Absorption Of Laser Beam Research Articles

Laser Powder Bed Fusion of Copper-Tungsten Powders Manufactured by Milling or Co-Injection Atomization Process.

The processing of pure copper (Cu) has been a challenge for laser-based additive manufacturing for many years since copper powders have a high reflectivity of up to 83% of electromagnetic radiation at a wavelength of 1070 nm. In this study, Cu particles were coated with sub-micrometer tungsten (W) particles to increase the laser beam absorptivity. The coated powders were processed by powder bed fusion-laser beam for metals (PBF-LB/M) with a conventional laser system of <300 watts laser power and a wavelength of 1070 nm. Two different powder manufacturing routes were developed. The first manufacturing route was gas atomization combined with a milling process by a planetary mill. The second manufacturing method was gas atomization with particle co-injection, where a separate W particle jet was sprayed into the atomized Cu jet. As part of the investigations, an extensive characterization of powder and additively manufactured test specimens was carried out. The specimens of Cu/W powders manufactured by the milling process have shown superior results. The laser absorptivity of the Cu/W powder was increased from 22.5% (pure Cu powder) to up to 71.6% for powders with 3 vol% W. In addition, a relative density of test specimens up to 98.2% (optically) and 95.6% (Archimedes) was reached. Furthermore, thermal conductivity was measured by laser flash analysis (LFA) and thermo-optical measurement (TOM). By using eddy current measurement, the electrical conductivity was analyzed. In comparison to the Cu reference, a thermal conductivity of 88.9% and an electrical conductivity of 85.8% were determined. Moreover, the Vickers hardness was measured. The effect of porosity on conductivity properties and hardness was investigated and showed a linear correlation. Finally, a demonstrator was built in which a wall thickness of down to 200 µm was achieved. This demonstrates that the Cu/W composite can be used for heat exchangers, heat sinks, and coils.

Nonlinear absorption of Hermite cosh-Gaussian laser beam in an assembly of cylindrical carbon nanotubes embedded with static magnetic field

Nonlinear absorption of Hermite cosh-Gaussian laser beam in an assembly of cylindrical carbon nanotubes embedded with static magnetic field

Electrostatic wave assisted super-gaussian laser beam absorption in collisional plasma embedded with static magnetic field

Electrostatic wave assisted super-gaussian laser beam absorption in collisional plasma embedded with static magnetic field

Review on Laser Welding Technology on Aluminum and Copper Dissimilar Materials for Secondary Batteries Assembly

Electric vehicles have recently taken center stage, driven by growing environmental concerns, with a key focus on ensuring the reliability of their batteries. The materials used in the electrical connections of secondary batteries predominantly consist of Al and Cu alloys, necessitating the joining of both similar and dissimilar materials. This paper presents a technical review on laser welding technology applied to the challenging Al/Cu dissimilar metal combination. The complexity arises from the formation of brittle phases in intermetallic compounds and the low absorption of laser beams by the base materials. The characteristics of various laser welding modes and their effective monitoring through deep learning are explained in detail. The development of real-time laser welding monitoring technology is crucial to increase productivity and enable the mass production of reliable electric vehicle battery systems.

The effect of Nd:YAG laser parameters on the microstructure, hot cracking susceptibility and elemental evaporation of surface melted AZ80 magnesium-based alloy

Laser surface melting (LSM) is an environmentally-safe technique in which melting and solidification occur in a very short reaction time, without affecting the bulk of the material. On the other hand, today, the use of magnesium as a very light metal, with excellent specific resistance, excellent sound damping and good casting ability, etc., has been expanded. Also, having low absorption of laser beams, strong tendency to oxidation, high thermal conductivity, etc are important drawbacks of these alloys. Today there is widespread agreement on the better resistance to hot cracking of magnesium alloys with Nd:YAG laser due to its shorter wavelength by comparison with CO2 laser. This research aims to investigate the effect of Nd:YAG laser surface melting parameters on the microstructural, phase changes and hot cracking susceptibility of the AZ80 magnesium base alloy. The results of current research showed that sample 5 with voltage, frequency, pulse duration and gas flow rate of 250V, 10 Hz, 4 ms, 5lit/min, respectively, and constant speed of 1 mms showed a remelted zone with minimum hot cracks and other defects. In conclusion by controlling the laser parameters such as pulse duration, shielding gas flow rate, laser frequency, scanning rate and the laser power, the susceptibility to hot cracking including both solidification and liquation cracking are significantly decreased.

