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Article

Analysis of Droplet Transfer and Arc Swing in “TIG + AC” Twin-Wire Cross Arc Additive Manufacturing

by
Xueping Song
1,2,3,*,
Zhuoxuan Li
1,3,
Jiankang Huang
1,*,
Ding Fan
3,* and
Shurong Yu
4
1
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
2
Mechanical Engineering College, Lanzhou Petrochemical University of Vocational Technology, Lanzhou 730060, China
3
Materials Science and Engineering College, Lanzhou University of Technology, Lanzhou 730050, China
4
Mechanical and Electrical College, Lanzhou University of Technology, Lanzhou 730050, China
*
Authors to whom correspondence should be addressed.
Metals 2023, 13(1), 63; https://doi.org/10.3390/met13010063
Submission received: 29 November 2022 / Revised: 23 December 2022 / Accepted: 23 December 2022 / Published: 26 December 2022
(This article belongs to the Special Issue Microstructure and Mechanical Property Improvement of Welded Metal Joints)
Browse Figures
Review Reports Versions Notes

Abstract

:
Twin-wire and arc additive manufacturing (T-WAAM) has potential advantages in improving deposition efficiency and manufacturing functionally graded materials (FGMs), thus attracting much attention. However, there are few studies on the droplet transfer mode of T-WAAM. This paper analyzes the droplet transfer mode and arc swing in the “TIG + AC” twin-wire cross-arc additive manufacturing by in-situ observation with high-speed photography, revealing what factors influence the T-WAAM on deposition shaping the quality and what are the key mechanisms for process stability. Experiments show that with the main arc current provided by TIG 100 A and the twin-wire AC arc current 10 A, three different droplet transfer modes, namely the “free transfer + free transfer, bridge transfer + free transfer, bridge transfer + bridge transfer,” can be observed with the twin wires under different feeding speeds. The corresponding deposition and arc swing are quite different in quality. Through comparative analysis, it is found that the frequent extinguishment and ignition of the arc between electrode wires is the main factor for the instability in the additive manufacturing process. The “bridge transfer + free transfer” mode can obtain a large arc swing angle and a stable deposition, in which the cross arc has a significant stirring effect on the molten pool, and the deposition shape is well-made.

