Laser Energy Distribution Research Articles

Theoretical modeling of feedstock-laser energy interaction in directed energy deposition with simultaneous wire and powder feeding

The Laser-based Directed Energy Deposition (L-DED) process is widely employed for the fabrication of numerous metallic parts. It also stands out as a crucial additive manufacturing (AM) technique for creating metal matrix composites (MMCs) reinforced with ceramic particles. The simultaneous deposition of wire and powder feedstocks into the created melt pools on the substrate surface proves advantageous in overcoming the limitations associated with mixed powders in powder deposition alone. The strategy of coaxial wire and powder deposition (CWP-DED) allows for more efficient construction of MMCs with complex geometries and desirable properties. Nevertheless, CWP-DED is a complex multi-phase deposition process that involves highly dynamic interactions among laser beams, wire, and powder particles, influencing laser energy distribution and absorption during the process. This work aims to develop a theoretical model to advance the fundamental understanding of the absorbed and lost laser beam energy by the fed wire and powders in CWP-DED before entering the melt pool. Laser energy absorption coefficients can be calculated based on the Step Wise Heating method to determine the final temperature of the powder particles and wire before entering the melt pool. The experimental validation of the proposed model requires a power meter that fits the CWP-DED system with six inclined laser beams, which is not commercially available so far, to measure the actual output power with and without wire and powder feeding. Consequently, this work presents a detailed explanation of the experimental procedure desired to measure the absorption coefficients of the wire and powder feedstocks and how to validate the proposed model as a future research endeavor. This model plays a vital role in predicting and understanding the amount of heat transferred into the melt pool, which lays a foundation for understanding the complicated melt pool thermodynamics that directly govern the microscopic characteristics and macroscopic properties of the as-built parts.

Study on asymmetric forming and performance strengthening mechanisms of 6061-T6 aluminum alloy joints fabricated by oscillating laser based on keyhole characteristics and molten pool dynamic behaviors

The study intends to unravel the processes behind the particular forming and performance improvement of 6061-T6 aluminum alloy joints by getting a thorough understanding of keyhole features and dynamical characteristics of molten pools during the laser welding with circular oscillation (C-OLW) process. The C-OLW experiments were carried out considering the pivotal parameters, including oscillation frequency (f) and amplitude (A), and their impacts on joint formation, microstructure, and performance were analyzed. Increasing f from 200 Hz to 300 Hz results in a more homogeneous and centered laser energy distribution, reducing weld breadth and the maximum depth. Raising A from 1.2 mm to 1.5 mm concentrated energy along the workpiece surface, increasing weld breadth but decreasing the maximum weld depth. A three-dimensional transient numerical model was established and verified. The simulation results reveal that increasing f considerably improves keyhole stability by virtue of the beam's mixing effect on energy, resulting in a more evenly distributed temperature within the molten pool. The velocity of the beam's motion intensifies, leading to elevated local temperature and subsequently causing an escalation in splattering. When A grows, keyhole stability reduces, as does the intensity of molten pool flow. The major effect of increased frequency is refining the grain size while increasing amplitude primarily promotes the columnar-to-equiaxed transition (CET) process. The extracted results of temperature distribution illustrate that an increase in f leads to an increase in the solidification rate (R), and temperature gradient values (G) are similar or higher along the same isotherm. Therefore, the G/R ratio rises, promoting the formation of finer microstructures. Although increasing A diminishes G, the reduced overall temperature and molten pool volume contribute to the rise in R. Consequently, the G/R ratio declines, fostering equiaxed grain growth, in line with experimental observations. A higher G value on the left side corresponds to finer grain structures, corroborating experimental findings via Electron Backscatter Diffraction (EBSD). The keyhole stability and microstructure characteristics explain why increasing f or A can enhance joint performance. The research reveals that strategically augmenting both oscillation frequency and amplitude enables the achievement of enhanced performance in 6061-T6 aluminum alloy joints.

Laser contacting of hairpin windings in electrical vehicle motors using ring-mode laser beams based on angularly-uniform Lissajous trajectories.

