Study of magnesium and aluminum alloys absorption coefficient during Nd:YAG laser interaction

In laser processes, the absorption factor of laser Nd:YAG by metals plays a very important role. In order to model laser welding, we need to know its evolution during the process. The theoretical calculation does not enable the prediction of the absorption factor in the case of a keyhole mode. It is difficult to predict the effect of plasma and recoil pressure on the shape of the keyhole. In this paper, an integrating sphere is used to determine the absorption factor during the laser process, which is carried out on two types of magnesium alloys (WE43 and RZ5) and an aluminum alloy. We obtain the evolution in time of the absorption factor according to different steps of the evolution of the keyhole.     

Effects of temperature-dependent material properties and shielding gas on molten pool formation during continuous laser welding of AZ91 magnesium alloy

Laser welding processes are widely used for fabrications in many engineering applications such as aerospace and automotives. In this paper, a moving distributed heat source model based on Goldak's method [1] has been implemented into finite volume thermal simulations in order to predict temperature distributions during the welding process of a magnesium alloy and to study the effects of variations in thermal properties, absorption coefficient and gas shielding on the computed temperature distributions and weld pool dimensions. The main conclusion is the significant effects of varying the thermal conductivity and absorption coefficient of magnesium. Also, it has been seen that the shielding gas, besides its main role of protection against oxidation, has a significant effect on the width of the weld pool. Finally, the obtained results have been compared to the experimental ones and a satisfactory correlation has been observed, indicating the reliability of the model developed in this study.     

Optimization of Nd:YAG laser welding onto magnesium alloy via Taguchi analysis

The use of an Nd:YAG laser for thin plate magnesium alloy butt welding was optimized using the Taguchi analytical methodology. The welding parameters governing the laser beam in thin plate butt welding were evaluated by measuring of the ultimate tension stress. The effectiveness of the Taguchi method lies in clarifying the factor that dominates complex interactions in laser welding. The factors can be the shielding gas, laser energy, convey speed of workpiece, point at which the laser is focused, pulse frequency, and pulse shape. Furthermore, 18 combinations of these six essential welding parameters were set and Taguchi's method followed exactly. The optimal result was confirmed with a superior ultimate tension stress of 169 MPa, 2.5 times larger to that from original set for laser welding.     

Effects of heat input on the low power Nd:YAG pulse laser conduction weldability of magnesium alloy AZ61

The effects of heat input on the low power Nd:YAG pulse laser conduction weldability of magnesium alloy AZ61 plates were investigated. The results show that for a hot-extruded AZ61 magnesium alloy plate laser conduction welding, the penetration depth and area of welds cross-section increased with an increase of the heat input. The microstructure of a band zone, which is located in the fusion zone (FZ) and close to the fusion boundary, evolved with an increase of the heat input. Moreover, an increase of the heat input increased the tendency of the formation of solidification cracking and liquation cracking. The porosities and average diameters of pores increased with an increase of the heat input but reduced sharply when a relatively large heat input was achieved. In addition, the degree of formation of craters increased linearly with an increase of the heat input.     

脉宽易变的高功率Nd:YAG激光器

本文描述一台由两级Nd:YAG激光器组成的紧凑高功率激光系统的结构及其工作性能。激光系统分成三个部分:对撞脉冲式的主-被动锁模振荡器,单脉冲选择器和四程放大器。振荡器输出锁模脉冲列的包络幅度起伏小于±4％,其脉冲宽度可以在宽范围内(20ps～2.5ns)方便地切换。对于单个0.1mJ、200ps的激光脉冲,四程放大器能提供7×10~2的放大倍数。     

An automatically controlled Nd:YAG laser with variable pulse width

The development of a high-energy, pulsed Nd:YAG laser system for materials-processing and medical applications is reported here. A variable pulse width in the range of 0.3–10 ms and a variable pulse repetition rate up to 50 pps are provided. An automatic operation system using a microprocessor-based driver/ controller enables safe operation of the laser system and automatic material processing when integrated in a laser/robot system.     

Mathematical modelling of an Nd: YAG laser: peak power and pulse width

In a previous paper, the author presented an empirically derived mathematical model of a pulse pumped laser which allowed calculation of the output energy from the laser—from the mirrors-only resonator to the fully assembled Q-switched state. In this paper the model is developed further by deriving an expression for the peak output power. New coupling and loss time constant are stated. The validity of the model is demonstrated by the ability of calculate the laser parameters from a measured pulse shape, knowing the rod dimensions and the pump energy at which the pulse was measured.     

