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Influence of external magnetic field on twin-wire indirect arc surfacing stainless steel layer
Vacuum 169 (2019) 108958
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Influence of external magnetic field on twin-wire indirect arc surfacing stainless steel layer Dongting Wu a, Cong Hu a, Wei Zhao b, Yongang Zhang c, **, Yong Zou a, * a
MOE Key Lab for Liquid-Solid Structure Evolution and Materials Processing, Shandong University, Jinan, 250061, China School of mechanical and automotive engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, China c School of Materials Science and Engineering, Shandong University, Jinan, 250061, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Twin-wire indirect arc welding Surfacing welding Applied magnetic field Pitting corrosion
Twin-wire indirect arc welding(TWIAW)owns advantages including high deposition efficiency, cost-effective energy utilization and low dilution rate of the base metal. However, there are some disadvantages such as small depth of fusion and narrow process window during welding. This work studied the influence of the external magnetic field on the formation of surfacing stainless steel layer prepared by the TWIAW. The effect of external magnetic field on welding process and pitting corrosion of surfacing layer was studied. The results show that the applied magnetic field can significantly modified the indirect arc shape and the associated pitting corrosion performance of the surfacing layer. Under the condition of the external magnetic field in the negative x-direction, the arc became lengthened and the heat input to the workpiece was increased accordingly. This is beneficial for improving the weld shape and the pitting corrosion resistance of the surfacing layer.
1. Introduction The twin-wire indirect arc welding (TWIAW) is a newly developed welding technology. In the process, the two poles of the welding power source are connected with two welding wires respectively while the workpiece is not connected into the current circuit, as shown in Fig. 1. Therefore, the indirect arc is formed between the end of the twin wires and droplet transfer occurs simultaneously from both wire tips. Accordingly, the generated arc heat is mainly used to melt the welding wires instead of melting the workpiece. Several advantageous features are derived from the TWIAW technology, that is, high deposition effi ciency, high energy-utilization efficiency, low fusion ratio and low dilution rate of base metal. Cao et al. [1] investigated the droplet transfer process of twin-wire indirect welding and analyzed the force of the droplet. The results showed that the metal droplet transfer modes occurring at the anode and the cathode are significantly different. Shi et al. [2,3] explored the characteristics of the twin-wire indirect arc by combining numerical simulation with welding experiments. It was found that the arc behavior was mainly controlled by the welding cur rent. Nevertheless, it has been noticed that the weld penetration by TWIAW was shallow and the welding process window was narrow by
Zhang et al. [4], which greatly restricted the popularization and appli cation of this technology. In order to improve the properties of arc welding, the external magnetic field was studied by numerous researchers. It is found that the external magnetic field plays a positive role on the welding quality during arc welding, because it can modify the arc shape, droplet transfer mode, the resultant weld size and microstructure [5,6]. This in turn improves the mechanical and electrochemical properties of the welded joints. In GMAW, applying external magnetic field can change arc shape and arc motion, which has an important influence on arc temperature gradient and undercooling degree [7,8]. The change of undercooling degree plays a key effect on the solidification mode of weld [9],resulting in elimination of chemical inhomogeneity [10], refinement of grains [11], reduction of shrinkage tendency [12] and restraint of hump weld [13,14]. The refinement of grain and the decrease of defect will lead to the decrease of welding residual stress [15,16] and the improvement of mechanical properties [17]. For stainless steel GMAW, applying external magnetic field can not only improve the mechanical properties of the weld, but also enhance the corrosion resistance of the weld, which is effective for all of austenitic stainless steel [18,19] and duplex stainless steel [20,21].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Zou). https://doi.org/10.1016/j.vacuum.2019.108958 Received 27 July 2019; Received in revised form 24 August 2019; Accepted 19 September 2019 Available online 20 September 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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Vacuum 169 (2019) 108958
the cathode wire. The experimental schematic diagram of the applied magnetic field and the established coordinate direction were also depicted in Fig. 3. In order to generate the x-directional magnetic field at the end of the welding wire, both the electromagnet cores were located in the xOy plane with the cores positioned along the y-axis positive di rection. The distance between the end of the excitation coil and the intersection of the cathode and anode welding wires was measured to be 2.2 cm. The excitation coil consisted of six layers with 60 turns in each layer. The magnetic induction intensity produced at the intersection of the twin wires ends is shown in Table 2 (the measured position was in the middle of the wire ends). The welding process parameters are shown in Table 3. In addition, the MV-D1024E-160DE high-speed camera system was set up to visu alize the arc shape. The Potentiodynamic method was utilized to obtain the polarization curve of the surfacing layer for evaluating its pitting corrosion perfor mance. The polarization curves were measured in 3.5 wt % NaCl solu tion at room temperature. The testing was scanned forward from - 0.4 V with a speed of 0.5 mV/s applying a CS350 electrochemical workstation. The reference electrode was a saturated calomel electrode (SCE), and the counter electrode was a platinum electrode. When the current density of the anode reached 10 3A/cm2, the scanning begun reversed. Once the reverse scanning curve intersected with the forward scanning curve, the annular anodic polarization curve was achieved.
