Laser Heating Process Research Articles

Inverse estimation of the pulse parameters of a time-varying laser pulse to obtain desired temperature at the material surface

A proper selection of the pulse parameters is essential to achieve desired temperature at the material surface. This leads to obtain the required metallurgical changes in the surface vicinity when a time-varying laser pulse is used in a heating process such as surface modification. In this paper, the conjugate gradient method (CGM) for parameter estimation is successfully applied to estimate the unknown laser pulse parameters for those purposes during laser heating process. The determination of the pulse parameters is treated as a one-dimensional, transient, non-linear inverse heat conduction problem (IHCP). Based on a sensitivity analysis, the inverse problem is solved as an optimization problem comparing a desired temperature at the surface and a calculated one where the objective function is minimized by CGM. The method has been applied to a test case of a heating process on steel, appropriate pulse parameters and desired temperature distribution can also be returned.

S131014 レーザ加熱援用切削における工具切れ刃温度

Laser beam is prospective as a heat source in the heated machining process of 'difficult-to-cut materials'. Because the heating decreases the strength of material, therefore high productivity could be achieved if the thermal wear of tool would be reduced. In this study, laser heating is applied for the milling process of titanium alloy which is well known as one of difficult-to-cut materials for its low thermal conductivity causing the thermal tool wear. And temperature of cutting edge of tool is investigated by two-color pyrometer using optical fiber which realizes measuring of the rapid temperature change in small region such as cutting edge of the tool. The cutting force is also investigated to estimate the effect of softening of material caused by laser heating process.

Application of residual correction method to laser heating process

In this study, the residual correction method is employed to predict the temperature distribution during laser heating process. The energy transfer induced by laser irradiation in the solid is described by Fourier’s law of conduction with an energy source modeled by Beer’s law. The approximate solution of temperature field is obtained by residual correction. Using the residual correction method, the precise average value of upper and lower approximate solutions is obtained and the error range between the analytical solution and the numerical solution can be analyzed. The results reveal that this method is an effective numerical method with satisfactory accuracy.

Laser induced desorption as tritium retention diagnostic method in ITER

Measurement and control of long term tritium retention is one of the most critical issues for ITER and future fusion devices. Since the measurement of the hydrogenic retention by post mortem tile analysis becomes more and more difficult in future devices due to active water cooling and tile activation, Laser Induced Desorption Spectroscopy (LIDS) is under development in TEXTOR to provide a tool to measure the tritium inventory in situ without tile removal. The method is based on rapid spot laser heating and consecutive detection of thermally released hydrogen by spectroscopic detection of Balmer line emission. This contribution presents a short overview of the LIDS setup, the laser heating process, the optical detection and data evaluation as well as the application of laser desorption to map the hydrogen content on a TEXTOR ALT limiter tile with erosion dominated areas and deposition dominated areas with thick a-C:H layers in a laboratory setup, demonstrating the ability to create a 2D mapping of the hydrogenic inventory without damaging the substrate.

Laser gas‐assisted nitriding of Ti implant

PurposeThe purpose of this paper is to investigate the morphological and metallurgical changes of laser gas‐assisted nitriding of titanium implants.Design/methodology/approachLaser gas‐assisted nitriding of titanium implant is carried out and the metallurgical as well as the morphological changes in the nitride layer are examined using optical microscopy, SEM, XRD, and X‐ray photoelectron spectroscopy. Temperature and thermal stress fields are computed during the laser heating process adopting the finite element method. The residual stress formed in the nitride layer is measured using the XRD technique while micro‐indentation tests are carried out to determine the fracture toughness of the surface after the laser treatment process.FindingsIt is found that nitride depth layer extends to 40 μm below the surface and it is free from the cracks and micro‐voids. The residual stress formed on the surface region is higher than at some depth below the surface in the nitride layer, provided that the maximum residual stress is less than the elastic limit of the substrate material.Originality/valueThe paper contains original findings and the findings are not submitted any other journal for publication.

Raman spectra of hybrid materials based on carbon nanotubes and Cs 3PMo 12O 40

A route for the synthesis of hybrid materials using multiwalled carbon nanotubes (MWCNTS) and Cs 3PMo 12O 40 was confirmed. The Cs 3PMo 12O 40 salt was synthesized from H 3PMo 12O 40·14H 2O and CsCl. All compounds were characterized by Raman spectroscopy. The synthesis method can produce a chemically stable Cs 3PMo 12O 40 molecule and pure and oxidized MWCNTS. Raman spectroscopy showed that the chemical interaction between MWCNTS and Cs 3PMo 12O 40 was essentially of electrostatic nature. Raman spectra obtained with different wavelengths showed thermal decomposition of H 3PMo 12O 40·14H 2O (raw materials) from laser heating process. On the contrary, the synthesized compound (i.e. Cs 3PMo 12O 40) was stable under the experimental conditions.

