Laser Percussion Drilling Research Articles

Spray mist-assisted drilling of through silicon vias (TSV) using nanosecond laser: Influence of CNT nanofluid

Through Silicon Vias (TSV) are crucial in semiconductor technology for vertically connecting multiple stacks in 3D packaging. High aspect ratio, smooth and slightly tapered TSVs enhance layout efficiency and void-free filling. Being contact-less and dry-etching process, laser machining is a promising technique to fabricate TSVs. A percussion laser drilling approach was used to fabricate TSVs on a 500 μm thick silicon wafer with a 1064 nm ns laser. Four environments such as air, compressed air jet, water mist, carbon nanotube (CNT) fluid mist were employed to assess laser machining effects on TSV performance in terms of entrance and exit diameters, recast layer, side wall damage, and taper angle. Optimized process parameters (laser pulse energy and pulse number) were derived from an ANN prediction model. Laser drilling under CNT mist produced relatively circular (entrance diameter), straight, and clean TSVs. Reduced debris redeposition was observed under air jet, water mist, and CNT mist compared to air. Minimal sidewall damage occurred under water mist and CNT mist as compared to those machined in air and air jet. CNT mist yielded TSVs with less recast layer (∼7 μm), taper angle (∼1.05 deg.), and heat affected zone (HAZ) thickness (∼50 μm). Enhanced machining quality may be attributed to increased thermal conductivity of CNT nanofluid and pressurized mist flow rate, improving cooling and debris removal. Furthermore, a detailed investigation of TSV using SEM was performed and the findings were compared to those of other research groups.

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.

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.

Innovative Fabrication of Hollow Microneedle Arrays Enabling Blood Sampling with a Self-Powered Microfluidic Patch

Microneedles are gaining a lot of attention in the context of sampling cutaneous biofluids such as capillary blood. Their minimal invasiveness and user-friendliness make them a prominent substitute for venous puncture or finger-pricking. Although the latter is suitable for self-sampling, the impracticality of manual handling and the difficulty of obtaining enough qualitative sample is driving the search for better solutions. In this context, hollow microneedle arrays (HMNAs) are particularly interesting for completely integrating sample-to-answer solutions as they create a duct between the skin and the sampling device. However, the fabrication of sharp-tipped HMNAs with a high aspect ratio (AR) is challenging, especially since a length of ≥1500 μm is desired to reach the blood capillaries. In this paper, we first described a novel two-step fabrication protocol for HMNAs in stainless steel by percussion laser drilling and subsequent micro-milling. The HMNAs were then integrated into a self-powered microfluidic sampling patch, containing a capillary pump which was optimized to generate negative pressure differences up to 40.9 ± 1.8 kPa. The sampling patch was validated in vitro, showing the feasibility of sampling 40 μL of liquid. It is anticipated that our proof-of-concept is a starting point for more sophisticated all-in-one biofluid sampling and point-of-care testing systems.

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.

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.

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.

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.

Real-time observation with metallurgical examination for laser percussion drilling in stainless steel sheets using simultaneous magnetic-ultrasonic assistance

Using simultaneous assistance of the water-based ultrasonic vibrations and a side-loaded magnetic field, the transient real-time observation and metallurgical examination were carried out for laser percussion drilling in stainless steel sheets, aiming for demonstrating the effect of the magnetic-ultrasonic assistance on the hole-drilling process and the metallurgical alteration characteristics. The effect of the camera shooting distance on the signal detection below the hole exit was discussed for laser drilling of through holes. The laser drilling efficiency was characterized based on the real-time observation. It was found that the camera shooting distance affected the signal observation below the hole exit after a workpiece was drilled through due to an optical shield effect of its border side. The influence of the magnetic-ultrasonic assistance was much more notable when the dominant or sufficient heat input for drilling through a hole could not be provided by using a smaller pulse energy, a smaller pulse duration or a bigger laser fluence. The average recast layer thickness of the through-hole middle generated using a millisecond laser could be less than around 5μm when using the water-based ultrasonic assistance accompanied with the magnetic assistance. Compared to the corresponding case without external assistance, a reduction of 40μs was achieved for drilling through a hole by using the simultaneous magnetic-ultrasonic assistance. The average grain size number became bigger for validating the grain refinement in the heat affected zone after laser drilling using the water-based ultrasonic vibrations with and without magnetic assistance.

Molecular dynamics-guided quality improvement in the femtosecond laser percussion drilling of microholes using a two-stage pulse energy process

During percussion laser drilling, the taper of microholes is commonly considered to be difficult to control due to cluster re-solidification at the bottom of the hole. Due to the lack of detailed analysis and understanding of ablation clusters, accurate control of the cluster behaviour in the ablation plume has not been realized, resulting in limited taper control in femtosecond laser percussion drilling. To address this issue, molecular dynamics (MD) simulation is used to analyse the cluster behaviour during femtosecond laser ablation in detail. More importantly, the results provide guidance for the optimization of the femtosecond laser percussion drilling technology. The evolution of clusters of various sizes is described in detail at the atomic scale, and the effect of the energy on the cluster size and ablation ratio is discussed. Furthermore, based on the guidance of MD, a two-stage pulse energy percussion drilling process is proposed. The pulse sequence of femtosecond laser percussion drilling is divided into two stages, where the second stage energy is higher than the initial stage energy. For blind holes, an increase in the pulse energy reduces the average size of the clusters in the ablation plume and promotes the upward removal of clusters. A second increase in the pulse energy when the hole breaks through to the exit side of the sample promotes the downward removal of clusters. The two-stage pulse energy percussion drilling process provides a new method for controlling the behaviour of clusters during femtosecond laser percussion drilling, reducing the re-solidification of clusters and ultimately reducing the taper of microholes.

