<p>Water-assisted ultrafast laser (pulse duration: 8 ps; wavelength: 1030 nm; spot diameter: 20.4 μm) processing was proposed to solve the problem of surface quality in nickel-based superalloy (GH3536) during laser micro-hole drilling. The percussion drilling process with different pulse energy densities was tested under air and 10 mm hydrostatic water. An Alicona 3D surface profilometer was used to measure the ablation characteristics, and the morphologies of the laser induced plasma (LIP) were observed through a high-speed camera. Experimental results show that the debris, spatter recast or others caused by the laser energy deposition are quickly transferred away due to the surface water film, which is beneficial for the improvement in the surface quality. However, the drilling depth has been significantly reduced at all pulse energy (from 5 μJ to 100 μJ) owing to the shielding effect of water and LIP. But the micro-hole entrance diameter has increased several times after a sufficient pulse number, and a raised annular feature has exhibited. It is noteworthy that the pulse energy had a significant influence on the micro-hole depth during the underwater drilling process, but a much lesser influence on the micro-hole entrance diameter. When the pulse energy is less than 45 μJ, the volume of underwater ablation (4000 pulses) is larger than that under the air environment. Conversely, when the pulse energy is greater than 45 μJ, the volume of under air ablation is larger than that under water, and the removal volume difference between that two environment enlarges with increasing pulse energy. In order to explain the mechanism of underwater ablation characteristics, a rotational symmetry model combining the sample ionization, optical breakdown of water and LIP expansion was developed to simulate the laser beam attenuation and refraction by the LIP under water. Based on the simulation of the free electron number density, the absorptivity, refractivity and reflectivity of the LIP for the 1030nm wavelength laser were obtained. Through the Lambert-Beer law, the laser beam attenuation by the water film and LIP was acquired. The analysis results show that the underwater LIP has a more significant shielding effect on the incident laser beam, and causes the beam to defocus, both of which will reduce the laser energy density focusing on the sample surface. The higher the laser pulse energy, the greater the LIP natural frequency, and the more obvious its attenuation and scattering effect on the beam, which is basically consistent with the experimental results. Moreover, the underwater annular protrusions can be explained by the influence of the LIP on the laser energy distribution of the focal plane (the sample surface).</p>

Influence Of Laser Induced Plasma on Material Removal in Micro-Drilling Underwater

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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