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Microstructure and performance of hybrid laser-arc welded high-strength low alloy steel and austenitic stainless steel dissimilar joint
Optics and Laser Technology 122 (2020) 105878
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microstructure and performance of hybrid laser-arc welded high-strength low alloy steel and austenitic stainless steel dissimilar joint Xiong Zhang, Gaoyang Mi, Chunming Wang
T
⁎
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
H I GH L IG H T S
joint of EH36 and 316L with 20 mm thickness was joined. • Dissimilar between element distribution and phase composition was studied. • Relationship • Effects of uneven microstructure on performance of the welded joint were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid laser-arc welding Dissimilar joint Microstructure Performance
Effects of hybrid laser-arc welding process parameters on the weld formation of a dissimilar steel joint were studied. A full penetrated weld with minimum defects was obtained under the optimized parameters. Different thermal conductivities and phase transition temperature resulted in a wider heat affected zone (HAZ) of highstrength low alloy steel (EH36) than that of austenitic stainless steel (316L). Owing to the limited penetration of arc, the laser zone could be only heated by laser while the hybrid zone was heated by not only laser but also arc, resulting in higher heat input and larger grain size in hybrid zone than in laser zone. The addition of filler wire led to higher contents of chromium (Cr) and nickel (Ni) in hybrid zone than laser zone, which inhibited the formation of austenite and led to the formation of martensite in laser zone. Small grain size and existence of martensite resulted in higher hardness of laser zone than hybrid zone. Tensile samples of both laser zone and hybrid zone failed at the EH36 side with massive dimples distributing on the fracture surfaces, indicating good tensile property of the welds and typical characteristics of ductile fracture. Corrosion resistance of laser zone was weaker than that of hybrid zone, which was closely related to the formation of martensite with poor corrosion resistance instead of austenite with good corrosion resistance.
1. Introduction Welding of high-strength low alloy steel and austenitic stainless steel dissimilar joint with large thickness has been investigated with increasing demands in a lot of industries including naval architecture and ocean engineering. Common used multi-layer multi-pass traditional arc welding technology was simplicity, mobility and versatility in joining materials with varying thickness, shape and physical properties [1]. However, owing to the existing disadvantages including high heat input, shallow welding penetration and low welding efficiency, it is more and more difficult for traditional arc welding to completely meet the growing manufacturing requirements in these industries [2]. Recently, the emerging welding methods including the narrow-gap arc welding and the electron beam welding provided new approaches to thick plate welding. The narrow-gap arc welding decreased the welding ⁎
passes and partly increased the welding efficiency by reducing the groove size, but the welding efficiency and quality were still dissatisfactory due to the shallow welding penetration and instability of the arc [3]. The electron beam welding possessed high energy density and low heat input, producing deep penetration and minimal distortion compared to arc welding. However, the requirement of vacuum environment greatly limited the adaptability and promotion of this technology [4]. Thus, it is necessary for the thick plate welding to develop more effective and adaptable welding technology. Laser welding was emerging as a promising welding technique for thick plate due to low heat input, deep welding penetration, high welding efficiency and adaptability [5]. The EH36 and 316L dissimilar joint with 6 mm thickness was joined by Cao et al. [6] and effects of welding speed on weld formation, microstructure and mechanical properties of the welds were studied in details. Rong et al. [7] realized
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.optlastec.2019.105878 Received 3 August 2018; Received in revised form 25 June 2019; Accepted 27 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
adopted, and the welding head was tilted 5° along the welding direction to reduce the influences of high reflectivity and spatters. The 100% Ar was used as shielding gas with a flow rate of 25 L/min during the welding process. The metallurgical samples were extracted from the steady state region of the welds. Cross sections of the samples were ground and polished, following by etching with aqua regia for microstructural analysis. Microstructure of the welds was observed by Keyence VHX-1000C optical microscope and analysis of the crystallographic feature was conducted by FEI Sirion-200 scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) system. Distribution of chemical elements was analyzed by energy dispersive spectrum (EDS) test. Hardness was measured by DHV-1000 Vickers hardness machine with a test load of 300 g and a dwell period of 15 s. Tensile properties were measured by Shimadzu AG-100kN system and the fracture morphologies were further investigated by SEM. Electrochemical corrosion behavior were analyzed by CorrTest-CS310 workstation in a conventional three-electrode cell. All of the corrosion samples were processed into 5 mm × 5 mm × 5 mm and packaged in epoxy resin. Working surfaces of all samples were polished with abrasive paper and the corrosion tests were carried out in solution of 3.5 wt% NaCl at 30 °C with a scanning rate of 1.667 mV/s.
the joining of EH36 and 316L dissimilar joint with 4 mm thickness by laser welding process and researched the influence of misalignment on weld profile and alloy element distribution. Esfahani et al. [8] analyzed the effect of alloying composition on microstructure and mechanical performance of laser welded EH36 and 316L dissimilar joint with 0.9 mm thickness and proposed strategies to control the alloying composition of the welds. However, the existing researches of laser welding EH36 and 316L dissimilar joint mainly focused on the small thickness and the cracking tendency and porosity of laser welding were relatively high due to the quick cooling rate. Besides, dilution of alloying elements and diffusion of carbon during laser welding process resulted in formation of the secondary phase and deteriorated the performance of the welds. Though the laser welding with filler wire showed great advantages in reducing weld porosity and controlling elementary composition, the low welding efficiency caused by the shallow welding penetration and slow welding speed was less than satisfactory [9]. The hybrid laser-arc welding technology combined laser welding with arc welding and the two heat sources interacted in such a way to produce a single high intensity energy source and overcome problems such as shallow penetration, cracking, porosity and detrimental phase formation [10–12]. Joo et al. [13] joined the EH36 and 316L dissimilar joint with 13 mm thickness by optimized hybrid laser-arc welding process and further studied the fatigue property of the welds. However, the attention paid to joining EH36 and 316L dissimilar metal by hybrid laser-arc welding was not enough. The mechanism of microstructure formation and effects of microstructure on performance of the welds were still unclear. In this study, the hybrid laser-arc welding technology was adopted to weld the EH36 and 316L dissimilar joint with a thickness of 20 mm. Effects of welding parameters on weld formation were studied and the welding process was optimized. On this basis, element distribution, microstructure and performance of the welds were further investigated and relationships between element, microstructure and performance were established.
