- Email: [email protected]
Investigation of arc behaviour and metal transfer in cross arc welding
Journal of Manufacturing Processes 37 (2019) 124–129
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Investigation of arc behaviour and metal transfer in cross arc welding L. Zhang a b c
a,b,⁎
c
a,b
, S. Su , J. Wang
, S.J. Chen
T
c
Hebei Engineering Laboratory of Aviation Lightweight Composite Materials and Processing Technology, Hebei University of Science and Technology, Shijiazhuang, China Hebei Key Laboratory of Material Near-Net Forming Technology, Hebei University of Science and Technology, Shijiazhuang, China Beijing University of Technology, Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, Beijing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cross arc welding process Arc behaviour Metal transfer Arc force analysis Weld forming
In traditional welding processes, the electrode and the work-piece serve as the two terminals of the arc, cathode and anode, respectively. In addition, their heat and mass inputs are coupled and unable to be freely optimized to suit for the needs from specific applications. A modified arc welding process is thus proposed in this work to establish a cross arc process where two wires are charged by a second power supply and are fed into the main arc (gas tungsten arc (GTA) or plasma arc (PA)) which heats the work-piece. An inter-wire arc can be established between the two wires to heat both wires directly. The heat and mass inputs are decoupled. A high speed camera is used to analyze the metal transfer behavior. The main arc swings regularly among the two wires as the alternating inter-wire arc current switches its priority: the main arc deviates to the cathode of inter-wire arc. The main arc is deviated by the Lorentz force, which is generated by the inter-wire arc current. The PA force and the aerodynamic drag force, thus automatically acts on the droplet on the cathode wire to provide an additional detachment force. The parameters of the two coupled arcs can be adjusted separately to provide the needed deposition rate, force and heat input freely as the capability to increase the productivity while maintaining the base metal heat input at a desired level. Therefore, this new innovative hybrid arc welding process can be used for overlaying by the new heat transfer mechanism.
Introduction The rapid development of modern manufacturing, especially in aerospace, petrochemical, marine engineering and energy engineering industry, which puts forward the urgent requirement of high efficiency, high quality of the welding process. The welding process can not satisfy the modern industrial production. Manufacturing industries are always looking for novel arc welding process to increase the welding quality [1–3]. GMAW (Gas Metal Arc Welding) and GTAW (Gas Tungsten Arc Welding) are two major welding processes in manufacturing industries [4,5], lots of research works have been done on the two arcs welding processes to accelerate the deposition, much more new welding processes are invented, especially in the areas of the high efficiency and high speed, such as Tandem, plasma-GMAW, multi-wire GMAW. Despite these new hybrid arc welding processes showed some obvious advantages in the welding speed and deposition [6–9], but all of them are based on the conventional working mode: the arcs are established between the multi-wire and work-piece in parallel, the welding pool is not only heated by several arcs, but also it is a common object in which arcs coupling. As the new hybrid arc welding processes which are in the
⁎
fundamental principle, are limited in heat input, deposition and the arc force for welding pool. The current, needs to be increased to increase the deposition rate, increase the deposition rate and increase the heat input thus the distortion. It is difficult to freely deliver the heat, mass and force needed to produce desirable welds effectively and ecumenically [10]. The multi-electrode coupled arc welding processes are different from the conventional hybrid arc welding processes, which are not based on the fundamental principle, but in the new mode, multi-electrodes or pairs of electrodes are discharged in a common space, the welding pool is just only one polarity of many electrodes which is not a common polarity in the conventional mode, the arcs are coupled with each other. It is easier to achieve the new welding process with the new arc working mode, which can relieve the inherent constraints on heat input, deposition and the arc force, the advantages of the new welding process are the high efficiency and high welding speed and low heat input, increase the welding currents can get higher productivity and less distortion. The changes of the arc principle, the arc polarity is changed in the new coupled welding process and it is no longer on the work-piece, the behavior is changed significantly. DE-GMAW (Double Electrodes GMAW) [11–13], arcing-wire GTAW [14] and cross arc
Corresponding author at: No. 26 Yuxiang Street, Shijiazhuang, 050000, China. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.jmapro.2018.11.018 Received 31 July 2018; Received in revised form 14 November 2018; Accepted 20 November 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Fig. 2. Cross arc welding experimental system. Fig. 1. Principle of cross arc welding process. Table 1 The Experimental System Components.
[15,16] are three new multi-electrode arc welding processes, which bypass arc was divided from the electrode of the main arc to another electrode. As add a bypass arc, the distribution of arc energy changed, the two arcs coupled with each other and the arc behaviour changed obviously. The cross arc welding processes Cross arc welding process is a novel welding process: the plasma arc (PA) is established between the tungsten in the plasma torch and workpiece as the main arc to heat the work-piece; the inter-wire arc is established between the two wires, fed toward the PA from opposite directions, to melt the two wires by its anode and cathode simultaneously. The inter-wire current for the inter-wire arc is provided by an AC (alternating current) power source, as shown in Fig.1. The AC current waveform can allow a separate control on the melting speeds of the two wires. The inter-wire arc is crossed with the PA and one can consider that the PA penetrates through the inter-wire arc to reach the work-piece from the tungsten electrode. The heat input on the workpiece as well as the deposition into the work-piece can thus be controlled separately by the PA and inter-wire arc currents. Cross arc welding process is different from the conventional hybrid welding processes, the two arcs are not established parallel between wire and work-piece, but coupled with each other, the work-piece is no longer heated by all arcs, the new welding process can freely provide the needed deposition and heat input as desired by different applications. The two arcs are coupled and interacted as that many parameters can affect the stability of the coupled arc and thereby affect the quality of welds. In this paper, in order to explain the mechanism of the coupled arc and improve the arc stability, the experiments are focus on the arc behavior and metal transfer to analyze the stable/unstable process in the new multi-electrode arc welding processes. The arc behavior and parameters are discussed and outlined to help further develop the new process.
