Laser Focal Point Research Articles

Study on powder dynamics of ultrahigh-speed laser cladding

In view of the key technical problems of ultrahigh-speed laser cladding, this paper has studied the influence of different laser power, powder feeding rate, and carrier gas velocity on the formation of the cladding layer. A theoretical model of the powder flow field in the laser cladding process has been established. An energy attenuation algorithm based on the powder shading ratio has been designed, and the particle thermal field has been analyzed. We verified their feasibility with four radially symmetrical powder flow nozzles and extended them to the annular cladding nozzles. It is found that the powder spot temperature can be increased by increasing the laser power and setting powder focus point 0–2 mm above the laser focus point, and the utilization of laser energy in powder can be improved by reducing the particle size and increasing the powder feeding rate and carrier gas velocity. The results provide theoretical references for the application of ultrahigh-speed laser cladding on the surface strengthening of shaft components.

Laser ablation ignition of flammable gas

To reduce the minimum pulse energy required for laser ignition (MPE), thereby reducing the cost of the laser device and realizing multipoint ignition with the same laser energy, the ignition mechanism flammable gas using laser ablation was investigated by varying the distance between the target and the focusing lens, together with schlieren and direct color emission observation, which reveals the behavior of the ablative target material and initial flame kernel. It was found that the MPE in the ablative ignition was consistent with the ablation threshold energy of laser fluence on the target, except when the laser focal point was near the target surface. In this latter case, vortex flow induced by laser breakdown and successive collision of the initial flame kernel with the target was observed. The increase in MPE, in this case, was explained by heat loss in the initial flame kernel.

Optical Tweezer-Based Microrheology of Axonemal Dynein-Based Active Material Systems using Large Beads Exhibit Multiple Nonlinearities not Found in Measurements Made with Smaller Beads

Active matter models and microrheology-based characterization techniques collectively quantify active stresses generated by out-of-equilibrium materials and polymer networks in cellular biology and tissue engineering. Our optical tweezer-based microrheological measurements of reconstituted axonemal dynein-microtubule active networks made with beads 5 to 10-fold larger than the laser focal point indicated that nonlinearities and sensitivities in both bead-detection and force-bead displacement relationships violated assumptions of linearity inherent in standard optical tweezer-based microrheological measurements.

Principle and progress of attosecond pulse generation

<p indent=0mm>With the rapid development of laser technology, the vision of attosecond world has been unfolded. Since the first isolated attosecond pulses produced by high-order harmonic generation in 2001, the time-resolved spectroscopy has entered the attosecond domain and greatly strengthened our understanding of ultrafast electron dynamics in atoms, molecules and condensed matter. Since it approaches the time scale of inner shell electron motion, attosecond pulses become one of the most important tools to explore the electron dynamics. Therefore, developing techniques of attosecond pulse generation is not only essential in optical research field but also imperiously demanded in atomic and molecular physics, which has attracted great attention of researchers worldwide. After years of continuous innovation and breakthroughs, the shortest pulse of 43 attosecond has been achieved. Comparing with the ultrafast laser pulse generation techniques in picosecond or femtosecond regions, the generation of attosecond pulses is rather complex and challenging. From high-order harmonic generation to single attosecond pulse characterization, several crucial technical details need to be considered and investigated carefully, while attosecond gating is a vital link in the chain of technics required by attosecond pulse generation. Up to now, various gating methods have been developed, such as amplitude gating, which, with spectral filtering, takes advantage of the continuous spectrum near the cut-off. In addition, new gating techniques, such as polarization gating, double optical gating, interference polarization gating, polarization-assisted amplitude gating, and phase-matched time gating, also have been proposed or demonstrated to generate isolated attosecond pulses. Recently, spatial gating, which takes advantage of the variation on the driving laser wave front to produce isolated attosecond pulses, has also been developed. Meanwhile, as the intensity at a femtosecond laser focal point now easily exceeds <sc>10¹⁶ W/cm²,</sc> attosecond pulse generation via laser-plasma interaction has been investigated more intensively. Combined with atto-chirp compensation techniques, these methods are pushing the frontier of ultrafast optics toward shorter and stronger attosecond pulses, and achieving even better time resolution in a broader range of applications. In this article, starting with a general introduction of the principle of attosecond pulse generation, we review various gating techniques for attosecond pulse generation in gas targets, new development of attosecond pulses generated by laser-plasma interaction, as well as atto-chirp compensation for even shorter pulses, which are all crucial for the advancement of attosecond science.

