Laser-induced Temperature Field Research Articles

An equivalent numerical model for calculating shot peening bending deformation based on the laser forming method

Shot peen forming is an essential method for forming large-size thin-walled structural parts in the aerospace field. However, the complex elastic–plastic deformation involved in the forming process presents a challenge in calculating directly the deformation response under the specified peening parameters. The energy of the shot beam in the shot peen forming process is equivalently converted to that of the laser beam in the laser forming process, enabling the equivalent simulation of macroscopic deformation of the shot-peened workpiece using the laser-induced temperature field. It is proven that the essential factors affecting the laser forming process are the laser power, scanning speed, the width and thickness of the plate. When other parameters remain constant, the equivalent laser energy can be determined by the impact energy of the shot beam and the equivalence factor. The average error of the proposed model is 5.068%, with a maximum error of less than 10%, demonstrating a well agreement between the simulation and test results. Furthermore, the equivalent numerical model is used to examine the influence of process parameters on shot peen forming, indicating that the radius of curvature produced by shot peen forming increases with nozzle speed and plate thickness, but decreases with air pressure.

Laser-induced plasma and local temperature field for high-efficiency ammonia synthesis

The effective activation of chemically inert nitrogen under room temperature and normal pressure conditions has positive effects on the ammonia synthesis process. Herein, we propose a laser activated nitrogen to assist ammonia synthesis for the first time, which effectively activates nitrogen and facilitates the subsequent ammonia synthesis process. The nitrogen gas with a high bond dissociation enthalpy is ionized into nitrogen plasma on the local central region of catalyst tablet by pulsed laser, which can be controlled by the focused laser with high pulse energy but not be affected by the unfocus laser induced high temperature. Meanwhile, the ammonia synthesis is proceeded in the intermediate temperature annular region with Fe or iron nitride as catalyst, which is accelerated by pulsed laser produced nitrogen plasma. The laser-induced ammonia synthesis (LIAS) achieves the original ammonia yield of 70.8 μmol g−1 min−1 (28.32 μmol min−1) with a high hydrogen converting ratio of 9.5% by Fe as catalyst and a stable yield of 23.63 μmol g−1 min−1 (9.45 μmol min−1) with a hydrogen converting ratio of 3.2% by iron nitride as catalyst, which are 14.9 and 49.7 times than those of thermocatalysis using the same catalysts, respectively. As a precedent for laser-induced catalytic reactions, this work provides a perspective for nitrogen activation in the ammonia synthesis process.

Subsurface damage in laser-assisted machining titanium alloys

Laser-assisted machining is an effective to improve the machinability of hard-to-machine material which has been already applied to titanium alloys. To reveal the subsurface damage in laser-assisted machining, the molecular dynamic simulations were performed on a novel titanium polycrystal crystals model, and the effects of scratching velocity and laser power on cutting force, temperature distribution, material removal volume, subsurface damage depth, phase transition, dislocation were investigated and discussed. The results showed that laser assistance can enhance the machinability through cutting force and material removal volume, and reduce subsurface damage. Comparative analysis indicated that subsurface damage was the result of competition between the stress field generated by the cutting force and the laser-induced temperature field, which means excessive laser power will inhibit the improvement of laser assistance. Laser-assisted scratching experiments were also performed under stress field dominated condition on a TC17 material, and prominently reduced subsurface damage was achieved compared to traditional scratching. Multiple kinds of subsurface damage underneath the scratched surface were induced by TEM analysis. This research enhances the understanding of subsurface damage law under different processing parameters and provide a way to obtain optimal subsurface damage depth in laser-assisted machining.

Thermal stress characteristics of monocrystalline silicon induced by ns pulse laser under auxiliary heating

Aiming at the problems of unsmooth notch and even crack caused by thermal stress in the laser processing of monocrystalline silicon, the temperature and thermal stress under auxiliary heating (AH) induced by the ns pulse laser (NPL) and their distribution characteristics are studied. Based on the theory of heat conduction and elastic–plastic mechanics, a two-dimensional axisymmetric geometric model for the temperature and thermal stress distribution of monocrystalline silicon irradiated by the NPL under AH is established, and the effect of AH on laser-induced temperature and thermal stress field is calculated and analyzed. The results show that the temperature of monocrystalline silicon irradiated by the NPL increases over auxiliary heating temperature (AHT), but it is not a simple superposition between the AHT and the temperature induced by the NPL, but the temperature change rate gradually decreases. When the temperature is lower than the target melting point, the thermal stress at the target surface is always negative as compressive stress, and the change law is the same as that of the temperature. When the temperature is more than the target melting point, the thermal stress in the melting zone is released immediately. The thermal stress decreases with the AHT increasing, and the change rate gradually decreases too. The negative effect of thermal stress can be overcome by using the correlation between temperature and thermal stress. Finally, an experiment was introduced to validate the theoretical model and calculation result preliminary. This study can provide a theoretical basis for the new technology of laser processing-monocrystalline silicon.

