Picosecond Laser Drilling Research Articles

Fabrication and quality improvement of film cooling holes via picosecond laser drilling assisted by waterjet and post-electrochemical machining

Film cooling holes (FCHs) are of superior importance for blade protection and performance improvement in aircraft engine, while their fabrication requires high efficiency, high precision and extremely good surface quality, making it a challenging task. In this study, the ultrashort picosecond (ps) laser is employed to drill the FCHs on the DD6 single crystal superalloy, based on which the assisted waterjet is further introduced to reduce thermal damage, and the electrochemical machining (ECM) is also added as a post-processing manner for further quality improvement. It has been found through orthogonal tests that the pulse repetition frequency possessing most significant influence on hole taper during direct ps-laser drilling. The introduced low-pressure waterjet effectively eliminates the thermal damage such as slag attachment and recast layer, and the waterjet angle affects the hole profile obviously. After conducting post-ECM, the hole taper decreases continuously with an increase in processing time and applied voltage. In addition, the laser-induced periodic surface structure (LIPSS) has been observed on the sidewall, while an obvious decrease in LIPSS wavelength has been confirmed after post processing. Moreover, simulation study of temperature evolution is carried out by developing a two-temperature model, demonstrating that the lattice temperature surpasses the boiling point within 0.182 ps, and the ablation can be precisely localized within laser irradiation zone. The simulation of flow field on DD6 surface has also been conducted, and the distributions of flow velocity, pressure and thickness are obtained, which helps understand the effect of waterjet on the drilling process.

Picosecond laser drilling grids in aluminium foil at 532 and 355 nm wavelengths

Ultra-short laser drilling technique enables production of filters, membranes, microfluidic, photonic, and biomedical devices. Micro-hole grids were drilled in aluminum foil using a 28 picosecond Nd:YAG laser operated at wavelength of 532 and 355 nm with energy up to 18 and 12 mJ, respectively. Varying pulse energy and number, micro-holes obtained at 355 nm and 3.6–9 mJ had the appropriate mean diameter of 36–86 μm. However, the circularity changed in 0.5–0.94. For the 2nd harmonic the mean diameter varied 15–61 μm at 0.36–9 mJ with the circularity of 0.81–0.92. The morphology of the area surrounding micro-holes investigated by scanning electron microscopy exhibited distribution of droplets dominated size 0.9–10 μm, which is feasible for filtration of contaminated liquids. Local elemental composition of the area determined by energy dispersive spectroscopy discovered significant increase in the concentration of C and O, and also emergency of F, depending on laser fluence under vapor-dominated ablation mode, where nuclear reactions are excited in hot plasma.

Picosecond Laser Drilling of Carbon Fiber Reinforced Composite Laminates by Helical Drilling Process

Picosecond Laser Drilling of Carbon Fiber Reinforced Composite Laminates by Helical Drilling Process

Investigation on Thermal Barrier Coating Damages for Ultrafast Laser Drilling of Cooling Holes.

To mitigate the challenges pertaining to coating damage and processing defects arising from the utilization of ultrafast laser drilling for microhole creation in thermal barrier coatings (TBCs), thereby exerting substantial influence on the long-term durability of these microholes, the investigation proposes a comprehensive methodology. It encompasses the design of a two-factor four-level full factorial experiment and the execution of experimental research on picosecond laser drilling of TBC microholes. By meticulously analyzing the morphology of the microholes and the coating interface, the damage mechanisms associated with picosecond laser drilling of TBC microholes, as well as the influence of laser process parameters on coating damage, are revealed. The findings reveal that the optimal microhole entrance quality and the lowest roughness along the hole perimeter are attained at a laser power of 12 W and a scanning speed of 320 mm/s. Moreover, at a laser power of 30 W and a scanning speed of 320 mm/s, the minimal crack length on the blunt angle side of the hole and the highest machining quality are observed.

