Stabilization of the process of mechanized pulsed-arc welding

The main disadvantage of the mechanized arc welding process in shielding gases with short circuits is the spatter during melting of the electrode metal and its transfer to the weld pool, which affects the productivity of the process, reducing it. Its elimination is possible through the implementation of the controlled transfer of molten electrode metal into the weld pool. The implementation of such a transfer and the control of the processes that take place in the arc gap to a large extent determine the conditions for the qualitative formation of the deposited metal, the stability of the process, the magnitude of the loss of electrode metal and the manufacturability of the processes of arc welding in shielding gases. At the present stage of development of welding technologies, controlled transfer of electrode metal is possible due to the pulsed nature of arc burning. In this case, one of the main methods for increasing the efficiency of the process is to limit the maximum value of the short circuit current by increasing the inductive resistance of the welding circuit. The research aimed to determine the effect of the rate of rising of the welding current during a short circuit on the stability of the welding arc. It was found that an increase in the current growth rate, starting from 1.23 kA/s to 50 kA/ s, leads to a decrease in the average duration of short circuits by at least 10 times. At the same time, the average frequency of short circuits increases by more than 2 times, from 36...38 s to 80...86 s. The reason for this is the increase in the values of the electrodynamics’ Lorentz force, the action of which leads to the compression of the liquid metal bridge of the drop (pinch effect) due to an increase in the short circuit current. At the same time, there is a violation of the stability of the pulse process, and this is reflected in an increase in the average frequency of arc breaks by more than 30 times from 0.33 s to 10 s. An increase in the energy parameters of the welding process led to a decrease in the average frequency of short circuits (2...3 times) and their average duration (2 times). The reason for this should be considered a change in the type of transfer of liquid metal – the welding process with short circuits has turned into a mixed process in which, along with short circuits, a droplet transfer of electrode metal is observed. Sergey Maksimov Head of department of physical and mechanical researches of weldability of structural steels Dr.Tech.Sc., Snr.Res.Ass. Anatoly Gavrilyuk Junior Researcher of physical and mechanical researches of weldability of structural steels Denys Krazhanovskyi Junior Researcher of physical and mechanical researches of weldability of structural steels Engineering, Environmental Science Transfer of Innovative Technologies Vol.4, No.2 (2021), 41-52 42


Abstract.
The main disadvantage of the mechanized arc welding process in shielding gases with short circuits is the spatter during melting of the electrode metal and its transfer to the weld pool, which affects the productivity of the process, reducing it. Its elimination is possible through the implementation of the controlled transfer of molten electrode metal into the weld pool. The implementation of such a transfer and the control of the processes that take place in the arc gap to a large extent determine the conditions for the qualitative formation of the deposited metal, the stability of the process, the magnitude of the loss of electrode metal and the manufacturability of the processes of arc welding in shielding gases. At the present stage of development of welding technologies, controlled transfer of electrode metal is possible due to the pulsed nature of arc burning. In this case, one of the main methods for increasing the efficiency of the process is to limit the maximum value of the short circuit current by increasing the inductive resistance of the welding circuit.
The research aimed to determine the effect of the rate of rising of the welding current during a short circuit on the stability of the welding arc. It was found that an increase in the current growth rate, starting from 1.23 kA/s to 50 kA/ s, leads to a decrease in the average duration of short circuits by at least 10 times. At the same time, the average frequency of short circuits increases by more than 2 times, from 36...38 s -1 to 80...86 s -1 . The reason for this is the increase in the values of the electrodynamics' Lorentz force, the action of which leads to the compression of the liquid metal bridge of the drop (pinch effect) due to an increase in the short circuit current. At the same time, there is a violation of the stability of the pulse process, and this is reflected in an increase in the average frequency of arc breaks by more than 30 times from 0.33 s -1 to 10 s -1 . An increase in the energy parameters of the welding process led to a decrease in the average frequency of short circuits (2...3 times) and their average duration (2 times). The reason for this should be considered a change in the type of transfer of liquid metalthe welding process with short circuits has turned into a mixed process in which, along with short circuits, a droplet transfer of electrode metal is observed.

