Theoretical investigation of penetration characteristics in gas metal-arc welding using finite element method

In the modeling of the gas metal-arc (GMA) welding process, heat inputs to the workpiece by the arc and the metal transfers have been considered separately. The heat energy delivered due to the metal transfer has been approximated in the form of a cylindrical volumetric heat source, whose dimensions of the radius and the height are dependent on the molten metal droplet characteristics. The pinch instability theory (PIT) and the static force balance theory (SFBT) of drop detachment have independently been used to obtain the expressions for various characteristics of the drop,i.e., the drop radius, the drop velocity, and the drop frequency at various welding parameters. The occurrence or the nonoccurrence of finger penetration, routinely found in the GMA welding at high welding currents, has been satisfactorily explained by the cylindrical heat source model. The effect of various welding parameters,e.g., the welding current, the wire radiusetc., on the weld bead penetration characteristics has been investigated. In this modeling effort, the heat conduction equation has been solved in three dimensions.     

Transition metal joints for steam generators—An overview

The transition metal joint (TMJ) between an austenitic stainless steel and a chromium-molybdenum (Cr-Mo) ferritic steel used widely in steam generators of power plants has for a long time presented problems relating to premature failures in service. The direct (bimetallic) TMJ presently in use is designed for a service life of about 200,000 h; but such TMJs with iron-base weld metals have been failing in service within about one-third of their design lifetime, while their counterparts with nickel-base weld metals fail within about one-half of their design lifetime. The causes for such premature service failures of these TMJs are discussed in detail, leading to the development of improved TMJs. One of the improved TMJs with a trimetallic configuration of austenitic stainless steel/Alloy 800/Cr-Mo ferritic steel is discussed in detail, covering its development, characterisation and evaluation. Accelerated performance tests in the laboratory have indicated a four-fold improvement in the service life of the TMJ with this trimetallic configuration compared to the bimetallic configuration. The metallurgical details of these studies are also discussed in this paper.     

Characterization of tempered martensite embrittlement using hardness measurement

A series of experiments were carried out on three commercial steels to explore the possibility for characterizing tempered martensite embrittlement (TME) by Rockwell, macro-, and micro-Vickers hardness tests. The results indicate distinct hardness peaks in two steels and an inflection in the other around the TME temperature. A new analytical approach for examining the slope of hardness-tempering temperature plots appears to reveal the TME phenomenon more sensitively. Dilatometric examinations substantiate that the temperature of hardness peak/inflection occurs beyond the second stage of tempering.     

A local asynchronous distributed privacy preserving feature selection algorithm for large peer-to-peer networks

In this paper we develop a local distributed privacy preserving algorithm for feature selection in a large peer-to-peer environment. Feature selection is often used in machine learning for data compaction and efficient learning by eliminating the curse of dimensionality. There exist many solutions for feature selection when the data are located at a central location. However, it becomes extremely challenging to perform the same when the data are distributed across a large number of peers or machines. Centralizing the entire dataset or portions of it can be very costly and impractical because of the large number of data sources, the asynchronous nature of the peer-to-peer networks, dynamic nature of the data/network, and privacy concerns. The solution proposed in this paper allows us to perform feature selection in an asynchronous fashion with a low communication overhead where each peer can specify its own privacy constraints. The algorithm works based on local interactions among participating nodes. We present results on real-world dataset in order to test the performance of the proposed algorithm.     

Note: Compact high voltage pulse transformer made using a capacitor bank assembled in the shape of primary

The experimental results of an air-core pulse transformer are presented, which is very compact (<10 Kg in weight) and is primed by a capacitor bank that is fabricated in such a way that the capacitor bank with its switch takes the shape of single-turn rectangular shaped primary of the transformer. A high voltage capacitor assembly (pulse-forming-line capacitor, PFL) of 5.1 nF is connected with the secondary of transformer. The transformer output voltage is 160 kV in its second peak appearing in less than 2 μS from the beginning of the capacitor discharge. The primary capacitor bank can be charged up to a maximum of 18 kV, with the voltage delivery of 360 kV in similar capacitive loads.     

Valence band offset at GaN/β-Si3N4 and β-Si3N4/Si(111) heterojunctions formed by plasma-assisted molecular beam epitaxy

Ultra thin films of pure β-Si₃N₄ (0001) were grown on Si (111) surface by exposing the surface to radio- frequency nitrogen plasma with a high content of nitrogen atoms. Using β-Si₃N₄ layer as a buffer layer, GaN epilayers were grown on Si (111) substrate by plasma-assisted molecular beam epitaxy. The valence band offset (VBO) of GaN/β-Si₃N₄/Si heterojunctions is determined by X-ray photoemission spectroscopy. The VBO at the β-Si3N4 / Si interface was determined by valence-band photoelectron spectra to be 1.84 eV. The valence band of GaN is found to be 0.41 ± 0.05 eV below that of β-Si₃N₄ and a type-II heterojunction. The conduction band offset was deduced to be ~ 2.36 eV, and a change of the interface dipole of 1.29 eV was observed for GaN/β-Si₃N₄ interface formation.     

Carrier concentration dependence of donor activation energy in n-type GaN epilayers grown on Si (1 1 1) by plasma-assisted MBE

The n-type GaN layers were grown by plasma-assisted MBE and either intentionally doped with Si or unintentionally doped. The optical characteristics of a donor level in Si-doped, GaN were studied in terms of photoluminescence (PL) spectroscopy as a function of electron concentration. Temperature dependent PL measurements allowed us to estimate the activation energy of a Si-related donor from temperature-induced decay of PL intensity. PL peak positions, full width at half maximum of PL and activation energies are found to be proportional to the cube root of carrier density. The involvement of donor levels is supported by the temperature-dependent electron concentration measurements.     

Analysis of Elevated Temperature Flow Behavior of 316LN Stainless Steel Under Compressive Loading

To understand the flow behavior of a newly developed austenitic stainless steel 316LN with 0.14 wt% nitrogen, isothermal compression tests have been carried out in the hot working domain. From the analysis of flow behavior, it is observed that the nitrogen enhanced steel, in its hot working domain exhibits strain and strain rate hardening with thermal softening. The flow behavior analysis also demonstrates the coupled effect of strain–temperature and strain rate temperature on the flow stress. To depict the flow behavior of the material, strain compensated Arrhenius (SCA) equation and Model D8A have been used. The SCA predicts the flow curves with an average absolute error of 9.27% and a correlation coefficient of 0.977, whereas the prediction by Model D8A gives the average absolute relative error as 10.86% with a correlation coefficient of 0.966. For high temperature and intermediate strain rate, both Model D8A and SCA equation depict the flow behavior of 316L (0.14)N SS with good correlation and generalization. However, at low temperature and high strain rate domain, both the models are unable to depict the behavior; this is attributed to the fact that the material shows two slope behaviors where the constants have been calculated assuming a linear relationship between stress and strain rate.