High Pressure Compressor Rectifier Vacuum Brazing

---

Aero-engine is one of the most widely used fields of brazing. Aero-engine has large thrust and high fuel temperature. Most of the structural materials used are high-temperature alloys with high content of stainless steel, aluminum and titanium, and their welding performance is generally poor. Therefore, Brazing connections are mainly made by means of vacuum or protective atmosphere furnaces. Compressor, casing, combustion chamber and other components in aero-engines are all welded structures. The high-pressure compressor rectifier studied in this paper is a type of welded structure. The high-pressure compressor rectifier is welded by the outer ring and the blades. It is one of the important parts of the stator part of the high-pressure compressor casing in the engine. Its main function is to comb and diffuse the airflow to improve the working efficiency of the engine. The overall performance and stability of the engine require very high precision.

Vacuum brazing is a process in which the parts to be welded are placed in a vacuum atmosphere without applying flux to connect the parts. It can braze those materials and structures that are difficult to connect by ordinary methods, and obtain smooth and compact, with excellent mechanical properties and corrosion resistance. performance of brazed joints [3]. The brazing deformation is small, the welding seam is smooth and beautiful, and it is suitable for welding precision, complex and parts composed of different materials. Before brazing, the workpiece must be carefully processed and strictly cleaned to remove oil stains and oxide films to ensure the smoothness and gap of the weld. The gap is generally required to be between 0.01 and 0.1mm.

It consists of an outer ring and a blade. The welding position is the contact surface between the blade crown and the outer ring as shown in the figure. The outer ring material is precipitation hardening stainless steel, and the blade is martensitic stainless steel. When machining the outer ring, the blade groove is machined on the outer ring according to the blade assembly size. In order to ensure that the blade and the blade hole on the outer ring have a good fit, the machining accuracy of the blade groove on the outer ring is very high. The technical difficulties are: the structure of the parts is a cantilever structure, the connection method of the blade and the outer ring is lap joint, the dimensional tolerance of the lap joint of the blade is large, the gap is difficult to control within the range of the brazing gap during the assembly process, the structure is complex, and the filling is difficult. The brazing material is difficult, the melting temperature of the brazing material is high, the brazing deformation is difficult to control, and the brazing quality is difficult to guarantee. The heat treatment after welding needs to take into account the mechanical properties of two different materials.

According to the structural characteristics of the parts, brazing requirements and processing accuracy requirements, the main points of the brazing process include: surface cleaning, positioning method, gap control, and filling method of solder. Control → cleaning → assembly → argon arc welding tack welding → surface treatment → filler filler material → drying → vacuum brazing → heat treatment → inspection.

Assembly, inspection:

Assemble the parts according to the design requirements, and check that there should be no defects such as bumps, rust, etc. on the parts.

Brazing Gap Control:

The brazing gap is the assembly gap between the weldments. The size of the brazing gap not only determines the performance of the brazing seam to a large extent, but also affects the difficulty of the brazing process.

The gap of the weld can be controlled by the method of trial assembly. During trial assembly, a stainless steel sheet of ≤0.1mm is inserted between the blade edge plate and the rectifier outer ring to make the gap between the blade edge plate and the rectifier outer ring evenly distributed. When the gap exceeds 0.1mm, the blade needs to be re-selected; When the thickness of the stainless steel foil is adjusted, the smaller the gap, the better, which is conducive to improving the quality requirements of brazing and recording the assembly sequence of the blades.

Surface treatment:

Before assembly, removing oil and dirt on the surface of the parts is a prerequisite for the mutual wetting of the brazing material and the base metal, and the diffusion and filling of the brazing seam. The parts are prone to oil stains and dirt during the turnover process. If the surface is not clean, a layer of oxide is easily formed on the surface during the brazing heating process, which hinders the flow and wetting of the brazing material. Therefore, before brazing, ultrasonic cleaning is used, and then Scrub the entire body with a non-chlorinated solvent (eg, acetone, alcohol), then rinse with cold or hot water. The cleaned parts are dried on the surface of the parts with a drying box or a hair dryer.

assembly:

According to the blade sequence marked after trial assembly, assemble the blades to the outer ring of the commutator, check that the gap between the blade edge plate and the outer ring of the commutator is ≤ 0.1mm, and when the gap is ≤ 0.02mm, it is allowed to slightly file the blade edge plate.

