Metal Laser Powder Bed Fusion (PBF-LB/M) allows for high degrees of freedom and the manufacture of high temperature materials. The PBF-LB/M microstructure is dependent on process parameters, component geometry and build orientation among others. The large number of process parameters and the constantly changing thermal conditions during the build cause challenges regarding reliability and repeatability of the achieved component quality.
IN718 is one of the most frequently used nickel-based superalloys and is applied, for example, in the aerospace industry as a turbine disk material due to its good high-temperature strength and corrosion and oxidation resistance. IN718 is characterized by very good weldability and is thus ideally suited for processing with PBF-LB/M . The elimination or minimization of Nb segregations poses a major challenge in the manufacturing process of this alloy. In addition to macro-segregations, the microstructure after the casting, forging or the PBF-LB process can show an unfavorable degree of precipitation. By heat treating IN718 fabricated components, a reduction of residual stresses, segregations as well as phase formation and distribution can be modified. The proportion of hardening phases is reduced and the proportion of brittle phases (𝛿- and Laves phase) is increased.
It is generally accepted in literature that these brittle phases (𝛿- and Laves phase) are the root cause for the IN718 notch embrittlement. Figure 1 shows results of notched IN718 PBF-LB/M creep samples from this study. As can be seen in Figure 1, IN718 notch embrittlement is not affected by the scan strategy (P1-P4). Even different heat treatments (HTA, HTB) have a limiting effect in prolonging the creep life. Only an adjustment in creep testing parameters (reduction in applied normal stress) allowed for an increased notched creep life.
In this study, different build orientations (0°, 45° and 90°), print parameter settings, heat treatments and notch factors were considered and their effect on notched creep life was investigated in short term creep experiments. Since the manufacturing and thermal conditions between different build orientations differ, the microstructure is also expected to differ. Differences in oxygen content, grain size and phase distribution were found, which significantly affect notched creep life.