Laser Percussion Research Articles

Spatter reduction in nanosecond fibre laser drilling using an innovative nozzle

Pulsed wave fibre lasers are becoming a popular industrial tool in microprocessing due to their many positive features, such as high beam quality, high reliability and high productivity, which are fundamental to machining small, precise features of industrial applications. However, the lasers’ use in the machining of ultraprecise features, such as small holes, is hindered by the fact that commercial pulsed wave fibre lasers commonly operate with pulse durations in the nanosecond regime. Such long pulse durations mean that the material is thermally removed, which results in the production of a melted layer and thermal damage in the bulk material. Consequently, the typical thermal defects of the melting regime, such as spattering of recast material around the hole, taper, heat-affected zone and poor hole circularity, are found in materials machined with these lasers. This paper proposes a design for an innovative nozzle that combines the high productivity of nanosecond fibre lasers with an improvement in the quality of the machined holes by reducing the spatter production in titanium laser percussion drilling. The innovative nozzle is based on the suction effect created by the Venturi principle that prevents the deposition of melted and vaporised material on the workpiece surface. The influence of the nozzle configuration and shielding gas on hole quality is investigated after the laser percussion drilling of 0.5-mm-thick titanium sheets, in which the process conditions that allow maximum productivity are used. The innovative nozzle produces a remarkable decrease in spatter on the entrance hole surface without affecting the other quality features, such as hole diameter, circularity and taper, while preserving the high productivity obtainable with a standard nozzle.

An experimental and numerical study on laser percussion drilling of thick-section alumina

A major challenge in laser percussion drilling of thick-section ceramics is to obtain a low taper and low spatter deposition hole leading to high quality post-processing. In order to achieve the fine hole drilling, it is important to understand the mechanism of laser percussion drilling. In this paper, an experimental and numerical study on laser percussion drilling was carried out. A two-dimension (2D) axisymmetric finite element (FE) model for simulation of temperature field and proceeding of hole formation during percussion drilling was developed. The FE model was validated by the corresponding experiment. Furthermore, a theoretical model for evaluation of temperature at melt front and velocity of melt ejection was presented in order to further validate the FE model and study the spatter deposition. The effects of laser peak power, pulse duty cycle and pulse repetition rate on hole diameter and spatter deposition were investigated by the developed models and experiments, in which the simulated results were in good agreement with the experiments. The study indicated that the size and temperature of the melt front significantly affected the hole diameter formation and spatter deposition during laser percussion drilling. The characteristic of melt front was mainly determined by the employed laser peak power, pulse repetition rate and pulse duty cycle. Based on the experimental and numerical study, the process parameters were optimised and a drilled-hole with low taper and low spatter deposition was obtained using a 3.5kW CO2 laser. A microstructural and element compositional study was also performed in this work, by which the characteristics of microstructure and element composition in HAZ around laser drilled hole were revealed.

High-speed Observation of the Heat Flow in CFRP During Laser Processing

We present fundamental experiments on the heat flow in carbon fibre reinforced plastic (CFRP) during laser percussion drilling with a picosecond laser system. With high-speed imaging, the growing of the matrix evaporation zone (MEZ) during laser processing can be observed with a high temporal and spatial resolution. By varying the pulse energy and the repetition rate of the laser beam, the influence of these parameters on the extension of the heat affected zone can be investigated. Based on these methods, the question, whether high pulse energy or a high repetition rate is more suitable for laser processing of CFRP, has been examined.

Laser microvia formation in polyimide thin films for metallization applications

PurposeThe purpose of this paper is to demonstrate laser microvia drilling of polyimide thin films from multiple sources before metallic sputtering. This process flow reduces Flexible Printed Circuit Board (FPCB) material, chemical and operational costs by 90 per cent in the construction of flexible circuits.Design/methodology/approachThe UV laser percussion drilling of microvias in 25 μm thick polyimide films with low coefficients of thermal expansion (CTE) and elastic modulii was investigated. Results were obtained using Scanning Electron Microscopy and Surface Profilometry. Polyimide films tested included: Dupont™ Kapton® EN; Kolon® GP and LV; Apical® NPI; and Taimide™ TA‐T.FindingsThere was no direct relationship between the top and bottom diameters and ablation depth rates between the polyimide films tested using the same test conditions. There was a direct relationship with exit diameters and etch rates at different laser pulse frequency rates and fluence levels. Laser pulse rates at 30 kHz produced 20 per cent larger exit diameters than at 70 kHz, however at 70 kHz the first pulse etched 16.5 per cent more material. High fluence levels etched more material but with a lower etch efficiency rate. Other microvia quality concerns such as surface swelling, membrane residues on the bottom side and surface debris inside the microvias were observed. Nanoscale powder‐like surface debris was observed on all samples in all test conditions.Originality/valueThis is the first comparison of material specifications and costs for films from multiple polyimide manufactures and laser microvia drilling. The paper also is the first to demonstrate results using a JDSU™ Lightwave Q302® laser rail. The results provide the first insights into potential microvia membrane issues and debris characteristics.

