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Porosity and liquation cracking of dissimilar Nd:YAG laser welding of SUS304 stainless steel to T2 copper
Optics and Laser Technology 122 (2020) 105881
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Porosity and liquation cracking of dissimilar Nd:YAG laser welding of SUS304 stainless steel to T2 copper Jianmin Lia, Yuanzheng Caia, Fei Yanb, a b
⁎,1
, Chunming Wanga,
⁎,1
T
, Zhengwu Zhua, Chongjing Hua
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Hubei Key Laboratory of Advanced Technology for Automotive Components (Wuhan University of Technology), Wuhan 430070, China
H I GH L IG H T S
mechanism of laser welding defects in stainless steel to copper was investigated. • The model is proposed to illustrate the evolution process of liquation cracking. • APossible solutions were advised to inhibit laser welded defects in dissimilar metals. • It had a good industrial applicability in allowing high quality and efficiency. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Stainless steel/copper dissimilar materials Liquation cracking Polygonal porosity Grain boundary liquation Composition segregation
In this study, we present experimental laser welding SUS304 stainless steel to T2 copper. The forming mechanism of fusion zone polygonal porosity and heat affected zone (HAZ) liquation cracking were systemically investigated and possible solutions to the welding process were also proposed. We deemed that the generation of HAZ liquation cracking mainly underwent three stages: crack incubation, crack initiation and crack growth. Formation of HAZ liquation cracking was closely related to precipitation of Cu-Fe compounds at grain boundaries and grain boundary liquation. The occurrence of porosity was determined by the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. The susceptibility to HAZ liquation cracking can be effectively lowered by controlling the heat input during laser welding. In addition, the porosity in the fusion zone can be eliminated by reasonably adjusting laser deflection angle in the experiments, which greatly improved the microstructure and mechanical properties of welded joints.
1. Introduction In recent years, we have observed growing interest for the application of bi-metallic systems to realize material optimisation, weight reduction and durable product operation under special service conditions [1]. Among these, the hybrid structures of stainless steel and copper have attracted much attention and been extensively applied in power generation industries [2], nuclear industries [3], chemical and automobile sectors [4] on account of their high electrical and thermal conductivity and stiffness. Nevertheless, the discrepancy in physical and chemical properties between stainless steel and copper makes it very difficult to achieve a defect-free welded joint between them. Aiming to achieving a strong joint, widely spread welding methods including arc welding [5], explosive welding [6], induction welding
[1], friction stir welding [7] and so on, were attempted by many researchers to achieve the combination of stainless steel and copper. All of these techniques have some advantages and disadvantages, such as low productivity, high energy consumption, formation of intermetallic layers at interface and narrow applicability scope. In particular, the growth of intermetallic layers at high temperature is likely to induce the generation of intermetallic compounds at interface during welding of stainless steel/copper dissimilar materials, which can reduce the performance of welded joints seriously [8]. Compared to conventional arc welding, laser welding has been paid extensive attention to the joining of stainless steel to copper due to its lower heat input, smaller deformation, higher energy density and so on. At present, considerable research on the joining of stainless steel to copper concentrated mainly upon microstructures and mechanical
⁎
Corresponding authors. E-mail addresses: [email protected] (F. Yan), [email protected] (C. Wang). 1 Fei Yan and Chunming Wang contributed equally to this work. https://doi.org/10.1016/j.optlastec.2019.105881 Received 4 July 2019; Received in revised form 9 September 2019; Accepted 29 September 2019 Available online 19 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105881
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sandpaper. Before laser welding, the acetone was also used to remove dirt on the surface of specimens. The schematic of the laser welding setup is shown in Fig. 1. A high-power multimode fiber laser with a maximum laser power of 3 kW, RFL-C3000, was employed for welding under shielding with pure argon gas at 16.7 L/min flow rate. The specific parameters of laser were as follows: a focal length of 300 mm, a focus diameter of 0.3 mm and a wavelength of 1070 nm. Welding was conducted on SUS304 stainless steel/T2 copper using three variables: laser deflection angle, welding speed and focal point position. Process variables of continuous laser welding are listed in Table 2. In the experiments, the following symbols were defined for the sake of research convenience: “+”represents that laser beam was deflected to one side of the steel plate while “—” means that laser beam was biased to one side of the aluminum plate. Welded samples were sectioned transversely into three samples using a line cut machine. Metallographic samples were prepared by means of hot mounting press. All samples were carefully shaped using different types of sandpaper, then polished and etched with different etchants. The copper side was etched with nitric acid diluent including 50% HNO3 mixed with 50% distilled water. After that, the steel side was etched with the etchant involving 10% nitric acid, 20% hydrofluoric acid and 70% distilled water. Metallographic examinations were first conducted to examine microstructural features near the steel/ copper interface. Morphologies and the element distribution were also observed using a scanning electron microscope (SEM) associated with energy diffraction spectrum (EDS). The phase constitution of the joint was then analyzed by means of X-ray diffractometer (XRD). Based on the evolutional characteristics of welded defects, possible solutions for welded defects were also proposed in this study.
propertes. Chen et al. [9] found that intergranular liquid metal penetration was harmful to the integrity of welded structures, and suggested building an interfacial ferrite band to inhibit intergranular liquid Cu penetration of solid steel. Yao et al. [10] applied a scarf joint geometry to conduct the copper-steel CO2-laser welding and controlled the melting ratio of copper to steel by befittingly choosing the deviation of the laser beam. Suga et al. [11] investigated laser brazing of stainless steel/copper lap joints using Cu–Si-based filler metals, and improved the performance of welded joints by increasing the bonding width. Meng et al. [12] performed laser-arc hybrid welding stainless steel/ copper, and concluded that macrosegregations of Fe-rich peninsula and islands in the fusion zone worsened the weld performance while the Curich matrix and inner uniformly distributed Fe-rich particles strengthened the weld. There are certain tendencies that lead to welding defects in the weld such as shrinkage, porosity, cracks etc., when welding stainless steel/ copper dissimilar materials. The forming reasons and suppressing methods of welding defects have been initially discussed by other researchers. Magnabosco et al. [13] thought that porosity and microfissures in welded joints were attributed to the process and geometry parameters. Dinda et al. [14] considered that the average size and number of the pores in the welded joints were associated with welding, and the presence of hydrogen gas could be responsible for porosity formation in EBW dissimilar joints. Kar et al. [15] found that the application of beam oscillation led to less porosity and the average pore size in electron beam welded dissimilar copper–304SS joints. Further, it was observed that an optimum beam oscillation diameter can effectively control the porosity formation in the weld joint. Shen and Gupta [16] proved that the solidification cracking had close relation with the composition of the weld depending on the ratio of copper and stainless steel. Chen et al. [17] demonstrated that the occurrence of the microcracks was closely associated with the quantity of fused copper during the laser welding, and liquid separation and formation of microcracks in the fusion zone were as the result of superfluous copper melting. Baghjari et al. [18] performed dissimilar joining of SS 420 to Kovar alloy using a pulsed Nd:YAG laser welding and proved that offsetting laser beam position to SS 420 base metal can promote the susceptibility of fusion zone to cracking. Previous studies showed porosity and cracks were important factors that affected the quality and performance of welded joints. Although investigation on porosity and cracks of a copper/steel dissimilar joint was systematically conducted by laser welding, the forming mechanism of welded defects has been quite unclear. Therefore, it is very imperative to clarify the mechanism of welded defects and inhibit them effectively so as to achieve an acceptable weld with free of visible defect and good mechanical properties. In this study, the butt welding of copper and stainless steel using a continuous fiber laser beam was detailedly described. Laser deflection angle was effectively performed during the process of laser welding stainless steel to copper in order to reduce the defects and improve the quality of the weld. The forming mechanism of laser welding defects such as porosity and liquation cracking, was deeply investigated using a scanning electron microscope (SEM), energy diffraction spectrum (EDS) and X-ray diffractometer (XRD). A novel model was graphically proposed to expound the forming process of HAZ liquation cracking. Possible solutions for welded defects were carefully discussed with the purpose of suppressing the porosity and cracking in the weld and achieving a joint with good weld formation and performance.
