Yuan-Jen Chang

On-line depth measurement for laser-drilled holes based on the intensity of plasma emission

The direct time-resolved depth measurement of blind holes is extremely difficult due to the short time interval and the limited space inside the hole. This work presents a method that involves on-line plasma emission acquisition and analysis to obtain correlations between the machining processes and the optical signal output. Given that the depths of laser-machined holes can be estimated on-line using a coaxial photodiode, this was employed in our inspection system. Our experiments were conducted in air under normal atmospheric conditions without gas assist. The intensity of radiation emitted from the vaporized material was found to correlate with the depth of the hole. The results indicate that the estimated depths of the laser-drilled holes were inversely proportional to the maximum plasma light emission measured for a given laser pulse number.

Characteristics of the effect of swirling gas jet assisted laser percussion drilling based on machine vision

In this paper, a machine vision-based system is used for in-situ monitoring and characterization of the effect of swirling gas jet assisted laser percussion drilling. Feasibility of laser percussion drilling on highly reflective materials, i.e., Al5052 with the implementation of assisting swirling gas jet, was explored. The influence of the gas jet type on the mechanism of material removal and resulting spatter is reported. The authors conclude that the swirling gas jet method greatly reduces spatter formation around the hole exit during the laser-drilling process. Consequently, in order to understand and monitor the gas jet effects in laser drilling, real-time in situ metrology is employed to observe the details. Compared with the results obtained when using a straight gas jet, laser drilling with a swirling gas jet reduces the drilled hole debris by 0.69 and increases the drilled hole efficiency by a factor of up to 1.5. Experimental results show that both the drilling depth and machining quality can be...

External Electric Field-Assisted Laser Percussion Drilling for Highly Reflective Metals:

In this study, an external electric field was employed during the laser percussion drilling on highly reflective materials. The laser-produced plasma was sputtered substantially, and the charged ions in the plasma plume were drawn by the electrodes. Different configurations of plate electrodes were proposed and investigated in this work to provide a simple, low-cost method that allows expelling the laser-induced plasma during the percussion drilling process. The electric field resulted from the potential that was applied across the two electrodes. This electrical perturbation produced a uniform electric field when the laser-generated plasma was created in the plane plate-charged capacitor. The electric field with different electrode configurations applied to the charged particles that are carrying the electrons was also simulated in this work. All processing work was performed in air under standard atmospheric conditions and in the absence of assisting process gas. The depth of the holes drilled when various types of electrode configurations were used was measured, and the results were used to evaluate the percussion drilling rate. Results show that vaporized debris is expelled by the applied electric field; hence, in optimal configuration the penetration depth can be increased by up to 91.1%.

Laser induced plasma detection by flat and circular interdigital electrodes in laser material processing

In this paper, we propose two new interdigital electrode designs, flat and circular, to increase the signal level and the detection sensitivity of laser-induced plasma.

Experiment on Intermittent Gas Jet Assisted Modulated Fiber Laser Drilling

In this paper, intermittent gas jet assisted laser drilling on stainless steel (SUS304) with a fiber laser of wavelength 1090 nm is studied. Compared with the conventionally used continuous gas jets in assisting laser drilling, the intermittent gas jet assisting can effectively increase the material removal rate. The intermittent gas jet can be modulated with the frequency to effectively reduce the over-cooling effect by the assist gas. Experimental result shows that the drilling depth and machining time can be improved. The effects of the intermittent gas jet pressures and the synchronicity of gas and laser pulses on the laser drilling are investigated and discussed. It is observed that the intermittent gas jet method obviously reduces heat loss and increases the machining efficiency during the laser drilling. Compared with result of using the continuous gas jet, laser drilling with the intermittent gas jet at 40 Hz increases the drilling depth with an improvement of 10%. It is worth noting that the intermittent gas jet method can also reduce the quantity and cost of gas while the gases such as helium and argon gases are applied.

Magnetic Assisted Laser Percussion Drilling

In this paper, feasibility of laser percussion drilling on highly reflective materials i.e., Al5052 using an assisting magnetic field was explored. During the laser percussion drilling, the Lorentz force generated by the assisting magnetic field affects the laser-induced plasma and results in significant influence of the laser percussion drilling. The magnetic field simulation was performed to investigate the field strength of the circular hollow permanent magnet. The influences of magnetic field and the laser radiation energy on the penetration depth and inlet diameter were examined.

Real-Time Breakthrough Detection for Laser Drilling Based on Coaxial Visual Sensing Technology

As the production demand for laser drilled holes continues to grow, efficient and reliable process monitoring and control techniques are required in order to improve the manufacturing efficiency and reduce manufacturing cost for ensuring quality conformance. A robust, real-time monitoring system for breakthrough and the drilling depth detection in laser drilling is proposed. The system is based on based on coaxial visual sensing technology of the working surface using a CMOS camera, coupled with microcontroller control of the drilling laser operation. The camera is installed adjacent to the entry surface of the workpiece, and the spatial extend of the laser-induced plasma at the working site is monitored as the criterion for breakthrough detection. Therefore, on-line inspection system with low cost is developed to increase productivity. Experimental results show that the proposed system successfully detects when the laser beam has broken through a workpiece, and turns off the laser automatically by feedback control.