3. Results and discussion 3.1. Welding process The gap of the backing welding was narrowest among all layers and the required penetration of the backing welding was largest among all layers. Thus, realization of the backing welding was the hardest among all layers. A lot of previous work found that laser power and welding speed were the decisive factors of weld penetration and formation [15]. However, except for laser power and welding speed, the arc current could also directly influence the weld penetration and formation. In view of that numerous studies were focused on effects of laser power and welding speed, appearances of the backing welds under different arc current were analyzed to illustrate the effects of arc currents on weld formation. The experiment was conducted under a laser power of 4.0 kW, a welding speed of 1.0 m/min, a defocusing amount (Δf) of 0 mm and a distance between laser and arc (DLA) of 2 mm. Appearances of the welds under different arc currents were shown in Fig. 2. It was clear that the melting widths of the backside were not enough when the arc currents were 100A and 150A, resulting in the incomplete penetration as shown in Fig. 2a and the undercut as shown in Fig. 2b, respectively. Besides, excessive penetration and collapse occurred when the arc current reached 250A and acceptable weld formation could be obtained when the arc current was 200A. On this basis, the laser power of the subsequent layers decreased to 2.0 kW because of the reduced requirement of welding penetration and the arc current increased from lower layer to upper layer to adapt the growing width of the gap. As a consequence, the dissimilar joint of EH36 and 316L with 20 mm thickness was made by 5 layers hybrid laser-arc welding. The optimized welding parameters and the cross-sectional appearance of the welds were listed in Table 2.
2. Experimental procedure The base metals adopted in this study were the high-strength low alloy steel EH36 and the austenitic stainless steel 316L named by the ABS standard with dimensions of 250 mm × 150 mm × 20 mm. The 1.2 mm-diameter solid filler wire (ER316LSi) was used to fill the gap. Chemical composition of the base metals and the filler wire were given in Table 1. The hybrid laser-arc welding system was composed by an IPG YLR-4000 fiber laser system with a wavelength of 1070 nm and a maximum power output of 4000 W and a Fronius TransPuls Synergic4000 welding power source equipped with an ABB IRB-6400 robot. The narrow-gap Y-groove was prepared to obtain the full penetrated weld. As shown in Fig. 1, the groove angle, thickness of root face and width of platform was 20°, 5 mm and 2 mm, respectively. Owing to better reachability and flexibility of laser cleaning than that of mechanical abrading, the contamination and oxide which contributed to the formation of the interlayer defects including pores, inclusions and lack of fusion could be effectively cleared up by laser cleaning [14]. Thus, the laser cleaning method instead of the mechanical abrading was adopted in this study to preprocess the groove. Detailed parameters of laser cleaning were presented as follows: the laser power was 500 W, the pulse repetition rate was 100 kHz, the defocusing amount was 0 mm and the cleaning speed was 1.0 m/min. The laser leading mode was
3.2. Microstructure As shown in Fig. 3, it was clear that microstructure of 316L (Fig. 3a) consisted of massive blocky austenite with little ferrite distributing at the grain boundary while microstructure of EH36 (Fig. 3b) consisted of massive blocky ferrite with a small amount of pearlite. Due to lower thermal conductivity and higher phase-transition temperature of 316L than that of EH36, the degree of heating and recrystallization of 316L (Fig. 3c) and EH36 (Fig. 3d) nearby the fusion line were different, resulting in much narrower HAZ in 316L (maximum width of 80 μm) than in EH36 (maximum width of 900 μm). Besides, massive martensite
Table 1 Chemical compositions of base metal and filler wire (mass fraction: %).
EH36 316L Wire
Cr
Ni
Mo
Mn
Si
P
C
S
Cu
N
Fe
0.04 16.82 18.12
0.01 9.84 11.70
0.01 1.78 2.55
1.46 1.52 1.86
0.32 0.96 0.76
0.01 0.03 0.02
0.17 0.02 0.01
0.01 – 0.01
0.04 – 0.08
– – 0.05
Bal. Bal. Bal.
2
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
Fig. 1. Schematic of: (a) the groove and (b) the welding process.