Measurement of the anisotropic thermal conductivity of carbon-fiber/epoxy composites based on laser-induced temperature field: Experimental investigation and numerical analysis

In this paper, the authors proposed an inverse analysis using an in–house three-dimensional finite element method for the extraction of thermal conductivity tensor components of carbon-fiber/epoxy composite, based on the recorded thermographic data in the form of laser-induced temperature field. The core of the computational algorithm used for determining the thermal conductivity values (tensor components) consists of finite element solver, which operates in a hybrid mode with an analytical model. The presented method evaluates the thermal conductivity values by matching the calculated spatial and temporal temperature fields to the experimental data. Due to the specific nature of the measurement process and computational algorithm, the proposed technique is very efficient and allows for relatively rapid obtainment of the thermal conductivity values. For considered in this work composite material with 34% fiber volume fraction the experimental thermal conductivity values in transverse and parallel to the fiber directions were equal to 0.32 W/mK and 6.80 W/mK, respectively. Additionally, in order to verify the reliability of the presented methodology, the selected isotropic reference material was investigated, using the same testing procedure. The results obtained by using this novel approach are in good agreement with data measured using the conventional testing techniques.

A methodology to determine the thermal diffusivity of additively manufactured jet engine blades based on laser-induced temperature field

This paper presents a new methodology for the thermal diffusivity determination of additively manufactured jet engine blades by using active infrared thermography. The technique is combined with a dedicated procedure based on deep machine learning models. The investigated samples were fabricated from different metal alloy powders by using the Laser Engineered Net Shaping (LENS®) technique. The experimental data for model development were in the form of laser shot-induced temperature fields, obtained from an original, in-house developed test apparatus. The advantage of the proposed methodology is that it can be used for the non-destructive measurements of the additively manufactured jet engine blades, which distinguishes the presented approach from the traditional laser-flash technique or other well-established methods. The obtained experimental values of the thermal diffusivity are in good agreement with data measured using the ASTM standard test method. The presented non-destructive technique has significant implementation/commercialization potential when applied to quality control or diagnostic procedures of 3D-printed parts.

Flexible and Precise Droplet Manipulation by a Laser-Induced Shape Temperature Field on a Lubricant-Infused Surface

Light actuation on a lubricant-infused surface (LIS) has attracted great attention because of its flexibility and remote control of droplet motion. However, to actuate a droplet on a LIS flexibly and precisely by light, the key issue is to control two degrees of freedom of the droplet motion in real time. In this paper, we propose a C-shape temperature field (CSTF) induced by rapid and selective laser irradiation on a LIS. The CSTF could not only manipulate a single droplet precisely and flexibly but also process multiple droplets automatically and orderly in real time. The mechanism showed that the droplet was confined by the Marangoni force in two orthogonal directions. For single droplet manipulation, the CSTF had the capability of correcting the off-track droplet motion. Moreover, the droplet motion, including rectilinear motion and curvilinear motion, could be precisely and flexibly controlled by the motion of the CSTF. For manipulation of multiple droplets, coalescence of multiple droplets was successfully achieved by triple rotating CSTFs. Such a method was applied in the detection of 5 μL of bovine serum albumin (BSA) by triggering chromogenic reactions automatically and orderly, which improved the efficiency of the whole process. We believe that this method is a significant candidate for intelligent droplet manipulation.

