Vacuum brazing of honeycomb sealing structure

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The power turbine part of the FT8 gas turbine adopts a large number of honeycomb sealing structures. The brazing technology of honeycomb structure is a difficult new technology. Setting, control of brazing material, brazing process and equipment, inspection of brazing quality, etc.

1. Solder state and preset: Preset solder is one of the important processes of vacuum brazing, and it is also the main factor affecting the quality of brazing. FT8 honeycomb structure vacuum furnace brazing uses AMS4777 nickel-based brazing filler metal. The brazing filler metal is available in powder, paste, box and tape shapes. The presetting method of the brazing filler metal depends on the honeycomb sealing structure. The material state, the honeycomb core and the positioning method of the matrix are related.

When the honeycomb structure is brazed with powder or paste brazing filler metal, the brazing filler metal is firstly adjusted into a paste shape and evenly pre-placed on the substrate to be brazed, and then the honeycomb sandwich is positioned and assembled with a fixture for brazing. . The disadvantage is that the preset amount and uniformity of the brazing material are difficult to control, especially for large-area brazing, a brazing fixture must be used. Another presetting method for powder solder is: use the energy storage welding machine to position and assemble the honeycomb core and the base body, fix the positioned sealing assembly on the rotating mechanism and let it rotate, and use a powder feeder to remove the solder. Evenly sprinkle into the honeycomb, spray adhesive to fix the solder. After experiments, it is found that the brazing quality of this method is very ideal, but it is only suitable for the inner ring honeycomb structure or the plane honeycomb structure, and it is difficult to use the outer ring honeycomb structure and multiple honeycomb structures.

When the honeycomb sealing structure adopts box-shaped solder, there are two preset methods: one is to first position the foil-shaped solder on the base with an energy storage welding machine, then assemble the honeycomb core, and then fix it with a clamp. The other is to directly pre-install the solder between the dry honeycomb sandwich core and the substrate, and fix it with a clamp. Compared with powder or paste solder, when foil solder is used, the preset amount is easy to control and the method is simple. However, because it can only be fixed with a clamp, it is difficult to guarantee the solder for a large and complex sealing component such as FT8. Therefore, the foil solder is only suitable for the production of single or small batches of relatively simple segments or single honeycomb structures.

Tape-shaped solder is a kind of solder with uniform thickness formed by mixing powder solder and binder together. Secure with clamps. The other is to extrude the brazing filler metal into the honeycomb, and then use the energy storage welding machine to position the honeycomb core with the brazing filler metal on the substrate, and directly enter the furnace for brazing. The advantage of this preset method is that the brazing filler metal is Preset is uniform, suitable for sealing components of various structures, and it is easy to ensure the quality of brazing. The disadvantage is that it is difficult to extrude the solder, especially for the honeycomb structure of small honeycomb size.

2. Positioning of honeycomb core and matrix
The positioning and assembly of the honeycomb core and the matrix is ​​also one of the important processes of vacuum brazing. The test found that the fixture positioning has certain limitations. It is only suitable for simple honeycomb structures that are easy to design tooling, and the brazing quality is not easy to guarantee. The energy storage welding positioning is suitable for various honeycomb structures, and the brazing quality is easy to ensure. And it is easy to install and come out of the oven.

Due to the limitation and unreliability of fixture positioning, it is more appropriate to use energy storage welding to position and assemble the honeycomb core and the matrix.
For brazing in FT8 honeycomb structure furnace, using energy storage welding for positioning, two problems must be solved:

1) How firm is the solder joint. After repeated tests, it was found that the solder joints can meet the requirements as long as they cannot be pulled off with a little force.

2) How the positioning solder joints are distributed. Because the distribution of positioning solder joints is unreasonable, in the brazing process, some parts will be pulled apart from the matrix and the honeycomb sandwich due to thermal deformation and other reasons, resulting in too large brazing gap and cannot be brazed. In order to avoid this phenomenon, the method of denser distribution of edge positioning solder joints and less intermediate positioning solder joints can be adopted.

The use of energy storage welding positioning has many advantages: the quality of brazing is easy to ensure, and it is very convenient for brazing and welding of honeycomb structures of any shape and structure. Rarely occurs in some parts of the fixture positioning, especially the phenomenon that the matrix and the honeycomb core of the intermediate honeycomb are pulled apart during the brazing process and cannot be brazed, so the repair rate is much lower than that of the fixture positioning and the positioning accuracy is high. The disadvantage is that the depth of burning loss at the contact between the honeycomb core and the electrode is large.

Brazing process and equipment:
The vacuum brazing procedure of the honeycomb sealing assembly is similar to the general joint brazing procedure, that is, supporting grinding, cleaning, drying, pre-brazing, assembly, energy storage welding positioning of the matrix and the honeycomb core, drying, brazing in the furnace, Release, quality inspection, etc.

FT8 honeycomb sealing components are brazed in furnace, and AMS4777 nickel-based brazing filler metal is selected for the design, and the brazing process is carried out according to the PWA10 brazing technical specification. Since the honeycomb structure is different from the general joint, the brazing process parameters are different. After repeated tests, it is considered that the brazing temperature is 910~950℃, and it is more suitable to keep the temperature at 1030~1050℃ for 5~10min.

The equipment has a great influence on the brazing quality, especially the furnace temperature uniformity and vacuum degree of the equipment. The furnace temperature uniformity should reach ±10℃, the air leakage of the furnace body should be less than 60 microns per hour3, and the vacuum degree should be kept above 5*104 Torr during the heating and heat preservation process, so as to meet the guarantee of PWA-10, VIS -144 and other related brazing quality standards.

Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.