Room-temperature low-cycle fatigue behaviour of cast and additively manufactured IN939 superalloy

The present study compares the fatigue properties of IN939 superalloy manufactured conventionally by casting and additively by laser powder bed fusion (L-PBF). To investigate the influence of specimen orientation, i.e. load-to-build direction, and heat treatment on fatigue properties, 4 batches of additively manufactured specimens were prepared: vertically and horizontally oriented batches with either as-built microstructure (non-treated) or aged γ / γ’ microstructure. Cylindrical specimens were machined from the cast rod and printed blocks, and subjected to low-cycle fatigue loading at room temperature. Fatigue testing was conducted in total strain amplitude control within ε a = 0.27–1% with a constant strain rate of 2∙10 −3 s −1 . Scanning and transmission electron microscopy techniques were used to analyse the specimens before and after testing. Out of all tested batches, horizontal aged L-PBF IN939 exhibited the best stress-life fatigue properties, while vertical non-treated L-PBF specimens showed the largest resistance to cyclic plastic straining. Plastic strain localization occurred in the form of persistent slip bands, which led to fatigue crack initiation at the specimen surface. The high stresses associated with the L-PBF batches favour interdendritic and intergranular cracking, increasingly so with higher batch strength. However, the cracking mechanism does not significantly affect the low-cycle fatigue life, which is primarily determined by the crack initiation phase. • Cast IN939 compared with 4 batches of L-PBF IN939 under low-cycle fatigue load. • L-PBF IN939 provides a competitive low-cycle fatigue performance to cast IN939. • Horizontal aged L-PBF IN939 offers the best stress-life low-cycle fatigue properties. • Vertical non-treated L-PBF IN939 is the most plastic strain-resistant. • Crack initiation phase determines the low-cycle fatigue life of L-PBF IN939.