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Vacuum brazing technology of laminated aluminum evaporator
The evaporator is an important part of the automobile air conditioner, which forms the air conditioning component with the heater core, fan, plastic shell, etc., and is located at the lower part of the automobile dashboard. The automobile refrigeration system is composed of a condenser, a circulating liquid receiver, a compressor, a temperature control valve and a liquid pipe.
Because of its good heat transfer efficiency, the laminated aluminum evaporator is gradually replacing the tube and strip evaporator in cars. It forms a channel for the flow of automobile refrigeration liquid by the superposition of tubes, and increases the heat transfer efficiency through the interphase heat sink. This structure increases the length of the brazing seam. Because the volatility of the refrigerant is far greater than that of water, and the density of the refrigerant in the evaporator is large when it is in the gaseous state, the sealing requirements for the channel formed by the overlapping of the segments are very strict, and the requirements for the quality of brazing are very high.
Under the condition of large-scale production of automobile parts, there are two main brazing processes of laminated aluminum evaporator core: nitrogen shielded brazing (belonging to controllable atmosphere brazing) and vacuum brazing. The scrap rate of brazing of cascade evaporator with nitrogen shielded brazing furnace is relatively high. Therefore, vacuum brazing is generally adopted. The current vacuum brazing production line has been able to produce 200000 sets of laminated aluminum evaporator cores per year in double shifts. The vacuum brazing process plan is as follows:
Vacuum brazing scheme
Although vacuum brazing does not have the chemical action of the flux and the reduction effect of the reducing atmosphere, it can reduce the oxygen partial pressure in the brazing area, remove the oxide film on the surface of the weldment, and protect the weldment from oxidation.
2.Mechanism of oxide film on metal removal surface
a.The oxide film can decompose itself in high temperature and high vacuum: the decomposition pressure of oxide is higher than the partial pressure of oxygen in vacuum.
b.Magnesium vapor destroys the oxide film on the metal surface. In order to prevent the occurrence of corrosion and contamination of flux and cleaning after brazing, flux is generally not used in vacuum brazing. Instead, the oxygen is replaced by a metal that is more active than aluminum, and magnesium metal is mostly used. However, with the addition of Mg, the brazing filler metal has a serious corrosion effect on the base metal, and the magnesium content in the general brazing filler metal is controlled at 1.0%~2.5%.
c.Aluminum expansion forces Al2O3 to crack first, so that the liquid solder enters the oxide film from the crack and uncovers the oxide film.
3.Requirements for filler metal
The brazing filler metal selected for vacuum brazing is very similar to that for nitrogen shielded brazing. Both are silicon aluminum brazing filler metals, but the requirements for composition are more strict. The brazing filler metal should not contain elements with high vapor pressure or elements whose surface oxides decompose significantly under vacuum and high temperature. Otherwise, the brazing process and brazing quality will be affected.
The equipment of vacuum brazing process is generally composed of vacuum degreaser, vacuum brazing furnace, cooling room and a set of circulating loading and transmission system, which has a compact structure. The high vacuum system is mainly composed of mechanical pump, fan, support pump, diffusion pump and high vacuum pump valve.
De oiling process: load the evaporator into the loading frame of the transmission system, heat it to 330~380 ℃ at 180~200 ℃ in the vacuum de oiling chamber, and de oil it when the vacuum reaches 999.92Pa. Then enter N2 backfilling cycle, and load after pressurization and cooling.
The loading frame (with evaporator) after vacuum deoiling is transported into the vacuum brazing chamber, and the heating process of the furnace is shown in the figure below. The heating speed, stable temperature and holding time of the furnace temperature depend on the material, shape, structure, size, type of filler metal used and the range of filler metal crystallization temperature of the workpiece. The furnace shall be pumped to a certain pressure during temperature rise, and 0.0013~0.0133Pa is required for aluminum and its alloys. The high vacuum state lasts until the furnace is backfilled with dry air. When the continuous backfilling of dry air makes the pressure of the heating chamber reach atmospheric pressure, the air permit valve of the heating chamber will be opened. When the temperature of the workpiece drops to 200 ℃, the loading rack is taken out of the furnace, moved to the air cooling chamber, forced cooling with a fan, and unloaded out of the chamber. The empty loading rack is moved to the loading station before vacuum deoiling. Then proceed to the next cycle.
In addition to adopting correct technology, the quality of brazing furnace selected in vacuum brazing is also a crucial factor. The RHVB vacuum high-temperature brazing furnace produced by SIMUWU Company has the advantages of good temperature control accuracy, high heating uniformity, and is an excellent product for handling high-temperature brazing process. On the premise of selecting appropriate process parameters, high quality vacuum brazing can be achieved. So as to meet the production requirements of the laminated aluminum evaporator.
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