Pulsed Nd:YAG 레이저를 이용한 초박판용 금속 재료의 미세 천공 기술 개발에 관한 연구

Abstract

Laser drilling process has been widely used in various industrial areas, including automotive, aerospace and electronics, for the rapid fabrication of small holes. The aluminum sheet has been used as the material of mobile phone board and printed circuit board with various small holes. However, the aluminum sheet has several inherent limitations in terms of laser material processing such as the high reflectivity, the high thermal conductivity and the distinguished oxidation reaction. Hence, in order to manufacture rapidly precise micro-holes, a proper combination of process parameters for the laser drilling of the aluminum should be estimated. The objective of this research work is to investigate into the influence of process parameters, such as the focal distance, the pulse duration, the pulse frequency, and the nozzle diameter, on the quality of the drilled micro-hole in the laser drilling of a thin Al 1050 sheet using high-power pulsed Nd:YAG laser. In order to investigate the effects of process parameters on qualities of the drilled hole quantitatively, laser drilling experiments were carried out using PC-NC controlled laser drilling apparatus. The maximum peak power, the wavelength, the spot size, and the transverse mode were 150 Watt, 1,064 nm, 0.4 mm, and TEM00 mode, respectively. In order to minimize the oxidation reaction, He gas was used as the assisted gas. Through the preliminary experiments, it was shown that the focal distance of the practical drilling region lies in the range of 4-6 mm. In the main drilling experiments, the sectional width, the dross height, the normalized deviation of hole diameter and the taper angle were chosen as measures to estimate the quality of the drilled hole. The results of the main experiments showed that the normalized deviation of the hole diameter and the dross height area are minimized, but on the other hand, taper angle and the sectional width are maximized when the nozzle diameter, the pulse frequency and the pulse duration are 0.7 mm, 1.0 ms and 12 Hz, respectively. Hence, the optimum drilling condition was chosen as 0.7 mm of the nozzle diameter, 1.0 ms of the pulse frequency and 12 Hz of the pulse duration. In addition, it was shown that the mean hole diameter at the optimum drilling condition is nearly 586 μm. In order to simulate the heat transfer characteristics during the laser drilling, three-dimensional transient heat transfer analysis was performed using a commercial code ABAQUS V6.5. Laser beam was assumed as surface heat source model with Gaussian heat distribution in a plane. Time variation of the heat intensity was reflected in the FE analysis through the User subroutine program supported by ABAQUS. Through the comparison of the results of the experiments and those of analyses, it was shown that FE model can properly simulate the heat transfer phenomenon during the laser drilling of Al 1050 sheet using the pulsed Nd:YAG laser. From the results of the FE analyses, it was shown that the heat affected area creates within 1.0 mm from the center of the heat source and the rapid temperature variation takes place within 0.25 mm from the center of the heat source. In addition, it was found that the difference of the melted radius of the top surface and that of the bottom surface are minimized when the nozzle diameter, the pulse frequency and the pulse duration are 0.7 mm, 1.0 ms and 12 Hz, respectively. Based on the above results, the optimum laser drilling condition of Al 1050 sheet with 0.2 mm of thickness was obtained.