Continuous contact transfer behavior in bypass coupling triple-wire indirect arc welding

Continuous contact transfer mode in bypass coupling triple-wire indirect arc welding (BCTW-GIA) is proposed to pursue an optional way to ensure the high deposition rate and obtain stable depositing process. The arc behaviors, droplet transfer and process stability with different distance between the tips ( d 1 ) of the main wire and side wires and the feeding speed of main wire (FSM) were studied. With the increase d 1 and FSM, the droplet transfer mode of main wire varied from explosive transfer to short-circuit transfer to bridge transfer. When the d 1 was greater than 4 mm, the continuous contact transfer bypass coupling triple-wire indirect arc welding can be realized and the welding process stability was improved. With the d 1 adjusted from 0 mm to 6 mm, the standard deviation of the current dropped from 123.8 to 53.8 and its variation coefficient from 117.33% to 39.24% at FSM= 1 m/min. The weld penetration and weld width reduced with the FSM increasing, while the weld reinforcement gradually increased.


Introduction
That additive manufacturing (AM) is the process of joining materials in layer-upon-layer manner to build objects from models, has developed rapidly in recent years.As a classifications of AM, wire and arc additive manufacturing (WAAM) which uses arc as the heat source to melt filler wires realizing depositing process, has emerged as a highly promising technique for manufacturing large-scale structural elements due to its high efficiency, low equipment and energy consumption etc. [1][2][3].Common WAAM technology includes traditional arc welding methods such as gas metal arc-additive manufacturing (GMA-AM), gas tungsten arc-additive manufacturing (GTA-AM), and plasma arc-additive manufacturing (PA-AM).However, the deposition rate of the filler wire is directly related to the welding current for these traditional arc welding technologies.The greater the welding current is, the faster the deposition rate will obtain, which results in the excessive heat input of deposition component.The excessive heat input always lead to a series of issues, such as distortion and poor mechanical properties etc. [4].
Considering these limitations, it is necessary to develop a manufacturing technique which is capable of high deposition efficiency with small heat affected zone and low metal dilution rate.
Indirect arc welding technology is a novel modification to GMAW, in which the welding current does not pass through base metal and the arc was ignited between electrodes.During the welding process, the energy of arc column is mainly concentrated near the electrodes which results in the melting rate of consumable electrode improving, but a small amount of energy is used to melt base metal [5,6].As has been demonstrated, the technology possessed several remarkable advantages, such as high deposition coefficient, low heat input, and dilution ratio etc. [7].These advantages demonstrated that indirect arc welding will be a highly promising technology in additive manufacturing field.At present, some indirect arc additive manufacturing processes has been reported successfully.For example, Wang et al. [8] achieved the process of depositing CuSi3 Cu alloy onto 30CrMnSi steel plate with consumable and nonconsumable electrodes indirect arc welding (CNC-IAW) technology.Wu et al. [9,10] prepared the stain steel surfacing layer with twin-wire indirect arc welding (TWIAW) and investigated its corrosion resistance behavior.The result clarified that compared with MIG surfacing layer, that of TWIAW obtained a better intergranular corrosion resistance due to the faster cooling speed during TWIAW restraining Cr23C6 precipitation.An et al. [11] applied the bypass coupling TWIAW to achieve the production of high-temperature and wear-resistant austenitebased stainless steel surfacing layer.Wang et al. [12] proposed a novel heterogeneous multiwire indirect arc directed energy deposition method for in-situ synthesis of Al-Zn-Mg-Cu alloy components, and realized its composition control.As a novel indirect arc welding technology, triple-wire indirect arc welding (TW-GIAW) retains low heat input and obtains higher deposition coefficient compared with other indirect arc welding technology [13].The reason is that there are three wires melted at same time during the welding process.To some extent, TW-GIAW technology has great potential in the field of AM.
In recent years, the researches on TW-GIAW mainly focused on the medium and thick plate welding, droplet transfer and weld formation, but the field regarding the applicability of TW-GIAW for AM has not been reported [14][15][16][17].According to the research of Fang et al [15], there was a convex weld formed in the TW-GIAW, which was not conducive to the next weld layer complete fusion.Although the bypass coupling technology has been applied on TW-GIAW to solve this problem [18].However, the available parameter interval that the welding process of bypass coupling triple-wire indirect arc welding (BCTW-GIAW) maintained stable was limited due to lack a particular control system to reducing the interaction between direct arc and indirect arc.Only when the current flowing into base metal was adjusted at 35~50% of the total current, a stable welding process and good weld formation can be realized.
To widen the process window, the continuous contact transfer mode in bypass coupling triple-wire indirect arc welding is proposed through changing the droplet transfer mode of the main wire.In the contact transfer BCTW-GIAW, three consumable filler wires are fed simultaneously but the main wire droplet transfers to the molten pool with short circuit or bridge transfer mode.Compared with the BCTW-GIAW, the stability of process will be improved because of the elimination of short circuit explosion due to the contact the main wire droplet and side wire droplet, which is benefit to its application in the AM field.Prior to that, the arc characteristic and droplet transfer behaviors affected by the main wire feeding speed, the spatial relative position between main and side wires should be studied deeply, which is benefit to the process optimization.Therefore, in this study, a high-speed camera system was applied to observe the arc and droplet transfer behavior, and welding process analyzing and monitoring system was used to evaluate the process stability in bypass coupling triple-wire indirect arc welding in real time.Meanwhile, the influence mechanism of different parameters on weld formation was clarified.This study will provide a technological basis for BCTW-GIA-AM.