Nd:YAG板状激光器的理论和实验研究

本文对Nd:YAG板状激光器进行了理论和实验研究。给出了板状介质在重复率泵浦和连续泵浦情况下的温度分布和应力状态的解析解,对板的热效应进行了分析与讨论。并用一块9×30×70mm~3的Nd:YAG板状材料,对其热效应进行了测量与分析。实现了“之”字形光路板状激光器的高重复率运转,在50Hz时,输出能量约600mJ,平均功率30W。     

Experimental study of high-power pulse side-pumped Nd:YAG laser

The paper reports on the characterization of a compact and simple side-pumped 0.538 J×100 Hz pulse Nd:YAG laser. A side-pumping configuration with 100 laser diode bars is used in the laser head. We also experimentally studied the laser performance of the diode-pumped Nd:YAG laser head in the free running and Q-switched operation under different repetition rates.     

Q-switched and mode-locked diode-pumped Nd:YAG laser with an LT-GaAs

Simultaneous Q-switching and mode-locking (QML) is accomplished in a diode-pumped Nd:YAG laser using low-temperature GaAs (LT-GaAs) as the saturable absorber, which also acts as an output coupler at the same time. The repetition rate of the Q-switched envelope increased from 25 to 40 kHz as the pump power increased from 2.2 to 6.9 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of 714 MHz. A maximum average output power of 770 mW was obtained.     

Optical spectroscopic studies of titanium plasma produced by an Nd : YAG laser

We present optical emission characteristics of the titanium plasma produced by the fundamental (1064 nm) and second (532 nm) harmonics of a Q-switched Nd: YAG laser using laser induced breakdown spectroscopy (LIBS). The experimentally observed line profiles of neutral titanium (Ti I) have been used to extract the electron temperature (T _e ) using the Boltzmann plot method. The electron number density (N _e ) is calculated using the Stark broadening profile of 368.73 nm spectral line. Beside we have studied the spatial variation of electron temperature and number density as a function of laser energy for titanium plasma by placing the target material in air (at atmospheric pressure). We have determined the electron temperature and the electron number density along the axial position of the plasma plume.     

Passively Q-switched Nd:YAG ceramic laser towards large pulse energy and short pulse width

A large pulse energy and high peak power passively Q-switched ceramic neodymium-doped yttrium aluminum garnet (Nd:YAG) laser has been demonstrated with Cr⁴⁺:YAG crystals as the saturable absorbers. By employing a continuous wave (CW) laser-diode (LD) as the pump source, as high as 11.3 W CW output power of 1064 nm was obtained under the pump power of 21.6 W, with an optical conversion efficiency of 52.3%. Inserting different initial transmissions Cr⁴⁺:YAG as saturable absorbers, under the incident pump power of 15.6 W, the largest pulse energy, shortest pulse width, and highest peak power are measured to be 188 μJ, 3.16 ns, and 59.5 kW, respectively. As we has known, this is the best passively Q-switched results ever reported by Nd:YAG ceramic material.     

Nd:YAG激光诱导等离子体冲击波实验及对晶状体破坏的研究

测试了利用调Q的NdYAG脉冲激光在水中诱导金属钛产生等离子体冲击波的声压和速度.并利用自行研制的激光乳化系统,对离体猪眼晶状体经硬化处理摹拟白内障进行乳化试验.实验结果表明NdYAG激光等离子体冲击波能对一定硬度的晶状体进行乳化,有可能成为白内障乳化的一种能量源.     

High-power Nd:YAG laser picosecond pulse delivery by a polymer-coated silver hollow-glass waveguide

An oscillator-amplifier high-power Nd:YAG laser system was used for transmission of a single 50-ps-long pulse or a train of pulses through a cyclic olefin polymer-coated silver hollow-glass waveguide. The maximum energy that was transmitted was 150 mJ for the train of pulses and 40 mJ for the single pulse, from which followed a delivered power of 100 GW/cm(2) . The characteristics that were obtained make these waveguides promising for the delivery of high-power laser pulses in medical as well as other applications.     

带抗共振环的非稳腔对撞脉冲锁模Nd:YAP(Nd:YAL0_3)激光器

本文报道用Nd:YAP (Nd:YALO_3)激光介质在带抗共振环的平-凸非稳腔结构中,选用五甲川-1,2-二氯乙烷染料,实现对撞脉冲被动锁模,获得良好锁模脉冲波形的实验结果.在1Hz的重复频率下,锁模脉冲平均脉宽<10ps,输出脉冲系列平均能量80mJ,脉冲系列能量集中于中央的三个脉冲,锁模成功率100%.输出能量稳定性80±4mJ.     

