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Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel
Journal of Manufacturing Processes 50 (2020) 596–602
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel
T
Lijin Huanga,b,*, Peng Liub, Su Zhub, Xueming Huaa,*, Shengmu Donga a b
Shanghai Key Laboratory of Material Laser Processing and Modification (Shanghai Jiao Tong University), Shanghai 200240, China Shipbuilding Technology Research Institute, Shanghai 200032, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic field assisted Laser welding AH36 steel Keyhole-induced porosity
The dynamic keyhole behaviors are directly observed in laser welding of AH36 steel/glass, and Zirconia particle is used to observe the convective pattern. The weld bead formation, keyhole instability and porosity formation are analyzed. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed. With the magnetic field is added, the weld width, small concave, the formation of the keyhole collapse and the convective pattern are similar in laser welding of steel, and the generated Lorentz force hinders the flow of liquid metal. Widening of bottom weld width without undercut defect can be obtained with magnetic field assisted. The lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercuts and spatter defects contributes to the increase of the keyhole stability, and thus restrains the porosity formation. Besides, an easier keyhole open, the larger bubble escape velocity and the lower solidification rate also promote the reduction of the porosity formation.
1. Introduction Due to the large water force of hull travelling in the sea, cast iron structural members are used in many hull structures. For the purpose of reducing the weight of hull and ensuring the safety, AH36 replaces traditional cast iron, which has been widely applied in the field of hull structure and luxury liner owing to the high strength, toughness and excellent corrosion resistance [1]. In the conventional methods, the thick steel plates are usually required pre-processing and multi-pass welding [2]. Compared to traditional welding methods, laser welding has advantages of high welding speed, large weld depth and small deformation, which has aroused widespread concern [3]. However, due to the complex heat and mass transfer behavior in laser welding, the porosity is easily emerged in the weld, which can reduce the mechanical properties of weld joints and limit the weld quality [4]. There are lots of researches coupling the keyhole-induced porosity formation and elucidation mechanism in laser welding, such as sub-atmospheric forces [5], N2 shielding gas [6], high recoil force [7], laser-GMA welding [8], pulsed laser welding [9] and dual beam laser welding [10]. As pointed out by Huang et al. [11] and Wu et al. [12], the porosity formation was close related to weld pool convection, and many researchers has found that the melt flow behaviors closely connected with magnetic field assisted [13,14]. Previously investigation on magnetic ⁎
field assisted laser welding is mainly concentrated on root humps [15], weld microstructure [16,17] and mechanical properties [18]. Qi et al. [19] investigated the effect of electromagnetic force on root humps by high-speed camera, and pointed out that the additional steady electromagnetic force could effectively prevent periodic root humps. Qi et al. [20] investigated full-penetration laser welding of 316 L stainless steel by introducing high imposed direct current and steady magnetic field, and proposed that steady electromagnetic force could weaken hydrostatic force at the bottom part of the keyhole. Based on the results of these experiment, it was found that magnetic field assisted could suppress humping, increase process stability and obtain better welding quality significantly, however, the researches about effect of magnetic field assisted on keyhole behavior and porosity formation were almost always ignored in laser welding of steel. In this paper, direct observational method with transparent glasses was used to observe the keyhole collapse, and the Zirconia particles was applied to evaluate weld pool convective by a high-speed video camera in fiber magnetically supported laser welding of AH36 steel. The weld bead formation, keyhole and weld pool behaviors are analyzed carefully. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed.
Corresponding authors at: Shanghai Key Laboratory of Material Laser Processing and Modification (Shanghai Jiao Tong University), Shanghai 200240, China. E-mail addresses: [email protected] (L. Huang), [email protected] (X. Hua).
https://doi.org/10.1016/j.jmapro.2020.01.007 Received 20 August 2019; Received in revised form 23 December 2019; Accepted 6 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 50 (2020) 596–602
L. Huang, et al.
Fig. 1. Diagram of the experimental platform. Fig. 2. Schematic of the configuration of AH36 steel and transparent glass.
