Laser-induced Thermal Stress Research Articles

Analysis of the coupling efficiency of grain-oriented electrical steel using fiber laser

Laser scribing of electrical steel used for transformers improves their efficiency. In particular, the laser-induced thermal stress reduces the core losses of grain-oriented (GO) electrical steels by a refinement of the magnetic domains. A reliable process control needs precise information about the narrow temperature range, which ensures the desired transformations of the material. Therefore, optimized process designs benefit from thermal simulations of the temperature field during the laser treatment. The accuracy of such computations depends upon the knowledge of the coupling efficiency of laser radiation into the processed materials. Therefore, a combined approach of experimental measurements and numerical calculations is applied to get reliable coupling efficiency data for GO electrical steels. The results show very high coupling efficiencies in a range of about 80% for solid-state laser radiation with a wavelength of 1070 nm. Subsequent investigations on coated and uncoated sheets reveal potential reasons for the high coupling efficiency.

Study of CO2 Laser-induced Thermal Stress Mechanisms on Decorative Soda-lime Glass

Study of CO2 Laser-induced Thermal Stress Mechanisms on Decorative Soda-lime Glass

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.

Photoacoustic 2D actuator via femtosecond pulsed laser action on van der Waals interfaces

Achieving optically controlled nanomachine engineering can satisfy the touch-free and non-invasive demands of optoelectronics, nanotechnology, and biology. Traditional optical manipulations are mainly based on optical and photophoresis forces, and they usually drive particles in gas or liquid environments. However, the development of an optical drive in a non-fluidic environment, such as on a strong van der Waals interface, remains difficult. Herein, we describe an efficient 2D nanosheet actuator directed by an orthogonal femtosecond laser, where 2D VSe2 and TiSe2 nanosheets deposited on sapphire substrates can overcome the interface van der Waals forces (tens and hundreds of megapascals of surface density) and move on the horizontal surfaces. We attribute the observed optical actuation to the momentum generated by the laser-induced asymmetric thermal stress and surface acoustic waves inside the nanosheets. 2D semimetals with high absorption coefficient can enrich the family of materials suitable to implement optically controlled nanomachines on flat surfaces.

Laser-Induced Thermal Stresses in Dense and Porous Silicon Dioxide Films

The laser-induced thermal stresses in silicon dioxide films are calculated using molecular dynamics simulations. The absorption of the laser energy is simulated by the linear temperature growth from room temperature to 1300 K in a time equal to the laser pulse duration. The maximum values of stresses for picosecond pulses are approximately twice as high as for nanosecond pulses. The stresses in highly porous glancing angle deposited films are approximately two times lower than in dense films. Stress waves caused by picosecond pulses are observed in dense films. An increase in the heating temperature to 1700 K leads to an increase in the absolute stress values for picosecond pulses, and a decrease for nanosecond pulses.

Numerical study on thermal stress cutting of silicon wafers using two-line laser beams

We propose a method of cleaving silicon wafers using two-line laser beams. The base principle is separating the silicon wafer using crack propagation caused by laser-induced thermal stress. Specifically, this method uses two-line laser beams parallel to the cutting line such that the movements of the laser beam along the cutting line can be omitted, which is necessary when using a point beam. To demonstrate the proposed method, 3D numerical analysis of a heat transfer and thermo-elasticity model was performed. Crack propagation was evaluated by comparing the stress intensity factor (SIF) at the crack tip with the fracture toughness of silicon, where crack propagation is assumed begin when the SIF exceeds the fracture toughness. The influences of laser power, line beam width, and distance between two laser beams were also investigated. The simulation results showed that the proposed method is appropriate for cleaving silicon wafers without any thermal damage.

Numerical study of the effect of diode laser-induced optical, thermal, and residual stresses on CZTS thin-films