Ultra-short pulse laser ablation of copper, silver and tungsten: experimental data and two-temperature model simulations

Experimental results of femtosecond laser ablation of the metals copper, silver and tungsten are compared to simulations based on the two-temperature model. The comparison provides new information about the laser-heating process: For the noble metals (Cu, Ag), the energy transport via ballistic electrons must be included, while this effect is negligible for a transition metal (W). The comparison provides values for the range of ballistic electrons in the noble metals. The model calculation is also employed to investigate the dependence of the threshold fluence and melting depth on pulse duration. It is observed that for pulses shorter than approximately 1 ps the threshold fluence and melting depth are independent of the pulse duration, while they increase as τ0.47 and τ0.51, respectively, for pulses longer than ∼40 ps, in good agreement with approximate analytical expressions predicting a \(\sqrt{\tau}\) dependence.

Laser Assisted Turning of Sintered Silicon Nitride

Laser assisted turning is an effective method machining difficult-to-machine materials such as ceramics, which uses a high power laser to focally heat a workpiece prior to material removal with a traditional cutting tool. A transient, three-dimensional heat transfer model was developed for laser assisted turning of silicon nitride using Finite Element Method to understand the thermal process of laser heating and to optimize the operating parameters. A laser assisted turning experiment system was set up to investigate the thermal conditions and cutting process of laser assisted turning of sintered silicon nitride and the experiments were conducted on the system using selected parameters. Effects of cutting parameters on cutting forces and specific cutting energy were investigated. Forces on the chip and SEM micrographs of chip morphology were studied to discuss the material removal mechanism of laser assisted turning of silicon nitride. Tool wear, surface roughness of the machined surface and the quality of subsurface were investigated. The results showed that the heat transfer model could be used to optimize the cutting parameters and laser assisted turning method could increase the machining efficiency while maintaining machining quality and reasonable levels of tool wear. A method of optimizing LAM based on the thermal model was presented.

Laser surface treatment of Inconel 718 alloy: Thermal stress analysis

Laser heating of Inconel 718 alloy is considered and the resulting temperature and stress fields are predicted using the finite element method (FEM). An experiment is carried out to treat the alloy surface by a laser beam at high pressure nitrogen environment. The metallurgical and morphological changes in the irradiated region are examined using the Scanning Electron Microscope (SEM), optical microscope, and X-ray Diffraction (XRD). It is found that the surface hardness of the alloy improves after the laser heating process, which is due to the microstructural changes and γ-phase nitride formation in the surface region. The maximum value of the residual stress predicted in the irradiated region is close to the yielding limit of the alloy.

Laser pulse heating and phase change process: A comparison of volumetric heat source models

In the laser heating process, irradiated energy is absorbed on the surface skin of the substrate material. This results in excess computational efforts due to grid arrangement in the irradiated region and the remaining region in the solution domain due to the fine grid spacing in the irradiated region. However, consideration of the surface heat source minimizes this problem, since it does not require fine grid spacing in the skin of the surface. In the present study, laser heating and phase change in the irradiated region are modelled. The laser heating situation is modelled after considering the volumetric heat source incorporating an absorption process (Beer—Lambert's Law) and the surface heat source model. The temperature distribution, melt layer, and solid—liquid zone (mushy zone) formed in the heated region are predicted for the volumetric and surface heat source heating models. This study is extended to include the influence of spatial distribution of the laser pulse on temperature rise and phase change processes. It is found that the surface heat source model predicts higher values of temperature than those corresponding to the volumetric heat source in the surface vicinity. As the depth increases, temperature distributions predicted from both models become almost identical. In addition, the melt layer thickness and mushy zone predicted from both models are almost identical.

Simulation and Experimental Study on Temperature Fields for Laser Assisted Machining of Silicon Nitride

Laser assisted machining (LAM) is an effective method machining difficult-to-machine materials such as ceramics which uses a high power laser to focally heat a workpiece prior to material removal with a traditional cutting tool. A laser assisted machining experiment system was set up and a transient, three-dimensional heat transfer model was developed for LAM of silicon nitride using Finite Element Method to understand the thermal process of laser heating. The model was based on temperature-dependent thermophysical properties and the heat generated was neglected due to cutting which is assumed to be small compared to the heat generated by laser heating. The experiments were carried out to investigate the effects of operating parameters, such as laser power, laser translational speed, rotational speed, laser beam diameter and preheating time on temperature distribution. An infrared radiation thermometer was used to measure the surface temperature histories and the experimental results were in good agreement with predictions. The laser power and laser translational speed have the greatest influence on the temperature.

