Microstructure and mechanical properties of Cu-Cu brazed joints

Abstract

Silver brazing filler metals are well known as suitable materials for joining various alloys, including copper, mild steel, stainless steel. However, the use of the common silver-based cadmium-containing fillers for brazing high-precision copper joints is increasingly restricted due to the toxicity of cadmium vapors. As a result, there is significant interest in developing novel silver-containing, cadmium-free brazing filler metals. Recently, interest in Ag-Cu-Zn alloys has been growing rapidly in many branches of the industry, due to increasing demand in the production of heat pumps.To address sustainability and efficiency concerns, replacing the widely used heat source acetylene flame by hydrogen flame or even switching to induction heating has been promoted. Furthermore, manual brazing is gradually being replaced by automated methods, along with changes in cooling conditions. The influence of the heat source and the cooling profile on the microstructure and reliability of the brazing material has not yet been sufficiently investigated.This research focuses on developing an efficient method for evaluation of the brazing filler materials and brazing methods, aiming to improve the quality and reliability of the Cu-Cu brazed joints.For this study, specially designed Cu-Cu butt joints with defined gap widths were fabricated using two types of brazing filler materials: 40Ag-30Cu-28Zn-2Sn (40 pct. Silver containing brazing filler) and 5Ag-6P-Cu (Copper phosphorous brazing filler). The two sets of samples were either quenched in water or air-cooled after flame- or induction brazing to investigate the influence of cooling rate on the quality of the joints. Microstructural investigations and hardness tests were conducted to characterize the various materials and conditions. Subsequently the samples were subjected to a comparative analysis with bulk material.For determination of lifetime curves and strength of the joints, an ultrasonic resonance fatigue testing system as well as a static set-up based on the three-point bending (3PB) method were employed. In the ultrasonic method, samples were subjected to cyclic bending loads at high frequencies (20kHz) until failure. Fatigue curves of brazed joints were determined as a function of displacement amplitudes against loading cycles to failure. Fracture surface analysis was conducted to determine the crack path and failure mode.A notable enhancement in fatigue life was observed for the 40 pct. silver containing brazing alloy compared to the joints made from the copper phosphorous brazing alloy. Furthermore, fatigue tests showed a moderately improved lifespan for both alloys of water-cooled compared to air-cooled joints.