Ruham Pablo Reis

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.

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.

Arc Interruptions in Tandem Pulsed Gas Metal Arc Welding

In addition to electromagnetic attraction between the arcs in Tandem Pulsed gas metal arc welding (GMAW), arc interruptions, mostly in the trailing arc at low mean current levels, may also occur, which is a phenomenon not widely discussed in the welding field. These arc interruptions must be avoided, since they also represent interruptions in metal fusion and deposition during the welding process, leading to lack of fusion/penetration and/or deposition flaws, adding cost for repairing operations. To improve the understanding on arc interruptions in Tandem Pulsed GMAW and how the current pulsing synchronism between the arcs relates to this phenomenon, this work proposes to evaluate the influence of parameters of adjacent arcs (Tandem Pulsed GMAW) and also of a single arc (GTAW—gas tungsten arc welding), but similarly subjected to magnetic deflection, on the occurrence of arc interruptions/extinctions. High-speed filming was used to help understand the interruption/extinction mechanism. In the case of Tandem Pulsed GMAW, the pulses of current of the leading and trailing arcs need to be almost-in-phase to prevent interruptions in the trailing arc. The distance of 10 mm between the adjacent arcs helped reduce the incidence of trailing arc interruptions, yet keeping a sound weld visual quality. In the case of GTAW, the higher the electrical current flowing through the arcs and the shorter their lengths, the more they resist to the extinction. The trailing arc interruptions in Tandem Pulsed GMAW seem to be determined by the deflection and heat in this arc, and their prevention can be achieved by a balance between these two factors, which is reached by synchronized pulsing currents.

Investigation on Welding Arc Interruptions in the Presence of Magnetic Fields: Arc Length, Torch Angle and Current Pulsing Frequency Influence

Arc interruptions have been observed in tandem pulsed gas metal arc welding (GMAW). This fact, which is likely related to magnetic interaction between the arcs, motivated previous study concerning the influence of the welding current on this phenomenon. In order to promote further understanding, this paper investigates the effects of arc length, torch angle, and high-frequency current pulsing on the arc resistance to extinction. To mimic the situation found in tandem GMAW (magnetic field induced by one arc acting on the other arc), external magnetic fields were applied to gas tungsten arc welding arcs. It was verified that short arc lengths and torch angles set to push the weld pool increase the arc resistance to extinction, whereas the utilization of high-frequency current pulsing tends to weaken the arc resistance to extinction. According to a model devised from the results, the arc extinction takes place if the heat generated inside and the heat transferred into the arc column become insufficient to counterbalance the total heat loss in this arc region.

Preliminary evaluations on laser - Tandem GMAW

Recently there has been considerable research and development activity in the use of lasers for welding operations, turning this process into an important tool for a variety of applications. Although it is possible to use lasers as a unique source of heat to promote union of materials, the combination of the beam provided by a laser system with an arc welding process has been studied widely and applied in the so-called hybrid welding systems. Generally, the final result of such a combination is an increase in the weld penetration depth, width and welding travel speed. Despite these advantages, there are many issues still requiring further research and development concerning the use of hybrid welding using laser and arc welding, including a more comprehensive understanding of the various welding phenomena involved and the exploitation of new combinations. This paper describes an approach for hybrid welding combining a laser with tandem G MAW, in particular placing the laser beam between the tandem GMAW wires. This hybrid process variation is described and some basic aspects regarding its performance are discussed. The laser beam was found to have a positive effect on the appearance of the weld beads produced and best results are obtained if the laser is located halfway between the leading and trailing wires. A 10 mm inter-wire distance was found to be the most appropriate of the separation distances tried. The hybrid process approach was able to increase the welding travel speed or penetration depth significantly in comparison with tandem GMAW (operating in pulsed mode).

Initial assessment of a new approach for laser: tandem GMA welding

Recently a large number of research and development has been made regarding the use of Laser in welding operations, turning this process into an important tool for a variety of applications. Although it is possible to use Laser as a unique source of heat to promote union of materials, the combination of the beam provided by a Laser system with an arc welding process has become largely studied and applied in the so called hybrid welding. Generally, the final result of such combination is an increase in the weld penetration depth, width and welding travel speed. Despite all these facts, there are many issues still requiring further research and development concerning the use of hybrid welding using Laser and arc welding, including more comprehensive understanding on the various welding phenomena involved and the exploitation of new combinations. This paper describes a new approach for hybrid welding combining LBW with Tandem GMAW (Laser beam placed between the Tandem GMAW wires). The first view on such hybrid process variation is described and some basic aspects regarding its performance are discussed. The Laser beam had a positive effect on the aspect of the weld beads produced and a 10 mm inter-wire distance showed to be the most appropriate among the distances tried. Key words: hybrid welding, LBW, tandem GMAW.

Evaluation of the Shielding Gas Influence on the Weldability of Ferritic Stainless Steel

The use of stainless steels has been nowadays widespread in a number of industrial sectors. They usually offer exceptional performance regarding mechanical and corrosion properties, but according to Lee et al. (2008) stainless steels are considered as high cost materials as far as solutions for structural engineering are concerned. However, this material group can provide aesthetic characteristics as well as outstanding versatility, easy cleaning and maintenance conditions. Nevertheless, there are still plentiful possibilities for applying stainless steels in new situations or improving their use in current applications due to their appealing visual aspect and durability. In the automotive industry, for instance, parts of the exhaustion system are in general composed of tubes and blanks (stamped metal sheets) that usually are welded and have ferritic stainless steels as the main base material. According to Alves et al. (2002), the main ferritic stainless steels used in the hot portion of automotive exhaustion systems are the AISI 409 and 441. On the other hand, in the cold portion the AISI 409, 439 and 436 are normally utilized. Faria (2006) states that automotive exhaustion systems went through a number of changes along the last 20 years as a consequence of more restrict pollution policies, needs for longer durability and higher engine efficiencies as well as requirements for reduction in weight and costs. Stainless steels used in the hot parts of automotive exhaustion systems, according to Sekita et al. (2004), must be refractory, which can be accomplished by niobium additions, high levels of molybdenum and optimized silicon presence. The same authors also mention the importance of having a good formability in such hot parts. The market for stainless steels has experienced constant growth because of their excellent properties and continuous improvement in manufacturing of these materials, especially when issues like increase in process productivity and reduction in costs are taken into consideration. However, recently there has been a sharp increase in the international prices of alloying elements largely used in stainless steels, mainly nickel and molybdenum. As a result, the most traditional stainless steels class (austenitic) went through severe price rise worldwide. Fortunately, the ferritic class, which contains no nickel, emerges as an alternative for some applications, but sometimes some drawbacks have to be figured out before replacing the austenitic class.