Laser Incident Angle Research Articles

Multi-phase flow simulation of powder streaming in laser-based directed energy deposition

As a promising additive manufacturing technique, laser-based directed energy deposition has been widely applied in part fabrication, surface cladding, and part restoration. During the powder feeding process, the gas, particles, and laser interact and further affect the molten pool flow. To study the powder feeding process, a multi-physics gas-particle multi-phase flow model coupled with a laser ray-tracing heat source model is developed to predict the powder particle temperature, velocity, and concentration distribution as well as laser energy loss, which is then validated against experimental results. The single-nozzle simulation shows that powder particles out of the nozzle follow a radial Weibull distribution instead of a 2D Gaussian distribution from the inlet of the nozzle. In the four-nozzle powder feeding process, the particles converge further after passing the focal plane, and setting the focal plane above the molten pool surface could potentially increase the powder catchment. Moreover, the laser ray-tracing model incorporates laser reflection and laser incident angle into laser energy loss calculation, which would be more accurate than those models based on the Beer-Lambert law. Additionally, by increasing the powder feed rate or decreasing the carrier gas flow rate, the laser energy loss linearly increases. The predicted powder velocity, temperature distribution, and attenuated laser can be further implemented into the molten pool flow model to more comprehensively study the interaction between particles and the molten pool in the future.

Numerical simulation of molten pool flow behavior and keyhole evolution behavior in dual-laser beam oscillating bilateral synchronous welding of T-joints

In the context of global warming and energy shortage, the key means to increase product competitiveness is how to achieve lightweight manufacturing of large spacecraft skin-stringer structures by welding instead of riveting. In this work, we propose dual-laser beam oscillating bilateral synchronous welding (DLBOW) by combining dual-laser beam synchronous welding (DLBW) with the oscillating process to overcome the long-standing porosity problem in T-joint welds and investigate the effect of different laser incidence angles on the porosity of the weld during welding. At the same time, the corresponding numerical model was established by computational fluid dynamics to analyze the molten pool flow behavior and keyhole evolution under molten pool coupling. This is the first time that complete wire melting process has been achieved in the T-joint dual-laser beam synchronous welding numerical model, reproducing more realistically the influence of the wire fusion behavior on the molten pool flow. Both experimental and simulation results show that the porosity is significantly reduced under DLBOW compared to DLBW; in addition, the porosity is lower when the laser incidence angle is smaller. An in-depth analysis of the numerical model reveals the mechanism of this physical phenomenon: First, the oscillation process optimizes the laser energy density distribution and reduces the peak temperature of the T-joint molten pool. The impact of the metal flow on the keyhole is weakened, which reduces the probability of bubble formation and thus significantly decreases the porosity. Moreover, for the oscillation process, the keyhole effect is more obvious at large laser incidence angles, and the actual heat input to the molten pool is larger, which makes the slender keyhole more prone to collapse and more likely to generate pores. This work provides new insight into the interrelationship between keyhole and pore in T-joint dual-laser beam synchronous welding.

Generation of second harmonic terahertz surface plasmon wave over a rippled graphene surface

Abstract We propose a mechanism for the generation of second harmonic terahertz surface plasmon waves by incident terahertz electromagnetic radiation (ω, k 0) over a graphene surface deposited on the rippled dielectric substrate (SiO2). A p-polarized THz radiation incident obliquely on the graphene surface exerts a nonlinear ponderomotive force on free electrons in the rippled regime. This nonlinear ponderomotive force imparts oscillatory velocity to the electrons at frequency 2ω. Second harmonic oscillatory velocity couples with the modulated electron density and generates a nonlinear current density that drives second harmonic terahertz surface plasmon waves. Rippled surface provides an extra wave number for the phase matching condition to produce resonantly second harmonic at frequency 2ω and wavenumber (2k 0z + q). We examine the tunable response of second harmonic terahertz surface plasmon waves with respect to change in Fermi energy of graphene and laser incident angle. Second harmonic amplitude gets higher values by lowering the Fermi energy (E F) and increasing incident angle.

