Impacts of external longitudinal magnetic field on arc ...

Int J Adv Manuf Technol DOI 10.1007/s00170-013-5403-1

ORIGINAL ARTICLE

Impacts of external longitudinal magnetic field on arc plasma and droplet during short-circuit GMAW Yun Long Chang & Xiao Long Liu & Lin Lu & A. S. Babkin & Bo Young Lee & Feng Gao

Received: 14 April 2013 / Accepted: 4 October 2013 # Springer-Verlag London 2013

Abstract In gas metal arc welding, electromagnetic force, plasma stream force, gravity, and surface tension are the most important factors that affect metal transfer and spatter generation rate. In this paper, different kinds of external electromagnetic fields were introduced to gas metal arc welding (GMAW). The photos of arc plasma and droplet and electric signals covering welding current and arc voltage were acquired synchronously by an analysis and evaluation system based on LabView for GMAW. It was confirmed that the metal transfer frequency was improved, and spatter generation rate was diminished under controls of external electromagnetic fields. The influencing rules of external electromagnetic fields on electromagnetic force, the gravity, the plasma stream force, and surface tension were studied by three physical models, and the mechanism of external electromagnetic fields was revealed. This paper is for the purpose of discussing these

Y. L. Chang (*) : X. L. Liu : L. Lu : F. Gao School of Materials Science and Engineering, Shenyang University of Technology, Shenyang, People’s Republic of China 110870 e-mail: [email protected] X. L. Liu e-mail: [email protected] L. Lu e-mail: [email protected] F. Gao e-mail: [email protected] A. S. Babkin Metallurgical Institute, Lipetsk State Technical University, Lipetsk, Russia 398024 e-mail: [email protected] B. Y. Lee School of Aerospace and Mechanical Engineering, Korea Aerospace University, Goyang, Gyeonggi-do 412-791, Republic of Korea e-mail: [email protected]

factors and will make a profit for the application of electromagnetic coupling control to short-circuit GMAW. Keywords Longitudinal magnetic field . Electromagnetic force . Surface tension . Short-circuit transfer . Spatter

1 Introduction Though short-circuit gas metal arc welding (GMAW) is widely used in modern industry for its high productivity and low cost, it still has some defects such as too much spatter and poor weld metal. Now, these problems in the conditions of lower welding current have been solved by the digital control technologies such as STT, AC-CBT, EWM coldArc, RMD, and CMT. These methods reduce the current when the short circuit is detected to avoid an explosive rupture of weld metal [1–4]. The irregular oscillation phenomena of droplet which may produce spatter in the conditions of higher welding current has been well restrained by Tokihiko by means of applying welding current with high-frequency pulses [5], but the problems caused by the asymmetric distribution of the droplet which is resulted from the movement of arc plasma in medium welding conditions are not yet well solved [6]. In other words, a right way of improving short-circuit GMAW may be developed by the effective control on arc plasma and liquid metal. It was confirmed that the external magnetic field could provide benefits in improving characteristics of arc plasma and liquid metal [7]. Jiang [8] conducted an experiment of GMAW controlled by an external longitudinal magnetic field and found that the spatter could be reduced in certain welding conditions. Luo [9–11] investigated the impacts of external magnetic fields on the characteristics of arc plasma and metal transfer during GTAW and GMAW. Chen [12] studied the impacts of optimum conditions of magnetic field in accelerating the rupture of the liquid bridge and reducing the peak

Int J Adv Manuf Technol Table 1 Chemical composition of material (weight percent) Material

C

Si

Mn

S

P

Q235

0.150

0.230

0.450

0.028

0.020

value of welding current in the period of short-circuit transfer. In this paper, different kinds of external electromagnetic fields were introduced to short-circuit GMAW to develop a good way of controlling the arc plasma and liquid metal which may improve the weld metal appearance and reduce the spatter and make a profit for the development of synchronously electromagnetic control welding technology.