On the Analysis of Short-Pulse Laser Heating of Metals Using the Dual Phase Lag Heat Conduction Model

Transient heat conduction in a thin metal film exposed to short-pulse laser heating is studied using the dual phase lag heat conduction model. The initial heat flux distribution in the film, resulting from the temporal distribution function of the laser pulse, together with the zero temperature gradients at the boundaries normally used in literature with the presumption that they are equivalent to negligible boundary heat losses is analyzed in detail in this paper. The analysis presented here shows that using zero temperature gradients at the boundaries within the framework of the dual phase lag heat conduction model does not guarantee negligible boundary heat losses unless the initial heat flux distribution is negligibly small. Depending on the value of the initial heat flux distribution, the presumed negligible heat losses from the boundaries can be even way larger than the heat flux at any location within the film during the picosecond laser heating process. Predictions of the reflectivity change of thin gold films due to a laser short heat pulse using the dual phase lag model with constant phase lags are found to deviate considerably from the experimental data. The dual phase lag model is found to overestimate the transient temperature in the thermalization stage of the laser heating process of metal films, although it is still superior to the parabolic and hyperbolic one-step models.

Investigation on microstructure, surface properties and anti-wear performance of HVOF sprayed WC–CrC–Ni coatings modified by laser heat treatment

Laser heat (LH) treatment was found to be an effective method to alter the microstructure and mechanical properties of conventional coatings. In this investigation, the effects of laser scanning velocity on microstructure, microhardness and wear performance of high velocity oxygen fuel sprayed (HVOF) WC–CrC–Ni coatings during laser heating treatment were investigated. The results showed that the microstructure of the HVOF coatings changed greatly after the LH treatment. X-ray diffraction (XRD) patterns indicated that WC and Cr 3C 2, as primary phases, were present in both the HVOF- and LH-treated coatings, while only a small amount of Ni phase existed in the LH-treated coatings. With the post-treatment process of laser heating, a compact interface was found between the coating and substrate. When the laser scanning velocity decreased, the porosity and thickness of the LH-treated coatings decreased, leading to a gradual increase of microhardness. In the wear test, the friction and wear resistance of the coating were improved significantly with an optimal laser heating process (600 W power, 300 mm/min scanning velocity). The improvement can be attributed to the improved porosity and hardness, as well as the formation of oxide tribofilms.

Jet impingement onto a cylindrical cavity

PurposeThe paper's aim is to provide information on heat transfer and flow characteristics for a jet emerging from a conical nozzle and impinging onto the cylindrical, which resembles the laser heating process, for researchers and graduate students working in the laser processing area, which can help them to improve the understanding of the laser machining process.Design/methodology/approachA numerical scheme employing the control volume approach is introduced to model the flow and heating situations. The effect of jet velocity on the heat transfer rates and skin friction around the cylindrical cavity subjected to the jet impingement was investigated.FindingsIncreasing jet velocity at nozzle exit enhances the heat transfer rates from the cavity wall and modifies the skin friction at cavity wall, which is more pronounced as the cavity depth increases to 1 mm.Research limitations/implicationsThe effects of nozzle cone angle on the flow structure and heat transfer characteristics were not examined, which perhaps limits the general usefulness of the findings.Practical implicationsVery useful information are provided for the laser gas assisted processing, which has a practical importance in machining industry.Originality/valueThis paper provides original information for the effects of the gas jet velocity on the cooling rates of the laser produced cavity.

Effect of a pulsed Nd:YAG laser irradiation on multi-walled carbon nanotubes film

Multi-walled carbon nanotubes (MWCNTs) film have been analyzed by Raman spectroscopy to clarify the effect of a pulsed Nd:YAG laser heating. The MWCNTs film surface was flashed with the fundamental harmonic ( λ = 1064 nm) or the second harmonic ( λ = 532 nm) of a single pulse of Nd:YAG laser in the air. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. At the laser fluence of 500 mJ/cm 2 with Nd:YAG laser ( λ = 1064 nm), the surface reached the maximum temperature 1395 °C at 12 ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I D (defect-mode) and I G (graphite-mode) was measured by the Raman spectroscopy. The maximum surface temperature was calculated and compared with the I G/ I D ratio of MWCNTs film. The graphitization occurred on the sample after irradiation.