Effects of laser welding parameters on the porosity and acicular phase in SiCp/6092 aluminum matrix composite welded joints

Al matrix composites (AMCs) are preferred engineering materials for aerospace and automotive industries owing to their high specific strength, superior wear resistance and low thermal expansion. It is a challenge to achieve high-quality fusion welded joints of AMCs because of the occurrence of defects such as pores, particles segregation and acicular phases. In this study, the effects of laser welding parameters including laser power, welding speed and laser incident angle on the formation of porosity and acicular phase were investigated in SiCp/6092 aluminum matrix composite welded joints using orthogonal experimental method. Range analysis of the orthogonal test results showed that the laser power and the welding speed had the greatest influence on the porosity and acicular phase size during the laser welding of aluminum matrix composites. At a heat input of 60 J/mm, the porosity in the weld was only 1.6% while that at 190 J/mm was 15.2%. Higher heat input produced a much larger size of brittle Al4C3 phases. The size of the acicular phase Al4C3 at the top and bottom of the weld was larger than that in the middle. Active Ti was added to the weld zone to mitigate the adverse effect of Al4C3 brittle phase and porosity on mechanical properties. The ultimate tensile strength increased from 115 MPa in direct laser welding to 148 MPa for the laser welding process employing the Ti interlayer. This work will provide referenceable data for designing high-quality AMC welded joints for aerospace and automotive industries.

Effects of lead and lean in multi-axis directed energy deposition

The present study examines the effect of varying laser incidence angles on textural, microstructural, and geometric characteristics of directed energy deposition (DED) processed materials, providing a more comprehensive outlook on participating laser-matter interaction phenomena and ultimately devising strategies to ameliorate print performance. In this study, single-layer, single-/multi-track specimens were processed to examine the effect of non-orthogonal angular configurations on bead morphology, microstructure, phase composition, and textural representation of DED-processed 316L stainless steel materials. It was observed that bead size decreased at increasing lead and lean angles. Asymmetry in the distribution of the bead morphology as a function of lead angle indicates better catchment for acute lead angle configurations over obtuse configurations. No significant differences in phase composition, texture, and microstructure were observed in moderate off-axis configurations. When the penetration depth for the deposits was below 20 μm, columnar structures dominated the microstructure of the deposited material. At deeper penetration depths, columnar and equiaxed structures were observed at the bead-substrate interface and center of the bead, respectively. Compared to powder-blown DED, wire-DED dilution profiles were found to be asymmetric in both orthogonal and non-orthogonal wire DED samples.

Analysis of Laser Cell Response Characteristics under Different Irradiation Conditions

Although the application of laser wireless energy transmission technology in many fields such as UAV power supply is increasing, the laser incidence angle and beam shift remain the key factors limiting the efficiency of long-range laser wireless energy transmission. In this study, a laser cell response test platform was built to measure and analyze the response characteristics of a laser cell under different laser incidence angles and beam shifts. The results show that the increase in the incident angle intensifies the reflection on the irradiated surface, resulting in a linear decrease in the power density received by the laser cell, which eventually leads to a significant decrease in the output power, and the output power tends to be close to 0 when the incident angle exceeds 75°. The increase in the beam offset distance increases the reverse bias of the cell, which is the main reason for the significant decrease in the output power. The local irradiation also leads to an increase in the heat generation power; when the beam coverage is below 50%, the overall output power tends to be close to 0. This study provides a reference for improving the laser wireless energy transmission efficiency and laser cell optimization.

Multisource-Domain Adaptation-Based Damage Detection Method for Air-Conditioning Condenser Using Laser Ultrasonic Guided Wave

Air conditioning condenser (ACC) is a key component of air-conditioning equipment. ACC tube leakage damage will cause huge economic losses and energy waste. The laser ultrasonic guided wave detection method is suitable for ACC damage detection due to its high sensitivity and non-contact advantages. However, the ACC tube size is small and the weld joint is not standardized, which makes it difficult for the laser to aim at the tube. And the detection signals generated by laser excitation at different laser incident angles are different. In this paper, the Unsupervised Multi-source Adaptation Adversarial Network (UMAAN) is proposed to improve the generalization ability of the ACC detection model. Firstly, the feature diagrams of the signal are obtained by time-frequency analysis, and the diagrams are set to different source and target domains according to the different incident angles of laser excitation. Then, the shared feature extractor is used to obtain features of all samples. And the model parameters are optimized by adversarial network to make the features distance of the target and source domains closer. Among them, the adaptive weight coefficients are designed to adjust the loss value weights of the different source domains. And the label error weight factor is designed to improve the damaged signals detection accuracy by adjusting the loss weight of damaged samples and normal samples. Finally, the damage detection experiments of the ACC are carried out. The experimental results illustrate that the proposed UMAAN can achieve high-precision detection of ACC damages.

