Laser Beam Trajectory Research Articles

Time-efficient surrogate models of thermal modeling in laser powder bed fusion

Two time-efficient surrogate models are proposed to emulate the nonlinear heat equation in the context of laser powder bed fusion, the performance of which is compared in accuracy and online execution time. Fast-computed numerical solvers are critical in implementing the digital twin framework in the additive manufacturing process addressing one of its main open problems: lack of quality assurance. The first surrogate model is the reduced Gaussian process emulator. It is a data-driven model equipped with a nonlinear dimension reduction scheme and manages to predict temperature profiles almost instantly (around 0.036s on average) with an accuracy of 95% for 99.38% of tests. Another surrogate model is the sketched emulator with local projection. It projects the accurate but high-dimensional finite element method solution on a low-dimensional basis and then bypasses the majority of costly computations for the temperature-dependent matrices in the projected model by randomized sketching. It has higher accuracy (97.78% of tests with relative errors below 1%) while spending comparably more time online (around 42.23s on average). Although compared with the finite element model both surrogates promote time efficiency with some minor controlled compromise in accuracy, the reduced Gaussian process emulator enables real-time implementation while the sketched emulator with local projection offers comparably higher levels of accuracy. A series of numerical experiments are carried out, which assumes a three-layer printing process with a fixed laser beam trajectory using a small number of printing control parameters as inputs, namely the laser power, scan speed, and time coordinates. Both surrogates are also principally feasible in other thermal-driven additive manufacturing to obtain better quality assurance with techniques like uncertainty management and closed-loop control.

Effect of laser beam trajectory on pocket geometry in laser micromachining

Abstract The article presents the problem of planning the laser beam trajectory for the laser micromachining process. The article concerns on the ablative laser micromachining issues. Different effects of laser beam trajectory on pocket geometry in laser micromachining were investigated. The results of experimental tests are presented. Based on the research, potential causes of different effects of the laser beam for various trajectories were formulated. Several different types of trajectories for the assumed shape were developed for the purposes of the research. Laser micromachining was performed with fixed parameters of the laser device using different trajectories. The article indicates the significant impact of the laser beam trajectory on the effect of interaction on matter during the laser milling process, which is not often mentioned in scientific reports. The article presents the basic geometrical measurements indicating the need to determine the leading of the laser beam. Studieswere conducted using the microscopic observation methods and interferometric methods for estimating the surface condition. The article indicates the need for extensive research focusing on the mechanism of the impact of the laser beam scan strategy on the effect on the material during ablative laser machining. The article summarizes the analysis and discussion of research results.

Device for source position stabilization and beam parameter monitoring at inverse Compton X-ray sources.

Compact X-ray sources based on inverse Compton scattering provide brilliant and partially coherent X-rays in a laboratory environment. The cross section for inverse Compton scattering is very small, requiring high-power laser systems as well as small laser and electron beam sizes at the interaction point to generate sufficient flux. Therefore, these systems are very sensitive to distortions which change the overlap between the two beams. In order to monitor X-ray source position, size and flux in parallel to experiments, the beam-position monitor proposed here comprises a small knife edge whose image is acquired with an X-ray camera specifically designed to intercept only a very small fraction of the X-ray beam. Based on the source position drift recorded with the monitor, a closed-loop feedback stabilizes the X-ray source position by adjusting the laser beam trajectory. A decrease of long-term source position drifts by more than one order of magnitude is demonstrated with this device. Consequently, such a closed-loop feedback system which enables stabilization of source position drifts and flux of inverse Compton sources in parallel to experiments has a significant impact on the performance of these sources.

Method of crack-free laser writing of microchannels on glass substrates

Approaches to the problem of the formation of microchannels of arbitrary shape on optical glass substrates and other fragile materials without microdefects (cracks and chips) both on the surface of the substrate and inside the substrates are considered. Different approaches to the formation of laser beam trajectories within microchannels are considered. It was found that the raster scanning method improves the performance of the laser micromachining process and provides the ability to write structures of any length without longitudinal cracks with the energy characteristics of the laser spot in the focus much higher than those in the vector scanning method.

