Marco Boselli

Two-dimensional time-dependent modelling of fume formation in a pulsed gas metal arc welding process

Gas-metal arc welding (GMAW) is a process in which a non-constricted plasma arc is ignited between a workpiece that works as the cathode and a metal wire that works as the anode; the latter is melted by the heat from the arc and a metal transfer occurs from the wire to the workpiece. Following recent studies on health effects for welding workers exposed to fume inhalation, a huge effort is currently being devoted to the investigation of fume formation during the welding process and to the development of processes with lower fume production rate. In the welding process, fumes are generated by nucleation and growth of nanoparticles from the metal vapour coming from evaporating weld pool, droplets and metal wire.Even if experimental try and fail approaches have been adopted to develop operating conditions that induce a lower formation of fumes, modelling is a valuable tool that provides insight on those physical processes occurring during the formation phase that cannot be easily monitored by diagnostics. In this work, the simulation of fume formation in pulsed GMAW process is reported taking into account the metal transport and metal vapour formation in a self-consistent approach using the Volume-of-Fluid (VoF) method and modelling fume nanoparticles production using the method of moments (MoM) for the solution of the aerosol general dynamic equation. While this method has been widely used for the modelling of nanoparticle synthesis in thermal plasma reactors, this is the first attempt to implement the MoM approach in modelling fume formation in pulsed GMAW.

Characterization of a Cold Atmospheric Pressure Plasma Jet Device Driven by Nanosecond Voltage Pulses

The structure, fluid-dynamic behavior, temperature, and radiation emission of a cold atmospheric pressure plasma jet driven by high-voltage pulses with rise time and duration of a few nanoseconds have been investigated. Intensified charge-coupled device (iCCD) imaging revealed that the discharge starts when voltage values of 5-10 kV are reached on the rising front of the applied voltage pulse; the discharge then propagates downstream the source outlet with a velocity around 107-108 cm/s. Light emission was observed to increase and decrease periodically and repetitively during discharge propagation. The structure of the plasma plume presents a single front or either several branched subfronts, depending on the operating conditions; merging results of investigations by means of Schlieren and iCCD imaging suggests that branching of the discharge front occurs in spatial regions where the flow is turbulent. By means of optical emission spectroscopy, discharge emission was observed in the ultraviolet-visible (UV-VIS) spectral range (N2, N2+ , OH, and NO emission bands); total UV irradiance was lower than 1 μW/cm2 even at short distances from the device outlet (<;15 mm). Plasma plume temperature does not exceed 45 °C for all the tested operating conditions and values close to ambient temperature were measured around 10 mm downstream the source outlet.

Advances in Plasma Arc Cutting Technology: The Experimental Part of an Integrated Approach

The experimental part of an integrated approach to design and optimization of plasma arc cutting devices will be presented; in particular results obtained through diagnostics based on high speed imaging and Schlieren photography and some evidences obtained through experimental procedures. High speed imaging enabled to investigate start-up transition phenomena in both pilot arc and transferred arc mode, anode attachment behaviour during piercing and cutting phases, cathode attachment behaviour during start-up transient in PAC torches with both retract and high frequency pulse pilot arc ignition. Schlieren photography has been used to better understand the interaction between the plasma discharge and the kerf front. The behaviour of hafnium cathodes at high current levels at the beginning of their service life was experimentally investigated, with the final aim of characterizing phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert.

Two-temperature modelling and optical emission spectroscopy of a constant current plasma arc welding process

Summary form only given. Plasma arc welding (PAW) is a process where an electric arc is created between a pointed thoriated tungsten electrode and a workpiece to melt it. The electrode is positioned inside the torch body and plasma gas is fed around it. The arc is constricted through a copper nozzle in order to increase the plasma velocity and temperature. A sheath gas is injected axially (usually with a swirl component) and concentrically around the arc, allowing the protection of the weld pool from contamination of the surrounding oxidant atmosphere. The combination of constriction and convection stabilization provides a high speed focused plasma jet with deeper weld penetration and higher energy concentration and resistance to perturbations than the gas tungsten arc welding (GTAW) process.In this paper the Authors compare the results obtained by means of both LTE and non-LTE two-temperature (2T) numerical modelling with the results of OES diagnostics, proposing a method to extend the Boltzmann plot technique to regions where lines s/n is poor and discussing its validity in case of thermal non-equilibrium conditions; also demonstrating how this approach can be effectively used to characterize a plasma source of industrial interest. For this reason modelling and diagnostics activities have been performed on a commercial plasma source torch. A PAW process with constant current in the range 25-70 A operating in pure Ar has been characterized by means of both thermo-fluid-dynamic modelling under the assumption of local thermodynamic equilibrium (LTE) and two-temperature thermal non-equilibrium modelling (2T), allowing a comparison of the LTE temperature fields with electron and heavy particle temperature fields: thermal non-equilibrium is strongest in the fringes of the arc and upstream the plasma flow even though a temperature difference between electrons and heavy particles has been found also in the arc core in the nozzle orifice, due to the high velocity of the gas. Also, excitation temperature of Ar atoms has been obtained from optical emission spectroscopy measurements using a new method (called hybrid method) that extends the usability of the Boltzmann plot method to spatial regions where the signal to noise ratio of the spectral lines adopted in the calculation is poor. A good agreement has been obtained between the modelling predicted electron temperature and the measured excitation temperature in the whole investigated spatial region.

