Welding definition: Welding in which a metal material with a lower melting point than the base material is used as the solder, and the liquid solder is used to wet the base material and fill the gap between the workpiece interface and make it diffuse with the base material method.

When welding, only the noodles melt while the base material remains solid. This requires the melting point of the noodles to be lower than the melting point of the base material, and their composition is also different. The molten noodles are sucked in and held in the gap between the base materials by wetting and capillary action, and the mutual diffusion between the liquid noodles and the solid base materials forms a metallurgical bond.

Generally speaking, solder and flux are used for soldering. The solder whose melting point is below 450°C is called solder (alloy of tin and lead), and the solder whose temperature is higher than that is called brazing. In addition, , Bonding using polymer media is called BONDING, which is distinguished from brazing.

Introduction to aluminum brazing:

Aluminum welding began in the early 1930s. Many different soldering techniques have been used today. In the brazing device, atmosphere furnace brazing, vacuum brazing and immersion brazing account for a large part.

Characteristics of aluminum brazing: In order to make the soldering successful, the surface of the soldering joint must be clean and the surface must not be oxidized at the soldering temperature. Aluminum is very easy to oxidize in the air and during welding. The resulting aluminum oxide (Al2O3) has a high melting point, is very stable, and is not easy to remove. (If it exceeds 250℃, high-temperature oxides will be formed on the aluminum surface, these oxides are difficult to remove by Noclok flux) The oxide film hinders the melting and fusion of the solder, and the oxide film has a large proportion, which is not easy to float on the surface, and it is easy to generate slag inclusions. , Lack of fusion, lack of penetration and other defects. The oxide film on the surface of aluminum and the absorption of a large amount of moisture can easily cause pores in the weld. The surface oxide film should be removed during welding. (The flux reacts with the oxide and replaces the oxide at the same time, so as to prevent the weldment from contacting the air in the furnace, so that the brazing material melts and is drawn into the weld by capillary action.)

Just as water droplets are formed on the surface of grease stains due to the effect of surface tension, brazing on the oxide film, the brazing material cannot be uniformly combined with the base metal material (base metal), so it will cause defects Brazing surface.

Aluminum brazing can only use the following chemical and physical methods.

The main and subordinate order of the aluminum brazing method is FB→VB→NB, the physical reduction method VB method and VAW method do not require flux coating. However, the disadvantage of the VB method is that the reduction is not thorough enough, and a part of the oxide remains. In addition, the magnesium added to strengthen the aluminum material decomposes with the reduction reaction under the vacuum state to make it not resistant to corrosion. In addition, the medium gas of VAW method needs to use nitrogen with D.P (dew point) -70℃ and oxygen content of 6-8ppm, which is difficult to achieve. And metal surface treatment must be carried out before painting in the future.

The other is a chemical reduction method. The biggest difference between the FB method and the NB method is the type of flux. The FB method uses chloride based flux (ZnCl2, LiCl2). Material) but also cause environmental pollution and other public hazards. Therefore, subsequent cleaning operations are indispensable. And surface treatment must be done before painting. Compared with this, the NB method uses fluoride flux (KAlF4), which does not require cleaning after brazing, and has strong corrosion resistance. It does not require surface treatment during painting, although the flux will contaminate equipment like the FB method. , The corrosion of the equipment itself is smaller than that of chloride (the life of the furnace FB=1/2 year, NB=5-6 years). It's just that the fluorine (F) in the flux and the magnesium (Mg) in the base metal material (base metal) react to produce MgF2 that will affect the quality of brazing, so the content of Mg in the base metal material needs to be less than 1%. Another shortcoming is the problem of flux residue. However, this method has many advantages, so the NB furnace is still the mainstream in the brazing of heat exchangers.

Note:[The so-called NB method] NB is the abbreviation of “NOCOLOK (NON COLOSION[non-hygroscopic]BRAZING[brazing]”, and the name of “NOKOLOK” is American ALCA The company’s patent name (patents are limited to flux).

Aluminum brazing The material consists of a thin layer of aluminum-silicon alloy wrapped on one or both sides of the core alloy (AA3003). The low-melting aluminum-silicon alloy melts and flows during the brazing process, and forms a metallurgical bond between the components to be connected after cooling.

