Heating and Cooling Rate of Workpieces in Brazing Process

    • In order to prevent the stress deformation of the joint and the workpiece during the brazing process, it is very important to control the temperature rise and cooling speed of the workpiece. Prior to brazing, annealed materials should be used, + quenched materials. Cold worked materials should also be annealed first.
    • The rate of temperature rise is related to the thermal conductivity of the material and the size of the workpiece, in addition to the effect of adjusting the melting temperature range of the flux and the solder as described in Section 1.5. For those workpieces that are brittle, have low thermal conductivity, and are relatively small in size, it is not suitable to heat up too quickly. Otherwise, the material will be cracked, resulting in a difference between the internal Iftf and the internal stress and deformation. This is because, in addition to high frequency heating, the heating of the workpiece is performed by radiation and convection of the ambient heat source. Increasing the rate of temperature rise is often achieved by increasing the temperature of the heat source, which tends to cause a temperature gradient difference between the inside and the outside of the workpiece,
    • thereby causing the above disadvantages. However, it is a desirable method to increase the ambient temperature of the workpiece and enhance the convection and circulation of the atmosphere to enhance the heat transfer to improve the heating rate. This can also improve the uniformity of heating, which is often used in tunnel furnace heating furnaces. a way. Similar to salt bath brazing and metal bath brazing are preferred.
    • The cooling rate has a large influence on the structure of the brazing joint. After the brazing process is completed, rapid cooling facilitates the refinement of the brazing alloy structure in the brazing joint, thereby enhancing the various mechanical properties of the brazing joint. This is not a problem for thin-walled materials with high heat transfer coefficients and toughness. On the contrary, for those materials with thick walls, low thermal conductivity and brittleness, there are the same disadvantages as when heating is fast.
    • The slower cooling rate is beneficial to the homogenization of the brazing structure, which is more prominent in the case where some brazing filler metal and the parent metal can be solid solution. For example, Cu-P brazing brazing copper, the slow cooling rate makes the brazing The seam contains more Cu-P solid solution and less Cu3P compound eutectic.
    • In short, to choose the appropriate heating or cooling rate, the properties of the base metal, the shape and size of the workpiece, the characteristics of the solder and its interaction with the base metal should be considered.

Annealing for Brazed Joints and Structure Dispersion in the Fillet

    • After the brazing process is completed, the heat treatment at the appropriate temperature F sometimes makes the brazed joint structure uniform and increases the strength of the joint; however, sometimes improper handling may damage the joint structure, and the strength of the joint is greatly reduced, even Break on your own.
    • When the main component of the brazing filler metal is the same as the parent metal, and the other components have a small solidity in the parent metal, the brazing seam is most suitable for heat preservation treatment. For example, aluminum alloy 3A21 is brazed with A1-SH2, the melting temperature of the solder is 577T, and the brazing temperature is 600弋. After the welding is subjected to different holding time at the original temperature, the metallographic phase of the brazing joint is as shown in Figure 1-28. ]. Figure l-28a shows the metallographic joints without brazing. Figure l-28b, c, and d show the brazing joints after lmin, 3min and 7min respectively. It can be seen that with the prolongation of time, the molten eutectic solder deepens along the grain boundary, the brazing seam diffuses and widens, and the eutectic silicon tends to aggregate into larger grains. After 7 minutes of heat preservation, the eutectic silicon disappeared almost completely, and only the discontinuous silicon grains remained in the brazed joint structure, and the metallographic sense of the brazing seam actually no longer existed. This phenomenon occurs because the solid solubility of Si in AI at 600t is <1% (mass fraction>, and it is impossible for Si in the solder to form a solid solution in the solid solution of AI. At this time, the small grain due to surface energy Large and solid solution, larger grains are further deposited due to smaller surface energy. It is obvious that the joints having the structure of Figures 1-28d have better performance than those of Figure 1-28a.
    • Similar to the case of brazing aluminum with an Al-Si eutectic solder, a similar phenomenon was observed when the pure copper was brazed in a soot with a Cu-p solder, but the dispersed phase was Cu3P.If there is an intermetallic compound between the base metal and the brazing filler metal, the first main component of the intermetallic compound is the same as the parent metal, and the other main components and the base material have a certain solid solubility. Intermetallic compounds decrease or even disappear.
    • When the first main component of the solder is the same as the base material, and the other components and the base material have a large solid solubility, after the heat treatment, the brazing seam will be filled with the solid solution to varying degrees with the increase of the holding time. . For example, Vacuum brazing furnace copper with H62 (Cu62Zn) solder, and after 950T insulation, you can see this situation, the brazing joint is brazed by Cii-Zn aluminum, and it is kept at more than 500 ;; fflln brazing copper, is greater than TOOT insulation; brazing nickel with Cu, heat insulation at 1300 or more; brazing aluminum with 03, more than 30T insulation. In this case, you should try to reduce the temperature and shorten the operation time.
    • The above-mentioned insulation effect, whether positive or negative, is due to the necessary solid solubility of the main component of the solder and the base material. In the case where there is no solid solubility between the brazing filler metal and the base metal, even if the thermal insulation treatment is carried out for a long time, there is no structural dispersion phenomenon to produce Zn brazing aluminum, which is insulated at more than 500 ;; fflln brazing copper, It is larger than TOOT insulation; it is brazed with Cu, and it is kept at 1300 or more; it is brazed with 03, and it is more than 30T. In this case, you should try to reduce the temperature and shorten the operation time.
    • The above-mentioned insulation effect, whether positive or negative, is due to the necessary solid solubility of the main component of the solder and the base material. In the case where there is no solid solubility between the brazing filler metal and the base metal, even if the thermal insulation treatment is carried out for a long time, no structural dispersion occurs.