Uncovering the mechanism behind a two-step infiltration during layered wide-gap brazing of a Ni-base superalloy using a B-Containing filler metal

Abstract

This study investigates the mechanisms resulting in a two-step infiltration process during layered wide-gap brazing of the MAR-M247/BNi-9 system for repair of high-temperature Ni superalloy components. Differential scanning calorimetry was coupled with electron microscopy and 3D X-ray microscopy to better quantify the two-step infiltration process. It was found that significant diffusional homogenization between braze alloy and MAR-M247 particles, occurred prior to liquation. As a result, only about 15% of the braze melted during Stage 1 of layered wide-gap brazing as compared to approximately 49% when only the melting of pure BNi-9 was considered for the same temperature interval. Stage 1 melting began at the ternary eutectic transformation temperature, 1053 °C, with the complete melting of Ni₃B and partial melting of $\gamma$-Ni. Upon liquation, the braze liquid readily infiltrated the MAR-M247 skeleton via capillary action. As a result of rapid boron diffusion into the MAR-M247 skeleton from the infiltrating braze, extensive base metal dissolution and eventual diffusional solidification occurred, with Stage 1 infiltration terminating at 1073 °C. Stage 2 melting and infiltration began at 1102 °C with the re-melting of the partially infiltrated braze as well as melting of remaining un-infiltrated braze atop the layered brazement, with near complete infiltration attained by 1150 °C. Further, it was found that infiltration behavior varied between Stages 1 and 2, with 58% and 40% of the MAR-M247 skeleton infiltrated after 120 s and 228 s, respectively. Kinetic differences were likely influenced by: (i) transition from rapid to sluggish skeletal dissolution regimes, and an (ii) alteration of the MAR-M247 pore network structure