The contacting process of hairpin windings is deemed the most important and challenging process in manufacturing electric vehicle motors. This paper proposes a ring-mode laser contacting method for copper hairpin windings based on Lissajous-shaped cyclic scanning with a uniform angular speed. Two key variables, trajectory cycle n and angular multiplier k, which represent the laser energy input and distribution respectively, were investigated via single-factor experiments. Morphology, microstructure, mechanical and electrical properties were analyzed consequentially to show the influence on the contact quality. The results show that, as n increases, contact appearance is criticized for being under-contacted, well-contacted, or over-contacted. Molten pool behaviors affected by n variations determine morphology formation. Larger k facilitates molten pool evolution and improves laser contacting efficiency. Fusion zone (FZ) has directional columnar grains, while heat-affected zone (HAZ) has overgrown equiaxed grains coarser than those of base metal (BM). The microstructure of FZ becomes coarser if n increases, or finer if k increases. The highest tensile force of 712.5 N was reached when k = 4, n = 6. Ductile dimple fractures in FZ indicate the contacts' excellent strength and toughness. FZ and HAZ show lower microhardness than BM due to thermal softening. FZ is slightly harder than HAZ due to its densely-interlaced microstructure. Poor contacts cause reduction in their electrical conductivity and increase in their resistance.

Parametric study of the laser energy absorption in high-power laser beam welding

Laser energy absorption on the keyhole wall is decisive for the thermodynamic behavior and the resultant weld properties in the high-power laser beam welding process. However, its highly transient nature on a microsecond scale makes the quantitative analysis challenging. In this paper, the influence of the relevant welding parameters on laser energy absorption is studied statistically by utilizing multiphysical modeling, in which the three-dimensional transient keyhole dynamics and thermo-fluid flow are calculated. A dynamic mesh adaption technique and a localized level-set-based ray-tracing method are employed to improve the model accuracy further. The results show that the focus position has a remarkable effect on the time-averaged laser absorption, and in contrast, the laser energy distribution regime is only slightly influenced by the welding speed in the studied parameter range (1.5–3.0 m/min). The absorption ratio of the laser energy on the keyhole front wall decreases with increasing welding speed and increases with upward-moving focus positions. The comparison between the calculated results and the experimental measurements ensures the validity of the proposed model.

Implementation of energy interaction between laser beam and keyhole wall based on algebraically reconstructing free surface: An experimentally validated numerical framework

The interaction of the laser beam with the material is critical for determining laser processing quality, with ray tracing methods mostly used in energy transfer research. These techniques generally rely on only the volume of fluid (VOF) method or level set (LS) function to track the molten pool free surface, resulting in either accuracy loss or difficulty in ensuring mass conservation of the interface. The few coupled algorithms are built on geometric notions that are difficult to understand while being limited to structural grids. In this paper, we have proposed a revised ray tracing framework, so-called ARCLS, to derive the exact intersections of the rays with the keyhole wall which is represented by a continuous LS surface algebraically rebuilt on top of the VOF interface. It is easy to code and convenient for unstructured meshes. This framework is tested through a V-groove case and a set of Ti6Al4V titanium alloy laser welding examples. The findings reveal that the reconstructed LS surface perfectly matches the VOF interface, and the predicted molten pool profile agrees with the actual data well. The keyhole morphology and molten flow patterns are also consistent with the physical truths. Spattering, porosity, necking, and swelling are observed in the developing keyhole, which are the possible reasons for the potential welding defects. Further investigation suggested that the ARCLS-based algorithm collected more laser rays at the keyhole bottom compared to the standard technique, achieving a deeper keyhole profile and a reflection pattern that was more optically logical. The work presents a reliable approach for estimating laser energy distribution, which can be effectively applied to the heat-fluid coupling simulation for laser manufacturing processes in order to forecast the dynamic behaviors of keyhole and molten pool.

Modeling of Yb:YAG Laser Beam Caustics and Thermal Phenomena in Laser-Arc Hybrid Welding Process with Phase Transformations in the Solid State.

