Abstract: The technical development of aluminum and aluminum alloy brazing in recent years in three aspects of brazing methods, filler metals and fluxes is reviewed, and their respective development directions are introduced.
It is pointed out that the brazing of aluminum and aluminum alloys is one of the research fields that have been studied more and developed rapidly in recent years, and the brazing technology of aluminum and aluminum alloys has broad application prospects.
1. Research status of aluminum and aluminum alloy brazing
Aluminum alloys are widely used in automobiles, high-speed railway vehicles, aerospace and military industries because of their low density, high strength and corrosion resistance.
Related reading: Types of Aluminum and Aluminum Alloy
Due to its unique physical and chemical properties, it will encounter a series of difficulties in the welding process, such as oxidation, weld hot cracks and pores.
For the welding of aluminum alloy, the traditional method is mainly fusion welding, with complex equipment and strict technical requirements for welders.
Related reading: Aluminum Alloy Welding Method and Material Selection
As an important method of aluminum alloy connection, aluminum brazing has small deformation of brazed parts.
In recent years, it has been widely used in China because of its high dimensional accuracy.
The brazing technology of aluminum and aluminum alloys has been studied extensively in recent years.
With the continuous emergence of new materials and new methods, the brazing process of aluminum and aluminum alloys has also been developed rapidly, and its brazing methods, filler metals and fluxes have made great progress.
The brazing of aluminum and aluminum alloys is one of the fields that have been studied and developed rapidly in recent years.
This is mainly because it has a series of excellent properties, such as high strength, good corrosion resistance, high conductivity and thermal conductivity, so it is increasingly widely used in aerospace, aviation, electronics, metallurgy, machinery manufacturing and light industry.
Especially, with the substantial price increase of copper materials, and in order to reduce the quality, improve the efficiency and enhance the aesthetics, the technology of replacing copper with aluminum and replacing steel with aluminum has been successfully applied in some fields.
The most typical is that the copper water tank of the automobile is replaced by the aluminum water tank.
There are few large-scale aluminum flux manufacturers in China, and most of the aluminum flux currently used is imported from abroad.
Because aluminum and aluminum alloys have low melting point, strong chemical activity, high melting point and stability of oxide film, and can firmly and tightly adhere to the surface of aluminum or aluminum alloys, general brazing fluxes cannot meet the requirements for brazing aluminum and aluminum alloys, and special brazing fluxes – brazing fluxes for aluminum and aluminum alloys must be used.
In addition, the corrosion resistance of aluminum and its alloy brazed joints is easily affected by solder and flux.
This is mainly because the electrode potential difference between solder and base metal is great, which reduces the corrosion resistance of joints, especially for soft solder joints.
Generally, in order to remove the oxide film on the surface of aluminum and its alloys, most of the fluxes are added with materials with strong corrosivity.
Even if these materials are cleaned after brazing, it is difficult to completely remove the impact on the corrosion resistance of the joint.
2. Brazing method
Brazing of aluminum and aluminum alloys can be carried out by flame brazing, furnace brazing and salt bath brazing.
Flame brazing has simple equipment, wide gas source, great flexibility and wide application.
It is mainly used for brazing small weldments and single piece production.
There are many kinds of flame available.
It is reported that China has cooperated with other countries to produce Sharp gas between liquefied gas and oxyacetylene.
This kind of gas has a soft flame and its strength is between that of liquefied gas and oxyacetylene.
It is a good heating source for aluminum brazing.
However, compared with other connection methods, the heating temperature of aluminum and aluminum alloy flame brazing is difficult to master, and requires higher experience for the operator.
Salt bath brazing has the advantages of fast and uniform heating, difficult deformation of weldments, and sufficient film removal, so the weldments have good quality and high production efficiency.
It is especially suitable for mass production, especially for the welding of dense structure brazing seam.
Paste, foil solder or solder coating are generally used for aluminum salt bath brazing.
The solder coating is of Al Si eutectic composition or Ai Si hypoeutectic composition.
At present, brazing production mostly uses filler metal cladding, which can improve production efficiency and ensure brazing quality.
First of all, since the workpiece is heated and the oxide film is removed by molten salt, it is difficult to enter and exit salt for the workpiece with complex structure, which limits the structure design and process, complicates it, and it is not easy to ensure the welding quality.
Secondly, due to the specific use environment and service life requirements, some products have high requirements for corrosion resistance, while a large amount of flux remains in the workpiece after salt bath brazing, which requires a long cleaning time.
In addition, salt bath brazing equipment has large investment, complex process and long production cycle.
