Summary: Recent technical advances in brazing aluminum and aluminum alloys have been reviewed in terms of brazing methods, filler metals and fluxes, and their respective development directions have been introduced.
It is observed that brazing of aluminum and aluminum alloys is a rapidly developing field of research and has a wide range of applications. Brazing technology for aluminum and aluminum alloys is attracting more and more attention and appears to have significant potential.
1. Research status of aluminum and aluminum alloy brazing
Aluminum alloys are a popular choice in many industries due to their low density, high strength and excellent corrosion resistance. They are widely used in automobiles, high-speed rail vehicles, aerospace and military applications.
Related Reading: Types of Aluminum and Aluminum Alloy
The unique physical and chemical properties of aluminum alloys can result in several difficulties during the welding process, such as oxidation, hot cracking and pores in the weld. The traditional method of welding aluminum alloys is fusion welding, which requires complex equipment and highly qualified welders with strict technical requirements.
Related Reading: Aluminum Alloy Welding Method and Material Selection
Aluminum brazing is a crucial method of connecting aluminum alloys and is known for its minimal deformation of the welded parts. In recent years, it has gained widespread use in China due to its high dimensional accuracy.
Brazing technology for aluminum and aluminum alloys has been the subject of extensive research in recent years, leading to rapid advances in brazing methods, filler metals and fluxes.
Brazing of aluminum and aluminum alloys is a rapidly developing field due to its excellent properties such as high strength, good corrosion resistance, high conductivity and thermal conductivity. As a result, it is increasingly being used in a variety of industries, including aerospace, aviation, electronics, metallurgy, machine manufacturing and light industry.
In some cases, the use of aluminum has replaced copper and steel, driven by the substantial increase in the cost of copper materials and the desire to reduce weight, improve efficiency, and improve aesthetics. An example of this is the replacement of the copper water tank in cars with an aluminum water tank.
In China, there are only a few large-scale aluminum flux manufacturers, and most of the aluminum flux used is imported from abroad.
Aluminum and aluminum alloys have a low melting point, strong chemical reactivity and high melting point, which makes the use of traditional brazing fluxes difficult. Therefore, special brazing fluxes for aluminum and aluminum alloys must be used to ensure proper brazing.
Furthermore, the corrosion resistance of aluminum and aluminum alloy welded joints can be easily compromised by the use of solder and flux. This is because there is a significant difference in electrode potential between the weld and the base metal, which reduces the corrosion resistance of the joint, especially in the case of soft weld joints.
Most fluxes used to remove the oxide film from the surface of aluminum and its alloys contain highly corrosive materials. Even if these materials are cleaned after brazing, it is challenging to completely eliminate their impact on the corrosion resistance of the joint.
2. Brazing method
Aluminum and aluminum alloys can be brazed using flame brazing, furnace brazing, or salt bath brazing.
Flame brazing is a popular method due to its simple equipment, versatility in terms of gas source, and wide range of applications. It is mainly used for brazing small components and for producing single parts. There are many types of flames available, including a new type of gas called Sharp gas, which is the result of cooperation between China and other countries. This gas has a soft flame and is a good heating source for brazing aluminum, as it lies between the strengths of liquefied gas and oxyacetylene. However, compared to other connection methods, the heating temperature for flame brazing of aluminum and aluminum alloys is difficult to control, requiring higher levels of operator experience.
Salt bath brazing offers fast and uniform heating, minimal component deformation and effective film removal, resulting in high quality brazed components with high production efficiency. This method is particularly suitable for mass production and for welding dense structures. Solder paste, solder foil or solder coating are commonly used for salt bath brazing of aluminum. Weld cladding is typically composed of eutectic AlSi or hypoeutectic AlSi compositions.
At present, brazing production mainly uses filler metal cladding, which can improve production efficiency and ensure the quality of welded components.
Brazing has some limitations:
Firstly, the complex design of some components can make access to the salt bath difficult, limiting design options and complicating the brazing process. This can also make it difficult to guarantee the quality of the brazing.
Second, although salt bath brazing can meet stringent corrosion resistance requirements, it can result in a large amount of flux residue on the component, requiring extensive cleaning. Furthermore, salt bath brazing equipment is expensive and the process is complex, leading to a long production cycle.
Air furnace brazing offers a low-cost equipment investment and a simple, easy-to-manage brazing process. However, the heating process is slow and the component surface can oxidize when exposed to air, especially at high temperatures. This makes it difficult to remove the flux film and the flux may also fail due to moisture in the air during heating.
