Pulse Arc Welding in Narrow Gap

A.G.Krampit, N.Yu. Krampit, Yurga Technolog Institute (branch) of Tomsk Polytechnic University

(YuTI TPU, Yurga, Russia) Corresponding - akrampit@mail.ru

Abstract—The article shows the possibilities of the groove welding process in carbon dioxide under the impulse arc power supply. The method makes it possible to reduce the extent of splashing, enables the control of the transfer of electrode metal and weld formation, improvement of the technology of narrow-gap welding, the possibility of carrying and welding in all spatial position is, improvement of the quality of welded joints, and a decrease in the probability of the defect formation. In the application of powerful pulses of welding current the process of solidification of the welded joint is affected resulting in the refining of the structure of deposited metal.

Keywords-pulse arc power, weld joint, narrow gap

I. INTRODUCTION The main trend in welding engineering development is

solving the national problems of energy efficiency, efficient use of resources and solving the ecological problems through developing and implementing latest technologies [1]. That’s why the problem of developing highly efficient resource-saving technologies is one of the vital tasks of welding engineering. Welding technology in narrow gap is one of technologies kind as application of it methods allows to reduce the amount of weld metal and welding consumables.

The researches got interested in groove welding in the 1960s of the last century when they studied submerged arc welding of plate metal without edge preparation with root opening. Groove gas-shielded welding was first presented in work [2]. During 50 years central journals published articles on the topic [3-14].

Together with advantages groove welding has a number of significant disadvantages which reduce its wide adaptability for producing weld constructions: the necessity to keep the electrode exactly along the centre of the groove; the necessity to maintain constant stick-out distance while filling the narrow groove; the highest possibility of weld defects; reduction of arc stability; exclusive standards for the quality of joint; special requirements to the welding head design.

These shortcomings can be eliminated by the application of the method of consumable electrode pulsed-arc carbon dioxide welding using a long arc. The method makes it possible to reduce the extent of splashing, enables the control of the transfer of electrode metal and weld formation, improvement of the technology of narrow-gap welding, the possibility of carrying and welding in all spatial position is, improvement of

the quality of welded joints, and a decrease in the probability of the defect formation. The process of pulsed-arc welding with a long arc is also characterised by high spatial stability and low sensitivity to the effect of electromagnetic masses. In welding with the pulsed power supplied through the welding and, the liquid metal of the weld pool is subjected to the active defects, resulting in the vibration or movement of the molten metal in the pool which has a beneficial effect on the process of gas generation; the density of the welded joint is higher. In the application of powerful pulses of welding current the process of solidification of the welded joint is affected resulting in the refining of the structure of deposited metal.

The article shows the opportunity of application welding in narrow gap process under pulse arc power supply.

II. THE BASIC PART The essence of the welding process with the impulse arc

power supply is in piling up high-power (up to 1200 A) short-time impulses of welding current on the pilot arc current (30 A) (Fig. 1). As a result direct transfer of electrode metal from the electrode tip into the weld pool is completed. The duration of the piled-up impulses is between 2.5÷10 ms, the rate of the weld current increase is 50÷100 kA/s. The principle “pulse-bead” being followed in the area of controlled transfer the pulse recurrence frequency is 30÷130 Hz.

Figure 1. Oscillograms of current and voltage of the welding process under the impulse arc power supply

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The impulse power supply system (power supply + impulse source) must meet the following requirements:

- open-circuit voltage over 55 V; - ability to provide the high rate of the welding current

increase. When welding in a narrow gap, due to the peculiarities of

edge preparation, to film the process of movement of the molten metal weld pool in a longitudinal arrangement is impossible, therefore, to use high-speed shooting arc gap and the weld pool camera installed in front (Fig. 2).

The groove welding process in СО2 under pulse power supply goes on as follows: at the end of the pause the welding current is 10-50 A, thus, the contacted area and the action force of the arc effecting the weld pool surface are not large (Fig. 2, frame 1). As a result the weld pool surface has a slight indentation under the arc.

Figure 2. Film footage of the welding process in the narrow gap

With the increase of the welding current the arc column extends. Bead transfer from the electrode tip into the weld pool is completed during the high-power current impulses piled up on the arc and extending the arc column. As a result the axial component of the electrodynamic force appears and draws the bead from the electrode tip into the weld arc, thus, enabling its detachment and coaxial transfer into the weld pool. The increase of the welding current results in its growing impact upon the weld pool surface. The molten metal of the pool is forced out from under the arc. (Fig. 2, frames 2-4).

The arc column completely overtakes the bead and covers part of the electrode (Fig. 2, frames 5-7). Then the arc stretches very rapidly, at this moment the ligament of molten metal between the bead and the electrode tip and the arc overtaking the ligament can be clearly seen at the frame (Fig. 2, frame 8). At the moment of detachment and transfer the bead is deformed and stretched towards the weld pool, so its size exceeds the size of the visible part of the arc which is above the edge of the weld pool (Fig. 2, frames 9-10). The average rate of the bead transfer is 3 m/s.

During the piling up of the impulse the weld pool metal is forced out from under the arc into the tail part and sideward covering the unfused edges of the groove (Fig. 2, frames 5-7).

The welding arc extends with the increase in the welding current amplitude, covering the side surfaces of the groove, thus, improving the fusion of the weld and the base metal. During the pause under the return motion of the weld pool metal a concave meniscus is formed (Fig. 2, frames 12-16). The strongest stir of the weld pool surface occurs when the weld pool metal is forced out by the arc. After the bead detachment and transfer into the weld pool a wave moving towards the groove edges is formed.

Impulse power supply of the welding arc due to periodic power increase allows to change the heat input into the side edges of the weld joint and leads to formation of a dished face of the weld (meniscus). Such form of the weld is most favorable for further overlay depositing and eliminating such defects as slagging of the weld toe. The specific form of the weld after completing groove welding in carbon dioxide with impulse arc power supply is shown in Figure 3. The meniscus form of the weld root pass is clearly seen.

Figure 3. The weld form under groove welding in СО2 with impulse power supply.

For the groove welding the width of the groove is one of the main parameters. The study provided in work [15] showed that it possible to form a high-quality weld bead with 10 to 13 mm grooving. Welding in СО2 can be completed with application of usual welding consumables, for example, with Sv-08G2S electrode wire 1.2÷1.6mm in diameter.

Impulse arc power supply allows controlling not only fusion and bead transfer process but also forming the weld joint: controlling the weld joint geometry and weld joint structure. Periodical power changes and active stirring of the weld pool change the freezing conditions of the weld pool. The increase of impulse recurrence frequency results in structure refinement of the weld metal (Fig. 4) [16].

Figure 4. Metal structure under impulse power supply with various pulse recurrence frequency

Thus, consumable electrode welding in СО2 with impulse power supply has the following technological advantages in comparison to arc welding [17]: controlled and directed transfer of the electrode metal, lower waste of metal on loss and spitting, possibility of long arc welding under the decrease of current, elimination of weld defects and, as a result, improving the quality of weld joints.

CONCLUSION 1. We have discussed the peculiarities of electrode metal

fusion and transfer processes, as well as molten metal stir and weld formation when welding in narrow gap with the pulse arc.

2. Use of impulse power supply allows to increase welding arc stability and receive a fault-free weld joint under groove welding in СО2.

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