Studying Changes in the Microhardness of a Zirconium Surface Modified by Laser Processing

The results of an investigation of the process of laser-induced hardening of zirconium (E110 alloy) are reported. It has been established that the laser treatment in air results in the formation of a uniformly distributed microrelief characterized by the presence of microparticles with various configurations. Overlap of the laser beam trajectories leads to the formation of a layered structure. The composition of laser-modified surface depends on the regime of processing and mostly comprises a mixture of zirconium oxides and nitrides. In addition, the formation of diffuse transition zones between the bulk metal and modified surface layer is established. The dependence of the thickness of the hardened layer on the regime of pulsed laser action is determined.

Исследование изменения микротвердости поверхности циркония после лазерной модифицирующей обработки

AbstractThe results of an investigation of the process of laser-induced hardening of zirconium (E110 alloy) are reported. It has been established that the laser treatment in air results in the formation of a uniformly distributed microrelief characterized by the presence of microparticles with various configurations. Overlap of the laser beam trajectories leads to the formation of a layered structure. The composition of laser-modified surface depends on the regime of processing and mostly comprises a mixture of zirconium oxides and nitrides. In addition, the formation of diffuse transition zones between the bulk metal and modified surface layer is established. The dependence of the thickness of the hardened layer on the regime of pulsed laser action is determined.

Peculiarities of the Microstructure and Properties of Parts Produced by the Direct Laser Deposition of 316L Steel Powder

The direct laser deposition of metal powders is one additive method of producing functional materials. It consists of the melting of metallic powders by a laser beam in inert gas. The main process parameters are the laser-beam power, laser-beam speed and scanning trajectory, and powder consumption. Each parameter is selected depending on the alloy type, which in totality affects the structure and defect formation in products. In this study, experimental rectangular samples of 316L austenitic steel are fabricated by the direct laser deposition of the powder. The microstructure and fractures of samples are investigated using scanning electron microscopy in order to determine the structural features and reveal the defects (pores, holes, crystallization cracks, and oxide inclusions). Uniaxial tension tests and hardness tests are performed. The analysis of the influence of the laser beam scanning trajectory on the microstructure and properties of samples during melting is performed. It is found that a dispersed structure with an average crystallite size of 1.3–1.9 μm is formed at a laser power of 250 W and scanning rate of 16 mm/s, which results in a high level of mechanical properties of experimental samples. It is shown that, when using the lengthwise laser-beam trajectory (along the largest sample size), the tensile strength reaches 730 MPa with a relative elongation of 25%, which exceeds the level of mechanical properties of 316L steel by 110 MPa.

Microstructure and mechanical properties of hybrid LasTIG welds of HSLA steel

Laser welding accompanied by tungsten inert gas welding was applied for 3 mm thick high strength low alloy S460MC steel butt joining with the aim to study the effect of arc electric current on weld cooling rate and thus weld microstructure and mechanical properties. Preheating realized by the arc decreased microhardness peaks in coarse-grained heat affected zone while strength properties were not degraded. Dissimilar S460MC to DC01 mild steel welds were performed with different positions of tungsten electrode towards the laser beam trajectory to maintain uniform weld profile.

