Papers by Keyword: Helium

Abstract: The diffusion process of multiple He atoms in W is simulated by a molecular dynamics (MD) method with the W-H-He analytic bond-order potential. The diffusivities of different number of helium (He) atoms in W are determined by the mean squared displacement (MSD) method at different temperatures. The diffusivity-temperature (D-T) relationship is fitted to the Arrhenius equation to obtain the pre-factor and the diffusion barrier. Under the temperature of 1200K He atoms diffuse together, and above 1200K they separate from each other. When the number of He atoms is greater than three, all He atoms oscillate at the tetrahedral interstitial site (TIS) instead of diffusing under 400K. In the temperature range of 400-1200K, the diffusion barriers of He atoms, the number of which is from two to five, are 0.098, 0.170, 0.125 and 0.112eV, respectively. Contrasting with one He atom (0.058eV), the higher diffusion barriers reflect a greater difficulty in diffusion of multiple He atoms in W. In addition, when the number of He atoms is over five, vacancies are formed in W, and He atoms occupy the vacancies.

Abstract: Procedures for welding duplex stainless steel conventional arc welding methods were studied by several authors and nowadays and recently they are well handled. Todays practice requires application of processes providing excellent weld joint quality, quick production and possibility of automation. Therefore the application of laser beam welding has a great potential of application. This research paper describes weldability of DSS 2205 by laser beam. The main factor examined in the process of welding duplex steel is the shielding gas and his effect on weld joint properties. Welding was performed with gaseous CO₂ laser machine Ferranti Photonics AF 8 having 8 kW max. output power and wave length 10.6 µm. Microstructure of weld joint specimens was analysed in order to observe the phase composition ratio. Samples are welded with same process parameters except of shielding gas. Helium and nitrogen were applied as a shielding gas to examine their influence in comparison to weld joints welded without shielding gas. Since the nitrogen promotes the austenite formation, its application brought the best results having almost same phase composition as base material.

Abstract: First-principles calculations are performed to investigate the Young’s modulus, bulk modulus, shear modulus and Poisson’s ratio of hexagonal phase ScA_x (A=H, He), where x=0, 0.0313, 0.125 and 0.25, represent the ratio of interstitial atoms A (A=H, He) to Sc atoms. The influences of hydrogen concentrations and helium concentrations on elastic modulus and Poisson’s ratio of ScA_x (A=H, He) have been studied. The results indicate that hydrogen and helium have different effects on the elastic modulus of hexagonal phase scandium. The change mechanism of the Poisson’s ratio with the variation of the x ranging from 0 to 0.25 has also been studied in hexagonal phase ScA_x (A=H, He). In addition, the changes in the charge densities of ScA_x due to the presence of hydrogen and helium have been calculated.

Abstract: This study investigated the effects of carrier gases (He and N₂) on the microstructure and properties of Ti coating layers to manufacture high-density Ti layers using the cold spraying. Cold spray coating layers manufactured using He had denser and more homogenous microstructures than those produced using N₂. Layers manufactured using He gas showed porosity of 0.02% and hardness of 229HV, which are better properties than the porosity (0.4%) and hardness (177HV) of N₂ coating layers. The bond strength of coating layers manufactured using He recorded higher value of 74.28MPa than that manufactured using N₂. This could be attributed to the fact that, when coating layers are manufactured using He gas, the powder can be easily deposited because of its high particle impact energy, facilitating the plastic deformation of the particles.

Abstract: Helium is widely used as mixing with argon for a shielding gas in GMAW process of Aluminum in order to improve weld quality and increase heat transfer to the weld pool. It has been known that helium could affect metal transfer behavior; however, its behavior has not been well understood. In this study, an analysis of the metal transfer behavior in the GMAW of aluminum was studied. The main objective is to study the effect of Helium on metal transfer in two main regions, short circuit (low welding current region) and spray transfer (high current region). The composition of 5 types of shielding gases were pure argon, 75%Ar + 25%He, 50%Ar + 50%He, 25%Ar + 75%He and pure helium. The welding parameters were fixed at 90A/17.0V, 100A/18.2V, 140A/24.6V and 180A/27.6V. Aluminum plates were welded bead-on-plate in a flat position. The metal transfer behavior was analyzed by using acoustic signals and arc voltage signals. For the result, at low welding current of 90A and 100A with pure argon, short-circuit transfer mode was observed. Adding helium in gas mixture gave no effect in metal transfer mode in low welding current regions but the metal transfer rate was slightly increased. At high welding currents of 140A and 180A with pure argon, spray transfer mode was observed and when increasing helium in gas mixture resulted in changing from spray transfer to combined mode of spray-globular. In these high welding currents, adding helium in gas mixture resulted in decreasing the metal transfer rate since helium gas tended to promote globular metal transfer. Acoustic signal and arc voltage signal can be used effectively in determining modes of metal transfer.

