Diffusion Welding

 

1. DIFFUSION WELDING

Diffusion welding is a solid state joining process in which the strength of the joint results primarily from diffusion. Diffusion means movement of molecules or atoms from high concentration region to low concentration region.

In this welding process both the welding plates are placed one over other in high pressure and temperature for a long period of time. This high pressure force starts diffusion between interface surfaces. This diffusion can be accelerated by the application of high temperature. This temperature does not melt the welding plates. The temperatures of about 50-60% of melting temperature can be used in order to have a high diffusion rate between parts being joined.

In diffusion welding, the pressure may be applied by dead weights or by a press using differential gas pressure. The parts are usually heated in a furnace or by electrical resistance. Figure 2.1 illustrates the diffusion welding process using electrical resistance for heating.

The strength of the welding depends on pressure, temperature, time of contact, and the 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. The gold is then placed over copper and then weight is placed on top of it. The assembly is then placed in a furnace and left until a good bond is obtained.

The diffusion welding is suitable for dissimilar metals. It is also used in reactive metals such as, titanium, zirconium and refractory metal alloys. The diffusion welding process is slower process when compared to other welding processes.

Working principle of diffusion welding:

First both the welding plate surfaces prepared for welding. In this process, both the interface surfaces made equally flat which is basic requirement of diffusion process. The interface surfaces should be machined, cleaned and polished well which remove all chemical contaminants from the surface. Any contaminant particle can be reduced diffusion between welding plates.

Now both the plates are clamped and placed one over another. This assembly placed into a vacuum chamber or in an inert environment. This protects the welding surface from oxidation.

A high pressure and temperature applied on this assembly to start diffusion. The temperature applied by the furnace heating or electric resistance heating. The high pressure is applied by a hydraulic press, dead weight or by the differential gas pressure. This conditions are maintained for a long duration of time for proper diffusion.

At the starting stage of this process, local deformation at the interface surface due to creep and yield take place. Now the diffusion takes place which form an interface boundary.

After a long period of time, both the plates properly diffused into one another which makes a strong joint. The interface boundary disappear which form a clean joint. This joint has same properties or strength as the base material.

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 usually used on sheet metal structures such as nuclear and electronics industries.

3. It is used to weld titanium, zirconium and beryllium metals and its alloy.

Advantages of diffusion welding:

1. Plastic deformation at surface is minimal.

2. There is no limitation in the thickness of workpieces.

3. The bonded surface has the same physical and mechanical properties as the base material.

4. This process produces clean joint which is free from interface discontinuity and porosity.

5. Both similar and dissimilar material can be joint by diffusion bonding process.

6. Running cost is less.

7. The diffusion bonding is able to help us to build high precision components with good dimension tolerance and hence precision components can be produced.

8. It can weld complex shapes.

Limitations of diffusion welding:

1. It is a time consuming process due to low productivity.

2. Very thorough surface preparation is required prior to welding process.

3. The mating surfaces must be precisely fitted to each other.

4. It is relatively high initial investments in equipment.

5. This process is not suitable for mass production.

Adhesive Bonding

 Adhesive Bonding is the process of joining two surfaces together, usually with the creation of a smooth bond. This may involve the use of glue, epoxy, or one of a wide range plastic agents which bond either through the evaporation of a solvent or through curing via heat, time, or pressure.

Bonding with adhesives is a very popular method of joining whose history dates back to 3000 years. It finds the application in assembly technologies very rapidly and extensively used in some of labeling, packing, book binding and foot wear. Its usefulness came into light during World War II when it was applied for assembling the load bearing components in aircrafts.

1. Adhesive and Types of Adhesives

Adhesive is a filler material that binds parts together. It is a non-metallic substance, mostly a polymer. The parts being joined are called “adhenands”.

Adhesives are very many in number. They can be classified as follows.

1. According to their function.

(a) Structural adhesives

They perform the mechanical load bearing function (e.g. Thiokol - automobile rear window adhesives)

(b) Holding adhesives

They can bear only limited loads but mainly form unstressed joints. (e.g. House hold appliance)

(c) Sealing adhesives we

They perform the function of excluding gases or liquids from a joint. (e.g. Caulking components).

2. According to chemical structure

(a) Natural adhesives (Starch and Dextrin)

(b) Inorganic adhesives (Sodium silicate and Magnesium oxychloride)

(c) Synthetic organic adhesives

(i) Thermoplastics

(ii) Thermosets.

3. According to the parts being jointed. 

(a) Metal - metal adhesive 

(b) Metal-plastic adhesive.

(c) Plastic glass adhesive.

4. Classification of synthetic organic adhesives:

(a) Chemically attractive (e.g. Polyethylene, epoxies, modified acrylic and phenolics, polyamides)

(b) Pressure sensitive (e.g. Natural rubber, butyl rubber, nitride rubber and polyacrylates)

(c) Hot melt (e.g. Polyester, polymides, polyolefins and other thermoplastic elastomers)

(d) Evaporative or diffusion (e.g. Vinyls, acrylics, phenols, synthetic and natural rubber)

(e) Elastically and thermally conductive (e.g. Epoxies, polyurethenes, silicones, Fillers such as silver, copper aluminium and gold are used to get electrical conductivity)

5. High temperature applications (up to 260°C)

(a) Polyimides

(b) Polybenzimidazoles.

