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Definition of brazing

Brazing refers to using a metal material with a lower melting point than the base metal as the solder, heating the weldment and the solder to a temperature higher than the melting point of the solder, using the liquid solder to wet the base metal, filling the gap between the joints, and The method of interdiffusion of parent metals to realize the connection of weldments. Brazing can realize the connection of dissimilar metals or alloys, metals and non-metals. Brazed joints have low strength, poor heat resistance, and relatively high requirements for assembly.

Brazing Features

Brazing is an important method to complete material connection, together with fusion welding and pressure welding, it constitutes three important components of modern welding technology. Figure Schematic diagram of lap joint comparison of three welding methods is a schematic diagram of the comparison of the lap joints of the three types of welding methods, and Table Comparison of the main characteristics of the three welding methods lists the comparison of the main characteristics of the three types of welding methods.

Brazing Basics
Comparison of the main characteristics of the three welding methods

Compared with fusion welding and pressure welding, brazing has some similarities, but there are essential differences.

Compared with other fusion welding methods, brazing has the following characteristics:

  • When brazing, the heating temperature is lower than the melting point of the weldment metal, so the solder melts, the weldment does not melt, and the structure and properties of the weldment metal change less. After brazing, the stress and deformation of the weldment are also less, which can be used for welding weldments with high dimensional accuracy requirements.
  • It can weld several, dozens of welds, or even more at a time, so the productivity is high. For example, in the welding of bicycle frames, it can weld several seams at a time. It can also weld joints with complex structural shapes that cannot be welded by other welding methods, such as missile exhaust nozzles, honeycomb structures, closed structures, etc.
  • Brazing can not only weld the same metal, but also suitable for welding dissimilar metals, even metal and non-metal. For example, the brazing of metals and graphite in atomic energy reactors, so it has a wide range of applications.
  • It can not only braze extremely thin and thin parts, but also braze parts with great differences in thickness and thickness.

At present, brazing technology has been greatly developed, solving problems that cannot be solved by other welding methods. It has been widely used in industrial sectors such as electrical machinery, radio, and instrumentation, especially in aerospace and space technology, and has become an irreplaceable process method.

The main disadvantage of brazing is that: in general, the strength and heat resistance of the brazed joint are lower than those of the base metal. In order to make up for the shortcomings of low strength, it can be solved by increasing the overlapping contact area; brazing has high requirements on the cleaning work of the workpiece connection surface and the assembly quality of the workpiece.

Fundamentals of Brazing

When brazing, the formation process of the brazing joint is: the solder with a melting point lower than the metal of the weldment is heated to the brazing temperature at the same time as the weldment, and the solder and the flux melt and wet when the weldment does not melt The brazing contact surface relies on the diffusion of the two to form a new alloy, and the brazing material cools and crystallizes in the brazing seam to form a brazing joint, as shown in Figure Schematic diagram of the brazing process

Schematic diagram of the brazing process

Obviously, the above-mentioned processes are not completely separated, but intersected. From the analysis of the whole process of brazing, it can be seen that the key to obtaining a firm brazed joint: on the one hand, the molten solder can flow into the joint gap well; on the other hand, after the molten solder flows into the joint gap, it can be connected with the weld Metal interaction and subsequent crystallization on cooling form strong joints.

Filling process of molten solder

In order for the molten solder to flow into the gap well, certain conditions must be met, and wettability and capillary action are the most basic conditions for caulking.

Wettability

During brazing, the ability of liquid solder to infiltrate and adhere to the base metal is called wettability.

Wettability indicates whether the liquid solder can make good contact with the metal surface of the solid weldment. Therefore, in order to make the molten solder flow into the gap smoothly, the molten solder must first be able to adhere to the solid metal surface. If the liquid solder is spherical on the metal surface of the solid weldment, rolling around like water droplets on the lotus leaf, its wettability is poor or not wet. This requires that the liquid solder itself has a small surface tension, and at the same time, the atoms of the solid weldment metal have a greater atomic force (ie, adhesion) on the liquid solder. That is to say, the liquid solder must have good wettability and spreadability (the ability of the liquid solder to flow and expand on the surface of the base metal, usually measured by the area covered by a certain weight of solder after melting the base metal surface). Generally speaking, the wettability of the solder and the weldment metal can form a solid solution or compound with each other, otherwise its wettability is poor.

The oxide film on the surface of the solder and the brazing workpiece has a great effect on damaging the wetting, so cleaning must be done before welding. When brazing, use flux to remove the oxide film, and braze under the protection of molten flux, or braze under protective gas or vacuum conditions, so as not to oxidize the surface of the molten solder and weldment, so as to avoid Affect the quality of brazing.

Capillary action

Insert two glass tubes with different thicknesses into the liquid, the liquid will automatically rise along the glass tubes, the smaller the diameter of the tube, the higher the liquid rises, this phenomenon is called capillary action. The stronger the capillary action, the better the gap filling ability of the molten solder.

