**4. Conclusions**

The e ffects of DryLP on the hardness, residual stress, and fatigue performance of laser-welded 2024-T3 aluminum alloy were investigated. After DryLP treatment, the hardness of the softened WM recovered to that of the original BM, while tensile residual stress in the WM and HAZ changed to compressive stress. DryLP treatment improved the fatigue performances of welded specimens with and without reinforcement almost equally. Positive factors (hardening and introduction of compressive residual stress) induced by DryLP had a larger e ffect on the mechanical properties than negative factors (stress concentrations near defects at lower stress amplitudes). Therefore, DryLP is expected to be more e ffective in improving the fatigue performance of laser-welded specimens with weld defects at lower stress amplitudes. Combining high-speed laser welding with DryLP is expected to be a suitable strategy for replacing other welding processes, resulting in high productivity. This combination could be applied in various industrial fields, such as the automotive, rail, aircraft, and space industries.

**Author Contributions:** This work was supervised by T.S. (Tomokazu Sano). T.S. (Tomokazu Sano) and T.E. conducted all experiments. Y.K. and S.K. supported laser welding experiment. K.A. supported TEM observation. K.M. supported fatigue test. A.S. and T.S. (Takahisa Shobu) supported residual stress measurement. A.H. advised laser welding experiment. Y.S. advised laser peening experiment. T.S. (Takahisa Shobu) wrote the manuscript. All authors discussed the results and commented on the manuscript.

**Funding:** This work was supported in part by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118068348, and JSPS KAKENHI Grant Numbers JP16H04247, JP16K14417, and 19K22061. This work was funded in part by ImPACT Program of Council for Science, Technology and Innovation (Cabinet O ffice, Government of Japan).

**Acknowledgments:** The laser welding experiment was performed under the Joint Usage/Research Center on Joining and Welding, Osaka University. The synchrotron radiation experiments were performed under the Shared Use Program of JAEA (Proposal Nos. JAEA 2015B-E12, JAEA 2016A-E18, JAEA 2017B-E11, and JAEA 2019A-E07) and QST (Proposal Nos. QST 2016B-H14 and QST 2017A-H18) facilities with the approval of Nanotechnology Platform project supported by the Ministry of Education, Culture, Sports, Science and Technology (Proposal Nos. A-15-AE-0034, A-16-QS-0010, A-16-QS-0026, A-17-QS-0016, A-17-AE-0031, and A-19-AE-0007). The synchrotron radiation experiments were performed using a JAEA experimental station at JAEA beamline BL22XU, SPring-8, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2015B3782, 2016A3783, 2016B3786, 2017A3788, 2017B3737, and 2019A 3737).

**Conflicts of Interest:** The authors declare no conflict of interest.