2. Materials and experimental setup The schematic of the experimental setup is shown in Fig. 1. A continuous wave solid state high-power ytterbium fiber laser system (YLS-10000) is used in this experiment. The maximum power is 10 kW, the laser wavelength is 1.070 ± 0.010 mm, and the focus radius is 0.36 mm. The laser head is inclined backwards at an angle of 5° for protection of the optical components of the system. The laser power is 6 kW. The defocused distance is 0 mm. 99.99% argon shielding gas is applied at a flow rate of 20 L/min to protect the weld. Laser beam is kept quiescent in the experiment, and a programmable 2-axis linear operating system is carried out controlling the linear motion of beadon-plate welding to the speed of 1.5 m/min accurately. Magnetic field is generated by placing the permanent magnet under the welded test material. The base material used in this work is AH36 steel plate in the dimension of 100(L) × 30(W) × 10 mm(H), and the heat resistant transparent glass in the dimension of 100(L) × 10(W) × 10 mm(H), whose chemical compositions in weight percentage are shown in Table 1. Prior to welding, the specimen surface is brushed to get rid of the oxidation layers and then cleaned using acetone. During the welding process, a high-speed video (Photron VEO710S) together with a band pass filter near 640 nm and a low-power laser-assisted light (Cavilux Smart) is fixed to monitor the weld pool convective and keyhole behaviors at the speed of 5000 frames/s, and the schematic of the configuration of AH36 steel and transparent glass is shown in Fig. 2. The Zirconia particles in near-spherical shape and diameter of 0.5−1 mm are used to observe the convective pattern. The size of particle is selected to guarantee flow on the weld pool and avoid being melted by a strong hybrid heat source. After welding, the weld porosity is examined by X-ray radiography perpendicular to the specimen surface. The samples are cut along the longitudinal section direction and corrodes with 4 ml HNO3 + 96 ml C2H5OH solution, and then observes by optical microscope (Zeiss Stemi2000).
Fig. 3. Weld surface morphology in laser welding: (a) no magnetic field assisted; (b) magnetic field assisted.
3. Results and discussion 3.1. Weld bead formation The obtained joint weld surface morphology is depicted in Fig. 3. A uniform and continuous weld appearance is formed, and the top weld surface has no visible crack and porosity defects, but spatters are observed on the weld of samples. The weld surface morphology is wellprotected and has a bright metallic luster. With the addition of magnetic field assisted, no appreciable change in the weld surface width (width of approximately 3.40 mm) is observed, which shows that the magnetic field assisted has little impact on the weld width. The comparison of cross-sections with no magnetic field assisted and magnetic field assisted is illustrated in Fig. 4. It can be seen that, undercut defect appears with no magnetic field assisted, which is in accordance with the research put into force by many scholars [21–23]. The Lorentz force produced by the magnetic field has a stirring effect on the edge of the weld pool, and the temperature at the keyhole front increases, then the accumulated heat is conducted to the bottom of the weld pool, which promotes widening of bottom weld width.
Table 1 Chemical compositions of AH36 steel and glass (wt.%). Materials
AH36 Glass
Elements, mass% SiO2
C
Mn
Si
S
P
Cu
Fe
– 99.97–99.99
0.15–0.18 –
0.70–1.60 –
0.10–0.50 –
≤0.035 –
≤0.035 –
≤0.35 –
Bal. –
597
Journal of Manufacturing Processes 50 (2020) 596–602
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Fig. 4. Weld cross-section morphology (The red dashed line represents the boundary of the weld): (a) no magnetic field assisted; (b) magnetic field assisted (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
is similar in laser welding of steel. Fig. 7 represents the continuous change of the weld pool in fiber laser welding with no magnetic field assisted and magnetic field assisted by tracing the movement of Zirconia particles on the weld pool surfaces. Due to the lower density of the Zirconia particle, it can float on the surface of the weld pool during the welding process. As shown in Fig. 7, the molten metal directly flows to the rear part of weld pool from both sides of keyhole opening, and then the convective pattern on the top surfaces is obtained on the weld pool surfaces, which is in favor of transporting the molten metal to the rear part of weld pool. With the addition of magnetic field assisted, the motion of the weld pool moves from the center to the edge owing to the Lorentz force produced by the magnetic field. The liquid metal on the rear wall of the keyhole flows into the keyhole and forms weld seam. Comparing to no magnetic field assisted, the length and area of the weld pool are extended along the welding direction. The liquid metal and steel can be regarded as two different metals in laser welding of steel, respectively, and there is a large temperature
3.2. Keyhole and weld pool behaviors When the laser beam is irradiated on the metal surface directly, the metal surface has a great melting and local vaporization, and the recoil force generated by the metal vapor makes the weld pool sag to form a small concave, as shown in Fig. 5(b) and (d). As the welding process proceeds, the laser acts directly on the bottom of the concave so that the concave deepens rapidly. Whether or not there is a magnetic field assisted, the formation process of the keyhole during laser welding is similar. To clearly observe the dynamic keyhole behaviors, the welding experiment on the “glass-metal” hybrid sample is implemented. Inner keyhole behaviors are shown in Fig. 6. From the observation result, the rear wall of keyhole is relatively smooth, and there are more humps in the front wall. As seen in Fig. 6, the bulge is formed at the rear part of the keyhole wall, and the bulge contacts with the front part of the keyhole wall, which can cause the keyhole collapse. Whether or not there is a magnetic field assisted, the formation of the keyhole collapse
Fig. 5. Small concave formation: (a) (b) no magnetic field assisted; (c) (d) magnetic field assisted. 598
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Fig. 6. Inner dynamic keyhole behaviors: (a) (b) (c) no magnetic field assisted; (d) (e) (f) magnetic field assisted.