TIG welding:

Assemble the blades into the outer ring groove of the commutator according to the order of the blades marked during trial assembly, and add a stainless steel sheet of ≤0.1mm at the blade edge plate, which has improved the uniformity of the gap between the blade edge plate and the outer ring of the rectifier. After the assembly is completed , Use argon arc welding to position the blade in the outer ring groove of the rectifier to prevent loosening during the brazing process and affect the brazing quality. During argon arc welding, symmetrical welding is used to reduce welding deformation, and the size of the welding scar should be controlled within the range of 2 to 3 mm. Excessive welding tumor will affect the subsequent addition of brazing filler metal, resulting in insufficient brazing filler metal, affecting the quality of brazing and welding. The quality after machining; on the contrary, the weld bead is small and can not play the role of point positioning, the blade is easy to loosen and crack, which is not good for brazing and needs repair welding.

Cleaning:

Use a non-chlorinated solvent (such as acetone, alcohol) to scrub the blade and the outer ring of the rectifier for spot welding of argon arc welding, and then rinse with cold water or hot water.

Filling solder:

The filling principle of the solder is to use the gravity effect of the solder after melting and the capillary action of the weld as much as possible to promote the filling of the gap by the solder. Before filling the brazing filler metal, check that the brazing seam gap should meet the process requirements. Put the paste brazing filler metal or the sticky ribbon brazing filler metal diluted with acetone into the syringe and fill it to the welding point between the blade and the outer ring of the rectifier. It should be ensured that the solder is uniform and the filling amount of the solder should fill the gap and form a suitable and uniform height and width. After the brazing filler metal is filled, clean up the excess brazing filler metal in the non-brazing parts, and apply a flow blocking agent around the brazing part to prevent the brazing filler metal from flowing to the non-brazing parts during the brazing process.

drying:

Put the parts filled with the solder into the drying box to dry the adhesive glue of the solder, the temperature is 130 ~ 150 ℃, and the time is 1 ~ 2h, to prevent the glue used for bonding in the solder from being in the brazing process. Volatile, affecting the quality of brazing.

Vacuum Brazing:

Put the parts on the fixture with a flatness of ≤0.1mm, and apply a flow blocking agent on the contact surface of the fixture and the part to prevent the solder from flowing during the brazing process and stick the parts and the fixture together.

Brazing process parameters: After the parts are installed in the furnace, after vacuuming to ≤4×10-2P, start heating and heating, and then heat to a certain temperature between 500°C and 600°C at a heating rate of less than 10°C/min. For a certain period of time, it is conducive to the volatilization of the adhesive and the uniform temperature inside and outside the parts, reducing the deformation of the parts. After the heat preservation is completed, it is heated to a temperature between 900 °C and 1000 °C at a heating rate of about 10 °C/min. For a certain period of time, then heat up to the brazing temperature at a heating rate of about 10℃/min and hold for a certain period of time. After the brazing is fully melted and spread, it is cooled to about 950℃ with the furnace. Start the fan to quickly cool down to below 200 ℃ and release it for air cooling.

Heat Treatment:

The parts are made of stainless steel, brazing is equivalent to quenching, and should be tempered immediately. On the one hand, in order to eliminate the residual stress generated during brazing (quenching) and prevent deformation and cracking, on the other hand, adjust the hardness, strength, plasticity and toughness of the parts. In order to meet the requirements of performance, improve and improve processing performance, etc., tempering is an essential process for parts to obtain the required performance.

test:

After brazing, the brazing quality, deformation and hardness of the parts are inspected. The quality of the welds is very good, and the hardness meets the design requirements, but the overall deformation of the parts is very serious. Because the inner diameter of the parts has no machining allowance, it does not meet the design requirements. And the parts are partially adhered to the fixture, and the parts cannot be taken out normally. It is necessary to use the damaged fixture to protect the parts, which not only affects the delivery of the product, but also restricts the development of a certain type of machine. Analyze and improve the influencing factors, improve product quality and production efficiency.

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.