Experimental research on micro-drilling of refractory material tungsten by multi-pulse femtosecond laser ablation

Influences of the number of femtosecond laser pulses on micro-drilling quality of tungsten (W) were investigated. In addition, the morphologies of holes, splashes, residual stress on side walls, and microstructures and performance of side walls under different numbers of pulses were compared. Results show that the hole ablated by using a femtosecond laser with a wavelength of 1030 nm, a pulse width of 230 fs, a focused spot diameter of 10 μm, and a power of 5 W was composed of the inverted cone shaped upper part, cylindrical middle part, and fusiform lower part. When the number of pulses is 4 × 10⁶, the obtained hole is 476.2 μm deep and the upper surface diameter, diameter in the middle part, and maximum diameter in the lower part are separately 35.8, 17, and 32 μm, with a diameter to depth ratio of about 13.3. The surface of internal hole walls at the hole mouth is very smooth, without cracks. Typical laser induced periodic surface structures (LIPSSs) are present on the hole walls, with the characteristic size of 400 nm. Lots of residues are found on the bottom of the hole. The different energy densities of the Gaussian-distributed laser beam irradiating the sidewalls of the small holes at different locations and depths resulted in the different microscopic morphologies of the hole walls.The grain sizes in the ablation affected zones do not differ significantly under the three numbers of pulses. This indicates that there was no significant thermal accumulation leading to grain growth during the femtosecond laser ablation processing of W. The divergence angle of splashes is 68.2°. As the distance from the axis of laser beam enlarges, the splashes are successively lots of nano-sized floccules and spherical particles, lots of nano/micron-sized spherical particles, and a few micron-sized spherical particles. The nanoindentation hardnesses near and far from the hole are separately 12.73–13.55 GPa and 15.07–15.71 GPa. The residual tensile stress on the hole wall is several hundred megapascals higher than that in the area far from the hole. The research analyzed the hole forming mechanism of W during femtosecond laser ablation (FLA) combining with experimental phenomena, formed a set of analytical methods for femtosecond laser ablation processing of small holes, and provided reference for FLA of refractory materials.