Materials and methods
Fig. 1 showed a schematic of the experimental system, which consisted of three parts: (1) the bypass coupling triple-wire indirect arc welding system; (2) arc and droplet acquisition system; and (3) electrical signal acquisition system.Two power sources and three consumable torches were used in the BCTW-GIAW system.The main torch and workpiece were linked to two welding sources cathodes, and two side torches to anodes respectively.During the welding process, the welding current was divided into two branches.One branch flowed through base metal (Ib), and the other branch to the main wire (Im).The value of welding total current depends on two side wires current (left side wire current Il and right side wires current (Ir).As such, the current relation can be expressed as: In the experiment, a high-speed camera (MS50K) and optical interference filter (central wavelength: 670.3 nm) were utilized to capture the arc shape and droplet behavior with acquisition frequency 2000 f/s.The electrical signal acquisition system (ANALYSATOR HANNOVER AH-19) includes a set of current and voltage sensors and a computer with corresponding data acquisition card.During the welding process, the electrical signal acquisition system and high-speed photography system performed synchronously.Since the current magnitude and direction of two side wires was same, the arc voltage between either side wires and workpiece was collected.The change in the coefficient of variation of electrical parameters was used to reflect the stability of welding process.Three torches was placed above the workpiece at a specific spatial arrangement as illustrated in Fig. 2. The distance between the tips of the main wire and side wires (d1), and the feeding speed of main wire (FSM) are significant factors which determine whether the technology can be performed successfully or not.In this study, the effect of different d1 and FSM on the arc and droplet behaviors, and process stability was analyzed.The experiment process parameters were illustrated in Table 1.The shielding gas was 80% Ar + 20% CO2, and gas flows rate was 25 L/min.The base metal was Q345 plates with thickness of 14 mm.The main wires with a diameter of 1.6 mm and two side wires with diameter of 1.2 mm were ER50-6.The chemical compositions of the materials was listed in Table 2.

