Aluminum vacuum brazing furnace and welding technology

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Aluminum vacuum brazing furnace and welding technology introduction

In recent years, aluminum material vacuum brazing technology has been widely used. SIMUWU brand vacuum brazing furnace has the characteristics of good vacuum degree and excellent temperature uniformity, which are deeply loved by customers. The following is an introduction to the process of vacuum brazing aluminum products by the brazing engineer of Shanghai Gehang Vacuum Technology Co., Ltd.

1. Preparation of vacuum brazing
1) If the vacuum brazing furnace is shut down for more than 24 hours, it is required to bake at a temperature of not less than 450±20℃ for one hour (or 700±50℃ for 60 minutes) before vacuum brazing;
2) Before opening the vacuum brazing furnace, the diffusion pump must be preheated for at least 1 hour, and at least 70 minutes in winter;
3) The amount of magnesium powder added is proportioned according to the ratio of 15g/m3;
4) Perform debugging of each system in accordance with the aluminum vacuum brazing operating procedures, and keep the water, electricity, gas and other meters displayed normally, and the data is consistent.

2. Equipment data setting
1) Open the shut-off valves of the water cooling system to maintain the water pressure at 0.2～0.3MPa and keep the drainage unblocked. During the operation of the furnace, the inlet water temperature should not be lower than 25℃, and the outlet water temperature should not be higher than 30℃;
2) When the vacuum brazing furnace is usually out of service, the furnace needs to be vacuumed, and it should be kept in a vacuum state of not less than 5×10*-2Pa.

3. Loading to vacuum furnace chamber
1) Send the clamped product components to the central constant temperature zone of the vacuum brazing furnace with a forklift for neat stacking;
2) The stacking of workpieces should be stable, reliable and tidy;
3) Do not allow the workpiece to contact the heating element, if found, it should be eliminated and adjusted immediately;
4) Wipe clean the sealing ring and contact surface of the furnace door with alcohol or acetone, close the furnace door and tighten the buckle.

4. The method of vacuum brazing
The vacuum brazing process is in a vacuum environment (up to 6.5 × 10-3Pa). As the temperature rises above 577°C, the aluminum alloy surface oxide film gradually falls off and melts under the action of magnesium (Mg). The material forms a brazing weld. During this period, there are two processes, which can be summarized as the film removal process and the weld forming process, which can be elaborated as follows:

4.1. Vacuum brazing and film removal process
In addition to eliminating the harmful effects of oxygen and moisture in the environment, magnesium (Mg) also penetrates into the surface layer of the base material under the film, and together with the diffused silicon, the surface layer forms a low melting point Al-Si-Mg alloy and melts, thereby Destroy the combination of the surface oxide film and the base material, so that the molten solder can wet the base material, spread along the base material under the film, and float the surface film to be removed. However, the amount of Mg element in the brazing filler metal is fixed, and the film removal ability can meet the brazing requirements under the condition of a certain oxygen content. According to the test results, it is best to clean the qualified parts of the aluminum plate-fin heat exchanger within 24h. For brazing production, the longest cannot exceed 72h. Parts older than 72h need to be cleaned again. It should be avoided that the parts are placed for too long after cleaning, which will cause an oxide film on the surface of the aluminum alloy, (the thickness of the oxide film is too thick) will affect the vacuum brazing quality of the product. After the aluminum plate-fin radiator parts are cleaned, as the storage time of the parts increases, the surface oxygen content of the parts gradually increases, which will affect the quality of the vacuum brazing seam, and ultimately lead to the decline in the strength of the product and the quality and performance of the weld , Can not meet product design requirements.

Humidity is the main cause of vacuum brazing quality problems
The vacuum brazing process stipulates that the temperature of the assembly workshop is 10℃～34℃; the humidity is <50%. It is necessary to operate strictly in accordance with process specifications and strengthen process discipline inspections. After cleaning and assembling, products are dried before entering the furnace. The drying temperature is below 120°C and the drying time is 3 hours. And control to install the furnace within 24 hours for vacuum brazing to prevent the parts from being placed in a room temperature environment for too long and the surface is oxidized. The vacuum brazing furnace is closed on holidays or the vacuum brazing furnace stops working for more than 48 hours. In view of the environment and the low temperature in the furnace, the insulation layer is out of gas and moisture is returned. Arrange the aluminum alloy vacuum brazing furnace to burn for one day, if necessary Vacuum brazing should work continuously without stopping the furnace. After taking these measures, the quality of aluminum alloy vacuum brazing products is qualified and stable. For vacuum brazing operations, attention should also be paid to the working environment requirements to prevent the surface of the parts from being damp and the oxide film thickened. At the same time, the process control and process discipline inspection of the production process should be strengthened, so as to ensure stable and qualified product quality and ensure the smooth progress of enterprise production.

4.2. The forming process of vacuum brazing seam
The key to obtain good performance brazing joints is to make the molten liquid brazing material fully flow into the brazing gap, and perform physical and chemical interactions with the base material well to obtain satisfactory brazing joints after condensation and crystallization. The formation of aluminum alloy vacuum brazing brazing seam can be roughly divided into three steps:
One is the removal of the oxide film on the aluminum alloy surface;
The second is to fill the weld with solder;
The third is the interaction between the solder and the base metal.

5. Vacuum brazing technology
1) According to the corresponding temperature change curve, compile the control program of the holding temperature and the temperature rising and cooling speed in each period;
2) The heating program of the heating vacuum brazing furnace can be opened only when the vacuum degree reaches 1.6～2×10-2Pa;
3) Filling with nitrogen gas during the whole brazing process to accelerate the heating and cooling rate. It is necessary to observe the various instruments frequently and record the time interval.
Prevent abnormal occurrence;
4) The brazing temperature curve and the firing temperature below 500 ℃ must be strictly observed, and it is allowed to operate according to the amount of furnace installed and changes in environmental conditions.
Small changes.

Learn More : Aluminum Vacuum Brazing Furnace