Effect of Ni in pure Cu/304 stainless steel induction brazing joints

The joints of pure Cu and 304 stainless steel are widely used industrially as heat exchangers. Brazing joints between pure Cu and 304 stainless steel using AgCuZnMnNi as the filler metal were investigated herein. The brazing experiments were carried out in the temperature range of 710-810 degrees C for 15 s. Microstructural analysis showed the primary components of the joints were Ag-based (ss.) (where "ss." indicates a solid solution) and Cubased (ss.). Energy dispersive X-ray spectroscopy revealed a violent interdiffusion at the Cu interface during brazing, where Ni accumulated at the Cu interface at higher brazing temperature and the Ni content in the alpha-Cu (ss.) of the Cu interface increased with brazing temperature. Under tensile deformation, fracture initiated at the Cu and propagated along the alpha-Cu (ss.) of the Cu side, but the tensile rupture location transformed from the alpha-Cu (ss.) to the HAZ of the Cu side as the temperature increased. A maximum joint tensile strength of 210 MPa was obtained at 810 degrees C. Nanoindentation matrix testing indicated that the hardness of the alpha-Cu (ss.) was proportional to the content of elemental Ni in the alpha-Cu (ss.). The accumulation of Ni at the Cu interface induced an enhancement of nanoindentation hardness, which caused the transformation of the fracture location.