Temporal and spatial laser intensification within nodular defects overcoated with multilayer dielectric mirrors over a wide range of defect geometries.

The nodular defect shape and the laser incidence angle have a dramatic impact on the spatial distribution of light intensification within the nodule as well as how the laser light is drained from the defect. Nodular defect geometries unique to ion beam sputtering, ion-assisted deposition, and electron-beam (e-beam) deposition, respectively, are modeled in this parametric study over a wide range of nodular inclusion diameters and layer count for optical interference mirror coatings constructed with quarter-wave thicknesses and capped with a half wave of the low index material. It was found for hafnia (n=1.9) and silica (n=1.45) multilayer mirrors that the light intensification in nodular defects with a C factor of 8, typical of e-beam deposited coatings deposited with a wide range of deposition angles, was maximized for a 24-layer design. For intermediate size inclusion diameters, increasing the layer count for normal incidence multilayer mirrors reduced the light intensification within the nodular defect. A second parametric study explored the impact of the nodule shape on the light intensification for a fixed number of layers. In this case, there is a strong temporal trend for the different nodule shapes. Narrow nodules tend to drain more laser energy through the bottom of the nodule into the substrate while wide nodules tend to drain more laser energy through the top of the nodule when irradiated at normal incidence. At a 45° incidence angle, waveguiding is an additional method to drain laser energy from the nodular defect. Finally, laser light resonates within nodular defects longer than within the adjacent nondefective multilayer structure.

Copper laser patterning on a flexible substrate using a cost-effective 3D printer

We studied the cost effective direct laser patterning of copper (Cu) on thin polyimide substrates (PI thickness: 12.5–50 µm) using a 405 nm laser module attached to an inexpensive 3D printer. The focal length of the laser was intentionally controlled to reduce defects on patterned Cu and surface damage of PI under predetermined process conditions. The appropriate focal length was examined at various focal distances. Focal distances of − 2.4 mm and 3 mm were found for the shorter focal length (SFL) and longer focal length (LFL), respectively, compared to the actual focal length. This resulted in clean Cu line patterns without line defects. Interestingly, the SFL case had a different Cu growth pattern to that of LFL, indicating that the small difference in the laser incident angle could affect Cu precursor sintering. Cu square patterns had a lower resistivity of 70 μΩ·cm for an LFL after three or four laser scans, while the SFL showed a resistivity below 48 μΩ·cm for a one-time laser scan. The residues of the Cu precursor on PI were easily removed with flowing water and normal surfactants. However, the resistivity of the patterns decreased after cleaning. Among the scan gaps, the Cu square pattern formed at a 70 μm scan gap had the lowest sheet resistance and the least change in resistance from around 4 to 4.4 Ω/ϒ after cleaning. This result implies that the adhesion of the patterned Cu could be improved if the coated Cu precursor was well sintered under the proper process conditions. For the application of this method to bioelectronics, including biosensors, LEDs were connected to the Cu patterns on PI attached to the arm skin and worked well, even when the substrate PI was bent during power connecting.

Numerical simulation and experimental analysis of machining morphology with pulsed laser

In this study, simulation and experimental studies are used to evaluate the temperature field distribution and process law for the pulsed laser processing of stainless steel. The numerical simulation is based on the theory of heat transfer. The temperature field distribution and change process of the stainless steel surface under the influence of laser is explained subjectively, and the morphology evolution law of stainless steel during the entire process is studied. The influence of insufficient laser spot overlap ratio and the variation of laser power on the machining quality is analyzed by comparing with the experimental results. The evolution law of machining morphology and size with the laser incident angle, scanning speed, repetition frequency, and laser power are evaluated. The results provide theoretical support for determining the high-quality processing of stainless steel by a pulsed laser, which can be used for the parameter optimization of laser processing, such as metal material cutting, surface microstructure preparation, and two-dimensional code marking.

