Improving the machining performance characteristics of the µEDM drilling process by the online cryogenic cooling approach

Productivity and surface quality would significantly affect the performance of the micro electrical discharge machining process (µEDM). Thus, the machining performance would be enhanced by improving the material removal rate (MRR) and surface quality. In this investigation, cryogenic LN₂ cooling was introduced to the conventional µEDM setup for developing an innovative process of cryogenically cooled µEDM process (CµEDM). The favorable outcomes of this process were estimated by selecting discharge current (I_p) and pulse on duration (T_on) for determining the effects of the machining performance including MRR and surface integrity. Surface quality was also analyzed by microstructural analysis and a scanning electron microscope (SEM) for evaluating the effects of the cryogenically cooled µEDM process. The experimental result shows 54–62% improvement in MRR and 22–36% improvement in average roughness values. Hence, it is suggested that cryogenically cooled µEDM facilitates improvement in productivity and surface quality.     

Electrical and optical properties of µc-SiH films

Microcrystalline silicon films have been found quite useful in amorphous silicon solar cells as a contact material in n-i-p cells. Microcrystalline silicon films are obtained when amorphous silicon films are prepared by R.F. glow discharge of SiH₄ + H₂ at higher power ratings. These films possess higher conductivity as well as high transmission than amorphous silicon films. The present paper reports the preparation technique ofμc-SiH films using R.F. capacitive glow discharge of hydrogen-diluted silane. X-ray studies andtem studies of the films indicate microcrystallinity of the films. The electrical and optical properties are also reported.     

Rotary ultrasonic machining— a new cutting process and its performance

In conventional ultrasonic machining (USM), brittle materials are machined by using ultrasonic impacts on the workpiece, through a medium of abrasive slurry. In this paper a new cutting process that resulted due to introduction of an additional parameter, namely the rotation of the workpiece during the machining, is presented. This may be called ‘rotary ultrasonic machining’. The material removal rates (MRR) in rotary USM are up to four times those in conventional USM. The MRR increases with increase in speed of rotation of workpiece. An explanation for the superior performance of rotary USM is presented. The performance of rotary USM as a function of static load, abrasive grain size, concentration of abrasive slurry, diameter of tool and ratio of diameters of hollow tools, is studied and the parameters are optimized for minimum machining time or maximum material removal rate. Comparisons are made with conventional USM.     

Superior Carrier Lifetimes Exceeding 6 µs in Polycrystalline Halide Perovskites

Lead halide perovskite films have witnessed rapid progress in optoelectronic devices, whereas polycrystalline heterogeneities and serious native defects in films are still responsible for undesired recombination pathways, causing insufficient utilization of photon-generated charge carriers. Here, radiation-enhanced polycrystalline perovskite films with ultralong carrier lifetimes exceeding 6 μs and single-crystal-like electron–hole diffusion lengths of more than 5 μm are achieved. Prolongation of charge-carrier activities is attributed to the electronic structure regulation and the defect elimination at crystal boundaries in the perovskite with the introduction of phenylmethylammonium iodide. The introduced electron-rich anchor molecules around the host crystals prefer to fill the halide/organic vacancies at the boundaries, rather than form low-dimensional phases or be inserted into the original lattice. The weakening of the electron-phonon coupling and the excitonic features of the photogenerated carriers in the optimized films, which together contribute to the enhancement of carrier separation and transportation, are further confirmed. Finally the resultant perovskite films in fully operating solar cells with champion efficiency of 23.32% are validated and a minimum voltage deficit of 0.39 V is realized.     

Experimental investigation on low-frequency vibration-assisted µ-ED milling of Inconel 718

The microelectric discharge (µ-ED) milling is a competent process for the fabrication of complex 3-D shapes, but longer machining time limits the wide applications of the process. Therefore, in this work, a novel approach of low-frequency workpiece vibration-assisted µ-ED milling has used intending to improve the process performance. The experimental investigation has been performed using Taguchi L-16 orthogonal array to examine the effects of low-frequency workpiece vibration assistance in the µ-ED milling while fabricating microchannels on Inconel 718. The gap voltage, capacitance, and vibration frequency were selected as input parameters while material removal rate (MRR) and frontal electrode wear (FEW) were chosen as response variables. It has been observed that the MRR increases and FEW decreases with an increase in vibrational frequency up to an optimal value and then decreases for further increase in vibration frequency. The results disclose a positive influence of the low-frequency workpiece vibration of both MRR and FEW during µ-ED milling of Inconel 718. Finally, the effects of vibration and discharge energy on surface quality of fabricated microchannels were analyzed using field emission scanning electron microscopic images.     