Fig. 7. The convective pattern obtained by tracing the movement of Zirconia particles on the weld pool surfaces: (a) (b) (c) no magnetic field assisted; (d) (e) (f) magnetic field assisted.
difference between them. When the auxiliary magnetic field is added, the flowing molten metal cuts the magnetic induction lines, so the current in weld pool be described as [24]: j=σ(v B + α ∙ ▽T)
on the formation of the keyhole collapse. Fig. 7 shows that under the effect of strong clockwise flow, the molten metal at rear keyhole wall flows toward the front keyhole wall [25]. Fig. 8 summarizes the keyhole collapse formation mechanism in laser welding of steel. The process can be detailed as follows: at an initial stage, the lager recoil force is good for keeping keyhole open, shown as Fig. 8(a). Under the influence of strong clockwise flow, the bulge forms at the rear keyhole wall shown as Fig. 8(b). The recoil force cannot maintain the keyhole open, the rear keyhole wall is collapsed, and the keyhole is divided into two parts by weld pool shown as Fig. 8(c).
(1)
where j is current density, σis the metallic conductivity, v is the velocity of liquid metal, B is magnetic strength, α is Seebeck coefficient, and T is the temperature difference between liquid metal and steel. Based on Eq. (1), there is induced current and thermal current generated by temperature difference in the weld pool under the effect of magnetic field. Using left-hand rule to determine the direction of Lorentz force, the Lorentz force produced by induced current always hinders the flow of liquid metal. As seen in Fig.7, the velocity of liquid metal is 1.94 m/s and 1.80 m/s, respectively. This always indicates that the generated Lorentz force hinders the flow of liquid metal. Fig. 6 illustrates that the magnetic field assisted has almost no effect
3.3. Keyhole stability The longitudinal-section of weld joint is shown in Fig. 9. It can be figured out that between no magnetic field assisted and magnetic field assisted laser welding, the minimum and maximum weld depth varied 599
Journal of Manufacturing Processes 50 (2020) 596–602
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Fig. 8. Schematic diagram showing keyhole collapse formation mechanism: (a) keyhole formation, (b) bulge formation and (c) keyhole collapse.
respectively. The white and featheriness plumes observed above the weld pool were produced periodically, and there is no plume when the keyhole is closed, which can reveal the periodic closures of the keyhole. Meanwhile, when the auxiliary magnetic field is added, Lorentz force is produced by the the moving charged particles, which impels electrons to rotate back in a magnetic field, and increases the time of plasma plume above the weld pool. According to Wang and Thomy's study [27,28], the undercut is close related to unstable keyhole. An undercut defect indicate more unstable keyhole in no magnetic field assisted laser welding. With the addition of magnetic field assisted, the generated Lorentz force hinders the flow of liquid metal, and the plasma in the keyhole is subjected to the electric transmission effect, and an orderly migration movement occurs, which increases the stability of the keyhole, thus reduces the frequency of keyhole collapse. During laser welding of steel, the keyhole is much unstable, and lots of spatters are produced with no magnetic field assisted, as shown in Figs. 3, 10 and 11. It was referred that an unstable molten pool which could cause spatter [29]. So with the addition of magnetic field assisted, the keyhole is much stable.
within a range of 6.11–6.66 mm and 6.39–6.87 mm, and the average weld depth is 6.385 mm and 6.63 mm, respectively. Although the weld pool is in quasi-steady state, the weld depth is violently fluctuated, which is usually regarded as the result of keyhole instability. According to Pang's study [26], the keyhole stability is close related to fluctuation of the weld depth. The Lorentz force generated by the induced current prevents the liquid metal near the keyhole flows towards the rear part, which leads to heat concentration in front part of the weld pool, thus the accumulated heat is transferred to the bottom of the molten pool, so with the addition of magnetic field assisted, the keyhole depth increases. No cracks are discovered in two cross-sections of the weld, and a small amount of porosity was seen at random position with no magnetic field assisted laser welding of steel. In fiber laser welding of steel, the surface peak temperature of weld pool is very high and outstrip its boiling points, which leads to the generation of metal vapor. The metal vapor can be partially ionized and form plasma plume. The typical appearance of plasma plume with no magnetic field assisted and magnetic field assisted is shown in Fig. 10. As seen, during laser welding of steel, the plume behavior is very unstable in the width and height direction, and the ejected plume is in a instability state. The vapor plume is very small and blown out of the keyhole inlet, as presented in Fig. 10(a) and (f). It can be seen from Fig. 10(c) and (h) that the plasma plume displays amaximum brightness and stable shape owing to the large recoil pressure produced by violent local evaporations. As time goes by, the plasma plume reduces markedly and becomes more and more unstable and irregular due to the position and orientation of local evaporation rapidly change. The inlet of the keyhole in laser welding of steel with no magnetic field assisted and magnetic field assisted is closed at 553.0 ms and 520.8 ms,
3.4. Formation mechanism of porosity To study the influence of magnetic field assisted on porosity formation, the porosity in the laser weld is observed. Fig. 12 shows the porosity in the weld by X-ray detection. With the addition of magnetic field assisted, the porosity defect is eliminated obviously, which certificates that the magnetic field assisted is conducive to restrict the porosity formation (Figs. 9 and 12). As reported by Huang et al. [25], the porosity formation basically
Fig. 9. Weld longitudinal-section morphology: (a) no magnetic field assisted; (b) magnetic field assisted. 600
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Fig. 10. High-speed images showing plasma plume dynamics during the welding process: (a) (b) (c) (d) (e) no magnetic field assisted; (f) (g) (h) (i) (j) magnetic field assisted.
