1. Surface preparation of workpiece

In order to ensure the formation of uniform and high quality brazed joints, oil stains and oxides on the surface of the workpiece must be removed before welding. In order to improve the brazing ability of some materials or increase the wettability of brazing fillers to the base metal, metal plating is often required on the surface of the base metal.

(1) Cleaning up oil pollution

Commonly used organic solvents to remove oil pollution, such as alcohol, gasoline, trichloroethylene, carbon tetrachloride and so on. Mass production often degreases in organic solvent vapor. Mechanical stirring or ultrasonic vibration can be used to improve the cleaning effect in the bath. Wash and dry with water after degreasing.

(2) Removal of oxides

The removal of oxide on the surface of parts can be selected by mechanical method, chemical etching method and electrochemical etching method according to materials, production conditions and batches. After chemical or electrochemical etching, brightening or neutralization should be carried out, followed by water cleaning and drying.

A. Mechanical cleaning methods suitable for mass production include grinding wheel, metal brush, sand blasting, etc.

B. The removal of surface oxides by chemical etching started in batch production with high productivity. The choice of the etching solution depends on the properties of the base metal and its surface oxides. Aluminum and aluminium alloys can be prepared by using impregnating solution (10% NaOH, residual water or 10% H2SO4, residual water).

C. Electrochemical etching is also suitable for mass production and rapid removal of oxides. It is mostly used in the removal of oxides from stainless steel and carbon steel.

(3) Metal plating on base metal

The main purpose of metal plating on the base metal is to improve the solderability of brazing filler metal, increase the wettability of brazing filler metal to the base metal, and as a pre-brazing layer to simplify assembly and improve productivity.

Table 1-1 Aluminum base metal plating materials and plating methods

Function of Material Plating Method for Base Material

Aluminum-nickel, copper and zinc electroplating and electroless plating can improve the wettability of solder and the corrosion resistance of joints.

Al Si alloy

Coating

Used as filler metal

2. Pre-brazing agent and current blocking agent

Some welding methods require pre-positioning of solders and current blockers.  Preset fluxes are mostly paste-like fluids to ensure uniform coating on the two surfaces of the workpiece to be joined. Solder with low viscosity can be dipped, sprayed manually or sprayed automatically. When the flux with high viscosity is heated to 50 ~ 600C, its viscosity can be reduced without dilution. The surface tension of the hot flux is reduced and it is easy to adhere to metal.

It is used for brazing in gas flux furnace, flame brazing and brazing with self-fluxing filler metal without pre-brazing filler metal. Vacuum brazing also requires no flux.

Current blocking agent is an auxiliary material used to prevent filler metal flooding during brazing. Brazing and True in Gas Protected Furnace

Brazing in empty furnace is the most widely used. The current blocker is mainly composed of stable oxides (such as alumina, titanium oxide, magnesium oxide, etc.) and appropriate binders. Paste current blocking agent is coated on the base metal surface without brazing before welding. Because the flux does not wet these substances, it can prevent them from flowing. After brazing, the blocking agent is removed. 3. Assembly positioning and placement of solder

The best assembling method is that parts can be self-positioned and self-supporting, in addition, fixtures can be used for positioning and clamping. For the joint of flat tube and fin coated with metal on the base metal, the gap is not considered, but the joint must be pre-loaded by fixture to reduce the gap during brazing. The end cap of the collector pipe and the collector pipe are installed in the corresponding position and tightened with a wooden hammer head. Then the end cap and the collector pipe are joined together by spot welding on the spot welding machine together with the inlet joint and the collector pipe for brazing.

If there is a need to pre-position the filler metal, the filler metal will be placed in a predetermined position at the same time of assembly. 4. brazing

Aiming at aluminum brazing in nitrogen furnace: a brazing flux spraying device sprays the brazing flux onto the workpiece, dries the workpiece to 150 ~250 ~C through the drying furnace, brazes the workpiece when the temperature reaches 610 ~C in the brazing furnace with protective atmosphere, and then discharges the workpiece from the discharging platform after cooling by water and air. The process is as follows:

Welded workpiece flux spraying conveying device drying furnace conveying device heating furnace brazing furnace water cooling chamber gas cooling chamber conveying device.

(1) Flux Spraying

A conveyor chain carries the workpiece through a closed flux chamber in which the water-containing flux is sprayed onto the workpiece. After spraying, the excess flux on the workpiece is removed by an air knife, and then the workpiece is transferred to the drying furnace.

(2) Drying process

After spraying brazing flux, the parts need to be dried in the drying furnace. The temperature is usually about 200 C. Care should be taken to prevent the heat exchanger from overheating, because overheating (i.e. 250 C) may lead to the formation of high temperature oxides on the surface of aluminium.

(3) Continuous brazing furnace

The brazing furnace must ensure that the temperature of the workpiece is increased by more than 20 C per minute, so that the brazing filler metal on the workpiece surface can reach the melting point (591 C). The temperature balance of the workpiece is (+3 C) and the atmosphere of nitrogen protection is maintained in the furnace. The temperature control of the workpiece and the consistency of the temperature of each workpiece are required very high, so the heating chamber is divided into several control zones. The more the area, the better the temperature distribution control of the workpiece. Any large fluctuation will lead to insufficient or excessive brazing of the workpiece.