Predictive model and process parameters optimization of Nd:YAG laser micro-turning of ceramics

The present work highlights laser micro-turning operation of 10-mm diameter cylindrical-shaped alumina (Al₂O₃) ceramic using pulsed Nd:YAG laser. The paper also addresses development of mathematical models for correlating the various micro-machining parameters such as laser beam average power, pulse frequency, workpiece rotational speed, assist air pressure, and Y feed rate with the response criteria such as surface roughness and deviation in turned depth for achieving desired surface quality as well as dimensional accuracy during micro-turning operation using Nd:YAG laser system. Response surface methodology-based design of experiments has been adopted for the experimentation. This investigation also highlights the various test results that confirm the validity and correctiveness of the developed mathematical models through analysis of variance test. The test results were analyzed through various response surface plots to study the effect of the process parameters on the aforementioned responses. The results of validation experimentation show a good agreement for the developed empirical models. Sensitivity analyses of the developed models have been done to find out the variation in the output with respect to variations in the significant input process parameters. Moreover, multi-performance optimization has been done to find out the optimal parametric setting for achieving the desired process performances. Analysis also has been made based on scanning electron microscopy micrographs of the laser micro-turned surface achieved during machining at multi-criteria optimization setting.     

Enhanced processing speed in laser drilling of stainless steel by spatially and temporally superposed pulsed Nd:YAG laser radiation

Percussion drilling through holes in stainless steel (1.4301, 5, 8, and 10 mm in thickness) was performed with the superposed radiation of two pulsed Nd:YAG lasers. Holes were drilled with flash lamp pumped Nd:YAG slab-laser radiation with a pulse duration of 0.5 ms superposed with diode-pumped solid-state (DPSS) laser radiation with a pulse duration of 17 ns. The drilling efficiency is improved by the spatially and temporally superposed radiation of the two lasers. With the superposed laser radiation, drilling through stainless-steel samples at a maximum aspect ratio of 60 is performed up to four times faster with the reproducibility of the drilling time improved by a factor of six in standard deviation.     

Experimental analysis on Nd:YAG laser micro-turning of alumina ceramic

Laser micro-turning is a micro-machining strategy to machine cylindrical workpiece of hard-to-process materials such as ceramics. Laser micro-turning method is in high demand in the present high-precision manufacturing industries because of its wide and potential uses in various engineering fields such as automobile, electronics, aerospace, and biomedical applications, etc. In the present research, the experimental analysis of Nd:YAG laser micro-turning of cylindrical-shaped ceramic material has been made to explore the desired laser output responses, i.e., depth of cut and surface roughness by varying laser micro-turning process parameters such as lamp current, pulse frequency, and laser beam scanning speed. Single laser beam has been utilized for successful micro-turning operation. Experimental results revealed that the laser machining process parameters have great influences for achieving desired laser micro-turned depth and surface roughness characteristics during laser micro-turning of alumina ceramics. SEM and optical photographs have also been analyzed for better understanding of the laser micro-turning process for different parametric settings.     

Characterization of hole circularity in pulsed Nd:YAG laser micro-drilling of TiN–Al2O3 composites

In micro-manufacturing, circularity of a drilled hole at entry and exit are important attributes which greatly influence the quality of a drilled hole. This study investigates the effect of five parameters in the circularity of drilled holes in pulsed Nd:YAG laser micro-drilling process. The drilling operation has been carried out on titanium nitride–alumina (TiN–Al₂O₃) composite, an important electroconductive ceramics suitable for wear and heating applications. The effect of various process parameters like lamp current, pulse frequency, pulse width, air pressure, and focal length of Nd:YAG laser micro-drilling on hole circularity at entry and exit has been investigated through response-surface-methodology-based experimental study. The parametric combination for optimal hole circularity at entry and exit has also been evaluated.     

Crack separation mechanism in CO2 laser machining of thick polycrystalline cubic boron nitride tool blanks

An improved method for cutting thick polycrystalline cubic boron nitride (PCBN) tool blanks is explored because current methods of pulsed Nd:YAG laser cutting and wire electrical discharge machining (EDM) are constrained by low speed and low precision. We present a CO₂ laser/waterjet (LWJ) process to cut 4.8-mm-thick PCBN tool inserts by a crack separation mechanism. In LWJ, the PCBN blank is locally heated using a high-power continuous wave CO₂ laser to cause phase transition from cubic to hexagonal followed by water quenching to generate thermal stresses and form boron oxide leading to increased brittleness, subsequent cracking, and material separation. A 2³ fractional design of experiment (DOE) approach was employed to determine the factors of laser power, cutting speed, and waterjet pressure on the responses of phase transformation depth, taper, and surface roughness. A numerical heat flow model, based on Green’s function, was used to calculate the temperature distributions along the depth. Surface profilometer, scanning electron microscopy, and Raman spectroscopy were utilized to analyze the phase transformation and crack zones. Results from LWJ compared with pulsed Nd:YAG laser and laser microjet? methods indicate LWJ cuts 30 times faster; this was attributed to a nonconventional material removal (crack separation) mechanism. When LWJ was compared against nitrogen-assisted CO₂ laser cutting, improved cut quality (less taper and smaller heat-affected zone) was observed due to a greater control on phase transformation and crack propagation. DOE analysis revealed laser power and waterjet pressure, and the interactions among them are more significant factors than others.     