A study on UV laser drilling of PEEK reinforced with carbon fibers

This paper deals with the application of Nd-YAG laser emitting at 355 nm to the drilling process of Carbon Fiber Reinforced Polyetheretherkethone (PEEK-CF) laminates. The combination of a modern UV-laser source with a scanning technology enabling speeds up to 4 m/s, suppresses heat affected zones (HAZ) and consequently detachment of fibers from the polymer matrix. A removal technique based on the ablation of superimposed layers of composite material is proposed and analyzed in detail as a function of energy density delivered to the surface, hatch strategy and thickness of the laminate to be drilled. The hatching technique discussed in the paper is compared with conventional laser trepanning and percussion drilling through holes for the realization of 2 mm and 0.1 mm diameter, respectively. In both the cases benefits were noticed in terms of extension of HAZ and quality of the cut edges.

STATISTICAL ANALYSIS OF FEMTOSECOND PULSES LASER ON HOLE DRILLING OF SILICON WAFER

This study investigated the relationship and parameters interactions between five independent variables in laser percussion drilling of micro-hole. Experiments were conducted on single crystal silicon wafer with material thickness of 725 μm using femtosecond (fs) laser with wavelength of 775 nm and pulse duration of 200 fs. Laser power, focus position, number of pulses, workpiece temperature and assist liquid were selected as independent process variables. Taguchi L18 orthogonal array was applied to design the experiments. The drilling process was evaluated in terms of entrance hole diameter and exit hole diameter. The analysis of the variance (ANOVA) was used to determine the significant parameters that are affecting the entrance hole diameter and exit hole diameter. The result shows that laser power and focus position has highly significant effect on entrance hole diameter. The number of pulses, focus position and assist liquid however has highly significant effect on exit hole diameter.

The effect of assist liquid on the hole taper improvement in femtosecond laser percussion drilling

Absence of debris or spatters on the periphery of laser micromachining area is crucial in semiconductor applications due to reduction of line width and line space of silicon (Si) devices. We studied the role of different types of volatile liquid films in reducing debris formation and taper improvement during femtosecond (fs) laser drilling of Si. It was found that more volatile liquids, i.e. liquids with lower boiling points were more effective in reducing debris formation during the fs laser drilling process. Plasma confinement in liquid and shock waves are believed to be the main causes for the reduction of debris formation. The more volatile liquids also led to reduction in hole taper angles.

Very large spot size effect in nanosecond laser drilling efficiency of silicon

The effect of the spot diameter in nanosecond excimer laser percussion drilling of through via in silicon wafer is presented. Experimental results show a ten fold increase of the ablation efficiency when decreasing the spot diameter from 220 μm to 9 μm at constant fluence in the range 7.5 J/cm(2) to 13.2 J/cm(2). Such effect is absent when using 60 ps deep-UV laser pulses. A model is developed that explain the findings in terms of plume shielding effect on the laser pulse. The model is successfully applied also on previously published data on deep-UV laser drilling of Polyamide and Alumina.

In situ 24 kHz coherent imaging of morphology change in laser percussion drilling

We observe sample morphology changes in real time (24 kHz) during and between percussion drilling pulses by integrating a low-coherence microscope into a laser micromachining platform. Nonuniform cut speed and sidewall evolution in stainless steel are observed to strongly depend on assist gas. Interpulse morphology relaxation such as hole refill is directly imaged, showing dramatic differences in the material removal process dependent on pulse duration/peak power (micros/0.1 kW, ps/20 MW) and material (steel, lead zirconate titanate PZT). Blind hole depth precision is improved by over 1 order of magnitude using in situ feedback from the imaging system.