Micro-hole drilling of 2.5D C/SiC composite with picosecond laser: Numerical modeling and experimental validation on hole shape evolution

Micro-holes of 2.5D C/SiC composites produced by picosecond laser are widely used in aerospace, military, and automotive industries. An accurate numerical model is crucial to illustrate the formation mechanism of hole shape and ablation characteristics. This paper proposes a two-dimensional transient numerical model based on solid-liquid-gas coupling in micro-hole drilling of 2.5D C/SiC composites with a picosecond laser. In the stage of solid-liquid phase transition, original mass, energy, and momentum conservation equations are optimized by considering surface tension, gravity, and hydrostatic pressure. In the stage of liquid-gas phase change, the mass transfer during the material evaporation is considered and used as a source term to optimize the level set equation. The effectiveness of the proposed model is validated by picosecond laser micro-hole drilling experiments on 2.5D C/SiC composites. Compared with the existing research, the results show that the proposed can accurately depict hole shape evolution in picosecond laser drilling of 2.5D C/SiC composites, not only including entrance, exit diameter, and hole taper, but also the hole profile and its dynamic variation. In addition, the model effectively simulates the thermal effect phenomena, such as thermal damage, pressure concentration, and recast layer. And the thermal damage is found to be the main cause of the hole quality variation, which changes from severe to smooth along the hole entrance to exit. The research provides important support for high-quality micro-hole machining of 2.5D C/SiC composites with the picosecond laser.

Defocusing effect and energy absorption of plasma in picosecond laser drilling

Plasma is commonly existing in pulsed laser drilling and it has an important impact on drilling results. The effects include plasma absorption and defocusing effect caused by plasma. This paper studied the influence of plasma on laser drilling by a combination method of numerical simulation and experiment. Based on the combination of computational fluid dynamics (CFD), fluid–solid coupling theory, double-temperature model, C ++ programming and OpenFOAM simulation, this paper established and added the defocusing effect and energy absorption of plasma as sub-modules. Finally, this paper conducted a series of laser drilling experiments with different parameters such as repetition rate, laser power and pulse number. Comparison of the results of simulations and experiments were conducted to verify the accuracy of the improved model. The comparison results proved that plasma play a significant role in pulsed laser drilling and the accuracy of the modificated model is very high. There is another conclusion that the influence of defocusing effect on laser drilling is greater than that of energy absorption.

Experimental investigation on thermal effects in picosecond laser drilling of thermal barrier coated In718

Multilayer material drilling is an important technology in the machining of film cooling holes. In this paper, trepanning drilling of thermal barrier coated In718 with 532 nm picosecond laser was investigated with emphasis on thermal effects. We discussed the relationship between heat accumulation and processing parameters as well as the formation mechanism of resolidified layer. According to the experimental result, the repetition rate is the most significant factor to affect heat accumulation. By discussing the influence of repetition rate on plasma reheating and heat transfer-dissipation of the lattice, the formation mechanism of molten material accumulation around the entrance as well as the mechanism of the cylinder at the center of the hole was explained. Further study on defects of the side wall revealed the existence of a thin intermittent resolidified layer, which is quite different from the recast layer in long pulsed laser drilling. The difference in material removal mechanisms was the primary cause of this phenomenon. Discussion of this difference can provide references to the formation of defects as well as improve processing quality in the process of picosecond laser drilling on thermal barrier coated alloy.

