Problems that should be paid attention to in vacuum brazing

Vacuum heating is accompanied by the release of surface adsorbed gas, volatilization and release of water, decomposition and release of organic matter such as oil, decomposition or reduction of oxide film and oxide, precipitation and release of gas elements in metal, volatilization of metal elements, changes in material structure, physical and chemical reactions between materials, etc. The decomposition and release of gas, water, organic matter, dirt, oxides, etc. will reduce the vacuum degree, cause oxidation of workpieces and brazing materials, hinder the smooth progress of brazing, and pollute the vacuum furnace, reducing the service life of the vacuum furnace, which should be avoided or reduced as much as possible.
There is also the problem of volatilization of elements in alloys during vacuum heating. The volatilization of elements in alloys is related to the saturated vapor pressure of alloy elements. As the temperature increases, the saturated vapor pressure of elements increases. When the saturated vapor pressure of elements is greater than the gas pressure in the vacuum chamber, the metal elements begin to volatilize strongly. The volatilization of metal elements will cause the element depletion of the parent material, affect the performance of the parent material, and cause serious pollution to the vacuum furnace, which should be limited as much as possible.
When vacuum brazing, you should try to avoid using base materials containing elements with high vapor pressure at the brazing temperature, and avoid choosing brazing materials containing elements with high vapor pressure. In order to reduce the volatilization of metal elements, a certain amount of inert gas can be introduced to increase the partial pressure of the inert gas in the vacuum chamber, that is, a brazing process with partial pressure control can be adopted. Special vacuum brazing furnaces can also be used, such as special brazing furnaces for batch brazing production of stainless steel heat exchangers with copper as brazing material.

Since there are few gas molecules in a vacuum, heat transfer is mainly radiation, and the heating process of parts is mainly achieved by receiving heat radiation from heating elements, heat conduction inside parts, and heat conduction from tooling contact. The law of heat transfer in a vacuum has an important influence on the actual temperature of parts when heated in a vacuum. In the case of single radiation heat transfer, the heat transfer capacity is related to the grayscale coefficient and temperature of the parts, and the heat transfer capacity is proportional to the fourth power of the absolute temperature. It can be seen that the heat transfer capacity at different temperatures is also very different. Rough surfaces and bright surfaces, thin-walled parts and thick-walled parts, surface area, different material properties, etc. may all cause differences in heating and cooling speeds and actual parts temperatures. The furnace temperature measured and controlled is actually the temperature of the actual structure of the hot end of the temperature-controlled thermocouple, which is often inconsistent with the temperature of the workpiece. Since the heat storage capacity of the vacuum furnace insulation layer is small, when the vacuum furnace temperature-controlled thermocouple rises to the set temperature, the heated workpiece is far from reaching the temperature. This is the so-called “hysteresis phenomenon” during vacuum heating. Test results show that the time for the core of a φ50mm×l50mm bearing steel rod to reach temperature during vacuum heating is 6 times that of salt bath heating and 1.5 times that of an air furnace. The burn-through time of a vacuum furnace at 850°C can be 1.5min/mm, and the burn-through time from 850~1280°C can be 0.45~0.52min/mm. The uniformity of furnace temperature actually reflects the uniformity of heat radiation in the furnace, and does not reflect whether the temperature of the actual workpiece is consistent. Therefore, the characteristics of vacuum heating should be considered when determining the brazing temperature. It is best to use a workpiece thermocouple in close contact with the workpiece (such as welding) to measure the workpiece temperature, or determine the controlled brazing temperature and insulation time through experiments. During vacuum brazing, there are also problems such as the flow and restriction of the brazing material, adhesion and reaction between materials, which can be solved by coating the flow inhibitor, selecting the appropriate materials and tooling, etc. Under vacuum heating at a higher temperature, the oxide film on the surface of the parts or tooling materials may be reduced, destroyed or volatilized, etc. Therefore, when the metals are in close contact, sometimes undesirable diffusion adhesion reactions will occur, affecting the brazing operation. In severe cases, the materials will react and melt, damaging the parts or equipment. For example, when brazing titanium alloy parts, if nickel-based alloy tooling is used in close contact with it, reaction melting will occur when it exceeds 942℃.

Vacuum Brazing Furnace