Equipment and Accessories
Model, material or Size
PA power supply GTA power supply GMA power supply Diameter of tungsten High-speed camera Shielding gas of GTAW
PLASMA-400 Miller Dynasty 700 Miller XMT 350 Φ5 mm IDT motion Y4 Pure Argon
camera (without optical filter) is used to record the arc at 3000 frames per second. High-speed camera and arc signals were synchronized to observe the behaviors of the multi-electrode coupled arcs. The recorded current waveforms and voltage waveforms (at the sampling rate 150 K Hz per channel) and images used to analyze and judge the behavior of coupled arcs. It should be noted that waveforms and the images are synchronous.
Experimental results In the previous preliminary study, the main arc is a GTA/PA, the inter-wire current is alternating current (AC). While using two wires provides a way to melt the wire fast without imposing heat on the workpiece or the tungsten, the operation of the proposed cross wire arc welding process relies on the balance between the melting of the two wires and the sustaining of the cross arc between the two wires. Since sustaining the cross arc depends on the melting balance, the melting can certainly be balanced. This study tries to verify the feasibility of the proposed idea, firstly without the coupling from the melting balance. The wires are first replaced by two non-consumable electrodes. To maximize the detachment ability, a constrained arc (plasma arc) should be more effective than a free GTA. Hence, it is the first time realizes a real cross arc welding process whose success depends on a successful metal transfer in this study, the authors propose to use a plasma arc (PA) to replace the GTA. In this paper, there are four group experiments, experiments 1# and 2# are the preliminary study by GTA and carbon electrodes, the purpose of the two experiments is to verify the feasibility of cross arc welding process, it is found that there is a new phenomenon: GTA is deviated to the cathode of inter-wire arc, the changes of arc behavior can modify arc force and metal transfer. Moreover, in experiments 3# and 4, the research is focus on the effect of arc behavior on the droplet transfer, especially the study on process of droplet detachment by the main arc. PA might even replace GTA as the main arc for the smooth metal transfer, in order to distinguish how the main arc can have effect on the metal transfer, there are two different modes: PA and PA with pulse in experiments 3# and 4# respectively. Based on the analysis of the experiments, stet the following parameters of cross arc welding experiments as shown in Table 2.
Experimental system The multi-electrode coupled arc welding processes are at least two arcs coupled with each other. This study tries to find the mechanism of the coupled arcs, in the research of the behaviors of the arcs, the experimental system should record all the signals of the arcs for the welding processes. In the experimental system, a GTA/PA is established using a DC-CC (direct current-constant current) power supply and GMA is established using an AC-CV (alternating current-constant voltage) power supply. An experimental system and system components are shown in Fig. 2 and Table 1 respectively, there are four sensors to detect the voltages and currents of the two arcs. The voltage sensors, current sensors and high speed camera can be realized the acquisition. The high-speed 125
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Table 2 The Parameters of the Cross Arc Welding Experiments. Welding Process
Experiments
GTA(PA) Current
GMA Current
Wire Feed Speed
GTA + Inter-wire arc
1# 2# 3# 4#
100 A 100 A 100 A 100 A(P)
50 A 150 A 100 A 100 A
0 0 3.5 m/min 3.5 m/min
PA + Inter-wire arc
Fig. 5. High-speed video in experiment #2.
considered to cross each other. The feasibility of the proposed cross arc process is thus experimentally demonstrated. When the inter-wire current further increased to 150 A in experiment #2. As can be seen from the Fig. 5, the GTA established between a tungsten tip (relatively thin) and the work-piece is more concentrated and should be in general brighter than the inter-wire arc established between two large carbon electrodes if the currents are the same. The GTA can also swing regularly between the two carbon electrodes, the deviation of GTA is rather obvious and the two arc spots of inter-wire arc are brighter than that in experiment #1. The voltages are measured and given in Fig.6, the GTA voltage waveform are not smooth, the stability of the GTA may also be affected by the inter-wire arc. However, it is also established as desired, the change is in the swing by the increased inter-wire arc. In the previous preliminary study, the cross arc can be established, as the new welding process is feasible, successful established of the desirable cross arc. The two arcs (GTA and inter-wire arc) are both stable, however, the main arc (GTA) is established between tungsten and work-piece, and the swing regularly between the two carbon electrodes, which is deviated to the cathode of inter-wire arc. The arc behaviors are different from the traditional arc characteristics, for this new phenomenon, the arc energy and arc force are redistributed by the new hybrid mode, the two wires can be melt by the two arcs, the main energy for melting wires is the polarity region energy of inter-wire arc as general arc. Moreover, the main arc is deviated and act on the wires, the changes of arc behavior break the function of arc energy and its column energy can be added on the two wires, therefore, the utilization rate of arcs energy to melt wires can be improved in cross arc welding process.
Fig. 3. High-speed video in experiment # 1.