Brazing in air furnace has small equipment investment, simple brazing process and convenient operation.
However, this method is slow in heating, and the workpiece surface is easy to oxidize when heated in air, especially when the temperature is high, which is not conducive to the removal of the flux film.
In addition, the flux will fail due to the moisture in the air during heating.
In view of this situation, brazing in dry air furnace has been developed.
Vacuum brazing and furnace brazing in protective atmosphere, due to their unique and excellent processes, have been widely used in the brazing of aluminum and aluminum alloys, and have developed rapidly.
2.1 Vacuum brazing
Aluminum is relatively active and easy to form a dense oxide film on the surface.
During brazing, it is difficult to remove oxides only depending on vacuum conditions, and some metal activators, such as Mg, Bi, must be used at the same time.
It is generally believed that the membrane removal mechanism of the activator is:
On the one hand, the activator reacts with the residual O and HO in vacuum, eliminating their harmful effects on aluminum brazing;
On the other hand, Mg vapor infiltrates into the surface material layer under the film and forms a low melting point Al Si Mg alloy together with the diffused Si.
During brazing, the alloy melts to destroy the bonding between the oxide film and the base material, so that the melted solder can wet the base material, spread on the base material under the film, and float the surface facial mask to remove it.
When vacuum brazing aluminum alloy, vacuum furnace should be selected according to productivity, cost, weldment size and structure.
The weldment shall be cleaned carefully before brazing.
The oxide on the surface can be washed away with acid or alkali.
If there is oil stain on the surface, it can be wiped with alcohol.
For the treatment of filler metal, sand paper is generally used to remove the surface oxide film, and then alcohol is used to remove the oil stain.
For larger workpieces, preheat them before welding to ensure that all parts are heated evenly before the weldment temperature reaches the brazing temperature.
As the vacuum brazing of aluminum alloy mainly depends on Mg activator to remove the film, for weldments with complex structure, in order to ensure that the base metal is fully affected by Mg vapor, some domestic units have adopted the supplementary process measures of local shielding, and achieved better results.
The general method is to put the workpiece into the stainless steel cover, put Mg chips in it, and then put it into the vacuum brazing furnace for brazing, which can greatly improve the brazing quality.
The most important and difficult process parameter to control in vacuum brazing is the vacuum degree.
To obtain high-quality joints, the vacuum degree largely depends on the size.
According to years of experience of some staff, if the brazing equipment has not been used for a long time, the vacuum furnace should be operated for several hours before use.
In use, especially in batch production, the time interval between two uses should be as short as possible, so that the vacuum degree of the vacuum furnace can easily and quickly meet the requirements.
Vacuum brazing is an excellent brazing method, but it also has some shortcomings, such as complex and expensive equipment, and difficult maintenance technology of vacuum system.
2.2 Brazing in protective atmosphere
Due to the expensive equipment and complex technology, the application of aluminum vacuum brazing is limited.
To solve this problem, we can replace vacuum with a neutral atmosphere.
In this way, the requirements for system leakage rate are reduced, the equipment is relatively simple, and the equipment maintenance problems caused by volatile element deposition are reduced, so the production cost is low.
The heating of this method mainly depends on the current.
The heating speed is fast and uniform, which can not only ensure the product quality but also improve the productivity.
In recent years, neutral gas shielded aluminum brazing has been paid more attention and developed rapidly.
It is a promising aluminum brazing method.
For gas shielded brazing of aluminum alloy, the film removal mechanism is the same as that of aluminum vacuum brazing, and it is mainly completed by Mg activator.
I will not repeat here.
The brazing quality can be improved by adding Bi to the filler metal.
Argon and pure nitrogen are generally used as the atmosphere for aluminum alloy gas shielded brazing, and the purity of nitrogen must be greater than 99.99%.
It is reported that for Al/Al, Al/Cu and other joints, a bonding method is to use the principle of diffusion brazing, spray a mixed powder composed of Si and potassium aluminum fluoride brazing flux on the aluminum surface in nitrogen atmosphere close to atmospheric pressure for brazing, and the effect is ideal.
Si can be replaced by Cu, Ge, Zn and other low melting point eutectic metals formed with Al.
During brazing, the weldments are connected by solidification of melted solder.
Therefore, the quality of the weld depends on the filler metal to a large extent.
Al Si alloy is the main aluminum filler metal.
Sometimes Cu, Zn, Ge and other elements are added to meet the requirements of process performance.
After years of practice and exploration, several series of aluminum brazing filler metals have been formed, many of which have achieved satisfactory brazing results with appropriate processes.
