Effects of processing parameters on hole circularity in laser percussion drilling – A statistical model

This is a study of laser percussion drilling optimization by combining the neural network method with the genetic algorithm. First, optimum input parameters of the process were obtained in order to optimize every single output parameter (response) of the zprocess regardless of their effect on each other (single criterion optimization). Then, optimum input parameters were obtained in order to optimize the effect of all output parameters in a multicriteria manner. Artificial neural network (ANN) method was employed to develop an experimental model of the process according to the experimental results. Then optimum input parameters (peak power, pulse width, pulse frequency, number of pulses, assist gas pressure, and focal plane position) were specified by using the genetic algorithm (GA). The output parameters include the hole entrance diameter, circularity of hole entrance and hole exit, and hole taper. The tests were carried out on mild steel EN3 sheets, with 2.5 mm thickness. The sheets were drilled by a 400 w pulsed Nd:YAG laser emitting at 1.06 µm wave length. Oxygen was employed as the assist gas. Considering the accuracy of the optimum numerical results and the high capability of the neural network in modeling, this method is reliable and precise and confirms the qualitative results in the previous studies. As a result, one can use this method to optimally adjust input parameters of the process in multicriteria optimization mode, which indicates substitute application of the method for optimizing the laser percussion drilling process.

The present study investigates the effects of four controllable variables on the metallurgy and geometry of acute (30° to surface) laser percussion drilled holes in 2 mm thick flat pieces of nickel superalloy: CMSX-4. Design of experiment (DOE) and statistical modelling were used in the experimental study to understand parameter interactions. Assist gas pressure, nozzle stand-off distance, focal plane position (FPP), and number of pulses were chosen as the independent process variables. A central composite design (CCD) was chosen as it offered the most economical number of experiments for the required information. The response surface method (RSM) was used to develop three models. The significant factors in each model were identified based on the analysis of variance (ANOVA), and the models were checked by complete residual analysis. It was found that a high FPP and high nozzle stand off gave the minimum entrance oxide layer thickness whereas the recast layer was minimized for a zero FPP and high pulse number.

The effects of process parameters on spatter deposition in laser percussion drilling

This study investigates the effect of five parameters on circularity and taper of drilled holes in pulsed Nd:YAG laser microdrilling process. The effect of various process parameters like lamp current, pulse frequency, pulse width, air pressure, and focal length of Nd:YAG laser microdrilling on hole circularity at entry, exit, and taper has been investigated through response surface methodology (RSM)–based experimental study. The significant parameters have been selected based on the analysis of variance (ANOVA). The parametric combination for optimal hole circularity and hole taper has also been evaluated. In micromanufacturing, circularity of a drilled hole at entry, exit, and taper are important attributes which greatly influence the quality of a drilled hole. The drilling operation has been carried out on titanium nitride-alumina (TiN-Al2O3) composite, an important electroconductive ceramic suitable for wear and heating applications.

This paper presents the optimal conditions for the ultraviolet laser percussion drilling of alumina materials intended for use in heat sinks. The Taguchi method and grey relational analysis, along with the consideration of multiple quality characteristics, were applied for determining the optimal parameters. The entrance diameter and taper angle of the drilled hole were affected by the material processing parameters, including laser power, pulse duration, focal plane position, and number of pulses. The Taguchi method and grey relational analysis were used for assessing the effects of the operational parameters on multiple performance characteristics. Nine experiments based on an orthogonal array were performed. According to the results, the optimal process parameters were as follows: laser energy density, 3.82 J/cm2; focal plane position, 0.1 mm; number of pulses, 20 shots; and single pulse duration, 3 ms. Analysis of the grey relational grade revealed that the focal plane position was the most dominant parameter.

