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Effect of axial external magnetic field on cold metal transfer welds of aluminum alloy and stainless steel
Materials Letters 152 (2015) 29–31
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of axial external magnetic field on cold metal transfer welds of aluminum alloy and stainless steel Yibo Liu a,b, Qingjie Sun a,b,n,1, Jinping Liu b, Shijie Wang b, Jicai Feng a,b a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 January 2015 Accepted 16 March 2015 Available online 28 March 2015
External magnetic field (EMF) is favored for application in welding, due to their positive effects on joint quality. This paper studies the effects of EMF on cold metal transfer welds of aluminum alloys and stainless steel. Results showed that an axial EMF with various magnetic induction intensities and alternating frequency influenced the morphology of a weld arc and molten drop, and ultimately changed the weld microstructure and tensile properties. Application of EMF could suppress the diffusion of Fe to the weld and increased the Si content in the IMCs layer, consequently reducing the thickness of brittle Al/Fe IMCs layers. When the EMF parameter was 2.5 A (17.8 mT) 0 Hz, the maximum load force reached 2.018 kN, which strengthened the joint 45% over an Al/steel joint without the EMF. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic field Welding Microstructure Mechanical property
1. Introduction Joining aluminum and steel by various methods has been increasingly used to construct automotive and aerospace body structures in order to reduce fossil-fuel consumption and obtain weight-optimized bodies with high stiffness [1]. However, significant differences in chemical and physical properties between the two materials, especially the brittle intermetallic compounds (IMCs) which form at the Al-steel interface, can severely degrade the mechanical properties of the joint [2]. Arc welding is currently the primary method utilized in practice, due to its flexibility and low cost. In this process, sheets and filler metal are efficiently heated or melted by high-temperature welding arc. But in the Al/steel joining process, the exorbitant heat input also goes against the limitation of growth of brittle IMC. To date, there are two approaches typically adopted in an effort to solve this problem. The first is to add alloy elements to suppress the formation of brittle Fe2Al5 and FeAl3 phases; for example, Si, Ni and Zn have been reported to significantly reduce the thickness of the Fe-Al IMC layer by formed ternary Fe–X–Al phases, or weaken the diffusion of Fe to the weld [2–5]. The second method uses a specialized drop transition mode or welding power source to
n
Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ 86 13863114355. E-mail address: [email protected] (Q. Sun). 1 Postal address: Harbin Institute of Technology at Weihai, No. 2 West Culture Road, Weihai 264209, China. http://dx.doi.org/10.1016/j.matlet.2015.03.077 0167-577X/& 2015 Elsevier B.V. All rights reserved.
decrease the heat input, such as cold metal transfer (CMT) welding [6,7] or alternate-current double-pulse gas metal arc welding [8]. In present study, an external magnetic field is utilized in Al/ Steel arc welding process. Under a given axial or radial magnetic field, metal fluids are impacted by Lorentz forces produced by currents in the moving fluid that interact with magnetic field lines. This force has been shown to suppress natural convection and homogenize temperature distribution [9,10], and consequently influence solidification and crystallization processes. In welding, sound weld appearance with refining grain can be obtained by applying a magnetic field during the joining of aluminum alloy, magnesium alloy and titanium [11–14]. Previous studies have focused primarily on the joining of homogeneous metals, but typically neglect the effects of magnetic fields on the joining of dissimilar metals. Therefore, Al and steel are selected and joined by CMT welding under magnetic fields in this study. The weld appearance and molten drop transfer was observed, and the interface microstructure and mechanical properties of Al/steel under the magnetic field are thoroughly investigated.
2. Materials and methods The materials used were 2 mm thick 304 stainless steel and a 5A06 aluminum alloy sheet. AlSi5 aluminum alloy wire with a diameter of 1.2 mm was used as filling wire. CMT welding was performed on a welding platform. Fig. 1 shows a schematic diagram of the welding process with an applied external magnetic field (EMF). The EMF was generated through two identical tandem annular coils and a tapered
30
Y. Liu et al. / Materials Letters 152 (2015) 29–31
iron core mounted coaxially to the weld torch. The strength and frequency of the EMF was controlled by adjusting the rotary knobs for current coils and frequency on a magnetic control power source. The magnetic induction intensity above the plate was measured in realtime by an A PG-5A Teslameter. The behavior of the arc and the molten drop were observed using a mono high-speed camera. During welding, the platform was driven by a motor and the relative positions of the weld torch and magnetic pole remained unchanged. Before welding, Nocolok brazing flux was precoated on the steel surface. Metallographic cross-sections of the specimens were prepared from the welded joints. Ground and polished specimens were observed and analyzed by scanning electron microscope (SEM3400) in backscattered electron mode, plus a LEO Gemini 1530 microscope equipped with an energy dispersive X-ray spectrometer (EDS). Tensile-shear tests were performed on an Instron-1186 mechanical testing machine at room temperature. Shear strength testing was conducted at a constant speed of 0.5 mm/min, and
Fig. 1. Schematic of welding process with an applied EMF.
average strength was determined using three samples produced under identical conditions.
