Factors Influencing the Brazing Quality of Vacuum Brazing Furnace

In addition to the influence of vacuum brazing parameters on the quality of brazed joints, the following important factors also have a great influence on the quality of brazing in vacuum brazing furnace.
(1) Leakage rate of vacuum brazing furnace
The leakage rate of vacuum brazing furnace is one of the important factors affecting the quality of brazing joints. When the leakage rate of vacuum brazing furnace is too large to meet the maximum allowable leakage rate or the leakage occurs in vacuum brazing furnace, it will cause the oxidation of welding parts. Therefore, it is very important to control the leakage rate of vacuum brazing furnace below the maximum allowable leakage rate (usually less than 10-6Pa.m3/S).
In addition, many vacuum equipment manufacturers also adopt the control equipment backup rate, that is, the pressure rise rate in the furnace after the main valve of the vacuum system closes is not allowed to exceed the standard of 0-75 Pa/h, in order to ensure the minimum leakage rate of the furnace.
(2) Purity of working gas
In vacuum brazing, argon or nitrogen are usually used as working gases. On the one hand, they are used as forced cooling gases, on the other hand, they are used as protective gases for brazing fillers (such as copper or copper alloys) with high evaporation pressure. In order to reduce the oxygen content in the gas, the higher the purity, the better. This is because even if the oxygen content is very small at high temperature, it will oxidize the components and make them discolored.
(3) Cleanliness of parts
Before brazing, the surface cleanliness of parts not only needs to be carried out according to the requirements of oil removal and cleaning, but also must be prevented from secondary pollution after oil removal. This is because filler metal and elbow usually re-pollute parts, so clean cotton gloves should be worn when contacting parts to prevent parts from contaminating again due to hand contact. In addition, it is strictly forbidden to knock with aluminium hammer during assembly to avoid low melting point metal pollution on brazing surface. At the same time, we should pay attention to the pollution of the furnace to the parts. When the furnace has just finished processing the copper-based solder, the furnace wall will be more or less polluted by copper (when the pressure in the furnace is regulated by micro-argon, the copper solder is seldom volatilized, but it is impossible to avoid it altogether). If the nickel-based material parts are brazed next, it may cause the corrosion of copper to nickel, so the vacuum furnace should be cleaned up to make the pollution furnace. The low melting point alloy of the wall evaporates again and is pumped out.
(4) Gap size during assembly
The size of joint clearance can directly determine the compactness and strength of the brazing seam. The large gap can weaken the capillary effect, make filling difficult, and weaken the alloying effect, resulting in poor mechanical properties of the joint. Especially for open joints at both ends, the surface tension between the liquid solder and the parts is less than the gravity of the solder, and the liquid solder can not maintain its own quality, which leads to the soldering can not be carried out. The small gap will hinder filling of filler metal, and it is not easy to form joints with good penetration. The surface tension between the liquid filler metal and parts is less than the gravity of the filler metal, and the liquid filler metal can not maintain its own quality, which leads to the failure of brazing. The smaller the gap is, the better the filling gap is. In this way, the alloying effect of brazing filler metal is stronger, which is conducive to diffusion and less metal compounds. More importantly, the brazing filler metal in the narrow gap is limited by the brazing metal with high surrounding strength when plastic deformation occurs. Complex stress is formed in the joint, which greatly improves the strength of the joint.
In production practice, joint clearance is guaranteed by machining accuracy, and is generally not re-checked. For copper and copper alloy brazing stainless steel and stainless steel, the assembly gap is 0.03-0.10 mm (the same thermal expansion coefficient of similar materials, assembly gap is brazing gap). For nickel-based brazing alloy, the assembly gap is 0.013-0.076mm.

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