This paper focuses on the mathematical and numerical modeling of the electric arc + laser beam welding (HLAW) process using an innovative model of the Yb:YAG laser heat source. Laser energy distribution is measured experimentally using a UFF100 analyzer. The results of experimental research, including the beam profile and energetic characteristics of an electric arc, are used in the model. The laser beam description is based on geostatistical kriging interpolation, whereas the electric arc is modeled using Goldak's distribution. Hybrid heat source models are used in numerical algorithms to analyze physical phenomena occurring in the laser-arc hybrid welding process. Thermal phenomena with fluid flow in the fusion zone (FZ) are described by continuum conservation equations. The kinetics of phase transformations in the solid state are determined using Johnson-Mehl-Avrami (JMA) and Koistinen-Marburger (KM) equations. A continuous cooling transformation (CCT) diagram is determined using interpolation functions and experimental research. An experimental dilatometric analysis for the chosen cooling rates is performed to define the start and final temperatures as well as the start and final times of phase transformations. Computer simulations of butt-welding of S355 steel are executed to describe temperature and melted material velocity profiles. The predicted FZ and heat-affected zone (HAZ) are compared to cross-sections of hybrid welded joints, performed using different laser beam focusing. The obtained results confirm the significant influence of the power distribution of the heat source and the laser beam focusing point on the temperature distribution and the characteristic zones of the joint.

Effect of energy distribution on laser cleaning quality of 30Cr3 ultra-high strength steel

30Cr3 is a novel ultra-high strength steel (UHSS) with excellent strength and plastic toughness, which has been mainly used in aerospace shells and stressed parts. Currently, there is still a lack of systematic research and comprehensive evaluation on the laser cleaning of this particular material. In this study, a nanosecond pulsed laser was employed for the cleaning process. First, the energy distribution model was derived, the influence of the spatial and temporal distribution of laser energy on the cleaning quality was discussed, and the cleaning experiment was conducted. The results showed that a laser energy density of 15 J/cm2, combined with the beam overlap rate 52% in the x-direction and 61% in the y-direction, can effectively remove the oxide layer and thoroughly expose the natural metal matrix. Then, under the optimum parameters, the oxygen content decreased from 22.3% to 3.4%, the Fe content increased from 54.5 to 83.4%, the Fe3+ peak disappeared in the X-ray photoelectron spectroscopy (XPS), a remelted layer of only 1 μm was observed. Last, the roughness was slightly reduced from 3.573 μm to 2.985 μm, the hardness was restored to the original hardness level of the substrate (∼260 HV), and the electron back scatter diffraction (EBSD) showed that the structure hardly changed after cleaning, the grain size was comparable to that of the matrix. These findings indicated that laser cleaning can effectively remove the oxide layer of 30Cr3 UHSS without damaging the properties of the base material, which has promising application in the cleaning of UHSS.

Influence of Scanning Strategy and Post-Treatment on Cracks and Mechanical Properties of Selective-Laser-Melted K438 Superalloy

The feasibility of manufacturing high-performance components with complex structures is limited due to cracks in some superalloys fabricated by selective laser melting (SLM). By controlling the main process parameters such as scanning strategy, the adverse effects of cracks can be effectively reduced. In this paper, the effects of two different SLM scanning strategies with island and ‘back-and-forth’ and post-heat treatment on the cracks and mechanical properties of selective-laser-melted (SLMed) K438 alloy were investigated. The results show that the SLM method of the ‘back-and-forth’ scanning strategy had better lap and interlayer rotation angles and a more uniform distribution of laser energy compared with the island scanning strategy. The residual stress accumulation was reduced and crack formation was inhibited under this scanning strategy owing to the cooling and shrinkage process. In addition, the dislocation motion was hindered by the formation of uniformly dispersed MC carbides and γ’ phases during the SLM K438 alloy process, which resulted in the density of the as-built SLMed K438 alloy being up to 99.34%, the hardness up to 9.6 Gpa, and the tensile strength up to 1309 MPa. After post-heat treatment, the fine secondary γ’ phases were precipitated and dispersed uniformly in the Ni matrix, which effectively improved the Young’s modulus and tensile strength of the alloy by dispersing the stress-concentrated area.