different phase composition between laser zone and hybrid zone was thought to be greatly influenced by the element distribution of Cr and Ni [17,18]. Thus, distributions of Cr and Ni across the laser zone and hybrid zone were analyzed to elucidate the cause of different phase composition between laser zone and hybrid zone. As presented in Fig. 5, it was clear that contents of Cr (square) and Ni (circle) across the laser zone and hybrid zone showed an common increasing trend from the EH36 side to the 316L side. However, contents of Cr and Ni in laser zone were much lower than that in 316L while contents of Cr and Ni in hybrid zone (black line) were approximately equal to 316L. Different contents of Cr and Ni between laser zone and hybrid zone was considered to be closely related to the percentage contribution of molten EH36 in laser zone and hybrid zone. Contents of alloying elements in EH36 was much lower than that of 316L, that’s to say, the molten EH36 diluted the concentration of alloying element in molten pool. Molten pool of laser zone was composed of EH36 and 316L while molten pool of hybrid zone contained not only EH36 and 316L but also a mass of molten filler wire which had the same chemical component as 316L. Therefore, addition of filler wire greatly reduced the percentage contribution of molten EH36 and maintained the concentration of alloying elements in the hybrid zone. Thus, the element contents of Cr and Ni in hybrid zone were much higher than that of laser zone, which led to uneven phase composition between laser zone and hybrid zone. In order to quantificationally study the relationship between element composition and microstructure, the element composition of laser zone and hybrid zone was measured and shown in Fig. 6. It was obvious that contents of Cr and Ni in laser zone were 12.50% and 7.20% respectively while contents of Cr and Ni in hybrid zone were 18.49% and
formed in HAZ of EH36 while no obvious phase transformation could be observed in HAZ of 316L since 316L was not transformable [16]. Owing to the large penetration of backing welding and the limited action range of arc, fusion zone of the multi-layer welds could be divided into two different zones, namely the laser zone at the downside and the hybrid zone at the upside. Microstructure of laser zone and hybrid zone was significantly different: microstructure of laser zone (Fig. 3e) consisted of massive acicular martensite while microstructure of hybrid zone (Fig. 3f) consisted of dendritic austenite from the fusion line to the weld center and blocky austenite at the weld center. Consequently, microstructure of the multi-layer welds was inhomogeneous, the phase composition and gain size of laser zone and hybrid zone showed significant differences. In order to further investigate the microstructural inhomogeneity of the welds, the phase composition and gain sizes of laser zone and hybrid zone were quantitatively analyzed by the EBSD test. As shown in Fig. 4a and b, microstructure of laser zone was much finer than that of hybrid zone. The average grain sizes of laser zone and hybrid zone were 1.5 μm2 and 7.6 μm2, respectively. As shown in Fig. 4c and d, the phase composition of laser zone was 37% austenite (the blue phase) and 63% martensite (the red phase) while the phase composition of hybrid zone was 96% austenite (the blue phase) and 4% ferrite (the red phase). Different grain sizes between the laser zone and the hybrid zone were considered to be closely related to different thermal cycles. The hybrid zone was heated by both laser and arc while the laser zone was only heated by laser. Thus, the heat input of hybrid zone was higher than that of laser zone, which resulted in lower cooling rate and larger grain size of hybrid zone than laser zone. In addition to the grain size,
Fig.2. Appearances of backing welds under: (a) 100A (b) 150A (c) 200A (d) 250A. 3
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
Table 2 Optimized parameters and cross-sectional macro appearance of the welds. Layer
Laser power (kW)
Current (A)
Voltage (V)
Speed (m/min)
Δf (mm)
DLA (mm)
1 2 3 4 5
4.0 2.0 2.0 2.0 2.0
200 225 225 250 250
22.0 23.2 23.2 24.4 24.4
1.0
0
2
Weld appearance
was much higher than that of hybrid zone. Different hardness distributions between laser zone and hybrid zone were thought to be closely related to the formation of massive martensite which possessed higher hardness than austenite and ferrite. In addition to the phase composition, different grain sizes between laser zone and hybrid zone were the secondary cause of different hardness distributions. The average grain size of laser zone was much smaller than that of hybrid zone, resulting in smaller dislocation and higher harness of laser zone than hybrid zone [19]. Thus, different phase composition and grain sizes led to significantly different hardness distributions between laser zone and hybrid zone. Besides, tensile properties of laser zone (sample 1) and hybrid zone (sample 2) were also studied in details. Fig. 8 suggested that the necking and fracture of both sample 1 and sample 2 occurred at the EH36 side. The ultimate tensile strength of sample 1 and sample 2 were 441.3 MPa and 442.3 MPa, respectively. The tensile elongations of sample 1 and sample 2 were 29.7% and 29.6%, respectively. In addition, large amounts of cup-like dimples formed on the fracture surfaces of both sample 1 and sample 2, indicating typical characteristics of ductile fracture [20]. Thus, tensile properties of both laser zone and hybrid zone were better than that of EH36.
10.96% respectively. Thus, the proportion of Cr and Ni in laser zone was less than that in hybrid zone. Besides, the cooling rate of laser zone was higher than that of hybrid zone due to the lower heat input of laser zone than hybrid zone. Thus, the formation of ferrite and austenite in laser zone was inhibited which led to high formative tendency of martensite. Microstructure of hybrid zone consisted of austenite and ferrite while microstructure of laser zone preferentially transformed to massive martensite instead of austenite and ferrite. In conclusion, different element composition and thermal cycles between laser zone and hybrid zone resulted in different phase composition between laser zone and hybrid zone. 3.3. Mechanical properties In order to study the relationship between microstructure and mechanical properties of the welds, hardness distributions of laser zone and hybrid zone were measured and analyzed, respectively. As shown in Fig. 7, the average hardness values of EH36 and 316L were about 170 HV0.3 and 185 HV0.3, respectively. The average hardness value of hybrid zone was about 250 HV0.3 while the average hardness value of laser zone reached 350 HV0.3. Thus, hardness of both laser zone and hybrid zone were higher than the base metal and hardness of laser zone
Fig. 3. Microstructure of: (a) 316L, (b) EH36, (c) HAZ of 316L, (d) HAZ of EH36 (e) laser zone and (f) hybrid zone. 4
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
Fig. 4. EBSD test: (a) grain size of laser zone; (b) grain size of hybrid zone; (c) phase composition of laser zone; (d) phase composition of hybrid zone.
Fig. 5. Distributions of Cr and Ni across laser zone and hybrid zone.
3.4. Corrosion behavior
Fig. 6. Element composition of laser zone and hybrid zone.