Abstract: Polycrystalline Fe 99.95 and 99.5 samples were implanted with helium at 8 keV and 3 MeV. Thermal Desorption Spectroscopy (TDS) and Nuclear Reaction Analysis (NRA) provided a complementary set of techniques to characterize helium-materials interactions within two different implantation depths, respectively close to the surface and in the bulk. Using TDS, it was possible to get information about the nature and the states of the structures where helium was trapped in radiation damaged Fe specimens. Activation energies for every trapping site (mono-vacancies, clusters) have been determined from conventional reaction model. The effect of interstitial carbon was also discussed, and compared with previous ab-initio studies. Moreover, the helium bulk diffusion constants in radiation damaged structures could be derived from non destructive ³He depth profiling. Preliminary observations highlighted that a few part of He remained trapped while helium bubbles migrated in the bulk.

Abstract: High helium contents will be generated within minor actinide doped uranium dioxide blankets which could be used in fourth generation reactors. In this framework, it is essential to improve our understanding of the type of damage which a pellet could incur as a result of extensive helium build-up. This paper is an attempt at tackling this issue. Sintered uranium dioxide disks have been implanted with helium ions then annealed at various temperatures. Above a concentration of 0.4 at.% and above 1000°C, optical images of the sample surface revealed swollen grains and extensive areas which have exfoliated. Nuclear reaction microanalyses and atomic force microscopy observations were performed to demonstrate that helium has substantially precipitated within the swollen grains. Massive precipitation of the gas leads under these conditions to sample surface blistering which appears to precede flaking. Deuterium ion irradiations have also been performed at ambient and a direct flaking of the sample surface was observed, but for this phenomenon to be observed required much higher doses than in the He study, indicating that temperature could be an essential ingredient for gas to migrate and cause extensive precipitation. Such phenomena could possibly lead to degradation of the fuel.

Abstract: The detrimental laves formation in fusion zone during welding of Inconel 718 is controlled with compound current pulsing technique along with helium shielding gas. Also solid solution filler wire is used to minimize the niobium segregation. Welds were produced in 2mm thick sheets by GTA welding process and subjected to the characterization techniques. The results show, refined fusion zone microstructure, reduced amount of laves phase, minimum niobium segregation and softer fusion zone in the as welded condition.

Abstract: In this paper, the Gas Metal Arc Welding (GMAW) process is studied and temperature field is gained in this process. Thermal effects of Gas Metal Arc (GMA) and temperature field from it on workpiece (copper) and shielding gas type, is the main key of process optimization for GMAW. Energy source properties of GMA strongly depend on physical property of a shielding gas. In this paper, carbon dioxide (CO₂) was used as an alternative gas for its low cost. The basic energy source properties of CO₂ GMA were numerically predicted ignoring the oxidation of the electrodes. It was predicted that CO₂ GMA would have excellent energy source properties comparable to that of He, Ar GMA. The numerical results show the time-dependant distributions of arc pressure, current density, and heat transfer at the workpiece surface are different from presumed Gaussian distributions in previous models.

Abstract: Low-temperature diffusion of Cr, Mo, Ni, Pd, Pt, and V in silicon diodes is compared in the range 450 - 800 oC. Before the diffusion, the diodes were implanted with high-energy He2+ to assess, if the radiation defects enhance the concentration of metal atoms at electrically active sites and what is the application potential for carrier lifetime control. The devices were characterized using AES, XPS, DLTS, OCVD carrier lifetime and diode electrical parameters. The metal atoms are divided into two groups. The Pt, Pd and V form deep levels in increased extent at the presence of radiation defects above 600 oC, which reduces the excess carrier lifetime. It is shown as a special case that the co-diffusion of Ni and V from a NiV surface layer results fully in the concentration enhancement of the V atoms. The enhancement of the acceptor level V-/0 (EC 0.203 eV) and donor level V0/+ (EC 0.442 eV) resembles the behavior of substitutional Pts. The second group is represented by the Mo and Cr. They easily form oxides, which can make their diffusion into a bulk more difficult or impossible. Only a slight enhancement of the Cr-related deep levels by the radiation defects has been found above 700 oC.