Bonding Process

 

Principle

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

Assembly.. The cladding plate is positioned parallel to and above the base plate, at a standoff distance 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.

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.

Industrialisation

The explosion cladding process is commonly used for flat plates. The process can also be used for the manufacture of concentrically bonded tubes and pipes. In tube cladding, the explosive can be place inside the bore, or on the outside of the outer tube, dependent upon diameters, wall thicknesses and other factors. The process is not suitable for cladding of complex contoured surfaces. When shaped products are required, such as heads, the clad is produced as flat plate and formed into the required product configuration after bonding.

Cladding and Base metals

The illustration above shows the wide range of combinations that can be done with explosion bonding.

Global dimensions of cladding sheets

The cladding global dimensions are typically limited only by the availability of flat component metal sheet or plate and transport constraints. Maximum plate size may also be limited by explosive detonation limits, such a noise and environmental constraints. Rarely is size limited by cladding know-how. 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

Fabrication

Clad metal can be readily formed and welded as needed to construct process equipment. Many equipment fabricators worldwide have extensive expertise and know how in this area. Head and shell forming and welding of clad plates is well known by fabricators specializing in clad.

General Considerations

In many applications, particularly for large pressure vessels designed for high temperature and pressure, titanium or zirconium clad steel construction can be very economical compared to solid construction. Tantalum cost is so high that clad construction is the only economic alternative for most process equipment. Also, tantalum is not recognized by at least the ASME Code as a structural material, which would limit its application even if the economics of solid construction were favorable.

Clad should be more economical than solid construction where wall thickness is in excess of 19 mm to 32 mm in titanium, and 16 mm to 19 mm in zirconium.

With titanium or zirconium, a minimum lining thickness of 2.0 mm is usually specified based on concern for iron contamination from the backer material due to welding heat or burn through if the welder is careless. Use of a thinner liner is certainly feasible with careful welding process selection and welding parameters chosen to minimize penetration, but the cost saving is so minimal that this practice has largely been discontinued.

Tantalum is typically used in thickness of 1.0 mm. Due to high cost and higher melting temperatures, tantalum clad often utilizes a copper interlayer which carries away heat and minimizes the risk of weld contamination even with very thin liners.

Clad construction becomes relatively more expensive if there are more details like nozzles and penetrations that require significant detailed fabrication work. Cladding can be very low cost for large uninterrupted surfaces.

Solid construction may be superior where there is a requirement for a smooth interior surface since the normal detail of clad batten strip construction results in an uneven surface.

Advantages of Clad Construction

• The main reason for clad construction is economy.
• Cladding also allows application of titanium or zirconium at temperatures in excess of their design allowable in code construction.

Disadvantages of Clad Construction

• There is an inherent structural weakness in the fillet welds used to make joints in the liner itself. The biggest drawback is that a failure of one of these welds releases corrosive compounds throughout the backing material. This can lead to undetected corrosion. Further, fluid contaminants behind a lining make high quality repairs very difficult if not impossible. Loose linings are not generally suitable under vacuum.
• Clad construction is inherently complex compared to solid construction, particularly in structures with many nozzles, attachments, or complex internals.
• At least in titanium, clad equipment may be heavier than solid construction, which can increase the cost of foundations and supports, and may be a consideration in certain weight critical applications on offshore platforms, for example.
• The exterior of a clad vessel may require painting and field touch up as well as continuing maintenance of the paint system.

Reference.. Titanium Fabrication Corporation

Large Titanium Clad pressure vessels

Image..www.coek.be