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Brazing knowledge Summary(3)

Jan 28, 2019

Wetting and spreading

When brazing, only the molten liquid solder wets the surface of the base material well to fill the braze. The measurement of the wetting ability of the brazing filler metal to the base metal can be expressed by the contact angle of the brazing material (liquid phase) in contact with the base material (solid phase). The main factors affecting the solder wetting base metal are:

1. Ingredients of solder and base material

If the solder and the base material do not undergo physicochemical action in both solid and liquid state, the wetting effect between them is very poor, such as lead and iron. If the brazing filler metal and the base material can dissolve each other or form a compound, it is considered that the brazing filler metal can better wet the base material, for example, silver to copper or copper is commonly used as a brazing filler metal.

2. Brazing temperature

The increase of the brazing heating temperature will improve the wettability of the brazing filler metal to the base metal due to the decrease of the surface tension of the brazing filler metal, etc., but the brazing temperature may not be too high, otherwise the brazing material loss, grain growth and the like may be caused.

3. Base metal oxide

If an oxide is present on the surface of the base metal, the liquid solder tends to condense into a spherical shape and does not wet with the base material. Therefore, the oxide must be sufficiently removed before brazing to ensure good wetting.

4. Base material surface roughness

When the action between the brazing material and the base metal is weak, the groove on the surface of the base material has a special capillary action, which can improve the wetting and spreading of the brazing material on the base material.

5. flux

The brazing agent can remove the oxides on the surface of the brazing filler metal and the base material during brazing, and improve the wetting action. Borax can be used.

Capillary flow

In brazing, the liquid solder fills the braze along the gap, and because the gap is small, like a capillary, it is called capillary flow. The size of the capillary flow ability determines whether the solder fills the gap between the brazes.

There are many factors affecting the capillary flow of liquid solder, mainly the wetting ability of the solder and the joint gap size. For example, if the solder has good wettability to the base material and the joint has a small gap, good solder flow can be obtained. With padding performance.


The liquid solder interacts with the base metal during the capillary caulking process. These interactions have a great influence on the performance of the brazed joints. They can be divided into two types:

1. The dissolution of the base metal into the solder

In the brazing process, the dissolution process of the base material into the liquid solder generally occurs, and the solder component can be alloyed, which is advantageous for improving the joint strength. However, excessive dissolution of the base metal causes the melting point and viscosity of the liquid solder to increase, and the fluidity is deteriorated, which often results in the inability to fill the gap of the brazing seam, and may cause defects such as dents due to excessive dissolution of the surface of the base material.

2. The diffusion of the solder component to the base metal

During brazing, the diffusion of the solder component to the base material also occurs. The diffusion is carried out in two ways: one is that the solder component diffuses into the entire parent crystal grain, and a solid solution is formed on the side of the parent material adjacent to the brazing seam. The layer does not adversely affect the joint. The other is that the solder component diffuses to the grain boundaries of the parent metal, which often makes the grain boundary brittle, especially in the case of thin brazing.

In order to make the brazing joints firmly connected, the adhesion of the brazing filler metal is enhanced, and a flux is used for brazing. Its function is to remove the oxides on the surface of the solder and the base metal, protect the weldment and the liquid solder from oxidation during the brazing process, and improve the wettability of the liquid solder to the weldment.

Common solder

There are generally two types. One type is a brazing material, and the melting point is above 450 ° C. The commonly used brazing filler metal is a copper-based, silver-based, aluminum-based, nickel-based alloy. The flux is usually borax, boric acid, chloride, fluoride, and the like. The heating sources for brazing include torch flame, electric resistance heating, induction heating, salt bath heating and furnace heating. Brazed joints have high strength and are suitable for brazing workpieces with high force or high working temperature, such as carbide cutters, bicycle frames, etc. These brazing are usually referred to as brazing; the other is Solder, melting point below 450 ° C, the most widely used solder is tin-based alloy, most solders suitable for soldering temperature of 200-400 ° C, flux is rosin, rosin alcohol solution, zinc chloride solution, The heating method is usually heated by a soldering iron. Brazed joints are low-strength and are suitable for workpieces with low or low operating temperatures, such as containers, instrument components, etc. These types of brazing are often referred to as soldering. Soldering is a type of soldering.

The brazing filler metal is the filler metal that forms the brazed joint, and the quality of the brazed joint depends largely on the brazing filler metal. The brazing filler metal should have a suitable melting point, good wettability and caulking ability, and can diffuse with the parent metal. It should also have certain mechanical properties and physical and chemical properties to meet the performance requirements of the joint.

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