Vacuum brazing process of aluminum alloy radiator for BMW engine

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Aluminum alloy is widely used in aerospace, construction, electrical appliances, automobiles, ships and other sectors due to its light weight, good corrosion resistance, and excellent thermal and electrical conductivity, and its usage is increasing. It has long been considered as a heat exchanger, Structural materials for waveguides and many other complex components. The research on aluminum alloy brazing initially focused on its metallurgical properties, and then stayed on the effect of added elements on the brazing process. At present, it is mainly focused on special-purpose aluminum alloy welding. Many special-purpose aluminum alloys are constantly being researched and developed and used in engineering practice. LT-3 aluminum alloy is specially designed for aluminum alloy vacuum brazing. It is made by double-sided cladding and rolling 4004 aluminum alloy on the base of 3003 aluminum alloy. The cladding material is used as brazing filler metal in the brazing process. This material is currently widely used in the manufacture of radiators and condensers. . The vacuum brazing scheme of LT-3 coated aluminum alloy is introduced below.

1. Introduction of aluminum alloy vacuum brazing scheme
1.1 Principle
Vacuum brazing is carried out in a vacuum furnace, and no flux is used in the brazing process. Although there is no chemical effect of the flux and the reducing effect of the reducing atmosphere, because the oxygen partial pressure in the brazing area is reduced, the oxide film on the surface of the weldment can also be removed to protect the weldment from oxidation.

1.1.1 Mechanism of removing oxide film on metal surface
a. The oxide film can decompose by itself in high temperature and high vacuum: the decomposition pressure of oxide>the oxygen partial pressure of the vacuum atmosphere.

b. Magnesium vapor destroys the oxide film on the metal surface. In order to prevent the occurrence of problems such as the corrosion of the flux and the cleaning after brazing, the vacuum brazing is generally not used to replace the oxygen with a more active metal than aluminum, and magnesium is mostly used. However, with the addition of Mg, the brazing filler metal has a serious corrosion effect on the base metal. Generally, the control range of magnesium content in the filler metal is 1.0% to 2.5%.

c. The expansion of aluminum forces the aluminum oxide to rupture first, so that the liquid solder enters the oxide film from the crack, and the oxide film is uncovered.

1.1.2 Requirements for solder
The brazing filler metal used for vacuum brazing is silicon-aluminum filler metal, which has strict requirements on the composition. The filler metal should not contain elements with high vapor pressure, and do not contain elements whose surface oxides are significantly decomposed under vacuum and high temperature, otherwise It will affect the vacuum brazing process and the quality of vacuum brazing.

1.2 Vacuum brazing process
The weldments need to be carefully cleaned before vacuum brazing. Under normal circumstances, it is sufficient to treat the weldment with oil stains, but in order to remove the thick oxide film generated during the heat treatment, it is sometimes necessary to clean with a cleaning agent. Among the factors, the temperature cycle mainly depends on the shape of the weldment, because heat is transmitted in the form of radiation in a vacuum, and the parts with different working thicknesses will be unevenly heated. Because the difference between the solidus of aluminum alloy and the brazing temperature is often small, the thinner part of the workpiece is easy to overheat, causing the surface to melt. Therefore, it is necessary to preheat the workpiece, especially the larger workpiece, to ensure that the weldment temperature reaches the brazing temperature before each part is evenly heated. Because the removal of the oxide film on the surface of aluminum alloy by vacuum brazing mainly relies on Mg activator, for weldments with complex structures, in order to ensure that the base metal is fully affected by the Mg vapor, some domestic research units often adopt supplementary process measures of partial shielding. Very good results.. The most common method is to put the weldment in a stainless steel box (called a process box), and then place it in a vacuum furnace for heating and brazing, which can significantly improve the quality of brazing. The formation of Mg vapor can be added to the solder Solid Mg particles or direct introduction of Mg steam. This can greatly reduce the contamination of the furnace chamber caused by elements such as Mg during furnace loading and discharge, thereby reducing the number of cleaning up the vacuum brazing furnace and increasing the service life of the vacuum brazing furnace.

The most important and difficult to control process parameter in vacuum brazing is the degree of vacuum. To obtain a good joint, to a large extent depends on the degree of vacuum in the vacuum brazing furnace, so the leakage rate of the vacuum equipment is very low. During the heating process, the furnace is pumped to a certain degree of vacuum, which requires 0.0013Pa to 0.0133Pa for aluminum and its alloys. It is generally believed that the vacuum brazing furnace absorbs moisture in the air at room temperature when it is opened, which greatly extends the vacuuming time before the next vacuum brazing temperature rises. According to experience, generally if the vacuum brazing furnace has not been used for a long time, the vacuum brazing furnace should be used for baking later. When in use, especially in mass production, the time interval between entering and exiting the furnace should be as short as possible, so that the vacuum degree of the vacuum brazing furnace can easily reach the required requirements quickly, thereby improving production efficiency. The heating rate, stable temperature and holding time of the vacuum brazing furnace temperature depend on the material, shape, structure, size of the vacuum brazing workpiece, the form of the solder used and the range of the crystallization temperature of the solder.