Experimental characterization and real-time monitoring for laser percussion drilling in titanium alloy using transverse electric field assistance and/or lateral air blowing

A novel laser percussion drilling technology assisted by an electric field with lateral air blowing is reported. The lateral air blowing is first proposed for assisting the E-field assisted laser drilling for improving hole-drilling performance, quality and efficiency by blowing away the vapor plume and cooling the target workpiece. A high-speed camera is first used for observing and characterizing the transient laser percussion drilling process with and without an electric field and/or a lateral airflow. It was indicated that the transverse electric force and/or the side air blowing expedited the spatial motion with dispersion and density reduction of the vapor-plasma plume for enhancing laser drilling efficiency via weakening the plume-induced shield effect. The recast layer was reduced by using the transverse E-field assistance with and without lateral air blowing, and the combined assistance of a transverse E-field together with a lateral airflow was more effective for reducing the recast layer. Interestingly, as a result of the dynamic air cover across the sample surface induced by the lateral air blowing, the laser-vapor interaction inside a drilling hole was greatly enhanced together with the laser-material interaction, leading to the rapidly-accumulated expansion and then the sudden explosion of the laser-induced plume inside a drilling hole. The enhanced vertical scouring and flowing effect resulting from the accelerated vertical plume-motion inside a drilling hole, which was caused by the accumulated pressure difference inside and outside of the hole due to the enhanced outside motion-dispersion effect (especially induced by the electric force) of the plasma-vapor plume, expedited the uniform molten material removal and distribution along the hole sidewall. Importantly, a transverse E-field with and without a side airflow effectively refined the grains of the heat affected zone (HAZ) near the fusion zone of a through hole via increasing the average grain size number. The micro hardness was improved after laser hole-drilling by loading a transverse E-field with and without a side airflow due to a grain refinement effect with and without an airflow cooling effect. It was demonstrated that the lateral air blowing used was more effective to increase laser drilling efficiency for drilling through the workpiece than the E-field used, and the hole-drilling efficiency was further increased if using the combined assistance of a transverse E-field and a side airflow. After the workpiece was drilled through, the hybrid plume signal was effectively suppressed primarily above the hole entrance by using a transverse E-field and/or a side airflow, especially in case of using the transverse E-field assistance.

DD6 single-crystal superalloy with thermal barrier coating in femtosecond laser percussion drilling

In recent years, single-crystal superalloy and thermal barrier coating (TBC) have attracted considerable attentions in femtosecond laser drilling. Surface morphology changes in DD6 single-crystal superalloy and TBC induced by incident laser pulses with a wide range of powers (2.28–11.18 W) at 50 kHz are studied. The ablation thresholds of DD6 single-crystal superalloy and TBC are calculated, and the ablation mechanisms of different femtosecond laser spot regions are discussed in details. Results show that the ablation threshold of DD6 single-crystal superalloy is 1.16 J/cm2, which is close to that of TBC (0.998 J/cm2). The femtosecond laser spot regions in the DD6 single-crystal superalloy and TBC can be divided into four (ablation, melting, laser-induced, and irradiated regions) on the basis of the energy distribution of the Gaussian laser beam. However, the physics behind femtosecond laser on single-crystal superalloy is laser-induced periodic surface structures (LIPSS), where is laser-induced cracks (LIC) on TBC. The dominant mechanism differs in different regions. The reason for cracks on the TBC may be the combined action of thermal stress and mechanism stress under high fluence and high repetition rate femtosecond laser.

Neural-network-assisted in situ processing monitoring by speckle pattern observation.

We propose a method to monitor the progress of laser processing using laser speckle patterns. Laser grooving and percussion drilling were performed using femtosecond laser pulses. The speckle patterns from a processing point were monitored with a high-speed camera and analyzed with a deep neural network. The deep neural network enabled us to extract multiple information from the speckle pattern without a need for analytical formulation. The trained neural network was able to predict the ablation depth with an uncertainty of 2 μm, as well as the material under processing, which will be useful for composite material processing.

Task-Point Sequencing and Trajectory Generation/Optimization with Benchmarking for Multi-axis Percussion Laser Drilling of Jet Engine Combustion Chamber Panels

Percussion drilling is a type of laser drilling operation for which optimized 5-axis trajectory planning has been developed. While percussion drilling is less advantageous in terms of local thermal loading and achievable part quality, it is used extensively in industry, due to its simplicity of automation compared to on-the-fly drilling. Thus, a TSP-style trajectory planning algorithm has been developed for percussion laser drilling. The novelty, in this case, is concurrent planning of 5-axis time-optimal point-to-point movements within the sequencing algorithm, and direct minimization of the total travel time, rather than just distance (in two Cartesian axes), as is the method for which a significant portion of TSP solvers and trajectory planners in the literature has been developed. Compared to currently applied methods, 32–36% reduction in the beam positioning time has been achieved. Also, 39–45% reduction in the peak magnitude of vibration has been realized. Limited benchmarking with the state-of-the-art TSP solvers from combinatorial mathematics, considering only 2-axis Euclidean distance as the objective function, indicates that the proposed sequencing algorithm for percussion drilling is sub-optimal by 9–12%. Thus, it can still use further improvement in future research. Nevertheless, the trajectory planners that have been developed in this paper for percussion drilling have experimentally demonstrated very promising improvements in terms of motion time and smoothness. As more advanced laser control electronics with deterministic execution and rapid synchronization capability become available, such algorithms are expected to facilitate significant production gains in laser drilling processes used in different industries.