Metal transfer characteristics of tandem narrow gap GMAW for flat welding position*

Cai Xiaoyu，Lin Sanbao，F(xiàn)an Chenglei，Yang Chunli，Xu Wanghui and Zhang Wei

蔡笑宇，林三寶，范成磊，楊春利，徐望輝，張 威**

Metal transfer characteristics of tandem narrow gap GMAW for flat welding position*

Cai Xiaoyu，Lin Sanbao，F(xiàn)an Chenglei，Yang Chunli，Xu Wanghui and Zhang Wei

蔡笑宇，林三寶，范成磊，楊春利，徐望輝，張 威**

A series of experiments of tandem narrow gap GMAW for flat welding position were carried out.The arc behavior and metal transfer process were observed by a high-speed photography system.The effects of the welding parameters on the metal transfer were investigated.The results show that the arc behavior and metal transfer process of tandem narrow gap GMAW are different from these of bead-on-plate tandem GMAW.The arc behavior and metal transfer process are influenced by the distance between the two wires，the peak voltage，the pulse frequency and the peak time.With the increase of the distance between the two wires，the metal transfer mode gradually transforms from one pulse-multi droplets into one pulse-one droplet，and the average welding current increases.With the increase of the peak voltage，pulse frequency or peak time，the metal transfer mode transforms from one pulse-one droplet into one pulse-multi droplets，and the arc tends to occur between the wire and the sidewall.

metal transfer，arc behavior，narrow gap welding，tandem GMAW

0 Introduction

Thick-walled plates are widely used in the ship building industry，pressure vessel，pipeline engineering.The conventional welding processes in use today require longer welding time and more filler metal，and more energy input may result in greater weld distortion.Narrow gap（NG）welding is used increasingly in the manufacture field of the thick plates structure because of its considerable productivity.But incomplete sidewall fusion is the most frequent defect innarrowgapgas metal arcwelding（NGGMAW）［1-2］.

Several methods were developed to ensure the sufficient penetration of sidewall in NG-GMAW，for example rotating arc GMAW［3-5］，oscillating arc GMAW［6-7］and tandem GMAW［8］.NG welding and tandem GMAW are both high efficient welding methods，so tandem NGGMAW，which combines the advantages of the two welding methods has potential applications.

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［9］.However，the metal transfer characteristics and arc behavior of tandem NGGMAW were rarely reported.

In this paper，the metal transfer and arc behavior of tandem NG-GMAW for flat welding position were studied by a high-speed photography system.The emphasis was placed on the analysis of the metal transfer characteristics and the effects of the welding parameters on the metal transfer，especially the distance between the two wires（DW），the peak voltage（UP），the pulse frequency（FP）and the peak time（TP）.Appropriate welding parameters were selected，and a welding process of the plates with a thickness of 60 mm was conducted，and a sound and macro-defect-free joint was obtained.

1　Experimental apparatus

The schematic of the experimental apparatus is pres-ented in Fig.1.The two wires are fed through two electrically isolated contact tips into a single molten pool.The wires can be controlled independently by two CLOOS 503 power sources.The power sources are operated in pulsed mode in which the peak voltage and the base current are constant.As shown in Fig.2，the ends of the two contact tips are bent to direct the two wires toward the opposite sidewalls to ensure the sufficient sidewall penetration.The U-groove with the bottom width of 10 mm，top width of 12 mm and depth of 25 mm is applied during the welding process.A CamRecord 5 000×2 high-speed video camera（2 000 frames/s）is used to observe the molten pool behavior and metal transfer.A laser illuminator is used as light source.

Fig.1 Schematic of the experimental apparatus

Fig.2 The schematic of groove and contact tips

Q235 and H08Mn2Si are used as base metal and filler wire respectively.The chemical compositions of base metal and filler wire are shown in Table 1.The diameter of the wire is 1.2 mm.92%Ar+8%CO2with a flow rate of 50 L/min is used as shielding gas.