Fig. 1. Schematic diagram of the twin-wire indirect arc welding.
However, the study focused on applying the external magnetic field to TWIAW is very limited. In order to improve the fusion depth during TWIAW, Zhang et al. conducted a preliminary study by introducing the external magnetic field to TWIAW process and found that the shape of the welding arc was related to the difference of internal and external magnetic field intensity in the area surrounded by two welding wires [22]. With the applied transverse magnetic field, the larger the intensity of the forward magnetic field was, the smaller the droplet volume and the higher the droplet transition frequency were [23]. Considering the high depositing efficiency and low dilution rate of base metal of the TWIAW process, it becomes suitable for surfacing manufacture. Wu et al. [24] employed the TWIAW technique to prepare a Fe-based stainless steel layer and successfully obtained the austenite þ ferrite composite structure with excellent corrosion resistance. However, the role of external magnetic field on the formation of the surfacing layer during TWIAW could be different from that of the traditional arc surfacing layer because of the different melting speed and droplet transfer modes of the anode and cathode wires. The study focused on the effect of external magnetic field on the surfacing layer prepared through TWIAW tech nique is limited. In this paper, the effect of external magnetic field on the stainless steel surfacing layer prepared through TWIAW technique was investi gated, in particular the modification of the arc shape and the pitting corrosion resistance.
3. Results and discussion 3.1. Effect of magnetic field on arc state Fig. 4 shows the arc shape captured by the high-speed camera with the magnetic field direction along the x-axis under the condition of magnetic induction intensity 0 mT, 3.2 mT, 5 mT and 7.8 mT, respec tively. It can be seen that under the same shooting distance and filter condition, the arc brightness increased and the volume of arc decreased with the increase of the magnetic induction intensity, indicating that the arc became compressed by the x-direction magnetic field and the arc current density increased accordingly. Fig. 5 shows the arc shape captured by the high-speed camera with the magnetic field direction along the negative x-axis under the condition of magnetic induction intensity 0 mT, 3.2 mT, 5 mT and 7.8 mT, respectively. As shown in Fig. 5, with the increase of the magnetic induction intensity, the brightness of the arc decreased gradually while the arc tends to be stretched. Overall, the current density decreased. Arising from the gas ionization inside the arc space and the electron emission at the end of the cathode welding wire, the twin-wire indirect arc was composed of a great number of charged particles. These moving charged particles would be affected by the Lorentz force in the magnetic field [25]. The direction of the Lorentz force is perpendicular to the motion direction of these charged particles and its value can be calcu lated by Eq. (1). The generated Lorentz force would alter the moving direction of the moving charged particles.
2. Experimental procedure Q235 was selected as the base metal in this study. The welding material was ER308 stainless steel wire with a diameter of 1.2 mm. The chemical composition of the base metal and the welding wire is shown in Table 1. The surfacing welding was carried out using a lab-designed twinwire indirect arc welding system, as presented in Fig. 2. The welding power supply was NBC-350 inverted gas shielded welding power source with flat external characteristics. Two sets of AS-35 wire feeder were used to feed the cathode and anode wires respectively and the wire feeding speed could be automatically adjusted. The cathode wire was set in the vertical position while anode wire had an angle of 30� inclined to
F ¼ qvB
where F is the Lorentz force with the unit Newton, q is the charge of charged particles with the unit Coulomb, v is the velocity of the charged particles with the unit meter per second and B is the magnetic induction intensity with the unit tesla. During the twin-wire indirect arc welding, the welding current flew from the anode to the cathode. The cathode current transmitted upward vertically while the anode current passing through the anode wire owned a certain angle to the vertical direction. When the direction of magnetic field was along the positive x-axis direction, the applied magnetic field was perpendicular to the outside of paper. The Lorentz force for the charged particles moving inside the arc between the anode and cathode poles is shown in Fig. 6 a.
Table 1 Chemical composition of base metal and welding wire(wt. %). Q235 ER308
C
Si
Mn
P
S
Cr
Ni
Fe
0.20 0.03
0.25 0.60
0.50 1.80
0.03 0.015
0.03 0.008
– 20
– 10
Re Re
(1)
2
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Vacuum 169 (2019) 108958
Fig. 2. Equipment of the twin-wire indirect arc welding system.
Fig. 3. Welding torch structure with twin wires angle 30� and x direction magnetic field to produce.
