Effect of pulse M-Arc frequency on Tri-Arc DE droplet transfer and weld forming

Zheng Jia，Wang Tianqi，Zhong Pu and Cheng Liqiang

Tianjin Key Laboratory of Modern Mechatronics Equipment Technology，Tianjin Polytechnic University，Tianjin 300387，China

Abstract In view of the unstable welding process of Tri-Arc DE, surfacing test with Q235 steel plate was completed with the help of the self-built high-speed camera and waveform synchronous acquisition system using the Tri-Arc DE technology. The effects of pulsed M-Arc frequency on Tri-Arc DE droplet transfer and weld formation were analyzed. The results show that while the gradual increase of pulse frequency, the droplet transfer frequency gradually decreases, which is followed by several drops per pulse, one drop per pulse, and one drop within several pulses. The most ideal transfer form is one drop per pulse, of which the welding process is the most stable, and the quality of weld formation is the most satisfied.

Key words Tri-Arc double electrode, pulse frequency, the metal transfer behavior, weld formation

0 Introduction

Tri-Arc DE is a new type of high-efficiency welding method. In 2010, Geng[1]from Harbin Institute of Technology proposed the welding process of Tri-Arc DE and applied for its patent; the welding method has made some progress in theory and practice. Yang[2]from Harbin Institute of Technology conducted experiments on different parameters of M-Arc and studied the influence of different M-Arc pulse frequency on welding spatter and penetration; Liu[3]from Shenyang University of Technology studied the welding process of 5356 aluminum alloy fusion of Tri-Arc DE. Although through unremitting efforts, Shenzhen Ruiling Company has produced its welding machine. Because of the serious splash in the process of welding, the problems of poor welding and M-Arc instability have caused the welding machine cannot mass-produce, and need to be further developed and perfected[4].

The paper has collected and arranged on the research progress and application of Tri-Arc DE technology[5]. Pulse frequency is a key parameter for the weld-

ing process; the appropriate pulse frequency increases the penetration rate and improves the microstructure of the welded joint[6]. By studying the influence of pulse frequency on droplet transfer and weld formation, the in-depth analysis of the droplet transfer process of Tri-Arc DE can be carried out, and provides a theoretical basis for the improvement of the welding power source and the optimization of the welding process[7].

In this paper, the effect of pulsed M-Arc frequency on the droplet transfer form of Tri-Arc DE and the weld formation are studied.

1 Test method

The wire used for the test is 1.2 mm in diameter. The material is ER50-6, and the mixed gas 82% Ar+18% CO2with a gas flow rate of 15 L/min serves as the shielding gas. The work piece to be welded is Q235 steel and its size is 200 mm200 mm6 mm.The welding speed is 1.4 m/min and the wire feed rate is 7 m/min. Other parameters are shown in Table 1.The surface of the work piece needs to be polished to remove the oxide layer for better welding effect.

Table1Weldingtestparameters

Welding voltage/VWire feed rate/(m·min-1)Welding speed/(m·min-1)Wire extension/mmWire diameter/mm307+71.4201.2

The test welding machine is produced by Shenzhen Ruiling company, equipped with Tri-Arc welding system. The test control platform is controlled by the automatic welding machine tool controller. Using neon light as background light, the high-speed camera is a German-made Optronis camera with a maximum shooting speed of 10 000 frames per second. The high-speed camera and current-voltage waveform acquisition synchronization analysis software developed by Labview is used to analyze the waveform of the electrical signal. The schematic diagram of the pulse current waveform of the front wire arc and M-Arc is shown in Fig.1. In a single pulse periodT, the duration of the pulse peak currentIpisTpand the duration of the pulse base currentIbisTb[8].

Fig.1Tri-ArcDEfrontwireandM-Arcpulsecurrent

2 Influence of pulse frequency on droplet transfer behavior

The experiment was carried out by using the standard parameters and the droplet transition image was captured by high-speed camera during the experiment. The pulse frequency was 120 Hz， 150 Hz and 180 Hz. The picture interval was 25 ms.

Due to the most stable welding process of the Tri-Arc DE one drop per pulse, the welding spatter is the least and the welding forming is the best[9-10]. Therefore, in order to obtain the transfer form of the droplet transfer effect, the paper summarizes the relationship of pulse frequencies, Wire feed rate and welding wire radius under the situation. The formula (1) for calculation is as follows:

(1)

whereris the radius of the wire,fis the pulse switching frequency of the M-Arc, andvfis the wire feed rate. Since the diameter of the droplet is about 1.1 times the diameter of the welding wire in actual production. When the current exceeds a critical value, a jet transition occurs. In contrast, the current is very small, it is not enough to complete a droplet transition in one pulse period, so the use of this formula requires experimental exploration[11-12].

This test is carried out when the wire diameter is 1.2 mm. According to formula (1), the corresponding relationship between wire feed rate and pulse frequency is shown in Table 2.

Table2Matchingrelationshipbetweenwirefeedrateandpulsefrequency

vf(m·min-1)5678910f/Hz104125146167188208

2.1 Pulse frequency is 120 Hz

When the M-Arc pulse frequency is 120 Hz and the pulse frequency is small. In a pulse peak current periodT, after completing a droplet transfer, the pulse current is still at the peak currentIp, and the end of the wire is subjected to multiple droplet transition. Therefore, at this pulse frequency, it is several drops per pulse form, and its waveform diagram and droplet transfer process are shown in Fig.2.

