Laser Incident Angle Research Articles

An Investigation on Laser Welding Parameters on the Strength of TRIP Steel

The energy required for joining steel segments by using laser welding is relatively very low compared with arc welding, gas welding, or any other conventional welding techniques. Moreover, the rapid cooling may create a significant effect on different regions, such as the fusion zone (FZ), heat affected zone (HAZ), and base metal (BM), and in turn affect different parameters. In this study, the characteristics of the laser-welded joint were investigated by varying laser power, welding velocity and incident angle, and tensile strength. In our, experiments. the microhardness was increased by varying the power of laser welding. The strength of the joint was increased to 549 MPa with 2200 W high power, 30 mm/s velocity, and 80º laser incident angle. By increasing the power and velocity of the laser, the welding gun strength was improved; conversely, the angle of laser incident on the welding location decreased while its strength was increased.

Comprehensive Evaluation of the ICESat-2 ATL08 Terrain Product

Current spaceborne lidar Ice, Cloud, and Land Elevation Satellite (ICESat)-2 provides ATL08 product for global terrain height, whose quality properties are yet to be fully understood. This article performs a comprehensive evaluation on its quality by using 3-D elevation program (3DEP) digital elevation model (DEM) and hundreds of survey marks of two counties in the USA. The evaluation is carried out in terms of data specification, survey marks, land cover, season and time (day or night) of acquisition, incidence angle, and terrain slope. The ATL08 height errors are further modeled as a function of laser incidence angle and canopy coverage. It is found out the height from ATL08 product lies between the 3DEP DEM and true ground surface. The uncertainty of ATL08 height is 0.2 m for plain terrain, and 2 m for mountainous terrain where the majority of ATL08 segments are not useful for terrain extraction. The terrain height gets more underestimated by ATL08 products in regions with large terrain slope or incidence angle and more overestimated where the terrain is covered by dense canopy. Furthermore, seasonal variation of terrain height error can be as high as 0.8 m, while the impact of acquisition time is less than 0.3 m. We expect these findings to be informative for the utilization of ATL08 terrain product by worldwide users, and for the improvement of future ATL08 product.

Influence of incident angle and laser footprint on precision and level of detail in terrestrial laser scanner measurements

Terrestrial laser scanners (TLS) are used for a variety of applications, e.g., surveying, forestry, cultural heritage preservation, mining, topographic mapping, urban planning, forensics etc. This technology has made a huge shift in 3D spatial data collection due to much faster speed compared to other techniques. In the absence of guiding principles for positioning TLS relative to an object, surveyors collect data at maximum arrangements of scanning geometry elements due to fear of incomplete data of TLS. In 3D spatial data acquisition, positional accuracy and Level of Detail (LOD) are major considerations and are dependent on laser incident angle, footprint size, range and resolution. Mathematical models have been developed relating range, incident angle and laser footprint size for different surface configurations. These models can be used to position TLS to collect data at required positional accuracy and LOD. Models have been verified by deriving one model from other surface models by changing parameters. Effects of incident angle and footprint size have been studied mathematically and experimentally on a natural sloping surface. From the results, surveyors can plan the positioning of the scanner so that data is collected at the required accuracy and LOD.

Investigation of Mechanical Behaviour of Laser Welded Butt Joint of Transformed Induced Plasticity (TRIP) Steel with effect Laser Incident Angle

Investigation of Mechanical Behaviour of Laser Welded Butt Joint of Transformed Induced Plasticity (TRIP) Steel with effect Laser Incident Angle

The Effect of the Laser Incidence Angle in the Surface of L-PBF Processed Parts

The manufacture of multiple parts on the same platform is a common procedure in the Laser Powder Bed Fusion (L-PBF) process. The main advantage is that the entire working volume of the machine is used and a greater number of parts are obtained, thus reducing inert gas volume, raw powder consumption, and manufacturing time. However, one of the main disadvantages of this method is the possible differences in quality and surface finish of the different parts manufactured on the same platform depending on their orientation and location, even if they are manufactured with the same process parameters and raw powder material. Throughout this study, these surface quality differences were studied, focusing on the variation of the surface roughness with the angle of incidence of the laser with respect to the platform. First, a characterization test was carried out to understand the behavior of the laser in the different areas of the platform. Then, the surface roughness, microstructure, and minimum thickness of vertical walls were analyzed in the different areas of the platform. These results were related to the angle of incidence of the laser. As it was observed, the laser is completely perpendicular only in the center of the platform, whilst at the border of the platform, due to the incidence angle, it melts an elliptical area, which affects the roughness and thickness of the manufactured part. The roughness increases from values of Sa = 5.489 μm in the central part of the platform to 27.473 μm at the outer borders while the thickness of the manufactured thin walls increases around 40 μm.

