Design of higher temperature copper brazing filler metals with reduced brittle phase content

Anneal-resistant copper alloys have been developed to address the need for stronger yet lighter fin and tube parts in heat exchangers, in order to facilitate greater efficiency and increased temperature capability. As such, brazing as opposed to traditional soldering has become the preferred joining method for these materials, due to the enhanced high-temperature properties of such joints. Copper-based filler metals are used extensively for the brazing of these materials, generally based on the copper-phosphorus system with alloying additions. In the present study, the microstructural issues arising from the current commercially available filler metals are discussed and addressed through CALPHAD-supported alloy design. Three selected novel compositions were used in the brazing of pure copper as a test of the design concept. Alloy compositions in weight percent of Cu(bal.)− 7Ni-4Sn-5 P and Cu(bal.)-10Ni-2Sn-6.5 P were found to result in a reduction of brittle Cu3P phase while increasing the fraction (0.78 and 0.77 respectively) of copper-rich solid solution in joints post-braze, comparing favourably to commercial Meta-Braze™ 077 under similar conditions (0.61). Furthermore, the copper-rich solid solution was leaner in tin, predicted to impart an increased melting temperature while microhardness profiles indicate increased joint ductility. The implications for mechanical properties of filler metals in this alloy family are discussed.