共查询到20条相似文献,搜索用时 0 毫秒
1.
An investigation into melt flow dynamics during repetitive pulsed laser drilling of transparent media 总被引:1,自引:0,他引:1
This paper presents an investigation into the dynamics of repetitive pulsed laser drilling of a visually transparent media using a CO2 laser source. This enabled the use of a high-speed imaging system for observing, in real time, the behaviour of the drilling process in the laser drilled cavity of 1.5 mm diameter holes of up to 18.5 mm in depth. The work revealed that the instantaneous drilling velocity within each laser pulse can vary considerably from the average drilling velocity as a result of the non-uniform temporal pulse shape and the oscillation of the melt ejection rate. During beam breakthrough, both upward and downward melt ejections were observed to occur inside the drilled hole for a short period of time, after which the material was ejected through the exit end of the holes. It has been shown in this work that the downward melt flow velocity increases with hole depth for a positively tapered hole (from 0.09 to 1.43 m/s) and decreases with hole depth for a negatively tapered hole geometry (from 0.4 to 0.1 m/s), as a result of the change in the assist gas velocity inside the drilled hole with respect to the hole taper geometry. The mechanisms of forming the positively and negatively tapered holes in the transparent media have been correlated with the hole geometry and melt flow velocity. The work has demonstrated a new method of studying the melt dynamics in laser drilling. 相似文献
2.
3.
Three quantitative methods, namely profilometry, high speed imaging and recoil momentum measurements using a ballistic pendulum, are used to determine the interplay of vaporization, melt displacement and melt ejection on nanosecond laser induced material removal. At low to moderate fluences (<7 J cm−2) material removal occurs via vaporization and melt displacement in aluminium. At high fluences (>7 J cm−2), material removal occurs predominantly via the explosive ejection of liquid droplets from the melt pool. 相似文献
4.
Results are presented on the dynamics of the laser drilling process for a single hole using a periodically pulsed nanosecond laser with a repetition rate up to 60 kHz. The intensity dependence of drilling velocity was determined by measuring the delay time between the beginning of exposure and the moment when the hole opens. The hole collapse phenomenon was observed in our experiments. The competition process between material ejection and the flow of the liquid phase into the ejected area is supposed to be responsible for the hole collapse. 相似文献
5.
The temporal pulse train modulation during laser percussion drilling was found to effect significant changes to the material ejection processes. In particular, distinct differences in the material ejection processes have been observed between a temporal pulse train shaping technique termed as sequential pulse delivery pattern control (SPDPC) and the normal delivery pattern (NDP), wherein the parameters of successive laser pulses were constant. Due to the reduced upward material removal fractions in SPDPC drilling, the spatter deposition area was reduced from approximately 6.7 to 2.7 mm2. In addition, the melt layer thicknesses at the hole bottom were significantly increased from 11–61 to 18–369 μm. Such changes were identified as being due to the low laser pulse intensities before beam breakthrough associated with the SPDPC method. It was observed that the use of the linearly increasing SPDPC method increased the downward material removal fractions, from 20% to 28% observed in NDP drilling, to 34%–39%. Such an increase in the downward material ejection mechanism in SPDPC drilling was identified as being primarily due to the pointed blind-hole profile generated before the onset of beam breakthrough. The work has shown that modulating the entire pulse train in laser percussion drilling could control the material ejection processes. Furthermore, the fundamental elements of the SPDPC technique are given in terms of the rate of energy deposition and total pulse train energy. 相似文献
6.
In this work, the two main factors that influence the repeatability of the laser percussion drilling process are identified. Experimental parametric analysis was carried out to correlate the laser parameters with the repeatability of a laser percussion drilling process. The experiment was conducted using a flash lamp pumped Nd:YAG laser to drill 2 mm thick mild steel sheets. The relationship between the percentage standard deviation (PSD) of entrance hole diameter, hole circularity and the operating parameters is established. Thirty-five holes were drilled and analysed for each set of identical laser parameters. The PSD of entrance hole diameter ranges between 1.47% and 4.78% for an operating window of 3.5–7 kW peak power, and 1–3 ms pulse width. The circularity of the entrance hole (defined as the ratio between the minimum and maximum diameters of the hole) ranges from 0.94 to 0.87, and is found to correlate with repeatability. The work shows that higher peak power, and shorter pulse width gives better hole geometry repeatability. The effect of melt ejection on hole geometry repeatability is also investigated. Melt ejection and spatter formation have been found to contribute to the poor repeatability of the process. 相似文献
7.