1. Introduction

Additive manufacturing (AM), a revolutionary manufacturing method for parts, has been widely used in the field of industrial manufacturing and research. This technique, based on the principle of “discrete-deposition” [1], builds materials layer by layer for components through CAD design data. The traditional method of manufacturing components is subtractive manufacturing, a “bottom-up” deposition [2]. In terms of energy sources, AM is generally classified into three categories [3]: laser additive manufacturing (LAM), electron beam additive manufacturing (EBAM), and wire arc additive manufacturing (WAAM). Because of their high energy density and good deposition accuracy, LAM and EBAM have been studied and applied in complex component shaping. Still, both have shortcomings, such as complex equipment, high manufacturing cost, low deposition efficiency, etc. It is especially difficult to manufacture large-sized structural components, and the cost is high [4,5]. In contrast, the low-cost WAAM can achieve efficient deposition and utilization and is applicable in in-situ and large-size component manufacturing [6,7,8].
With the increasing industrial manufacturing requirements for arc additive processing, the WAAM using a single wire and single arc is no longer acceptable. Therefore, efficient deposition methods such as single-arc and multi-wire [9], multi-arc and single-wire [10], and multi-wire and arc [11] come into being. They improve cladding efficiency and regulate the microstructure composition of additive manufacturing components. Technologies such as twin-wire welding [12,13,14], twin-wire welding bypass-arc [15,16], and twin-wire cross-arc welding [17,18] have laid a good foundation for the development of T-WAAM technology. Wu [19] et al. used double-wire indirect arc surfacing, using ER308 stainless steel wire on Q235 mild steel. Compared with the MIG surfacing layer, layers made by twin-wire indirect arc welding (TWIAW) are ready for intergranular corrosion but are spot-corrosion resistant. Since dual or multiple wires can be used to change the wire feed rate for composition control by using different wires, different functionally graded materials [20,21] even 4D functional components can be obtained [22,23]. Thus, scholars have widely been concerned about it [24,25,26,27]. Feng [28] et al. fabricated stainless steel workpieces with an average 10.2% increase in ultimate tensile strength and a maximum elongation increment of 176% using a dual-wire arc additive. Gu [29] et al. manufactured Al-Cu-Sn alloys using single-wire and dual-wire arc additive techniques. In addition, they found that the deposited walls made by dual-wire had better surface shape and mechanical microstructure than those by a single wire. There are also a series of studies using WAAM to produce gradient-structured materials. Chen [30] et al. used TC4 and 316 stainless steel dual welding wire to fabricate workpieces with a maximum microhardness of ~945 HV0.2. According to Zhang [31] et al., gradient material was made by ER2594 alloy wire whose microstructure was adjusted. Reisgen [32] et al. used multi-wire arc additive manufacturing by varying the wire feeding speed. In addition, Pan [33] et al. investigated twin-wire arc additive manufacturing of gradient materials using aluminum alloy wire and stainless steel wire with a TIG arc as the heat source. Wang [34] et al. manufactured Ni-Ti alloys rich in nickel for the first time using Ti/Al dual wire deposition on pure Ni substrates. However, there are few studies on improving the molding quality of additively manufactured parts, especially on the stability of the dual wire WAAM deposition process and molding quality in terms of heat and mass transfer.
The current research on functional gradient materials focuses on how to make high-quality parts by multi-wire arc additive manufacturing [35]. However, due to the difference in melting speeds of dissimilar wires, double wires in the manufacture of heterogeneous wires, the transfer of mass and heat are not synchronous, which leads to uneven microstructure in manufacturing workpieces [36]. Thus, it is unsatisfactory to consider tissue, structure, and function in the design of the additional manufacturing parts. Our research team [37] proposed a dual-wire arc additive manufacturing technology assisted by AC current. The obtained additive manufacturing parts have uniform microstructure, fine grain, reduced porosity, and obviously improved deposition quality. It has been found that heat and mass transfer behavior is a key factor in the microstructure and properties of twin-wire arc additive manufacturing workpieces, so it is particularly important to study the process’s physical behavior and forming mechanism.
To further reveal the reasons for the differences in stack morphology from the perspective of heat and mass transfer, this paper builds a “TIG + AC” twin-wire arc additive manufacturing experiment system, in which three different melt droplet transfer modes are obtained with adjustment in wire feeding speed. Therefore, comparative studies are made on the relationship among the melt droplet transfer modes, arc swing behavior, process stability, and deposition quality. This study may contribute to the selection and optimization of twin-wire arc additive manufacturing parameters for quality purposes and the tissue homogenization of metal gradient functional materials by WAAM.

2. Deposition Experiment of “TIG + AC” Twin-Wire Arc Additive Manufacture

An experimental system was built, as shown in Figure 1, in which the base material is 304 stainless steel with a size of 150 mm × 100 mm × 3 mm. Two filler wires are also 304 stainless steel with a diameter of 1.2 mm and are independently controlled by two wire feeders. 99% argon gas is used as the shielding gas at the speed of 10 L/min. In the experiment, high-speed photography is used to record the additive manufacturing process, including the arc morphology and droplet transfer, with a frame rate of 1000 frames/second. The main arc current is 100 A, supplied by TIG power with direct current straight polarity (DCSP). AC arc is generated between two filler electrodes. The current is set at 10 A, the frequency at 10 Hz, supplied by an AC welding power source. An AC arc can generate an alternating magnetic field to cause the TIG arc to swing periodically with the change of double-wire polarity. The heat of melting the wire in the dual wire cross arc additive manufacturing consists of TIG arc heat, AC arc heat, and resistance heat. Meanwhile, due to the decisive role played by the spacing between the ends of the twin wires in arc stability, it is considered as the length of the interwire AC arc in this paper.
Experiments show that the transfer mode of the twin-wire droplet can be controlled by changing the speed of twin-wire feeding (V1 on the left, V2 on the right). In the deposition, the droplet transfer modes present three types:
  • When the wire feeding speed is V1:V2 = 110 mm/min:120 mm/min, it takes the mode of “free transfer + free transfer.” The molten droplets on both sides of the TIG torch fall freely into the molten pool with no contact.
  • When the wire feeding speed is V1:V2 = 150 mm/min:120 mm/min, it is in the mode of “bridge transfer + free transfer.” That is, when the welding wires on both sides of the TIG welding torch stacks, the length of the droplet formed at one end is in contact with the molten pool, thus forming a bridge for transfer, and the droplet at the other end is in the form of free transfer.
  • When the wire feeding speed is V1:V2 = 150 mm/min:160 mm/min, it is in the form of “bridge transfer + bridge transfer,” that is, the welding wire on both sides of the TIG welding torch contacts the molten pool before the droplet grows up to form bridge transfer.

