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Gas shielded arc welding (MIG, MAG and TIG)
Published in Andrew Livesey, Alan Robinson, The Repair of Vehicle Bodies, 2018
The shielding gases used in the MIG/MAG and TIG welding processes perform several important functions: Protection from atmospheric contaminationArc support and stabilizationControl of weld bead geometryControl of weld metal properties. It is necessary to prevent contamination of the weld pool by atmospheric gases which cause deterioration of the weld bead quality, by surface oxidation, porosity or embrittlement. In the consumable electrode process MIG/MAG, it is also necessary to consider the potential loss of alloying elements in the filler wire owing to preliminary oxidation in the arc atmosphere. In TIG welding, oxidation of the non-consumable tungsten electrode must be prevented. For these reasons most welding shielding gases are based on the inert gases argon (Ar) and helium (He). Active gases such as carbon dioxide (CO2), oxygen (O2) and hydrogen (H2) may be added to the shielding gas to control one or another of the functions stated, but the gas chosen must be compatible with the material being welded.
Surface Preparation
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
The other welding defect that should be noted is welding porosity (see Figure 2.13), meaning the presence of cavities in the welds caused by freezing the gas from the weld pool. The definitions of different surface preparations with regard to the ISO 8501-3 standard and weld porosity are as follows: P1 grade means that the weld porosities will remain as they are without any surface treatment. P2 grade surface preparation means that the surface pores should be open and visible, as illustrated in number 1 in Figure 2.13. Having open and visible pores allows for the penetration of paint. The highest grade of steelwork preparation is P3, in which the metal surface should be free from porosity, including visible pores.
Formation of Weld and Deposited Metal
Published in German Deyev, Dmitriy Deyev, Surface Phenomena in Fusion Welding Processes, 2005
As follows from Equation (1.1), duration of weld-pool existence, τex, becomes longer with the increase of weld-pool length. On the other hand, the weld-pool length depends primarily on thermo-physical properties of the metal being welded and the power of the used heat source. Therefore, in welding of a certain metal, the duration of weld-pool existence may be increased by using multi-electrode welding, preheating the part, or lowering the welding speed. The first of these techniques is used, mainly, in welding of very long rectilinear welds, for instance in the case of large-diameter pipes, the second, for comparatively small items, and the third technique is not optimal, as it lowers the process efficiency.
A robust PIλDµ controller for enhancing the width of the molten pool and the tracking of welding current in gas metal arc welding (GMAW) processes
Published in International Journal of Modelling and Simulation, 2023
Noureddine Hamouda, Badreddine Babes, Amar Boutaghane, Cherif Hamouda
The considered GMAW system consists of six basic pieces of equipment including the welding power supply, the wire feeding unit, the shielding gas, the welding torch, the workpiece, the welding arc, and the material transfer process. A schematic diagram of such a system is depicted in Figure 1. As shown, the process is considered having four inputs: the open-circuit voltage (Voc), the wire feed speed (ve), the contact tube to the workpiece (H), and the torch travel speed (v), respectively, and two measured outputs: the arc current (I) and the weld pool width (w), respectively. The process utilizes a continuous metal wire electrode coiled on a spool that is fed through the wire feeder to the welding torch. At this point, the electrical current from the power source is transferred to the electrode through the contact tube. The wire then encounters the electric arc. The arc is maintained between the electrode and the workpiece and is controlled by the inverter power source. The heat produced from the arc melts both the workpiece and the electrode, creating a molten weld pool. Because the process is traveling along the workpiece, the weld pool solidifies once it is out of the heating influence of the arc.
Analysis of mechanical properties and optimization of tungsten inert gas welding parameters on dissimilar AA6061-T6 and AA7075-T6 by a response surface methodology-based desirability function approach
Published in Engineering Optimization, 2023
Md Saquib Bin Reyaz, Amar Nath Sinha
The purpose of PE is to understand how much a joint can be stretched as a percentage of its original dimensions before it breaks during a tension test. It ensures the ductility of the joint. The more the~joint is elongated, the more ductile it becomes. In general, the ductility of welded plates permits the redistribution of loads prior to breakage (Koli, Yuvaraj, and Aravindan 2021). Figure 9 illustrates the 3D response surface graphs and contour plots for PE. The impact of welding current (I) and travel speed (TS) on PE is depicted in Figure 9(a). It is evident that as the intensity of the welding current is increased, the PE of the welded joint increases. However, PE is decreased with the increase in travel speed. The results for PE are consistent with recent work from Koli, Yuvaraj, and Aravindan (2021), in which a welding process was optimized and validated to obtain superior joint quality. As the welding current varies with heat input directly and travel speed indirectly, an increase in the welding current or a decrease in travel speed enhances the heat input. Thus, more energy is transmitted per unit length to the welded joint. This increased energy leads to an increase in the weld penetration depth, which results in enhanced ductility. Moreover, an increased heat input helps to avoid or reduce the formation of voids (Adalarasan and Santhanakumar 2015), which results in easier deformation of the material when a tensile load is applied, and a larger percentage of elongation. Figure 9(b) illustrates the variation in welding current (I) and gas flow rate (Q) with PE, while Figure 9(c) demonstrates the effects of TS and Q on PE. The impact of flow rate on PE is similar to that seen in the case of UTS, i.e. with an increase in the gas flow rate, the PE of the weld joint increases significantly. By increasing the gas flow rate during welding, the weld pool can be better shielded, minimizing the weld defects and improving the arc stability. This, in turn, facilitates a more uniform penetration depth for the weld. This ensures the ductility of the welded joint. It can be deduced that increasing the welding current led to a higher PE for the joint, whereas increasing the travel speed had the opposite effect. Also, PE increased marginally as the gas flow rate increased and vice versa.
The influence of the welding circuit magnetic field on the formation of the joint at unsupported welding
Published in Welding International, 2019
B. V. Sitnikov, V. P. Marshuba
The use of external magnetic fields for retaining the weld pool and for producing welds with a specified geometry, without a sharp transition from the base metal to the weld, for increasing the thickness of components welded in one pass and for increasing welding productivity, is of considerable interest. The welding current passes through the liquid metal of the weld pool. If a transverse magnetic field, penetrating the liquid metal, is introduced into the weld pool, the interaction of the welding current with the magnetic field will create bulk electrodynamic forces in it, directed upwards or downwards depending on the relative direction of the magnetic field and the welding current. However, practical implementation of this technique is difficult, because the transverse magnetic field introduced into the weld pool acts upon the arc, and at a low magnitude of magnetic field intensity, insufficient for retaining the liquid metal, extinguishes the arc. To prevent this, two transverse magnetic fields are introduced into the weldment: one behind the arc, into the weld pool region, and the other, identical, in front of the arc, but of the opposite direction [4]. The device for applying these fields consists of two electromagnets, positioned above the weldment. When welding with direct current, the windings of the electromagnets are supplied with direct current, and when welding with alternating current they are supplied with industrial-frequency alternating current. The windings of the electromagnets may also be powered by the welding current. The magnetic field intensity, and therefore also the magnitude of the bulk electrodynamic forces in the weld pool are regulated by varying the current in the windings of the electromagnets, or when supplying the windings of the electromagnets with the welding current – by varying the number of turns of the windings. Owing to absence of direct contact of the devices for magnetic retention of the liquid metal with the blanks being welded, this method is not sensitive to the presence of differences in height of the edges, curvature of the blanks and other factors that hamper the use of spacers, chambers etc. However, the conventional designs of torches for welding with magnetic retention of the liquid metal have relatively large dimensions, impairing the view of the welding zone, hampering the manoeuvrability of the welding tool and limiting the overall potential of the method.