Experimental research on electrical discharge machining characteristics of engineering ceramics with different electrical resistivities

Electrical discharge machining (EDM) is the extensively used nonconventional material removal process for machining engineering ceramics provided they are electrically conductive. However, the electrical resistivity of the popular engineering ceramics is higher, and there has been no research on the relationship between the EDM parameters and the electrical resistivity of the engineering ceramics that can be machined effectively by EDM. This paper investigates the effects of the electrical resistivity and the EDM parameters on the EDM performance of ZnO/Al₂O₃ ceramic in terms of the machining efficiency and the quality. The experimental results showed that the electrical resistivity and the EDM parameters such as pulse on-time, pulse off-time, and peak current had the great influence on the machining efficiency and the quality during electrical discharge machining of ZnO/Al₂O₃ ceramic. Moreover, the electrical resistivity of the ZnO/Al₂O₃ ceramic, which could be effectively machined by EDM, increased with increasing the pulse on-time and peak current and with decreasing the pulse off-time, respectively. Furthermore, the ZnO/Al₂O₃ ceramic with the electrical resistivity up to 3,410 Ω cm could be effectively machined by EDM with the appropriate machining condition.     

Dry machining of aluminum for proper selection of cutting tool: tool performance and tool wear

Machining of aluminum and its alloy is very difficult due to the adhesion and diffusion of aluminum, thus the formation of built-up edge (BUE) on the surface. The BUE, which affects the surface integrity and tool life significantly, affects the service and performance of the workpiece. The minimization of BUE was carried out by selection of proper cutting speed, feed, depth of cut, and cutting tool material. This paper presents machining of rolled aluminum at cutting speeds of 336, 426, and 540 m/min, the feeds of 0.045, 0.06, and 0.09 mm/rev, and a constant depth of cut of 0.2 mm in dry condition. Five cutting tools WC SPUN grade, WC SPGN grade, WC + PVD (physical vapor deposition) TiN coating, WC + Ti (C, N) + Al₂O₃ PVD multilayer coatings, and PCD (polycrystalline diamond) were utilized for the experiments. The surface roughness produced, total flank wear, and cut chip thicknesses were measured. The characterization of the tool was carried out by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) pattern. The chip underface was analyzed for the study of chip deformation produced after machining. The results indicated that the PCD tool provides better results in terms of roughness, tool wear, and smoother chip underface. It provides promising results in all aspects.     

Measuring solid-state quantum yields: The conversion of a frequency-doubled Nd:YAG diode laser pointer module into a viable light source

This article outlines the difficulties associated with measuring quantum yields for solid-state samples using a high-pressure mercury arc lamp as the irradiation source. Details are given for the conversion of an inexpensive frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) diode laser pointer module into a viable irradiation source. The modified Nd:YAG laser was incorporated into a computer-controlled system, which allowed for the simultaneous irradiation and spectroscopic monitoring of the sample. The data obtained with the Nd:YAG diode laser system show far less scatter than data obtained with a high-pressure Hg arc lamp, and consequently the degradation rates obtained with the laser system could be calculated with far greater accuracy.     

Fabrication of polycrystalline diamond microtool using a Q-switched Nd:YAG laser

A Q-switched Nd:YAG laser (1,064 nm, 100 ns) was used to machine 2?×?1.5?×?0.5-mm rhombus-shaped tool inserts from a 60?×?0.5-mm circular disk of polycrystalline diamond. A systematic experimental study was undertaken to examine the effects of pulse repetition rate, feed rate, and number of laser passes on kerf, material removal rate, recast layer, surface morphology, and surface roughness. The optimal laser parameters for generating two-dimensional tool profiles were an average power of 3 W, a pulse repetition rate of 2 kHz, a feed rate of 1 mm/s, and a total of 45 laser passes. The beneficial results were a material removal rate of 0.02 mm³/min, kerf width of 27 μm, cutting edge radius of 6 μm, and surface roughness (Ra) of 0.625 μm. Recast layer formation, undulations, and striations were observed in the laser-cut regions. These features were attributed to the presence of a molten layer of cobalt binder, and amorphous carbon and graphite transitioned from diamond. An intriguing feature is the presence of fine particulate matter ranging in size from nanometers to a few micrometers in the laser-cut regions. It is believed that phase transition of diamond and cobalt during laser machining created thermal expansion mismatch stresses sufficient to fracture the solid into fine fragments.     

