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.     

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 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.     

外部流场对激光加热运动目标影响的数值模拟

 考虑激光与运动目标相互作用的基础上,利用有限元方法分析了亚声速条件下运动目标在激光辐照全过程的温度场和热应力场的分布与演化规律。结果表明：高速流场的存在,导致了明显的冷却效应;加热过程中目标材料出现了屈服,导致激光熄灭后结构内出现残余应力和变形;激光辐照区边缘产生很高的温度梯度和应力梯度,并且由于气流影响,受辐照区域前后两端应力分布不对称。     

Laser bending of AISI 304 steel sheets: Thermal stress analysis

Laser induced bending of steel sheet is carried out and thermal stress developed in the heated region is examined. Temperature and stress fields are predicted using the finite element model. The microstructural changes in the melted region are investigated through scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The residual stress developed at the surface vicinity of the laser treated region is measured using the X-ray diffraction technique, which is then compared with its counterpart predicted from the simulations. It is found that the residual stress at the surface vicinity is compressive and the prediction of the residual stress agrees well with that obtained from the X-ray diffraction technique. In addition, surface temperature predictions are in good agreement with the thermocouple data. The laser treated region is free from major cracks and large cavities.     

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 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 pulse heating of steel surface and flexural wave analysis

Laser gas-assisted material processing finds wide application in industry. The modelling of heating, elastic response of the substrate material, and the wave analysis gives insight into the laser workpiece interaction. In the present study, laser gas-assisted heating of steel is considered. The normal component of the thermal stress is taken as the source of load for the flexural wave generation in the material. The flexural wave generated is simulated and the wave characteristics are analyzed at four locations at the workpiece surface. The numerical scheme employing a control volume approach is introduced when solving the governing equations of flow and heat transfer while finite element and spectran element methods are used when solving the stress and wave equations. It is found that the normal component of the stress is tensile. The dispersion effect of the workpiece material, interference of the reflected beam, and partial overlapping of second mode of the travelling wave enable to identify a unique pattern in the travelling wave in the substrate.     

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.     

Laser short pulse heating: Influence of pulse intensity on temperature and stress fields

Laser short-pulse heating of gold surface is considered and the influence of laser pulse intensity on the temperature and stress fields is investigated. Laser step input pulses with different pulse lengths and the same energy content are employed in the simulations. The electron kinetic theory approach employing thermomechanical coupling is introduced to model the non-equilibrium energy transport in the electron and lattice sub-systems. Thermal stress development in the lattice sub-system and temperature rise in the lattice and electron sub-systems are computed. It is found that electron temperature rises rapidly while lattice site temperature rise is gradual in the early heating period, which is more pronounced for high intensity pulses. Thermal stress component in the axial direction is compressive and its magnitude is considerably less than the yielding limit of the substrate material.     

Repetitive laser pulse heating analysis: Pulse parameter variation effects on closed form solution

Laser conduction limited heating finds wide application in surface processing industry. Modelling of the heating process gives insight into the physical processes involved in conduction limited heating. Moreover, analytical solutions provide functional relation among the parameters that influence the heating process. In the present study, repetitive laser pulse heating of a solid substrate is considered. A closed form solution for the temperature rise including the cooling cycle is obtained using a Laplace transformation method. It is found that the results obtained from the closed form solution agree well with the numerical predictions. The maximum surface temperature rises rapidly once the cooling period between the consecutive pulses reduces.     

Surface Morphological Evolution of Thin Films Under Stress and Capillary Forces

Holes and hillocks can commonly be observed on the surface of thin films after thermal processing. For films deposited on a substrate with a different coefficient of thermal expansion, strains due to thermal expansion mismatch can produce very large stresses. While capillary forces tend to produce a thermal groove at a grain boundary (GB), compressive and tensile stresses can form, respectively, a ridge or a canal at the GB. These phenomena can strongly influence mobility of a GB. The formation of a canal enhances the potential for pinning the GB, whereas the formation of a ridge tends to repel the GB.After a short overview of the theory, analytical and numerical solutions for surface profiles of static and travelling GBs under stress are presented. The results of the computed profiles are compared to experimental surface morphologies in aluminum thin film.     

Exact solution for temerature field due to non-equilibrium heating of solid substrate

Non-equilibrium energy transfer between electron and lattice sub-systems due to short-pulse heating is formulated and the closed form solution for electron and lattice site temperatures is presented. The electron kinetic theory approach is incorporated to formulate non-equilibrium energy transfer in the electron and lattice sub-systems. The method of Lie point symmetries is used in the exact solution of governing energy equation. In the analysis, the volumetric heat source, representing the laser heating pulse, and surface heat source, corresponding to short thermal contact of the surface, are incorporated and the analytical solutions for each heating source are presented. Electron temperature distribution obtained from the closed form solution is compared with its counterpart predicted from the numerical simulation. It is found that the results obtained from the closed form agree well with electron temperature predictions obtained from numerical simulation.     

