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Analysis of Defects in Vacuum Brazing of Aviation Aluminum Alloy Chassis

Due to its special working environment, aviation airborne power supply chassis requires some special requirements such as heat dissipation, weight reduction, anti-electromagnetic interference, anti-vibration, and corrosion resistance. The rust-proof aluminum alloy 3A21 has mature and good brazing performance. At present, the vacuum brazing process of 3A21 aluminum alloy is generally used to manufacture the box body, and its main production process is one-time brazing forming. It is difficult to avoid defects in the brazing process, which often affect the quality and delivery of products. Therefore, it is of great significance to analyze the defects and their causes in production to reduce and eliminate defects.

1 Major Flaw

In the process of vacuum brazing of airborne power supply chassis, due to the combined effect of various complex factors such as brazing materials, technology and environment, various defects often occur in the joints. According to the location and nature of defects, it is mainly divided into external defects and internal defects. External defects mainly include insufficient melting of brazing filler metal, lack of brazing, dissolution, and loss of brazing filler metal; internal defects mainly include inclusions, pores and microstructure segregation. The existence of defects in brazing joints directly affects the brazing quality and reliability of products.

1.1 External Defects

1.1.1 Insufficient melting of solder

Insufficient melting of brazing filler metal generally means that some components of the brazing filler metal are melted and some components with high melting point are not melted. At the same time, if the workpiece is loaded with a large amount of furnace and the heat absorption of the fixture is large, the heating rate of the weldment in the solid-liquidus range of the solder will be slow, and the low melting point components of the solder will be vaporized in a vacuum environment. The melting point of the solder increases, and eventually the defect of insufficient melting of the brazing material occurs. In addition, the poor surface quality of the product will also lead to incomplete removal of the film on the joint surface, poor wettability of the solder and the base metal, and the two cannot have physical and chemical interaction, and finally the incompletely melted solder remains in the brazing joint.

1.1.2 Not brazed

In the process of vacuum brazing, the molten liquid solder wets the base metal and forms a concave surface in the brazing seam, and the surface tension of the liquid will form additional pressure at the concave surface, and the liquid solder is precisely here. Under the action of additional pressure, it automatically flows into the brazing seam. The formula for calculating the additional pressure Pk is:

Pa=2σ/r

Where: σ is the surface tension of the liquid solder; r is the radius of curvature of the liquid surface. Because the radius of curvature formed by the liquid surface in the small end gap is small, the additional pressure generated by the surface tension of the liquid is large. For a piece of liquid solder, the sum of the additional pressures in all directions will point to the small end gap. It will automatically move in the direction of the small end gap (capillary action), so that sufficient brazing material cannot be obtained in the place where the brazing gap is large, thereby forming a non-brazing defect. In the same way, the larger the brazing gap, the smaller the surface tension additional pressure, and the smaller the height that the liquid solder can climb, so the larger brazing gap needs more solder to fill, when the additional pressure is less than the liquid solder. When the gravity of the brazing filler metal is applied, the surface tension between the liquid brazing filler metal and the parts will be less than the gravity of the brazing filler metal, and the liquid brazing filler metal will not be able to fill the gaps and form unbrazed defects.

Non-brazing is one of the more serious brazing defects, which has adverse effects on the air tightness, electromagnetic compatibility, moisture resistance and salt spray corrosion resistance of the product. However, due to the limitation of parts manufacturing accuracy and assembly accuracy, the two parts The gap formed between them is difficult to be uniform, and often there is a large gap at one end and a small gap at the other end on the cross section of the end face joint.

In addition to the above-mentioned main reasons for the uneven gap of the brazing seam, the unreasonable design of the fixture, the lack of consideration of the clamping method and the clamping position will all cause non-brazing defects.

1.1.3 Dissolution

Erosion is a defect formed by the dissolution of the surface of the base metal by the molten solder. When the brazing holding time is constant and the brazing temperature is too high, the fluidity and filling performance of the liquid brazing filler metal are good at high temperature, so that the brazing filler metal accumulates at the mounting screws of the bottom plate and the side panel, resulting in the Si element in the brazing filler metal passing through the transition layer to the parent At the same time, the base metal also dissolves into the brazing material. The diffusion of Si element changes the composition of the base metal and reduces the melting point, so that the base metal transition layer at the brazing temperature is almost completely melted, while the nearby base metal There is also a little melting, that is, corrosion behavior occurs. The occurrence of corrosion reduces the strength and fatigue resistance of the brazing joint, and changes the mechanical properties. In addition, if multiple batches of products have corrosion defects and no improvement is seen after adjusting the temperature, the matching of the brazing filler metal and the base metal should be considered, and the test should be re-tested by changing the brazing filler metal grade and adjusting the element ratio.

1.2 Internal Defects

Since the microscopic appearance inside the weld cannot be observed by ordinary visual inspection, and microscopic defects are the origin of macroscopic defects, it is often found in the test that the appearance of the visual weld is full and defect-free, while the internal inspection of the weld is found to have microcracks. Defects such as uneven tissue.

The uneven organization and segregation become the main factors affecting the internal quality of the product. Although the appearance of the weld area is plump, the dendritic and flaky Si segregation appears in the structure, which seriously splits the structure, affects the uniformity of the weld structure and reduces the strength of the product. Seriously, the segregation polymer will form cracks at the interface, resulting in weld cracking. Therefore, it is believed that with the extension of the brazing holding time, the Si-rich segregated polymer becomes less and less. When the holding time is short, the Si element does not have time to diffuse into the base metal and the brazing filler metal, so it is related to the AI in the brazing filler metal and so on. Other elements generate eutectic structures or form Si segregated structures. As the holding time is extended to 7min, it can be seen that the Si segregation structure in the weld becomes very little, and the weld is almost a solid solution structure. Due to the prolongation of the holding time, the Si element has enough time to diffuse and evenly disperse in the aluminum matrix to form a solid solution structure, which avoids the Si segregation structure.

2 Conclusion

(1) The defects of aluminum vacuum brazing are divided into internal defects and external defects, and different defect types have an impact on the reliability of the joint.

(2) The quality of brazing is the result of the selection of comprehensive factors such as brazing filler metal, brazing temperature, brazing time, furnace loading, and fixtures.

(3) Under the premise that other factors remain unchanged, appropriately prolonging the brazing holding time can reduce the segregation of Si in the brazing joint, form a solid solution structure with stable performance, and effectively improve the quality of the brazing joint.

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