Mechanism analysis of free formation of backing weld by the pulsed MAG-TIG double arc tandem welding

Liu Liming,Zhou Yanbin

School of Materials Science and Engineering,Dalian University of Technology,Dalian 116024,China

Abstract Because of the presence of lower-level automation and efficiency in the backing welding of medium-thick plates,pulse-metalactive-gas-metal-inert-gas(MAG-MIG)dual-arc welding was applied to backing welding of 24-mm-thick Q235-B plate and the process and mechanism of root welding with back formation were investigated.The heating position of the MAG-arc at the front of the molten pool could be adjusted by using the electromagnetic force between the MAG-arc and the MIG-arc,and part of the arc energy could work on the root face directly.By combining the arc-discharge behaviour and analysis of flow in the molten pool,the shear stress of a tungsten inert gas(TIG)arc to the molten pool could make the liquid metal flow backwards.Thus,the quality of the front and bottom liquid metal were reduced,which favored the balance and stability.Continuous and stable back formation with uniform penetration could be achieved by using the pulse MAG-TIG dual-arc welding technology.

Key words pulse MAG-TIG dual arc,backing weld,one-side welding with back formation,high-efficiency welding,mechanism analysis

0 Introduction

As equipment becomes larger and heavier in the shipbuilding,pressure vessel,heavy machinery and other industries,butt welding of medium-thick plates is applied universally.The improvement efficiency of root welding is particularly significant and affects the manufacturing cycle[1].Tungsten inert gas(TIG),metal-inert-gas-metal-active-gas(MIG-MAG)and submerged arc welding(SAW)are applied frequently in root welding with back formation.The TIG process is stable for root welding,but inefficient with a low speed because of the limited melting ability[2].Ceramic backing can be introduced into MIG-MAG technology,which increases the process margin of the root welding parameters.However,preparation and removal requires additional procedures and is restricted under certain specific conditions(such as the welding of small-diameter pipes and box structures).Therefore,the development of a new welding process to achieve high-efficiency root welding without essential auxiliary procedure for back formation is in great demand and is of significance[3].

Yamane reported a MIG-MAG process of “switch back welding”,in which the torch oscillates forward and backwards to achieve root welding with back formation[4-6].The poor efficiency is attributed to a lower welding speed of 1.7 mm/s and the back bead morphology is approximately similar to the overlap of arc welding pots which indicates a poor uniformity of back formation.Yang and Zhang proposed another method of asymmetrical double-side double arc welding technology without setting up the backing plate[7-8].The method can be applied to the specific welding positions(horizontal and vertical position)but strenuously for the flat position.

Dual-arc hybrid welding was verified to be a high-quality and efficient technology.Extensive studies have been conducted on the shielding gas and current waveform of the MAG-MIG and TIG process,and have been combining with laser welding for high-efficiency modification[9].Li et al.applied MIG and TIG power to construct bypass-coupled double electrodegas metal arc welding(DE-GMAW),which achieved high-efficiency droplet transfer under a lower heat input by using the shunting effect of tungsten electrodes of TIG welding[10].TIG-MIG hybrid welding was selected by Kanemaruet al.to improve the arc stability of MIG in pure Ar.The quality and efficiency were enhanced[11].Menget al.achieved high-speed welding of butt joints and overlay by introducing a TIG-MAG composite heat source,which suppressed hump and undercut defects[12].No reports exist on the application of pulse MAG-TIG dual-arc with communal molten pool heat source welding technology to root welding with the back formation of medium and thick plates.

Root welding with a back-formation technology was investigated by applying the pulse MAG-TIG dual-arc with a communal molten pool heat source process to the medium-thick plate.For further study,a high-speed camera-vision monitoring system was used to collect the liquid metal flow morphology in the molten pool and the MAG arc plasma before and after the TIG arc operation.The reason for the increased root welding stability with a back formation procedure that was operated by the pulse MAG-TIG dual-arc with a communal molten pool heat source technology on the medium and thick plate was studied.The partial arc energy could work on the root face by adjusting the electromagnetic force between the MAG-arc and MIG-arc,which was achieved by changing the heating position of the MAG-arc at the front of the molten pool.Heating and vibration could promote liquid metal movement in the molten pool towards the back and upper directions,by which liquid metal flow in the molten flow was regulated.The stability improvement of root welding with back formation was achieved.

