Américo Scotti

The effect of wollastonite on operational characteristics of AWS E6013 electrodes

The main objective of this work was to assess the operational behavior of ANSI/AWS A5.1-91 E6013 type electrodes when 0, 8 and 16 % of quartz (100 % SiO2) is replaced with wollastonite (Calcium Silicate, 50 % SiO2 - 50 % CaO) in the coating composition. The electrodes were tested through bead-on-plate welds in flat position on DC, both polarities, and on AC currents. Arc stability, fusion rate and deposition rate were used as operational characteristics evaluation criteria. The results suggested that the replacement of quartz with wollastonite, that increased slag basicity, kept, or even improved, the typical excellent operational characteristics of E6013 type electrodes.

Identification and control of metal transfer in pulsed GMAW using optical sensor

Abstract The aim of the present work was to design and develop an identification device for droplet detachments during pulsed GMAW and to assess the possibility of implementing a metal transfer control system. The developed identification device was based on a luminescence sensor and an electronic interface, capable of sensing the arc light flux. A basic control system for the pulse parameters was implemented using an A/D–D/A board for receiving signals from the sensor and sending instructions to an electronic power source and a trained neural network for the metal transfer recognition. Alternatively, the signal from the electronic interface could be directed to a microcomputer with a dedicated monitoring program. Automated pulsed welding was carried out on plain carbon steel, aluminium and stainless steel plates in the flat position. The metal transfers were recorded using a high speed camera and applying the shadowgraphy technique to validate the device performance. The results showed that the detachment device was efficient and consistent for aluminium (best results), stainless steel and low carbon welding. Moreover, the control system was capable of setting parameters that provided stable transfer at one drop per pulse rate, from welds starting at inadequate pulse parameter levels.

Proposal for a modified fowler-milne method to determine the temperature profile in TIG welding at low currents

It is possible to assess the thermal efficiency of welding shielding gases by means of the arc temperature field analysis. Since this approach opens a remarkable study field to assess different shielding gases, giving support for dealing with advantages and disadvantages of commercial gas mixtures, there is a research line under development, which goal is to find techniques to measure arc temperatures. This work describes a proposed method containing different procedures to quantify plasma jet temperature profiles from experimental data. This method was applied on data taken from TIG welding arc, at low current (40 A). This low current was selected to contrast to and amplify the current literature focus, i.e., high currents. The experiment was conducted using emission spectroscopy, picking punctual luminescence from the plasma through an optic system. The TIG arc was stroked on a water-cooled copper plate and shielded by pure argon. The temperature field was determined through the modified Fowler-Milne method. The introduced modification aimed to overcome the limitation that this method has for low radiation intensity presents in low current arcs: the Fowler-Milne method has an intrinsic threshold of 10,000 - 25,000 K. For a 40-A arc, the lower 10,000-K limiting isotherm is reached close to the cathode, restricting the analysis field, especially for the anode region. The proposed modification suggests a linear distribution of the particle density instead of a Maxwellian one, at temperatures below 12,500 K. The experimental temperature field was compared to a previous publication that deals with numerical simulation and the results were found in good agreement, what indicates the supposition of a linear distribution it is not far from the reality.

Models to describe Plasma Jet , Arc Trajectory and arc blow formation in arc welding

Several researchers have devoted efforts on studying physics of arc and descriptive models are used to explain many arc welding related phenomena. However, due to the subject complexity, doubts still emerge about the mechanisms of some phenomena related to the arc. For instance, the description about electromagnetic interactions with the arc, which governs the arc trajectory and lead to plasma jet and arc blow formation, seems to be yet controversial. Thus, the present study aimed a better understanding of these phenomena. An e-mail survey was carried out to confirm discordant descriptions of the phenomena. Some points were raised about the actual physical explanation used in the current literature. Some experimental work with arcs at different levels of current was carried out and shows that plasma jet has an important role in the welding arc trajectory, but there are other factors acting at the same time. Using electromagnetic theory and flow mechanics explanation, more comprehensive models were eventually presented to explain plasma jet formation and the arc blow phenomenon.

A quality and cost approach for welding process selection

The aim of this work was to propose, apply and evaluate a methodical approach to select welding processes in a productive environment based on market requirements of Quality and Costs. A case study was used. The welds were carried out in laboratory, simulating the joint conditions of a manufacturer and using several welding processes: SMAW, GTAW, pulsed GTAW, GMAW with CO2 and Ar based shielding gases and pulsed GMAW. For Quality analysis geometrical aspects of the beads were considered and for Cost analysis, welding parameters and consumable prices. Quantitative indices were proposed and evaluated. After that, evaluation of both Quality and Costs was done, showing to be possible to select the most suitable welding process to a specific application, taking into account the market conditions of a company.

