Nickel electrochemical alloys like constantan and monel contain iron and copper. They have a chemically unstable oxide film on their surface, easily reduced in gas environments, which is removed by fluxing in a vacuum during high-temperature brazing, decomposing into metal and oxygen, so brazing such alloys does not cause any particular difficulties.

Fluxes

Fluxes, gas media, and solders that are recommended for copper and steels can be used for brazing. Nickel alloys require special fluxes because the surface of nickel alloys, such as nichromes (nickel-chromium alloys), is covered by a very resistant chromium oxide-based film. When nichrome is alloyed with titanium and aluminum, the chemical resistance of the oxide film increases, which leads to some difficulty in soldering.

Gas media

When brazing nickel and nickel alloys, it is necessary to ensure that the used gaseous media have no sulfur compounds, because the interaction of sulfur and nickel forms a fusible eutectic that penetrates deep into the grain boundaries and causes embrittlement of the metal. Brazing of heat-resistant nickel-based alloys in a gas reducing environment requires careful cleaning of the oxygen residue with a dunite or platinum catalyst and additional drying to the dew point (-70 ° C). When brazing such alloys in a vacuum or in neutral gas environments - they should be thoroughly dried with barium peroxide, phosphorus anhydride or zeolite. Before soldering, nichromes are covered with a 15 µm thick copper or nickel layer that provides wetting of the soldering surfaces in neutral environments and vacuum without using flux.

Solders

When brazing nickel at low temperature, lead-tin brazing alloys that contain 40 to 60% Sn are used, as well as the recommended fluxes for steel brazing. Silver solders are used for structures made of nickel alloys that operate at temperatures of 350-500°C. Copper solder intended for brazing heat resistant nickel alloys are rarely used because of their low corrosion resistance and heat resistance. In addition, nickel dissolves in the brazing alloy during the copper brazing process. Therefore, strict control of soldering temperature and precise dosage of solder is required. When brazing nickel alloys alloyed with beryllium and boron, brazed metal also dissolves in the solder, so it is necessary to conduct brazing without overheating and quickly, strictly observing the regime of brazing.

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