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Vacuum Heat Treatment Furnace

Vacuum Sintering Furnace

Vacuum Brazing Furnace

Vacuum Brazing Defects and Solutions

Vacuum brazing is to heat and heat the structural parts in a vacuum state, so that the brazing filler metal melts within a suitable temperature and time range, and fully infiltrate, dissolve, diffuse and weld with the solid metal under the action of capillary force, so as to achieve An advanced welding method for welding purposes. The outstanding advantages of vacuum brazing are that it can connect different metals and realize simultaneous welding of complex structures. The welded joint after welding is smooth and compact, with small deformation and excellent mechanical properties and corrosion resistance.

1 Solder layer thickness

When the thickness of the solder layer is too thin, it is easy to cause welding defects such as low welding strength, weak welding, and substandard pressure; when it is too thick, it will cause the thickness of the core layer alloy to be too thin, the pressure does not meet the standard, and even corrosion occurs. phenomenon causing leakage. Therefore, the thickness and uniformity of the brazing filler metal layer is an important index to measure its quality, and it is also one of the important factors affecting the quality of brazing.

2 Other quality requirements

Intrinsic defects such as pores, slag inclusions in the core layer alloy, and poor welding with the brazing filler metal layer, etc.; in addition to unclean surface treatment, external defects include bumps and scratches during processing. When the thickness of the material layer is increased, the continuity of the metal will be directly damaged, resulting in a decrease in the pressure bearing capacity.

3 Vacuum brazing process system

In a vacuum brazing furnace, the workpiece is heated mainly by thermal radiation. At high temperature, even a small temperature difference requires high heat conduction, that is, the higher the vacuum heating temperature, the greater the heat that needs to be transferred. It shows that the heating rate in the vacuum furnace is much slower than other heating methods under the same conditions. The time required for vacuum heating is about 3 times that of an air furnace and 6 times that of a salt bath furnace. Therefore, when formulating the heating process system of vacuum brazing furnace, the heating process system of air furnace, salt bath furnace and atmosphere furnace cannot be copied. In the process of vacuum brazing, heating should be as slow as possible to keep the temperature inside and outside the product to be brazed consistent, otherwise the brazing quality will be directly affected. Preheating and setting temperature, heat preservation, energy storage, temperature setting, heat preservation, brazing temperature setting, heat preservation and power-off cooling in industrial production are effective technological processes that can not only achieve the above purpose but also improve production efficiency. Among them, vacuum brazing temperature And holding time is the key to affecting the quality of brazing.

Vacuum Brazing Defects

1 Drift

Flood flow is the thin coating of brazing filler metal formed on the base metal at the brazing joint when the brazing filler metal flows through it during brazing.

(1) Reasons for flooding

The fixture should have a certain degree of elasticity and rigidity at the brazing temperature, so that there is a suitable gap at the joint of the weld to form a rough.

The fine phenomenon attracts the molten solder. The tooling is not tightly clamped, and the brazing component gap is too large to hold the solder, resulting in overflow defects.

The rigidity of the tooling is low, and the thermal deformation and gravity after heating cause the joint gap of the brazed components to increase, and the capillary phenomenon of the brazing material cannot be formed.

The phenomenon also leads to flooding of the solder.

Vacuum brazing is radiative heat transfer, the heat capacity of the fixture is large, the heating rate of the brazed parts is small, the residence time in the solid-liquid phase temperature range of the solder is long, the low melting point components of the solder are more volatilized, and the flux of the solder is volatile. The action time is also long, and the two further destroy the surface tension of the liquid solder, and excessively improve the wettability of the solder to the base metal. Large furnace loading, small heating and cooling rate, long holding time, etc. are the same as the large heat capacity of the tooling fixture. The solder stays in the liquid state for a long time, and the slow cooling rate is equivalent to prolonging the holding time of the solder liquid, which will also cause flooding.

The brazing holding time is short, and the diffusion time of a certain component to the base metal is short. In the solid-liquid phase range of the solder, the heating time is too long, and some components of the solder are more volatilized. The vacuum pressure is too high or the leakage rate of the vacuum furnace is large, and the brazing filler metal or base metal is partially oxidized during heating. The oxide film is not completely removed. The brazing filler metal or base metal is corroded during alkaline cleaning and changes the filler metal composition. The amount of flux is small, and the wettability of the solder is not good.

