Brazing of Reactive Metals: Explained

1. Brazing Materials

(1) Soft brazing materials are rarely used for brazing titanium and its alloys. The main types of hard brazing materials used are silver-based, aluminum-based, titanium-based, or titanium-zirconium-based solders.

Silver-based solders are mainly used for components with operating temperatures below 540℃. Joints made with pure silver solder have low strength and are prone to cracking. They also have poor corrosion resistance and oxidation resistance.

The brazing temperature of Ag-Cu solder is lower than that of silver, but its wetting ability decreases with increasing copper content. Ag-Cu solders with a small amount of lithium can improve wetting ability and enhance the alloying between solder and base material.

Ag-Li solder has a low melting point and strong reducing properties, making it suitable for brazing titanium and titanium alloys in a protective atmosphere.

However, vacuum brazing can contaminate the furnace due to the evaporation of lithium. Ag-5Al-(0.5~1.0)Mn solder is the preferred solder for thin-walled titanium alloy components, with good oxidation and corrosion resistance of the brazed joints. The shear strength of titanium and titanium alloy joints brazed with silver-based solders is shown in Table 12.

Table 12: Brazing Process Parameters and Joint Strength of Titanium and Titanium Alloy

Aluminum-based solders have a low brazing temperature, which does not cause β-phase transformation in titanium alloys, reducing the requirements for brazing fixture materials and structures.

These solders have a low level of interaction with the base material, and dissolution and diffusion are not significant.

However, the solder has good plasticity and can easily be rolled together with the base material. Therefore, it is suitable for brazing titanium alloy radiators, honeycomb structures, and sandwich structures.

Titanium-based or titanium-zirconium-based solders generally contain elements such as copper (Cu) and nickel (Ni). During brazing, these elements can quickly diffuse into the base material and react with titanium, causing dissolution of the base material and the formation of a brittle layer.

Therefore, it is necessary to strictly control the brazing temperature and holding time and avoid using them for brazing thin-walled structures. B-Ti48Zr48Be is a typical titanium-zirconium solder. It has good wetting ability on titanium and does not promote grain growth during brazing.

For brazing zirconium and its alloys, commonly used solders include B-Zr50Ag50, B-Zr76Sn24, B-Zr95Be5, which are widely used for brazing zirconium alloy pipes in nuclear power reactors.

For titanium, zirconium, and their alloys, satisfactory results can be obtained in vacuum or inert gas atmospheres such as helium or argon. High-purity argon should be used for argon gas protection, and the dew point should be -54℃ or lower. Special fluxes containing metal Na, K, and Li should be used for flame brazing.

2. Brazing Techniques

Thorough surface cleaning, degreasing, and oxide film removal are necessary before brazing. Mechanical methods, sandblasting, or molten salt baths can be used to remove thick oxide films. Thin oxide films can be eliminated in a solution containing 20% to 40% nitric acid and 2% hydrofluoric acid.

For titanium, zirconium, and their alloys, the joint surfaces should not be exposed to air during brazing. Brazing can be performed in vacuum or under inert gas protection. Induction heating or protective gas heating methods can be used. Induction heating is suitable for small symmetric parts, while furnace brazing is more advantageous for large and complex components.

For heating elements used in brazing titanium, zirconium, and their alloys, materials such as Ni-Cr, W, Mo, and Ta are preferred. Equipment with exposed graphite heating elements should not be used to avoid carbon contamination.

Brazing fixtures should be made of materials with good high-temperature strength, similar thermal expansion coefficients to titanium or zirconium, and minimal reactivity with the base metal.