Laser Percussion Research Articles

Laser and mechanical micro-drilling for aerospace applications

Traditional laser percussion drilling encounters challenges related to poor geometry and thermal defects. At the same time, mechanical micro-drilling while generating high-quality holes, faces issues such as premature drill breakage and difficulty drilling at acute angles. This review paper introduces a novel approach to micro-drilling Inconel 718 alloy sheets at acute angles by combining sequential laser and mechanical drilling. The study showcases the feasibility and fundamental characteristics of this innovative technique. Results indicate that the sequential laser-mechanical micro-drilling approach mitigates the drawbacks associated with laser-drilled holes, leading to reduced burr size, decreased machining time, and increased tool life when compared to conventional mechanical drilling methods. This advancement holds promise for enhancing precision and efficiency in aerospace applications, paving the way for improved manufacturing processes in the aerospace industry.

Millisecond fibre laser drilling of alumina-alumina CMC

Alumina-Alumina (OX-Ox)CMCs are a technical ceramic composed of alumina fibres in an alumina matrix, with a primary usage case in the aerospace industry. Ox-Ox CMCs offer the traditional benefits of high toughness and bending strength, along with resistance to oxidation and corrosion associated with monolithic alumina, coupled with low brittleness and crack arresting behaviour of fibre in matrix composites. Whilst these properties are highly desirable, CMCs present significant difficulties for conventional mechanical machining, with laser machining offering potential solutions providing the impetus to the work presented in this paper. This work presents a parametric study of the effects of frequency and pulse energy for 10mm deep laser percussion drilled holes and a response surface study for these two variables, using an industrially representative 20kW peak power laser. Results show that for an 800µm diameter hole, a taper angle of <1⁰ and average recast layer thickness of <40µm are achievable, while the response surface study highlights the importance of considering input variables as a complex system, particularly for recast layer thickness.

High-speed X-ray imaging of bulge formation during laser percussion drilling with various polarizations in stainless steel

Achieving high-quality and high-aspect-ratio micro holes with ultrashort pulse laser percussion drilling remains a challenge. Lateral extensions or bulges in dependence on the polarization of the laser beam have so far only been observed by analyzing metallographically prepared cross-sections and the outlets of the holes. For the first time, we present the use of in-situ synchrotron high-speed X-ray imaging to quantitatively capture the time-resolved growth of these bulges in metals. A laser with a pulse duration of 1 ps and a wavelength of 1030 nm was used to investigate the influence of linear and circular polarization as well as the pulse energy on the shape and orientation of the bulges at increasing depths during the drilling process. The bulges were observed to form perpendicular to the polarization at the start of the drilling process. The observations from X-ray images were confirmed by µCT images of the drilled samples.

Design of a Femtosecond Laser Percussion Drilling Process for Ni-Based Superalloys Based on Machine Learning and the Genetic Algorithm.

Femtosecond laser drilling is extensively used to create film-cooling holes in aero-engine turbine blade processing. Investigating and exploring the impact of laser processing parameters on achieving high-quality holes is crucial. The traditional trial-and-error approach, which relies on experiments, is time-consuming and has limited optimization capabilities for drilling holes. To address this issue, this paper proposes a process design method using machine learning and a genetic algorithm. A dataset of percussion drilling using a femtosecond laser was primarily established to train the models. An optimal method for building a prediction model was determined by comparing and analyzing different machine learning algorithms. Subsequently, the Gaussian support vector regression model and genetic algorithm were combined to optimize the taper and material removal rate within and outside the original data ranges. Ultimately, comprehensive optimization of drilling quality and efficiency was achieved relative to the original data. The proposed framework in this study offers a highly efficient and cost-effective solution for optimizing the femtosecond laser percussion drilling process.