Development of a multi-spectral thermal imager for measurement of the laser-induced damage temperature field

In order to analyze the damage effectiveness of the high-energy laser beam damage to the target, it is important to accurately measure the laser-induced damage temperature field. However, the obtained data cannot meet the requirements of damage effectiveness evaluation, as most of these data are from simulated temperature measurements in the laser-induced damage temperature field. Moreover, traditional temperature measuring instruments can only measure temperature points, and cannot obtain the multi-spectral temperature field of the laser-induced damage. In response to the above problems, this paper has developed a multi-spectral thermal imager for measurement of the laser-induced damage temperature field. Using a five-channel four-wavelength filter photometer, the instrument makes the fifth channel as a calibration channel so that the infrared thermal imager can accurately collect data under multi-spectral channels. Temperature fields corresponding to the spectral data can be inverted by multi-spectral thermometry method. The temperature results of the multi-spectral thermal imager are compared with that of thermocouples, and the measurement error is<1.5%. According to the experimental data of laser-induced damage field measurements, it is shown that the laser-induced damage temperature field can be acceptably measured with the multi-spectral thermal imager.

A Methodology to Determine the Thermal Diffusivity of Additively Manufactured Jet Engine Blades Based on Laser-Induced Temperature Field

A Methodology to Determine the Thermal Diffusivity of Additively Manufactured Jet Engine Blades Based on Laser-Induced Temperature Field

Ultrastrong pure aluminum structure with gradient nanocrystals via selective pulsed laser melting: Computation framework and experiments

Controlling the transient temperature field on the selective laser melting (SLM) process is important not only for quality assurance, by reducing the occurrence of voids, cracks, and excessive deformations, but also for controlling the microstructure within the heat-affected zone. Microstructures with grains sizes gradients are known to have a synergy between the high yield strength of its fine-grained volume and the high ductility of its coarse-grained volume. This synergy attributes to the geometrically necessary dislocations present at the interface between these volumes. This work presents a systematic philosophy that uses temperature field control via pulsed lasers to induce gradient heterostructures on a pure aluminum part fabricated by SLM. Pulsed lasers have the advantage of enabling rapid and localized heating and cooling, which makes the formation of ultrafine grains possible. Therefore, it is a valuable tool to locally melt and re-solidify a metal into a refined microstructure and generate the desired gradient nanostructure in additive manufacturing of metal parts. In this work, we developed a multiscale computational framework to simulate the laser processing, by determining a transient temperature field induced by each laser pulse, microstructure evolution through the thermal history of the pulse over the treated surface, and heterostructure's mechanical properties, for the first time. This framework consists of modules of pulsed laser interaction with Aluminum parts, including Finite Element Analysis (FEA), microstructure evolution modeling via Molecular Dynamics (MD) simulations, and mechanical property prediction via MD by simulating stress-strain tests and nanoindentation tests. These modules determine how the pulsed laser-induced transient temperature field affects the heterostructure formation and mechanical strength. This comprehensive simulation framework provides means to determine selective laser melting parameters to enhance material properties. EBSD revealed that ultrafine resulting surface microstructure was formed, as predicted by the modeling framework, and this microstructure is due to the high cooling rates after short pulse durations, at the nanosecond scale. The process employed in this work can produce nanograined structures with yield strengths as high as 250.8 MPa and indentation strengths as high as 725 MPa, which are comparable to some steel's mechanical properties. This work provides a valid computational framework to build gradient heterostructure in many 3D metallic parts with superior mechanical properties using pulsed laser-induced selective laser melting.

Towards fast nanopattern fabrication by local laser annealing of block copolymer (BCP) films

The nanopatterning of surfaces and thin films with pattern dimensions of less than 100 nm is challenging for laser processing in particular in the case of large-area, low-cost fabrication. Self-assembly processes, however, provide a mechanism of pattern generation in this dimensional range offering an alternative fabrication method. The current work focuses on high-temperature, short-time laser annealing of PS-b-PMMA block copolymer (PS-b-PMMA BCP: poly(styrene-block-methyl methacrylate)) films on fused silica samples to achieve self-assembly into vertical lamellas with periods of approximately 50 nm. BCP samples were irradiated with a focussed CO2-laser beam for studying the influence of the laser power PL and the scanning speed vs on the lamellae formation in BCP films. The formation of lamellae is observed in the centre of the laser track at sufficient laser irradiation (PL: 2 to 15 W, vs: 1 to 250 mms−1). With increasing laser irradiation, first the quality of the lamellas improves to a certain point but thereafter a partial degradation of the BCP and dewetting of the BCP film occurs. The partial degradation of the PMMA micro-phase of the ordered BCP results in a local self-developing process reducing the processing steps for nanopattern formation. The experimental results on laser-induced local self-assembly with irradiation times below 0.1 s are discussed in relation to laser-induced temperature field simulations. Combining self-assembly capabilities of BCP with local heating by laser beams can provide a tool for direct writing of hierarchical nano-/microscale patterns that is useful for various applications mimicking bio-inspired structures.