3. Results and discussion 3.1. Morphology characteristics of the defects The microstructure of weld metal and heat affected zone (HAZ) in the joint can be observed through scanning electron microscope. Microstructural examination of welded samples displayed the occurrence of fusion zone porosity and HAZ liquation cracking. The weldmetal porosity observed in the experiment usually occurs in the fusion zone independently of HAZ liquation cracking in Fig. 2a. The fusion zone porosity with irregular morphology was detected in the lower weld where the mixed structure involving spherical structures and small columnar structures formed at the final solidification stage of weld pool in Fig. 2b. The area of the porosity approximately equals to 926.81 μm2. These local uneven structures are mainly associated with the segregation of the alloying elements. According to Fe-Cu alloy phase diagram, the Fe-rich phase happens at temperatures ranging from 1535 °C to 1083 °C. However, during the process of solidification, the precipitation of iron element alters the composition of the molten pool, leading to the occurrence of the Cu-rich phase at temperatures ranging from 1083 °C to 950 °C. General speaking, these precipitates appear near the interface of Cu matrix. In addition, these phases can be also observed inside iron-rich phases under high power electron microscopy. The results of closer SEM examination, displayed in Fig. 3, demonstrate that these cracks occurred along liquated grain boundaries in the HAZ, with an irregular morphology that is the typical characteristic of liquation cracking. The HAZ crack was intergranular, which extended along the grain boundary of austenite. The length and width of the crack in HAZ were 32 μm and 4 μm, respectively. Liquation is a metallurgical factor that induces HAZ cracking during the process of welding heat cycle. Generally speaking, the occurrence of the HAZ cracking is mainly associated with the liquation of some grain boundary constituents such as γ-γ′ eutectics, and carbides [19]. Liquated grain boundaries survive to lower temperatures during cooling, resulting in reducing the cohesion between grains. Liquation cracking occurs when
2. Materials and methods The experimental materials used in the experiments were the rolled SUS304 stainless steel and soft T2 copper, the chemical compositions of which are listed in Table 1. The materials were supplied in the forms of 100 mm × 50 mm × 2 mm plates. The specimens with no grooves were firmly clamped in butt-weld geometry after a careful polishing with 2
Optics and Laser Technology 122 (2020) 105881
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Table 1 Chemical compositions of the experimental materials in the tests (wt %). Materials
Cu + Ag
Sn
Pb
Fe
Sb
Bi
S
O
As
T2
≥99.90
≤0.002
≤0.005
≤0.005
≤0.002
≤0.001
≤0.005
≤0.06
≤0.002
Materials
C
Si
Mn
P
S
Cr
Ni
SUS304
≤0.08
≤1.00
≤2.00
≤0.045
≤0.03
18.0–20.0
8.0–10.0
Fig. 1. The schematic of the laser welding setup.
the tensile stresses produced during weld cooling failed to be neutralized by the cohesion. Therefore, it is also vital to take fully into account the mechanical aspects that can affect the susceptibility of the HAZ cracking.
Table 2 Laser welding process variables. Equipment
Units
Variables
Levels
RFL-C3000
Laser power laser deflection angle Welding speed Focal point position
kW ° m/min mm
3.0 −15, −10, 0, +10, +15 1.8, 2.4, 3.0, 3.6 −5, −3, 0, 3, 5
3.2. Formation mechanism of welded defects The above SEM micrographs indicated clearly that porosity and HAZ liquation cracking were major welding defects in the joint, which
Fig. 2. SEM micrographs showing the porosity with irregular morphology in the weld-metal zone. 3
Optics and Laser Technology 122 (2020) 105881
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Table 3 SEM/EDS sem-quantitative analysis of some typical particles (at. %). Element
O
Si
Cr
Mn
Fe
Ni
Cu
Point B Point C
38.70 58.36
1.72 2.43
4.86 1.74
0.25 0.16
14.71 5.56
1.53 0.65
38.23 31.00
observed that Fe(Cr, Ni)-rich zones are formed in Point B while Fe(Cr, Ni)-poor zones are generated in Point C. Migration of alloying elements can induce the disturbance of the molten pool because of the concentration difference and lead to the keyhole instability. The trapped metallic vapor in the keyhole fails to be expelled and induces the formation of porosities near the central parts of the keyhole. The SEM/EDS results of HAZ precipitates at grain boundary are shown in Fig. 5 and Table 4. The occurrence of HAZ grain boundary cracking is closely associated with the formation of liquid phases along the intergranular region and subsequent decohesion at intergranular solid/liquid interface under tensile stresses. The HAZ grain boundary liquation is diffusely accepted to arise either by non-equilibrium subsolidus melting or equilibrium supersolidus melting [21]. Especially, the subsolidus melting is more harmful because it enlarges the actual melting temperature range and extends the size of crack impressionable region. As demonstrated in Table 1, it can be deduced that precipitated phases at grain boundary are Cu-rich phases. Under the action of welding heat cycle, Cu atoms can spread easily along the grain boundary instead of the interior of the grain because of having a smaller resistance. Owing to the interdiffuse between Fe and Cu, multiple Fe-Cu intercompounds are easily generated at Fe/Cu interface. In addition, the existence of Cr at the grain boundary can effectively improve intercrystalline corrosion resistance of the materials. However, the existence of these compounds can reduce seriously the plasticity in materials due to its high hardness. Moreover, the impurity phases at the grain boundary generally have low point, which may be liquefaction during weld cooling and become the source of crack initiation. The interface bonding force can be dramatically weakened. In this case, the thermal stresses fail to be fully neutralized by the cohesion among grains so as to induce the occurrence of hot cracking.
