Adjacent Laser Pulses Research Articles

Acoustic emission monitoring and heat-affected zone evaluation of CFRP laser cutting

Acoustic emission (AE) technology can be used to analyze the mechanism and evaluate the size of the heat-affected zone (HAZ) during laser cutting carbon fiber-reinforced plastics (CFRP). A hardware acquisition platform and data collection system for collecting AE signals were first built. Experiments of 17 groups with different laser repetition rates and 13 groups with different galvanometer scanning speeds were carried out in CFRP laser processing. Through the spectrum analysis of fast Fourier transform, it can be observed that the frequency distribution of the AE signal was located only at the integer multiple of the laser repetition rate, and the maximum frequency of the AE signal was the product of the laser repetition rate and the number of AE oscillation cycles between two adjacent laser pulses; The critical state of laser ablation and denudation (plasma impact) was described by root-mean-square (RMS) characteristics of the AE signal. When the laser repetition rate is 50 kHz and the scanning speed is 2500 mm/s, laser ablation and denudation are both very intense, which is helpful to improve the processing efficiency; As the laser repetition rate increases, the RMS value of AE increases first and then decreases with the synchronization of the HAZ. By monitoring the RMS value, the value of HAZ can be obtained clearly.

Pulse picking of a fiber laser enables velocimetry of biomass-laden jets at low and ultra-high repetition rates

The introduction of fibrous aspherical biomasses for the substitution of coal as a sustainable solid fuel causes complex particle-turbulence interactions, which need to be captured using minimally invasive laser-based diagnostics. High-resolution two-phase particle image velocimetry (PIV) of high-velocity biomass-laden turbulent flows require laser inter-pulse times of only a few microseconds, which is usually achieved with separate laser systems for low-speed statistical and time-resolved measurements. In this work, we introduce a novel approach to achieve low-speed dual-pulse and ultra-high-speed time-resolved two-phase PIV using a single laser system. The flexible repetition rate laser output is realized by coupling a continuously pulsing fiber laser to an acousto-optic deflector (AOD) for pulse picking. The selective deflection of two adjacent laser pulses into the measurement volume is presented at a camera repetition rate of 25 Hz and laser repetition rate of 312.5 kHz resulting in an inter-pulse time of 3.2 µs. Temporal limitations of the AOD pulse picking system are evaluated beyond the access time limit for partially overlapping pulse pairs. To demonstrate the feasibility of the diagnostic system, low-speed measurements and additional ultra-high-speed measurements at 200 kHz are performed at high spatial resolutions to resolve individual biomass particles and their surrounding flow field in a non-reacting turbulent round jet. Simultaneous diffuse back-illumination measurements using a pulsed LED for particle size measurements of a high-aspect-ratio biomass (miscanthus) are conducted. The diagnostic system offers the capability of determining the slip velocity field and projected particle size, enabling a direct measurement of the local particle Reynolds number under high-velocity turbulent conditions at low and ultra-high repetition rates.

Non‐damage deep etching of SiC by hybrid laser‐high temperature chemical processing

AbstractSiC is considered as preferred material for micro‐electro‐mechanical system in the future. The excellent mechanical property and chemical stability make it difficult to perform deep etching. The hybrid laser‐high temperature chemical etching is investigated to realize non‐damage deep etching of SiC. The influences of defocus, laser pulse interval, laser intensity, and pulse number on etching depth are researched. The optimized laser parameter for SiC non‐damage deep etching is laser intensity of 10 × 109 W/cm2 with a pulse interval of 10 ms. In order to analyze the interaction mechanism, the temperature field and laser‐induced liquid jet in the liquid environment are calculated numerically. It is concluded that the material removal mechanism consists of laser heating vaporization during laser pulse, mechanical effect of laser‐induced liquid jet impact between two adjacent laser pulses and chemical etching in laser‐induced local high‐temperature environment. The chemical reaction between SiC and mixture of HF, and HNO3 solution produces gases and fluosilicic acid and effectively reduces the roughness of the modified layer making the surface smoother, and also removes the microcracks on the side wall of the etched region.

Effect of Laser Peening on the Mechanical Properties of Aluminum Alloys Probed by Synchrotron Radiation and X-Ray Free Electron Laser

Synchrotron radiation (SR) and X-ray free electron laser (XFEL) are indispensable tools not only for the exploration of science but also for the evolution of industry. We used SR and XFEL to elucidate the mechanism and the effects of laser peening without coating (LPwC) which enhances the durability of metallic materials. X-ray diffraction (XRD) employing SR revealed that the residual stress (RS) in the top surface became compressive as the laser pulse irradiation density increased with appropriate overlapping of adjacent laser pulses. SR-based computed tomography (CT) was used to nondestructively reconstruct three-dimensional (3D) images of fatigue cracks in aluminum alloy, revealing that LPwC retarded crack propagation on the surface and inside of the sample. SR-based computed laminography (CL) was applied to friction stir welded (FSWed) aluminum alloy plates to visualize fatigue cracks propagating along the welds. The fatigue crack had complicated shape; however, it became a semi-ellipsoid once projected onto a plane perpendicular to the fatigue loading direction. Ultra-fast XRD using an XFEL was conducted to investigate the dynamic response of aluminum alloy to an impulsive pressure wave simulating the LPwC condition. The diffraction pattern changed from spotty to smooth, implying grain refinement or subgrain formation. Shifts in diffraction angles were also observed, coinciding with the pressure history of laser irradiation. The durations of the dynamic phenomena were less than 1 µs; it may be possible to use high-repetition lasers at frequencies greater than kHz to reduce LPwC processing times.

