Lasers ?" The Second High
Power Density Welding Systems.
By: Robert Holland
The first high power density welding systems
were electron beam. The generation of a high
power electron beam in a vacuum environment,
accelerating this stream of electrons via high
voltage applied between the cathode and anode,
and then electromagnetically focusing that total
power to a very small spot on the piece to be
welded. The power density (power per unit area)
was so intense that it immediately vaporized the
metal being welded, which then solidified behind
the progression of the weld as the aligned weld
joint of the part was being moved at a constant
velocity under the focused beam. The welding
parameters were power, speed, and focus which
provided the weld penetration and weld
properties desired. The characteristics of an
electron beam weld are excellent because the
welding takes place in a vacuum environment and
oxidation of the weld area is not a problem
since any significant oxygen is absent.
Other considerations which detract from electron
beam welding are that the part or the electron
beam gun has to be manipulated in a large vacuum
chamber in order for the part to be welded.
These large vacuum chambers require large
mechanical and oil vapor diffusion pumps to draw
down the vacuum to the high vacuum operating
level required. Pump down times can be 20
minutes to a few hours for large systems. In the
case of welding the large titanium wing box for
the Grumman F15 Fighter the vacuum chamber was
extremely large and the vacuum pumps were also
large and numerous.
A second consideration is the fact that electron
beam welders produce intense X-Rays. The high
voltage electron beam welders (150,000 volts)
with stationary electron beam guns produced the
highest penetrating X-Rays and thus the vacuum
chambers and guns had to be lined with 1/8”
thick lead. The lower voltage (60,000 volts)
movable gun systems had to use 1 ¼ inch thick
steel in the construction of the vacuum chamber
to attenuate the X-Rays. Both types of electron
beam welders had to use thick leaded glass for
the viewing ports.
The next iteration of electron beam welders
where called non-vacuum welders. With this
process the electron beam was generated in a
small high vacuum chamber around the electron
beam gun only and the electron beam was passed
through a small pressure differential orifice
into normal atmospheric pressure. The electron
beam immediately collided with air molecules and
dispersed rapidly. The weld joint needed to be
extremely close to the exit orifice (1/8” to
3/8”) or the electron beam spot diameter was too
large to do any effective work. Even at a close
distance from the exit orifice the weld
characteristics lost the deep depth to width
ratios associated with high vacuum electron beam
welding. The exit orifices were expensive and
eroded fairly quickly. The non-vacuum electron
beam welder was also a large X-Ray producer and
any automation or production material handling
equipment had to be baffled and placed in a lead
room. Several electron beam in air systems were
produced but this technology did not last long.
Electron Beam Welding then progressed into the
production welding environment with the advent
of “Soft-Vacuum” electron beam welding. With
this technology the electron beam is generated
in a small high vacuum chamber, directed through
a small pressure differential orifice and into a
partial vacuum generated by only mechanical
displacement vacuum pumps. The electron beam
still maintains its deep penetrating narrow weld
characteristics, while a smaller vacuum chamber
closely sized to the parts to be welded, can be
evacuated to the operating vacuum level in a
matter of seconds rather than minutes. Many
automotive and other parts i.e. flywheels,
transmission planet carriers, catalytic
converters, hydraulic pistons, hacksaw blades,
band saw blades, commutator blanks, torque
converters, and spark plugs etc. have been
produced on a production basis with “Soft
Vacuum” electron beam welding equipment. With
some of these systems a unique dial feed table
with a sliding vacuum seal was incorporated
which allowed the part to be pre-evacuated in
the station before the weld station. This set
up, totally eliminated the vacuum pump down time
from the machine cycle time. Production rates up
to 3,000 parts per hour have been achieved using
this welding technique.
With the advent of higher power lasers many of
the applications that were accomplished by
electron beam are now being processed by laser
systems. However, many of the close tolerance
high value aircraft engine and aerospace
components that require deep penetrating non
contaminated welds are still being processed by
electron beam. The laser is also a much more
versatile tool. The various wave lengths
available with lasers can offer some very
selective results depending on the interaction
of that wavelength with the material being
processed. For instants, a Nd:YAG laser with a
wavelength 1,060 nm can weld a clear piece of
plastic to an opaque piece of plastic by passing
directly through the clear piece without
affecting it and then impinging on the opaque
piece heating and melting its surface and
effectively producing a leak-tight weld at the
interface of the two materials.
Pulsed Nd:YAG lasers are used for intricate
highly controlled low penetration welding. This
would be the welding of heart pacemakers, heart
valves, medical instruments, orthopedic
implants, small hermetically sealed electronic
enclosures, spot welding razor blades, jewelry
welding applications etc. These lasers produce
high energy pulses of short duration so a seam
weld needs to be produced by overlapping these
pulses approximately 75%.
High Power CO2 Lasers have been used for
numerous welding applications, many of which
have replaced electron beam welding for the same
applications. However, the future for narrow
deep penetrating electron beam type welds will
fall to the new High Power Fiber Lasers. These
lasers are extremely efficient, have a long
expected solid state pumping diode lifetimes of
over 100,000 hours, are virtually maintenance
free, and can deliver this laser power through a
small flexible fiber optic cable. Manufacturers
can use robots to manipulate the fiber optic
delivery system to weld a wide variety of large
production parts or incorporate it into high
speed automated production lines.
The laser beam quality of the fiber laser
(ability to focus to the smallest spot diameter)
is superior to other high power lasers and
provides faster welding speeds for a given power
level or increased production rates. Also
because of the better beam quality the stand-off
distances of the focusing optics to the work
piece can be extended to two to four feet for
some applications. Great strides in the number
of new laser welding applications can be
expected in the near future. The high power
fiber laser can be an expensive commodity but
should be able to greatly increase productivity
and equipment up time and easily justify their
cost. Remember; automation, robots, and reliable
lasers level the playing field with low labor
costs when competing on the world stage.
Information about various laser systems and
laser component manufacturers can be located at
http://www.allthelasers.com .
Article Source: http://www.articlesnatch.com
About the Author:
Bob Holland is the Technical Consultant for
allthelasers .com. Allthelasers.com is a
depository for information and links to help
users find the appropriate laser systems and
component vendors to fill their laser
requirements. Bob has held various high power
density welding management, engineering, and
sales positions for several manufacturers since
entering the industry in 1972. He has a thorough
knowledge of industrial high production laser
and electron beam systems and their
applications.
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