Effect of bonding temperature on the precipitation of δ phase in diffusion bonded inconel 718 joints

Precipitation of δ phase during solid-state diffusion bonding of Inconel alloy 718 to itself was studied in the present investigation. The joining was carried out from 910 to 1060°C, in steps of 30°C, with time and pressure fixed at 60 min and 32 MPa, respectively. In the joint bonded at 910°C, transforming of γ″ to δ was observed. δ phases had two kinds of morphology: platelet-like in the bond line area and needle-like in the bulk material. In both shapes, δ appeared intergranular and intragranular. Widmanstätten structure was formed. For a joint bonded at 940°C, only platelet δ phases precipitated, both intergranularly and intragranularly. No Widmanstätten structure was observed. When bonding temperature was increased beyond 970°C, δ phase was not observed in the as-bonded joints. To avoid the transformation of γ″ to δ, the bonding temperature should be higher than the solvus of γ″. Higher cooling rate was also found helpful in this regard, and a cooling rate faster than 5°C/min seemed adequate in suppressing δ precipitation.