(2) Elimination measures

Extend the holding time so that the diffusion is fully completed. The temperature is increased in different intervals, and the temperature is rapidly increased in the solid-liquid phase of the solder to reduce the volatilization of the low melting point components of the solder. Reduce the vacuum pressure to prevent re-oxidation of the solder or base metal during heating. Check the pressure rise rate of the device. Increase the concentration or temperature of the lye solution, or prolong the time of alkaline etching to completely remove the oxide film. Reduce the concentration or temperature of the lye solution (generally controlled at 60°C), or shorten the alkaline etching time, and separate the solder or base metal for alkali cleaning to prevent the composition of the solder from being changed during alkali cleaning. Increase the amount of flux to improve the wettability of the solder.

2 missing solder

Leakage welding is the unwelded gap formed by the lack of solder or the melting and loss of solder at the brazing joint of the brazing parts.

(1) Reason

The amount of solder is insufficient or the connection gap is large. The brazing heating rate is too high, which leads to the deformation of the parts, and the joint gap increases, and the capillary phenomenon cannot be formed. The amount of flux used is large, and the wettability of the solder is too good, resulting in the loss of the solder or the brazing seam being too wide. In the solid-liquidus range of the solder, the heating rate is slow, and the volatilization of the low melting point components of the solder mostly changes the composition of the solder, increases the melting point of the remaining part of the solder, and reduces the mutual diffusion between the solder and the base metal. . The furnace capacity is large or the tooling design is unreasonable. The tooling is too heavy and absorbs too much heat, resulting in a slow heating rate. If the holding time is long or the cooling rate is slow, etc., the low melting point components of the solder will volatilize more. The solder is over-corroded, changing its composition and thus its melting point.

(2) Elimination measures

Increase the amount of solder, increase the clamping force of the tooling and reduce the gap at the joint. Before brazing, increase the stress relief annealing process of the brazed components, or increase the temperature in stages and set up an isothermal stage to rapidly increase the temperature above 500 °C. Reduce the amount of flux used and improve the wettability of the solder to the base metal. The amount of flux used is large, and the wettability of the solder to the base metal is too good, causing the solder to flow. The high holding temperature, the small surface tension of the liquid solder, and the enhanced effect of the flux to reduce the surface tension cause the solder to flow. The workpiece stays in the solid-liquid temperature range of the solder for a long time, which leads to flooding.

3 Dissolution

Erosion is a depression formed on the surface of the base metal by the melting of the molten brazing filler metal.

(1) Causes of corrosion

The brazing filler metal does not match the brazing base metal, and the brazing filler metal forms a low melting point phase with a certain component in the base metal, which reduces the solidus temperature in some areas of the base metal. The heating rate of the parts is slow due to the large heat capacity of the tooling or the large amount of furnace loading, and the residence time in the solid-liquid phase temperature range of the solder is too long. At a certain temperature, the solder and a component in the parent phase are complexed to form a low melting point The melting point of some regions of the parent phase alloy decreases and melts. The furnace temperature is not uniform, the local temperature of the brazing parts is too high, and the brazing temperature is too high, which leads to the melting of the low-melting components of the base metal through the diffusion area of the brazing filler metal. The temperature rises and falls slowly in the solid-liquidus range of the solder. Brazing holding time is too long.

(2) Elimination measures

The solution is generally to replace the solder grade. Or quickly heat up when it is close to the melting point of the solder, reduce the furnace load, reduce the weight of the tooling, reduce the brazing temperature, and shorten the holding time of vacuum brazing.

4 The brazing is not penetrated or the penetration distance is not enough

The brazing time is too short and the flow rate is not reached.

5 Through the above analysis, the following work should be done:

(1) The raw materials used should ensure product quality and should be purchased from regular and professional manufacturers;

(2) Carry out material preparation, surface cleaning, assembly, and brazing in strict accordance with the vacuum brazing process;

(3) In practice, the brazing process system such as vacuum brazing temperature, holding time, and vacuum degree should be optimized and strictly controlled.

(4) Control the environmental humidity or pre-dry the product: dry the product according to the general drying process, and the drying speed is 200±50r/min, drying temperature 200±10℃.