3. Results and discussion Effects of EMF on weld appearance and the morphology of molten drop: Fig. 2 shows the weld appearance of the Al/steel lap joint when the weld current was 96 A and the weld speed was 10 mm/s. With the absence of EMF, the weld surface was smooth. With the applied of the EMF, the weld surface showed wave-like characteristics, implying that the molten pool had fluctuated to some extent during the welding process. Besides that, high-speed photography of combined weld arc and molten drop morphologies indicated that an axial magnetic field made the welding arc rotate within a certain range. During the welding process, the turning radius (R) increased the heating area of the welding arc on the base metal. At the same heat input, this not only decreased the unit area heat of the interface, but also preheated the ambient base metal, which was beneficial for the spreading of the weld metal. Additionally, the EMF also changes the movement of molten drop on the tip of the wire. Both tabular and rotated molten drops were found in the drop transition process, which more easily triggered fluctuation when they transferred to the molten pool. Because of the rapid cooling in a welding process, the fluctuated molten pool solidified before the molten pool returned to the steady state, forming the resultant wave-like surface. Effects of EMF on weld microstructure: Fig. 3 shows the microstructure of the lap joint with and without EMF application. The observed position is at the middle of the weld. As shown, the IMC layer was thinner (decreased to 1 μm,) under the EMF. Additionally, the morphology of the IMC layer was more uniform and flat under EMF application. Table 1 lists the EDS results of selected points from Fig. 3, which suggest that in the absence of EMF, the IMC layers were mainly composed of Al8Fe2Si þ(Al, Si)13Fe4 phases.
Fig. 2. Weld appearance and morphologies of weld arc and molten drop.
Fig. 3. Microstructure of the Al/steel lap joint interface (a) without EMF (b) with EMF.
Y. Liu et al. / Materials Letters 152 (2015) 29–31
Table 1 EDS results of selected points from Fig. 3. Points inFig. 3
Al
Fe
Si
Possible phase
1 2 3 4
92.41 77.64 78.60 75.44
3.05 12.48 13.16 11.89
4.54 9.88 8.23 12.67
α-Al solid solution Al8Fe2Si þ(Al, Si)13Fe4 Al8Fe2Si þ(Al, Si)13Fe4 Al9Fe2Si2
31
flow of molten metal, and the decreased thickness of the brittle Fe/Al IMC layers. As shown in Fig. 4, when the EMF parameter was 2.5 A (17.8 mT) 0 Hz, the maximum load force reached 2.018 kN, which enhanced 45% over an Al/steel joint welded without an EMF.
4. Conclusion (1) Under the effect of an axial EMF, both the welding arc and the molten drop were rotated by Lorenz force. This forced rotational movement homogenized the heat distribution on the base metal and changed the metal flow in the welding process. (2) The EMF influenced the growth of the Al/Fe IMC layers during Al/steel welding. Under EMF application, the diffusion of Fe to the weld was suppressed and the Si content in the IMC layers increased, which restrained the growth of brittle Al/Fe IMC phases. (3) The weld joint formed with an EMF exhibited higher tensile shear force compared to the weld joint formed without the EMF. At EMF frequencies of 0 Hz and 5 Hz, stronger joints were obtained, and with increased of coil current, the joint strength increased even further.
Fig. 4. Tensile shear force of the Al/steel lap joint with various magnetic parameters.