In view of the marine environment, the potentiodynamic polarization tests were conducted to research the corrosion behavior at different locations and illuminate the effects of microstructure on corrosion resistance. Test locations and potentiodynamic polarization curves of 316L, EH36, laser zone and hybrid zone were revealed in Fig. 9. It was clear that the corrosion behavior of laser zone showed similar characteristics with EH36 and the corrosion behavior of hybrid zone showed similar characteristics with 316L. In addition, the self-corrosion
potential (Ecorr), the corrosion current density (Icorr) and the corrosion rate (K) were fitted to evaluate the corrosion behavior of different zones, as presented in Table 3. Zhang et al. [21] demonstrated that lower Ecorr and higher Icorr corresponded to faster corrosion rate and poorer corrosion resistance. Accordingly, the corrosion resistance of 316L was the best of all zones while the corrosion resistance of EH36 was the worst of all zones. Moreover, the corrosion resistance of hybrid zone was much better than that of laser zone. Different corrosion 5
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
Table 3 Self-corrosion potential, corrosion current densities and corrosion rates of different zones. Zone
Ecorr/V
Icorr/A·cm−2
K/mm·a−1
316L Hybrid zone Laser zone EH36
−0.21 −0.27 −0.39 −0.44
1.29 × 10−6 1.55 × 10−6 6.53 × 10−6 8.89 × 10−6
0.01 0.02 0.08 0.11
96% austenite and 4% ferrite. The corrosion resistances of ferrite and martensite were worse than that of austenite [22], leading to the decrease of corrosion resistance from 316L to hybrid zone to laser zone to EH36. 4. Conclusions (1) An accepted weld of EH36 and 316L dissimilar joint with 20 mm thickness was obtained by optimized narrow-gap hybrid laser-arc welding process. (2) Molten EH36 diluted the concentration of alloying element in molten pool. High melting rate of EH36 in laser zone led to the reduction of stabilized alloying element of austenite and ferrite, facilitating the formation of martensite in laser zone. Addition of the filler wire substantially reduced the melting rate of EH36 and led to the formation of massive austenite in hybrid zone. (3) Large amounts of martensite and small grain size resulted in higher harness of laser zone than that of hybrid zone. Tensile properties of the fusion zone were better than that of EH36 and showed typical characteristics of ductile fracture. (4) Corrosion resistances of hybrid zone and laser zone were better than that of EH36 but worse than that of 316L and corrosion resistance of hybrid zone was better than that of the laser zone, which was attributed to the increasing content of the perishable phase from EH36 to laser zone to hybrid zone to 316L.
Fig. 7. Hardness distributions across laser zone and hybrid zone.
Acknowledgments Fig. 8. Tensile properties and fracture morphologies of laser zone and hybrid zone.
This work was supported by the National Natural Science Foundation of China (Grant No. 51705173) and National Program on Key Basic Research Project (Grant No. 2014CB046703). We would like to express our deep gratitude to the Analysis and Test Center of HUST (Huazhong University of Science and Technology) and the State Key Laboratory of Material Processing and Die & Mould Technology of HUST, for their friendly cooperation. References [1] B.S. Huang, J. Yang, D.H. Lu, W.J. Bin, Study on the microstructure, mechanical properties and corrosion behaviour of S355JR/316L dissimilar welded joint prepared by gas tungsten arc welding multi-pass welding process, Sci. Technol. Weld. Joining 21 (5) (2016) 1–8. [2] R. Nivas, P.K. Singh, G. Das, S.K. Das, S. Kumar, B. Mahato, K. Sivaprasad, M. Ghosh, A comparative study on microstructure and mechanical properties near interface for dissimilar materials during conventional V-groove and narrow gap welding, J. Manuf. Processes 25 (2017) 274–283. [3] J. Feng, W. Guo, N. Irvine, L. Li, Understanding and elimination of process defects in narrow gap multi-pass fiber laser welding of ferritic steel sheets of 30mm thickness, Int. J. Adv. Manuf. Technol. 88 (5–8) (2016) 1–10. [4] D.J. Smith, G. Zheng, P.R. Hurrell, C.M. Gill, B.M.E. Pellereau, K. Ayres, D. Goudar, E. Kingston, Measured and predicted residual stresses in thick section electron beam welded steels, Int. J. Pressure Vessels Piping s 120–121 (1) (2014) 66–79. [5] M. Zhang, Z. Zhang, K. Tang, C. Mao, Y. Hu, G. Chen, Analysis of mechanisms of underfill in full penetration laser welding of thick stainless steel with a 10kW fiber laser, Opt. Laser Technol. 98 (Supplement C) (2018) 97–105. [6] C. Longchao, S. Xinyu, J. Ping, Z. Qi, R. Youmin, G. Shaoning, M. Gaoyang, Effects of welding speed on microstructure and mechanical property of fiber laser welded dissimilar butt joints between AISI316L and EH36, Metals 7 (7) (2017) 13 270. [7] Y. Rong, J. Xu, T. Lei, W. Wang, A.A. Sabbar, Y. Huang, C. Wang, Z. Chen, Microstructure and alloy element distribution of dissimilar joint 316L and EH36 in laser welding, Sci. Technol. Weld. Joining (2017) 1–8.
Fig. 9. Potentiodynamic polarization curves of different zones.
