A solid-state welding process that produces a weld by high velocity impact of the workpieces as the result of controlled detonation.
Explosion welding is a solid-state welding process that produces a weld by high-velocity impact of the workpieces as the result of controlled detonation. The explosion accelerates the metal to a speed at which a metallic bond will form between them when they col- lide. The weld is produced in a fraction of a second without the addition of filler metal. This is essentially a room-temperature process in that gross heating of the workpieces does not occur. The faying surfaces, however, are heated to some extent by the energy of the collision, and welding is accomplished through plastic flow of the metal on those surfaces.
Welding takes place progressively as the explosion and the forces it creates advance from one end of the joint to the other. Deformation of the weldment varies with the type of joint. There may be no noticeable deformation in some weldments, and there is no measurable loss of metal. Welding is usually done in air, although it can be done in other atmospheres or in a vacuum when circumstances dictate. Most explosion welding is done on sections with relatively large surface areas, although there are some applications for sections with small surface areas.
A typical arrangement of the components for explosion welding is shown in Figure E-10.
Fundamentally, there are three components: base metal, prime or cladding metal, and explosive. The base component remains stationary as the prime component is welded to it. The prime component is usually positioned parallel to the base component; however, for special applications it may be at some small angle with the base component. In the parallel arrangement, the two are separated by a specified spacing, referred to as the standoff distance. The explosion locally bends and accelerates the prime component across the standoff distance at a high velocity so that it collides at an angle with and welds to the base component. This angular collision and welding front progresses across the joint as the explosion takes place.
The explosive, almost always in granular form, is distributed uniformly over the top surface of the prime component. The force which the explosion exerts on the prime component depends on the detonation characteristics and the quantity of the explosive. A buffer layer, such as a neoprene material, may be required between the explosive and the prime component to protect the surface of that component from erosion by the detonating explosive. The action that occurs during explosion welding is illustrated in Figure E-11.
There are three important interrelated variables of the explosion welding process: collision velocity, collision angle, and prime component velocity. The intense pressure necessary to make a weld is generated at the collision point when any two of these three variables are within certain well defined limits. These limits are determined by the properties of the particular metals to be joined. Pressure forces the surfaces of the two components into intimate contact and causes localized plastic flow in the immediate area of the collision point. At the same time, a jet is formed at the collision point, as shown in Figure E-11. The jet sweeps away the original surface layer on each component, along with any contaminating film that might be present. This exposes clean underlying metal which is required to make a strong metallurgical bond. Residual pressures within the system are maintained long enough after collision to avoid release of the intimate
contact of the metal components and to complete the weld.
Capabilities and Limitations
One attribute of the explosion welding process is its ability to join a wide variety of similar and dissimilar metals. The dissimilar metal combinations range from those that are commonly joined by other welding processes, such as carbon steel to stainless steel, to those that are metallurgically incompatible for fusion welding or diffusion bonding processes, such as aluminum or titanium to steel.
The process can be used to join components of a wide range of sizes. Surface areas ranging from less than 6.5 cm2 (1 in.2) to over 37 m2 (400 ft2) can be welded. Since the base component is stationary during welding there is no upper limit on its thickness. The thickness of the prime component may range from .25 to 31.8 mm (0.001 to 1.25 in.) or more depending on the material.
Geometric configurations that can be explosion welded are those which allow a uniform progression of the detonation front and, hence, the collision front. These include flat plates as well as cylindrical and conical structures. Welds may also be made in certain complex configurations, but such work requires thorough understanding and precise control of the process.
Applications
As a general rule, any metal can be explosion welded if it possesses sufficient strength and ductility to withstand the deformation required at the high velocities associated with the process. Metals that will crack when exposed to the collision of the two components cannot be explosion welded. Metals with elongations of at least 5% to 6%(in a 51 mm [2 in.] gauge length), and Charpy V-notch impact strengths of 13.65
(10 ft-lb) or better can be welded with this process. The commercially significant metals and alloys that can be joined by explosion welding are given in Figure E-12. Metallurgical and mechanical properties of the materials must be considered when selecting EXW as a welding process and specifying welding conditions.
Cladding. The cladding of plate constitutes the major commercial application of explosion welding. It is customary to supply explosion clad plate in the as-welded condition because the hardening which occurs immediately adjacent to the interface does not significantly affect the bulk engineering properties of the plate. Despite this, some service requirements may demand postweld heat treatment. Clad plates are usually distorted somewhat during explosion welding and must be straightened to meet standard flatness specifications. Pressure vessel heads and other components can be made from explosion clad plates by conventional hot or cold forming techniques.
Explosion welding can be used to clad the inside and outside surfaces of cylinders. Transition joints between two incompatible metals can be made with EXW techniques. In electrical systems, aluminum, copper, and steel are the most commonly used materials, and joints between them are often necessary to take advantage of the special properties of each. Transition joints cut from thick explosion welded plates of
aluminum and copper, or aluminum and steel, provide efficient conductors of electricity. This concept is routinely used in the fabrication of anodes for the primary aluminum industry.
Tubular transition joints in various configurations can be machined from thick clad plate. While the majority of explosion welded tubular transition joints are aluminum to steel, other metal combinations for this type of joint include titanium to stainless steel, zirconium to stainless steel, zirconium to nickel base alloys, and copper to aluminum.
Explosion welding can be used to make tube-to- tube sheet joints in heat exchanger fabrication. Most applications of these joints involve tube diameters in the range of 13 to 38.1 mm (0.5 in. to 1.5 in.). Metal combinations include steel to steel, stainless steel to stainless steel, copper alloy to copper alloy, nickel alloy to nickel alloy clad steel, and both aluminum and titanium to steel.
Electric utilities and petro-chemical companies use explosion welding to plug leaking tubes in heat exchangers; however only qualified, trained technicians should implement it. An explosive handling permit is required.
Explosion welding is also used to join lengths of large diameter gas and oil transmission pipelines. It is also used for buildup and repair of worn components, particularly repair of inside and outside surfaces of cylindrical components.
Safety
Explosives and explosive devices are a part of explosion welding. Such materials and devices are inherently dangerous. Safe methods for handling them do exist. However, if the materials are misused, they can kill or injure anyone in the area and destroy or damage property.
Explosive materials should be handled and used by competent people who are experienced in that field. Handling and safety procedures must comply with all applicable federal, state, and local regulations. Federal jurisdiction over the sale, transport, storage, and use of explosives is through the U.S. Bureau of Alcohol, Tobacco, and Firearms; the Hazardous Materials Regulation Board of the U.S. Department of Transportation, the Occupational Safety and Health Agency; and the Environmental Protection Agency. Many states and local governments require a blasting license or permit, and some cities have special explosive requirements.
Reference: American Welding Society, Welding Handbook, Vol. 2, 8th Edition. Miami, Florida: American Welding Society, 1991.