who know their way around metalworking are no doubt familiar with
peening—using a ball-peen hammer to pound a piece of metal
into shape and strengthen it against fatigue failure. For the past
50 years, an industrialized equivalent has been shot peening, in
which metal or ceramic beads as large as marbles or as small as
salt and pepper grains pneumatically bombard a metal surface. Laser
peening, a process based on a superior laser technology developed
at Lawrence Livermore, replaces the hammer blows and streams of
beads with short blasts of laser light. The end result is a piece
of metal with significantly improved performance.
Lawrence Livermore and Metal
Improvement Company, Inc., won a coveted R&D 100 Award for their
laser-peening process in 1998 (see S&TR, October
of Light to Strengthen Metals"). Since that time, they've been
developing uses for the technology with a number of industries,
including automotive, medical, and aerospace. They've also developed
an offshoot technique—laser peenmarkingSM—which provides a
way to easily and clearly identify parts with a mark that is extremely
difficult to counterfeit. Another outgrowth is a new peen-forming
technology that allows complex contouring of problematic thick metal
components such as the thick sections of large aircraft wings. There
have also been spinback applications to the Department of Energy's
programs for stockpile stewardship, fuel-efficient vehicles, and
long-term nuclear waste storage.
The concept of laser peening
is not new, but it took a DOE Cooperative Research and Development
Agreement (CRADA) between Livermore and Metal Improvement Company
to develop a machine that makes laser peening a cost-effective option.
The resultant LasershotSM Peening System uses a solid-state, high-energy
(50-joule), neodymium-doped glass laser, which pulses at a rate
20 times faster than other available systems and can peen about
1 square meter of metal per hour. With each pulse of the laser,
an intense shock wave is created over a roughly 5-millimeter by
5-millimeter area and drives in a residual compressive stress about
1 to 2 millimeters deep into metal. In conventional peening, this
compressed layer is only about 0.25 millimeter deep. The added depth
is key to laser peening's superior ability to keep cracks from propagating
and extends the life of parts three to five times over that provided
by conventional treatments.
jet engine fan blade that is being peened by a laser shot.
Fan Blades and Knee Implants
Shot peening has long been
used on automobile springs and transmissions because the treatment
increases resistance to cracks, corrosion, and fatigue. Physicist
Lloyd Hackel, who heads the Livermore side of the joint development
effort, says that the automotive industry is now interested in applying
the depth compression afforded by laser peening to automobile frames.
have added mass to the entire frame structure to achieve the required
fatigue lifetime and keep high-stress areas in frames from cracking.
Now, laser peening can extend fatigue lifetime and allow manufacturers
to cut back on the weight of the frame. By one company's calculations,
laser peening would improve the fatigue lifetime of a 200-kilogram
frame by a factor of two, allowing them to lessen the frame weight
by about 20 kilograms. This 20-kilogram weight savings translates
into gas savings as well. Laser peening 8 million automobile frames
could save about 285 million liters of gasoline per year. "So this
technology has two big benefits: it makes the car lighter and cheaper
to build, and it results in more fuel efficiency," says Hackel.
Livermore is also working
with the Biomechanics Department of the University of California
at Los Angeles to use laser peening for knee implants. "The biggest
concern in this area is pediatric knee replacement," says Hackel.
"A surgeon puts in a small knee joint, the child grows, so the knee
is loaded with more stress, which can lead to joint failure. What
do you do? Until now, the answer has been to undertake a painful
and risky operation every few years to replace the knee with a larger
model." In contrast, a laser-peened metal joint would be strong
enough to last nearly a decade. The aerospace industry also sees
major applications for laser peening, particularly in jet engines.
"If you look at a modern turbo jet engine such as those used in
a Boeing 777," says Hackel, "you'll see that it's essentially a
giant propeller engine, with the fan blades in the front and the
compressor blades inside." These blades get hit by a variety of
debris including nuts and bolts, seagulls, sand, and rocks, that
can cause cracks and failure. Laser peening adds safety while also
lowering the life-cycle cost of each fan blade.
Another use of laser peening
for aerospace and other industries involves metal shaping. For instance,
the leading edge of an airplane wing is basically a big piece of
curved metal. "Aerospace and other industries bend metal all the
time, but it's difficult to bend very thick pieces and get certain
complex shapes. And when you do bend metal, its surface is under
tension—think of the metal as being 'stretched' around that bend.
