Vacuum Brazing Technology of Stainless Steel Radiator Core

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The stainless steel high-pressure radiator of the fuel cooling lubricating oil system in the aircraft engine is an indispensable part of the engine. The radiator has a tubular structure. In order to meet the heat exchange requirements, hundreds of 0.2mm thin-walled thin tubes are used, and they are welded together with a 3.0mm thick end plate with a tube spacing of 1mm to form a group of dense plug-in joints, which withstand high pressure, vibration and thermal shock loading. This structural feature brings a series of difficulties to the brazing process, mainly including: thin tube wall, which requires the use of brazing filler metal with weak corrosion resistance and small intergranular penetration; many joints, which require successful welding at one time, otherwise repair welding will cause the tube to be damaged. Deformation affects the performance of the radiator; the tube spacing is small, which makes assembly difficult, and appropriate solder types and special assembly processes must be selected; the joint is subject to complex stress, requiring joint sealing, pressure resistance and shock resistance. Therefore, it is required to use high-strength solder, and to form the joint on both sides during the welding process, so as to form an excellent joint with continuous round roots. Therefore, the brazing process of the radiator core is studied, and small batch production is carried out, and the process document is proposed. The nearly 100 sets of products submitted have passed the air tightness, pressure resistance, earthquake resistance and thermal shock tests, and have been installed for long-term test run and flight test, which have met the design requirements, proving that this research is successful.

1 Board  2 Tube  3 Solder

Process flow

1. Materials

The pipe material is 1Cr18Ni9Ti of φ2mm×0.2mm, and the plate is 1Cr18Ni9Ti of thickness 3.0mm and 1.5mm. The final selected brazing filler metal is HBNi82CrSiB, and the particle size is 150 mesh and 300 mesh.

2. Solder

Brazing stainless steel is commonly used with copper-based, silver-based and nickel-based solders. Due to the high strength of nickel-based solders, good air-tightness of joints, and low cost, it is suitable for vacuum brazing. Therefore, nickel-based solders are used for testing. and analyse.

The core of the radiator is a welded joint of a thin-walled tube and a thick end plate. When selecting a solder, in addition to the general requirements for brazing process, the melting point, corrosion resistance and intergranular infiltration tendency of the solder should be considered. The melting point of HBNi82CrSiB is the lowest, and the brazing temperature can be controlled below 1080℃, which can prevent the grain growth of the 1Cr18Ni9Ti tube and control the deformation of the product. No corrosion was found in the test, and the intergranular infiltration was very small. By adjusting the brazing temperature, holding time and the amount of solder added, the intergranular infiltration and corrosion of the solder can be controlled, so that the radiator can meet the requirements of use.

There are many kinds of brazing filler metal, including powder, sticky tape and amorphous tape. The test found that it is very difficult to assemble the powder solder between pipes with a spacing of 1mm, and it is easy to leak the solder into the pipe, and it is difficult to control the amount and uniformity of the solder. It is required to successfully solder thousands of joints at one time. difficult to achieve. Amorphous tape can overcome the problem of uneven solder, but due to the thin tape, it is necessary to use multiple layers, and it is necessary to locate and drill thousands of φ2.1 holes, which is obviously unrealistic. The thickness of the adhesive tape can be selected arbitrarily. Once the thickness is determined, the amount and uniformity of the solder added can be controlled. The adhesive tape solder is soft and has a certain stiffness, which is convenient to assemble and easy to grasp. After testing, one-time welding of high-quality joints and thousands of joints can be obtained. It is determined from this that the HBNi82CrSiB-ZD adhesive tape solder with a thickness of 0.3mm is selected for the product.

3. Brazing process

Regarding the determination of the heating rate and cooling rate, the main consideration is that the adhesive tape solder is used, and it needs to be kept for about 20 minutes (or slowly heating) at 400-600 °C, so that the binder can be completely volatilized to maintain a high degree of vacuum. Keep the temperature at 950°C for 15~20min to ensure the temperature of the parts is uniform. After this temperature, the temperature rises rapidly to reach the brazing temperature. At 1060~1070°C, keep the temperature for 7~10min, which can shorten the high temperature residence time and reduce intergranular infiltration and corrosion. , which is conducive to improving the quality of the joint. The optimal value of the brazing gap is determined to be 0.1mm, and the gap strength is the highest at this time, reaching the level of 191MPa.

The RHVB-H high temperature vacuum brazing furnace produced by SIMUWU is a high-quality product for this type of brazing process. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. With more than ten years of sales and manufacturing experience, it is exported to developed regions in Europe, America and Asia. It is a widely acclaimed vacuum furnace product.