The macroscopic expression of the Lorentz force imposed on the charged particles in arc space was Ampere force. The direction of the Ampere force acting on the moving charged particles near the welding wires is shown in Fig. 6 a. The formed Ampere force F1 and F2 com pressed the charged particles in the arc space into a smaller space. Macroscopically, the arc got shrunken and the current density in the arc space increased. With the increase of magnetic induction intensity, the magnetic induction intensity of the magnetic field present in the forward direction along the x-axis increased. Therefore, the arc continued to the compressed further. When the magnetic field direction was imposed along the negative direction of x-axis, the magnetic field became inside-facing of vertical paper. Considering that the current direction was the same, the direction of Ampere Force F1’and F2’ imposed to the moving charged particles became opposite to F1 and F2, as displayed in Fig. 6 b. The present F1’and F2’ forced the charged particles moving outside of the arc space, and the welding arc would be lengthened. As a result,the space current density of the arc would be reduced. With the increase of magnetic
Table 2 Magnetic induction strength on the tip of twin wires. Excitation voltage (V)
0
1.6
3
4
Excitation current (A) Magnetic induction intensity (mT)
0 0
4 3.2
7 5
10 7.8
Table 3 Welding parameters of different external magnetic field.
a b c d
Angle between twin-wire (� )
Welding current (A)
Arc voltage (V)
Excitation current(A)
Magnetic induction intensity (mT)
Velocity (mm⋅min1)
30
190
25
0 4 7 10
0 3.2 5 7.8
187
3
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Vacuum 169 (2019) 108958
Fig. 4. Arc shape with applying x positive magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT.
Fig. 5. Arc shape with applying negative x magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT.
Fig. 6. Force of arc under x direction magnetic field (a) x positive direction (b)x negative direction.
induction intensity, the increased magnetic induction intensity along the negative direction of x-axis would further elongate the welding arc. For the case of positive x-axis direction magnetic field, the twin-wire indirect arc got compressed and concentrated at the end of the anode
and cathode poles. However, under the condition of the negative x-axis direction magnetic field, the twin-wire indirect arc became elongated from the wire tips towards the workpiece. Accordingly, more heat was transferred to the substrate, increasing the weld penetration.
Fig. 7. Weld profile of x negative magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT. 4
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3.2. Effect of magnetic field on weld formation
Table 4 Ferrite content for surfacing layers in different external magnetic field.
As shown in Fig. 5, the welding arc became elongated when the magnetic field was exerted along the negative x-axis direction. Accordingly, more arc heat could be transferred to the surface of the workpiece to form a weld. Fig. 7 demonstrates the appearance of the weld under the condition of 190 A welding current, 25 V arc voltage, 187 mm/min welding speed and the negative x-axis direction magnetic field with different magnetic induction intensity. The negative magnetic field along the x-axis extended the twin-wire indirect arc towards the welded workpiece. The number of charged particles in the lower part of the arc became larger compared that in the absence of magnetic field. As a result, the heat input to the welded workpiece increased, promoting the weld width.
a b c d
Welding current (A)
Arc voltage (V)
Excitation current (A)
Magnetic field intensity (mT)
Ferrite content (%)
190 190 190 190
25 25 25 25
0 4 7 10
0 3.2 5 7.8
4.02 4.30 5.89 6.51
formation of chromium-poor layer of austenite grain. 3.4. Effect of magnetic field on pitting corrosion resistance Fig. 9 illustrates the cyclic polarization curves of stainless steel surfacing layers for pitting corrosion experiments under the condition of the negative x-axis direction magnetic field with different magnetic field
3.3. Effect of magnetic field on ferrite content and distribution Fig. 8 gives the ferrite distribution of the surfacing layer under the condition of the negative x-axis direction magnetic field with different magnetic induction intensity. The changes of the ferrite content derived from the metallographic images are summarized in Table 4. It can be seen that more ferrite microstructure appeared with the increased magnetic induction intensity. For the case of none external magnetic field, only a small fraction of ferrite dots scattered on the austenite matrix, as shown in Fig. 8 a. With the external magnetic field increased to 3.2 mT, more ferrites started to precipitate along the austenite subgrain boundary as shown in Fig. 8 b. Once the external magnetic field increased to 5.0 mT and 7.8 mT, the content of ferrite precipitates occupied around 6%, distributing at the austenite grain boundary to form interconnected network as shown in Fig. 8c and d. In addition, compared to the weld microstructure without magnetic field, the so lidification process of weld metal became equilibrium due to the elec tromagnetic stirring by the external magnetic field. In generally, in order to improve the intergranular corrosion resistance of austenite welding layer, it is desirable to retain a certain amount of ferrite phase in welding layer. Its function is as follows: (1) it can disturb the orientation of the columnar crystal of single austenite phase, so as not easy to form a continuous chromium-poor layer; (2) the ferrite phase is rich in chro mium and has good chromium supply conditions, which can reduce the
Fig. 9. Cyclic polarization curves of surfacing layer in different external mag netic field.