Effect of d1 on arc and droplet transfer behavior
The appropriate d1 is the key to obtain a stable contacting transfer BCTW-GIAW process.
Too large or too small d1 will result in the change of indirect and direct arc state [19].In this section, the arc and droplet transfer behavior with d1 = 0 mm, 2 mm, 4 mm, 6 mm, and 8 mm respectively was discussed when the main wire feeding speed was kept 3 m/min constant.
When d1 was 0 mm, the arc and droplet characteristic was shown in Fig. accompanied by quantities of spatter producing, which was unfavorable for obtaining good welding stability and weld formation [20].After maintaining the stable combustion state about 25 ms, the direct arc was extinguished.Because of the main wire continuous feeding, the extinguished indirect arc was reignited when the main wire was in contact with side wires.
Meanwhile, the direct arc was extinguished and the value of Ib returned to 0 again as shown in Fig. 3 (e).On this occasion, the main wire metal transfer mode became repelled globular transfer and that of side wires was globular transfer.When the d1 increased to 4 mm, the arc behavior and droplet transfer was illustrated in Fig. 5. From the fluctuation of the arc current and voltage curve, it can be preliminarily evaluated that the arc stability has been improved slightly compared with the situation when d1 was 0 mm or 2 mm.The frequency of 0 A current decreased, indicating that the extinction frequency of the direct arc decreased during the welding process.The direct arc was also the first to be ignited when the d1 was 4 mm as shown in Fig. 5 (a).The metal at the root of the main wire was growing with the main wire feeding as illustrated in Fig. 5 (b)-(c).Only the direct arc was burning till the main wire droplet was in contact with that of side wires and the indirect arc was ignited as illustrated in Fig. 5 (d).When the droplet suspended on the tip of main wire contacted with the molten pool completing its transfer process, the indirect arc was extinguished and the direct arc reignited as shown in Fig. 5 (e).This transfer mode was classified as contact transfer.However, it was different from the short circuiting transfer mode, in which the droplet would be detached from the consumable electrode after finishing transfer.As can be seen from Fig. 5 (e), the droplet was not detached from the wire.As such, this contact transfer mode was similar to bridging transfer always appearing in the cold-wire GTA welding process, in which the wire was inserted into molten pool and melted by the molten pool heat.The transfer mode was considered ideal because of its good stability and small amount of spatter.When the d1 further increased to 6 and 8 mm, the arc behavior and droplet transfer was illustrated in Fig. 6 and Fig. 7.It can be seen that the situation of both was basically similar.
The Ib was kept at 160 A and the voltage was kept at 24 V in the whole welding process.The direct arc was burning continuously with no extinguished as shown in Fig. 6 (a)-(f) and Fig. 7 (a)-(f).That there was no 0 A value in the current waveform also can prove this point.The main wire has been in the contact transfer mode mentioned above to complete its melting process.
Therefore, it can be considered that the stable contact transfer in BCTW-GIA, when the d1 was kept at 6~8 mm.

