Effects of Ultrasonic Vibration on the Transport Coefficients in Plasma Arc Welding

In ultrasound assisted plasma arc welding (U-PAW), the exerted ultrasonic vibration on the tungsten electrode interacts with the plasma arc and changes its heat-pressure characteristics. It is of great significance to investigate the underlying interaction mechanism. In this study, the calculation method of transport coefficients in U-PAW is developed. Translational thermal conductivity (including electrons thermal conductivity and the thermal conductivity of heavy particles) and electrical conductivity are calculated by considering the second-order approximation of Maxwell velocity distribution function, while the method of Butler et al. is adopted to calculate the reaction thermal conductivity in U-PAW. The effective value of the ultrasound velocity gradient tensor is employed to describe the effects of ultrasonic vibration on transport coefficients in ultrasound assisted plasma arc. The calculation results show that when the ultrasound is applied, the thermal conductivity of heavy particles in the plasma increases significantly and the electron thermal conductivity increases within some extent. The thermal conductivity of the reaction also increased to a great extent, and the electrical conductivity decreases a little bit. Although the thermal diffusion coefficient also has some increase, but the ordinary diffusion coefficient is obviously reduced due to the application of the ultrasound. With the updated transport coefficients, the plasma arc pressure on the anode surface is numerically computed, and the predicted pressures of PAW and U-PAW can be consistent with the measured ones

Dislocation strain energy based modeling for ultrasonic effect on friction stir lap welding process of dissimilar Mg/Al alloys

Understanding of the ultrasonic effect on the process variables has important significance for the process optimization in ultrasonic vibration enhanced friction stir lap welding (UVeFSLW) of dissimilar Mg/Al alloys. In this study, the dislocation strain energy in both grain interior and grain boundary is considered to improve the formula for determing threshold thermal stress, and the constitutive equation is modified and used to develop the model for Mg/Al UVeFSLW process. The influences of ultrasonic vibration on heat generation, temperature profile, material flow and intermixing are quantitatively analyzed. It is found that the ultrasonic vibration induced the flow stress reduction gets larger, and the interfacial heat generation and viscous dissipation heat generation in the shear layer drop, which results in lower temperature at the tool/workpiece interface, intensifier material flow and larger material intermixed region in Mg/Al UVeFSLW. The interface temperature between the tool/workpiece and the profile of thermo-mechanically affected zone are experimentally measured, and compared with the predicted ones. The results show that the calculation accuracy of the modified model is improved

Numerical Investigation of Arc-Pool-Metal Vapor Behavior in GTAW with an External Magnetic Field

Gas tungsten arc welding with an external magnetic field is proven to suppress weld defects while improving welding speed. A three-dimensional numerical model that considers interactions among the arc plasma, weld pool, metal vapor, and external magnetic field is developed. The influences of the external magnetic field and metal vapor on arc and weld pool behaviors are investigated. The external magnetic field has an important influence on the arc shape and the weld pool flow field. The metal vapor increases the arc radiation loss but decreases the conductivity and local current density

Experimental investigation and 3D-simulation of the ablated morphology of titanium surface using femtosecond laser pulses

The femtosecond laser ablated morphology on titanium surface is investigated theoretically and experimentally. A three dimensional two temperature model (3D-TTM) is used to simulate the surface morphology of titanium sample which is irradiated by femtosecond laser pulses. The electron heat capacity and electron-phonon coupling coefficient of titanium (transition metal) are complex temperature dependent, so the two parameters are corrected based on the theory of electron density of states (DOS). The model is solved by the finite difference time domain (FDTD) method. The 3D temperature field near the target surface is achieved. The radius and depth of the ablated crater are obtained based on the temperature field. The evolutions of the crate’s radius and depth with laser fluence are discussed and compared with the experimental results. It is found that the back-flow of the molten material and the deposition of the material vapor should be responsible for the little discrepancy between the simulated and experimental results. The present work makes a better understanding of the thermodynamic process of femtosecond laser ablating metal and meanwhile provides an effective method tool to predict the micro manufacturing process on metals with femtosecond laser

Investigation on the Metal Transfer and Cavity Evolution during Submerged Arc Welding with X-ray Imaging Technology

The physical phenomena of submerged arc welding (SAW) conducted with a 1.6 mm flux-cored wire were investigated using X-ray imaging technique. Three kinds of metal transfer modes were confirmed in this paper, namely the front flux wall-guided droplet transfer, back flux wall-guided droplet transfer, and repelled droplet transfer, of which the corresponding percentages were 47.65%, 45.29%, and 7.06%, respectively. Although the average sizes of the droplets for SAW and FCAW (flux-cored wire welding) were 2.0 mm and 1.9 mm with an average droplet transfer time of 90.3 ms, it required 36.4% more time for the droplet of SAW to finish one metal transfer than it did in FCAW. In addition, the volume of the cavity was not constant but repeated a cycle mode of “expansion and contraction” during the whole process. Thus, the dynamics of the cavity and viscous resistance caused by the flux collectively slowed down the velocity of the droplets from the wire to the weld pool in SAW. Compared with FCAW, a smoother weld without pits and pores was manufactured during the SAW process. Due to the compression effect of the flux, the 14.5 mm average weld width of SAW was 2.9 mm shorter than that of the FCAW. Furthermore, the thickness of slag with a porous structure in SAW was 2.7 times of that in FCAW, indicating that it could provide better protection to the weld of SAW