That stretching weakens it and makes it more vulnerable to cracking."
Laser peening just one side of a metal
piece will make it naturally bend, which places both peened and unpeened sides under
compression and makes the part more resistant to failure. The deep compressive stress
and the precise placement of the stress afforded by the laser-peening process allows
forming of thick, complex shapes never before possible.
peenmarking prints a complete high-quality, machine-readable
matrix mark that could deter the counterfeiting of metal parts.
This data matrix represents the number string "123456."
Another recently developed
application involving industries using or manufacturing metal parts
is laser peenmarkingSM, in which a high-resolution mark is imprinted
into the metal. This identification mark can take any form, for
example, as alpha numeric characters, a logo, or a data matrix.
This development is particularly timely for aerospace industries
facing a new marking requirement from the Aerospace Transportation
Association, called the ATA 2000. An ATA mark, in a matrix form
that can be read by barcode machines, must be set into each part
early in the manufacturing process so that the part can be tracked
throughout its lifetime.
Normal marking methods—scribing,
etching, or stamping—remove material or impart tensile stresses
that can leave the part weakened at the marked spot. But laser peenmarking
adds a strengthening residual compressive stress. Peenmarks are
also of very high resolution, similar to the watermark on currency,
and thereby provide a barrier to counterfeiting. Counterfeit, substandard
parts are a major concern, notes Hackel. For example, the U.S. Coast
Guard prosecutes approximately 20 cases each year involving the
fraudulent use of counterfeit parts. "Laser peenmarking could be
an enormous deterrent to criminals and really put a dent in the
counterfeit metal parts racket," says Hackel.
Lawrence Livermore and Metal
Improvement Company have been working with other organizations,
including the National Aeronautics and Space Administration, to
determine the efficiency of laser peenmarking. In June 2001, three
laser peenmarked parts are tentatively scheduled to ride on the
NASA shuttle to the international space station. The parts will
be bolted onto the space station to face the slipstream solar wind.
After three years, they'll be retrieved and examined to see how
well they held up in the hostile space environment.
test the resistance of laser-peened welds to corrosion, the
team took two welded pieces of 304 stainless steel and bathed
them in a 40-percent solution of magnesium chloride, a highly
corrosive salt, at 160°C. Cracks developed in the unpeened weld
within 24 hours, whereas the laser-peened weld showed no observable
cracks after a week of exposure.
The laser-peening technology
is a spinoff of high-energy lasers developed in the DOE Inertial
Confinement Fusion program. Those lasers were brought to high average
power with Department of Defense funding. The technology is spinning
back home as it becomes clear that peening has relevant applications
for DOE's Yucca Mountain Nuclear Waste Disposal and Stockpile Stewardship
For Yucca Mountain, laser
peening could be used to prevent stress corrosion cracking in the
final closure welds of 6-meter by 1.5-meter nuclear waste storage
canisters. Such canisters must completely contain waste for a minimum
of 10,000 years. Analyses show that stress corrosion in some of
the canister welds could cause the canisters to fail prematurely.
Experiments show that laser peening the welds would keep corrosion
and cracking at bay, allowing the canister to remain intact for
10,000 years and more.
In the Stockpile Stewardship
Program, one research area seeks to determine the effect of intense
strain on various materials. The laser-peening team discovered that
it could generate meaningful strain rates and effects through shock
waves created by the laser-peening process. "We can give stockpile
stewardship scientists 10 laser shots a minute, providing them with
an enormous amount of data and information," says Hackel. The process,
he adds, can give these scientists exquisite control over test parameters,
including the intensity, duration, and profile of the desired shock
As for DOE's efforts in promoting
fuel efficiency in vehicles, Hackel says, "I see peening as another
spinback for the DOE—particularly the Office of Transportation
Technology—in terms of reducing the weight of vehicles. DoD would
also benefit, from getting better fuel efficiency in the field and
also for airlift capability."
Going from ball-peen hammers
to laser light takes a big jump in technology. The applications
of laser peening—some known years ago, others newly discovered—are
just as far-reaching. "What we've come to," says Hackel, "is an
active CRADA that's working to field the technology for specific
industries and spinning it back with important benefits to Laboratory
and DOE work."
Laser peening, Stockpile Stewardship Program, Yucca Mountain Nuclear
Waste Disposal Program.
information contact Lloyd Hackel (925) 422-9009 (email@example.com).