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Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint
ARTICLE IN PRESS Optics and Lasers in Engineering 47 (2009) 807–814
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint Chengwu Yao a,b,, Binshi Xu c, Xiancheng Zhang a, Jian Huang a,b, Jun Fu a,b, Yixiong Wu a,b a b c
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, China Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200030, China National Key Laboratory for Remanufacturing, Beijing 100072, China
a r t i c l e in fo
abstract
Article history: Received 25 September 2008 Received in revised form 12 December 2008 Accepted 2 February 2009 Available online 18 March 2009
Relatively the high reflectivity of copper to CO2 laser led to the difficulty in joining copper to steel using laser welding. In this paper, a new method was proposed to complete the copper–steel laser butt welding. The scarf joint geometry was used, i.e., the sides of the copper and steel were in obtuse and acute angles, respectively. During the welding process, the laser beam was fixed on the steel side and the dilution ratio of copper to steel was controlled by properly selecting the deviation of the laser beam. The offset of laser beam depended on the scarf angle between the copper and steel, the thickness of plate and the processing parameters used in the laser welding. The microstructure near the interface between Cu plate and the intermixing zone was investigated. Experimental results showed that for the welded joint with high dilution ratio of copper, there was a transition zone with numerous filler particles near the interface. However, if the dilution ratio of copper is low, the transition zone is only generated near the upper side of the interface. At the lower side of the interface, the turbulent bursting behavior in the welding pool led to the penetration of liquid metal into Cu. The welded joint with lower dilution ratio of copper in the fusion zone exhibited higher tensile strength. On the bases of the microstructural evaluation at the interface of the welded joint, a physical model was proposed to describe the formation mechanism of the dissimilar joint with low dilution ratio of copper. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Dissimilar joint Laser welding Dilution ratio Microstructural evaluation
1. Introduction In the power-generation industries, the copper–steel combinations have often been widely used due to their high electrical conductivity and stiffness. However, the joining of copper to steel has become a challenging task facing modern manufactures. The butt welding of copper to carbon steel belongs to joining of dissimilar metals. However, the obvious material mismatches, such as the chemical properties and thermomechanical properties, between the copper and carbon steel make the dissimilar welding of these materials difficult. Hence, it is difficult to achieve defect-free copper–steel dissimilar joints using the conventional methods, such as shielded metal arc, gas tungsten arc, gas metal arc, and submerged arc, etc. In the past decades, the electronbeam welding was often used in the fabrication of the dissimilar joints [1,2]. However, the quality of the joints was often influenced by the vacuum conditions used in the welding process. By comparing with electron-beam welding, the laser welding is not restricted by vacuum condition. Hence, using the laser welding to
Corresponding author.
E-mail address: [email protected] (C. Yao). 0143-8166/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.02.004
fabricate the dissimilar joints has been attractive in the recent years [3–5]. However, the detailed research on the microstructure and mechanical properties of the laser-welded copper–steel dissimilar joints were relatively few. Mai and Spowage [6] proposed a processing map in which butt joints were fabricated by focusing the laser beam of 0.2 mm into the steel. In such a case, most of the laser energy was absorbed in the steel and the amount of copper dissolved in the molten steel was very limited. However, the complete metallurgical bonding could not be achieved at the interface between copper plate and molten steel. Phanikumar et al. [7] have studied Fe–Cu dissimilar couple, and found the interface near the copper side was in the jagged shape. Using the commonly welded structure for copper–steel dissimilar joints, in which laser beam is focused on the center of butt welding joint, the following problems may occur. First, the reflectivity of copper to 10.6 mm wavelength CO2 laser is up to 98.4% [8], leading to the low absorptivity of copper. Second, the thermal expansion coefficient and thermal conductivity of copper are significantly higher than those of the low-carbon steel [8,9]. Hence, during the welding process, the large misfit strain and the residual stresses will be inevitably generated in the joint, leading to solidification cracking of it. Third, the porosity is a common defect originating from hydrogen which is highly soluble in liquid
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C. Yao et al. / Optics and Lasers in Engineering 47 (2009) 807–814
copper. Fourth, in laser welding, the high-temperature gradients may result in obvious phase transformation, leading to excessive hardness in the fusion zone. The modified structural design of the copper–steel dissimilar joint was necessary to avoid the occurrence of the above problems. In this paper, the butt welding of copper and low-carbon steel (i.e., E235A) using a continuous CO2 laser beam was investigated. A scarf structure was designed for Cu–Fe dissimilar joint. The fusion-control process of copper and the microstructure near the Cu–steel interface, and the mechanical properties of joints were experimentally investigated. The formation mechanism of the joint was described. The topics in this paper could provide some important insights on the development of designing methodology and process optimization for laser welding dissimilar joints.
2. Experimental procedures
porosity in the intermixing zone [11]. Hence, the cooling temperatures of Cu–Fe joints containing Cu lower than 7.2 wt% should be controlled from 1485 to 1538 1C, while those of the joints containing Cu higher than 96.5 wt% should be controlled from 1096 to 1084 1C. But, in the laser welding process, the amount of soluble hydrogen in liquid welding metal containing Cu higher than 96.5 wt% should be higher than the one containing Cu lower than 7.2 wt%. In addition, higher laser power should be used for the melting of joint with higher amount of Cu. Hence, controlling the dilution ratio of Cu to be lower than 7.2 wt% Cu is possible to improve the quality of the laser welded Cu–Fe dissimilar joint. Considering the high-temperature gradients in the joint through the thickness direction during the laser-welding process, the scarf geometry of the joint was used, as shown in Fig. 2, where a and b denote acute and obtuse angles at the sides of Fe and Cu plates, respectively, d is the offset of laser beam on the steel plate, and t denotes the defocusing amount. The sum of a and b angles is
2.1. Materials and structural design of joints The binary phase diagram of Fe–Cu is shown in Fig. 1 [10]. It can be seen that there exists one peritectic reaction point in the iron-rich side. The melting temperature of pure Fe is 1538 1C and the peritectic point is 1485 1C. At the peritectic point, dFe is in equilibrium with gFe containing 7.2 wt% Cu and L containing 11.5 wt% Cu. In the copper-rich side, there exists another peritectic reaction point (i.e., 1096 1C) existing, where the maximal solubility of Fe in Cu is about 3.5%. Generally, if the temperature interval between liquidus and solidus temperature is narrow, dwell time of liquid weld metal becomes relatively short. In such a case, it is possible to minimize the gas porosity since the gas is under restraint over a short time during weld metal solidification. In contrast, if the temperature interval is wide, a large amount of gas will be dissolved into the molten weld metals, leading to high porosity. Moreover, the solidification of the molten pool will be in mushy solidification mode in the subsequent cooling process, leading to the increment of the possibility of cracks and shrinkage
Fig. 1. Phase diagram of Fe–Cu.
