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Vacuum brazing of the rear passage assembly of the flame tube

In a conventional combustion chamber, the flame tube mainly acts as an air distributor, which can divide the air entering the combustion chamber into several strands and distribute the air to a suitable position to organize combustion or cool the wall of the flame tube. In the next-generation combustion chamber design using staged combustion, the combustion organization inside the combustion chamber will be gradually undertaken by the Air Module in the head. The key point of the flame tube engineering is to ensure its structural strength. Design, material selection and manufacturing process can make the wall of the flame tube obtain reasonable durability under the corresponding aerodynamic load, and provide an effective heat capacity space for combustion.

The rear passage assembly of the flame tube is a key component in the flame tube, and its success directly affects the overall quality. The structure of the channel assembly in the rear section of the flame tube is shown in the figure (hereinafter referred to as the channel assembly). Type, resistance spot welding, and then vacuum brazing. After brazing, the components must undergo 100% visual inspection and X-ray inspection, and the brazing area of the joint surface is required to reach 97% (generally brazed parts only require 70% to 80% of the brazing area). The hole exceeds the standard defect, and there should not be too many pores.

The structure diagram is as follows:

Vacuum brazing process:

1.Selection of solder

Correct selection of brazing filler metal is one of the most important factors in ensuring high quality brazed joints. During brazing, the weldment is wetted by the liquid brazing filler metal, fills the joint gap, diffuses with the base metal, and connects with the base metal after solidification. Therefore, the quality of the brazed joint is largely determined by the Choice of solder. This process uses imported Amdry 915 nickel-based solder.

2.Assembly and positioning

The structure and size of the weldment are shown in the figure above. The outer diameter of part 1 is larger than the inner diameter of part 2. Therefore, the expansion process must be used during assembly. In addition, in order to prevent the sheet metal parts from rebounding due to the material, and to ensure that the brazing gap of the brazing joint is less than or equal to 0.127mm, it is necessary to use the resistance point positioning. Swelling and spot welding are the characteristics of this brazed part, of course, it will bring a lot of trouble to the brazing process.

3.Solder state and preset

The available solder states are: powder, paste, sticky tape and foil. The solder in different states will affect the quality of the welded joint and whether it will block the pores. After the presetting of the solder, the combination of adhesive tape + paste + powder solder is finally selected; the presetting of the powder solder should be controlled powder to prevent clogging the air intake holes.

4.Preparation before welding

Vacuum brazing is an automatic brazing process, which puts forward higher requirements on the preparation of parts before entering the furnace. If the parts are not properly prepared before entering the furnace, it may lead to the failure of the brazing. In the process of processing, transportation and storage before brazing, the surface of the parts to be brazed is easy to be stained with grease and dust, etc., which will prevent the liquid brazing filler metal from spreading and caulking on the base metal.

(1) The cleaning of the flame cylinder before welding is divided into parts soaking and degreasing and wiping.

(2) The first step in cleaning the parts is degreasing. The method is: directly soak the parts in the degreaser, scrub with a silk cloth or brush, and then wipe with a silk cloth with alcohol or acetone. The specific method is: soaking and degreasing → storing in a clean plastic bag → directly presetting the adhesive tape brazing material → expanding → storing and rotating in a bag.

(3) The second step of cleaning the parts is wiping. Although some anti-fouling measures have been taken, in order to ensure the brazing quality of the parts, the parts should be cleaned again before the parts are put into the furnace, that is, wipe with acetone or alcohol to ensure the cleanliness of the brazing parts.

5.Antifouling of solder

Conventional parts can be brazed in the furnace by pre-setting the brazing filler metal at the place to be brazed before the parts are put into the furnace.

Due to the special structure of the channel assembly, the adhesive tape solder must be preset in the interlayer of the two parts before the expansion, and then the resistance spot welding positioning is performed. Therefore, the anti-fouling of the solder during the expansion and after the expansion to the tack welding is extremely important. In order to reduce the contamination of solder, the following measures have been taken:

(1) Shorten the interval time between the pre-adhesive brazing filler metal of the gas film ring and the expansion process

(2) Before expanding, the expanding mold must be cleaned and degreasing; 3. During the expanding process, use disposable plastic gloves to take parts to prevent contamination of parts and solder.

