Solid state bonding of graphite to Inconel 718

Graphite was bonded to Inconel 718 in a vacuum using an RF-induction furnac e. The influence of joining conditions on the bending strength of the graph ite/Inconel 718 joint, and changes in the microstructure and hardness of In conel 718 near the joining interface, were investigated. Thermal stress ind uced in the joint was estimated using the finite element method. Good solid -state bonding becomes feasible by annealing at temperatures higher than 11 73 K under compressive stress of 35 MPa. The adequate joining temperature a nd joining compressive stress in graphite/Inconel 718 bonding are higher th an those in graphite/nickel bonding. This is because fracture of passive-ox ide film on the surface due to plastic deformation of Inconel 718 and the r esulting 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 stres s 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 treatmen t is required to recover the strength of Inconel 718 since intermetallic co mpounds precipitated in Inconel 718 dissolve in matrix during annealing and age hardening disappears.