Research method

Establishment of Laser Peening Technology forImproving Surface Quality of Metallic Products

Imaging
X-ray diffraction/
scattering

Optimization of laser irradiation conditions to enhance the effect of surface-quality improvement

Role of SPring-8

Achievements

Establishment of optimal laser irradiation conditions for laser peening* toimprove the surface quality by measuring the depth profiles of residual stress**on a metal surface nondestructively
Confirming the suppression of the fatigue-crack growth on metals by laser
peening through 3D imaging of the cracks
Establishment of laser peening technology for suppressing the initiation anddevelopment of cracks in the nuclear-reactor structures
R&D facility: Toshiba Corporation*Laser peening: Peening is similar to the hammering process used by a swordsmith. Compressive residual stress is imparted to a metal surface by peening
to cancel the tensile stress inside the metal, changing it into compressive stress. Hence, the prevention of fatigue and stress corrosion cracks fromdeveloping in welded parts is realized. In laser peening, a metal surface is compressed by the shock of plasma (high-pressure gas) that is generated byirradiating an intense pulsed laser onto the metal surface, and introduces compressive residual stress inside the metal. Laser peening technology has beenused for nuclear reactors and is considered for application to the components of airplanes, automobiles, and bridges.
**Residual stress: Stress retained inside an object even after the removal of the external forces such as tension, compression, bending, and heat treatment.

BackgroundResults

Shot peening with fine metallic balls being
shot at materials is a well-known peening
technology. However, this method is not suitablefor peening in some cases, such as the inside ofnuclear reactors, because of its large-scale setup
and the difficulty in retrieving the metallic balls.
There has been demand for a new peening
technology with a compact setup.
With this background, laser peening using ahigh-intensity pulsed laser has come into thespotlight. However, it is difficult to observe theinterior and fine regions of materials and measure
the residual stress on sites very close to the surface
using a conventional X-ray.

The synchrotron radiation X-ray of SPring-8 has ahigh transmittance, enabling us to nondestructively
measure the states of materials processed by laser
peening from their surface to their interior. We
demonstrated that the residual tensile stress on thesurface and interior is changed into compressive
stress by laser irradiation. Moreover, the depth
profiles of residual stress were compared
between different laser irradiation conditions todetermine the optimal conditions.
In addition, the images obtained from X-ray
computerized tomography (CT) clearly revealed
that the growth of fatigue cracks was markedly
suppressed for materials processed by laser
peening.

Publication: Y. Sano and K. Akita; SPring-8 Research Frontiers 2005, 127-128 (2006) Y. Sano and K. Masaki; SPring-8 Research Frontiers 2006, 151-152 (2007)
K. Masaki et al.; Journal of Solid Mechanics and Materials Engineering 2 (8), 1104-1113 (2008)

Depth profile of residual stress

NuclearreactorTensile stress remains on the surface of sample B after the sparse laser
irradiation to the target surface. Sufficient compressive stress is applied to thesurface and inside of sample A after dense laser irradiation to the target surface.

During laser irradiation
Laser

The growth of fatigue cracks (the white parts in the center)is suppressed on the specimen processed by laser peening.
Not processed

Tensile stress

Laser-peening setup
PlasmaWaterStructure

Tensile stress

After laser irradiation

Compressive stress

Surface36

X-ray CT image of fatigue crack generated
on aluminum alloyLens

Sample A
Sample B

Processed by laser peening

Compressive stress

The surface layer of a metal inwater changes into plasma upon
the irradiation of a high-intensity
pulsed laser. Because plasma
expansion is prevented in water,
the laser energy becomes locallyconcentrated on the metal surface,
generating shock waves. The
metal surface is slightly deformedby these shock waves; however, its
further deformation is hamperedby the surrounding parts. Thus,
the material is strengthened by
the compressive stress remaining
on its surface.

Residual stress (Mpa)

Principle of laser peening

Depth from surface (μm)

37