tical to the base material and have no discernable microstructural discontinuity at the bond line (Refs. 12, 18). Despite extensive application of IN718 in various industries, there are few published works (Ref. 9) on diffusion brazing of this superalloy. There are some modeling efforts regarding isothermal solidification time of wrought IN718 alloy (Ref. 9); however, there is limited information regarding microstructure development and mechanical properties during diffusion brazing of this superalloy. Therefore, this paper aims at investigating the metallurgy of diffusion brazing of cast IN718 nickelbased superalloy using Ni-Cr-Fe-B-Si filler metal. Experimental Procedure A cast IN718 nickel-based superalloy was used as the base metal in this investigation. The chemical composition of the base metal is Ni-20Fe-18Cr-5Nb-3Mo- 0.2Si (wt-%). A 50-μm-thick amorphous Ni-based filler metal of BNi-2 (Ni-7Cr- 4.5Si-3.2B-3Fe) was used as the interlayer for diffusion brazing. In this filler metal, both B and Si play important roles, being very effective melting-point depressant elements that facilitate wetting. Coupons sized at 10 × 5 × 5 mm were sectioned from the base metal using an electrodischarge machine. Thereafter, in order to remove the oxide layer, contacting surfaces were ground using 600-grade SiC paper and then ultrasonically cleaned in an acetone bath. An interlayer was then inserted between two base metal coupons. A brazing operation was carried out in a vacuum furnace under a vacuum of approximately 10–5 torr. The brazing temperature was selected as 1050°C. The brazing time was varied from 10 to 40 min. Specimens were sectioned perpendicular to the bond and then microstructural observations were made on cross sections using an optical microscope and a field emission scanning electron microscope (FESEM). For microstructural examinations, specimens were etched using 10 mL HNO3–10 mL C2H4O2–15 mL HCl. Semiquantitative chemical analyses of phases formed in the centerline of the bond region and adjacent to base metal were conducted using a JEOL 5900 FESEM equipped with an ultrathin window Oxford energy-dispersive X-ray spectrometer (EDS). Element distribution across the joint region was analyzed using a JEOL JXA-8900R electron probe X-ray microanalyzer equipped with line scan wavelength dispersive spectrometry (WDS). Microhardness testing was used to determine the joint region hardness profile. The test was conducted on sample cross sections using a 10-g load on a Buehler microhardness tester. To evaluate the mechanical strength of TLP bonds, shear testing was used instead of tensile testing. Tensile testing is not strict to the bond line. Indeed, a minimal amount of the bond line is oriented on the plane experiencing the maximum resolved shear stress (i.e., plane that oriented 45 deg to the tensile axis) (Ref. 22). Therefore, it can be deduced that tensile testing of a TLP bond with a thin interlayer (i.e., 50 μm) does not effectively test the bond line. Therefore, a fixture was designed for shear testing — Fig. 1. The designed fixture subjects the sample to a pure shear stress at the bond line. Despite the fact that this testing method is not a standard one and is primarily comparative, the results are sensitive to the joint microstructure. Room-temperature shear testing was performed employing an Instron tensile machine with a cross-head speed of 2 mm/min. The edge effects were eliminated by machining before the shear test. Results and Discussion Typical Microstructure of Diffusion Brazed IN718 During diffusion brazing, the following metallurgical phenomena occurs: 1. Melting of filler metal 2. Dissolution of the base metal 3. Diffusion of MPD elements into the base metal 4. Solidification of liquid phase, which can occur via two different mechanisms, isothermal solidification at the brazing temperature and athermal solidification upon cooling. When the base metal/filler metal assembly is heated to the brazing temperature, the filler metal melts. This is because the liquidus temperature of the filler metal is lower than the brazing temperature. To achieve equilibrium at the solid/liquid interface, the composition of the solid base metal is brought to the solidus at brazing temperature via dissolution of the base metal (Ref. 18). This process modifies the chemical composition of the liquid phase adjusting it to the liquidus composition at brazing temperature. Once local equilibrium is achieved, WELDING JOURNAL 61-s WELDING RESEARCH Fig. 1 — Schematic of shear test fixture (Ref. 10). Fig. 2 — Microstructure of diffusion brazed IN718/Ni-Cr-Si-B-Fe/IN718 after partial isothermal solidification. A — Backscattered electron micrograph of joint cross section indicating four distinct zones: ASZ, ISZ, DAZ, and BM; B — optical micrograph showing detailed view of ASZ microconstituents; C — SEM micrograph of Ni-rich boride and Cr-rich boride (M and N indicate Ni-rich boride and Cr-rich boride, respectively); D — SEM micrograph showing detailed view of Ni-Si-B ternary eutectic (R and S indicate Ni-rich silicide and Ni-rich boride, respectively).
Welding Journal | February 2014
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