Influence of oscillation parameters on energy distributions and dynamic behaviors during laser welding of aluminum alloy T-joints assisted with solder patch

The laser energy distribution on base material (BM) surface plays an important role in the welding quality. To reduce pore and spatter welding defects during laser welding of aluminum alloy T-joints, a circular shaped oscillating laser beam welding (OLBW) method for aluminum alloy T-joints assisted with solder patch is proposed in this paper. An energy distribution model and a dynamic model for OLBW of aluminum alloy T-joints assisted with solder patch are established to analyze the influence of oscillation parameters on energy distributions of BM surface and dynamic behaviors of molten pool and keyhole. The energy distribution characteristics are obtained by the energy distribution model and the dynamic behaviors of molten pool and keyhole are calculated by the dynamic model for OLBW of aluminum alloy T-joints assisted with solder patch. The results show that the uniformity of energy distribution is improved with the increase in oscillation amplitude or frequency. The oscillation parameters can be adopted to adjust the keyhole stability and the temperature distributions, flow velocities and morphologies of molten pool, which is of great importance for aluminum alloy T-joints welding with high quality.

Simulation of evaporation ablation dynamics of materials by nanosecond pulse laser of Gaussian beam and flat-top beam

Based on the established two-dimensional asymmetric model of the interaction between a nanosecond pulse laser and metallic aluminum, the effect of beam shaping on the evaporation ablation dynamics during the ablation of metallic aluminum by a nanosecond pulse laser is simulated. The results show that plasma shielding, which has a significant influence on the ablation properties of the target, occurs mainly in the middle phase and late phase of the pulse. Among the three laser profiles, the Gaussian beam has the strongest shielding effect. As the diameter of the reshaped flat-top beam increases, the shielding effect gradually weakens. The two-dimensional spatial distribution of target temperature is relatively different between ablation by a Gaussian beam and that by a flat-top beam. For the Gaussian beam, the center of the target is first heated, and then the temperature spreads in radial direction and axial direction. For the flat-top beam, due to the uniform energy distribution, the target is heated within a certain radial range simultaneously. Beam shaping has a great influence on the evaporation ablation dynamics of the target. For the Gaussian beam, the center of the target is first ablated, followed by the radial ablation. For the flat-top beam, the evaporation time of the target surface is delayed due to the lower energy density after the beam has been shaped. In addition, the target evaporates simultaneously in a certain radial range due to the more uniform distribution of laser energy. For each of the three laser profiles, the evaporation morphology of the target resembles the intensity distribution of the laser beam. The crater produced by the Gaussian beam is deep in the center and shallow on both sides, while it becomes relatively flat by the flat-top beam.

Effect of variable laser incident angles on microstructure and mechanical properties of laser-cladded Inconel 718

In the laser cladding process of complex components, laser deflection is a significant issue that greatly affects manufacturing quality. This study investigates the influence of different incident angles on the microstructure and wear resistance of laser-clad layers. Firstly, an Inconel 718 coating was prepared on an Inconel 718 substrate using an IPG fiber laser. Next, the macrostructure and microstructure of the coating were observed using electron microscopy. Finally, the microhardness of the coating was measured, and the underlying mechanisms affecting wear resistance due to incident angle were revealed. The results show that with an increase in the laser incident angle, the laser energy distribution becomes more divergent, leading to a shift in the peak energy density and changes in the macroscopic morphology of the melt pool, exhibiting a non-Gaussian distribution and resulting in increased cellular dendrite count and coarsened grains. Laser deflection causes variations in microstructure and properties between inner and external angles, where higher laser energy density and temperature gradients at the inner angle result in finer grains and more numerous smaller secondary dendrites on the dendritic arms, along with less severe element segregation and higher microhardness at the inner angle.

