Brazing of Refractory Metals: Explained

1. Brazing Materials

Brazing materials with a melting point below 3000°C can be used for tungsten (W) brazing. Copper-based or silver-based solders are suitable for components operating below 400°C.

For components operating between 400°C and 900°C, gold-based, manganese-based, molybdenum-based, palladium-based, or diamond-based solders are typically used.

For components operating above 1000°C, pure metal solders such as niobium (Nb), tantalum (Ta), nickel (Ni), platinum (Pt), palladium (Pd), and molybdenum (Mo) are commonly used. Pt-based solders can withstand working temperatures up to 2150°C. With diffusion treatment at 1080°C after brazing, the maximum working temperature can reach 3038°C.

Most of the brazing materials used for tungsten (W) can also be used for molybdenum (Mo) brazing. Copper-based or silver-based solders are suitable for Mo components operating below 400°C.

For electronic devices and non-structural components operating between 400°C and 650°C, solders such as Cu-Ag, Au-Ni, Pd-Ni, or Cu-Ni can be used. For components operating at higher temperatures, titanium-based or other high-melting-point pure metal solders should be used.

However, manganese-based, cobalt-based, and nickel-based solders are not recommended to avoid the formation of brittle intermetallic compounds in the brazed joint.

For tantalum (Ta) or niobium (Nb) components operating below 1000°C, copper-based, manganese-based, cobalt-based, titanium-based, nickel-based, gold-based, and palladium-based solders can be used. Solders such as Cu-Au, Au-Ni, Pd-Ni, Pt-Au-Ni, and Cu-Sn have good wetting ability on Ta and Nb, and they can form high-strength joints.

However, the use of silver-based solders, which tend to make the brazed metal brittle, should be avoided as much as possible.

For components operating between 1000°C and 1300°C, solders such as pure metals Ti, V, Zr, or alloys based on these metals that form an infinite solid-liquid solution with Ta and Nb can be used. For higher operating temperatures, solders containing hafnium (Hf) can be selected.

Table 13 shows high-temperature brazing materials for refractory metals such as W, Mo, Ta, and Nb.

Table 13: Brazing Materials for High-Temperature Brazing of Refractory Metals

2. Brazing Technique

Before brazing, it is important to carefully remove the oxide layer on the surface of refractory metals. This can be achieved through mechanical grinding, sandblasting, ultrasonic cleaning, or chemical cleaning.

Once the cleaning process is completed, brazing should be performed immediately. Due to the inherent brittleness of tungsten (W), W components should be handled with caution during assembly to avoid breakage.

To prevent the formation of brittle tungsten carbide, direct contact between W and graphite should be avoided. Pre-existing stresses resulting from machining or welding operations should be eliminated prior to brazing.

As tungsten is highly prone to oxidation at elevated temperatures, high vacuum conditions are required for brazing. When brazing within the temperature range of 1000-1400℃, the vacuum level should not be lower than 8×10-3Pa.

To enhance the remelting temperature and operational temperature of joints, it is recommended to combine brazing with diffusion treatment.

For instance, using B-Ni68Cr20Si10Fel brazing filler metal to braze W at 1180℃, followed by three diffusion treatments at 1070℃/4h, 1200℃/3.5h, and 1300℃/2h, the usage temperature of the brazed joint can exceed 2200℃.

When brazing molybdenum (Mo) joints, it is important to consider the low coefficient of thermal expansion as a characteristic. The gap between joints is recommended to be within the range of 0.05-0.13mm.

If using fixtures, materials with a low coefficient of thermal expansion should be selected. Flame brazing, controlled atmosphere furnaces, vacuum furnaces, induction heating furnaces, and resistance heating above the recrystallization temperature or the diffusion-induced lower recrystallization temperature can cause recrystallization in Mo.

Therefore, when the brazing temperature is close to the recrystallization temperature, shorter brazing times are preferred. When brazing above the recrystallization temperature of Mo, it is necessary to control the brazing time and cooling rate to avoid cracking.

When using an oxyacetylene flame for brazing, it is ideal to use a mixed brazing flux, such as industrial borate flux or a high-temperature brazing flux containing calcium fluoride, to ensure good protection.

The method involves initially coating the Mo surface with a silver brazing flux, followed by applying the high-temperature brazing flux. Silver brazing flux possesses activity at lower temperature ranges, while the activity temperature of the high-temperature brazing flux can reach 1427℃.

For tantalum (Ta) or niobium (Nb) components, it is preferable to perform brazing under vacuum conditions with a vacuum level not lower than 1.33×10-2Pa. If brazing is conducted under inert gas protection, it is necessary to thoroughly remove gas impurities such as carbon monoxide, ammonia, nitrogen, and carbon dioxide.

When brazing in ambient air or through resistance heating, specialized brazing filler metals with suitable flux should be used. To prevent Ta or Nb from coming into contact with oxygen at high temperatures, a layer of copper or nickel should be plated on the surface, followed by appropriate diffusion annealing treatment.