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Relationship between material surface roughness and vacuum brazing

Surface roughness is one of the important factors affecting the quality of vacuum brazing, especially in terms of the flow and wetting behavior of the brazing material and the performance of the joint. Vacuum brazing is a brazing process performed in a vacuum environment and is widely used to connect materials such as ceramics and metals. Its purpose is to achieve high-quality joints while avoiding oxidation and contamination. The following will analyze in detail the relationship between surface roughness and vacuum brazing, including its influencing mechanism, research examples and optimization methods.

Effect of surface roughness on vacuum brazing
It has a significant impact on vacuum brazing through the following aspects:
1. Solder flow and wetting Surface roughness directly affects the flow and wetting behavior of the solder on the joint surface:
Rougher surface: may cause the solder to gather in the depressions and be difficult to distribute evenly, thus affecting the sealing and strength of the joint. Too smooth surface: may reduce the attachment points of the solder, reduce the wetting effect, and lead to poor joint bonding.
2. Contact area Rough surfaces usually have a larger actual contact area:
Advantageous effect: Increase the contact between the solder and the substrate, which helps the formation of the joint.
Adverse effect: If the roughness is too large, the solder may not be able to completely fill the groove, resulting in pores or inclusion defects, and reducing the quality of the joint.
3. Capillary action In vacuum brazing, capillary action is the main driving force for solder flow:
Moderate roughness: By forming tiny grooves and protrusions, the capillary action is enhanced, which promotes the flow and filling of the solder on the joint surface. Improper roughness: Too high or too low may weaken the capillary action and affect the uniformity of solder distribution.
4. Joint strength There is a certain correlation between surface roughness and joint strength: Moderate roughness: can improve the shear strength of the joint. For example, studies have shown that in stainless steel vacuum brazing, the joint strength is best when the surface roughness Ra is 0.8-1.6μm. Too high or too low roughness: may lead to uneven distribution of the brazing material or insufficient wetting, reducing the joint performance.

Research examples on the impact of vacuum brazing:
1. Ceramic-metal brazing case: In the vacuum brazing of alumina ceramics and Kovar alloys, the roughness of the ceramic surface has a significant effect on brazing filler metal wetting and joint strength. Findings: By optimizing the roughness of the ceramic surface (such as Ra 0.5-1.0μm) and combining it with molybdenum-manganese metallization, the sealing and strength of the joint can be significantly improved, with a leakage rate as low as 8.0 x 10⁻⁹ Pa m³/s. Application: This technology is often used for ceramic-metal seals in nuclear power equipment.
2. Stainless steel brazing case: In the vacuum brazing of stainless steel, surface roughness is critical to brazing filler metal flow and joint quality. Findings: When the surface roughness Ra is controlled at 0.8-1.6μm, the brazing filler metal wettability is optimal and the joint strength is highest. Too high or too low roughness will lead to performance degradation. Application: Suitable for industrial parts that require high strength and sealing.

Improve the quality of vacuum brazing
In order to improve the quality of vacuum brazing, the surface roughness can be optimized by the following surface treatment methods:
1. Mechanical processing method: grinding, polishing, etc. Purpose: to control the surface roughness within an appropriate range (such as stainless steel recommended Ra 0.8-1.6μm, ceramic recommended Ra 0.5-1.0μm). Effect: improve the flow and wettability of the brazing material.
2. Chemical treatment method: acid washing, alkali washing, etc. Purpose: remove the surface oxide layer and pollutants, and improve the surface activity. Effect: enhance the bonding ability between the brazing material and the substrate.
3. Coating treatment method: metallization treatment on the ceramic surface (such as molybdenum manganese method). Purpose: improve the wettability of ceramics and brazing material. Effect: improve the strength and sealing of the joint.

Conclusion
There is a close relationship between surface roughness and vacuum brazing. It directly determines the sealing and strength of the joint by affecting the flow, wetting behavior and capillary action of the brazing filler metal. Moderate surface roughness can optimize the brazing effect, while too high or too low roughness may lead to defects. In practical applications, appropriate surface treatment methods should be selected according to specific materials and brazing processes to control the surface roughness within the optimal range to achieve high-quality vacuum brazed joints.