Experimental and parametric studies of Nd:YAG laser drilling on austenitic stainless steel

To ensure overall quality of a precision large-scale component, a tool condition monitoring (TCM) technique for multi-step form milling is presented. The form milling of fir tree slots for a steam turbine rotor is an appropriate example that requires a fine surface finish and high dimensional accuracy. Therefore, we propose a novel TCM system based on a multi-sensor fusion strategy which utilises the combination of spindle motor current and acoustic emission (AE) as well as adaptive thresholding for multiple manufacturing steps (roughing, semi-finishing and finishing). To investigate the tool deterioration process, tool longevity tests using a test piece are carried out for each step. With the aid of qualitative inspection, it is found that AE signals provide comprehensive tool state information regarding tool flank wear, crack propagation and severe adhesive wear. In addition, by intentionally adding a bundle of chips to the surface, bursts of AE of large amplitudes occur in finishing, which provides the possibility of discovering anomalous events related to surface quality. By careful consideration of such characteristics, provisional alert levels are determined using a two-dimensional diagram with respect to both sensors. The strategy is verified throughout the actual manufacturing processes of the rotors. The proposed TCM system shows not only an excellent ability to prevent catastrophic tool failure and surface irregularities in form milling but also acceptable expendability for various groove specifications.     

Surface texturing by pulsed Nd:YAG laser

Introducing specific textures on a tribological surface can contribute to friction reduction in sliding contacts. In the present paper, a pulsed Nd:YAG laser emitting at 1064 nm, was used against 100Cr6 steel samples in order to produce well-defined surface micro-pores, which can act as lubricant reservoirs, micro-hydrodynamic bearings as well as traps for wear debris. Due to the high flexibility of the laser system, structural features such as shape, size, density and depth can be varied easily by changing the laser parameters. To optimize the parameters of the laser surface texturing process, an investigation was performed using different pulse numbers, various pulse energies and two different modes (single- and multi-mode). The microtextures were characterized with optical microscopy, scanning electron microscopy (SEM) and by topography techniques. The relationship between the laser processing parameters and qualitative and quantitative profile of the micro-pores was studied. Tribological testing of laser textured surfaces was performed in a low frequency–long displacement reciprocating sliding wear tester under boundary lubrication and results compared to un-textured case. Tribological comparison of textured, textured and lapped, and untextured surfaces shows only minimal influence of texturing for contact conditions investigated.     

Analysis of pulsed Nd:YAG laser welding of AISI 304 steel

Pulsed laser welding of AISI 304 stainless steel plate was simulated using commercial finite element software to determine the optimal welding conditions. Due to geometric symmetry, only one plate was modeled to reduce the simulation computation time. User subroutines were created to account for a moving three-dimensional heat source and to apply boundary conditions. The material properties such as conductivity, specific heat, and mass density were determined as functions of temperature. The latent heat was considered within the given temperature range. The three-dimensional heat source model for pulsed laser beam butt welding was designed by comparing the finite element analysis results and experimental data. This successful simulation of pulsed Nd:YAG laser welding for AISI 304 stainless steel will prove useful for determining optimal welding conditions.     

钢-铝异种合金脉冲激光焊焊缝表征分析

采用Nd:YAG激光焊对304不锈钢和5052铝合金进行异种金属焊接,分别以峰值功率、焊接速度、离焦量和脉冲频率等工艺参数设计24组工艺试验,并对比分析未熔合、熔合和焊穿3种焊缝表面表征。运用激光点位移传感器测量焊缝高度,探索焊缝高度随激光功率等工艺参数的变化趋势,得出钢-铝焊缝3种表面形貌的工艺参数区间。分析结果表明,焊缝的表面形貌是由激光单点能量、离焦量和脉冲频率等因素共同决定,焊缝高度与峰值功率、焊接速度、离焦量和脉冲频率等工艺参数有一定的变化规律。     

Numerical analysis of Nd:YAG pulsed laser polishing CVD self-standing diamond film

