Fri, 26 Oct 2018 06:15:34 +0000
Vacuum brazing technology
(1) vacuum brazing temperature
Brazing temperature is the most important technical parameter in vacuum brazing. The important basis for determining the vacuum brazing temperature is the melting point of the selected solder and the heat treatment criterion of the brazed base metal. The vacuum brazing temperature should be appropriately higher than the melting temperature of the solder, so as to reduce the surface tension of the liquid solder, improve the humidity and filling seam, and make the solder and base metal have full effect on each other and improve the strength of the joint. At the same time, if the melting temperature of the solder is appropriately higher than that of the solder and the temperature space is set aside, the defect of bad melting of the solder caused by inaccurate temperature control of the equipment and uneven temperature of the workpiece can be prevented. However, excessive brazing temperature is harmful, which may result in evaporation of high vapor pressure elements in the solder, growth or over-firing of base metal grains, dissolution of solder and base metal due to the effect of overheating, brittle compound layer and intergranular entry, which may severely weaken the function of the base metal.
Generally speaking, the vacuum brazing temperature should be 20-60 degrees higher than the liquidus temperature of the solder, but for different solders, the temperature scale of the liquidus of the solder itself is different, and sometimes even need to be brazed below the liquidus temperature of the solder.
Welding. As for the solder with strong mutual effect with the base metal, a new alloy is formed by a great change in its composition during filling. The determination of the brazing temperature should be based on the melting point of the new alloy formed in the brazing seam. For solders with a wide melting temperature range, the selected brazing temperature can also be lower than the liquidus temperature of the solder because liquids already exist above the solidus temperature and have certain fluidity. As for eutectic reactive brazing, the temperature of brazing is only slightly higher than that of eutectic reaction temperature.
Considering the influence of vacuum brazing thermal cycle on the base metal function, it is necessary to consider the influence of brazing temperature selection on the base metal function, brazing must be carried out under the temperature of the base metal occurring intense grain growth or overheating. The preparation of brazing temperature needs to be in harmony with the heat treatment criterion after brazing. For example, the brazing temperature can be selected at the same temperature as the solid solution treatment temperature of the material, and even the brazing-heat treatment integration technology can be used. After brazing insulation is completed, the brazing and heat treatment technology can be completed at the same time in a thermal cycle. When heat treatment is carried out after brazing, the heat treatment temperature selected is best not to cause the solder to remelt.
(2) vacuum brazing heat preservation time
Brazing heat preservation time is also an important parameter in the brazing process, a certain heat preservation time is necessary to complete the brazing process between the solder and the base metal, forming a solid connection. The extension of the insulation time may make the strength of the joint improved, but sometimes the joint function will be severely reduced.
The main basis for determining the heat preservation time is the mutual effect between filler metal and base metal. When the mutual effect between base metal and brazing filler metal will cause intense corrosion, intergranular penetration and brittle phase increase, the brazing holding time should be shortened as far as possible. On the contrary, the holding time should be appropriately extended and, if necessary, the holding time should be greatly extended to disperse the solder and the base metal, so as to improve the joint function.
It is also necessary to consider the size, structure and the influence of furnace loading on the workpiece. In order to ensure that the brazed parts can reach the required brazing temperature everywhere, the large and thick parts have longer holding time than the thin and small parts, and the holding time is longer when the furnace is loaded more than when the furnace is loaded less. Insulation time is also related to the selection of temperature measurement methods, thermocouple placement and other elements, the operation should be summarized thinking. When brazing in furnace, it is necessary to ensure that the welding piece reaches a mild and satisfactory completion time of the brazing seam. When the welding piece is not too large and the furnace is not too large, the usual brazing holding time is 5-30 minutes. It should be pointed out that the brazing holding time and brazing temperature should not be determined separately from each other, there is a certain compensation relationship between them,’can be compensated for each other within a certain scale, detailed selection, but also through experiments to determine.
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