Working with
Explosive Bonded Parts
Machining techniques are adapted for specialized manufacturing
technology
 By
Stephanie Gooch
"Mid-event" diagram
of explosive bonding
Design engineers must continually
consider new materials and methods when designing today's high-tech equipment,
whether for factory automation, medical devices--or the specialized flywheel
technology discussed on page 22. Understanding the new methods of bonding
metals, as well as the challenges in machining such components, helps an
engineer to provide the ultimate in design while maintaining costs and meeting
deadlines, an edge that makes explosive bonded materials worth exploring.
Background
Explosive bonding (also known as explosive welding or explosive cladding)
is a metallurgical technique used for bonding dissimilar metals into a highly
strong joint. The resulting transition is ultra-high vacuum (UHV) tight
and can withstand drastic thermal incursions. This "cold-welding"
process, which allows metals to be joined without losing their pre-bonded
properties, can introduce thin, diffusion-inhibiting layers, such as tantalum
and titanium, to allow conventional weld-up installation.
Controlled detonations are used to create an atomic-level bond between
the metals. The force of the explosion creates an angular collision, which
in turn produces an ejected plasma. This plasma jet cleanses both surfaces
of impurities immediately prior to fusion. The high pressures--up to 600,000
psi--squeeze the metals into behaving like viscous fluids, thereby causing
a distinctive wave pattern bond line.
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Aluminum end panel for FESM, as used in satellite applications,
machined on a 4-axis milling machine
Same panel showing the explosion- bonded copper layer
Same panel on the underside.
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Explosive bonding causes two deformation aftereffects: metal thinning
and reduced flatness. Thus, it is recommended that starting material be
thicker and that the materials be machined to final tolerances after bonding.
Most materials require mechanical flattening after bonding. For materials
with crack sensitivity--such as molybdenum, titanium or tungsten--this flattening
may be performed at elevated temperatures or after thermal stress treatment.
Secondary machining expertise
There is a learning curve when performing secondary machining on explosive
bonded parts. High Energy Metals, Inc., Port Townsend, WA, relies on a strategic
alliance with J&S Fabrication Inc., Port Townsend, WA, to provide all
of their secondary machining operations. According to David Brasher, Chief
Metallurgist with High Energy Metals, "There are a variety of challenges
to overcome when machining materials with such different characteristics
as, say, copper and steel. J&S has years of experience working with
explosive bonded parts and can handle any combination we throw at them."
Through the use of CNC and conventional equipment, J&S exploits a combination
of ISO 9002 quality practices along with a variety of innovative manufacturing
techniques gained over 73 cumulative years of metalworking experience.
The company is able to machine into and across bonded stock with extreme
accuracy, even when there is a high variance between material characteristics
that affect the machining properties.
Critical to the machining process is identifying the exact bond-line
of the materials, which is done during the initial inspection process and
through the use of ultrasonic thickness measurements. The bond lines, used
as control points for the machining process, govern cut depths and cutter/insert
speeds, depending upon the specific characteristics of the two materials
used in the bond.
Additionally, heat transfer also becomes an important guideline, because
excessive heat from one material can actually cause some bonded materials
to soften. For example, to machine over a bond of copper alloy, tantalum
and stainless steel, heat transfer, cutter speed and insert depth must be
controlled precisely to eliminate the chances of gouging and thereby destroying
the designed part. Exact settings are a proprietary secret, but a typical
cutting operation might be done at 4500 rpm. Across the bond line there
must be a slowdown so that when the cutter gets to the bond joint it is
running less aggressively. Not only can J&S Fabrication machine the
bonded materials without such complications, the company's years of experience
in working with explosive fusion bonded materials allows them to provide
standard tolerances of 0.002 in., and possibly tight tolerances of 0.0002
in.
The bond line is also an important feature in the end-user's design.
Strict tolerances between the two materials used are a byproduct of designing
within space and weight limitations. Most equipment designers are concerned
with the rising material costs, as well as material characteristics, when
they design-in an explosive fusion bonded component. In a tight space, and
using as little overall material as necessary for the application, the bond
line can become a critical transition point, especially if used to insulate
one machine component from another. Without a strict understanding of the
materials used and continual attention to the control over the machining
processes, thousands of dollars in scraps can result. Further, for every
lost part, time-to-market schedules suffer, as well.
For more information:
Circle 608 - High Energy Metals Inc, or connect directly
to their website via the Online Reader Service Program at http://www.OneRS.net/107df-608
Circle 609 - J&S Fabrication Inc, or connect directly
at http://www.OneRS.net/107df-609
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