Recast and oxide formation in laser-drilled acute holes in CMSX-4 nickel single-crystal superalloy

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.

Sequential laser and mechanical micro-drilling of Ni superalloy for aerospace application

Laser percussion drilling is inherently associated with poor geometry and thermal defects. While mechanical micro-drilling produces good quality holes, premature drill breakage often occurs and it is difficult to drill holes at acute angles. This paper presents the feasibility and basic characteristics of a new approach for micro-drilling In718 alloy sheets at an acute angle, using sequential laser and mechanical drilling. The results demonstrate that sequential laser-mechanical micro-drilling alleviates the defects associated with laser-drilled holes, reduces burr size and machining time and increases the tool life compared with mechanical drilling.

Laser percussion drilling modeling utility

Laser percussion drilling is widely used in the aerospace industry to produce cooling holes in jet engine components. A variety of theoretical models have been developed to predict the outcome of the drilling process. However, to utilize these models often requires sophisticated software to perform the required numerical analyses. A simple and easy-to-use computer utility Drilling Routine for Estimating, Analyzing, and Modeling (DREAM) was developed. DREAM is a free-standing MATLAB-based design that offers a convenient and flexible way to predict a variety of drilling process outcomes and can be executed on any Windows-based platform. This paper discusses the features that have been incorporated into the latest release.

Statistical analysis of recast formation in laser drilled acute blind holes in CMSX-4 nickel superalloy

The present study investigates the effects of four controllable variables on the metallurgy and drilling velocity of laser percussion drilled acute (30° to surface) blind holes in 2 mm thick flat pieces of nickel superalloy—CMSX-4 material. Design of experiment and statistical modelling were used in the experimental study to understand parameter interactions. Pulse energy, pulse width, pulse frequency and gas pressure were chosen as the independent process variables. The response surface method was used to develop the models for each of the responses: physical characteristics of wall and bottom recast layer and drilling velocity (function of material removal rate). A central composite design was chosen as it offered the most economical number of experiments for the required information. The significant process factors in each model were identified based on the analysis of variance and the models were checked by complete residual analysis. It was found that high pulse energy and short pulse width gave the maximum drilling velocity and lowest recast layer thickness at the hole bottom, whereas low pulse energy and short pulse width minimised the wall recast layer thickness.

LASER PERCUSSION DRILLING ON THIN MILD STEEL SHEET BASED ON THE DUTY CYCLE VARIATIONS OF A PULSED RF SLAB CO2 LASER

In this work, the influence of laser pulse trains on thin mild steel sheet blades is investigated. The method is based on varying the duty cycle (DC) of a pulsed RF slab CO 2 laser during the process of laser percussion drilling. Here, an experimental methodology to correlate laser parameters and their effects on the characteristics of the melting material ejected was carried out. In the experiments, the interaction time such that of 5%, 10%, and 15% DC were considered for the pulse trains. Results showed a significant effect on the characteristics of melt material ejection during laser percussion. We demonstrate that it was not evident the influence and molten material for pulse width of less than 500 μs and larger than 4500 μs, respectively, when having 5% DC; spatter formation (droplets) for 10% and 15% DC with pulse periods > 2000 μs; debris formation (re-solidified material) at pulse periods from 200 to 1250 μs at 15% DC.

Optimisation of effective factors in geometrical specifications of laser percussion drilled holes

Abstract Nowadays, laser percussion drilling is finding increasingly widespread application in industry. Precise modeling has not yet been achieved due to the complexity of this process. The neural network has been used in this study for process modeling. Approximate experimental models of the process have been developed by the neural network (Generalized Regression Neural Network—GRNN) according to the results of the experiments. Then the optimum input parameters (peak power, pulse time, pulse frequency, number of pulses, gas pressure and focal plane position) were specified using the genetic algorithm (GA) method, the results of which are optimum output parameters. The output parameters include the hole entrance diameter, circularity of entrance and exit holes, hole exit diameter and taper angle of the hole. The tests were carried out on stainless steel 304 sheets with a thickness of 2.5 mm. A Nd:YAG laser machine was employed with a wavelength of 1.06 μm. Oxygen was used as an assist gas. Diameter of the central nucleus of laser beam was 600 μm. Considering the precision of the optimum numerical results and the high speed of the neural network in modeling, this method is reliable and economical and also confirms the qualitative results of the previous studies. Therefore, one can use this method to optimally adjust input parameters of the process in multipurpose and single purpose optimisation modes, which indicates substitute application of the method for optimising the laser percussion drilling process.

