Fatigue strength of additive manufactured Mar-M-509 superalloy

In a recent paper, we have reported successful additive manufacturing of cobalt-based superalloy Mar-M-509® via laser powder bed fusion. The work examined effects of build orientation and subsequent heat treatments on the evolution of microstructure to tailor monotonic strength and ductility of the alloy. In this paper, we present the main results from an experimental investigation into the low cycle fatigue (LCF) and high cycle fatigue (HCF) behavior of the alloy in its optimized condition for strength and ductility. To this end, strain-controlled LCF tests were conducted under strain amplitudes ranging from 1% to 2% without the mean strain component. Stress amplitudes showed small hardening with cycles after an initial transient in rapid cyclic hardening followed by rapid cyclic softening. Some anisotropy was observed in the LCF behavior of the alloy. The amplitudes were higher for the samples tested perpendicular to the build direction (BD) than along the BD, consistent with static strength. In contrast, the LCF life was longer for the samples tested long the BD, consistent with ductility. The measured data was used to delineate the strain-life curves for the alloy per test direction based on the Coffin-Manson model. Additionally, stress-controlled rotary bending HCF fatigue tests were carried out under stress amplitudes ranging from 150 MPa to 750 MPa at room temperature and at 760 °C. The HCF life of the alloy was longer at room temperature than at 760 °C for all stress amplitudes. In contrast to static strength and LCF, the HCF behavior was found isotropic. The observed fatigue characteristics are correlated with the initial microstructure of the alloy.