Propagation properties and radiation forces of the chirped Pearcey Gaussian vortex beam in a medium with a parabolic refractive index

In our study, an analytical expression of the second order chirped Pearcey Gaussian vortex beam (CPGVB) in a medium with a parabolic refractive index is derived. We demonstrate its propagation behaviors as well as the effects of the parameters on the propagation properties. Expectedly, the CPGVB performs periodically automatic Fourier transform in the specific positions, but unexpectedly the off-axis autofocusing position can be generated and controlled without changing the laser incident angle. Furthermore, we investigate the radiation forces of the CPGVB, and thus notice that trapping positions form in the beam-propagation direction.

Surface residual stress, micro-hardness and geometry of TC6 titanium alloy thin-wall parts processed by multiple oblique laser shock peening

The titanium alloy thin-walled part with light in weight and high specific strength is widely used in the aviation engine fan, turbine and compressor. The fatigue damage of the thin-walled part often happens under the action of the cyclic loads and high temperatures. Laser shock peening(LSP) is an innovative surface treatment process, which has been used to improve the fatigue life of metallic materials. Considering the shape and size of the thin-walled parts, the TC6 titanium alloy thin-wall parts were treated by the oblique incident laser beam in this work. The purpose of this study is to investigate the effects of multiple oblique LSP on TC6 titanium alloy thin-wall parts. Nd: YLF laser with a wavelength of 1053 nm was applied. The laser incidence angle was determined first by experiment. The effects of laser energies, impact times, pulse durations and pulse frequencies on the surface residual stress, micro-hardness and geometry of TC6 titanium alloy thin-wall parts were investigated. The results showed that by the use of oblique LSP, the higher surface residual stress and surface micro-hardness could be induced on the surface of treated specimens. However, surface deformation and surface roughness of treated specimens can also increase.

Characteristic analysis of light and heat transfer in photothermal therapy using multiple-light-source heating strategy

Nanoparticles enhanced laser-induced thermal therapy (LITT) is an emerging method for early stage tumor treatment. One of the major issues challenging this technique is the accurate heating of the cancerous tissue without influencing the nearby healthy tissue. Owning to its outstanding optical, chemical, and biological properties, gold nanoparticles (GNPs) have been frequently applied to act as nanosource of heat for selective heating of tumor. In the present work, we studied the GNPs enhanced LITT theoretically. The bioheat transfer model under laser irradiation was established by the finite element method. The influences of laser intensity, GNPs volume fraction, anisotropic scattering characteristics of nanoparticles, convective heat transfer coefficient, and laser incident angle were investigated, respectively. On this basis, different treatment strategies, i.e. single heat source and multiple heat sources, were also explored. It is found that for single dose therapy, i.e. the whole tumor was damaged in one-time treatment, the efficacy is almost the same for oblique and vertical laser incident. However, for fractionated therapy, i.e. the tumor was totally damage after a series of treatments, the efficacy of oblique laser irradiation is better than that of vertical laser irradiation. Furthermore, the hyperthermia efficacy of multiple heat sources is much better than that of single heat source, which means that it can be applied in GNPs enhanced LITT to improve the photothermal therapy efficacy.

All-Dielectric Nanophotonics Enables Tunable Excitation of the Exchange Spin Waves.

Launching and controlling magnons with laser pulses opens up new routes for applications including optomagnetic switching and all-optical spin wave emission and enables new approaches for information processing with ultralow energy dissipation. However, subwavelength light localization within the magnetic structures leading to efficient magnon excitation that does not inherently absorb light has still been missing. Here, we propose to marriage the laser-induced ultrafast magnetism and nanophotonics to efficiently excite and control spin dynamics in magnetic dielectric structures. We demonstrate that nanopatterning by a 1D grating of trenches allows localization of light in spots with sizes of tens of nanometers and thus launch the exchange standing spin waves of different orders. The relative amplitude of the exchange and magnetostatic spin waves can be adjusted on demand by modifying laser pulse polarization, incidence angle, and wavelength. Nanostructuring of the magnetic media provides a unique possibility for the selective spin manipulation, a key issue for further progress of magnonics, spintronics, and quantum technologies.