Fig. 8. Ferrite for surfacing layers of x negative direction magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5 mT and (d) 7.8 mT. 5
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intensities. For the different magnetic induction intensity, the corrosion current density did not change significantly with the increased polari zation potential at the initial stage due to the presence of a passive film formed on the stainless steel surfacing layer surface as shown in Fig. 9. Meanwhile, the free corrosion potential Ef were similar. When the corrosion potential was further increased, the passive film on metal surface was broken down and the corrosion current density increased sharply. Therefore, the inflection point on the curve where the current started to increase could be correlated with the occurrence of pitting corrosion (point Eb as marked in Fig. 9). With the increase of the external magnetic field intensity, the pitting breakdown potential Eb shifted to ward the high potential value indicating the improved pitting resistance of the surfacing layer. However, when the magnetic induction intensity rose up to 7.8 mT, there were two inflection points when the potential went from Ef to Eb, implying the pitting corrosion resistance didn’t become better, but got worse compared with that of the 5.0 mT magnetic field condition. The reason of the lower corrosion resistance was the increasing of ferrite content caused by higher dilution rate with the increasing of magnetic field intensity. Fig. 10 shows the surface morphologies of the stainless steel surfacing layers after cyclic polarization pitting tests. Increasing the magnetic induction intensity made a significant contribution in decreasing the number of corrosion pits and therefore facilitated the pitting resistance of the surfacing layer. The corrosive pitting on the metal surface is mostly induced by the incomplete passivation film or non-uniform surface structure. Even though both welding wires were melted simultaneously during the twinwire indirect arc welding process, their melting speed and the metal droplet transfer modes were completely different. The wire feeding speed and melting speed of the cathode welding wire were nearly twice as fast as that of the anode welding wire. Consequently, the metal droplets were transferred from the anode welding wire in the large droplet transition stage, while the metal droplets were transferred from the cathode welding wire in the droplet transition state. The heat input from the welding arc to the workpiece was small and the welding pool existed for a short time in the welding process. The fast solidification was detrimental for ensuring a homogeneous elemental distribution on the surfacing layer. Therefore, the passivation film was prone to form
local rupture, leading to the pitting corrosion. The pitting corrosion potential was slightly smaller without applying the magnetic field. With the negative x-axis direction magnetic field,the shape of welding arc and the distribution of charged particles inside the arc space changed. More heat was transferred from the welding arc to the workpiece, which extended the existence time of the welding pool. In addition, the elec tromagnetic stirring induced by the external magnetic field could enable the elements distributing more uniformly inside the surfacing welding layer. This improved the stability of passive metal film on the surfacing layer, increasing the pitting corrosion potential and enhancing the pitting corrosion resistance. 4. Conclusions In this paper, the austenitic stainless steel surfacing layer was pre pared by the twin-wire indirect arc welding on the Q235 low carbon steel substrates. During the welding process, an external magnetic field was applied surrounding the twin wire indirect arc. The effects of the external magnetic field on the arc shape, the evolution of the ferrite content and the pitting corrosion resistance of the surfacing layer were studied. The following conclusions are drawn: (1) External magnetic field modified the shape of the twin-wire in direct arc. The arc became compressed by the positive x-axis di rection magnetic field. The arc got elongated. Under the condition of the negative x-axis direction magnetic field, which was beneficial for promoting the heat input to the workpiece and increasing the fusion depth. (2) When negative x-axis direction magnetic field was applied, more ferrite precipitated inside the surfacing layer with the increase of the magnetic field intensity. This is correlated with that the increased heat input postponed the solidification of the molten pool. (3) Under the condition of the negative x-axis direction magnetic field, the pitting corrosion potential of surfacing layer increased with the increase of magnetic field intensity. This is due to that the external magnetic field enabled the passive film of the
Fig. 10. Corrosion morphology after epr tests for surfacing layers in different external magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5 mT and (d) 7.8 mT. 6
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surfacing layer form more homogeneously by the electromag netic stirring effect.
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Acknowledgements This project is supported by National Natural Science Foundation of China (No. 51975331), Shandong Province Key Research and Develop ment Program (Public Welfare Project) (2018 GGX103031), National Natural Science Foundation of China Youth Fund Project (51805285). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.vacuum.2019.108958. References [1] M.Q. Cao, Z.D. Zou, S.S. Zhang, S.Y. Qu, Metal transfer of twin-wire indirect arc argon welding, Rare Metal Mater. Eng. 40 (2011) 156–159. [2] C. Shi, Z. Yong, Z. Zou, D. Wu, Twin-wire indirect arc welding by modeling and experiment, J. Mater. Process. Technol. 214 (2014) 2292–2299. [3] C. Shi, Z. Yong, Z. Zou, D. Wu, Electromagnetic characteristic of twin-wire indirect arc welding, Chin. J. Mech. Eng. 28 (2015) 123–131. [4] Z. Zhang, D. Wu, Z. Yong, Effect of bypass coupling on droplet transfer in twin-wire indirect arc welding, J. Mater. Process. Technol. 262 (2018) 123–130. [5] P. Sharma, S. Chattopadhyaya, N.K. Singh, A review on magnetically supported gas metal arc welding process for magnesium alloys, Mater. Res. Express 6 (2019). [6] H. Wu, Y.L. Chang, L. Lu, J. Bai, Review on magnetically controlled arc welding process, Int. J. Adv. Manuf. Technol. 91 (2017) 4263–4273. [7] Z.Q. Guan, H.X. Zhang, X.G. Liu, A. Babkin, Y.L. Chang, Effect of magnetic field frequency on the shape of GMAW welding arc and weld microstructure properties, Mater. Res. Express 6 (2019). [8] J. Lueg-Althoff, J. Bellmann, S. Gies, S. Schulze, A.E. Tekkaya, E. Beyer, Influence of the flyer kinetics on magnetic pulse welding of tubes, J. Mater. Process. Technol. 262 (2018) 189–203. [9] M. Bachmann, V. Avilov, A. Gumenyuk, M. Rethmeier, About the influence of a steady magnetic field on weld pool dynamics in partial penetration high power laser beam welding of thick aluminium parts, Int. J. Heat Mass Transf. 60 (2013) 309–321. [10] O. Ustundag, V. Avilov, A. Gumenyuk, M. Rethmeier, Improvement of filler wire dilution using external oscillating magnetic field at full penetration hybrid laserarc welding of thick materials, Metals 9 (2019) 10.