Study on the influence of local optical field amplification effect on laser-induced damage of fused silica materials

Fused silica is an important part of optical components in large laser systems. Due to the limitation of manufacturing process, impurities and defects in the optical element would greatly reduce the service life of the optical element, and significantly reduce the final output of the laser performance. Aiming at the modulation effect of the internal defect of the component on the internal optical field of the component, the theoretical, simulation and experimental research are carried out. The results show that in the double-bubble impurity coupling, under the same radius R, when the impurity spacing is 1 λ, the local optical field amplification has a maximum value. The effect is equivalent to single-bubble modulation with radius 2 R–3 R. There is an extreme point of the modulated optical field between the air and fused silica crossing line for bubble impurities of different radii. The optical field modulation of small radius impurities is distributed behind the bubble impurities, and the modulation effect of large radius impurities is the maximum when the spacing is 2 λ. The temperature distribution curve of the fused silica element modulated by bubble impurities is consistent with the optical field distribution curve, showing a trend of decreasing slope. The presence of bubble impurities will cause the surface combustion wave of the component to flash and accelerate, and the bubble impurities will increase the generation and expansion rate of the surface combustion wave. This study provides a basis for reducing the uneven distribution of laser energy during the interaction between laser and fused silica, improving the lifetime of the overall optical system, and experimental measurement and analysis.

Numerical study on multiphase evolution and molten pool dynamics of underwater wet laser welding in shallow water environment

Underwater wet laser welding (UWLW) is a promising and labor-saving in-situ repair technique in aquatic environments, which simplifies the arduous dismantling and transportation processes of repairing underwater equipment on land. However, the thermo-physical mechanism during UWLW is still unclear. This study presents, for the first time, a thermal multi-phase flow model to study the heat transfer, fluid dynamics, and phase transitions involved in the UWLW process at shallow water column depths (water depth ≤ 4 mm). A dual-cone volumetric heat source model considering the laser energy distribution in both the water and metal is proposed. The driving forces, including gravity, surface tension, and the water/metal vapor recoil pressure are taken into account. The simulated weld profiles are in good agreement with the experimental results. The results indicate that in the UWLW process, laser heating creates a water keyhole, forming a localized dry region (LDR) above the molten pool. The LDR isolates the water from the molten pool and protects the laser-metal interactions, thereby making the welding environment similar to that of in air laser welding (IALW). Due to the water evaporation, the upper rear and sides of the molten pool during the UWLW pocessess a higher cooling rate compared to the IALW case, resulting in a higher temperature gradient and smaller molten pool dimensions. The results contribute to a better understanding of the complex interactions in UWLW process.

A novel strategy for promoting corrosion and wear resistance of Mg-Li alloys: Gradient eutectic high-entropy alloy coating induced by in-situ bidirectional diffusion

In this work, a novel surface strengthening strategy for Mg-Li alloys was proposed, called cold spraying assisted high-speed laser cladding. CuAl9 aluminum bronze coating was firstly deposited on Mg-Li alloy by cold spraying, and then CoCrFe0.5Ni1.5Mo0.1Nb0.68 eutectic high-entropy alloy (EHEA) coating was prepared on the CuAl9 coating utilizing high-speed laser cladding. A gradient coating consisted of in-situ transition region and EHEA layer formed by bidirectional diffusion. The in-situ transition region was composed of β-Li, Cu2Mg and Cu3Al2 phases. TEM analysis indicated that Cu2Mg and Cu3Al2 phases were well matched with the β-Li matrix phase. The EHEA coating had a nano-lamellar eutectic microstructure with relatively small lamellar-spacing (< 100 nm). Metallurgical bonding interfaces formed between the EHEA coating, transition region and Mg-Li substrate. The evolution mechanism of the coating was revealed from the perspectives of mixing enthalpy, atomic radius difference and laser energy distribution. In 3.5 wt.% NaCl solution, the corrosion potential of the EHEA coating (-24 mVSHE) was 1345 mVSHE higher than that of Mg-Li alloy (-1369 mVSHE), while the corrosion current density of the EHEA coating (3.13 × 10−7 A·cm−2) was almost three orders of magnitude lower than that of Mg-Li alloy (1.25 × 10−4 A·cm−2). The wear rate of Mg-Li alloy (1.11 × 10−3 mm3/N·m) was about 36 times higher than that of the EHEA coating (3.05 × 10−5 mm3/N·m).