2 Welding test 2.1 Test materials and equipments Bead-on-plate welding was performed on a mild steel sheet Q235 in the size of 120 mm×50 mm×6 mm whose chemical composition was shown in Table 1. Welding wire of H08Mn2SiA with a diameter of 1.2 mm was used. As shown in Fig. 1, the equipment system covering a welder of Lincoln Invertec V350-PRO, a wire feeder of S86A, a self-developed magnetic field generator of MCWE-10/100, and an analysis and evaluation system based on LabView for GMAW which was mainly composed of a high-speed camera of MotionPro and a data acquisition card of PCL1800.

magnetic fields on the frequency of metal transfer f w, arcburning time t a, and short-circuit time t s in a cycle of metal transfer, the photos of arc plasma and droplet and electric signals covering welding current I w and arc voltage U w shown in Fig. 2 were acquired synchronously by the analysis and evaluation system, and the working parameters were shown in Table 4. The four photos from the left to the right in Fig. 2 were, respectively, the last image of arc-burning phase, the first and the last image of short-circuit phase, and the first image of arc-burning phase. The physical information covering the angle of the welding current conduction zone at the anode β 1, the radius of droplet r 1 acquired from the last image of arc-burning phase, the radius of equatorial plane of liquid bridge r 2, and the radius of bottom of liquid bridge r 3 acquired from the last image of short-circuit phase was used for the mechanical analysis of liquid metal. Equations 1 and 2 based on a weighing method were applied to calculate the average value of spatter generation ratio δ under some certain magnetic fields parameters. To be specific, the δ i =1, 2, 3 was the spatter generation ratio of three welding tests in the same conditions. The m i0, m i1, and m i2 during test i (i =1, 2, 3) were, respectively, the actual consumption of welding wire in 60 ms, the quality of an unwelded original specimen, and the quality of a welded specimen. δi¼1;2;3 ¼ δ¼

mi0 þ mi1 −mi2 100% mi0

δ1 þ δ 2 þ δ 3 3

ð1Þ ð2Þ

3 Results and analysis

2.2 Test procedure

3.1 Test results

The DC longitudinal magnetic fields of different intensities were introduced to GMAW by attaching a cylindrical wire solenoid on the torch. The terminals of the solenoid were connected with the magnetic field generator. The welding parameters and magnetic field parameters were shown in Tables 2 and 3, respectively. To study the impacts of external

The impacts of magnetic frequency f m on the average value of spatter generation ratio δ were shown in Fig. 3. The δ was maximally reduced by 3.20 and 4.98 % under low- and highfrequency magnetic fields with an exciting current I m =6 A. According to the results, in the conditions of spatter-reduced δ decreased with the increasing of f m under both low- and highfrequency magnetic fields and after f m exceeded 20 or 1, 000 Hz under two different magnetic fields, the δ tended toward stability.

Wire feeder Hall voltage sensor

Hall current sensor

Table 2 Parameters of welding test

Solenoid DAQ card

PC

Magnetic field generator

Torch

High speed camera

Xenon lamp Specimen

Fig. 1 Equipment system of GMAW

Welding power source

Arc voltage U w (V) Welding current I w (A) CO2 gas flow rate v g (l/min) Wire feed speed v wf (m/min) Travel speed v ts (cm/min)

21.4 150 15 3.5 24

Int J Adv Manuf Technol Table 3 Parameters of external magnetic field

Types of magnetic field

Low frequency

Exciting current I m (A) Magnetic frequency f m (Hz) Duty radio (wt%) Turns per coil Radius of cooper wire r w (mm)

2 5 50 390 0.45

320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 -20 -1

320 300 280 Welding current 260 240 220 200 180 160 140 120 Short-circuit phase 100 Arc burning phase 80 60 40 20 0 -20 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Arc voltage

0

Timet [ms]

Fig. 2 Process of metal transfer without magnetic field

Arc voltageUw [V]

Welding currentIw [A]

The weld metals with magnetic fields of I m =6 A or without magnetic field were shown in Fig. 4. Compared with the weld metal without magnetic field, the appearance of weld metals controlled by external magnetic fields was more regular, and the weld toes were straighter. Furthermore, the reinforcements were well lowered. Specially speaking, the reinforcements of the left one, middle one, and right one were 1.66, 1.84, and 2.15 mm, respectively. In short-circuit GMAW, the shape of weld metal is closely related to the frequency of metal transfer f w. The higher the f w, the better the shape is. In this paper, the f w was calculated by the analysis and evaluation system mentioned above. The impacts of magnetic frequency f m on frequency of metal transfer f w were shown in Fig. 5. Compared with the f w without external magnetic field, the f w controlled by external magnetic field was improved obviously. To be specific, when f m varied in the range of 5–25 Hz, f w slightly decreased with the increasing of f m, and on the other side, f w slightly increased with the increasing of f m when f m varied in the range of 500–2,000 Hz, but when f m exceeded 1,000 Hz, the f w tended toward stability. The impacts of I m on the δ and f w were studied, and it was shown that in the conditions of spatter reduced and f w improved, when I m =2 A, the change of f m did not make a

