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.     

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.     

Destruction of a solid film under the action of ultrashort laser pulse

Molecular-dynamics (MD) simulation of the destruction of a crystal film heated by a femtosecond laser pulse was carried out. Heating is assumed to be instantaneous, because there is no time for the material to be displaced during the pulse. Film destruction is caused by the interaction of unloading waves. It can be considered as a model of a more complex process of splitting out of a thin surface layer from a massive target in the case where the layer remains solid after heating. It was found that the crystal order is broken due to the stretching strains and to the strong anisotropy of residual stress, resulting in a bipartition of the layer separating from the target. The lattice stretching and the formation of anisotropic stresses are due to the expansion of a heated lattice.     

Laser cutting of steel and thermal stress development

In laser cutting of sheet metals, thermal stresses are developed in the region of the cutting section. Depending on the cutting conditions and substrate material properties, the thermal stress levels can attain high values. In the present study, thermal stress developed in the region of the laser cut edges is modeled and temperature as well as stress fields are predicted. Temperature predictions are validated through the experimental results. It was found that the temporal variation of the maximum temperature along y-axis follows the laser heating source. However, temporal variation of von-Mises stress deviates slightly from the temporal variation of temperature along the cutting direction. Increase in scanning speed enhances the von-Mises stress levels due to the attainment of high temperature gradients in the substrate material.     

Laser heating: an electron-kinetic theory approach and induced thermal stresses

Laser heating offers considerable advantages for laser treatment of mechanical parts. The laser heating initiates the thermal stresses and expansion of the substrate. In the present study, laser heating of steel substrate is considered. Three-dimensional electron-kinetic theory approach is introduced when simulating the heating process. A moving laser source is taken into account provided that the laser scans the surface at a constant speed. Thermal stresses due to laser heating are computed and thermal expansion along the laser beam axis is predicted. It is found that the thermal stress increases considerably as the heating progresses. Moreover, the expansion of the free surface reaches a value of the order of a nanometer.     

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.     

Experimental simulation and theoretical analysis of the thermal deformation of dielectric plates under submicrosecond radiation heating

The flexural deformation of dielectric plates subjected to submicrosecond thermal stresses generated by volume and surface sources is studied. The action of such stresses is simulated by laser irradiation of colored glass plates with different optical absorption factors. Such an approach to simulating the thermomechanical action of radiation on dielectric materials allows researchers to visualize the qualitative and quantitative thermoelastic response of the plates to the action of pulsed sources of thermal stresses with different spatial parameters. For volume sources of thermal stresses, it is shown that the thermal deformation of a dielectric plate can be viewed as a set of quasi-harmonic extension-compression wave processes combined with the quasi-static bend of the plate. Under the action of surface sources of submicrosecond thermal stresses, the deformation mechanism is a superposition of the thermal deformation of a thin surface layer and a pulse wave process resulting in the bend of the plate when the pulses reverberate between the surfaces of the plate. Approximate analytical models of thermal deformation due to pulsed thermal disturbances are suggested that make it possible to predict the extent of bend versus the absorbed energy dose under the action of both volume and surface sources of thermal stresses.     

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.     

脉冲激光辐照硅材料引起表面波纹的特性研究

叙述了激光与材料相互作用过程中引起相干受激光散射的机制,以及形成材料表面波纹的特性。在激光波长1．06μm、能量15mJ、光斑直径2mm、脉冲半峰全宽约10ns和入射方向为布儒斯特角的条件下,进行了脉冲激光辐照硅材料形成表面波纹的实验研究。在脉冲激光辐照硅材料表面功率密度略大于材料损伤阈值的条件下,发现了硅材料表面形成的平行等间距直线条纹结构。用光学显微镜和原子力显微镜分别测量了被辐照硅材料表面的波纹形貌特征。在假设硅材料表面波纹的产生与声波在材料中的传播速度有关的条件下,由声波传播速度和激光辐照硅材料的脉冲宽度较好地解释了材料表面形成条纹的宽度．并认为在形成表面波纹的过程中,热应力起主要作用。     

