Microstructure and Properties of Vacuum-Brazed Joints of 3D-Structured Ni718 High-Temperature Alloy Prepared by Selective Laser Melting

The 3D-structured IN718 surfaces have been designed and printed by SLM for vacuum brazing with BNi2 filler. The effect of brazing temperature and time on microstructure and mechanical property of structured IN718 joints is investigated. The results reveal that the joint consists of ISZ, ASZ, and DAZ, with ISZ consisting of γ solid solution; ASZ consisting of CrB, NiB NiSi, and NiSiB and the intermetallic compounds are concentrated in the crest and trough; DAZ has a large amount of (Cr, Nb, Mo)-rich borides. The different degrees of enrichment of the elements affect the changes of the microstructure and morphology in DAZ. With the increase in brazing temperature, the ASZ gradually becomes smaller, and more CrB is observed. At 1110 °C, the reduced isothermal solidification rate leads to the re-formation of intermetallic compounds in the narrower gap zone. With the increase in holding time, the intermetallic compounds gradually decrease to disappear, but in the narrower gap zone, Kirkendall holes appeared. The highest shear strength of the joint is 676.5 MPa along the groove direction and 624.4 MPa in the vertical groove structure direction at 1080 °C/40 min, 27.5 to 38.1% improvement in strength compared with flat joints. The fracture mechanism is mixed with brittle and ductile fracture.