Picosecond laser drilling of silicon with applied voltage

The short pulse duration of a picosecond laser (ps) leads to high peak power and ablates materials with much reduced thermal effects. Commercial ps lasers generate high-repetition-rate pulses and become more accepted as a major tool for material removal applications. However, most of the commercial ps lasers emit pulses at infrared (IR) wavelengths with photon energy close to the Si band gap energy of 1.1 eV. The corresponding optical absorption coefficient is low. To improve the laser beam absorption in Si, several methods have been investigated. For instance, it is reported that the material removal efficiency has been improved by raising the substrate temperatures during laser drilling due to the enhancement of the Si absorption coefficient. However, such approach may result in reduction in machining accuracy due to thermal expansion of the substrate. In this paper, we propose a new method by applying a direct current (DC) across a silicon substrate during the ps laser drilling process. The externally applied voltage potential would lead to more aligned movement of free electrons and therefore increase electrical current flow in the silicon substrate. The hypothesis of this study is that more free electrons are made available in the Si substrate for collisions with the laser photons, which increases the Si absorption coefficient of the laser beam. It was found that the material removal is markedly improved with the assistance of the electrical current flow. The entrance hole diameter increased by 14% and the exit hole diameter increased by 90% when the current in the Si substrate was subjected to a fixed current of 0.5 A. However, a larger amount of material debris covering an enlarged surface area was observed under the applied DC voltage. The possible reasons for such observations are discussed based on the enhanced laser energy absorption as the result of the presence of electrical current in the Si substrate.

Repetition Rate Effects in Picosecond Laser Microprocessing of Aluminum and Steel in Water.

Picosecond laser drilling was studied in the case of industrial steel and aluminum, which are difficult to microprocess by conventional methods. The dependence of hole morphology and dimensions on the pulse repetition rate and number of pulses in water and air were ascertained. For both materials, the diameter of the hole is larger in water than in air. In water, the diameter is larger at higher repetition rates than at lower ones, and increases with the number of pulses. In air, the hole diameter is not affected by the repetition rate, and remains constant from 100 to 100,000 pulses. Overall, material removal is more efficient in water than in air. The shape of the hole is generally more irregular in water, becoming more so as the number of pulses is increased. This is probably due to debris being trapped in the hole, since water flowing over the target surface cannot efficiently remove it. In aluminum, the depth of the hole is smaller at higher repetition rates. By scanning the beam over the aluminum target in water, the laser penetrates a 400-μm thick workpiece, generating a line with comparable widths at the entrance and exit surfaces.

Experimental investigation on low cycle fatigue of DZ125 with film cooling holes in different processes of laser drilling

Due to the different low cycle fatigue (LCF) properties and fatigue fracture behavior around film cooling holes on DZ125, the LCF tests are carried out using tension cycling under stress control conditions (stress ratio R=0.1) at 900°C. The specimens were designed as thin-wall plate with single hole and multi holes under picosecond and nanosecond laser drilling processes. Comparative analyses of the differences between fatigue life and microscopic fracture morphology are conducted. It is shown that under the same stress condition, the relationship between fatigue life is as follows: picosecond laser single-holed specimen>nanosecond laser single-holed specimens>picosecond laser multi-holed specimens>nanosecond laser multi-holed specimens. Scanning electron microscope (SEM) analyses of the fracture revealed that the crack initiates from the film cooling holes where fatigue source zone, fatigue crack propagation zone and fatigue fracture zone can be found. However, the different processes lead to slightly different fracture morphology: radial-type ridge centering on the fatigue source zone is more apparent and uniform in picosecond laser drilling specimens than in the nanosecond laser drilling ones. On the other hand, the radial-type ridge is biased toward large-aperture side with nanosecond laser drilling.

An investigation on the hole quality during picosecond laser helical drilling of stainless steel 304

Precision drilling with ultra-short pulse lasers (e.g., picosecond and femtosecond) has been advocated to significantly improve the quality of the micro-holes with reduced recast layer thickness and no heat-affected zone. However, a combination of high-power picosecond laser with helical drilling strategy in laser drilling has rarely been reported in previous studies. In the present study, a series of micro-holes with circular, triangular, rectangular, and rhombic shapes (diameter 0.6 mm) were manufactured on stainless steel 304 using a newly developed laser drilling system which incorporated a picosecond laser and a high-speed laser beam rotation apparatus into a five-axis positioning platform. The quality of the helical drilled holes, e.g., recast layer, micro-crack, circularity, and conicity, were evaluated using an optical microscope, an optical interferometer, and a scanning electron microscope. In addition, the microstructure of the samples was investigated following etching treatment. It was demonstrated that the entrance ends, the exit ends, and the side walls of the micro-holes were quite smooth without accumulation of spattering material and formation of recast layer and micro-crack. No tapering phenomenon was observed, and the circularity of the holes was fairly good. There was no distinctive difference with regard to the microstructure between the edges of the holes and the bulk material. Picosecond laser helical drilling can be an effective technique for manufacturing of micro-holes with very high quality. The development of high-power picosecond laser would promote picosecond laser drilling to be more industrial relevance in the future.