d
Laser beam
α
β
Fig. 2. Scheme of butt welding by laser.
t
ARTICLE IN PRESS C. Yao et al. / Optics and Lasers in Engineering 47 (2009) 807–814
809
Table 1 Chemical compositions of the materials used (wt%). Name
C
Si
Mn
P
S
Ni
Cu
Fe
Total of
T1 copper E235A steel
– 0.14–0.22
– p0.30
– 0.30–0.65
p0.001 p0.045
p0.002 p0.050
p0.002 –
X99.95 –
p0.005 Bal.
p0.05 –
Table 2 Processing condition. Name
Thickness (mm)
Power (KW)
t (mm)
d (mm)
a (deg.)
S1 S2
7.0 10.0
8.0 11.0
3.0 4.0
0.5 1.0
84 85
kept to be 1801. To assure the full meting of Cu near the Cu–Fe interface, the magnitudes of a and b vary along with the thicknesses of the plates. The dilution ratios of the copper to steel are controlled by properly selecting the deviation of the laser beam to the Cu plate. In the paper, the E235A steel is used as the low-carbon steel in laser welding. The chemical compositions of the materials used are listed in Table 1.
(K) and Cu (K). It can be seen that there are a few gas pores and cracks existing in the intermixing zone between the Cu and Fe plates, as indicated by the arrows in Fig. 3a. The width of the intermixing zone in joint S1 ranges from 1819.8 to 2029.7 mm, indicating that the copper is obviously penetrated into the steel. However, in the intermixing zone of joint S2, the cracks and pores are almost eliminated, as shown in Fig. 3c. By comparing with joint S1, the intermixing zone in joint S2 is significantly narrow. Fig. 3d shows the EDS intensity profiles along the lines denoted in Fig. 3c. By comparing the results in Fig. 3b with those in Fig. 3d, it can be seen that the plate thickness and the laser power have important influences on the distribution of the Cu element in the intermixing zone. The dilution ratios of copper in joints S1 and S2 are listed in Table 3. It can be seen that the large amount of copper is dissolved in the molten steel in joint S1 during the laser welding.
2.2. Processing parameters used in laser welding 3.2. Interface microstructure A CO2 Laser system (Trumpf TCF15000) with a maximum output power of 15 kW and laser beam diameter of 0.7 mm was used. The pure helium used as protective gas for all the experiments was supplied by an inert-gas shroud. The processing parameters used on the laser-welding process were listed in Table 2. For the specimen with a code S1, the thicknesses of the Cu and Fe plates are 7.0 mm and the power of the laser used in the welding was 8 kW. For the specimen S2, the plate thickness was 10 mm and the laser power was 11 kW. 2.3. Characterization of the joints The laser welded joints were differentially etched with 4 pct Nital in the region near the interface and with 25 ml HCl+5 g FeCl3+100 ml H2O solution on the copper side to reveal the crosssectional microstructure through the lateral direction of the joint. The entire morphology of the weldment was observed by using optical microscopy (OM). The microstructure and the element distribution at the interface between Cu and Fe plates were characterized by using scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) analysis system. The microstructure in upper and lower sides at the interface near the Cu plate was also observed by using transmission electron microscopy (TEM). Thin discs with 3 mm in diameter are punched from the upper and the lower sides near the interface in the joint. The thicknesses of the samples were reduced by planar grinding, and then by using a dimple grinder and furthermore by ion milling with argon ions to prepare the samples for TEM observations.
3. Results and discussion 3.1. Dilution ratio of copper Fig. 3a shows the OM cross-sectional morphology of the joint S1, where the dash lines denote the locations for the EDS linescan, and Fig. 3b shows the corresponding intensity profiles for Fe
Fig. 4a shows a cross-sectional image of the microstructure near the interface between intermixing zone and Cu plate. Fig. 4b shows the higher-magnification view near the interface region. It can be seen that there is a transition zone near the Cu plate. A large amount of granular phases with the diameters around 0.1–8.0 mm are existed in the transition zone. The EDS analysis result shows the compositions of such phases are mainly Fe and Cu, as shown in Fig. 4c. According to Fe–Cu phase diagram, the component Fe may be the Fe-rich bcc solid solution (aFe) and Cu is the Cu-rich fcc solid solution (often called the e phase). For joint S2, the microstructure near the interface between the intermixing zone and Cu plate shows different feature, as shown in Fig. 5. Fig. 5a shows the microstructure in the upper side of the intermixing zone, which is plotted by the rectangular mark A in Fig. 3c. The transition zone filtered with a larger amount of granular phases is also found. However, by comparing with the granular phases shown in Fig. 4b, the dimensions of the phase in joint S2 are significantly lower. Fig. 5b shows the microstructure in the lower side of the intermixing zone, which is plotted by the rectangular mark B in Fig. 3c. Fig. 5c shows the highermagnification view near the interface region. It can be seen that the liquid metal spread along the pool boundary and penetrates into copper side with turbulent flow morphology. Hence, the turbulent bursting in the welding pool is generated during the laser-welding process. The similar phenomena can be seen elsewhere [12–19]. Fig. 6 show the distribution map of elements Fe, Cu near the interface in joint S2. At the lower side of the joint, the interface between Cu and intermixing zone is relatively smooth, while it appears in jagged shape at the upper side, as shown in Fig. 6a. The EDS mapping shows that only a little amount of Cu is diffused into the intermixing zone, as shown in Fig. 6b and c. This indicates that the dilution ratio of copper is very limited. Moreover, the amount of Cu decreases gradually from the lower side of intermixing zone to upper side of it, which indicates that the diffusion of Cu into the intermixing zone becomes difficult due to the generation of turbulent bursting in the welding pool.