Numerical analysis of the effect of an oscillating metal vapor plume on the keyhole and molten pool behavior during deep penetration laser beam welding

The effect of the oscillating metal vapor plume on the keyhole and molten pool behavior during the laser beam welding of AlMg3 aluminum alloys is investigated by experimental and numerical methods. The real-time height of the metal vapor plume is measured by high-speed camera observation. The obtained experimental results are used to evaluate the additional heating source and laser beam attenuation caused by the scattering and absorption based on the Beer–Lambert theory. Furthermore, the dynamic behavior of the metal vapor plume is incorporated into a 3D transient heat transfer and fluid flow model, coupled with the ray tracing method, for the laser beam welding of the AlMg3 alloy. It is found that additional heating resulting from the scattered and absorbed laser beam energy by the metal vapor plume significantly expands the shape of the molten pool on the top region. Moreover, the oscillating metal vapor plume caused the fluctuation of the high-temperature region in the molten pool. The probability of keyhole collapse at the bottom increases 17% due to the oscillating laser power induced by the laser beam attenuation. The internal interplay between the metal vapor plume, molten pool shape, and keyhole collapse is obtained. The developed model has been validated by experiments, which shows a good agreement.

Laser light absorption of high-temperature metal surfaces

Laser beam absorption is the basic effect to enable many high-temperature applications and processes. However, high temperature absorption data of metals is often not available or based on theoretical assumptions. In this work, using a newly developed experimental arrangement to measure laser light absorption on liquid metal surfaces even above boiling temperature enabled the derivation of absorption values in those regimes. Results indicate that interband absorption must be considered even at such high temperatures against common theoretical predictions. It is shown that the simulated nearly constant absorption depth and absorption values between melting and boiling temperatures indicate that the increased atom distance due to thermal expansion, denoting a reduced absorption volume, is counterbalanced by the increased statistical availability of conduction electrons due to Fermi band broadening.

One camera-based laser keyhole welding monitoring system using deep learning

The laser-beam absorptance changes dynamically during laser keyhole welding due to unstable keyhole movements, and monitoring the absorptance can provide a deep understanding of the process. Recently, Kim et al. [1,2] developed a deep-learning-based method to monitor the absorptance by detecting the top and bottom keyhole apertures and estimating the absorptance from the reconstructed keyhole shape based on the detected apertures. However, this method was limited in that it required simultaneously observing the top and bottom keyhole apertures using two cameras. In this study, we proposed a novel deep-learning-based method to monitor the laser-beam absorptance in a keyhole using only one camera during laser keyhole welding of Al 5052-H32 alloy. In this method, both the top and bottom keyhole apertures were simultaneously detected from the images coaxially obtained from the top side. Although part of the bottom apertures may be sometimes obscured when viewed from above, this study demonstrated that the predicted absorptance was accurate enough and sufficient for monitoring laser welding processes of aluminum alloys. Using the developed method, changes in welding mode and generation of welding defects were successfully detected.

Electron Bernstein wave aided Hermite cosh-Gaussian laser beam absorption in collisional plasma

In this paper, we theoretically study the electron Bernstein wave aided Hermite-Cosh-Gaussian laser beam absorption in collisional plasma with a static magnetic field. This laser beam may have the potential to couple the pre-existing electron Bernstein wave with the coupled wave frequency and wave number where and are the electron Bernstein wave and laser beam frequencies, respectively, and and are the electron Bernstein wave and laser beam wave numbers, respectively. The plasma electron oscillatory velocities associated with the coupled wave mode produce the linear and nonlinear current density. An expression for the nonlinear absorption coefficient of an electron Bernstein wave aided Hermite cosh-Gaussian laser beam is analytically derived and more concisely discussed via different relative graphs. The graphical results promise that the absorption coefficient is strongly dependent on Hermite mode index m, beam decentered parameter d associated with the cosh term, laser beam width, laser beam frequency, electron Bernstein wave frequency, electron cyclotron frequency, electron thermal velocity and collisional frequency. Nonlinear coupling, collision phenomena and laser beam decentered parameter (very sensitive parameter) play an important role in absorption processes. The association of a very small electron Bernstein wave frequency with the laser beam causes too much of an increase in the absorption coefficient compared to only the laser beam. This efficient nonlinear absorption process may be applicable to plasma electron heating and the harmonic generation process.