Fig. 3. HAZ liquation cracks associated with precipitated phases at grain boundaries.
can affected seriously the performance and service cycle of welded joints. Therefore, it is highly desirable to explore formation mechanism of defects during the process of laser welding, aiming at improving the quality of the joint. Based on this reason, SEM associated with energy diffraction spectrum (EDS) analysis was conducted to further investigate the element segregation characteristic and microstructural evolution behavior near the defects in the joint. The polygonal porosity in the weld, shown in Fig. 4, belongs to the keyhole mode porosity. It is believed that the occurrence of porosities is as a consequence of the keyhole instability correlated with fluid flow and keyhole free surface evolutions in a welding process. The keyhole instability was closely related to the behaviour of humps on the keyhole wall [20]. During the process of laser welding, the effects of recoil pressure, surface tension and impacting pressure of fluid flow can actuate periodically the humps downwards and collapse the keyhole. In addition, the composition segregation is also an important factor that affected liquid metal flow. As shown in Fig. 4 and Table 3, it can be
3.3. XRD analysis near the interface To further investigate the phase near the interface of stainless steel/
Fig. 4. SEM and spot EDS spectrum near interface (a) spectrum A, (b) spectrum B, (c) spectrum C. 4
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Fig. 5. Image (SEM) of HAZ realing cracks and associated features.
of Cu atoms can be seriously prevented during the cooling process by reason of the presence of greater subcooling. Fe-Cu compounds at grain boundaries are easily produced because Cu atoms fail to fully diffuse to Fe crystal structure. The plastic deformation of grains can be severely hindered owing to these compounds having a high hardness. The existence of Fe-Cu compounds greatly weakens the cohesion between grains and increases the cracking susceptibility. Therefore, it is very imperative to reduce the HAZ cracking by inhibiting the diffusion of Cu element along the grain boundary.
Table 4 SEM/EDS sem-quantitative analysis of some typical particles (at. %). Element
O
Si
Cr
Mn
Fe
Ni
Cu
Point 1 Point 2
3.36 6.29
0.36 0.46
4.49 7.54
1.04 1.22
13.88 21.60
2.39 2.06
74.47 60.83
3.4. Liquation cracking model It is generally believed that the occurrence of HAZ liquation cracking is closely associated with the impurity phases or low melting eutectic phases that precipitated at grain boundaries. The existence of these phases weakens the cohesion between adjacent grains and induces the occurrence of liquation cracking. However, it is found that the occurrence of liquation cracking is closely connected with the diffusion of Cu atoms along the grain boundary. Based on evolution characteristics of liquation cracking, a new model is presented to study crack formation. In Fig. 7, we exhibit graphically that formation of liquation cracking experiences mainly three stages in a welding process. The first stage is the process of crack incubation at grain boundaries. During laser welding of stainless steel to copper, Cu atoms permeates promptly along the grain boundary because of a small resistance. At high temperature, α solid solution can be generated on account of the reaction between Fe atoms and Cu atoms. The presence of α phase is quite beneficial to improving the grain boundary performance. However, a small number of copper atoms participate in the above reactions and the rest are propelled to form Fe-Cu compounds by reason of laser welding a fast cooling speed. During this stage, although grain boundary liquation is likely to occur, the thermal stress is very small so that it can be counteracted by plastic deformation of grains. Thus, no liquation cracking can be formed. The second stage is the process of crack initiation. It is obviously observed that Fe-Cu compounds enriched at grain boundaries. The cohesion between the grains is greatly reduced so that the thermal stress formed in the weld fails to be thoroughly neutralized. Furthermore, under the action of welding heat cycle, the impurity phases at grain boundary can easily induce the grain boundary liquation occurs. With increasing liquefied zone continuously, the binding force between grains can be further weakened. The synergistic action, involving Fe-Cu compounds and the impurity
Fig. 6. XRD diffraction pattern of the stainless steel/copper welded joint.
copper dissimilar alloys laser welding joint with liquation cracking, XRD analysis was systematically performed on prepared samples under specified conditions. Fig. 6 schematically illustrates the XRD results of experimental stainless steel/copper welded samples. The results of closer XRD analysis, shown in Fig. 6, revealed that the laser welding seam region involves mainly α-Fe phase and Cu(Fe2O4) phase. In addition, Cu-Fe compounds, such as Cu40Fe60 and CuxFe1−x were also detected in the joint, which proved convincingly the correctness of above EDS analysis results. Formation of α solid solution is as a result of atomic diffusion during the solidification of weld cooling, which is extremely beneficial to improving the joint performance. General speaking, atomic diffusion can be greatly affected by the actual cooling rate. α solid solutions can be generated because of the sufficient diffusion of Cu atoms at high temperature. However, the solubility between Fe and Cu is very small below the melting point. The diffusion 5
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Fig. 7. Liquation cracking model (a) crack incubation, (b) crack initiation, (c) crack growth.
phases, induces crack initiation at local grain boundaries. The third stage is the process of crack growth. With increasing the heat input in a welding process, the thermal stresses increase sharply. Meanwhile, microcracks can become a new crack source and extend quickly along the grain boundary. Thus, microcracks are easily connected in series so as to form a big crack.
3.5. Possible solutions for welded defects The constitutional liquation of the HAZ grain boundary is a requisite factor for formation of liquation cracking. The degree of grain boundary liquefaction depends mainly on the heat input during the process of laser welding. The result of HAZ crack length measurements in the welded joints along the grain boundary is diagrammatized in Fig. 8 and displays that the tendency of HAZ liquation increased with increasing the welding heat input. It can be seen that the total crack length is closely associated with the welding heat input. Nevertheless, it does not keep the direct proportion relationship between them. At first, the crack length increased with the heat input increasing during laser welding process. However, when it exceeded 125 kJ/m, the crack length began to decline unexpectedly. It may mainly attribute to the self-healing capacity of liquation cracks. It means that cracks filled with the copper inside the fusion zone have a self-healing property, which might improve the adverse influence of the cracks on the mechanical property of welded joints. As seen from Fig. 9, it can be proved that cracks can be effectively prevented at liquefied grain boundary because of its self-healing capacity. Although the crack susceptibility can be effectually lowered via the self-healing, the process parameters are rigorously demanded for good weld quality and subsequent industrial applications are greatly limited. Therefore, for the sake of controlling availably the liquation cracks in the joints, it is very imperative to reduce the heat input by improving welding speed or decreasing laser power during the process
Fig. 9. HAZ microstructure and grain boundary liquation of laser welded samples.