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Fig. 3. Copper–steel joints welded by using laser focused on the steel side: (a) macrostructure of the joint S1, (b) Fe(K) and Cu(K) intensity profiles of the joint S1 by EDS line-scan, (c) macrostructure of the joint S2, and (d) Fe(K) and Cu(K) intensity profiles of the joint S2 by EDS line-scan.
Table 3 Dilution ratios of copper in weld (at%). Name
S1 S2
Position i
ii
iii
37.2 o1
36.5 o1
33.2 o1
Fig. 7 shows a TEM bright-field image at the interface between the Cu plate and intermixing zone in joint S2, where the insert shows the corresponding electron diffraction pattern. Fig. 7a shows the microstructure at the lower side of the interface and Fig. 7b shows the higher magnification of region plotted by the rectangular mark in Fig. 7a. The electron diffraction results indicate that the compositions of dark phase surrounded by copper matrix are mainly Fe. Fig. 8a shows the TEM bright-field image at the upper side of the interface between the Cu plate and intermixing zone in joint S2. A large amount of particles disperse in the copper matrix. The EDS analysis result shows that and the main composition in this region is also Fe, as shown in Fig. 8b.
3.3. Tensile properties Fig. 9a and b shows the stress–strain curves of joints S1 and S2, respectively. The tensile strengths of joints S1 and S2 are 168.5 and 233.4 N/mm2, and the elongations of them are 1.28% and 29.8%, respectively. Hence, the tensile strength of joint S2 is significantly higher than that of joint S1. Moreover, joint S1 is
broken in the intermixing zone and joint S2 is broken in the Cu site. The fracture surface morphologies of joints S1 and S2 are, respectively, shown in Fig. 10a and b. It can be seen that the failure of joint S1 is due to brittle fracture, since the fracture surface is relatively flat and perpendicular to the direction of the applied tensile stress. There are a few pores and cracks existing in the intermixing zone in joint S1 (as seen in Fig. 3a). During the tensile process, the stress concentrations near these micro-defects, leading to the crack propagation in the intermixing zone. However, on the fracture surface of joint S2, numerous dimples can be found, indicating that the joint is subjected to large plastic deformation prior to failure. Hence, the failure of joint S1 is due to ductile fracture.
3.4. Formation mechanism of interface The dilution ratio of Cu in the laser-welded copper–steel dissimilar joint can be controlled by properly selecting the processing parameters and the location of laser beam. The formation mechanism of the interface between the Cu plate and intermixing zone is discussed on the basis of Fe–Cu phase diagram and the theory of deep-penetration laser welding. Fig. 11 schematically shows the physical model that describes the formation process of the interface. The formation mechanism of the interface can be reflected by the following procedures, i.e., the formation of weld pool on steel side, the formation of the fusion zone on copper side, and the formation of the welding interface. It is well known that the high reflectivity of copper to laser makes the fabrication of laser-welded dissimilar Cu–Fe joints difficult. If the laser beam is fixed on the side of the steel plate with some distance from copper plate, the weld pool with keyhole will be generated inside the steel plate, as seen in Fig. 11a. Once
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Cu
811
α ε
Weld
Cu
Weld 80μm c:\edax32\genesis\genmaps.spc 18-Jul-2007 10:23:38 LSecs : 83
1.3
KCnt
1.1
Fe
0.8 Fe 0.5 0.3
Cu
Cu
0.0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Energy - keV Fig. 4. Microstructure of the weld interface of joint S1 on the copper side: (a) microstructure near the interface between intermixing zone and Cu plate, (b) highermagnification image of (a), and (c) EDS result of granular phase.
a
b Cu α
ε
Cu Weld
Weld 80μm
c
Cu
Weld
α
ε
Fig. 5. Microstructure of the weld interface of joint S2 on the copper side: (a) microstructure in the upper part of the intermixing zone, (b) microstructure in the lower part of the intermixing zone, and (c) microstructure of S2 near the interface region.
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a
b
c
FeK
CuK
Interface
Weld
Weld
Cu
Weld
Cu
Cu
Fig. 6. Distribution maps of the elements Fe, Cu near the interface in joint S2; t: (a) interface shape, (b) map scanning of Fe, and (c) map scanning of Cu.
a
b
312 222 110
111 200 111
[011] Cu [112]
Fe
Fig. 7. TEM bright-field image at the lower side of the interface between the Cu plate and intermixing zone in joint S2: (a) microstructure at the lower side of the interface, inset: SAED pattern of weld growing into copper matrix, and (b) magnification of (a), inset: SAED pattern of copper matrix.
Spectrum 3
Fe
Cu Fe
Cu Fe 0
Fe Cu 2
4
6
8
10
12
Full Scale 67 cts Cursor : -0.197 keV (0 cts)
14
16
18 keV
Fig. 8. TEM bright-field image at the upper side of the interface between the Cu plate and intermixing zone in joint S2: (a) microstructure at the upper side of the interface, and (b) analysis result of energy dispersive spectrometer (EDS).
