Surface preparation of workpiece
In order to ensure the formation of uniform and high-quality brazed joint, the oil and oxide on the surface of the workpiece must be removed before welding; in order to improve the solderability of some materials or increase the wettability of the filler metal to the base metal, it is often necessary to coat the surface of the base metal.
(1) Oil removal
Commonly used organic solvents to remove oil, such as alcohol, gasoline, trichloroethylene, carbon tetrachloride, etc. In mass production, degreasing is often carried out in organic solvent vapor. Mechanical stirring or ultrasonic vibration can be used to improve the cleaning effect in the bath. After degreasing, wash with water and dry.
(2) Removal of oxides
According to the material, production conditions and batch size, the surface oxide removal of parts can be selected from mechanical method, chemical etching method and electrochemical etching method. After chemical etching or electrochemical etching, it is necessary to carry out light treatment or neutralization treatment, and then wash and dry with water.
a. There are grinding wheel, metal brush, sand blasting and so on.
b. Chemical etching for surface oxide removal starts from batch production with high productivity. The choice of etching solution depends on the properties of the base metal and its surface oxides. For aluminum and aluminum alloy, etching solution composition (10% NaOH, residual water or 10% H2SO4, residual water) can be selected.
c. Electrochemical etching is also suitable for large-scale production and rapid removal of oxides. It is mostly used in the oxide removal process of stainless steel and carbon steel.
(3) The base metal is coated with metal
The main purpose of coating metal on the surface of base metal is to improve the solderability of solder, increase the wettability of solder to the base metal, and use it as preset solder layer to simplify assembly and improve productivity.
Table 1-1 metal materials and plating methods of aluminum base metal
Function of base metal plating method
Al Ni, Cu, Zn Electroplating, electroless plating can improve the wettability of solder and improve the corrosion resistance of joint , Al Si alloy , Coating , As filler metal .
Preset flux and flow inhibitor
Some welding methods need to place flux and flow inhibitor in advance. The preset flux is mostly paste liquid to ensure uniform coating on the two surfaces of the workpiece to be connected. The flux with low viscosity can be dipped, sprayed manually or automatically. When the flux with high viscosity is heated to 50 ~ 600C, the viscosity can be reduced without dilution. The surface tension of hot flux decreases and it is easy to stick to metal.
Flow inhibitor is an auxiliary material used to prevent solder flooding during brazing. Brazing in gas shielded furnace
Brazing in an empty furnace is the most widely used. The flow inhibitor is mainly composed of stable oxides (such as alumina, titanium oxide, magnesium oxide, etc.) and appropriate binder. Before welding, paste flow inhibitor is applied on the surface of base metal without brazing. Because the flux does not wet these substances, it can prevent their flow. Remove the blocking agent after brazing.
Assemble, locate and place solder
The best assembly method is that the parts can be self positioned and self supported, in addition, the fixture can be used for positioning and clamping. For the flat tube and fin coated with metal on the base metal, the gap size is not considered at this time, but a certain pressure must be added to the clamp to reduce the joint clearance during the brazing process. Install the end cover of the collector pipe and the collector pipe according to the corresponding position and knock it tightly with a wooden hammer, and then connect them together with the inlet joint and the collector pipe on the spot welding machine by spot welding, so as to carry out brazing.
If it is necessary to place the solder in advance, put the solder in the predetermined position while assembling.
For aluminum brazing in nitrogen furnace: a flux spraying device sprays flux onto the workpiece, and heats the workpiece to 150 ℃ ~ 250 ℃ for drying through the drying furnace. When the temperature in the brazing furnace with protective atmosphere reaches 610 ℃, the workpiece is brazed. After cooling by water cooling and air cooling, the workpiece is unloaded from the unloading table. The process is as follows:
Welding parts → flux spraying → conveying device → drying furnace → conveying device → heating furnace → brazing furnace → water cooling chamber → air cooling chamber → conveying device.
(1) Flux spraying
A conveyor chain carries the workpiece through a closed flux chamber, in which water containing flux is sprayed on the workpiece. After spraying, the excess flux on the workpiece is removed by an air knife, and then the workpiece is transported to the drying oven.
(2) Drying process
After spraying the flux, the parts need to be dried in the drying oven, and the temperature is usually about 200 ℃. Care should be taken to prevent the heat exchanger from overheating, which may lead to the formation of high-temperature oxides on the aluminum surface.
(3) Continuous brazing furnace
The brazing furnace must ensure that the temperature of the workpiece is increased above 20 ℃ every minute, so that the surface coating solder reaches the melting point (591 ℃). The temperature balance of the workpiece is ± 3 ℃, and the protective atmosphere of nitrogen is maintained in the furnace. The control of workpiece temperature and the consistency of each workpiece temperature are very high, so the heating chamber is divided into several control areas. The more areas, the better the temperature distribution control of the workpiece. Any large fluctuation will lead to insufficient or excessive brazing of the workpiece.