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.     

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.     

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.     

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.     

Laser-induced thermal stresses on steel surface

In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.     

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 melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface (β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.     

Thermoelastic wave induced by pulsed laser heating

In this work, a generalized solution for the thermoelastic plane wave in a semi-infinite solid induced by pulsed laser heating is developed. The solution takes into account the non-Fourier effect in heat conduction and the coupling effect between temperature and strain rate, which play significant roles in ultrashort pulsed laser heating. Based on this solution, calculations are conducted to study stress waves induced by nano-, pico-, and femtosecond laser pulses. It is found that with the same maximum surface temperature increase, a shorter pulsed laser induces a much stronger stress wave. The non-Fourier effect causes a higher surface temperature increase, but a weaker stress wave. Also, for the first time, it is found that a second stress wave is formed and propagates with the same speed as the thermal wave. The surface displacement accompanying thermal expansion shows a substantial time delay to the femtosecond laser pulse. On the contrary, surface displacement and heating occur simultaneously in nano- and picosecond laser heating. In femtosecond laser heating, results show that the coupling effect strongly attenuates the stress wave and extends the duration of the stress wave. This may explain the minimal damage in ultrashort laser materials processing. Received: 23 May 2000 / Accepted: 26 May 2000 / Published online: 20 September 2000     

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.     

Laser control melting of alumina surfaces and thermal stress analysis

Laser gas assisted melting of alumina surface is carried out and temperature as well as stress fields developed in the irradiated region are predicted using the finite element method (FEM). An experiment is conducted resembling the simulation conditions. Optical and scanning electron microscope (SEM) are used to examine the morphological and the metallurgical changes in the laser treated region. The X-ray diffraction (XRD) technique is used to determine the residual stress developed in the irradiated region. It is found that the residual stress predicted agreed with the measurement result. High heating and cooling rates result in high von Mises stress levels in the surface region.     

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.     

High temperature thermal behaviour modeling of large-scale fused silica optics for laser facility

High temperature annealing is often used for the stress control of optical materials.However,weight and viscosity at high temperature may destroy the surface morphology,especially for the large-scale,thin and heavy optics used for large laser facilities.It is necessary to understand the thermal behaviour and design proper support systems for large-scale optics at high temperature.In this work,three support systems for fused silica optics are designed and simulated with the finite element method.After the analysis of the thermal behaviours of different support systems,some advantages and disadvantages can be revealed.The results show that the support with the optical surface vertical is optimal because both pollution and deformation of optics could be well controlled during annealing at high temperature.Annealing process of the optics irradiated by CO2 laser is also simulated.It can be concluded that high temperature annealing can effectively reduce the residual stress.However,the effects of annealing on surface morphology of the optics are complex.Annealing creep is closely related to the residual stress and strain distribution.In the region with large residual stress,the creep is too large and probably increases the deformation gradient which may affect the laser beam propagation.     

Irradiation effects of CO2 laser parameters on surface morphology of fused silica

To understand the surface morphology evolution of fused silica induced by 10.6-μm CO2 laser irradiation at different parameters,this paper reports that optical microscopy,profilometry,and hydrophilicity tests are utilized to characterize the surface structure and roughness of the laser irradiated area. The results show that three typical surface morphologies and two typical hydrophilicity test images are observed at different laser powers and pulse durations. The correlations between surface temperature and surface morphology as well as hydrophilicity behaviours are presented. The different hydrophilicity behaviours are related to surface structures of the laser-induced crater and thermal diffusion area. The thermal diffusion length monotonously increases with increasing laser power and pulse duration. The crater width is almost determined by the laser beam size. The crater depth is more sensitive to the laser power and pulse duration than the crater width.     

Laser repetitive pulse heating of tool surface

Laser heating of a cemented carbide tool is considered and the temperature field as well as phase changes in the heated region is modeled. Temperature rise, liquid layer thickness, and mushy size are predicted numerically. A control volume approach is introduced to solve the governing equations of heat transfer and phase change. Consecutive pulses with the duty cycle of 60% are accommodated in the simulations in line with the experimental conditions. An experiment is carried out to treat the cemented carbide tool surfaces using the CO₂ laser delivering consecutive pulses. The treated surfaces and their cross-sections are examined using the scanning electron microscope (SEM). It is found that the temperature gradient is high along the laser beam axis resulting in cracks at the irradiated surface. The rapid solidification of the surface causes compact structures with very fine grains in the surface region of the laser irradiated spot.     