3D Numerical Modeling of Laser Assisted Tape Winding Process of Composite Pressure Vessels and Pipes-Effect of Winding Angle, Mandrel Curvature and Tape Width.

Advanced thermoplastic composites manufacturing using laser assisted tape placement or winding (LATP/LATW) is a challenging task as monitoring and predicting nip point (bonding) temperature are difficult especially on curved surfaces. A comprehensive numerical analysis of the heat flux and temperature distribution near the nip point is carried out in this paper for helical winding of fiber reinforced thermoplastic tapes on a cylindrically shaped mandrel. An optical ray-tracing technique is coupled with a numerical heat transfer model in the process simulation tool. The developed optical-thermal model predictions were compared with experimental data available in literature to validate its effectiveness. The influences of winding/placement angle, mandrel curvature and tape width on the incident angles, the laser absorbed intensity, and the process temperature distribution are studied extensively using the validated model. Winding/placement angle has a considerable effect on the temperature distribution. Increase in winding angle results in a higher temperature for tape due to more reflections coming from the substrate. On the other hand, substrate temperature decreases as the winding angle increases due to a decrease in the laser incident angles based on the local surface curvature. An increase in mandrel curvature results in higher nip point temperatures for substrate and lower one for tape. Different mandrel sizes for 90 placement path do not have a strong effect on the substrate process temperature as for other winding angles because of less curvature change of the corresponding irradiated area. Tape width causes local temperature variations at the edges of the tape/substrate. In order to obtain the desired process temeprature during LATW or LATP processes, the laser intensity distribution on the tape and substrate surfaces should be regulated.

Effect of welding parameters on weld formation quality and tensile-shear property of laser welded SUS301L stainless steel lap filet weld

Laser welding of lap-filet joints SUS301L austenitic stainless steel plates were carried out with filler wire. Results showed that the weld zone was composed of columnar austenite and a small amount of δ-ferrite. The effect of feed direction, wire distance (H), laser incident position (L) and laser incident angle on the wire melting mechanism, weld pool geometry, metal transfer modes and heat distribution were studied with the assistance of the high speed camera recording the welding process. Better weld quality was obtained under the circumstances of leading direction, wire distance 0.5mm, laser incident position 0.5mm, and laser incident angle 0̊. The fracture was always located in the weld where the thickness of it was the smallest. There was a linear relationship between the weld width and the smallest weld thickness. The maximum tensile-shear force was 24.46kN. The fracture mode was ductile fracture with fracture surface dimples.

The effect of spot overlap ratio on femtosecond laser planarization processing of SiC ceramics

Silicon carbide ceramics are widely used in many fields owing to their excellent mechanical properties such as good wear resistance, thermal stability, and chemical corrosion resistance. The laser planarization processing can effectively achieve the functional, practical, and engineering process, satisfying the demands of the assembly precision. Therefore, different spot overlap ratios were utilized to investigate the effect of femtosecond laser planarization processing on silicon carbide ceramic samples. Various scanning speeds and laser repetition frequencies were matched to achieve different spot overlap ratios under the premise of fixed laser incident angle and single pulse energy. Under different parameters, the ablation depth, surface roughness, surface morphology, oxidation, and residual stress were compared. The result showed that at the same spot overlap ratio, the ablation depth and surface roughness are basically maintained at a stable value despite the difference in the scanning speed and repetition rate. The surface oxidation phenomenon remarkably decreased at the spot overlap ratio less than 90%. The decrease in the overlap ratio leads to the transition of the low frequency nano-ripple and the high frequency nano-ripple, and the surface residual stress converted from tensile stress to compressive stress. Combining processing requirements with ablation quality, the parameters are divided into three parts to fit three processes based on the spot overlap ratios. The variation in the spot overlap ratios as the guidelines had good potential in guiding the parameter range selection.