Dual-beam laser welding of ultra-thin AA 5052-H19 aluminum

Welding of 0.05 mm (0.002 inch) thin AA 5052-H19 aluminum samples in lap-joint configuration was conducted autogenously (no filler metal) using dual lasers that included Nd:YAG and diode with a zero inter-beam spacing. The 70-ns pulsed Nd:YAG (1064 nm) laser acted as the welding tool while the continuous wave diode (810 nm) laser with interaction times of 40–120 ms served to improve the light absorption characteristics of aluminum through preheating and oxidation effects. The microstructure, composition, flaws, and hardness of the joint were evaluated by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and micro-indentation hardness test. The dual-beam welding technique was also compared with single-beam (Nd:YAG) welding. Results of parametric effects are displayed in the form of processing maps. Deeper penetration, better weld quality (less humping and cutting), and increased hardness were observed in dual-beam welds when compared with single-beam welds. The most astounding result was a nearly 200% increase in hardness over the base metal in dual-beam welding. This can be explained by the oxygen pickup in a dual-beam weld due to longer heating and amorphous microstructure of aluminum oxide as revealed by energy dispersive X-ray spectrum and X-ray diffraction respectively.     

Initial operation of a pulse-burst laser system for high-repetition-rate Thomson scattering

A pulse-burst laser has been installed for Thomson scattering measurements on the Madison Symmetric Torus reversed-field pinch. The laser design is a master-oscillator power-amplifier. The master oscillator is a commercial Nd:YVO(4) laser (1064 nm) which is capable of Q-switching at frequencies between 5 and 250 kHz. Four Nd:YAG (yttrium aluminum garnet) amplifier stages are in place to amplify the Nd:YVO(4) emission. Single pulses through the Nd:YAG amplifier stages gives energies up to 1.5 J and the gain for each stage has been measured. Repetitive pulsing at 10 kHz has also been performed for 2 ms bursts, giving average pulse energies of 0.53 J with ΔE/E of 4.6%, where ΔE is the standard deviation between pulses. The next step will be to add one of two Nd:glass (silicate) amplifier stages to produce final pulse energies of 1-2 J for bursts up to 250 kHz.     

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.     

氧化铝陶瓷基板紫外纳秒激光打孔工艺研究

氧化铝陶瓷基板作为雷达微波组件的核心部件,其硬脆特性使得传统加工方法在导通孔加工中存在很多限制。激光加工作为一种非接触式高能束加工方法,是氧化铝陶瓷表面孔加工的最优选择。文中主要研究了紫外纳秒激光氧化铝陶瓷表面孔加工中的激光加工参数（包括激光平均功率、扫描速度和扫描次数等）对孔的特征尺寸（包括入口直径、出口直径和锥度）的影响规律,并分析了各种规律产生的相关机理。此研究为雷达微波用电子陶瓷基板的导通孔加工提供了有力的理论依据和技术支持。     

DUCTILE-REGIME MACHINING OF PARTICLE-REINFORCED METAL MATRIX COMPOSITES

ABSTRACT

This paper presents research results on ultraprecision machining of metal matrix composite (MMC) composed of aluminum matrix and either SiC or A1₂ 0₃ particles. Ductile-regime machining of both SiC and aluminum was evaluated to improve the surface integrity of the composite. Both polycrystal-line diamond (PCD) and single crystalline diamond (SCD) tools were used to ultraprecision machine the composites at a depth of cut ranging from 0 to 1μm using a taper cut. The feedrate was normalized to the tool nose radius. A model is proposed to calculate the critical depth of cut for MMCs reinforced with either A1₂0₃ or SiC. The critical depths of cut were found to be 1 p.m and 0.2 u.m for MMCs reinforced with A1₂ 0 or SiC₃, respectively. Both depth of cut and crystallographic direction of the ceramic particles are the sufficient conditions for ductile-regime machining. Although both tools produce similar surface finish, a SCD tool removed the MMC as chips while a PCD tool simply smeared the surface. A diffusion-abrasion mechanism was suspected to cause the surprising wear of the SCD tools when machining the aluminum/SiC composite.     

长脉宽脉冲激光硅片弯曲成形试验

利用毫秒脉宽Nd:YAG激光对硅片进行了弯曲试验,给出了长脉宽脉冲激光弯曲硅片的能量阈值条件。研究了长脉宽Nd:YAG激光脉冲频率和脉冲宽度参数对硅片弯曲角度的影响,同时说明了脉冲频率和脉冲宽度参数对弯曲角度的影响可以转换成扫描速度和功率密度对弯曲角度的影响,并对试验结果进行了分析,引入了脉冲占空比来表征能量的时域分布对弯曲现象的影响。试验结果表明,采用毫秒量级脉冲激光可以对硅片进行弯曲加工,弯曲角度可达20°以上。     

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.     

