Multi-objective optimization of Nd:YAG laser cutting of thin superalloy sheet using grey relational analysis with entropy measurement

This paper presents a hybrid optimization approach for the determination of the optimum laser cutting process parameters which minimize the kerf width, kerf taper, and kerf deviation together during pulsed Nd:YAG laser cutting of a thin sheet of nickel-based superalloy SUPERNI 718 (an equivalent grade to Inconel 718). A hybrid approach of Taguchi methodology and grey relational analysis has been applied to achieve better cut qualities within existing resources. The input process parameters considered are oxygen pressure, pulse width, pulse frequency, and cutting speed. A higher resolution based L₂₇ orthogonal array has been used for conducting the experiments for both straight and curved cut profiles. The designed experimental results are used in grey relational analysis and the weights of the quality characteristics are determined by employing the entropy measurement method. The significant parameters were obtained by performing analysis of variance (ANOVA). The optimized parameters for straight and curved laser cut profiles have been compared. On the basis of optimization results it has been found that the optimal parameter level suggested for straight cut profiles are not valid for curved cut profiles. The application of the hybrid approach for straight cuts has reduced K_t and K_d by 52.37% and 17%, respectively. For curved cuts the approach has reduced K_w and K_t by 8.45% and 44.44%, respectively. The results have also been verified by running confirmation tests.     

高功率高光束质量脉冲Nd: YAG激光器

以实现高功率、高光束质量的脉冲激光输出为目的,对非对称平-凹谐振腔的结构进行了理论分析。设计出了高功率、高光束质量非对称放置的平-凹谐振腔、双氙灯泵浦的脉冲Nd: YAG激光器。当占空比为9%时,实现输出激光平均功率近480 W,光束参数积优于12.7 mm·mrad,电光转化效率近4%,与理论分析吻合,可用芯径300 μm的光纤传输,不稳定性优于±1%。加工实验证明有较好的质量：切割材料为不锈钢,厚度为3 mm时、切割速度为0.6 m/min和厚度为1.5 mm、切割速度为1.2 m/min时,两种情况下所得切缝宽度均为250 μm,且切割上下沿光滑。     

高功率高光束质量脉冲Nd: YAG激光器

 以实现高功率、高光束质量的脉冲激光输出为目的,对非对称平-凹谐振腔的结构进行了理论分析。设计出了高功率、高光束质量非对称放置的平-凹谐振腔、双氙灯泵浦的脉冲Nd: YAG激光器。当占空比为9%时,实现输出激光平均功率近480 W,光束参数积优于12.7 mm·mrad,电光转化效率近4%,与理论分析吻合,可用芯径300 μm的光纤传输,不稳定性优于±1%。加工实验证明有较好的质量：切割材料为不锈钢,厚度为3 mm时、切割速度为0.6 m/min和厚度为1.5 mm、切割速度为1.2 m/min时,两种情况下所得切缝宽度均为250 μm,且切割上下沿光滑。     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.     

Gas protection optimization during Nd:YAG laser welding

Many laser processes, such as welding or surface treatments are associated with an undesired phenomenon, which is oxidation. The solution commonly employed to solve this problem approaches the shielding gas and/or the shielding gas device. What we propose in this paper is a methodology with the goal to optimize the protection gas device design as well as the gas flow in the case of laser welding and surface treatments. The pressure created by the gas flow on the sample surface is recorded and analysed together with the operating parameters influence in order to reach the objectives. The nozzle system designed and presented below assures the protection against material oxidation using minimal gas flow rates and increases the welding penetration in the case of high-power Nd:YAG laser welding.     

Multi-objective optimization of Nd:YAG laser cutting of nickel-based superalloy sheet using orthogonal array with principal component analysis

Laser cutting of nickel-based superalloy sheets, having wide applications in aircraft and rocket industries, is important from the quality of cut point of view. Keeping this in view, a hybrid approach of Taguchi method (TM) and principal component analysis (PCA) has been applied for multi-objective optimization (MOO) of pulsed Nd:YAG laser beam cutting (LBC) of nickel-based superalloy (SUPERNI 718) sheet to achieve better cut qualities within existing resources. The three-quality characteristics kerf width, kerf deviation (along the length of cut), and kerf taper have been considered for simultaneous optimization. The input parameters considered are assist gas pressure, pulse width, pulse frequency, and cutting speed. Initially, single-objective optimization has been performed using TM and then the signal-to-noise (S/N) ratios obtained from TM have been further used in PCA for multi-objective optimization. The results of MOO include the prediction of optimum input parameter level and their relative significance on multiple quality characteristics (MQC). The responses at predicted optimum parameter level are in good agreement with the results of confirmation experiments conducted for verification tests.     

