Theoretical and experimental investigation into laser melting of steel samples

Among the available laser applications, laser melting has turned out to be a powerful technique for the production of mechanically improved surfaces. To enhance the understanding of the laser melting process investigations into modeling of the heating mechanism initiating the laser melting are necessary. In the present study, a mathematical modeling of the laser melting process is introduced and power require ments for the laser melting are predicted as functions of workpiece properties and velocity. Maximum melt width is predicted analytically and compared with the experimental results. In this regard, an experiment is conducted to melt the mild steel samples with a cw CO₂ laser at different power settings and workpiece velocities. It is found that the melt variables predicted from theory are in agreement with the experimental results.     

Mathematical modeling of laser induced heating and melting in solids

An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.     

Principles of alignment of multibeam lasers for thermonuclear purposes

The concrete object of the investigation is the alignment of the high-power 12-channel Del'fin neodymium-glass laser facility. The objects of the alignment are the optical channel of the laser installation, the system for pointing and focusing the laser radiation on the target, the system for positioning the target in the focal volume, and the system for monitoring the optical quality of the elements of the laser installation and laser beams and also the conditions of the target irradiation in the vacuum chamber. The list of requirements that must be satisfied by the alignment beams, the possible sources of alignment beams, the makeup of the apparatus, and the methods of aligning the laser facility are considered. The principles and the actual systems of automatic alignment of the optical elements are described, as are the operating models of the automatized units. The problem of simulating the working laser beams by the alignment beams and the possibility of automatizing the control of the spatial and angular characteristics of the laser radiation are discussed. The system for controlling the alignment processes by means of a laser is considered, as is also a scheme for incorporating the automatized alignment subsystem in the overall system for the automation of the Del'fln facility.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 103, pp. 52–83, 1978.     

Computer simulation of pulsed laser processing of amorphous Si

The paper introduces a 3D computer simulation model of the melting and recrystallization process of amorphous Si induced by pulsed laser irradiation. The model takes into account the temperature dependence of thermal and optical properties of crystalline, amorphous and liquid Si. The melting process is described by introducing for each volume element of melt pool the characteristic times of beginning of melting, end of melting and nucleation of a stable nucleus. The solution of heat equations of liquid and solid phases also provides one with the nucleation rates and temperatures. These data enable one to discriminate whether amorphous or crystalline phases are really allowed to be formed. Two examples of computer simulation are carried out to show the outputs of the model. Received: 7 February 2000 / Accepted: 28 March 2000 / Published online: 9 August 2000     

高功率激光表面大气击穿阈值的波长关系

通过对大气击穿的物理机制、低空大气中的自由电子及其寿命和电离机制进行讨论,给出了高功率激光大气击穿较为明晰的物理图像。并通过理论分析,给出了激光大气击穿阈值的波长关系,对给定波长激光的大气击穿阈值可以作出迅速的估值,是一种较为简捷的方法     

超短脉冲激光诱导单晶铜熔化的物理机制

 采用校正的分子动力学方法研究了超短脉冲熔化单晶铜的动力学微观机制,建模时将熔化潜热的消耗及自由电子的热传导均考虑在内,使熔化过程的模拟更加真实。皮秒激光熔化单晶铜是一种过热熔化,可归因于液相在固相中的均匀形核。熔沿传播的速度高达5.8 nm/ps ,高于铜中声速。熔化发生在热约束区域内部,导致温度分布不太复杂,且卸载波对应力波的影响与应力约束区域相比较弱。     

Laser heating of a moving slab: Influence of laser intensity parameter and scanning speed on temperature field and melt size

Laser melting of a moving slab is considered, and the temperature field and the phase-change in the heated region are simulated in-line with experimental conditions. The influence of laser power intensity parameter (β) and laser scanning speed on temperature field and melt depth is examined. An experiment is carried out to compare the melt layer thickness with the predictions. It is found that increase in laser power intensity parameter enhances melt size along the x-axis, despite the fact that peak temperature in the melt-pool reduces. This is more pronounced at a low laser scanning speed (0.05 m/s).     

