A laser-irradiated surface is the paradigm of a self-organizing system, as coherent, aligned, chaotic, and complex patterns emerge at the microscale and even the nanoscale. A spectacular manifestation of dissipative structures consists of different types of randomly and periodically distributed nanostructures that arise from a homogeneous metal surface. The noninstantaneous response of the material reorganizes local surface topography down to tens of nanometers scale modifying long-range surface morphology on the impact scale. Under ultrafast laser irradiation with a regulated energy dose, the formation of nanopeaks, nanobumps, nanohumps and nanocavities patterns with 20–80 nm transverse size unit and up to 100 nm height are reported. We show that the use of crossed-polarized double laser pulse adds an extra dimension to the nanostructuring process as laser energy dose and multi-pulse feedback tune the energy gradient distribution, crossing critical values for surface self-organization regimes. The tiny dimensions of complex patterns are defined by the competition between the evolution of transient liquid structures generated in a cavitation process and the rapid resolidification of the surface region. Strongly influencing the light coupling, we reveal that initial surface roughness and type of roughness both play a crucial role in controlling the transient emergence of nanostructures during laser irradiation.

The Zener pinning dynamics of a moving boundary interacting with one or more particles is described by a three-dimensional (3D) finite-element model. The model, based upon a variational formulation for boundary motion by viscous drag, is solved by a finite-element method to obtain the velocity at each node of triangular linear elements on the grain boundary. It is first applied to relatively simple and validated cases, for which analytical and numerical results are available. These cases correspond to an axisymmetrical geometry, in which the grain boundary interacts with a centred particle. A simple analytical pinning criterion is derived from these simulations. The model is then applied to general 3D cases, in which the grain boundary interacts with arbitrarily localized and sized particles. The aim of these 3D simulations is to quantify the influence of the position and the number of particles on the average grain-boundary velocity. It is shown, for example, that the drag effect is enhanced when the particle, or the cluster of particles, is off-centre and that pinning is less efficient with several particles than with a single particle producing the same Zener force.

Ultrafast laser was recently used to modify the surface integrity and peen the surface region of aluminum based alloy 2024-T351 without a sacrificial layer prior to the process. We show that controllable laser parameters such as fluence and pulse duration have a significant influence on peening qualities, such as the compressive residual stress, hardness, and surface roughness of peened parts. The residual stress profile was analyzed by x-ray diffraction. By controlling the laser fluence and pulse duration, it was possible to obtain 200 MPa of compressive residual stresses close to the surface and 100 MPa of compressive residual stresses at 50 $\mu$ m depth. Moreover, micro-hardness was increased from 2.1 to 2.5 GPa in the near-surface region. In addition, the dislocation densities were evaluated from high-resolution x-ray diffraction peaks. The increase of the dislocation density indicates that plastic deformation occurred, which generated compressive residual stresses and hardness enhancement. Plastic deformation is considered to be created by an ultrafast laser-induced pressure wave. The correlation between laser parameters and modified surface properties is interpreted by the complex interplay between laser excitation, material relaxation, and pressure waves. A pulse duration in the picosecond range and a relatively low fluence is possibly the optimal condition for a best peening quality with small surface roughness, which could potentially be used to reduce surface cracking and associated failures of additively manufactured parts.