Study of the TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes

To protect superalloys under extremely high temperatures in aero-engine core components, thermal barrier coatings (TBC) are sprayed on the material surface and thousands of film cooling holes are drilled which produce a cooling air film. A large proportion of these holes must be drilled at inclined angles relative to the surface in order to provide optimal cooling effects. Laser drilling has been widely applied in the industrial fabrication of film cooling holes due to its high precision, high efficiency, flexibility in drilling angle, and ability to drill all kinds of materials. In particular, nanosecond lasers have attracted lots of interest because they can achieve a good balance between drilling quality and efficiency. However, TBC is prone to delaminate during laser drilling, which severely reduces the lifetime of aero-engine components. In this study, we investigate TBC delamination in nanosecond laser percussion drilling of inclined film cooling holes in TBC-coated cobalt-based superalloy. It was found that the leading edges of the inclined holes suffer from more severe TBC delamination than the trailing edges. By studying the effects of laser focus position, pulse width, pulse energy, repetition rate and average power, we found that faster hole breakthrough and less heat accumulation were essential for the reduction of TBC delamination. Through multi-physical numerical simulations, it was demonstrated that the asymmetric heat transfer effect in laser inclined drilling causes much larger interfacial thermal stress on the leading edge than the trailing edge, qualitatively explaining our experimental observations. The results of this study contribute to the understanding of TBC delamination in laser drilling of inclined film cooling holes.

Picosecond laser microvia drilling of ABF material using MHz burst mode

We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is being evaluated by the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are being discussed on the ablation threshold, penetration depth and laser microvia drilling quality. We find that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer. Therefore, the parameter clearance is introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication.

Laser percussion drilling of transparent hard and brittle materials with ring-shaped electrodes

This paper presents an innovative approach to detect plasma induced by laser drilling in a laser processing system using an external sensing electric field. The system uses a sensing electrode to detect the plasma signal, which is then sent to the controller in real-time via an embedded signal sampling rate system. The system is capable of accurately sensing the pulse repetition rate from 0 to 500 kHz during laser processing, with a signal sampling rate of up to 1 MHz. The controller analyzes the plasma signal and adjusts the laser focus position by controlling the servo motor to achieve real-time detection of laser drilling. The paper compares several configurations of electric field assistance, including parallel type interdigitated electrodes, mask type interdigitated electrodes, ring shape electrodes, and parallel type interdigitated electrodes without a steel plate. The electric field strength and power line distribution of the electric field were simulated using the COMSOL Multiphysics finite element software, and the system was tested on sapphire and quartz glass. The experimental results showed that in the presence of the electric field, the opening diameter decreased by 8 % and 12.13 %, respectively, for sapphire and quartz glass, compared to the case without the electric field. Moreover, the exit diameter increased by more than 100 % and 49.79 %, the ring shape taper angle decreased by 23.31 % and 33.74 %, the machining time decreased by 66.22 % and more than 40 %, and the opening heat affected zone decreased by 24.45 % and more than 50 %, for sapphire and quartz glass, respectively.

Pulse-on-Demand Operation for Precise High-Speed UV Laser Microstructuring

Laser microstructuring has been studied extensively in the last decades due to its versatile, contactless processing and outstanding precision and structure quality on a wide range of materials. A limitation of the approach has been identified in the utilization of high average laser powers, with scanner movement fundamentally limited by laws of inertia. In this work, we apply a nanosecond UV laser working in an intrinsic pulse-on-demand mode, ensuring maximal utilization of the fastest commercially available galvanometric scanners at scanning speeds from 0 to 20 m/s. The effects of high-frequency pulse-on-demand operation were analyzed in terms of processing speeds, ablation efficiency, resulting surface quality, repeatability, and precision of the approach. Additionally, laser pulse duration was varied in single-digit nanosecond pulse durations and applied to high throughput microstructuring. We studied the effects of scanning speed on pulse-on-demand operation, single- and multipass laser percussion drilling performance, surface structuring of sensitive materials, and ablation efficiency for pulse durations in the range of 1–4 ns. We confirmed the pulse-on-demand operation suitability for microstructuring for a range of frequencies from below 1 kHz to 1.0 MHz with 5 ns timing precision and identified the scanners as the limiting factor even at full utilization. The ablation efficiency was improved with longer pulse durations, but structure quality degraded.

