Laser Induced Plasma Micromachining Research Articles

Adaptive Online Continual Learning for In-Situ Quality Prediction in Manufacturing Processes

Abstract Manufacturing processes undergo continuous changes in order to meet various requirements, such as process/product changes and variations in tool/workpiece conditions, leading to mixed, heterogenous, or anomalous data. As a result, a quality prediction model trained from previous data may not perform well when new tasks emerge. In order to achieve in-time and accurate product quality prediction, it is crucial to develop a predictive method that adapt to variations in the manufacturing system, capable of learning from new tasks without forgetting previous ones and detecting unknown tasks. This study proposes a deep learning method integrated with continual learning for in-situ quality prediction that is capable of learning from new tasks without forgetting previous ones. To demonstrate this idea, deep CNNs are designed to analyze in-process sensor data, which consist of shared layers to capture the common underlying features across all tasks, and task-specific layers that capture specific characteristics of each individual task. In order to identify the task to which incoming product belongs, a task prediction approach based on task relevancy using filter subspace distance is proposed. When new data come in, the model first identifies the task, followed by predicting the quality of the current product. The proposed method is demonstrated on two case studies, including quality prediction of the workpiece using acoustic emissions during the laser-induced plasma micro-machining process, and quality prediction of the product through thermal images during the hot stamping process.

Wettability, droplet impact and anti-icing performance of micro-nano hierarchical structure on Ti6Al4V surface via integrating of chemical modification and laser-induced plasma micromachining

Superhydrophobic functional surfaces are widely used in medical treatment, pipeline transportation, and ship corrosion protection and have great potential in the field of anti-icing. Ultrafast laser-induced plasma micromachining (LIPMM) combined with a chemical modification of a fluoroalkyl low surface-energy substance (Novec) was used to fabricate superhydrophobic surfaces with periodic layered micro-nano arrays on Ti6Al4V. The wettability, droplet collision behavior and wear resistance of surfaces with different physical and chemical properties were analyzed. In terms of static anti-icing, based on the classical nucleation theory, the icing-delay performance of different surfaces was analyzed. Ice adhesion strength and frosting properties were also studied. It was found that the micro-nano structure surface fabricated by LIPMM had strong hydrophobicity (contact angle of 164°, roll-off angle of 1.0°), high icing delay time (3072 s, 4 μL of water droplet on the surface at −10 °C), low ice adhesion strength (38.2 kPa), and better durability. Additionally, the droplets bounced off this surface after colliding at different angles (0°, 5°, 10°, and 20°). This study provides a feasible solution for fabricating anti-icing surfaces on Ti6A14V.

Laser-induced plasma micromachining on surfaces parallel to the incident laser in different solutions

Laser-induced plasma micromachining (LIPMM) is an advanced technology that utilizes the plasma generated from laser breakdown to remove material, thereby facilitating the fabrication of microstructures. This paper explores the use of LIPMM on 304 stainless steel surfaces parallel to the laser beam in different solutions, focusing on the impact of the liquid environment on the machining process. It presents a theoretical analysis of the material removal mechanisms unique to this orientation and experimentally investigates how water, a salt solution, and ethanol affect plasma shockwave characteristics. Notably, the plasma shockwave in the salt solution demonstrates the most significant peak pressure and energy, enhancing the micromachining efficiency. These findings suggest that varying the liquid environment can significantly influence LIPMM's effectiveness, offering potential improvements in precision and control. This study broadens the understanding of LIPMM applications, especially in orientations not commonly explored, and opens new possibilities for advanced micromachining techniques in various industrial applications.

Transfer Learning For Predictive Quality In Laser-Induced Plasma Micro-Machining

Abstract In laser-induced plasma micro-machining (LIPMM), a focused, ultrashort pulsed laser beam creates a highly localized plasma zone within a transparent liquid dielectric. When the beam intensity is greater than the breakdown threshold in the dielectric media, plasma is formed which is then used to ablate the workpiece. This paper aims to facilitate in-situ process monitoring and quality prediction for LIPMM by developing a deep learning model to (1) understand the relationship between acoustic emission data and quality of micro-machining with LIPMM, (2) transfer such understanding across different process parameters, and (3) predict quality accurately by fine-tuning models with a smaller dataset. Experiments and results show that the relationship learned from one process parameter can be transferred to other parameters, requiring lesser data and lesser computational time for training the model. We investigate the feasibility of transfer learning and compare the performance of various transfer learning models: different input features, different CNN structures, and the same structure with different fine-tuned layers. The findings provide insights into how to design effective transfer learning models for manufacturing applications.