(4) Put the parts into bags for storage and transfer immediately after the expansion is completed;

(5) Do not touch the parts with bare hands during resistance tack welding and transfer parts to prevent contamination of the solder.

6.Brazing process route

According to the above analysis, the finalized brazing process route is as follows: matching → polishing → cleaning → presetting solder → assembling, expanding → cleaning → assembling, positioning welding → pliers processing → vacuum brazing (contents include: wiping → pre-processing at the front Placing paste solder → drying or drying → presetting powder solder on the leading edge → brazing in furnace) → visual inspection → X-ray inspection.

7.Choice of vacuum degree

The selection of vacuum degree is based on: the type of base metal, the type of brazing, the size of the brazing area, whether a fixture is used or not, and the degree to which the gas is discharged from the base metal during the entire brazing cycle. For the structure of the flame tube, nickel-based superalloy material AMS5536, nickel-based solder, no brazing fixture), according to the previous experience of brazing similar superalloy parts, as long as the vacuum degree is not lower than 6.67×10-2Pa during brazing, the parts Oxidation and discoloration will not occur. In the trial production, considering the outgassing of the parts and the graphite plate during the heating process, as well as the volatilization of the binder in the solder, the cold vacuum degree is required to be higher than 4X10-2Pa.

8.Heating speed and stable temperature

The brazing heating speed should be able to ensure that the gas precipitated from the parts is fully pumped away, and at the same time, the welding parts should be heated evenly, so as to reduce or prevent the deformation caused by the stress caused by the sudden heating of the welding parts. Due to the use of adhesive tape and paste solder, in order to fully volatilize the adhesive and prevent the solder splashing caused by it, the heating speed (less than 10°C/mm) below 900°C is slow, and the premise of ensuring the vacuum degree When heated to 1121 °C (stable temperature), keep the temperature for 10~20mm, on the one hand, to restore the vacuum degree, on the other hand, to make the temperature of the parts uniform before the solder melts. After the heat preservation is over, in order to prevent the growth of the base metal grains and avoid staying at a high temperature for too long, the temperature should be raised to the selected brazing temperature at the fastest speed for brazing.

9.Brazing temperature and brazing holding time

The brazing temperature is one of the most important process parameters in the brazing process. The main basis for determining the brazing temperature is the melting point of the selected brazing filler metal. The brazing temperature should be appropriately higher than the melting point of the brazing filler metal, so as to reduce the surface tension of the liquid brazing filler metal, improve the wetting and filling, and make the filler metal and the base metal better. It can fully interact with each other, which is beneficial to improve the joint strength. For nickel-based solder, the brazing temperature is preferably about tens of degrees higher than the liquidus of the solder. The second is to consider the material and structure of the brazing parts. In order to prevent excessive growth of the base metal grains of the flame tube and avoid corrosion, etc., after comprehensively considering various factors, the selected brazing temperature is 1177℃±4℃.

Brazing holding time is an important stage for solder filling and alloying control. The effect on joint strength is similar to that of brazing temperature. A certain holding time is necessary for the mutual diffusion of the solder and the base metal to form a firm bond. Excessive holding time will cause the growth of the base metal grains. Therefore, the holding time is selected to be within 30min. Generally, when there are many brazing parts, the holding time is longer and the upsetting value is larger).

10.Cooling rate

The cooling rate should be selected to meet the heat treatment requirements of the base metal, so the cooling rate should be cooled to 954°C after the furnace is cooled to 954°C, and then recharged with 0.13Pa argon for cooling, and cooled to below 100°C. The brazing process curve finally determined by the test is shown in the figure.

Process technology must be matched with excellent equipment to play a role, and the brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, the workpiece can be brazed evenly and evenly, so as to achieve high-level brazing standards, reduce scrap rates, increase production efficiency and reduce costs.