Surface treatment to form a dispersed Y2O3 layer on Zircaloy-4 tubes

Zircaloy-4 is a traditional zirconium-based alloy developed for application in nuclear fuel cladding tubes. The surfaces of Zircaloy-4 tubes were treated using a laser beam to increase their mechanical strength. Laser beam scanning of a tube coated with yttrium oxide (Y2O3) resulted in the formation of a dispersed oxide layer in the tube’s surface region. Y2O3 particles penetrated the Zircaloy-4 during the laser treatment and were distributed uniformly in the surface region. The thickness of the dispersed oxide layer varied from 50 to 140μm depending on the laser beam trajectory. The laser treatment also modified the texture of the tube. The preferred basal orientation along the normal to the tube surface disappeared, and a random structure appeared after laser processing. The most obvious result was an increase in the mechanical strength. The tensile strength of Zircaloy-4 increased by 10–20% with the formation of the dispersed oxide layer. The compressive yield stress also increased, by more than 15%. Brittle fracture was observed in the surface-treated samples during tensile and compressive deformation at room temperature; however, the fracture behavior was changed in ductile at elevated temperatures.

FEA simulation of thermal processes during the direct metal laser sintering of Ti64 titanium powder

Abstract With regard to the fact that laser sintering belongs to the high-temperature processes in which metal particles are sintered by a high-power laser, forming a homogenous structure, it is necessary and important to know the characteristics and the mechanism of these thermal processes. A high-power laser system produces three forms of heat that include convection, conduction, and radiation. These thermal processes affect the formation of internal stresses and tension that lead to deformations and rapidly influence the resulting quality, dimensions, density, micro-structure, and mechanical properties of fabricated parts. In response to this fact, it was important to analyse these heat transfer methods instantly during the direct metal laser sintering (DMLS) process simulation and subsequently monitor the parameters and settings of the sintering equipment in order to obtain acceptable manufacture outputs intended for further use. This work is focused on the creation of a FEA simulation model and the simulation of thermal processes across an object during and after the sintering process in the cooling stage, when it is important to consider a laser beam trajectory, temperatures of individual elements affected by the laser beam, and current laser energy in time. A 3D FEA simulation model was created in order to represent actual behaviour of a part during the sintering process. The simulation model consisted of two sub-models, particularly the building platform model with the dimensions of 250 mm × 250 mm × 22 mm, with stainless steel as the selected material, and the model of individual layers of sintered titanium powder with the dimensions of 10 mm × 10 mm × 0.03 mm. The total number of used layers was 12, which represents the total thickness of 0.36 mm. Applied power was P = 170 W. The simulation as such was carried out using the FEA software, Simulia Abaqus supported on the Windows x86-64 platform, which uses an integrated solver to make thermal and mechanic calculations. The calculations included also the impact of the protective argon atmosphere located in the process chamber. Mutual impact between individual layers was also considered. The simulation results were confronted with the results of already performed experimental studies of other scientific works, with the compliance and confirmation of assumptions being on a very good level.

Laser-welded fused silica substrates using a luminescent fresnoite-based sealant

The laser welding of two fused silica substrates using a fresnoitic glass thin film as a sealant by irradiation with a ns laser is studied. Two different laser parameter sets were compared in terms of bond quality, which include two different laser beam trajectories: linear and wobble (circular) trajectory. The composition of the glass sealant changes with the course of the laser welding, incorporating silica from the substrates. After joining, the bonded samples were exposed to UV light and a very intense emission in the blue spectral range is observed by naked eye, which is due to the crystallization of the fresnoite glass upon the laser irradiation. EDX analysis confirms the crystallization of fresnoite, together with a great enrichment in silica. The formation of a eutectic between both is very plausible. Bond quality and bond strength were evaluated by scanning acoustic microscopy (SAM) and tensile test, which results in a tensile stress of 7MPa.

The effect of laser beam wobbling mode in welding process for structural steels

A laser welding process using a 30-kW fiber laser with scanning mode optics is investigated in the paper. Welding is conducted in two ways: constant laser beam trajectory and wobbling trajectory with the use of lower speed and power. The main goal was to investigate the influence of the second wobbling laser welding pass on microstructure and mechanical properties of structural steel. The following parameters were monitored: visual control and mechanical properties (microhardness, three-point bend, and Charpy impact V-notch test); metallographic analysis and 2D and 3D computer tomography (CT) were also done. The results show that after the second welding pass, with wobbling trajectory of laser beam, middle and cap parts of the seam have a lower microhardness, in relation to the root part. It can be explained by annealing influence of the second wobbling pass at weld metal.

