ANALYSIS OF SPARK PROFILES DURING EDM PROCESS

A fundamental study of electro discharge machining (EDM) based on the physics of an arc and heat transfer theory has been carried out. The field equations for electric potential and temperature in the spark region are simultaneously solved by employing the finite element method. Using the criterion of constant current at any cross section of a spark, the arc radii at different cross sections are corrected until convergence. The final spark shape obtained is noncylindrical, and has different radii at different cross sections. Also, the percent of heat input absorbed by cathode, anode, and dielectric has been calculated. The computed relative electrode wear has been compared with experimental results.     

Evaluation of AC Stabilization of a Dc Arc Lamp for Spectroscopic Applications

ABSTRACT

Spectroscopic applications of the recently described method of arc lamp stabilization by the addition of a small (<2v) alternating current (AC) to a direct current (DC) arc lamp are discussed. A possible explanation for the improved arc stability is presented. Evaluation of arc formation and spatial behavior is monitored by a series of still photographs of the arc image. Observation of the electrode surfaces in similar photographs provide insight into the distribution of heat in the inter-electrode region. A photodiode array imaging device is used to accurately detect lateral movement of the arc image over prolonged time periods. Advantages of AC stabilized lamps in spectroscopic applications are also presented.     

Alpha-particle spectrometry using position-sensitive solid-state track detectors

A precision technique for measuring the angular distribution of differential cross sections for elastic scattering of charged particles using position-sensitive solid-state track detectors has been developed. The technique is based on the freedom of choice of the angle pitch under the condition of the minimum angular resolution of the spectrometric system, which is determined by the geometry of the experimental setup. The differential cross sections in the reaction ²⁷Al(α, α)²⁷Al have been measured at an α particle energy of 5.5 MeV.     

ANALYSIS OF SPARK ERODED CRATER FORMED UNDER GROWING PLASMA CHANNEL IN ELECTRO-DISCHARGE MACHINING

In electro-discharge machining, the occurrence of sparks cause material removal in the form of craters. These craters are due to melting and vaporization of workpiece over a localized area under the spark, which acts as the heat source. The crater under a single spark has been predicted by theoretical models adopting different approaches in solving the transient heat conduction equation, considering suitable assumptions with appropriate initial and boundary conditions. In the present work, a transient thermal model for a very large solid cylinder has been used to predict the size of the crater obtained under a single spark by determining the melting isotherm in both the axial and radial directions. An analytical study of the effect of plasma channel radius, heat flux, and pulse duration on the size of the crater has been made. Experiments are conducted using a commercial electro-discharge machine. The craters are measured under microscope and a comparison with theoretical results is presented. Subsequently, the nature of variation of crater diameter, crater depth and volume of material removed with respect to different machining parameters such as 'ON' time, 'OFF' time, and current have been explained by the theoretical results and it has been concluded that the plasma channel grows with respect to pulse duration such that at the end of the pulse the plasma channel radius becomes equal to the crater radius.     

Laminar flow through a curved channel

A complex variable method has been used to analyse the steady flow of a viscous incompressible fluid through a curved channel whose cross section is formed by the intersection of two hyperbolas at unequal distances from the axis which is also the origin for the Cartesian system (x,y). An expression for the velocity distribution was derived and the flux discharge per second, the average velocity and the coefficients k and k′ were calculated. Variations of velocity along three directions are tabulated and discussed. The variations of velocity along circular arcs of different radii and the ratios of the velocities in the directions θ = 0, θ = 90° and θ = 30° where θ is the polar coordinate are calculated.     

Studying the dependence of the ratio of the prompt neutron yields in the symmetric and asymmetric fission of <Superscript>235</Superscript>U and <Superscript>238</Superscript>U nuclei on the energy of neutrons inducing this fission

The ratios have been measured of the prompt neutron yields in the symmetric and asymmetric fission of ²³⁵U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors, as well as in the ²³⁸U fission induced by 14.7-MeV neutrons. The well-known measured integrated cross sections of the system of dosimetric nuclear reactions are compared to the cross sections calculated using the differential cross sections from the data libraries known worldwide, the three-component representation of fission neutron spectra from ²³⁵U and ²⁵²Cf nuclei, and the KASKAD spectrum reconstruction program.     

Molecular Beam Analyzer for Identifying Transient Intermediates in Gaseous Reactions

SUMMARY

When using a mass spectrometer to monitor free radical and atom intermediates in rapid gas-phase reactions, identification of the parent species of each ion is made difficult by fragmentation processes that occur at electron energies necessary for optimum ionization cross sections. Molecular beam analysis is a new approach to this problem whereby molecular beam measurements are used to characterize neutral species before they are ionized. Thus, the trajectory of a particle in an inhomogeneous magnetic field shows whether or not it has a large magnetic moment characteristic of a free radical, its motion in an inhomogeneous electric field indicates its symmetry and the magnitude of its electric dipole moment, and its velocity distribution determines its mass when the source temperature is known. Using these techniques, various atom, free-radical, and unstable molecular species have been identified in gases excited by microwave discharge or as products of atom-molecule reactions.     

