Fabrication of micro holes with high surface quality and mechanical properties by high-speed laser trepanning and electrochemical post-treatment

Laser processing could drill micro holes with very high efficiency, but the micro holes suffer from recast layers, microcracks and heat-affected zones, which has limited its wide application in engineering. In this paper, the high-speed laser drilling and electrochemical post-treatment has been combined to improve the machining efficiency and surface quality. To reduce the surface sputtering and redeposit in laser drilling, avoid the stray current induced corrosion and improve the machining precision in electrochemical post-treatment, the double-side coating has been applied to the workpiece before processing. Influences of laser power ratio, frequency, duty cycle and defocus amount on the diameter and taper of the laser drilled micro holes were explored experimentally. Results showed that the micro hole diameter increased with the increase of laser power ratio, duty cycle and defocus amount and decreased with the increase of pulse frequency. The micro hole taper increased with the increase of frequency, duty cycle and defocus amount. A laser power ratio of larger than 70% was recommended to reduce the taper of the laser drilled micro holes. Additionally, orthogonal experiments were employed to study the effects of laser power ratio, frequency, duty cycle and defocus on laser drilled micro hole diameter, taper, and recast layer thickness. Results show that duty cycle has the greatest impact on aperture, and power ratio has the greatest impact on taper, and frequency has the greatest impact on the thickness of the recast layer. Micro holes with an average diameter of 0.58 mm, a taper angle of 1.77°, and an average recast layer thickness of 25 µm have been obtained by laser drilling utilizing the optimal parameters. Finally, electrochemical post-processing was adopted to improve the surface quality and mechanical properties of the laser drilled micro holes. Results had shown that micro holes without recast layer and micro cracks were obtained by electrochemical posttreatment with a processing time of 20 s. Moreover, the inner hole surface roughness has been reduced by 68.16% and the micro hardness was reduced by 63.40%. A surface roughness of Ra 0.71 μm has been achieved. The proposed laser and electrochemical machining could be applied to the fabrication of micro holes with high surface quality and high efficiency.