1 Experimental procedures

The composite heat source was constructed by a tandemway of the pulse MAG-TIG dual-arc with a communal molten pool,as shown in Fig.1.The welding system was composed of a S8-500A-type pulse MAG welding machine,a YC-500WXN-type TIG welding machine and an ER50-C20 welding robot system.A pulse MAG arc was used with a negative polarity direct-current(DC)method followed by a TIG arc with a straight polarity DC mode.

Fig.1 Schematic diagram of system of pulse MAG-TIG double arc tandem welding

Q235-B plate with 300 mm×150 mm×24 mm was used as the base metal.A V-shaped groove of 40° was machined with a 2-mm root face and the plates were set up with a root gap of 1.0-1.2 mm,as shown in Fig.2.A welding wire was selected with a 1.2 mm-diameter ER50-6 low-alloy steel wire,and Ar-rich shield gas for MAG welding was mixed with 80% Ar and 20% CO2with a flow rate of 20 L/min.TIG welding was protected by using 99.9% pure Ar gas at a flow rate of 6 to 8 L/min.Rust,oil and other impurities within 20 mm from the groove were cleaned before welding.An angle of 30° was maintained between the TIG and MAG gun during the welding procedure.The pulse frequency of MAG welding was 166.7 Hz with a peak,a base current of 440 A and 120 A and a duty cycle of 0.56.The welding parameters are listed in Table 1.

Fig.2 Sketch map of specimen and groove size

To study the arc plasma morphology and the molten pool flow state during the pulse MAG-TIG dual-arc with communal molten pool heat source welding,a high-speed camera(2 000 frame/s)was applied for real-time monitoring.Before data collection,a narrow band filter with acentre wavelength of 659.5 nm and a full width half maximum of 9.4 nm was set up in front of the high-speed camera lens so that an Ar spectrum could pass through it.The arc height was fitted flush with the high-speed camera lens.A xenon lamp with an adjustable intensity was used for auxiliary lighting to obtain a clear molten pool view.The camera lens was placed higher than the molten pool downward with an inclination of～45° and vertically with the welding direction.

Table 1 Welding parameters

2 Results and discussion

2.1 Back bead formation by root welding

The back bead formation morphology that was achieved by the individual MAG root welding is indicated in Fig.3a.Related welding parameters are detailed in the caption,whereIMPandIMbare the peak and base current of MAG welding,andUMis the arc voltage.As illustrated in Fig.3a,for a heat input of 853.2 J/mm,an uneven bead of back formation was observed.The back reinforcement was relatively small and discontinuous at the initial stage,and it is was too large and overlapped at the end.The back formation of this root welding did not meet the technical requirements.However,as shown in Fig.3b,when root welding was carried out by a pulse MAG-TIG dual-arc with a communal molten pool heat source for a heat input of 826.4 J/mm,the back bead showed a good penetration with a continuous,uniform and stable formation(ITandUTare the TIG welding current and voltage,respectively).The continuous and stable back formation with a uniform penetration can be achieved by the pulse MAG-TIG dual-arc with a communal molten pool heat source,even by using a lower heat input compared with single MAG welding.

Fig.3 Weld back bead formation in single MAG welding and pulsed MAG-TIG double arc welding(a)Single MAG welding(IMp=440 A,IMb=120 A,UM=31.6 V,v=600 mm/min)(b)Pulsed MAG-TIG double arc welding(IMp=440 A,IMb=120 A,UM=30.0 V,IT=100 A,UT=11 V,DMT=20 mm,v=660 mm/min)

A cross-sectional image of the root weld that was finished by the pulse MAG-TIG dual-arc with a communal molten pool heat source is shown in Fig.4.No formation defects existed,such as an undercut,incomplete fusion,porosity and slag inclusion.The root pass thickness reached 7.0 mm and was accomplished with a back reinforcement of 2.3 to 2.5 mm and a width of 6.6 to 7.7 mm.High-efficiency root welding(welding speed of 660 mm/min)with back formation was achieved on medium-thick plates.

