Evaluation of copper brazed joint failure by thermal-fatigue test applicable in heat exchangers

Ag-based brazing filler (Ag-15Cu-16Zn-24Cd) was applied for bonding copper specimens. The brazing temperatures and holding times employed in this study were 660 °C-720 °C and 10-20 min, respectively. The joints mainly consisted of Ag-rich matrix along with some intermetallic compounds like CuZn5 and Cu solid solution. Optimum shear strength achieved was measured to be 73.6 MPa. Thermal-fatigue test was performed on copper brazed specimens within the designed temperature range of 320, 420 and 580 °C. Fracture strength values at three different temperatures were determined at a constant number of 100 thermal cycles. Fractography analysis revealed that all cracks propagated along the center regions which adopted ductile morphology changing to brittle one by increasing the test temperature.