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Oil cooler vacuum brazing

Oil cooler, also known as oil cooler, is used to cool engine lubricating oil, automatic transmission oil, torque converter oil or hydraulic oil of hydraulic device to ensure reliable operation of the engine, etc. It is also the “core auxiliary component” of the engine. The service life and quality of oil coolers often depend on the quality of their brazing. Take the disc type oil cooler (hereinafter referred to as the cooler) of the British Perkins engine engine as an example, it adopts the vacuum brazing process, the welding seam quality is high and the service life is long, which plays an important role in improving the brazing quality of the oil cooler. played a vital role.

The cooler is mainly composed of a bottom plate, a cover plate, a composite assembly, an oil inlet pipe and an oil outlet pipe. A copper foil with a thickness of 0.1mm or a nickel-based foil with a thickness of 0.04mm is placed inside and between the sheets, and the thickness of the base material to be brazed is 0.4mm. The schematic diagram of the structure of the cooler is shown in Fig.

The performance requirement of the cooler is that the cooling effect is good, so the brazing seam is required to be dense and the appearance is neat. The base material of the cooler is austenitic stainless steel, and its joint design is a flat superimposed type. Correct selection of brazing filler metal is an important factor in ensuring a high-quality brazed joint. The base metal to be brazed is austenitic stainless steel, so copper brazing filler metal should be the first choice. The determination of the weld gap value of the cooler is relatively complicated. According to the base metal material, joint structure and brazing process, and referring to the experience and data summarized in production practice, when pure copper solder is selected, the brazing gap is 0.03mm.

Process flow:

(1) Preparation of parts before welding. Since the base material is stainless steel, it is only necessary to carefully remove the surface oil, that is, soak and clean in a mixed solution of NaOH, NaSiO, Na3PO4, and OPL emulsifier.

(2) Assembly and positioning of parts. Due to the special shape of the cooler, a special brazing fixture is designed to ensure the brazing position. When designing the fixture, the assembly stress should be reduced to a lower limit, which is conducive to the flow of solder.

Vacuum brazing process parameters

(1) Degree of vacuum. The degree of vacuum depends on the vapor pressure of the metal elements of the brazing filler metal and the base metal to be brazed. When brazing stainless steel with pure copper solder, the vacuum degree in cold state can be pumped to 5×10-2Pa. Copper is a highly volatile element, with a vapor pressure of 1Pa at 1100°C; when the temperature rises to 1120°C and the vacuum degree is in the order of 1*10-2Pa, copper will volatilize in large quantities, adhere to the surface of the product, and form a “copper spray” phenomenon. It will also cause a short circuit in the vacuum furnace. In order to suppress copper evaporation, the thermal vacuum degree can be reduced, that is, high-purity argon or nitrogen gas is introduced into the furnace, so that the furnace pressure is about 5Pa at the brazing temperature.

(2) Heating rate. When the temperature is between 500 and 700 °C, the heating and cooling speed should be faster to avoid the precipitation of chromium carbide and intergranular corrosion. In the temperature range of 50~100℃ below the solidus temperature of the solder, the heating rate should not be too fast to ensure that the temperature inside and outside the component is basically the same when the solder melts, and the capillary effect is fully exerted.

(3) Stable temperature and holding time. Due to the relatively poor thermal conductivity of stainless steel components, when heating to near the brazing temperature, the holding time can be appropriately extended to reduce the temperature gradient of each part of the component.

(4) Brazing temperature. The brazing temperature is the most important technological parameter in the brazing process, and its selection is mainly based on the melting point of the brazing filler metal. Pure copper is a single-element solder with good fluidity, and the preferable brazing temperature range is 1110~1120℃.

(5) Cooling method. In order to prevent the air flow from disturbing the liquid solder, during brazing, the solder is vacuum cooled until it reaches 950°C, and then high-purity argon is introduced to start air cooling.

(6) The temperature of the furnace. When the furnace temperature drops below 150°C, it is cooled to room temperature again with ordinary argon gas, and the parts are taken out.

Brazing quality judgment

(1) Observation of the surface of the cooler. After brazing, the surface is bright, indicating that the vacuum furnace has good air tightness during the brazing process, and the base metal is not oxidized.

(2) Weld observation. The brazing seam is round, the root is full, the welding seam is continuous, and the surface of the base metal is not cracked or eroded, indicating that the brazing temperature is appropriate and the amount of brazing material is appropriate. Pure copper brazing filler metal is fuller than Ni-based brazing filler metal, has better wettability to stainless steel, and has stronger fluidity.

(3) Carry out the air tightness test on the cooler. The air pressure is 1MPa, and the pressure is maintained for 1min without leakage, indicating that the joint of the brazing seam is good.

(4) Mechanical analysis. The joint obtained by brazing stainless steel with pure copper has a shear strength of 370~500MPa. The brazing joint obtained by brazing pure copper is a single-phase structure, and the joint strength is relatively high.

Vacuum brazing furnace equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for vacuum brazing of auto parts. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.