Technical Overview of Explosive Metal Welding
Explosive cladding, or explosive metal welding, has been theoretically understood for decades. Although academia has acknowledged explosive welding as a novel and fascinating process, with several specific exceptions, historically, industry has been slow to realize the potential composites this process makes available. Explosive welding manufacturers, such as Hermetic Solutions Group’s Bonded Metals Division, formerly known as Northwest Technical Industries (NTI), have characterized and defined many aspects of the process and have been informing design engineers of the many composite options that the explosive bonding process allows.
Explosively welded composites may be designed and fabricated to combine desirable properties of very different metals, allowing designers to optimize the composite’s performance for high temperature, cryogenic, high strength, thermal or electrical conductivity, enhanced mechanical properties or corrosion resistance.
Welding Incompatible Materials
Explosive bonding is a solid-state welding process that uses controlled, explosive energy to force two or more metals together at high-pressures.
The resulting composite metal pieces are joined with a high-quality metallurgical bond. The duration involved in the explosive welding means the reaction zone between the constituent metals is microscopic. During the bonding process, several atomic layers on each metal’s surface become plasma. The collision angle (typically less than 30°) between the two surfaces forces the plasma to jet ahead of the collision front, effectively scrubbing both metal surfaces and leaving virgin metal.
The remaining thicknesses remain at near ambient temperature and acts as a huge heat sink. The resulting bond line is an abrupt transition from the clad metal to the base metal with virtually no degradation of their original physical and mechanical properties. Conventional cladding methods that use heat may cause brittle inter-metallic compounds to form.
Explosively welding multi-laminates requires a working knowledge of the process phenomena and the ability to use them efficiently to create quality composites. The variables impacting a weld’s formation must be tightly controlled in order to produce a high-quality weld. The periodicity and amplitude of the wave pattern formed during explosive welding process is controlled by adjusting three major parameters: detonation velocity (Vd), explosive load, and interface spacing. The wave pattern formed at the bond line results from a fluid-flow collision. The two constituent metals act as viscous fluids in the reaction zone and, just as in describing laminar or turbulent flow, a Reynolds number for the system can be determined.
In a fluid-flow collision, the interface turbulence is controlled by the detonation velocity and the collision angle. The interface morphology is important for some specific applications.
For example: it may be desirable to attain a wavy interface to increase transition joint’s shear strength. It also may be desirable to attain a flat interface in a system, where a reaction zone must be minimized for thermal reasons, or where it is necessary to know the depth of a bond line on a microscopic level.
It is also important to know the metallurgy involved in a particular system when selecting bonding parameters. In very turbulent wave patterns, localized melt pockets can occur at the “crests” of the waves. These melt pockets can contain a variety of binary alloys, rapidly-solidified micro-structures and inter-metallic compounds. Some systems that form a very stable inter-metallic compound may form a continuous layer of that compound at high bonding pressures. Such a bond, with a continuous inter-metallic layer, usually shows very high tensile strength, but low ductility and impact resistance. It will also react poorly to thermal cycling.
The problems of extreme metallurgical incompatibility may be overcome with the addition of an interlayer. The interlayer is chosen for improved compatibility with both of the constituent metals or because it allows thermal excursions which otherwise may lead to service problems. High melting temperature interlayers allow transition joints to be conventionally welded to their respective parent metals without the concern of diffusion related failures or bond degradation.
Explosively welded multi-laminates come very close to achieving ideal composite conditions i.e. a sharp transition between layers; physical and mechanical properties which are constant or enhanced throughout individual layer thickness’; and a metallurgical bond between layers. These composites are available for a wide variety of industrial and strategic applications. The high integrity of the bond allows design engineers to utilize the specific desirable properties of metals more efficiently.
Transition joints between metals with widely differing melting temperatures can be produced with the appropriate diffusion barrier interlayer. Thin, exotic metals with unique desirable properties can be metallurgically incorporated externally or within a metal matrix. This process allows the economical use of strategic metals, while mitigating design constraints common with mechanical joining methods.
Explosive bonding is used in several different geometries. Flat sheets or tubes and rods can be bonded. The geometry used for any given product depends on the end requirements for the material.
Explosive cladding offers advantages over other coating technologies, because after sheets of material are bonded together, they retain essentially 100% of their theoretical density. Other coating techniques, which employ spray or vapor deposition, have much higher porosity and, as a result, do not protect the substrate as well.
Explosive welding is a proven process that has gained Navy approval for joining aluminum to steel (MIL-J-24445). Further explosion bonded multi-laminates are certified for manned space flight and both civilian and military aerospace applications.