ARTICLE IN PRESS C. Yao et al. / Optics and Lasers in Engineering 47 (2009) 807–814
the keyhole is generated, a large amount of heating will be absorbed since the laser beam is trapped inside the hole, leading to the increment of the heating output and width of the welding pool. In such a case, the copper near the welding pool will be melted and leads to the formation of a thin fusion zone, which is denoted by the shaded area in Fig. 11. Due to the temperature gradient in the welding pool, the widths of the fusion zones at the upper and lower sides are different. Moreover, at the bottom side of the interface between the Cu plate and intermixing zone, the turbulent-bursting behavior in the welding pool may lead to the penetration of Fe into Cu matrix. Due to the movement of the laser beam, the keyhole will be closed and the temperature of the weld pool decreases gradually. Since the thermal conductivity of copper is relatively high, the solidification of the liquid metal in the weld pool will start from the fusion zone on the copper side, leading to the formation of the interface between the Cu plate and intermixing zone.
813
The main reason is associated with slope butt joint and focusing the laser beam on the steel side. Temperature inhomogeneity in thickness can be avoided by using a new welding structure where the butt joint was slope, and dilution ratios between copper and steel in the weld can be controlled through focusing the laser beam proper deviation to the copper plate. A copper–steel dissimilar joint free of defects can be obtained when the amount of copper dissolved in the molten steel is very limited. The tensile properties of copper–steel dissimilar joint are excellent. The microstructure analyses of the weld interface with copper melting little show that there are transition zones with a large amount of gradual phases at the upper part of the interface and the weld metal with turbulent flow morphology at the lower part of the interface. The formation of the joint with copper melting little is made up of the formation of weld pool on the steel side, the formation of the fusion zone on the copper side and the formation of the interface between the Cu plate and intermixing zone.
4. Conclusions
Laser beam
A complete metallurgical bond was obtained at the interface between the copper plate and the steel plate in the present study.
Stress in N/mm
2
butt welding face 250 200 150 100 50 0
2
Cu plate
fusion zone 0
Stress in N/mm
Steel plate
joint S1
5
10
15 Strain in %
20
25
penetrating into copper
30
matrix
joint S2
250 200 150 100 50 0
Intermixing zone Steel plate 0
5
10
15 Strain in %
20
Fig. 9. Stress–strain curves of joints S1 and S2.
25
Cu plate
30 Fig. 11. Physical model of interface formation of joint welded by laser focused on steel side: (a) formation of weld pool on steel side, (b) formation of the fusion zone on copper side, and (c) formation of the welding interface.
Fig. 10. Fracture surface morphologies of joints S1 and S2: (a) S1, and (b) S2.
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Acknowledgement Funding support from Shanghai Key Laboratory of Materials Laser Processing and Modification No. 06DZ22102, it is gratefully acknowledged. References [1] Magnabosco I, Ferro P, Bonollo F, Arnberg L. An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel. Materials Science and Engineering A 2006;424(1–2):163–73. [2] Sun Z, Karppi R. The application of electron beam welding for the joining of dissimilar metals: an overview. Journal of Materials Processing Technology 1996;59(3):257–67. [3] Xiu-Bo Liu, Ming Pang, Zhen-Guo Zhang, et al. Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo. Optics and Lasers in Engineering 2007;45(9):929–34. [4] Jose´ RB, Wagner dR, Maurı´cio DMN, Ivan AA, Nilson DVJ. Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels. Optics and Lasers in Engineering 2007;45(9):960–6. [5] Alexandre M, Rajashekar S, Alexis D. Dissimilar material joining using laser (aluminum to steel using zinc-based filler wire). Optics and Laser Technology 2007;39(3):652–61. [6] Mai TA, Spowage AC. Characterisation of dissimilar joints in laser welding of steel–kovar, copper–steel and copper–aluminium. Materials Science and Engineering A 2004;374(1–2):224–33. [7] Phanikumar G, Manjini S, Dutta P, Mazumder J, Chattopadhyay K. Characterization of a continuous CO2 laser-welded Fe–Cu dissimilar couple. Metallurgical and Materials Transactions A 2005;36A:2137–47. [8] ASM International Handbook Committee. Properties and selection: Nonferrous alloys and special-purpose materials. Vol. 2. ASM Metal Handbook, 1992.
[9] ASM International Handbook Committee. Properties and selection: irons, steels, and high performance alloys. Vol. 1. ASM Metal Handbook, 1993. [10] ASM International Handbook Committee. Alloy phase diagrams. Vol. 3. ASM Metal Handbook, 1993. [11] ASM International Handbook Committee. Metallography and microstructures. Vol. 9. ASM Metal Handbook, 1993. [12] Chakraborty N, Chakraborty S. Modelling of turbulent molten pool convection in laser welding of a copper–nickel dissimilar couple. International Journal of Heat and Mass Transfer 2007;50:1805–22. [13] Campana G, Ascari A, Fortunato A, Tani G. Hybrid laser-MIG welding of aluminum alloys: The influence of shielding gases. Applied Surface Science 2009;255(10):5588–90. [14] Zhou Qi, Liu Fang-jun. Behaviors of element’s density distribution and melting metal flow in electron beam deep penetration welding. Transactions of the China Welding Institution 2001;22(5):17–20. [15] Chakraborty N, Chakraborty S. Thermal transport regimes and generalized regime diagram for high energy surface melting processes. Metallurgical and Materials Transactions B 2007;38(1):143–7. (5). [16] Chakraborty N, Chakraborty S. Distinct influences of turbulence in momentum, heat and mass transfers during melt pool convection in a typical laser surface alloying process. European Physical Journal: Applied Physics 2006;39:71–89. [17] Chatterjee D, Chakraborty S. Large eddy simulation of laser induced surface tension driven flow. Metallurgical and Materials Transactions B 2006;36B: 143–5. [18] Chakraborty N, Chatterjee D, Chakraborty S. A scaling analysis of turbulent transport in laser surface alloying process. Journal of Applied Physics 2004;96:4569–77. [19] Mohanraj P, Sarkar S, Chakraborty S, Dutta P. Three dimensional computational modelling of momentum, heat and mass transfer in laser surface alloying with distributed melting of alloying element. International Journal of Numerical Methods for Heat and Fluid Flow 2001;11:576–99.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint Chengwu Yao a,b,, Binshi Xu c, Xiancheng Zhang a, Jian Huang a,b, Jun Fu a,b, Yixiong Wu a,b a b c
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, China Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200030, China National Key Laboratory for Remanufacturing, Beijing 100072, China
a r t i c l e in fo
abstract
Article history: Received 25 September 2008 Received in revised form 12 December 2008 Accepted 2 February 2009 Available online 18 March 2009