Control of Laser Focal Point by Using an Electrically Tunable Lens in Laser-induced Plasma Optical Emission Spectrometry

Control of Laser Focal Point by Using an Electrically Tunable Lens in Laser-induced Plasma Optical Emission Spectrometry

Manufacturing of anti-fogging super-hydrophilic microstructures on glass by nanosecond laser

In this paper, a nanosecond laser processing method with the support of light-absorption auxiliary materials was developed to fabricate anti-fogging super-hydrophilic microstructures on glass substrate surfaces. Through adjusting the focal point offset, the laser was focused on the auxiliary material layer, thus the laser energy required for micromachining was precisely controlled. As a processing example, a bionic honeycomb structure was successfully manufactured by this method. The effects of laser processing parameters on the size and integrity of the fabricated microstructures and on the light transmission of the machined surface were investigated through laser machining experiment. The results indicate that lower power and frequency is the key to obtaining regular honeycomb structures in laser machining. The laser focal point significantly affects the light transmittance of glass, while the feed rate has little effect. The water droplet contact angle was measured to evaluate the hydrophilicity of glass specimens with different dimensions of microstructure. It was found that the contact angle decreased with reduction of the honeycomb structure size.

Feasibility of laser induced jets in needle free jet injections.

This paper reports a detailed characterization of laser-induced micro-jets, including ex vivo experiments using skin as a target substrate to check the feasibility in terms of needle-free jet injection. The actuation technique comprised a Nd-YAG based laser system to superheat the liquid locally, which creates a pressure wave actuating the motion of the liquid as a jet. Typical jet speeds were ~O(102) m/s and diameters ~O(101-102) μm and Re ~O(102-104). We studied the effect of various system parameters related to both geometry (e.g., capillary diameter, laser focal point) and fluid properties (e.g., viscosity). To advance the understanding of liquid delivery into tissue, transient penetration dynamics and dispersion patterns were studied. Stand-off distance, mechanical strength of the proxy gel, and laser pulse energy were also found to affect the penetration of liquid jets into the skin models.

Two-dimensional thermal finite element model of directed energy deposition: Matching melt pool temperature profile with pyrometer measurement

An open source two-dimensional (2D) thermal finite element (FE) model of the Directed Energy Deposition (DED) process is developed using the Python-based FEniCS framework. The model incrementally deposits material ahead of the laser focus point according to the geometry of the part. The laser heat energy is supplied by a Gaussian-distributed heat source while the phase change is represented by increased heat capacity around the solidus-liquidus temperature range. Experimental validation of the numerical model is performed by matching with the melt pool temperature measurements taken by a dual wavelength pyrometer during the build process of a box-shaped Ti--6Al--4V part with large geometrical voids. Effects of large geometrical voids on the melt pool shape and maximum melt pool temperature are examined. Both the numerical and experimental data show an increase in the melt pool size and temperature during deposition above large voids. The trailing edge of the melt pool's temperature profile obtained using the developed numerical model closely matches pyrometer measurements.

Pulsed mode selective laser melting of porous structures: Structural and thermophysical characterization

In this paper, the potential of selective laser melting (SLM) of stainless steel CL 20ES powder was investigated with a focus on controlled fabrication of porous structures with strongly reduced pore sizes, i.e. feature sizes significantly below conventional minimum SLM feature sizes. By controlling laser scan properties interacting with the powder bed directly, porous structures can be generated by selectively sintering powder particles. A wide range of porous samples was manufactured following this strategy, aiming to increase porosity while keeping pore sizes low. The effect of process parameters, including laser power and focal point positioning, was evaluated for a fibre laser operated in pulsed wave (PW) emission mode. The first part of this study focuses on characterization of key porous structure properties, i.e., porosity, average mass density, average pore sizes and structures at microscopic scales. The second part deals with the influence of porosity and pore sizes on thermal and fluid properties, i.e., the effective thermal conductivity (ETC) and wettability. We have quantified the directional dependence (build direction plane and scan direction plane) off the structural and thermophysical properties of porous structures. For a range of porosities and pore sizes, we have observed that porosity and surface morphology influence the thermal properties and contact angle of droplets on the printed surface. Thermal conductivity was measured and the associated analysis was compared with available models and correlations in literature. The average thermal conductivity of fabricated porous structures was determined between 6−14 W/m K and found to be a function of porosity. Furthermore, the capillary wicking performance of additively manufactured stainless steel porous structures having an average pore radius from 9 to 23 μm was determined.