Investigations on laser percussion drilling of some thermoplastic polymers

Experimental investigation of cutting parameters during laser beam machining of aluminum alloy 7075

Results are presented for micromachining of single crystal ( orientation) of silicon using a femtosecond Ti:Sapphire laser from Spectra Physics of 2mJ energy, 100 fs pulse width, 10 kHz repetition rate and wavelength of 790 nm. Some of the results include: measurements of femtosecond laser characteristics, effect of beam focus position on micromachined line width, dependence of ablated volume on number of laser pulses and process parameters for achieving sub micron line widths.Results are presented for micromachining of single crystal ( orientation) of silicon using a femtosecond Ti:Sapphire laser from Spectra Physics of 2mJ energy, 100 fs pulse width, 10 kHz repetition rate and wavelength of 790 nm. Some of the results include: measurements of femtosecond laser characteristics, effect of beam focus position on micromachined line width, dependence of ablated volume on number of laser pulses and process parameters for achieving sub micron line widths.

This paper reports a novel technique for controlling hole taper in laser percussion drilling. The method is termed Sequential Pulse Delivery Pattern Control (SPDPC) and it is based on the alteration of the overall laser pulse pattern during a pulse train required to drill a hole. The work has shown that SPDPC is effective in producing through holes with good overall parallelism in Ni alloy (2.65 mm thick), giving an average hole taper of 12 μm for 0.55 mm holes. Experiments were conducted to understand the taper formation mechanisms. The effectiveness of the SPDPC method in reducing hole taper has been identified as being primarily due to the reduction of upward liquid melt ejection which in turn reduced the erosion of hole due to melt ejection, as well as the optimisation of the laser pulse intensity distribution during hole propagation.

Dynamic aortomyoplasty using Latissimus Dorsi muscle (LDM) has been shown to improve myocardial function. However, systematic examination of the effects of stimulation parameters on aortic wrap function has not been done. Thus, the present study measures the direct effect of stimulation voltage, pulse train duration, frequency of the pulses, and the duration of the stimulation delay from R wave on the aortic wrap function. In eight female goats, the left LDM was wrapped around the descending aorta. The muscle was then subjected to electrical stimulation, altering frequency of stimulation pulses (16.6, 20, 25, 33 and 50 Hz), amplitude (2, 4, 6, 8 and 10 V), and number of pulses (2, 4, 6, 8 and 10 pulses) in a train stimulation. Left ventricular, aortic pressure, and pressure generated by LDM on aorta (wrap pressure) was measured. The changes in hemodynamic parameters mentioned above were calculated and compared for different stimulation parameters during unassisted and assisted cardiac cycles. Aortomyoplasty counterpulsation using LDM provided significant improvement in wrap pressure (78 mmHg +/- 2), aortic diastolic pressure, and changes in aortic diastolic pressure from 2 to 4 V (P < 0.05). Further increase in amplitude did not make any significant improvements of the above mentioned parameters. Significant augmentation of wrap pressure (82 mmHg +/- 2), aortic diastolic pressure (79 mmHg +/- 3) and changes in aortic diastolic pressure (12 mmHg +/- 1) occurred at 6 pulses (P < 0.05). Other changes in number of pulses did not show any significant improvements. Significant improvement of wrap pressure (80 mmHg +/- 2), aortic diastolic pressure (73 mmHg +/- 3) and changes in aortic diastolic pressure (12 mmHg +/- 1) was observed with a frequency of 33 Hz. To examine a wide range of delays from the onset of the QRS complex to LDM stimulation, stimulation was delivered randomly. The exact delay was determined from the ECG signal and superimposed LDM stimulation pulses. In this study we present a new measurement, wrap pressure. We also present that in aortomyoplasty using LDM, the most significant improvement in wrap pressure, aortic diastolic pressure and changes in aortic diastolic pressure occurs when the stimulation consists of an amplitude of 4 V, a frequency of 33 Hz and a train stimulation of 6 pulses.

