Analytical investigation into laser pulse heating and thermal stresses

Laser pulse heating of metallic surfaces results in rapid rise of temperature in the region irradiated by the laser beam. This in turn results in high temperature gradient in this region. The irradiated substrate material expands as a response to the temperature gradient. Consequently, high thermal stress levels are developed in the region of the high temperature gradient. In the present study, closed form solutions for temperature and stress fields due to a laser pulse decaying exponentially in time are presented. A Laplace transformation method is employed in the analysis. The resulting equations are non-dimensionalized with the appropriate parameters. It is found that temperature rises rapidly during the early heating period in the surface region. In this case, internal energy gain dominates the conduction losses from the surface vicinity. The thermal stress levels attain high values in the surface region. The stress wave developed is compressive and it propagates with a wave speed c₁ inside the substrate.     

Comparison of Cut Path Deviation Between CO2 and Diode Lasers in Float-Glass Cutting

Laser cutting of glass using the controlled fracture technique leads to cut path deviation at the leading and trailing edges of the float glass sheet. In this technique, thermal stresses are used to induce the crack, and the material is separated along the cutting path by extending the crack. We show that the cut path deviation is partly due to high magnitudes of thermal stresses generated near the sheet edges. The absorption of intense radiation from the CO₂ and diode laser beams in the glass causes local temperature increases and consequently generates different thermal fields and stress distributions due to surface and volumetric heat absorption. In this paper, we report the effect of the CO₂ and diode laser wavelength interaction with the float glass and its effect on the magnitudes of thermal stresses generated near the edges of the glass sheet. We simulate the distribution of the thermal stress and temperature using finite-element analysis software Abaqus and validate it against the experimental data. We show that the CO₂ laser produces a lower surface quality and a larger cut path deviation at the leading and trailing edges of the glass sheet as compared to the diode laser.     

Cemented carbide cutting tool: Laser processing and thermal stress analysis

Laser treatment of cemented carbide tool surface consisting of W, C, TiC, TaC is examined and thermal stress developed due to temperature gradients in the laser treated region is predicted numerically. Temperature rise in the substrate material is computed numerically using the Fourier heating model. Experiment is carried out to treat the tool surfaces using a CO₂ laser while SEM, XRD and EDS are carried out for morphological and structural characterization of the treated surface. Laser parameters were selected include the laser output power, duty cycle, assisting gas pressure, scanning speed, and nominal focus setting of the focusing lens. It is found that temperature gradient attains significantly high values below the surface particularly for titanium and tantalum carbides, which in turn, results in high thermal stress generation in this region. SEM examination of laser treated surface and its cross section reveals that crack initiation below the surface occurs and crack extends over the depth of the laser treated region.     

Cutting glass substrates with dual-laser beams

In order to improve the cutting quality, a dual-laser-beam method was proposed to cut glass substrates in the current study, where a focused CO₂-laser beam was used to scribe a straight line on the substrate, and a defocused CO₂-laser beam was used to irradiate on the scribing line to generate a tensile stress and separate the substrate. The finite-element-method (FEM) software ANSYS was applied to calculate the temperature distribution and the resulting thermal stress filed. Through experimental study, it concluded that the glass substrate can be separated along an expected path with dual-laser beams and the cutting quality can be improved comparing with the cutting using a defocused laser beam alone. The relation between the cutting speed and the defocused laser power was also investigated in cutting glass with this method.     

Closed form solutions for thermal stress field due to non-equilibrium heating during laser short-pulse irradiation

Non-equilibrium heating in the lattice sub-system results in high temperature gradients in the surface region. This in turn causes thermal stress waves propagating into the substrate material. In the present study, a closed form solution for thermal stress developed in the substrate material due to volumetric pulse heating is presented. The stress free and stress continuity boundary conditions at the surface are incorporated in the closed form solutions. It is found that thermal stress wave is tensile in the surface region and it becomes compressive at some depth below the surface for stress free condition at the surface; however, it remains compressive for the condition of stress continuity at the surface.     

