A B Fig. 9 — A — Variation of width of isothermally solidified gamma solid solution with root of bonding time; B — microstructure of isothermally solidified diffusion brazed IN718 at 1050°C for 40 min showing a eutectic-free joint centerline. sion in the base metal. Increasing brazing time reduces the volume fraction of eutectic type microconstituent in the joint centerline. When brazing time increased to 40 min, no eutectic structure was observed in the bond region — Fig. 9B. Therefore, it is concluded that a holding time of 40 min at 1050°C is sufficient for isothermal solidification completion. Figure 10 shows the hardness distribution across the braze region for brazes made at 1050°C for 10 min. The hardness distribution across the joint can be well correlated to the microstructural gradient across the braze region. The average base metal hardness is 230 HV, which corresponds to the γ matrix of cast IN718. The average hardness of the ASZ is about 700 HV, which is related to the formation of intermetallic containing eutectic-type solidification products. The hardness of ISZ averaged 120 HV, which is lower than the base metal. According to the EPMA line scan (Fig. 3), the ISZ exhibits a lack of sufficient Nb, Al, Ti, Cr, and Mo compared to the base metal, which explains its lower hardness. The average hardness of the DAZ is 440 HV. The higher hardness of the DAZ compared to the base metal is directly associated to the formation of Cr-Mo-Nb rich boride phase in the γ matrix. The effect of isothermal solidification on the hardness profile is also superimposed in Fig. 10. Since the isothermal solidification eliminates the eutectic-type microconstituent, it is not surprising that the peak hardness in the joint centerline is not present. The hardness of ISZ after completion of isothermal solidification is increased compared to ISZ of joints made at 1050°C for 10 min. This can be related to more diffusion of alloying elements (e.g., Nb, Cr, Mo, Al, and Ti) from base metal into the bond region with increasing brazing time enhancing the solid solution strengthening contribution. However, completion of isothermal solidification does not influence the peak hardness in DAZ. This is due to the fact that boride precipitates in DAZ are still stable even after isothermal solidification completion. Shear Strength The mechanical properties of the diffusion brazed joint are described in terms of maximum shear strength, total strain at failure point (fracture strain), and fracture energy (defined as the area under stressstrain curve up to failure point). The joint strength and fracture of diffusion brazed joints depend on the bond microstructure. Since the microstructure development is significantly affected by brazing time (Fig. 9A), it is expected that the brazing time may have a significant effect on the mechanical properties of the joints. Effect of brazing time on the shear strength, failure elongation, and failure energy of joined cast IN718 is shown in Fig. 11. As can be seen, the shear strength, failure elongation, and failure energy of joints made at 1050°C for 10 min are the lowest. Metallographic examination of the cross section of the fractured sample (Fig. 12A, B) showed the failure occurred via crack propagation through the ASZ. High hardness of eutectic products (Fig. 10) coupled with the fact that nickel boride phase forms an interlinked network, provide a metallurgical notch that significantly decreases the load-carrying capacity of the joint. The low ductility of the intermetallic leads to low fracture strain and fracture energy of partially isothermally solidified joints. Scanning electron micrograph fractorgraphy of the fracture surface along with an X-ray map are shown in Fig. 12C–E. As can be seen, the fracture surface exhibits a semi-cleavage morphology that confirms the low failure energy of this joint. The X-ray map of the fracture surface (Fig. 12E) also indicates crack propagation through athermal solidification products. Based upon the X-ray elemental map (Fig. 12E) and SEM-EDS chemical analysis (not shown here), it can be inferred that the locations marked as X, Y, and Z (Fig. 12D) are Cr-rich boride, eutectic γ and Ni-rich boride precipitates. As can be seen, the interlinked network of Nirich boride is the main source for the fracture of the joint. Therefore, it is necessary to eliminate the eutectic products in order to improve joint strength. According to Fig. 11, increasing brazing time improves the mechanical properties of the joints in terms of shear strength, fracture strain, and fracture energy. This can be related to the decrease in the width FEBRUARY 2014, VOL. 93 66-s WELDING RESEARCH Fig. 10 — Hardness distribution across the bond region for diffusion brazed IN718 at 1050°C for 10 min with partial isothermal solidification and 1050°C for 40 min with complete isothermal solidification.
Welding Journal | February 2014
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