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Aluminum alloy chassis vacuum brazing process

1 Aluminum alloy chassis

An aluminum alloy chassis is an important carrier of a certain equipment, and its quality directly affects the performance of the entire equipment. With product batches

Put into production, although the traditional manual tungsten argon arc welding process can meet the product quality requirements, but the production cycle is long and the efficiency is low. Therefore, we have been exploring various advanced technological measures to improve product production efficiency and ensure product quality. One of the main technological measures is to use vacuum brazing process to replace the traditional manual tungsten argon arc welding process.

Vacuum brazing is a new welding process developed to meet the needs of aerospace and electronics industries. Compared with traditional manual argon arc welding, it has the following advantages:

a) The heating is uniform, the deformation of the weldment is small, and the production efficiency is high;

b) The welding process does not require flux, the weld is dense and bright, and the mechanical properties are excellent;

c) It is easy to realize automation and has good product consistency.

Although vacuum brazing has the advantage of high efficiency for mass production, whether vacuum brazing can be used to replace traditional manual tungsten

The key to realizing the batch production of aluminum alloy chassis by the argon arc welding process is the strength of the weld.

2 Vacuum brazing equipment and principle

2.1 Vacuum brazing equipment

2.2 The principle of vacuum brazing

In a vacuum environment, a filler material (brazing filler metal) with a melting point lower than that of the base material (the material of the chassis) is used. At a temperature lower than the melting point of the base material but higher than the melting point of the filler metal, the capillary force is used to make the liquid solder. The material wets and spreads on the surface of the substrate to fill the gaps between the substrates, and dissolves and diffuses with the substrate to realize the connection. The brazing process does not use flux. Although there is no chemical effect of flux and reducing effect of reducing atmosphere, it also removes the oxide film on the surface of the weldment due to reducing the oxygen partial pressure in the brazing area to protect the weldment from being oxidized.

(1) Mechanism of removing oxide film on metal surface

a) The oxide film decomposes by itself in high temperature and high vacuum: oxide decomposition pressure > oxygen partial pressure in vacuum.

b) Magnesium vapor destroys the oxide film on the metal surface. In order to prevent the occurrence of problems such as flux corrosion and pollution and cleaning after brazing, vacuum brazing generally does not use flux but uses metal more active than aluminum to replace oxygen, and magnesium metal is mostly used. However, with the addition of magnesium, the solder has a serious corrosion effect on the base metal. Generally, the magnesium content in brazing is controlled at 1.0% to 2.5%.

c) Aluminum expansion forces Al2O3 to rupture, so that the liquid solder enters the oxide film from the crack and uncovers the oxide film.

(2) Capillary action gap requirements

Brazing relies on capillary force to fill the gap with solder, and the size of the gap affects the tightness of the brazing seam and the strength of the joint. The figure below shows the relationship between the joint clearance and the brazing rate. If the gap is too small, due to the uneven contact surface, it is difficult for the solder to flow in, and slag inclusions or incomplete brazing are formed in the brazing seam, resulting in a decrease in the strength of the joint; if the gap is too large, the capillary action is weakened, and the solder cannot fill the gap. The compactness of the joint weld is deteriorated and the strength is reduced. 3A21 aluminum alloy brazing gap is controlled at 0.10 mm ~ 0.25 mm.

3 Vacuum brazing process

3.1 Process flow

Cleaning→Drying→Assembling→Placing the Brazing Material→Into the Furnace→Vacuum Brazing→Out of the Furnace

3.2 Process method and parameters

(1) Cleaning before welding

When 3A21 aluminum alloy is vacuum brazed, the surface cleanliness of the workpiece is required to be higher. To obtain good quality, it is necessary to clean before brazing, which mainly consists of the following three steps:

a) Remove oil stains on the surface: wash with Na2CO3 aqueous solution at a temperature of 60°C to 70°C for 5 min to 10 min, and then rinse with clean water;

b) Remove the oxide film on the surface: heat it with 10% NaOH aqueous solution to 20 ℃ ~ 50 ℃, etch the parts for 2 min ~ 4 min, and then rinse with hot water and cold water;

c) Gloss treatment: neutralize with 15% HNO3 aqueous solution for 2 min ~ 5 min, then rinse with cold water and hot water.

(2) Drying

It is carried out in a clean oven, the temperature is controlled at 80 ℃ ~ 100 ℃, and the holding time is subject to the drying of the parts.

(3) Assembly

The staff should put on clean work clothes and spun yarn gloves before assembling the parts and filling the solder. Before assembling, use a small file to grind the surface to be welded mechanically. The assembly tools must not be oily to avoid affecting the wetting and spreading of the solder. , resulting in brazing defects.

(4) Place the solder

The brazing filler metal should be placed as far as possible by the gravity effect of the brazing filler metal and the capillary effect of the brazing gap. The location of the solder is shown in the figure below.

(5) into the furnace

Before the assembly is put into the furnace, it should be dried in a clean oven, and it can be put into the furnace after inspection.

(6) Vacuum brazing

Vacuum brazing process parameters mainly include vacuum degree, heating rate, brazing temperature, holding time and cooling rate.

a) Vacuum degree

Before brazing heating, first vacuumize, the degree of vacuum should not be lower than 4 × 10-3 Pa, start heating, but continue to vacuumize until the end of brazing.

b) Heating speed

Using the step heating method, when the temperature rises to about 500 ℃, the heat preservation and the uniform heating of the workpiece are ensured. Then continue to heat up to the brazing temperature, and heat preservation brazing.

c) Brazing temperature

Mainly based on the condition of the solder, generally higher than the liquidus temperature 25 ℃ ~ 50 ℃, below this temperature, although the solder has melted, the solid crystal has not been completely disintegrated, and the fluidity is poor; if the temperature is too high, due to the 3A21 liquidus The temperature is 654 ℃, which will cause over-burning and even melting. In addition, the base metal and brazing material may also be eroded.

d) holding time

The holding time is mainly determined by the complexity of the structure. It is necessary that the brazing filler metal can be fully wetted in the weld of the weldment, the strength after welding is good, and defects such as corrosion are not generated.

e) cooling rate

The cooling speed has an impact on the quality of brazing. If the cooling is too fast, it is easy to generate stress, and it may even cause cracks in the joint. If it is too slow, the grains will grow.

(7) Released

When the furnace temperature dropped to 200°C, the furnace door was opened to take out the workpiece, and air-cooled to room temperature.

4 Conclusion

Practice has proved that the vacuum brazing process of the aluminum alloy chassis is reasonable, and the tensile strength of the welding seam is high. In the batch production of aluminum alloy chassis, this process can replace the traditional manual tungsten argon arc welding process and improve the production efficiency. In addition, through the research on the vacuum brazing process of aluminum alloy chassis, the experience of processing aluminum alloy vacuum brazing was obtained, and the welding process technology was further improved.