WELDING PIPELINES WITH MODIFIED MIG/MAG PROCEDURES
Marko Horvat, Veljko Kondić, Marko Đunđer, Ivan Samardžić
Keywords: pipeline welding, MIG/MAG process
ABSTRACT:/strong>
The MIG/MAG (GMAW) welding process is an electric arc fusion welding process in which an electric arc is formed between a continuously fed wire (electrode) and the base material in a protective atmosphere of inert/active gases or gas mixtures. The paper presents the basics of the MAG/MIG welding process, some of the modern modifications of the MAG/MIG process, and examples of adapted MIG/MAG processes for welding pipe joints.
1. INTRODUCTION
MIG/MAG – Metal Inert Gas/Metal ActiveGas (eng. GMAW – Gas Metal Arc Welding) is an electric arc welding procedure with a fusible electrode in a protective atmosphere of inert or active gas, and is considered one of the most common welding procedures. Although the history of the development of this welding process goes back to the 40s of the 20th century, the need to increase the productivity and quality of welding, i.e. welded structures, contributed to the active development and modification of all welding processes, especially the MIG/MAG process. [1-6] Investing in the development of welding procedures, which also results in an increase in productivity, is a consequence, according to source [7], of the scope of production processes related to welding (it is believed that in Europe, almost 2,000,000 jobs are related to welding) , ieproduction processes that annually create an added value of 1600 billion Euros/p>
2. THEORETICAL BASIS OF MIG/MAG WELDING
The conventional MIG/MAG process is an electric arc welding process, in which an electric arc is established between a fusible, continuous electrode in the form of a wire and a workpiece, as a rule, connected to the + (plus) pole of a direct current source for welding. The welding process takes place in a protective gas atmosphere provided by inert gases (Ar, He –MIG process) or active gas CO2 and gas mixtures, for example 82% Ar + 18% CO2 (MAG process). [1-6] Basic process in MIG /MAG process is the transfer of metal (molten droplets) from the tip of the electrode (additional material) to the base metal (welded joint). There are four basic mechanisms of metal transfer, and all today's modified procedures of this welding method are based on one of them. [1-6] the classification of metal transfer mechanisms is shown in figure 1.[2]
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Figure 1. Basic mechanism of metal transfer in MIG/MAG welding
The transfer of metal is achieved either by a short circuit between the electrode and the work piece, or the droplet separates before establishing a short circuit, which is made possible by the large amount of energy introduced into the process. Application, productivity and quality of the welded joint in MIG/MAG welding didirectly depends on the applied metal transfer mechanism.
2.1 Transfer of metal by arcing
In metal transfer by arcing, the molten droplet from the tip of the electrode is transferred in free flight through the atmosphere of the electric arc. Transfer is a consequence of a large amount of energy in the process (transfer takes place at high welding currents, eg with conventional MIG/MAG processes at currents greater than 180 A). A higher welding current facilitates the realization of the "pinch-effect" and reduces the forces in the electric arc that have an unfavorable influence on the separation of the droplet and its transfer (reactive forces, surface tension forces). Also, this type of metal transfer takes place exclusively in gas mixtures with a maximum active gas content of 18%, which enable adequate ionization in the electric arc column. The main characteristic of this type of metal transfer is the large amount of molten material per unit of time. The transfer of metal by the sparking arc is shown in figure 2.[1-6]
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Figure 2. Representation of metal transfer by a sparking arc
2.2 Metal transfer by pulsed arc/strong>
Metal transfer by pulsed arc in Figure 3 is characterized by non-contact metal transfer with reduced heat input. The amount of the average welding current is less than the amount of jakosand arc welding currents. The welding current source alternates the current between a maximum value (which allows the droplet to separate at the tip of the electrode) and a minimum value (the value which keeps the arc stable) at regular time intervals. The stated values are also the welding parameters that change according to the construction requirements, and the main advantage of this type of metal transfer is the lower heat input in the process, which enables the possibility of application in thinner positions (compared to the sparking arc). [1-6]
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