To overcome these challenges, dry air furnace brazing and vacuum brazing in a protective atmosphere were developed and gained widespread use in the brazing of aluminum and aluminum alloys. These methods offer improved processes and have seen rapid growth in recent years.
2.1 Vacuum brazing
Aluminum is known to be active and easily forms a dense oxide film on its surface.
During the brazing process, it can be challenging to remove oxides through vacuum conditions alone. As a result, metallic activators such as Mg and Bi must be used.
It is widely accepted that the activator removal mechanism works as follows:
Firstly, the activator reacts with residual O and HO in a vacuum, neutralizing their harmful effects on aluminum brazing.
Secondly, the Mg vapor penetrates the layer of material below the film and forms a low-melting AlSiMg alloy together with diffused Si.
During brazing, melting of the alloy breaks the bond between the oxide film and the base material, allowing the molten solder to wet the base material, spread over it under the film, and lift the oxide film from the surface, effectively removing it.
When vacuum brazing aluminum alloys, the vacuum furnace should be chosen based on factors such as productivity, cost, weld size and structure.
It is important to thoroughly clean the solder before brazing. Surface oxide can be removed with acid or alkali, and oil stains can be removed with alcohol.
To prepare the filler metal, sandpaper is often used to remove the oxide film from the surface, followed by cleaning with alcohol to remove oil stains.
For larger parts, preheating before welding is recommended to ensure uniform heating of all parts before reaching brazing temperature.
Vacuum brazing of aluminum alloys relies heavily on Mg activator to remove the oxide film. To ensure that the base metal is fully exposed to Mg vapor in welding with complex structures, some national units have adopted complementary measures such as local shielding, resulting in improved brazing quality.
A common method is to place the part inside a stainless steel lid with Mg chips and then into the vacuum brazing furnace for brazing. This can significantly improve the brazing quality.
The degree of vacuum is the most crucial and challenging process parameter to control in vacuum brazing. To obtain high-quality joints, the degree of vacuum largely depends on the size of the workpiece.
Based on years of experience of some experts, it is advised that if the brazing equipment has not been used for a long period, the vacuum furnace should be operated for several hours before use. In regular use, especially for batch production, it is recommended to keep the time interval between uses as short as possible to ensure that the vacuum degree of the vacuum oven meets the requirements easily and quickly.
Although vacuum brazing is an effective brazing method, it also has some limitations, such as complex and expensive equipment and the difficulty of maintaining the vacuum system.
2.2 Brazing in a protective atmosphere
The use of aluminum vacuum brazing is limited due to the expensive equipment and complex technology involved. To solve this problem, a neutral atmosphere can replace the vacuum. This reduces requirements for system leak rate and equipment complexity. Furthermore, it reduces equipment maintenance problems caused by the deposition of volatile elements, resulting in lower production costs.
Heating in this method is achieved mainly through current and is rapid and uniform. This not only guarantees product quality but also improves productivity.
Neutral gas shielded aluminum brazing has gained more and more attention and has seen rapid development in recent years. It is considered a promising method of aluminum brazing.
The film removal mechanism for gas shielded brazing of aluminum alloys is similar to that of vacuum brazing of aluminum and is mainly carried out using Mg activator. Brazing quality can be improved by adding Bi to the filler metal.
Pure argon and nitrogen, with purity greater than 99.99%, are commonly used as atmosphere for gas shielded brazing of aluminum alloy.
For Al/Al and Al/Cu joints, it has been reported that an effective bonding method is to use the principle of diffusion brazing. A mixed powder composed of potassium aluminum fluoride brazing flux is sprayed onto the surface of aluminum in a nitrogen atmosphere close to atmospheric pressure for brazing. Si can be replaced by other low-melting eutectic metals, such as Cu, Ge or Zn, which form with Al.
3. Soldering
During brazing, the connection between the welds is made by the solidification of the molten solder. As a result, the quality of the weld largely depends on the filler metal used.
The main filler metal of aluminum is Al Si alloy, but sometimes Cu, Zn, Ge and other elements are added to improve process performance.
With years of experience and experimentation, several series of aluminum brazing filler metals have been developed, many of which have produced satisfactory results with the correct processes.
In the following, we will introduce some of the most commonly used aluminum alloy brazing filler metals.