Chemical vapor deposited(CVD) diamond film has broad application foreground in high-tech fields.But polycrystalline CVD self-standing diamond thick film has rough surface and non-uniform thickness that adversely affect its extensive applications.Laser polishing is a useful method to smooth self-standing diamond film.At present,attentions have been focused on experimental research on laser polishing,but the revealing of theoretical model and the forecast of polishing process are vacant.The paper presents a finite element model to simulate and analyze the mechanism of laser polishing diamond based on laser thermal conduction theory.The experimental investigation is also carried out on Nd:YAG pulsed laser smoothing diamond thick film.The simulation results have good accordance with the results of experimental results.The temperature and thermal stress fields are investigated at different incidence angles and parameters of Nd:YAG pulsed laser.The pyramidal-like roughness of diamond thick film leads to the non-homogeneous temperature fields.The temperature at the peak of diamond film is much higher than that in the valley,which leads to the smoothing of diamond thick film.The effect of laser parameters on the surface roughness and thickness of graphite transition layer is also carried out.The results show that high power density laser makes the diamond surface rapid heating,evaporation and sublimation after its graphitization.It is also found that the good polish quality of diamond thick film can be obtained by a combination of large incident angle,moderate laser pulsed energy,large repetition rate and moderate laser pulse width.The results obtained here provide the theoretical basis for laser polishing diamond film with high efficiency and high quality.     

Scanning electron microscopy and transmission electron microscopy microstructural investigation of high-speed tool steel after Nd:YAG pulsed laser melting

This article presents the microstructure of a pulsed Nd:YAG laser‐melted high‐speed steel, namely HS6‐5‐2. The high chemical homogeneity and fine structure of the melted zone was attributed to high cooling rates due to the short duration of interaction with the Nd:YAG pulsed laser radiation and the relatively small volume of the melted material. The structure obtained in the surface layer after laser melting has a high level of hardness and shows improved wear resistance.     

Using a Nd:YAG laser and six axes robot to cut zinc-coated steel

Zinc-coated steel sheets are important materials in the automobile and home appliance industries. Currently, lasers are the preferred tools for metal cutting because of their good cutting quality, flexibility and excellent features and results, as compared to traditional tools. The solid-state Nd:YAG laser has successfully replaced the gaseous CO₂ laser for metal cutting; its small size and short wavelength makes it suitable for cutting bright and metal-coated materials, as well as being able to be transmitted via optical fibers and robots to cut complicated three dimensional and curved shapes. In this work, the Nd:YAG laser is used to cut 1 mm zinc coated steel sheets. We demonstrate the effects of different cutting parameters such as laser power, cutting speed, different gas types and pressures, and focus position on the cutting quality characteristics of attached dross, kerf width and cut surface roughness. Using a six axes robot, cutting speed was limited to 6 m/min because of the noticeable vibration at higher speeds. Results showed that the cutting surfaces achieved were very sharp and smooth. In cutting, Nd:YAG required less power and attained higher speeds than the published results of a CO₂ laser, which makes Nd:YAG an economical alternative to cut zinc and metal-coated materials. In addition, laser cutting using robots provided efficient and consistent cutting quality, especially in the case of 3D and countered cutting. Apart from using low speed, robots proved to be more economical than costly, specially designed CNC tables.     

A pulsed two-channel Nd:YAG laser with an improved spatial structure of IR radiation for prompt recording and development of photothermoplastic holograms

A two-channel Nd:YAG laser for high-speed recording and development of photothermoplastic holograms has been created. In two independent channels, the laser generates monopulse radiation of nanosecond duration (30 ns) at the second harmonic frequency (λ ₂ = 0.532 μm) and IR radiation (λ ₁ = 1.064 μm) produced by quasi-stationary free-running lasing of millisecond duration (4 ms). The radiations with wavelengths λ ₂ and λ ₁ are used, respectively, for exposure and development of photothermoplastic holograms. A method based on the use of a passive resonator is proposed to improve the spatial structure of IR radiation during lasing.     

Nano-second pulsed DPSS Nd:YAG laser cutting of CFRP composites with mixed reactive and inert gases

Superior structural capabilities and lightweight of carbon-fibre-reinforced polymer composites have made their applications increasingly noticeable particularly in the aerospace and automotive industries for reduced fuel consumption. Anisotropic and heterogeneous features of these materials, however, have been prohibiting the application of laser cutting on these materials in industrial scale. In the present study the thermal degradation characteristics in laser cutting of these materials are investigated with a nano-second pulsed diode pumped solid state Nd:YAG. A statistical analysis was performed for the optimisation of the process parameters. Furthermore, quality improvement was achieved by the use of low oxygen content assistant gas simultaneously with the inert gas shield. The controlled presence of oxygen as a burning mechanism reduced the fibre pull out up to 55% at the same time with a high processing rate.     

