Effects of High-Temperature Oxidation on Fatigue Life of Additive-Manufactured Alloy 625

The effect of isothermal oxidationIsothermal oxidation on the fatigue performanceFatigue performance of differently processed Alloy 625Alloy 625 was studied (wrought 625, laser powder bed fusionLaser powder bed fusion, direct energy deposition). Uniaxial fatigue tests at room temperature were conducted after prior exposures at 800 °C for 24 h, 300 h, and 1000 h in either air or argon. Exposures in air resulted in chromia-scale formation, internal attack, and the formation of subsurface precipitates (i.e., δ-phase and σ-phase). Fatigue results indicated a consistent life reduction of up to 96% for the oxidized additive-manufactured test bars compared to their counterpart aged in back-filled argon. The fatigue life decreases as the oxidation exposure time increases. By contrast, any of the prior high-temperature exposures were not detrimental to the performance of the wrought Alloy 625Alloy 625. Microstructural analysis of the after-testing oxidized AM-processed bars indicated that the failure mode was attributed to the exacerbation of interfacial and subsurface defects from the oxidation exposure (i.e., internal attack, decohesion of the scale, and subsurface precipitates). These defects acted as preferential crack-initiation sites, leading to a reduction in fatigue life. On the other hand, the failure mode for the thermal-aged bars, without superficial degradation, involved fragmentation of (δ + σ) precipitate clusters favored by the Nb and Mo segregation from the as-built microstructureMicrostructures of both AM processes (LPBF and DED). A large precipitate fraction depletes the matrix, facilitating crack formation.