Oil Cooler Vacuum Brazing Process

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The oil cooler is mainly used to cool the engine lubricating oil to stabilize the lubrication system, improve the power and economy of the automobile engine, and is an important part of the automobile heat exchange system to ensure the safe operation of the engine and its service life. In recent years, with the development of automobile lightweight, aluminum alloy has gradually replaced copper and steel, becoming the most widely used oil cooler material. Aluminum alloy plate-fin oil coolers generally use multi-layer staggered zigzag fins, and the core body and flange plates are brazed at one time. Traditional oil cooler flange plates usually use 3003 aluminum alloy or its modified aluminum alloy, but the 6-series Al-Mg-Si series aluminum alloy containing Mg has a certain natural aging strengthening effect after brazing, and its strength after welding is relatively low. It has gradually replaced the traditional 3-series aluminum alloy as flange material, among which 6063 and 6A02 are the most commonly used 6-series aluminum alloy flange materials.

Since the 6-series aluminum alloy contains Mg element, in order to avoid the reaction of Mg element with O2, CO2 and other gases in the brazing furnace during brazing and affect the surface quality, the 6-series aluminum alloy plate is usually used as the flange plate of the oil cooler. The vacuum brazing method is used for welding. In application, it is found that the surface color of 6-series aluminum alloy is blue after vacuum brazing, and it is also very easy to oxidize the surface and turn black. Therefore, a layer of 3003 aluminum alloy is usually compounded on the upper and lower surfaces of the 6-series aluminum alloy flange plate to improve the surface color. In practical application, it is also found that the outer surface around the punching hole on the flange plate often has the phenomenon of solder accumulation, which makes the surface dark and uneven. This defect is generally called flashing. This phenomenon greatly affects the assembly of the oil cooler and other parts and the appearance of the weldment, which reduces the brazing yield. Severe flashing will cause corrosion and shorten the service life of the oil cooler. Analyze the flash phenomenon, explore its causes, and try to improve the flash defect through the improvement of raw materials.

Improvement of Vacuum Brazing Process for Oil Cooler

The influencing factors of solder flow mainly include alloying elements, thickness of brazing layer, brazing temperature, brazing time and material grain size, etc. In industrial production, when the alloy structure and vacuum brazing process are not easy to change, it is a more economical and widely applicable solution to improve the grain size of the material by adjusting the production process. The flow of solder must pass through grain boundaries, the smaller the grain size, the more grain boundaries, the more flow channels. Therefore, the direction of process improvement is to increase the grain size, thereby reducing grain boundaries and reducing solder fluidity.
Production process of 3003/6063/3003 three-layer aluminum alloy composite material before and after process improvement. Compared with before improvement, a cold rolling and intermediate annealing process is added, and the intermediate annealing temperature is 380 ℃.

The optical microscopic phase structure diagram of the 6063 aluminum alloy layer before and after process improvement shows that the grains are all completely recrystallized grains. The grain size before improvement is about 275 μm, and the grain size after improvement is about 476 μm. Compared with the improvement before, there is a significant increase, indicating that the increase in the intermediate annealing process has played a role in increasing the grain size of the finished product, which may be related to the change in the deformation energy storage of the material. After the material is hot-rolled, the microstructure is a deformed structure with sub-grains, dislocations, and stresses inside, and the deformation energy storage is relatively high. After subsequent multi-pass cold rolling, the deformation energy storage is further improved. The larger the deformation storage energy, the larger the recrystallization driving force and the smaller the corresponding grain size. Before improvement, from hot rolling to finished product annealing, the total deformation of the material is 50%. After the intermediate annealing process is added, the material is softened, sub-grains, dislocations, stress, etc. are partially eliminated, and the deformation storage energy is released. From the end to the finished product annealing, the total deformation of the material is 38%, the total deformation decreases, the deformation storage energy decreases, and the recrystallization driving force decreases, so the grain size increases compared with that before improvement.

Improvement conclusion of vacuum brazing process for oil cooler

(1) The flange plate of the oil cooler produces flash after vacuum brazing, mainly because the melted solder flows during the vacuum brazing process, and accumulation is formed on the surface near the punching hole of the flange plate.
(2) The intermediate annealing during the cold rolling process of the material reduces the total deformation energy storage, and the recrystallization driving force decreases when the finished product is annealed, which is conducive to the increase of the grain size of the finished product.
(3) The grain size of the material increases, and the flashing after vacuum brazing is significantly improved, mainly because the number of grain boundaries decreases after the grain size increases, and the solder flows along the grain boundaries to the surface of the flange plate during vacuum brazing The tendency is weakened and the solder build-up is reduced.

Vacuum brazing equipment selection：SIMUWU high-temperature brazing vacuum furnace can be used for vacuum brazing with this oil cooler, and has the following advantages:

1) The equipment adopts external circulation cooling mode, and the heating and cooling are separated.

2) The heat preservation adopts a full metal screen to ensure the high vacuum degree required for the brazing of the workpiece.

3) Heating adopts multi-zone temperature control method to ensure good temperature uniformity in the temperature zone.

Learn More:

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The importance of brazing holding time

Effect of Magnesium on Aluminum Alloy Vacuum Brazed Joints