Influence of initial micro-porosity of target on material ejection under nanosecond laser pulses

Heating and explosive destruction of the near surface volume of metal with initial micro-porosity under nanosecond laser pulse is simulated. It is shown that presence of initial micro-porosity may influence and modify the ejection mechanisms from melt and from solid phase. Thermal and gas dynamic processes in a gas bubble growing in laser irradiated melt are analysed. Experiments show rather different surface morphology defined by micro-explosions and melt ejection for different metals. Possible explosion of micro-pores and solid particles release under nanosecond laser irradiation is analysed for metals with high melting point. It is shown that the stress attains the values that are close to the damage threshold.     

Sensitivity of a neodymium-glass laser amplifier

The results of an experimental study of the sensitivity of a neodymium-glass laser amplifier are presented. The sensitivity of a traveling wave laser amplifier is shown to be close to the theoretical limit, being 1.7 · 10^–6 W for a pass band of 6 · 10¹² cps.     

Laser Accelerated, High Quality Ion Beams

Intense beams of protons and heavy ions have been observed in ultra-intense laser-solid interaction experiments. Thereby, a considerable fraction of the laser energy is transferred to collimated beams of energetic ions (e.g. up to 50 MeV protons; 100 MeV fluorine), which makes these beams highly interesting for various applications. Experimental results indicate very short pulse duration and an excellent beam quality, leading to beam intensities in the TW range. To characterize the beam quality and its dependence on laser parameters and target conditions, we performed experiments at several high-power laser systems. We found a strong dependence on the target rear surface conditions allowing to tailor the ion beam by an appropriate target design. We also succeeded in the generation of heavy ion beams by suppressing the proton amount at the target surface. We will present recent experimental results demonstrating a transverse beam emittance far superior to accelerator-based ion beams. Finally, we will discuss the prospect of laser-accelerated ion beams as new diagnostics in laser-solid interaction experiements. Special fields of interest are proton radiography, electric field imaging, and relativistic electron transport inside the target.     

Tapered laser cooling of stored coasting ion beams

We propose a scheme for tapered laser cooling of coasting ion beams in storage rings. Tapered cooling has recently been shown to be crucial for attaining crystalline ion beams. The scheme proposed here, based on a relative displacement of a co- and a counterpropagating Gaussian laser beam, gives a radial variation in the equilibrium velocities to which particles are cooled. The variation is approximately linear in a relatively large range transverse to the laser beams. Expressions for the spatially dependent equilibrium velocities and the range of the tapered cooling forces are derived. We discuss the dependence on laser beam parameters as well as the limitations of this cooling scheme.     

Thermal melting and ablation dynamics on a femtosecond laser-heated highly-oriented pyrolytic graphite surface

Time-resolved optical reflection microscopy studies demonstrate spatiotemporal dynamics of melting and ablation of graphite surface molten by single IR femtosecond laser pulses, which are revealed by monitoring picosecond oscillations of the probe reflectivity modulated by transient acoustic reverberations in the surface melt. Temporal periods and amplitudes of the reverberations are affected through transient variations of melt thickness and acoustic impedance by melting, thermal expansion, spallation and fragmentation processes, thus enabling quantitative evaluation of their contributions and basic parameters.     

Spatial amplification of a laser wave and the transverse spread of electrons in an undulator with a noncollinear configuration

The spatial amplification of a wave in a magnetostatic undulator with noncollinear electron and laser beams is studied in the framework of the dispersion relation for single-frequency and collective regimes. The dependence of the gain on the electron beam width is estimated with regard to the spatial boundedness of the beams. The laser power threshold at which the selection with respect to the transverse velocity is possible is obtained for a free-electron laser without inversion.     

