**1. Introduction**

Laser peening (LP), or laser shock peening (LSP), is a surface treatment method used to improve mechanical properties, such as fatigue performance and corrosion resistance, by hardening the material and adding compressive residual stresses on the surfaces via a laser-driven shock wave that causes plastic deformation of the material [1–5]. In recent years, the application of LP has been extended to the aerospace, nuclear, automotive, and biomedical industries [6]. The LP process can efficiently induce plasticity because of the high-strain-rate deformation caused by shock compression. Unlike other peening processes, such as shot peening, hammer peening, and ultrasonic peening, LP is a noncontact process that does not contaminate the sample.

During LP processing, the material surface is irradiated with a laser pulse passed through a transparent plasma confinement medium, such as water or glass, resulting in explosive ablation of the material. The material is plastically deformed by a shock wave formed by the recoil force of plasma expansion during ablation, which propagates into the material. An opaque overlay, such as paint, black tape, or metal foils, is conventionally applied to the material surface to avoid thermal effects, such as excessive temperature increase or melting, due to the high energy of the laser itself or laser-generated plasma. A LP method that does not require such a coating has been developed using an irradiation source formed by overlapping low-energy pulses [7,8]. Furthermore, a dry LP (DryLP) technique has been developed that does not require a sacrificial overlay and is performed under ambient conditions, where the material is directly irradiated using femtosecond laser pulses [9–11]. Although femtosecond laser pulses have a small pulse energy, the electric field is so strong that the material is explosively ablated without requiring the opaque overlay or the plasma confinement medium [12,13]. This induces a plastic shock wave strong enough to cause shock e ffects in the material [14–19].

Here, we proposed the application of DryLP to improve the mechanical properties of laser-welded 2024 aluminum alloy. LP is generally e ffective for improving the fatigue performance of arc-welded [20] and friction stir-welded joints [21–23]. The fatigue performance of welded precipitation-strengthened aluminum alloys, such as the 2000, 6000, and 7000 series, were worse than the corresponding base material (BM) because of the softening of the weld metal (WM), heat-a ffected zone (HAZ), and residual tensile stress on the surface after welding [24,25]. Therefore, in recent years, friction stir welding (FSW) has been widely used to join precipitation-strengthened aluminum alloys because it results in only a small decrease in the strength of the weld joint and small distortion of the joint after welding [26–28], 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 [29,30]. 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 [31]. However, areas of WM with reduced strength exist, and avoiding generation of blowholes in the laser-welded joints is di fficult. Although the thickness with the compressive residual stress induced by DryLP process is almost one-tenth of conventional LP methods [9,10], this method was shown to be e ffective for FSW-processed 7075-T73 aluminum alloy, where the stir zone, thermo-mechanically a ffected 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. However, the e ffectiveness of DryLP on welded precipitation-strengthened aluminum alloy containing welding defects has never been investigated. Hence, the purpose of the present study is to verify the e ffectiveness of DryLP for laser-welded 2024 aluminum alloy containing welding defects by investigating the mechanical properties.