Cross‐Coupled Ratio Control Structure

Abstract

In this paper, a cross‐coupled ratio control structure with two blend stations has been proposed. The main feature of the proposed structure is to give a good ratio performance for both set‐point changes and load disturbances that might occur in any one of the two loops. A simple tuning procedure for the inner controllers of the two loops and an easy design methodology for the blend stations have been proposed in order to avoid any significant tuning and design effort for the implementer. The proposed structure is realized on a microcontroller based system. This cross‐coupled ratio control structure that has been embedded into the microcontroller is then used to maintain the prescribed ratio “a” of the two processes, namely heat transfer process trainer (PT 326‐Feedback) and process control simulator (PCS 327‐Feedback). The same two processes are run under another alternative ratio control structure and the results are compared with each other.     

复杂航天筒体结构件的焊接应力应变演变规律

采用有限元分析手段对复杂航天筒体结构件的纵向和环向焊缝温度场和应力场进行模拟。航天筒体焊接采用变极性等离子弧焊工艺,根据变极性等离子弧焊工艺特点使用面高斯热源和圆柱热源的组合热源模型作为变极性等离子弧焊的热源。采用ANSYS软件中ANSYS参数化设计语言(ANSYS parametric design language, APDL)编写程序,实现变极性等离子弧焊接热源在筒体结构上加载和移动,完成四道纵缝和两道环缝的焊接热力过程仿真。通过对模拟结果分析,解释复杂筒体焊接应力应变的演变过程。发现在焊接过程中纵缝两端有明显的应力-应变释放,交叉部位的变形直接影响到结构整体的尺寸精度。     

Tribological behaviours of ultrafine Sn/heat treated serpentine powder in grease

Abstract

In order to improve the tribological performance of traditionally based lithium grease, ultrafine serpentine powder of 0·33 μm and Sn powder of 120 nm were prepared by ball mill and direct current arc plasma evaporation respectively. The milled serpentine powder was heat treated at 200, 500, 600 and 800°C in a muffle furnace separately with a four-ball wear test. Then, the tribological behaviours of lithium grease modified with complex powder with different ratios of ultrafine Sn/heat treated serpentine were investigated, and the effects of total concentration and load were discussed too. The optimal performance was achieved by the compound grease with 1 wt-% complex powder and 1∶1 Sn/serpentine powder (heated at 200°C). The friction coefficient and wear scar diameter decreased 25·5 and 42·5% respectively under 392 N compared with pure grease. The compound grease is much more suitable under high load rather than low load.     

用于电弧发射光谱的数字化自动引弧电路设计

转盘电极发射光谱分析技术在油液成分检测中有重要应用,研究表明,电弧光源的稳定性对光谱分析的结果有重要影响,传统的电弧发生器多使用火花放电器作为引弧电路的核心器件,在油液成分分析中引弧的稳定性和可靠性难以保证.文章提出了一种用于电弧光源激发的数字化自动引弧电路,建立了高频引弧电路拓扑结构模型,使用双路可控硅代替火花放电器...     

Performance and emission characteristics of a low heat rejection spark ignited engine fuelled with E20

In internal combustion engines, the concept of low heat rejection (LHR) using thermal barrier coating on the surface of combustion chamber is gaining attention. Thermal barrier coating reduces the heat transfer to the cooling system, protects engine components from peak heat flux and fluctuating temperature produced during combustion and improves the performance of the engine. Information in the literature is plentiful for LHR diesel engine and only few studies exist on LHR spark ignited engine. The application of thermal barrier coating in spark ignited engine is limited by pre-ignition and knocking due to elevated combustion chamber temperature. A spark ignited engine with moderate insulation on the combustion chamber and higher octane fuel can overcome this difficulty. The objective of the present experimental study is to quantify the changes in performance and emission characteristics brought by partial thermal insulation on the combustion chamber of a four stroke spark ignited engine fueled with E20 blend. Partial thermal insulation was created by coating 0.3 mm thick Alumina (Al₂O₃) on the cylinder head, inlet and exhaust valves. The changes are quantified with respect to unmodified engine fueled with gasoline. The combustion parameters such as flame development and rapid burn duration are also estimated and compared. The results indicate that partially insulated SI engine when fueled with E20 improves performance and reduces emission. A maximum of 48% reduction in THC and 50% reduction in CO emission at part load was achieved.

     

Shear-torsion of large curvature beams-Part II. Applications to thin bodies

The shear-torsion state of stress in a curved beam, whose cross sections is a thin rectangle with sides not parallel to the plane of the beam, are determined in closed form. The maximum value of the shear stress is attained at the concave boundary of the beam. The shear-torsion moment of inertia J_st, in multi-connected thin-walled cross sections is evaluated. Several examples of cross sections are discussed.     

