The operation steps of vacuum brazing furnace (aluminium alloy) are as follows:
Check whether the oil level of the vacuum pump meets the requirement, add 1 # vacuum pump oil into the oil cup, observe that the oil level line in the oil window of the pump cover should be located at 3/4 of the diameter of the oil window before and after, if not, add 1 # vacuum pump oil.
Check whether the cooling water pressure meets the required requirements.
Check the thermocouple before entering the furnace. After the workpiece enters the furnace, start the mechanical vacuum pump and preheat the diffusion pump at the same time. When the system pressure reaches – 0.098 MPa – 0.1 MPa, start the Roots pump. It is strictly forbidden to start Roots in advance, so that the vacuum in the furnace reaches 10-1 Pa. When the diffusion pump starts to work, open the high vacuum valve and pump the vacuum in the furnace to 1.6 x 10-2 Pa. Start heating.
Before 380 C, it can be heated at a faster speed, usually 20 C/min, and at 380 C, it should be kept for 60-90 min.
It can increase the vacuum requirement to 8. The heating rate should not be too fast in the range of 470 520 C, generally controlled at 10 15 C/min.
The temperature of the workpiece should be close to 510 C and the vacuum should be 6.5 *10-3. At 570 °C, the workpiece should be kept for 60-90 minutes.
The temperature difference between the surface and the center of the workpiece is reduced to 5.0*10-3 (up to 560 C) and the vacuum is reduced to 5.0*10-3.
After the end of insulation at 570 C (vacuum degree to 3.0*10-3), the heating rate should be accelerated, generally controlled in 20 minutes to rapidly rise to brazing temperature 620 (core temperature is mainly controlled in brazing, vacuum degree is not considered);
The brazing holding time is as short as possible under the condition of guaranteeing penetration, and the specific time depends on the quantity of the furnace and the structure of the parts.
After brazing, when the monitoring thermocouple on the workpiece reaches 595 C, turn off the heating power and start vacuum cooling; after brazing, vacuum degree of vacuum furnace is often checked to check the pressure rise rate. When the vacuum thermocouple on the workpiece drops to 540 C,
Close the diffusion pump and high vacuum valve. When the monitoring thermocouple on the workpiece drops to 400 C, the Roots pump is closed, the mechanical vacuum pump is closed when the oil temperature of the diffusion pump is below 100 C, and the total power supply is closed after the work is finished.
If the unit is to prevent frost cracking in winter, it is necessary to drain the cooling water in the pump and pay attention to anti-rust treatment.
Vacuum Brazing Furnace Vacuum Control:
The oxide film is compact and protective at temperatures below A. 400 °C. At this time, if the furnace has a certain temperature (80 °C), and the workpiece is baked from the low-temperature oven, because the furnace and the workpiece surface absorb less water vapor, can be quickly heated. If the temperature in the oven is lower (below 60 °C), and the workpiece has not been baked after assembly, so there is more water vapor adsorbed in the oven and on the surface of the workpiece, then the oven is baked at low temperature with a slower heating rate to avoid sudden large amount of ventilation, resulting in a significant decrease in vacuum.
B. From 400 to 551 °C, the original dense and smooth surface of the oxide film becomes porous under the action of magnesium. When the temperature rises to 551 °C of Al-Si-Mg ternary eutectic, the Mg2Si grains dissolve and the ternary liquid phase wets the porous oxide film surface. During this period, if the vacuum degree is less than 10-3 pa, or if the vacuum degree is good, but the leakage rate is high, a double oxide film will be formed on the surface of the material.
C. For Al-Si-Mg alloys containing about 10% silicon,
when the temperature reaches the solid line at 577 °C, the alloys begin to melt. When the temperature continues to rise to the liquid line temperature of 591 °C, all the alloys will melt. At the later stage of this temperature range, the heating rate should be slowed down to narrow the gap between the surface temperature and the central temperature of the workpiece.
D. 591 ~605 °C. In this temperature range, the heating rate should be slowed down to prevent temperature overshoot. The viscosity of liquid filler metal decreases with the increase of temperature. Under the action of wool suction, the fillet metal flows to the joint and forms the fillet. Then the cooling begins and the brazing is completed. In the meantime, the vacuum should be kept above 3 x 10-3 3a to prevent oxidation.
Vacuum brazing process requires that the controlled quantity should be changed within the temperature control range, and the precision should be strictly controlled within (+2 °C). When the temperature rising curves of t1, T3 and T5 enter the constant temperature range, the overshoot should not exceed the allowable range, otherwise the brazing quality of the workpiece will be affected.
Because of the large volume of the hot zone and the large number of heaters in the vacuum brazing furnace, the method of temperature control in different zones is needed to ensure the uniform temperature characteristics in the hot zone. At this time, each zone has the same structure, executes the same heating curve, and the temperature of each zone affects each other. The division of temperature zone should be rough or not fine, otherwise the system control will become very complex.
Vacuum brazing fixture consists of the following parts: upper and lower pressure plate, counterweight block, stainless steel screw, cast iron nut, stainless steel partition and so on. Among them, the upper and lower pressure plates and counterweight blocks have great influence on the uniformity of temperature and the size control of heat exchanger products during vacuum brazing process. Because the material of heat exchanger is aluminium alloy, its strength decreases with the increase of temperature. In order to ensure the uniform deformation of heat exchanger and good clearance during the heating process, the upper and lower pressure plates and counterweight blocks play a decisive role. The following factors should also be considered in the design of brazing fixture:
(1) Brazing fixture materials can withstand brazing temperature without losing strength, deforming and releasing gas, and it is not easy to react with the assemblies.
(2) Pay attention to the temperature expansion and shrinkage of fixture and parts. At brazing temperature, it is necessary to ensure that brazed parts have appropriate joint clearance.
(3) The fixture should ensure the heat conduction in the brazing zone, and the heat transfer caused by the fixture has the least interference to the stable heating and cooling of the parts, and does not hinder the flow of the brazing metal.