Vacuum brazing process for copper

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Copper has excellent electrical conductivity, thermal conductivity, corrosion resistance, ductility and certain strength. Adding various alloy elements to copper can improve its corrosion resistance, strength and improve machining performance.
Parent material:
1. Pure copper (divided into oxygen copper and oxygen-free copper)
Composition: Copper content ≥ 99.5%, may contain a small amount of impurities (such as oxygen, sulfur, lead).
Features: Excellent electrical conductivity (about 100% IACS) and thermal conductivity, but high oxygen content (the oxygen content of ordinary copper such as T2 is about 0.06%~0.1%).
Usage scenario: Suitable for general electrical or thermal conductivity scenarios (such as household cables, pipes, copper radiators, copper cold plates, etc.), and cost-sensitive.
The oxides Cu2O or CuO formed on the surface of pure copper are easy to remove, so the brazing of pure copper is very good.
There is copper oxide in pure copper oxygen copper. If brazing is carried out in a hydrogen-containing reducing atmosphere, hydrogen will diffuse into the copper, reduce the copper oxide, and form water vapor. In severe cases, the copper will break.
Pure copper cannot be strengthened by heat treatment. After cold working and strengthening, the strength of pure copper will decrease after brazing. Especially after brazing in the furnace, pure copper will become very soft. When choosing the brazing heating method, the softening problem of pure copper should be fully considered.

2. Oxygen-free copper:
Composition: Copper content ≥ 99.99%, oxygen content ≤ 0.003% (TU0, TU1, etc.).
Features: Almost no oxygen, higher conductivity (close to theoretical value), better resistance to hydrogen embrittlement and processing performance.
Application scenarios: Meet high purity and low oxygen requirements, such as: vacuum environment (oxygen impurities can easily cause discharge or contamination). High-frequency electronics (high conductivity reduces signal loss). Precision machining (avoid cracking caused by oxides).

Vacuum brazing process of copper:
1. Silver-copper eutectic brazing filler metal (AgCu28) process
Optimal parameters: Brazing temperature 810-850℃, holding time 5 minutes, joint shear strength reaches a maximum value of 240MPa; strength decreases after exceeding 850℃.
Microstructure: The joint is composed of copper-based solid solution (close to the parent material side) and silver-copper eutectic structure (brazing seam center).
When the temperature rises, the copper-based solid solution layer thickens, the eutectic layer thins, and the overall width of the brazing seam decreases.
Fracture mode: Fracture mostly occurs at the junction of the copper-based solid solution and the silver-copper eutectic structure.
Advantages of vacuum brazing: avoid oxidation, reduce residual stress, no need for flux, reduce pores and slag inclusion defects.

2. Amorphous copper-phosphorus solder (Cu-Ni-Sn-P) process
Process optimization: Compared with ordinary solder, amorphous solder has a melting point 4.5℃ lower, a crystallization range 3.5℃ smaller, and significantly improved wettability (larger spreading area).

The best wettability is achieved at 680℃ for 20 minutes, with a spreading area of ​​270mm².

Microstructure: The brittle phase Cu₃P in the weld decreases with the extension of the holding time, and the plastic Cu-based solid solution increases, which improves the joint strength.

Element diffusion: Sn, Ni, and P diffuse significantly with the base material, and the reaction wetting is the main reason for the tight interface bonding.+

3. Silver-copper-phosphorus solder
The phosphorus element in the solder helps to improve the self-fluxing ability of the solder, thereby eliminating the need for additional flux during the brazing process. The solder has good fluidity and wettability and is specifically used for brazing copper and copper alloys.
Application scenarios: Silver-copper-phosphorus solder is an ideal choice for connecting copper and copper alloys. It is very popular in the HVAC industry and is commonly used in the HVAC/R (heating, ventilation, air conditioning and refrigeration) industry to connect copper pipes and components in refrigeration systems, heat exchangers and air conditioning units; in the automotive field, it is used to weld fuel lines and transmission components; the low melting point and self-fluxing characteristics are also suitable for pipeline applications and other copper-based connections. Vacuum switches, contacts, and electrical material brazing.
BCuP-5 (15 silver-copper-phosphorus), BCuP8 (18 silver-copper-phosphorus) (form: solder paste, solder sheet) are also used as vacuum brazing solder for copper.
The actual application needs to be combined with the specific situation to select the type of solder and process parameters to optimize the performance.

Vacuum Brazing Furnace