A sound assessment of the loading capabilities of L-PBF materials is necessary for the design of machine parts and components, especially when aimed for high temperature applications. Two basic requisites for understanding the origin of properties exhibited by L PBF materials are the comparison to traditional metal processing results and a correlation between microstructural parameters and resulting mechanical properties.
In this study, additively manufactured batches of the nickel-based alloy IN718, made via Laser Powder Bed Fusion (L-PBF) process with variated volume energy density (VED), is compared to conventionally wrought and cast IN718 material. To isolate the processing influence, the same heat treatment according to AMS 5663 was applied to all material batches.
An initial characterization of the examined material states, including chemical analysis, microstructural investigations and tensile testing at room temperature and at the limit temperature of 650 °C was performed. This revealed significant differences in grain structure as well as deformation parameters.
The wrought material state shows a homogenous fine grain microstructure. Cast IN718 consists of an irregular microstructure with varying grain sizes and precipitations. The microstructure of L-PBF longitudinal to the build-direction can be divided into two different sections that are formed in the printing process by the hatch distance. This process influence can also be seen transversal to the build-up direction in the form of a chessboard-like microstructure.
While the tensile behaviour of the L-PBF variants is similar to the investigated wrought material at room temperature, a significant reduction of rupture elongation is observed along with the formation of micro-cracks on the fracture surface in the regions of melt pool overlap at 650 °C.
A more in-depth characterization of the cyclic mechanical behaviour at high temperature of the different material states includes incremental step tests under strain-control, as well as HCF notch life tests and fatigue crack growth investigations under force controlled conditions.
In incremental step tests, the maximum bearable strain energy density is measured for each material batch. Fatigue crack growth experiments show the initiation and propagation of cracks as function of the applied stress ratio. The results indicate that despite the reduced deformation capability at 650 °C observed in tensile tests, the L-PBF materials have a similar loading capacity and crack resistance under cyclic conditions.