Brazing of Superalloys

Brazing of Superalloys

(1) Brazing characteristics superalloys can be divided into three categories: nickel base, iron base and cobalt base. They have good mechanical properties, oxidation resistance and corrosion resistance at high temperatures. Nickel base alloy is the most widely used in practical production.

The superalloy contains more Cr, and Cr2O3 oxide film which is difficult to remove is formed on the surface during heating. Nickel base superalloys contain Al and Ti, which are easy to oxidize when heated. Therefore, to prevent or reduce the oxidation of superalloys during heating and to remove the oxide film is the primary problem during brazing. As borax or boric acid in the flux can cause corrosion of the base metal at the brazing temperature, the boron precipitated after the reaction can penetrate into the base metal, resulting in intergranular infiltration. For cast nickel base alloys with high Al and Ti contents, the vacuum degree in hot state shall not be less than 10-2 ~ 10-3pa during brazing to avoid oxidation on the alloy surface during heating.

For solution strengthened and precipitation strengthened nickel base alloys, the brazing temperature should be consistent with the heating temperature of solution treatment to ensure the full dissolution of alloy elements. The brazing temperature is too low, and the alloy elements cannot be completely dissolved; If the brazing temperature is too high, the base metal grain will grow up, and the material properties will not be restored even after heat treatment. The solid solution temperature of cast base alloys is high, which generally will not affect the material properties due to too high brazing temperature.

Some nickel base superalloys, especially precipitation strengthened alloys, have the tendency of stress cracking. Before brazing, the stress formed in the process must be fully removed, and the thermal stress should be minimized during brazing.

(2) Brazing material nickel base alloy can be brazed with silver base, pure copper, nickel base and active solder. When the working temperature of the joint is not high, silver based materials can be used. There are many kinds of silver based solders. In order to reduce the internal stress during brazing heating, it is best to choose the solder with low melting temperature. Fb101 flux can be used for brazing with silver base filler metal. Fb102 flux is used for brazing precipitation strengthened superalloy with the highest aluminum content, and 10% ~ 20% sodium silicate or aluminum flux (such as fb201) is added. When the brazing temperature exceeds 900 ℃, fb105 flux shall be selected.

When brazing in vacuum or protective atmosphere, pure copper can be used as brazing filler metal. The brazing temperature is 1100 ~ 1150 ℃, and the joint will not produce stress cracking, but the working temperature shall not exceed 400 ℃.

Nickel base brazing filler metal is the most commonly used brazing filler metal in Superalloys because of its good high temperature performance and no stress cracking during brazing. The main alloy elements in nickel base solder are Cr, Si, B, and a small amount of solder also contains Fe, W, etc. Compared with ni-cr-si-b, b-ni68crwb brazing filler metal can reduce the intergranular infiltration of B into the base metal and increase the melting temperature interval. It is a brazing filler metal for brazing high-temperature working parts and turbine blades. However, the fluidity of W-containing solder becomes worse and the joint gap is difficult to control.

The active diffusion brazing filler metal does not contain Si element and has excellent oxidation resistance and vulcanization resistance. The brazing temperature can be selected from 1150 ℃ to 1218 ℃ according to the type of solder. After brazing, the brazed joint with the same properties as the base metal can be obtained after 1066 ℃ diffusion treatment.

(3) Brazing process nickel base alloy can adopt brazing in protective atmosphere furnace, vacuum brazing and transient liquid phase connection. Before brazing, the surface must be degreased and oxide removed by sandpaper polishing, felt wheel polishing, acetone scrubbing and chemical cleaning. When selecting brazing process parameters, it should be noted that the heating temperature should not be too high and the brazing time should be short to avoid strong chemical reaction between the flux and the base metal. In order to prevent the base metal from cracking, the cold processed parts shall be stress relieved before welding, and the welding heating shall be as uniform as possible. For precipitation strengthened superalloys, the parts shall be subject to solid solution treatment first, then brazed at a temperature slightly higher than the aging strengthening treatment, and finally aging treatment.

1) Brazing in protective atmosphere furnace brazing in protective atmosphere furnace requires high purity of shielding gas. For superalloys with w (AL) and w (TI) less than 0.5%, the dew point shall be lower than -54 ℃ when hydrogen or argon is used. When the content of Al and Ti increases, the alloy surface still oxidizes when heated. The following measures must be taken; Add a small amount of flux (such as fb105) and remove the oxide film with flux; 0.025 ~ 0.038mm thick coating is plated on the surface of parts; Spray the solder on the surface of the material to be brazed in advance; Add a small amount of gas flux, such as boron trifluoride.

2) Vacuum brazing vacuum brazing is widely used to obtain better protection effect and brazing quality. See table 15 for the mechanical properties of typical nickel base superalloy joints. For superalloys with w (AL) and w (TI) less than 4%, it is better to electroplate a layer of 0.01 ~ 0.015mm nickel on the surface, although the wetting of solder can be ensured without special pretreatment. When w (AL) and w (TI) exceed 4%, the thickness of nickel coating shall be 0.020.03mm. Too thin coating has no protective effect, and too thick coating will reduce the strength of the joint. The parts to be welded can also be placed in the box for vacuum brazing. The box should be filled with getter. For example, Zr absorbs gas at high temperature, which can form a local vacuum in the box, thus preventing the oxidation of the alloy surface.

Table 15 mechanical properties of Vacuum Brazed Joints of typical nickel base superalloys

The microstructure and strength of the brazed joint of Superalloy change with the brazing gap, and the diffusion treatment after brazing will further increase the maximum allowable value of the joint gap. Taking Inconel alloy as an example, the maximum gap of Inconel joint brazed with b-ni82crsib can reach 90um after diffusion treatment at 1000 ℃ for 1H; However, for the joints brazed with b-ni71crsib, the maximum gap is about 50um after diffusion treatment at 1000 ℃ for 1H.

3) Transient liquid phase connection transient liquid phase connection uses the interlayer alloy (about 2.5 ~ 100um thick) whose melting point is lower than the base metal as the filler metal. Under a small pressure (0 ~ 0.007mpa) and an appropriate temperature (1100 ~ 1250 ℃), the interlayer material first melts and moistens the base metal. Due to the rapid diffusion of elements, isothermal solidification occurs at the joint to form the joint. This method greatly reduces the matching requirements of the base metal surface and reduces the welding pressure. The main parameters of transient liquid phase connection are pressure, temperature, holding time and composition of interlayer. Apply less pressure to keep the mating surface of the weldment in good contact. Heating temperature and time have a great impact on the performance of the joint. If the joint is required to be as strong as the base metal and does not affect the performance of the base metal, the connection process parameters of high temperature (such as ≥ 1150 ℃) and long time (such as 8 ~ 24h) shall be adopted; If the connection quality of the joint is reduced or the base metal cannot withstand high temperature, a lower temperature (1100 ~ 1150 ℃) and a shorter time (1 ~ 8h) shall be used. The intermediate layer shall take the connected base metal composition as the basic composition, and add different cooling elements, such as B, Si, Mn, Nb, etc. For example, the composition of Udimet alloy is ni-15cr-18.5co-4.3al-3.3ti-5mo, and the composition of intermediate layer for transient liquid phase connection is b-ni62.5cr15co15mo5b2.5. All these elements can reduce the melting temperature of Ni Cr or Ni Cr Co alloys to the lowest, but the effect of B is the most obvious. In addition, the high diffusion rate of B can rapidly homogenize the interlayer alloy and base metal.