2. Choosing an Adhesive

Choice of an adhesive is to be made taking into account various parameters. Some of them are as follows.

1. One-part adhesives are better than two parts. Two parts may result in improper metering and mixing human ignorance or negligence.

2. Tape and film adhesives are better than liquid and paste systems because of ease of handling. Also shrinkage problems are eliminated by resulting in strong bond.

3. Thermal expansion properties of metals being joined should be given utmost importance. If the difference of expansion is very high, the bond may fail.

4. Better to go for adhesives which are less critical about the cleanliness of the surfaces being joined.

5. Adhesives with harmful chemicals are to be avoided to protect human health.

6. A single adhesive cannot perform in the same way in all cases. An adhesive which is perfect for metal-metal may not be so for metal-plastic. Table below shows some commonly used adhesives for various materials.

3. Some Principles of Adhesive Bonding

Though the choice of adhesives differs greatly depending on applications, there are some general principles which are to be followed in any bonded joint. They are as follows.

1. Contact area:

Joint contact area is to be maximum. It will result in more load carrying capacity.

2. Region of failure:

If a bonded joint failure occurs, it should be confined only to the adherents not affecting the adhesive itself. It can be achieved by ensuring (i) chemical bonding, (ii) physical interactions, and (iii) mechanical interactions.

3. Surface preparation:

Surface of the parts on which adhesive is applied is to be clean and free from dust, oil and oxide films. If they are present, they would hinder intimate contact between parts and adhesives.

4. Types of Adhesive Joints

Various types of joints are used with adhesives. Some of them are shown in Figure 2.2.

Particularly the corner joints may be subjected to peel and cleavage stresses if they are wrongly loaded as shown in Figure 2.3. Figure 2.4 shows some ways how to minimize those stresses.

Adhesives with welding / riveted joints:

To overcome the cleavage/peel failure adhesives may be combine with traditional fastening methods such as riveting and welding. In the case of a rivet adhesive combination, the fatigue life of rivets is improved by the adhesive. On the other hand, rivets hold the ends of the bonds securely, thereby avoiding peeling. In the case of a weld adhesive combination, welding is done after the parts are jointed and cured by adhesive.

5. Surface Preparation for Adhesive Bonding

A foremost necessity in bonded joints is that the surfaces should be clean and free from any form of dirt. The possible impurities are water, metal oxides, adsorbed gases, lubricants and any such things. All these are to be removed and cleaned for getting a good durable joint. Some of the surface preparation methods are given below:

1. Abrasion:

Abrasion technique is used when the objects are large and thick layers of impurities are to be removed. They include:

(a) Dry blasting

(b) Wet blasting

(c) Sanding

(d) Brushing.

2. Solution cleaning:

Solution cleaning methods are normally used when the impurities are held loosely over the surface. They include:

(a) Hot alkaline washing

(b) Solvent wiping

(c) Vapour degreasing.

3. Conversion techniques:

In this technique, the impurities are removed using chemicals. The base metal is converted into a chemically different structure. They include:

(a) Anodizing

(b) Phosphating

(c) Chemical cross linking.

6. Applications, Advantages and Limitations of Adhesive Bonding

Applications:

1. Adhesive bonding is used across many industries including automotive, aerospace, medical and even textiles.

2. Bonding of metal to non-metals especially plastics is the major application of adhesive bonding.

3. Bonded joints are used as an alternative to riveting for aircraft structures.

4. Widely applicable in fastening of stiffeners to the aircraft skin and in assembling honeycomb structures in aircraft.

5. Extensively used in the fabrication of aircraft internal structures and providing the smooth surface for supersonic planes.

6. These joints are used in fabrication of railway coaches, boats, refrigerators, storage tanks, and microwave reflectors for radar and space communications.

Advantages:

1. The bond prevents electrochemical corrosion between dissimilar metals.

2. Vibration and noise are reduced because of internal damping provided by the adhesive.

3. Thin and fragile components can be joined without increasing the weight.

4. Stress concentration is minimized because the entire bond area is utilized.

5. Smooth contours are possible. Bolt, rivets heads or spot welds may increase wind resistance or make the component look unattractive. They can be reduced by using adhesives.

6. Easy assembling of parts.

7. Cost reduction may be achieved when used in combination with other joining methods.

Limitations:

1. They are not suitable for high temperature services.

2. Bonding and curing is a lengthy process.

3. Self-life of the adhesives is short and some may require special storage conditions such as refrigeration.

4. Surface preparation is essential.

5. Some adhesives may be toxic or inflammable. Thus, ventilation and fire extinguishing systems may be necessary.

6. It provides limited reliability.