4 10

High frequency

20

6 25

2 500 50 100 1.00

4 800

1,000

6 2,000

difference in improving the frequency of metal transfer, but when I m was increased to 4 or 6 A (in other words, when the magnetic induction B was enhanced [13]), the low-frequency magnetic field played a leading role in improving f w and reducing spatter. To make a further study on the impacts of external magnetic field on the frequency of metal transfer f w, the shortcircuit time t s was calculated by studying the scale of shortcircuit arc voltage signal in a cycle of metal transfer, and then, the arc-burning time t a in a cycle of metal transfer was obtained through the subtraction between the cycle time t and short-circuit time t s. The impacts of f m on t s and t a were shown in Figs. 6 and 7. According to Fig. 6, the t s was shortened by the external magnetic field. In the conditions of some exciting currents, the higher the f m, the shorter the t s was. That is to say, the highfrequency magnetic field played a leading role in improving the frequency of metal transfer f w just by shortening the shortcircuit time t s. On the other side, as shown in Fig. 7, the t a was also shortened when the f m varied in the range of 5–25 Hz, but inversely, the t a increased with the increasing of f m, and when f m varied in the range of 500–2,000 Hz, the t a controlled by high-frequency magnetic field was all longer than that without the control of external magnetic field, and the metal transfer was hindered. In other words, in this test, only the lowfrequency magnetic field could make a difference in improving the frequency of metal transfer just by shortening the arcburning time t a, and the lower the f m, the shorter the t a was. The process of metal transfer under the external longitudinal magnetic field of an exciting current of 6 A was shown in Fig. 8. Compared with Fig. 2, the shape of arc plasma and droplet during arc-burning phase were expanded by the lowfrequency magnetic field (5–25 Hz) in the horizontal Table 4 Working parameters of the analysis and evaluation system

Signal acquisition Acquisition time (ms) Numbers of samples Image capture Acquisition time (ms) Shutter speed (frame/ms) Time of exposure (μs) F number

60 245,760 60 500 330 11

9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5

Without external magnetic field

Exciting current Im=2A Exciting current Im=4A Exciting current Im=6A

0

5

10

15

20

Spatter generation ratio d [wt%]

Fig. 3 Impacts of magnetic frequency on spatter generation rate under low-frequency magnetic field (left) and highfrequency magnetic field (right)

Spatter generation ratio d [wt%]

Int J Adv Manuf Technol

25

9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5

Without external magnetic field Exciting current Im=2A Exciting current Im=4A Exciting current Im=6A

0

Magnetic frequency fm [Hz]

500

1000

1500

2000


Fig. 4 Contrast of weld metal appearances and their reinforcements under lowfrequency magnetic field of 5 Hz (left) and high-frequency magnetic field of 1,000 Hz (middle) and no magnetic field (right)

stage (assumed to correspond to the first image of short-circuit transfer process in this paper) of short-circuit phase, the liquid metal controlled by the low-frequency magnetic field spread out smoothly, but that controlled by the high-frequency magnetic field was hindered in the horizontal direction. In the final stage (assumed to correspond to the last image of short-circuit transfer process in this paper) of short-circuit phase, both the radius of equatorial plane of liquid bridge r 2 and the radius of

83.0

Frequency of metal transfer fw [Hz]

Fig. 5 Impacts of magnetic frequency on frequency of metal transfer under low-frequency magnetic field (left) and highfrequency magnetic field (right)

Frequency of metal transfer fw [Hz]

direction. Furthermore, the lower the magnetic frequency f m, the bigger the angle β 1 and radius r 1 were. When the f m varied in the range of 500–2,000 Hz, however, the shape of arc plasma and droplet during arc-burning phase was compressed by the high-frequency magnetic field in the horizontal direction. Before the f m reached 1,000 Hz, the angle β 1 and radius r 1 decreased with the increasing of f m, and after f m exceeded 1,000 Hz, the β 1 and r 1 tended toward stability. In the initial

82.5 82.0 81.5 81.0 80.5 80.0

Exciting current Im=2A

79.5


79.0


78.5 78.0


0

5

10

15

20


25

83.0 82.5


82.0


81.5


81.0 80.5 80.0 79.5 79.0 78.5 78.0


0

500

1000

1500

2000


3.25 3.20 3.15 3.10 3.05 3.00 2.95 2.90 2.85 2.80 2.75 2.70 2.65 2.60 2.55 2.50


Exciting current Im=2A Exciting current Im=4A Exciting current Im=6A

0

5

10

15

20

Short-circuiting time ts [ms]

Fig. 6 Impacts of magnetic frequency on short-circuit time under low-frequency magnetic field (left) and high-frequency magnetic field (right)

