Aluminum brazing knowledge - NOCOLOK brazing process

Definition of brazing: A welding method in which a metal material with a lower melting point than the base metal is used as the brazing material, and liquid brazing material is used to wet the base metal and fill the interface gap of the workpiece, and make it diffuse with the base metal.

When brazing, only the brazing material melts while the base material remains solid, which requires the melting point of the brazing material to be lower than that of the base material, and its composition also varies. The melted solder relies on wetting and capillary action to absorb and maintain in the gap between the base metal, and the mutual diffusion between the liquid solder and the solid base metal forms a metallurgical bond.

Introduction to aluminum brazing:

The brazing of aluminum began in the early 1930s. Nowadays, many different brazing techniques have been adopted. In brazing equipment, atmosphere furnace brazing, vacuum brazing, and immersion brazing account for a significant portion.

Aluminum brazing characteristics: In order to achieve successful brazing, the surface of the brazing joint must be clean and free of any oxidation at the brazing temperature. Aluminum is prone to oxidation in the air and during welding, resulting in high melting point and very stable alumina (Al2O3) that is difficult to remove. (If the temperature exceeds 250 ℃, high-temperature oxides will form on the surface of aluminum, which are difficult to remove by Noclok flux.) The oxide film hinders the melting and fusion of the brazing material, has a high specific gravity, and is not easy to surface, resulting in defects such as slag inclusion, incomplete fusion, and incomplete penetration. The surface oxide film of aluminum material and the adsorption of a large amount of water can easily cause porosity in the weld seam. During welding, the surface oxide film should be removed. (The flux reacts with the oxide and simultaneously replaces the oxide to avoid contact between the solder and the air in the furnace, so that the solder melts and is pulled into the weld seam through capillary action.)

Just like pouring water on the surface of oil stains can cause water droplets to form due to surface tension, brazing on the oxide film cannot evenly bond the brazing material with the base metal material (base metal), resulting in the formation of poor brazing surfaces.

The brazing of aluminum materials can only be carried out using the following chemical and physical methods.

The main and secondary order of aluminum brazing methods is FB → VB → NB, and the physical reduction methods VB and VAW do not require flux coating. However, the disadvantage of VB method is that the reduction is not thorough enough, leaving a portion of oxides. Additionally, it is to strengthen the decomposition of magnesium added to aluminum in vacuum with the reduction reaction, making it less corrosion resistant. In addition, the medium gas of the VAW method requires the use of nitrogen gas with a dew point of -70 ℃ and an oxygen content of 6-8ppm, which is difficult to achieve. And in the future, metal surface treatment must be carried out before painting.

Another method is chemical reduction. The biggest difference between FB method and NB method is the type of flux used. FB method uses chloride based flux (ZnCl2, LiCl2), which not only corrodes the base material (substrate) but also causes environmental pollution and other harmful issues. Therefore, subsequent cleaning operations are indispensable. And surface treatment must be carried out before painting. Compared to this, the NB method uses fluoride based flux (KAlF4), which does not require cleaning after brazing. It has strong corrosion resistance and does not require surface treatment during painting. Although the same flux as the FB method can contaminate the equipment, the corrosion of the equipment itself is smaller than that of chloride (the furnace has a lifespan of FB=1/2 years, NB=5-6 years). However, the MgF2 generated by the reaction between fluorine (F) in the flux and magnesium (Mg) in the base metal material (base metal) will affect the quality of brazing, so the content of Mg in the base metal material needs to be below 1%. Another deficiency is the problem of residual flux. However, this method has many advantages, so NB furnace is still the mainstream in the brazing of heat exchangers.

Note: [The so-called NB method] NB is the abbreviation for "NOCOLOK (non hygroscopic) BRAZING", and the name "NOKOLOK" is the patent name of ALCA Company in the United States (the patent is limited to flux).

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

Core alloy: NOCOLOK brazing requires the solidus temperature of the base material to not be lower than 615 ℃. At the same time, due to the limited dissolution of MgO on the alloy surface by the brazing flux, and the reaction of Mg and MgO with the brazing flux to generate MgF2, the melting point of the brazing flux increases and loses its activity. Therefore, the Mg content is required to not exceed 1% (at most not exceeding 0.5%). Therefore, aluminum alloys suitable for NOCOLOK brazing include industrial pure aluminum, Al Mn alloys with low 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 and can be used in all brazing methods.

