Residual stress (MPa)
1. 4 1. 2 1.0
0.8 0.6 0.4 0.2 0.0
www.industrial-lasers.com MAY/JUNE 2017 Industrial Laser Solutions 13
LSPT engineers now faced the challenge of maturing LSP
technology to its maximum effectiveness and throughput.
Higher repetition rates required more robust optical glass to
withstand cyclic heating from the flashlamps, so LSPT part-
nered with specialty glass manufacturer Schott AG to identify
stronger laser glass rods with tailored Nd doping concentra-
tions for laser peening. Deeper residual stress profiles required
more sophisticated modeling tools and process controls, while
complex surface geometries necessitated integrated robotics
for precise part manipulation. Throughout the technological
maturation of laser peening, LSPT developed and patented
numerous advancements, including innovative methods for
beam delivery, laser pulse slicing, overlay application, and
These technological improvements made laser peening more
efficient and less expensive, leading to new commercial applications and greater market demand. LSPT began performing
production laser peening services for customers in the aviation,
transportation, power generation, and tooling industries—all of
whom are benefiting from significant increases in the fatigue
strength of their laser-peened components. Laser peening was
proven to consistently extend the service life of metal compo-
nents by improving their resistance to fatigue, cracking, and
damage effects, while reducing mainte-
nance and inspection requirements for
How laser peening works
Just as the name implies, laser peening
uses high-energy laser pulses to peen
metal surfaces for improved material
characteristics. Peening is a cold-work-
ing, mechanical process that imparts
beneficial stresses into material through
compressive surface hardening. While
traditional peening methods rely on the
brute kinetic force of hammer blows or
projectile shots, laser peening employs
high-energy laser pulses to produce
confined plasma bursts that direct compressive stress waves into target material. As a mechanical cold-work process, laser peening is distinct from other
laser-material processes that rely on thermal energy to induce
heating or melting.
Laser peening is performed using a high-energy, short-
pulsed laser system and an assortment of process overlays.
An opaque overlay (typically vinyl tape or paint) is applied to the
target surface prior to processing to absorb the laser energy
and protect the part surface from thermal effects. A trans-
parent overlay (typically water) is applied over the opaque
overlay, and acts as a tamping medium to confine the rapidly
expanding plasma and amplify the pressure pulse at the sur-
face of the part.
When the laser pulse is directed to the target, the pulse
passes through the transparent overlay and strikes the opaque
overlay, generating a high-pressure, rapidly expanding plasma.
The high pressure of the confined plasma burst drives a
high-amplitude compressive stress wave deep into the target
surface, inducing plastic deformation and compressive residual stresses within the part (FIGURE 1).
The key to enhancing metals like titanium or steel is to produce a stress wave with an amplitude greater than the mate-
rial’s dynamic yield strength. If the stress wave amplitude
exceeds this value for a given metal, the material will undergo
plastic strain as dislocations in the microstructure produce
compressive residual stresses in the part.
Laser peening tailors compressive residual stress fields by controlling the intensity and coverage of the laser pulses. Different
metals require different laser peening power densities (GW/cm2),
Laser peening is the only industrial
material processing technology that utilizes high-energy laser pulses with subEffective laser peening coverage is achieved by overlapping
layers across the process area. Spots are typically applied in
rows with at least a 33% overlap, producing a uniform field of
compressive residual stresses that inhibit the propagation of
surface or near-surface cracks.
FIGURE 2. Higher power densities
generate deeper compressive residual
stresses in Ti-6Al-4V (a titanium alloy).