New developments in advanced welding

EXW applications

3.4.3 Explosion clad industry

The dominant application of EXW today is in the manufacture of large, flat clad plates. Table 8.1 presents a list of typical metal types which are supplied as clad plates.

Explosion clad plates are readily formed into cylinders and heads and fabricated into process equipment. Figures 8.2 and 8.3 show clad plates and pressure vessels fabricated from clad plates.

EXW is one of the three cladding technologies being used for equipment manufacture today, the others being hot rollbonding and weld overlay.8 Both

8.2 Titanium-steel EXW clad plates being prepared for shipment, 2100mm x 8000mm x (125mm steel + 8mm titanium Gr 17).

8.3 Stainless steel clad refinery column. 4.6m ID x 35.3m long x (100mm Cr-Mo steel + 3mm type 321 stainless steel).

rollbonding and overlay are limited to production of clad plates of compatible metal combinations, primarily stainless steels, nickel alloys and some copper alloys. Figure 8.4 presents the general competitive positions of the three technologies. EXW is generally the lowest cost technology when the cladding metal has a high cost and the base metal is in the mid-range (typically 50 to 100 mm) regarding thickness. When the cladding alloy and base are not metallurgically compatible, for example, titanium, zirconium, aluminum, or tantalum clad to steel, EXW is the only commercially significant technology for clad plate manufacture.

Explosion clad is used extensively in the manufacture of pressure vessels and heat exchangers for high pressure and/or high temperature corrosion - resistant processes. Applications are predominantly in the chemical, petrochemical, refinery, hydrometallurgy, and upstream oil and gas industries. For greater detail on these applications, equipment fabrication and clad performance issues, there are several good references.913

3.4.4 Welding transition joints

Welding transition joints, produced from EXW clad plates, provide a means for making fully welded dissimilar metal joints in normal fabrication environments. Figure 8.5 presents the transition joint concept. Transition

8.5 The transition joint (TJ) concept for using an EXW block to facilitate welding between aluminum and steel components.

joints are used extensively for joining aluminum to steel or to stainless steel. Other common metals combinations are aluminum-to-copper, aluminum-to - titanium, and titanium-to-steel or - to-stainless steel. Transition joints are primarily used as a replacement for mechanical connections in environments where mechanical joints have major technical weaknesses.

Primary applications include:

1 Ship construction for making high strength, crevice-free joints between aluminum bulkheads and steel decks. Corrosion maintenance costs are dramatically reduced.14

2 Truck and rail car installations for producing maintenance-free joints between light weight aluminum bodies and durable steel undercarriages.

3 Aluminum smelting plants for making resistance-free electrical connections between aluminum buss and steel anodes and cathodes.15

4 Other electrochemical systems for making maintenance-free electrical connections between aluminum and copper, aluminum and steel, or copper and steel.

5 Leak-free pipe couplings between aluminum and stainless steel, primarily for the cryogenic industries.

6 Leak-free pipe couplings between titanium and stainless steel for chemical process and aerospace applications.

7 Aerospace structural installations of high strength fully welded assemblies of Ti-6Al-4V joined to Inconel® 718.

3.4.5 Bipolar cell plates

Many compact electrochemical systems benefit from bimetallic, bipolar cell plates. Applications can range from chlorine and chlorate manufacture to speciality batteries and fuel cells. Titanium-steel and titanium-nickel bimetallic plates have been used extensively in chlorate and chlorine cells. EXW plates have generally been relatively thick and up to the present have found only limited uses in other bipolar cell equipment. In a variant of the EXW process, explosion clad plates are subsequently hot rolled to lighter gage plates. The technology, commonly referred to as ‘bang and roll’, is far more cost effective for manufacture of clad plate of total thickness less than 12 mm when large quantities are produced. The bang and roll technology has been used for production of large plates of titanium-steel, stainless and nickel alloys to steel, titanium-copper, and titanium-steel-copper.

8.5.4 Electronic packaging

Electronic component packaging often demands a number of unique performance requirements frequently not attainable in any single metal. Requirements can include electrical and thermal conductivity, thermal expansion, corrosion resistance and glass-sealing ability. EXW multilayer metal products offer an optimum technical solution in many situations. Examples include:

1 aluminum boxes with Kovar or stainless steel seal rings

2 copper boxes with Kovar or stainless steel seal rings

3 copper-Kovar or copper-molbydenum base plates

4 aluminum-Kovar glass sealing inserts

5 copper-Kovar bimetallic wire.

With the exception of the latter, all are manufactured by producing a relatively large clad plate, and then cutting it into many small, planar interface parts. Bimetallic wire has been manufactured by producing a fully bonded bimetallic cylinder and then by drawing to the required wire sizes.

8.5.5 Sputtering targets

The manufacture of modern electronic microprocessor circuits often includes sputtering processes. Typically the metal being sputtered, the target, is a high purity metal; examples include titanium, aluminum and tantalum. The targets are mounted in sophisticated machines, typically constructed of stainless steel, aluminum, and copper alloy components. Process efficiency necessitates that contact between the target plate and the base be electrically and thermally conductive, structurally sound and highly reliable. Explosion welding has been proved to be one of the most reliable target assembly processes. EXW manufacturing techniques are often specifically tailored to minimize consumption of the high cost target materials.

8.5.6 Thick plate manufacture

For some metal plate alloys, product thicknesses are limited by metallurgical processing considerations. For example, the relationship between centerline properties, plate thickness, and cooling rate considerations limits the thickness that can be produced in many aluminum and nickel alloys. Explosion welding of multiple plates together offers a way to produce a thicker product with reliable centerline properties. Production applications have included the production of a 300 mm thick plate of 6XXX-series aluminum produced by EXW welding six 50 mm plates together.

8.5.7 Partially welded plates: spot welds and line welds

Although EXW is typically used to produce large welded areas, the technology can be used for making line welds or spot welds. The explosive energy required is considerably lower. In the case of spot welds, techniques have been demonstrated for the manufacture of explosion welds under normal production shop conditions.16

8.5.8 Other applications

The applications presented above typify the broad range of products that have been produced using EXW and represent many commercially successful uses. Over the 40 years of its industrial development, EXW has been employed to obtain a much broader list of products. These range from a host of small, on-site applications to multilayer razor blades.