Crackless femtosecond laser percussion drilling of SiC by suppressing shock wave magnitude

Femtosecond lasers have attracted attention as tools for the microfabrication of various materials, such as semiconductors, insulators, and metals. When hard and brittle materials are processed by femtosecond lasers, shock waves that propagate during processing generate cracks around the processed area, making precision processing difficult. In this study, we propose and demonstrate multi-step drilling where, after drilling a pilot hole with a small pulse energy, the hole is expanded by increasing the pulse energy in a stepwise manner without generating strong shock waves. The proposed method suppresses cracks in silicon carbide by approximately 70% and could be an important option for femtosecond laser microdrilling.

Competitive mechanism of laser energy and pulses on holes ablation by femtosecond laser percussion drilling on AlN ceramics

In laser microstructure machining, results are derived through the action of multiple laser processing parameters (LPPs); therefore, multiple studies on the effect of LPPs on machining results are available. Further, few studies observed how competitive mechanism of each LPP affects machining results. As the most important LPPs, single-pulse energy and the number of pulses were varied to assess their effect on hole dimension and morphology. Additionally, competitive mechanism of LPPs on hole ablation was further explored. The study was based on femtosecond laser percussion drilling on aluminum nitride (AlN) ceramics surface. It was found that laser energy has a dominant role in initial ablation, determining both ablation and ablation mechanism. Additionally, it also determined hole inlet surface morphology and ablation mechanism remained the same regardless of increase in the number of pulses. Laser pulses assumed a dominant role after intersection point of LPPs acceleration rate and hole dimension acceleration rate. Hence, final hole dimensions and its sidewall morphology were defined. It was found that deep holes can be processed both in gentle ablation and strong ablation with lots of pulses. However, there are opposite mechanisms for inducing hole sidewall morphology; for example, in strong ablation, solidification effect is enhanced with increase of hole depth, while it is decreased in gentle ablation.

Optimization of multistage femtosecond laser drilling process using machine learning coupled with molecular dynamics

Although femtosecond laser percussion drilling is widely used in many key industrial manufacturing fields, the quality of micro-holes limits its service performance. The traditional trial and error method and physical model are always time and cost consuming for process optimization. To address this problem, a process optimization framework coupled with molecular dynamics simulation, machine learning and a high-throughput optimization algorithm is proposed. The physical information obtained by molecular dynamics enriches the data set used to train machine learning model, and thus reducing the amount of data required. Machine learning can quickly and accurately establish the regression model between laser parameters and target machining performance, and high-throughput optimization algorithm is responsible for determining the optimal process in the process space. For femtosecond laser percussion drilling, a new four-stage drilling process is proposed and optimized using the coupling process optimization framework to solve efficiency and quality problems. Finally, the experimental validation for Ni-based single crystal superalloy verifies the reliability of the optimized process. The framework can be extended to other complex systems to realize process optimization with high efficiency and low cost in laser material processing technology.

An in-depth investigation into high fluence femtosecond laser percussion drilling of titanium alloy

High fluence femtosecond laser percussion drilling has a potential to produce micro holes for cooling purpose in turbine blades made of titanium alloys due to its short interaction time with the material. For an in-depth investigation, numerical simulation is carried out using 2D axi-symmetric two-temperature and heat conduction models in tandem, mimicking the laser percussion drilling. Moving mesh approach in finite element based COMSOL Multiphysics software is used to arrive at the crater geometry during the ablation process. The heat conduction model provides the value of surface temperature after each pulse. Taking temperature-dependent thermo-physical and optical properties with intraband absorption effect, the simulations are carried out on a 2 mm thick Ti6Al4V plate for 1–15 pulses with fluence in the range of 0.84 to 8.4 J cm−2. For comparison, the simulation results based on room temperature properties are also included. Validation experiments are carried out and the crater morphology is measured using laser scanning confocal microscope. The overall average absolute deviations of the results for crater diameter and depth are within 20% and 40% respectively. The proposed simulation approach is robust and can be used to investigate multi pulses laser ablation process in any other applications.

