A B C of the eutectic-type microconstituents. According to Fig. 11A–C, there is an inverse relation between width of the ASZ and mechanical properties of the joint. Therefore, it can be deduced that in a brazing condition in which isothermal solidification is not completed, the extent of the eutectic constituent (ASZ) is the controlling factor for the shear strength, fracture strain, and fracture energy. When brazing time increased to 40 min at 1050°C, a joint with a shear strength of about 497 MPa was achieved. This can be related to the complete removal of the eutectic type microconstituents in the joint centerline producing a single-phase solidsolution microstructure across the joint region. The shear strength of base metal heat treated at 1050°C for 40 min is about 660 MPa. Therefore, a joint efficiency of 75% was obtained after completion of isothermal solidification. The lower shear strength of the joint in this condition compared to the base metal is related to the soft ISZ relative to the base metal. Higher joint efficiency can be achieved via designing a proper postbraze heat treatment to improve the homogenization across the braze region via enhancing the interdiffusion of alloying elements, particularly Nb, Cr, and Mo, and eliminating the boride precipitates in the DAZ. Conclusions For this study, microstructure development and mechanical properties of diffusion brazed IN718 nickel-based superalloy were investigated. The following conclusions can be drawn from this study: 1) The joint microstructure is influenced by complicated phase transformations including diffusion-induced isothermal solidification, cooling-induced athermal solidification, and difusioninduced solid-state precipitation. After partial isothermal solidification, three distinct microstructural zones were formed in the brazing-affected zone: isothermal solidification zone (ISZ), which consisted of Ni-rich γ solid solution; athermal solidification zone (ASZ), which consisted of eutectic type microconstituents; and diffusion affected zone (DAZ), which consisted of extensive boride precipitates distributed in the matrix. 2) When the brazing time is not sufficient to complete solidification of the liquid phase at the brazing temperature, the residual liquid is solidified on cooling. The solidification behavior of residual liquid is governed by low solubility of Si and B and their segregation behavior. Solidification of the residual liquid is started by solidification of Ni-rich γ dendrites enriching the remaining melt with B and Si. This follows by formation of binary eutectic of γ-Nirich and Ni-rich boride and then by formation of binary eutectic of γ-Ni-rich and Cr-rich boride. The formation of boride phases enriches the remaining melt in Si leading to formation of ternary eutectic of Ni-Si-B as the last solidified portion. 3) Extensive fine Ni-rich silicide (γ´- Ni3Si) formed within the eutectic γ by WELDING JOURNAL 67-s WELDING RESEARCH Fig. 11 — Effect of joining time and width of eutectic-type microconstituents (ASZ size) on the mechanical properties of diffusion brazed IN718: A — Shear strength; B — fracture strain; C — fracture energy. Fig. 12 — A — Fracture path during shear testing of cast IN718 diffusion brazed at 1050°C for 10 min; B — magnified view of fracture location in A; C — morphology of fracture surface; D — higher magnification view of fracture morphology; E — X-ray map of Ni, Si, and Cr from selected location (D) of fracture surface. The locations marked as X, Y, and Z are Cr-rich boride, eutectic-γ and Nirich boride.
Welding Journal | February 2014
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