164
T. Sano
laser pulse. After the laser treatment, the remaining coating needs to be removed.
laser peening without coating was developed using 532 nm wavelength lasers by
optimizing process conditions, which has been applied to practical uses in nuclear
industries [17]. However, the surface needs to be covered with a transparent medium
such as water to suppress the plasma expansion and obtain a high amplitude of the
shock wave sufficient to deform the material plastically for both wavelengths lasers.
Although a micro laser shock peening process has been developed using the shorter
wavelength of 355 nm with tens of nanosecond pulse width to suppress thermal
damage, this process also requires both a coating and water [14]. The nanosecond
laser process does not produce a sufficient shock wave without covering the surface
with a plasma confinement medium. Although the applicability of laser peening will
clearly be increased if a sacrificial overlay is not required, such a technique has yet
to be realized for the nanosecond laser process.
The intensity of an ultrashort laser pulse, which is equivalent to the energy per
unit time and unit area and is proportional to the square of the electric field intensity,
is extremely high even at a low energy because the pulse width is extremely short
[26]. Therefore, direct irradiation of a solid surface with an ultrashort laser pulse
drives an intense shock wave that propagates into the solid [27–44]. A shock wave
driven by the ultrashort laser pulse irradiated under atmospheric conditions deforms
a material plastically, resulting in quenching metastable high-pressure phases [45–
47] or forming a high density of dislocations [48–51]. Heat-affected and melted
zones formed by an ultrashort laser pulse are much smaller than those produced by
a nanosecond laser pulse due to its extremely short pulse width [52, 53]. Based on
these features, peening without a sacrificial overlay under atmospheric conditions
to improve mechanical properties was developed using an ultrashort laser pulse by
optimizing process conditions [54], which was named Dry Laser Peening (DryLP)
[55], although ultrashort laser peening of steel under water [56, 57] and ultrashort
laser peen forming of thin metal sheet in air [58, 59] have been reported so far.
Dry laser peening of base metal and laser welded 2024 aluminum alloy are
described in this chapter. Ultrashort pulsed laser ablation of solids and the accompanying shock wave plays an important role on the peening. Details of the ultrashort
pulsed laser ablation of solids and the ultrashort laser-driven shock wave in solids
are described in [60–76] and [77–83], respectively.
5.2 Dry Laser Peening of Base Metal of 2024 Aluminum
Alloy
5.2.1 Experimental Methods
The material used was a precipitation-hardened 2024 aluminum alloy which is
commercially used in the aerospace industry. A 2024-T351 aluminum alloy was used
except for the fatigue tests, where a 2024-T3 aluminum alloy was used. The proof
T. Sano
laser pulse. After the laser treatment, the remaining coating needs to be removed.
laser peening without coating was developed using 532 nm wavelength lasers by
optimizing process conditions, which has been applied to practical uses in nuclear
industries [17]. However, the surface needs to be covered with a transparent medium
such as water to suppress the plasma expansion and obtain a high amplitude of the
shock wave sufficient to deform the material plastically for both wavelengths lasers.
Although a micro laser shock peening process has been developed using the shorter
wavelength of 355 nm with tens of nanosecond pulse width to suppress thermal
damage, this process also requires both a coating and water [14]. The nanosecond
laser process does not produce a sufficient shock wave without covering the surface
with a plasma confinement medium. Although the applicability of laser peening will
clearly be increased if a sacrificial overlay is not required, such a technique has yet
to be realized for the nanosecond laser process.
The intensity of an ultrashort laser pulse, which is equivalent to the energy per
unit time and unit area and is proportional to the square of the electric field intensity,
is extremely high even at a low energy because the pulse width is extremely short
[26]. Therefore, direct irradiation of a solid surface with an ultrashort laser pulse
drives an intense shock wave that propagates into the solid [27–44]. A shock wave
driven by the ultrashort laser pulse irradiated under atmospheric conditions deforms
a material plastically, resulting in quenching metastable high-pressure phases [45–
47] or forming a high density of dislocations [48–51]. Heat-affected and melted
zones formed by an ultrashort laser pulse are much smaller than those produced by
a nanosecond laser pulse due to its extremely short pulse width [52, 53]. Based on
these features, peening without a sacrificial overlay under atmospheric conditions
to improve mechanical properties was developed using an ultrashort laser pulse by
optimizing process conditions [54], which was named Dry Laser Peening (DryLP)
[55], although ultrashort laser peening of steel under water [56, 57] and ultrashort
laser peen forming of thin metal sheet in air [58, 59] have been reported so far.
Dry laser peening of base metal and laser welded 2024 aluminum alloy are
described in this chapter. Ultrashort pulsed laser ablation of solids and the accompanying shock wave plays an important role on the peening. Details of the ultrashort
pulsed laser ablation of solids and the ultrashort laser-driven shock wave in solids
are described in [60–76] and [77–83], respectively.
5.2 Dry Laser Peening of Base Metal of 2024 Aluminum
Alloy
5.2.1 Experimental Methods
The material used was a precipitation-hardened 2024 aluminum alloy which is
commercially used in the aerospace industry. A 2024-T351 aluminum alloy was used
except for the fatigue tests, where a 2024-T3 aluminum alloy was used. The proof