Metallurgical Evaluation of an Additively Manufactured Nickel-Base Superalloy for Gas Turbine Guide Vanes

Recent advancements in additive manufacturing (AM) have challenged the tradition design and fabrication approaches for high temperature gas turbine components. One focus has been directed towards the ability of AM to create gas turbine guide vanes with advanced cooling features for improved efficiency. Significant studies on AM have demonstrated the capability to produce complex parts without cracks and defects using both selective laser melting (SLM) and electron beam (EB) methods. Materials such as IN-939, IN-738, GTD-111, MAR M247 and many other nickel base superalloys are used in high temperature gas turbines as both stationary guide vanes and rotating turbine blades and have now be produced by AM. These materials are used in part for their high temperature strength and resistance to creep deformation. Currently, there is a scarcity of data and research devoted to understanding the long-term high temperature creep properties in these alloys produced using AM methods. The Electric Power Research Institute (EPRI) and Power Systems Manufacturing (PSM) have collaborated to evaluate AM production of gas turbine guide vanes with advanced cooling features coupled with coating strategies made from alloy IN-939.  

In this paper, AM produced test samples were tested for tensile, fatigue and creep properties at temperatures up to 871°C. A literature review of microstructure and creep properties of traditionally cast IN-939 was carried out and compared to the results in this study. Initial results showed improved tensile and fatigue strength, but a significant reduction in both long-term creep rupture strength and ductility in the AM produced material. Microstructural observations in the AM produced material showed a significant difference in the overall metallurgical characteristics when compared to traditional casting material. To provide a direct comparison between AM and traditional castings, both AM and cast components were tested in live engine trials exceeding 4,000 hours.

Detailed characterization using confocal laser microscopy and scanning electron microscopy techniques was conducted for both mechanical test specimens and the removed engine trial guide vane components. The as-produced material showed a substantial difference in the overall primary carbide distributions, grain size, and grain texture. The traditional casting showed large grains and primary carbides (~10 to 20µm) in a blocky shape with many concentrated at grain boundaries. The AM produced material was finer in grain size and showed very fine primary carbides (<1µm) that were randomly dispersed in the matrix with very few at grain boundaries. Secondary carbides (such as M₂₃C₆) were also evaluated and compared between the two different methods of manufacturing after exposure to high temperature environments. Lastly, the gamma prime precipitate size was quantified and compared to estimate the actual temperatures in the engine trial components exposed to temperatures up to 760°C. The results from the AM produced material are discussed in comparison to expected properties and characteristics from traditional casting methods. Results have shown that material production and short-term metallurgical properties are sufficient to produce high quality high temperature stationary guide vanes, but additional research and development is needed to optimize the AM process to achieve high-temperature creep behavior comparable to castings.