GTA + inter-wire arc(carbon electrodes) There are two experiments in GTA + Inter-wire arc (carbon electrodes), the distance between tungsten and wire is fixed at 3 mm. The main arc is a GTA and the inter-wire arc is an AC GMA. The inter-wire distance is fixed at 2 mm while the currents vary. In experiment #1, GTA current and inter-wire arc current are fixed at 100 A and 50 A respectively. The desirable stability of the inter-wire arc can be seen in Fig.3, because of the relatively small currents of the GTA and inter-wire arc, the arc plasma for both arcs are not always seen clearly in the high-speed video. One pair of inter-wire arc spots (the cathode and anode) on the two carbon electrodes. However, it can still be observed that the GTA is not always straight. It may deviate either left or right from its straight trajectory. Analysis of the synchronized image and cross arc voltage and current, it shows that the GTA is straight when the cross arc is switching its polarity and the GTA deviates to the carbon electrode whichever is the cathode. The voltage waveforms are given in Fig.4, the GTA voltage (CH2) is at 22 V approximately, and the inter-wire arc voltage (CH4) is stable in AC square wave. Carefully observe the Fig.4, all the voltages are non-zero in the two voltage waveforms, hence, the GTA and inter-wire arc are
PA + inter-wire arc(wires) In experiment #1 and #2, it can be found that the GTA deviated to
Fig. 4. Voltage waveforms in experiment # 1.
Fig. 6. Voltage waveforms in experiment # 2. 126
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Fig. 9. The Lorentz force in cross arc welding process.
process is changed by the PA pulse duration. The smooth metal detachment can get the stable welding pool, and also result in the good weld formation.
Fig. 7. High-speed video in experiment #3.
the cathode wire and swung among the two wires as the polarity changes. In order to verify that is a unique phenomenon in the cross arc welding process, the two carbon electrodes are replaced by two wires, and GTA is replaced by PA in experiments #3 and #4. In order to get the metal transfer, the filter and aperture are both changed in the highspeed camera. As the PA is a restrain arc and the inter-wire arc is a free arc, the brightness and intensity of current are both different, therefore, the inter-wire arc is not caught by the high-speed camera. The experimental results are shown in Fig. 7, PA is a direct arc and established between tungsten and work-piece, which can automatically deviate to the cathode wire and switched among the two wires in experiment #3. Wires are melted by the anode and cathode of the interwire arc and the resultant droplets are not successfully detached from the two wires. By the careful observation of the high-speed videos in experiments #3, it is found that the force of PA can be acting on the droplet changes and the droplet is pushed to the opposite side of interwire arc. Although the droplet is also on the tip of the two wires, the cross arc is stable. The above observation and analysis clearly suggest that the PA pulse must be sufficient in order to detach the droplet and the sufficiency of the PA pulse. As such, the pulse duration must be sufficient to complete the detachment process. It appears that the detachment process would be fully completed in 3 ms for the aerodynamic drag force generated by 400 A plasma current in experiment #4. The experimental results are shown in Fig. 8, the PA pulse is applied and PA is larger in pulse duration. The metal transfer is stable and smooth. As the reason is that the force acting on the droplet changes and the droplet starts the detachment process. The dynamic development of the droplet detachment
Analysis and discussion Careful observation of the high-speed videos in experiments #1-4, it shows that GTA/PA can swing periodically with the polarity change among the wires (carbon electrodes). The GTA/PA deviates to the cathode electrode. The cathode switches from one carbon electrode to another, the GTA/PA switches accordingly. This is perfectly understandable, because the current polarity can be changed almost instantaneously while the ionization of the gas and temperature rise in the gas to establish the arc in a new region requires a time although it is short. As can be seen in the experimental results, the GTA/PA swing is apparent and it can be easily identified from the videos. The inter-wire current flowing from one electrode to another generates a magnetic field in the GTA/PA column. Fig. 9 shows the directions of this magnetic field, i.e., inward and outward the paper above and below the electrodes respectively. In accordance with the left-hand rule, the electrons from the tungsten electrode in such magnetic field will subject to an electromagnetic force [17–19], i.e., the Lorentz force, in the direction as shown in Fig. 9. That is, the force is toward the cathode of the electrodes. Hence, the GTA/PA must deviate periodically as the polarity of the inter-wire current changes. This swing may provide a mechanism to scan the GTA/PA on the work-piece, while the main current does increase force to increase the stability of droplet detachment, the parameters of GTA/PA are changed, resulting in changing the metal transfer, and the reason for the deep analysis from the changes, the forces acting on the droplet are related to GTA/PA. Fig. 10 shows the major forces acting on the droplet when the GTA/PA deviate to the droplet in the cross arc welding process, there
Fig. 8. High-speed video in experiment #4.
Fig. 10. The major forces acting on the droplet in cross arc welding process. 127
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
welding process. Fortunately, it is improved drastically in the cross arc welding process, the arc dividing relieves the inherent coupling between the heat input and deposition in conventional arc processes, and result in the advantages for high efficiency, high speed and low heat input with desirable coupled controls. To address this issue, the experiment research is carried out on carbon steel plate to analyze the form factor of weld, the plasma arc current is fixed at 45 A, just only adjust inter-wire arc current from 60 A to 120 A and change the wire feed speed accordingly. The weld cross sections are shown in Fig.13, as can be seen, in this case for the given work-piece and welding parameters, the weld with relatively low penetration is produced as desired, with the parameters changed, the weld width is increased gradually. The reason for this phenomenon is that heat input and deposition are decoupled in the novel arc welding process. Plasma arc and inter-wire arc can be adjusted separately, plasma arc as the main arc to heat the work-piece and maintain weld penetration at a certain level, as shown in Fig.13, however, the most obvious changes are in weld width and height, as the inter-wire arc welding current increases, the melting wire increases directly, which lead to excessive molten metal on the welds. Hence, the form factor of weld can be modified as demands. Cross arc welding process can decouple heat input and penetration for the capability to increase the productivity while maintaining the base metal heat input at a desired level.
Fig. 11. Weld appearance in experiments#3.
Fig. 12. Weld appearance in experiments#4.
