Numerical simulation of transient temperature field during laser keyhole welding of 304 stainless steel sheet

A three-dimensional transient numerical model was developed to study the temperature field and molten pool shape during continuous laser keyhole welding. The volume-of-fluid (VOF) method was employed to track free surfaces. Melting and evaporation enthalpy, recoil pressure, surface tension, and energy loss due to evaporating materials were considered in this model. The enthalpy-porosity technique was employed to account for the latent heat during melting and solidification. Temperature fields and weld pool shape were calculated using FLUENT software. The calculated weld dimensions agreed reasonable well with the experimental results. The effectiveness of the developed computational procedure had been confirmed.     

强脉冲离子束辐照金属材料表面热力学效应计算

简要讨论了强脉冲离子束与金属靶材料相互作用的理论模型,以此为基础应用数值计算的方法模拟计算了离子能量为1.0 MeV和300 keV,束流密度分别为10 A/cm2、50 A/cm2、100 A/cm2的质子束与金属铝靶材料相互作用时的热力学效应,给出了铝在辐照后内部的温度分布、温度梯度分布,以及温度变化速率（加热速率与冷却速率）、热激波的产生与传播过程及与应力等模拟计算结果。     

Simulation of temperature distribution in skin under laser irradiation with different wavelengths

With the help of numerical calculations of the finite element method, we simulate the distributions of the temperature in skin after laser irradiation with four wavelengths, 532 nm, 694 nm, 755 nm, and 800 nm, respectively. Combined metabolism of the organism with the traditional Pennes equation, the induced thermal effect in tissue is expressed. The simulated results show that the temperature in the irradiation center decreases as the increase of the wavelength for the same irradiation duration, and the relationships of the distribution of temperature and the wavelength along the radial and longitudinal directions are different.     

Synthesis of nanoporous structures in metallic materials under laser action

We defined conditions of the laser-aided formation of nanoporous structures with nanopores ranging in size from 40 to 50 nm using laser pulses of 10.6 μm wavelength at a pulse-repetition rate of up to (4-5)×10³ Hz for a model metallic material (a two-component alloy “brass of 62%”). It has been established that the exposure to a uniform laser light at depths of up to 25-30 μm results in the formation of nanopores with a relatively uniform distribution across the surface. The resulting pattern contains both solitary pores and ramified porous channels. The nanopores are uniformly distributed within a subgrain, being fairly stable in size and shape. The nanopore size and shape feature larger non-uniformity on the subgrain boundary. The resulting metallic structures show promise for use as catalysts and ultrafiltration membranes.     

Numerical calculation of the use of a concave–convex lens for shaping axisymmetric laser beams

Based on transformation matrices in meridian plane and sagittal plane, a geometrical lens design method is presented with the aim of obtaining an improved spot. A concave–convex convergent lens (CCCL) is employed as a shaping lens to focus a group of oblique laser beams from a CO₂ laser with axially symmetrical fold combination (ASFC) resonator. The cases including interference, noninterference, different wavelength, and misalignment have been analyzed. Through theoretical analysis and numerical calculation, a smaller beam radius in a specific reference plane and a smaller far-field divergence angle can be obtained.     

非稳态下翅片管气化器霜层内温度分布数值研究

为了分析非稳态下空温式翅片管气化器霜层内温度分布,采用数值模拟技术对多组影响因素进行数值计算。通过指数模型和多元线性回归模型,拟合出非稳态下霜层内温度分布关系式。结果表明该拟合关系式总体均方差小于10%,其精度在工程计算要求范围内,可用于工程计算。     

?500 mm阵列透镜波前采样器的研制

设计加工了一个由484块单透镜组成的大口径阵列透镜,并设计了一套光束波前采样及测量实验系统。在实验过程中,合理匹配了阵列透镜波前采样器、CCD、成像屏和图像采集处理各系统参数,并讨论了该系统的动态范围和采样精度。利用平行光标定了该系统,且测量了大口径波前的像差,测量值与理论值相吻合。实验证明了阵列透镜波前采样器用于波前采样的技术可行性。     

