Interfacial microstructure evolution and formation process of the joints prepared by diffusion bonding on DD6 nickel-based single crystal superalloy

Nickel-based single crystal superalloy DD6 was diffusion bonded in vacuum at 1170 °C under 55 MPa for 2–6 h. Two processes, direct bonding and bonding with interlayer, were designed for comparison. A pure 4 μm nickel (Ni) foil was selected for the interlayer. The joint strength was measured via shear test, whereas the joint microstructure and element distribution were analyzed via scanning electron microscope, electron probe analyzer and transmission electron microscope. The results indicated that the joint direct bonded for 6 h achieved the best joint quality at approximately 353 MPa of shear strength. In this case, the joint morphology and microstructure were characterized by microvoids and carbide precipitation. In contrast, in the joints bonded with interlayer, both carbide precipitation and microvoids disappeared. When 4 μm nickel foil was used as an interlayer, the foil/DD6 interface became less distinct as the bonding time prolonged. When the bonding time reached 6 h, the γ′ phase crystals connected each other with the misorientation distribution of the joint less than 5°, and maximum shear strength of 727 MPa was obtained. The fracture morphology has a mixed ductile-brittle feature.