Conclusion This paper has experimentally studied the arc behavior and metal transfer of cross arc welding process, as an innovative welding processes, the authors have found the following:
Fig. 13. Weld cross section in different inter-wire arc currents.
are six major forces: the gravitational force (Fg), the electromagnetic force of the inter-wire arc (Fem), GTA/PA aerodynamic drag forces (FP), the inter-wire arc aerodynamic drag forces (Fa), the surface tension of melt droplet (Fσ) and the vapor jet force (Fv). For the metal transfer, there are two groups forces: Fg, Fem, Fa and FP can promote the droplet detachment, nevertheless, Fσ is the dominant force retaining the droplet from being detached, Fv is the same as in the conventional GMAW, which only obviously influences the droplet detachment. The gravitational force (Fg) and the aerodynamic drag force from the PA (FP) are thus the two dominant forces facilitating the droplet detachment. The droplet could be detached from the two wires smoothly with a pulse PA in the cross arc welding process. The two welds of experiments #3 and #4 are shown in Figs. 11 and 12 respectively, the PA current changes and the droplet detachment can be changed accordingly, nevertheless, the more significant changes occurred in the welds, there is a big difference between the two welds forming, the weld appearance is rough and some pinholes/porosities on the weld surface, the weld width is variable, correspondingly. Several droplets detach from wire but fall at the boundary of the weld which instead of the center of weld, hence, with the unstable droplet detachment and transfer, the edge of weld toe is nonlinear, as shown in Fig. 11. Besides the factor of unstable droplet transfer, the flow behavior of weld pool is also unstable, which can get a humped bead. On the contrary, the stable droplet detachment and transfer can get a smooth weld, as shown in Fig.12, the significant differences in the two welds are the weld appearance. The main parameter which affects the welding quality is the form factor of weld (weld width/penetration), the welding parameters: arc current, arc voltage and welding speed, can affect the weld bead geometry and weld characteristics to a great extent. A quintessential example of welds should be cited is GMAW welds, the experiment results have previously shown that the form factor of weld is configured to permit adjustment within a fixed range, it is reasoned that the conventional arc welding processes distribute the anode and cathode of the arc on their respective electrode and the work-piece, it is difficult to separate the heat input and deposition rate in the traditional arc
• An • • •
experimental system has been established to experimentally demonstrate the feasibility of the proposed cross arc welding process with GTA/PA. The main arc can swing between regularly the two wires (carbon electrodes), which can deviate to the cathode of the inter-wire arc automatically as the Lorentz force. The PA force can act on the droplet on the cathode wire to provide an additional detachment force, droplets can be detached from wires. Cross arc welding process can decouple heat input and penetration for the capability to increase the productivity while maintaining the base metal heat input at a desired level.
Acknowledgements This work is supported by the Hebei Education Department (NO. ZD2018060) and the National Science Foundation of China (NO. 5177052363). References [1] 9th editionAnnette O, Brien G, editors. Welding handbook, Vol.2. Miami Fla: Welding Processes. American Welding Society; 2012. [2] Moniz BJ, Miller RT. Welding skills. 4th edition 2012. American Technical Publisher; 2010. [3] Key JF. 10th edition Arc physics of gas tungsten arc welding. ASM handbook vol.6. ASM International; 1993. [4] Yin SY. Fundamental and application of shield gas arc process. Beijing: China Machine Press; 2012. [5] Hasanniah A, Movahedi M. Welding of Al-Mg aluminum alloy to aluminum clad steel sheet using pulsed gas tungsten arc process. J Manuf Process 2018;33(6):96–110. [6] Ueyama T, Ohnawa T, Tanaka M, Nakata K. Occurrence of arc interaction in tandem pulsed gas metal arc welding. Sci Technol Weld Join 2007;12(6):523–9. [7] Lee HK, Park SH, Kang CY. Droplet transition for plasma-MIGwelding on aluminium alloys. J Mater Process Technol 2015;223(9):203–15. [8] Lassaline E, Zajaczkowski B. Narrow groove twin-wire GMAW of high- strength steel. Weld J 1989;68(9):53–8. [9] Ueyama T, Tong H, Harada S, et al. AC pulsed GMAW improves sheet metal joining.
128
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
2012;91(10):261–9. [15] Chen SJ, Zhang L, Wang XP, et al. Stability of cross arc process – a preliminary study. Weld J 2015;94(5):158–68. [16] Zhang L, Chen SJ, Song YX, et al. Metal transfer in cross arc welding process. Weld J 2016;95(9):340–56. [17] Shi Y, Liu XP, Zhang YM. Analysis of metal transfer and correlated influences in dual-bypass GMAW of aluminum. Weld J 2008;87(12):229–36. [18] J.F. Lancaster The physics of welding. Oxford, England: Pergamon Press. [19] Waszink JH, Piena MJ. Experimental investigation of drop detachment and drop velocity in GMAW. Weld J 1986;65(11):289–98.