Optimal cutter size determination for 2½-axis finish machining of NURBS profile parts

A curve model of non-uniform rational B-spline (NURBS) has been widely adopted in mainstream CAD/CAM software systems to design complicated geometries of mechanical parts, for example, the curved profiles of pockets, sides, and islands. NURBS profile parts (the profiles include NURBS curves for pockets and islands) are produced in 2½-axis rough and finish machining. In rough machining of the parts, several end-mills with different sizes are employed for high cutting efficiency, and in finish machining, a single end-mill is usually used to cut along the profiles for high surface quality. To accurately produce the geometries with NURBS curves in finish machining, the cutter size should be optimised in order to eliminate gouging and save machining time. Although this topic has been a research focus for a decade, optimal cutter size determination still remains as a technical challenge. To rise to this challenge, our work proposes a new approach to determining the largest allowable size for the cutter to move along all the profiles (including NURBS curves) in 2½-axis finish machining without global and local gouging. The salient feature of this approach is that an original model of the cutter size is formulated and an effective solver–the particle swarm optimisation method–is employed to compute the largest allowable cutter size. This intelligent approach is more efficient and accurate than the conventional computational method based on the test examples in this work. It can also be applied to global and local gouging detection for NURBS profile machining. Our research work has great potential to advance CNC machining techniques.     

Travelling-wave Brewster-angled Stripe InGaAsP Laser Amplifier at 1·3 µm

Abstract

Optical amplification of a Brewster-angled stripe travelling-wave laser amplifier operating at 1·3 µm is described. An actively mode-locked external cavity semiconductor laser generating pulses of duration from 11 to 30 ps was used as a signal source. Maximum single-pass gain of 32 dB was attained for an input of ?24 dB m and a maximum output peak power of 25 dB m was measured for an input power of 8 dB m. Narrowing in the amplified pulse was observed on a synchroscan streak camera when the amplifier gain is too weak to sustain constant temporal amplification across the whole pulse     

Macrochanneled poly (ɛ-caprolactone)/ hydroxyapatite scaffold by combination of bi-axial machining and lamination

A combination of bi-axial machining and lamination was used to fabricate macrochanneled poly (ɛ-caprolactone) (PCL)/hydroxyapatite (HA) scaffolds. Thermoplastic PCL/HA sheets with a thickness of 1 mm, consisting of a 40 wt% PCL polymer and 60 wt% HA particles, were bi-axially machined. The thermoplastic PCL/HA exhibited an excellent surface finish with negligible tearing of the PCL polymer and pull-out of the HA particles. The bi-axially machined sheets were laminated with a solvent to give permanent bonding between the lamina. This novel process produced three-directionally connected macrochannels in the dense PCL/HA body. The macrochanneled PCL/HA scaffold exhibited excellent ductility and reasonably high strength. In addition, good cellular responses were observed due to the osteoconductive HA particles.     

Small-bore hollow waveguides for delivery of 3-µm laser radiation

Flexible hollow glass waveguides with bore diameters as small as 250 μm have been developed for 3-μm laser delivery. All the guides exhibit straight losses between 0.10 and 1.73 dB/m, and the loss increases to between 2.4 and 5.1 dB/m upon bending 1 m of the guides into 15-cm-diameter coils. This behavior is shown to depend strongly on the launch conditions and mode quality of the input beam. The waveguides are capable of efficiently delivering up to 8 W of Er:YAG laser power with proper input coupling, and they are suitable for use in both medical and industrial applications.     