Influence of surface morphology and processing parameters on polishing of silicon carbide ceramics using femtosecond laser pulses

In this paper, silicon carbide (SiC) ceramics were polished using a high-frequency femtosecond laser (fs-laser). The influence of different laser incidence angles on the machined surface were investigated in detail, and the processing morphologies for different laser processing parameters were analyzed and optimized. The theoretical simulation and experimental results indicated that the original surface defects of pits prior to processing of the SiC surface remarkably affects the ablation effect. When fs-laser polishing at vertical incidence, the effect of light trapping occurred in the pits due to multiple reflections, enhancing the absorption of laser energy to enlarge the size of pits. The small incidence angle can eliminate the effects of light trapping in the pits and obtain the smooth surface of the material. In addition, the degree of oxidation and graphitization decrease significantly with a decrease of incidence angle. The optimal processing parameter combination under the incidence angle of 10°, laser frequency of 100 kHz, scanning speed of 200 mm/s, single pulse energy of 60 μJ was obtained. The polished SiC ceramic exhibited a smooth, flat and pit-free surface with the average Sa of 0.187 μm and Sz of 2.313 μm. Finally, the formation of typical features such as pits, particles and debris deposition, was explained in detail. Our work provides new research ideas for understanding the polishing mechanisms concerning the removal of surface defects and depositions from the SiC ceramics.

Research on two-dimensional laser warning integrated detection technology

Laser weapons are playing an increasingly important role in modern warfare. To address the growing laser threat, it is necessary to study high-performance two-dimensional laser warning integrated detection technology to achieve simultaneous measurement of two-dimensional directional angle, wavelength and pulse width of incoming laser. This is of great importance for rapid response to security threats. To this end, the paper investigates a two-dimensional laser warning integrated detection system. The measurement principle of the system is as follows: under the condition that the optical system is known, the incidence angle and wavelength of the incoming laser can be judged by the position of the zero-level and first-level diffraction spots, and the mathematical relationship is theoretically derived, so that the principle of laser pulse width measurement based on time-to-digital conversion can be analyzed. For this purpose, firstly, the pulsed laser detection system was designed, the photoelectric conversion of the pulsed laser signal and the time interval measurement were completed, and the pulse width measurement and control unit were experimentally verified. Secondly, the imaging control system of the surface array detector was designed and the imaging effect was verified, and the spot images of lasers with different central wavelengths incident from different angles were collected and the extracted spot center coordinate data were analyzed. Thirdly, the assembly and testing of the two-dimensional laser warning integrated detection system were completed, and the measured wavelength, azimuth, pitch angle and pulse width multi-parameters were tested, and the measurement results were analyzed. The test results show that the system has an azimuth angle of 95°, a pitch angle of 75°, an angular measurement resolution of ≤ 0.5°, a spectral measurement range of 400–1700 nm, and a minimum measurement pulse width of 5 ns, and the two-dimensional laser warning integrated detection system can meet the practical application requirements.

Order-disorder control of Si nanoparticles in laser-irradiated amorphous-Si thin film

Ordering of Si nanoparticles formed on an amorphous-Si thin film via laser irradiations was examined in terms of laser fluences, incident angles, and polarizations. The Si nanoparticles were created through two-dimensional scans of an ultraviolet nanosecond laser with a Gaussian beam. The arrangements of the nanoparticles varied significantly depending on the polarization at off-normal incidence. To perform quantitative analysis, an edge detection algorithm (the Hough transform method) was used to extract the nanoparticle size and position data from secondary electron microscopy images, and the nanoparticle distribution and ordering were estimated through the pair-correlation function. At normal incident angle, the Si nanoparticles were well ordered along the polarization direction, whereas for off-normal incidence at 30°, the Si nanoparticles were significantly disordered at polarization angles below 90° (out-of-plane) but ordered at the polarization angle of 90°. These features were interpreted in terms of the Rayleigh anomaly condition and competition between two wave vectors at off-normal incidence. The features can be applied to control the device performance in substrates for electronic or photovoltaic devices via nanoparticle ordering.