Table 1 Chemical compositions of base metal and filler wire（wt.%）

2　Results and discussion

2.1Metal transfer characteristics of tandem NGGMAW

In tandem NG-GMAW，the arcs work in the narrow and deep gap，and the further concentration of heat inevitably affects the metal melt and transfer characteristics. The welding parameters are given in Table 2.

Table 2 Welding parameters of comparison experiments

As presented in Fig.3（the left wire is lead wire and the right one is trail wire），in tandem NG-GMAW，the end of the lead wire is closer to the sidewall，so when the pulse peak comes，the lead arc burns mainly between the wire and the sidewall，and the lead arc is asymmetric.The trail wire is behind the lead wire，so there is a generated molten pool under the trial wire.The distance between the trail wire and the molten pool surface is close，so the trial arc burns at the corner between the sidewall and the molten pool，and the trial arc is relatively symmetric.

The metal transfer process of bead-on-plate welding is shown in Fig.4.It can be observed that，in pulse peak，the wire melts and the droplet grows.In pulse base，a necking appears between the end of the wire and the droplet（t=3 458.5 ms）.Before the droplet detaching from the wire，the droplet gets in touch with the molten pool because the arc length is short（t=3 459.0 ms），and a short circuit transfer occurs（t=3 459.5-3 460.0 ms）.The transfer mode is one pulse-one droplet.

Fig.3 Arc shape of tandem NG-GMAW

Fig.4 Metal transfer process of trail wire in bead-on-plate tandem GMAW

However，as shown in Fig.5，under the same conditions，the metal transfer mode in tandem NG-GMAW is one pulse-multi droplets，and a metal stream is generated. No short circuit transfer occurs，and the arc length in tandem NG-GMAW is longer than that in bead-on-plate GMAW.Theanalysis suggests that intandem NG-GMAW，because of the narrow and deep gap，the heat is more concentrative，and the temperature of the arc atmosphere is relatively higher，so the wire melt rate of tandem NG-GMAW is greater than that of bead-on-plate tandem GMAW.Metal transfer mode tends to be one pulse-multi droplets.

Fig.5 Droplets transfer process in tandem NG-GMAW

2.2Effects of welding parameters

2.2.1Effect of DWon the metal transfer

In tandem NG-GMAW，the two arcs are close，so the welding heat is concentrated.The heat concentration decreases with the increase of the DW，so the DWcan influence the metal transfer process.The welding parameters are given in Table 3.The DWis selected below 30 mm to ensure the two arcs work in the same molten pool.

Table 3 Welding parameters with different DW

The high-speed images are presented in Fig.6.When the DWis 8 mm，the metal transfer mode of the trail wire is one pulse-multi droplets.When the DWincreases，the metal transfer mode transforms from the one pulse-multidroplets to one pulse-one droplet gradually，and it totally turns into one pulse-one droplet when the DWis 22 mm.

The welding current is acquired by using welding electrical signals acquisition system，and the average welding currents of the two arcs are calculated as given in Table 4.In welding process，the heat concentration accelerates the melting rate of the two wires，so the average currents of the two wires decrease in order to maintain the equality of the wire melting rate and the wire feed speed. However，more of the lead arc heat is used for melting base metal，so the heat concentration effect is stronger to trail wire.Hence the average current of trail wire is less than that of lead wire.The heat concentration effect decreases with the increase of the DW，so the average currents of the two wires increase and the difference of the two currents reduces.

Fig.6 Metal transfer with different DW

Table 4 Average current of each wire

2.2.2Effect of UPon metal transfer

The value of the voltage determines the arc shape and also influences the metal transfer process.Different UPare selected to reveal the effect of UPon metal transfer.The experimental parameters are given in Table 5.The highspeed images are shown in Fig.7.

Table 5 Welding parameters with different UP

The arc length increases with the increase of UP，and the arc root tends to locate at the sidewall.When the UPis at low level（32 V），the arc length is short，and the distance between the end of the wire and the molten pool surface is closer than the distance between the end of the wire and the sidewall，so the arc root occurs at the molten pool surface.However，because arc length is short，short circuit transfer occurs.When the UPis raised to 34 V，the metal transfer mode is one pulse-multi droplets transfer，and the arc works at the corner between the molten pooland the sidewall.With the further increase of the UP（38 V，40 V），the distance between the end of the wire and the sidewall becomes closer，so the arc occurred between wire and sidewall.