Fig. 10. SEM image of laser weld profile showing no significant defects.
of laser welding. Laser deflection angle in this experiment was also adjusted to suppress the occurrence of defects in welded joints. The micrograph of Fig. 10 shows one of weld profiles with laser deflection angle of 10°. The experimental results testified that a good joint with no significant defects such as porosity, can be successfully achieved by means of altering laser deflection angle. It can mainly give the credit to laser stirring effect during the welding process. Laser deflection angle was tactfully performed on welded samples so as to alter the flow of liquid metal. As seen from Fig. 10, two molten pools can be formed on account of laser beam action. The interaction of molten pools can enhance the metallurgical reaction effectually and inhibit the occurrence of porosity
Fig. 8. HAZ crack susceptibility variation with welding heat intut. 6
Optics and Laser Technology 122 (2020) 105881
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Center of HUST (Huazhong University of Science and Technology), for their friendly cooperation. This work was supported by the Natural Science Foundation of Hubei Province (Grant No. 2019CFB210 and Grant No. 2018CFB210). It was also funded by the Fundamental Research Funds for the Central Universities (Grant No. 2018JYCXJJ033) and Science and Technology Planning Project of Guangdong Province (Grant No. 2017B090913001). References [1] B.M. Rk, P. Maji, A. Samadhiya, S.K. Ghosh, B.S. Roy, A.K. Das, S.C. Saha, A study on induction welding of mild steel and copper with flux under applied load condition, J. Manuf. Process. 34 (2018) 435–441. [2] M. Weigl, M. Schmidt, Influence of the feed rate and the lateral beam displacement on the joining quality of laser-welded copper-stainless steel connections, Phys. Proc. 5 (2010) 53–59. [3] J. He, D. Sirois, S. Li, M. Sullivan, C. Wikle, B.A. Chin, Ballistic impact welding of copper to low carbon steel, J. Mater. Process. Technol. 232 (2016) 165–174. [4] M.S. Akella, M.V. Harinadh, M.Y. Krishna, M.R.K. Buddu, Welding simulation of dissimilar materials SS304 and Copper, Proc. Mater. Sci. 5 (2014) 2440–2449. [5] Z. Cheng, J. Huang, Z. Ye, Y. Chen, J. Yang, S. Chen, Microstructures and mechanical properties of copper-stainless steel butt-welded joints by MIG-TIG doublesided arc welding, J. Mater. Process. Technol. 265 (2019) 87–98. [6] H. Zhang, K.X. Jiao, J.L. Zhang, J. Liu, Microstructure and mechanical properties investigations of copper-steel composite fabricated by explosive welding, Mater. Sci. Eng. A 731 (2018) 278–287. [7] T. Wang, S. Shukla, S.S. Nene, M. Frank, R.W. Wheeler, R.S. Mishra, Towards obtaining sound butt joint between metallurgically immiscible pure Cu and stainless steel through friction stir welding, Metall. Mater. Trans. A 49 (7) (2018) 2578–2582. [8] W.W. Zhang, S. Cong, Y. Huang, Y.H. Tian, Microstructure and mechanical properties of vacuum brazed martensitic stainless steel/tin bronze by Ag-based alloy, J. Mater. Process. Technol. 248 (2017) 64–71. [9] S. Chen, X. Yu, J. Huang, J. Yang, S. Lin, Interfacial ferrite band formation to suppress intergranular liquid copper penetration of solid steel, J. Alloys Compound 773 (2019) 719–729. [10] C. Yao, B. Xu, X. Zhang, J. Huang, J. Fu, Y. Wu, Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint, Opt. Lasers Eng. 47 (7–8) (2009) 807–814. [11] T. Suga, Y. Murai, T. Kobashi, K. Ueno, M. Shindo, K. Kanno, K. Nakata, Laser brazing of a dissimilar joint of austenitic stainless steel and pure copper, Weld. Int. 30 (3) (2016) 166–174. [12] Y. Meng, X. Li, M. Gao, X. Zeng, Microstructures and mechanical properties of laserarc hybrid welded dissimilar pure copper to stainless steel, Opt. Laser Technol. 111 (2019) 140–145. [13] I. Magnabosco, P. Ferro, F. Bonollo, L. Arnberg, An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel, Mater. Sci. Eng. A 424 (1–2) (2006) 163–173. [14] S.K. Dinda, J.M. Warnett, M.A. Williams, G.G. Roy, P. Srirangam, 3D imaging and quantification of porosity in electron beam welded dissimilar steel to Fe-Al alloy joints by X-ray tomography, Mater. Des. 96 (2016) 224–231. [15] J. Kar, S.K. Dinda, G.G. Roy, S.K. Roy, P. Srirangam, X-ray tomography study on porosity in electron beam welded dissimilar copper–304SS joints, Vacuum 149 (2018) 200–206. [16] H. Shen, M.C. Gupta, Nd: yttritium–aluminum–garnet laser welding of copper to stainless steel, J. Laser Appl. 16 (1) (2004) 2–8. [17] S. Chen, J. Huang, J. Xia, H. Zhang, X. Zhao, Microstructural characteristics of a stainless steel/copper dissimilar joint made by laser welding, Metall. Mater. Trans. A 44 (8) (2013) 3690–3696. [18] S.H. Baghjari, M. Gholambargani, S.A. Mousavi, Application of the pulsed Nd: YAG laser welding to investigate the effect of laser beam position on weld characteristics of AISI 420 stainless steel to Kovar alloy, Lasers Manuf. Mater. Process. 6 (1) (2019) 14–25. [19] M. Montazeri, F.M. Ghaini, The liquation cracking behavior of IN738LC superalloy during low power Nd: YAG pulsed laser welding, Mater. Charact. 67 (2012) 65–73. [20] S. Pang, L. Chen, J. Zhou, Y. Yin, T. Chen, A three-dimensional sharp interface model for self-consistent keyhole and weld pool dynamics in deep penetration laser welding, J. Phys. D: Appl. Phys. 44 (2) (2010) 025301. [21] O.A. Ojo, N.L. Richards, M.C. Chaturvedi, Contribution of constitutional liquation of gamma prime precipitate to weld HAZ cracking of cast Inconel 738 superalloy, Scr. Mater. 50 (5) (2004) 641–646.
Fig. 11. Tensile curves under different process conditions.