depended on the instability of the keyhole and the bubble captured by the solidification interface. With the addition of magnetic field assisted, the lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercut and spatter defects are the three effective measures for improving the keyhole stability. Meanwhile, the Lorentz force has a strong stirring effect on the weld pool, which can decrease the temperature gradient. Fig. 13 shows the force balance of the keyhole collapse. According to the pressure equilibrium at the keyhole wall, when the keyhole is collapsed, the Lorentz force produced by the magnetic field and the smaller surface force caused by reduced temperature gradient are in favour of keyhole open. According to Stokes equation:
η= η0 exp
Ve =
Eμ RT
2( ρ− ρG)gr 2 9η
Fig. 12. Radiographs of laser weld: (a) no magnetic field assisted; (b) magnetic field assisted.
(2)
(3)
where η is the viscosity of liquid metal; η0 is the viscosity at the reference temperature; Eμ is the temperature coefficient of viscosity; R is the universal gas constant and T is the weld pool temperature in kelvin, ρ and ρG are the densities of liquid metal and gas inside the bubble, g is the acceleration of gravity; r is the radius of the bubble. With the addition of magnetic field assisted, the generated Lorentz force hinders the flow of liquid metal, the temperature of the inner keyhole increases. From Eqs. (2) and (3), it can be gotten that the bubble escape velocity is larger in magnetic field assisted laser welding. The mixing and stirring effects produced by the magnetic field accelerates uniform temperature distribution in weld pool, and slows down the cooling rate of the weld pool. Therefore, it promotes the reduction of solidification rate, at the same time, the solidification rate of the weld pool decreases with the increase of the area of the weld pool. As proposed in Zhu’s study [30], If G < 0, the bubble escapes from
Fig. 13. Force balance of the keyhole collapse: (a) no magnetic field assisted; (b) magnetic field assisted.
Fig. 11. High-speed photographs of spatters: (a) no magnetic field assisted; (b) magnetic field assisted. 601
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the weld pool. The bubble leaves the weld pool, and the equation can be given by Eq. (4):
G= t e − t s
[5]
(4)
where te is the time of the bubble escape to the weld pool, and ts is the solidification time for the solidification interface to reach the bubble. With the addition of magnetic field assisted, te is much smaller, and ts is much larger, so the porosity has more time to escape from the weld pool.
[6]
[7]
[8]
4. Conclusions [9]
In this work, the keyhole collapse is observed with transparent glasses, and the convective pattern on the weld pool surfaces is analyzed by tracing the movement of Zirconia particles. Furthermore, the relationship between the keyhole instability and porosity formation with no magnetic field assisted and magnetic field assisted laser welding are investigated. The main conclusions from this paper are summarized as follows: (1) By analysis and comparison of magnetic field assisted laser welding, the magnetic field assisted has little impact on the weld width, no undercut defect can be got with magnetic field assisted, and the magnetic field assisted promotes widening of bottom weld width. (2) Although the magnetic field is added, small concave, the formation of the keyhole collapse and the convective pattern are similar. (3) With the addition of magnetic field assisted, the fluctuation of the weld depth is lower, the time of plasma plume above the weld pool is longer, and fewer undercuts and spatter defects can be obtained, which are more instrumental in improving the keyhole stability. (4) The porosity formation basically depended on the keyhole instability and the bubble captured by the solidification interface. With the addition of magnetic field assisted, the more stability of the keyhole, much easier keyhole open, the larger bubble escape velocity and the lower solidification rate of the weld pool are the main effective means for restraining the porosity formation.
[10]
[11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19] [20] [21]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22]
Acknowledgments
[24]
This research was financially supported by the Super Postdoc of Shanghai (No. 2019080).
