Effect of additive manufacturing on fatigue crack propagation of a gas turbine superalloy

Additive manufacturing (AM) offers new possibilities in gas turbine technology by, for example, allowing for more complex internal cooling channels. One such application, where AM can improve the function by new designs, is burners. However, the fatigue performance, especially the fatigue crack propagation of AM gas turbine material, is not fully known. In the present study, an AM adopted nickel-based superalloy Hastelloy X was subjected to low-cycle fatigue (LCF) loading at room temperature. The LCF tests were conducted in strain control on additive manufactured smooth bars with two different build orientations (with an angle of 0° and 90° relative to the building platform). During the the manufacturing process, an AM component often solidifies with a dendritic structure. Initial fractography of the ruptured LCF specimens revealed that the dendritic structure was visible on the fracture surface. It was noted that the dendritic structure could easily be mistaken for regular striations although they represent a different fracture mechanism. The fracture surfaces were therefore cross sectioned and possible correlations between fracture surface characteristics and underlying microstructure were studied using electron backscatter diffraction and electron channelling contrast imaging. The outcome was used to discuss the effect of AM microstructure on the LCF crack propagation.