Metallurgical and Fractographic Characterization of Prematurely Fractured PBF-LB/M Built Short-time Creep Rupture Specimens Made of IN939AM

Abstract

An Original Equipment Manufacturer (OEM) produced row 1 turbine vanes by additive manufacturing (AM), using the Powder Bed Fusion-Laser Beam/Metal (PBF-LB/M) process. It is mandatory in such cases to co-print specimens for mechanical testing, such as short-time creep rupture testing. Some specimens were found to have fractured prematurely. The specimens representing the worst and best cases were examined at the authors' laboratory to determine the metallurgical causes of the premature failure. The main difference was found to be in the grain size. While the prematurely fractured specimen exhibited a finer grain size of approximately 6–8 per ASTM E112, the other specimen, having exhibited two and a half times the lifetime of the prematurely failed specimen, exhibited a coarser grain size of approximately 4–5. The difference in grain sizes was attributed to the pinning of grain boundaries by secondary carbide precipitates in the prematurely fractured sample, which restricted grain-boundary mobility and prevented full recrystallization.