Relatively the high reflectivity of copper to CO2 laser led to the difficulty in joining copper to steel using laser welding. In this paper, a new method was proposed to complete the copper–steel laser butt welding. The scarf joint geometry was used, i.e., the sides of the copper and steel were in obtuse and acute angles, respectively. During the welding process, the laser beam was fixed on the steel side and the dilution ratio of copper to steel was controlled by properly selecting the deviation of the laser beam. The offset of laser beam depended on the scarf angle between the copper and steel, the thickness of plate and the processing parameters used in the laser welding. The microstructure near the interface between Cu plate and the intermixing zone was investigated. Experimental results showed that for the welded joint with high dilution ratio of copper, there was a transition zone with numerous filler particles near the interface. However, if the dilution ratio of copper is low, the transition zone is only generated near the upper side of the interface. At the lower side of the interface, the turbulent bursting behavior in the welding pool led to the penetration of liquid metal into Cu. The welded joint with lower dilution ratio of copper in the fusion zone exhibited higher tensile strength. On the bases of the microstructural evaluation at the interface of the welded joint, a physical model was proposed to describe the formation mechanism of the dissimilar joint with low dilution ratio of copper. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Dissimilar joint Laser welding Dilution ratio Microstructural evaluation
1. Introduction In the power-generation industries, the copper–steel combinations have often been widely used due to their high electrical conductivity and stiffness. However, the joining of copper to steel has become a challenging task facing modern manufactures. The butt welding of copper to carbon steel belongs to joining of dissimilar metals. However, the obvious material mismatches, such as the chemical properties and thermomechanical properties, between the copper and carbon steel make the dissimilar welding of these materials difficult. Hence, it is difficult to achieve defect-free copper–steel dissimilar joints using the conventional methods, such as shielded metal arc, gas tungsten arc, gas metal arc, and submerged arc, etc. In the past decades, the electronbeam welding was often used in the fabrication of the dissimilar joints [1,2]. However, the quality of the joints was often influenced by the vacuum conditions used in the welding process. By comparing with electron-beam welding, the laser welding is not restricted by vacuum condition. Hence, using the laser welding to
Corresponding author.
E-mail address: [email protected] (C. Yao). 0143-8166/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.02.004
fabricate the dissimilar joints has been attractive in the recent years [3–5]. However, the detailed research on the microstructure and mechanical properties of the laser-welded copper–steel dissimilar joints were relatively few. Mai and Spowage [6] proposed a processing map in which butt joints were fabricated by focusing the laser beam of 0.2 mm into the steel. In such a case, most of the laser energy was absorbed in the steel and the amount of copper dissolved in the molten steel was very limited. However, the complete metallurgical bonding could not be achieved at the interface between copper plate and molten steel. Phanikumar et al. [7] have studied Fe–Cu dissimilar couple, and found the interface near the copper side was in the jagged shape. Using the commonly welded structure for copper–steel dissimilar joints, in which laser beam is focused on the center of butt welding joint, the following problems may occur. First, the reflectivity of copper to 10.6 mm wavelength CO2 laser is up to 98.4% [8], leading to the low absorptivity of copper. Second, the thermal expansion coefficient and thermal conductivity of copper are significantly higher than those of the low-carbon steel [8,9]. Hence, during the welding process, the large misfit strain and the residual stresses will be inevitably generated in the joint, leading to solidification cracking of it. Third, the porosity is a common defect originating from hydrogen which is highly soluble in liquid
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C. Yao et al. / Optics and Lasers in Engineering 47 (2009) 807–814
copper. Fourth, in laser welding, the high-temperature gradients may result in obvious phase transformation, leading to excessive hardness in the fusion zone. The modified structural design of the copper–steel dissimilar joint was necessary to avoid the occurrence of the above problems. In this paper, the butt welding of copper and low-carbon steel (i.e., E235A) using a continuous CO2 laser beam was investigated. A scarf structure was designed for Cu–Fe dissimilar joint. The fusion-control process of copper and the microstructure near the Cu–steel interface, and the mechanical properties of joints were experimentally investigated. The formation mechanism of the joint was described. The topics in this paper could provide some important insights on the development of designing methodology and process optimization for laser welding dissimilar joints.
2. Experimental procedures
porosity in the intermixing zone [11]. Hence, the cooling temperatures of Cu–Fe joints containing Cu lower than 7.2 wt% should be controlled from 1485 to 1538 1C, while those of the joints containing Cu higher than 96.5 wt% should be controlled from 1096 to 1084 1C. But, in the laser welding process, the amount of soluble hydrogen in liquid welding metal containing Cu higher than 96.5 wt% should be higher than the one containing Cu lower than 7.2 wt%. In addition, higher laser power should be used for the melting of joint with higher amount of Cu. Hence, controlling the dilution ratio of Cu to be lower than 7.2 wt% Cu is possible to improve the quality of the laser welded Cu–Fe dissimilar joint. Considering the high-temperature gradients in the joint through the thickness direction during the laser-welding process, the scarf geometry of the joint was used, as shown in Fig. 2, where a and b denote acute and obtuse angles at the sides of Fe and Cu plates, respectively, d is the offset of laser beam on the steel plate, and t denotes the defocusing amount. The sum of a and b angles is
2.1. Materials and structural design of joints The binary phase diagram of Fe–Cu is shown in Fig. 1 [10]. It can be seen that there exists one peritectic reaction point in the iron-rich side. The melting temperature of pure Fe is 1538 1C and the peritectic point is 1485 1C. At the peritectic point, dFe is in equilibrium with gFe containing 7.2 wt% Cu and L containing 11.5 wt% Cu. In the copper-rich side, there exists another peritectic reaction point (i.e., 1096 1C) existing, where the maximal solubility of Fe in Cu is about 3.5%. Generally, if the temperature interval between liquidus and solidus temperature is narrow, dwell time of liquid weld metal becomes relatively short. In such a case, it is possible to minimize the gas porosity since the gas is under restraint over a short time during weld metal solidification. In contrast, if the temperature interval is wide, a large amount of gas will be dissolved into the molten weld metals, leading to high porosity. Moreover, the solidification of the molten pool will be in mushy solidification mode in the subsequent cooling process, leading to the increment of the possibility of cracks and shrinkage
Fig. 1. Phase diagram of Fe–Cu.