Fig.4 Cross section of backing weld

2.2 Arc morphology analysis

To investigate the electromagnetic intersection and the morphology of the arc plasma during pulse MAG-TIG dual arc welding,the arc morphology data of the single MAG welding and pulse MAG-TIG dual arc welding were collected by using a high-speed camera system.Fig.5 shows the arc plasma pattern of one cycle during single MAG welding(the same parameters with Fig.3a).A bell-jar-shaped arc was found when the welding current peaked,as indicated in Fig.5b and Fig.5c.For the base current,the arc was beamed and tilted slightly backwards,as shown in Fig.5e.The discharge combustion position of the MAG arc plasma coincided with the molten pool because a conductive channel that was formed by the charged particles dominated the thermo-ionization during single MAG welding.The droplet transition of the liquid metal lags behind the rear of the arccentral line,as shown in Fig.5a-Fig.c.Under the effect of inertia,the end of the arc plasma and the liquid droplet lagged slightly behind the central line of the wire.

Fig.5 Arc morphology of single MAG welding(a)t=1.0 ms(b)t=2.0 ms(c)t=3.0 ms(d)t=4.0 ms(e)t=5.0 ms(f)t=6.0 ms

Fig.6 Weld arc morphology of pulse MAG-TIG double arc tandem welding(a)t=1.0 ms(b)t=2.0 ms(c)t=3.0 ms(d)t=4.0 ms(e)t=5.0 ms(f)t=6.0 ms

The arc plasma morphology of one cycle of pulse MAG current was studied during the pulse MAG-TIG dual-arc with communal molten pool heat source welding(parameters in Fig.3b),as shown in Fig.6.An obvious rejection occurred between the pulse MAG arc and the DC TIG arc.The TIG arc makes the MAG arc during the peak current work below the wire centre or slightly ahead,which reduces the induction of arc plasma discharge in the high-temperature zone of the molten pool[13].Electromagnetic repulsion causes the end of the TIG arc to shift backwards,as indicated in Fig.6b-Fig.6c.When the pulse MAG arc is at the base current period,the discharge position is pushed by the TIG arc to the front of the molten pool,as shown in Fig.6e-Fig.6f.Therefore,the welding heat source of the highfrequency pulse MAG-TIG oscillation arc with a communal molten pool is formed.The electromagnetic force between the TIG and MAG arcs makes the MAG arc work steadily at the front of the molten pool.Part of the arc energy can heat the root face,which guarantees the penetration uniformity of root welding.At this time,the liquid metal in the molten pool has a lower temperature with a larger surface tension,which favors the stable and controllable molten pool.Consequently,the heat position of the MAG arc can be regulated by the TIG arc,which makes the temperature distribution reasonable at the front of the molten pool.Thus,a continuous and stable back bead with a uniform penetration can be obtained during root welding.

2.3 Molten-pool morphology and analysis

An observation and investigation of the liquid metal flow state in the molten pool were implemented by using a high-speed camera system for the MAG arc welding and the pulse MAG-TIG dual arc welding.

Fig.7 shows the weld pool morphology of the single MAG welding in a cycle of the pulse welding current.When the pulse current is at the base period,the flow pattern at the front of the molten pool can be observed more clearly,as shown in Fig.7a-Fig.7f.At this state of single MAG welding,the weld pool width is 9-10 mm and the length is 20-25 mm.Liquid metal flows backwards under the combined effect of an arc plasma shear stress,Lorenz force,buoyant force and shear stress that is caused by the gradient of surface tension.However,because of the energy density distribution characteristics of the single MAG welding heat source,the flow range and the liquid metal that flowed backwards in the MAG weld pool were limited.Therefore,more liquid metal existed at the front of the molten pool,which is not conducive to stable control of the free back formation during root welding[14].