7
Contents lists available at ScienceDirect
Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Influence of external magnetic field on twin-wire indirect arc surfacing stainless steel layer Dongting Wu a, Cong Hu a, Wei Zhao b, Yongang Zhang c, **, Yong Zou a, * a
MOE Key Lab for Liquid-Solid Structure Evolution and Materials Processing, Shandong University, Jinan, 250061, China School of mechanical and automotive engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, China c School of Materials Science and Engineering, Shandong University, Jinan, 250061, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Twin-wire indirect arc welding Surfacing welding Applied magnetic field Pitting corrosion
Twin-wire indirect arc welding(TWIAW)owns advantages including high deposition efficiency, cost-effective energy utilization and low dilution rate of the base metal. However, there are some disadvantages such as small depth of fusion and narrow process window during welding. This work studied the influence of the external magnetic field on the formation of surfacing stainless steel layer prepared by the TWIAW. The effect of external magnetic field on welding process and pitting corrosion of surfacing layer was studied. The results show that the applied magnetic field can significantly modified the indirect arc shape and the associated pitting corrosion performance of the surfacing layer. Under the condition of the external magnetic field in the negative x-direction, the arc became lengthened and the heat input to the workpiece was increased accordingly. This is beneficial for improving the weld shape and the pitting corrosion resistance of the surfacing layer.
1. Introduction The twin-wire indirect arc welding (TWIAW) is a newly developed welding technology. In the process, the two poles of the welding power source are connected with two welding wires respectively while the workpiece is not connected into the current circuit, as shown in Fig. 1. Therefore, the indirect arc is formed between the end of the twin wires and droplet transfer occurs simultaneously from both wire tips. Accordingly, the generated arc heat is mainly used to melt the welding wires instead of melting the workpiece. Several advantageous features are derived from the TWIAW technology, that is, high deposition effi ciency, high energy-utilization efficiency, low fusion ratio and low dilution rate of base metal. Cao et al. [1] investigated the droplet transfer process of twin-wire indirect welding and analyzed the force of the droplet. The results showed that the metal droplet transfer modes occurring at the anode and the cathode are significantly different. Shi et al. [2,3] explored the characteristics of the twin-wire indirect arc by combining numerical simulation with welding experiments. It was found that the arc behavior was mainly controlled by the welding cur rent. Nevertheless, it has been noticed that the weld penetration by TWIAW was shallow and the welding process window was narrow by
Zhang et al. [4], which greatly restricted the popularization and appli cation of this technology. In order to improve the properties of arc welding, the external magnetic field was studied by numerous researchers. It is found that the external magnetic field plays a positive role on the welding quality during arc welding, because it can modify the arc shape, droplet transfer mode, the resultant weld size and microstructure [5,6]. This in turn improves the mechanical and electrochemical properties of the welded joints. In GMAW, applying external magnetic field can change arc shape and arc motion, which has an important influence on arc temperature gradient and undercooling degree [7,8]. The change of undercooling degree plays a key effect on the solidification mode of weld [9],resulting in elimination of chemical inhomogeneity [10], refinement of grains [11], reduction of shrinkage tendency [12] and restraint of hump weld [13,14]. The refinement of grain and the decrease of defect will lead to the decrease of welding residual stress [15,16] and the improvement of mechanical properties [17]. For stainless steel GMAW, applying external magnetic field can not only improve the mechanical properties of the weld, but also enhance the corrosion resistance of the weld, which is effective for all of austenitic stainless steel [18,19] and duplex stainless steel [20,21].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Zou). https://doi.org/10.1016/j.vacuum.2019.108958 Received 27 July 2019; Received in revised form 24 August 2019; Accepted 19 September 2019 Available online 20 September 2019 0042-207X/© 2019 Elsevier Ltd. All rights reserved.