INTRODUCTION
It is known that mechanized arc welding in shielding gases with short circuits (s.c.) is performed at moderate values of the welding current (up to 180...220 A) and a relatively low voltage (18...24 V) on the arc. The main disadvantage of the process is spattering during melting of the electrode metal and its transfer to the weld pool, which affect the productivity of the process, reducing it [1,2]. Authors of publications [3 -6], devoted to the improvement of technological processes of shieldedgas arc welding, based on theoretical and practical searches, came to the conclusion that the elimination of drawbacks is possible through the implementation of the controlled transfer of molten electrode metal into the weld pool. The implementation of such a transfer and the control of the processes that take place in the arc gap to a large extent determine the conditions for the qualitative formation of the deposited metal, process stability, the amount of electrode metal losses and the manufacturability of the processes of arc welding in shielding gases [7 -9]. At the present stage of development of welding technologies, controlled transfer of electrode metal is possible due to the pulsed nature of arc burning [10 -13].
When pulsed-arc welding, one of the main methods for increasing the efficiency of the process is to limit the maximum value of the short circuit current by increasing the inductive resistance L in the welding circuit [14 -16]. The parameters of the inductive resistance of the welding circuit determine the current growth rate с during short-circuit, on which depends max s.c.
I , the stability of the welding process and spatter of the electrode metal [1,17,18]. Under the stable behaviour of the pulse-arc welding process, we will consider such a process in which there is no violation of the welding arc burning. A sign of violation of the arc burning will be the transition of the power source to open-circuit voltage, which will be recorded by the information-measuring system when registering the instantaneous values of current and voltage on the arc.

PURPOSE AND METHODS
Based on the features of mechanized arc welding, the aim of the research was to determine the influence of the value of the welding current growth rate during a short circuit on the stability of the welding arc. It should be noted that in a pulse power supply, there is a structurally absent inductor that regulates the value of с, and, accordingly, the maximum value of the short circuit current max s.c.
I . To control these parameters, it is provided that the so-called virtual inductance LV is numerically laid in the controller at the program level, which determines the reaction rate of the source to a change in current in the "source-arc" circuit.
In connection with this feature, before performing experimental studies, the relationships between LV and с were determined using a computerized information-measuring system (ІMS) [19]. For this, the inverter [20] was connected to the ballast rheostat according to the circuit in Fig. 1. When the circuit breakers were closed at 50 A, and then at 100 A at different LV values, the analogue-digital converter ІMS recorded a current jump from 50 A to 150 A using a connected current transformer.  Table 1. The implementation of the experimental work involved surfacing on a plate with programming the inverter operation mode at different values of LV= 9, 12, 15, 18, 21, 24, 27, 30. For this purpose, current-voltage characteristics (CVC) No.1 and No.2 were placed in the inverter (Fig. 3) and set the pulse mode with a frequency of f = 25 Hz and a duty cycle of C = 2.
Plate materialsteel of strength class X70, wire -Sv08G2S with a diameter of 1.2 mm, wire feed speed VW = 5.1 m/min., shielding gas -Ar + CO2, welding speed V = 30 cm/min. The heat input Q for each experiment was calculated using the well-known formula in which the values of IAV and UAV were determined by statistical processing of instantaneous values of current and voltage by the information-measuring system IMS 2007: where Vwelding speed (cm/min.), η = 0.7.