This paper presents a numerical study of the continuous wave (CW) diode laser processing of Cu2ZnSnS4 (CZTS) thin-film. The CZTS film’s structural, optical, surface morphological, and electrical properties improvement due to the localized and instantaneous heating provided by the diode laser mandates a detailed understanding of how laser processing affects the CZTS thin-film during thermal treatment to ensure a high-quality processing result as well as optimization and characterization. However, understanding of such process suffers from the restricted accessibility as most of the important phenomena occur inside the film during a very short duration time, where numerical simulations can serve as a valuable alternative to gain accessibility. This contribution, therefore, presents a numerical approach to examine the CW diode laser-induced thermal effect on the CZTS thin-film. A numerical model in OpenFOAM has been developed to elucidate the influence of laser processing parameters on the optical, heat transfer, and residual stress, as well as design architecture, of the CZTS thin-films. The model results predicted that the inappropriate settings of the laser parameters, as well as the thickness of the CZTS films, generate thermal gradient within the film and hence induce delamination and propagation of crack at the interfaces. The developed model helped in understanding the laser-induced thermal behavior of the CTZS thin-film and can be utilized as a guide to using lasers as an effective tool for fabricating the high-efficiency CZTS thin-film based solar cells.

Surface morphology and straight crack generation of ultrafast laser irradiated β-Ga2O3

Single crystal (010) β-Ga2O3 was irradiated by a Ti:sapphire ultrafast laser (150 fs pulse width) with varying fluences and a number of pulses in air ambient. Femtosecond laser-induced damage threshold of β-Ga2O3 is reported. Single pulse exposure results in surface morphological changes above a threshold laser fluence of 1.11 J/cm2. Laser-induced straight cracks aligned to the [001] crystallographic direction are observed in the laser irradiated regions, which are believed to be caused by laser-induced thermal stress, due to the unique low thermal conductivity and anisotropy associated with β-Ga2O3. Multiple pulse irradiation below the single pulse damage threshold fluence exhibited the formation of high spatial frequency laser-induced periodic surface structures. Electron backscattering diffraction and Raman spectroscopy suggested that there was no apparent phase transition of the irradiated β-Ga2O3 material for either single pulse or multiple pulse irradiation. This work serves as a starting point to further understanding the material properties of β-Ga2O3 and to unlock the potential for ultrafast laser material processing of β-Ga2O3.

Ultrashort pulse induced elastodynamics in polycrystalline materials. Part II: Thermal–mechanical response

Near-field thermal–mechanical responses in polycrystalline gold films considering grain size effects and temperature-dependent thermophysical properties are examined using the generalized thermo-elastodynamic model formulated in Part I. Ultrashort laser-induced transverse and radial thermal stress waves are attenuative, broad in bandwidth, and dispersive with extremely high frequencies. Grain size effects on thermal response are found to be so prominent that the stress fields induced in the polycrystalline film considered for the study are consistently more intense with decreasing averaged grain diameter. The normal and shear stress fields induced by a below melting threshold fluence are low in amplitude but extremely high in frequency response and power density. With the former not exceeding 20 MPa and the latter of the order of 1018-to-1019 [W/m3] in magnitude, non-melting ultrafast heating that generates no plastic deformation is profound with the potential for initiating fatigue micro-cracking in near-field and for inflicting physical damage.

Development of Mirror Surface Groove Processing Technology of Glass by Laser Induced Thermal Stress

Development of Mirror Surface Groove Processing Technology of Glass by Laser Induced Thermal Stress

Laser-induced cracks in ice due to temperature gradient and thermal stress

This work presents the experimental and theoretical investigations on the mechanism of laser-induce cracks in ice. The laser-induced thermal gradient would generate significant thermal stress and lead to the cracking without thermal melting in the ice. The crack density induced by a pulsed laser in the ice critically depends on the laser scanning speed and the size of the laser spot on the surface, which determines the laser power density on the surface. A maximum of 16 cracks within an area of 17 cm × 10 cm can be generated when the laser scanning speed is at 10 mm/s and the focal point of the laser is right on the surface of the ice with a laser intensity of ∼4.6 × 107 W/cm2. By comparing the infrared images of the ice generated at various experimental conditions, it was found that a larger temperature gradient would result in more laser-induced cracks, while there is no visible melting of the ice by the laser beam. The data confirm that the laser-induced thermal stress is the main cause of the cracks created in the ice.

