1. Brazability
The brazing of aluminum and aluminum alloys is low, mainly because the oxide film on the surface is difficult to remove. Aluminum has a strong affinity for oxygen and easily forms a dense, stable, high-melting-point Al2O2 oxide film on the surface.
Magnesium-containing aluminum alloys also form a very stable Mgo oxide film. They seriously impair the wetting and spreading of the solder and are difficult to remove. Only using suitable flux can the brazing process be carried out.
In addition, the operation difficulty of brazing aluminum and aluminum alloys is high. The melting point of aluminum and aluminum alloys is not very different from the melting point of the hard solder used, and the temperature range available for brazing is very narrow.
Inadequate temperature control can easily cause the base material to overheat or even melt, complicating the brazing process. Some heat-treated aluminum alloys may suffer from excessive aging or annealing softening due to brazing heat, resulting in a decrease in weld joint performance.
In flame brazing, it is not easy to judge the temperature due to the unchanged color of the aluminum alloy during heating, which also increases the demand on the operator's skill level.
Furthermore, the corrosion resistance of aluminum and aluminum alloy welded joints is easily affected by the solder and flux used. The electrode potential of aluminum and aluminum alloys differs greatly from that of welding, which reduces the corrosion resistance of the joint, especially for soft welded joints.
Furthermore, most fluxes used in brazing aluminum and aluminum alloys are highly corrosive, and even if they are cleaned after brazing, the impact of the flux on the corrosion resistance of the joint cannot be completely eliminated.
2. Brazing Materials
(1) Welding:
Soft welding of aluminum and aluminum alloys is not commonly used because the difference in composition and electrode potential between the weld and the base material in soft welding can easily cause electrochemical corrosion of the joint.
Zinc-based solders and tin-lead solders are mainly used for soft welding, which can be divided into low temperature soft solders (150-260°C), medium temperature soft solders (260-370°C) and high . soft temperature welds (370-430°C) according to the temperature range of use.
When tin-lead solder is used for brazing and copper or nickel are pre-coated on the aluminum surface, corrosion at the interface can be prevented, thereby improving the corrosion resistance of the joint.
Hard welding of aluminum and aluminum alloys is widely used, such as filter guides, evaporators, heat sinks and other components.
Only aluminum-based solder can be used for hard welding of aluminum and aluminum alloys, among which aluminum-silicon solder is the most widely used. The specific application range and shear strength of welded joints are shown in Table 8 and Table 9, respectively.
However, the melting point of these solders is close to that of the base material, so the heating temperature must be strictly and precisely controlled during brazing to avoid overheating or melting of the base material.
Table 8: Applicable range of hard welds for aluminum and aluminum alloys
| Brazing Material Class | Brazing temperature /℃ |
Brazing Methods | Aluminum and aluminum alloys suitable for brazing |
| B-Al92Si | 599~621 | Dip, Oven | 1060-8A06,3A21 |
| B-Al90Si | 588~604 | Dip, Oven | 1060-8A06, 3A21 |
| B-Al88Si | 582~604 | Dip, Oven, Flame | 1060-8A06, 3A21,1F1,LF2,6A02 |
| B-Al86SiCu | 585~604 | Dip, Oven, Flame | 1060-8A06,3A21,1F1,5A02,6A02 |
| B-Al76SiZnCu | 562~582 | Flame, Furnace | 1080-8A06,3A21,LF1,5A02,6A02 |
| B-Al67CuSi | 555~576 | Flame | 1060-8A06,3A21,LF1,5A02,6A02,2A50,2L102,ZL202 |
| B-Al90SiMg | 599~621 | Vacuum | 1060-8A06、3A21 |
| B-Al88SiMg | 588~604 | Vacuum | 1060-8A06,3A21,6A02 |
| B-Al86SiMg | 582~604 | Vacuum | 1060-8A06,3A21,6A02 |
Table 9: Shear strength of aluminum and aluminum alloy joints welded with aluminum-silicon solder
| Brazing Material Class | Tensile strength /MPa |
||
| Pure Aluminum | 3A21 | 3A12 | |
| B-A188Si | 59~78 | 98~118 | – |
| B-A167CuSi | 59~78 | 88~108 | 118~196 |
| B-A186SiCu | 59~78 | 98~118 | – |
| B-A176SiZnCu | 59~78 | 98~118 | – |
Aluminum-silicon brazing materials are typically supplied in the form of powder, paste, wire or metal foil. In some cases, a composite brazing plate is used, which consists of an aluminum core and an aluminum-silicon brazing material as the cladding layer. This composite plate is produced using hydraulic methods and is commonly used as a component of welded assemblies.