Effect of cathodic arc PVD parameters on roughness of TiN coating on steel substrate

Titanium nitride which is widely used as a hard coating material was coated on tool steel, by physical vapor deposition method. Surface roughness was investigated as a function of deposition rate, substrate bias and temperature, nitrogen flow rate and metal ion etching. The study showed that increase in surface roughness mainly depends on the condition of sample preparation, surface treatment, macro-droplets, pitting defects, rise in compressive stress at higher coating thickness, growth defects and to a lesser extent selection of surface under testing. It was observed that chromium ion etching significantly reduced the surface roughness compared to titanium ion etching.     

Investigation of the local structure variance of water molecules in laser-induced thermal desorption process

This paper presents the use of molecular dynamics simulation in the study of laser-induced thermal desorption (LITD) of water molecules adjacent to a laser-heated Au substrate. The local structure of the water molecules is investigated by considering the densities of the oxygen and hydrogen atoms, the average number of neighbors, n_NN, and the average number of H-bonds, n_HB. At an equilibrium temperature of 300 K, the simulation results show that three adsorption water layers are formed in the immediate vicinity of the Au surface, and that each four-fold hollow site on the uppermost Au(0 0 1) surface is occupied by a single water molecule. Following laser-induced heating of the Au substrate with a sub-picosecond laser pulse of 350 fs, the substrate temperature increases to 1000 K. This causes a gradual heating of the adjacent water film, which is accompanied by a decrease in the values of n_NN and n_HB. Hence, it can be concluded that an increase in the water film temperature destroys the hydrogen-bonding network throughout the water film. Although the maximum local temperature of the water film occurs in the region immediately adjacent to the Au substrate, it is determined that the attractive energy between the Au atoms and the water molecules in this region causes the water molecules to aggregate together to form three-dimensional water clusters. Furthermore, this energy prevents the hydrogen bonds in this region from breaking apart as violently as those within the phase explosion region. Finally, it is observed that the phase explosion phenomenon occurs in the region of the water film where the values of n_NN and n_HB are at a minimum.     

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.     

窗口区域烧蚀瞬态热响应耦合计算与温升规律研究

本文针对航天器窗口区域局部防热结构，进行了数值计算和研究，成功地解决了材料热解、烧蚀以及复杂形状和边界传热的耦合计算问题。本文还针对防热结构均匀加热与表面变化加热量两种不同的加热条件；计算分析了有关温升规律的差别及其物理原因，为防热设计提供参考。     

THE INFLUENCE OF COMPRESSIVE STRESS ON THE FORMATION OF CUBIC BORON NITRIDE THIN FILM

The elastic strain energy and Gibbs free energy of cubic BN (cBN) thin film in biaxial stress field are calculated. The results show that the stress in cBN thin films has an impact on the formation of cubic phase. It is concluded that the high compressive stress in the cBN thin films is not the cause of cBN formation. This conclusion is different from that predicted by compressive stress model; however, it could well account for the experimental results. At a given substrate temperature, there is a compressive stress threshold, below which cBN phase is thermodynamically stable and above which hexagonal BN(hBN) phase is thermodynamically stable. At room temperature the compressive stress threshold is calculated to be 9.5 GPa.     

Strength evaluation of brittle ceramics with surface defects subjected to thermal shock

The thermal shock strength of a ceramic material plate with a semi-elliptical surface crack is evaluated in this paper. The thermal stress distribution at the plate without crack is expressed by a polynomial form. The calculated thermal stresses are used to obtain the thermal stress intensity factors of semi-elliptical surface cracks using the geometric functions. Variations of thermal stress intensity factor with crack depth and time are obtained. For the strength evaluation of the material, crack growth analysis is conducted. In addition, the thermal shock resistance of the ceramic material plate is evaluated by the stress-based failure criterion and the toughness-based failure criterion. The critical temperature at which crack propagation starts is obtained. Applicability of the two failure criteria in thermal shock resistance evaluation of ceramic is identified and the importance of the semi-elliptical surface defects on the thermal shock resistance behaviour of ceramic materials is justified.     

毫秒脉冲激光致硅光电二极管电学损伤的有限元分析及实验研究

为了研究毫秒脉冲激光致硅基PIN光电二极管电学损伤,基于热传导及弹塑性力学理论,在光电二极管内部材料各向同性并且P-I-N三层结构之间满足温度连续和热流平衡条件下,建立毫秒脉冲激光辐照硅基PIN光电二极管二维轴对称模型,采用有限元方法模拟分析了1064 nm Nd:YAG毫秒量级脉冲激光辐照硅基PIN光电二极管的温度场与应力场分布,并实验测量了硅基PIN光电二极管实验前后的电学参数.结果表明,激光辐照硅基PIN光电二极管时,温升使材料表面熔融、烧蚀,并且在空间上存在温度梯度变化,即激光辐照产生的热与应力使光敏面及硅晶格晶键损伤,最终造成光电探测器的探测性能下降.研究结果可为毫秒脉冲激光辐照硅基PIN光电二极管电学损伤机理奠定基础.