Effects of temperature on laser diode ignition

In this paper, the effects of temperature on laser diode ignition and the resulting consequences were discussed in detail through theoretical analysis, experiments and numerical calculations. The results indicated that the output power of laser diode decreases and the wavelength of laser redshifts with elevated working temperature under a certain condition. The threshold conditions of ignition for powders are easily satisfied with increase in ambient temperature. While the temperature reaches a high enough level, ignition can occur and also the self-combustion or thermal induced explosion can do, even if laser power is very low. Therefore, it is of great importance to carefully control the working temperature of laser diode and the ambient temperature of powder system, and in the meanwhile, to install necessary insurance apparatus in order to ensure the normal and safe operation of the ignition system.     

Numerical study of picosecond soft x-ray enhancement from porous targets irradiated by double laser pulses

Simulations show enhanced free-free and free-bound x-ray emission from laser-produced plasmas for both porous and solid targets irradiated by picosecond laser pulses under different prepulse conditions. The porous targets are modeled as a thick solid substrate over-coated with a thin porous layer. Using porous targets and prepulses shows that x-ray yields can be enhanced significantly over single pulses on solid density targets. The optimum conditions of prepulse and porous layer density are investigated by simulations with a fluid and atomic physics code.     

Numerical modeling and experimental investigation of TiC formation on titanium surface pre-coated by graphite under pulsed laser irradiation

A model for carbonization of titanium surface by pulsed Nd:YAG laser was developed. The Ti substrate was covered with a relatively thick graphite layer prior to be processed under the laser beam. The experiments were performed at 15 J pulse energy with various pulse durations and overlapping factor to validate the results obtained from the numerical calculations. The model results such as temperature gradient, surface temperature, and the cooling rate were correlated with the micro-hardness of the alloyed layer. Higher pulse durations and overlapping factors which lead to the heat input increasing will result in significant rising in the micro-hardness values. The hardness values of the processed layer partially containing TiC, increased up to 10 times of the Ti substrate.     

The temperature profile in the molten layer of a semi-infinite target induced by irradiation using a pulsed laser

The two-dimensional Laplace integral transform technique has been used to compute the temperature profile of the molten layer on the surface of a semi-infinite target when irradiated by a pulsed laser. Mathematical expressions for the temperature distribution in the molten layer thickness and the solid part of the target, taking cooling and the temperature-dependent absorption coefficient of the irradiated surface into account, were obtained. As an illustrative example computations were carried out on a semi-infinite aluminum (Al) target.     

Improving wear resistance of pure copper by laser surface modification

With the aim of improving surface strength on copper with the electrical conductivity of the integral bulk retained, laser surface modification on copper was carried out using powder preplacement. The microstructures, hardness, wear resistance and electrical conductivity of the sample was investigated. It was shown that the modified layer of sample had crack-free, fine and homogeneous microstructures. Compared with pure copper, the average hardness of the sample was enhanced by a factor of six (about HV_0.1650), and the wear mass loss was reduced by 4/5, but the electrical conductivity of the integral bulk was only slightly decreased. The improvement of hardness and wear resistance could be attributed to the precipitation hardening of boride and carbide, grain refinement and solid solution strengthening in the layer.     

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.     

高功率环形激光二极管阵列重复脉冲抽运激光器中棒的瞬态温度分布

 采用光线追迹方法和有限元方法，对高功率环形激光二极管阵列（LDA）重复脉冲抽运Nd:YAG激光器中，棒的瞬态温度分布进行了详细的计算模拟，分析比较了不同抽运阶段、抽运频率及占空比的情况下棒内的瞬态温度分布。结果表明：高功率环形LDA重复脉冲抽运时，棒中心处温度随时间变化成锯齿形分布，棒内温度随抽运频率和占空比的增大而增大，最后温度随时间成周期性变化。     

A study of electron beams extracted from a plasma produced by laser interaction with a solid target

In this work, wave formation in laser-produced plasma is investigated by an analysis of time-of-flight signal of the electron pulse. Electrons are extracted from a non-equilibrium plasma, generated by pulsed laser ablation on a solid Ge target. The process is represented by ion-acoustic waves, which are generated from an external perturbation, given by the positive bias voltage of a Faraday cup. The characteristics of the waves depend substantially on the geometry of the plasma expansion chamber and on laser fluence, but are independent on bias potential. A KrF excimer UV laser was employed for plasma generation. Measurements were performed at two different laser fluences, 4 and 7 J/cm². The plasma created propagates with a mean velocity of about 1.1?×?10⁴ m/s. A movable Faraday cup was employed in order to collect electrons at different bias voltage values.     

