What is diffusion bonding?
Diffusion bonding or welding is a solid state joining process wherein coalescence of the faying surfaces is produced by the application of pressure and elevated temperature to carefully cleaned and mated metal surfaces so that they actually grow together by atomic diffusion. Diffusion welding takes longer welding time.
History of Diffusion Bonding:
The process developed in the early 1970’s. The process created based on the goldsmith’s gold and copper foil bonding to produce “filled gold”. At first, gold foil placed above the copper foil. Simultaneously a weight is placed over the coil and from the furnace constant heat is applied to make the strong bond.
Diffusion welding process involves two steps or stages:
To accomplish the task of diffusion bonding, metal to metal contacts of the two pieces to be diffused. This is done by the application of pressure that deforms the substrate roughness and disrupts and disperses the above mentioned surface layers and contaminants.
It involves diffusion and grain growth to complete the weld and ultimately eliminate the interface formed in the previous stage. The second stage induces complete metallic bonding across the area of contact.
In order to increase diffusion rate, moderate heating temperatures (usually below 1100°C) are used. Without applying heat if it takes many hours to perform a certain bonding, with the application of heat, the time element will be cut to a few hours or minutes.
Diffusion welding process steps:
- Two typical work piece surfaces to be diffusion welded can be seen
- The individual peaks which make up the roughness are deformed by the application of increasing pressure
- At places where the surfaces move together under shear, the oxide films are disrupted and metal to metal contact takes place
- After metal to metal contact is established, the atoms are within the attractive force fields of each other and hence a high strength joint is generated. The joint resembles a grain boundary
- A planer interracial boundary being thermodynamically unstable tends to migrate to a more stable configuration if conditions permit
Diffusion welding methods:
- Gas pressure bonding
- Vacuum fusion bonding
- Eutectic fusion bonding
Gas pressure bonding:
Parts to be joined are placed together in intimate contact and then heated to around 815°C. During heating, an inert gas pressure is built up over all the surfaces of the parts to be welded. Non ferrous metals are joined with the help of gas pressure bonding method.
Vacuum fusion bonding:
Parts to be joined are pressed together mechanically or hydraulically. A hydraulic press used for diffusion welding resembles that employed in forging and is equipped to pressurize from three directions.
Eutectic fusion bonding:
It is a low temperature diffusion welding process. A thin plate of filler metal is kept between the base metal to be joined. As the pieces are heated to an elevated temperature, the filler material diffuses and forms a eutectic compound with the parent metals.
Advantages of diffusion welding:
- Continuous, leak tight welds can be formed
- The process is well suited for welding dissimilar metals and ceramics
- Number of welds can be made simultaneously
- Weld-ability is largely independent of material thickness
Limitations of diffusion welding:
- Opposing surfaces must be mated in size to within a few angstroms of each other in order to achieve a satisfactory metal bond
- Diffusion welding requires a relatively long, time consuming thermal cycle
- Diffusion welding is not classified as a mass production process
- The cost of the machine is usually high. Simultaneous application of heat and compressive force in vacuum environment are the major problem in constructing the machine.
Applications of diffusion welding:
- Fabrication of composite materials
- Copper liner fabrication in the liquid rocket combustion chamber
- Stainless steel window frame fabrication in the land based missile
- Internal channel liners for the liquid hydrogen cooling