defects. It can be observed in Fig. 11 that the tensile strength of the joint with a deflection angle of 10° can reach 278 Mpa, which was apparently higher than that with porosity defects. To some extent, the mechanical properties of the joint can be significantly enhanced by controlling porosity defects and improving the microstructure and performance. 4. Conclusions Laser beam welding SUS304 stainless steel to T2 copper was carefully investigated in order to explore formation mechanism of porosities and HAZ liquation cracking. Possible solutions for laser welded defects are effectively proposed based on evolution characteristics of the defects. The results of this study are generalized as follows: (1) The polygonal porosity and liquation cracking are main welding defects in joining stainless steel to copper. The porosity appeared in the middle weld where mixed structures involving spherical structures and small columnar structures are formed. Meanwhile, liquation cracking, with an irregular morphology, occurred along liquated grain boundaries in the HAZ. (2) The occurrence of porosity is as a consequence of the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. Formation of HAZ liquation cracking is closely associated with migration of copper atoms along the grain boundary and grain boundary liquation. (3) The results of closer XRD analysis demonstrated that the laser welding seam region involves mainly α-Fe phase and Cu(Fe2O4) phase. In addition, Cu-Fe compounds, such as Cu40Fe60 and CuxFe1x were also detected in the joint. (4) A model was schematically presented involving crack incubation stage, crack initiation stage and crack growth stage to illustrate formation of HAZ liquation cracking. (5) The porosity in the weld can be availably eliminated by adjusted laser deflection angle in the experiments. And the susceptibility of HAZ liquation cracking can be effectively lowered by reducing the heat input during the process of laser welding. Acknowledgments We would like to express our deep gratitude to Analysis and Test
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Full length article
Porosity and liquation cracking of dissimilar Nd:YAG laser welding of SUS304 stainless steel to T2 copper Jianmin Lia, Yuanzheng Caia, Fei Yanb, a b
⁎,1
, Chunming Wanga,
⁎,1
T
, Zhengwu Zhua, Chongjing Hua
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China Hubei Key Laboratory of Advanced Technology for Automotive Components (Wuhan University of Technology), Wuhan 430070, China
H I GH L IG H T S
mechanism of laser welding defects in stainless steel to copper was investigated. • The model is proposed to illustrate the evolution process of liquation cracking. • APossible solutions were advised to inhibit laser welded defects in dissimilar metals. • It had a good industrial applicability in allowing high quality and efficiency. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Stainless steel/copper dissimilar materials Liquation cracking Polygonal porosity Grain boundary liquation Composition segregation
In this study, we present experimental laser welding SUS304 stainless steel to T2 copper. The forming mechanism of fusion zone polygonal porosity and heat affected zone (HAZ) liquation cracking were systemically investigated and possible solutions to the welding process were also proposed. We deemed that the generation of HAZ liquation cracking mainly underwent three stages: crack incubation, crack initiation and crack growth. Formation of HAZ liquation cracking was closely related to precipitation of Cu-Fe compounds at grain boundaries and grain boundary liquation. The occurrence of porosity was determined by the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. The susceptibility to HAZ liquation cracking can be effectively lowered by controlling the heat input during laser welding. In addition, the porosity in the fusion zone can be eliminated by reasonably adjusting laser deflection angle in the experiments, which greatly improved the microstructure and mechanical properties of welded joints.
1. Introduction In recent years, we have observed growing interest for the application of bi-metallic systems to realize material optimisation, weight reduction and durable product operation under special service conditions [1]. Among these, the hybrid structures of stainless steel and copper have attracted much attention and been extensively applied in power generation industries [2], nuclear industries [3], chemical and automobile sectors [4] on account of their high electrical and thermal conductivity and stiffness. Nevertheless, the discrepancy in physical and chemical properties between stainless steel and copper makes it very difficult to achieve a defect-free welded joint between them. Aiming to achieving a strong joint, widely spread welding methods including arc welding [5], explosive welding [6], induction welding
[1], friction stir welding [7] and so on, were attempted by many researchers to achieve the combination of stainless steel and copper. All of these techniques have some advantages and disadvantages, such as low productivity, high energy consumption, formation of intermetallic layers at interface and narrow applicability scope. In particular, the growth of intermetallic layers at high temperature is likely to induce the generation of intermetallic compounds at interface during welding of stainless steel/copper dissimilar materials, which can reduce the performance of welded joints seriously [8]. Compared to conventional arc welding, laser welding has been paid extensive attention to the joining of stainless steel to copper due to its lower heat input, smaller deformation, higher energy density and so on. At present, considerable research on the joining of stainless steel to copper concentrated mainly upon microstructures and mechanical
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Corresponding authors. E-mail addresses: [email protected] (F. Yan), [email protected] (C. Wang). 1 Fei Yan and Chunming Wang contributed equally to this work. https://doi.org/10.1016/j.optlastec.2019.105881 Received 4 July 2019; Received in revised form 9 September 2019; Accepted 29 September 2019 Available online 19 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 122 (2020) 105881
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sandpaper. Before laser welding, the acetone was also used to remove dirt on the surface of specimens. The schematic of the laser welding setup is shown in Fig. 1. A high-power multimode fiber laser with a maximum laser power of 3 kW, RFL-C3000, was employed for welding under shielding with pure argon gas at 16.7 L/min flow rate. The specific parameters of laser were as follows: a focal length of 300 mm, a focus diameter of 0.3 mm and a wavelength of 1070 nm. Welding was conducted on SUS304 stainless steel/T2 copper using three variables: laser deflection angle, welding speed and focal point position. Process variables of continuous laser welding are listed in Table 2. In the experiments, the following symbols were defined for the sake of research convenience: “+”represents that laser beam was deflected to one side of the steel plate while “—” means that laser beam was biased to one side of the aluminum plate. Welded samples were sectioned transversely into three samples using a line cut machine. Metallographic samples were prepared by means of hot mounting press. All samples were carefully shaped using different types of sandpaper, then polished and etched with different etchants. The copper side was etched with nitric acid diluent including 50% HNO3 mixed with 50% distilled water. After that, the steel side was etched with the etchant involving 10% nitric acid, 20% hydrofluoric acid and 70% distilled water. Metallographic examinations were first conducted to examine microstructural features near the steel/ copper interface. Morphologies and the element distribution were also observed using a scanning electron microscope (SEM) associated with energy diffraction spectrum (EDS). The phase constitution of the joint was then analyzed by means of X-ray diffractometer (XRD). Based on the evolutional characteristics of welded defects, possible solutions for welded defects were also proposed in this study.
propertes. Chen et al. [9] found that intergranular liquid metal penetration was harmful to the integrity of welded structures, and suggested building an interfacial ferrite band to inhibit intergranular liquid Cu penetration of solid steel. Yao et al. [10] applied a scarf joint geometry to conduct the copper-steel CO2-laser welding and controlled the melting ratio of copper to steel by befittingly choosing the deviation of the laser beam. Suga et al. [11] investigated laser brazing of stainless steel/copper lap joints using Cu–Si-based filler metals, and improved the performance of welded joints by increasing the bonding width. Meng et al. [12] performed laser-arc hybrid welding stainless steel/ copper, and concluded that macrosegregations of Fe-rich peninsula and islands in the fusion zone worsened the weld performance while the Curich matrix and inner uniformly distributed Fe-rich particles strengthened the weld. There are certain tendencies that lead to welding defects in the weld such as shrinkage, porosity, cracks etc., when welding stainless steel/ copper dissimilar materials. The forming reasons and suppressing methods of welding defects have been initially discussed by other researchers. Magnabosco et al. [13] thought that porosity and microfissures in welded joints were attributed to the process and geometry parameters. Dinda et al. [14] considered that the average size and number of the pores in the welded joints were associated with welding, and the presence of hydrogen gas could be responsible for porosity formation in EBW dissimilar joints. Kar et al. [15] found that the application of beam oscillation led to less porosity and the average pore size in electron beam welded dissimilar copper–304SS joints. Further, it was observed that an optimum beam oscillation diameter can effectively control the porosity formation in the weld joint. Shen and Gupta [16] proved that the solidification cracking had close relation with the composition of the weld depending on the ratio of copper and stainless steel. Chen et al. [17] demonstrated that the occurrence of the microcracks was closely associated with the quantity of fused copper during the laser welding, and liquid separation and formation of microcracks in the fusion zone were as the result of superfluous copper melting. Baghjari et al. [18] performed dissimilar joining of SS 420 to Kovar alloy using a pulsed Nd:YAG laser welding and proved that offsetting laser beam position to SS 420 base metal can promote the susceptibility of fusion zone to cracking. Previous studies showed porosity and cracks were important factors that affected the quality and performance of welded joints. Although investigation on porosity and cracks of a copper/steel dissimilar joint was systematically conducted by laser welding, the forming mechanism of welded defects has been quite unclear. Therefore, it is very imperative to clarify the mechanism of welded defects and inhibit them effectively so as to achieve an acceptable weld with free of visible defect and good mechanical properties. In this study, the butt welding of copper and stainless steel using a continuous fiber laser beam was detailedly described. Laser deflection angle was effectively performed during the process of laser welding stainless steel to copper in order to reduce the defects and improve the quality of the weld. The forming mechanism of laser welding defects such as porosity and liquation cracking, was deeply investigated using a scanning electron microscope (SEM), energy diffraction spectrum (EDS) and X-ray diffractometer (XRD). A novel model was graphically proposed to expound the forming process of HAZ liquation cracking. Possible solutions for welded defects were carefully discussed with the purpose of suppressing the porosity and cracking in the weld and achieving a joint with good weld formation and performance.