Under EMF application, the Si content increased in the IMC layers and the Fe content decreased. This demonstrates the EMF’s ability to suppress the diffusion of Fe to the weld; further, the increased Si content in the resultant IMC layers can also inhibit the growth of the brittle Fe/Al IMC phase [1]. Effects of EMF on weld mechanical properties: Fig. 4 shows the tensile properties of the Al/steel lap joint with various coil currents and frequencies. When the EMF was not applied, the peak load was 1.4 kN. With the EMF, most of the specimens’ peak loads were beyond this value. When the frequency of EMF was 0 Hz or 5 Hz, it was tended to obtain a stronger joint strength, and with the increase of the coil current, the joint strength presented an increased trend. Combining the weld appearance and molten drop morphology shown in Fig. 2, it indicated that effects of the EMF on molten drop transfer and fluctuation of the molten pool effectively improved overall joint strength. This improvement can be attributed to the redistribution of heat and element composition caused by the forced
Acknowledgments This project is support by the National Natural Science Foundation of China (NSFC, Grant nos. 51475104, 51435004). References [1] Song JL, Lin SB, Yang CL. J Alloys Compd 2009;488:217–22. [2] Dong HG, Hu WJ, Duan YP, Wang XD, Dong C. J Mater Process Technol 2012;212:458–64. [3] Zhang HT, Liu JK. Mater Sci Eng, A 2011;528:6179–85. [4] He H, Lin SB, Yang CL, Fan CL, Chen Z. J Mater Eng Perform 2013;22:3315–23. [5] Cao R, Sun JH, Chen JH, Wang PC. Weld J 2014;93:193–204. [6] Cao R, Yu G, Chen JH, Wang PC. J Mater Process Technol 2013;213:1753–63. [7] Lin J, Ma N, Lei YP, Murakawa H. J Mater Process Technol 2013;213:1303–10. [8] YC Su, XM Hua, Wu YX. Mater Sci Eng, A 2013;578:340–5. [9] Samanta D, Zabaras N. Int J Heat Mass Transfer 2006;49:4850–66. [10] Bouabdallah S, Bessaih R. Int J Heat Fluid Flow 2012;37:154–66. [11] Bachmann M, Avilov V, Gumenyuk A, Rethmeier M. Int J Heat Mass Transfer 2013;60:309–21. [12] Li Y, Luo Z, Yan FY, Duan R, Yao Q. Mater Des 2014;56:1025–33. [13] Sundaresan S, Janaki Ram GD. Sci Technol Weld Joining 1999;4:151–60. [14] Lim YC, Yu X, Cho JH, Sosa J, Farson DF, Babu SS, et al. Sci Technol Weld Joining 2010;15:400–6.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of axial external magnetic field on cold metal transfer welds of aluminum alloy and stainless steel Yibo Liu a,b, Qingjie Sun a,b,n,1, Jinping Liu b, Shijie Wang b, Jicai Feng a,b a b
State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China
art ic l e i nf o
a b s t r a c t
Article history: Received 27 January 2015 Accepted 16 March 2015 Available online 28 March 2015
External magnetic field (EMF) is favored for application in welding, due to their positive effects on joint quality. This paper studies the effects of EMF on cold metal transfer welds of aluminum alloys and stainless steel. Results showed that an axial EMF with various magnetic induction intensities and alternating frequency influenced the morphology of a weld arc and molten drop, and ultimately changed the weld microstructure and tensile properties. Application of EMF could suppress the diffusion of Fe to the weld and increased the Si content in the IMCs layer, consequently reducing the thickness of brittle Al/Fe IMCs layers. When the EMF parameter was 2.5 A (17.8 mT) 0 Hz, the maximum load force reached 2.018 kN, which strengthened the joint 45% over an Al/steel joint without the EMF. & 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic field Welding Microstructure Mechanical property
1. Introduction Joining aluminum and steel by various methods has been increasingly used to construct automotive and aerospace body structures in order to reduce fossil-fuel consumption and obtain weight-optimized bodies with high stiffness [1]. However, significant differences in chemical and physical properties between the two materials, especially the brittle intermetallic compounds (IMCs) which form at the Al-steel interface, can severely degrade the mechanical properties of the joint [2]. Arc welding is currently the primary method utilized in practice, due to its flexibility and low cost. In this process, sheets and filler metal are efficiently heated or melted by high-temperature welding arc. But in the Al/steel joining process, the exorbitant heat input also goes against the limitation of growth of brittle IMC. To date, there are two approaches typically adopted in an effort to solve this problem. The first is to add alloy elements to suppress the formation of brittle Fe2Al5 and FeAl3 phases; for example, Si, Ni and Zn have been reported to significantly reduce the thickness of the Fe-Al IMC layer by formed ternary Fe–X–Al phases, or weaken the diffusion of Fe to the weld [2–5]. The second method uses a specialized drop transition mode or welding power source to
n
Corresponding author at: State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ 86 13863114355. E-mail address: [email protected] (Q. Sun). 1 Postal address: Harbin Institute of Technology at Weihai, No. 2 West Culture Road, Weihai 264209, China. http://dx.doi.org/10.1016/j.matlet.2015.03.077 0167-577X/& 2015 Elsevier B.V. All rights reserved.