behavior between different weld locations was related to the phase composition of different zones: microstructure of 316L consisted of almost fully austenite while microstructure of EH36 consisted of massive ferrite, microstructure of laser zone was composed of 37% austenite and 63% martensite while microstructure of hybrid zone was composed of 6
Optics and Laser Technology 122 (2020) 105878
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[16] M.J. Torkamany, J. Sabbaghzadeh, M.J. Hamedi, Effect of laser welding mode on the microstructure and mechanical performance of dissimilar laser spot welds between low carbon and austenitic stainless steels, Mater. Des. 34 (2012) 666–672. [17] M.F. Mamat, E. Hamzah, Z. Ibrahim, A.M. Rohah, A. Bahador, Effect of filler metals on the microstructures and mechanical properties of dissimilar low carbon steel and 316L stainless steel welded joints, Mater. Sci. Forum 819 (2015) 57–62. [18] F. Wang, X. Zhang, L. Liu, Y. Chen, X. Zhang, H. Liu, One-side welding procedure and joint properties of Q345R/316L clad plate, Electric Welding Mach. 48 (1) (2018) 47–51. [19] X. Zhang, G. Mi, L. Chen, P. Jiang, X. Shao, C. Wang, Microstructure and performance of hybrid laser-arc welded 40mm thick 316L steel plates, J. Mater. Process. Technol. 259 (2018) 312–319. [20] Q. Jia, W. Guo, W. Li, Y. Zhu, P. Peng, G. Zou, Microstructure and tensile behavior of fiber laser-welded blanks of DP600 and DP980 steels, J. Mater. Process. Technol. 236 (2016) 73–83. [21] Z. Xiong, M. Gaoyang, X. Lingda, W. Chunming, Effects of interlaminar microstructural inhomogeneity on mechanical properties and corrosion resistance of multi-layer fiber laser welded high strength low alloy steel, J. Mater. Process. Technol. 252 (2018) 81–89. [22] R. Chen, P. Jiang, X. Shao, G. Mi, C. Wang, Effect of magnetic field applied during laser-arc hybrid welding in improving the pitting resistance of the welded zone in austenitic stainless steel, Corros. Sci. 126 (2017) 385–391.
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7
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microstructure and performance of hybrid laser-arc welded high-strength low alloy steel and austenitic stainless steel dissimilar joint Xiong Zhang, Gaoyang Mi, Chunming Wang
T
⁎
State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China
H I GH L IG H T S
joint of EH36 and 316L with 20 mm thickness was joined. • Dissimilar between element distribution and phase composition was studied. • Relationship • Effects of uneven microstructure on performance of the welded joint were analyzed.
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid laser-arc welding Dissimilar joint Microstructure Performance
Effects of hybrid laser-arc welding process parameters on the weld formation of a dissimilar steel joint were studied. A full penetrated weld with minimum defects was obtained under the optimized parameters. Different thermal conductivities and phase transition temperature resulted in a wider heat affected zone (HAZ) of highstrength low alloy steel (EH36) than that of austenitic stainless steel (316L). Owing to the limited penetration of arc, the laser zone could be only heated by laser while the hybrid zone was heated by not only laser but also arc, resulting in higher heat input and larger grain size in hybrid zone than in laser zone. The addition of filler wire led to higher contents of chromium (Cr) and nickel (Ni) in hybrid zone than laser zone, which inhibited the formation of austenite and led to the formation of martensite in laser zone. Small grain size and existence of martensite resulted in higher hardness of laser zone than hybrid zone. Tensile samples of both laser zone and hybrid zone failed at the EH36 side with massive dimples distributing on the fracture surfaces, indicating good tensile property of the welds and typical characteristics of ductile fracture. Corrosion resistance of laser zone was weaker than that of hybrid zone, which was closely related to the formation of martensite with poor corrosion resistance instead of austenite with good corrosion resistance.
1. Introduction Welding of high-strength low alloy steel and austenitic stainless steel dissimilar joint with large thickness has been investigated with increasing demands in a lot of industries including naval architecture and ocean engineering. Common used multi-layer multi-pass traditional arc welding technology was simplicity, mobility and versatility in joining materials with varying thickness, shape and physical properties [1]. However, owing to the existing disadvantages including high heat input, shallow welding penetration and low welding efficiency, it is more and more difficult for traditional arc welding to completely meet the growing manufacturing requirements in these industries [2]. Recently, the emerging welding methods including the narrow-gap arc welding and the electron beam welding provided new approaches to thick plate welding. The narrow-gap arc welding decreased the welding ⁎
passes and partly increased the welding efficiency by reducing the groove size, but the welding efficiency and quality were still dissatisfactory due to the shallow welding penetration and instability of the arc [3]. The electron beam welding possessed high energy density and low heat input, producing deep penetration and minimal distortion compared to arc welding. However, the requirement of vacuum environment greatly limited the adaptability and promotion of this technology [4]. Thus, it is necessary for the thick plate welding to develop more effective and adaptable welding technology. Laser welding was emerging as a promising welding technique for thick plate due to low heat input, deep welding penetration, high welding efficiency and adaptability [5]. The EH36 and 316L dissimilar joint with 6 mm thickness was joined by Cao et al. [6] and effects of welding speed on weld formation, microstructure and mechanical properties of the welds were studied in details. Rong et al. [7] realized
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.optlastec.2019.105878 Received 3 August 2018; Received in revised form 25 June 2019; Accepted 27 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105878
X. Zhang, et al.
adopted, and the welding head was tilted 5° along the welding direction to reduce the influences of high reflectivity and spatters. The 100% Ar was used as shielding gas with a flow rate of 25 L/min during the welding process. The metallurgical samples were extracted from the steady state region of the welds. Cross sections of the samples were ground and polished, following by etching with aqua regia for microstructural analysis. Microstructure of the welds was observed by Keyence VHX-1000C optical microscope and analysis of the crystallographic feature was conducted by FEI Sirion-200 scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) system. Distribution of chemical elements was analyzed by energy dispersive spectrum (EDS) test. Hardness was measured by DHV-1000 Vickers hardness machine with a test load of 300 g and a dwell period of 15 s. Tensile properties were measured by Shimadzu AG-100kN system and the fracture morphologies were further investigated by SEM. Electrochemical corrosion behavior were analyzed by CorrTest-CS310 workstation in a conventional three-electrode cell. All of the corrosion samples were processed into 5 mm × 5 mm × 5 mm and packaged in epoxy resin. Working surfaces of all samples were polished with abrasive paper and the corrosion tests were carried out in solution of 3.5 wt% NaCl at 30 °C with a scanning rate of 1.667 mV/s.
