Comparison of fatigue crack growth behaviour in electron-beam and laser powder-bed-fusion Inconel 718

A comparative study on fatigue crack growth behaviour was made between the electron-beam powder-bed-fusion (PBF-EB) and laser powder-bed-fusion (PBF-LB) Inconel 718. The crack followed a transgranular path with a faster propagation rate for the PBF-EB Inconel 718, whereas a combination of intergranular and transgranular path was observed for the PBF-LB, and its slower rate being comparable to the wrought counterpart. The main fatigue crack in the PBF-EB Inconel 718 exhibited a sawtooth shaped path at the micro-scale, with intensive slip traces close to the crack surfaces owing to the very low work hardening rate. Based on the digital image correlation (DIC) analysis of the crack tip field, fatigue-crack sawtooth path in the PBF-EB Inconel 718 can be successfully predicted by using a strain energy density criterion, which dictates the crack growth follows the direction of minimum distance from the crack tip to elasto-plastic boundary. For the PBF-LB Inconel 718, the predominant fatigue crack was straight at low Delta K, but severe deflections occurred at the medium and high Delta K regimes. A clear correlation exists between the incipient intergranular cracks and the main crack path deflection. This suggests that once the accumulated damage in the plastic zone around the crack tip reaches the critical value, intergranular cracks can form the new front of the fatigue crack, causing the main crack path deflection. Elasto-plastic fracture mechanics parameters of rp and Delta CTOD, derived from the DIC-based crack tip field analysis, can qualitatively predict the lower crack growth rate of the PBF-LB Inconel 718.