Several new and commonly used aluminum alloy brazing filler metals are mainly introduced below.
3.1 Al Si solder
Al Si series solders are based on Al Si eutectic composition, and also include hypoeutectic, hypereutectic and Al Si alloys with no more than 5% added elements.
This series of solders has good solderability, strength, color and luster of base metal, plating and corrosion resistance, and is a good solder.
Moreover, this series of solders can also be modified, which can greatly increase the toughness and bending performance of solders and solder joints.
Now, a new type of Al Si alloy brazing filler metal is developed by using rapid solidification technology.
Compared with ordinary crystalline brazing filler metal, the liquid phase point of this brazing filler metal is about 3-5 ℃ lower than that of ordinary crystalline brazing filler metal with the same composition, the wettability coefficient is increased by 18%, and the strength is increased by 28.4%, and its fluctuation is small, so it has a certain processing flexibility.
3.2 Copper solder
Copper is welded according to the principle of contact reactive brazing.
At present, aluminum contact reactive brazing is considered to be an ideal method to solve the aluminum brazing problem.
This method has the following advantages:
① No flux, no pollution to the environment, no need to clean the brazing products, no chemical corrosion in the brazing seam;
② Selecting appropriate eutectic reactive alloy layer can reduce the brazing temperature, which not only reduces the energy consumption, makes the brazing process easy to control, but also has low requirements for equipment.
The contact reaction of Cu on aluminum substrate has obvious surface preferential spreading, which can break the oxide film, and is conducive to the formation of uniform liquid phase filler layer between the joint interfaces in the process of contact reactive brazing;
On the other hand, the grain boundary with contact reaction in the depth direction of aluminum matrix preferentially penetrates, which can ensure the bonding strength of the brazed joint.
Some data show that the appropriate process parameters for aluminum contact reactive brazing with Cu as the interlayer material are: brazing temperature 570-580 ℃, holding time 15-20 min.
However, the electrochemical corrosion resistance of Cu welded joint is poor, and the eutectic reaction layer is brittle.
In order to improve the performance of Cu as filler metal alone, some other elements can be added.
The filler metal for reactive brazing with aluminum alloy includes Ag, Ni, Si, Zn, Ti, etc.
3.3 Copper and zinc composite layer as reactive filler metal
In order to make up for the shortcomings of Zn and Cu as filler metal alone, the composite layer of both can be used.
Contact eutectic reaction brazing is carried out with Cu and Zn composite layers.
Peritectic reaction occurs at the Cu/Zn interface, eutectic reaction occurs at the Cu/Al interface, and eutectic liquid phase is formed to break the oxide film on the aluminum surface.
As a reactive filler metal for aluminum brazing, the Cu and Zn content of the Cu and Zn composite layer must be appropriate.
It has been proposed that the brazing result is ideal when δ(Zn)=0.2mm，δ(Cu)＜0．1 mm.
At this time, the reaction layer not only can break the oxide film, but also has strong electrochemical corrosion resistance and high shear strength.
3.4 Al Si Cu Zn solder
The temperature range of the liquid phase point of the solder is 500-577 ℃.
When Cu is added to Al Si solder, the fluidity of the solder is significantly enhanced.
Because of the high content of CuAl2 intermetallic compound, this ternary eutectic solder is very brittle, and is only suitable for casting into strips, so it is difficult to process into wire or foil.
The wettability and fluidity of Al Si filler metal are enhanced after adding Zn.
With the increase of Zn concentration, the solubility of Si decreases rapidly.
As there is no compound in the filler metal, its hot workability is better than that of Al Si Cu system.
3.5 Al Cu Ag Zn series solder
The liquid-phase temperature range of the solder is 400-500 ℃, which is close to the range of aluminum alloy solder.
The Al Cu Ag ternary eutectic composition makes its color very close to that of the Al base metal.
The fluidity of brazing is good, but it is relatively brittle.
The other ternary system is Al Cu Zn. The color of this filler metal is also close to that of the base metal, and the machined parts can be better.
Adding 0.05% – 0.08% (mass fraction, the same below) Mg, 0.05% Ni or Cr to the filler metal can improve the corrosion resistance of the filler metal.
There are many more ideal filler metals for aluminum.
In general, the melting point of most existing aluminum brazing filler metals is close to that of aluminum alloys.
Therefore, it is a task for the majority of welding workers to find a filler metal with lower melting point and better technological performance.
4. Flux for aluminum
Aluminum is relatively active, and its surface is easy to generate a layer of dense and chemically stable oxide, which is one of the main obstacles in aluminum and aluminum alloy brazing.
