Published by Maney Publishing (c) IOM Communications Ltd

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in the downward motion process is larger than that in the upward motion process. The rotating arc process can improve the metal transfer. The results indicated that the centrifugal force induced by the arc rotating has great effect on the globular transfer mode which has lower transfer frequency and larger droplet size. Regarding the spray transfer mode, it has limited effect.

Keywords: Metal transfer, Rotating arc welding, Horizontal welding, Narrow gap welding

Introduction

The horizontal gas metal arc welding (GMAW) technology is used increasingly, especially in the manufacture field of the large and heavy structure parts because of its considerable flexibility and productivity.¹ The rotating arc process was used to improve the sidewall penetration in the flat position,^2,3seam tracking^4–6and prevent the molten pool sagging during the horizontal welding.⁷

However, a better understanding of the metal transfer mechanisms involved in the GMAW process is still imperative and would be most useful for precise control of the geometry and quality of the weld bead,^8–11as well as for control of heat input in the substrate. The metal transfer in rotating arc welding process was studied only by the computed simulation method. The experimental results of rotating arc narrow gap welding were rarely reported, especially in horizontal position.

With the development of welding technology, a number of sensing techniques have been used, e.g.

sensing of airborne sound, sensing of welding arc voltage, sensing of arc light and high speed photography. Compared with other methods, photography can directly provide information on the droplet transfer rates and droplet shapes.¹² In this paper, the metal transfer of rotating arc narrow gap horizontal welding was studied by the high speed photography system. The emphasis was placed on the analysis of the metal transfer characteristics and effect of the rotating speed on the metal transfer.

Experimental apparatus and procedure

Generally, in narrow gap welding, the fume fills the narrow groove. It induces that it is very difficult to observe the metal transfer process. In this work, a FastCAM Super 10K high frame rate digital camera (3000 frames s²¹) is used to obtain the images of the droplet detachment process in real time. A continuous xenon lamp is used as the backlighting source. The schematic of images collection system is shown in Fig. 1.

The schematic of the structure of the welding torch in the present study is shown in Fig. 2. The formation principle of the rotating arc is that the nozzle and wire are clockwise rotated with the conductor tube which is connected with the eccentric sleeve driven by the motor.

There is no relative motion between the nozzle and wire, so it could reduce wear and prolong the service life of the nozzle. The welding power source is Kemppi ProMIG 5000. The power source is operating in constant wire feed and constant voltage mode. The square groove with the width of 9 mm and depth of 20 mm is applied during the welding process. The chemical compositions of base metal and weld metal are shown in the Table 1. The diameter of the wire is 1?6 mm. Ar₂z5%CO₂ with a constant flowrate of 20 L min²¹ is used for shielding.

The welding parameters are given in Table 2.

Results and discussion

In this paper, in all the figures related to metal transfer, the left side is corresponding to the upper side and right side is corresponding to the lower side in horizontal welding. Figure 3 shows the metal transfer in a rotating period at rotating frequency of 5 Hz. In this welding parameter, a shapely horizontal weld can be obtained.⁷ It can be seen that the wire presents oscillation in the weld width direction. Actually, there is a motion in the direction which is vertical to the paper surface. When the wire is near the upper groove sidewall, the linear velocity

State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, Harbin 150001, China

*Corresponding author, email gn21c@126.com

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direction of the rotating points to the inner side. When it is near the lower groove sidewall, the direction points to the outer side. It should be noted that the metal transfer mode is different with the various wire locations. When the wire is near the upper sidewall, the metal transfer presents spray transfer mode. With the downward motion of the wire, the spray transfer frequency decreases and the size of droplet increases. When the wire crosses the centreline and continues downward motion, the spray transfer frequency increases again. A liquid column is formed on the tip of the wire.

Additionally, the molten surface is downward sloping due to gravity. So it makes the arc length too short and makes a little short circuiting transfer happened during this process. Then the molten pool surface depresses with the effect of the arc force and impact of the droplet.

The short circuiting transfer disappears. The metal transfer still presents spray transfer mode. The depres- sing level reaches maximum when the wire moves to the location near the lower sidewall. When the wire moves to the upper side, the metal transfer frequency decreases rapidly. When the wire moves to the centre of the groove, the metal transfer transforms to globular transfer mode. The size of droplet is larger than the diameter of the wire. When the wire moves upwards to the location near the upper sidewall, the metal transfer transforms to spray transfer mode again. The short circuiting transfer does not occur during the upward motion process. The metal transfer frequency and droplet size of different locations in a rotating period are summarised in Fig. 4. It indicates that the metal transfer has certain regularity in a rotating period. In the weld width direction, the transfer frequency in the area of near both sidewalls is larger than that in the centre of the groove. In a rotating period, the transfer frequency of the downward motion is larger than that in the upward motion process.