3.1 Al Si Welding
Al Si series solders are based on the eutectic Al Si composition and also include hypoeutectic, hypereutectic and Al Si alloys with a maximum of 5% added elements. These welds are highly weldable, strong, have a similar color and luster to the base metal, offer plating and corrosion resistance, and are considered a good choice for welding.
Furthermore, this series of welds can be modified, which significantly improves its strength and flexural performance in weld joints.
Recently, a new type of Al Si alloy brazing filler metal was developed using rapid solidification technology. This brazing filler metal has a lower liquid phase point of around 3-5°C compared to common crystalline brazing filler metals of the same composition. Its wettability coefficient increased by 18% and its resistance increased by 28.4%. Its fluctuations are also minimal, providing a certain degree of processing flexibility.
3.2 Copper soldering
Copper welding is carried out based on the principle of contact reactive brazing. Currently, aluminum contact reactive brazing is considered the ideal solution to aluminum brazing problems.
This method offers several benefits, including:
① No flux required, making it environmentally friendly and avoiding contamination of brazing products. There is no need to clean the welded products, and there is no chemical corrosion on the brazing seam.
② Selection of appropriate eutectic reactive alloy layer can lower the brazing temperature, reducing energy consumption, making the brazing process easier to control and having low equipment requirements.
The contact reaction of Cu on the aluminum substrate shows a notable preferential surface spreading, disrupting the oxide film and promoting the formation of a uniform layer of liquid phase filler between the joint interfaces in the contact reactive brazing process. On the other hand, the grain boundary with contact reaction in the direction of the depth of the aluminum matrix penetrates preferentially, guaranteeing the bond strength of the welded joint.
The data shows that the appropriate process parameters for reactive contact brazing of aluminum with Cu as interlayer material are a brazing temperature of 570-580°C and a holding time of 15-20 minutes. However, the electrochemical corrosion resistance of Cu welded joints is poor and the eutectic reaction layer is brittle.
To improve the performance of Cu as a filler metal, other elements can be added, such as Ag, Ni, Si, Zn, Ti, etc. The filler metal for reactive brazing with aluminum alloys includes these elements.
3.3 Composite layer of copper and zinc as reactive filler metal
To solve the limitations of using Zn and Cu as filler metals separately, a layer composed of both can be used. Brazing by contact eutectic reaction is carried out using the layer composed of Cu and Zn.
A peritectic reaction occurs at the Cu/Zn interface, while a eutectic reaction occurs at the Cu/Al interface, forming a eutectic liquid phase that disrupts the oxide film on the aluminum surface.
When using Cu and Zn as reactive filler metal for aluminum brazing, the appropriate content of both metals in the composite layer is crucial. It has been suggested that the best brazing results are achieved when the Zn layer thickness is 0.2 mm and the Cu layer thickness is less than 0.1 mm.
At this point, the reaction layer not only breaks the oxide film, but also provides strong electrochemical corrosion resistance and high shear strength.
3.4 Al Si Cu Zn Welding
The temperature range of the solder liquid phase point is between 500-577°C. When Cu is added to Al Si solder, its fluidity is greatly improved.
However, due to the high content of CuAl2 intermetallic compound, this ternary eutectic solder is very brittle and is only suitable for strip molding, making it difficult to process into wire or sheet form.
Adding Zn to Al Si filler metal increases its wettability and fluidity. As the concentration of Zn increases, the solubility of Si decreases rapidly. Since there are no compounds in the filler metal, its hot workability is better compared to the Al Si Cu system.
3.5 Al Cu Ag Zn Series Weld
The temperature range of the liquid phase of the weld is 400-500°C, which is close to the range of aluminum alloy weld. The ternary eutectic composition Al Cu Ag gives the filler metal a color very close to the base metal Al.
This filler metal has good fluidity but is relatively brittle. Another ternary system is the Al Cu Zn filler metal, which is also close in color to the base metal and can produce better machined parts.
Adding 0.05% – 0.08% (by mass) Mg, 0.05% Ni or 0.05% Cr to the filler metal can improve its corrosion resistance.
There are many other ideal filler metals for aluminum, but in general, most existing aluminum brazing filler metals have a melting point close to that of aluminum alloys.
As a result, it is a challenge for most welding workers to find a filler metal with a lower melting point and better technological performance.
4. Flux for aluminum
Aluminum is relatively active, and its surface easily forms a dense and chemically stable oxide layer, which is a major obstacle in brazing aluminum and aluminum alloys. To obtain high-quality joints, surface oxide must be removed.