An Analysis of the Mechanical and Metallurgical Behavior of AISI 4130 Steel After Co2 Laser Beam Welding Process

Alloy metal has received special attention in the aerospace and defense areas. The AISI 4130 alloy steel had been also considered, since it is applied in landing gears, small aircrafts engine cradles, and besides general industries. The Laser Beam Welding of high strength metals obtained small Weld Zone and better quality with good appearance. In this research work, a Laser Beam Welding (LBW) is used to weld AISI 4130. The experiments are conducted accordingly combination of Taguchi L25 based 5 levels of Laser Power, Speed, Angle, Focal Length and Focal Point Position. The AISI 4130 weld joint Bead Width and tensile strength are measured and analysed by ANOVA. Microstructure and SEM with EDAX are using to analysis the AISI 4130 weld joint.

Second harmonic generation in zinc sulfide by femtosecond laser irradiation

Second harmonic generation (SHG) has been observed from zinc sulfide (ZnS) under linearly polarized and circularly polarized femtosecond laser excitations. The output power of SHG signal at 513 nm exhibits a quadratic relationship to the input power of femtosecond laser at 1026 nm, which can figure out the effective SHG response of 2.12 pm/V in ZnS. Additionally, the total, vertical, and horizontal SHG signals all demonstrate well defined cosine-like curves with approximate periods of 40°, indicating a textured crystallized material rather than randomly orientated crystals. Finally, the relative position between the laser focus point and ZnS surface could be detected by the relationship between the SHG power and sample step, due to the regulation of the SHG signal by the power density of incident laser, which is also determined by the relative position. This work may pave the way towards nonlinear optical architectures and femtosecond laser machining.

Ablation characteristics of picosecond laser single point drilling of Si3N4 under dry and water medium

Single point laser ablation of Si3N4 under dry and water medium is investigated and analysed. The experiments were conducted based on the one-factor-at-a-time strategy considering various input parameters such as piercing time, focus point position and water layer thickness. The effects of process inputs on the ablated hole’s geometrical features including the ablation depth and diameter, hole taper, and hole edge roundness under presence and absence of water as an intermediate medium are studied. The laser focus point position was determined and varied during the investigation. It was found that, compared to the dry condition, the maximum ablation depth under wet condition is much more sensitive to the position of the laser focus point. It was also observed that the ablation depth is directly influenced by the piercing time in both dry and under water conditions. Picosecond laser single point ablation under a water layer with a thickness of 0.6 mm could increase the ablation depth up to 23%, compared to the dry condition. However, the ablation depth was significantly decreased by increasing the water layer thickness. Additionally, the hole entrance edge was rounded with increasing the water layer thickness. The hole taper was significantly influenced by the water layer thickness.

Parametric optimization for autogenous butt laser welding of sub-millimeter thick SS 316 sheets using central composite design

Laser Beam Welding (LBW) is now well known for its high precision and greater power intensities capable of welding any type of material. LBW finds wide applications in numerous areas ranging from simple cooking utensils to complex automotive and aeronautical components. In the present study, an attempt was made to characterize the LBW process used to join sub-millimeter thick sheets of SS 316. For this purpose Ytterbium Fiber laser having 2 kW power was used and SS 316 sheets of 0.85 mm, 0.56 mm, and 0.2 mm thickness were welded in butt joint configuration. For optimizing the selected input parameters laser power, laser scan speed and focal point position, central composite design technique was used. The responses considered are ultimate tensile strength, weld width, and micro hardness. The analysis of variance of the selected quadratic models shows that, the model can adequately predict correct results within the selected limits. The microstructural, fractographic and tensile behavior analysis clearly shows the effect of base material composition and heat input on the weld quality. The input parameters are optimized to give highest tensile strength and minimum weld width and were confirmed using confirmatory experiments.