Transfer learning for predictive quality in laser-induced plasma micro-machining

In laser-induced plasma micro-machining (LIPMM), a focused, ultrashort pulsed laser beam creates a highly localized plasma zone within a transparent liquid dielectric. When the beam intensity is greater than the breakdown threshold in the dielectric media, plasma is formed which is then used to ablate the workpiece. This paper aims to facilitate in-situ process monitoring and quality prediction for LIPMM by developing a deep learning model to (1) understand the relationship between acoustic emission data and quality of micro-machining with LIPMM, (2) transfer such understanding across different process parameters, and (3) predict quality accurately by fine-tuning models with a smaller dataset. Experiments and results show that the relationship learned from one process parameter can be transferred to other parameters, requiring lesser data and lesser computational time for training the model. We investigate the feasibility of transfer learning and compare the performance of various transfer learning models: different input features, different CNN structures, and the same structure with different fine-tuned layers. The findings provide insights into how to design effective transfer learning models for manufacturing applications.

Investigation of Overflow-Water-Assisted Femtosecond Laser-Induced Plasma Modulation of Microchannel Morphology

Laser-induced plasma micromachining (LIPMM) process is an effective approach to create microfeatures with high aspect ratio (AR) and reduced heat affected zone (HAZ). Therefore, LIPMM plays a crucial role in improving the morphology of microchannels. In this study, microchannels were fabricated using a femtosecond laser with two distinct sets of process parameters under three different processing methods: overflow-water-assisted laser-induced plasma micromachining (OF-LIPMM), laser direct writing (LDW), and static water laser-induced plasma micromachining (S-LIPMM). Furthermore, single-factor experiments were conducted to systematically analyze the effects of four parameters, namely single-pulse energy, scanning speed, scanning times, and frequency, on the HAZ, AR, and material removal rate (MRR) of the microchannels. Finally, the optimized parameters determined from the single-factor experiments were applied for large-scale grid fabrication on a surface. The experimental results revealed that OF-LIPMM enables the creation of two different kinds of microchannel surfaces: one microchannel was fabricated with a higher AR of 3:1 and a larger HAZ, while another microchannel was created with a lower AR of 1:1 and a reduced HAZ. Moreover, the parameters investigated in the single-factor experiments can be applied to large-scale processing. The results also indicate that variations of the scanning speed, frequency, and single-pulse energy have similar effects on the machining characteristics of the three processing methods. The findings enable the generation of microchannels with favorable morphological characteristics and have significant implications for the large-scale production of both types of microchannels.

Fabrication and droplet impact performance of superhydrophobic Ti6Al4V surface by laser induced plasma micro-machining

Titanium alloy Ti6Al4V super-hydrophobic surface plays a key role in enhancing the water repellency and anti-condensation of the fuselage. In this study, laser-induced plasma micromachining (LIPMM) is used for processing an array of cone-shaped protrusions on the surface with a micro-nano hierarchical structure. The relationship between surface wettability, micro-nano structure, and surface chemical composition was studied, meanwhile, the droplet impact behavior was experimented and simulated. The study found that with the aging treatment surface C element distribution and content increased significantly, and the wettability changed from superhydrophilic to hydrophobic. As the scan line spacing (50, 40, 30 μm) decreases, the contact angle increased and the hysteresis contact angle decreased. At 50 times and 30 μm line spacing, the contact angle 151.209°>150°, and the hysteresis contact angle 2.619°<10°. The collision experiments reveal that with the increase of the collision velocity (0.44 m/s to 0.98 m/s), the spreading diameter and the rebound height increases. The comparative results of experiments and simulations of collisions present that as the contact angle increases, the spreading diameter decreases, and the rebound height increases. Therefore, the LIPMM is feasible for the fabrication of a superhydrophobic surface and has a wide range of application prospects such as waterproofing, anti-fouling, and anti-icing.