Experimental study on influence of shielding gas pressure on the quality of pulsed laser welding of thin walled T91 steel clad tube with end plug

Fabrication of metal fuel pins for test irradiation in fast breeder test reactor requires welding of thin wall clad tube with solid end plug of modified 9Cr-1Mo steel material under high pressure helium atmosphere. Autogenous pulsed Gas Tungsten Arc Welding [GTAW] technique is currently used for end cap welding of metal fuel pins inside the glove box. Pulsed laser welding of end plug with clad tube of modified 9Cr-1Mo steel material under inert atmosphere at 1.2 bar gas pressure is under development.Pulsed laser beam welding [PLBW] is considered as alternate welding technique to replace the pulsed tungsten inert gas welding.In the current study the influence of shielding gas pressure on the weld joint characteristics like depth of penetration, bead width, and heat affected zone were investigated to optimize laser welding parameters. Initially experiments were conducted under helium gas at atmosphere pressure to optimize the process parameters to obtain the desired depth of penetration and then repeated the experiment with the optimized parameters at 0.5 bar pressure to study the effect of shielding gas pressure. Laser beam welding parameters like peak power − 1800 to 2400 W, pulse frequency – 5 to 8 Hz and pulse duration – 6 to 10 ms were varied during welding sample coupons. Welded sample coupons were subjected to various NDT techniques and subsequently metallographic examination was carried out to measure the bead width, depth of penetration and delta ferrite distribution in the weld metal. Laser beam welding parameters were optimized at atmosphere pressure to meet the stringent acceptance criteria. Further study was carried out under helium gas atmosphere at 0.5 bar using optimized welding parameters and it reveals that the drastic reduction in heat input due to laser beam attenuation and absorption by quartz window.

A coupled ray-tracing based CFD and cellular automaton model for predicting molten pool formation and microstructure evolution in narrow gap laser welding

Laser beam reflection plays a significant role in the temperature development history during various laser material processes. Different from other processes where laser beam reflection main happens inside the keyhole, in narrow gap laser welding (NGLW), the laser beam would interact with the sidewalls multiple times before existing the narrow groove, which would have a great impact on the defect formation as well as the grain structure development. However, the underlying mechanism of this process remains unknown in multi-pass NGLW. In this study, an integrated ray-tracing based CFD and cellular automaton (CA) modelling framework is established to investigate the impact of laser beam reflection on molten pool formation and microstructure evolution in multi-pass NGLW. From the CFD simulation, it is found that the overall laser beam absorption of NGLW is greater than laser metal deposition (LMD) while smaller than laser powder bed fusion (LPBF) process. Besides, the overall absorptivity in NGLW is increased 28% when considering the ray reflection during the first welding pass. The absorptivity would increase and then stabilise through the multi-pass welding process, which is attributed to the change in incident angle and number of reflections. Additionally, it is found that the undercut defect is more likely to occur with ray reflection due to the excessive temperature rise on the sidewalls, while lack of fusion is observed in the case without ray reflection. By incorporating the thermal history of each welding pass with CA, the impact of laser beam reflection on grain structure development is evaluated. Columnar grains would govern the microstructure evolution in the case with ray reflection, which grow epitaxially from the fusion line to the molten pool centre. Grain coarsening would occur around the lack of fusion region in the case without ray reflection. Moreover, the analysis on grain size distribution is conducted and a greater maximum grain size is obtained in the case without ray reflection as a result of lower cooling rate caused by lack of fusion. However, a higher grain size increase is obtained in the case with ray reflection during multiple welding passes, and larger grains are expected to form if more welding passes are to be conducted. The simulated molten pool morphology and grain structure are compared with the experimental observation, and a good agreement is achieved.

Control of low-mode drive asymmetry in an efficient long-pulse low gas-fill density Hohlraum

