Diffusion bonding, also known as diffusion welding, is a welding technique which is primarily used in metal working. By using diffusion bonding, it is possible to join two different metals through the principle of solid-state diffusion. This technique is usually performed at 50 – 70 % of the melting temperature of the materials. After the desired temperature is reached, high pressure is applied to the materials. Diffusion bonding is widely applied in nuclear industries and aerospace.
The actual process is centuries old and was used to bond gold and copper for jewelry. The soviet scientist N. F. Kazakov described modern methods of diffusion bonding in 1953.
Principle of diffusion bonding
Diffusion bonding operates on the principle of diffusion, which is the movement of atoms or molecules from a region with high concentration to a region with low concentration. To set the process in motion, high pressure is applied on both materials, which are placed one over the other. Heat accelerates the process of diffusion bonding. However, the temperature must not be too high and ranges between 50 – 70 % of the melting temperature of the metal.
Process of diffusion bonding
When diffusion bonding is performed on two materials with similar crystalline structure, the workpieces are clamped together with the metal surface facing each other. A main prerequisite for a successful process of diffusion bonding are the surfaces of the bonded materials. The metal surfaces have to be machined to be as smooth as possible and kept free from residues and chemical impurities. To successfully complete the procedure of diffusion bonding, every material located between the workpieces must be removed.
After the clamping of the materials, heat and pressure are applied on the workpieces for many hours. Normally, the surfaces of the workpieces are heated in a furnace or through electrical resistance. To achieve the required pressure, a hydraulic press can be used. This allows an accurate measure of the pressure during the process. If no temperature change is intended, the differential thermal expansion can be utilized to apply load on the workpieces. Further, pressure can be applied through differential gas pressure between the workpieces, dead weight, or high pressure autoclaves.
When using metals that have strong oxide layers, diffusion bonding must take place in a vacuum or inert gas environment to ensure a proper process. This protects the welding surface from oxidation.
Advantages and disadvantages of diffusion bonding
While diffusion bonding have many advantages, there are a few downfalls to the solid-state welding technique.
- The bonded materials show the same physical and mechanical characteristics as the base material. The bonded surface can be tested through a tensile testing.
- Joints created by diffusion bonding are clean, of high quality, and free from discontinuity and porosity.
- Diffusion bonding allows joining similar and dissimilar materials. For this reason, it is widely applicable.
- Due to a good dimension tolerance, diffusion bonding is used to produce high precision components and complex shapes.
- The running costs are rather low, and the process is easy to approach.
- Plastic deformation can be reduced under diffusion.
- Although the running costs are low, the initial setup cost is high.
- Compared to other welding techniques, diffusion bonding is very time-consuming.
- The preparation of the workpieces is critical and may be difficult.
- The available equipment limits the size of the weld.
- Diffusion bonding is not applicable for mass production.
- The outcome is highly dependent on the welding parameters, such as temperature, pressure, metal surface finish and the welding material.