d
Laser beam
α
β
Fig. 2. Scheme of butt welding by laser.
t
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Table 1 Chemical compositions of the materials used (wt%). Name
C
Si
Mn
P
S
Ni
Cu
Fe
Total of
T1 copper E235A steel
– 0.14–0.22
– p0.30
– 0.30–0.65
p0.001 p0.045
p0.002 p0.050
p0.002 –
X99.95 –
p0.005 Bal.
p0.05 –
Table 2 Processing condition. Name
Thickness (mm)
Power (KW)
t (mm)
d (mm)
a (deg.)
S1 S2
7.0 10.0
8.0 11.0
3.0 4.0
0.5 1.0
84 85
kept to be 1801. To assure the full meting of Cu near the Cu–Fe interface, the magnitudes of a and b vary along with the thicknesses of the plates. The dilution ratios of the copper to steel are controlled by properly selecting the deviation of the laser beam to the Cu plate. In the paper, the E235A steel is used as the low-carbon steel in laser welding. The chemical compositions of the materials used are listed in Table 1.
(K) and Cu (K). It can be seen that there are a few gas pores and cracks existing in the intermixing zone between the Cu and Fe plates, as indicated by the arrows in Fig. 3a. The width of the intermixing zone in joint S1 ranges from 1819.8 to 2029.7 mm, indicating that the copper is obviously penetrated into the steel. However, in the intermixing zone of joint S2, the cracks and pores are almost eliminated, as shown in Fig. 3c. By comparing with joint S1, the intermixing zone in joint S2 is significantly narrow. Fig. 3d shows the EDS intensity profiles along the lines denoted in Fig. 3c. By comparing the results in Fig. 3b with those in Fig. 3d, it can be seen that the plate thickness and the laser power have important influences on the distribution of the Cu element in the intermixing zone. The dilution ratios of copper in joints S1 and S2 are listed in Table 3. It can be seen that the large amount of copper is dissolved in the molten steel in joint S1 during the laser welding.
2.2. Processing parameters used in laser welding 3.2. Interface microstructure A CO2 Laser system (Trumpf TCF15000) with a maximum output power of 15 kW and laser beam diameter of 0.7 mm was used. The pure helium used as protective gas for all the experiments was supplied by an inert-gas shroud. The processing parameters used on the laser-welding process were listed in Table 2. For the specimen with a code S1, the thicknesses of the Cu and Fe plates are 7.0 mm and the power of the laser used in the welding was 8 kW. For the specimen S2, the plate thickness was 10 mm and the laser power was 11 kW. 2.3. Characterization of the joints The laser welded joints were differentially etched with 4 pct Nital in the region near the interface and with 25 ml HCl+5 g FeCl3+100 ml H2O solution on the copper side to reveal the crosssectional microstructure through the lateral direction of the joint. The entire morphology of the weldment was observed by using optical microscopy (OM). The microstructure and the element distribution at the interface between Cu and Fe plates were characterized by using scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) analysis system. The microstructure in upper and lower sides at the interface near the Cu plate was also observed by using transmission electron microscopy (TEM). Thin discs with 3 mm in diameter are punched from the upper and the lower sides near the interface in the joint. The thicknesses of the samples were reduced by planar grinding, and then by using a dimple grinder and furthermore by ion milling with argon ions to prepare the samples for TEM observations.
3. Results and discussion 3.1. Dilution ratio of copper Fig. 3a shows the OM cross-sectional morphology of the joint S1, where the dash lines denote the locations for the EDS linescan, and Fig. 3b shows the corresponding intensity profiles for Fe
Fig. 4a shows a cross-sectional image of the microstructure near the interface between intermixing zone and Cu plate. Fig. 4b shows the higher-magnification view near the interface region. It can be seen that there is a transition zone near the Cu plate. A large amount of granular phases with the diameters around 0.1–8.0 mm are existed in the transition zone. The EDS analysis result shows the compositions of such phases are mainly Fe and Cu, as shown in Fig. 4c. According to Fe–Cu phase diagram, the component Fe may be the Fe-rich bcc solid solution (aFe) and Cu is the Cu-rich fcc solid solution (often called the e phase). For joint S2, the microstructure near the interface between the intermixing zone and Cu plate shows different feature, as shown in Fig. 5. Fig. 5a shows the microstructure in the upper side of the intermixing zone, which is plotted by the rectangular mark A in Fig. 3c. The transition zone filtered with a larger amount of granular phases is also found. However, by comparing with the granular phases shown in Fig. 4b, the dimensions of the phase in joint S2 are significantly lower. Fig. 5b shows the microstructure in the lower side of the intermixing zone, which is plotted by the rectangular mark B in Fig. 3c. Fig. 5c shows the highermagnification view near the interface region. It can be seen that the liquid metal spread along the pool boundary and penetrates into copper side with turbulent flow morphology. Hence, the turbulent bursting in the welding pool is generated during the laser-welding process. The similar phenomena can be seen elsewhere [12–19]. Fig. 6 show the distribution map of elements Fe, Cu near the interface in joint S2. At the lower side of the joint, the interface between Cu and intermixing zone is relatively smooth, while it appears in jagged shape at the upper side, as shown in Fig. 6a. The EDS mapping shows that only a little amount of Cu is diffused into the intermixing zone, as shown in Fig. 6b and c. This indicates that the dilution ratio of copper is very limited. Moreover, the amount of Cu decreases gradually from the lower side of intermixing zone to upper side of it, which indicates that the diffusion of Cu into the intermixing zone becomes difficult due to the generation of turbulent bursting in the welding pool.