Generation of highly efficient terahertz radiation in ferromagnetic heterostructures and its application in spintronic terahertz emission microscopy (STEM)

The laser terahertz emission microscopy (LTEM) technique, which breaks through the resolution limitation of terahertz waves from millimeter to micrometer scales, has been widely used in many real application circumstances, such as contactless chip nondestructive testing, biosensing, imaging, and so on. Recently developed spintronic terahertz emitters featuring many unique properties such as high efficiency, easy integration, low cost, large size and so on, may also have great applications in LTEM, which can be called spintronic terahertz emission microscopy (STEM). To achieve high efficiency and good performance in STEM, we propose and corroborate a remnant magnetization method to radiate continuous and stable terahertz pulses in W/CoFeB/Pt magnetic nanofilms without carrying magnets on the transmitters driven by nJ femtosecond laser pulses. We systematically optimize the incidence angle of the pumping laser and find the emission efficiency is enhanced under oblique incidence, and we finally obtain comparable radiation efficiency and broadband spectrum in W/CoFeB/Pt heterostructures compared with that from 1 mm thick ZnTe nonlinear crystals via optical rectification under the same pumping conditions of 100 fs pulse duration from a Ti:sapphire laser oscillator, which was not previously demonstrated under such long pulse duration. We believe our observations not only benefit for a deep insight into the physics of femtosecond spin dynamics, but also help develop novel and cost-effective broadband spintronic terahertz emitters for the applications in STEM.

In situ laser reflectivity to monitor and control the nucleation and growth of atomically thin 2D materials*

The growth of atomically thin two-dimensional (2D) layered and other quantum materials is typically performed without in situ monitoring or control. Here, a simple laser reflectivity approach is demonstrated to provide in situ control over sub-monolayer thickness and growth kinetics during pulsed laser deposition (PLD) of MoSe2 layers. First, the general technique is presented with emphasis on designing the maximum sensitivity of the optical contrast through consideration of Fresnel’s equations with proper choice of layer thickness, substrate, and laser monitoring wavelength, incidence angle, and laser polarization. Then the 632.8 nm optical reflectivity of MoSe2 layers on SiO2/Si substrates was predicted and compared with in situ monitoring of MoSe2 growth by PLD under actual growth conditions using a probe HeNe laser beam. The measurements showed high sensitivity and excellent agreement with MoSe2 surface coverages calculated from atomic resolution STEM analysis of 2D layers deposited in arrested growth experiments. Growth kinetics revealed by these measurements showed sigmoidal nucleation and growth stages in the formation of the 2D MoSe2 layers that are described by a simple model, indicating the promise of the laser reflectivity technique for in situ monitoring and control of 2D materials deposition.

Colorful and superhydrophobic titanium surfaces textured by obliquely incident femtosecond laser induced micro/nano structures

Multiscale micro/nano structures (e.g. large period ripples and sputtered particles) on titanium surfaces by obliquely incident femtosecond laser texturing. The effects of laser incident angle on surface structures, wettability and structural color were studied. As a result, sputtered particles are formed on the titanium surface for all incident angle. Fine ripples are appeared on the large period ripples, when the incident angle is greater than 50°. However, there is no fine ripples structure when the incident angle is less than 50°. Meanwhile, the structured surfaces appear structural color and superhydrophobicity for all laser angle. The formation mechanism of hybrid micro/nano structures on the titanium surface is also discussed.

Study of the intensity asymmetry in Brillouin light scattering from magnons in FeNi thin films

The Stokes–anti-Stokes intensity asymmetry, observed by Brillouin light scattering from surface magnetic excitations in thin FeNi films, has been studied by replacing substrates with different dielectric constant as well as varying the applied magnetic field and the laser incident angle. The experimental results show that the Stokes–anti-Stokes intensity ratio depends upon the applied magnetic fields, the wave vectors and also the material of the substrates used to support the FeNi films. Most interesting is that for a fixed magnetic field and a fixed wave vector, the intensity ratio is much more significant for the FeNi film deposited on Nb-doped SrTiO3 substrate than that deposited on surface oxidized Si substrate. The phenomenon of the different asymmetry is depending on the dielectric constant of the substrates. The tendency is in agreement with the theory.