High-speed micro-particle manipulation in a microfluidic chip by directional femtosecond laser impulse

We report a high-speed manipulation method to control flow direction of micro-particles in microfluidic chip activated by a femtosecond (fs) laser impulse. When the femtosecond laser pulse is focused in the channel, a micro-explosion is induced and generates an impulsive force at the laser focal point. In this method, the impulsive force initiated inside a micro-structure in the channel is applied to manipulate micro-particles flowing in the channel. Specifically, a directional force loading to the particle is realized by an additive pocket structure fabricated in the channel. When the laser is focused at the entrance of the pocket fabricated perpendicular to the channel, we could enhance directionality and intensity of the impulsive force and realize an effective particle manipulation compared with the chip without assistance of the pocket structure. The enhancement was apparently observed in a longer and wider pocket, especially whose entrance width is narrower than the pocket end width. The manipulation process occurs in less than 10 micro-seconds which is a potential high-speed manipulation method for single micro-particles.

Nd:YAG laser hardening of AISI 410 stainless steel: Microstructural evaluation, mechanical properties, and corrosion behavior

In this paper, a pulsed Nd:YAG laser with a maximum power of 700 W, was utilised to investigate the laser surface hardening of AISI 410 martensitic stainless steel. Focal point position, scanning speed and pulse width were considered as process variable parameters. Corrosion behavior of laser surface hardened samples were investigated by IVIUMSTAT apparatus in a 3.5 wt% NaCl solution. Maximum microhardness, depth, and width of hardness and percentage of ferrite phase of metallographic and FESEM pictures were evaluated. Results show that surface hardness increased up to 762 HV. Results also reveal that the laser focal point position and pulse width are effective parameters in laser hardening process. In potentiodynamic polarization tests potential stated to increase at a rate of 1 mV/s from −0.4 V to 0.2 V. Results indicate that the corrosion resistance increased due to laser hardening process.

Focal Point Sequestration for Operando Raman Confocal Micro-Spectroscopy of Fuel Cell Catalytic Layers

Intense laser powers used in confocal Raman micro-spectroscopy can photoablate thermally sensitive samples. This has precluded study of polymer electrolyte fuel cell catalytic layers. We now describe how thermal damage is mitigated by lifting the focal point above a transversely homogeneous sample surface. This results in a circular conical illumination volume below the focal point. The geometric probability of photons generated in this volume, having pathways to the detector pinhole, is at least two orders of magnitude lower than those emerging from a focal point. However, excellent signal to noise is recovered by sequestering the laser focal point in an isotropic medium that has no interfering Raman bands.

SA49THE EFFECT OF CHILDHOOD TRAUMA ON BLOOD EXPRESSION OF MED22 IN PATIENTS WITH MAJOR DEPRESSIVE DISORDER IS MEDIATED BY CIS-ACTING SNPS

The quality of robotic laser welded parts is related to the joint location, the trajectory of the laser focal point and the process parameters. By performing in-process monitoring, it is possible to acquire sufficient process knowledge for post-inspection to evaluate the geometrical weld quality. The existing solutions for such systems operate along linear welds. This paper contributes with a quality inspection system for robot laser welding, that can handle double-curved geometries. The data acquisition system includes a CMOS camera, which is mounted such that it looks through the laser optics, external LED illumination and matching optical filters. During the process, the area around the moving laser focal point is captured, resulting in a sequence of images. The trajectory of the focal point is determined by estimating the 2D displacement field between each image using template matching and subsequently filtering the data through a Kalman filter to improve the accuracy and robustness of the system. The joint location is determined by applying a Canny edge detector and a standard Hough transform within a specified region of interest. As this paper deals with double-curved geometries, the region of interest is moved in relation to the laser trajectory, such that it always contains the visible part of the joint, that is closest to the focal point. The developed post-inspection system evaluates the quality of the weld by comparing the estimated trajectory relative to the determined location of the joint. The performance of the proposed quality inspection system was validated empirically on 18 samples. The tests showed promising results, as the system was able to accurately detect changes in the welding trajectory relative to the location of the joint with an accuracy of ± 0.2 mm.