金属材料在冲击载荷下的动态响应在许多民用工程、航空航天等领域都有重要的应用背景.而金属材料在冲击载荷下的微喷形成过程,包括微射流、碎裂以及微层裂的物理过程的研究中尚存在许多空白.介绍了国内首次在神光Ⅲ原型激光装置上开展的金属材料微喷回收实验,实现了激光加载下低密度泡沫材料对微喷颗粒的回收,对回收样品进行了X光CT分析,通过图像重建,获得了回收微喷颗粒的三维图像,以及颗粒不同形态分布、颗粒尺寸、颗粒质量等定量结果. 相似文献
8.
Repeatability characteristics of laser percussion drilling of stainless-steel sheets 总被引:1,自引:0,他引:1
An analysis on the repeatability of a laser percussion drilling process is conducted using a flash lamp pumped Nd:YAG laser on 2 mm thick stainless-steel sheets. Laser drilling process is finding increasingly widespread application in the industry and has continually attracted new interests to the industry in recent years. However, the inherent problem of hole geometry repeatability associated with laser percussion drilling is likely to limit the extent of industrial applications of the process. The characteristic of melt ejection is found to be dependent on the parameter setting and is shown to have a significant influence on entrance hole geometry and hence repeatability. The relationship between the percentage standard deviation of entrance hole diameter and the operating parameters is established, and varies between 1.8% and 5.6% in the operating range under this study. 相似文献
9.
Yuan Qin Gang Dai Bin Wang Xiao Wu Ni Juan Bi Xi He Zhang 《Optics & Laser Technology》2011,43(3):563-569
With the aim of improving the efficiency of laser drilling, an upward drilling method is proposed. In the experiment, a long pulsed laser beam was arranged to propagate upwards, in the opposite direction to gravity, and was used to drill hole at the bottom of an aluminum slab. A semi-infinite axisymmetric model of this system was also established. The analytical solution for the hole shape was derived by assuming that material, once it melted, was removed from hole with the aid of gravity. The calculation results agreed well with the experimental results. For further verification of the effects of gravity, the removed molten material and the hole shape for the downward (along the gravity direction) and the upward drilling cases were compared experimentally. In addition, the relationships between gravity, the inertia force, the surface tension and the viscosity were discussed. The results show that more molten material is expelled with the assistance of the gravity, and the laser energy is used more efficiently to melt the aluminum slab in the upward drilling. 相似文献
10.
A threefold study combining profilometry, high speed imaging and recoil momentum measurements is used to deconvolve the relative contributions to material removal attributable to vaporisation, melt displacement and explosive melt ejection. The interplay of these three mechanisms is studied as a function of the number of laser pulses incident on an aluminium target and pulse repetition frequency. This study shows cumulative heating affects matter removed as both vapour and liquid melt, and highlights the influence of the vapour plume and ablation crater morphology on the proportions of material removed as melt displacement and melt ejection. 相似文献
11.
12.
13.
采用分子动力学方法对熔体初始温度热历史条件对液态金属Na凝固过程中微观结构的影响,进行了模拟研究,并采用双体分布函数g(r)曲线、键型指数法和原子团类型指数法对凝固过程中的微观结构进行了分析.结果表明:液态金属Na在不同熔体初始温度条件下以1×1011K/s冷速凝固时,均形成晶化结构,其中1661和1441键型或体心立方基本原子团(14 6 0 8)在凝固过程中对微观结构的转变起决定性作用.同时发现:熔体初始温度对凝固微结构有显著影响,而对液态和过冷态的微观结构影响并不明显,只有在晶化起始温度Tc附近才充分地展现出来.不同熔体初始温度对凝固结构的晶化程度有不同的影响,虽其影响程度是随着熔体初始温度的下降呈非线性变化关系的,但仍表明是可以通过改变熔体初始温度来加以控制的.原子团类型指数法(比键型指数法)更进一步表征了晶化体系中原子团的结构特征,将有利于对液态金属凝固过程中微观结构的转变机理进行更为深入的研究. 相似文献
14.
An investigation of long pulsed laser induced damage in sapphire 总被引:1,自引:0,他引:1
The formation of keyhole and transverse section of a laser-cut kerf with slight stripe undulations by a 1064 nm ms pulse laser on (0 0 0 1) sapphire was investigated. The morphologies of keyhole and transverse section surfaces were evaluated by SEM, and the composition of transverse section of laser-cut kerf was evaluated by EDS, XRD and XPS. The time scale for onset of vaporization and the keyhole depth with different laser pulse energies were calculated. The result suggests that the depth of keyhole is approximately directly proportional to laser pulse energy. On sapphire transverse section surface, the element ratio of Al to O deviates from the stoichiometry of sapphire, perhaps due to the oxygen removal from surface. 相似文献
15.
