Effect of P particle size on brazability of Cu-P composite plating film

To apply a Cu-P composite plating film as a brazing material for Cu plates of a heat exchanger, the effect of a phosphorous particle size in the Cu-P composite plating film on brazability has been investigated. For Cu-P composite plating films using phosphorous particles with average sizes of 7.5mum and 3mum, solidus temperatures were investigated to be 765.0 deg C and 715.6 deg C, respectively. These temperatures are lower than 800 deg C, which is conventional brazing temperature for Cu and Cu alloys. In the case of phosphorous particle with an average size of 1.5mum, however, obvious solidus temperature was not observed by differential scanning calorimetry (DSC). In brazed Cu joints using Cu-P composite plating films with 7.5mum and 3mum size phosphorous particles, Cu phases and eutectic phases were observed. The eutectic phases consist of fine Cu and Cu3P phases. A similar microstructure was observed in a brazed Cu joint with conventional Cu-7 mass%P alloy foil. In contrast, brazing was not completed when the average phosphorous particle size is 1.5mum. In the brazed interface, a brazed microstructure of Cu-7 P alloy was not observed but a crack was found. In the brazed joint, phosphorus was dispersed uniformly in Cu plates. Since fine phosphorous particles can diffuse easily into Cu plates in heating process of brazing and thus the Cu-P composite film does not remain in the brazed interface, such a Cu-P composite plating film makes brazing difficult.