It can be concluded from Fig.2 that the change in welding current is stable, there is no droplet when timet=13 150 ms. When the timet=13 175 ms, the front wire completes the first droplet transfer of the cycle. With the accumulation of energy, the droplet is still in transfer during the pulse time. At timet=13 200 ms, the front wire completes the second and third droplet transfer of the cycle. As time increases, the back wire begins to transfer. When the timet=13 225 ms, the back wire also completes the same transition. Therefore, the wires are subjected to a multi-drop droplet transfer in one pulse period. In theory, many transformations can be completed in one pulse period, but in actual production welding, it is difficult to control the droplet transfer process of several drops per pulse period. The droplet transition process is unstable and the spatter is serious in the welding process.

Fig.2f= 120Hzwaveformanddroplettransferprocess(a)waveform(b)transferprocess

2.2 Pulse frequency is 150 Hz

According to the matching relationship between the wire feed rate and the pulse frequency in Table 2, in the case of the wire feed rate in this paper, when the pulse frequency is 150 Hz, it is a one-pulse and one-drop transfer form. At the beginning of the welding process, the current fluctuates greatly. As the welding process proceeds, the peak value of the current pulse and the base value are gradually stabilized. In one pulse periodT, the droplets reach the energy required for the transition to make a one-drop transition. Only one melt is formed in one pulse peak phase of the front wire. When the transition to the molten pool is completed, the pulse enters the base stage, and the other wire undergoes the same transition and achieves the ideal welding transition form of one pulse and one drop. The waveform diagram and the droplet transfer process are shown in Fig.3.

It can be concluded from Fig.3 that when the timet=13 100 ms, the front wire pulse is at its peak stage and the formed droplet is about to be separated from the welding wire. When the timet= 13 125 ms, the formed droplet is separated from the welding wire and transfer to the molten pool. When the time ist=13 150 ms, the back wire pulse is at the peak stage, and the formed droplet is almost separated from the welding wire. When the time ist=13 175 ms, the droplet is separated from the welding wire and transfer to the molten pool. At this pulse frequency, the welding process is stable and the quality of the welded joint is good.

Fig.3f= 150Hzwaveformanddroplettransferprocess(a)waveform(b)transferprocess

2.3 Pulse frequency is 180 Hz

When the pulse frequency is 180 Hz, the droplettransfer is one drop within several pulses form, which is due to the inability to reach the energy required for a droplet to complete the transition during the peak time of a current pulse. With the increasing of time, shortening arc length at the welding wire end and finally achieve transition by voltage principle. Therefore, at this pulse frequency, a droplet transfer in the form of multi-pulse and one drop occurs and the waveform diagram and droplet transfer process are shown in Fig.4.

It can be concluded from Fig.4 that the peak value of the M-Arc and the base value fluctuate greatly. It takes several cycles to complete a droplet transfer at this frequency. At timet=13 100 ms, the energy of the front wire droplets is not enough to cause the arc to make a transition. As time increases, a pulse cycle is completed and the droplets are still not transited. At timet=13 175 ms, the front wire droplets reach the transition energy and transit to the molten pool. The back wire undergoes the same the stage to complete the transition. At timet=13 200 ms, a large drop transition is formed. The droplet transfer process at this pulse frequency is very instable and the splash is obvious, leading to poor quality of the welded joint.

Fig.4f= 180Hzwaveformanddroplettransferprocess

(a)waveform(b)transferprocess

In summary, as the pulse frequency of the M-Arc increases, the pulse peak time also decreases gradually from the peak time of the pulse frequency of 120 Hz is 83.3 ms to the peak time of the pulse frequency of 150 Hz is 66.7 ms and then to the peak time of the pulse frequency of 180 Hz is 55.6 ms. The frequency of droplet transfer is decreases gradually, which is from several drops per pulse, one drop per pulse to several pulses per drop.

Fig.5MainarcpulsepeakcurrentatdifferentM-Arcpulsefrequencies

3 Influence of pulse current frequency on weld formation

3.1 Pulse frequency is 120 Hz

The macroscopic topography of the weld at the M-Arc pulse frequency of 120 Hz is shown in Fig.6. The hump bead is formed at the front end of the weld. The transition form of the front and back wires is one pulse and multiple drops. The penetration depth is deep. The drop transition process is unstable and the welded joints are poor quality.

Fig.6f= 120Hzweldappearanceandweldcrosssection

3.2 Pulse frequency is 150 Hz

The macroscopic topography of the weld at the M-Arc pulse frequency of 150 Hz is shown in Fig.7. During the welding process, with the droplet transfer process is the most stable and the droplet transfer of the front and back wires is a one-pulse transition, and the penetration depth becomes small. There is no hump phenomenon in the weld formation, the quality of the welded joint is the best, and the weld uniformity is good.

Fig.7f= 150Hzweldappearanceandweldcrosssection

3.3 Pulse frequency is 180 Hz

The macroscopic topography of the weld at the M-Arc pulse frequency of 180 Hz is shown in Fig.8. The droplet transfer process during the welding process at this pulse frequency is unstable, and the droplet trans-fer patterns of the front and back wires are all multi-pulse. The arc is unstable, the penetration depth is the smallest, the quality of the weld is poor, and the weld uniformity is poor.

Fig.8f= 180Hzweldappearanceandweldcrosssection

4 Conclusions

(1) With the gradual increase of the pulse frequency of the M-Arc, the form of the droplet transition gradually changes, which in turn becomes several drops per pulse, one drop per pulse to several pulses per drop. This is mainly because the pulse frequency of the M-Arc increases continuously.As the peak current time gradually decreases, the transition frequency also gradually decreases.

(2) At different M-Arc pulse frequencies, the weld profile is ideal in the case of one drop per pulse, during which the welding process is stable and the quality of the welded joint is good. When the pulse frequency is 120 Hz, The weld is irregular in shape and poor in the weld formation; when the pulse frequency is 150 Hz, there is no hump phenomenon in the weld formation, and the weld uniformity is best; when the pulse frequency is 180 Hz, the macroscopic shape of the weld is very poor.