3. Analysis of Arc Swing and Droplet Transfer in “TIG + AC” Twin-Wire Cross Arc Additive Manufacturing

3.1. Arc Swing Behavior in the Model of “Free Transfer + Free Transfer”

A high-speed camera is used to capture the welding wire melting and arc swing by “TIG + AC” twin-wire cross-arc additive manufacturing in the mode of “free transfer + free transfer,” as shown in Figure 2. It is found that the spacing between the droplet and the molten pool at the end of the right welding wire is larger than that on the left, so is the size of the droplet on the right, and the swing angle of the arc to the right welding wire is 32.955°, which is much larger than that at the end of the left welding wire whose angle is 10.755°. When the droplets at the two ends are in the form of free transfer, the arc extinction and the restart of the arc between the wires are prone to occur because of the impact of the transfer and swing of the droplets on the spacing between the wires. As shown in the 60–80 ms high-speed photographs, the arc current after extinction is much larger than that in additive manufacturing, in which the wire is molten quickly and the droplet swings significantly because of the arc force, resulting in unstable changes in the spacing between the double wires. The arc will be extinguished when the distance becomes too large between the tips at 100 ms. As the welding wire continues to be fed, it will repeat itself again and again, making the whole process unstable, as shown in Figure 3. In turn, the swing and tremor of the molten droplet caused by repeated arc extinguishing and restarting results in the failure of the molten droplet to fall into the pool. Thus, the molding morphology is seriously affected.
Figure 4 shows the change of droplet size in 25 arc swing cycles in the mode of “free transfer + free transfer.” The right side welding wire only completed one transfer in 25 cycles, which lasted 1600 ms, and the maximum diameter of the droplet growth was 3.77 mm. Two transfers were completed on the left side the last 900 ms, with the maximum diameters of droplet growth 2.61 mm and 2.70 mm, respectively. It can be seen that the droplet size is related to its transfer period. The longer the transfer period, the lower the frequency and the larger the droplet size. Therefore, the droplet size on the left side is smaller than that on the right. The droplet transfer frequency on the left side is higher than that on the right side because the deflection angle of the arc to the left is larger than that to the right. The smaller the deflection angle, the stronger the TIG main arc thrust in the direction of the droplet transfer and the more frequent the transfer. In “free transfer + free transfer,” the droplet has enough space to fully grow up before falling under gravity and arc force because of the slow speed of wire feeding, the large spacing between the wires, and the tip’s long distance from the arc and the molten pool.