Simultaneous compression of the pulsewidth and pulse train of a passively mode-locked Nd:YAG laser

Simultaneous compression of the pulsewidth and the pulse train of a passivelymode-locked Nd:YAG laser has been achieved by using a plano-convex unstable resonatorwith a nonlinear Sagnac ring interferometer.A single pulse energy of >30mJ and apulsewidth of ≤10ps have been obtained.Using this system,the LAGEOS and ETALONsatellites laser ranging have been performed successfully.     

Preliminary studies of material surface cleaning with a multi-pulse passively Q-switched Nd:YAG laser

A multi-pulse Q-switched (MPQ) Nd:YAG laser system for surface material cleaning has been developed. Trains of pulses having total output energy of as much as 1.05 J, 10–30 pulses/train, 20–70 ns individual pulse-width, and about 100 μs whole duration were generated. Ablation threshold fluence and ablation rate for limestone substrate have been measured and compared with the theoretical calculations. Safe surface cleaning of limestone samples was demonstrated thanks to the high gap between the ablation threshold fluence of the substrate and black crust. Tests of surface cleaning for samples of stone, ceramics, and metal were performed.     

Numerical and experimental study of the Ti6Al4V macrostructure obtained by Nd:YAG laser

Titanium and its alloys (Ti6Al4V) have been widely used in the biomedical field; nevertheless, they should be subject to specific surface treatments, before being implanted, in order to improve bio-integration. Although laser processing is a useful technique for this purpose, different aspects of the basic mechanisms of this process are still in progress, with special emphasis on the modeling structure formation on the irradiated surface. For this research, the finite element method was used to study the generation of a macrostructure on the Ti6Al4V surface using a Nd:YAG laser. The temperature profiles, estimated during the extremely high heating and cooling rates caused by the output power of the laser beam, allowed us to analyze, among other things, the melting depth and the heat affected zone, in order to optimize the process. Moreover, the experimental results (SEM data) were positively compared with the numerical model, and a relationship of the crater profile formation (depth to width ratio) was determined.     

Structure formation on titanium during oxidation induced by cumulative pulsed Nd:YAG laser irradiation

We investigated local oxidation and surface structure development of Ti targets under multi-pulse, high-repetition-rate Nd:YAG ( =1.064 m, 300 ns, =30 kHz) laser irradiation in air at atmospheric pressure. The experiments were performed at laser intensity levels below the single-laser-pulse melting threshold of Ti. The morphology of the irradiated areas was studied by scanning electron microscopy and profilometry. The variation of the oxide compositions and the crystalline state with increasing laser pulse number was analysed by X-ray diffractometry. Besides the known phenomena related to laser irradiation and oxidation of metal surfaces (micro-crack or pore formation), we evidenced new morphological features such as droplet-like structures inside the surface micro-cracks and micro-columns, and with increasing laser pulse number the formation of a dome-shaped structure over the whole irradiated zone. The occurrence of melting under multi-pulse irradiation was associated with the rise in the surface temperature enhanced by the oxidation, and the progressive evolution of the surface structures was associated with the consecutive melting–solidification processes as well as with the different stages of oxidation. PACS 61.80.Ba; 68.37.Hk; 81.65.Mq     

Numerical solution and experiment of a self-Q-switched 946 nm Cr,Nd:YAG laser

A LD-pumped self-Q-switched 946 nm laser by using a co-doped Cr,Nd:YAG crystal as a gain medium as well as a saturable absorber is studied. The 946 nm self-Q-switched rate equations of co-doped crystal are solved numerically by Runge–Kutta method directly. The important parameters of lasers, such as average output power, threshold pump power, pulse width, pulses repetition rate, and optimal transmission of output coupler are obtained numerically. Experimentally, the maximum average output power up to 2.61 W, corresponding to a slope efficiency of 23.43%, was obtained in a simple and compact linear cavity. The optical-to-optical efficiency is 17.3% and the peak power is 7.57 kW with repetition rate of 23.78 kHz and pulse width of 14.5 ns. In the experiments, a high-quality fundamental transverse mode can be preserved in a large range of incident pump power. The numerical results of the 946 nm self-Q-switched Cr,Nd:YAG laser are in good agreement with the experimental results.