Laser cutting of holes in thick sheet metals: Development of stress field

Laser cutting of hole in a mild steel thick sheet metal is investigated. Temperature and stress fields developed around the cutting section are simulated using the finite element method. An experimental is carried out accommodating the simulation parameters. The residual stress developed in the cutting section is measured using the XRD technique and findings are compared with the predictions. Optical microscopy and SEM are carried out to examine the morphological changes in the cutting sections. It is found that temperature decays sharply in the region of the laser heat source, which results in high temperature gradient in this region. This causes the development of high stress levels around the cut edges. The residual stresses predicted are in agreement with the measured results.     

Laser cutting of Kevlar laminates and thermal stress formed at cutting sections

Laser cutting of Kevlar laminates is carried out and thermal stress field developed in the cutting region is predicted using the finite element code. Temperature predictions are validated through the thermocouple data. The morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that temperature predictions agree well with the thermocouple data. High values of von Mises stress are observed at the cutting edges and at the mid-thickness of the Kevlar laminate due to thermal compression formed in this region. The laser cut edges are free from whiskers; however, striation formation and some small sideways burning is observed at the kerf edges.     

Improved formulation of electron kinetic theory approach for laser shortpulse heating: Thermal stress consideration

Nonequilibrium energy transport between excited electrons and lattice site is re-formulated after considering the ballistic contribution of the electron energy to the energy transport process. The improved formulation of the electron kinetic theory predictions are compared with the previously obtained electron kinetic and two-equation models. Thermal stress developed in the region irradiated by a laser beam is formulated during the heating pulse. Copper with variable properties is used in the simulations. It is found that improved electron kinetic theory model predicts less temperature rise than that corresponding to previously formulated electron kinetic theory and two equation models in the surface region; in this case, electron temperature attains high values. Thermal stress developed is compressive and attains the maximum at some depth below the surface. The thermal stress level is well below the yielding limit of the substrate material.     

Simulation of elastic displacement of surface during laser short pulse heating of gold

The laser short pulse heating initiates nonequilibrium heating of the substrate material, which in turn results in the thermal stresses developing in the region below the surface. The surface temperature can be measured possibly through the monitoring of the resulting surface displacement. This requires in detail investigation into the surface displacement and surface temperature rises across the heated spot during the laser short pulse heating process. In the present study, the laser short pulse heating of gold surface is considered and the temperature rise at the surface and elastic displacement of the surface are investigated. The spatial and temporal distributions of surface displacement and surface temperature are predicted and the elastic response of the substrate material due to temperature rise is explored. It is found that the temporal and spatial distributions of the surface displacement do not follow the temperature rise at the surface. Consequently, care should be taken when measuring the temperature rise at the surface by means of monitoring the surface displacement during a laser short pulse heating process.     

Laser carbonitriding of alumina surface

Laser carbonitriding of alumina surfaces is examined. Temperature and stress fields developed during the laser heating of the substrate surface are predicted using the finite element method in line with the experimental conditions. The formation of Al(C, N) and AlN compounds in the surface region of irradiated workpiece is examined using X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD). The microstructural and morphological changes in the laser irradiated region are examined using Scanning Electron Microscope (SEM). The microhardness of the resulting surface is measured and compared with the base material hardness. It is found that high temperature gradient is developed in the irradiated region, which in turn, results in high residual stress levels in this region. XPS and XRD data reveal the presence of Al (C, N) and AlN compounds in the surface region. The microhardness in the surface region of the laser treated workpiece increases significantly.     

Three-dimensional consideration of jet impingement onto the kerf in relation to laser cutting process: Effect of jet velocity on heat transfer rates

In laser cutting process, an assisting gas is used to improve the mass removal rate from the cutting kerf and protect the kerf surfaces from the high temperature exothermic reactions, such as oxidation reactions, during the cutting process. Therefore, heat transfer rates from the kerf wall and the skin friction along the kerf surface are important for quality cutting. In the present study, jet emerging from a conical convergent nozzle and impinging onto the kerf surface is investigated in relation to the laser cutting process. The flow field in the kerf, the heat transfer rates from the kerf wall, and the skin friction along the kerf surface are computed for four average jet velocities at the nozzle exit and two kerf wall wedge angles. The ratio of the stand-off-distance (distance between the nozzle exit and the kerf top surface) to nozzle diameter is selected as H/D=2.2., where H is the stand-off-distance and D is the nozzle exit diameter. The kerf wall temperature is kept at 1500 K to resemble the laser cutting process. It is found that the Nusselt number increases sharply at the kerf inlet and decreases towards the kerf exit for the kerf wall angle of 0°. However, it increases gradually in this region for the kerf wedge angle of 4°. The skin friction decreases along the kerf surface.     