3.2. Arc Swing Behavior in the Model of “Bridge Transfer + Free Transfer”

By increasing the wire feeding speed of the left side welding wire, a mixed double-wire droplet transfer can be observed in a bridge transfer mode on the left and a free transfer mode on the right side, in which the feeding speed of the former and latter are 150 mm/min and 120 mm/min respectively. Observed by high-speed photography, the arc swing is shown in Figure 5. The arc swing is relatively stable in that the swing angle to the left (25.165°) is greater than that to the right (12.575°). In particular, because the left droplet is closer to the deposition layer and accessible to the molten, the swing to the left tends to move toward the pool center. Moreover, the welding wire moves to the side with better conductivity, that is, the side of the deposition layer. In this mode, therefore, the arc can stir the molten pool more effectively during the deposition. The inclination of the arc swing to the bridge transfer end on the left produces droplets less affected by arc force and stable arc length between the wires and light swing. In other words, the deposition-forming effect is much better.
Figure 6 shows the change of droplet size during 25 arc swing cycles in the “bridge transfer + free transfer” when the welding wire on the right experiences two complete droplet transfers taking 1100 ms and 1200 ms, respectively. On the left side, four times are completed with a transfer period of 500 ms. Compared with the “free transfer + free transfer” mode, the size of the droplet is smaller. The maximum diameter of the droplet growth on the left is within 1.80 mm~2.40 mm, and that on the right is within 3.25 mm~3.50 mm. The main reason for these differences is the increased feeding speed on the left side, the reduced spacing between wires, and the closer location of the wire tip to the core area of the arc and pool with larger flow force. The drop transfer frequency is higher than that of “free transfer + free transfer,” and the period is shorter. In addition, the transfer frequency on the left side is higher than that on the right side as the former is a bridge transfer, in which the liquid bridge accumulates more heat than the right side at a faster formation speed.

3.3. Arc Swing in the Model of “Bridge Transfer + Bridge Transfer”

Both the left and right welding wire droplets are in the mode of bridging transfer, where the feeding speed on the right is increased to 160 mm/min and the left at 150 mm/min, as shown in Figure 7. The deflection angle of the TIG main arc to the left is 7.125° and 10.135° to the right; therefore, little difference can be observed. In the bridging transfer, the droplet contacts the molten pool before it grows because of its fast feeding speed. The molten metal bridge is subjected to a large electromagnetic contraction force and surface tension, so the molten droplet quickly transfers into the pool. Even with repetition, the process is relatively stable. However, a short circuit occurs when the current in the AC circuit between the two wires flows through the molten pool, in which the arc between the AC output wires disappears, and the effect of the alternating magnetic field is weakened. This results in an insignificant swing of the TIG main arc and a poor stirring effect on the pool. In addition, it can also be found that the molten pool volume in this mode is significantly smaller than that of the above two free transfer modes, as the overall fluidity of the molten pool is poor.
Figure 8 shows the droplet size changes in 25 arc swing cycles in the mode of “bridge transfer + bridge transfer.” As the double wire is fed at high speed, the droplet contacts the molten pool before it grows to form a bridge with high transfer frequency in 500 ms on the left and 600 ms on the right. Meanwhile, the sizes of the right and left droplets are both small, where the maximum diameters of each growth are 1.9 mm~2.1 mm and 1.6 mm~1.8 mm, respectively.