3. Results and discussion 3.1. Morphology characteristics of the defects The microstructure of weld metal and heat affected zone (HAZ) in the joint can be observed through scanning electron microscope. Microstructural examination of welded samples displayed the occurrence of fusion zone porosity and HAZ liquation cracking. The weldmetal porosity observed in the experiment usually occurs in the fusion zone independently of HAZ liquation cracking in Fig. 2a. The fusion zone porosity with irregular morphology was detected in the lower weld where the mixed structure involving spherical structures and small columnar structures formed at the final solidification stage of weld pool in Fig. 2b. The area of the porosity approximately equals to 926.81 μm2. These local uneven structures are mainly associated with the segregation of the alloying elements. According to Fe-Cu alloy phase diagram, the Fe-rich phase happens at temperatures ranging from 1535 °C to 1083 °C. However, during the process of solidification, the precipitation of iron element alters the composition of the molten pool, leading to the occurrence of the Cu-rich phase at temperatures ranging from 1083 °C to 950 °C. General speaking, these precipitates appear near the interface of Cu matrix. In addition, these phases can be also observed inside iron-rich phases under high power electron microscopy. The results of closer SEM examination, displayed in Fig. 3, demonstrate that these cracks occurred along liquated grain boundaries in the HAZ, with an irregular morphology that is the typical characteristic of liquation cracking. The HAZ crack was intergranular, which extended along the grain boundary of austenite. The length and width of the crack in HAZ were 32 μm and 4 μm, respectively. Liquation is a metallurgical factor that induces HAZ cracking during the process of welding heat cycle. Generally speaking, the occurrence of the HAZ cracking is mainly associated with the liquation of some grain boundary constituents such as γ-γ′ eutectics, and carbides [19]. Liquated grain boundaries survive to lower temperatures during cooling, resulting in reducing the cohesion between grains. Liquation cracking occurs when
2. Materials and methods The experimental materials used in the experiments were the rolled SUS304 stainless steel and soft T2 copper, the chemical compositions of which are listed in Table 1. The materials were supplied in the forms of 100 mm × 50 mm × 2 mm plates. The specimens with no grooves were firmly clamped in butt-weld geometry after a careful polishing with 2
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Table 1 Chemical compositions of the experimental materials in the tests (wt %). Materials
Cu + Ag
Sn
Pb
Fe
Sb
Bi
S
O
As
T2
≥99.90
≤0.002
≤0.005
≤0.005
≤0.002
≤0.001
≤0.005
≤0.06
≤0.002
Materials
C
Si
Mn
P
S
Cr
Ni
SUS304
≤0.08
≤1.00
≤2.00
≤0.045
≤0.03
18.0–20.0
8.0–10.0
Fig. 1. The schematic of the laser welding setup.
the tensile stresses produced during weld cooling failed to be neutralized by the cohesion. Therefore, it is also vital to take fully into account the mechanical aspects that can affect the susceptibility of the HAZ cracking.
Table 2 Laser welding process variables. Equipment
Units
Variables
Levels
RFL-C3000
Laser power laser deflection angle Welding speed Focal point position
kW ° m/min mm
3.0 −15, −10, 0, +10, +15 1.8, 2.4, 3.0, 3.6 −5, −3, 0, 3, 5
3.2. Formation mechanism of welded defects The above SEM micrographs indicated clearly that porosity and HAZ liquation cracking were major welding defects in the joint, which
Fig. 2. SEM micrographs showing the porosity with irregular morphology in the weld-metal zone. 3
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Table 3 SEM/EDS sem-quantitative analysis of some typical particles (at. %). Element
O
Si
Cr
Mn
Fe
Ni
Cu
Point B Point C
38.70 58.36
1.72 2.43
4.86 1.74
0.25 0.16
14.71 5.56
1.53 0.65
38.23 31.00
observed that Fe(Cr, Ni)-rich zones are formed in Point B while Fe(Cr, Ni)-poor zones are generated in Point C. Migration of alloying elements can induce the disturbance of the molten pool because of the concentration difference and lead to the keyhole instability. The trapped metallic vapor in the keyhole fails to be expelled and induces the formation of porosities near the central parts of the keyhole. The SEM/EDS results of HAZ precipitates at grain boundary are shown in Fig. 5 and Table 4. The occurrence of HAZ grain boundary cracking is closely associated with the formation of liquid phases along the intergranular region and subsequent decohesion at intergranular solid/liquid interface under tensile stresses. The HAZ grain boundary liquation is diffusely accepted to arise either by non-equilibrium subsolidus melting or equilibrium supersolidus melting [21]. Especially, the subsolidus melting is more harmful because it enlarges the actual melting temperature range and extends the size of crack impressionable region. As demonstrated in Table 1, it can be deduced that precipitated phases at grain boundary are Cu-rich phases. Under the action of welding heat cycle, Cu atoms can spread easily along the grain boundary instead of the interior of the grain because of having a smaller resistance. Owing to the interdiffuse between Fe and Cu, multiple Fe-Cu intercompounds are easily generated at Fe/Cu interface. In addition, the existence of Cr at the grain boundary can effectively improve intercrystalline corrosion resistance of the materials. However, the existence of these compounds can reduce seriously the plasticity in materials due to its high hardness. Moreover, the impurity phases at the grain boundary generally have low point, which may be liquefaction during weld cooling and become the source of crack initiation. The interface bonding force can be dramatically weakened. In this case, the thermal stresses fail to be fully neutralized by the cohesion among grains so as to induce the occurrence of hot cracking.
