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Vacuum Brazing of EGR Stainless Steel Cooler
EGR car coolers are exhaust gas recirculation coolers that reduce vehicle exhaust emissions. The exhaust gas that has been burned can effectively reduce the oxygen content and temperature after passing through the EGR cooler, and mix and burn with fresh gas in the cylinder, reducing the amount of NO generated and making the vehicle exhaust emission limit meet the standard. Modern engines have EGR exhaust gas recirculation systems to achieve maximum benefits and balance between environmental protection and power.
EGR is a heat exchanger, the working environment is very harsh, and it is a high temperature corrosive gas. The working medium is high temperature exhaust gas, and the external environment is rainwater, air pollution, sea salt and dust. ECR has to bear a large vibration fatigue load during the working process, so the requirements for the mechanical properties of the weld are stricter. If argon arc welding is used, it is easy to produce time-dependent deformation and thermal cracks; if controlled atmosphere brazing is used, the surface oxide film is difficult to remove, and the brazing quality is poor. Vacuum brazing can solve the connection of such alloy materials. Based on the EGR cooler for heavy-duty vehicles, this paper determines the appropriate process parameters for vacuum brazing, and then extends it to coolers of other structures, aiming to find the best brazing process parameters and further improve product quality and service life.
1 Test materials and methods
The EGR cooler is mainly composed of connecting flanges, inlet and outlet water pipe joints and core components. The core assembly consists of a shell, a connecting plate, a cooling pipe and a helical fin. High temperature exhaust gas is passed through the core, the inlet temperature is 500 e, the outlet target temperature is 200 e, and the external medium is downstream cooling water. The thickness of the base metal to be brazed is 15 mm, and the thickness of the paste coating is 1 mm. Structural materials of each part of EGR: the two connecting flanges are made of ZGOCrl8N i9 stainless steel, the two water pipe joints and the shell are made of 304 stainless steel, and the connecting plates, cooling pipes and spiral fins are all made of 316 stainless steel.
Correct selection of solder is an important factor to ensure high-quality brazing joints. According to the matching table of base metal and solder, if the base material is stainless steel, copper solder should be the first choice, that is, pure copper with a copper content of 99.95%. Again choose Ag-Cu-Ni, Ni-based, Co-based solder. Due to cost factors, copper brazing filler metal and Ni-based brazing filler metal BNi-5 were selected for testing. Brazing joint form of EGR pipe fittings: the determination of the weld gap value is more complicated, and should be selected according to the base metal material, joint structure and brazing process. is 0.10 mm.
2 Preparation before welding
1) Prepare the surface for degreasing before welding: the base metal is stamped and formed stainless steel, so only ordinary degreasing is required. The composition of the etching liquid used: 10% H2SO4 + 15% HCl + 3% HNO3 + balance water, temperature 100 e , Ultrasonic cleaning for 30 s and drying.
2) Assembly and positioning of parts Due to the special shape of ECR parts, it is necessary to design a special hanging fixture to ensure the vertical brazing position of the parts, that is, the fixture should be designed in a point positioning mode.
Using a vacuum brazing furnace, the furnace has very good air tightness, the ultimate vacuum can reach 6×10-4Pa, and the temperature uniformity is ±5 e. Cool to room temperature through pure argon, and remove the parts.
3 vacuum brazing process parameters
4 EGR cooler inspection and analysis
The quality inspection, performance test and metallographic analysis of the EGR coolers of the two kinds of brazing materials were carried out to verify the rationality of the process parameters.
4.1 Quality Check
Geometric tolerance inspection: Check the flatness and verticality of the flange, which meet the tolerance requirements of 0.10 mm and 0.08 mm respectively. The vacuum brazing deformation is small and meets the design requirements.
Visual inspection: The surface of the brazing seam is bright, the brazing traces are continuously distributed along the joint, and the brazing penetration rate is good, indicating that the brazing material is appropriate; there is no corrosion near the brazing seam, but the surface is slightly oxidized, and the brazing temperature is appropriate.
Observing the welding seam of red copper brazing filler metal and nickel-based brazing filler metal, it is found that the red copper brazing seam is fuller, indicating that the wettability of red copper to stainless steel is better than that of nickel-based brazing filler metal.
1) It can be seen from the test that copper and BNi5 can be selected as brazing materials for successful brazing of EGR stainless steel cooler. The red copper weld is fuller, indicating that the wettability of red copper to stainless steel is better than that of nickel-based solder BNi5, but because BNi5 contains more nickel elements, its high temperature performance of the weld is better than that of red copper.
2) If red copper is used as vacuum solder, attention should be paid to the high evaporation pressure characteristics of copper element. At 1100 e, the vapor pressure of pure copper is 1 Pa, and it is easy to escape under vacuum. Therefore, the partial pressure starts when the temperature is stable, and the self-cooling Below 1050 e, the partial pressure is terminated to limit the escape of copper element. If BNi5 brazing is selected, no partial pressure is required.
3) BNi5 brazing filler metal is easy to form nickel-based solid solution when brazing stainless steel due to its nickel content as high as 70%. If the weld gap increases, the silicon-nickel compound NiSi2 will appear, forming a brittle structure, which greatly reduces the joint strength, especially the impact toughness.
4) In addition to BNi5 solder, other nickel-based solders of the same series can also be used, such as BNi2, BNi7, etc.
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