- Email: [email protected]
Characterization of laser beam offset welding of titanium to steel with 38Zn-61Cu alloy filler
Optics and Laser Technology 127 (2020) 106195
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Characterization of laser beam offset welding of titanium to steel with 38Zn61Cu alloy filler
T
⁎
Yan Zhanga, , YanKun Chena, JianPing Zhoua, RuiLei Xueb, DaQian Sunb, HongMei Lib a b
School of Mechanical Engineering, Xinjiang University, Wulumuqi 830000, China Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, Changchun 130022, China
H I GH L IG H T S
possibility of laser welding-diffusion bonding for Ti alloy-SS joint with Cu-base filler metal was studied. • The joint based on two different joining mechanisms. • AThecomposite Ti alloy acted as a barrier to mixing of the base materials. • The unmelted • unmelted Ti alloy and filler metal prevented the formation of brittle Ti-Fe intermetallics in the joint.
A R T I C LE I N FO
A B S T R A C T
Keywords: TC4 Ti alloy 304 austenitic stainless steel Laser welding Diffusion welding Microstructure Filler metal
Laser welding of TC4 Titanium (Ti) alloy to 304 austenitic stainless steel (SS) has been applied using 38Zn-61Cu alloy as filler metal. A new welding process for SS-Ti alloy joint was introduced on the basis of the controlling the formation of Ti-Fe intermetallics in the joint. One pass welding involving creation of a joint with one fusion weld and one diffusion weld separated by remaining unmelted Ti alloy. When laser beam on the Ti alloy side was 1.5 mm, Ti alloy would not be completely melted in joint. Through heat conduction of unmelted Ti alloy, the atomic diffusion occurred at the SS-Ti alloy interface. A diffusion weld was formed at the SS-Ti alloy interface with the main microstructure of β-CuZn + Fe3Zn7, β-CuZn and Ti2Zn3 + Ti3Cu4. The joint fractured at the diffusion weld with the maximum tensile strength of 128 MPa.
1. Introduction The aerospace and nuclear industries have strong demand for Titanium (Ti) alloy and stainless steel (SS) applications due to the potential advantages of Ti alloys in terms of material cost savings, weight reduction, design flexibility and complexity, and improved product functionality [1,2]. As reported by Gang Li et al. [3], the welding processes and their effects on materials were also subjected to tremendous research efforts to understand the connection and interaction between processing, microstructure and mechanical properties. However, the joint between Ti alloy and SS is facing great difficulties due to the poor metallurgical compatibility between Ti alloy and SS, and the serious mismatch of physical and mechanical properties [4]. The research of TIG welding between pure Ti and pure Fe showed that the continuous distribution of TiFe and TiFe2 was the fundamental cause of joint embrittlement. The hardness of weld metal was within the range of HV740-HV1324 [5]. The presence of these brittle phases reduced the
⁎
strength and ductility of the joint. In this case, complex intermetallic compounds were formed between Ti, Fe, Cr and Ni, making the weld more brittle [6]. Therefore, the resulting weld was very brittle and easy to crack. Due to the physical properties of the Ti alloy-SS joint, such as the significant difference in thermal expansion coefficient and melting point, the joint is excessively deformed and residual stress, are clamped each other. Current research indicates that conventional fusion welding would result in the formation of a thick and brittle intermetallic compound layer and the accumulation of residual stress at the joint. Due to the metallurgical incompatibility of Ti alloy and SS, the direct fusion welding method is not suitable for the welding of Ti alloy and SS. At present, indirect joining is usually achieved by adding intermediate layers (such as Cu, Ni or Ag) to prevent atom diffusion between Ti and Fe, Cr or Ni [7]. The tensile strength of pulsed laser welding dissimilar joint of TC4 Ti alloy to 301L SS using Cu as an interlayer reached 340 MPa, as found by XiaoYan Gu et al. [8]. Using Cu as interlayer can greatly reduce the brittleness of the weld. As their studies
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.optlastec.2020.106195 Received 8 December 2019; Received in revised form 23 February 2020; Accepted 29 February 2020 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
suggest, the amount of brittle Ti-Fe intermetallics decreased in the joint. Yet brittle Cu-Ti intermetallics, such as CuTi2 were formed. Though these Cu-Ti intermetallics were found less brittle than the Ti-Fe intermetallics, they still adversely impacted the ductility of the joint. Since the intermediate layer formed an intermetallic phase with Ti and Fe, the strength of this weld depended on the brittleness of TixMy and FexMy (M-metal of the intermediate layer) relative to TixFey and the spatial distribution of intermetallic compounds in the joint [9]. Therefore, as long as the intermediate layer was completely melted, the Ti and Fe elements reacted in the molten pool to produce a Ti-Fe intermetallic compound during the welding process. According to the study of G. Casalino et al., the feasibility of using a fiber laser to perform a dissimilar metal joining was explored. AZ31B magnesium and 316 stainless steel were autogenously joined in butt configuration [10]. The metallurgy of fusion zone indicated the effectiveness of phases coalescence, without mixing at liquid states. As reported by Gang Li et al. [3], the feasibility of fiber laser butt joining AZ31B alloy to 304 stainless steels with copper foil is investigated. The microstructure and mechanical properties of AZ31B/304SS butt joints during laser welding are studied. J.P Oliveira et al. [11] studied the laser welding of a precipitation strengthened Ni-rich NiTiHf high temperature shape memory alloy is reported for the first time. By one pass welding, Yan zhang et al. [12–13] studied pulsed laser welding of SUS 301L SS to TC4 Ti alloy via pure Nb interlayer. The joint was formed