Introduction of Diffusion Bonding
Diffusion bonding is a joining process where in the principal mechanism for joint formation is solid state diffusion. The solid phase diffusion bonding of the product is used in various industrial fields to make making to high performance and making to a high function near-net shape in addition. Diffusion bonding offers many advantages, mainly the strength of the bonding line, which is equal to the base metals. The microstructure at the bonded region is exactly the same as the parent metals. On the other hand, this advantage joining process requires several strictly controlled condition: clean and smooth contacting surfaces which are free from oxides, etc., high temperature condition to promote diffusion process. In diffusion bonding, the bond strength is achieved by pressure, temperature, time of contact, and cleanness of the surfaces. The strength of the bond is primarily due to diffusion rather than any plastic deformation. Diffusion bonding is an attractive manufacturing option for joining dissimilar metals and for making the component with critical property continuity requirements. Unlike other joining processes the diffusion bonding process preserves the base metal microstructure at the interface. More importantly no localized thermal gradient is present to induce distortion or to create residual stresses in the component.
A three stages mechanistic model, adequately describes weld formation :
- In the first stage, deformation of the contacting asperities occurs primarily by yielding and by creep deformation mechanism to produce intimate contact over a large fraction of the interfacial is. At the end this stage, the joint is essentially a grain boundary at the areas of contact with voids between these areas.
- During the second stage, diffusion become more important than deformation, and many of the voids disappear as grain boundary diffusion of atoms continues.
- In the third stage, the remaining voids are eliminated by volume diffusion of atoms to the void surface.