Weld J 2005;84(2):40–6. [10] Chen KH, Li HQ, Li CX. Progress in variable polarity plasma arc welding. Trans China Weld Inst 2004;25(1):124–8. [11] Zhang YM, Jiang M, Lu W. Double electrodes improve GMAW heat input control. Weld J 2004;83(11):39–41. [12] Shi Y, Li J, Zhang G, et al. Corrosion behavior of aluminum-steel weld-brazing joint. J Mater Eng Perform 2016;25(5):1–8. [13] Zhou X, Zhang G, Shi Y, et al. Microstructures and mechanical behavior of aluminum-copper lap joints. Mater Sci Eng A 2017:705. [14] Chen JS, Lu Y, Li XR, et al. Gas tungsten arc welding using an arcing wire. Weld J
129
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Technical Paper
Investigation of arc behaviour and metal transfer in cross arc welding L. Zhang a b c
a,b,⁎
c
a,b
, S. Su , J. Wang
, S.J. Chen
T
c
Hebei Engineering Laboratory of Aviation Lightweight Composite Materials and Processing Technology, Hebei University of Science and Technology, Shijiazhuang, China Hebei Key Laboratory of Material Near-Net Forming Technology, Hebei University of Science and Technology, Shijiazhuang, China Beijing University of Technology, Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, Beijing, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cross arc welding process Arc behaviour Metal transfer Arc force analysis Weld forming
In traditional welding processes, the electrode and the work-piece serve as the two terminals of the arc, cathode and anode, respectively. In addition, their heat and mass inputs are coupled and unable to be freely optimized to suit for the needs from specific applications. A modified arc welding process is thus proposed in this work to establish a cross arc process where two wires are charged by a second power supply and are fed into the main arc (gas tungsten arc (GTA) or plasma arc (PA)) which heats the work-piece. An inter-wire arc can be established between the two wires to heat both wires directly. The heat and mass inputs are decoupled. A high speed camera is used to analyze the metal transfer behavior. The main arc swings regularly among the two wires as the alternating inter-wire arc current switches its priority: the main arc deviates to the cathode of inter-wire arc. The main arc is deviated by the Lorentz force, which is generated by the inter-wire arc current. The PA force and the aerodynamic drag force, thus automatically acts on the droplet on the cathode wire to provide an additional detachment force. The parameters of the two coupled arcs can be adjusted separately to provide the needed deposition rate, force and heat input freely as the capability to increase the productivity while maintaining the base metal heat input at a desired level. Therefore, this new innovative hybrid arc welding process can be used for overlaying by the new heat transfer mechanism.
Introduction The rapid development of modern manufacturing, especially in aerospace, petrochemical, marine engineering and energy engineering industry, which puts forward the urgent requirement of high efficiency, high quality of the welding process. The welding process can not satisfy the modern industrial production. Manufacturing industries are always looking for novel arc welding process to increase the welding quality [1–3]. GMAW (Gas Metal Arc Welding) and GTAW (Gas Tungsten Arc Welding) are two major welding processes in manufacturing industries [4,5], lots of research works have been done on the two arcs welding processes to accelerate the deposition, much more new welding processes are invented, especially in the areas of the high efficiency and high speed, such as Tandem, plasma-GMAW, multi-wire GMAW. Despite these new hybrid arc welding processes showed some obvious advantages in the welding speed and deposition [6–9], but all of them are based on the conventional working mode: the arcs are established between the multi-wire and work-piece in parallel, the welding pool is not only heated by several arcs, but also it is a common object in which arcs coupling. As the new hybrid arc welding processes which are in the
⁎
fundamental principle, are limited in heat input, deposition and the arc force for welding pool. The current, needs to be increased to increase the deposition rate, increase the deposition rate and increase the heat input thus the distortion. It is difficult to freely deliver the heat, mass and force needed to produce desirable welds effectively and ecumenically [10]. The multi-electrode coupled arc welding processes are different from the conventional hybrid arc welding processes, which are not based on the fundamental principle, but in the new mode, multi-electrodes or pairs of electrodes are discharged in a common space, the welding pool is just only one polarity of many electrodes which is not a common polarity in the conventional mode, the arcs are coupled with each other. It is easier to achieve the new welding process with the new arc working mode, which can relieve the inherent constraints on heat input, deposition and the arc force, the advantages of the new welding process are the high efficiency and high welding speed and low heat input, increase the welding currents can get higher productivity and less distortion. The changes of the arc principle, the arc polarity is changed in the new coupled welding process and it is no longer on the work-piece, the behavior is changed significantly. DE-GMAW (Double Electrodes GMAW) [11–13], arcing-wire GTAW [14] and cross arc
Corresponding author at: No. 26 Yuxiang Street, Shijiazhuang, 050000, China. E-mail address: [email protected] (L. Zhang).
https://doi.org/10.1016/j.jmapro.2018.11.018 Received 31 July 2018; Received in revised form 14 November 2018; Accepted 20 November 2018 1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Fig. 2. Cross arc welding experimental system. Fig. 1. Principle of cross arc welding process. Table 1 The Experimental System Components.
[15,16] are three new multi-electrode arc welding processes, which bypass arc was divided from the electrode of the main arc to another electrode. As add a bypass arc, the distribution of arc energy changed, the two arcs coupled with each other and the arc behaviour changed obviously. The cross arc welding processes Cross arc welding process is a novel welding process: the plasma arc (PA) is established between the tungsten in the plasma torch and workpiece as the main arc to heat the work-piece; the inter-wire arc is established between the two wires, fed toward the PA from opposite directions, to melt the two wires by its anode and cathode simultaneously. The inter-wire current for the inter-wire arc is provided by an AC (alternating current) power source, as shown in Fig.1. The AC current waveform can allow a separate control on the melting speeds of the two wires. The inter-wire arc is crossed with the PA and one can consider that the PA penetrates through the inter-wire arc to reach the work-piece from the tungsten electrode. The heat input on the workpiece as well as the deposition into the work-piece can thus be controlled separately by the PA and inter-wire arc currents. Cross arc welding process is different from the conventional hybrid welding processes, the two arcs are not established parallel between wire and work-piece, but coupled with each other, the work-piece is no longer heated by all arcs, the new welding process can freely provide the needed deposition and heat input as desired by different applications. The two arcs are coupled and interacted as that many parameters can affect the stability of the coupled arc and thereby affect the quality of welds. In this paper, in order to explain the mechanism of the coupled arc and improve the arc stability, the experiments are focus on the arc behavior and metal transfer to analyze the stable/unstable process in the new multi-electrode arc welding processes. The arc behavior and parameters are discussed and outlined to help further develop the new process.