Bubble Behavior and Its Effect on Surface Integrity in Laser-Induced Plasma Micro-Machining Silicon Wafer

Abstract Laser-induced plasma micro-machining (LIPMM) process does well in fabricating high-quality surface microstructures of hard and brittle materials. However, the liquid medium is overheated to induce lots of bubbles to defocus the laser beam, reducing machining stability, and explosive behavior of bubbles destroys the surface quality. Thus, the static and dynamical behaviors of bubbles in LIPMM are comprehensively investigated in this article. First, a series of mechanisms including bubble generation and growth, bubble motion and explosion, and the effect of bubbles behavior on machining characteristics were explained. Second, a volume of fluid (VOF) model of bubble motions in laser-induced plasma micro-machining was established to simulate the dynamical behavior of bubbles under different depths of water layer, which reflect the growth of microbubbles, the aggregation of multiple bubbles, and the floating movement of bubbles. Then, a series of experiments were carried out to reveal bubble static behaviors, and further bubble explosion behaviors on surface integrity, surface defects, and hardness were analyzed. The increase of laser frequency leads to the increase of the maximum attached bubble size. Obstructed by bubble dynamical behaviors, a discontinuous section and the unablated area are observed in the microchannel. The elastic modulus and surface hardness of surface impacted by explosion bubbles are reduced. This research contributes to better understanding bubble behavior related to machining performances in LIPMM of single-crystal silicon.

Cavitation bubble removal by surfactants in Laser-Induced Plasma Micromachining

The Laser-Induced Plasma Micromachining (LIPMM) process results in higher material removal rates, lower heat affected areas, and is less influenced by materials properties in comparison to direct laser ablation. Despite these improved benefits, cavitation bubbles generated during the process that reflect, refract, and scatter the laser beam pose a challenge for processing materials in precision engineering applications. This work reports an effective solution to remove surface bubbles in LIPMM using surfactants. The force of repulsion between the surfactant molecules aids the buoyancy force in the bubbles to overcome the surface tension which causes the bubbles to detach from the surface.

Comparative Analysis of Bubbles Behavior in Different Liquids by Laser-Induced Plasma Micromachining Single-Crystal Silicon

Laser-induced plasma micromachining (LIPMM) can be used to fabricate high-quality microstructures of hard and brittle materials. The liquid medium of the LIPMM process plays a key role in inducing the plasma and cooling the materials, but the liquid medium is overheated which induces lots of bubbles to defocus the laser beam and reduce machining stability. In this paper, a comparative investigation on bubble behavior and its effect on the surface integrity of microchannels in three types of liquids and at different depths during LIPMM has been presented. Firstly, the formation mechanism of microbubbles was described. Secondly, a series of experiments were conducted to study the number and maximum diameter of the attached bubbles and the buoyancy movement of floating bubbles in the LIPMM of single-crystal silicon under deionized water, absolute ethyl alcohol, and 5.6 mol/L phosphoric acid solution with a liquid layer depth of 1–5 mm. It was revealed that the number and maximum diameter of attached bubbles in deionized water were the highest due to its high tension. Different from the continuous rising of bubbles at the tail of the microchannels in the other two liquids, microbubbles in 5.6 mol/L phosphoric acid solution with high viscosity rose intermittently, which formed a large area of bubble barrier to seriously affect the laser focus, resulting in a discontinuous microchannel with an unablated segment of 26.31 μm. When the depth of the liquid layer was 4 mm, absolute ethyl alcohol showed the advantages in narrow width (27.15 μm), large depth (16.5 μm), and uniform depth profile of the microchannel by LIPMM. This was because microbubbles in the anhydrous ethanol quickly and explosively spread towards the edge of the laser processing zone to reduce the bubble interference. This research contributes to a better understanding of the behavior and influence of bubbles in different liquid media and depths in LIPMM of single-crystal silicon.

Fabrication and Droplet Impact Performance of Superhydrophobic Ti6al4v Surface by Laser Induced Plasma Micro-Machining

Fabrication and Droplet Impact Performance of Superhydrophobic Ti6al4v Surface by Laser Induced Plasma Micro-Machining

Investigation on the evolution and distribution of plasma in magnetic field assisted laser-induced plasma micro-machining