Comparison of temperature field due to laser step input and time exponentially varying pulses

Laser evaporative heating of the solid surface is considered and the effect of temporal variation of laser pulse shape on temperature rise is examined. In the analysis, time exponentially varying and step input pulses are employed and closed-form solutions for temperature rise are presented. Comparison of temporal variation of surface temperature is carried out for various laser pulse parameters of exponential and step input pulses. The pulse energies are kept the same for all pulses used in the comparison. It is found that temperature distributions corresponding to pulses used in the simulations are different and temperature decay in cooling cycle (after ending of the laser pulse) is clearly evident for step input pulses; however, this is not clearly identified for exponential pulses.     

Theory of Thermal Fatigue Caused by RF Pulsed Heating

The normal conducting electron-positron Linear Collider projects imply that accelerating structures and other RF components will undergo an action of extremely high RF fields. Except for the RF breakdown threat, there is an effect of the copper surface being damage due to multi-pulse mechanical stress caused by Ohmic losses in the skin layer. In this paper we would like to introduce a new “grain” model of the processes responsible for the fatigue of the metal surface. This model is based on the quasi-elastic interaction between neighboring grains in the metal due to the thermal expansion of the skin layer. This mechanism of fatigue is compared with another, where stresses are generated by the temperature gradient towards the bulk of the material. With the proposed formalism one can estimate the total number of the RF pulses required to fracture the surface depending on the temperature rise, pulse length and steady state temperature. The parameters necessary to finalize the proposed approach were found through the comparison of experimental data obtained at 11.424 GHz.     

Effect of laser surface hardening on the microstructure, hardness and residual stresses of austempered ductile iron grades

A study of the laser surface hardening process of two austempered ductile iron grades, with different austempering treatments has been carried out. Hardening was performed with an infrared continuous wave Nd:YAG laser in cylindrical specimens. The microstructure of the laser hardened samples was investigated using an optical microscope, microhardness profiles were measured and surface and radial residual stresses were studied by an X-ray diffractometer. Similar results were achieved for both materials. A coarse martensite with retained austenite structure was found in the treated area, resulting in a wear resistant effective layer of 0.6 mm to 1 mm with a microhardness between 650 HV and 800 HV. Compressive residual stresses have been found at the hardened area being in agreement with the microhardness and microstructural variations observed. The achieved results point out that the laser surface hardening is a suitable method for improving the mechanical properties of austempered ductile irons.     

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)     

纳秒激光脉冲在空气中聚焦的临界自由电子密度问题

实验研究了高功率纳秒量级激光脉冲在空气中聚焦时的能量透过率随输入激光脉冲能量变化的规律,发现在纳秒激光脉冲聚焦半径相同的情况下,激光脉冲的能量透过率随入射激光脉冲能量的变化可分为三种情况：当入射激光脉冲能量较低时,激光脉冲能量全部通过;当入射激光脉冲能量增大后,激光脉冲的能量透过率由近100%迅速减小;当入射激光脉冲的能量进一步增加时,激光脉冲的能量透过率继续缓慢变小.用临界自由电子密度以及所对应的临界时间点对上述实验现象进行了理论分析得到了如下结论：当自由电子密度未达到临界自由电子密度时,多光子电离过程起主要作用,而当自由电子密度超过临界自由电子密度后,逆韧致吸收过程起主要作用,临界时间点是入射激光脉冲与空气作用过程中自由电子密度达到临界自由电子密度的时刻.入射激光脉冲能量决定了临界时间点在脉冲作用时间上的位置,临界时间点的位置决定了激光脉冲的能量透过率.可以通过测量激光脉冲的能量透过率来计算出临界自由电子密度,从而确定出激光脉冲在空气中聚焦时的能量透过特性. 关键词： 临界自由电子密度 临界时间点 多光子电离 逆韧致吸收     

Thermal stresses in box-type laser packages

The effect of Au coating on thermally induced stresses in box-type semiconductor laser packages was investigated by a finite-element method (FEM). The simulated results showed that Invar–Invar joints with Au coating have maximum stresses two times higher than joints without Au coating. This is due to the different coefficients of the thermal expansion (CTE) between dissimilar materials of Invar and Au, resulting in higher stresses. Maximum stresses were also found to be increased as the Au thickness increased. This effect is attributed to the increase in the thermal gradient in the welded region provided by the increased thermal conduction of the thicker Au coating layer. These results suggest that both the difference in CTE between dissimilar materials and higher thermal conduction of Au coating layer have an important impact on thermally induced stresses for optoelectronic packages having laser-welded Au coated materials.     