激光照射下铝靶表面汽化压力的测量

 本文分析了会聚自由振荡钕玻璃脉冲激光束辐照平面铝靶的汽化特性。在激光功率密度约10 MW/cm²下，由拍摄激光与靶相互作用产生射流发光的时间积分照片测得靶面的汽化压力约3.6 GPa。把时间积分照片与高速分幅照片比较，证明在激光与靶相互作用一段时间后确实存在着马赫盘。     

New dye solutions for mode-locking infrared lasers

The recovery time of saturable absorbers is shortened externally by electronically active solvents, e.g., iodoethane, methyl sulfoxide, or ethyl sulfide. With a highly stable dye in such a solvent, very reproducible mode-locking of the neodymium-glass laser is achieved. The recovery time of the dye and hence the degree of mode-locking can be varied readily through the choice of solvent or solvent mixture.     

Phase Transitions of Colloidal Suspensions Under External Potential

We review phase transitions in a system of charge-stabilized colloids subject to laser beams with wavevectors tuned to the ordering wavevector of the liquid. Density-functional theory shows that the first-order freezing transition becomes continuous for large strengths of the modulation potential, if the wavevectors satisfy certain symmetry requirements. Computer simulations indicate that, apart from laser induced freezing (LIF), there is laser induced melting (LIM) as well. Recent experiments have shown the existence of both transitions. Theoretical arguments, based on the dislocation-mediated melting scenario in two dimensions; confirm the LIM phenomenon.     

Temperature-dependent absorptance of painted aluminum, stainless steel 304, and titanium for 1.07 μm and 10.6 μm laser beams

We measured the temperature-dependent absorptance of metals (Al, Ti, SS304) for continuous beams from 1.07 μm fiber laser and 10.6 μm CO₂ laser using power sensors and infrared (IR) pyrometers. The absorptance measurements were repeated for metals with three different paint coatings. For measurements at elevated temperatures up to the melting point, integrating sphere is not practical since high temperature radiation from a heated target disturbs weak output from the sphere considerably. Our results provide how each metal, whether coated or uncoated, absorbs the infrared beams as temperature is elevated to a melting point. A polynomial approximation to the measured absorptance of each target is provided for modeling of the laser-metal interaction at elevated temperatures.     

Process monitoring of powder pre-paste laser surface alloying

Instead of the continuous powder delivery method using a powder feeder for thick layer laser cladding, pre-pasting of the alloying powder on the substrate is a widely used method to supply the coating powders into the melt pool for LSA. A method to monitor the process of laser surface alloying based on the infrared emission from the melt pool using infrared photodiodes was developed. The technique is solely aimed at the process of laser surface alloying using pre-paste metal powder on the substrate surface prior to laser melting. This monitoring technique is able to distinguish the existence or the absence of the pre-paste powder and the consistency of the laser surface alloying process. The technique is of low cost and is simple to implement into the process.     

多模激光束通过时间变化相位片变换的数值分析

根据衍射积分公式,对连续多模激光束通过随时间变化相位片的变换特性进行了数值模拟,详细地讨论了相位片单元尺寸和相位变化频率对多模光束光滑和能量分布的影响。数值模拟结果很好地解释了实验结果。     

Characterization of polycrystalline Ge thin films fabricated by short-pulse XeF excimer laser crystallization

XeF excimer laser-induced melting and recrystallization dynamics of amorphous germanium are investigated using time-resolved optical reflection and transmission measurements with a nanosecond time resolution, field-emission scanning electron microscopy, and micro-Raman spectroscopy. It is found that the disc-shaped grain with a diameter of approximately 0.8 μm is located in the complete melting regime with a melt phase duration of approximately 141–200 ns. The significant change of transmissivity is a key phenomenon revealing the excessive excimer laser fluence during excimer laser crystallization by in-situ optical measurements. Differences between the melting and recrystallization phenomenon for Si and Ge thin films are also discussed.