1206 Numerical Modeling of Picosecond Laser Drilling of Glass

We discuss the laser-processing conditions for creating deep holes on glass substrates and build a numerical model for picosecond laser drilling based on the energy absorption. We obtained numerical and experimental results that agreed with each other, revealing that the depth of the holes is increased by increasing the repetition rate. Furthermore, we revealed that when the repetition rate is too high, the generation of cracks prevents the precise creation of holes.

A Study of Picosecond-Laser Drilling Technique in Ceramics

In this paper, the technique of drilling process with picosecond(ps) laser is studiedinAl2O3 ceramics. The defocus method is used to drill the ceramic slice, which is able to improve thedrilling ratio of depth to diameter.It is found out that the best location ofthis ps-laser is about0.15mm under the lower surface of workpiece,demonstrating the difference from nanosecond(ns)-laser machinein the respective ofdefocusing.The impact of the power and the processing speed isinvestigated on the quality of the drilling. For ps-laser machine,a smaller and high qualitythrough-holeis achievable by increasing the power of laser and reducing the speed of process, withthe ratio of depth to diameter from 3.12 to 4.27.The ratio can be further increased upto 6.25,throughoptimization of these three parameters: defocusing amount, laser power and laser processing speed.

A Comparison in laser precision drilling of stainless steel 304 with nanosecond and picosecond laser pulses

Precision drilling with picosecond laser has been advocated to significantly improve the quality of micro-holes with reduced recast layer thickness and almost no heat affected zone. However, a detailed comparison between nanosecond and picosecond laser drilling techniques has rarely been reported in previous research. In the present study, a series of micro-holes are manufactured on stainless steel 304 using a nanosecond and a picosecond laser drilling system, respectively. The quality of the micro-holes, e.g., recast layer, micro-crack, circularity, and conicity, etc, is evaluated by employing an optical microscope, an optical interferometer, and a scanning electron microscope. Additionally, the micro-structure of the samples between the edges of the micro-holes and the parent material is compared following etching treatment. The researching results show that a great amount of spattering material accumulated at the entrance ends of the nanosecond laser drilled micro-holes. The formation of a recast layer with a thickness of ∼25 μm is detected on the side walls, associated with initiation of micro-cracks. Tapering phenomenon is also observed and the circularity of the micro-holes is rather poor. With regard to the micro-holes drilled by picosecond laser, the entrance ends, the exit ends, and the side walls are quite smooth without accumulation of spattering material, formation of recast layer and micro-cracks. The circularity of the micro-holes is fairly good without observation of tapering phenomenon. Furthermore, there is no obvious difference as for the micro-structure between the edges of the micro-holes and the parent material. This study proposes a picosecond laser helical drilling technique which can be used for effective manufacturing of high quality micro-holes.

Femtosecond and picosecond laser drilling of metals at high repetition rates and average powers

The influence of pulse duration on the laser drilling of metals at repetition rates of up to 1 MHz and average powers of up to 70 W has been experimentally investigated using an ytterbium-doped-fiber chirped-pulse amplification system with pulses from 800 fs to 19 ps. At a few hundred kilohertz particle shielding causes an increase in the number of pulses for breakthrough, depending on the pulse energy and duration. At higher repetition rates, the heat accumulation effect overbalances particle shielding, but significant melt ejection affects the hole quality. Using femtosecond pulses, heat accumulation starts at higher repetition rates, and the ablation efficiency is higher compared with picosecond pulses.