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Vacuum Brazing of Aviation Diffuser Components
With the upgrading of aero-engine technology, the structure of the diffuser assembly is becoming more and more complex and the processing difficulty is becoming more and more difficult, and the processing quality requirements are also getting higher and higher. The diffuser assembly consists of a diffuser and a diffuser cover, and the axial vanes of the diffuser need to pass through the leaf-shaped holes on the diffuser cover for vacuum brazing connection.
Both the diffuser and the diffuser cover are thin-walled parts, and the base material is martensitic stainless steel, which is easily deformed during the vacuum brazing process. The brazing seam of the diffuser assembly is a T-shaped joint of “mortise + corner joint”, which not only ensures the welding rate of the corner joint brazing seam and the sealing performance of the tenon and groove brazing seam, but also ensures that the brazing seam fillet in the flow channel after welding (R ) in the range of (0.45±0.25) mm, the vacuum brazing welding structure is complex and extremely difficult. In this regard, in the vacuum brazing process, the vacuum brazing quality of the parts is guaranteed by adjusting the process, controlling the amount of brazing filler metal, optimizing the process route, and improving the brazing fixture, and through the system integration of the above processes, stable and reliable production.
Technical Difficulties in Vacuum Brazing of Thin-walled Stainless Steel Diffuser Components
Partial leakage of brazing seam
Dozens of brazing seams of the diffuser assembly need to be brazed successfully at the same time. If there is leakage of one brazing seam, it needs to be rewelded in a vacuum furnace as a whole. In actual production, after vacuum brazing, some brazing seams are likely to fail the inspection of kerosene leakage, and repair welding is required, and there are many repair welding failures.
The fillet of the brazing seam is too large
In the design drawings, the size of the brazing fillet after vacuum brazing is relatively strict, the maximum is only 0.7mm, and the brazing fillet after vacuum brazing is too large, reaching more than 1mm.
Parallelism of parts out of tolerance after vacuum brazing
The design drawing requires that the parallelism of the final flow path of the part relative to the A reference be 0.05, which must be controlled in the vacuum brazing process, and the parallelism after vacuum brazing is above 0.9, far exceeding this design requirement.
General idea of technical improvement
Research on surface modification of the part to be welded
The airfoil hole of the diffuser cover is processed by laser cutting, and there is a recast layer and surface oxidation on the surface of the airfoil hole, which will affect the wetting of the solder. Therefore, it is necessary to use a grinder to remove the recast layer and scale on the surface, but a large amount of grinding removal by the grinder will cause the local brazing gap to be too large, resulting in failure of the leakage inspection after vacuum brazing. However, if the sandpaper is cut into strips and penetrated into the airfoil hole for grinding, the removal margin is small and the impact on the gap between the brazing joints is small. Therefore, use sandpaper to grind and clean the leaf hole.
Research on feeding amount control
Since the design pattern has relatively high requirements on the fillet of the brazing seam after vacuum brazing, it is necessary to control the feeding amount at the brazing seam. The thicknesses of the two commonly used sticky solders are 0.15mm and 0.25mm respectively. In order to control the feeding amount, the adhesive tape solder with a thickness of 0.15mm is selected on the end surface of the blade (in the flow channel), and the adhesive tape solder with a thickness of 0.25mm is selected at the brazing seam (outer surface) where the blade protrudes from the leaf hole. And add a certain amount of paste solder to ensure that the brazing seam fillets that meet the design requirements are formed after brazing.
Research on pre-vacuum brazing process
Add single-piece pre-brazing process before the vacuum brazing of the diffuser assembly, and make the pre-brazing temperature equal to the vacuum brazing thermal cycle temperature, both of which are set at 1030-1060 °C. Adding single-piece pre-brazing is theoretically conducive to fully releasing the stress of the parts at high temperatures and reducing the deformation of the parts during subsequent vacuum brazing.
The overall technological process is single-piece rough machining, stabilization treatment, single-piece pre-brazing, single-piece finishing, component vacuum brazing, and component finishing.
Improved Vacuum Brazing Fixture
Due to the deformation of the diffuser assembly during the vacuum brazing process, the deformation of the largest mounting edge exceeds 1mm, and the remaining machining allowance is only 0.5mm, so it is necessary to improve the fixture to limit the deformation of the mounting edge. out of shape. One part of the fixture is fitted to the part, and the other part is left with a certain gap with the part. When the part is deformed greatly, the gap left by the fixture can limit the deformation of the part to the direction of the fixture.
The parts are inspected according to the relevant standards, among which there is no leakage in the kerosene leakage inspection, and the fillet of the brazing joint is controlled at 0.25-0.65mm, which meets the design requirements. The vacuum brazing deformation of parts is controlled within the range of parallelism 0.2, and the parts are qualified for inspection, with a pass rate of 100%. The qualified rate of one-time brazing of parts has increased from 32.3% to 87.5%; the scrap rate has been reduced from 11.8% to 0; the processing cycle has been shortened by more than 40.8%.
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