Effect of laser-arc synergy on melting energy in laser-CMT hybrid welding of aluminum alloy

The physical process of laser-arc synergy in laser-cold metal transfer (CMT) hybrid welding of aluminum alloys is extremely complex and difficult to observe, which limits our understanding of laser-arc synergy and the industrial application of laser-CMT hybrid welding. In this paper, the laser-arc synergistic effect is studied quantitatively by the change of melting energy. Therefore, the dimensionless parameter (ψ) is introduced to discuss the effect of heat source guidance, energy distribution and distance of laser and arc (DLA) on melting energy increment. The results show that the novel laser-arc simultaneous co directional hybrid welding (no-guided) process increases ψ by 4.3 times and 1.8 times compared to CMT-guided and laser-guided processes, respectively. The laser-arc synergy is strongest when the laser power is 80% of the total power and DLA is 2 mm. The quantitative analysis method in this paper can be extended to the study of other hybrid heat sources, and this research can facilitate the optimization of the welding process and a better understanding of the hybrid welding mechanism.

Predicting temperature field in keyhole-mode selective laser melting with combined heat sources: a rapid model

Predicting the temperature field during selective laser melting (SLM) is crucial for improving the performance of printed parts. However, there is still a lack of an efficient and accurate model for predicting the temperature field of keyhole-mode melting in SLM. Based on the physical phenomena of keyhole-mode melting observed in experiments and simulations, this study proposes an analytical model for rapidly predicting the temperature distribution during SLM keyhole-mode melting. The model considers vapor depression in the molten pool and the interaction between the laser and molten pool during keyhole-mode melting. The model was validated using numerical simulations and experimental data. The variation trend of the laser energy distribution and molten pool size with respect to the laser energy density was revealed. As the laser energy density increased, the depth of the molten pool and the vapor depression increased linearly, and the molten pool width increased to a peak and then remained constant. The process parameter window to avoid a lack-of-fusion was also investigated. With a computation time of 15 s and a prediction error of less than 10%, this model is an effective way to simulate SLM processes and guide the optimization of process parameters.

Analysis and Experiment of Laser Energy Distribution of Laser Wireless Power Transmission Based on a Powersphere Receiver

Laser wireless power transmission (WPT) is one of the most important technologies in the field of long-range power transfer. This technique uses a laser as a transmission medium instead of conventional physical or electrical connections to perform WPT. It has the characteristics of long transmission distance and flexible operation. The existing laser wireless power transmission system uses photovoltaic cells as a receiver, which convert light into electricity. Due to the contradiction between the Gaussian distribution of laser and the uniform illumination requirements of photovoltaic cells, the laser wireless power transmission technology has problems such as low transmission efficiency and small output power. Therefore, understanding the energy distribution changes in the laser during transmission, especially the energy change after the laser is transmitted to each key device, and analyzing the influencing factors of the energy distribution state, are of great significance in improving the transmission efficiency and reducing the energy loss in the system. This article utilizes the optical software Lighttools as a tool to establish a laser wireless power transmission model based on a powersphere. This model is used to study the energy distribution changes in the laser as it passes through various components, and to analyze the corresponding influencing factors. To further validate the simulation results, an experimental platform was constructed using a semiconductor laser, beam expander, Fresnel lens, and powersphere as components. A beam quality analyzer was used to measure and analyze the laser energy distribution of each component except for the powersphere. The output voltage and current values of various regions of the powersphere were measured using a multimeter. The energy distribution of the powersphere was reflected based on the linear relationship between photo-generated current, voltage, and light intensity. The experimental results obtained were in good agreement with the simulation results. Simulations and experiments have shown that using a beam expander can reduce divergence angle and energy loss, while employing large-aperture focusing lens can enhance energy collection and output power, providing a basis for improving the efficiency of laser wireless power transfer.

