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Vacuum brazing of elliptical corrugated horn
Elliptical corrugated horn is an important part of the radar feeder system. As a device for transmitting high-frequency electromagnetic waves, it has complex structure and high accuracy requirements. The traditional processing method is mechanical processing, but this is not only time-consuming and material consuming, but also the parts are heavy and the deformation is not easy to control. Therefore, vacuum brazing of multilayer elliptical corrugated horn is adopted to solve the above problems.
Because no flux is needed in the vacuum brazing process, the tedious post welding cleaning is omitted, and the problem that the brazing seam of the workpiece is corroded by flux residues is fundamentally solved. In addition, during vacuum brazing, the whole part is heated evenly, the thermal stress is small, and the deformation can be controlled to the minimum, which is suitable for brazing precision products.
The figure shows the structural diagram of elliptical corrugated horn (small section). The part material is LF21 aluminum alloy.
The whole horn consists of two parts: large section and small section, which are made of multi-layer elliptical rings (10.5mm thick) and elliptical plates (1.85mm thick) by superposition and welding, and then fastened by flange screws.
Due to the characteristics of the horn structure, the overall dimensions of each layer of elliptical ring and elliptical plate are different. To ensure the accuracy, the parts are processed by wire cutting and leveled by heat treatment. After welding, the elliptical circumferential seams are required to be dense and watertight, the tolerance of the distance between layers is not more than ± 0.15 mm, and the effective surface area of the brazing seam is not less than 90%.
The aluminum base brazing filler metal for vacuum brazing is mostly composed of aluminum silicon eutectic and a small amount of magnesium which can improve the wettability. Because magnesium is a strong activator, during brazing, magnesium vapor can not only combine with the remaining oxygen in the brazing atmosphere or the oxygen in the water vapor to protect the parts from re oxidation, but also penetrate into the oxide film on the part surface that has not been cleaned to remove it.
In addition, in order to improve the leaching and caulking ability of the filler metal, a proper amount of magnesium chips are placed on the bottom plate of the inner cavity of the horn by using the “process cover” formed by the horn itself after assembly to obtain a good brazed joint.
Vacuum brazing process
(1) Cleaning of materials before welding
Vacuum brazing process does not allow oil stain on the surface of parts. If oil stain exists, it will remain on the metal surface after heating, which will hinder the role of brazing flux and damage the wetting and spreading of liquid brazing filler metal to the base metal and good brazing seam formation.
The cleaning steps are as follows:
Parts: alkali washing (degreasing) → cold water washing → pickling (photochemical) → cold water washing → hot water washing → drying
(2) Assembly and positioning of parts
The assembly of oval horn is carried out on the bench bench, and the operation is carried out by a specially assigned person wearing gloves to prevent the sweat stains on the hands from polluting the parts to be welded. The assembly of parts adopts the self clamping method, which is fixed by the self weight of locating pins and parts. To facilitate assembly, the annular filler metal is cut according to the dimension line of the part in advance. The positioning block with corner grinding is evenly placed between the elliptical plates, which can reduce deformation and improve the accuracy of the inner cavity.
(3) Process parameters and processes
In order to obtain a good brazing seam and control the deformation of the workpiece, the brazing process parameters must be selected reasonably. The figure shows the vacuum brazing process curve of elliptical corrugated horn. The main parameters are as follows
Vacuum degree: 1.0 × 10-2～1.0 × 10-3Pa
Brazing temperature 580-604 ℃
Cooling mode: air cooling in furnace
Discharge temperature:<100 ℃
Due to the large volume of the workpiece, the gas output of the workpiece increases significantly at 520 ℃, which leads to the decrease of the vacuum degree in the furnace and affects the overflow of the filler metal. In order to improve the brazing seam forming, the holding time at 400 ℃ should be extended, and the holding time at 500 ℃ should be reduced. The vacuum is cooled to 450 ℃, the furnace is filled with high-purity nitrogen, and the fan circulation system is started to cool rapidly to below 100 ℃ for discharging.
After welding, the appearance of the brazed joint of the elliptical corrugated horn was inspected to be uniform, full and bright, and the electrical performance test and mechanical dimension accuracy measurement showed that it met the design requirements.
Vacuum brazing equipment selection: In addition to adopting correct technology, the quality of brazing furnace selected in vacuum brazing is also a crucial factor. The RVB vacuum brazing furnace produced by SIMUWU Company has the advantages of good temperature control accuracy and high heating uniformity. It is an excellent product for processing the brazing process of aluminum alloy electronic components. On the premise of selecting appropriate process parameters, high quality vacuum brazing can be achieved.
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