The process of explosive bonding, or explosive welding, has been
understood for decades. Although
academia has acknowledged explosive
welding as a novel and fascinating process,
with several specific exceptions, industry
has been slow to realize its potential and
the possible composites that it makes
available. More recently, explosive welding
manufacturers, such as SOURIAU PA&E's Bonded Metals Division, formally known as Northwest Technical Industries (NTI), have characterized and
defined many aspects of the process and
have made efforts to inform design
engineers of the many composite
possibilities that explosive bonding allows.
A composite can be designed and
fabricated to combine desirable properties
of very different metals. This process allows
the designers to optimize the performance
of the composite for high temperature,
cryogenic, high strength, thermal or
electrical conductivity, enhanced
mechanical properties, corrosion
resistance, or any other application.
Welding Metallugically Incompatible Materials
Explosive bonding
is considered to be a
solid-state welding process that uses
controlled explosive energy to force two or
more metals together at high pressures.(see below)
The resultant composite system
is joined with a high-quality metallurgical
bond. The time duration involved in the
explosive welding event is so short, that the
reaction zone between the constituent
metals is microscopic. During the bonding
process, several atomic layers on the
surface of each metal become plasma. The
collision angle between the two surfaces
(typically less than 30°) forces the plasma
to jet ahead of the collision front,
effectively scrubbing both surfaces and
leaving virgin metal.
The remaining
thickness remains near ambient
temperature and acts as a huge heat sink.
Therefore, the bond line is an abrupt
transition from the clad metal to the base
metal with virtually no degradation of their
initial physical or mechanical properties.
The obvious benefit from this process is the
joining of metallurgically incompatible
systems. Any conventional joining method,
which uses heat, may cause brittle
intermetallic compounds to form.
Process Control
The fabrication of multilaminates by explosive
welding involves a working knowledge of the
process phenomena and the ability to utilize
them efficiently to create quality composites.
In order to produce a quality weld, the
variables affecting the weld formation must be
tightly controlled. The amplitude and
periodicity of the wave pattern formed during
explosive welding can be controlled by
adjusting three major parameters: detonation
velocity (Vd), explosive load, and the interface
spacing. The wave pattern formed at the bond
line is most often described as resulting from
a fluid-flow collision. The two constituent
metals can be considered to act as viscous
fluids in the reaction zone and, just as in
describing laminar or turbulent flow, a
Reynolds number can be determined for the
system.
In a fluid-flow collision, the interface
turbulence is controlled by the detonation
velocity and the collision angle (Figure 2). 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 microstructures
and intermetallic compounds. Some systems that
form a very stable intermetallic compound may form
a continuous layer of that compound at high
bonding pressures. Such a bond, with a continuous
intermetallic layer, usually shows very high tensile
strength, but low ductility and impact resistance. It
will also react poorly to thermal cycling.
Interlayers
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.
Summary
Explosively welded multilaminates 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 can be bonded as shown in
Figure 1 or as tubes and rods. 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
multilaminates are certified for manned space
flight and both civilian and military aerospace
applications.
