Tailoring boride formation and strength–toughness balance in dissimilar superalloy joints with Ni–Pd-based brazing filler metals

The excessive formation of borides in brazed joints deteriorates the base metal and restricts joint performance. To address this issue, a Ni-Pd-based brazing filler metal (BFM) was developed using Ni-40Pd as the primary alloy system, while Si and B were rationally adjusted to suppress boride precipitation. The resulting Inconel 718/NP-BFM/Inconel 625 joints exhibited reduced boride content with a dispersed distribution. The strength degradation in the DAZ was limited to less than 15 %, significantly lower than the observed in conventional BNi-2 joints (>40 %). Interface defects were eliminated, and the joints demonstrated excellent forming quality, achieving a tensile shear strength of 443 MPa and an elongation of 18 % at room temperature. The formation and failure mechanisms of the joints were elucidated. Comparative analysis with high-Si and B BFMs highlighted the superior microstructural control and performance of the NP-BFM. The relationship between Si content and B diffusion rate was quantified, revealing the fundamental reason for the suppressed boride formation. Furthermore, the influence of boride types on strength enhancement and toughness degradation was modeled, clarifying the relative contributions of various boride types to overall joint performance.