Active control of electromagnetic radiation through an enhanced thermo-optic effect.

The control of electromagnetic radiation in transformation optical metamaterials brings the development of vast variety of optical devices. Of a particular importance is the possibility to control the propagation of light with light. In this work, we use a structured planar cavity to enhance the thermo-optic effect in a transformation optical waveguide. In the process, a control laser produces apparent inhomogeneous refractive index change inside the waveguides. The trajectory of a second probe laser beam is then continuously tuned in the experiment. The experimental results agree well with the developed theory. The reported method can provide a new approach toward development of transformation optical devices where active all-optical control of the impinging light can be achieved.

Micromagnetization patterning of sputtered NdFeB/Ta multilayered films utilizing laser assisted heating

A process for magnetizing small areas of a multilayered several micrometer thick NdFeB/Ta permanent magnet (PM) deposited by magnetron sputtering was developed. In this process, areas of the PM film heated by laser beam scanning were magnetized under an external DC magnetic field. First, by measuring the demagnetization curve at room temperature and 300°C, the target temperature and external DC magnetic field were determined. These were found to be 300°C and 0.8–0.9T, respectively. Then, in order to satisfy these requirements, the heat conduction during laser heating was simulated, resulting in glass being chosen as the substrate material and, in addition, determining the laser beam scanning conditions. We also fabricated a DC magnetic field generator consisting of an array of bulk magnets which was designed using a magnetic field simulator. By adjusting the laser beam trajectories, magnetization patterns consisting of stripes with a width of 100μm were experimentally generated in the 4.5μm thick PM film. Agreement between the measured and simulated magnetic flux densities over the PM film was obtained. Furthermore, a 70μm wide striped pattern, a 100μm diameter dot pattern and a pattern consisting of a set of letters were also generated in the PM film. The desired magnetic field pattern could be successfully observed.

The Research for Control of the Laser Beam Trajectory on Surface Hardening Using NC

Laser surface hardening is widely used. The complexity of the surface restricts its application. To solve this problem, it is necessary to study the path of the laser spot. By comparison the difference of CNC cutting machine and laser processing machines, the control system of laser spot is built. And the method of alignment for work coordinate system is obtained. NC programming model of the laser beam is established using CAM software. The change of area of spot when laser beam projected to the different normal angles surface was analyzed, and it is get the relationship of normal angles and feed rate. The complex surface laser surface hardening is realized.

Performance analysis of continuous tracking laser Doppler vibrometry applied to rotating structures in coast-down

In this paper a performance analysis of the so-called tracking continuous scanning laser Doppler vibrometry (TCSLDV) exploited in coast-down has been performed. This non-contact measurement system is able to scan continuously over a rotating surface during coast-down and to determine vibration operational deflection shapes (ODSs) and natural frequencies in short time, i.e. the temporal extent of the coast-down. The method is based on a laser Doppler vibrometer (LDV) whose laser beam is driven to scan continuously over the whole rotor surface synchronously with its rotation, so that the LDV output is modulated by the structure's ODSs. This technique has a full-field nature that enables it to measure simultaneously the time and spatial dependence of the vibration in a unique measurement. However, the TCSLDV presents some criticalities in practical applications, especially when applied to rotary transient and fast processes. In fact, if the vibration is transient and decays very fast, then the laser beam could not have had the time to scan the complete structure surface and the modulation of the ODS could be partial. An analytical model reproducing a representative experiment has been developed in order to evaluate the sensitivity of results to testing conditions. The laser beam trajectory in both the fixed and rotating reference systems has been synthesized showing its dependence on experimental parameters as the rotation speed variation during coast-down. It has been demonstrated the decrease in speed induces the deformation of the laser trajectory influencing the LDV output time history, spectrum and consequently the recovered ODS.

