Investigation of microstructure evolution and strength mechanism of the Cu coated BNi-7 core-shell structure filler metal

In this paper, a novel method of using the Cu coated BNi-7 core-shell structured composite filler metals to braze was adopted to alleviate the residual thermal stress in the joints of Inconel 718. A pure Cu layer coated with the BNi-7 powders was successfully prepared by electroless plating to fabricate ductile filler metal with excellent mechanical properties. The obtained BNi-7 powders with 2 μm thick Cu coating exhibit dense and uniform structure through the elemental mapping analysis of the composite filler metal. The thickness and the content of the Cu layer could be determined by electroless plating time and the pH value of the plating solution. Meanwhile, the thermal physical parameters of the obtained composite filler were characterized by DSC analytical method, and the result indicated that the melting point was higher than the BNi-7 filler metal. The Inconel 718 were brazed without any defects using the composite filler metal in a vacuum. Compared to the BNi-7 filler metal, the Cu coated BNi-7 composite filler metal could form plenty of uniform reticular solid solutions during the high-temperature brazing process instead of acicular eutectic compounds. When adding the Cu powders mechanically into the original BNi-7 filler metals, voids and cracks could be found through the joints due to the uneven distribution of the Cu. The typical interfacial sequence of the joints was Inconel 718/Ni(Cu)_ss+ σ-FeCr/Ni₅P₂ + Ni₃P + Ni(Cr)_ss + Ni₂Cr + Ni(Cu)_ss + σ-FeCr + Ni(Cu, Cr, Fe)_ss/Ni(Cu)_ss+ σ-FeCr/Inconel 718. The formation of the reticular Ni(Cu, Cr, Fe)_ss phase released the stress in the brittle phase adjacent leading to the increasing ductility of the joints. The addition of the Cu-coated layer significantly enhanced the strength of the joint at room temperature, and the strengthening mechanisms were proposed.