利用半导体激光放大器放大半导体激光超短脉冲及选取单模

本文对半行波半导体激光放大器的超短光脉冲响应进行了研究,结果表明:脉宽为27ps,重复频率为1GHz的超短光脉冲通过放大器后,脉冲没有展宽,并且由于半行波半导体激光放大器对波长的选择放大性,在输入为多模的情况下,获得了单模超短脉冲输出。     

Thermal deformation of a solid surface under laser irradiation

A calculation is presented for the deformation of a solid surface when subjected to laser irradiation. The elastic response of the target is given in terms of the stress and displacement field due to thermal expansion, thus allowing an explicit evaluation of the shape of the deformed target surface. Quantitative results are obtained for the height and shape of the induced bump as a function of the laser pulse and target properties. The thermal load threshold is specified for the onset of plastic yield.     

激光钝化对熔石英修复后损伤性能影响的实验研究

采用10.6 μupm 的 CO₂激光, 对单次激光脉冲辐照修复熔石英存在的烧蚀采用大光斑钝化去除. 经过辐照修复的区域置于前表面测试初始损伤阈值, 结果表明调制造成的损伤得到了一定程度的抑制; 辐照区域置于后表面修复后 熔石英的初始损伤阈值超过了基底的初始损伤阈值. 实验观察到了应力分布外扩, 同时明显减弱. 对损伤增长的测试说明, 经过激光熔融辐照后的损伤点, 当应力释放以后, 损伤扩展初期表现出指数增长趋势, 后期随着辐照次数的增加, 损伤增长不再明显, 并且趋于恒定值.     

脉冲LDA侧面泵浦大能量Nd:YAG激光器时变热效应分析

建立了激光二极管阵列(LDA)侧面泵浦棒状Nd:YAG增益介质时变热效应理论计算模型。采用有限元Ansys软件模拟分析了脉冲LDA侧面泵浦大能量固体激光器的时变热效应特性。研究结果表明,所研究的脉冲LDA侧面泵浦大能量Nd:YAG激光器热效应具有时变特性,介质横截面内中心点处的稳态温度场分布随时间呈锯齿形周期变化,锯齿形变化频率为LDA泵浦频率,脉冲LDA泵浦参数对介质稳态温度场分布有较大影响。分析和计算了介质内热梯度、应力双折射以及激光晶体端面效应等导致的晶体热透镜焦距。计算表明,介质的热焦距主要来源于介质内温度梯度引起的热透镜焦距。     

Simulation of the stresses produced in large-diameter silicon wafers during thermal annealing

The three-dimensional problem of stress and strain fields in dislocation-free silicon wafers 200 and 300 mm in diameter placed horizontally on three symmetrical supports and subjected to is gravitational forces and thermal stresses formulated and solved in the isotropic approximation. Under the action of gravitational forces, a wafer is shown to have the lowest total stress when the supports are positioned at a distance of 0.6–0.7R from the center of the wafer. The shear-stress fields are calculated for all possible slip systems. The elastic-stress field in a 300-mm wafer is found to be induced mainly by gravitational forces even at a radial temperature gradient of 10 K. At this temperature gradient, the contribution from thermal stresses in a 200-mm wafer is comparable to the contribution from gravitational forces. At radial temperature gradients lower than 5 K, the contribution from thermal stresses can also be neglected for a 200-mm wafer. The maximum shear stresses calculated indicate that one should not neglect possible dislocation generation in the zone of contact between a wafer and the supports during high-temperature annealing of 200-mm and, especially, 300-mm wafers.