[25]
[23]
[26]
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Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental research on formation mechanism of porosity in magnetic field assisted laser welding of steel
T
Lijin Huanga,b,*, Peng Liub, Su Zhub, Xueming Huaa,*, Shengmu Donga a b
Shanghai Key Laboratory of Material Laser Processing and Modification (Shanghai Jiao Tong University), Shanghai 200240, China Shipbuilding Technology Research Institute, Shanghai 200032, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic field assisted Laser welding AH36 steel Keyhole-induced porosity
The dynamic keyhole behaviors are directly observed in laser welding of AH36 steel/glass, and Zirconia particle is used to observe the convective pattern. The weld bead formation, keyhole instability and porosity formation are analyzed. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed. With the magnetic field is added, the weld width, small concave, the formation of the keyhole collapse and the convective pattern are similar in laser welding of steel, and the generated Lorentz force hinders the flow of liquid metal. Widening of bottom weld width without undercut defect can be obtained with magnetic field assisted. The lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercuts and spatter defects contributes to the increase of the keyhole stability, and thus restrains the porosity formation. Besides, an easier keyhole open, the larger bubble escape velocity and the lower solidification rate also promote the reduction of the porosity formation.
1. Introduction Due to the large water force of hull travelling in the sea, cast iron structural members are used in many hull structures. For the purpose of reducing the weight of hull and ensuring the safety, AH36 replaces traditional cast iron, which has been widely applied in the field of hull structure and luxury liner owing to the high strength, toughness and excellent corrosion resistance [1]. In the conventional methods, the thick steel plates are usually required pre-processing and multi-pass welding [2]. Compared to traditional welding methods, laser welding has advantages of high welding speed, large weld depth and small deformation, which has aroused widespread concern [3]. However, due to the complex heat and mass transfer behavior in laser welding, the porosity is easily emerged in the weld, which can reduce the mechanical properties of weld joints and limit the weld quality [4]. There are lots of researches coupling the keyhole-induced porosity formation and elucidation mechanism in laser welding, such as sub-atmospheric forces [5], N2 shielding gas [6], high recoil force [7], laser-GMA welding [8], pulsed laser welding [9] and dual beam laser welding [10]. As pointed out by Huang et al. [11] and Wu et al. [12], the porosity formation was close related to weld pool convection, and many researchers has found that the melt flow behaviors closely connected with magnetic field assisted [13,14]. Previously investigation on magnetic ⁎
field assisted laser welding is mainly concentrated on root humps [15], weld microstructure [16,17] and mechanical properties [18]. Qi et al. [19] investigated the effect of electromagnetic force on root humps by high-speed camera, and pointed out that the additional steady electromagnetic force could effectively prevent periodic root humps. Qi et al. [20] investigated full-penetration laser welding of 316 L stainless steel by introducing high imposed direct current and steady magnetic field, and proposed that steady electromagnetic force could weaken hydrostatic force at the bottom part of the keyhole. Based on the results of these experiment, it was found that magnetic field assisted could suppress humping, increase process stability and obtain better welding quality significantly, however, the researches about effect of magnetic field assisted on keyhole behavior and porosity formation were almost always ignored in laser welding of steel. In this paper, direct observational method with transparent glasses was used to observe the keyhole collapse, and the Zirconia particles was applied to evaluate weld pool convective by a high-speed video camera in fiber magnetically supported laser welding of AH36 steel. The weld bead formation, keyhole and weld pool behaviors are analyzed carefully. Moreover, the differences in keyhole instability and porosity formation between no magnetic field assisted and magnetic field assisted laser welding are analyzed.
Corresponding authors at: Shanghai Key Laboratory of Material Laser Processing and Modification (Shanghai Jiao Tong University), Shanghai 200240, China. E-mail addresses: [email protected] (L. Huang), [email protected] (X. Hua).
https://doi.org/10.1016/j.jmapro.2020.01.007 Received 20 August 2019; Received in revised form 23 December 2019; Accepted 6 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 50 (2020) 596–602
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Fig. 1. Diagram of the experimental platform. Fig. 2. Schematic of the configuration of AH36 steel and transparent glass.
