COPPER EMBRITTLEMENT BY SILVER BRAZING ALLOYS

Intergranular void formation and accompanying embrittlement, following the brazing of tough-pitch copper with silver alloys, is well known. Hydrogen embrittlement has been suggested as the main mechanism leading to these detrimental conditions. In this study, commercial brazing alloys containing zinc, cadmium, or phosphorous were found to cause embrittlement by migration of (copper) oxide to the grain boundaries, causing void formation and rupture of grain boundaries. (Oxides do not migrate as such, but rather dissolve in the grains. The oxygen diffuses to the grain boundaries, where it recombines, forming less-stressed particles.) The brazing was performed both by the conventional fluxed and fluxless methods without the presence of any source of hydrogen. But this still resulted in the same embrittlement. Therefore, it was concluded that the influence of flux is insignificant, since embrittlement also persisted in joints brazed without flux under argon. The mechanism suggested by this study is embrittlement induced by the coexistence of copper oxides in the base material together with certain alloying additions in the filler metal, such as cadmium, zinc, or phosphorus, capable of reducing the copper oxides.