Diffusion Bonding | Working , Advantages and Application
It is a solid state welding process that uses heat and pressure, usually in a controlled atmosphere, with sufficient time for diffusion and coalescence to occur. In this process, moderate pressure of about 10 MPa is applied to carefully cleaned surfaces of the workpieces at an elevated temperature below the melting point of the metals to result primarily from diffusion. Atoms diffuse across the interface to form the bond. Diffusion involves the migration of atoms across the joint due to concentration gradients. This process requires temperatures of about 0.6Tm (Tm is the melting temperature of the metal) in order to have a high diffusion rate between parts being joined. Here, plastic deformation at surfaces is minimal and primary coalescence mechanism is solid state diffusion.
When joining two materials of similar crystalline structure occurs, diffusion bonding is performed by clamping two pieces to be welded with their surfaces abutting each other. Prior to welding, these surfaces must be machined to smooth finish and kept free from chemical contaminants or other debris. Mostly, surface treatment including polishing, etching and cleaning as well as diffusion pressure and temperature are important factors regarding to process of diffusion bonding. Surface roughness value, Ra of less than 2 micron and waviness of less than 400 micron are preferred. Oxides need to be removed.
Diffusion bonding is performed under controlled atmosphere to prevent oxidation. Sometimes, a layer of filler material is needed to achieve good bonding. Any intervening material between two metallic surfaces, as shown in Figure 3.5, may prevent adequate diffusion of material because the strength of the welding depends on pressure, temperature, time of contact and cleanliness of the metal.
The example for diffusion welding is bonding of gold over copper. First, a thin layer of gold foil is obtained by hammering. Once clamped, the gold foil is then placed over copper and then the weight is placed on top of it, usually for many hours. In diffusion welding, the pressure may be applied by dead weights or by a press using differential gas pressure or by high-pressure autoclaves because this method allows for exact measurements of load on the parts. The assembly is then placed in a furnace and left until a good bond is obtained. The parts are usually heated in a furnace or by electrical resistance. Figure 3.6 illustrates the heating of diffusion bonding by using electrical resistance method. Diffusion bonding must be done in a vacuum or inert gas environment when using metals that have strong oxide layers such as copper.
Stages in Diffusion Welding
Diffusion bonding occurs in three simplified stages as follows:
Stage 1: Deformation and interfacial boundary formation
Before the surfaces come to complete contact, very small surface defects (asperities) on the two surfaces will contact and plastically deform. As these small surface defects deform, they interlink for forming interfaces between two surfaces.
Stage 2: Grain boundary migration and pore elimination
Both elevated temperature and pressure cause an accelerated creep in the materials, grain boundaries and raw material migrate. The gap between two surfaces is reduced to isolated pores.
Stage 3: Volume diffusion pore elimination
Material begins to diffuse across the boundary of the abutting surfaces, blends this material boundary and creates a bond.
Advantages, Limitations and Applications of Diffusion Welding
Advantages of Diffusion Welding :
1. Plastic deformation at surface is minimal.
2. Dissimilar materials may be welded.
3. Welds of high quality are obtained.
4. There is no limitation in the thickness of workpieces.
5. The bonded surface has the same physical and mechanical properties as the base material.
6. The diffusion bonding is able to help us to build high precision components with complex shapes. Also, the diffusion is flexible.
7. The diffusion bonding method can be used wildly, joining either similar or dissimilar materials and also important in processing composite materials.
8. The process is not extremely hard to approach and the cost to perform the diffusion bonding is not high.
9. The diffusion bonding process is able to produce high quality joints in which no discontinuity and porosity exist in the interface.
Limitations of Diffusion Welding :
1. It is a time consuming process due to low productivity.
2. Time required for diffusion can range from seconds to hours.
3. Very thorough surface preparation is required prior to welding process.
4. The mating surfaces must be precisely fitted to each other.
5. It requires relatively high initial investments in equipment.
Applications of Diffusion Welding :
1. It is used in joining of high-strength and refractory metals based on titanium in aerospace and nuclear industries.
2. Diffusion welding is most commonly used to join sheet metal structures in nuclear and electronics industries.
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