Effect of thermal stresses on chip-free diode laser cutting of glass

In laser cleaving of brittle materials using controlled fracture technique, thermal stresses are used to induce a crack and the material is separated along the cutting path by extending this crack. In this study, a glass sheet is stressed thermally using a 808-940 nm diode laser radiation. One of the problems in laser cutting of glass with controlled fracture technique is the cut deviation at the leading and the trailing edges of the glass sheet. In order to avoid this damage it is necessary to understand the stress distributions which control crack propagation. A study is conducted here to analyse the cut deviation problem of glass by examining the stress fields during diode laser cutting of soda-lime glass sheets. Optical microscope photographs of the breaking surface are obtained to examine the surface quality and cut path deviation while the latter is explained from the results of the stress fields which are obtained from a finite element simulation.     

The effect of material thickness,laser power and cutting speed on cut path deviation in high-power diode laser chip-free cutting of glass

In the laser cleaving of brittle materials, using controlled fracture technique, thermal stresses are used to induce the crack and the material is separated along the cutting path by extending the crack. One of the problems in laser cutting of glass with this technique is the cut path deviation at the leading and the trailing edges of the glass sheet. Previous work has shown this deviation to be partly due to the high magnitudes of thermal stresses generated near the edges of the sheet. This paper reports on the experimental results of the effects of glass thickness, laser power and the cutting speed on cut path deviation in diode laser cutting of glass. Finite element modelling of the cutting process has also been used to simulate the transient effects of the moving beam and predict thermal fields and stress distributions. These predictions are validated against the experimental data and are used to explain the process mechanisms. It is shown that an increase in the thickness of the glass sheet for the same power and cutting speed or an increase in the cutting speed with constant power and a given sheet thickness results in smaller cut path deviations at the leading and trailing edges of the glass sheet.     

Laser cutting of sharp edge: Thermal stress analysis

Laser cutting of sharp edge and thermal stress development in the cutting section is examined. The finite element method is used to predict temperature and stress fields while the X-ray diffraction (XRD) technique is used to measure the residual stress around the cut edges. A mild steel sheet with 5 mm thickness is used in the simulations and the experiment. The morphological and metallurgical changes around the edges are examined using the optical microscopy and scanning electron microscopy (SEM). It is found that temperature remains high at the sharp edge when the laser beam is located in this region. This, in turn, lowers the cooling rate and reduces von Mises stress in this region. The magnitude of the residual stress is about 90 MPa at the sharp corner while the maximum von Mises stress is in the order of 280 MPa, which occurs away from sharp corner. In addition, the residual stress predicted agrees with the experimental data.     

High-speed laser cutting of superposed thermoplastic films: thermal modeling and process characterization

Common thermoplastic films used in the packaging industry have a thickness lower than 100 μm, and present low absorption to CO₂ laser radiation. This characteristic renders the use of cutting parameters, predicted by models developed for thicker thermoplastics inappropriate. In addition, the usual procedures involve the use of an assisting gas, responsible for removing the melted material, which, when processing thin films, induces changes in position in the material. A new theoretical model describing the temperature distribution on thin thermoplastic material during laser cutting was later developed. The heat conduction was solved analytically by the Green function method and heating and cooling thermal stress evolution was taken into consideration. The laser beam diameter over the samples provides the possibility of obtaining two cut operations: a simple cut, on beam focus, and a cut with welding, defocusing the beam. Engineering parameters predicted by the model were applied to cutting superposed high- and low-density polyethylene and polypropylene samples, transparent and white, with thicknesses between 10 and 100 μm, and experimentally validated.Proper modeling and the introduction of a reflective substrate under the samples allowed the improvement of process efficiency and the achievement of cutting operations up to 20 m s⁻¹, and cut with welding up to 14 m s⁻¹; an order of magnitude of improvement on industrial speeds previously attained for this operation.     