Laser-driven Hohlraums filled with gas at lower densities (&amp;lt;0.6 mg/cc) have higher efficiency compared to original ≥ 0.96 mg/cc fill because of reduced backscatter losses [Hall et al., Phys. Plasmas 24, 052706 (2017)]. However, using low-density filled Hohlraums with longer drive required for lower adiabat implosions, and hence potentially higher inertial confinement fusion gain designs, has been challenging since the Hohlraum wall blow-off is less tamped, thus altering the laser beam absorption regions and drive symmetry. A series of NIF experiments using optimized pulse shaping, beam pointing, and temporal phasing have demonstrated, through imaging of the Hohlraum and capsule dynamics, that a symmetric implosion using a 14-ns low-adiabat drive pulse {2× longer than high-density-carbon ablator designs using low gas-fill density Hohlraums [Divol et al., Phys. Plasmas 24, 056309 (2017)]} is possible in a low backscatter loss 0.45 mg/cc He-filled Hohlraum. The ingress of the Hohlraum walls was mitigated by revisiting the adiabat-shaped design [Clark et al., Phys. Plasmas 21, 112705 (2014)] that uses a low-power (1 TW) trough that delays the wall expansion. Low-mode P2 and P4 drive asymmetry swings caused by the drift of the laser spots were essentially zeroed out by employing temporal beam phasing between cones of beams [Turner et al., Phys. Plasmas 7, 333 (2000)]. The results also indicate an improved coupling efficiency of ∼30% compared to an earlier design using higher density filled Hohlraums and pave the way for revisiting low-adiabat, high convergence drives using CH ablators.

Laser beam absorption measurement at molten metal surfaces

Laser light absorption is one of the elementary effects of laser material processing. Absorption values are relevant to calculate the process efficiency and predict the impact on the material for the increasingly used laser processes. However, absorption measurement can be a complex task. At high temperatures of metals, only limited experimental data is available due to the dynamic surfaces and the often unknown emissivity needed for the temperature measurement. Models were created to predict the absorption at different temperatures, which are successful with assumptions in some regimes, but often fail in others. For improving the theoretical models, an experimental measurement of high-temperature metal surfaces is desired. Therefore, a radiometric measurement method is proposed in this work using a heating laser to create a metal melt pool, while measuring temperature and reflection of its surface by a second measuring laser beam. General tendencies known from literature could be confirmed by the measurements, while absorption values tend to scatter at increasing temperature. However, trends could be observed. Between melting and boiling temperature, a slight absorption increase was seen in the range between 35% and 38%. Those values indicate that both interband and intraband absorption must be considered to explain the absorption in this regime. At increased temperatures, the intraband absorption becomes the dominating absorption mechanism, reaching absorption values above 45% at very high temperatures.

Effects of Shielding Gases on the Weldability of Laser Welded Austenitic Stainless Steel

In laser welding of austenitic stainless steels, shielding gases have great potential for improving the mechanical properties of the welds, suppressing plasma plumes, and stabilizing the keyhole. Previous studies have shown that shielding gases can alter the weld geometry and microstructural constituents of the welds. Disturbances of laser beam absorption by laser-induced plasma can be controlled through shielding gas parameters such as the shielding gas composition, flow rate, and supplying direction. In addition, surface-active elements added to the shielding gas can alter the behavior of the molten pool. In this paper, previous studies are reviewed to assess the shielding gas effects on the characteristics of laser-welded austenitic stainless steel. The activated mechanism of molten pool flow by additional surface tension active elements is also discussed.

Electron Bernstein wave aided cosh-Gaussian laser beam absorption in plasma

In this paper, we investigate the electron Bernstein wave aided cosh-Gaussian laser beam absorption in a plasma with static magnetic field. The cosh-Gaussian laser beam has potential to couple with pre-existed electron Bernstein wave and the coupled wave has frequency ω1=ω0+ω and wave number k1=k0+k where ω0 and ω are the laser and electron Bernstein wave frequencies respectively and k0 and k are the wave numbers of laser and electron Bernstein wave respectively. The oscillatory velocity due to laser couples with the density perturbation due to electron Bernstein waves to produce nonlinear current density driving the electromagnetic sideband wave. An expression of absorption coefficient is analytically derived when the nonlinear current density is in phase with cosh-Gaussian laser field. The graphical discussion promises the more absorption in plasma by varying laser beam decentered parameter, beam width, laser frequency, electron cyclotron frequency, parameter b, and electron Bernstein wave frequency. The laser beam decentered parameter behaves as a very sensitive parameter. The absorption process is found to be very robust even for very small change in electron Bernstein wave frequency as compared with laser frequency. This tunable and efficient absorption theory might be applicable in heating of plasma.