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the cathode wire. The experimental schematic diagram of the applied magnetic field and the established coordinate direction were also depicted in Fig. 3. In order to generate the x-directional magnetic field at the end of the welding wire, both the electromagnet cores were located in the xOy plane with the cores positioned along the y-axis positive di rection. The distance between the end of the excitation coil and the intersection of the cathode and anode welding wires was measured to be 2.2 cm. The excitation coil consisted of six layers with 60 turns in each layer. The magnetic induction intensity produced at the intersection of the twin wires ends is shown in Table 2 (the measured position was in the middle of the wire ends). The welding process parameters are shown in Table 3. In addition, the MV-D1024E-160DE high-speed camera system was set up to visu alize the arc shape. The Potentiodynamic method was utilized to obtain the polarization curve of the surfacing layer for evaluating its pitting corrosion perfor mance. The polarization curves were measured in 3.5 wt % NaCl solu tion at room temperature. The testing was scanned forward from - 0.4 V with a speed of 0.5 mV/s applying a CS350 electrochemical workstation. The reference electrode was a saturated calomel electrode (SCE), and the counter electrode was a platinum electrode. When the current density of the anode reached 10 3A/cm2, the scanning begun reversed. Once the reverse scanning curve intersected with the forward scanning curve, the annular anodic polarization curve was achieved.
Fig. 1. Schematic diagram of the twin-wire indirect arc welding.
However, the study focused on applying the external magnetic field to TWIAW is very limited. In order to improve the fusion depth during TWIAW, Zhang et al. conducted a preliminary study by introducing the external magnetic field to TWIAW process and found that the shape of the welding arc was related to the difference of internal and external magnetic field intensity in the area surrounded by two welding wires [22]. With the applied transverse magnetic field, the larger the intensity of the forward magnetic field was, the smaller the droplet volume and the higher the droplet transition frequency were [23]. Considering the high depositing efficiency and low dilution rate of base metal of the TWIAW process, it becomes suitable for surfacing manufacture. Wu et al. [24] employed the TWIAW technique to prepare a Fe-based stainless steel layer and successfully obtained the austenite þ ferrite composite structure with excellent corrosion resistance. However, the role of external magnetic field on the formation of the surfacing layer during TWIAW could be different from that of the traditional arc surfacing layer because of the different melting speed and droplet transfer modes of the anode and cathode wires. The study focused on the effect of external magnetic field on the surfacing layer prepared through TWIAW tech nique is limited. In this paper, the effect of external magnetic field on the stainless steel surfacing layer prepared through TWIAW technique was investi gated, in particular the modification of the arc shape and the pitting corrosion resistance.
3. Results and discussion 3.1. Effect of magnetic field on arc state Fig. 4 shows the arc shape captured by the high-speed camera with the magnetic field direction along the x-axis under the condition of magnetic induction intensity 0 mT, 3.2 mT, 5 mT and 7.8 mT, respec tively. It can be seen that under the same shooting distance and filter condition, the arc brightness increased and the volume of arc decreased with the increase of the magnetic induction intensity, indicating that the arc became compressed by the x-direction magnetic field and the arc current density increased accordingly. Fig. 5 shows the arc shape captured by the high-speed camera with the magnetic field direction along the negative x-axis under the condition of magnetic induction intensity 0 mT, 3.2 mT, 5 mT and 7.8 mT, respectively. As shown in Fig. 5, with the increase of the magnetic induction intensity, the brightness of the arc decreased gradually while the arc tends to be stretched. Overall, the current density decreased. Arising from the gas ionization inside the arc space and the electron emission at the end of the cathode welding wire, the twin-wire indirect arc was composed of a great number of charged particles. These moving charged particles would be affected by the Lorentz force in the magnetic field [25]. The direction of the Lorentz force is perpendicular to the motion direction of these charged particles and its value can be calcu lated by Eq. (1). The generated Lorentz force would alter the moving direction of the moving charged particles.
2. Experimental procedure Q235 was selected as the base metal in this study. The welding material was ER308 stainless steel wire with a diameter of 1.2 mm. The chemical composition of the base metal and the welding wire is shown in Table 1. The surfacing welding was carried out using a lab-designed twinwire indirect arc welding system, as presented in Fig. 2. The welding power supply was NBC-350 inverted gas shielded welding power source with flat external characteristics. Two sets of AS-35 wire feeder were used to feed the cathode and anode wires respectively and the wire feeding speed could be automatically adjusted. The cathode wire was set in the vertical position while anode wire had an angle of 30� inclined to
F ¼ qvB
where F is the Lorentz force with the unit Newton, q is the charge of charged particles with the unit Coulomb, v is the velocity of the charged particles with the unit meter per second and B is the magnetic induction intensity with the unit tesla. During the twin-wire indirect arc welding, the welding current flew from the anode to the cathode. The cathode current transmitted upward vertically while the anode current passing through the anode wire owned a certain angle to the vertical direction. When the direction of magnetic field was along the positive x-axis direction, the applied magnetic field was perpendicular to the outside of paper. The Lorentz force for the charged particles moving inside the arc between the anode and cathode poles is shown in Fig. 6 a.