the joining of EH36 and 316L dissimilar joint with 4 mm thickness by laser welding process and researched the influence of misalignment on weld profile and alloy element distribution. Esfahani et al. [8] analyzed the effect of alloying composition on microstructure and mechanical performance of laser welded EH36 and 316L dissimilar joint with 0.9 mm thickness and proposed strategies to control the alloying composition of the welds. However, the existing researches of laser welding EH36 and 316L dissimilar joint mainly focused on the small thickness and the cracking tendency and porosity of laser welding were relatively high due to the quick cooling rate. Besides, dilution of alloying elements and diffusion of carbon during laser welding process resulted in formation of the secondary phase and deteriorated the performance of the welds. Though the laser welding with filler wire showed great advantages in reducing weld porosity and controlling elementary composition, the low welding efficiency caused by the shallow welding penetration and slow welding speed was less than satisfactory [9]. The hybrid laser-arc welding technology combined laser welding with arc welding and the two heat sources interacted in such a way to produce a single high intensity energy source and overcome problems such as shallow penetration, cracking, porosity and detrimental phase formation [10–12]. Joo et al. [13] joined the EH36 and 316L dissimilar joint with 13 mm thickness by optimized hybrid laser-arc welding process and further studied the fatigue property of the welds. However, the attention paid to joining EH36 and 316L dissimilar metal by hybrid laser-arc welding was not enough. The mechanism of microstructure formation and effects of microstructure on performance of the welds were still unclear. In this study, the hybrid laser-arc welding technology was adopted to weld the EH36 and 316L dissimilar joint with a thickness of 20 mm. Effects of welding parameters on weld formation were studied and the welding process was optimized. On this basis, element distribution, microstructure and performance of the welds were further investigated and relationships between element, microstructure and performance were established.
3. Results and discussion 3.1. Welding process The gap of the backing welding was narrowest among all layers and the required penetration of the backing welding was largest among all layers. Thus, realization of the backing welding was the hardest among all layers. A lot of previous work found that laser power and welding speed were the decisive factors of weld penetration and formation [15]. However, except for laser power and welding speed, the arc current could also directly influence the weld penetration and formation. In view of that numerous studies were focused on effects of laser power and welding speed, appearances of the backing welds under different arc current were analyzed to illustrate the effects of arc currents on weld formation. The experiment was conducted under a laser power of 4.0 kW, a welding speed of 1.0 m/min, a defocusing amount (Δf) of 0 mm and a distance between laser and arc (DLA) of 2 mm. Appearances of the welds under different arc currents were shown in Fig. 2. It was clear that the melting widths of the backside were not enough when the arc currents were 100A and 150A, resulting in the incomplete penetration as shown in Fig. 2a and the undercut as shown in Fig. 2b, respectively. Besides, excessive penetration and collapse occurred when the arc current reached 250A and acceptable weld formation could be obtained when the arc current was 200A. On this basis, the laser power of the subsequent layers decreased to 2.0 kW because of the reduced requirement of welding penetration and the arc current increased from lower layer to upper layer to adapt the growing width of the gap. As a consequence, the dissimilar joint of EH36 and 316L with 20 mm thickness was made by 5 layers hybrid laser-arc welding. The optimized welding parameters and the cross-sectional appearance of the welds were listed in Table 2.
2. Experimental procedure The base metals adopted in this study were the high-strength low alloy steel EH36 and the austenitic stainless steel 316L named by the ABS standard with dimensions of 250 mm × 150 mm × 20 mm. The 1.2 mm-diameter solid filler wire (ER316LSi) was used to fill the gap. Chemical composition of the base metals and the filler wire were given in Table 1. The hybrid laser-arc welding system was composed by an IPG YLR-4000 fiber laser system with a wavelength of 1070 nm and a maximum power output of 4000 W and a Fronius TransPuls Synergic4000 welding power source equipped with an ABB IRB-6400 robot. The narrow-gap Y-groove was prepared to obtain the full penetrated weld. As shown in Fig. 1, the groove angle, thickness of root face and width of platform was 20°, 5 mm and 2 mm, respectively. Owing to better reachability and flexibility of laser cleaning than that of mechanical abrading, the contamination and oxide which contributed to the formation of the interlayer defects including pores, inclusions and lack of fusion could be effectively cleared up by laser cleaning [14]. Thus, the laser cleaning method instead of the mechanical abrading was adopted in this study to preprocess the groove. Detailed parameters of laser cleaning were presented as follows: the laser power was 500 W, the pulse repetition rate was 100 kHz, the defocusing amount was 0 mm and the cleaning speed was 1.0 m/min. The laser leading mode was
3.2. Microstructure As shown in Fig. 3, it was clear that microstructure of 316L (Fig. 3a) consisted of massive blocky austenite with little ferrite distributing at the grain boundary while microstructure of EH36 (Fig. 3b) consisted of massive blocky ferrite with a small amount of pearlite. Due to lower thermal conductivity and higher phase-transition temperature of 316L than that of EH36, the degree of heating and recrystallization of 316L (Fig. 3c) and EH36 (Fig. 3d) nearby the fusion line were different, resulting in much narrower HAZ in 316L (maximum width of 80 μm) than in EH36 (maximum width of 900 μm). Besides, massive martensite
Table 1 Chemical compositions of base metal and filler wire (mass fraction: %).
EH36 316L Wire
Cr
Ni
Mo
Mn
Si
P
C
S
Cu
N
Fe
0.04 16.82 18.12
0.01 9.84 11.70
0.01 1.78 2.55
1.46 1.52 1.86
0.32 0.96 0.76
0.01 0.03 0.02
0.17 0.02 0.01
0.01 – 0.01
0.04 – 0.08
– – 0.05
Bal. Bal. Bal.