Magnetically Controlled Laser-induced Plasma Micro-Machining (MC-LIPMM), compared to traditional direct laser ablation, has the ability of achieving better machining characteristics including larger aspect ratios of machined features, a smaller heat affected zone and more diverse micro-structures. To reveal the process mechanisms affected by the magnetic field, this paper studies the evolution and distribution of plasma in MC-LIPMM by analytical and experimental methods. An analysis of the confinement effect and enhanced collision of electrons-ions in the plasma in the presence of a magnetic field is performed for revealing the influence of the magnetic field on its diffusion and geometric size. To validate our theoretical predictions, experiments were conducted to analyze the effects of key process parameters on the geometrical size, shape and fluence of the plasma. Edge contour extracting and elliptic formula fitting algorithms were applied for quantitatively evaluating the changes. The experimental results show that transverse magnetic fields generate a larger plasma plume and change its shape from spherical to ellipsoidal compared to that without a magnetic field. The eccentricity of the ellipsoidal plasma exhibits a parabolic growth while the cross-sectional area of the fitted ellipse shows a linear growth as the transverse magnetic field strength increases. Greater growth in geometric size and intensity of the plasma was obtained under a longitudinal compared to a transverse magnetic field, and its shape changed into an elongated ellipsoid. Moreover, a two-dimensional magnetic field, combining both the transverse and the longitudinal magnetic field component, was shown to produce a larger area and intensity of spherical plasma under an appropriate transverse and longitudinal magnetic field strength compared to that without a magnetic field.

Surface microfabrication using coaxial waterjet assisted laser-induced plasma micromachining

In this study, a novel coaxial water assisted laser-induced plasma micromachining (CW-LIPM) method was studied to process microstructures on the workpiece surface. In CW-LIPM, a pulsed laser beam with a wavelength 532 nm was focused within a coaxial water jet, leading to the optical breakdown and the subsequent formation of the localized plasma plume. Compared with conventional laser processing, a cleaner surface could be acquired by CW-LIPM, while maintaining the machining resolution and processing accuracy. The mechanism of CW-LIPM and the effects of microbubble explosions on the surface roughness of the workpiece in proximity to the entrance of the processing area have been studied. With the developed experimental setup, the influence of single laser pulse energy, pulse repetition frequency, and water-jet velocity on the plasma distribution was also researched. Moreover, the effects of waterjet speed on the distribution of cavitation bubbles and the relative position on the machining quality of the processed microchannels were also studied, considering the explosions of the micro cavitation bubbles near the workpiece surface due to the pressure difference. Compared with laser micromachining in the air, CW-LIPM could process microstructures with no protrusions and better precision. High-quality surface microstructures could be obtained by combining the higher-velocity waterjet, the lower single pulse energy, multiple scanning passes, and the processing positions 2–3 mm down from the plasma center and 4–5 mm above the bottom. The laser-induced plasma distribution was asymmetric, and the lower half plasma was more suitable for precision processing due to its focused shape, which is preferred to enhance the localization of the proposed CW-LIPM process.

Investigation of the Capabilities of Transverse Magnetic Field Controlled Laser-Induced Plasma Micro-Machining

Abstract Laser-induced plasma micro-machining (LIPMM) has proven a number of advantages in micro-machining due to reduced thermal defects, smaller heat-affected zones, and larger aspect ratios when compared with conventional laser ablation. The present work explores the use of external magnetic fields to further enhance process outcomes in LIPMM. Specifically, machining characteristics and outcomes including plasma intensity, attainable aspect ratios, and surface quality will be explored through a theoretical and experimental study in different classes of materials in a transverse magnetic field controlled LIPMM. First, process improvement mechanisms are illustrated in terms of plasma confinement and laser absorption in transverse magnetic fields. A magnetic field redistribution analysis is performed to reveal the differences in the achievable enhancements in machining characteristics in terms of material characteristics. Second, a set of single-factor experiments is conducted to investigate the effects of the strength and direction of the magnetic field on machining capabilities in magnetic and nonmagnetic materials (410, 304 stainless steels and silicon). The experimental results show that plasma intensity and aspect ratios can be significantly increased in the presence of transverse magnetic fields. The greatest influence on machining capability is achieved in a magnetic material. In this case, plasma intensity and aspect ratios were increased by about 176% and 160%, respectively, when compared with other materials with a magnetic field strength of 0.1 T and a magnetic field direction parallel to the processing direction. Finally, the morphology and cross-section profiles of micro-channels have been measured for verifying the impact on the surface quality of transverse magnetically controlled LIPMM.