Thermographic investigation of laser-induced temperature fields in selective laser beam melting of polymers

Selective laser beam melting of polymers is one of the most promising additive manufacturing technologies. Still, the main challenge is to enhance the reproducibility of part properties. To achieve this, better understanding and adapting the basic material beam interactions is crucial for the resulting temperature fields and local temperature development in xy- and z-direction. Therefore, thermographic infrared measurements were conducted within a selective beam melting system to determine the resulting temperature fields during laser exposure in xy-direction. The present results indicate a strong influence of process parameters, such as scan speed, laser power and scan vector length on resulting temperatures during and after exposure. A major influence of scan speed and therefore impact time on resulting maximum temperature was found. The shown results indicate the strongly time- and geometry- dependent influence of scan parameters on the melting and aging behavior of PA 12. For generating optimal properties of laser beam molten parts, the exposure parameters should be adapted according to the parts geometry.

Mechanisms of laser activation of chondrocytes in osteoarthritis healing

Lasers offer new possibilities in the treatment of such widespread diseases as osteoarthritis, with both direct and indirect effects on cell metabolism. Cyclic hydrostatic pressure is one of the main natural stimuli of cartilage chondrocytes. The present work shows that hydrostatic stimulation with magnitudes of up to 20 MPa can be realized locally through infrared impact on the neighboring media of chondrocytes. We compare indirect (thermomechanical, λ = 1560 nm) and direct (photo-modulation, λ1 = 1560 nm, λ2 = 670 nm) laser effects on the synthetic activity of chondrocytes in cultures within a 1 min exposure time limit, to study separately the photo-modulation and thermomechanical components of laser impact. The chondrocyte activity was monitored by immunohistochemical analysis in normoxic and hypoxic conditions. Collagen II and proteoglycan accumulation increased significantly (up to 70%) after a pulsed thermomechanical laser impact. Thermomechanical laser irradiation showed the more pronounced stimulation in both normoxic and hypoxic conditions, while the effect of photo-modulation was inhibited by oxygen concentration increase. Theoretical calculations of the laser-induced temperature and stress fields show that the spreading of the stress field with a maximum at 19.2 MPa is approximately three times greater than that of appreciable (>1 °C) heating. Thus, thermomechanical infrared stimulation of chondrocytes can be a perspective method for the restoration of hyaline-type cartilage.

Varied laser induced damage phenomena of gold coated gratings for pulse compression

In this paper, gold-coated gratings for pulse compression have been prepared and their laser damage experiments have been performed. Varied laser damage morphologies have been observed: when a 60fs-pulsed laser with energy density slightly higher than the damage threshold was used, damage morphology with a characteristic of discrete distribution of small pits was appeared. These damage pits are linearly distributed at the junction of ridges and grooves. If the laser energy density is much higher than the damage threshold, the gold films was overall ablated and the grating structure disappeared. Besides, if the gold film has poor adhesion, it was peeled off. When a 450ps-pulsed laser with energy density slightly higher than the damage threshold was used, part of grating ridges will be ablated and an obvious line exists between the ablated area and the unchanged area. In theory, the laser induced temperature field and stress field in gold-coated gratings were calculated based on the electromagnetic field using the finite element method. It is demonstrated that the temperature and thermal stress distribution characteristics are affected by the laser heating rate and the heat diffusion time (the calculated diffusion time ranges from 6fs to 450ps), which determines the laser damage characteristics. The possible damage drivers have electron hydrodynamic pressure, thermal ablation and thermal stress.

Influence of heat generated by a Raman excitation laser on the structural analysis of thin amorphous silicon film

In the present work we investigate thin amorphous silicon film fabricated by plasma enhanced chemical vapor deposition. In particular, we analyze changes in the recorded Raman spectra caused by excitation laser irradiation. Solid phase crystallization, hydrogen diffusive outflow and Raman spectra peak shifts have been observed experimentally and analyzed numerically. The role of film thickness on all these features is pointed out. The study involves laser powers between 0.1mW and 10mW focused to a spot diameter of ∼1μm and film thicknesses between 50 and ∼2000nm. Additionally, the laser induced temperature fields were analyzed by means of numerical simulation and the Raman spectral shift trough Balkanski model. Results are correlated to structural analysis by Raman spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. It was found that the hydrogen content and solid phase fraction identified by Raman spectroscopy are highly sensitive to the applied excitation laser power.

