vacuum brazing heating and cooling process parameters

---

The vacuum degree, working pressure, heating rate, stable temperature and holding time, temperature, holding time, cooling rate and outlet temperature of vacuum brazing furnace have direct influence on vacuum brazing process.

(1) The selection of cold vacuum mainly depends on the type of base metal.

(2) Working pressure

1. The working pressure of vacuum brazing furnace is also called hot point porosity, and its selection depends on the type of solder. When heating up the lifting tool

2. The filler metal will release large amount of gas, which makes the cold degree constantly change, but at the brazing temperature of the vacuum brazing furnace, the vacuum brazing furnace is required to be clean, and basically return to the cold vacuum degree. But there are some exceptions, such as copper in copper base solder, the vapor pressure is 1pA at 940 ℃, so the working pressure is not allowed to be less than 1pA. Therefore, high purity nitrogen and argon can be introduced into the furnace to maintain the working pressure in the furnace.

(3) Heating rate during brazing

The heating rate of the base metal shall be able to ensure that the released gas is pumped away, and at the same time, the components shall be heated evenly, so as to reduce or prevent the deformation caused by the stress caused by the sudden heating of the components. The material, shape, structure and size of components should be considered in determining the heating rate of vacuum brazing furnace. For copper and copper alloy, it should be heated at a faster rate between 250-500 ° C; for heat-resistant alloy or austenitic stainless steel, it should be heated rapidly in the dangerous temperature zone of carbide precipitation; for components with complex shape and large assembly prestress, it should be heated slowly; for thick components, the heating rate should not be too fast.

The type of solder and its crystallization temperature range. For pure metal solder, the heating rate can be faster; for alloy solder, the heating rate should be faster within the melting temperature range, so as to avoid the increase of liquidus temperature due to solder segregation; for paste solder, the heating rate should be slower below 500C, so as to avoid the spatter of solder due to violent volatilization of binder. However, no matter what kind of solder is used, the heating rate of the vacuum brazing furnace should not be too fast within the range of 50-100 ° C below the solidus temperature of the solder, so as to ensure that the internal and external temperatures of the components are basically the same when the solder melts, so that the capillary effect can be brought into full play. In order to prevent the base metal from being eroded by the filler metal, the heating rate should be as fast as possible on the premise of controlling the deformation of the base metal in the vacuum brazing furnace.

(4) Stable temperature and holding time

The stable temperature and holding time refer to the temperature close to the solder solidus when brazing in the vacuum brazing furnace, and the heating is suspended, and maintained for a certain time at this temperature. The purpose is to reduce the temperature gradient of the module and make the temperature of each part of the module uniform. When brazing stainless steel, heat-resistant alloy and other components with poor thermal conductivity in the vacuum brazing furnace, if the furnace temperature of the vacuum brazing furnace is heated from room temperature to brazing temperature, a large temperature difference will be caused in each part of the components. The temperature difference is related to material type, structure and wall thickness. At this time, the outer layer of solder will flow along the higher temperature zone after melting; however, due to the low temperature in the joint, the solder can not be well filled, resulting in incomplete brazing and reducing the quality of the joint. Therefore, the stable temperature and holding time must be correctly selected according to the workpiece condition of the component.

Generally, the brazing temperature of vacuum brazing furnace should be 30 ~ 100 ° C higher than the liquid temperature line of solder. However, the larger the crystallization temperature range of different solders and resident solders is, the higher the brazing temperature is than the melting point of solder. For single solder, ~ 70c is enough; for multi alloy solder, it must be 60 ~ 120 bar higher than liquidus.

(5) Holding time

The holding time is related to the following factors

① When the interaction between filler metal and base metal will produce strong dissolution, brittle phase and intergranular infiltration, the holding time should be shortened as far as possible; on the contrary, when the diffusion between filler metal and base metal is beneficial to eliminate the brittle phase and low melting eutectic structure of brazing seam, the holding time of vacuum brazing furnace should be increased.

② The dimension, structure and brazing gap of workpiece affect the holding time. Generally, the insulation time of large and thick parts is longer than that of thin and small parts; the insulation time of parts with large braze joint is longer; the heating capacity is more and the insulation time is longer.

③ The weldment is not large, and the furnace charge is not large. Generally, the brazing holding time is 5 ~ 10 min.

(6) Cooling rate of vacuum brazing furnace

The cooling rate depends on the following factors:

① When the solder is in liquid state, do not ventilate or turn on the fan for cooling;

② For example, 1 crl8ni9 is forced to cool by fan in order to prevent carbide precipitation at grain boundary;

③ For thin, long and complex components, the cooling rate is slower.

(7) Outlet temperature of vacuum brazing furnace

The temperature of general stainless steel and heat-resistant alloy is lower than 150 ° C; the temperature of aluminum and aluminum alloy is below 300 ° C; the temperature of carbon steel and alloy structural steel is below 100 ° C.