Laser cutting of rectangular geometry into alumina tiles

Laser cutting of rectangular geometry into the 5 mm thick alumina tiles is carried out. Temperature and stress fields, which are developed during the cutting process, are simulated in line with the experimental conditions. The morphological changes in the cutting sections are examined using optical and electron scanning microscopes, energy dispersive spectroscopy, and X-ray diffraction technique. The predictions of surface temperature and the residual stress are validated through the experimental data. It is found that von Mises stress attains high values in the region of the mid-thickness of the workpiece. The laser cut sections are free from major cracks and large scale sideways burning. The predictions of surface temperature and residual stress agree well with their counterparts obtained from the experiment.     

Luminescence of metals excited by fast nondestructive loading

The paper reports a study of the luminescence excited on the back side of metal targets irradiated with laser pulses of energy substantially below the plasma-flare formation threshold, and calculation of the temporal and spatial distributions of temperature, thermal stresses, and rate of thermal-stress variation in a sample. The evolution of the luminescence pulse is compared with that of the laser pulse, sample temperature, thermal stresses, and rate of thermal-stress variation. It has been established that the luminescence is excited as soon as the stresses at the sample surface become approximately equal to the yield point of the sample material, its intensity grows as long as the rate of stress rise increases, after which the process decays. The temporal and spatial distributions of temperature, thermal stresses, and rate of thermal-stress variation have also been calculated for the experiments, in which anomalous electron emission from the back side of laser-pulse irradiated metal targets was detected, and which were described in the literature but not appropriately explained. The dynamics of experimental and calculated relations have been compared. A correlation closely similar to that found for mechanoluminescence has been established. Fiz. Tverd. Tela (St. Petersburg) 40, 957–965 (June 1998)     

Estimating cutting front temperature difference in disk and CO2 laser beam fusion cutting

A three-dimensional, semi-stationary, simplified thermal numerical model was developed. The average cutting front temperature difference in disk and CO₂ laser beam fusion cutting of 90MnCrV8 was estimated by computing the conductive power loss. Basing on heat affected zone extension experimentally measured and using an inverse methodology approach, the unknown thermal load on the cutting front during laser cutting was calculated. The accuracy of the numerical power loss estimation was evaluated comparing the results from simulation with the ones from analytical models. A good agreement was found for all the test cases considered in this study. The conduction losses estimation was used for justifying the lower quality of disk laser cuts due to the lower average cut front temperature. This results in the increase of viscosity of molten material and in the subsequent more difficult ejection of the melted material from the cut kerf.     

YAG物理特性参量对LD泵浦固体激光器热效应的影响

针对LDA三角均布侧泵结构激光棒,采用有限元方法,在YAG介质热传导率和热膨胀系数的温度无关和相关条件下,比较分析了激光介质的温度和热应力分布.数值模拟结果表明,考虑YAG介质特性参量的温度相关性时介质的热效应将变得更加严重,并且泵浦越强,YAG特性参量的温度相关性对热效应的作用越明显.因此在设计激光器,特别是大功率激光器时应对激光介质物性参量的温度相关性加以考虑.     

Thermal modeling of GaAs-based semiconductor disk lasers

Thermal characteristics including the temperature, the heat flux, and the temperature gradient of GaAs-based semiconductor disk lasers (SDLs) under various conditions are modeled using the finite element method. The effects of the substrate thickness, the pump spot radius, the heatspreader, and the pump power on the thermal properties of laser are simulated, and the maximum temperature rise in active region is highlighted. Numerical analysis predicts that SiC is an ideal substitute for diamond as a heatspreader. Instead of the sophisticated completely-etched substrate, the use of a high-thermal-conductivity heatspreader along with partly-etched substrate can provide sufficient thermal management to the laser.