TEMPERATURE DISTRIBUTION DURING ELECTRO-DISCHARGE ABRASIVE GRINDING

The objective of this work is to develop a finite element method (FEM) based mathematical model to simulate the hybrid machining process of grinding and electric discharge machining (EDM), named as Electro-discharge abrasive grinding (EDAG), for temperature distribution in the workpiece. Two different finite element codes have been developed to calculate the temperature distribution due to grinding heat source and EDM heat source separately. The transient temperature field within workpiece due to cut-off grinding is determined due to moving rectangular heat source. Gaussian heat distribution of power within a spark has been considered in the calculation of temperature distribution due to EDM. Temperature distribution in the workpiece due to combined process is obtained by using superposition. The simulation shows a sudden rise in temperature at the spark location. The predicted results can be used for calculation of thermal stresses, which play a major role as far as high-quality product requirements are concerned.     

TEMPERATURE DISTRIBUTION DURING ELECTRO-DISCHARGE ABRASIVE GRINDING

The objective of this work is to develop a finite element method (FEM) based mathematical model to simulate the hybrid machining process of grinding and electric discharge machining (EDM), named as Electro-discharge abrasive grinding (EDAG), for temperature distribution in the workpiece. Two different finite element codes have been developed to calculate the temperature distribution due to grinding heat source and EDM heat source separately. The transient temperature field within workpiece due to cut-off grinding is determined due to moving rectangular heat source. Gaussian heat distribution of power within a spark has been considered in the calculation of temperature distribution due to EDM. Temperature distribution in the workpiece due to combined process is obtained by using superposition. The simulation shows a sudden rise in temperature at the spark location. The predicted results can be used for calculation of thermal stresses, which play a major role as far as high-quality product requirements are concerned.     

The effect of arc behavior on weld geometry by high-frequency pulse GTAW process with 0Cr18Ni9Ti stainless steel

Based on 0Cr18Ni9Ti austenitic stainless steel plate (h?=?6 mm), a study on arc behavior by ultrasonic frequency pulse gas tungsten arc welding (GTAW) process has been carried out. The results show that with the increasing pulse frequency, an obvious pinch effect of arc plasma has been detected and also the increment of arc voltage, stiffness, and force. Then, the method, combining weld appearance and numerical simulation, has been adapted for weld behavior on the basis of arc behavior by ultrasonic frequency pulse GTAW process. As a result of arc shrinkage, the root radius of arc decreased, which caused narrower weld bead. The larger arc force led to more depression of pool surface that made the downward heat source and external force point, which had been important to increasing weld penetration. Meanwhile, the mobility of molten pool was enhanced by weld behavior compared with conventional GTAW process.     

A differential time-of-flight spectrometer of very slow neutrons

A time-of-flight spectrometer of neutrons with energies of 0.05–2.50 μeV is described. This spectrometer has been tested by measuring the total and differential neutron cross sections for a number of materials—aluminum, copper, silicon, zirconium, polyethylene, ⁶LiF, and fluoropolymers—that are essential for experiments in physics of ultracold neutrons.     

Low-energy electron/atom elastic scattering cross sections from 0.1–30 keV

Empirical forms for electron/atom scattering cross sections predict backscattering factors that compare well with those calculated using tabulated Mott data from 0.1 to 30 keV. The form of the empirical total cross section is similar to the screened Rutherford cross section. The fit to the tabulated differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section (σ_R), and the second part being an isotropic distribution (σ_I). The ratio of the total cross sections (σ_R/σ_I) between the screened Rutherford part of the differential scattering cross section and the isotropic part of the distribution is fitted to give the same ratio of forward to backscattered currents as the tabulated Mott differential cross sections. The three equations, one for the total elastic cross section and two describing the differential cross section—one for the Rutherford screening parameter and one for the ratio σ_R/σ_I—give backscattering results covering all the major trends with energy and atomic number compared with the backscattering coefficients calculated using tabulated Mott cross sections. However, agreement with experiment is poor for some well-researched examples such as Au. Monte Carlo calculations using the empirical cross sections show that surface effects may be critical in interpreting experimental results.     

Modification of a Hilger E612 Raman Spectrometer to a Laser Excitation Source

SUMMARY

Since the advent of the continuous wave laser, it has been advantageous to employ lasers as excitation sources for Raman spectroscopy [1]. This communication reports details of an arrangement being used in our laboratory whereby a laser replaces the conventional mercury arc source with a Hilger E612 Raman Spectrometer.     

An arc generator of a broad plasma stream

The design and basic parameters of an arc plasma generator based on a combined cathode are described. The cathode consists of a hot tungsten filament located in the hollow cathode. A plasma stream with a cross section of 150×10 cm² and a density of ∼10¹⁰ cm⁻³ at a pressure of 0.1–1 Pa is generated at a discharge current of up to 60 A without a cathode spot. The plasma generator can be utilized for final cleaning and activation of surfaces of materials and articles before depositing functional coatings on them and in plasma-assisted deposition by using either vacuum arc or magnetron discharges.