Simulation of Ultrashort Laser Pulse Absorption at the Water–Metal Interface in Laser-Induced Plasma Micromachining

Abstract This work proposes a physically consistent numerical model to simulate ultrashort laser absorption by a metallic workpiece at the water–metal interface when optical breakdown of the dielectric occurs. The simulation couples the framework of the finite difference time-domain method used in computational electromagnetics with the constitutive relation derived from both the model of direct ablation of metals and the first-order model of water breakdown. The simulation is used to describe interface ablation processes such as laser-induced plasma micromachining (LIPMM). Applied to the water–aluminum interface, the model is able to describe the metal absorption and the dielectric breakdown threshold in three-dimensional (3D) geometry. It is an extensible monolithic approach in which the absorption by different materials can be described by simply changing the constitutive relations.

Surface Morphology and Wall Angle Comparison of Microchannels Fabricated in Titanium Alloy Using Laser-Based Processes

Abstract The increase in the usage of titanium alloys for micro-engineering applications has driven the demand for improved micromanufacturing processes. Laser-based microfabrication processes such as direct laser ablation (DLA), laser-induced plasma micromachining (LIPMM), and magnetically controlled laser-induced plasma micromachining (MC-LIPMM) are promising technologies to fill this technological gap. In this paper, we evaluate microchannels fabricated in Ti6Al4V substrates using laser ablation, LIPMM, and MC-LIPMM. Scanning electron microscope (SEM) images and 3D scans of the channels were used to compare the surface morphology and channel geometry for different feed rates and number of laser passes. Wall angle measurements show that the LIPMM processes yield channels with steeper walls and smoother walls in comparison with the channels fabricated using direct ablation. The clear morphological differences on the surface finish of the walls made by direct ablation and using laser-induced plasmas hint at the differences in material removal mechanisms between these manufacturing methods.

Experimental investigation and optimization of laser induced plasma micromachining using flowing water

In this study, laser induced plasma micromachining using flowing water (F-LIPMM) was performed to create micro-channels on stainless steel surface. The effects of process parameters (water speed, laser pulse energy, frequency and scanning speed) on the responses of micro-channel width, depth, material removal rate (MRR) and heat affected zone (HAZ) were investigated based on response surface methodology (RSM). The regression models for the machined width, depth, MRR and HAZ were developed, and the adequacies of the developed models were subsequently verified by analysis of variance (ANOVA) method. Finally, grid structures consisting of desired micro-channels were created with the optimum process parameters: water speed of 8 m/s, pulse energy of 10.4 μJ, frequency of 15.8 kHz and scanning speed of 2.7 mm/s. The uniform micro-channels with smooth bottoms, side-walls and small HAZ were obtained, and the results showed that the predicted responses using the developed models were comparable with the experimental results.

Fabrication and cutting performance of bionic micro-serrated scalpels based on the miscanthus leaves

The purpose of this study is to improve the cutting performance of the scalpel by imitating the micro-structures of miscanthus leaves. Four different types of micro-serrated edges were designed, and these micro-serrations were then successfully processed on the edges of the scalpels by laser induced plasma micromachining (LIPMM). Subsequently, cutting tests were carried out to explore the effect of micro-serrations on the cutting performance. Compared to the unprocessed commercial scalpel, the micro-serrated scalpels could significantly reduce the cutting force and actual cutting depth. The type of blended serration without spacing scalpel had the best performance during cutting soft tissue due to the induced serration effect, which was also verified by the cutting process images and finite element simulation.

短/超短脉冲激光诱导等离子体微加工研究进展

短/超短脉冲激光诱导等离子体微加工研究进展

Comparative assessment of picosecond laser induced plasma micromachining using still and flowing water

Laser induced plasma micromachining (LIPMM) process is an effective approach to create micro-features with high aspect ratio and reduced heat affected zone. However, the throughput of this process is limited by the low scanning speed and low frequency used in the conventional underwater condition due to the disturbance of water layer, generation of bubbles and ablation debris. In this study, a new alternate process using flowing water was developed to provide a stable and thin water layer on the top workpiece surface. Thereafter, the influences of laser parameters like scanning speed, frequency and pulse energy on the machining characteristics between LIPMM using still and flowing water were experimentally investigated and comparatively evaluated. It was found that the influences of scanning speed, frequency and pulse energy on the machining characteristics between two processes were similar. However, LIPMM using flowing water created micro-channels with smaller heat affected zone, smoother surface, higher aspect ratio and higher MRR in comparison with that of using still water.