LIFT of cell spheroids: Proof of concept

The application of spheroids in tissue engineering has a number of advantages over conventional cell suspensions and 2D cultures. One of the methods for tissue and organ fabrication from spheroids is bioprinting. As one of bioprinting methods, laser-induced forward transfer (LIFT) has received much attention in terms of cell printing, while its potential has not been realized for spheroid patterning yet. In this paper, the authors have shown for the first time the practical applicability of LIFT for spheroid transfer with high survival rates and printing precision. For this, a special optical device, a piShaper, was used to change the laser energy distribution to non-Gaussian profile which allowed for mitigating the negative effects of laser radiation on the spheroids during LIFT. The authors showed that non-Gaussian energy distribution in the laser spot in the form of double ring led to higher post-printing viability of spheroids than in case of conventional Gaussian energy distribution in laser beam. Subsequently, using the double ring laser spot geometry, the spheroids were bioprinted in the form of simple geometric figures: line, triangle, and square. Overall, LIFT bioprinting of spheroids has demonstrated a strong potential as the precise, safe, and reproducible method for biofabrication that can be potentially used for making tissue-engineered bioequivalents or building specific organ-on-a-chip platforms.

Ultra-precision cutting characteristics of binderless tungsten carbide by in-heat-process laser-assisted diamond machining

The study systematically investigates the ductile-brittle transition behavior, material removal mechanism, and end-face turning performance of binderless tungsten carbide using in-process-heat laser-assisted diamond machining. Both numerical simulation analysis and experimental cutting experiments were conducted. To ensure that the laser loading condition and energy distribution were consistent with the ray-tracing simulation and spatial power density distribution measurement, a practical thermal simulation model was created with a semicircular Gaussian distribution. This model was then used to study the temperature distribution, and the material's thermal transient response to laser radiation was also analyzed in detail. The high-temperature nanoindentation revealed increased ductile machinability due to plastic deformation, microhardness reduction, and Young's modulus decrease at elevated temperatures, providing fundamental support for the ductile-brittle transition behavior. The ductile-brittle transition depth increased by 247.20% compared to conventional groove cutting without laser assistance. The cutting forces during groove cutting decreased gradually with increasing laser power, but excess laser power may cause fluctuations in cutting forces due to surface high-temperature oxidation and potential thermal expansion. The ductile-brittle transition dynamic behavior analysis, based on the discrete wavelet transform algorithm, showed good consistency with the conventional groove profile analysis. The friction coefficient between the tool rake face and chip decreased with increasing laser power, which facilitated improving the tool life. The statistical analysis of end-face turning experiments based on the Taguchi method indicated that laser power was the most significant factor affecting surface quality. The verification turning experiments validated the determined optimal parametric combination. Ultra-high-quality spherical lenses were obtained in ductile cutting mode with slightly less tool wear, a more homogeneous finish (less than 3 nm in Sa), and more outstanding form accuracy (133 nm in PV) compared to conventional turning without laser assistance.

Propagation and focusing dependency of a laser beam with its aberration distribution: understanding of the halo induced disturbance

In various applications, it is necessary to understand laser field dynamics during its propagation, especially at the focal position including the dispersed energy surrounding the main pulse, called the halo effect. For instance, the properties of electron beams produced by laser wakefield acceleration (LWFA) strongly depend on the laser energy distribution and its halo in the vicinity of the focus. Indeed, under certain conditions, this halo, or even its internal structures, can propagate and be self-focused independently of the main pulse in the plasma. This paper aims to provide sufficient tools to properly describe the behavior of a focused laser beam, including the halo. Subsequently, an optical description regarding the source of this halo is provided. A more accurate estimation of the input laser beam that should be used in simulations of high-power laser applications may now be obtained. Finally, one may also find ways to positively manipulate the laser beam. Using Fresnel diffraction theory, the propagation and focusing of an experimental high-power (sub-petawatt) aberrated beam is numerically calculated. The shape of the focused beam pattern within a few Rayleigh lengths is analysed as a function of main aberrations (up to the 14 th term of Zernike polynomials). Furthermore, at the focus position, the spreading of the energy is compared to both the case of a perfect diffraction-limited Gaussian and a super-Gaussian beam.