Explosive welding | principle , Advantages and Disadvantages
Explosive welding (EXW) is a solid state (solid-phase) welding process that uses a controlled application of large pressure generated by the detonation of applied explosives. In explosive welding, welded parts (plates) are metallurgically bonded as a result of oblique impact pressure exerted on them by a controlled detonation of an explosive charge.
The following terms are frequently used in the explosive welding.
(i) Cladding metal or cladder is the thinner plate that is either in direct contact with the explosive or it is shielded by a flyer plate from the explosive.
(ii) Flyer plate is a sacrificial plate placed between cladder and explosive to protect the cladder metal.
(iii) Interlayer is a thin metal layer which is sometimes placed between cladder and base plate to enhance joining.
(iv) Base plate or backer is the plate that the cladder is being joined.
(v) Anvil is the surface on which backer rests during the joining operation.
(vi) Standoff is the distance between cladder and base plate prior to the joining operation.
(vii) Bond window is the range of process variables such as velocity, dynamic bend and standoff distance that-result in a successful weld.
(viii) Bonding operation is the detonation of the explosive which results in the weld.
Before welding, the surface to be welded must be cleaned. To carry out the welding process, one of the parts to be welded is kept as stationary and the other one is made as movable. The movable part is called flayer plate. The base plate kept as stationary is rested on an anvil and the flyer plate is located above the base plate with an angled or constant interface clearance as shown in Figure 3.8 (a). On top of the flyer plate, the rubber spacer is placed to avoid the rapid effect of burnt explosives. Explosives are placed on this rubber spacer with a detonator.
Detonation starts at an edge of the plate and propagates at high velocity along the plate. The flyer plate moves towards the base plate at very high velocity (4 to 5 km/s) due to the impact of kinetic energy in the form compressive stress during detonation of explosives to collide with a stationary part to be joined. The maximum detonation velocity is about 120% of the material sonic velocity. The compressive stress is in the order of thousands of MPa. The material at the intersection points behaves similar to a viscous fluid after explosion.
Advantages, Limitations and Applications of Explosive Welding
Advantages of Explosive Welding :
1. It ensures high quality bonding such as high strength, no distortions, no porosity and no change of the metal microstructure.
2. There is no heat-affected zone (HAZ) other than weld surface.
3. There is no diffusion.
4. Only, minor melting occurs.
5. Differences in irtateriai melting temperatures and coefficients of thermal expansion do not affect the final product.
6. Combination of dissimilar metals, copper to stainless steel, aluminium to steel or titanium to steel can be easily welded.
7.Explosive welding is much suited to cladding application.
8. Process is simple and rapid. It also gives close thickness tolerance.
Limitations of Explosive Welding :
1. Brittle materials cannot be processed.
2.Only, simple shape parts may be bonded.
3. Thickness of flyer plate is limited.
4. Safety and security aspects of storage and using explosives are difficult.
5. Metals must have high enough impact resistance and ductility.
6. The cladding plate cannot be too large.
7. Noise and blast can require worker protection, vacuum chambers and buried in sand/water.
8. The use of explosives in industrial areas will be restricted by the noise and ground vibrations caused by the explosion.
9. Area should be cleaned and sound grounded for explosion.
10. Licenses are necessary to hold and use explosives
Application of Explosive Welding :
1. This process is applied to welding of tubes and tube plates in heat exchangers, feedwater heaters and boiler tubes to clad tube plates.
2. The tubes may be of steel, stainless steel or copper, aluminium brass and bronze tubes in naval brass tube plates are welded.
3. It is used for manufacturing clad tubes and pipes, pressure vessels, aerospace structures, heat exchangers, bi-metal sliding bearings, ship structures and weld transitions.
4. It is used to clad thick plates with corrosion resistant layers where other techniques (e.g. roll bonding) are not practical.
5. It is used in tube plugging.
6. It is used in remote joining in hazardous environments.
7. It is used in fixing cooling fins.
8. It is also used in cryogenic industries.
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