Table 1 Chemical composition of base metal and welding wire(wt. %). Q235 ER308
C
Si
Mn
P
S
Cr
Ni
Fe
0.20 0.03
0.25 0.60
0.50 1.80
0.03 0.015
0.03 0.008
– 20
– 10
Re Re
(1)
2
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Fig. 2. Equipment of the twin-wire indirect arc welding system.
Fig. 3. Welding torch structure with twin wires angle 30� and x direction magnetic field to produce.
The macroscopic expression of the Lorentz force imposed on the charged particles in arc space was Ampere force. The direction of the Ampere force acting on the moving charged particles near the welding wires is shown in Fig. 6 a. The formed Ampere force F1 and F2 com pressed the charged particles in the arc space into a smaller space. Macroscopically, the arc got shrunken and the current density in the arc space increased. With the increase of magnetic induction intensity, the magnetic induction intensity of the magnetic field present in the forward direction along the x-axis increased. Therefore, the arc continued to the compressed further. When the magnetic field direction was imposed along the negative direction of x-axis, the magnetic field became inside-facing of vertical paper. Considering that the current direction was the same, the direction of Ampere Force F1’and F2’ imposed to the moving charged particles became opposite to F1 and F2, as displayed in Fig. 6 b. The present F1’and F2’ forced the charged particles moving outside of the arc space, and the welding arc would be lengthened. As a result,the space current density of the arc would be reduced. With the increase of magnetic
Table 2 Magnetic induction strength on the tip of twin wires. Excitation voltage (V)
0
1.6
3
4
Excitation current (A) Magnetic induction intensity (mT)
0 0
4 3.2
7 5
10 7.8
Table 3 Welding parameters of different external magnetic field.
a b c d
Angle between twin-wire (� )
Welding current (A)
Arc voltage (V)
Excitation current(A)
Magnetic induction intensity (mT)
Velocity (mm⋅min1)
30
190
25
0 4 7 10
0 3.2 5 7.8
187
3
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Fig. 4. Arc shape with applying x positive magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT.
Fig. 5. Arc shape with applying negative x magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT.
Fig. 6. Force of arc under x direction magnetic field (a) x positive direction (b)x negative direction.
induction intensity, the increased magnetic induction intensity along the negative direction of x-axis would further elongate the welding arc. For the case of positive x-axis direction magnetic field, the twin-wire indirect arc got compressed and concentrated at the end of the anode
and cathode poles. However, under the condition of the negative x-axis direction magnetic field, the twin-wire indirect arc became elongated from the wire tips towards the workpiece. Accordingly, more heat was transferred to the substrate, increasing the weld penetration.
Fig. 7. Weld profile of x negative magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5.0 mT and (d) 7.8 mT. 4
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3.2. Effect of magnetic field on weld formation
Table 4 Ferrite content for surfacing layers in different external magnetic field.
As shown in Fig. 5, the welding arc became elongated when the magnetic field was exerted along the negative x-axis direction. Accordingly, more arc heat could be transferred to the surface of the workpiece to form a weld. Fig. 7 demonstrates the appearance of the weld under the condition of 190 A welding current, 25 V arc voltage, 187 mm/min welding speed and the negative x-axis direction magnetic field with different magnetic induction intensity. The negative magnetic field along the x-axis extended the twin-wire indirect arc towards the welded workpiece. The number of charged particles in the lower part of the arc became larger compared that in the absence of magnetic field. As a result, the heat input to the welded workpiece increased, promoting the weld width.
a b c d
Welding current (A)
Arc voltage (V)
Excitation current (A)
Magnetic field intensity (mT)
Ferrite content (%)
190 190 190 190
25 25 25 25
0 4 7 10
0 3.2 5 7.8
4.02 4.30 5.89 6.51
formation of chromium-poor layer of austenite grain. 3.4. Effect of magnetic field on pitting corrosion resistance Fig. 9 illustrates the cyclic polarization curves of stainless steel surfacing layers for pitting corrosion experiments under the condition of the negative x-axis direction magnetic field with different magnetic field
3.3. Effect of magnetic field on ferrite content and distribution Fig. 8 gives the ferrite distribution of the surfacing layer under the condition of the negative x-axis direction magnetic field with different magnetic induction intensity. The changes of the ferrite content derived from the metallographic images are summarized in Table 4. It can be seen that more ferrite microstructure appeared with the increased magnetic induction intensity. For the case of none external magnetic field, only a small fraction of ferrite dots scattered on the austenite matrix, as shown in Fig. 8 a. With the external magnetic field increased to 3.2 mT, more ferrites started to precipitate along the austenite subgrain boundary as shown in Fig. 8 b. Once the external magnetic field increased to 5.0 mT and 7.8 mT, the content of ferrite precipitates occupied around 6%, distributing at the austenite grain boundary to form interconnected network as shown in Fig. 8c and d. In addition, compared to the weld microstructure without magnetic field, the so lidification process of weld metal became equilibrium due to the elec tromagnetic stirring by the external magnetic field. In generally, in order to improve the intergranular corrosion resistance of austenite welding layer, it is desirable to retain a certain amount of ferrite phase in welding layer. Its function is as follows: (1) it can disturb the orientation of the columnar crystal of single austenite phase, so as not easy to form a continuous chromium-poor layer; (2) the ferrite phase is rich in chro mium and has good chromium supply conditions, which can reduce the
Fig. 9. Cyclic polarization curves of surfacing layer in different external mag netic field.