Measurement of the temperature distribution on the surface of the laser heated specimen in a diamond anvil cell system by the tandem imaging acousto-optical filter

We demonstrate that combining the laser heating (LH) system in a diamond anvil cell (DAC) with a tandem acousto-optical tunable filter (TAOTF) allows measurement of the temperature distribution (TD) under infrared (IR, 1064?nm) LH of a specimen under high pressure in a DAC. The main component of the system is a TAOTF synchronized with a video camera. The system allows TD mapping within a temperature range of approximately 1000–2000?K on the surface of a laser-heated object (Fe plates) at high pressure in a DAC with a spatial resolution 2?µm by fitting the actual signal to Planck’s equation at each point.     

激光辐照下旋转柱壳温度场的数值模拟

 采用有限元方法数值模拟在连续激光辐照下旋转柱壳温度场的变化和分布情况,并分析了热性能参数对温度场造成的影响,同时还比较分析了不同旋转频率对柱壳温度场分布的影响。结果表明,激光作用下旋转柱壳的温升大大低于静止柱壳的温升, 外表面温度呈现出与旋转频率相符的周期性上升过程,而内表面温升由于热传导的原因在较小频率下才表现出这种周期性,当频率增大到一定值时,内表面温升不出现周期性的台阶而是曲线上升。     

Studies on surface pitting during laser assisted removal of translucent ellipsoidal particulates from metallic substrates

We report on the manifestation of field enhanced surface absorption during laser assisted removal of translucent particulates of ellipsoidal geometry from a metallic substrate surface. The surface pitting caused due to this effect has been experimentally probed as a function of the ratio of minor to major axis of the ellipsoid and the behavioral trend has been theoretically interpreted by invoking the principle of geometrical optics. The study also includes the effect of fluence and wavelength of the incident coherent radiation on the surface pitting. Probing of the surface topography has helped gain insight into the formation of multiple pits by a single particulate following its removal post laser exposure.     

Generation of rotating intensity blades by superposing optical vortex beams

A kind of optical beam with controllable rotating intensity blades is generated by coaxially superposing two optical vortex beams with frequency difference (Δω), different topological charges (m₁ and m₂) and equal amplitude. It is shown theoretically that the number of the blades is determined by the subtraction of topological charges (m₁ − m₂) and the angular velocity of the rotating pattern is equal to Δω / (m₁ − m₂). In our experiment, the rotating beams were generated by two optical vortex beams with opposite topological charges, where the frequency difference is acquired by a rotating plate glass. The results are quite in accordance with the simulations.     

Porous microstructures induced by picosecond laser scanning irradiation on stainless steel surface

A study of porous surfaces having micropores significantly smaller than laser spot on the stainless steel 304L sample surface induced by a picosecond regenerative amplified laser, operating at 1064 nm, is presented. Variations in the interaction regime of picosecond laser pulses with stainless steel surfaces at peak irradiation fluences(F_pk=0.378–4.496 J/cm²) with scanning speeds(v=125–1000 μm/s) and scan line spacings(s=0–50 μm) have been observed and thoroughly investigated. It is observed that interactions within these parameters allows for the generation of well-defined structured surfaces. To investigate the formation mechanism of sub-focus micropores, the influence of key processing parameters has been analyzed using a pre-designed laser pulse scanning layout. Appearances of sub-focus ripples and micropores with the variation of laser peak fluence, scanning speed and scan line spacing have been observed. The dependencies of surface structures on these interaction parameters have been preliminarily verified. With the help of the experimental results obtained, interaction parameters for fabrication of large area homogeneous porous structures with the feature sizes in the range of 3–15 μm are determined.