In order to obtain high-quality joints, the oxide on the surface must be removed.
When brazing aluminum and its alloys, the use of brazing flux can remove the oxide film on the aluminum surface and reduce the interfacial tension between the filler metal and the base metal.
The brazing flux for aluminum is divided into soft soldering flux and brazing flux.
Generally, the brazing flux used for brazing temperature higher than 450 ℃ is brazing flux, and the brazing flux used for brazing temperature lower than 450 ℃ is soft soldering flux.
The aluminum brazing flux, especially Nocolok flux, which is developing rapidly, is introduced below.
The traditional aluminum brazing flux is mainly chlorine salt brazing flux, which is generally based on LiCl-KCl or LiCl-KCl-NaCl system.
This kind of flux has the advantages of high activity, stability during heating and not easy to lose efficacy.
The utility model can use various heating sources, which is convenient and cheap.
The disadvantage of this flux is that the existence of Cl ions will cause strong electrochemical corrosion to the base metal, with strong moisture absorption, and it is difficult to preserve the flux.
Therefore, it is important to clean the residues when using this type of flux for brazing.
In the late 1970s, a non corrosive and insoluble brazing flux was rapidly developed.
The flux is synthesized by using A-KF eutectic, and its solubility in water is very small.
It avoids the disadvantage of chloride flux that is easy to absorb moisture, and its corrosivity is also very small, so it is also called Nocolok flux.
4.1 Properties of flux
Nocolok flux is a fine white powder. Its main component is a mixture of potassium fluoaluminate, which may contain a crystal water.
The molten flux will dissolve the oxides on the aluminum surface and prevent re oxidation.
Under the action of the flux, the filler metal freely penetrates into the joint surface through capillarity.
After cooling, the flux forms a paste film with strong adhesion on the surface of the component.
The residual layer of the flux is non hygroscopic, non corrosive, and insoluble in aqueous solvents.
Although the solubility of potassium fluoaluminate flux in water is very small, its thermal stability is not good, and chemical reaction will occur when heated in air.
4.2 Improvement and new progress of Nocolok flux
In recent years, a large number of literatures have reported on the improved Nocolok method, mainly in two aspects:
One is to add a third or more salts in potassium fluoaluminate flux to increase the activity and other properties of the flux;
The other is to develop new application methods of potassium fluoaluminate flux.
Si can improve the activity of potassium fluoaluminate flux.
The ideal way is to add it in the form of K2SiF6, but the amount of excess KF should be calculated.
When W (Si)>2%, it can self drill.
Adding K2GeF6, SnF2, ZnF2, etc. can improve the activity of the flux, especially K2GeF6.
In the improvement of Nocolok, someone mixed the filler metal powder with this kind of flux.
Others regard KAlF4 as the method of gas phase brazing:
One is to directly mix KAlF4 vapor into the low-pressure non oxygen atmosphere for aluminum alloy brazing;
The other is to vacuum deposit a layer of KA1F4 on the outside of aluminum parts, and then assemble and re braze as required.
The composite solder formed by depositing a layer of KAlF4 flux on the surface of Al Si eutectic solder powder can be mixed into solder paste with organic solvent.
The brazing of aluminum and aluminum alloys is one of the research fields that have been studied more and developed rapidly in recent years.
In recent years, foreign scholars have explained the special bonding strength of Sn Zn eutectic solder (8.9%) when brazing aluminum alloy below 350 ℃ by studying the interface reaction between liquid phase Sn Zn eutectic alloy and Al.
Diffusion brazing of aluminum has also received great attention in recent years.
One of them is to spray a mixed powder composed of Si and potassium aluminum fluoride flux on the surface of Al, and braze in N2 atmosphere close to atmospheric pressure.
Among them, Si can be replaced by Cu, Ge, Zn and other metals that can form low melting point eutectic with aluminum.
This method can be used to braze Al/Al, Al/Cu, Cu/Cu and Cu/brass joints.
Diffusion brazing is also used to weld Al Si alloy castings, which solves the problem that Al alloy castings are easy to be corroded and not well wetted in molten solder.
There is still a lot of research progress in aluminum and aluminum alloy brazing technology, and some have been applied to actual production.
The application of aluminum and aluminum alloy brazing technology is mainly aimed at aluminum radiator, aluminum stainless steel dissimilar materials, aluminum alloy door frame of microwave and other products.
Another research and application field is the brazing of aluminum stainless steel composite pot bottom.
Aluminum and aluminum alloy brazing is an excellent joining technology, but there are still many problems to be solved.