The brazing properties of copper and copper alloys

The brazing properties of copper and copper alloys mainly depend on the following factors:

1. The stability of the oxide formed on the surface. 2. The influence of brazing heating process on material properties. 3. The sensitivity of the material to stress cracking. Two oxides of Cu2O and CuO may be formed on the surface of pure copper. The copper surface is covered by Cu2O at room temperature; the oxide scale at high temperature is divided into two layers, the outer layer is CuO, and the inner layer is Cu2O. Copper oxides are easy to remove, so the brazing properties of pure copper are very good. Oxygen copper is copper refined by fire and electrolytic toughening grade copper. It contains oxygen with a mass fraction of 0.02% to 0.1%. Oxygen exists in the form of copper oxide and forms a eutectic structure with copper. This eutectic structure is distributed in the copper matrix in the form of small balls. If aerobic copper is brazed in a reducing atmosphere containing hydrogen, hydrogen rapidly diffuses into the metal, reducing oxides to produce water vapor. This water vapor forms cavities in the copper crystals and expands rapidly, resulting in hydrogen embrittlement. In severe cases, the copper material will break. If the atmosphere contains carbon monoxide and moisture, carbon monoxide can reduce water vapor to hydrogen, and then diffuse into the metal, forming hydrogen embrittlement. Therefore, aerobic copper can not be brazed in the decomposing ammonia, endothermic and exothermic reducing atmospheres. Long-term heating of aerobic copper above 920°C will cause copper oxide to accumulate on the grain boundaries, reducing the strength and plasticity of the copper. Therefore, when brazing aerobic copper, it is necessary to avoid the material staying at a temperature above 920°C for a long time.