Ultrashort Pulse Laser Drilling Research Articles

Improving the fatigue property of diffusive film cooling holes in nickel-based single crystal superalloy via ultrashort pulse laser drilling coupled with abrasive flow machining

A novel hole-making technique, which combines ultrashort pulse (UP) laser drilling with abrasive flow machining (AFM), has been developed to enhance the surface quality and fatigue resistance of diffusive holes in nickel-based single crystal (NBSC) superalloys. This study conducted a comprehensive analysis of the surface morphology and metallurgical characteristics of the hole wall, evaluated by fatigue testing at elevated temperatures and fractography analysis. The findings demonstrate that AFM can effectively eliminate the solidified debris generated during UP laser drilling, significantly reducing surface roughness and inducing a rounded effect at the outlet acute zone of the diffusive hole. Such improvements have been shown to increase the fatigue life of the holes by up to 50.6 % compared to those without polish. Furthermore, the crystal plasticity finite element method (CPFEM) was employed to investigate the localized stress concentration and the accumulation of plastic slip around the diffusive hole, elucidating the mechanisms behind fatigue failure in NBSC superalloys. The study also discusses the influence of the different hole-making technology on the fatigue properties of diffusive holes, integrating CPFEM results with analyses of surface quality and fatigue fractography. The study conducted in this study can provide valuable guidance for the fabrication of diffusive holes in turbine blades. Furthermore, it can also improve our understanding of the fatigue failure mechanism of diffusive holes in NBSC superalloy.

Surface integrity of drilling Ti-6Al-4V micro-holes using the ultrashort pulse laser with different three-dimensional paths planning

The improvement of aero-engine performance posed a challenge to the heat dissipation ability of blades, resulting in a substantial increase in the demand for micron-sized (≤1 mm) film cooling holes. In this paper, an improved three-dimensional path machining method for ultrashort pulse laser drilling of Ti-6Al-4V titanium alloy is studied. The influence of four representative paths that are contour offset path, broken line path, raster path and jagged path on hole quality is described and discussed in detail. By establishing a three-dimensional two-temperature model in cylindrical coordinates, the defocusing control of the machining process is carried out, and the temperature field distribution with different drilling paths is simulated and analyzed. Finally, the experimental study of drilling round holes with a diameter of 800 μm is carried out on the titanium alloy Ti-6Al-4V plate with a thickness of 1 mm, and the wear resistance of the workpieces is tested. The experimental results show that, compared with the other three paths, the temperature field uniformity of laser processing with the contour offset path is the best, the quality of the processed holes is significantly improved, the outlet roundness is increased by 26.54 %–36.59 %, the cone angle is reduced by 12.7° and the wear resistance is increased by >1.3 times. Moreover, there is no obvious recast layer and micro cracks on the pore wall, and dense nano oxide particles are produced on the surface. In summary, the contour offset path controlled by three-dimensional defocusing is an effective hole processing path, which provides the possibility for machining efficient and high-quality film cooling holes.

A deep learning-based predictive simulator for the optimization of ultrashort pulse laser drilling

Ultrashort pulse laser drilling is a promising method for the fabrication of microchannels in dielectric materials. Due to the complexity of the process, there is a strong demand for numerical models (simulators) that can predict structures produced under specific processing conditions in order to rapidly find optimal processing parameters. However, the validity of conventional laser drilling simulators for dielectrics has been confined to a range of strict interpolations of the data used during the construction of the model, and thus, their usefulness is limited. Here, we demonstrate simulator-based optimization for ultrashort pulse laser drilling in dielectrics based on an iterative deep neural network which is trained to predict microchannel structure after a small number of irradiated pulses. Our approach predicts the development of hole shapes over a wide variety of conditions and allowed the discovery of 20% more energy efficient processing strategies than in the initial experimental data. More broadly, our approach can address realistic problems in laser processing considering a variety of processing parameters, and thus enabling improved performance of next-generation smart laser processing systems.