Influence of micro-grooves processed by laser with different incident angle on the properties of TiAlSiN coatings

In order to investigate the effect of micro-grooves geometry on coatings properties, the micro-grooves were prepared on the TiC-based cermets surface using laser with different laser incident angle, and TiAlSiN coatings were fabricated on the micro-grooved surfaces by PVD. The microstructure, surface roughness, hardness and phase composition of grooved surface and coatings were obtained. In addition, the adhesion strength and tribological properties of the coatings were studied by scratch tests and friction tests, respectively. The residual stress distribution and the influence mechanism of the micro-grooves on the properties of the coatings were discussed. The results illustrated that as the laser machining angle reducing, the surface roughness and the width of the grooves were elevated, and the depth of grooves was decreased. Compared with the coatings deposited on smooth surface, the tribological properties, adhesion strength, residual stress and contact area ratio of the coatings deposited on the micro-grooved surface machined by laser with incident angle of 90° were effectively improved. However, the coatings on the S-edge of the micro-grooves were flaked off, which was mainly due to the lager residual stress concentrated on the S-edge. Reasonable micro-grooves geometric parameters can effectively improve the adhesion strength and tribological properties of the coatings.

Effects of incidence angle and optimization in femtosecond laser polishing of C/SiC composites

In this article, C/SiC ceramic matrix composites (CMC) were polished by femtosecond laser. The effects of different laser incidence angles on the machined surface were studied in detail, and the process parameters were optimized. The results have indicated that different laser incident angles influence the machined surface of CMC. With the increase of laser incident angle, the defects and oxidation degree decrease significantly, the carbon (C) content gradually decreases, silica (Si) content gradually increases, and oxygen (O) content is almost unchanged. Under an incident angle of 70°, the surface carbon fiber along the axial direction was neatly cut, and the bump structure was eliminated. The laser's incident angle greatly influences the corrugated structure of the carbon fiber section. The response surface method establishes the quadratic response model of four parameters and surface roughness. The small-scale optimization method predicted the optimal expected value as follows: single pulse energy is 71.53 μJ, defocus amount is −0.13 mm, incident angle is 20.61°, Y-axis spot overlap rate 77.50%, and the optimal surface roughness (Ra) is predicted as 1.03 μm. The experimental results show that the average surface roughness of 0.92 μm is within the 95% confidence interval mean (95% CI Mean), indicating that the experimental results are reliable. It is 51.98% lower than the original surface roughness value of 1.92 μm. The optimal surface roughness value is 0.71 μm, 63.02% lower than the original surface roughness value.

Ultrafast Laser Texturing of Stainless Steel in Water and Air Environment

Ultrafast laser texturing allows the generation of micro- and nanostructures on steel substrates. Laser-ablated textures show a wide range of structure geometries, from the micro to the nanoscale, which can enable plastic product functionalization. Polymer processing technologies are used to replicate mold textures on a large manufacturing scale. To enable new product functionalities, developing novel texture geometries is critical. The laser-ablated texture dimensions are primarily linked to the laser light properties, such as the laser wavelength, thus limiting the achievable structure shapes. This work uses ultrafast laser to manufacture textures in air and water environments. The effect of the different mediums on structures formation is characterized. The irradiation is performed over a wide range of fluence values. The texture geometry and characteristics are evaluated by scanning electron microscopy. For decreasing fluence values, the structures transitioned from micro bumps, to LIPSS, to nanostructures, regardless of the irradiation environment. Conversely, structure morphology is affected by the irradiation environment. The LIPSS pitch is lower for the underwater environment due to the change in the laser angle of incidence, which changes with the refraction index ratio of the air and water. A novel nano-lamellae texture was generated when irradiating the steel surface underwater at relatively low fluence. The dynamics of different LIPSS generations are discussed, considering the irradiation medium’s optical, thermal, and physical properties.