Fig.7 Weld pool morphology of single MAG welding(a)t=1.0 ms(b)t=2.0 ms(c)t=3.0 ms(d)t=4.0 ms(e)t=5.0 ms(f)t=6.0 ms

Fig.8 Weld pool morphology of pulse MAG-TIG double arc tandem welding(a)t=1.0 ms(b)t=2.0 ms(c)t=3.0 ms(d)t=4.0 ms(e)t=5.0 ms(f)t=6.0 ms

The flowing weld pool morphology of the pulse MAG-TIG dual-arc welding during one pulse period is shown in Fig.8 and can be observed during the base pulse current period,as shown in Fig.8a and Fig.8f.Under the double-arc state of the pulse MAG-TIG hybrid welding,the formated weld pool width was 9-10 mm and the estimated pool length was 35-40 mm.Because the MAG arc is a highfrequency pulse current,the electromagnetic force between these two arcs makes the TIG arc oscillate backwards and forwards with a high frequency.Thus,the length and duration of the molten pool by the MAG-TIG dual arc heat source increased significantly.The pool surface area and surface tension were also enlarged.Under the action of arc plasma shear stress,the high-temperature liquid metal at the front of the molten pool starts to flows towards the back end of the pool,which decreases the front liquid metal and is conducive to stable control of the molten pool for free formation of root welding[15].

A comparison of the morphology of the front molten pool between the single MAG welding and pulse MAG-TIG dual-arc hybrid welding,shows that the centre of the welding wire is～4.0 mm from the front edge of the molten pool,as observed with the pool image of single MAG welding in Fig.9a.It can be inferred that the pool radius of single MAG welding is～4.0 mm.When applied to pulse MAG-TIG dual arc welding,a specific offset forward occurs for the MAG arc because of the electromagnetic repulsion between the two arcs,as shown in Fig.6e-f.The distance increases up to 6.0 mm from the front pool to the wire centre,which is 2.0 mm further than the single MAG process,as shown in Fig.9b.

Fig.9 Comparison of front of weld pool(a)Single MAG welding(b)Pulsed MAG-TIG double arc welding

2.4 Offset calculation and mechanism investigation

To simplify,the profile of the front molten pool that is formed by the pulse MAG arc is treated as a circle,as sketched in Fig.10.The X-axis in the figure is the welding direction.CircleO1is the front profile of the molten pool that is formed before the MAG arcshifting,whereasO2is the profile after shifting.dis the forward displacement of the molten pool,Ris the pool radius,S1is the forward offset region area of the molten pool andS2is the area of the shaded area as indicated in Fig.10.The ratio of areaS1to the whole pool area,K,can be calculated from Equations(1)and(2).

Fig.10 Schematic diagram of weld pool migration in pulse MAG-TIG double-arc tandem welding

Under optimal root welding conditions with back formation for the medium-thick plate,a substitution of the parameters ofR(4.0 mm)andd(2.0 mm)into the equations results inK=31.5%.

The analysis implies that the difference between the single MAG welding and pulse MAG-TIG dual-arc with communal molten pool heat source welding is as follows.For single MAG welding,the arc plasma heat works almost entirely directly above the liquid metal in the molten pool,and the root face is melted by means of thermal conduction through the weld pool energy.As a result,incomplete root fusion occurs easily if a lower heat input processis used,and back bead overlap may result for a higher heat input.Moreover,in this thermal conduction mode,the molten pool temperature is higher and the surface tension is smaller,which is harmful to welding stabilization and control.The superior parameter interval is narrow for stable root welding.Nevertheless,when the pulse MAG-TIG dual-arc is applied with a communal molten pool heat source welding technology,the presence of an opposite polarity between the power source of the MAG arc and TIG arc results in the production of mutually exclusive electromagnetic effects between them.The use of a repulsive force of the TIG arc with the pulse MAG arc allows the heating position of the MAG arc to be moved forward in the pulse MAG-MIG hybrid welding process.Thus,approximately 31.5% of the MAG arc energy can be imposed directly on the root face at the front of the molten pool.The liquid metal of the back weld pool is formed through the flow behaviour towards the bottom pool from the face side liquid metal,which is the mixed melted root face and the liquid droplets.Sufficient root welding penetration and a reduction in liquid metal temperature in the weld pool is achieved.For the pulse MAG-MIG dual-arc hybrid welding,because of the lower weld pool temperature and because～68.5% of the energy is imposed on the pool,the surface tension of the liquid metal in the molten pool is relatively large.Thus,control of the formation of the back weld pool helps to yield a continuous and stable back bead with a uniform penetration.