Equipment and Accessories
Model, material or Size
PA power supply GTA power supply GMA power supply Diameter of tungsten High-speed camera Shielding gas of GTAW
PLASMA-400 Miller Dynasty 700 Miller XMT 350 Φ5 mm IDT motion Y4 Pure Argon
camera (without optical filter) is used to record the arc at 3000 frames per second. High-speed camera and arc signals were synchronized to observe the behaviors of the multi-electrode coupled arcs. The recorded current waveforms and voltage waveforms (at the sampling rate 150 K Hz per channel) and images used to analyze and judge the behavior of coupled arcs. It should be noted that waveforms and the images are synchronous.
Experimental results In the previous preliminary study, the main arc is a GTA/PA, the inter-wire current is alternating current (AC). While using two wires provides a way to melt the wire fast without imposing heat on the workpiece or the tungsten, the operation of the proposed cross wire arc welding process relies on the balance between the melting of the two wires and the sustaining of the cross arc between the two wires. Since sustaining the cross arc depends on the melting balance, the melting can certainly be balanced. This study tries to verify the feasibility of the proposed idea, firstly without the coupling from the melting balance. The wires are first replaced by two non-consumable electrodes. To maximize the detachment ability, a constrained arc (plasma arc) should be more effective than a free GTA. Hence, it is the first time realizes a real cross arc welding process whose success depends on a successful metal transfer in this study, the authors propose to use a plasma arc (PA) to replace the GTA. In this paper, there are four group experiments, experiments 1# and 2# are the preliminary study by GTA and carbon electrodes, the purpose of the two experiments is to verify the feasibility of cross arc welding process, it is found that there is a new phenomenon: GTA is deviated to the cathode of inter-wire arc, the changes of arc behavior can modify arc force and metal transfer. Moreover, in experiments 3# and 4, the research is focus on the effect of arc behavior on the droplet transfer, especially the study on process of droplet detachment by the main arc. PA might even replace GTA as the main arc for the smooth metal transfer, in order to distinguish how the main arc can have effect on the metal transfer, there are two different modes: PA and PA with pulse in experiments 3# and 4# respectively. Based on the analysis of the experiments, stet the following parameters of cross arc welding experiments as shown in Table 2.
Experimental system The multi-electrode coupled arc welding processes are at least two arcs coupled with each other. This study tries to find the mechanism of the coupled arcs, in the research of the behaviors of the arcs, the experimental system should record all the signals of the arcs for the welding processes. In the experimental system, a GTA/PA is established using a DC-CC (direct current-constant current) power supply and GMA is established using an AC-CV (alternating current-constant voltage) power supply. An experimental system and system components are shown in Fig. 2 and Table 1 respectively, there are four sensors to detect the voltages and currents of the two arcs. The voltage sensors, current sensors and high speed camera can be realized the acquisition. The high-speed 125
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Table 2 The Parameters of the Cross Arc Welding Experiments. Welding Process
Experiments
GTA(PA) Current
GMA Current
Wire Feed Speed
GTA + Inter-wire arc
1# 2# 3# 4#
100 A 100 A 100 A 100 A(P)
50 A 150 A 100 A 100 A
0 0 3.5 m/min 3.5 m/min
PA + Inter-wire arc
Fig. 5. High-speed video in experiment #2.
considered to cross each other. The feasibility of the proposed cross arc process is thus experimentally demonstrated. When the inter-wire current further increased to 150 A in experiment #2. As can be seen from the Fig. 5, the GTA established between a tungsten tip (relatively thin) and the work-piece is more concentrated and should be in general brighter than the inter-wire arc established between two large carbon electrodes if the currents are the same. The GTA can also swing regularly between the two carbon electrodes, the deviation of GTA is rather obvious and the two arc spots of inter-wire arc are brighter than that in experiment #1. The voltages are measured and given in Fig.6, the GTA voltage waveform are not smooth, the stability of the GTA may also be affected by the inter-wire arc. However, it is also established as desired, the change is in the swing by the increased inter-wire arc. In the previous preliminary study, the cross arc can be established, as the new welding process is feasible, successful established of the desirable cross arc. The two arcs (GTA and inter-wire arc) are both stable, however, the main arc (GTA) is established between tungsten and work-piece, and the swing regularly between the two carbon electrodes, which is deviated to the cathode of inter-wire arc. The arc behaviors are different from the traditional arc characteristics, for this new phenomenon, the arc energy and arc force are redistributed by the new hybrid mode, the two wires can be melt by the two arcs, the main energy for melting wires is the polarity region energy of inter-wire arc as general arc. Moreover, the main arc is deviated and act on the wires, the changes of arc behavior break the function of arc energy and its column energy can be added on the two wires, therefore, the utilization rate of arcs energy to melt wires can be improved in cross arc welding process.
Fig. 3. High-speed video in experiment # 1.
