samples in the first build and up to twelve (12) refined evaluation samples in the follow up build for the Concept Laser M2 machine which has 250 x 250 x 285 mm build envelope and similar evaluations for the EOS M100 which has a 100 mm diameter x 85 mm build envelope.

Inverted pyramid shaped “Gao Block” in the size of 16 x 16 mm with saw tooth features and down surface were used in producing metallography examination coupons. The 16 mm block provides a large cross sectional area that is representative to typical industrial applications. Chemical composition of the as built AM C-103 is monitored with LECO specimens in distributed across the build plate. 

Optical metallography examination of SLM pass (bead) profile in perpendicular cross section and longitudinal section is measured, width and depth ratio, defect formation mechanism are examined. Metallographic and mechanical specimens will be produced using the optimized parameters by Castheon and MSFC. In order to investigate the orientation-dependent initial microstructure and mechanical properties micro-tensile and micro-compression specimens will be printed at different orientations with respect to the build layer. 

Stress relief and vacuum anneal (recrystallization) schedules have been identified and will be applied to printed specimens at the MSFC heat treatment laboratory. If time allows Hot Isostatic Press (HIP) schedules will be investigated. In the previous study, exact onsite recrystallization temperature was not determined, in this study, DTA/DSC will be used in addition to optical metallography to precisely determine the onsite temperature.

As-built, stress-relived, and annealed specimens will undergo metallographic evaluation to characterize the evolution of the microstructure with respect to post-processing. In-situ chemical analysis will identify potential contamination or compositional changes during the AM process. Castheon and MSFC will conduct optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS).

Specimens will be evaluated at each material condition to determine yield strength, ultimate tensile strength and elongation. Specimens in XY plane and Z direction (three tensile bars minimum in each direction) will be produced and evaluated. MSFC will conduct tensile and compression testing at room temperature and ATI will conduct testing as a function of temperature up to 1200 °C (2200 °F). Results will be compared to wrought C103 data. Compression test results provide a comprehensive understanding of stress asymmetry while the mechanical anisotropy will be assessed by testing specimens with different build orientations (XY plane and Z direction).

A model will be developed by Castheon to understand cost, schedule, and material usage efficiency difference between additive and traditional subtractive manufacture techniques for typical C-103 applications in aerospace industries and space applications.