Nanoscale Measurement of Laser-Induced Temperature Rise and Field Evaporation Effects in CdTe and GaN

The measurement of laser-induced temperature change and its influence on the field evaporation behavior in nanoscale CdTe and GaN specimens was assessed through systematic studies using laser pulsed atom probe tomography. For CdTe, the laser was shown to induce a linear thermal response in the material. Using the determined relationships, a phase map of the field evaporation behavior was created. This shows that at high base temperatures, high laser energies, or low fields, significant Cd sublimation occurs, leading to apparently Te-rich measured compositions. In contrast, the highest fields result in simultaneous evaporation of multiple Te species, leading to apparently Cd-rich measured compositions. For GaN, increasing laser energy reduced the applied bias necessary for a given detection rate, whereas base temperature changes produced no significant effect on the evaporation behavior, indicative of a largely athermal evaporation mechanism. Similarly, the laser energy and bias affected the measured compo...

Laser-Induced Hydrothermal Growth of Heterogeneous Metal-Oxide Nanowire on Flexible Substrate by Laser Absorption Layer Design.

Recent development of laser-induced hydrothermal growth enabled direct digital growth of ZnO nanowire array at an arbitrary position even on 3D structures by creating a localized temperature field through a photothermal reaction in liquid environment. However, its spatial size was generally limited by the size of the focused laser spot and the thermal diffusion, and the target material has been limited to ZnO. In this paper, we demonstrated a next generation laser-induced hydrothermal growth method to grow nanowire on a selected area that is even smaller than the laser focus size by designing laser absorption layer. The control of laser-induced temperature field was achieved through adjusting the physical properties of the substrate (dimension and thermal conductivity), and it enabled a successful synthesis of smaller nanowire array without changing any complex optics. Through precise localized temperature control with laser, this approach could be extended to various nanowires including ZnO and TiO2 nanowires even on heat sensitive polymer substrate.

Cooling profiles of laser induced temperature fields for superconducting vanadium nitrate products

The flexibility of vanadium nitrate makes it a good constituent for emerging superconductors. Its thermal instability engenders a disordered structure when doped by insulating constituents. The physics of the heat source i.e. the probe laser was theoretical derived to avoid deficiency of the superconducting material at low laser energy density. The mathematical experimentation was accomplished by queering the energy balance and heat conductivity of the individual constituents of the reagent. In-depth analysis of the layered distribution of laser induced temperature fields was carried out by cooling the compound via the forced convective cooling technique to about 150 °C. The material was gradual heated via the laser probe to its superconducting state. The structural defect which explained different state of the thermal outcomes were explained and proven to correspond with experimental outcomes. The temperature distribution under the irradiating laser intensity (0.45 W) shows an effective decay rate probability density function which is peculiar to the concept of photoluminescence. The dynamics of the electronic structure of thermally-excited superconducting materials is hinged on the complementary stoichiometry signatures, thermal properties amongst others. The maximum possible critical temperatures of the inter-layer were calculated to be about 206 K.

Profiling Laser-Induced Temperature Fields for Superconducting Materials Using Mathematical Experimentation

The equations of energy balance and heat conductivity are queried by introducing known parameters using virtual mathematical experimentation. Distribution of temperature by layers in depth for samples of superconducting material is obtained under different conditions. The structural defect at different state-liquid and solid was analyzed. The theoretical model generated was validated by experimental data. The temperature distribution under the irradiating laser intensity (0.45 W) shows an effective decay rate probability density function that is peculiar to the concept of photoluminescence.

Effect of laser-induced temperature field on the characteristics of laser-sintered silver nanoparticle ink

Laser sintering of metal nanoparticles is a key technology for high-performance printed electronics fabricated on heat-sensitive substrates such as glass or plastic. Although laser-sintered electronic devices have been successfully fabricated, the role of the induced temperature field in the laser sintering process has not been reported thus far. In this work, the effect of temperature on the laser sintering process is described for the first time using a two-dimensional transient heat conduction equation for inkjet-printed silver nanoparticle ink. The in situ electrical resistance was measured to estimate the transient thermal conductivity and hence the temperature of the sintered ink during the laser sintering process. To verify the estimated laser sintering temperature, the morphology of furnace-sintered silver nanoparticle ink was compared with that of laser-sintered ink. The electrical characteristics and surface morphology of laser-sintered ink are found to be related to the process temperature.