Effect of FSM on arc and droplet transfer behavior
As reported in ref. [21], the main wire feeding speed is also a significant factor to the welding stability of BCTW-GIAW.Too fast or too slow feeding speed of main wire (FSM) results in the welding stability getting worse.In this section, the effect of different FSM on arc and droplet transfer behavior was investigated when the d1 was kept at 6 mm constant.
When the FSM was 1 m/min, the arc behavior and droplet transfer was illustrated in Fig. 8. Before the main wire contacting, the direct arc kept a stable burning state and the droplet suspended on the main wire was growing gradually with the feeding of the main wire as shown in Fig. 8 (a)-(d).The bypass current was also steady at about 210 A. When the main wire was fed further, the direct arc tended to expand to the main wire side and the arc current was rising as illustrated in Fig 8 (e).After that, the indirect arc was ignited, the direct arc extinguishing.
Both the main wire and side wire droplet transfer mode manifested globular transfer as shown in Fig. 8 (f).When the FSM increased to 2 m/min, the arc behavior and droplet transfer was illustrated in Fig. 9.It can be found that the direct arc behavior was not affected by the droplet transfer of the main wire.Similar to that of 1 m/min, the droplet was suspended on the main wire and growing gradually with the feeding of the main wire in the early transfer process as shown in Fig. 9 (a)-(c).However, the difference was that the droplet would contact with the base metal, completing its transfer process, which was typical shorting circuit transfer as illustrated in Fig. 9 (d)-(e).In addition, the bypass current decreased to about 180 A compared with that of 1 m/min.The arc behavior and droplet transfer with FSM of 3 m/min has been mentioned above as shown in Fig. 6, so it would not be repeated in this section.When the FSM further increased to 4 m/min, the arc behavior and droplet transfer was illustrated in Fig. 10.
Similar to that of d1=6 mm, the main wire droplet completed its transfer process with the continuous contact transfer mode after it entering direct arc as shown in Fig. 10 (a)-(e).The bypass current further reduced to about 110 A. The main reason was that the feeding speed of the main wire decided the current value flow through the main wire (IM).With the higher the main wire feeding speed, the greater IM was.As such, when the FSM increased gradually, the bypass current decreased.and FSM, the droplet transfer mode of the main wire can be varied from unstable explosive transfer to unstable repelled globular transfer, stable short-circuiting transfer and continuous bridge transfer, as shown in Fig. 11.
For given a FSM, the d1 decided probability of the indirect arc being ignited.A large d1 resulted in the main wire droplet being further away from that of side wire, which was in favor of minimize the interference between the indirect arc and direct arc, thus avoiding the occurrence of unfavorable phenomena in the welding process, such as spatters, short-circuit explosion etc..When the d1 was too small, the side wire first contacted with the main wire, which led to the indirect arc being ignited first.In this situation, the main wire was easier to realize projected transfer.However, with the side wire in contact with base metal, the current tended to flow through the base metal first, resulting in the direct arc being ignited and the indirect arc extinguished.The major reason for this was that the main wire and workpiece were similar electron work function, but the size of the base metal was much larger than that of the main wire.Because of the continuous feeding of the main wire, the extinguished indirect arc was reignited with the droplet short-circuit explosion when the main wire droplet touched that of side wires, as shown in Fig. 11 (a).By increasing the d1, the main wire touched the direct arc resulting in the indirect arc being ignited, and the main wire droplet was located below that of side wires, which was beneficial to avoid the short-circuit caused by the contact of anode and cathode droplets.However, the main wire droplet was not separated from the wire tip till it was in contact with the molten pool and by the surface tension of the molten pool, which led to the short-circuit transfer, as shown in Fig. 11 (b).After further increasing the d1, the main wire absorbed the slight direct arc heat and directly inserted into the molten pool without the formation of the indirect arc and droplet, as illustrated in Fig. 11 (c).The reason for no indirect arc forming was that the d1 exceeded the formation distance of indirect arc discharge channel.For a given d1, the FSM decided the creation point of the main wire droplet.A large FSM not only led to a longer preheated process of the main wire, but also to a low direct arc current, which resulted in the main wire inserting into to molten pool and further heated by the heat of molten pool before completely melted.Since no the main wire droplet was formed in this process, the negative influence of the droplet on arc was avoided and a stable welding process can be obtained.When the FSM became too slow, it took long time to get the main wire tip close to the side wires tip, which led to the coarse liquid metal suspend on wire and unable to detach till the distance between two wire droplet tips decreased to the indirect arc ignited.The transfer mode was typical repelled globular transfer.With the FSM increasing, the droplet was inclined into the direct arc but not detached from the wire.When the droplet touched the molten pool, it was pulled away from the main wire by the surface tension of the molten pool and a droplet transfer period was completed.After sufficiently increasing the FSM, the main wire tip metal was preheated briefly by the self-resistance heat and indirect arc heat, then directly inserted into to molten pool without the droplet formed, as illustrated in Fig. 12 (c), which resulted in continuous contact transfer.( ) Where xi represents the i-th sample value, ̅ is the average value and n represents the total number of samples.
According to the equation, it is clear that the smaller standard deviation and coefficient of variation indicates the better stability.In contacting transfer BCTW-GIAW, the welding stability was affected by the main wire feeding speed and the distance between wires tip.Fig. 13 and Fig. 14 plotted the s and ν obtained for different d1 and FSM, respectively.From the variation trend, the value of current and voltage standard deviation decreased significantly with the increase of the distance between wires tip d1, when the FSM remained constant.This phenomenon was particularly prominent at high FSM.In addition, higher wire feeding speed made it easier to obtain the smaller standard deviation at the same d1.When the d1 was greater than or equal to 4 mm, the standard deviation of welding electrical parameter was not affected basically by the main wire feeding speed.Although the variation law was basically the same, the voltage variation coefficient was much smaller than that of current.The main reason was that the constant voltage welding power sources mode was applied in the welding process.The characteristic of current and voltage variation coefficient with different parameters was essentially similar to that of standard deviation.With the d1 increasing, the variation coefficient of electrical parameter has markedly dropped.For example, when the FSM was 1 m/min, the minimum value of current variation coefficient can reach 31.43%, which was obtained at d1 =6 mm.Compared with the maximum value 117.33% at d1 =0 mm, the current variation coefficient was stepped down by 3 times.Similarly, the variation coefficient of current and voltage tended to be stable, when the d1 was greater than or equal to 4 mm.Based on the results of standard deviation and variation coefficient, the larger d1 was good for improving the stability of BCTW-GIAW.Considering that higher deposition rate can promote the efficiency of AM, the FSM should be more than 3 m/min, and the d1 be at least 4 mm.providing the same heat for melting base metal.As such, the variation of the weld penetration, weld width and reinforcement was not obvious, as illustrated in Fig. 17 (a).When d1 the kept 4 mm consistent, the weld appearance and cross section morphologies with different FSM was illustrated in Fig. 16.It can be found that the good weld formation can be obtained at different FSM.Although the indirect arc extinguishing also happened occasionally with the FSM=1 m/min as mentioned above 3.2 section, the weld appearance was not affected as shown in Fig. 16 (a).With the FSM increasing, the weld penetration (WP) and weld width (WW) had a downward trend while the weld reinforcement (WR) gradually increased as show in Fig. 17 (b).
The reason for the increase of reinforcement was that the weld wire deposition rate became fast with the FSM increase.However, the decrease of weld penetration and width was attributed to the decrease of direct arc current (Ib) caused by the FSM increase.According to the research of Cao [23], the weld penetration is nearly proportional to the arc current and the weld width is affected by the arc column diameter.When the direct arc current decreased, the arc column diameter also reduced resulting in the weld width decreasing.

