Fatigue behavior and microstructural evaluation of Inconel 718 gas tungsten arc welds

Microstructural and fatigue behavior of Inconel 718 weldments performed using gas tungsten arc welding process (GTAW) were studied. Ti and Nb segregation tends to form brittle phases (Laves, NbC and TiC), which were identified in the interdendritic region of the weld metal (WM). The heat generated to weld Inconel 718 produced a microstructural transformation of the γ´´ (Ni3Nb), decreasing the microhardness in comparison to the base metal (BM). The welded joints were heat treated by a solubilization and aging treatment. Fatigue life behavior of the welded joints was measured. Curves of stress amplitude versus number of cycles to failure were obtained for the welded joints. Nevertheless, considering the difference in tensile properties of the welded joints and BM, similar fatigue life resistance was found. The fatigue crack initiation was located close to the WM, coinciding with softer zone as identified by microhardness. Fatigue crack growth rate da/dN as a function of stress intensity factor range ΔK was determined. The slower da/dN as a function of ΔK corresponds to the WM in the as-welded condition, followed by the HAZ, WM after the heat treatment, and finally the BM. This phenomenon was analyzed in terms of the crack closure effect induced by the plasticity zone ahead of the crack tip.