Brazing of high temperature alloys: explained

1. Brazing Characteristics

High temperature alloys can be divided into three main categories: nickel-based, iron-based and cobalt-based. They exhibit good mechanical properties, oxidation resistance and corrosion resistance at high temperatures. Nickel-based alloys are the most commonly used in practical applications.

High-temperature alloys contain a significant amount of chromium (Cr), which forms a difficult-to-remove Cr ₂ Ó ₃ oxide film on the surface during heating. Nickel-based high-temperature alloys also contain aluminum (Al) and titanium (Ti), which are prone to oxidation during heating.

Therefore, preventing or minimizing oxidation and oxide film removal is crucial in brazing high-temperature alloys. For nickel-based cast alloys with high Al and Ti content, it is necessary to ensure a vacuum level of not less than 10-2 to 10-3 Pa during heating to prevent surface oxidation of the alloy.

For solid solution strengthened and precipitation strengthened nickel-based alloys, the brazing temperature should be selected to be consistent with the heating temperature during solid solution treatment to ensure complete dissolution of the alloying elements.

Too low a temperature can result in incomplete dissolution, while too high a temperature can cause grain growth in the base material, making it impossible to restore the material's properties even with subsequent heat treatment. Cast alloys generally exhibit high solid solution temperatures that are not significantly affected by brazing temperatures.

Some nickel-based high-temperature alloys, especially precipitation-strengthened alloys, are prone to stress cracking.

Therefore, it is necessary to remove the stresses formed during processing before brazing and minimize thermal stresses during brazing.

2. Brazing Materials

Silver-based, pure copper, nickel-based and active brazing alloys can be used for brazing nickel-based alloys.

When the operating temperature of the joint is not high, silver-based materials can be used. There are a wide variety of silver-based brazing materials available, and to minimize internal stress during heating, it is advisable to choose alloys with a low melting temperature.

For brazing precipitation-strengthened high-temperature alloys with high aluminum content, FB102 brazing flux should be used and 10-20% sodium fluoroaluminate or aluminum flux (such as FB201) should be added. When the brazing temperature exceeds 900°C, brazing flux FB105 should be chosen.

In vacuum brazing or in a protective atmosphere, pure copper can be used as brazing material. The brazing temperature is in the range of 1100-1150 ℃ and no stress cracking occurs in the joint. However, the operating temperature must not exceed 400°C.

Nickel-based brazing alloys have excellent high-temperature properties and are commonly used for brazing high-temperature alloys.

The main alloying elements in nickel-based brazing alloys are chromium (Cr), silicon (Si), and boron (B), with a small amount of iron (Fe), tungsten (W), and other elements. B-Ni68CrWB brazing alloy is suitable for brazing high-temperature components and turbine blades as it reduces the intergranular penetration of boron into the base material and has a wider melting temperature range.

However, brazing alloys containing tungsten have reduced fluidity, making it more difficult to control joint clearance.

Active diffusion brazing alloys do not contain silicon and have excellent resistance to oxidation and sulfidation. The brazing temperature can be selected between 1150-1218°C depending on the type of brazing alloy. After brazing, diffusion treatment at 1066°C can result in welded joints with properties consistent with the base material.

3. Brazing Process

Nickel-based alloys can be brazed using methods such as protective atmosphere furnace brazing, vacuum brazing, and liquid phase transient bonding.

Before brazing, it is necessary to remove surface grease and oxides by sanding, polishing with a polishing wheel, cleaning with acetone and chemical cleaning.

When selecting brazing process parameters, it is important to avoid excessively high heating temperatures and keep the brazing time short to avoid strong chemical reactions between the brazing filler metal and the base material.

To avoid cracks in the base material, pre-stress relief treatment must be carried out on parts that have been subjected to cold working before brazing. Heating during welding must be as uniform as possible.

For precipitation-strengthened high-temperature alloys, the parts must first be solution treated, then welded at a temperature slightly higher than the aging treatment temperature, and finally subjected to aging treatment.

1) Brazing in a Furnace with Protective Atmosphere

Brazing in a furnace with a protective atmosphere requires high purity of the protective gas. For high-temperature alloys with W (Al) or W (Ti) content less than 0.5%, when using hydrogen or argon gas, the dew point must be below -54°C.

As the content of Al and Ti increases, oxidation still occurs during heating of the alloy surface. The following measures should be taken: add a small amount of brazing flux (such as FB105) to remove the oxide film; application of a coating of 0.025 to 0.038 mm thick on the surface of the parts; pre-application of brazing filler metal on the surface of the material to be brazed; using a small amount of gas flow such as boron trifluoride.

2) Vacuum Brazing

Vacuum brazing is widely used and provides better protection and brazing quality. The mechanical properties of typical nickel-based high-temperature alloy joints are shown in Table 15.

For high-temperature alloys with w(Al) or w(Ti) less than 4%, brazing filler metal can wet the surface even without special pretreatment, but it is preferable to electroplate a nickel layer of 0.01 to 0.015 mm thick on the surface.

When w(Al) or W(Ti) exceeds 4%, the thickness of nickel plating should be 0.020.03mm. A coating that is too thin does not provide sufficient protection, while a coating that is too thick can reduce the strength of the joint.

Vacuum brazing can also be carried out by placing the parts to be welded in a box together with an absorber, such as Zirconium (Zr), which absorbs high-temperature gases and creates a partial vacuum inside the box, thus preventing surface oxidation. of the league. .

Table 15: Mechanical properties of typical vacuum brazed joints in high temperature nickel-based alloys

The microstructure and strength of high-temperature alloy welded joints may vary with the brazing gap. Post-braze diffusion treatment can further increase the maximum allowable joint gap value.

Taking Inconel alloy as an example, for Inconel joints welded with B-Ni82CrSiB, the maximum gap value after diffusion treatment at 1000°C for 1 hour can reach about 90um. On the other hand, for joints welded with B-Ni71CrSiB, the maximum gap value after diffusion treatment at 1000°C for 1 hour is about 50um.

3) Liquid phase transient connection

Liquid-phase transient bonding involves the use of an alloy interlayer with a lower melting point than the base material (thickness approximately 2.5 to 100um) as the brazing filler metal. Under low pressure (0 to 0.007 MPa) and suitable temperature (1100 to 1250°C), the middle layer material first melts and wets the base material.

Due to the rapid diffusion of the elements, the joint solidifies isothermally to form the joint. This method significantly reduces the surface fit requirements of the base material and reduces welding pressure. The key parameters for transient binding of the liquid phase include pressure, temperature, retention time, and interlayer composition.

The application of low pressure ensures good contact between the articular surfaces. Temperature and heating time greatly influence joint performance. If the joint needs to have similar strength to the base material without affecting its properties, high temperature (e.g. ≥1150°C) and a long time (e.g. 8 to 24 hours) should be used as process parameters. collage.

If a slight decrease in joint quality is acceptable or if the base material cannot withstand high temperatures, lower temperatures (1100 to 1150°C) and shorter times (1 to 8 hours) should be used. The composition of the intermediate layer should be based on the composition of the base material to be joined, with additional alloying elements such as B, Si, Mn, Nb, etc.

For example, if the composition of the Udimet alloy is Ni-15Cr-18.5Co-4.3Al-3.3Ti-5Mo, the composition of the interlayer used for liquid phase transient bonding would be B-Ni62.5Cr15Co15Mo5B2.5. These added elements can lower the melting temperature of Ni-Cr or Ni-Cr-Co alloys, with B having the most significant reducing effect.

Furthermore, B has a high diffusion rate, allowing rapid homogenization of the intermediate layer alloy and the base material.