Fig.7 Metal transfer with different UP

2.2.3Effect of FPon metal transfer

Different FPis selected to investigate its effect on the metal transfer.The welding parameters are given in Table 6.The high-speed images are shown in Fig.8.

Table 6 Welding parameters with different FP

When the FPincreases while the TPremains constant，the total peak time has an increasing proportion in total welding time，which accelerates the melting rate of the wire，so the arc length increases.When the FPis at low level（90 Hz，120 Hz），the droplet transfer mode is short circuit transfer.With the increase of FP，spray transfer occurs.

With a higher FP，the wire melting rate can be accelerated and the distance between the wire and the molten pool surface（DWP）is greater.Fig.9 indicates the relationship of the FPand DWP，and each value of DWPis the average of many measurement data.It can be seen that when the FPis at high level（210 Hz），the arc still occurs at the corner between the molten pool and the sidewall.

Fig.8 Metal transfer with different FP

Fig.9 The relationship of the FPand DWP

2.2.4Effect of TPon metal transfer

Different TPare selected to investigate its effect on the metal transfer.The welding parameters are presented in Table 7.The high-speed images are shown in Fig.10.The arc length increases with the increase of the TP.The metal transfer mode is one pulse-one droplet when the TPis 2.0 ms.The metal transfer mode transforms to one pulse-multi droplets with the increase of TP，and when the TPis 4.0 ms，the droplet transfer mode of the lead wire is rotating spray transfer.

Table 7 Welding parameters with different TP

Fig.10 Metal transfer with different TP

When the TPincreases while the FPremains constant，the total peak time also has an increasing proportion in total welding time，and it accelerates the wire melting rate，so the arc length increases.Fig.11 shows the relationship of the TPand DWP，and each value of DWPis the average of many measurement data.The higher TP（≥3.5 ms）leads to the greater distance between the wire and the molten pool surface，and the arc tends to occur between wire and the sidewall.Compared with Fig.9，it can be concluded that TPhas a more significant effect on the DWP.

2.3Single pass multilayer welding

Based on the above results，the appropriate welding parameters were selected，and a single pass multilayer welding process was carried out.The parameters are given in Table 8.The distance between two wires is set to 5 mm.The U-groove with the bottom width of 14 mm，top width of 16 mm and depth of 60 mm is applied during the welding process.Fig.12 shows a cross section of the multi-layer weld.It can be seen that a high quality and macro-defect-free multi-layer weld joint was acquired.

Fig.11 The relationship of the TPand DWP

Fig.12 Cross section of the multi-layer weld

Table 8 Parameters of single pass multilayer welding

3　Conclusions

（1）In tandem narrow gap GMAW，the arcs work in the narrow and deep gap，and the further concentration of heat inevitably affects the metal melt and transfer characteristics.Metal transfer mode tends to be one pulse-multi droplets transfer.The welding parameters influence the metal transfer process.

（2）With the increase of the distance between the two wires，the metal transfer mode transforms from one pulse-multi droplets to one pulse-one droplet gradually.

（3）When the peak voltage，pulse frequency or peak time are at low level，the arc length is short，and short circuit transfer occurs.The metal transfer mode transforms from one pulse-one droplet to one pulse-multi droplets with the increase of peak voltage，pulse frequency or peak time.

（4）In appropriate conditions，a butt joint with the depth of 60 mm can be acquired，and the joint is sound and macro-defect-free.

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*This research is supported by National Natural Science Foundation of China（Grant No.51275109）.

**Cai Xiaoyu，Lin Sanbao，F(xiàn)an Chenglei，Yang Chunli and Xu Wanghui，State Key Laboratory of Advanced Welding and Joining，Harbin Institute of Technology，Harbin，150001. Zhang Wei，Xi’an Space Engine Factory，Xi’an，710100. Lin Sanbao，Corresponding author，E-mail：sblin@hit.edu.cn