Developments in explosion welding technology
J. BANKER, Dynamic Materials Corporation, USA
Explosion welding technology (EXW) utilizes the energy of a detonating explosive to create conditions which result in welding between metal components. The technology is typically considered to be a cold welding, non-fusion process. For practical purposes, the process is typically limited to creating welds between the faces, or planar surfaces, of metal components. It is generally not applicable for production of traditional butt welds. It is most suitable for producing large area planar welds that are typical in clad metals. There are many good publications discussing EXW technology to which the reader is referred for an in-depth discussion; only primary basic process information, new developments and practical applications will be reported in this chapter.1-6
EXW is a highly versatile technology from the materials applicability
perspective. It is suitable for joining both metal combinations of similar composition and metal combinations of highly dissimilar composition. The latter has been the primary impetus for the commercial development of the technology. Today two of the primary applications of explosion welding are the production of reliable weld joints between aluminum and steel, and between titanium and steel. The success with these metal systems results from the absence of any significant bulk heating of the metals. The temperaturetime conditions that cause the formation of brittle intermetallic compounds do not exist. Consequentially, explosion welding can be used to join almost any combination of metals. The factors limiting the suitability of EXW are primarily mechanical. During the EXW process, the metals are subject to high impact loading and significant cold deformation. A minimum ductility of 15% and a minimum fracture toughness of 50 J are generally considered the practical limits for successful EXW welds.
EXW is ideally suited for making planar welds between metal plates or sheets. The thickness of the flyer plate, often called the cladder, can range from 0.1mm to about 50 mm. For practical commercial reasons, costs are minimized when the flyer plate thickness is about 2-3 mm. The thickness of the base plate can range from 0.1mm to over 1m. In the case of the base metal, costs are minimized when the base plate thickness is around 12 mm.
The lateral size of clad plates, length and width, is primarily limited by the size of metal sheets or plates that are commercially available, not by the technical limits of the process. The commercial metal sizes available vary considerably between the different metal types. For most common commercial flyer metals, widths at 3mm and less are limited to 1.2m; thicker gages are commonly produced in widths of 2.5 m to 3.5 m dependent upon alloy type. For many metal types, two or more sheets can be edge butt seam welded using common fusion processes to increase plate size options. For example, for production of clad plates of 3 mm nickel alloy onto 25 mm thick steel, up to 4 m x 10m plate size is not uncommon.
Because of the direction of the jet in the EXW process, it cannot be used for welding onto three dimensional contoured surfaces. EXW is limited to cladding of flat plate surfaces or to cladding of concentric circular surfaces of straight bars, tubes or pipes.
The amount of explosive required for production of most components is considerable. Although very small welds can be made under typical shop conditions, most explosion welds are produced under conditions where hundreds or thousands of kilograms of explosive can be detonated without damage to surroundings. Remote open air detonating sites or massive shooting chambers are commonly used. Restrictions on the commercial availability of the explosive products further limit broad applications of the process. At the time of writing there are about 30 to 40 companies commercially using EXW worldwide.
For well over a century, there have been infrequent reports of metal pieces being ‘welded’ together during military detonation situations. During the late 1950s several institutions worldwide increased R&D activities in the realm of explosion metalworking. In 1960, DuPont filed the first internationally recognized patent on EXW.7 During the ensuing 20 years there was extensive research concerning the technology. In 1962 DuPont commercialized the explosion cladding industry, with the first major application being production of tri-layer coinage for the US government. During the late 1960s DuPont codified the production processes and licensed manufacturers in many developed regions of the world. In parallel with this effort, a number of institutions independently developed variants of the technology, the most extensive and best known being operations within the former Soviet Union.
During those development years, EXW solutions were developed for a broad range of welding situations. These included micro-sized spot welds for the electronics industry, pipe-to-pipe butt welds for the gas transmission industry, tube-to-tube sheet welds for the power industry, simultaneously formed and cladded pots and pans, and several kinds of spot welds and overlapping butt welds. Although the EXW technology proved highly versatile technically, most of the applications did not prove to be commercially viable. The primary exceptions were clad plate manufacture and a variant process to make welding transition joints.
In today’s industries, around 80% of the world’s EXW production is clad plates, primarily used in the process industries for corrosion or wear-resistant equipment. The balance is mainly bimetal transition joints, used as junctions for making commercial fusion welds between ‘non-weldable’ metal components, predominantly aluminum-to-steel.
Figure 8.1 presents a schematic description of the EXW process which uses high energy from explosive detonation to produce a metallurgical weld between metal plates. The basic sequence of process operations is as follows:
1 Surfaces of metals are ground and the metals are fixed parallel with a predefined separation distance.
2 Especially formulated explosive powder is placed on the cladder surface.
3 Detonation front travels uniformly across the cladder surface from the initiator.
4 Cladding metal collides with backer at a specific velocity and impact angle.
5 Momentum exchange causes a thin layer of the mating surfaces to be spalled away as a jet.
6 Jet carries spalled metal and oxides from the surfaces ahead of the collision point.
7 Thin layer of ‘micro-fusion’ 0.1 micron thick is formed at the characteristic wavy weld line.
8 Pressures exceeding 10,000 MPa hold the metals in intimate contact while metallurgical weld solidifies across the complete surface.
9 Rapid heating and cooling at the interface does not allow time for bulk heating of the metal. Total time above the melting point is in the range of 10 microseconds.
10 The EXW process assures that the backer materials retain specified physical properties and the cladding material retains the specified corrosion resistance properties.
The current understanding of the EXW technology was relatively well developed by 1980. Blazynski1 presents an excellent compilation of the technology developments through 1983. Since that time, the primary EXW process advances have been in the areas of safety, facilitation and industrialization.
The initial EXW development and production was performed using classical explosives, such as amatol or dynamite, which are relatively easily initiated (Class A in US-DOT terminology). These explosives exhibited good detonation characteristics and easy detonation velocity control. They had the downside of being initiation sensitive and required extensive safety measures during manufacture, transit, storage and use. By 1980, manufacturers were beginning to develop ANFO (ammonium nitrate fuel oil) blasting agents for large area EXW work. ANFO is more difficult to initiate than are Class A explosives. In US-DOT regulations it is a blasting agent, not an explosive. ANFO offers far greater safety and reduced regulatory control. Further, ANFO is less costly. The detonation characteristics of ANFO necessitated process modifications. Today, most commercial explosion welding companies use ANFO as their primary production explosive. Although ANFO is less easily initiated, when detonated the energy release can be highly destructive. Safety remains a major issue.
Owing to the large amounts of explosive involved in production work, often exceeding 1000 kg, for cladding large plates, EXW must be performed in an isolated and controlled environment. Traditional options were to work out - of-doors in a very isolated location or to work in an underground facility. In recent years, some practitioners have constructed vacuum chambers for EXW production. Working under vacuum conditions offers several operational and technical benefits. Further EXW production can potentially be performed in typical industrial facilities. Capital and maintenance costs have caused this variant of EXW production to be limited.
The major advances in EXW over the past two decades have been in the area of industrial application development. The development of increasingly broader applications for EXW clad plate products and the development of products derived from clad plates has resulted in a growing EXW industry worldwide. These products and their areas of application are described in the following section.