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Fig. 3. Copper–steel joints welded by using laser focused on the steel side: (a) macrostructure of the joint S1, (b) Fe(K) and Cu(K) intensity profiles of the joint S1 by EDS line-scan, (c) macrostructure of the joint S2, and (d) Fe(K) and Cu(K) intensity profiles of the joint S2 by EDS line-scan.
Table 3 Dilution ratios of copper in weld (at%). Name
S1 S2
Position i
ii
iii
37.2 o1
36.5 o1
33.2 o1
Fig. 7 shows a TEM bright-field image at the interface between the Cu plate and intermixing zone in joint S2, where the insert shows the corresponding electron diffraction pattern. Fig. 7a shows the microstructure at the lower side of the interface and Fig. 7b shows the higher magnification of region plotted by the rectangular mark in Fig. 7a. The electron diffraction results indicate that the compositions of dark phase surrounded by copper matrix are mainly Fe. Fig. 8a shows the TEM bright-field image at the upper side of the interface between the Cu plate and intermixing zone in joint S2. A large amount of particles disperse in the copper matrix. The EDS analysis result shows that and the main composition in this region is also Fe, as shown in Fig. 8b.
3.3. Tensile properties Fig. 9a and b shows the stress–strain curves of joints S1 and S2, respectively. The tensile strengths of joints S1 and S2 are 168.5 and 233.4 N/mm2, and the elongations of them are 1.28% and 29.8%, respectively. Hence, the tensile strength of joint S2 is significantly higher than that of joint S1. Moreover, joint S1 is
broken in the intermixing zone and joint S2 is broken in the Cu site. The fracture surface morphologies of joints S1 and S2 are, respectively, shown in Fig. 10a and b. It can be seen that the failure of joint S1 is due to brittle fracture, since the fracture surface is relatively flat and perpendicular to the direction of the applied tensile stress. There are a few pores and cracks existing in the intermixing zone in joint S1 (as seen in Fig. 3a). During the tensile process, the stress concentrations near these micro-defects, leading to the crack propagation in the intermixing zone. However, on the fracture surface of joint S2, numerous dimples can be found, indicating that the joint is subjected to large plastic deformation prior to failure. Hence, the failure of joint S1 is due to ductile fracture.
3.4. Formation mechanism of interface The dilution ratio of Cu in the laser-welded copper–steel dissimilar joint can be controlled by properly selecting the processing parameters and the location of laser beam. The formation mechanism of the interface between the Cu plate and intermixing zone is discussed on the basis of Fe–Cu phase diagram and the theory of deep-penetration laser welding. Fig. 11 schematically shows the physical model that describes the formation process of the interface. The formation mechanism of the interface can be reflected by the following procedures, i.e., the formation of weld pool on steel side, the formation of the fusion zone on copper side, and the formation of the welding interface. It is well known that the high reflectivity of copper to laser makes the fabrication of laser-welded dissimilar Cu–Fe joints difficult. If the laser beam is fixed on the side of the steel plate with some distance from copper plate, the weld pool with keyhole will be generated inside the steel plate, as seen in Fig. 11a. Once
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Cu
811
α ε
Weld
Cu
Weld 80μm c:\edax32\genesis\genmaps.spc 18-Jul-2007 10:23:38 LSecs : 83
1.3
KCnt
1.1
Fe
0.8 Fe 0.5 0.3
Cu
Cu
0.0 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Energy - keV Fig. 4. Microstructure of the weld interface of joint S1 on the copper side: (a) microstructure near the interface between intermixing zone and Cu plate, (b) highermagnification image of (a), and (c) EDS result of granular phase.
a
b Cu α
ε
Cu Weld
Weld 80μm
c
Cu
Weld
α
ε
Fig. 5. Microstructure of the weld interface of joint S2 on the copper side: (a) microstructure in the upper part of the intermixing zone, (b) microstructure in the lower part of the intermixing zone, and (c) microstructure of S2 near the interface region.
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a
b
c
FeK
CuK
Interface
Weld
Weld
Cu
Weld
Cu
Cu
Fig. 6. Distribution maps of the elements Fe, Cu near the interface in joint S2; t: (a) interface shape, (b) map scanning of Fe, and (c) map scanning of Cu.
a
b
312 222 110
111 200 111
[011] Cu [112]
Fe
Fig. 7. TEM bright-field image at the lower side of the interface between the Cu plate and intermixing zone in joint S2: (a) microstructure at the lower side of the interface, inset: SAED pattern of weld growing into copper matrix, and (b) magnification of (a), inset: SAED pattern of copper matrix.
Spectrum 3
Fe
Cu Fe
Cu Fe 0
Fe Cu 2
4
6
8
10
12
Full Scale 67 cts Cursor : -0.197 keV (0 cts)
14
16
18 keV
Fig. 8. TEM bright-field image at the upper side of the interface between the Cu plate and intermixing zone in joint S2: (a) microstructure at the upper side of the interface, and (b) analysis result of energy dispersive spectrometer (EDS).
