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Vacuum Brazing Furnace

Vacuum brazing furnace applications

1.EGR car cooler vacuum brazing

EGR car cooler is an exhaust gas recirculation cooler that reduces car exhaust emissions. The burned exhaust gas can effectively reduce the oxygen content and temperature after passing through the ECR cooler, and is mixed with fresh gas for combustion in the cylinder, reducing the amount of NOx generated and making the vehicle exhaust emission limits meet the standards.

EGR is a heat exchanger, and the working environment is very harsh and contains high-temperature corrosive gases: the working medium is high-temperature exhaust gas, and the external environment is rainwater, air pollution, sea salt, and dust. During the working process of EGR, it has to bear a large vibration fatigue load, so the requirements for the mechanical properties of the weld are stricter. If argon arc welding is used, aging strain and thermal cracks are likely to occur; if controlled atmosphere brazing is used, the surface oxide film is difficult to remove, and the brazing The quality is poor, and vacuum brazing can solve the connection of this type of alloy materials.

The correct selection of brazing material is an important factor in ensuring a high-quality brazing joint. According to the matching table of the base material and the brazing material, if the base material is stainless steel, the first choice is copper brazing material, which is pure copper containing 99.5% copper. , again choose Ag-Cu-NiNi-based and Co-based solder.

EGR pipe fittings brazing joint form: According to the inner and outer diameters of the connecting flange and core assembly shell after connection, the difference is allowed. The designed joint form is as shown in the figure. The determination of the weld gap value is more complicated and should be selected based on the base material material, joint structure and brazing process. When copper brazing filler metal is used, the brazing joint gap is 0.02 mm; when nickel-based brazing filler metal is used, the brazing joint gap is 0.10 mm.

The vacuum brazing process of the EGR cooler is: stamping, cleaning, core component fin clamping, machine application of paste solder resist, drying, shell assembly, core component cooling tube expansion, assembly flange, argon arc Welding water pipes, overall vacuum brazing, pressure testing, inspection, packaging, the important part is vacuum brazing.

2.Preparation before welding

1) Preparation of parts before welding

Surface degreasing: The base material is stamped stainless steel, so ordinary degreasing is enough. The composition of the etching liquid used: 10% H2 SO4 + 15% HCl + 5% HNO3 + remaining water, temperature 100 e, ultrasonic cleaning 30 s and then dried.

2) Assembly and positioning of parts

Due to the special shape of ECR parts, a special hanging fixture needs to be designed to ensure the vertical brazing position of the parts, that is, the fixture should be designed in a point positioning mode.

The vacuum brazing furnace produced by SIMUWU Company is used. The furnace has very good air tightness and the ultimate vacuum degree can reach 6×10^-4Pa. Cool to room temperature with pure argon gas and take out the parts.

When brazing with red copper, the stable temperature is 1050 e.

3 Vacuum brazing process parameters

The process parameters of vacuum brazing mainly include vacuum degree, heating rate, stable temperature and holding time, brazing temperature, brazing holding time, cooling rate and furnace temperature, etc. Selecting reasonable process parameters can ensure sufficient brazing penetration, reasonable alloying reaction, avoid the occurrence of brazing defects, and obtain a brazing joint with good performance.

a) Vacuum degree

It mainly depends on the vapor pressure of the metal elements of the brazing material and the base material to be brazed. The material to be brazed is stainless steel, so the thermal vacuum degree is desirable (2~5)x10^-2Pa. Red copper is an element with high evaporation pressure. According to the volatilization characteristic curve of copper, when the pressure is 1332×10^-1 Pa and the temperature is 1035e, the copper element evaporates significantly. When choosing copper brazing, in order to prevent large amounts of copper from volatilizing, partial pressure brazing must be used. Pump the furnace cavity to a vacuum degree of (2~5)x10^-2Pa in advance. When the temperature rises to 1050 e, an inert gas slightly higher than the equilibrium pressure of copper volatilization should be passed into the furnace so that the partial pressure vacuum degree is not greater than 10 Pa. To limit the phenomenon of copper spraying and improve the surface cleanliness of the product. After the brazing is completed, argon or nitrogen with a purity greater than 99.96% and a dew point lower than -67e can be introduced into the furnace so that the pressure in the furnace is not greater than 350 Pa.

When brazing with BNi5, the cold vacuum degree is 50 Pa. No partial pressure is used during brazing. The hot vacuum degree is (2~5)x10^-2 Pa. This can not only effectively remove the oxide film on the surface of the base material, but also inhibit the base metal Evaporation of Cr element.

b) Heating rate

It can be selected based on the heating rate recommendations of common metal components. Within the temperature range of 50~100e below the solidus temperature of the solder, the heating rate should not be too fast to ensure that the temperature inside and outside the component is basically the same when the solder is melted, and the capillary action can be fully exerted to penetrate the base material.

c) Stable temperature and holding time

Since the thermal conductivity of stainless steel components is relatively poor, when heated to near the brazing temperature, the holding time can be appropriately extended to reduce the temperature gradient in each part of the component.

d) Brazing temperature

It is the most important process parameter in the brazing process, and the main basis for selection is the melting point of the solder. Red copper is a single-element solder material with no crystallization intervals and good fluidity. Through the slow flow test of copper solder, the acceptable brazing temperature range is 1112~1120 e; BNi5 is a high-temperature solder, and the acceptable brazing temperature range is 1150~1190 e;

e) Cooling method:

In order to prevent the air flow from disturbing the liquid solder, cool it in the vacuum furnace until 930 e, then fill it with high-purity argon (99.96% A r), and start air cooling.

f)Out of the furnace temperature When the furnace temperature drops below 80 e, fill it with ordinary pure argon gas again and cool it to room temperature, then take out the parts.

It is worth noting that when brazing with red copper, the partial pressure starts when the stable temperature is 1050e, and the vacuum degree of 10 Pa is self-cooled to below 1050e, and the partial pressure ends; while when brazing with BNi5, since there is no high evaporation pressure element, it is not used partial pressure.

4 Precautions for vacuum brazing

1) It can be seen from the test that red copper and BN i5 can be used as the solder for successfully brazing the EGR stainless steel cooler. The red copper weld is fuller, indicating that the wettability of red copper to stainless steel is better than that of nickel-based solder BNi5. However, because BNi5 contains more nickel elements, its high-temperature performance of the weld is better than that of red copper.

2) If copper is used as vacuum solder, attention should be paid to the high evaporation pressure of the copper element. The vapor pressure of pure copper is l Pa at 1100 e, and it is easy to escape under vacuum. Therefore, partial pressure begins when the temperature is stable, and the vacuum degree 10 Pa self-cools to below 1050 e, ending the partial pressure to limit the escape of copper elements. If you choose BNi5 brazing, there is no need to use partial pressure.

3) Because BNi5 brazing filler metal contains up to 70% nickel, it is easy to form a nickel-based solid solution when brazing stainless steel. If the weld gap increases, the silicon-nickel compound Ni Si2 will appear, forming a brittle structure, which will greatly reduce the joint strength, especially the impact toughness.

4) In addition to BNi5 solder, other nickel-based solders of the same series can also be used, such as BNi2, BNi7, etc.