3.4. Comparison and Analysis of Three Droplet Transfer Models

Statistics were made on the unstable times of arc extinction and restarting, and the number of arc swings within 10 s in the 100 arc swing cycles under the three droplet transfer modes. The results are shown in Figure 9. Statistics show that within 100 cycles, arc instability occurs 64 times and 5 times in the mode of “free transfer + free transfer” and “bridge transfer + free transfer,” respectively. However, there is no arc extinguishing and restriking in the bridge + bridge transfer mode. As 100 arc swing cycles should be included in 10 s, the arc is quite unstable under the mode of “free transfer + free transfer” with 87 completed swings. In contrast, 99 and 100 arc swings are completed stably in the “bridge transfer + free transfer” and “bridge transfer + bridge transfer” modes, respectively. The main reason for arc extinguishment and restarting between AC double wires is due to the change in wire spacing. When the distance is too large, the arc between the AC double wires cannot be maintained, and the heat received by the welding wire decreases. If the melting speed exceeds the feeding speed, the dry elongation of the welding wire will increase, and restriking will occur. Hence, the instantaneous melting speed of the wire will be accelerated because of the strike current, and the spacing between the wires will increase. Arc extinguishing and restriking caused by the secondary ignited AC arc is easy to extinguish. Obviously, in the mode of “free transfer + free transfer,” the manufacturing suffers the worst stability, which thus has a bad impact on the stirring of the arc in the molten pool and the shape of the stacked layer.
We investigate the maximum diameter of droplet growth and the average transfer period of three different droplet transfer modes in the 10 droplet transfer periods. The results are shown in Figure 10. It can be seen that the droplet size and droplet transfer period of the right welding wire are larger than those of the left, as shown in all modes. As for the “free transfer + free transfer” mode and the “bridge mode + free transition” mode, the wire droplet on the right is in the mode of free transfer, but the diameters of the droplet and the transfer periods are different because of the difference in the arc deflection angles. The arc in the latter mode is prone to the left while the former mode is closer to the right, and the melting speed of the wire on the right is faster, so the droplet is relatively far away from the molten pool. Therefore, the droplet diameter is larger, and the droplet transfer period is longer. The early contact with the pool before the droplet grows up results in its significantly smaller size and longer bridge transfer period.
Figure 11 displays the arc swing angles under different droplet transfer modes within 10 arc swing cycles. The ranges of the arc swing angles are “37.5°~50°, 35°~40°, 15°~20°” in the “free transfer + free transfer, bridge transfer + free transfer, bridge transfer + bridge transfer, “respectively, and the arc swing amplitude decreases obviously in turn. This is since the spacing between the wires limits the arc swing to a certain extent with the smallest spacing and swing angle in the “bridge transfer + bridge transfer” mode. However, the swing angle fluctuates greatly in the mode of “free transfer + free transfer.” In the 2nd, 5th, and 8th cycles, the arc current is relatively large when the arc between the wires extinguishes and restarted. The increased spacing results in the widening of the arc swing angle.
Figure 12 shows the macroscopic morphology of the stacked layer in the single channel and single layer under different droplet transfer modes, and the height-width ratio of the stacked layer is shown in Figure 13. In the mode of “free transfer + free transfer,” the uneven layer width is caused by the instability in the manufacturing process. The droplet with a large initial kinetic energy in the free transfer mode shows more defects and poorer surface morphology, with lower layer height and larger width [38,39]. For the mode of “bridge transfer + bridge transfer,” which involves no cross-arc action, the melting rate of the welding wire decreases, and the welding wire inserts into the molten pool, resulting in the bending of the stacked layer. In contrast, under the mode of “bridge transfer + free transfer,” the deposition layer is straight with good deposition efficiency, in which the additive manufacturing process is stable. Thus, the obtained deposition shape is better.
Figure 14 shows the arc and droplet swing characteristics under the three droplet transfer modes. In the mode of “free transfer + free transfer” (see Figure 14a), repeated extinguishing and restarting of the indirect arc are caused by the frequent changes in the spacing between the two welding wire tips due to the sloshing and dripping of larger droplets. For the sake of good additive manufacturing quality, this mode should be avoided. In the mode of “bridge transfer + bridge transfer” (see Figure 14c), the swing of the TIG arc is not obvious because of the disappearance of the indirect arc and the small spacing between the two wires. The stirring effect on the molten pool is relatively small as well. In the mode of “bridge transfer + free transfer” (see Figure 14b), however, the left droplet has not fully grown up before contacting the molten pool for a liquid bridge, and the arc moves towards the side of the liquid bridge with better conductivity. With the left side of the arc swing angle larger than the right side, the electromagnetic force on the right droplet is relatively small, with no swings. Thus, the transfer in this mode is quite stable.

4. Conclusions

In this paper, we analyze the droplet transfer and arc swing in the twin-wire AC cross WAAM by high-speed photography in situ observation, and the following main conclusions are found:
The research shows that by changing the wire feeding speed, the control of the droplet transfer mode during the deposition process can be realized.
Through high-speed photographic observation, the repeated arc extinguishing and re-starting in the AC arc between wires is the main reason for the stability of the deposition and the deterioration of the deposition parts.
Comparative analysis reveals that the arc in the mode of “free transfer + free transfer” is poor in stability and deposition shape. In addition, in the mode of “bridge transfer + free transfer,” the arc swing angle is larger, and the stirring effect of the arc on the molten pool is enhanced. In this sense, the deposition process is stable with a good deposition shape.

