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Vacuum Brazing Furnace

The Basics of Aluminum alloy vacuum brazing

Vacuum brazing uses an alloy material with a lower melting point than the base metal as the filler metal. The base metal and the filler metal are clamped together with a clamp, and heated in a vacuum furnace to melt the filler metal into a liquid state. The liquid filler metal is in the gap or between the base metal. Wet on the surface, flow, fill and spread under the action of capillary force, and interact with the base metal (dissolve, diffuse or produce intermetallic compounds) to achieve welding metallurgical bonding. The factors that affect the quality of aluminum alloy vacuum brazing include: the applicability of the filler metal and the base metal, the surface condition of the base metal (oxide film cleaning state, roughness, etc.), the clamping of the base metal and the filler metal, the brazing vacuum degree, liter Cooling rate, brazing temperature and holding time, etc.
3A21 aluminum alloy is an AI-Mn aluminum alloy with high plasticity, good corrosion resistance and weldability. Its melting temperature range is between 643~654℃. Generally, Al-Si solder is mainly used for vacuum brazing aluminum alloys. This solder has good wettability and fluidity and is based on the eutectic composition of Al and Si (the mass fraction of Si is 7%~12%). It is the most widely used solder material. However, this solder also has the disadvantage of high melting point. The liquidus temperature is 577°C, and the brazing temperature reaches 600°C. The brazing temperature is close to the melting point of the base metal, which can easily cause grain growth in the base metal. If the heating temperature is not controlled properly during brazing, the base metal may be overburned. Even melt.
The antenna assembly is one of the key components of a certain type of product. Among 100 aluminum alloy antenna assemblies, 21 were found to have solder leakage and accumulation.
Phenomenon. The antenna assembly is brazed from 5 layers of 3A21-H112 raw material plates. The processing technology is: blanking → heat treatment (290℃, 4~10h) → CNC processing → cleaning → vacuum brazing (425~600℃, 4h) → CNC processing. Due to different degrees of solder spreading, part of the antenna solder accumulates on the solder toes and spreads along the surface of the plate, resulting in insufficient effective surface area of the antenna, affecting the electromagnetic wave reflection signal and causing failure.
Possible factors affecting the quality of vacuum brazing are:
1) The applicability of filler metal and base metal. According to the Al-Si-Mg ternary phase diagram, it can be seen that the solder is mainly composed of the eutectic of the (Al) phase and the Si phase. Generally, vacuum brazing aluminum alloys mainly use Al-Si based solder, but the solder also has a melting point. The high disadvantage is that the brazing temperature is close to the melting point of the base metal, which can easily cause grain growth in the base metal. Heat treatment experiments also show that heat treatment at 450°C has a greater impact on the rolling structure, and the structure recrystallizes and strips form. The grains evolve into equiaxed grains; in heat treatment at 500°C and above, as the heat treatment temperature increases, the grains continue to grow. In addition, the hardness of plates with larger grain sizes is lower, indicating that the melting point and brazing temperature of the filler metal are relatively high compared to the 3A21 base metal, and the welding temperature will have a certain impact on the performance of the base metal. In order to lower the melting point of Al-Si solder, alloying is one of the main methods. Research shows that elements such as Ge, In, Yb, and Cu can lower the melting point of solder. The addition of rare earth elements can improve the wettability of Al-Si-based solder and improve the welding quality. However, considering the material cost, adding rare earth elements is currently mainly used. A certain content of Cu element is used to reduce the melting point of the solder.
2) Vacuum brazing process. If the process is not properly controlled during vacuum brazing (the brazing temperature is too high or the temperature rise and fall rate is too fast), Si and other alloying elements in the brazing material will easily form intermetallic compounds and segregation of alloying elements during the brazing process, thereby reducing the welding quality. joint performance. In addition, if the brazing temperature is too high and the solder melts too fully, it is easy to cause the loss of solder and may lead to oxidation of the solder. Failures due to improper brazing temperature control often occur.
3) Surface condition of the plate. Surface roughness has an important influence on the diffusion of interface elements and the wetting and spreading of the solder during the brazing process! A certain surface roughness has a capillary effect on the molten solder. If the surface of the base metal is too smooth, the capillary effect will be poor and the brazing will be poor. It is difficult for the material to spread throughout the entire weld, and the resulting gaps will reduce the strength of the joint. In order to ensure that the brazing material is fully spread, the brazing surface of the weldment should be roughened. In addition, there are obvious differences in the grain size of the plate after heat treatment. This may be due to the heat treatment process not being well controlled or the large difference in grain size before the plate is heat treated. It is recommended to strictly control the heat treatment process of the raw material rolled plate and control the raw material. Quality and acceptance process. In the later period, by strictly controlling the heat treatment temperature of the plate below the recrystallization temperature and the grain size of the plate, this type of failure did not occur again.