Nd3＋：YAG脉冲固体激光器的研制

本论文结合实验需要,根据激光器的基本原理设计了一套脉冲固体激光系统。通过本论文。对固体脉冲激光器的一些基本原理。基本组成。调Q理论和选模理论有了更清晰的认识,并基本掌握了Nd3＋：YAG脉冲激光器选择输出模式的方法。     

Process parameters selection for laser polishing DF2 (AISI O1) by Nd:YAG pulsed laser using orthogonal design

Mechanisms of laser polishing metals in a continuous scanning mode, as envisaged and analyzed in this paper, are rather complex, and experimental optimization of laser polishing metallic material is very time-consuming and difficult. Aiming at shortening the experimental time in achieving a better surface finishing of DF2 (AISI 01) tool steel by pulse Nd:YAG laser, we used the orthogonal experimental design for selecting the laser operational parameters. The results showed that the orthogonal design (OD) allowed the optimum technological parameters for the laser polishing to be obtained quickly and effectively. According to the OD analysis and experimental data, the attainable optimum laser smoothening/polishing parameters from this study are pulse energy (P)?=?1?J, feed rate?=?300–400?mm/min, pulse duration?=?3?ms, and pulse frequency (f)?=?20–25?Hz. The work in this paper demonstrates the relative superiority of orthogonal design in saving experimental times and providing good laser polishing surface in surface texturing and the smoothening process.     

The laser dressing of resin-bonded CBN wheels by a Q-switched Nd: YAG laser

Resin-bonded cubic boron nitride (CBN) grinding wheels are widely used in industry due to their good grinding ability. There is, therefore, a need to find a good method suitable for the dressing of these wheels. This paper presents a new method dressed by an acoustic-optic Q-switched Nd: YAG laser based on thermal interaction, which is unlike the conventional mechanical dressing methods based on force interaction. The mechanism of the selective removal of the bond was analysed. Experiments of a single pulse ablation on the resin bond and the CBN grains, and the laser dressing of resin-bonded CBN wheels with different dressing parameters have been carried out. The grinding force has been compared for the conventional mechanical dressed wheel and the laser dressed wheel. It is shown that the resin-bonded CBN grinding wheel is suitable for an acoustic-optic Q-switched Nd: YAG laser dressing with radial irradiation, which is better than a continuous wave (CW) laser dressing with radial irradiation and conventional mechanical dressing methods.     

Wavelength optimization for machining metals with the harmonic generations of a short pulsed Nd:YAG laser

The wavelength conversion for a short pulsed Nd:YAG laser has been implemented from infrared to visible and to ultraviolet spectra by using nonlinear optical crystals. The analytical method of wavelength optimization for machining metals with various harmonic generations of a Nd:YAG laser is presented in this paper. Combining the absorptivity of metal and the conversion efficiency of laser apparatus, the absorption efficiency is proposed to select an optimum machining wavelength. Various metals have different optimum machining wavelengths. The optimum machining wavelengths for gold, silver, and copper are in the third-, fourth- and second-harmonic generations of a Nd:YAG laser, respectively. For other metals, such as nickel, their optimum machining wavelengths are all in the fundamental wavelength of a Nd:YAG laser.     

应用Nd∶YAG激光加工连杆初始裂解槽

为改善连杆裂解槽加工精度和质量,进而提高裂解连杆的成品率,通过试制两种连杆产品,确定了激光加工参数。采用Nd∶YAG固体激光器对两种类型的连杆进行了裂解槽激光加工试验,分析了采用不同切割参数加工的裂解槽质量,并对激光切割参数进行了优化。结果表明:激光峰值功率、离焦量、切割速度、脉冲频率、辅助气体压力、激光入射角等对裂解槽的加工质量均有很大的影响,其中激光峰值功率2.4kW,脉冲时间0.4ms时,切槽深度为0.453～0.457mm,当脉冲频率与切割速度比值约为3,在非正离焦量状态下,所加工的裂解槽对连杆的裂解非常有利,能够满足M0406连杆的裂解质量要求。     