A Finite Element Model for Temperature Prediction in Laser-Assisted Milling of AerMet100 Steel

Laser-assisted milling (LAM) represents an innovative process to enhance productivity in comparison with conventional milling. The workpiece temperature in LAM not only affects the cutting performance of materials, but also the machined surface quality of the part. This paper presents a 3D transient finite element (FE) model for workpiece temperature prediction in LAM. A moving Gaussian laser heat source model is implemented as a user-defined subroutine and linked to ABAQUS. The thermal model is validated by machining AerMet100 steel under different process parameters (laser power, spindle speed and feed per tooth). Good agreement between predicted and measured workpiece temperatures indicates that the FE model is feasible. In addition, the effects of laser spot size and incident angle on workpiece temperature are analyzed based on the proposed model. This work can be further applied to optimize process parameters for controlling the machined surface quality in LAM.

Optimization of optical control of nitrogen vacancy centers in solid diamond

The nitrogen-vacancy (NV) centers in diamond have the advantages of stable triaxial structure, ultra-long electron spin coherence time and simple optical readout at room temperature. A nitrogen atom in the diamond crystal replaces a carbon atom and a vacancy is generated at the adjacent position, forming a point defect in the <i>C</i><sub>3<i>v</i></sub> space group structure. Its ground state and excited state are both spin triplet states. It is the key to achieving efficient preparation of optical initial state and extracting NV color center’s information in the researches of highly sensitive sensing magnetic detection, temperature detection, biological imaging, quantum computing, etc. However, there was no systematic study on relevant parameters of laser for high-concentration NV color center’s samples in previous experimental studies. Based on a high concentration diamond NV ensemble, we use pulsed optical detection magnetic resonance (ODMR) technology to systematically study the relationship among laser initial polarization time, information reading time and laser power, and the influence of laser incident polarization angle on the accuracy of sensing information. The effects of various laser parameters on the NV1 peak of ODMR on the [111] axis of the NVs of diamond are also investigated. The contrast of ODMR increases firstly with a sigmoid function and then decreases with an e-exponential function as the information reading time increases. The incident polarization angle of the laser is sinusoidal, with a period of 90°. According to the above experimental results, we finally choose the appropriate experimental parameters at 45.8 W/cm<sup>2</sup> (300 μs of polarization, 700 ns, reading time, laser incident angle is 220°) for ODMR test. Compared with previous experimental parameters (polarization time was 50 us, read the time of 3000 ns, laser incident angle was 250°), the experimental results show that the contrast of ODMR increases from 2.1% to 4.6%, and the typical magnetic sensitivity is improved from 21.6 nT/Hz<sup>1/2</sup> to 5.6 nT/Hz<sup>1/2</sup>. The optimization of the optical control of NVs in solid diamond is realized. The above results provide an effective support for the detection of high-sensitivity manipulation sensing based on high-concentration NV ensemble.

Process variation in Selective Laser Melting of Ti-6Al-4V alloy

The present work explores the variation in Ti-6Al-4V part quality introduced by the key process operations of Selective Laser Melting (SLM) process, the recoating, the gas flow, and the laser beam irradiation. Novel specimens and experiments were designed to characterize the differences in surface quality and thermal history as a function of part geometry and location on the build platform. The variation in the roughness of inclined surfaces shows a clear dependency on the laser incidence angle and the influence of gas flow on process by-products. The direction in which the laser beam traverse across the build area with respect to the gas flow direction also affects the surface quality. Thermal profiles were recorded by attaching thermocouples to the surface of the built part with various geometries. The measured temperature profiles show intense local fluctuations due to the rapid movement of the laser beam. The parts also experience a continuous heat treatment throughout the SLM process due to the low effective conductivity of the powder bed and continuous heat input by the laser.

Relativistic electron acceleration by surface plasma waves excited with high intensity laser pulses

The process of high energy electron acceleration along the surface of grating targets (GTs) that were irradiated by a relativistic, high-contrast laser pulse at an intensity $I=2.5\times 10^{20}~\text{W}/\text{cm}^{2}$ was studied. Our experimental results demonstrate that for a GT with a periodicity twice the laser wavelength, the surface electron flux is more intense for a laser incidence angle that is larger compared to the resonance angle predicted by the linear model. An electron beam with a peak charge of ${\sim}2.7~\text{nC}/\text{sr}$ , for electrons with energies ${>}1.5~\text{MeV}$ , was measured. Numerical simulations carried out with parameters similar to the experimental conditions also show an enhanced electron flux at higher incidence angles depending on the preplasma scale length. A theoretical model that includes ponderomotive effects with more realistic initial preplasma conditions suggests that the laser-driven intensity and preformed plasma scale length are important for the acceleration process. The predictions closely match the experimental and computational results.