脉冲Nd:YAG激光熔覆修复塑料模具工艺参数的优化

激光器的电压、电流、脉冲宽度、脉冲频率与激光的扫描速度、光斑直径等工艺参数直接影响着脉冲Nd:YAG激光熔覆质量。为了简化工艺调整过程,提出了重叠率的概念,并进行了理论推导,建立了重叠率与脉冲频率、激光扫描速度、激光光斑直径相互关系的方程式。利用预置法在低碳钢基体上熔覆Ni基合金粉末,获得了2组最佳工艺参数,即单脉冲能量、重叠率分别为6.7 J,97.4%和21 J,69.4%。研究表明：在单脉冲能量和重叠率一定的条件下,改变电流、脉宽、脉冲频率及熔覆速度不会影响熔覆质量。     

脉冲Nd:YAG激光熔覆修复塑料模具工艺参数的优化

 激光器的电压、电流、脉冲宽度、脉冲频率与激光的扫描速度、光斑直径等工艺参数直接影响着脉冲Nd:YAG激光熔覆质量。为了简化工艺调整过程，提出了重叠率的概念，并进行了理论推导，建立了重叠率与脉冲频率、激光扫描速度、激光光斑直径相互关系的方程式。利用预置法在低碳钢基体上熔覆Ni基合金粉末,获得了2组最佳工艺参数，即单脉冲能量、重叠率分别为6.7 J,97.4%和21 J,69.4%。研究表明：在单脉冲能量和重叠率一定的条件下，改变电流、脉宽、脉冲频率及熔覆速度不会影响熔覆质量。     

Experimental study of electro-optical-switched pulsed Nd:YAG laser

We report the specification of a compact and stable side diode-pumped Q-switched pulsed Nd:YAG laser. We experimentally study and compare the performance of the pulsed Nd:YAG laser in the free-running and Q-switched modes at different pulse repetition rates from 1 Hz to 100 Hz. The laser output energy is stabilized by using a special configuration of the optical resonator. In this laser, an unsymmetrical concave–concave resonator is used and this structure helps the mode volume to be nearly fixed when the pulse repetition rate is increased. According to the experimental results in the Q-switched operation, the laser output energy is nearly constant around 70 m J with an FWHM pulse width of 7 ns at100 Hz. The optical-to-optical conversion efficiency in the Q-switched regime is 17.5%.     

355 nm nanosecond pulsed Nd:YAG laser profile measurement, metal thin film ablation and thermal simulation

Laser ablation of nickel, gold and copper thin film on glass substrates has been investigated using a nanosecond pulsed Nd:YAG laser operating at 355 nm in air with a Gaussian intensity profile. The exact beam profile was measured through mechanical scanning with a photodiode. A small beam defect was observed, which can affect the machining performance at higher pulse energies. The ablation thresholds of the films were calculated from the crater diameter values. The effect of the pulse repetition rate and the film thickness was also studied. At high pulse repetition rates heat accumulation was observed and the ablation threshold decreased with the film thickness. Both cases resulted in higher diameters.     

Direct-liquid-cooled Nd:YAG thin disk laser oscillator

A Nd:YAG thin disk laser oscillator convection-cooled directly by the liquid is demonstrated. Lasing performance at different flow patterns and different flow rates is studied in detail. Micro-scale eddies that appear at the disk edge can induce decrease of the output power and the degradation of beam quality. 17.1 W of output power is realized at the maximum pump power of 54.8 W, corresponding to the optical–optical efficiency of 31.2 %, and the slope efficiency of 37.5 %.     

Hollow-fiber delivery of high-power pulsed Nd:YAG laser light

We propose hollow fibers for delivery of high-peak-power pulsed Nd:YAG laser light. Hollow fibers with an internal polymer layer were fabricated by a liquid-phase coating technique. We reduced the losses of the fibers in the near-infrared region by producing a silver film that was very smooth owing to use of an ultrasonic wave for mixing of the silver and the reducer solutions in the silver-plating process. The straight losses of the 1-m-long polymer-coated fibers were 0.3 dB for the 700-mum bore size and 0.1 dB for the 1000-mum bore fiber.     

Experimental study of electro-optical Q-switched pulsed Nd:YAG laser

We report the specification of a compact and stable side diode-pumped Q-switched pulsed Nd:YAG laser. We experimentally study and compare the performance of the pulsed Nd:YAG laser in the free-running and Q-switched modes at different pulse repetition rates from 1 Hz to 100 Hz. The laser output energy is stabilized by using a special configuration of the optical resonator. In this laser, an unsymmetrical concave-concave resonator is used and this structure helps the mode volume to be nearly fixed when the pulse repetition rate is increased. According to the experimental results in the Q-switched operation, the laser output energy is nearly constant around 70 mJ with an FWHM pulse width of 7 ns at 100 Hz. The optical-to-optical conversion efficiency in the Q-switched regime is 17.5%.     