Fig. 8. Ferrite for surfacing layers of x negative direction magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5 mT and (d) 7.8 mT. 5
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intensities. For the different magnetic induction intensity, the corrosion current density did not change significantly with the increased polari zation potential at the initial stage due to the presence of a passive film formed on the stainless steel surfacing layer surface as shown in Fig. 9. Meanwhile, the free corrosion potential Ef were similar. When the corrosion potential was further increased, the passive film on metal surface was broken down and the corrosion current density increased sharply. Therefore, the inflection point on the curve where the current started to increase could be correlated with the occurrence of pitting corrosion (point Eb as marked in Fig. 9). With the increase of the external magnetic field intensity, the pitting breakdown potential Eb shifted to ward the high potential value indicating the improved pitting resistance of the surfacing layer. However, when the magnetic induction intensity rose up to 7.8 mT, there were two inflection points when the potential went from Ef to Eb, implying the pitting corrosion resistance didn’t become better, but got worse compared with that of the 5.0 mT magnetic field condition. The reason of the lower corrosion resistance was the increasing of ferrite content caused by higher dilution rate with the increasing of magnetic field intensity. Fig. 10 shows the surface morphologies of the stainless steel surfacing layers after cyclic polarization pitting tests. Increasing the magnetic induction intensity made a significant contribution in decreasing the number of corrosion pits and therefore facilitated the pitting resistance of the surfacing layer. The corrosive pitting on the metal surface is mostly induced by the incomplete passivation film or non-uniform surface structure. Even though both welding wires were melted simultaneously during the twinwire indirect arc welding process, their melting speed and the metal droplet transfer modes were completely different. The wire feeding speed and melting speed of the cathode welding wire were nearly twice as fast as that of the anode welding wire. Consequently, the metal droplets were transferred from the anode welding wire in the large droplet transition stage, while the metal droplets were transferred from the cathode welding wire in the droplet transition state. The heat input from the welding arc to the workpiece was small and the welding pool existed for a short time in the welding process. The fast solidification was detrimental for ensuring a homogeneous elemental distribution on the surfacing layer. Therefore, the passivation film was prone to form
local rupture, leading to the pitting corrosion. The pitting corrosion potential was slightly smaller without applying the magnetic field. With the negative x-axis direction magnetic field,the shape of welding arc and the distribution of charged particles inside the arc space changed. More heat was transferred from the welding arc to the workpiece, which extended the existence time of the welding pool. In addition, the elec tromagnetic stirring induced by the external magnetic field could enable the elements distributing more uniformly inside the surfacing welding layer. This improved the stability of passive metal film on the surfacing layer, increasing the pitting corrosion potential and enhancing the pitting corrosion resistance. 4. Conclusions In this paper, the austenitic stainless steel surfacing layer was pre pared by the twin-wire indirect arc welding on the Q235 low carbon steel substrates. During the welding process, an external magnetic field was applied surrounding the twin wire indirect arc. The effects of the external magnetic field on the arc shape, the evolution of the ferrite content and the pitting corrosion resistance of the surfacing layer were studied. The following conclusions are drawn: (1) External magnetic field modified the shape of the twin-wire in direct arc. The arc became compressed by the positive x-axis di rection magnetic field. The arc got elongated. Under the condition of the negative x-axis direction magnetic field, which was beneficial for promoting the heat input to the workpiece and increasing the fusion depth. (2) When negative x-axis direction magnetic field was applied, more ferrite precipitated inside the surfacing layer with the increase of the magnetic field intensity. This is correlated with that the increased heat input postponed the solidification of the molten pool. (3) Under the condition of the negative x-axis direction magnetic field, the pitting corrosion potential of surfacing layer increased with the increase of magnetic field intensity. This is due to that the external magnetic field enabled the passive film of the
Fig. 10. Corrosion morphology after epr tests for surfacing layers in different external magnetic field:Magnetic induction intensity (a) 0 mT, (b) 3.2 mT, (c) 5 mT and (d) 7.8 mT. 6
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surfacing layer form more homogeneously by the electromag netic stirring effect.
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