Vacuum brazing of locomotive aluminum exchanger

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With the development of economy, my country has increased the speed of train transportation five times in a row, so the requirements for the heat exchanger of locomotives are getting higher and higher. The traditional locomotive heat exchanger is made of copper alloy. Since the density of aluminum is only 30% of that of copper, replacing copper alloy with light material aluminum can greatly reduce the mass of the heat exchanger. Although the thermal conductivity of aluminum is quite different from that of copper, the heat exchange performance can be improved by designing a reasonable structure to meet the requirements of heat exchangers.

The brazing of aluminum and aluminum alloys mainly adopts vacuum brazing and NOCOLOK brazing. Combined with practice, the vacuum brazing of locomotive aluminum heat exchanger (hereinafter referred to as heat exchanger) is introduced here.

heat exchanger material
The schematic diagram of the structure of the heat exchanger is shown in the figure. The heat exchanger is mainly composed of air channel fins, oil channel fins, I-type air channel seals, rectangular oil channel seals and intermediate partitions.

The I-type air duct seals and rectangular oil duct seals are rolled from industrial pure aluminum, the oil duct fins and air duct fins are stamped from 3A21 (LF21) plates with a thickness of 0.2mm, and the intermediate partitions are vacuum brazed Use composite aluminum foil. This composite aluminum foil is composed of core material and skin layer by cold rolling. The strength is mainly guaranteed by the core material, which is generally Al-Mn (3A21) alloy, and the skin layer is melted during the brazing process to provide brazing material.

Vacuum brazing process:
1. Pre-welding treatment of workpiece
There are oil stains and oxide films on the surface of seals, fins and composite aluminum foils, which must be removed before vacuum needle welding to ensure the welding quality. To remove grease, trichloroethane and trichloroethylene are commonly used for dipping and steam cleaning. However, trichloroethane destroys the ozone layer, and trichloroethylene is highly toxic, so it is not used in terms of environmental protection and labor environment. Soaking and cleaning with a mixed solution of NaOH, NaSiO3, Na3PO4, and OPL emulsifier can also effectively remove grease, so the latter is used. Since there is a thick oxide film on the surface of the material, it is soaked in w (NaOH) 10% solution. After soaking, in order to reduce the surface roughness, each part is treated with gloss—soaked in HNO3 solution for 2~5min.

2. Vacuum brazing process
The main technical indicators of vacuum needle welding furnace are as follows:
Effective heating zone size: 500mm×500mm×750mm; temperature control accuracy: ±1℃; maximum furnace temperature: 800℃; ultimate vacuum degree: ≤8×10-4Pa; rated working temperature: 650℃; heating power: 90kW; furnace Temperature uniformity: ±3℃.

After the workpiece is loaded into the vacuum chamber, a magnesium block is placed in the furnace, and the heating is started after vacuuming to 6×10-3Pa. During the heating process, each part of the brazing will release a large amount of adsorbed gas (the residual moisture during cleaning is vaporized). When the temperature rises to 350 ℃, in order to ensure the vacuum degree, the temperature is kept for 30 minutes. Due to the large volume of the heat exchanger, the melting point of the base metal and the brazing material is close, so that the holding time at the brazing temperature will not be too long, the temperature is kept at 550 ℃for 20 minutes. When the brazing temperature is 605℃, keep the temperature for 10 minutes, then turn off the heater to stop heating. When the temperature drops to the solidus temperature of the brazing filler metal, in order to speed up the cooling rate, the vacuum chamber is filled with nitrogen with a purity of 99.999%, and the cooling fan is turned on. Cool to room temperature.

3. Post-weld inspection and analysis
(1) The surface of the heat exchanger after brazing is bright and there is no obvious oxidation phenomenon. Before the furnace is installed, the oxide film on the surface is effectively removed. During the brazing process, magnesium blocks are placed in the furnace to effectively reduce the effect.

(2) After the heat exchanger is brazed, the smooth transition of the brazing corners at the brazing joints is observed, indicating that during the brazing process, the brazing filler metal on the intermediate spacer has good wettability with the fins after melting.

(3) The air pressure test is carried out on the heat exchangers after welding one by one, which can meet the requirement that the oil passage and the air passage do not leak from each other when the pressure is less than 520 MPa. When the workpiece was blasted by the hydraulic test, it was found that the oil channel fins were evenly fractured from the middle, indicating that the brazing part in the oil channel was well bonded.

(4) The heat exchange efficiency of the brazed heat exchanger is tested, and the heat exchange efficiency can reach 95% of the use requirements.

4. Device selection:
The RHVB-H high temperature vacuum brazing furnace produced by SIMUWU is a high-quality product for this type of brazing process. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. With more than ten years of sales and manufacturing experience, it is exported to developed regions in Europe, America and Asia. It is a widely acclaimed vacuum furnace product.