2
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Fig. 1. Schematic of: (a) the groove and (b) the welding process.
different phase composition between laser zone and hybrid zone was thought to be greatly influenced by the element distribution of Cr and Ni [17,18]. Thus, distributions of Cr and Ni across the laser zone and hybrid zone were analyzed to elucidate the cause of different phase composition between laser zone and hybrid zone. As presented in Fig. 5, it was clear that contents of Cr (square) and Ni (circle) across the laser zone and hybrid zone showed an common increasing trend from the EH36 side to the 316L side. However, contents of Cr and Ni in laser zone were much lower than that in 316L while contents of Cr and Ni in hybrid zone (black line) were approximately equal to 316L. Different contents of Cr and Ni between laser zone and hybrid zone was considered to be closely related to the percentage contribution of molten EH36 in laser zone and hybrid zone. Contents of alloying elements in EH36 was much lower than that of 316L, that’s to say, the molten EH36 diluted the concentration of alloying element in molten pool. Molten pool of laser zone was composed of EH36 and 316L while molten pool of hybrid zone contained not only EH36 and 316L but also a mass of molten filler wire which had the same chemical component as 316L. Therefore, addition of filler wire greatly reduced the percentage contribution of molten EH36 and maintained the concentration of alloying elements in the hybrid zone. Thus, the element contents of Cr and Ni in hybrid zone were much higher than that of laser zone, which led to uneven phase composition between laser zone and hybrid zone. In order to quantificationally study the relationship between element composition and microstructure, the element composition of laser zone and hybrid zone was measured and shown in Fig. 6. It was obvious that contents of Cr and Ni in laser zone were 12.50% and 7.20% respectively while contents of Cr and Ni in hybrid zone were 18.49% and
formed in HAZ of EH36 while no obvious phase transformation could be observed in HAZ of 316L since 316L was not transformable [16]. Owing to the large penetration of backing welding and the limited action range of arc, fusion zone of the multi-layer welds could be divided into two different zones, namely the laser zone at the downside and the hybrid zone at the upside. Microstructure of laser zone and hybrid zone was significantly different: microstructure of laser zone (Fig. 3e) consisted of massive acicular martensite while microstructure of hybrid zone (Fig. 3f) consisted of dendritic austenite from the fusion line to the weld center and blocky austenite at the weld center. Consequently, microstructure of the multi-layer welds was inhomogeneous, the phase composition and gain size of laser zone and hybrid zone showed significant differences. In order to further investigate the microstructural inhomogeneity of the welds, the phase composition and gain sizes of laser zone and hybrid zone were quantitatively analyzed by the EBSD test. As shown in Fig. 4a and b, microstructure of laser zone was much finer than that of hybrid zone. The average grain sizes of laser zone and hybrid zone were 1.5 μm2 and 7.6 μm2, respectively. As shown in Fig. 4c and d, the phase composition of laser zone was 37% austenite (the blue phase) and 63% martensite (the red phase) while the phase composition of hybrid zone was 96% austenite (the blue phase) and 4% ferrite (the red phase). Different grain sizes between the laser zone and the hybrid zone were considered to be closely related to different thermal cycles. The hybrid zone was heated by both laser and arc while the laser zone was only heated by laser. Thus, the heat input of hybrid zone was higher than that of laser zone, which resulted in lower cooling rate and larger grain size of hybrid zone than laser zone. In addition to the grain size,
Fig.2. Appearances of backing welds under: (a) 100A (b) 150A (c) 200A (d) 250A. 3
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Table 2 Optimized parameters and cross-sectional macro appearance of the welds. Layer
Laser power (kW)
Current (A)
Voltage (V)
Speed (m/min)
Δf (mm)
DLA (mm)
1 2 3 4 5
4.0 2.0 2.0 2.0 2.0
200 225 225 250 250
22.0 23.2 23.2 24.4 24.4
1.0
0
2
Weld appearance
was much higher than that of hybrid zone. Different hardness distributions between laser zone and hybrid zone were thought to be closely related to the formation of massive martensite which possessed higher hardness than austenite and ferrite. In addition to the phase composition, different grain sizes between laser zone and hybrid zone were the secondary cause of different hardness distributions. The average grain size of laser zone was much smaller than that of hybrid zone, resulting in smaller dislocation and higher harness of laser zone than hybrid zone [19]. Thus, different phase composition and grain sizes led to significantly different hardness distributions between laser zone and hybrid zone. Besides, tensile properties of laser zone (sample 1) and hybrid zone (sample 2) were also studied in details. Fig. 8 suggested that the necking and fracture of both sample 1 and sample 2 occurred at the EH36 side. The ultimate tensile strength of sample 1 and sample 2 were 441.3 MPa and 442.3 MPa, respectively. The tensile elongations of sample 1 and sample 2 were 29.7% and 29.6%, respectively. In addition, large amounts of cup-like dimples formed on the fracture surfaces of both sample 1 and sample 2, indicating typical characteristics of ductile fracture [20]. Thus, tensile properties of both laser zone and hybrid zone were better than that of EH36.
10.96% respectively. Thus, the proportion of Cr and Ni in laser zone was less than that in hybrid zone. Besides, the cooling rate of laser zone was higher than that of hybrid zone due to the lower heat input of laser zone than hybrid zone. Thus, the formation of ferrite and austenite in laser zone was inhibited which led to high formative tendency of martensite. Microstructure of hybrid zone consisted of austenite and ferrite while microstructure of laser zone preferentially transformed to massive martensite instead of austenite and ferrite. In conclusion, different element composition and thermal cycles between laser zone and hybrid zone resulted in different phase composition between laser zone and hybrid zone. 3.3. Mechanical properties In order to study the relationship between microstructure and mechanical properties of the welds, hardness distributions of laser zone and hybrid zone were measured and analyzed, respectively. As shown in Fig. 7, the average hardness values of EH36 and 316L were about 170 HV0.3 and 185 HV0.3, respectively. The average hardness value of hybrid zone was about 250 HV0.3 while the average hardness value of laser zone reached 350 HV0.3. Thus, hardness of both laser zone and hybrid zone were higher than the base metal and hardness of laser zone
Fig. 3. Microstructure of: (a) 316L, (b) EH36, (c) HAZ of 316L, (d) HAZ of EH36 (e) laser zone and (f) hybrid zone. 4
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Fig. 4. EBSD test: (a) grain size of laser zone; (b) grain size of hybrid zone; (c) phase composition of laser zone; (d) phase composition of hybrid zone.