Observation of damage generation induced by electron excitation and stress wave propagation during ultrashort pulse laser drilling of sapphire

Observation of damage generation induced by electron excitation and stress wave propagation during ultrashort pulse laser drilling of sapphire

High-speed observation of damage generation during ultrashort pulse laser drilling of sapphire

Ultrashort pulse laser processing attracts attention as a microfabrication method for transparent materials such as sapphire. However, during processing, numerous damages are formed around the processed area, which inhibits precision processing. In this study, to elucidate the damage generation mechanism, we combined a pump–probe imaging method with a high-speed camera and investigated the relationship between high-speed phenomena during processing and damage generation. Using the imaging system, a simultaneous observation of ultrafast phenomena on a timescale of picoseconds to nanoseconds and relatively slow phenomena on a timescale of milliseconds was achieved with a high spatial resolution, and the results clarify that stress wave propagation caused damage generation. In addition, the results showed that as the number of pulses increased, both positions of the stress wave generation and damage generation changed. Furthermore, we evaluated the pulse width dependence and demonstrated that the heat affected the damage generation when the pulse width was long. The damage generation mechanism clarified in this study will contribute to the development of methods for the precision processing of sapphire.

High-speed observation of stress wave propagation and damage generation during ultrashort pulse laser drilling of sapphire

High-speed observation of stress wave propagation and damage generation during ultrashort pulse laser drilling of sapphire

High-speed observation of pulse energy and pulse width dependences of damage generation in SiC during ultrashort pulse laser drilling

Ultrashort pulse laser processing is a potential method for microfabricating silicon carbide (SiC). However, it is difficult to precisely process SiC using this method because numerous damages are generated around the processed region. Although various studies have investigated the optimal processing conditions, the effects of the conditions on the high-speed phenomena during processing and the effects of these phenomena on the processed shapes and generated damages remain largely unclear. In this study, we combine a high-speed camera with ultrashort laser probe pulses to visualize the phenomena driven by each pulse irradiation and investigate the pulse energy (50, 100, 200 μJ) and pulse width (180 fs, 1 ps, 10 ps) dependencies. The results clarify that for larger pulse energy, stronger stress waves propagate inside SiC, causing greater damages. In addition, we find that for a pulse width of 10 ps, thermal damage occurs near the hole entrance. The clarified effects of the processing conditions on the stress wave propagation, damage generation, and processed shapes will contribute significantly to developing methods for precision processing of SiC.

Simulation of ultrashort pulse laser drilling of glass considering heat accumulation.

In accordance with the increasing demand for high-speed processing, the repetition rate of ultrashort pulse lasers has continued to increase. With the development of these lasers, there is a growing demand for the prediction of shapes processed at high repetition rates. However, the prediction of these shapes is a major challenge, because of the difficulty associated with the estimation of heat accumulation. In this study, we developed a simulation of ultrashort laser drilling in glass including heat accumulation calculation between pulses. In this simulation model, temperature is considered as an additional criterion of material removal, thus, the dependency of the repetition rate can be estimated. Two model parameters of laser absorption at high temperatures are investigated and determined by experiments under high environmental temperatures. Using the simulation model, high shape-prediction accuracy at high repetition rates was achieved and validated by comparison with experiments. This study may contribute to broadening the applications of high-repetition-rate ultrashort pulse lasers.

Analysis of energy consumption in micro-drilling processes

The paper analyzes energy consumption as a performance indicator for micro-manufacturing. A comparative analysis between micro-EDM and ultrashort pulse laser drilling is performed on the basis of similar process outputs like drilled thickness of 350 μm, hole diameter of 180 ± 3 μm, drilling time, micro-hole shape and inner surface quality. Experimental data revealed that ultrashort pulse laser drilling enabled root mean squared surface roughness of about 100 nm in a drilling time of 2.5 s, whereas using a conventional micro-EDM drilling operation with a 10 s duration yielded surface roughness of 380 nm.For ultrashort pulse laser ablation the specific energy consumption is increased, as the drilling time is decreased due to an increase of the adopted pulse energy up to 50 μJ but the surface roughness and the capability of obtaining very sharp edges remain unchanged. With a calculated energy intensity in the order of 1010 J/kg and a process rate in the order of 10−6 kg/h, ultrashort pulse laser drilling can be considered as a valid alternative from both technology point of view and specific energy consumption to similar drilling process, like micro-EDM which is characterized by a lower process rate and a higher energy intensity.