Simulation of surface heat treatment with inclined laser beam

The beam inclination leads to a change in the laser spot size on the material surface. The higher the inclination, the larger the irradiated area and the lower the laser intensity. Moreover, if the material surface is outside of the beam focal plane, the intensity distribution profile becomes asymmetric. In this study, a heat source model, which calculates the intensity distribution on the workpiece surface as a function of beam parameters (beam waist, divergence half-angle) and process parameters (laser power, incidence angle, and distance to focal plane) was developed. The applicability of the heat source model was demonstrated by simulating 4 different laser hardening regimes. Once the heat efficiency coefficient had been calibrated the developed finite-element model allowed computation of temperatures while hardening with perpendicular laser beams as well as with inclined beams. The open-source software FEniCSx was used for the finite element computations. The mathematical formulation, required for performing temperature simulations with FEniCSx was briefly introduced.

Stress sensitivity analysis of laser quenching based on regression model

In the laser quenching process, quantitative evaluation of the influence of different process parameters on quenching stress is the key to achieve quenching process optimization. A variety of different process parameters are selected as research objects, and a parameterized thermomechanical coupling model of the disk laser hardening process for 45 steel is established. Through the calculation of the model, the instantaneous change law of stress field and the phase change field under different process parameters are obtained, and the response surface equation between the multiphysics solution results and process parameters is established. The Monte Carlo method is used to randomly sample the response equation repeated times, and the sensitivity of quenching process parameters is calculated through sampling results. The calculation result shows that hollow teardroplike stress distribution is formed in the center of the spot during quenching, and the stress around the spot is larger. Thermal stress concentration appears in the reverse direction of quenching scanning, with a maximum value of 850 MPa, and a cometlike stress zone appears in the reverse direction of quenching scanning. The diameter of the laser spot is negatively correlated with quenching stress and has the greatest influence on it. The laser incidence angle has a great influence on the depth and width of the phase change layer and is positively correlated. This model can reveal the mechanism of multifield coupling changes in the material during quenching and provide technical and theoretical support for the reasonable selection of process parameters.

Megahertz ultrasonic source induced by femtosecond laser irradiation of graphene foam

The performance of a photo-thermo-acoustic (PTA) ultrasonic source can be significantly improved by utilizing graphene-based materials. Graphene is an excellent PTA material owing to its wide electromagnetic absorption spectrum, low heat capacity per unit area, and high thermal conductivity. In this study, a broadband graphene-foam-based PTA ultrasonic source covering the frequency range of 50 kHz to 1.8 MHz was excited by a near-infrared femtosecond laser beam. The lower and upper frequency limits of the acoustic wave that can be detected in the experiments were determined by the responsivity of the microphone and the attenuation of the acoustic wave in air, respectively. The experimental results show that the sound pressure of this ultrasonic source was independent of the laser polarization and incident angle. The peak-to-peak magnitude of the ultrasonic wave was proportional to laser energy when the single-pulse energy of the femtosecond laser varied from 0.4 to 1.0 mJ. Therefore, the sound intensity of the ultrasonic source could be easily controlled by modulating the laser energy. The experimental results also show that the ultrasonic wave emitted from the graphene foam had a dipole-like acoustic pressure distribution, and its principal emission direction was normal to the sample surface, regardless of the laser incidence angle. This characteristic may benefit future applications in directed message transfer/acquisition and nondestructive testing/imaging.

Three-dimensional echo distribution analysis of multi-band and oblique Gaussian beams propagating through cat-eye optical system

The cat-eye effect in optical systems enables active laser detection at long distances and high orientation precision. However, few studies focus on the influence of a multi-band laser with a large incident angle on the echo distribution. This study proposes a three-dimensional echo distribution calculation model of an obliquely incident multi-band laser based on the Collins diffraction integral formula and aperture function expansion as the sum of complex Gaussian functions. The echo distributions for different detection ranges are calculated by coupling the size of the detector in a cat-eye optical system into the model. In the numerical simulation, the influences of laser wavelength, incident angle, and target detector parameter are quantitatively analysed by introducing a target-missing quantity of echo spot and peak light intensity. The results show that the peak light intensity decreases with an increase in incident laser wavelength. When the incident angle increases to a particular value, the peak light intensity decreases sharply, and the target-missing quantity of the echo spot centroid is positively correlated with the incident angle. We can effectively extract important information, such as the working band and detector size, from the cat-eye target echo using this model and provide a trajectory prediction theory for the active detection of a cat-eye target moving at high speed.