Investigation of the transient coupling between the dynamic laser beam absorptance and the melt pool - vapor depression morphology in laser powder bed fusion process

A predictive powder-bed model has been developed using open-source environments to investigate the coupling between the optical and the thermo-hydrodynamical phenomena for the laser powder bed fusion process. Firstly, discrete element modeling has been carried out to predict the configuration of powder particles over a substrate. Then, thermo-fluidic simulations for both the stationary and the moving laser irradiation have been carried out to investigate the melt pool dynamics, the vaporization induced keyhole depression instabilities, and the multiple reflections of the laser beam in the powder bed and the keyhole. The local absorptance and the reflectance of the laser beam at the static curvature of the solid powder particles and the dynamic interface of the vaporization-induced keyhole depression have been incorporated using a parallelized ray-tracing algorithm. The influences of laser power, scanning speed, beam spot diameter, and powder layer thickness on the melting mode and the transient beam absorption have been investigated. During the initial phase of melting in the powder bed, it was found that the laser absorption is very high (up to 73%) due to multiple reflections induced beam trapping. The absorption behavior for the conduction mode of melting was found to be dependent on the scanning speed of the laser beam. For the keyhole mode of melting, the temporal variation of the laser beam absorption shows a highly non-linear fluctuating behavior that is directly related to the vapor depression morphology of the melt pool. The numerically predicted attributes such as the initial laser beam absorptance, the transient melt pool-vapor depression morphology for the stationary laser irradiation, and the melt pool dimensions for the single-track deposition matches well with the findings of the published powder calorimetric measurement, published high-speed transmission X-ray imaging and the measured single-track deposited using the in-house laser powder bed fusion setup, respectively.

Modeling Laser Beam Absorption of Metal Alloys at High Temperatures for Selective Laser Melting

Adv. Eng. Mater. 2021, 23, 2100137 10.1002/adem.202100137 After publication of the original article it came to the authors’ attention that there is a mistake in Equation (9), the “cos” should be “sin”.

Apparatus and its principle for thermal aberration compensation.

Thermal aberrations caused by absorption of laser beams degrade the image quality of exposure tools during the working process. Many compensators, such as lens movement or lens deformation, are used to compensate for low-order thermal aberrations of optical systems. In this paper, an apparatus with higher-order aberration correction capability is presented. The main principle of the apparatus is to actively heat and cool the lens near the pupil to generate a desired temperature profile to compensate for thermal aberrations. We first introduce the basic concept of the apparatus. Then we establish an analytical model to describe the lens temperature of the apparatus based on its working principle and demonstrate its compensation capability. Finally, an algorithm for dynamic thermal aberrations compensation is proposed to overcome the time lag effects of a thermally controlled lens.

Laser Additive Manufacturing of Bulk Silicon Nitride Ceramic: Modeling versus Integral Transform Technique with Experimental Correlation

A semi-analytical-numerical solution is theorized to describe the laser additive manufacturing via laser-bulk ceramic interaction modeling. The Fourier heat equation was used to infer the thermal distribution within the ceramic sample. Appropriate boundary conditions, including convection and radiation, were applied to the bulk sample. It was irradiated with a Gaussian spatial continuous mode fiber laser (λ = 1.075 µm) while a Lambert-Beer law was assumed to describe the laser beam absorption. A close correlation between computational predictions versus experimental results was validated in the case of laser additive manufacturing of silicon nitride bulk ceramics. The thermal field value rises but stays confined within the irradiated zone due to heat propagation with an infinite speed, a characteristic of the Fourier heat equation. An inverse correlation was observed between the laser beam scanning speed and thermal distribution intensity. Whenever the laser scanning speed increases, photons interact with and transfer less energy to the sample, resulting in a lower thermal distribution intensity. This model could prove useful for the description and monitoring of low-intensity laser beam-ceramic processing.

Mathematical model to study the keyhole formation in pulsed Nd:YAG laser welding of SS 316L material and its experimental verification

A mathematical model to study keyhole formation and its propagation in the material is developed for laser welding performed in an open atmosphere. The present model overcomes the limitations of existing models in assuming sonic vapor jet velocity to calculate vaporization-induced recoil pressure responsible for keyhole formation. In the present model, the exact value of vapor jet velocity is calculated using gas dynamics equations. The minimum threshold value of absorbed laser beam intensity required to perform keyhole welding irrespective of laser pulse duration for laser beam radius of 0.6 mm has been found to be 0.8 × 105 W/cm2 and is in good agreement with the experimental value. In between conduction mode welding and keyhole mode welding, a transition mode exists where a keyhole mechanism develops itself and melt displacement is not considerable in this zone. Weld penetration occurs mainly through heat diffusion in this transition mode. The predicted values for keyhole penetration velocity are also in good agreement with the experimental values. At a longer pulse duration, the model over-predicts the keyhole penetration velocity as compared to the experimental value due to nonconsideration of vapor plasma absorption of the laser beam.