Improvement of focused ultrasonic beam generated by laser phased array: Theoretical analysis

To improve the focused ultrasonic beam induced by laser phased array (LPA), the superiority of an improved LPA distribution with conjunction of geometric attenuation and directivity functions of the stimulated ultrasonic beams are investigated theoretically instead of only considering the directivity function. Numerical simulations for the generation of focused longitudinal waves in the thermoelastic regime were implemented to reveal the advantages of the improved LPA design. It is shown that the amplitude of the focused beam increased by 42.1%, and the rise time reduced by 25.0%, as well as spatial sizes narrowed by 50.6% and 41.9% in longitudinal and transverse directions, respectively. In addition, the thermal expansion superposition caused by adjacent laser pulses, which deteriorates the focusing features of the focused beam, should be avoided by setting proper spacing of LPA.

A universal theoretical model for thermal integration in materials during repetitive pulsed laser-based processing

In laser-based techniques such as laser processing, knowledge of the thermal integration process and temperature distribution in materials is the basis for process parameters optimization and product quality control. This work presents a universal theoretical model to study the repetitive pulsed laser-induced thermal integration in materials. Using the Green function method, the analytical solution formula for temperature field induced by laser pulses is mathematically deduced based on the Fourier heat transfer theory. Effects of two key parameters of laser pulses, i.e. the pulse spacing to pulse width ratio (tc/th) and the intensity ratio (I/I0) on thermal integration are studied. Results reveal that for a given I/I0, thermal integration is mainly controlled by the tc/th ratio, and the peak temperature difference induced by adjacent laser pulses drops exponentially as the tc/th ratio (<25) increases linearly. For a given cooling period (tc/th=1), the peak temperature difference induced by adjacent laser pulses changes linearly with the ratio of I/I0. Therefore, temperature distribution in target materials can be tuned by adjusting I/I0 or tc/th of laser pulses. Furthermore, the analytical solution formula is applied to model temperature distribution in SiGe thin solid films and fused silica irradiated by pulsed laser.

Laser Assisted Patterning of a-Si:H: Detailed Investigation of Laser Damage

A detailed investigation of the laser damage to amorphous silicon (a-Si:H) layers patterned by laser ablation (LA) and wet chemical etching is presented. This approach can be applied to pattern the rear side of silicon heterojunction interdigitated back-contact solar cells. Only the top sacrificial a-Si:H laser-absorbing layer of an a-Si:H/SiOx/a-Si:H/c-Si stack is ablated. Laser damage in the bottom a-Si:H layer and a-Si:H/c-Si interface is analyzed by both scanning electron microscopy and transmission electron microscopy. We show that the a-Si:H/c-Si passivation is degraded by laser damage and that this degradation can be diminished by increasing laser processing speed. This is attributed to a decrease of laser-irradiated area, and particularly smaller overlapping zones of adjacent laser pulses. The re-passivation quality after LA and wet etching is similar to that of as-passivated samples. This indicates that laser damage is not present in the bulk c-Si substrate but only in the a-Si:H passivation layer, which is removed during subsequent wet etching, thus allowing high quality repassivation.

Fabrication of Micro-Scale Gratings by Nanosecond Laser and Its Applications for Deformation Measurements

This paper experimentally investigated the fabrication and optimization of micro-scale gratings formed by nanosecond laser etching. The mechanism of nanosecond laser processing and the geometric phase analysis (GPA) are discussed, and the factors influencing the fabrication process including laser energy, laser fluence, and ablation threshold of material, are experimentally studied. In order to eliminate the dependence of the processing parameters on the samples, depositing Al film on a sample before laser processing is proposed for the fabrication of high-quality gratings. The energy of the laser pulse is optimized for clear line etching on Al film considering the distance between adjacent lines of parallel gratings. The optimal energy of the laser pulse is 9.8 μJ, and the optimum fluence is 9.5 J/mm2 with the waist radius of the laser beam 25.7 μm. With the optimal parameters, experimental results indicate that the highest frequency of parallel gratings is about 30 lines/mm, with a line width of 29 μm, and the distance between two adjacent laser pulses being of 10 μm. By performing tensile tests, micro-scale gratings fabricated on specimens are experimentally verified. The verification tests prove that the proposed fabrication method for the micro-scale gratings in GPA measurements is reliable and applicable, and the micro-scale gratings can be fabricated in many areas of interest, such as the crack tip, for deformation measurements. Furthermore, the adhesion between the Al film and the tested sample is strong enough so that the pattern sticks well to the sample.