Fig. 3. HAZ liquation cracks associated with precipitated phases at grain boundaries.
can affected seriously the performance and service cycle of welded joints. Therefore, it is highly desirable to explore formation mechanism of defects during the process of laser welding, aiming at improving the quality of the joint. Based on this reason, SEM associated with energy diffraction spectrum (EDS) analysis was conducted to further investigate the element segregation characteristic and microstructural evolution behavior near the defects in the joint. The polygonal porosity in the weld, shown in Fig. 4, belongs to the keyhole mode porosity. It is believed that the occurrence of porosities is as a consequence of the keyhole instability correlated with fluid flow and keyhole free surface evolutions in a welding process. The keyhole instability was closely related to the behaviour of humps on the keyhole wall [20]. During the process of laser welding, the effects of recoil pressure, surface tension and impacting pressure of fluid flow can actuate periodically the humps downwards and collapse the keyhole. In addition, the composition segregation is also an important factor that affected liquid metal flow. As shown in Fig. 4 and Table 3, it can be
3.3. XRD analysis near the interface To further investigate the phase near the interface of stainless steel/
Fig. 4. SEM and spot EDS spectrum near interface (a) spectrum A, (b) spectrum B, (c) spectrum C. 4
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Fig. 5. Image (SEM) of HAZ realing cracks and associated features.
of Cu atoms can be seriously prevented during the cooling process by reason of the presence of greater subcooling. Fe-Cu compounds at grain boundaries are easily produced because Cu atoms fail to fully diffuse to Fe crystal structure. The plastic deformation of grains can be severely hindered owing to these compounds having a high hardness. The existence of Fe-Cu compounds greatly weakens the cohesion between grains and increases the cracking susceptibility. Therefore, it is very imperative to reduce the HAZ cracking by inhibiting the diffusion of Cu element along the grain boundary.
Table 4 SEM/EDS sem-quantitative analysis of some typical particles (at. %). Element
O
Si
Cr
Mn
Fe
Ni
Cu
Point 1 Point 2
3.36 6.29
0.36 0.46
4.49 7.54
1.04 1.22
13.88 21.60
2.39 2.06
74.47 60.83
3.4. Liquation cracking model It is generally believed that the occurrence of HAZ liquation cracking is closely associated with the impurity phases or low melting eutectic phases that precipitated at grain boundaries. The existence of these phases weakens the cohesion between adjacent grains and induces the occurrence of liquation cracking. However, it is found that the occurrence of liquation cracking is closely connected with the diffusion of Cu atoms along the grain boundary. Based on evolution characteristics of liquation cracking, a new model is presented to study crack formation. In Fig. 7, we exhibit graphically that formation of liquation cracking experiences mainly three stages in a welding process. The first stage is the process of crack incubation at grain boundaries. During laser welding of stainless steel to copper, Cu atoms permeates promptly along the grain boundary because of a small resistance. At high temperature, α solid solution can be generated on account of the reaction between Fe atoms and Cu atoms. The presence of α phase is quite beneficial to improving the grain boundary performance. However, a small number of copper atoms participate in the above reactions and the rest are propelled to form Fe-Cu compounds by reason of laser welding a fast cooling speed. During this stage, although grain boundary liquation is likely to occur, the thermal stress is very small so that it can be counteracted by plastic deformation of grains. Thus, no liquation cracking can be formed. The second stage is the process of crack initiation. It is obviously observed that Fe-Cu compounds enriched at grain boundaries. The cohesion between the grains is greatly reduced so that the thermal stress formed in the weld fails to be thoroughly neutralized. Furthermore, under the action of welding heat cycle, the impurity phases at grain boundary can easily induce the grain boundary liquation occurs. With increasing liquefied zone continuously, the binding force between grains can be further weakened. The synergistic action, involving Fe-Cu compounds and the impurity
Fig. 6. XRD diffraction pattern of the stainless steel/copper welded joint.
copper dissimilar alloys laser welding joint with liquation cracking, XRD analysis was systematically performed on prepared samples under specified conditions. Fig. 6 schematically illustrates the XRD results of experimental stainless steel/copper welded samples. The results of closer XRD analysis, shown in Fig. 6, revealed that the laser welding seam region involves mainly α-Fe phase and Cu(Fe2O4) phase. In addition, Cu-Fe compounds, such as Cu40Fe60 and CuxFe1−x were also detected in the joint, which proved convincingly the correctness of above EDS analysis results. Formation of α solid solution is as a result of atomic diffusion during the solidification of weld cooling, which is extremely beneficial to improving the joint performance. General speaking, atomic diffusion can be greatly affected by the actual cooling rate. α solid solutions can be generated because of the sufficient diffusion of Cu atoms at high temperature. However, the solubility between Fe and Cu is very small below the melting point. The diffusion 5
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Fig. 7. Liquation cracking model (a) crack incubation, (b) crack initiation, (c) crack growth.
phases, induces crack initiation at local grain boundaries. The third stage is the process of crack growth. With increasing the heat input in a welding process, the thermal stresses increase sharply. Meanwhile, microcracks can become a new crack source and extend quickly along the grain boundary. Thus, microcracks are easily connected in series so as to form a big crack.
3.5. Possible solutions for welded defects The constitutional liquation of the HAZ grain boundary is a requisite factor for formation of liquation cracking. The degree of grain boundary liquefaction depends mainly on the heat input during the process of laser welding. The result of HAZ crack length measurements in the welded joints along the grain boundary is diagrammatized in Fig. 8 and displays that the tendency of HAZ liquation increased with increasing the welding heat input. It can be seen that the total crack length is closely associated with the welding heat input. Nevertheless, it does not keep the direct proportion relationship between them. At first, the crack length increased with the heat input increasing during laser welding process. However, when it exceeded 125 kJ/m, the crack length began to decline unexpectedly. It may mainly attribute to the self-healing capacity of liquation cracks. It means that cracks filled with the copper inside the fusion zone have a self-healing property, which might improve the adverse influence of the cracks on the mechanical property of welded joints. As seen from Fig. 9, it can be proved that cracks can be effectively prevented at liquefied grain boundary because of its self-healing capacity. Although the crack susceptibility can be effectually lowered via the self-healing, the process parameters are rigorously demanded for good weld quality and subsequent industrial applications are greatly limited. Therefore, for the sake of controlling availably the liquation cracks in the joints, it is very imperative to reduce the heat input by improving welding speed or decreasing laser power during the process
Fig. 9. HAZ microstructure and grain boundary liquation of laser welded samples.