2. Materials and experimental setup The schematic of the experimental setup is shown in Fig. 1. A continuous wave solid state high-power ytterbium fiber laser system (YLS-10000) is used in this experiment. The maximum power is 10 kW, the laser wavelength is 1.070 ± 0.010 mm, and the focus radius is 0.36 mm. The laser head is inclined backwards at an angle of 5° for protection of the optical components of the system. The laser power is 6 kW. The defocused distance is 0 mm. 99.99% argon shielding gas is applied at a flow rate of 20 L/min to protect the weld. Laser beam is kept quiescent in the experiment, and a programmable 2-axis linear operating system is carried out controlling the linear motion of beadon-plate welding to the speed of 1.5 m/min accurately. Magnetic field is generated by placing the permanent magnet under the welded test material. The base material used in this work is AH36 steel plate in the dimension of 100(L) × 30(W) × 10 mm(H), and the heat resistant transparent glass in the dimension of 100(L) × 10(W) × 10 mm(H), whose chemical compositions in weight percentage are shown in Table 1. Prior to welding, the specimen surface is brushed to get rid of the oxidation layers and then cleaned using acetone. During the welding process, a high-speed video (Photron VEO710S) together with a band pass filter near 640 nm and a low-power laser-assisted light (Cavilux Smart) is fixed to monitor the weld pool convective and keyhole behaviors at the speed of 5000 frames/s, and the schematic of the configuration of AH36 steel and transparent glass is shown in Fig. 2. The Zirconia particles in near-spherical shape and diameter of 0.5−1 mm are used to observe the convective pattern. The size of particle is selected to guarantee flow on the weld pool and avoid being melted by a strong hybrid heat source. After welding, the weld porosity is examined by X-ray radiography perpendicular to the specimen surface. The samples are cut along the longitudinal section direction and corrodes with 4 ml HNO3 + 96 ml C2H5OH solution, and then observes by optical microscope (Zeiss Stemi2000).
Fig. 3. Weld surface morphology in laser welding: (a) no magnetic field assisted; (b) magnetic field assisted.
3. Results and discussion 3.1. Weld bead formation The obtained joint weld surface morphology is depicted in Fig. 3. A uniform and continuous weld appearance is formed, and the top weld surface has no visible crack and porosity defects, but spatters are observed on the weld of samples. The weld surface morphology is wellprotected and has a bright metallic luster. With the addition of magnetic field assisted, no appreciable change in the weld surface width (width of approximately 3.40 mm) is observed, which shows that the magnetic field assisted has little impact on the weld width. The comparison of cross-sections with no magnetic field assisted and magnetic field assisted is illustrated in Fig. 4. It can be seen that, undercut defect appears with no magnetic field assisted, which is in accordance with the research put into force by many scholars [21–23]. The Lorentz force produced by the magnetic field has a stirring effect on the edge of the weld pool, and the temperature at the keyhole front increases, then the accumulated heat is conducted to the bottom of the weld pool, which promotes widening of bottom weld width.
Table 1 Chemical compositions of AH36 steel and glass (wt.%). Materials
AH36 Glass
Elements, mass% SiO2
C
Mn
Si
S
P
Cu
Fe
– 99.97–99.99
0.15–0.18 –
0.70–1.60 –
0.10–0.50 –
≤0.035 –
≤0.035 –
≤0.35 –
Bal. –
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Fig. 4. Weld cross-section morphology (The red dashed line represents the boundary of the weld): (a) no magnetic field assisted; (b) magnetic field assisted (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
is similar in laser welding of steel. Fig. 7 represents the continuous change of the weld pool in fiber laser welding with no magnetic field assisted and magnetic field assisted by tracing the movement of Zirconia particles on the weld pool surfaces. Due to the lower density of the Zirconia particle, it can float on the surface of the weld pool during the welding process. As shown in Fig. 7, the molten metal directly flows to the rear part of weld pool from both sides of keyhole opening, and then the convective pattern on the top surfaces is obtained on the weld pool surfaces, which is in favor of transporting the molten metal to the rear part of weld pool. With the addition of magnetic field assisted, the motion of the weld pool moves from the center to the edge owing to the Lorentz force produced by the magnetic field. The liquid metal on the rear wall of the keyhole flows into the keyhole and forms weld seam. Comparing to no magnetic field assisted, the length and area of the weld pool are extended along the welding direction. The liquid metal and steel can be regarded as two different metals in laser welding of steel, respectively, and there is a large temperature
3.2. Keyhole and weld pool behaviors When the laser beam is irradiated on the metal surface directly, the metal surface has a great melting and local vaporization, and the recoil force generated by the metal vapor makes the weld pool sag to form a small concave, as shown in Fig. 5(b) and (d). As the welding process proceeds, the laser acts directly on the bottom of the concave so that the concave deepens rapidly. Whether or not there is a magnetic field assisted, the formation process of the keyhole during laser welding is similar. To clearly observe the dynamic keyhole behaviors, the welding experiment on the “glass-metal” hybrid sample is implemented. Inner keyhole behaviors are shown in Fig. 6. From the observation result, the rear wall of keyhole is relatively smooth, and there are more humps in the front wall. As seen in Fig. 6, the bulge is formed at the rear part of the keyhole wall, and the bulge contacts with the front part of the keyhole wall, which can cause the keyhole collapse. Whether or not there is a magnetic field assisted, the formation of the keyhole collapse
Fig. 5. Small concave formation: (a) (b) no magnetic field assisted; (c) (d) magnetic field assisted. 598
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Fig. 6. Inner dynamic keyhole behaviors: (a) (b) (c) no magnetic field assisted; (d) (e) (f) magnetic field assisted.