During brazing, the brazing material in the composite plate melts and flows to fill joint gaps, aided by capillary action and gravity.
(2) Flux and shielding gas are commonly used in the soft brazing of aluminum and aluminum alloys.
Brazing aluminum and aluminum alloys often requires the use of specialized fluxes to remove oxide films. Triethanolamine-based organic fluxes such as FS204 are used with low temperature brazing alloys.
These fluxes have the advantage of minimal corrosion in the base material, but generate a large amount of gas during the flow, which can affect the wetting and filling of the brazing material.
Zinc chloride based reactive fluxes such as FS203 and FS220A are used with medium and high temperature brazing alloys. Reactive fluxes have strong corrosive properties and their residues must be carefully cleaned after brazing.
Hard brazing of aluminum and aluminum alloys still relies on flux removal. Brazing fluxes used include chloride-based fluxes and fluorine-based fluxes. Chloride-based fluxes have strong deoxidizing ability and good fluidity, but they have a significant corrosive effect on the base material, so their residues must be completely removed after brazing.
Fluorine-based fluxes are a new type of flux with good deoxidizing effects and no corrosive effects on the base material. However, they have a high melting point and low thermal stability, and can only be used in combination with aluminum-silicon brazing materials.
In hard brazing of aluminum and aluminum alloys, vacuum, neutral or inert atmospheres are commonly used. In vacuum brazing, the vacuum level should generally reach the order of 10-3 Pa. When using nitrogen or argon shielding, high purity and low dew point below -40 ℃ are required.
3. Brazing Techniques
Brazing aluminum and aluminum alloys requires high cleanliness of the part surface. To obtain good quality, the surface oil and oxide films must be removed before brazing. Surface oil can be removed by washing with sodium carbonate (Na 2 CO 3 ) water solution at a temperature of 60-70°C for 5-10 minutes, followed by rinsing with clean water.
Surface oxide films can be removed by immersion in an aqueous sodium hydroxide (NaOH) solution at a temperature of 20-40°C for 2-4 minutes, followed by rinsing with hot water.
After removing the surface oil and oxide films, the part must be treated with nitric acid (HNO 3 ) water solution for 2 to 5 minutes, rinsed in running water and air dried. After these treatments, the part must not be touched by hand or contaminated with other dirt, and brazing must be carried out within 6 to 8 hours, preferably immediately if possible.
Soft brazing methods for aluminum and aluminum alloys mainly include flame brazing, soldering iron brazing and furnace brazing. These methods generally use flux and have strict requirements on heating temperature and retention time.
When brazing with a flame and soldering iron, direct heating of the flux must be avoided to avoid overheating and failure of the flux. Because aluminum can dissolve in solder with a high zinc content, heating must be stopped as soon as the joint is formed to prevent dissolution of the base material.
In some cases, soft brazing of aluminum and aluminum alloys is performed without flux, using ultrasonic or friction methods to remove the oxide film. When using friction stripping for brazing, the workpiece is first heated to brazing temperature, and then the end of the brazing rod (or friction tool) is used to scrape the brazing area off the workpiece . This breaks the surface oxide film and allows the brazing material to melt and wet the base material.
Hard brazing methods for aluminum and aluminum alloys include flame brazing, furnace brazing, immersion brazing, vacuum brazing, and gas shield brazing. Flame brazing is commonly used for small parts and single-piece production.