Ultrashort Pulsed Laser Drilling of Printed Circuit Board Materials

We report on a comprehensive study of laser percussion microvia drilling of FR-4 printed circuit board material using ultrashort pulse lasers with emission in the green spectral region. Laser pulse durations in the pico- and femtosecond regime, laser pulse repetition rates up to 400 kHz and laser fluences up to 11.5 J/cm are applied to optimize the quality of microvias, as being evaluated by the generated taper, the extension of glass fiber protrusions and damage of inner lying copper layers using materialography. The results are discussed in terms of the ablation threshold for FR-4 and copper, heat accumulation and pulse shielding effects as a result of pulse to pulse interactions. As a specific result, using a laser pulse duration of 2 ps appears beneficial, resulting in small glass fiber protrusions and high precision in the stopping process at inner copper layer. If laser pulse repetition rates larger than 100 kHz are applied, we find that the processing quality can be increased by heat accumulation effects.

Process Parameter Prediction and Modeling of Laser Percussion Drilling by Artificial Neural Networks.

Finding process parameters for laser-drilled blind holes often relies on an engineer’s experience and the trial-and-error method. However, determining such parameters should be possible using methodical calculations. Studies have already begun to examine the use of neural networks to improve the efficiency of this situation. This study extends the field of research by applying artificial neural networks (ANNs) to predict the required parameters for drilling stainless steel with a certain depth and diameter of blind holes, and it also pre-simulates the drilling result of these predicted parameters before actual laser processing. The pre-simulated drilling results were also compared with real-world observations after drilling the stainless steel. These experimental findings confirmed that the proposed method can be used to accurately select laser drilling parameters and predict results in advance. Being able to make these predictions successfully reduces time spent, manpower, and the number of trial-and-error shots required in the pre-processing phase. In addition to providing specific data for engineers to use, this method could also be used to develop a set of reference parameters, greatly simplifying the laser drilling process.

Comparative Analysis of Grey Relational Analysis Integrated with the Principal Component Analysis and Analytic Hierarchy Process for Multiobjective Optimization of Inclined Laser Percussion Drilling in Carbon Fiber Reinforced Composites

Multiobjective optimization (MOO) helps to achieve simultaneous improvement of more than one output characteristic in machining processes where complex interaction between the input parameter exists. This study focuses on the comparative analysis of design of experiment (DoE)-based grey relational analysis (GRA) combined with principal component analysis (PCA) and analytic hierarchy process (AHP). Experiments were conducted with millisecond (ms) duration pulsed Nd: YAG laser using the Box–Behnken design (BBD) approach of the response surface methodology (RSM) at three different levels of input parameters. The output parameters, i.e., hole circularity at top (HCT), hole circularity at bottom (HCB), and hole taper (HT), were determined for various input parameters like pulse current (I), pulse width ([Formula: see text]), gas pressure ([Formula: see text]), workpiece thickness ([Formula: see text]), and incidence angle ([Formula: see text]) during laser percussion inclined hole drilling (LPIHD) in the carbon fiber reinforced polymer (CFRP) of three different thickness, i.e., 1[Formula: see text]mm, 3[Formula: see text]mm, and 5[Formula: see text]mm at incidence angles of 0, 10, and 20 degrees. Multiobjective function based on RSM has been developed for GRA-PCA and GRA-AHP and further optimizations were performed using the desirability approach of RSM. The analysis revealed that the angle of incidence is the most significant factor for controlling the output parameters. Interaction of pulse current and thickness ([Formula: see text]) has a major impact on output responses. The GRA-PCA approach gives the average improvement of 2%, 9%, and 37%, respectively, for HCT, HCB, and HT, whereas in the case of GRA-AHP, the corresponding improvements are only 1%, 6%, and 11%. Therefore, the GRA-PCA approach is a more effective tool for the MOO of LPIHD in CFRP.

