The common brazing defects of aluminum heat exchanger mainly include lack of penetration, corrosion, bending and lodging of fins, solder loss, gray and black color of heat exchanger, poor adhesion of flux residue affecting paint adhesion and peculiar smell of heat exchanger.
There are many factors that affect the quality of brazing in the actual production process. The main factors are discussed here.
1. Dimensional tolerance of parts
The key dimensional tolerances of the parts must be ensured during processing, such as the dimensions of the elbow after shaping the serpentine flat tube of the tube ribbon condenser, the necking size of the brazed joint between the serpentine flat tube and the collector pipe, the dimension tolerance of the fin height, the punching size of the composite header, the forming size of the plug, and the necking size of the flat tube of the parallel flow condenser are very heavy Otherwise, the clearance of the core cannot be guaranteed after assembly.
2. Degreasing of parts surface
The degreasing requirement of aluminum brazing on the surface of parts is higher than that of vacuum brazing. The dirt such as lubricating oil (grease) on the surface of parts must be thoroughly removed to obtain the surface wettable by water, so as to evenly apply flux. The degreasing of parts surface can be carried out by steam cleaning with organic solvent (trichloroethane or trichloroethylene), water-based cleaning agent or alkali washing.
If no oil is applied or volatile lubricating oil is used in raw material processing and volatile lubricating oil is used in parts processing, cleaning or hot air drying can be avoided according to the volatility of lubricating oil. In order to improve the wettability of flux suspension, 0.1% surfactant (NCH) can be added into the suspension. Too much surfactant will reduce the solderability and corrosion resistance of the product.
3. Assembly of parts
It is beneficial to obtain good brazing quality by ensuring the brazing gap after the core is assembled. It is recommended to fit the fin (composite aluminum foil) with flat tube without gap. The skin of composite aluminum foil serves as the gap. If the local gap is too large, solder foil can be inserted, but local corrosion and local solder joint may be too large.
The larger the brazing gap is, the larger the flux loading is, and it is better not to exceed 0.1 mm. The NOCOLOK brazed core is usually clamped with clamps after assembly. Due to the different thermal expansion coefficients of aluminum and stainless steel clamps, the clamping force is too large, which may lead to the bending and lodging of fins after brazing and the size of the core is out of tolerance; if the clamping force is too small, the fins are prone to false welding and loosening.
4. Preparation and coating of flux suspension
The flux is prepared into suspension with distilled water, deionized water or pure drinking water. The recommended surface flux loading is 5g / m2.
The concentration of flux suspension, the cleaning quality of parts, the coating method, the blowing out of excess flux, the complexity of the workpiece and the speed of the mesh belt of the coating device will affect the actual flux loading. Generally, the concentration of flux suspension is 5-10%. For important brazing joints such as flat pipe and collector pipe, the suspension with concentration of 30-50% can be swept separately to ensure the brazing quality.
Excessive flux loading (more than 5g / m2) will cause the core color to turn gray and black after brazing, poor adhesion of flux residue, affect paint adhesion, heat exchanger odor, etc., and reduce the service life of brazing furnace muffle. The residual flux has a certain smell of H2S, and the presence of flux residue affects the passivation and hydrophilic treatment of evaporator surface.
5. Drying of brazing flux
The workpiece coated with flux shall be dried thoroughly to remove moisture. During the brazing process, moisture can react with kalf4 to form HF, which affects brazing, increases the corrosion of muffle and mesh belt, and increases the workload of HF waste gas treatment.
The drying degree of the workpiece is affected by the drying temperature and mesh speed. Generally, the temperature is controlled at about 200 ℃ and can not exceed 250 ℃. Otherwise, the oxidation of the workpiece is serious and the flux cannot remove the oxide film.
6. Brazing temperature and mesh speed
The key to ensure the quality of brazing is to establish appropriate brazing temperature and mesh speed. With aa4045, the temperature of brazing zone is controlled at 600-610 ℃; with aa4343, the temperature of brazing zone is controlled at 610-620 ℃.
The mesh speed condenser is 750-850mm / min. If the brazing temperature is too high and the mesh belt speed is too slow, it is easy to cause solder loss, corrosion, fin bending and other defects; if the brazing temperature is too low, the mesh belt speed is too fast, the workpiece has not reached the necessary brazing temperature, and the temperature of each part is uneven, which is easy to cause defects such as false welding, leakage and discontinuous brazing seam.
The brazing temperature and mesh speed should be determined by measuring the actual temperature curve of the workpiece in the furnace. The temperature of each part of the workpiece should be heated to the brazing temperature for 3-5 minutes.
7. Brazing atmosphere
It is recommended to use high purity liquid nitrogen gasified nitrogen. The purity of high purity liquid nitrogen produced by steel works is 99.9995%, in which the oxygen content is less than 3ppm and the dew point is lower than – 73 ℃.
However, in a typical tunnel furnace, the atmosphere in the furnace is always higher than this level due to the moisture and air brought into the furnace by workpieces, fixtures and continuous operation mesh belt, and the imbalance of nitrogen intake and exhaust leads to the backflow of workshop atmosphere.
In order to ensure the brazing effect, it is necessary to ensure that the atmosphere dew point of the brazing section of the tunnel furnace does not exceed – 40 ℃, and the oxygen content does not exceed 100 ppm. If the brazing atmosphere is poor, it is necessary to increase the flux loading.
8. Uniformity of composite skin
The nonuniformity of composite skin will cause defects such as false welding, burning, discontinuous brazing seam and leakage. The uniformity of domestic composite skin is inferior to that of imported material.
9. Bending resistance of composite aluminum foil
The bending resistance of composite aluminum foil at brazing temperature is a very important performance index to measure the quality of aluminum foil. The bending resistance of composite aluminum foil is poor, and the fins bend, lodging and adhesion during brazing seriously affect the appearance quality and heat transfer performance of heat exchanger.
The results show that this is due to the small grain size of the core material of composite aluminum foil, and the silicon atom in the skin material diffuses along the grain boundary to the center of the core material.
The cold work rolling rate and annealing process are strictly controlled during the rolling of composite aluminum foil, which is beneficial to obtain the core material with coarse grain and excellent bending resistance.
10. Arrangement in furnace
The placement of the products in the furnace is unreasonable. The collector tube skin may flow downward under the action of gravity after melting, which is easy to cause solder loss and the solder joint is not compact. If the filler metal flows to the fin, the fin will be completely dissolved.