To prevent acetylene gas impurities from coming into contact with the flow and causing flow failure, it is advisable to use a compressed air flame with gasoline and reduce the flame slightly to prevent oxidation of the base material.
In specific brazing processes, the flux and brazing material may be pre-placed in the joint, heated together with the workpiece, or the workpiece may be heated first to the brazing temperature and then the brazing material. Flux brazing can be applied to the brazing area.
After the flux and brazing material are melted and the brazing joint is evenly filled, the heating flame can be gradually removed.
When brazing aluminum and aluminum alloys in an air furnace, the brazing material must be pre-charged and the flux must be melted in distilled water to form a concentrated solution with a concentration of 50%-75%.
This solution can then be applied or sprayed onto the brazing surface, or a suitable amount of powder flow can be applied to the material and the brazing surface. The assembled part is then placed in the furnace for heating and brazing. To avoid overheating or even melting of the base material, the heating temperature must be strictly controlled.
Dip brazing of aluminum and aluminum alloys typically uses paste or foil brazing materials. The assembled part is preheated to a temperature close to the brazing temperature before being immersed in the brazing flux.
During brazing, the temperature and brazing time must be strictly controlled. If the temperature is too high, the base material will be subject to dissolution and the brazing material will be subject to loss.
If the temperature is too low, the brazing material may not melt properly, resulting in a lower brazing rate.
The brazing temperature should be determined based on the type and size of the base material, composition and melting point of the brazing material and other specific factors, generally ranging between the liquidus temperature of the brazing material and the solidus temperature of the base material .
The time the part is immersed in the flux bath must ensure that the brazing material is fully melted and flows. The time should not be too long, as the silicon element in the brazing material can diffuse into the base metal, causing embrittlement in the vicinity of the joint.
Vacuum brazing of aluminum and aluminum alloys generally uses metal activators to transform the surface oxide film of aluminum, ensuring wetting and spreading of the brazing material.
Magnesium can be placed directly on the workpiece in granular form, introduced as vapor into the brazing zone, or added as an alloying element to aluminum-silicon brazing material.
For complex structures, to ensure the full effect of magnesium vapor on the base material and improve the brazing quality, local shielding measures are often taken.
This involves placing the part in a stainless steel box (commonly known as a process box) and then heating it in a vacuum furnace for brazing.
Vacuum brazing of aluminum and aluminum alloy joints produces smooth surfaces, dense brazing seams and there is no need for post-braze cleaning.
However, vacuum brazing equipment is expensive and magnesium vapor can seriously contaminate the furnace, requiring frequent cleaning and maintenance.
When brazing aluminum and aluminum alloys in a neutral or inert atmosphere, activators or magnesium flux can be used for oxide film removal. When using magnesium activators, the required amount of magnesium is much less compared to vacuum brazing, generally around 0.2%-0.5% (by weight).
A higher magnesium content can decrease the quality of the joint. The Nocolok brazing method, which uses fluorine-based flux and nitrogen gas shielding, has been rapidly developed in recent years. Fluorine-based flux residues do not absorb moisture and are not corrosive to aluminum.
Therefore, the step of removing flux residue after brazing can be omitted. With nitrogen gas protection, a small amount of fluorine-based flux can be applied and the brazing material can wet the base material well, resulting in high-quality welded joints. This Nocolok brazing method has been widely used in batch production of components such as aluminum radiators.
For aluminum and aluminum alloys brazed with fluxes other than fluorine-based fluxes, flux residue must be completely removed after brazing. Waste organic flux for aluminum can be washed with organic solvents such as methanol or trichloroethylene, followed by neutralization with aqueous sodium hydroxide solution and finally rinsed with hot and cold water.
Residues of chloride-based flux from hard brazing of aluminum can be removed by soaking them in hot water at 60-80°C for 10 minutes, carefully scrubbing the residues into the welded seam with a brush and rinsing with cold water. Then soak in a 15% aqueous solution of nitric acid for 30 minutes and rinse with cold water.