Short-circuiting time ts [ms]

Int J Adv Manuf Technol

25

3.25 3.20 3.15 3.10 3.05 3.00 2.95 2.90 2.85 2.80 2.75 2.70 2.65 2.60 2.55 2.50


0


bottom of liquid bridge r 3 controlled by the external magnetic field decreased with the increasing of f m. The impacts of external magnetic field on the β 1, r 1, r 2, and r 3 were shown in Figs. 9, 10, 11, and 12, respectively, based on the physical information reported by the photos. Similar to the impacts of I m on the δ and f w, the impacts of I m on the β 1, r 1, r 2, and r 3 also showed that the higher the I m, the stronger the impacts were.

3.2 Mechanism of promoting metal transfer In short-circuit GMAW, the stability of metal transfer which greatly determine the performance of weld metal and quality of spatter was closely related to the forces acting on the arc plasma and droplet such as electromagnetic force F em, plasma stream force F p, gravity F g, and surface tension F σ [14]. In this paper, a series of mechanical analysis was conducted to study the mechanism of external magnetic field in improving the GMAW. Three physical models shown in Fig. 13 were used to imitate the droplets and arc plasma during the arcburning phase (left), initial stage (middle), and final stage (right) of short-circuit phase, respectively. The positive direction of coordinates stood for the positive direction of the resultant forces.

1500

2000

In short-circuit GMAW, the short-circuit phenomenon is mainly a result of a resultant force of electromagnetic force F em1, plasma stream force F p, gravity F g, and surface tension F σ1. That is to say, the mechanical analysis on the droplet during arcburning phase will develop a way to study the mechanism of external magnetic field in influencing the arc-burning time t a. To be specific, in bead-on-plate welding, the plasma stream force F p and gravity F g were downward. The surface tension F σ1 was upward and prevented the droplet from being detached. The impact of F em1 was more complex. It was confirmed that when the welding arc was expanded, the F em1 was upward, and when the welding arc was compressed, the F em1 was downward [15]. So, the resultant forces F r on the droplet in the axial direction under low- and high-frequency magnetic fields could be expressed in Eqs. 3 and 4, respectively. The F em1, F p, F g, and F σ1 could be expressed in Eqs. 5–8 where μ 0 is the magnetic permeability in free space, I w1 is the welding current in arc-burning phase, r 0 is the radius of welding wire, C d is the aerodynamic drag coefficient, ρ p is the density of arc plasma, v f is the plasma fluid velocity, ρ d is the density of droplet during the arc-burning phase, g is the acceleration of gravity, and σ 1 is the coefficient of surface tension [16, 17]. According to the waveform of welding current acquired by the


9.8


9.7


9.6 Without external magnetic field

9.5 9.4 9.3 9.2 9.1

Arc burning time ta [ms]

10.0

9.9

9.0

1000

3.2.1 Mechanical analysis during arc-burning phase

10.0

Arc burning time ta [ms]

Fig. 7 Impacts of magnetic frequency on arc-burning time under low-frequency magnetic field (left) and high-frequency magnetic field (right)

500


9.9 9.8 Without external magnetic field 9.7 9.6 9.5 9.4


9.3


9.2


9.1 0

5

10

15

20


25

9.0

0

500

1000

1500

2000


Int J Adv Manuf Technol Fig. 8 Process of metal transfer under the external longitudinal magnetic field of an exciting current of 6 A

84 82 80 78 76 74 72 70 68 66 64 62


0

5

10

15

20

25

Angle of the welding current conduction zone at the anode β1 [°]

Fig. 9 Impacts of magnetic frequency on angle β 1 under lowfrequency magnetic field (left) and high-frequency magnetic field (right)

Angle of the welding current conduction zone at the anode β1 [°]

Int J Adv Manuf Technol 84 82 Exciting current 80 Exciting current 78 Exciting current 76 74 Without external magnetic field 72 70 68 66 64 62 0 500 1000 1500

0.72 0.71 0.70 0.69 0.68 0.67 0.66 0.65 0.64 0.63 0.62 0.61


0

5

10

15

20

25

F em1

1 2 2 π ⋅ r1 − r0 ⋅ C d ⋅ ρ f ⋅ v2f 2 4 F g ¼ π ⋅ r31 ⋅ ρd ⋅ g 3 F σ1 ¼ 2π ⋅ r0 ⋅ σ1

ð6Þ ð7Þ ð8Þ

Generally speaking, if the F r >0, drop transfer will be carried out, and if F r