What is Explosion Cladding or explosion Welding process?

Explosion Bonding or Explosion Cladding or Explosion Welding is surface depositing welding process. The process uses an explosive detonation as the energy source to produce a metallurgical bond between two materials laid on each other parallelly. Theoretically, It can be used to join virtually any material combination, having metallurgically in-compatible and those that are known as non-weldable by other conventional welding processes. Furthermore, this process can clad one or more layers onto one or both faces of a base metal, with the potential for each to be a different metal type or alloy.

Explosion welding/ cladding Principle

  1. Preparation: The first step of the explosion cladding operation is the preparation of the two surfaces that are to be bonded together. These surfaces are ground or polished to obtain a uniform surface finish throughout with a roughness value of Ra 3 µm (140 RMS) or below to bring the closeness of the two materials, dependent upon the metals combination and thicknesses.

2. Setup: The cladding plate is placed parallelly and above the substrate material or base plate as shown in below figure, at a standoff distance (space between plates) which has been predetermined for the specific metals combination being bonded. This distance is selected to assure that the cladding plate collides with the base plate after accelerating to a specific collision velocity. The standoff distance typically varies from 0.5 to 4 times the cladder sheet thickness dependent upon the choice of impact parameters. The limited tolerance in collision velocity results in a similar tolerance control of the standoff distance.

The standoff distance is controlled by support spacers at the plate edges and internally as needed. Internal standoff devices are designed to be consumed by the jet.

An explosive containment frame is placed around the edges of the cladding metal plate. The height of the frame is set to contain a specific amount of explosive providing a specific energy release per unit area.

3. Bonding Operation: The explosive composition and type is selected to yield a specific energy release and a specific detonation rate (the speed at which the detonation front travels across the explosive layer). The detonation rate must be subsonic to the acoustic velocities of the metals.

The explosive, which is generally granular, is uniformly distributed on the cladding plate surface filling the containment frame. It is ignited at a predetermined point on the plate surface using a high velocity explosive booster. The detonation travels away from the initiation point and across the plate surface at the specified detonation rate. The gas expansion of the explosive detonation accelerates the cladding plate across the standoff gap resulting in an angular collision at the specified collision velocity. The resultant impact creates very high-localized pressures at the collision point.

These pressures travel away from the collision point at the acoustic velocity of the metals. Since the collision is moving forward at a subsonic rate, pressures are created at the immediately approaching adjacent surfaces, which are sufficient to spall a thin layer of metal from each surface and eject it away in a jet. The surface contaminants, oxides and impurities are stripped away in the jet. At the collision point, the newly created clean metal surfaces impact at high pressures of several GPa. Although there is much heat generated in the explosive detonation, there is no time for heat transfer to the metals. The result is an ideal metal-metal bond without melting or diffusion.

Materials types & combinations welded with Explosion Welding

Explosive welding process can be used to join together a variety of material irrespective of huge difference between them. For example, with explosive welding, aluminum can be bonded to the carbon steel or stainless steel. This combination is not possible to achieve with conventional welding process.

As you can notice is above pictures, where orange color represents the substrate material and the blue color represents the various clad chemistry or materials that can be bonded with Explosive Welding process.

Metallurgy of the explosive cladding

Explosive cladding works on the principle of sold state bonding where an external high force is applied to join two materials. The explosive bonded parts exhibits high bond strength, uniform joining, and uniform clad thickness. Below micrograph shows the visual appearance of the explosive bonded aluminum material on carbon steel substrate as plate.

Standard dimensions of cladding sheets in market

The size of cladded plates typically limited only by the availability of flat component metal sheet or plate and transportation constraints. Maximum plate size may also be limited by explosive detonation limits, such a noise and environmental constraints. The following maximum plate sizes can typically be produced:

  • Length 12000 mm
  • Width 5000 mm
  • Base thickness 500 mm
  • Cladding thickness 25 mm
  • Surface 35 m²

Welding of Cladded plates

Cladded sheets or plates are readily formed and welded in the industries. They are used extensively to fabricate process equipment. In oil & gas field, static equipment’s such as columns, vessels, and exchangers heads are made from cladded plate along with conventional weld overlay applications. Clad material behave alike to similar material in terms of weldability without any concern.

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