decrease the heat input, such as cold metal transfer (CMT) welding [6,7] or alternate-current double-pulse gas metal arc welding [8]. In present study, an external magnetic field is utilized in Al/ Steel arc welding process. Under a given axial or radial magnetic field, metal fluids are impacted by Lorentz forces produced by currents in the moving fluid that interact with magnetic field lines. This force has been shown to suppress natural convection and homogenize temperature distribution [9,10], and consequently influence solidification and crystallization processes. In welding, sound weld appearance with refining grain can be obtained by applying a magnetic field during the joining of aluminum alloy, magnesium alloy and titanium [11–14]. Previous studies have focused primarily on the joining of homogeneous metals, but typically neglect the effects of magnetic fields on the joining of dissimilar metals. Therefore, Al and steel are selected and joined by CMT welding under magnetic fields in this study. The weld appearance and molten drop transfer was observed, and the interface microstructure and mechanical properties of Al/steel under the magnetic field are thoroughly investigated.
2. Materials and methods The materials used were 2 mm thick 304 stainless steel and a 5A06 aluminum alloy sheet. AlSi5 aluminum alloy wire with a diameter of 1.2 mm was used as filling wire. CMT welding was performed on a welding platform. Fig. 1 shows a schematic diagram of the welding process with an applied external magnetic field (EMF). The EMF was generated through two identical tandem annular coils and a tapered
30
Y. Liu et al. / Materials Letters 152 (2015) 29–31
iron core mounted coaxially to the weld torch. The strength and frequency of the EMF was controlled by adjusting the rotary knobs for current coils and frequency on a magnetic control power source. The magnetic induction intensity above the plate was measured in realtime by an A PG-5A Teslameter. The behavior of the arc and the molten drop were observed using a mono high-speed camera. During welding, the platform was driven by a motor and the relative positions of the weld torch and magnetic pole remained unchanged. Before welding, Nocolok brazing flux was precoated on the steel surface. Metallographic cross-sections of the specimens were prepared from the welded joints. Ground and polished specimens were observed and analyzed by scanning electron microscope (SEM3400) in backscattered electron mode, plus a LEO Gemini 1530 microscope equipped with an energy dispersive X-ray spectrometer (EDS). Tensile-shear tests were performed on an Instron-1186 mechanical testing machine at room temperature. Shear strength testing was conducted at a constant speed of 0.5 mm/min, and
Fig. 1. Schematic of welding process with an applied EMF.
average strength was determined using three samples produced under identical conditions.
3. Results and discussion Effects of EMF on weld appearance and the morphology of molten drop: Fig. 2 shows the weld appearance of the Al/steel lap joint when the weld current was 96 A and the weld speed was 10 mm/s. With the absence of EMF, the weld surface was smooth. With the applied of the EMF, the weld surface showed wave-like characteristics, implying that the molten pool had fluctuated to some extent during the welding process. Besides that, high-speed photography of combined weld arc and molten drop morphologies indicated that an axial magnetic field made the welding arc rotate within a certain range. During the welding process, the turning radius (R) increased the heating area of the welding arc on the base metal. At the same heat input, this not only decreased the unit area heat of the interface, but also preheated the ambient base metal, which was beneficial for the spreading of the weld metal. Additionally, the EMF also changes the movement of molten drop on the tip of the wire. Both tabular and rotated molten drops were found in the drop transition process, which more easily triggered fluctuation when they transferred to the molten pool. Because of the rapid cooling in a welding process, the fluctuated molten pool solidified before the molten pool returned to the steady state, forming the resultant wave-like surface. Effects of EMF on weld microstructure: Fig. 3 shows the microstructure of the lap joint with and without EMF application. The observed position is at the middle of the weld. As shown, the IMC layer was thinner (decreased to 1 μm,) under the EMF. Additionally, the morphology of the IMC layer was more uniform and flat under EMF application. Table 1 lists the EDS results of selected points from Fig. 3, which suggest that in the absence of EMF, the IMC layers were mainly composed of Al8Fe2Si þ(Al, Si)13Fe4 phases.