Biocompatible film deposition by using Nd:YAG pulsed laser

A Nd:YAG laser operating at the fundamental wavelength (1064 nm) and at the second harmonic (532 nm), with 9 ns pulse duration, 100–900 mJ pulse energy, and 30 Hz repetition rate mode, was employed to ablate in vacuum (10^?6 mbar) biomaterial targets and to deposit thin films on substrate backings. Titanium target was ablated at the fundamental frequency and deposited on near-Si substrates. The ablation yield increases with the laser fluence and at 40 J/cm ² the ablation yield for titanium is 1.2×10¹⁶ atoms/pulse. Thin film of titanium was deposited on silicon substrates placed at different distance and angles with respect to the target and analysed with different surface techniques (optical microscopy, scanning electron spectrosopy (SEM), and surface profile).

Hydroxyapatite (HA) target was ablated to the second harmonic and thin films were deposited on Ti and Si substrates. The ablation yield at a laser fluence of 10 J/cm ² is about 5×10¹⁴ HA molecules/pulse. Thin film of HA, deposited on silicon substrates placed at different distance and angles with respect to the target, was analysed with different surface techniques (optical microscopy, SEM, and Raman spectroscopy).

Metallic films show high uniformity and absence of grains, whereas the bio-ceramic film shows a large grain size distribution. Both films found special application in the field of biomaterial coverage.     

Investigation of Cr:YAG passive mode-locking in a pulsed Nd:YAG laser

Passive mode-locking of a pulsed Nd:YAG laser using a Cr⁴⁺:YAG saturable absorber was realized for the first time in a nearly critical stable resonator containing an antiresonant ring structure. The output energy and pulse duration are 13.5 mJ and 180 ps, respectively. The recovery time and saturable intensity for excited-state absorption of Cr⁴⁺:YAG under the action of strong laser pulses were calculated from rate equations.     

用倍频晶体KTP实现Nd：YAG激光锁模的研究

报道用倍频晶体KTP实现Nd:YAG激光锁模的系统研究,给出了该锁模激光器输出脉宽的表达式,并和实验测量值相比较,计算值跟实验结果符合较好.文中还分析了各种参数对该锁模激光输出性能的影响.     

Quality of the beam produced a pulsed Nd:YAG laser

The quality of the beam produced by a Nd:YAG laser was investigated. A hemispherical Nd:YAG laser resonator was developed. A flashlamp driver was designed to pump a Nd:YAG laser crystal. The output of the laser was characterized via the variation of the capacitor voltage. A phosphor card was used to detect the invisible of the infrared beam. Exposed photographic paper was utilized to examine the energetic beam after interaction. The brightness and strength of the beams were analyzed using the Matrox Inspector and VideoTest 5 software packages. The intensity of the beam produced and the volume loss after interaction were found to linearly increase with respect to the input energy.     

UV laser micromachining of piezoelectric ceramic using a pulsed Nd:YAG laser

UV laser (=355 nm) ablation of piezoelectric lead zirconate titanate (PZT) ceramics in air has been investigated under different laser parameters. It has been found that there is a critical pulse number (N=750). When the pulse number is smaller than the critical value, the ablation rate decreases with increasing pulse number. Beyond the critical value, the ablation rate becomes constant. The ablation rate and concentrations of O, Zr and Ti on the ablated surface increase with the laser fluence, while the Pb concentration decreases due to the selective evaporation of PbO. The loss of the Pb results in the formation of a metastable pyrochlore phase. ZrO₂ was detected by XPS in the ablated zone. Also, the concentrations of the pyrochlore phase and ZrO₂ increase with increasing laser fluence. These results clearly indicate that the chemical composition and phase structure in the ablated zone strongly depend on the laser fluence. The piezoelectric properties of the cut PZT ceramic samples completely disappear due to the loss of the Pb and the existence of the pyrochlore phase. After these samples were annealed at 1150 °C for 1 h in a PbO-controlled atmosphere, their phase structure and piezoelectric properties were recovered again. Finally, 1–3 and concentric-ring 2–2 PZT/epoxy composites were fabricated by UV laser micromachining and their thickness modes were measured by impedance spectrum analysis and a d₃₃ meter. Both composites show high piezoelectric properties. PACS 81.40.Gh; 77.84.-s     

Dynamic phenomena in laser cutting and cut quality

Coherent emission of light in the uv or vuv range can be obtained from an undulator or optical klystron by harmonic generation using an external laser. We show that if the input is a multimode laser (instead of an ideal monochromatic coherent wave) the kinetics of growth of the rms beam modulation is completely different from the one described by the usual Webster formula, and we find a simple analytical expression for a Gaussian input (large number of modes). Numerical calculations show that with 3 modes the result is already close to the Gaussian case, and that the frequency bandwidth increases as the square root of the harmonic number.