Numerical Simulation of Arc and Droplet Behaviors in TIG-MIG Hybrid Welding

Tungsten inert gas-metal inert gas hybrid welding (TIG-MIG) combines the advantages of tungsten and metal inert gas welding. It can efficiently produce high-quality weld joints that meet modern manufacturing quality and efficiency requirements. Based on heat transfer, fluid dynamics, and electromagnetic theory, a three-dimensional coupled transient model of arc-droplet interactions in TIG-MIG hybrid welding was established. In this study, the temperature field, flow field, electromagnetic force, pressure, and current density parameters were analyzed in the arc space. The results show that introducing TIG welding has a significant impact on MIG welding

Enhanced Uptake of Iodide from Solutions by Hollow Cu-Based Adsorbents

Cu2O exhibits excellent adsorption performance for the removal of I− anions from solutions by doping of metallic Ag or Cu. However, the adsorption process only appears on the surface of adsorbents. To further improve the utilization efficiencies of Cu content of adsorbents in the uptake process of I− anions, hollow spheres of metallic Cu, Cu/Cu2O composite and pure Cu2O were prepared by a facile solvothermal method. Samples were characterized and employed for the uptake of I− anions under various experimental conditions. The results show that Cu content can be tuned by adjusting reaction time. After the core was hollowed out, the uptake capacity of the samples increased sharply, and was proportional to the Cu content. Moreover, the optimal uptake was reached within only few hours. Furthermore, the uptake mechanism is proposed by characterization and analysis of the composites after uptake. Cu-based adsorbents have higher uptake performance when solutions are exposed to air, which further verified the proposed uptake mechanism. Finally, hollow Cu-based adsorbents exhibit excellent selectivity for I− anions in the presence of large concentrations of competitive anions, such as Cl−, SO42− and NO3−, and function well in an acidic or neutral environment. Therefore, this study is expected to promote the development of Cu-based adsorbents into a highly efficient adsorbent for the removal of iodide from solutions

Impoundment led to spatial trophic segregation of three closely related catfish species in the Three Gorges Reservoir, China

The construction of the Three Gorges Reservoir (TGR), the largest dam in the world, created novel spatial and seasonal fluctuations in food availability along the Yangtze River. We studied changes in diet and niche overlap of three co-occurring catfish species between the upper and lower sections of the TGR and across four hydrological seasons to investigate whether the diets of native catfish species reflect the fluctuations in food availability. Zoobenthos dominated the diets of the three species in the upper section, but shrimp made the bulk of the diet in the lower section. This was the case in every season except during the rising-water season when all three species increased the consumption of allochthonous foods. Diet breadth was significantly broader in the upper section relative to the lower section, and expanded during the rising-water season in the lower section. Niche overlap was significantly lower in the upper section than in the lower section. A non-significant trend of reduced niche overlap during the rising-water season was observed in both sections. Our results suggest that, in less than a decade, dietary plasticity facilitated the adaptation of these species to novel habitats and a shifted food-resource base formed by the impoundment

Spatial and interspecific comparisons of the reproductive biology of two species of co-occurring freshwater shrimps (Decapoda: Caridea: Palaemonidae) in the Three Gorges Reservoir, China

Exopalaemon modestus (Heller, 1862) and Macrobrachium nipponensis (De Haan, 1840) (Palaemonidae) are ecologically and commercially important fishery resources in the Three Gorges Reservoir (TGR), China, and co-occur in demersal habitats. Little information is available on their biology. We compared the reproductive traits of both species on the upper, middle, and lower sections of the TGR. Both species differed in mean size at maturity, which increased from upper to middle and lower sections. Sex ratios were significantly biased toward females, but no significant differences were observed among the three sites for either species. Both species displayed an obvious trade-off between embryo size and fecundity, with the size of embryos being the smallest but with the highest fecundity at the upper section, followed by the middle and lower sections. Exopalaemon modestus spawned from early April to late September, with a peak in May and June, whereas M. nipponensis spawned from late April to early October with a peak in June and July. The spatially distinct reproductive traits, which were concordant across species, are likely related to differences in habitat characteristics such as water flow. We suggest that management strategies should account for the reproductive biology of the two species in order to maintain a sustainable fishery in the reservoir

Effect of ambient pressure on a femtosecond laser induced titanium plasma

Femtosecond laser induced Ti plasma has been characterized as a function of pressure by means of femtosecond laser induced breakdown spectroscopy (fs-LIBS). Experiments were performed with a Ti: sapphire laser system (100 fs, 800 nm), in an air pressure from 10 Pa to 104 Pa. The time-resolved spectrum has been acquired and the spectral intensities of different plasma species have been investigated with a changing ambient pressure. The Ti atomic lines decay while the ionic ones grow with an increasing pressure. The enhancement of nitrogen ionic line has also been observed. The time of flight spectroscopy is adopted to measure the expanding velocity of the plasma plume. The increasing pressure slows the plasma expansion along both axial and radial directions. The electron density and temperature are measured by means of Boltzmann plot method and Stark width method, respectively. It is concluded that higher pressure will increase the energy absorption and retard the plasma expansion, leading to larger electron density and temperature