Fig. 2. Weld appearance and morphologies of weld arc and molten drop.
Fig. 3. Microstructure of the Al/steel lap joint interface (a) without EMF (b) with EMF.
Y. Liu et al. / Materials Letters 152 (2015) 29–31
Table 1 EDS results of selected points from Fig. 3. Points inFig. 3
Al
Fe
Si
Possible phase
1 2 3 4
92.41 77.64 78.60 75.44
3.05 12.48 13.16 11.89
4.54 9.88 8.23 12.67
α-Al solid solution Al8Fe2Si þ(Al, Si)13Fe4 Al8Fe2Si þ(Al, Si)13Fe4 Al9Fe2Si2
31
flow of molten metal, and the decreased thickness of the brittle Fe/Al IMC layers. As shown in Fig. 4, when the EMF parameter was 2.5 A (17.8 mT) 0 Hz, the maximum load force reached 2.018 kN, which enhanced 45% over an Al/steel joint welded without an EMF.
4. Conclusion (1) Under the effect of an axial EMF, both the welding arc and the molten drop were rotated by Lorenz force. This forced rotational movement homogenized the heat distribution on the base metal and changed the metal flow in the welding process. (2) The EMF influenced the growth of the Al/Fe IMC layers during Al/steel welding. Under EMF application, the diffusion of Fe to the weld was suppressed and the Si content in the IMC layers increased, which restrained the growth of brittle Al/Fe IMC phases. (3) The weld joint formed with an EMF exhibited higher tensile shear force compared to the weld joint formed without the EMF. At EMF frequencies of 0 Hz and 5 Hz, stronger joints were obtained, and with increased of coil current, the joint strength increased even further.
Fig. 4. Tensile shear force of the Al/steel lap joint with various magnetic parameters.
Under EMF application, the Si content increased in the IMC layers and the Fe content decreased. This demonstrates the EMF’s ability to suppress the diffusion of Fe to the weld; further, the increased Si content in the resultant IMC layers can also inhibit the growth of the brittle Fe/Al IMC phase [1]. Effects of EMF on weld mechanical properties: Fig. 4 shows the tensile properties of the Al/steel lap joint with various coil currents and frequencies. When the EMF was not applied, the peak load was 1.4 kN. With the EMF, most of the specimens’ peak loads were beyond this value. When the frequency of EMF was 0 Hz or 5 Hz, it was tended to obtain a stronger joint strength, and with the increase of the coil current, the joint strength presented an increased trend. Combining the weld appearance and molten drop morphology shown in Fig. 2, it indicated that effects of the EMF on molten drop transfer and fluctuation of the molten pool effectively improved overall joint strength. This improvement can be attributed to the redistribution of heat and element composition caused by the forced
Acknowledgments This project is support by the National Natural Science Foundation of China (NSFC, Grant nos. 51475104, 51435004). References [1] Song JL, Lin SB, Yang CL. J Alloys Compd 2009;488:217–22. [2] Dong HG, Hu WJ, Duan YP, Wang XD, Dong C. J Mater Process Technol 2012;212:458–64. [3] Zhang HT, Liu JK. Mater Sci Eng, A 2011;528:6179–85. [4] He H, Lin SB, Yang CL, Fan CL, Chen Z. J Mater Eng Perform 2013;22:3315–23. [5] Cao R, Sun JH, Chen JH, Wang PC. Weld J 2014;93:193–204. [6] Cao R, Yu G, Chen JH, Wang PC. J Mater Process Technol 2013;213:1753–63. [7] Lin J, Ma N, Lei YP, Murakawa H. J Mater Process Technol 2013;213:1303–10. [8] YC Su, XM Hua, Wu YX. Mater Sci Eng, A 2013;578:340–5. [9] Samanta D, Zabaras N. Int J Heat Mass Transfer 2006;49:4850–66. [10] Bouabdallah S, Bessaih R. Int J Heat Fluid Flow 2012;37:154–66. [11] Bachmann M, Avilov V, Gumenyuk A, Rethmeier M. Int J Heat Mass Transfer 2013;60:309–21. [12] Li Y, Luo Z, Yan FY, Duan R, Yao Q. Mater Des 2014;56:1025–33. [13] Sundaresan S, Janaki Ram GD. Sci Technol Weld Joining 1999;4:151–60. [14] Lim YC, Yu X, Cho JH, Sosa J, Farson DF, Babu SS, et al. Sci Technol Weld Joining 2010;15:400–6.