In this study, the vibration phenomenon during pulsed laser forming of thin metal plates was investigated numerically and experimentally. The metal plates were made of 304 stainless steel and were heated by a CO2 Gaussian laser beam with a long pulse duration of 0.1 s, which incited vibration due to the elastic–plastic deformation of the specimen. This uncoupled thermal–mechanical problem was solved using a three-dimensional finite element method and was subsequently satisfactorily verified with experiments. Using the superposition method with multiple laser pulses, the non-linear vibration phenomenon during pulsed laser forming has also been observed experimentally for thin plates. 相似文献
16.
采用量子 Sutton-Chen多体势, 对熔体初始温度热历史条件对液态金属Ni快速凝固过程中微观结构演变的影响进行了分子动力学模拟研究. 采用双体分布函数g(r)曲线、键型指数法、原子团类型指数法和三维可视化等分析方法对凝固过程中微观结构的演变进行了分析. 结果表明: 熔体初始温度对凝固微结构有显著影响, 但在液态和过冷态时的影响并不明显, 只有在结晶转变温度Tc附近才开始充分显现出来. 体系在1×1012 K/s的冷速下, 最终均形成以1421和1422键型或面心立方(12 0 0 0 12 0)与六角密集(12 0 0 0 6 6) 基本原子团为主的晶态结构. 末态时, 不同初始温度体系中的主要键型和团簇的数目有很大的变化范围, 且与熔体初始温度的高低呈非线性变化关系. 然而, 体系能量随初始温度呈线性变化关系, 初始温度越高, 末态能量越低, 其晶化程度越高. 通过三维可视化分析进一步发现, 在初始温度较高的体系中, 同类团簇结构的原子出现明显的分层聚集现象, 随着初始温度的下降, 这种分层现象将被弥散开去. 可视化分析将更有助于对凝固过程中微观结构演变进行更为深入的研究.
关键词:
液态金属Ni
熔体初始温度
微观结构
分子动力学模拟 相似文献
17.
B. S. Yilbas 《Optics & Laser Technology》1986,18(1)
Laser produced plasma plays an important role in the laser drilling of sheet metals as it can partially block and absorb the incident laser beam. A previous study of the transient properties of charged particles in the plasma plume has shown that, at low electron densities with high electron temperatures, laser drilling improves. This suggests that measurement of the absorption of the plasma plume is essential.The present study covers measurement of the absorption of a HeNe beam passing transversely through the plasma plume. The measurement was carried out using two fast response photodiodes and was repeated for sub-atmospheric pressures of air.The results obtained show that drilling is best at a pressure of 200 torr (2.7 x 104 Pa) and rapid expansion of the flares is favourable at 2 mm above the surface. Coupling of absorption and heating is also best at this pressure. 相似文献
18.
19.
Mechanisms of melt droplets and solid-particle ejection from a target surface by pulsed laser action
A. B. Brailovsky S. V. Gaponov V. I. Luchin 《Applied Physics A: Materials Science & Processing》1995,61(1):81-86
We suggest an explanation of the effect of melt droplets and solid particle ejection from a target surface under the impact of laser radiation with intensity 108–1010 W/cm2. We consider the capillary wave instabilities on the evaporating surface of melt, which lead to growth of large-scale surface structures and ejection of macroparticles. The instability increments and characteristic droplet sizes are determined. Conditions are found for droplet-free evaporation in terms of the dynamic pressure of evaporated matter. 相似文献
20.
Deep drilling of through holes in stainless steel (1.4301, sample thicknesses 5, 8 and 10 mm) has been performed with the superposed radiation of two pulsed Nd:YAG lasers with pulse duration of 0.5 ms superposed by 17 ns pulses. The drilling efficiency is improved by the spatially and temporally superposed radiation of the two lasers. The enhanced drilling speed and the larger reproducibility of the drilling time are explained by a modified formation of closures in the hole during percussion drilling which are recorded by high-speed photography. The metallographic hole analysis exhibits high-temperature oxidation marks. The development of these marks is described by a ray tracing of the incident beam within the hole and the resulting intensity distribution at the hole wall. 相似文献