GTA + inter-wire arc(carbon electrodes) There are two experiments in GTA + Inter-wire arc (carbon electrodes), the distance between tungsten and wire is fixed at 3 mm. The main arc is a GTA and the inter-wire arc is an AC GMA. The inter-wire distance is fixed at 2 mm while the currents vary. In experiment #1, GTA current and inter-wire arc current are fixed at 100 A and 50 A respectively. The desirable stability of the inter-wire arc can be seen in Fig.3, because of the relatively small currents of the GTA and inter-wire arc, the arc plasma for both arcs are not always seen clearly in the high-speed video. One pair of inter-wire arc spots (the cathode and anode) on the two carbon electrodes. However, it can still be observed that the GTA is not always straight. It may deviate either left or right from its straight trajectory. Analysis of the synchronized image and cross arc voltage and current, it shows that the GTA is straight when the cross arc is switching its polarity and the GTA deviates to the carbon electrode whichever is the cathode. The voltage waveforms are given in Fig.4, the GTA voltage (CH2) is at 22 V approximately, and the inter-wire arc voltage (CH4) is stable in AC square wave. Carefully observe the Fig.4, all the voltages are non-zero in the two voltage waveforms, hence, the GTA and inter-wire arc are
PA + inter-wire arc(wires) In experiment #1 and #2, it can be found that the GTA deviated to
Fig. 4. Voltage waveforms in experiment # 1.
Fig. 6. Voltage waveforms in experiment # 2. 126
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
Fig. 9. The Lorentz force in cross arc welding process.
process is changed by the PA pulse duration. The smooth metal detachment can get the stable welding pool, and also result in the good weld formation.
Fig. 7. High-speed video in experiment #3.
the cathode wire and swung among the two wires as the polarity changes. In order to verify that is a unique phenomenon in the cross arc welding process, the two carbon electrodes are replaced by two wires, and GTA is replaced by PA in experiments #3 and #4. In order to get the metal transfer, the filter and aperture are both changed in the highspeed camera. As the PA is a restrain arc and the inter-wire arc is a free arc, the brightness and intensity of current are both different, therefore, the inter-wire arc is not caught by the high-speed camera. The experimental results are shown in Fig. 7, PA is a direct arc and established between tungsten and work-piece, which can automatically deviate to the cathode wire and switched among the two wires in experiment #3. Wires are melted by the anode and cathode of the interwire arc and the resultant droplets are not successfully detached from the two wires. By the careful observation of the high-speed videos in experiments #3, it is found that the force of PA can be acting on the droplet changes and the droplet is pushed to the opposite side of interwire arc. Although the droplet is also on the tip of the two wires, the cross arc is stable. The above observation and analysis clearly suggest that the PA pulse must be sufficient in order to detach the droplet and the sufficiency of the PA pulse. As such, the pulse duration must be sufficient to complete the detachment process. It appears that the detachment process would be fully completed in 3 ms for the aerodynamic drag force generated by 400 A plasma current in experiment #4. The experimental results are shown in Fig. 8, the PA pulse is applied and PA is larger in pulse duration. The metal transfer is stable and smooth. As the reason is that the force acting on the droplet changes and the droplet starts the detachment process. The dynamic development of the droplet detachment
Analysis and discussion Careful observation of the high-speed videos in experiments #1-4, it shows that GTA/PA can swing periodically with the polarity change among the wires (carbon electrodes). The GTA/PA deviates to the cathode electrode. The cathode switches from one carbon electrode to another, the GTA/PA switches accordingly. This is perfectly understandable, because the current polarity can be changed almost instantaneously while the ionization of the gas and temperature rise in the gas to establish the arc in a new region requires a time although it is short. As can be seen in the experimental results, the GTA/PA swing is apparent and it can be easily identified from the videos. The inter-wire current flowing from one electrode to another generates a magnetic field in the GTA/PA column. Fig. 9 shows the directions of this magnetic field, i.e., inward and outward the paper above and below the electrodes respectively. In accordance with the left-hand rule, the electrons from the tungsten electrode in such magnetic field will subject to an electromagnetic force [17–19], i.e., the Lorentz force, in the direction as shown in Fig. 9. That is, the force is toward the cathode of the electrodes. Hence, the GTA/PA must deviate periodically as the polarity of the inter-wire current changes. This swing may provide a mechanism to scan the GTA/PA on the work-piece, while the main current does increase force to increase the stability of droplet detachment, the parameters of GTA/PA are changed, resulting in changing the metal transfer, and the reason for the deep analysis from the changes, the forces acting on the droplet are related to GTA/PA. Fig. 10 shows the major forces acting on the droplet when the GTA/PA deviate to the droplet in the cross arc welding process, there
Fig. 8. High-speed video in experiment #4.
Fig. 10. The major forces acting on the droplet in cross arc welding process. 127
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
welding process. Fortunately, it is improved drastically in the cross arc welding process, the arc dividing relieves the inherent coupling between the heat input and deposition in conventional arc processes, and result in the advantages for high efficiency, high speed and low heat input with desirable coupled controls. To address this issue, the experiment research is carried out on carbon steel plate to analyze the form factor of weld, the plasma arc current is fixed at 45 A, just only adjust inter-wire arc current from 60 A to 120 A and change the wire feed speed accordingly. The weld cross sections are shown in Fig.13, as can be seen, in this case for the given work-piece and welding parameters, the weld with relatively low penetration is produced as desired, with the parameters changed, the weld width is increased gradually. The reason for this phenomenon is that heat input and deposition are decoupled in the novel arc welding process. Plasma arc and inter-wire arc can be adjusted separately, plasma arc as the main arc to heat the work-piece and maintain weld penetration at a certain level, as shown in Fig.13, however, the most obvious changes are in weld width and height, as the inter-wire arc welding current increases, the melting wire increases directly, which lead to excessive molten metal on the welds. Hence, the form factor of weld can be modified as demands. Cross arc welding process can decouple heat input and penetration for the capability to increase the productivity while maintaining the base metal heat input at a desired level.
Fig. 11. Weld appearance in experiments#3.
Fig. 12. Weld appearance in experiments#4.