The metal transfer characteristics of rotating arc narrow gap horizontal welding are mainly induced by the different arc lengths at various locations caused by the molten pool surface shape. The schematic of molten pool surface during the welding is shown in Fig. 5. The

fusion metal is pushed backwards in the molten pool because of the arc force and impact of the droplet. It makes the height of rear part of molten pool higher than that of fore part. Additionally, in the weld width direction, the height of molten pool in the centre of the groove is lower than that in both sidewall areas because of the arc force and impact of droplet as well as additional pressure caused by the surface tension.

During the welding process, the welding torch position is fixed and the wire extension is nearly unchanged. So different molten pool heights induce different arc lengths: A,B,C,D. In this paper, the constant wire feed system is applied. Because of the arc self-regulating effect, the shortening of the arc length induces the increase in the welding current. So the relationship of welding current at the locations A, B, C and D is A.B.C.D. The relationship of wire feeding rate and welding current in normal (non-rotating) welding process is shown in Fig. 6. The variation of the welding current with the electrode rotating time is shown in Fig. 7. So the periodical change of welding current with the wire rotating motion is the reason that the metal transfer characteristics exist.

The metal transfers with different rotating frequencies present the same transfer characteristics. However, the rotating frequency has great effect on the metal transfer.

It is embodied in the metal transfer in the centre of the groove during the wire upward motion. Figure 8 shows the metal transfer in the centre of the groove with different rotating frequencies. It can be seen that the transfer mode transforms from globular transfer at the rotating frequency of 5 Hz to spray transfer at the rotating frequency larger than 20 Hz. The size of the droplet decreases obviously. The relationship of droplet transfer frequency and size with different rotating speeds

2 Schematic of welding torch in rotating arc horizontal welding

Table 1 Chemical composition of base metal and weld metal, wt-%

Material C Mn Si S P Ni Cr Cu

Base metal 0?17 0?80 0?30 0?035 0?035 0?30 0?30 0?30 Weld metal 0?09 1?97 0?84 0?019 0?027 0?26 0?18 0?27

Table 2 Welding parameters used during welding

Arc voltage, V 28

Wire feeding speed, m min²¹ 5

Welding speed, m min²¹ 0?23

Arc rotation frequency, Hz 5–50

Rotating radius, mm 2?0

1 Schematic diagram of image collection system used in rotating arc narrow gap horizontal welding

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Published by Maney Publishing (c) IOM Communications Ltd is shown in Fig. 9. The rotating frequency has limited effect on the spray transfer in the area near the both sidewalls. It indicates that the centrifugal force induced by the arc rotating has great effect on the globular

3 Droplet transfer of rotating arc process with rotating frequency of 5 Hz

4 Droplet transfer frequency and droplet size in one

rotating period 5 Schematic of molten pool surface during welding

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transfer mode which has lower transfer frequency and larger droplet size. Regarding the spray transfer mode, it has limited effect.

The schematic of the force affecting the droplet is shown in Fig. 10. The force which accelerates the droplet detachment includes:

(i) centrifugal force Fc~16

3 p³R³rr0f (1)

where R is the radius of droplet, r is the density of the fusion metal, r0is the arc rotating radius and f is the arc rotating frequency

(ii) electromagnetic force F_e~m_mI²

4p lnr sin a R {1

4{ 1

1{ cos az

2 1{ cos a

ð Þ²ln 2

1z cos a

#

(2) where m_mis magnetic permeability, I is welding current and a is half cone angle of the arc

(iii) plasma flow force Fd~CdAp

r_fv²

2 (3)

Ap~p R ²{r²

(4) where Cdis the coefficient of the plasma flow, Ap

is the affected area of plasma flow, r_f is the density of the plasma flow, v is the velocity of the plasma flow and r is the radius of the wire.

The main baffle force of the droplet detachment is surface tension. The equation is given below

Fc~2prc (5)

where c is the coefficient of surface tension.

The effect of gravity on the droplet detachment is quite different with the flat position welding. When the wire tip is under the centre of the groove, the wire is downward sloping. The gravity of the droplet presents as an acceleration force of the droplet detachment.

When the wire tip is upon the centre of the groove, the gravity presents a baffle force.

So when the metal transfer is in spray transfer mode, the electromagnetic force and plasma flow force are quite large. The change of the centrifugal force has little effect on

6 Relationship of wire feeding rate and welding current in non-rotating welding process

7 Variation of welding current with electrode rotating time

8 Droplet transfer in centre of groove with different arc rotating frequencies

9 Droplet transfer frequency and droplet size with different arc rotating frequencies

10 Schematic of force affecting droplet

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groove present different modes. Generally, in the weld width direction, the transfer frequency in the area of near both sidewalls is larger than that in the centre of the groove. In a rotating period, the transfer frequency in the downward motion process is larger than that in the upward motion process.

3. The centrifugal force induced by the arc rotating has great effect on the globular transfer mode which has lower transfer frequency and larger droplet size.

Regarding the spray transfer mode, it has limited effect.

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