Main points of stainless steel EGR brazing process

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1. Brazing joint connection method:

Brazed joints are available in two ways: butt joint and lap joint.

Butt joints are used, because the strength of the brazing filler metal and the brazing seam is generally lower than that of the base metal, so the butt joint cannot guarantee that the joint has the same bearing capacity as the base metal, so the brazed joints are mostly in the form of lap joints. Improve the strength of the brazed joint by changing the lap length.

For lap joints brazed with high-strength copper-based and nickel-based solders, the lap length is usually 2 to 3 times the thickness of the thin-walled parts. Due to the different shapes of the workpieces, the specific shapes of the lap joints are also different. For thin-walled parts, lap joints in the form of overlocks are often used to improve the strength of the brazed joints.

2. Placement of the solder

The principle of brazing filler metal placement is that the gravitational action of the brazing filler metal and the capillary action of the brazing seam should be used as much as possible to promote the filling of the gap by the brazing filler metal.
The brazing of the EGR cooler will use both nickel-based solder paste and amorphous ribbon. Paste solder should be applied directly to the brazing joint, and

The standard of amorphous thin strip solder has different thicknesses such as 0.0254mm-0.0381mm.

According to the requirements of the workpiece, it is processed into different shapes and placed at the brazing seam. In short, there is still a lot of work to be done for us to use the nickel-based solder reasonably, such as the surface treatment of the solder, the coating method of the paste, the amount of the solder, etc., and further improve it according to the actual requirements.

3. Workpiece surface treatment

Before brazing, the oxide, oil and dirt on the surface of the workpiece should be thoroughly removed, which is the prerequisite for the mutual wetting of the brazing material and the base metal, and the diffusion and filling of the weld.

The main methods of cleaning the workpiece surface are as follows:

3.1 Remove oil stains

Organic Solvents, Metal Detergents, Alkaline Solutions:

3.2 Removal of oxides

Mechanical methods, chemical cleaning, electrochemical cleaning

According to the degree of brightness of the surface of the sample, its surface treatment should have two processes of degreasing and chemical cleaning.
The EGR cooler has a tubular structure, which is a thin-walled part brazing with more than 200 solder joints. It is difficult to meet the requirements of air tightness, corrosion resistance, and strength. Therefore, it is particularly important to completely remove the oil and oxides on the surface of the workpiece.

4. Argon partial pressure control process:

Vacuum brazing can be divided into high vacuum brazing and partial vacuum brazing according to different vacuum conditions.

High vacuum brazing is suitable for brazing filler metals and base metals that contain less volatile elements and are difficult to decompose oxides. If the solder contains more volatile elements, in order to reduce the vapor pressure of these elements in the solder and base metal at the brazing temperature and improve the quality of brazing, a partial vacuum with a certain partial pressure of argon can be used brazing technology.

During the heating process of brazing, the minimum pressure at which the brazing filler metal and base metal can maintain the solid or liquid phase is determined by calculation or experience.

In the heating process of brazing in high vacuum state, a certain amount of high-purity argon is introduced when the temperature is just lower than the temperature at which the vaporization starts (liquidus temperature). in order to balance the vapor pressure of volatile metals at the temperature at which vaporization begins. With this technology, the evaporation of solder and base metal can be reduced, thereby greatly expanding the range of materials that can be soldered by vacuum brazing. In particular, it contains volatile elements such as manganese and phosphorus, and the effect is more obvious.

The partial vacuum brazing process can be summarized as two times of argon filling. The first filling of argon (argon partial pressure control), when the temperature is close to the welding temperature

Fill with argon for 10 to 15 minutes, and the argon pressure is 40kPa. The second time to fill with argon (quick cooling), when the temperature drops to 500°C, start the fast cooling, and the argon pressure is 70~80kPa.

Vacuum Brazing Furnace