166
T. Sano
Based on the relationship between the removed depth and the pulse energy, five pulse
energies of 5, 30, 75, 200, and 600 μJ, which corresponded to spot diameters of 12,
30, 40, 60, and 70 μm, respectively, and two different coverages of 692 and 2768%
were chosen for peening treatment.
Surface morphology was observed using a scanning electron microscope (SEM,
HITACHI S-3000H). Microstructure was observed using a transmission electron
microscope (TEM, JEOL JEM-2010). For TEM observations, a small piece of
the cross section was thinned by a 30 keV focused Ga-ion beam (HITACHI FB2000). The residual stress on the laser-irradiated surface was measured from the
Al(222) diffraction peak of CrKα X-rays (2.2897 Å) using a stress constant of −
96.89 MPa/degree, which was calculated using the Kröner model [84] with a singlecrystal elastic stiffness (C 11 = 106.78 GPa, C 12 = 60.74 GPa, and C 44 = 28.21 GPa)
[85]. Thin layers of the surface were successively removed by electrolytic polishing
to obtain the depth profile of the residual stress. The hardness of the cross section was
measured using a nanoindentation system (ELIONIX ENT-1100a) with the applied
load of 1 mN. Before the nanoindentation test, the cross section was polished by a
5 keV Ar-ion beam (JEOL SM-09010) to remove the work-hardened layer.
The shape and dimensions of the fatigue specimens of the 2024-T3 aluminum
alloy are shown in Fig. 5.1c. The thickness of the specimen was 3 mm. Both top
and bottom surfaces were mirror-finished in the same manner as the 2024-T351
specimens. Dry laser peening treatments were performed for both surfaces. Picture
of fatigue test specimen after the dry laser peening treatment is shown in Fig. 5.1d.
Plane bending tests were conducted at a cyclic speed of 1400 cycles/min with a
constant strain amplitude and a stress ratio of R = −1 in the air at room temperature.
5.2.2 Results and Discussion
The relationships between the removed depth per pulse and the pulse energy is shown
in Fig. 5.2. The gradient above 30 μJ is larger than that below 30 μJ, suggesting that
a stronger shock pressure is driven above 30 μJ because the larger volume of the
removed material creates a larger recoil force. Therefore, pulse energies of 5, 30, 75,
200 and 600 μJ, which are below, at, and above 30 μJ, were chosen for the peening
experiments to confirm the existence of the threshold.
Figure 5.3 shows the SEM images of the laser-irradiated surface for the pulse
energies of 30, 75, and 600 μJ and coverages of 692 and 2768%. Regardless of the
condition, droplets are not observed, indicating that the femtosecond laser treatment
creates a negligibly small molten layer.
The results of the residual stress measurements for surfaces of the femtosecond
laser irradiated material with coverage of 692 and 2768% are shown in Fig. 5.4.
Compressive residual stress is achieved above 30 μJ, which corresponds to the point
where the gradient of the removed depth per pulse energy changes. This means
that a pulse energy above 30 μJ sufficiently drives a shock wave to induce plastic
deformation. A larger pulse energy gives a larger compressive stress for a given
T. Sano
Based on the relationship between the removed depth and the pulse energy, five pulse
energies of 5, 30, 75, 200, and 600 μJ, which corresponded to spot diameters of 12,
30, 40, 60, and 70 μm, respectively, and two different coverages of 692 and 2768%
were chosen for peening treatment.
Surface morphology was observed using a scanning electron microscope (SEM,
HITACHI S-3000H). Microstructure was observed using a transmission electron
microscope (TEM, JEOL JEM-2010). For TEM observations, a small piece of
the cross section was thinned by a 30 keV focused Ga-ion beam (HITACHI FB2000). The residual stress on the laser-irradiated surface was measured from the
Al(222) diffraction peak of CrKα X-rays (2.2897 Å) using a stress constant of −
96.89 MPa/degree, which was calculated using the Kröner model [84] with a singlecrystal elastic stiffness (C 11 = 106.78 GPa, C 12 = 60.74 GPa, and C 44 = 28.21 GPa)
[85]. Thin layers of the surface were successively removed by electrolytic polishing
to obtain the depth profile of the residual stress. The hardness of the cross section was
measured using a nanoindentation system (ELIONIX ENT-1100a) with the applied
load of 1 mN. Before the nanoindentation test, the cross section was polished by a
5 keV Ar-ion beam (JEOL SM-09010) to remove the work-hardened layer.
The shape and dimensions of the fatigue specimens of the 2024-T3 aluminum
alloy are shown in Fig. 5.1c. The thickness of the specimen was 3 mm. Both top
and bottom surfaces were mirror-finished in the same manner as the 2024-T351
specimens. Dry laser peening treatments were performed for both surfaces. Picture
of fatigue test specimen after the dry laser peening treatment is shown in Fig. 5.1d.
Plane bending tests were conducted at a cyclic speed of 1400 cycles/min with a
constant strain amplitude and a stress ratio of R = −1 in the air at room temperature.
5.2.2 Results and Discussion
The relationships between the removed depth per pulse and the pulse energy is shown
in Fig. 5.2. The gradient above 30 μJ is larger than that below 30 μJ, suggesting that
a stronger shock pressure is driven above 30 μJ because the larger volume of the
removed material creates a larger recoil force. Therefore, pulse energies of 5, 30, 75,
200 and 600 μJ, which are below, at, and above 30 μJ, were chosen for the peening
experiments to confirm the existence of the threshold.
Figure 5.3 shows the SEM images of the laser-irradiated surface for the pulse
energies of 30, 75, and 600 μJ and coverages of 692 and 2768%. Regardless of the
condition, droplets are not observed, indicating that the femtosecond laser treatment
creates a negligibly small molten layer.
The results of the residual stress measurements for surfaces of the femtosecond
laser irradiated material with coverage of 692 and 2768% are shown in Fig. 5.4.
Compressive residual stress is achieved above 30 μJ, which corresponds to the point
where the gradient of the removed depth per pulse energy changes. This means
that a pulse energy above 30 μJ sufficiently drives a shock wave to induce plastic
deformation. A larger pulse energy gives a larger compressive stress for a given