172
T. Sano
distortion of the joint after welding [98–100], although the welding speed is relatively low. Laser welding is a high-speed welding method that has been used for
achieving high-productivity welding of precipitation-strengthened aluminum alloys
[101, 102]. Although the weldability of 2024 aluminum alloy is generally low, fast
full-penetration welding of this alloy using highly focused fiber laser achieved weld
joints with smaller HAZ regions and no cracking [103]. However, areas of WM with
reduced strength exist, and avoiding generation of blowholes in the laser-welded
joints is difficult. Although the thickness with the compressive residual stress induced
by DryLP process is almost one-tenth of conventional LP methods [54, 104], this
method was shown to be effective for FSW-processed 7075-T73 aluminum alloy,
where the stir zone, thermo-mechanically affected zone, and HAZ were softened,
but no welding defects occurred, confirming that the fatigue performance was better
than that of the BM at lower stress amplitude after DryLP treatment.
5.3.2 Experimental Methods
A 2024-T3 aluminum alloy with thickness of 3 mm was used. The original alloy
had a 0.2% proof stress of 334 MPa, tensile strength of 464 MPa, and elongation of
21.8%.
A single-mode fiber laser (IPG Photonics, YLS-2000-SM, Japan, wavelength:
1070 nm, CW) was used for full-penetration bead-on-plate welding of the aluminum
alloy, as shown in Fig. 5.9a. The fiber diameter was 14 μm and we used a laser power
of 2.0 kW. The laser was focused on the alloy surface with a spot size of 54 μm. Ar
was used for shielding gas with a flow rate of 30 L/min. A welding speed of 2.5 m/min
was used. The top and bottom surfaces of the laser-welded specimens were observed
using an optical digital microscopy (Hirox, KH-7700, Japan). The cross-section of
the weld bead was observed using optical microscopy (Olympus, SZX7, Japan).
Then, the laser-welded specimens were subjected to DryLP in air. The peening
was performed 15 months after welding to allow the completion of natural aging. As
shown in Fig. 5.9b, c, femtosecond laser pulses with a wavelength of 800 nm, pulse
duration of 130 fs, and pulse energy of 0.6 mJ (Spectra-Physics, Spitfire, Japan) were
focused using a plano-convex lens with focal length of 70 mm onto the specimen.
The laser pulses were overlapped, with a coverage of 692%, which was shown to be
the most effective condition for DryLP of 2024-T3 aluminum alloy [54].
For the preparation of specimens for hardness tests, the weld reinforcement was
removed and electropolished in 20% sulfuric acid–methanol electrolyte for 30 s to
remove the work-strained layer before DryLP treatment. The hardness of the top
surface was measured using a Vickers hardness tester (Mitsutoyo, HM-221, Japan)
with a load of 1.96 N and loading time of 15 s.
For the preparation of specimens for residual stress measurement, DryLP treatment was conducted on as-welded specimens without removing the weld reinforcement. Depth profiling of the residual stress which was normal to the weld bead in the
specimens was conducted nondestructively using the BL22XU beamline at SPring-8
T. Sano
distortion of the joint after welding [98–100], although the welding speed is relatively low. Laser welding is a high-speed welding method that has been used for
achieving high-productivity welding of precipitation-strengthened aluminum alloys
[101, 102]. Although the weldability of 2024 aluminum alloy is generally low, fast
full-penetration welding of this alloy using highly focused fiber laser achieved weld
joints with smaller HAZ regions and no cracking [103]. However, areas of WM with
reduced strength exist, and avoiding generation of blowholes in the laser-welded
joints is difficult. Although the thickness with the compressive residual stress induced
by DryLP process is almost one-tenth of conventional LP methods [54, 104], this
method was shown to be effective for FSW-processed 7075-T73 aluminum alloy,
where the stir zone, thermo-mechanically affected zone, and HAZ were softened,
but no welding defects occurred, confirming that the fatigue performance was better
than that of the BM at lower stress amplitude after DryLP treatment.
5.3.2 Experimental Methods
A 2024-T3 aluminum alloy with thickness of 3 mm was used. The original alloy
had a 0.2% proof stress of 334 MPa, tensile strength of 464 MPa, and elongation of
21.8%.
A single-mode fiber laser (IPG Photonics, YLS-2000-SM, Japan, wavelength:
1070 nm, CW) was used for full-penetration bead-on-plate welding of the aluminum
alloy, as shown in Fig. 5.9a. The fiber diameter was 14 μm and we used a laser power
of 2.0 kW. The laser was focused on the alloy surface with a spot size of 54 μm. Ar
was used for shielding gas with a flow rate of 30 L/min. A welding speed of 2.5 m/min
was used. The top and bottom surfaces of the laser-welded specimens were observed
using an optical digital microscopy (Hirox, KH-7700, Japan). The cross-section of
the weld bead was observed using optical microscopy (Olympus, SZX7, Japan).
Then, the laser-welded specimens were subjected to DryLP in air. The peening
was performed 15 months after welding to allow the completion of natural aging. As
shown in Fig. 5.9b, c, femtosecond laser pulses with a wavelength of 800 nm, pulse
duration of 130 fs, and pulse energy of 0.6 mJ (Spectra-Physics, Spitfire, Japan) were
focused using a plano-convex lens with focal length of 70 mm onto the specimen.
The laser pulses were overlapped, with a coverage of 692%, which was shown to be
the most effective condition for DryLP of 2024-T3 aluminum alloy [54].
For the preparation of specimens for hardness tests, the weld reinforcement was
removed and electropolished in 20% sulfuric acid–methanol electrolyte for 30 s to
remove the work-strained layer before DryLP treatment. The hardness of the top
surface was measured using a Vickers hardness tester (Mitsutoyo, HM-221, Japan)
with a load of 1.96 N and loading time of 15 s.
For the preparation of specimens for residual stress measurement, DryLP treatment was conducted on as-welded specimens without removing the weld reinforcement. Depth profiling of the residual stress which was normal to the weld bead in the
specimens was conducted nondestructively using the BL22XU beamline at SPring-8