Experiment Research On Low Vacuum Brazing Stainless Steel Oil Cooler With Pure Copper Solder

In this paper, the reason of the failure problems of ISX stainless steel plate-fin oil cooler was studied. The results show that as the main source of the corrosion, the fillet brazing between the fins and plates and the copper deposition on the fins from evaporation will act with the active sulfide which comes from the gear oil. And the corrosion products into the engine and accumulation in the oil cooler will have a negative impact on the mechanical properties. And the possibility of solving the problems of product obsolescence caused by copper corrosion by chemical cleaning was studied. The results show that the method of chemical cleaning has made significant effects in reducing the source of corrosion. However, considering the difficulties in the control of stability of the chemical cleaning fluid, the risk of reducing the life of products and the increase in production costs, the program application has been limited.And to reduce the corrosion source, the technology research on brazing stainless steel oil cooler with low copper was proposed. By improving the accuracy of components manufacturing mold, the product assembly precision was improved. The upper and lower solder foil thickness was respectively reduced from 0.12mm and 0.08mm to 0.08mm and 0.06mm so as to achieve the purpose of reducing the amount of solder. And study shows that corrosion products from oil cooler was decreased almost proportional with the reduced amount of solder. In addition, according to structural features of the plate-fin oil cooler, the rational allocation of solder was realized by removing the solder foil which was non-contact with the fin after assembly .And this approach further reduced the amount of solder. The size of the solder joints and the thickness of copper deposition decreased significantly. So the source of corrosion was effectively controlled. And the significantly reducing of the solder amount, not only did not affect the quality of brazing, but increased burst strength of the product.Meanwhile, for the characteristics of the stainless steel plate-fin oil cooler brazing with copper in the low vacuum in the actual production, the mechanism of the oxide film removal from the surface of stainless steel and the mechanism of the filler metal spreading were studied, and it provides a theoretical basis to optimize the production process. The experiment research and thermodynamics show that the oxide film will change and loose when the carbon removal from stainless steel and the carbon evaporated from the graphite rod which was in the vacuum furnace react with Cr₂O₃. In the process of oxide film removal from the stainless steel, the copper will evaporate and deposit on the surface of stainless steel effectively. And study shows that, the copper deposition from evaporation will diffuse to the stainless steel at 1050℃and form Solid-phase connection with the stainless steel matrix. Therefore, the formation of stainless steel solder joints by low vacuum brazing with copper was a process of melting solder mixing with liquid copper deposition and spreading on the matrix and filling the gap between the base metal.When the temperature is lower than the melting point of the solder, the copper deposition has already diffused to the stainless steel, and it alloyed the surface of the stainless steel. Thus the interfacial tension between the solder and the base metal were effectively reduced. And it promote the process of melting solder mixing with liquid copper deposition and spreading on the matrix and filling the gap between the base metal. In addition, in the process of melting solder filling the gap, the solder and the base metal will interact with each other relatively strongly under the conditions of a large number of dislocation net existing in base metal and under the driven of nickel atoms in the stainless steel diffusing priority. The solder and the base metal thence form metallurgical bond and high-quality solder joints.