Effect of machining parameters on turning of VAT32® superalloy with ceramic tool

The objective of this research was to study the machining of superalloy VAT32® using alumina-based ceramic tool without cutting fluid, applying different machining parameters to evaluate the surface finish of parts, vibration and main wear of tools. For this, a turning process with a linear trajectory of 30 mm was performed, in which were collected data vibration and surface roughness of the stretch, as well as wear and damage in the tools. The turning tests were performed utilizing cutting speeds of 270, 280 and 300 m/min, a feed of 0.10, 0.18 and 0.25 m/rev and a cutting depth of 0.50 mm. With results, it was identified that the feed influenced significantly both the vibration and the system, since the cutting speed influenced only the vibration. Being that the best results happened for the smaller feed and greater cutting speed. It concludes that the machining of superalloy VAT32® with ceramic tool introduced herself promising.     

Electrolytic concentration effect on the abrasive assisted-electrochemical machining of an aluminum–boron carbide composite

All engineering materials can be machined by one or combination of processes in such a way that the material’s potential is fully exploited. Electrochemical machining is found to be a most promising process that produces various components from the hard-to-machine materials for the various applications. Electrolyte concentration is playing a positive role by improving the electrolyte conductivity, but negatively forming the passivation layer on the cut surfaces. In order to improve the surface finish and removal of generated residual materials from the cut surfaces, abrasive particles were fed along with electrolyte into the machining zone. This present paper investigates the sodium chloride (NaCl) electrolyte with varied concentration (10–30%) in association with SiC abrasive particles on the material removal rate, surface roughness, and radial overcut while machining of aluminum 6061–boron carbide (5–15?wt%) composites. This study conclusively derived that electrolyte concentration up to 20% exhibited a positive role in the material removal rate for the machining of composites because the rate of dissolution was of higher magnitude. Externally supplied abrasive particles along with electrolyte reduced the surface roughness and radial over cut to an extent. Conversely, at higher electrolyte concentration, the externally supplied abrasive particles have a little effect on the removal of the formed passivation layer as confirmed by SEM analysis.     

Single-photon detection beyond 1 µm: performance of commercially available germanium photodiodes

Germanium avalanche photodiodes (APD's) working biased above the breakdown voltage detect single optical photons in the near-infrared wavelength range. We give guidelines for the selection of devices suitable for photon-counting applications among the commercial samples, and we discuss in detail how the devices should be operated to achieve the best performance, both in terms of noise-equivalent power (NEP) and the timing-equivalent bandwidth. We introduce the driving electronics and we show that, in the measurements of fast optical signals, the adoption of single-photon techniques is very favorable, notwithstanding that presently available photodiodes are not designed for this purpose. On the contrary, in the detection of cw signals, the lower NEP values achieved in photon counting may not be sufficient to justify the replacement of conventional analog p-i-n germanium detectors, which offer comparable performance with a definitely larger sensitive area. Finally, we show that, by properly choosing theoperating conditions, some selected APD's achieve an 85-ps time resolution in the detection of optical photons at a 1.3-μm wavelength, which corresponds to a timing-equivalent bandwidth of 1.8 GHz. To the best of our knowledge, this time resolution is the lowest reported to date for single-photon detectors in the near infrared.     

Silicon grating-based mirror for 1.3-µm polarized beams: MATLAB-aided design

A dielectric lamellar-grating layer-substrate structure is proposed to be capable, under some conditions, of acting as a 100% efficiency mirror when operated at fixed wavelengths and incidence angles. The design of such mirrors for 1.3 μm and near normal incidence is achieved with silicon as the grating-layer material and glass substrates of two types. The study is based on a new matrix-vector procedure for the solution of rigorous coupled-wave equations. The computations use MATLAB, and, in particular, its goal-attainment routine. Design parameter tolerances are also discussed.     

Refractive index of ice in the 1.4-7.8-µm spectral range

New accurate values of the imaginary part of the refractive index k of polycrystalline ice at T = -22 °C are reported. The k spectrum in the 1.43-2.89-μm region was found to be in excellent agreement with the most recent study, and the data in the 3.35-7.81-μm range eliminate the large existing uncertainty in the 3.5-4.3-μm region.     