Fig. 7. The convective pattern obtained by tracing the movement of Zirconia particles on the weld pool surfaces: (a) (b) (c) no magnetic field assisted; (d) (e) (f) magnetic field assisted.
difference between them. When the auxiliary magnetic field is added, the flowing molten metal cuts the magnetic induction lines, so the current in weld pool be described as [24]: j=σ(v B + α ∙ ▽T)
on the formation of the keyhole collapse. Fig. 7 shows that under the effect of strong clockwise flow, the molten metal at rear keyhole wall flows toward the front keyhole wall [25]. Fig. 8 summarizes the keyhole collapse formation mechanism in laser welding of steel. The process can be detailed as follows: at an initial stage, the lager recoil force is good for keeping keyhole open, shown as Fig. 8(a). Under the influence of strong clockwise flow, the bulge forms at the rear keyhole wall shown as Fig. 8(b). The recoil force cannot maintain the keyhole open, the rear keyhole wall is collapsed, and the keyhole is divided into two parts by weld pool shown as Fig. 8(c).
(1)
where j is current density, σis the metallic conductivity, v is the velocity of liquid metal, B is magnetic strength, α is Seebeck coefficient, and T is the temperature difference between liquid metal and steel. Based on Eq. (1), there is induced current and thermal current generated by temperature difference in the weld pool under the effect of magnetic field. Using left-hand rule to determine the direction of Lorentz force, the Lorentz force produced by induced current always hinders the flow of liquid metal. As seen in Fig.7, the velocity of liquid metal is 1.94 m/s and 1.80 m/s, respectively. This always indicates that the generated Lorentz force hinders the flow of liquid metal. Fig. 6 illustrates that the magnetic field assisted has almost no effect
3.3. Keyhole stability The longitudinal-section of weld joint is shown in Fig. 9. It can be figured out that between no magnetic field assisted and magnetic field assisted laser welding, the minimum and maximum weld depth varied 599
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Fig. 8. Schematic diagram showing keyhole collapse formation mechanism: (a) keyhole formation, (b) bulge formation and (c) keyhole collapse.
respectively. The white and featheriness plumes observed above the weld pool were produced periodically, and there is no plume when the keyhole is closed, which can reveal the periodic closures of the keyhole. Meanwhile, when the auxiliary magnetic field is added, Lorentz force is produced by the the moving charged particles, which impels electrons to rotate back in a magnetic field, and increases the time of plasma plume above the weld pool. According to Wang and Thomy's study [27,28], the undercut is close related to unstable keyhole. An undercut defect indicate more unstable keyhole in no magnetic field assisted laser welding. With the addition of magnetic field assisted, the generated Lorentz force hinders the flow of liquid metal, and the plasma in the keyhole is subjected to the electric transmission effect, and an orderly migration movement occurs, which increases the stability of the keyhole, thus reduces the frequency of keyhole collapse. During laser welding of steel, the keyhole is much unstable, and lots of spatters are produced with no magnetic field assisted, as shown in Figs. 3, 10 and 11. It was referred that an unstable molten pool which could cause spatter [29]. So with the addition of magnetic field assisted, the keyhole is much stable.
within a range of 6.11–6.66 mm and 6.39–6.87 mm, and the average weld depth is 6.385 mm and 6.63 mm, respectively. Although the weld pool is in quasi-steady state, the weld depth is violently fluctuated, which is usually regarded as the result of keyhole instability. According to Pang's study [26], the keyhole stability is close related to fluctuation of the weld depth. The Lorentz force generated by the induced current prevents the liquid metal near the keyhole flows towards the rear part, which leads to heat concentration in front part of the weld pool, thus the accumulated heat is transferred to the bottom of the molten pool, so with the addition of magnetic field assisted, the keyhole depth increases. No cracks are discovered in two cross-sections of the weld, and a small amount of porosity was seen at random position with no magnetic field assisted laser welding of steel. In fiber laser welding of steel, the surface peak temperature of weld pool is very high and outstrip its boiling points, which leads to the generation of metal vapor. The metal vapor can be partially ionized and form plasma plume. The typical appearance of plasma plume with no magnetic field assisted and magnetic field assisted is shown in Fig. 10. As seen, during laser welding of steel, the plume behavior is very unstable in the width and height direction, and the ejected plume is in a instability state. The vapor plume is very small and blown out of the keyhole inlet, as presented in Fig. 10(a) and (f). It can be seen from Fig. 10(c) and (h) that the plasma plume displays amaximum brightness and stable shape owing to the large recoil pressure produced by violent local evaporations. As time goes by, the plasma plume reduces markedly and becomes more and more unstable and irregular due to the position and orientation of local evaporation rapidly change. The inlet of the keyhole in laser welding of steel with no magnetic field assisted and magnetic field assisted is closed at 553.0 ms and 520.8 ms,
3.4. Formation mechanism of porosity To study the influence of magnetic field assisted on porosity formation, the porosity in the laser weld is observed. Fig. 12 shows the porosity in the weld by X-ray detection. With the addition of magnetic field assisted, the porosity defect is eliminated obviously, which certificates that the magnetic field assisted is conducive to restrict the porosity formation (Figs. 9 and 12). As reported by Huang et al. [25], the porosity formation basically
Fig. 9. Weld longitudinal-section morphology: (a) no magnetic field assisted; (b) magnetic field assisted. 600
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Fig. 10. High-speed images showing plasma plume dynamics during the welding process: (a) (b) (c) (d) (e) no magnetic field assisted; (f) (g) (h) (i) (j) magnetic field assisted.