Conclusion This paper has experimentally studied the arc behavior and metal transfer of cross arc welding process, as an innovative welding processes, the authors have found the following:
Fig. 13. Weld cross section in different inter-wire arc currents.
are six major forces: the gravitational force (Fg), the electromagnetic force of the inter-wire arc (Fem), GTA/PA aerodynamic drag forces (FP), the inter-wire arc aerodynamic drag forces (Fa), the surface tension of melt droplet (Fσ) and the vapor jet force (Fv). For the metal transfer, there are two groups forces: Fg, Fem, Fa and FP can promote the droplet detachment, nevertheless, Fσ is the dominant force retaining the droplet from being detached, Fv is the same as in the conventional GMAW, which only obviously influences the droplet detachment. The gravitational force (Fg) and the aerodynamic drag force from the PA (FP) are thus the two dominant forces facilitating the droplet detachment. The droplet could be detached from the two wires smoothly with a pulse PA in the cross arc welding process. The two welds of experiments #3 and #4 are shown in Figs. 11 and 12 respectively, the PA current changes and the droplet detachment can be changed accordingly, nevertheless, the more significant changes occurred in the welds, there is a big difference between the two welds forming, the weld appearance is rough and some pinholes/porosities on the weld surface, the weld width is variable, correspondingly. Several droplets detach from wire but fall at the boundary of the weld which instead of the center of weld, hence, with the unstable droplet detachment and transfer, the edge of weld toe is nonlinear, as shown in Fig. 11. Besides the factor of unstable droplet transfer, the flow behavior of weld pool is also unstable, which can get a humped bead. On the contrary, the stable droplet detachment and transfer can get a smooth weld, as shown in Fig.12, the significant differences in the two welds are the weld appearance. The main parameter which affects the welding quality is the form factor of weld (weld width/penetration), the welding parameters: arc current, arc voltage and welding speed, can affect the weld bead geometry and weld characteristics to a great extent. A quintessential example of welds should be cited is GMAW welds, the experiment results have previously shown that the form factor of weld is configured to permit adjustment within a fixed range, it is reasoned that the conventional arc welding processes distribute the anode and cathode of the arc on their respective electrode and the work-piece, it is difficult to separate the heat input and deposition rate in the traditional arc
• An • • •
experimental system has been established to experimentally demonstrate the feasibility of the proposed cross arc welding process with GTA/PA. The main arc can swing between regularly the two wires (carbon electrodes), which can deviate to the cathode of the inter-wire arc automatically as the Lorentz force. The PA force can act on the droplet on the cathode wire to provide an additional detachment force, droplets can be detached from wires. Cross arc welding process can decouple heat input and penetration for the capability to increase the productivity while maintaining the base metal heat input at a desired level.
Acknowledgements This work is supported by the Hebei Education Department (NO. ZD2018060) and the National Science Foundation of China (NO. 5177052363). References [1] 9th editionAnnette O, Brien G, editors. Welding handbook, Vol.2. Miami Fla: Welding Processes. American Welding Society; 2012. [2] Moniz BJ, Miller RT. Welding skills. 4th edition 2012. American Technical Publisher; 2010. [3] Key JF. 10th edition Arc physics of gas tungsten arc welding. ASM handbook vol.6. ASM International; 1993. [4] Yin SY. Fundamental and application of shield gas arc process. Beijing: China Machine Press; 2012. [5] Hasanniah A, Movahedi M. Welding of Al-Mg aluminum alloy to aluminum clad steel sheet using pulsed gas tungsten arc process. J Manuf Process 2018;33(6):96–110. [6] Ueyama T, Ohnawa T, Tanaka M, Nakata K. Occurrence of arc interaction in tandem pulsed gas metal arc welding. Sci Technol Weld Join 2007;12(6):523–9. [7] Lee HK, Park SH, Kang CY. Droplet transition for plasma-MIGwelding on aluminium alloys. J Mater Process Technol 2015;223(9):203–15. [8] Lassaline E, Zajaczkowski B. Narrow groove twin-wire GMAW of high- strength steel. Weld J 1989;68(9):53–8. [9] Ueyama T, Tong H, Harada S, et al. AC pulsed GMAW improves sheet metal joining.
128
Journal of Manufacturing Processes 37 (2019) 124–129
L. Zhang et al.
2012;91(10):261–9. [15] Chen SJ, Zhang L, Wang XP, et al. Stability of cross arc process – a preliminary study. Weld J 2015;94(5):158–68. [16] Zhang L, Chen SJ, Song YX, et al. Metal transfer in cross arc welding process. Weld J 2016;95(9):340–56. [17] Shi Y, Liu XP, Zhang YM. Analysis of metal transfer and correlated influences in dual-bypass GMAW of aluminum. Weld J 2008;87(12):229–36. [18] J.F. Lancaster The physics of welding. Oxford, England: Pergamon Press. [19] Waszink JH, Piena MJ. Experimental investigation of drop detachment and drop velocity in GMAW. Weld J 1986;65(11):289–98.
Weld J 2005;84(2):40–6. [10] Chen KH, Li HQ, Li CX. Progress in variable polarity plasma arc welding. Trans China Weld Inst 2004;25(1):124–8. [11] Zhang YM, Jiang M, Lu W. Double electrodes improve GMAW heat input control. Weld J 2004;83(11):39–41. [12] Shi Y, Li J, Zhang G, et al. Corrosion behavior of aluminum-steel weld-brazing joint. J Mater Eng Perform 2016;25(5):1–8. [13] Zhou X, Zhang G, Shi Y, et al. Microstructures and mechanical behavior of aluminum-copper lap joints. Mater Sci Eng A 2017:705. [14] Chen JS, Lu Y, Li XR, et al. Gas tungsten arc welding using an arcing wire. Weld J
129