Mapping transfer modes for stainless steel gas metal arc welding

Abstract A methodology for the construction of transfer mode maps for stainless steel gas metal arc welding, with argon and argon–oxygen shielding gases, is presented. A back lighting laser and high speed video camera were used for visualisation and measurement of droplets and electrode extension. The reasons for the use of a groove, instead of the traditional bead on plate method, and of same volume beads are discussed and the results assessed. Unlike in other mapping procedures, mapping was conducted as a function of welding current and arc length. In addition, transfer rate v. welding current or wire feedrate curves were plotted. The results show the importance of the use of both maps and curves for identification and quantification of the shielding gas effects on the transfer mode. The results also suggest that an increase in oxygen content in the shielding gas reduces the values of transition current and transition wire feedrate (as expected), but also that it reduces the transfer rate and droplet size at the globular–spray transition.

Performance assessment of the (Trans)Varestraint tests for determining solidification cracking susceptibility when using welding processes with filler metal

This work is aimed at verifying if the Varestraint and Transvarestraint tests are adequate to assess solidification cracking susceptibility in welding of aluminium when a process with filler metal is employed. The main goal was to compare the performance of a reasonably new process (double pulsed GMAW) for welding aluminium alloys. Both tests and three cracking quantification criteria were analysed and compared to determine which was the most robust and reliable. The results showed that the most adequate ones were the Transvarestraint test and the maximum cracking length criterion. Some steps must be taken so as to guarantee reliability of the results if a small number of experiments are expected. The application of this methodology in comparison between a conventional pulsed GMAW and a double pulsed GMAW still showed that the latter reduces cracking susceptibility, appearing as an alternative solution for this critical phenomenon in aluminium welding.

Investigation on Welding Arc Interruptions in the Presence of Magnetic Fields: Welding Current Influence

Arc interruptions and, therefore, oscillation in the amount of energy and molten wire delivered to the plate have been observed during tandem pulsed gas metal arc welding (GMAW). It appears that these instabilities are related to the magnetic interaction between the arcs. In order to clarify the possible mechanisms involved, this paper tries to mimic the tandem GMAW arc interruptions. External magnetic fields were dynamically applied to GTAW arcs in constant current mode to verify their resistance to extinction as a function of current level and direction of deflection. High-speed filming was carried out as an additional tool to understand the extinction mechanism. The influence of the welding current level on the arc resistance to extinction was established: The higher the welding current, the more the arc resists to the extinction. The arc deflection direction has minor effect, but arcs deflected backward have more resistance to extinction.

Influence of welding current in plasma–MIG weld process on the bead weld geometry and wire fusion rate

One of the versions of the plasma–metal inert gas (MIG) process is basically a combination of a plasma arc with a MIG/metal active gas (MAG) arc in a single torch. With this association, the advantages of each arc are combined. The main characteristic of this is the independence between the heat input by the process and the deposited material, resulting in greater facility to control bead weld geometry. In the current literature, there is a shortage of information related to the process, and most of this goes back to the 1970s and 1980s when the technology available was not able to make the process viable for industry. However, in recent years, the use of the diffusion of new electronic power sources used in welding has sparked up again the interest in plasma–MIG process. In this context, this paper aims to contribute to the studies related to the influence of the MIG and plasma current balance on the geometry of the bead weld and wire fusion rate. Bead-on-plate welds were carried out with plasma and MIG/MAG current combinations at three levels each, keeping, by welding speed corrections, the bead volume the same. It was observed that the introduction of the plasma current over the MIG/MAG current reduces penetration and dilution and leads to convex beads. On the other hand, the use of plasma current increases the MIG/MAG wire fusion rate. However, it seems that the intensity of the plasma current is not the governing parameter of those changes.

A Study on out-of-phase current pulses of the double wire MIG/MAG process with insulated potentials on coating applications: part II

The part 1 of this work presents a study about the use of the out-of-phase current pulses technology applied to the Double Wire MIG/MAG welding process with Insulated Potentials on welded coating. High-speed digital back-lighted images were recorded to evaluate the influence of the levels of out-of-phase current pulses on the behavior of both metallic transfer modes and voltaic arcs configurations. The results showed that, with the electrodes positioned side by side and the mean currents below the transition current, the out-of-phasing parameter setting reduces the deviations of the arcs and changes the paths of the droplets. However, no significant effect was noticed on bead finishing or arc stability.