Core alloy:NOCOLOK brazing requires that the solidus temperature of the base metal is not less than 615℃. At the same time, the flux has a certain limit on the dissolution of MgO on the surface of the alloy, and Mg and MgO It reacts with the flux to generate MgF2, which causes the flux to increase its melting point and lose its activity. Therefore, the Mg content is required to not exceed 1% (preferably not more than 0.5%). Therefore, the aluminum alloys suitable for NOCOLOK brazing include industrial pure aluminum and Al-Mn alloys with lower Mg content (such as AA3003, AA3102, AA3005, AA3105, etc.) and Al-Mg-Si alloys (such as AA6063, AA6951, etc.). AA3003 is a traditional alloy that can be considered for automotive heat exchangers. It can be used in all brazing methods.

Note: Most aluminum alloys can be brazed but not all brazing techniques can be used. The main part is protective atmosphere brazing (CAB), which is commonly referred to as NB brazing. How to choose the alloy for vacuum brazing (VB). For air furnace welding, the selection of alloy can be the same as that of CAB.

AA3003 is a traditional alloy that can be considered for automotive heat exchangers. It can be used in all brazing methods. In order to obtain higher strength, AA3005 alloy for vacuum brazing and aging strengthening alloys AA6060, AA6063, AA6951 can also be used for vacuum brazing. In order for 6000 series alloys to be aging strengthened, the cooling rate after brazing must be fast. In order to maximize the strength after aging, the cooling rate must be> 1°C/sce in the temperature range of 400°C to 200°C.

5000 series alloys can also be used for VB soldering. It is difficult to treat this series of alloys with flux soldering. When the alloy contains high Mg content, the flux cannot remove oxides. For NB (CAB) brazing, the Mg content is objectively limited to about 0.4%. The same is true for other Mg-containing alloys such as the 6000 series.

1000, 2000 and 7000 series alloys are less used for brazing. The 1000 series and the other two series with low alloy composition have relatively low post-weld strength. When the alloy composition is high, its soldering melting point is low and can not provide good soldering.

Main aluminum-silicon alloy (a brazing filler metal in aluminum brazing): 4000 series

The filler alloy is 4000 series. The melting temperature of this series alloy is close to 600℃ and contains 7-13% Si. For vacuum brazing (VB), 1.2% Mg is added. In order to have better wettability and fluidity of the filler during VB soldering, 0.1% bismuth can be added.

For reference, if the NB brazing material does not contain magnesium materials: the coating amount of flux is about 2g/m2 (oxygen concentration is about 50ppm) if the magnesium content is 0.2-0.25 g/m2 material: the coating amount of flux is about 6-8g/m2 (oxygen concentration is about 50ppm)

If the content of magnesium (Mg) is above 2g/m2, NB brazing cannot be performed.

When the protective atmosphere brazing (CAB) brazing method is used, the temperature rise of thick-walled parts is significantly slower than that of thin-walled parts, so the alloys used for different parts of the heat exchanger have different Si contents. As the Si content increases, the melting point of the alloy decreases, so for different parts, different Si contents should be used to make the filler and metal melt almost at the same time The filler starts to flow at about the midpoint of the minimum and maximum temperature of the melting range, which is sufficient for brazing.

Zn in AA4343 and AA4045 plays an anodic protection role. The Zn content of commercial solders is generally 1 to 1.5%. Excessive Zn content reduces the fluidity and brazing properties of the solder. AA4047 is a eutectic solder. Its fluidity is too strong and difficult to control at the recommended brazing temperature. It has a higher dissolution tendency than other solders. It is generally used for flame brazing. The brazing filler metal is generally used as the skin material of the composite material, and the skin material adopts double-sided composite or single-sided composite according to different uses, and the composite rate (single-sided) is 5-10%. For the two parts connected by brazing, only one part is made of composite materials. When it is not convenient to use composite materials, solder foil, plate or wire can be used.

Note: The basic requirements for brazing materials:

·The appropriate melting point should be tens of degrees lower than the melting point of the base material. The two melting points are too close, which will make the brazing process difficult to control, and even lead to the growth of the base metal grains, overburning and partial melting.

·With good wettability, it can fully fill the gap between the seams.

·The diffusion effect with the base material ensures the formation of a firm bond between them.

·It has a stable and uniform composition to minimize the segregation phenomenon and the loss of volatile elements during the brazing process.

·Meet the mechanical and physical properties of the product.

·Economy.

·The melting point is lower than 450 ℃ is fusible noodles, commonly known as soft noodles; higher than 450 ℃ is refractory noodles, commonly known as hard noodles.　　　</