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- soli d-state bonding becomes feasible with annealing at temperatures higher than 1173 K under compressive stress of 35 MPa. The adequate joining temperatur e 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 th e resulting direct contact of graphite with matrix of Inconel 718 are requi red on solid-state bonding. The bending strength of the joints is nearly eq ual to or greater than that of graphite. This is because the compressive st ress induced on the surface of graphite by the plastic deformation of Incon el 718 after bonding and the resulting elastic deformation of graphite in t he radial direction relaxes the tensile thermal stress induced on the surfa ce of graphite during cooling or the compressive stress remains. Heat treat ment is required to recover the strength of Inconel 718 since intermetallic compounds precipitated in Inconel 718 dissolve in the matrix during anneal ing and age hardening disappears.