RESULTS AND EXPLANATIONS
The results of the analysis of the data recorded by the computerized ІMS, and conclusions regarding the stability of the pulse process are shown in Table 2.
Evaluation of all recorded oscillograms of the arc voltage and their statistical processing shows that the value of LV significantly affects the stability of the pulse process during the transfer of metal with short circuits (Fig. 4). Leads to the fact that the process of arc burning is much more stable, almost without breaks (Fig. 4, e-k). On the histogram, this is displayed by a sharp decrease in the number of instantaneous values of the power supply operation at UOC.
When analysing the oscillograms of the welding current and the statistical processing of the I-U characteristics of the process (Fig.  5), it was found that a decrease in the value of с leads to a significant limitation of the maximum value of the short circuit current. So, for example, at с = 35.7 kA/s (Fig.5, a), there is, in addition to the already identified violations in the stability of continuous arc burning (IW = 0, UA = UOC), a wide range of the spread of the No arc interruption а b Fig. 4 (beginning). Oscillograms  A gradual increase in virtual inductance to LV = 24 ... 30 (с = 0.35 ... 0.06 kA/s) led to a significant change in the quality of the pulse process (Fig. 5, f-h) Statistical processing of instantaneous values of the welding current showed that an increase in the current growth rate с, starting from 1.23 kA/s and up to 50 kA/s, leads to a decrease in the average short-circuit duration by at least 10 times (Fig. 6, a). At the same time, the average frequency of short circuits increases more than 2 timesfrom 36... Statistical analysis also confirmed the conclusion (Fig. 6, b) that the increase in с leads to a violation of the stability of the pulse process and this is reflected in an increase in the average frequency of arc breakage by more than 30 times from 0.33 s -1 (for с = 0.06...1.23 kA/s) to 10 s -1 (for с = 50 kA/s).
To determine how the growth of the welding current с affects the stability of the pulsed process if it is necessary to increase the heat input Q, additional experimental and theoretical studies were carried out. To do this, the I-V characteristic was placed in the inverter (Fig.  7), in which the falling sections of the I-V characteristics No.1 and No.2 (in the range of 40 -11 V) were shifted in the direction of increasing the welding current by 100 A compared to the previous version programming the power source (see Fig. 3). The pulse process was carried out with a frequency f = 25 Hz, the welding speed V = 30 cm/min., the wire feed speed VW = 7.7 m/h, the shielding gas -Ar + CO2.
The results of processing these data by a computerized information-measuring system are shown in Table 3.
An analysis of the obtained data and their comparison with the results of previous experiments (Fig. 6, a) showed that an increase The reason for this should be considered that the increase in energy indicators changed the type of transfer of liquid metalthe welding process with short circuits (Fig. 8a) turned into a mixed process [15], in which, along with short circuits, a droplet transfer of electrode metal is observed (Fig. 8b). In the latter case, part of the molten metal flows into the weld pool in small drops, while the short circuit time is much shorter (3...5 times) than with a conventional short circuit. As a result of this, the arc voltage remains at the level of USC > 12...15 V and does not have time to decline to the accepted values of USC = 5...10 V.
Since the IMS 2007 statistically calculates the cases of short circuit for the condition USC = 5...10 V and does not take into account larger voltage values, the calculated average frequency of short circuits is lower.
Processing oscillograms of the welding current and voltage showed that pulse-arc welding proceeds stably without disturbances in the welding arc burning in the entire range of the control value LV = 9...30 (с = 50.0...0.06 kA/s).

CONCLUSIONS
1. The increase in the inductance of the power source, which due to the control system of the inverter significantly reduces the slew rate of the short circuit current, leads to stabilization of the pulse-arc welding process with short circuits.
2. An increase in the growth rate of the short circuit current с, starting from 1.23 kA/s to 50 kA/s, leads to a decrease in the average duration of the short circuit by at least 10 times. At the same time, the average frequency of short circuits increases more than 2 times.
3. An increase in the energy indices of pulse-arc welding led to a sharp decrease in the average frequency of short circuits (2...3 times) and their average duration (2 times). As a result, the process proceeds stably without disturbances in the burning of the welding arc in the entire range of changes in the inductance of the power source.