Quality Inspection System for Robotic Laser Welding of Double-Curved Geometries

The quality of robotic laser welded parts is related to the joint location, the trajectory of the laser focal point and the process parameters. By performing in-process monitoring, it is possible to acquire sufficient process knowledge for post-inspection to evaluate the geometrical weld quality. The existing solutions for such systems operate along linear welds. This paper contributes with a quality inspection system for robot laser welding, that can handle double-curved geometries. The data acquisition system includes a CMOS camera, which is mounted such that it looks through the laser optics, external LED illumination and matching optical filters. During the process, the area around the moving laser focal point is captured, resulting in a sequence of images. The trajectory of the focal point is determined by estimating the 2D displacement field between each image using template matching and subsequently filtering the data through a Kalman filter to improve the accuracy and robustness of the system. The joint location is determined by applying a Canny edge detector and a standard Hough transform within a specified region of interest. As this paper deals with double-curved geometries, the region of interest is moved in relation to the laser trajectory, such that it always contains the visible part of the joint, that is closest to the focal point. The developed post-inspection system evaluates the quality of the weld by comparing the estimated trajectory relative to the determined location of the joint. The performance of the proposed quality inspection system was validated empirically on 18 samples. The tests showed promising results, as the system was able to accurately detect changes in the welding trajectory relative to the location of the joint with an accuracy of ± 0.2 mm.

Effect of rotor angle to characteristic amplitude vibration shaft on wind turbine horizontal axis testing

The vibrations that arise in the axis of the wind turbine is a phenomenon that results from the unstable wind velocity coming from the angle of the wind attacked by the blade of the wind turbine itself. Vibration is widely used as a tool to analyze both machines with rotation and translational motion. Knowledge of vibration and the resulting data is very important for maintenance and repair analysis. This understanding can help the technician to reduce downtime and can increase profits both in terms of production and from longer wind turbine life. Vibrations arising from cyclic forces through existing machine elements, where they interact with each other and energy is amplified through the structure in the form of vibration. The effects of vibration are the occurrence of noise, the decrease in performance and performance of wind turbines and can damage components in wind turbines especially on shafts and bearings. In this study varied angle blade (blade / rotor) wind turbine to the shaft as well as variations in the coming wind speed. With the variation can be observed and known behavior of vibration that occurs by measuring by using vibration vibrometer VQ-400-A OMETRON connected with Labjack U3-LV forwarded to the PC in the form of a digital power voltage to an analog voltage. This model measures vibration in the horizontal or x-axis direction where the laser focus point on the rotating wind turbine shaft. To display the measurement results are used labjack connected to the laptop PC. This research resulted that for wind speeds between 3- 4.5 m/s which has the smallest deviation using 30º blade angle to minimize the vibration that is between 0.2353425 mm to 0.356839 mm. Based on the calculation of critical frequency occurs if the frequency of turbine rotation up to the frequency of 9.464226 Hz and the simulation using ANSYS software apply frequency of 4.1854 Hz.

Micro-nano 3D printing of electronically conductive polymers as a new process for achieving higher electronic conductivities

We have already reported multi-photon excitation of tris(2,2′-bipyridyl)ruthenium complex [Ru(bpy) 3 2+ ] by femtosecond pulse laser with near infrared wavelength as the light sources, the multi-photon sensitized polymerization of pyrrole and the 3D printing (photo-fabrication) system of polypyrrole (PPy) by 3D scanning of the laser focal point. In the present study, the 3D printing of PPy into a Nafion sheet was carried out to evaluate the electronic conductivity of the product micro-structures formed by multi-photon sensitized polymerization. When the PPy straight line structure was drawn in the direction of the optical axis (vertical), relatively higher conductivity values (Max 410 Scm−1) were obtained in the fully-doped state. The values of over 102 Scm−1 indicated the Drude model of electrical conduction, therefore, the micro-structures would be applied to a metamaterial. On the other hand, 1 or 2 orders of magnitude lower conductivity values were obtained from the crooked line structures drawn in the vertical and horizontal directions. This difference in the conductivity would be affected by the distorted cross section of the PPy deposition and the electrically overlapping of each deposition particle.