Fig. 5. Distributions of Cr and Ni across laser zone and hybrid zone.
3.4. Corrosion behavior
Fig. 6. Element composition of laser zone and hybrid zone.
In view of the marine environment, the potentiodynamic polarization tests were conducted to research the corrosion behavior at different locations and illuminate the effects of microstructure on corrosion resistance. Test locations and potentiodynamic polarization curves of 316L, EH36, laser zone and hybrid zone were revealed in Fig. 9. It was clear that the corrosion behavior of laser zone showed similar characteristics with EH36 and the corrosion behavior of hybrid zone showed similar characteristics with 316L. In addition, the self-corrosion
potential (Ecorr), the corrosion current density (Icorr) and the corrosion rate (K) were fitted to evaluate the corrosion behavior of different zones, as presented in Table 3. Zhang et al. [21] demonstrated that lower Ecorr and higher Icorr corresponded to faster corrosion rate and poorer corrosion resistance. Accordingly, the corrosion resistance of 316L was the best of all zones while the corrosion resistance of EH36 was the worst of all zones. Moreover, the corrosion resistance of hybrid zone was much better than that of laser zone. Different corrosion 5
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Table 3 Self-corrosion potential, corrosion current densities and corrosion rates of different zones. Zone
Ecorr/V
Icorr/A·cm−2
K/mm·a−1
316L Hybrid zone Laser zone EH36
−0.21 −0.27 −0.39 −0.44
1.29 × 10−6 1.55 × 10−6 6.53 × 10−6 8.89 × 10−6
0.01 0.02 0.08 0.11
96% austenite and 4% ferrite. The corrosion resistances of ferrite and martensite were worse than that of austenite [22], leading to the decrease of corrosion resistance from 316L to hybrid zone to laser zone to EH36. 4. Conclusions (1) An accepted weld of EH36 and 316L dissimilar joint with 20 mm thickness was obtained by optimized narrow-gap hybrid laser-arc welding process. (2) Molten EH36 diluted the concentration of alloying element in molten pool. High melting rate of EH36 in laser zone led to the reduction of stabilized alloying element of austenite and ferrite, facilitating the formation of martensite in laser zone. Addition of the filler wire substantially reduced the melting rate of EH36 and led to the formation of massive austenite in hybrid zone. (3) Large amounts of martensite and small grain size resulted in higher harness of laser zone than that of hybrid zone. Tensile properties of the fusion zone were better than that of EH36 and showed typical characteristics of ductile fracture. (4) Corrosion resistances of hybrid zone and laser zone were better than that of EH36 but worse than that of 316L and corrosion resistance of hybrid zone was better than that of the laser zone, which was attributed to the increasing content of the perishable phase from EH36 to laser zone to hybrid zone to 316L.
Fig. 7. Hardness distributions across laser zone and hybrid zone.
Acknowledgments Fig. 8. Tensile properties and fracture morphologies of laser zone and hybrid zone.
This work was supported by the National Natural Science Foundation of China (Grant No. 51705173) and National Program on Key Basic Research Project (Grant No. 2014CB046703). We would like to express our deep gratitude to the Analysis and Test Center of HUST (Huazhong University of Science and Technology) and the State Key Laboratory of Material Processing and Die & Mould Technology of HUST, for their friendly cooperation. References [1] B.S. Huang, J. Yang, D.H. Lu, W.J. Bin, Study on the microstructure, mechanical properties and corrosion behaviour of S355JR/316L dissimilar welded joint prepared by gas tungsten arc welding multi-pass welding process, Sci. Technol. Weld. Joining 21 (5) (2016) 1–8. [2] R. Nivas, P.K. Singh, G. Das, S.K. Das, S. Kumar, B. Mahato, K. Sivaprasad, M. Ghosh, A comparative study on microstructure and mechanical properties near interface for dissimilar materials during conventional V-groove and narrow gap welding, J. Manuf. Processes 25 (2017) 274–283. [3] J. Feng, W. Guo, N. Irvine, L. Li, Understanding and elimination of process defects in narrow gap multi-pass fiber laser welding of ferritic steel sheets of 30mm thickness, Int. J. Adv. Manuf. Technol. 88 (5–8) (2016) 1–10. [4] D.J. Smith, G. Zheng, P.R. Hurrell, C.M. Gill, B.M.E. Pellereau, K. Ayres, D. Goudar, E. Kingston, Measured and predicted residual stresses in thick section electron beam welded steels, Int. J. Pressure Vessels Piping s 120–121 (1) (2014) 66–79. [5] M. Zhang, Z. Zhang, K. Tang, C. Mao, Y. Hu, G. Chen, Analysis of mechanisms of underfill in full penetration laser welding of thick stainless steel with a 10kW fiber laser, Opt. Laser Technol. 98 (Supplement C) (2018) 97–105. [6] C. Longchao, S. Xinyu, J. Ping, Z. Qi, R. Youmin, G. Shaoning, M. Gaoyang, Effects of welding speed on microstructure and mechanical property of fiber laser welded dissimilar butt joints between AISI316L and EH36, Metals 7 (7) (2017) 13 270. [7] Y. Rong, J. Xu, T. Lei, W. Wang, A.A. Sabbar, Y. Huang, C. Wang, Z. Chen, Microstructure and alloy element distribution of dissimilar joint 316L and EH36 in laser welding, Sci. Technol. Weld. Joining (2017) 1–8.
Fig. 9. Potentiodynamic polarization curves of different zones.
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