Densification behavior, microstructural evolution, and mechanical properties of TiC/316L stainless steel nanocomposites fabricated by selective laser melting

Here we investigate the influence of the volumetric laser energy density (η) utilized in selective laser melting (SLM) on the phase evolution, densification behavior, microstructure evolution, and mechanical properties of TiC/316L stainless steel nanocomposite parts. The η values, which was controlled by adjusting the laser scanning speeds, was found to strongly correlate with the metallurgical mechanisms of the fabricated parts. During the SLM process, low η values induced disordered liquid solidification fronts with significant balling and pore chain defects, which arose from the low-viscosity and highly unstable flow behavior caused by densification-limiting Marangoni convection. Conversely, high η values in SLM significantly enhance densification but induce fine spherical pores and thermal microcracks by increasing liquid lifetimes and thermal stresses. The diffraction angles of the γ-Fe peaks were shifted from their standard locations by the lattice distortion from laser-induced thermal stress. The samples processed at η of 67J/mm3 showed the most refined microstructures formed under exposure to the highest cooling rates, which led to higher compressive yield strengths than those of samples processed at η of 300J/mm3. Pole figures obtained for the γ-Fe phase depicted no significant directionality in the texture of any of the components; however, a significant texture intensity increase was reported with increasing η.

Analysis of Cutting Glass Sheets by Laser-Induced Thermal Stress

Scribing, or the full-body cutting of glass substrates, can be achieved by thermal stress that is induced by laser heating. In such processing, the substrate’s temperature distribution is controlled by scanning the heat source or a pair of heat and cooling sources for generating sufficiently high tensile stress to propagate a crack. As a result of crack propagation that follows the laser beam, the substrate can be divided along the laser beam scanning path. By analyzing the temperature and the thermal stress in the numerical simulations, we can estimate suitable processing conditions. How the crack propagates can also be estimated by analyzing the stress intensity factors. Numerical simulations supported clarifying the mechanism of crack propagation and devising new processing methods for controlling crack propagation.

An experimental investigation of underwater pulsed laser forming

Laser forming is a new forming technology, which deforms a metal sheet using laser-induced thermal stresses. This paper presents an experimental investigation of pulsed laser forming of stainless steel in water and air. The effects of cooling conditions on bending angle and morphology of the heat affected zone (HAZ) are studied. It is shown that the case of the top surface in air and the bottom surface immersed in water has the greatest bending angle based on the forming mechanism of TGM. The water layer above the sample decreases the coupling energy, leading to a small bending angle. For a thin water thickness (1mm), the water effects on the HAZ are limited. As water layer thickness increases (5mm), the concave shape of the HAZ is more remarkable and irregular because the shock waves by high laser energy heating water are fully developed. However, the area and the depth of the HAZ become less significant when water thickness is 10mm due to the long pathway that laser undergoes.

Analysis of pulsed laser bending of sheet metal using neural networks and neuro-fuzzy system

Laser bending is a nontraditional forming process, where sheet metal gets plastically deformed by laser-induced thermal stresses. The objective of this study is to establish the relationships between bending angles and process parameters in a pulsed laser bending process using soft computing–based methods, that is, neural networks and neuro-fuzzy system. Laser power, scan speed, spot diameter and pulse duration were considered as inputs, and bending angle was taken as output for modeling the bending angle (called forward analysis). In the case of inverse analysis or process synthesis (i.e. to determine the process parameters in order to achieve the desired outputs), bending angle and pulse duration were considered as inputs, and laser power, scan speed and spot diameter were treated as outputs. For both forward and inverse analyses, neural networks and neuro-fuzzy systems were trained in a batch mode with experimental data using two different algorithms, that is, genetic algorithm and back-propagation algorithm. The optimized networks were used for the predictions of bending angles and process parameters for some test cases. All the developed models were found to be satisfactory for both the analyses. Genetic algorithm was found to perform better than the back-propagation algorithm for both the networks in terms of prediction accuracy but at the cost of computational time. Neural networks trained with genetic algorithm were seen to perform better than the other models in predicting bending angles and process parameters. The developed models might be helpful in automating the pulsed laser forming process.

Predictive Modeling for Glass-Side Laser Scribing of Thin Film Photovoltaic Cells

Laser scribing of multilayer-thin-film solar cells is an important process for producing integrated serial interconnection of mini-modules, used to reduce photocurrent and resistance losses in a large-area solar cell. Quality of such scribing contributes to the overall quality and efficiency of the solar cell, and therefore predictive capabilities of the process are essential. Limited numerical work has been performed in predicting the thin film laser removal processes. In this study, a fully-coupled multilayer thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress responsible for SnO2:F film removal. A plasma expansion induced pressure model is also investigated to simulate the nonthermal film removal of CdTe due to the micro-explosion process. Corresponding experiments of SnO2:F films on glass substrates by 1064 nm ns laser irradiation show a similar removal process to that predicted in the simulation. Differences between the model and experimental results are discussed and future model refinements are proposed. Both simulation and experimental results from glass-side laser scribing show clean film removal with minimum thermal effects indicating minimal changes to material electrical properties.