Author Contributions

Conceptualization, J.H. and D.F.; methodology, J.H. and D.F.; software, J.H.; validation, J.H. and D.F. and X.S.; formal analysis, X.S. and Z.L.; investigation, X.S.; resources, J.H.; data curation, X.S. and Z.L.; writing—original draft preparation, X.S. and Z.L.; writing—review and editing, X.S. and J.H.; visualization, Z.L.; supervision, S.Y.; project administration, J.H.; funding acquisition, J.H. and X.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 52175324” and “The APC was funded by he Innovation Capability Improvement Project of higher education institutions in Gansu Province of China in 2019 (No.2019-198A).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental system of the “TIG + AC” twin-wire cross arc additive manufacturing.
Figure 1. Experimental system of the “TIG + AC” twin-wire cross arc additive manufacturing.
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Figure 2. Arc swing in the model of “free transfer + free transfer”.
Figure 2. Arc swing in the model of “free transfer + free transfer”.
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Figure 3. Periodic extinguishment and ignition of AC arc in the mode of “free transfer + free transfer”.
Figure 3. Periodic extinguishment and ignition of AC arc in the mode of “free transfer + free transfer”.
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Figure 4. Changes in droplet size in the model of “free transfer + free transfer”.
Figure 4. Changes in droplet size in the model of “free transfer + free transfer”.
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Figure 5. Arc swing in the model of “bridge transfer + free transfer”.
Figure 5. Arc swing in the model of “bridge transfer + free transfer”.
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Figure 6. Changes in droplet size in the mode of “bridge transfer + free transfer”.
Figure 6. Changes in droplet size in the mode of “bridge transfer + free transfer”.
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Figure 7. Arc swing in the model of “bridge transfer + bridge transfer”.
Figure 7. Arc swing in the model of “bridge transfer + bridge transfer”.
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Figure 8. Changes in droplet size in the mode of “bridge transfer + bridge transfer”.
Figure 8. Changes in droplet size in the mode of “bridge transfer + bridge transfer”.
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Figure 9. Stability comparison of three droplet transfer modes.
Figure 9. Stability comparison of three droplet transfer modes.
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Figure 10. Comparison of three droplet transfer periods and droplet sizes.
Figure 10. Comparison of three droplet transfer periods and droplet sizes.
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Figure 11. Comparison of the arc swing angles under three droplet transfer modes.
Figure 11. Comparison of the arc swing angles under three droplet transfer modes.
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Figure 12. Macroscopic morphology of deposition layer (a) free transfer + free transfer (b) bridge transfer + free transfer (c) bridge transfer + bridge transfer.
Figure 12. Macroscopic morphology of deposition layer (a) free transfer + free transfer (b) bridge transfer + free transfer (c) bridge transfer + bridge transfer.
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Figure 13. Deposition layer height-width ratio under three droplet transfer modes.
Figure 13. Deposition layer height-width ratio under three droplet transfer modes.
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Figure 14. Arc swing in deposition process in three droplet transfer modes. (a) free transfer + free transfer (b) bridge transfer + free transfer (c) bridge transfer + bridge transfer.
Figure 14. Arc swing in deposition process in three droplet transfer modes. (a) free transfer + free transfer (b) bridge transfer + free transfer (c) bridge transfer + bridge transfer.
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MDPI and ACS Style

Song, X.; Li, Z.; Huang, J.; Fan, D.; Yu, S. Analysis of Droplet Transfer and Arc Swing in “TIG + AC” Twin-Wire Cross Arc Additive Manufacturing. Metals 2023, 13, 63. https://doi.org/10.3390/met13010063

AMA Style

Song X, Li Z, Huang J, Fan D, Yu S. Analysis of Droplet Transfer and Arc Swing in “TIG + AC” Twin-Wire Cross Arc Additive Manufacturing. Metals. 2023; 13(1):63. https://doi.org/10.3390/met13010063

Chicago/Turabian Style

Song, Xueping, Zhuoxuan Li, Jiankang Huang, Ding Fan, and Shurong Yu. 2023. "Analysis of Droplet Transfer and Arc Swing in “TIG + AC” Twin-Wire Cross Arc Additive Manufacturing" Metals 13, no. 1: 63. https://doi.org/10.3390/met13010063

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