This study aims to investigate the influence of the pulse duration on the mechanical properties and dislocation density of an aluminum alloy treated using dry laser peening (DLP), which is a laser peening technique that uses ultrashort pulsed laser-driven shock wave to eliminate the need for a sacrificial overlay under atmospheric conditions. The results of the micro-Vickers hardness test, residual stress measurement, and dislocation density measurement demonstrate that over a pulse duration range of 180 fs to 10 ps, the maximum peening effects are achieved with a pulse duration of 1 ps. Moreover, the most significant DLP effects are obtained by choosing a pulse duration that achieves a laser intensity that simultaneously generates the strongest shock pressure, suppresses optical nonlinear effects, and realizes the least thermal effects, which weaken the shock effects. Shock temperature calculations based on thermodynamic equations also suggest that a laser intensity driving a shock pressure less than 80 GPa, as in the case of a pulse duration of 1 ps in this study, maintains the solid state of the material throughout the process, resulting in significant DLP effects.

We found an optimum pulse duration for dry laser peening in the femtosecond-to-picosecond regime, in which the laser intensity exceeds the air breakdown threshold. A pulse duration of 1 ps produced the most effective peening effects under conditions wherein the laser energy was constant; this was caused by a decrease in the laser fluence due to a beam expansion of less than 1 ps, in addition to an increase in the thermal effect above 1 ps. When the laser intensity exceeds the air breakdown threshold, it is necessary to select the pulse duration while considering laser–air and laser–metal interactions.

The utilization of ultrashort pulse lasers surpassing the air ionization threshold may result in detrimental focusing due to nonlinear optical phenomena. In the context of ultrashort pulse laser processing, alterations in focusing characteristics can lead to reduced processing efficiency and quality. In this study, numerical simulations were conducted to visualize the focusing characteristics across pulse durations ranging from femtoseconds to picoseconds. The distribution of fluence and the position of maximum focus during laser focusing are found to be dependent on the pulse duration, and correction of the irradiation position is crucial for achieving proper processing. The intensity and fluence achieved under high numerical aperture (NA) conditions are determined by the combination of NA and pulse duration. These findings are crucial in selecting optimal laser conditions and achieving optimal control of the processing position in high-energy laser processing applications.

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