Study on the machining of Al–SiC functionally graded metal matrix composite using die-sinking EDM

Functionally graded aluminum matrix composites (FGAMCs) are new materials with excellent capabilities for design and development of complex engineering works. In this work, FGAMCs are machined using electrical discharge machining (EDM) with the process input parameters such as pulse current, pulse on time, and zone position in brake disk. Design of experiments is used for the experimental planning with full factorial method. The selected input process parameters are optimized using gray relational analysis to minimize the electrode wear ratio, overcut, power consumption, and surface roughness. The influential studies of input process parameters on the output responses are also conducted. The optimal EDM parameter setting for achieving better output parameters is pulse current at 5 A, pulse on time at 50?µs and 45?mm zone position distance in the brake disk. The pulse current (39.40%) contributed the maximum in minimizing the output responses. Further, the surface morphology is also analyzed on the material to observe the crater formation and the erosion mechanism.     

Machinability analysis in drilling woven GFR/epoxy composites: Part I – Effect of machining parameters

The present paper deals with the effect of machining parameters (feed, speed and drill diameter) on the thrust force and machinability of woven glass fiber-reinforced epoxy (GFRE) composites. The selected machinability parameters were delamination size, surface roughness, and bearing strength. The results show that, delamination-free in drilling GFRE composites was not observed, in the range of the investigated cutting parameters. Surface roughness instrument can be used as an indication for the position of the internal delamination damage in drilling GFRE composites. The high values of correlation coefficients between thrust force and the machinability parameters confirm the importance of reducing the thrust force to improve the load carrying capacity of composite structure assembled by rivets or bolted joints.     

A New Approach of Tool Wear Monitoring and Compensation in RµEDM Process

This paper presents a new approach of tool wear compensation based on “volume removal per discharge” (VRD) in “reverse-micro-electrical-discharge machining.” In this process, a plate with predrilled microhole is used as a tool, the erosion of which limits the fabrication of desired height of microrod(s). Therefore, in order to achieve the dimensional accuracy of this microrod(s), such tool plate wear needs to be compensated. Since this approach is based on the real-time estimation of volume removal from workpiece, which is obtained by multiplying the number of “contributing” pulses with VRD, an accurate estimation of VRD is very much essential for availing correct tool wear compensation. In this work, VRD is estimated by considering two new aspects: the number of actual “contributing” pulses and the variation of VRD with machining depth. These pulses are identified by a “pulse discriminating” system developed in house. The real-time material removal volume from workpiece is then used to estimate the “real-time height” of microrods after considering the over cut and taperness of each microrod. The proposed method is also compared with the normal machining method and “uniform wear method” and found to be more accurate with a negligible error of maximum 1.7%.     

1.2-µm transitions in erbium-doped fibers: the possibility of quasi-distributed temperature sensors

We propose the principle of a high-dynamic, quasi-distributed temperature sensor based on the behavior of the 1.13- and the 1.24-μm emission lines in erbium-doped silica fibers. The ratio of fluorescent intensity of these lines presents a temperature dynamic of more than 11 dB between room temperature and 600 °C. As the lower level of these transitions is not the fundamental, the emission lines are absorption free, and no dependence of the intensity ratio of the two lines has been observed, with power and wavelength pump variations permitting the realization of self-calibrated quasi-distributed sensors.     

Direct observation of transient fluorine atoms with 25-µm wavelength-stabilized diode laser absorption

Through the use of continuous diode laser absorption, detection of transient fluorine atoms with an initial number density in the range of 10(14) cm(-3) has been demonstrated. A crucial part of the continuous-detection technique was laser frequency stabilization with a reference cell of atomic fluorine with Zeeman modulation of the absorption lines to generate a feedback signal. Long-term wavelength stability was demonstrated with second-harmonic phase-sensitive detection of the second-derivative signal for periods up to several hours. For determination of the short-term wavelength stability in the range of microseconds to seconds, a transient signal was generated by photolysis of F(2) with an excimer laser at 308 nm. The initial diode laser absorption was compared to a calculated value obtained from the measured excimer laser fluence, the known dissociation cross section of F(2), and the atomic fluorine absorption cross section, which included a statistical population distribution, the finite bandwidth of the laser dode, and the effects of pressure broadening. The observed absorption was approximately 33% less than the calculated value, possibly because of the diode laser's wavelength instability on the time scale of a few seconds, which is consistent with an observed amplitude instability from pulse to pulse when pulsed at 1-10 Hz.