Laser-Pulsed Heating of Aluminum: Cavity Formation at the Surface

Laser evaporative heating of aluminum surface is modeled numerically and mushy zone formation across the solid-liquid and the liquid-vapor phases is introduced in the model study. Temperature rise in the irradiated region and the cavity formation during the laser heating pulse are predicted. Temperature-dependent properties are introduced during the simulations. Oxygen diffusion into the substrate material is considered during the laser heating process while employing temperature-dependent diffusion coefficient. An experiment resembling the simulation conditions is carried out and the morphology of the cavity produced at the surface of the laser-irradiated region is examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). It is found that the depth of liquid phase increases significantly with the progressing time and the size of the mushy zone across the solid-liquid phase is smaller than that of across the liquid and the vapor phases. The cavity depth, observed from the SEM and AFM micrographs, is shallow and similar to those predicted from the numerical simulations.

Treatment of the interfacial temperature jump condition with non-Fourier heat conduction effects

Transient energy transport with non-Fourier heat conduction effects in a two-layer structure of dissimilar materials subject to ultra-fast laser heating is studied using the proper interfacial temperature jump condition. The solution obtained is compared with solutions available in literature that use diffusion-type interfacial conditions in conjunction with non-Fourier heat conduction effects. The dual-phase lag heat conduction model is used in this work as it includes both the temporal and spatial non-Fourier effects. It is found that the diffusion-type interfacial temperature jump condition with non-Fourier heat conduction models can lead to discrepancies and erroneous trends in theoretical predictions. Moreover, a comparison between the functional forms of the two solutions obtained utilizing both interfacial conditions shows that implementing the proper interfacial temperature jump condition does not add any complexity to the solution obtained. This study – implementing the proper interfacial temperature jump condition – is further extended to show the strong effects of the thermal contact conductance and the surface layer thickness on the transient thermal response of a two-layer material in a semi-infinite domain subject to ultra-fast laser heating processes in terms of the reflectivity change of the surface layer, the temperature distribution in the two-layer structure as well as the temporal variation of the interfacial temperature difference.

Laser pulse heating and phase changes in the irradiated region: Temperature-dependent thermal properties case

Laser heating and phase change processes in the substrate material are formulated using the energy method. Constant and temperature-dependent thermal properties (variable thermal properties) of the substrate material are considered in the simulations for comparison. Temperature rise in the substrate material and mushy zones formed across liquid–solid and liquid–vapor phases are predicted for constant and variable thermal properties cases. Laser pulse intensity resembling the actual laser pulse is accommodated in the analysis. The recession velocity predicted from the present simulation is compared with the results of analytical solution obtained previously for constant properties case. The cavity depth predicted is compared with the experimental results. It is found that the simulation for constant properties results in early evaporation of the surface and deeper cavities as compared to variable thermal properties case. The prediction of cavity depth for variable thermal properties case agrees well with the experimental results.

Laser optoacoustic study of near‐critical states and phase transitions in metals

Pulsed laser optoacoustic technique is used for generation and study of near‐critical states and phase transitions in metals. Metal surface is confined by a layer of transparent dielectric. In this case the efficiency of pressure generation is much greater than in case of the free surface so it is possible to achieve states of metals with relatively high thermodynamic parameters: P ∼104 atm and T ̃ 104 K with a table‐top laser system. The experimental setup for simultaneous measurements of pressure, temperature, and reflectivity of metal surface with nanosecond temporal resolution was assembled. Q‐switched Nd:YAG laser with pulse duration ̃ ∼10 ns and pulse energy ∼1 J was used. Pressure was measured using LiNbO3 piezotransducer. Temperature was obtained with optical pyrometer. Lead and mercury were chosen as test metals. Pressure pulses up to 1 kbar in lead and up to 7 kbar in mercury (with T ∼2400 K ‐ super‐critical area of the phase diagram for Hg) were obtained. The curve of laser heating process in P‐T coordinates was plotted. The measurements of optical properties showed considerable decrease of surface reflectivity both for lead and mercury at high laser fluences due to increase of temperature and density decrease.

Pulsed laser heating process of multi-walled carbon nanotubes film

The prepared multi-walled carbon nanotubes (MWCNTs) film was mounted on the holder and the film surface was flashed with a single pulse of Nd:YAG laser ( λ = 532 nm) in the air. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. The finite element method (FEM) was applied to solve the equation. At the laser fluence of 1 J/cm 2 with Nd:YAG laser, the surface reached the maximum temperature 1503 °C at 13 ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. The intensity of the two characteristic Raman shifts I D (defect-mode) and I G (graphite-mode) was measured by the Raman spectroscopy. The maximum surface temperature was calculated and compared with the I G/ I D ratio of MWCNTs film. The graphitization occurred on the sample after irradiation.