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Applied Crack Repair Technology of Vacuum Brazing

Speaking of the perception of soldering, the scene of connecting electronic components with electric soldering iron and tin wire will appear in everyone’s mind, which is the most common welding method in soldering connection. Brazing is divided into hard brazing and soft soldering according to the different melting points of the solder: that is, brazing with a melting point temperature lower than 450°C is called soft soldering, and tin brazing belongs to soft soldering; Brazing with a melting point temperature greater than 450°C is called brazing, and flame brazing, induction brazing, and vacuum brazing are all brazing.

In the field of aero-engine processing and manufacturing, vacuum brazing technology is widely used due to its incomparable advantages, such as vacuum brazing of engine honeycomb rings, vacuum brazing of turbine guide vane guide tubes, and blade brazing. Restoration and many other cases.

The principle of vacuum brazing: Vacuum brazing generally uses a vacuum brazing furnace as a heating device, and places the parts to be welded with assembled solder in a vacuum brazing furnace. Keep warm until the brazing temperature point is reached. When the brazing temperature point is reached, the brazing material starts to melt but the base metal does not melt. The liquid metal wets the part to be welded, and the molten metal fills the entire weld through capillary flow, thus forming Strong brazed joints.

Advantages of vacuum brazing:

After understanding the principle of vacuum brazing, we can find that its biggest advantages include almost no stress deformation, excellent welding performance of dissimilar materials, oxidation resistance, etc., so it is widely used in parts with complex structures, internal channels and thin-walled features of welding.

After understanding the advantages of vacuum brazing, we found that the existence of vacuum brazing technology has solved many problems in the field of processing and manufacturing aviation parts. One typical application is the repair of cracks in blades. Cracks are one of the main damage forms of failure of engine hot-end components after long-term operation.

Usually we have two ways to repair cracks, one is welding repair, and the other is vacuum brazing repair. Since fusion welding uses local heating in the repair process, large deformations will occur in the weld seam and the heat-affected zone, and new cracks may also be generated for thin-walled parts. The vacuum brazing repair adopts overall heating, and the deformation of the parts is small. Therefore, the main method of repairing the hot end parts is vacuum brazing repair.

Vacuum brazing repair process:

It mainly includes four processes of cleaning, crack filling, vacuum brazing and post-welding treatment.

Parts cleaning methods include pickling to peel off the protective aluminum-silicon coating, fluoride ion cleaning to remove oxides at residual cracks, and high-temperature purification treatment to remove residual carbon deposits on the surface to keep the surface of the parts clean.

The solder used for crack filling generally uses low-melting-point alloy powder similar to the blade base material, adding strengthening elements such as chromium, tungsten, tantalum, and cobalt, and boron elements for lowering the melting point. Dilute the solder with acetone and assemble it into a syringe, aim the needle of the syringe at the crack site, and slowly cover the solder on the surface of the crack.

Then put the assembled parts in the vacuum furnace. By turning on the mechanical pump, rotary vane pump and Roots pump in turn, the vacuum in the furnace is pumped down to less than 2×10^-2Pa. The furnace body starts to heat up at a constant speed, and the temperature in the furnace reaches vacuum. At the brazing heat preservation temperature point, the brazing filler metal reaches the liquidus temperature and melts into a liquid metal, which fills the entire crack space under the capillary action of the crack site. This uniform diffusion process allows the filler metal to diffuse into the superalloy powder and the surrounding base metal while allowing residual boron to diffuse away from the weld zone. After the vacuum brazing is completed, it is restored to the size close to the drawing by fitter trimming.

Vacuum Brazed Joints Hot end parts – guide vanes

The application of vacuum brazing crack repair technology can not only reduce operating costs, but also improve the reliability of aero-engines, and plays an important role in the field of aero-engine processing and manufacturing.