Investigation of the antimicrobial activity at safe levels for eukaryotic cells of a low power atmospheric pressure inductively coupled plasma source

Low power atmospheric pressure inductively coupled thermal plasma sources integrated with a quenching device (cold ICP) for the efficient production of biologically active agents have been recently developed for potential biomedical applications. In the present work, in vitro experiments aimed at assessing the decontamination potential of a cold ICP source were carried out on bacteria typically associated with chronic wounds and designed to represent a realistic wound environment; further in vitro experiments were performed to investigate the effects of plasma-irradiated physiological saline solution on eukaryotic cells viability. A thorough characterization of the plasma source and process, for what concerns ultraviolet (UV) radiation and nitric oxide production as well as the variation of pH and the generation of nitrates and nitrites in the treated liquid media, was carried out to garner fundamental insights that could help the interpretation of biological experiments. Direct plasma treatment of bacterial cells, performed at safe level of UV radiation, induces a relevant decontamination, both on agar plate and in physiological saline solution, after just 2 min of treatment. Furthermore, the indirect treatment of eukaryotic cells, carried out by covering them with physiological saline solution irradiated by plasma, in the same conditions selected for the direct treatment of bacterial cells does not show any noticeable adverse effect to their viability. Some considerations regarding the role of the UV radiation on the decontamination potential of bacterial cells and the viability of the eukaryotic ones will be presented. Moreover, the effects of pH variation, nitrate and nitrite concentrations of the plasma-irradiated physiological saline solution on the decontamination of bacterial suspension and on the viability of eukaryotic cells subjected to the indirect treatment will be discussed. The obtained results will be used to optimize the design of the ICP source for an effective production of reactive species, while keeping effluent temperature and UV radiation at values compatible with biomedical treatments.

High-Speed and Schlieren Imaging of a Low Power Inductively Coupled Plasma Source for Potential Biomedical Applications

High-speed and Schlieren imaging have been used to visualize ignition transients, discharge behavior and flow fields of a plasma device integrating a low-power inductively coupled plasma torch, generating a high temperature thermal plasma, with a quenching device, able to cool the gaseous effluent down to biocompatible temperatures for effective use in biomedical applications.

Investigation of Thermal Nonequilibrium in a Plasma Arc Welding Process: Modeling and Diagnostics

In this paper, a plasma arc welding (PAW) process operating in pure Ar with constant current at 40 A has been characterized by means of a thermofluid-dynamic modeling and optical emission spectroscopy (OES). Excitation temperature of Ar atoms in the fringes of the arc measured using the OES has been found higher than temperature calculated from local thermodynamic equilibrium modeling, even if radiation reabsorption is taken into account using a discrete ordinate model; on the contrary, excitation temperature of Ar atoms is in a good agreement with electron temperature calculated using a two-temperature model for thermal nonequilibrium neglecting radiation reabsorption. Results reported suggest that for the investigated PAW process, thermal nonequilibrium due to steep radial temperature gradients induces a Boltzmann distribution of excited states at the electron temperature.

Comparing the effect of different atmospheric pressure non-equilibrium plasma sources on PLA oxygen permeability

Plasma technology is widely adopted for polymer surface modification. In this work polylactide (PLA) samples have been exposed to the plasma region generated by three different plasma sources operating at atmospheric pressure: a floating electrode dielectric barrier discharge (FE-DBD), a novel linear corona discharge and a DBD roller. The sources have been supplied with a high voltage generator capable of producing pulses with a rise rate in the order of several kV/ns in order to obtain diffuse plasma and avoid local damage to the membrane; air and argon have been used as working gases. Pure oxygen permeation tests in PLA films have been carried out by means of a closed-volume manometric apparatus working at 35°C with a pressure difference of pure O2 of about 1 bar applied across the membrane. Tests have been performed shortly after the plasma treatment and also replicated at different times in order to investigate the durability of surface modification. The effects of voltage, pulse repetition frequency (PRF) and exposure time on the membrane surface characteristics and barrier property have been studied.

Time-Dependent Modeling of Droplet Detachment in GMAW Including Metal Vapor Diffusion

The most advanced models for gas-metal arc welding (GMAW) take into account either the time-dependent droplet formation using volume-of-fluid models or the diffusion of metal vapors evaporating from a fixed wire under steady-state conditions. In this paper, both aspects have been included in an axisymmetric 2-D time-dependent model for a GMAW torch operating in globular transfer mode in an Ar atmosphere at 160 A dc current. Numerical results have been compared with experimental high-speed-camera images.

High-Speed Imaging in PAC: Multiple View and Tomographic Reconstruction of Pilot Arcing Transients

High-speed imaging of the pilot arc transient phase in plasma arc cutting has been carried out synchronizing the use of two high-speed cameras in conjunction with an optical system suitably designed for producing multiple synchronized views of the same phenomenon. Such imaging techniques have allowed a deeper understanding of the pilot arcing process, owing to the simultaneous visualization of anode and cathode attachments and through the development of tomographic reconstruction of the pilot arc.