A source identification problem related to mathematical model of laser surface heating

A mathematical model of two-dimensional laser surface heating for the hardening of metallic materials is proposed. The model is governed by the heat equation u t − Δ u = m ( t ) δ γ ( x − ω ( t ) ) , ( x , t ) ∈ Ω , with the pointwise source term δ γ ( y ) , satisfying the initial u ( x , 0 ) = g ( x ) and boundary u ( x , t ) = 0 , x ∈ ∂ Ω , conditions. The pair of source terms 〈 m ( t ) , ω ( t ) 〉 is assumed to be unknown. The two-valued ( m ( t ) = 0 or m ( t ) = m 0 > 0 ) function m ( t ) is treated as the intensity of the laser beam, and the function ω ( t ) describes the laser beam trajectory. The identification problem consists of determining the pair of source terms 〈 m ( t ) , ω ( t ) 〉 such that the corresponding heat function u ( x , t ) satisfies the condition ‖ u − v ‖ L 2 ( Ω ) ≤ ε , where the smooth function v ( x , t ) is assumed to be known (experimentally), and ε > 0 is a given-in-advance parameter. Besides the existence result, the structure of the optimal trajectory is also described.

Effect of Laser Beam Trajectory on Donor Plate in Laser Induced Thermal Printing Process

Organic (Alq3) film, which was coated on a donor plate, was transferred to an organic light emitting diode (OLED) substrate with help of heat generated by a dithering laser beam. The laser beam was diffracted in an acousto-optic modulator (AOM), then focused on the laser-to-heat converting layer of the donor plate; the focused spot followed trajectories guided by rotation of a Galvano-mirror. Three different functional waveforms, sine wave, square wave, and saw tooth wave were applied to the AOM as modulation signal to generate the dithering beam. The fluorescence microscope images of the donor plate showed that the patterns of removed Alq3 film were affected considerably by the modulation waveforms and the phase difference between adjacent dithering beams. Further, the printed images of Alq3 film on the OLED substrate were different from the patterns of removed Alq3 film. Atomic force microscope images indicated that not only direct transfer but also deposition by sublimated vapor of Alq3 contributed to the pattern formation. Printed patterns affected considerably the electricity-to-light conversion characteristics of OLEDs. For uniform transfer, not only the phase relation of dithering beam lines but also adequate waveform were important.

Product Model Preparation and Processing for Micromanufacturing

Based on the two-photon polymerization technique, an analysis of product shapes is performed so that their digital manufacturing models can be efficiently processed for micromanufacture. To describe microstructures, this analysis shows that nonmanifold models are of interest. These models can be intuitively understood as combinations of wires, surfaces, and volumes. Minimum acceptable wall thickness, wire dimension, and laser density of energy are among the elements justifying this category of models. Taking into account this requirement, a model preparation and processing scheme is proposed that widens the laser beam trajectories with a concept of extended layer manufacturing technique. A tessellation process suited for non-manifold models has been developed for computer-aided design models imported from standard for the exchange of product files. After tessellation, several polyhedral subdomains form a nonmanifold polyhedron. To plan the trajectories of the laser beam, adaptive slicing and global 3D hatching processes as well as a “welding” process (for joining subdomains of different dimensionality) have been combined. Finally, two nonmanifold microstructures are fabricated according to the proposed model preparation and processing scheme.

Closed laser-beam trajectories in plano–spherical resonators with Gaussian apertures

The closed trajectories of a Gaussian beam in plano–spherical degenerate and nondegenerate resonators with two Gaussian apertures (with two pump regions with the Gaussian distribution of the gain) are described analytically and the existence conditions are found for such trajectories. It was assumed that Gaussian apertures (gain regions) were located in the plane of a flat mirror at equal distances on both sides of the resonator axis and the size of apertures restricted their action by the corresponding half-plane.