Modeling of the material removal and heat affected zone formation in CFRP short pulsed laser processing

Short pulsed laser milling is a novel method to process the carbon fiber reinforced plastics (CFRP) which has bad machinability. This paper presents a numerical model studying the material removal mechanism of CFRP laser milling. It is confirmed by both the experiment and the simulation that laser ablation and mechanical erosion caused by the polymer pyrolysis are all involved in the material removal. Because the heating and cooling rate in short pulsed laser milling is high, ablation of two adjacent laser pulses almost has little influence on each other. By conducting the parametric analysis, it was found that the spacing distance under which the matrix between two adjacent laser pulses was completely degraded should be adopted to utilize the mechanical erosion effectively. Laser milling experiments of CFRP laminates were performed using a nanosecond pulsed laser system. The established model could predict the average ablation depth per scanning pass at an optimal spacing distance.

Tilting of adjacent laser-induced liquid jets

The effects of time delay and spatial separation between two adjacent laser pulses in blister-actuated laser-induced forward transfer are studied experimentally and computationally. Each laser pulse creates a blister that expands into a liquid film, forming liquid jets to transfer material from a donor substrate to an acceptor substrate. For a fixed separation between the two laser pulses, time-resolved imaging reveals a tilting of the second liquid jet toward or away from the first jet, depending on the time delay between pulses. Simulations of the same process reveal that the first jet perturbs the ink−air interface around it, and the initial angle of the liquid−air interface below the second laser pulse is shown to be a good predictor of the angle of the second jet. The time evolution of the initial interface angle at a fixed separation is then investigated analytically in terms of a viscously damped cylindrical capillary wave, initiating once the jet retracts or pinches off. This two-jet setup can be considered as a model system for high repetition rate printing, so these results reveal limits on the repetition rate and separation between pulses for LIFT such that materials are printed in desired patterns.

Single step channeling in glass interior by femtosecond laser

Channeling inside a transparent material, glass, by femtosecond laser was performed by using a single step process rather than hybrid processes that combine the laser irradiation with an additional tool or step to remove the material. Tightly focusing of a single femtosecond laser pulse using proper optical and laser processing parameters could induce the micro-explosion and could create voids inside transparent materials, and the effects of these parameters on the resultant feature geometry and channel length were studied. Understanding of the channel length variation at different locations from the specimen surface could enhance prediction capability. Taking into account of the laser, material, and lens properties, numerical models were developed to predict the absorption volume shape and size at different focusing depths below the surface of a specimen. These models will also be validated with the variation in feature and channel lengths inside the specimen obtained from the experiments. Spacing between adjacent laser pulses and laser parameters was varied to investigate effects of channel overlapping and its influence on long channel formation.

Coded excitation for photoacoustic imaging using a high-speed diode laser

A Q-switched Nd:YAG laser providing nanosecond pulse durations and millijoule pulse energies is suitable for typical biomedical PA applications. However, such lasers are both bulky and expensive. An alternative method is to use a diode laser, which can achieve a higher pulse repetition frequency. Although the energy from a diode laser is generally too low for effective PA generation, this can be remedied by using high-speed coded laser pulses, with the signal intensity of the received signal being enhanced by pulse compression. In this study we tested a version of this method that employs coded excitation. A 20-MHz PA transducer was used for backward-mode PA detection. A frequency-coded PA signal was generated by tuning the interval between two adjacent laser pulses. The experimental results showed that this methodology improved the signal-to-noise ratio of the decoded PA signal by up to 19.3 dB, although high range side lobes were also present. These side lobes could be reduced by optimizing the compression filter. In contrast to the Golay codes proposed in the literature, the proposed coded excitation requires only a single stimulus.

Thermal analysis for graphitization and ablation depths of diamond films

In the present study, a thick diamond film deposited on a 6″ silicon wafer was planarized by an ArF excimer laser. In order to predict the graphitization thickness formed in the diamond film by a laser beam moving with a sliding velocity as well as a fluence, the general solution of the three-dimensional temperature distribution is successfully developed to be a function of the fluence and pulse duration of the laser beam. A reported model developed for the graphitization probability of diamond film is adopted in the present study to determine the graphitization thickness if the temperature solutions are substituted into this model. The ablation thickness in the graphitization layer can be determined so long as the local temperature in the specimen is higher than the ablation temperature of the graphitization layer. Then, the thickness of graphite residual on the working surface after ablation can be obtained by the subtraction of the ablation thickness from the graphitization thickness. This graphite thickness is proved to be very close to the value obtained from scratching tests. This implies that the temperature solutions predicted by the present model are trustworthy. The effect of the scanning velocity is insignificant to the surface temperature if the pulse duration time is much shorter than the time period between two adjacent laser pulses.