Microstructural and mechanical characterizations on Cu machined and sidewise polished by femtosecond laser

Femtosecond laser (fs-laser) micro-machining has been increasingly utilized to efficiently fabricate microscale specimens for mechanical tests. However, the potential damages caused by the fs-laser ablation are still the greatest concerns, making it doubtful whether accurate mechanical properties can be obtained by the fs-laser machined specimens. Herein, the surface roughness and sub-surface microstructure of pure ultra-fine-grained (UFG) and coarse-grained (CG) Cu machined and repeated sidewise polished by fs-laser with optimized parameters are characterized in detail by scanning electron microscopy, confocal laser scanning microscopy, electron backscatter diffraction, and transmission electron microscopy. The results demonstrate that the application of fs-laser beam with a shorter wavelength (343 nm), circular polarization, and lower fluences for cutting and polishing can enhance the cut surface quality (roughness ≈ 0.15 μm without obvious ripples) and effectively remove the damage layer induced by heat effect and plastic dislocation injections. No evident recrystallization or recovery of the dislocation microstructures was detected in the UFG Cu, whereas the dislocation damages were locally observed to extend to a depth below 0.8 μm in the CG Cu. Microscale tensile tests with samples prepared by the fs-laser machining well reproduce the mechanical responses of the macroscale counterparts, demonstrating the feasibility of fs-laser micro-machining for efficient and reliable mechanical evaluations.