The laser shaping of ceramic by a fracture machining technique

A new laser machining technique for ceramic shaping, based on the concept of fracture mechanics, is proposed in this paper. The principle of fracture machining technique is investigated. A focused laser is used to scribe two groove-cracks at the two intersecting surfaces of the rectangular substrate. Then, a defocused laser beam is applied throughout the length of the groove-cracks to generate a great thermal stress, which makes the two groove-cracks link together. The material removal is due to the linkage of the groove-cracks. Conventional laser machining requires high laser power to evaporate the materials. The high temperature gradient would induce the formation of micro-cracks, which leads to a remarkable reduction in strength. The laser power required in the method proposed is only a tenth of what is required in the conventional method under the same material removal rate, and in addition, the amount of the micro-crack is smaller. The experimental specimens are alumina ceramics, and the laser sources are a CO₂ laser and a Nd:YAG laser. The fracture machining technique can be successfully employed for step shaping and blind corner shaping for a thick ceramic substrate. The relationships of machining parameters such as groove-crack depth, material removal rate, laser scanning speed, and laser power are discussed. Finally, the measurement of the surface roughness and the inspection of crack defects are examined thoroughly.     

Dimensional analyses and surface quality of pulsed UV laser micro-machining of STAVAX stainless steel mold inserts

Laser micro-machining is a new, precise, and very flexible process in micro-mold manufacturing, especially for difficult to machine material, i.e., hardened steel. The aim of the work reported in this paper was to utilize response surface methodology to optimize the dimensional accuracy and surface finish for STAVAX stainless steel mold inserts in the pulsed UV laser micro-machining. The influence of laser machining parameters on the ablated depth and surface roughness of the machined mold inserts have been experimentally investigated. The parameters of insert quality are analyzed under varying laser power, pulse frequency, hatched spacing, scan rate, and number of passes. The settings of the laser micro-machining parameters are determined by using design of experiments method. The analysis of variance, and regression analyses are employed to find the optimal levels and to analyze the effect of the parameters on the depth accuracy values and surface finish. Confirmation experiments with the optimal levels of micro-machining parameters are carried out in order to illustrate the effectiveness of the multi-optimization method. The validity of regression approach to process optimization is well established.     

The drilling of Al2O3 using a pulsed DC supply with a rotary abrasive electrode by the electrochemical discharge process

The electrochemical discharge machining (ECDM) process has the potential to machine electrically non-conductive high-strength, high-temperature-resistant (HSHTR) ceramics, such as aluminum oxide (Al₂O₃). However, the conventional tool configurations and machining parameters show that the volume of material removed decreases with increasing machining depth and, finally, restricts the machining after a certain depth. To overcome this problem and to increase the volume of material removed during drilling operations on Al₂O₃, two different types of tool configurations, i.e., a spring-fed cylindrical hollow brass tool as a stationary electrode and a spring-fed cylindrical abrasive tool as a rotary electrode, were considered. The volume of material removed by each electrode was assessed under the influence of three parameters, namely, pulsed DC supply voltage, duty factor, and electrolyte conductivity, each at five different levels. The results revealed that the machining ability of the abrasive rotary electrode was better than the hollow stationary electrode, as it would enhance the cutting ability due to the presence of abrasive grains during machining.     

Erosive smoothing of abrasive slurry-jet micro-machined channels in glass,PMMA, and sintered ceramics: Experiments and roughness model

Abrasive slurry jet micro-machining (ASJM) was used to machine channels in glass, PMMA, zirconium tin titanate, and aluminum nitride. The channel roughness was measured as a function of the ASJM process parameters particle size, dose, impact velocity, and impact angle. The steady-state roughness of the channels was reached relatively quickly for typical ASJM abrasive flow rates. The roughness of channels having depth-to-width aspect ratios up to about 0.25 could be reduced by approximately 35% compared to the roughest channel by decreasing particle impact velocity and angle. However, machining at such conditions reduced the specific erosion rate by 64% on average. It was therefore quicker to post-blast reference channels (225 nm average root mean square (R_rms) roughness) using process parameters selected for peak removal. It was also found that the roughness of reference channels could be reduced by about 78% by post-blasting using 3 μm diameter silicon carbide particles at 15° jet incidence. The smoothest post-blasted channels had an R_rms roughness of about 23 nm in glass, PMMA, and zirconium tin titanate, and 170 nm in aluminum nitride. Computational fluid dynamics was used to predict the particle impact conditions that were used in a model to predict the steady-state roughness due to ductile erosion with an average error of 12%.