Reliable Joining Technology for Oxygen-Free Copper Materials

Oxygen-free copper is a high-purity copper material with extremely low oxygen content, usually less than 0.001%. Compared to regular electrolytic copper, oxygen-free copper offers higher electrical conductivity (above 101 % IACS), superior thermal conductivity (~390 W/m·K at 20 °C), and improved ductility and fracture resistance. These properties make Oxygen-free copper ideal for high-precision electrical components, vacuum systems, heat exchangers, aerospace parts, and electronic packaging.

Although oxygen-free copper has excellent physical properties, joining this material is technically challenging. The melting point of oxygen-free copper is about 1084 °C, while its coefficient of thermal expansion is approximately 16.5*10^-6/°C. Its very high thermal conductivity causes rapid heat dissipation from the welding area, making it difficult to maintain sufficient heat input during welding. If heat input is not properly controlled, defects such as lack of fusion, excessive distortion, and uneven weld penetration may occur. These characteristics require precise control of welding parameters and careful selection of the joining process.

That’s why Vacuum brazing is preferred for joining Oxygen-free copper and also widely used for complex assemblies and precision components because it provides a clean, oxygen-free environment throughout the heating process. Uniform furnace heating reduces thermal gradients and distortion, while controlled temperature cycles allow precise control of brazing filler flow and diffusion.

Ni or Ag-Cu-based brazing alloys are commonly used for vacuum brazing of oxygen-free copper, depending on service temperature and joint requirements. The filler material is selected to have good wettability and a suitable melting range, typically between 780 and 1150 °C. Joint clearances are usually controlled within 0.02 to 0.10 mm to ensure effective capillary action and complete filling of the joint during brazing.

Before assembly, the oxygen-free copper surfaces are thoroughly cleaned to remove oil, grease, oxide films, and other contaminants. Mechanical cleaning followed by chemical degreasing is commonly used to ensure clean and active surfaces. Clean surface conditions improve wetting behavior and promote strong metallurgical bonding between the base material and the brazing filler.

Technical Details

The assembled oxygen-free copper components are placed in the uniform temperature zone of a vacuum brazing furnace and evacuated to a vacuum level typically below 5 *10^-4Pa. The furnace temperature is raised gradually at a controlled rate to allow sufficient degassing of the base material and filler alloy. A preheating stage is usually applied between 400 and 600 °C to remove residual gases. The brazing temperature is commonly maintained in the range of 1050 to 1150 °C, depending on the filler alloy used, with a holding time of 10 to 30 minutes to ensure complete melting and diffusion. After brazing, controlled furnace cooling is applied to reduce residual stress and distortion.

Vacuum-brazed oxygen-free copper joints typically exhibit high density, low porosity, and excellent metallurgical bonding. The absence of flux and oxidation results in clean joint interfaces with high electrical and thermal conductivity. These joints show good mechanical strength and long-term stability, even under thermal cycling and vacuum service conditions.

The Normantherm VF1300-644 vacuum brazing furnace is the most suitable model for this application and is widely used for precision vacuum brazing of copper, stainless steel, and other metals because it is designed for high-vacuum brazing applications and can reliably operate at brazing temperatures of 1050 to 1150 °C, with a maximum temperature up to 1300 °C. It provides a stable high-vacuum environment (down to about 10⁻⁴ Pa), precise temperature control, and good heating uniformity, which are essential to prevent oxidation, ensure proper degassing, and achieve clean, dense brazed joints in oxygen-free copper. For smaller components or lower production volumes, the VF1300-422 can also be used, but the VF1300-644 offers better overall process stability and flexibility for industrial vacuum brazing.

Edited by: Shristi Paudyal
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