半导体量子点材料在Nd:YAG激光辐照下的非线性光学效应

使用10 n,脉冲调Q Nd:YAG激光器Z-scan技术测量了化学合成的无掺杂硫化锌量子点(QDs)以及掺Mn2+硫化锌量子点(QDs)的非线性光学特性,并使用透射电镜技术(TEM)以及X射线衍射法(XRD)表征合成材料的纳米结构.在室温下,分别利用UV-VIS分光光度计和分光荧光计测量了人工合成QDs胶体溶液的线性光学吸收特性以及光致发光的发射特性.样品的吸收特性表明,由于量子限制效应的影响,样品的截止吸收低于硫化锌的截止吸收.样品的光致发光特性显示,掺Mn2+的硫化锌样品显示出明显的光致发光现象,发射峰大约在580 nm;而无掺杂的硫化锌样品在紫外区辐射,发射峰大约在365 nm.对样品的UV-VIS吸收特性分析和TEMXRD分析表明,硫化锌样品的平均粒度(半径)大约为1.2 nm.分析开放光圈(OA) Z-scan技术得到的实验数据,发现在1 064 nm处两种试验样品都会发生四光子吸收(FPA)现象.拟合实验数据得到了两种试验样品的FPA系数以及FPA横截面,结果表明,ZnS QD的FPA横截面的计算值是4.9×1-106cm8·s3·photon-3,比硫化锌的FPA横截面大了5个数量级,而且人工合成的ZnS QD也有光学限制的性质.掺Mn2+离子的样品具有大的FPA横截面和在可见光区有高的发光效率这两个特点,使得该材料适合用于多光子荧光成像.     

Development of controlled Nd:YAG laser for medical applications

Several medical fields are concerned with applications of thermal lasers such as neodymium-doped, yttrium aluminum garnet (Nd:YAG), argon, and CO2. However, quantification of the necrotic volume of Nd:YAG laser-induced damage is not possible at the time of treatment. Mathematic models and feedback control can help to optimize Nd:YAG laser treatments. We therefore formulated mathematic models for coagulation processes and developed an intelligent Nd:YAG laser system with closed-loop feedback control. Surface temperature evolution proved to be valuable data for real-time control of coagulation and ablation. Infrared thermometry provided the noncontact measurement of temperature. A computer stored the temperature data calculated by the mathematic model. Deviations of surface temperature during the treatment beyond established tolerances causes the Nd:YAG laser system to adjust the laser power automatically.     

染料调Q的Nd3+:YAG激光器偏振性测试

为了确定染料调Q的Nd3+:YAG激光器发射的单次激光脉冲的偏振性,利用偏振分束立方体和双通道激光能量计,实验测量了激光脉冲经偏振分束后P偏振光和S偏振光的能量比,分析总结了激光器的偏振性.结果表明:激光器偏振性个体差异很大,不存在统一性;每一台激光器都有各自的偏振性,同一台激光器前后发射的激光脉冲的偏振性可能一致,也可能变化较小,也可能完全随机.产生偏振性的具体原因有待进一步研究.     

Pulsed Nd:YAG laser treatment of monocrystalline silicon substrate

To minimize reflection and increase the opportunity for photovoltaic (PV) devices to absorb incident light, we produced rough-textured surfaces using a pulsed Nd:YAG laser. We investigated the effect of various operating parameters on surface features and optical performance. The parameters investigated were pulse repetition frequency, pulse energy, and textured surface exposure time. Following laser surface treatment, no chemical solution etching or anti-reflective coatings were necessary. Reflectance values were measured by spectrophotometer. Structural properties including surface morphology and surface roughness of the textured surfaces were analyzed by a three-dimensional confocal laser scanning microscope. The resulting surface reflection curves indicated that, of the different laser machining paths, the vertical and circular paths produced identical optical properties in the samples. Surface reflection values decrease as the pulse energy and exposure times increase. By contrast, surface roughness increases with exposure time. An increase in pulse frequency reduces surface roughness, although the extent of the reduction depends upon the pulse energy. Following surface texturing of the monocrystalline silicon samples, the total reflection was reduced from 40% to approximately 10% of incident light. Our experimental results indicate that the rougher surfaces attained by laser surface treatment provide better light-trapping properties for PV devices.