Joining of Graphite to Copper with Nb Interlayer: Microstructure and Mechanical Properties

Reliable joining of graphite and copper is achieved by means of a multilayer structure: niobium, which is used as stress-relief interlayer, is joined to copper on one side by transient liquid phase (TLP) bonding and to graphite on the other side, using Ti/Cu foils as a filler alloy. The joints prepared by different parameters are analyzed in terms of microstructure, joint strength, and fracture behavior. With low joining temperature and low Ti content, a ductile banded Cu/TiCu4 layer forms between graphite and Nb by peritectic reaction. The dissolution-precipitation of Nb in TiC particles can help to reduce the brittleness on graphite interface. The TLP bonding between Nb and copper results in a narrow diffusion layer in copper substrate. The maximum shear strength of joints reaches 32 MPa as using 11.3 wt% Ti at 1000 degrees C. The fracture modes change from bowl shape cracks in graphite/Cu joints to multi-step cracks in graphite/Nb/Cu joints.