Laser pulse heating: Cavity formation into steel, nickel and tantalum surfaces

The cavity formation during laser pulse heating of steel, nickel, and tantalum is examined and evaporation rate from the cavity surface is predicted. The mushy zones generated across the vapor–liquid and liquid–solid phases are modeled using the energy method. Temperature-dependent thermal properties are accommodated in the analysis and the laser pulse shape resembling the actual laser pulse is employed in the simulations. A numerical scheme using the control volume method is used to predict the cavity size, recession velocity of the vapor front, and temperature field in the laser irradiated region. It is found that cavity depth for steel is the largest, then follows nickel and tantalum. The recession velocity of the vapor front is high for steel due to the low evaporation temperature and latent heat of evaporation of steel.     

超快脉冲激光辐照金属薄膜热-力效应的模拟研究

基于双曲双温两步热传导和热电子崩力模型,考虑到超快脉冲激光辐照金属薄膜材料过程中的热-力耦合效应,得到了完全耦合的、非线性的超快热弹性模型.运用具有人工粘性和自适应步长的有限差分算法,以脉宽为100 fs的脉冲激光辐照200 nm厚金膜为例,对薄膜体内的电子-晶格温度及温度梯度、热应力和电子热流进行了数值模拟研究.结果表明：脉冲辐照早期为明显的非平衡加热阶段,同时形成较大的热电子崩力;电子热流出现双峰现象;超快加热引起的热应力是导致薄膜力学损伤的主要原因.     

Laser short pulse heating of metal nano-wires

Non-equilibrium energy transfer takes place in a solid substrate during a short-pulse laser irradiation and temperature field can be obtained analytically in the irradiated region. In the present study, laser short-pulse heating of metal nano-wire is considered and the analytical solution for two-dimensional axisymmetric nano-wire is presented. Since the absorption of the incident beam takes place in the skin of the irradiated surface, a volumetric heat source resembling the absorption process is incorporated in the analysis. Three different nano-wire materials are introduced in the analysis for the comparison reason. These include silver, chromium, and copper. It is found that temperature decay is gradual on the surface vicinity and temporal variation of the surface temperature follows almost the laser pulse intensity profile at the irradiated center.     

Entropy generation rate during laser pulse heating: Effect of laser pulse parameters on entropy generation rate

Laser pulse heating of solid surface and entropy generation during the heating process are considered. Time exponentially decaying pulse is accommodated in the analysis and the laser pulse parameter (β₁/β₂) resulting in minimum entropy generation rate is computed. Analytical solutions for temperature rise are presented and volumetric entropy generation rate is formulated. Two laser pulses resulting in low volumetric entropy generation rate are examined in detail and volumetric entropy generation rate is associated with the laser pulse parameter (β₁/β₂). It is found that volumetric entropy generation rate attains high values in the early heating period due to large (1/T²). Moreover, the laser pulse with high-peak intensity results in lower volumetric entropy generation rate than that corresponding to the low-intensity laser pulse with the same energy content.     

10.6μm激光诱导扩散中热致破坏的抑制

在半导体激光诱导扩散实验中,用连续波CO2 10．6μm激光聚焦后照射基片表面。为实现局部区域的选择扩散,激光光斑半径仅数十微米。要使曝光区温度达到扩散实验要求,必须使曝光区功率密度很高。另一方面,Si、InP等半导体材料对10．6μm波长激光的吸收系数随温度的升高而增大,这导致实验时容易产生热致破坏,损伤基片。在分析热致破坏的产生机理后,提出了在聚焦激光束照射下,曝光区温度的数值计算方法。计算结果表明,在半导体基片初始温度为室温时,以恒定功率的激光束照射基片,曝光区温度不能稳定在扩散试验需要的温度范围。在此基础上,提出了预热基片及对曝光区温度进行实时控制等抑制热致破坏的方法,有效地克服了这一困难。这对于用激光微细加工制作出高性能的单片光电集成电路(OEICs)器件有重要意义。     

Modeling of femtosecond laser damage threshold on the two-layer metal films

The heating processes of the single-layer gold thin film and the two-layer film assembly of gold padded with other metal (silver, copper and nickel) irradiated by femtosecond laser pulse are studied by the two-temperature model. It is found that the substrate metal can change energy transport, which is corresponding to the temperature changing process, and the thermal equilibrium time. Compared with the single-layer gold film at the same laser fluence, the two-layer film structure can change the damage threshold of the gold surface. Our results indicate that we can maximize the damage threshold of the gold film surface by altering the thickness ratio of the gold layer and the substrate layer in the two-layer film assembly.