Vacuum Brazing Process of Slide Valve Sub-assemblies

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Vacuum brazing is a process method for welding parts without applying flux in vacuum. It can be used for materials and structures that are difficult to weld. It can obtain smooth and compact brazed joints with excellent mechanical properties and corrosion resistance, and the thermal stress of the joint is small. , the deformation amount can be controlled within a very small range.

The pressure ratio regulator of the accessories of the afterburner control system of the aircraft engine realizes the adjustment function by driving the air piston to rotate (700r/min) through the slide valve sub-assembly. Therefore, the slide valve sub-assembly carries a large torque during work. The welding of the slide valve body and the tube of the slide valve sub-assembly is realized by a vacuum brazing process.

Solder selection:

Under the condition that the brazing filler metal meets the strength of the joint after brazing, the brazing temperature of the brazing filler metal should be lower than the solution temperature of the pipe (1030°C), and the brazing filler metal should also satisfy the quenching-tempering and tempering treatment of the spool valve body after welding. During the process (holding at 910 ℃ for 30 to 40 minutes), the brazing seam is not remelted. Therefore, the selected brazing temperature should be between 910 and 1000 °C. Within this range, the brazing filler metal can easily fill the brazing gap under the capillary action, and has a good alloying effect with the base metal, thereby forming a joint with sufficient strength. In order to prevent contamination of the vacuum system, the solder components are not allowed to contain volatile elements such as Zn, Cd, Li and other volatile elements or pure metals with high vapor pressure.

Brazing process

In addition to the correct selection of process parameters such as vacuum degree, brazing temperature, holding time, etc., which are decisive factors, factors such as brazing gap, argon gas purity, cleanliness of parts, heating rate, cooling rate, etc. will also seriously affect the brazing quality.

Brazing gap

There are various factors affecting the brazing gap. Process tests are often used to determine the results in production, rather than calculating the results through formulas. Therefore, the brazing gap of silver-based solder should mainly consider several aspects:

When silver-based solder is used, the brazing gap is usually selected between 0.02 and 0.15mm. When the assembly gap is less than 0.04mm, the shear strength is large. The physical and chemical properties of the solder to the base metal and the interaction characteristics during the brazing process. The solder has good wettability to the base metal, and the brazing gap can be smaller. For brazing dissimilar materials, the selection of the brazing gap is suitable for brazing. Based on materials with poor properties. If the surface of the brazing body is smooth and clean, the capillary caulking effect will be poor, and the liquid solder is difficult to flow on the entire joint surface, and it is easy to generate holes and reduce the strength of the joint. Therefore, the brazing surface should be appropriately roughened. For brazing joints with long brazing channels, the brazing gap should be appropriately increased, and there should be a better matching value.

Through experiments, it is determined that the vacuum brazing gap between the slide valve body and the tube is between 0.06 and 0.1mm, and the ideal range should be between 0.04 and 0.08mm. Reaming is required to make this gap more reasonable.

Purity of Argon

High-purity argon is used as a medium to reduce the evaporation pressure and forced cooling of the liquid solder during the vacuum brazing process. At high temperature, a small amount of oxygen and moisture will oxidize and discolor the brazing surface, increase the surface tension of the liquid brazing filler metal, reduce the wetting performance, and weaken the capillary action, thereby affecting the filler metal performance. Under normal circumstances, the cooling vacuum should be 5×10~Pa, the furnace temperature should be above 1000℃, the oxygen content in argon should be less than 4ppm, and the moisture should not be more than 10ppm. At present, 99.99% high-purity argon is used, which can meet the process requirements.

Cleanliness

Be sure to do a good job of degreasing, dehydration, and cleaning processes before brazing, and do not allow re-contamination before entering the furnace. When assembling parts and placing solder rings, be sure to wear clean cotton gloves.

Furnace temperature control

The heating temperature is closely related to the structure of the parts to be brazed, the solder used and the placement. Generally, it includes heating rate, brazing temperature, etc. The setting of each stage has an extremely important effect on the entire process.

Heating power

The welding material (base metal) is composed of lCrl8Ni9Ti stainless steel and alloy steel 25Cr3MoA, and this austenitic stainless steel above 900P is a dangerous area for carbide precipitation, so it should be heated quickly in this area during the heating process: but when assembling The brazed joints are almost completely shielded and cannot be directly heated by radiation. The holding time should be appropriately extended to reduce the surface temperature difference of the parts. The two heating methods cause process contradictions. Therefore, when the brazing is heated to a temperature close to the solidus line of the solder, the heating is suspended and the temperature is kept stable for a period of time. The purpose is to reduce the temperature gradient between the parts and make the welding parts. The temperature of each part is consistent, which better solves the contradiction between the poor thermal conductivity of stainless steel and the complete shielding of brazing joints. The stable temperature is set at (850 ± 10) degrees Celsius, and high-purity argon is filled after holding for 25 to 30 minutes, so that the pressure in the furnace rises to more than 10Pa. Prevent the precipitation of base metal carbides and the evaporation of the main elements of the solder under high vacuum and high temperature.

Brazing temperature and time

The brazing temperature and brazing time are the key process parameters of the whole process. Generally, for austenitic stainless steel, it is better to choose 30-50 ℃ higher than the liquidus phase of the brazing filler metal, and keep the temperature for 5-7 minutes. The temperature will evaporate the components of the low melting point of the solder, and also make the grains of the base metal grow, which will destroy the strength of the base metal.

Cooling temperature

The cooling temperature also has a great influence on the quality of the brazed joint. Too slow cooling will cause the strength of the base metal to decrease. In general, accelerated cooling is beneficial to improve the strength of the joint. However, because the thermal expansion coefficients of the two base metals are different, excessive cooling is prone to cracks and pores.

Device Selection

The RHVB-H high temperature vacuum brazing furnace produced by SIMUWU is a high-quality product for this kind of brazing process. It adopts PLC automatic control, and the good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU has more than ten years of experience in vacuum furnace sales and manufacturing, and is exported to developed regions in Europe, America and Asia. It is a well-received vacuum furnace manufacturer.