Structural changes during femtosecond laser percussion drilling of high-aspect-ratio diamond microholes

We studied the structural and elemental evolutions during the femtosecond laser percussion drilling of high-aspect-ratio diamond microholes. Microholes 225-μm-deep having an aspect ratio of 15 were drilled with an exposure time of 100 s and a laser power of 60 mW. It is found that when the specimen was machined by femtosecond laser at low power and short exposure time, the laser-affected zone (LAZ) may be still solid in the interior rather being void, even it reached the bottom of the diamond. A clear crack appeared between the solid portion and pristine diamond. Elemental analysis revealed that oxygen was incorporated into the solid portion of the LAZs, and its atomic percentage reached 6.5% for a laser power of 10 mW at initial position and decreased as the depth increased in the solid portion. The wall of the void contained nearly no oxygen. Furthermore, nanoripples were observed on the sidewall surface of the hole.

Parametric Analysis and Optimization of Inclined Laser Percussion Drilling of Carbon Fiber Reinforced Plastic Using Solid-State Nd: YAG Laser

Parametric Analysis and Optimization of Inclined Laser Percussion Drilling of Carbon Fiber Reinforced Plastic Using Solid-State Nd: YAG Laser

Parametric analysis of hole circularity for laser percussion drilling of carbon fibre reinforced plastic using Nd: YAG laser

Holes of diameter less than 1 mm has wide application of manufacturing sector. Laser beam can be conveniently used for creation of such hole in different materials. The Carbon Fiber Reinforced Composite (CFRP) is quickly replacing the conventional materials in most of the engineering applications. Therefore, a detail parametric study to analyse the influence of factors like laser current, laser interaction time, pressure of assist gas, workpiece thickness and incidence angle on the geometrical accuracy of hole in terms of hole circularity at top (HCT) has been explored by changing the individual contributing factors. Small holes (< 1mm) have been fabricated with Nd: YAG laser of millisecond pulse duration in CFRP workpiece of thickness 1mm, 3mm, and 5 mm. It was observed that hole circularity at top surface is highest for zero angle of incidence. The increase of laser current, interaction time, and pressure of assist gas increases the HCT when the laser direction is normal to the workpiece whereas it shows a decline trend when the angle of incidence of 10° and 20°g;. The increase of workpiece thickness reduces the HCT irrespective of angle of incidence.

Performance comparison of laser-etched microstructures on K9 glass and PMMA light guide plate

PMMA is widely used to make light guide plates (LGP) of backlight module (BLM), but it is easy to deform at high temperatures, which makes the display effect worse. Due to K9 glass has similar optical properties with PMMA and high temperature-resistance, it is proposed to replace PMMA to make LGP. In this paper, femtosecond laser-etched microstructures and BLM optical performances of K9 glass and PMMA were compared, and it provides that it is feasible to use K9 glass LGP in BLM. The microstructures of LGP are obtained by laser percussion and rotary cutting respectively. It was found that K9 glass has a higher laser-induced damage threshold, ablation threshold, and desired micro-holes morphology than PMMA. Then, the luminance uniformity of K9 glass LGP is 90% by simulation, which is greater than that of PMMA LGP. In the future, this research of K9 glass LGP will improve the quality of liquid crystal display and provide a reference for the development of glass LGP.

Analytical model for the depth progress of percussion drilling with ultrashort laser pulses

A simplified analytical model is presented that predicts the depth progress during and the final hole depth obtained by laser percussion drilling in metals with ultrashort laser pulses. The model is based on the assumption that drilled microholes exhibit a conical shape and that the absorbed fluence linearly increases with the depth of the hole. The depth progress is calculated recursively based on the depth changes induced by the successive pulses. The experimental validation confirms the model and its assumptions for percussion drilling in stainless steel with picosecond pulses and different pulse energies.