depended on the instability of the keyhole and the bubble captured by the solidification interface. With the addition of magnetic field assisted, the lower fluctuation of the weld depth, the longer time of plasma plume above the weld pool, fewer undercut and spatter defects are the three effective measures for improving the keyhole stability. Meanwhile, the Lorentz force has a strong stirring effect on the weld pool, which can decrease the temperature gradient. Fig. 13 shows the force balance of the keyhole collapse. According to the pressure equilibrium at the keyhole wall, when the keyhole is collapsed, the Lorentz force produced by the magnetic field and the smaller surface force caused by reduced temperature gradient are in favour of keyhole open. According to Stokes equation:
η= η0 exp
Ve =
Eμ RT
2( ρ− ρG)gr 2 9η
Fig. 12. Radiographs of laser weld: (a) no magnetic field assisted; (b) magnetic field assisted.
(2)
(3)
where η is the viscosity of liquid metal; η0 is the viscosity at the reference temperature; Eμ is the temperature coefficient of viscosity; R is the universal gas constant and T is the weld pool temperature in kelvin, ρ and ρG are the densities of liquid metal and gas inside the bubble, g is the acceleration of gravity; r is the radius of the bubble. With the addition of magnetic field assisted, the generated Lorentz force hinders the flow of liquid metal, the temperature of the inner keyhole increases. From Eqs. (2) and (3), it can be gotten that the bubble escape velocity is larger in magnetic field assisted laser welding. The mixing and stirring effects produced by the magnetic field accelerates uniform temperature distribution in weld pool, and slows down the cooling rate of the weld pool. Therefore, it promotes the reduction of solidification rate, at the same time, the solidification rate of the weld pool decreases with the increase of the area of the weld pool. As proposed in Zhu’s study [30], If G < 0, the bubble escapes from
Fig. 13. Force balance of the keyhole collapse: (a) no magnetic field assisted; (b) magnetic field assisted.
Fig. 11. High-speed photographs of spatters: (a) no magnetic field assisted; (b) magnetic field assisted. 601
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the weld pool. The bubble leaves the weld pool, and the equation can be given by Eq. (4):
G= t e − t s
[5]
(4)
where te is the time of the bubble escape to the weld pool, and ts is the solidification time for the solidification interface to reach the bubble. With the addition of magnetic field assisted, te is much smaller, and ts is much larger, so the porosity has more time to escape from the weld pool.
[6]
[7]
[8]
4. Conclusions [9]
In this work, the keyhole collapse is observed with transparent glasses, and the convective pattern on the weld pool surfaces is analyzed by tracing the movement of Zirconia particles. Furthermore, the relationship between the keyhole instability and porosity formation with no magnetic field assisted and magnetic field assisted laser welding are investigated. The main conclusions from this paper are summarized as follows: (1) By analysis and comparison of magnetic field assisted laser welding, the magnetic field assisted has little impact on the weld width, no undercut defect can be got with magnetic field assisted, and the magnetic field assisted promotes widening of bottom weld width. (2) Although the magnetic field is added, small concave, the formation of the keyhole collapse and the convective pattern are similar. (3) With the addition of magnetic field assisted, the fluctuation of the weld depth is lower, the time of plasma plume above the weld pool is longer, and fewer undercuts and spatter defects can be obtained, which are more instrumental in improving the keyhole stability. (4) The porosity formation basically depended on the keyhole instability and the bubble captured by the solidification interface. With the addition of magnetic field assisted, the more stability of the keyhole, much easier keyhole open, the larger bubble escape velocity and the lower solidification rate of the weld pool are the main effective means for restraining the porosity formation.
[10]
[11]
[12]
[13]
[14]
[15] [16]
[17]
[18]
[19] [20] [21]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[22]
Acknowledgments
[24]
This research was financially supported by the Super Postdoc of Shanghai (No. 2019080).
[25]
[23]
[26]
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