Solidification of a two-dimensional high-Reynolds-number flow and its application to laser percussion drilling

The competition between inertia and solidification for the high-Reynolds-number flow of molten aluminium across a cool solid aluminium surface is investigated. A two-dimensional molten aluminium droplet is of finite extent and is surrounded by a passive gas. The droplet initially freezes due to rapid thermal conduction into the solid. Depending on the initial velocity of the molten aluminium, one of two situations may develop: (i) If the molten aluminium has a non-decreasing initial velocity profile, solidification continues until the passing of the trailing edge of the liquid/gas interface or the flow is engulfed; (ii) If the molten aluminium has a decreasing initial velocity profile, the droplet narrows and thickens resulting in a reduction in the heat flux and in the rate of solidification; this will eventually lead to fluid clumping and shock formation. The rate of solidification may also be reduced by increasing the ambient temperature. The results are interpreted in terms of the recast observed during the solidification phase of laser percussion drilling.

Influence of microsupersonic gas jets on nanosecond laser percussion drilling

This paper reports on etching rates and hole quality for nanosecond laser percussion drilling of 200-μm thick 316L stainless steel performed with micro supersonic gas jets. The assist-gas jets were produced using nozzles of 200, 300 and 500 μm nominal throat diameters. Air and oxygen were used separately for the process gas in the drilling trials and the drilling performance was compared to drilling in ambient conditions. The highest etch rate of 1.2 μm per pulse was obtained in the ambient atmosphere condition, but this was reduced by about 50% with assist-air jets from the 200 μm nozzle. Increasing the jet diameter and/or using oxygen assist gas also decreased the etching rate and increased the hole diameter. The 200 μm nozzle using air-assist jets produced the least amount of recast and gave the best compromise for etching rate. A combination of plasma shielding and different gas dynamic conditions inside the holes and at the surface are correlated to the observations of different drilling rates and hole characteristics.

Percussion drilling with laser: hole completion criterion

The criterion for ensuring formation of a through hole is an important aspect of laser drilling. Close observation of the process has indicated that drilling of the substrate is achieved in six successive microstages. The stages have been time resolved and the behaviour of laser induced plasma (LIP) during the microstages of drilling has been carefully examined. The LIP behaviour during the percussion laser drilling has been explained with the help of the contemporary understanding of plasma. This paper proposes a drilling completion criterion that is based on the behaviour of LIP during laser percussion drilling of SUPERNI 263A with a Nd-doped yttrium aluminium garnet laser. The completion of formation of a hole is marked by the momentarily increased luminescence of the plasma plume emanating from the laser beam exit side of the workpiece during drilling. The criterion has been experimentally verified. The criterion is versatile and can help in augmenting productivity during manufacturing.

Investigations on laser percussion drilling of some thermoplastic polymers

Experimental investigations have been carried out to find the optimum ranges of parameters for drilling of polymers using a carbon dioxide (CO 2) laser-cutting machine. A few important factors that might affect the drilling process have been selected for the study on three different polymers. An attempt is made to plan the experiment using Taguchi's method. Matrix experiment-L 9 orthogonal array was used to conduct the experiments with four factors and three levels of each factor. Through experiments the effects of number of pulses, standoff distance, assist gas pressure and nozzle diameter on the circularity of hole, spatter thickness, hole taper and material removal rate was assessed. The optimum conditions obtained from the analysis show the combination of parameters that improves hole quality.

YAG laser percussion drilling of a functional multi-layer thin plate

An Au-coated Fe–Ni alloy thin plate was laser drilled by a pulsed Nd:YAG laser. The influences of laser parameters on the diameter of perforation, the outer diameter of crater and the roundness of the perforation have been investigated by employing scanning electron microscopy. The diameter of perforation increases gradually with an increase in pulse width from 0.3 to 8 ms at the fixed average laser power and frequency, and increases with an increase in average laser power from 10 to 25 W at the fixed pulse width and frequency. Some craters are found around the perforations because of the heat affecting of laser beam to the non-irradiated zone. The diametric difference of the perforation between the incident and the effluent sides is very small under the shortest pulse width of 0.3 ms. Good roundness of the perforation can be achieved at either the lowest pulse width or the lowest laser power.