Note: Most aluminum alloys can be brazed, but not all brazing techniques can be utilized. They are mainly divided into protective atmosphere brazing (CAB), commonly known as NB brazing, and how to choose alloys for vacuum brazing (VB). For air furnace brazing, 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 and can be used in all brazing methods. In order to obtain higher strength, vacuum brazing uses AA3005 alloy, and aging strengthened alloys AA6060, AA6063, and AA6951 can also be used for vacuum brazing. In order for the 6000 series alloys to have the potential for aging strengthening, the cooling rate after brazing must be fast. To achieve the highest strength after aging, the cooling rate must be>1 ℃/sce within the temperature range of 400 ℃ to 200 ℃.

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

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

Main aluminum silicon alloys (brazing materials in aluminum brazing): 4000 series

The filler alloy is the 4000 series, which has a melting temperature close to 600 ℃ and contains 7-13% Si. For vacuum brazing (VB), add 1.2% Mg. In order to ensure good wetting and flowability of the filler during VB brazing, an additional 0.1% bismuth can be added.

As a reference, for materials without magnesium in NB brazing, the coating amount of the flux is about 2g/m2 (with an oxygen concentration of around 50ppm). For materials with a magnesium content of 0.2-0.25g/m2, the coating amount of the flux is about 6-8g/m2 (with an oxygen concentration of around 50ppm)

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

When using the protective atmosphere brazing (CAB) brazing method, the temperature rise of thick walled components is significantly slower than that of thin-walled components, so different alloys used for heat exchangers have different Si contents. Due to the decrease in the melting point of the alloy as the Si content increases, different Si contents should be selected for different components to ensure that the filler and metal melt almost at the same time. The filler begins to flow at approximately the midpoint temperature between the lowest and highest temperatures in the melting zone, which is sufficient for brazing.

Aluminum silicon alloy

AA-4343

AA-4045

AA-4047

Silicon calibration value (%)

seven point five

ten

eleven

Liquidus

613℃

591℃

582℃

Solidus line

577℃

577℃

577℃

The Zn in AA4343 and AA4045 serves as anode protection, and the Zn content of commercial brazing materials is generally 1-1.5%. Excessive Zn content reduces the flowability and brazing ability of the brazing materials. AA4047 is a eutectic brazing material that has strong flowability at the recommended brazing temperature and is difficult to control. It has a higher corrosion tendency than other brazing materials and is generally used for flame brazing. Filler metal is generally used as the skin material of composite materials, and according to different purposes, the skin material adopts double-sided composite or single-sided composite, with a composite rate (single side) of 5-10%. Two parts connected by brazing each other only need one part to be made of composite material. When it is not convenient to use composite materials, brazing foil, plate, or wire can be used.

Note: Basic requirements for brazing materials in brazing:

·The appropriate melting point should be several tens of degrees lower than the melting point of the base material. If the melting points of the two are too close, it will make the brazing process difficult to control, and even lead to grain growth, over burning, and local melting of the base material.

·It has good wettability and can fully fill the gap between brazing joints.

·The diffusion effect with the base material ensures a solid bond between them.

·Having stable and uniform composition, minimizing segregation and loss of volatile elements during the brazing process.

·Meet the mechanical and physicochemical performance requirements of the product.

·Economy.

·Fusible solder, commonly known as soft solder, has a melting point below 450 ℃; Above 450 ℃ is a refractory solder, commonly known as hard solder.

NOCOLOK ®   Welding flux and its brazing process

The world's first NB furnace was invented by Alcan Company in Canada in 78, with its registered trademark being NOCOLOK ®。 

In 78-79, SOLVAY Company developed Flux, which is mainly a mixture of KF and AIF3, namely potassium fluoaluminate (KAIF4).

NOCOLOK flux (Flux) does not react with Al at room temperature and brazing temperature, but only has reactive activity at melting (at least partially melting). After melting, the flux dissolves Al2O3 on the surface of Al, wets the joint surface, reduces the surface tension of the liquid solder, allows the liquid solder to freely flow into the joint surface through the action of fine particles, and prevents surface re oxidation. After cooling, the flux forms a layer of 1-2 on the surface of the component μ The residue of m (when the flux load is 5g/m2) has strong adhesion, is not hygroscopic, has no corrosiveness, and will not break during the heat exchange process. It does not need to be removed and can be directly sprayed.

△ Flux brazing characteristics:

1. Flux is a mixture of potassium fluoroaluminate salts;

2. The melting point range is 565 ° C-572 ° C, which is lower than the melting point of the solder (577 ° C);

3. Particle size range 0.2-0.5 μ m. Good paste properties;

4. Non hygroscopic, infinitely long storage period;

5. Non corrosive;

6. The solubility is 0.2-0.4%, with an unlimited shelf life.

Function of flux:

·Remove the oxides on the surface of the base metal and solder, creating necessary conditions for the liquid solder to spread and fill the joints on the base metal.

·Cover the surface of the base metal and solder with a thin liquid layer to isolate air and provide protection.

