The effect of active elements on the microstructure and properties of graphite and copper-brazed joints

Purpose To obtain better properties of joints of graphite/Cu brazing, brazing of graphite to oxygen-free copper with different coating has been employed.
Methods Graphite is brazed to copper with surface coatings containing Cr, Ni or no element at 860 ℃ for 10 min.
Results The shear strength of all joints exceeds 20 MPa. In addition to the interaction between the Ti element in the filler and the C element leading to the formation of TiC, the coating materials Cr and Ni also form Cr₇C₃ and Ni₃C, respectively. Compared to the Cr-coated and no-coated brazed samples, the filler is distributed more evenly in the Ni-plated brazed sample. TEM diffraction patterns have been utilized to calibrate the granular metal phase forming at the brazing filler metal boundary, revealing its composition Ti (Cu, Ni)₂. The thermal conductivity of the brazed samples has been tested at different temperatures; the Cr-coated samples show significantly lower thermal conductivity than the uncoated samples. Conversely, the thermal conductivity of the Ni-coated samples is significantly higher than that of the uncoated samples.
Conclusion Graphite and copper with different coatings can be successfully joined by using AgCuTi at 860 ℃ for 10 min. The graphite/Cu joint with Ni coating achieves better thermal conductivity with a more even distribution of the filler.