Generally speaking, solder has good wettability on solid metal, and its capillary action is also strong. However, the size of the gap also has a great influence on the capillary action. Small gaps, strong capillary action, and sufficient caulking. But it does not mean that the smaller the gap, the better, because during brazing, the metal of the weldment is heated and expands. If the gap is too small, it will make it difficult to fill the gap.

From a lot of practical experience, it is found that the factors affecting the wettability of solder include the following aspects:

Influence of brazing filler metal and weldment metal composition

The composition of the solder and the weldment has a great influence on the wettability. When the liquid solder and the weldment do not work in liquid or solid state, the humidity adjustment between them is very poor. If the liquid solder can dissolve or form a compound with the weldment, the solder can better wet the weldment. For example, silver and iron do not interact with each other, and the wettability of silver on iron is extremely poor; silver is slightly soluble in nickel (30%) at 1000°C, so silver can wet nickel; and silver can dissolve in nickel at 779°C Steel (about 8%), so the wettability of silver on copper is good. Another example is that lead, copper and steel are incompatible with each other, so the wettability of lead on copper and steel is very poor. However, after adding tin, which can form solid solutions and compounds with copper and steel, to lead, the wettability of the solder is improved, and the higher the tin content, the better the wettability. Therefore, for solders that do not interact with the weldment (base metal) and have poor wettability, a third substance that can form a common phase with the weldment can be added to improve the wettability of the weldment.

Influence of brazing temperature

An increase in the brazing temperature helps to improve the wettability of the solder to the weldment. However, if the brazing temperature is too high and the wettability of the solder is too good, fluidization often occurs. More importantly, if the temperature is too high, the corrosion of the solder to the weldment will increase. Therefore, the brazing temperature must be selected reasonably.

Effect of metal surface oxides

The existence of oxides on the metal surface prevents the atoms of the solder from directly contacting the weldment, making the liquid solder agglomerate into a ball, forming a non-wetting phenomenon. Therefore, when brazing, the oxides on the metal surface must be fully removed.

The influence of the surface state of the brazing weldment.

The roughness of the weldment surface has a significant impact on the wettability of the solder that interacts weakly with it. The wettability of the solder on the rough surface is better than that on the smooth surface. This is because the criss-cross grooves have a special capillary effect on the liquid solder, which promotes the spread of the solder along the brazing surface.

Brazing Interaction with Weldment Metal

During brazing, the molten solder interacts with the weldment metal in the process of filling the gap. This effect can be summarized into two types: one is that the solid weldment metal dissolves into the liquid solder; the other is The liquid solder diffuses into the solid weldment metal. These two effects have a great influence on the performance of brazed joints.

The metal of the weldment is dissolved in the liquid solder

When brazing, if the solder and the weldment metal can dissolve each other in the liquid state, the weldment metal will dissolve in the liquid solder during the brazing process. For example, when a copper radiator is immersed in liquid tin solder for brazing, it is found that the copper content in the liquid solder increases as the number of brazing increases and the brazing temperature increases. As another example, when brazing steel with copper brazing filler metal, after holding the temperature at 1150°C for 2 minutes, the iron content of the brazing filler metal in the brazing seam increases from zero to 4.7%. This shows that the dissolution of the weldment metal exists during the brazing process. The effect of this dissolution is equivalent to “cleaning” the surface of the weldment, so that the molten solder has good contact with the weldment, which is conducive to improving the wettability. At the same time, it plays an alloying role on the brazing material, which can improve the strength of the brazing joint.

However, it should be pointed out that during the brazing process, if the metal of the weldment is easily dissolved, the capillary action of the molten solder will be destroyed, making brazing difficult. If the metal of the weldment dissolves too much, defects such as “melting erosion” and “burning through” will appear, and sometimes it will cause intergranular corrosion of the brazing seam. Therefore, it is necessary to control the solder composition, brazing temperature, heating time, gap size and solder filling amount, so as to achieve the purpose of controlling the amount of metal dissolution in the weldment and prevent the occurrence of the above-mentioned defects.

Diffusion of liquid solder to weldment metal

During the brazing process, while the weldment metal dissolves in the liquid solder, there is also diffusion of the solder to the weldment metal. For example, when brazing copper with brass, a solid solution of zinc in copper is found on the contact surface of the weldment close to the liquid solder. Similarly, when copper and copper alloys are brazed with tin solder, intermetallic compounds are found to form at the interface between the weldment and the brazing seam. This all proves that the diffusion process of the solder to the weldment occurs during brazing.

During the brazing process, while the weldment metal dissolves in the liquid solder, there is also diffusion of the solder to the weldment metal. For example, when brazing copper with brass, a solid solution of zinc in copper is found on the contact surface of the weldment close to the liquid solder. Similarly, when copper and copper alloys are brazed with tin solder, intermetallic compounds are found to form at the interface between the weldment and the brazing seam. This all proves that the diffusion process of the solder to the weldment occurs during brazing.

In summary, during brazing, the solder and the weldment dissolve and diffuse each other, resulting in the formation of a brazing seam.