·Activate the interface and improve the wetting of the base metal by liquid solder.

△NOCOLOK ® Advantages of brazing process:

1. Flux is inert and non corrosive;

2. Large allowable clearance;

3. Good gap filling performance;

4. Strengthen corrosion resistance (synthetic aluminum, Zn deposition treatment);

5. The brazed surface is suitable for chrome treatment/spraying;

6. Allow for imperfect cleaning;

7. Cleaning is not required after brazing;

8. Extremely strong ability to remove oxide film;

9. Simple storage methods;

10. Do not remove brazing residues.

Continuous nitrogen protection brazing furnace

The continuous nitrogen protection brazing furnace is a static atmosphere tunnel furnace.

The brazing furnace is generally composed of a flux coating device, a drying furnace, a brazing chamber, a water cooling chamber, and an air cooling chamber. The flux coating device relies on conveyor belt transportation to spray flux suspension on the heat exchanger and then blow off excess liquid. Dry the brazing flux in a drying chamber at around 200 ℃. The brazing chamber adopts an integral stainless steel muffle structure, with the inlet end of the muffle floating and the outlet end fixed. The stainless steel mesh belt passes through the muffle, and the muffle is protected by nitrogen gas. The workpiece is brazed inside the muffle. Nitrogen enters the muffle from the section where the workpiece heats up to the brazing temperature and is discharged towards the inlet and outlet of the brazing chamber. The muffle is equipped with electric heating components arranged above and below, with PID control in different zones, surrounded by insulation layer and external steel shell. The water-cooled cover chamber and the air-cooled chamber are located at the tail of the brazing chamber. The brazed heat exchanger passes through the water-cooled cover chamber and the air-cooled chamber, and is cooled to room temperature.

Brazing involves three processes: firstly, the filling process of the brazing flux. When the workpiece is heated to around 560 ° C, the brazing flux begins to melt, dissolve the oxide film on the Al surface, and wet the joint surface; The second is the process of filling the brazing seam with solder. When the workpiece is heated to around 577 ° C, the solder (composite layer) begins to melt, and the liquid solder freely flows into the welded joint surface through capillary action; The third is the process of interaction between the solder and the base metal.

After the workpiece comes out of the heating zone, it enters the cooling zone. The workpiece cools in this area, the weld solidifies, and the brazing flux also solidifies and remains on the surface of the component.

The entire welding process is carried out under nitrogen protection with a purity of 99.9995% and a dew point of -40 ℃ (within the range of 300-560 ℃, a small amount of KALF4 evaporates and reacts with the existing moisture to generate a small amount of HF, which greatly damages the muffle furnace and other facilities). During the welding process, the oxygen content in the muffle chamber is required to be less than 100PPM.

Note: Whether the solder can fill the weld seam depends on its capillary flow characteristics in the gap between the base metal.

Factors affecting the wettability of solder:

·The composition of solder and base metal has low interfacial tension and good wettability.

·The influence of temperature leads to an increase in temperature, a decrease in surface tension, and an increase in the wettability of the solder. If the temperature is too high, the brazing material will lose and the grain size of the base material will grow.

·The effect of metal surface oxides: Remove oxides from the surface of the solder and base metal to improve wetting.

·The effect of brazing flux: Destroying the surface oxide film.

·The influence of the surface state of the base metal: The crisscrossing fine grooves on the rougher surface have a special capillary effect on the liquid solder.

·The influence of surface active materials: Any substance that can significantly reduce the surface tension of the solution and thus undergo positive adsorption is called a surface active material.

Composition and function of each section of the continuous nitrogen protection brazing furnace:

This type of brazing furnace generally has a total length of over 20-40m and is composed of the following parts:

(1) Spray soldering flux area (residual oil stains should be removed before spraying soldering flux on the workpiece)

The workpiece is placed on the conveyor belt, which drives at a constant speed. When the workpiece enters the spraying flux area, the nozzle sprays the mixed flux water mixture downwards at a certain angle. Noclok non corrosive brazing flux is generally used as the brazing flux. The flux concentration ranges from 5% to 25%, and the liquid flux infiltrates the brazing gap under capillary action. The excess flux falls off from the non connecting surface of the workpiece under gravity.

(2) Air blowing area

The workpiece enters this area with the movement of the mesh belt. Due to the limited detachment of excess flux on the workpiece solely by gravity. Use a fan to blow air onto the workpiece in this area to remove excess flux. The blown flux from these two areas falls into the receiving water tank and flows to the flux storage tank. Recycling.

(3) Dry area

The function of this area is to remove moisture from the brazing flux and the air moisture adsorbed on the surface of the workpiece, preventing the workpiece from carrying air