Fig. 10. SEM image of laser weld profile showing no significant defects.
of laser welding. Laser deflection angle in this experiment was also adjusted to suppress the occurrence of defects in welded joints. The micrograph of Fig. 10 shows one of weld profiles with laser deflection angle of 10°. The experimental results testified that a good joint with no significant defects such as porosity, can be successfully achieved by means of altering laser deflection angle. It can mainly give the credit to laser stirring effect during the welding process. Laser deflection angle was tactfully performed on welded samples so as to alter the flow of liquid metal. As seen from Fig. 10, two molten pools can be formed on account of laser beam action. The interaction of molten pools can enhance the metallurgical reaction effectually and inhibit the occurrence of porosity
Fig. 8. HAZ crack susceptibility variation with welding heat intut. 6
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Center of HUST (Huazhong University of Science and Technology), for their friendly cooperation. This work was supported by the Natural Science Foundation of Hubei Province (Grant No. 2019CFB210 and Grant No. 2018CFB210). It was also funded by the Fundamental Research Funds for the Central Universities (Grant No. 2018JYCXJJ033) and Science and Technology Planning Project of Guangdong Province (Grant No. 2017B090913001). References [1] B.M. Rk, P. Maji, A. Samadhiya, S.K. Ghosh, B.S. Roy, A.K. Das, S.C. Saha, A study on induction welding of mild steel and copper with flux under applied load condition, J. Manuf. Process. 34 (2018) 435–441. [2] M. Weigl, M. Schmidt, Influence of the feed rate and the lateral beam displacement on the joining quality of laser-welded copper-stainless steel connections, Phys. Proc. 5 (2010) 53–59. [3] J. He, D. Sirois, S. Li, M. Sullivan, C. Wikle, B.A. Chin, Ballistic impact welding of copper to low carbon steel, J. Mater. Process. Technol. 232 (2016) 165–174. [4] M.S. Akella, M.V. Harinadh, M.Y. Krishna, M.R.K. Buddu, Welding simulation of dissimilar materials SS304 and Copper, Proc. Mater. Sci. 5 (2014) 2440–2449. [5] Z. Cheng, J. Huang, Z. Ye, Y. Chen, J. Yang, S. Chen, Microstructures and mechanical properties of copper-stainless steel butt-welded joints by MIG-TIG doublesided arc welding, J. Mater. Process. Technol. 265 (2019) 87–98. [6] H. Zhang, K.X. Jiao, J.L. Zhang, J. Liu, Microstructure and mechanical properties investigations of copper-steel composite fabricated by explosive welding, Mater. Sci. Eng. A 731 (2018) 278–287. [7] T. Wang, S. Shukla, S.S. Nene, M. Frank, R.W. Wheeler, R.S. Mishra, Towards obtaining sound butt joint between metallurgically immiscible pure Cu and stainless steel through friction stir welding, Metall. Mater. Trans. A 49 (7) (2018) 2578–2582. [8] W.W. Zhang, S. Cong, Y. Huang, Y.H. Tian, Microstructure and mechanical properties of vacuum brazed martensitic stainless steel/tin bronze by Ag-based alloy, J. Mater. Process. Technol. 248 (2017) 64–71. [9] S. Chen, X. Yu, J. Huang, J. Yang, S. Lin, Interfacial ferrite band formation to suppress intergranular liquid copper penetration of solid steel, J. Alloys Compound 773 (2019) 719–729. [10] C. Yao, B. Xu, X. Zhang, J. Huang, J. Fu, Y. Wu, Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint, Opt. Lasers Eng. 47 (7–8) (2009) 807–814. [11] T. Suga, Y. Murai, T. Kobashi, K. Ueno, M. Shindo, K. Kanno, K. Nakata, Laser brazing of a dissimilar joint of austenitic stainless steel and pure copper, Weld. Int. 30 (3) (2016) 166–174. [12] Y. Meng, X. Li, M. Gao, X. Zeng, Microstructures and mechanical properties of laserarc hybrid welded dissimilar pure copper to stainless steel, Opt. Laser Technol. 111 (2019) 140–145. [13] I. Magnabosco, P. Ferro, F. Bonollo, L. Arnberg, An investigation of fusion zone microstructures in electron beam welding of copper–stainless steel, Mater. Sci. Eng. A 424 (1–2) (2006) 163–173. [14] S.K. Dinda, J.M. Warnett, M.A. Williams, G.G. Roy, P. Srirangam, 3D imaging and quantification of porosity in electron beam welded dissimilar steel to Fe-Al alloy joints by X-ray tomography, Mater. Des. 96 (2016) 224–231. [15] J. Kar, S.K. Dinda, G.G. Roy, S.K. Roy, P. Srirangam, X-ray tomography study on porosity in electron beam welded dissimilar copper–304SS joints, Vacuum 149 (2018) 200–206. [16] H. Shen, M.C. Gupta, Nd: yttritium–aluminum–garnet laser welding of copper to stainless steel, J. Laser Appl. 16 (1) (2004) 2–8. [17] S. Chen, J. Huang, J. Xia, H. Zhang, X. Zhao, Microstructural characteristics of a stainless steel/copper dissimilar joint made by laser welding, Metall. Mater. Trans. A 44 (8) (2013) 3690–3696. [18] S.H. Baghjari, M. Gholambargani, S.A. Mousavi, Application of the pulsed Nd: YAG laser welding to investigate the effect of laser beam position on weld characteristics of AISI 420 stainless steel to Kovar alloy, Lasers Manuf. Mater. Process. 6 (1) (2019) 14–25. [19] M. Montazeri, F.M. Ghaini, The liquation cracking behavior of IN738LC superalloy during low power Nd: YAG pulsed laser welding, Mater. Charact. 67 (2012) 65–73. [20] S. Pang, L. Chen, J. Zhou, Y. Yin, T. Chen, A three-dimensional sharp interface model for self-consistent keyhole and weld pool dynamics in deep penetration laser welding, J. Phys. D: Appl. Phys. 44 (2) (2010) 025301. [21] O.A. Ojo, N.L. Richards, M.C. Chaturvedi, Contribution of constitutional liquation of gamma prime precipitate to weld HAZ cracking of cast Inconel 738 superalloy, Scr. Mater. 50 (5) (2004) 641–646.
Fig. 11. Tensile curves under different process conditions.
defects. It can be observed in Fig. 11 that the tensile strength of the joint with a deflection angle of 10° can reach 278 Mpa, which was apparently higher than that with porosity defects. To some extent, the mechanical properties of the joint can be significantly enhanced by controlling porosity defects and improving the microstructure and performance. 4. Conclusions Laser beam welding SUS304 stainless steel to T2 copper was carefully investigated in order to explore formation mechanism of porosities and HAZ liquation cracking. Possible solutions for laser welded defects are effectively proposed based on evolution characteristics of the defects. The results of this study are generalized as follows: (1) The polygonal porosity and liquation cracking are main welding defects in joining stainless steel to copper. The porosity appeared in the middle weld where mixed structures involving spherical structures and small columnar structures are formed. Meanwhile, liquation cracking, with an irregular morphology, occurred along liquated grain boundaries in the HAZ. (2) The occurrence of porosity is as a consequence of the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. Formation of HAZ liquation cracking is closely associated with migration of copper atoms along the grain boundary and grain boundary liquation. (3) The results of closer XRD analysis demonstrated that the laser welding seam region involves mainly α-Fe phase and Cu(Fe2O4) phase. In addition, Cu-Fe compounds, such as Cu40Fe60 and CuxFe1x were also detected in the joint. (4) A model was schematically presented involving crack incubation stage, crack initiation stage and crack growth stage to illustrate formation of HAZ liquation cracking. (5) The porosity in the weld can be availably eliminated by adjusted laser deflection angle in the experiments. And the susceptibility of HAZ liquation cracking can be effectively lowered by reducing the heat input during the process of laser welding. Acknowledgments We would like to express our deep gratitude to Analysis and Test
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