Effect of pulse to pulse interactions on ultra-short pulse laser drilling of steel with repetition rates up to 10 MHz

We report on the effect of pulse to pulse interactions during percussion drilling of steel using high power ps-laser radiation with repetition rates of up to 10 MHz and high average powers up to 80 W. The ablation rate per pulse is measured as a function of the pulse repetition rate for four fluences ranging from 500 mJ/cm2 up to 1500 mJ/cm2. For every investigated fluence an abrupt increase of the ablation rate per pulse is observed at a distinctive repetition rate. The onset repetition rate for this effect is strongly dependent on the applied pulse fluence. The origin of the increase of the ablation rate is attributed to the emergence of a melt based ablation processes, as Laser Scanning Microscopy (LSM) images show the occurrence of melt ejected material surrounding the drilling holes. A semi empirical model based on classical heat conduction including heat accumulation as well as pulse-particle interactions is applied to enable quantitative conclusions on the origin of the observed data. In agreement with previous studies, the acquired data confirm the relevance of these two effects for the fundamental description of materials processing with ultra-short pulsed laser radiation at high repetition rates and high average power.

In situ imaging of hole shape evolution in ultrashort pulse laser drilling

For the first time, in situ the hole shape evolution during ultrashort pulse laser drilling in semiconductor material is imaged. The trans-illumination of the sample at a wavelength of 1.06 µm is projected onto a standard CCD camera during the ablation, providing an image of the contour of the ablated structure perpendicular to the irradiation for drilling. This demonstrated technique enables a direct, high resolution investigation of the temporal evolution of the drilling process in the depth of the material without complex sample preparation or post processing.

Influence of electrostatic fields and laser-induced discharges on ultrashort laser pulse drilling of copper

We evaluated the application-oriented suitability of electrostatic fields and laser-induced discharges to enhance both debris removal and ablation rate during ultrashort pulse laser drilling of copper. The electric field was varied to either induce or prevent laser-induced electric discharges. Results show that debris is expelled to a much greater distance by applying an electric field. If a laser-induced electric discharge is induced, greater heat-affected zone and greater amounts of debris are observed, pointing to an increased amount of ablated material. Results were independent on the applied voltage polarity, suggesting that there is no significant charge separation in the material plasma.

Ultrashort Pulsed Laser Machining for Biomolecule Trapping

Ultrashort pulse laser drilling of polycarbonate track-etched membrane (pTEM) material was used to fabricate a mouse embryo cell trapping device. Holes with a diameter of <TEX>$2{\mu}m$</TEX> to <TEX>$5{\mu}m$</TEX> were fabricated on a <TEX>$10{\mu}m$</TEX> thick membrane using a femtosecond laser with a 150 fs pulse width and 775 nm wavelength and multiple-pulse irradiation. In cell trapping tests, the overall cell occupancy of the machined holes in the fabricated pTEM was found to be more than 80%. The results of a single pulse and multiple pulse irradiation were compared in terms of the surface quality. It was generally found that a single pulse with high energy was less desirable than irradiation with multiple pulses of lower energy.

The precision of the femtosecond-pulse laser ablation of TiN films on silicon

Ti:sapphire laser pulses of 130 fs and 800 nm were focused on 3.2-μm-thick TiN films by a 60-mm focal length lens in air. The morphology of the ablated areas generated by laser pulses at a fluence slightly above the ablation threshold was characterized in dependence on the pulse number by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The cavity profiles, depths, diameters, and volumes were quantitatively evaluated by AFM. The polarization state of the laser light is discussed as a further parameter, in addition to fluence and pulse number, that influences and controls the ablation precision of these materials. It was observed that circularly polarized radiation enhances the average ablation rates and reduces the roughness in the cavities by a factor of 2–3 as compared to linearly polarized radiation of the same incident laser fluence. Special attention was paid to the interfacial region between the coating and substrate. Ultrashort-pulse laser drilling into the Si substrate revealed the generation of columnar features which even may surmount the original coating under laser conditions.

Material effects in ultra-short pulse laser drilling of metals

Although hole drilling using ultra-short laser pulses has been shown to produce holes of excellent quality, we have observed that during the evolution of the hole, the morphology of the hole bottom has a dependence on fluence, number of shots, and polarization. We describe the nature of this structure within the hole and under what conditions it is observed.

Polarization effects in ultrashort-pulse laser drilling

A strong influence of the polarization of the laser radiation on the geometry of laser-machined microdrillings has been observed for ultrashort pulses. For drillings with a certain aspect ratio, reflections at the hole walls take place, leading to a non-uniform intensity distribution deep inside the formed hole. Experimental and theoretical results on this subject are discussed. It is shown that a rotation of the polarization during the drilling process (“polarization trepanning”) significantly improves the quality of the produced holes.