Laser-induced front side etching of CaF2 crystals with KrF excimer laser

The laser-induced front side etching (LIFE) of amorphous materials like fused silica was manifold studied and the LIFE process was sufficient optimized for the fabrication of well-defined etching trenches with a very low surface roughness. The LIFE process is an indirect laser-induced ablation process, the – for the used laser wavelength – transparent substrate was covered by a highly absorbing material and the absorbing process causes a transfer of the laser energy into the substrate and, finally, to an ablation process of the substrate surface.However, the structuring of crystalline materials like CaF2 is a great challenge for the LIFE process. The properties of CaF2(111) and CaF2(001) surfaces etched by KrF excimer laser pulses (pulse duration Δtp=25ns, wavelength λ=248nm) were analysed by white light interferometry (WLI) as well as scanning electron microscopy (SEM). The surface morphologies of laser etched CaF2 surfaces depend on the laser parameters and on the crystal orientation and are frequently characterized by microcracks and flake spallation. The most probable reasons therefore are laser-induced thermal stress or laser-induced shock waves.

Deformation behavior of laser bending of circular sheet metal

The application of a thermal source in non-contact forming of sheet metal has long been used. However, the replacement of this thermal source with a laser beam promises much greater controllability of the process. This yields a process with strong potential for application in aerospace, shipbuilding, automobile, and manufacturing industries, as well as the rapid manufacturing of prototypes and adjustment of misaligned components. Forming is made possible through laser-induced non-uniform thermal stresses. In this letter, we use the geometrical transition from rectangular to circle-shaped specimen and ring-shaped specimen to observe the effect of geometry on deformation in laser forming. We conduct a series of experiments on a wide range of specimen geometries. The reasons for this behavior are also analyzed. Experimental results are compared with simulated values using the software ABAQUS. The utilization of line energy is found to be higher in the case of laser forming along linear irradiation than along curved ones. We also analyze the effect of strain hindrance. The findings of the study may be useful for the inverse problem, which involves acquiring the process parameters for a known target shape of a wide range of complex shape geometries.

Image-guided genomic analysis of tissue response to laser-induced thermal stress

The cytoprotective response to thermal injury is characterized by transcriptional activation of "heat shock proteins" (hsp) and proinflammatory proteins. Expression of these proteins may predict cellular survival. Microarray analyses were performed to identify spatially distinct gene expression patterns responding to thermal injury. Laser injury zones were identified by expression of a transgene reporter comprised of the 70 kD hsp gene and the firefly luciferase coding sequence. Zones included the laser spot, the surrounding region where hsp70-luc expression was increased, and a region adjacent to the surrounding region. A total of 145 genes were up-regulated in the laser irradiated region, while 69 were up-regulated in the adjacent region. At 7 hours the chemokine Cxcl3 was the highest expressed gene in the laser spot (24 fold) and adjacent region (32 fold). Chemokines were the most common up-regulated genes identified. Microarray gene expression was successfully validated using qRT- polymerase chain reaction for selected genes of interest. The early response genes are likely involved in cytoprotection and initiation of the healing response. Their regulatory elements will benefit creating the next generation reporter mice and controlling expression of therapeutic proteins. The identified genes serve as drug development targets that may prevent acute tissue damage and accelerate healing.

3D laser-forming strategies for sheet metal by geometrical information

Forming sheet metal by laser-induced thermal stress (laser forming) is considered to offer great potential for rapid prototyping and flexible manufacturing. Accordingly, many studies have been carried out in different areas of laser forming. However, in order to apply the laser-forming process to real 3D products, a method that encompasses the whole process planning is required, including the laser irradiation patterns, laser power, and travel speed, when the target shape is given. In this work, a new method for 3D laser forming of sheet metal is proposed. This method uses geometrical information rather than a complicated stress–strain analysis. Using this new method the total calculation time is reduced considerably while affording strong potential for enhanced accuracy. Two different target shapes were formed by laser irradiation with the proposed procedure to validate the algorithm.