Conclusion
To widen the stability process interval of bypass coupling triple-wire indirect arc welding (BCTW-GIAW), the contact transfer BCTW-GIAW was established through adjusting the distance (d1) between tips of the main wire and side wires in this study.The effects of d1 and the main wire feeding speed (FSM) on droplet transfer mode, arc behaviors, welding stability and weld formation were studied.The main conclusions are as follows.
(1) The d1 range for obtaining contact transfer BCTW-GIAW was determined.When the d1 was greater than 4 mm, the probability of explosive transfer caused by the contact between main wire droplet and side wire droplet was reduced and the stable contact transfer of the main wire can be realized.
(2) With the d1 and FSM increasing, the droplet transfer mode of main wire varied from projected transfer to unstable explosive transfer, to stable short-circuit and finally to stable bridge transfer.The stable short-circuit and bridge transfer were desirable for welding process stability.
(3) When the d1 was smaller than 4 mm, the higher FSM was inclined to achieve the lower standard deviation and variation coefficient of welding electrical signal.With the d1 adjusted from 0 to 6 mm, the standard deviation of the current dropped from 123.8 to 53.8 and its variation coefficient from 117.33% to 39.24% at FSM= 1 m/min.
(4) Weld penetration and width decreased with the FSM increasing but weld reinforcement increased.However, the d1 had little effect on the cross section geometry.

Fig. 2 -
Fig. 2-The spatial position arrangement of wiresTable 1-Parameters of the welding test

3 .
Fig. 3 (b), the indirect arc was extinguished while the direct arc between side wires and workpiece was ignited with a sharp increase of Ib as shown in Fig 3 (c).This process was always

Fig. 3 -
Fig.3-Arc behavior and droplet transfer with d1 of 0 mm Fig.4showed that the arc behavior and droplet transfer with d1 of 2 mm.On the whole, the arc behavior was similar to that with d1 of 0 mm when the d1 was adjusted to 2 mm.But, the obvious difference was that the direct arc between side wires and workpiece was ignited first in this situation as illustrated in Fig.4 (a).The bypass current maintained around 160 A. With the main wire continuously feeding, the distance from the root of main wire to the side wire decreased and the direct arc tended to deflect in the direction of the main wire as illustrated in Fig.4 (b).As the distance further decreased, the indirect arc was ignited.The direct arc was not extinguished immediately, but integrated with the indirect arc as shown in Fig.4 (c).As such, it can be observed that the current waveform was a downward trend due to the arc length increased.After two types of arcs existed in a coupled state for 10 ms, the direct arc was extinguished as shown in Fig.4 (d).Both the main wire and side wire metal transferred to the molten pool with the globular transfer mode.The indirect arc was extinguished with the main wire droplet completing the transfer process and the direct arc was reignited as shown in Fig.4(e).

Fig. 11 -
Fig. 11-Schematic of metal transfer and arc behavior with different d1: (a) unstable explosive transfer (b) stable short-circuiting transfer (c) stable continuous contacting transfer

Fig. 12 -
Fig. 12-Schematic of metal transfer and arc behavior with different FSM: (a) unstable repelled globular transfer (b) stable short-circuiting transfer (c) stable continuous contacting transfer According to Luksa [22], the standard deviation (s) and variation coefficient (ν) of arc voltage and current can be applied to evaluate the arc stability.The definition of standard deviation and variation coefficient can be expressed by Eq. (2) and Eq.(3), respectively.

Fig. 15 Fig. 17 -
Fig. 15 Weld formation and its cross section with different d1

Table 2 -
Compositions of the base metal and filler wires (Wt%)