2001 Volume 42 Issue 9 Pages 1945-1951
Graphite was bonded to Inconel 718 in a vacuum using an RF-induction furnace. The influence of joining conditions on the bending strength of the graphite/Inconel 718 joint, and changes in the microstructure and hardness of Inconel 718 near the joining interface, were investigated. Thermal stress induced in the joint was estimated using the finite element method. Good-solid-state bonding becomes feasible with annealing at temperatures higher than 1173 K under compressive stress of 35 MPa. The adequate joining temperature and joining compressive stress in graphite/Inconel 718 bonding are higher than those in graphite/nickel bonding. This is because fracture of passive-oxide film on the surface due to plastic deformation of Inconel 718 and the resulting direct contact of graphite with matrix of Inconel 718 are required on solid-state bonding. The bending strength of the joints is nearly equal to or greater than that of graphite. This is because the compressive stress induced on the surface of graphite by the plastic deformation of Inconel 718 after bonding and the resulting elastic deformation of graphite in the radial direction relaxes the tensile thermal stress induced on the surface of graphite during cooling or the compressive stress remains. Heat treatment is required to recover the strength of Inconel 718 since intermetallic compounds precipitated in Inconel 718 dissolve in the matrix during annealing and age hardening disappears.
