High frequency welding solution for air conditioner accessories

High frequency welding solution for air conditioner accessories

The composition of air conditioning accessories

  Air-conditioning accessories include circuit control boards, wire controllers, manifolds, refrigerants, electronic expansion valves, motors, compressors, liquid mirrors, and ball valves. High-frequency induction brazing technology is mainly used for high-frequency welding in automotive air-conditioning pipe fittings.

Accessories that need to be brazed

Introduction to induction brazing

  Brazing is different from ordinary welding. The base material does not melt during brazing. It uses a solder with a lower temperature than the base material. The heating temperature is lower than the solidus of the base material and higher than the liquidus of the solder. When the connected parts and the brazing material are heated until the brazing material is melted, the capillary action and wetting effect are used to dissolve and diffuse with the base material, and then fill the gap between the base material and the connected part.

Brazing process

The first step: surface preparation, remove the oxide film and oil film on the surface of the workpiece;

Step 2: Positioning the workpiece. The workpiece is positioned, supported, and fixed to ensure that the relative position of the workpiece and the sensor remains unchanged during mass production, and the brazing filler metal is made into a ring and added to the part to be welded;

The third step: heating parameter setting, by adjusting the heating current and heating time to control the brazing temperature and heating rate;

The fourth step: post-brazing treatment, directly put the workpiece in water or clean it with other chemicals;

The main points of brazing---pre-welding treatment

  Before brazing, the oxides, grease, dirt, etc. on the surface of the workpiece must be carefully removed, because the molten solder cannot wet the surface of the uncleaned parts, nor can it fill the joint gap. Cleaning the surface of the part includes:

 Clean up oil stains: oil stains can be removed with organic solvents. Commonly used organic solvents are alcohol, carbon tetrachloride, gasoline, trichlorinated alkene, dichloroethane and trichloroethane, etc.

  Removal of oxides: Mechanical methods can be used to remove oxides on the surface of parts, such as files, metal brushes, sandpaper, sandblasting, etc.; chemical erosion methods; electrochemical erosion methods.

Introduction of solder

In order to meet the requirements of joint performance and brazing process, brazing filler metal generally meets the following basic requirements;

1) Suitable melting temperature range, under normal circumstances, its melting temperature is lower than that of the base material;

2) It has good wetting performance and spreading performance at brazing temperature, and can fully fill the joint gap;

3) The physical and chemical interaction with the base metal should ensure that they form a firm bond;

4) The composition is stable, minimize the burning or volatilization of the elements at the brazing temperature, and contain little or no rare metals and precious metals;

5) It can meet the requirements of physical, chemical and mechanical properties of brazed joints.

According to the different solders used, brazing is generally divided into:

(1) Soft soldering-the liquidus temperature of the solder is lower than 450℃;

(2) Brazing-the liquidus temperature of the solder is higher than 450°C.

Flux

  The role of the flux is to remove oxides on the surface of the base material and the liquid solder, protect the base material and the solder from further oxidation during the heating process, and improve the wetting performance of the solder on the surface of the base material. Therefore, the flux must have sufficient ability to remove oxides on the surface of the base material and the solder; the melting temperature and the lowest active temperature should be slightly lower than the melting temperature of the solder, and have sufficient wetting ability at the brazing temperature.

According to the melting point of the flux, it is divided into soft flux and brazing flux:

(1) Flux—The liquidus temperature of the flux is lower than 450℃; it is mainly divided into non-corrosive and corrosive flux.

(2) Brazing flux-the liquidus temperature of the flux is higher than 450℃; it is mainly composed of borax, boric acid and their mixtures, but sometimes some fluorides and chlorides of alkali metals and alkaline earth metals are added to improve the flux performance.

Fit clearance

  In the brazing process, the choice of joint gap is very important. A reasonable joint gap is not only related to the capillary action of the molten solder, and then affects the flow in the weld, but also the structural formula formed by the solidification of the molten solder in different gaps is different. In the case of a large gap, the crystal grains will grow without directionality, which will affect the strength of the joint; when a small gap is used, there is a layer of crystal grains along the width of the brazing joint, the gap is reduced, and the amount of solidified metal is reduced. , So that smooth flat crystal grains can be formed after solidification, and the weld strength is improved.

  According to the material of the workpiece and the selected brazing filler metal, the gap of the liquid-dispensing joint welding is selected from 0.05 mm to 0.15 mm, which can give full play to the capillary function and ensure the strength of the weld joint.

Placement of solder

  In most brazing, the solder must be pre-installed on the joint. When placing the solder, the gravity effect of the solder and the capillary action of the gap should be used as much as possible to promote the filler to fill the gap. The solder must be close to the joint. Can rely on capillary action to suck into the gap. For the joints with tight fit and large overlap length, the brazing material should be opened on the joints.

Apply flux

  In order to increase the fluidity of the solder, it is sometimes necessary to apply flux. The function is to remove the oxides on the surface of the solder and the base material, and to protect the welding parts and the liquid solder from oxidation during the brazing process, and to improve the wettability of the liquid solder to the welding parts.

Assembly and positioning

  There are many ways to braze and fix parts. For parts with small size and simple structure, simple fixing methods can be used, such as relying on self-weight, tight fit, spring clips, positioning pins, etc.

  For parts with complex structures, special fixtures are generally used for positioning. The requirement for brazing fixtures is that the fixture materials should have good high temperature resistance and oxidation resistance, fixtures and parts materials should have similar thermal expansion coefficients, fixtures should have sufficient rigidity, but the structure should be as simple as possible, and the size should be as small as possible. The fixture not only works reliably, but also guarantees higher production efficiency.

Brazing process parameters (multi-stage temperature heating)

  The main technological parameters of the brazing process are the brazing temperature and holding time. The brazing temperature is usually selected to be 25-60°C higher than the liquidus temperature of the solder to ensure that the solder can fill the gap, but sometimes there are exceptions. For example, for some brazing filler metals with a wide interval of crystallization temperature, since a considerable amount of liquid phase exists below the liquidus temperature, it has a certain degree of fluidity. This is the brazing temperature can be equal to or slightly lower or slightly lower than the liquidus temperature of the solder.

  The holding time of brazing depends on the size of the workpiece and the intensity of the interaction between the brazing filler metal and the base metal. The heat preservation time of large parts should be longer to ensure uniform heating. If the brazing filler metal has a strong effect on the base material, the heat preservation time should be short. Generally speaking, it is necessary to promote the mutual diffusion of the brazing filler metal and the base material during a certain holding time to form a firm bond, but an excessively long holding time will cause defects such as dissolution.

Clean after welding

  Most of the flux residues corrode the brazed joints and hinder the inspection of the welds, so they need to be cleaned up. The rosin of the flux will not corrode, so it does not need to be removed. The rosin-containing active flux residue is insoluble in water and can be removed with organic solvents such as isopropanol, alcohol, gasoline, and trichloroethylene. The flux composed of organic acid and salt is generally soluble in water and can be washed with hot water. The flux composed of inorganic acid is soluble in water and can also be washed with hot water. The borax and boric acid flux residues used for brazing are basically not soluble in water and difficult to remove. Generally, sandblasting is used to remove them.

The most important issues in high frequency induction brazing

Possible problems include: excessive spreading, outflow, cut-off inside the weld, etc.

However, the above problems have their reasons and can be solved.

The most difficult problem to solve is the problem of additives.

High-frequency welding is different from flame welding in that there is no flow aid effect of flame welding. Must rely on additives for flow.