When brazing aluminum and its alloys, the use of a brazing flux can remove the oxide film from the aluminum surface and reduce the interfacial tension between the filler metal and the base metal.
Brazing flux for aluminum is divided into soft soldering flux and brazing flux, the latter being used for brazing temperatures above 450°C and the former for temperatures below 450°C.
The rapidly developing Nocolok aluminum brazing flux is presented below. Traditional aluminum brazing flux is mainly chlorine salt brazing flux, generally based on the LiCl-KCl or LiCl-KCl-NaCl system. This flux has the advantages of high activity, stability during heating and not easily losing its effectiveness. It can be used with a variety of heating sources, making it convenient and inexpensive.
However, the disadvantage of this flux is that the presence of Cl ions causes strong electrochemical corrosion in the base metal, has strong moisture absorption and is difficult to preserve.
Therefore, it is essential to clean residues when using this type of flux for brazing.
By the late 1970s, development of a non-corrosive, insoluble brazing flux was underway. This flux is synthesized using the A-KF eutectic and its solubility in water is minimal.
It avoids the disadvantage of chloride flux, which easily absorbs moisture and has very little corrosiveness, hence its nickname Nocolok flux.
4.1 Flow properties
Nocolok flux is a fine white powder, mainly composed of a potassium fluoaluminate mixture that may contain crystal clear water.
The molten flux dissolves the oxides on the aluminum surface and prevents reoxidation. Under the influence of flow, the filler metal freely penetrates the joint surface through capillary action.
After cooling, the flux forms a pasty film with strong adhesion on the surface of the component. The residual flux layer is non-hygroscopic, non-corrosive and insoluble in aqueous solvents.
Although the solubility of potassium fluoaluminate flux in water is minimal, its thermal stability is not strong, and chemical reactions will occur when heated in air.
4.2 Improvement and further progress of the Nocolok flow
In recent years, many studies have focused on improving the Nocolok method in two main ways: adding additional salts to the potassium fluoaluminate stream to increase its activity and other properties, and developing new methods of using the potassium fluoaluminate stream. potassium.
Si can increase the flux activity of potassium fluoaluminate.
The ideal way is to add it in the form of K 2 SiF 6 but the value of the excess KF must be calculated.
When W(Si) > 2%, it can drill automatically.
Adding K 2 GeF 6 SnF 2 ZnF 2 etc. can improve flow activity, especially K 2 GeF 6 .
In improving Nocolok, someone mixed filler metal powder with this type of flux.
Others consider KAlF 4 as a gas phase brazing method:
One is to directly mix KAlF 4 steam into low-pressure oxygen-free atmosphere for aluminum alloy brazing;
The other is to vacuum deposit a layer of KA1F 4 on the outside of the aluminum parts, then assemble and weld again as needed.
The composite solder formed by depositing a layer of KAlF 4 flux on the surface of the Al Si eutectic solder powder can be mixed with solder paste with organic solvent.
5. Conclusion
Brazing of aluminum and aluminum alloys has been extensively studied and rapidly developed in recent years.
Foreign scholars demonstrated the exceptional bond strength of Sn-Zn eutectic solder (8.9%) when brazing aluminum alloys below 350°C, investigating the interface reaction between the Sn-Zn eutectic alloy in liquid phase and Al .
Diffusion brazing of aluminum has also received considerable attention in recent years.
One approach involves spraying a mixed powder composed of Si and potassium aluminum fluoride flux onto the Al surface and brazing in a N2 atmosphere near atmospheric pressure.
Among the materials used, Si can be replaced by Cu, Ge, Zn and other metals that form low melting point eutectics with aluminum.
This method can be used to weld Al/Al, Al/Cu, Cu/Cu and Cu/brass joints.
Diffusion brazing is also used to weld Al-Si alloy castings, solving the problem of corrosion and poor wetting of Al alloy castings in molten solder.
There is still much progress to be made in aluminum and aluminum alloy brazing technology, and some advances have already been applied to practical production.
The application of aluminum and aluminum alloy brazing technology mainly focuses on aluminum radiators, aluminum-stainless steel non-materials, aluminum alloy microwave door frames and other products.
Another area of research and application is the brazing of pan bottoms composed of aluminum and stainless steel.
Although brazing aluminum and aluminum alloys is an excellent joining technology, there are still many challenges to be faced.