A schematic of the flow state in the molten pool is given in Fig.11 under optimal conditions for the MAG-TIG dual-arc tandem welding pulse.The TIG arc acts at the tail of the molten pool and vibrates backwards and forwards,which lengthens the weld pool and changes the temperature distribution state of the dual-arc welding process.The shear stress of the surface-tension gradient(Marangoni effect)facilitates the flow of high-temperature liquid metal in the molten pool backwards and up[16],as shown in Fig.11a.Hence,the liquid metal gravity at the front of the back weld pool is reduced,which provides control for free back formation of root welding.The centre temperature at the end of the molten pool is increased by the heating effect of the TIG arc.Flow behaviour in the molten pool under negative surface tension conditions is shown in Fig.11b,which shows the temperature gradient alteration of the weld pool from heating the TIG arc.The liquid metal flows towards the groove sides with the Marangoni effect.Pulse MAG-TIG dual-arc root welding takes advantage of the Lorenz force between the two arcs and the heat effect of the TIG arc(Marangoni effect),which makes a continuous and stable back bead with a uniform penetration achievable for the root welding of medium-thick plates[17].

Fig.11 Schematic diagram of molten pool flow of pulse MAG-TIG double arc tandem welding(a)Welding pool longitudinal section(b)Welding pool cross section

In summary,the best condition of the pulse MAG-TIG dual-arc with communal molten pool heat source root welding process is that the TIG arc imposes a heating effect at the tail of the molten pool.By using the electromagnetic repulsion force between the two arcs,the discharge combustion position of the MAG arc can be adjusted to heat part of the arc plasma energy directly on the root face.The lower temperature of the liquid metal in the molten pool and the larger surface tension are conducive to a good weld bead formation with full root penetration[18].The reheating effect of the TIG arc can modify the temperature gradient of the weld pool.The Marangoni effect makes the liquid metal flow backwards and upwards.As a result,the decreasing liquid metal mass at the front of the molten pool helps to control and stabilise root welding with the back-formation procedure[19].

3 Conclusions

(1)A pulse MAG-TIG dual-arc with a communal molten pool heat source root welding process was investigated,and root welding with back formation was achieved even with the lower welding heat input compared with single MAG arc welding.A continuous and stable back bead with uniform penetration was obtained.The deposited metal of the root pass was as high as 6.8-7.0 mm with a high welding speed of 600 mm/min.The width and reinforcement size of the back bead reached the technical requirements of the root welding procedure.A high efficiency and high-quality root welding technology was achieved for the mediumthick plate.

(2)The TIG arc is of considerable significance to the MAG arc in the electromagnetic repulsion force when using pulse MAG-TIG dual-arc hybrid welding.The distribution of the arc energy on the root face and molten pool can be regulated by adjusting the heating position of the MAG arc by controlling the intensity of the electromagnetic force.For the best state of the dual-arc welding process,in the molten pool.

(3)In terms of the pulse MAG-TIG dual-arc with a communal molten pool heat source root welding process,when the back formation is in the optimal status,the TIG arc acts at the tail of the molten pool.The temperature gradient of the weld pool can be modified by using the reheating effect of the TIG arc.The shear impact towards the liquid metal from the Marangoni effect and the arc plasma makes the liquid metal flow backwards and upwards.Therefore,the decrease in bottom liquid metal mass at the front of the molten pool favors penetration control of the root welding with back formation.