2. The welding process of vacuum brazing
2.1 Weldability analysis of vacuum brazing
The LT-3 aluminum composite panel is manufactured through a cold rolling process. First, the core layer metal 3003 is rolled into shape, and then the upper skin layer metal 4004 is rolled on the core layer metal. The core metal 3003 is an Al-Mn series alloy, which has good corrosion resistance and pressure workability. The skin layer metal 4004 is based on the eutectic Al-Si alloy, which has good wettability and fluidity, and contains 1.0-2.0% Mg element, which is helpful to the removal of oxide film. Vacuum brazing composite board is much more convenient than wire, strip or powder to use 304, and can be conveniently used to make complex parts. When the clad metal on the brazing plate melts, it can directly wet the bonding base material and fill the gap immediately. The joint can be formed with only a slight spread. The chemical composition and melting point of LT-3 aluminum composite sheet are shown in the table below.
LT-3 Aluminum Alloy Core and Skin Chemical Composition Table
Material Si Fe Cu Mn Mg Zn Al Melting temperature/℃
Core layer (3003 aluminum alloy) 0.6 0.7 0.05～0.2 1.0～1.5 / 0.1 margin 643～654
Skin layer (4004 aluminum alloy) 9.0～10.5 0.8 0.25 0.1 1.0～2.0 0.2 margin 559～591

2.2 The welding process of vacuum brazing
2.2.1 Cleaning before vacuum brazing
The brazing of aluminum and aluminum alloys has higher requirements for the cleanliness of the surface of the workpiece. To obtain good quality, the oil stain and oxide film on the surface must be removed before brazing. To remove oil stains on the surface, wash it with a Na2CO3 aqueous solution at a temperature of 60-70°C for 5-10 minutes, and then rinse it with clean water; to remove the surface oxide film, it can be etched with a NaOH aqueous solution at a temperature of 20-40°C for 2-4 minutes, and then rinse it with hot water ; After removing the surface oil and oxide film, wash the workpiece in running water and finally air dry. After treatment, brazing should be carried out within 6-8h, and brazing should be done as soon as possible. Because of the active nature of aluminum, it is easy to regenerate an oxide film on the surface of the metal if it stays for too long. Do not over-grind the surface of the workpiece before welding to avoid damage to the skin metal. It is also easy to damage the flatness of the workpiece surface during grinding, which causes difficulties in brazing.

2.2.2 Vacuum brazing welding process

Generally, vacuum brazing adopts stage heating method to ensure that the workpiece is heated evenly. The process parameters of vacuum brazing are: brazing temperature control meter set temperature 620℃, heat preservation 15min-30min, vacuum degree 3×10-3Pa, heating rate 10℃/min, adopt the method of controlled cooling at the beginning of cooling stage, rate 10℃/min, the temperature reaches 450℃, then it is cooled to room temperature with the furnace. Passed test: The aluminum alloy radiator was vacuum brazed with six different brazing heat preservation times of 15min-90min and six different heat preservation temperatures of 595℃-620℃, through optical microscope, scanning electron microscope, X-ray energy spectrometer, hardness Experiments and other studies determine the influence of vacuum brazing time and temperature on the weld structure. The results show that with the extension of the vacuum brazing holding time and the appropriate increase of the holding temperature, the element diffusion between the welds becomes more and more sufficient, and the Si segregation shape changes from the original dendritic shape to the point shape. When the vacuum brazing process is When held at 615°C for 75 minutes, there is only a small amount of point-like Si segregated structure in the weld structure, most of which are solid solution. The brazing filler metal has a small amount of dissolution on the base material, and the bonding is stronger. The vacuum brazing process is better.

The requirements for vacuum brazing furnaces are generally:
① The vacuum brazing furnace needs to have a sufficient heating rate and a great pumping speed, the ultimate vacuum is required to be less than 4×10-4Pa, the working vacuum is less than 3.0×10-3 Pa, and the brazing temperature fluctuation in the soaking zone does not exceed 3℃;

② The vacuum brazing furnace should have an advanced, reliable and safe electrical control and protection system;

③ The size of the heating chamber of the vacuum brazing furnace can accommodate the components to be brazed;

④ The vacuum brazing furnace should have a forced cooling function, one can meet the requirements of the vacuum brazing process of the material, and the second can shorten the vacuum brazing cycle. To prevent the occurrence of brazing defects such as incomplete penetration, erosion, and loss of solder.

Pay attention to the following matters before and during vacuum brazing:
① Strictly control the vacuum brazing temperature and holding time to ensure that the system vacuum and temperature uniformity of the vacuum brazing furnace meet the standard requirements.

②Before vacuum brazing, ensure that the oil stains and oxide film on the surface of the workpiece are thoroughly cleaned, so that the liquid solder can completely wet the core layer.

③The workpiece assembly should be reasonable to ensure the normal filling of the liquid solder. The vacuum brazing assembly tooling fixture needs to be designed reasonably to reduce the heat capacity of the tooling fixture as much as possible to prevent the product from brazing deformation.

The following suggestions are put forward: The internal and external quality of domestic composite aluminum foil and composite plate needs to be improved. At present, domestically produced composite materials have neither price advantage nor quality advantage. Companies mostly use imported composite materials for mass production.

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