The electric breakdown characteristics in microcavity structure under repeated frequency pulse (RFP) were studied, and the physical mechanism was investigated quantitatively based on the full statistical distribution of breakdown time delay obtained in step rectangular pulse (SRP). Experimentally, microcavity heights of 300, 800, and 2000 μm were used. In RFP, the occurrence of breakdown becomes probabilistic when the time delay t d and pulse width t PW satisfy the condition t s-min < t PW < t s-max. The breakdown probability increases with pulse width, and the probability distributions are roughly exponential and Gaussian at pulse frequencies of 3 and 1000 Hz, respectively. We found the results are attributed to the similar distributions of time delay in RFP and SRP with similar afterglow time and pulse voltage, and the equal distributions of breakdown probability (with pulse width) and cumulative probability of t d in RFP. The microcavity effect will decrease the breakdown probability under given pulse width and voltage. Additionally, it is found that in RFP the increase of pulse width from 1 to 1000 μ s will decrease the threshold voltages at 0% and 100% breakdown probabilities, and the threshold voltage difference will decrease simultaneously to around 0, which results in the transition of breakdown feature from probability to certainty. This phenomenon is due to that the reduction of pulse voltage will increase the time delay significantly and meanwhile the variation rate of time delay with pulse voltage Δt d/ΔU w decreases sharply. The microcavity effect will cause the increase of threshold breakdown voltages at a given pulse width and frequency. Finally, it is found that in RFP the breakdown voltage will decrease with the rise of pulse frequency from 10° to 104 Hz, which is consistent with the variation of time delay with afterglow time (from 10−1 to 103 ms) in the memory curve measured in SRP under similar afterglow time. Overall, the microcavity effect will enhance the adsorption of charged and excited species by dielectric walls during afterglow period and enlarge the time delay in the following pulse breakdown, and then influence the RFP breakdown characteristics.

Characteristics of spatter formation under the effects of different laser parameters during laser drilling

Plasma electron density and temperature were characterized in a continuous flowing gas-liquid film reactor with argon carrier gas by time-resolved optical emission spectroscopy. The plasma parameters were studied as a function of time for varying pulse widths and frequencies. Pulse frequency was varied between 1 and 10 kHz at 16 kV (input voltage) and 40 ns (pulse width) using an Eagle Harbor Technologies, Inc. (EHT) power supply and 5–100 kHz using an Airity Technologies, LLC (AT) power supply. The pulse width was varied between 40 and 200 ns at 16 kV, 2 kHz with the EHT power supply. Optimal frequencies of 5 and 20 kHz were observed for peak electron density with EHT and AT power supplies, respectively. The peak electron density increased with increasing pulse width between 40 and 200 ns using the EHT power supply. Hydrogen peroxide exiting the reactor in the liquid phase increased with discharge power irrespective of the power supply or pulse parameters. Mineralization of 12.5, 50, and 200 ppm perfluorooctanoic acid (PFOA) dissolved in DI water to fluoride (F−) correlated to the peak electron density. Glycerol, a liquid-phase hydroxyl radical scavenger, depleted hydrogen peroxide but did not affect PFOA mineralization. CO, a gas-phase hydroxyl radical scavenger, led to a reduction in the formation of F− production, suggesting hydroxyl radicals in the gas-liquid film play a necessary, but not singular, role in mineralization of PFOA.

In the present work, basalt–glass hybrid composite has been fabricated and machined using laser beam drilling, to predict a safe machining zone pertaining to high drill quality with minimum heat-affected zone and maximum hole circularity. The prediction of the zone has been done by mathematical modeling using response surface methodology. The obtained zone has also been validated by performing more experiments. Moreover, the dependency of hole circularity and heat-affected zone on input parameters has also been discussed. From the results, it is evident that the obtained zone is capable of minimizing the heat-affected zone with acceptable hole circularity. Moreover, the behavior of input parameters is non-monotonic in nature.

The process parameter selection in any machining process is one of the most challenging criteria for effective machining. Selection of an optimal parametric combination always results in efficient machining. In the current work, a fuzzy TOPSIS method for selection of optimal laser beam micro-marking process parameters, such as current, pulse frequency, and scanning speed, during laser beam marking on gallium nitride is adopted. Response surface methodology (central composite design) is adopted for the experimental design. The mark width, mark depth, and mark intensity are considered as the process responses. The mark width and mark depth are measured with the help of a precision optical microscope. Microscopic images are then treated using image processing software to calculate the mark intensity. A multi-criteria decision-making model is developed considering the effect of process parameters on process response for selection of optimal process parameter in the laser beam micro-marking process. The triangular fuzzy number is used to calculate the weight of each performance criteria, and the fuzzy TOPSIS hybrid approach is used for selection of optimal parametric combination. The results indicate that a small pulse frequency and high current and scanning speed lead to increase in mark intensity.