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the keyhole is generated, a large amount of heating will be absorbed since the laser beam is trapped inside the hole, leading to the increment of the heating output and width of the welding pool. In such a case, the copper near the welding pool will be melted and leads to the formation of a thin fusion zone, which is denoted by the shaded area in Fig. 11. Due to the temperature gradient in the welding pool, the widths of the fusion zones at the upper and lower sides are different. Moreover, at the bottom side of the interface between the Cu plate and intermixing zone, the turbulent-bursting behavior in the welding pool may lead to the penetration of Fe into Cu matrix. Due to the movement of the laser beam, the keyhole will be closed and the temperature of the weld pool decreases gradually. Since the thermal conductivity of copper is relatively high, the solidification of the liquid metal in the weld pool will start from the fusion zone on the copper side, leading to the formation of the interface between the Cu plate and intermixing zone.
813
The main reason is associated with slope butt joint and focusing the laser beam on the steel side. Temperature inhomogeneity in thickness can be avoided by using a new welding structure where the butt joint was slope, and dilution ratios between copper and steel in the weld can be controlled through focusing the laser beam proper deviation to the copper plate. A copper–steel dissimilar joint free of defects can be obtained when the amount of copper dissolved in the molten steel is very limited. The tensile properties of copper–steel dissimilar joint are excellent. The microstructure analyses of the weld interface with copper melting little show that there are transition zones with a large amount of gradual phases at the upper part of the interface and the weld metal with turbulent flow morphology at the lower part of the interface. The formation of the joint with copper melting little is made up of the formation of weld pool on the steel side, the formation of the fusion zone on the copper side and the formation of the interface between the Cu plate and intermixing zone.
4. Conclusions
Laser beam
A complete metallurgical bond was obtained at the interface between the copper plate and the steel plate in the present study.
Stress in N/mm
2
butt welding face 250 200 150 100 50 0
2
Cu plate
fusion zone 0
Stress in N/mm
Steel plate
joint S1
5
10
15 Strain in %
20
25
penetrating into copper
30
matrix
joint S2
250 200 150 100 50 0
Intermixing zone Steel plate 0
5
10
15 Strain in %
20
Fig. 9. Stress–strain curves of joints S1 and S2.
25
Cu plate
30 Fig. 11. Physical model of interface formation of joint welded by laser focused on steel side: (a) formation of weld pool on steel side, (b) formation of the fusion zone on copper side, and (c) formation of the welding interface.
Fig. 10. Fracture surface morphologies of joints S1 and S2: (a) S1, and (b) S2.
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Acknowledgement Funding support from Shanghai Key Laboratory of Materials Laser Processing and Modification No. 06DZ22102, it is gratefully acknowledged. References [1] Magnabosco I, Ferro P, Bonollo F, Arnberg L. An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel. Materials Science and Engineering A 2006;424(1–2):163–73. [2] Sun Z, Karppi R. The application of electron beam welding for the joining of dissimilar metals: an overview. Journal of Materials Processing Technology 1996;59(3):257–67. [3] Xiu-Bo Liu, Ming Pang, Zhen-Guo Zhang, et al. Characteristics of deep penetration laser welding of dissimilar metal Ni-based cast superalloy K418 and alloy steel 42CrMo. Optics and Lasers in Engineering 2007;45(9):929–34. [4] Jose´ RB, Wagner dR, Maurı´cio DMN, Ivan AA, Nilson DVJ. Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels. Optics and Lasers in Engineering 2007;45(9):960–6. [5] Alexandre M, Rajashekar S, Alexis D. Dissimilar material joining using laser (aluminum to steel using zinc-based filler wire). Optics and Laser Technology 2007;39(3):652–61. [6] Mai TA, Spowage AC. Characterisation of dissimilar joints in laser welding of steel–kovar, copper–steel and copper–aluminium. Materials Science and Engineering A 2004;374(1–2):224–33. [7] Phanikumar G, Manjini S, Dutta P, Mazumder J, Chattopadhyay K. Characterization of a continuous CO2 laser-welded Fe–Cu dissimilar couple. Metallurgical and Materials Transactions A 2005;36A:2137–47. [8] ASM International Handbook Committee. Properties and selection: Nonferrous alloys and special-purpose materials. Vol. 2. ASM Metal Handbook, 1992.
[9] ASM International Handbook Committee. Properties and selection: irons, steels, and high performance alloys. Vol. 1. ASM Metal Handbook, 1993. [10] ASM International Handbook Committee. Alloy phase diagrams. Vol. 3. ASM Metal Handbook, 1993. [11] ASM International Handbook Committee. Metallography and microstructures. Vol. 9. ASM Metal Handbook, 1993. [12] Chakraborty N, Chakraborty S. Modelling of turbulent molten pool convection in laser welding of a copper–nickel dissimilar couple. International Journal of Heat and Mass Transfer 2007;50:1805–22. [13] Campana G, Ascari A, Fortunato A, Tani G. Hybrid laser-MIG welding of aluminum alloys: The influence of shielding gases. Applied Surface Science 2009;255(10):5588–90. [14] Zhou Qi, Liu Fang-jun. Behaviors of element’s density distribution and melting metal flow in electron beam deep penetration welding. Transactions of the China Welding Institution 2001;22(5):17–20. [15] Chakraborty N, Chakraborty S. Thermal transport regimes and generalized regime diagram for high energy surface melting processes. Metallurgical and Materials Transactions B 2007;38(1):143–7. (5). [16] Chakraborty N, Chakraborty S. Distinct influences of turbulence in momentum, heat and mass transfers during melt pool convection in a typical laser surface alloying process. European Physical Journal: Applied Physics 2006;39:71–89. [17] Chatterjee D, Chakraborty S. Large eddy simulation of laser induced surface tension driven flow. Metallurgical and Materials Transactions B 2006;36B: 143–5. [18] Chakraborty N, Chatterjee D, Chakraborty S. A scaling analysis of turbulent transport in laser surface alloying process. Journal of Applied Physics 2004;96:4569–77. [19] Mohanraj P, Sarkar S, Chakraborty S, Dutta P. Three dimensional computational modelling of momentum, heat and mass transfer in laser surface alloying with distributed melting of alloying element. International Journal of Numerical Methods for Heat and Fluid Flow 2001;11:576–99.