with one welding zone and a reaction layer separated by residual unmelted Nb interlayer. The unmelted Nb interlayer served as a diffusion barrier between Ti and Fe to restrain the formation of Ti-Fe intermetallics. The mechanical performance of the joint was determined by the reaction layer at the Nb-SS interface with a tensile strength of 370 MPa. Yet Fe-Nb intermetallics were formed in the reaction layer, which decreased the mechanical property of the joints. As reported by Shailesh N et al. [14], the effects of the heat source position offset on mechanical properties of dissimilar joints where in experimental investigations have been carried out. A theoretical approach to decide optimum heat source position offset is presented here and validated with published literature results for dissimilar Al to Cu welded joints. In terms of mechanical properties, both the laser offset welding and the use of an Ni interlayer, were seen to improve the tensile strength of the dissimilar joints (above 400 MPa) compared to the centerline welding condition (around 200 MPa). Hence, LOW was confirmed to be an effective method to laser weld the NiTi/stainless steels [15]. In fact, butt welding of Ti alloy and SS is technically convenient. Moreover, laser welding as an efficient and flexible non-contact welding technology, has made major achievements in the connection of refractory materials and dissimilar materials [16]. Laser welding was particularly suitable for welding of materials with high thermal diffusivity and conductivity, crack-sensitive, different melting points [17]. Therefore, the laser is selected as a welding heat source for Ti alloy and stainless steel. As reported by Giuseppe Casalino [18], the laser offset selection is fundamental to control the heat propagation and mixing. The base metals liquid-state mixing of the Ti alloy-SS joint should be avoided during the welding process. Based on the above analysis, the welding mechanism of fusion welding and diffusion welding are combined to avoid melting and liquid mixing of the base metal during welding. The laser was used as the welding heat source, and the Cu-based solder was used as the intermediate layer material. The two welding mechanisms (fusion welding and diffusion welding) are combined to avoid melting and liquid mixing of the Ti alloy and SS during welding, and the advantages of the two welding methods are complementary. Based on the above analysis, this paper proposed to use laser welding and diffusion welding to develop a welding process to ensure partial melting of Ti alloy and avoid mixing of Ti alloy and SS. The melted Ti alloy formed a fusion weld, and a diffusion weld was formed at the interface of the unmelted Ti alloy and SS. A special kind of joint was obtained by this method, and since the unmelted Ti alloy acted as a diffusion barrier, the Ti-Fe intermetallic compound in the joint could be
Fig. 1. Schematic diagram of the welding process.
completely avoided. The relationship between joint structures, mechanical properties and fracture modes was discussed in detail. 2. Experimental The base materials used are 1 mm plates of 304 stainless steel (68 wt % Fe, 19.5 wt% Cr and 9.25 wt% Ni), TC4 Ti alloy (88 wt% Ti, 6.06 wt % Al and 4.03 wt% V). The specimens for butt welding experiments were machined into 100 mm × 80 mm × 1 mm plates. The filler metal used was 0.2 mm plate of (melting point 820 °C) Cu-base filler metal (61.2 wt% Cu, 37.2 wt% Zn, 0.28 wt% Si and 0.89 wt% Sn). Before welding, the specimens were mechanically and chemically cleaned. The gap between the edges of the Ti alloy and SS was very important to adequate heat transfer and prevent porosity formation. The specimens are clamped each other tightly in order to get the minimum gap formation between the edges. CW laser was used with average power of 1.20 kW, wavelength of 1080 nm and beam spot diameter of 0.1 mm. A schematic diagram of the welding procedure is shown in Fig. 1. In order to ensure that SS was not completely melted, the laser beam was focused on the SS plate 1.5 mm away from the Ti alloy interface. The welding parameters were: laser beam power of 480 W, defocusing distance of +5 mm, welding speed of 650 mm/min. Argon gas with the purity of 99.99% was applied as a shielding gas with total flow of 20 L/ min at top of the joint. The cross sections of joints were polished and etched in the reagent with 2 ml concentrated HNO3 and 6 ml concentrated HF. The microstructure of joints were studied by optical microscopy (Scope Axio ZEISS), scanning electron microscope SEM (S-3400) with fast energy dispersion spectrum EDS analyzerand and selected area XRD (X’Pert3 Powder) analysis. Vickers microhardness tests for the weld carried out with a 10 s load time and a 200 g load. Tensile strength of the joints was measured by using universal testing machine (MTS Insight 10 kN) with cross head speed of 2 mm/min. WRN-191 K sheathed thermocouple (measuring range −250 to 1350 °C) as the temperature sensor. 3. Results and discussion The optical microscopy image of the cross section of the joint is shown in Fig. 2a. The joint can fall into three parts: the fusion weld formed at the Ti alloy side, unmelted Ti alloy and the diffusion weld formed at the SS-Ti alloy interface. The fusion weld did not form Ti-Fe intermetallics due to the presence of unmelted Ti alloy. The average width of fusion weld, unmelted Ti alloy and diffusion weld was 2.1 mm, 0.37 mm and 0.16 mm, respectively. The unmelted part of Ti alloy acted as a heat sink absorbing a significant amount of energy from the welding pool and transferring it to the SS side [19,20]. Because the microstructure of the fusion weld is quite different from that of the diffusion weld, the diffusion weld becomes black after etching. Fig. 2b presents the optical image before corrosion of the diffusion weld. It does not present such defects as pores and macro-cracks. To simplify the test and make the test more precise, this paper studied the heat cycle test from the side slot of the test plate. The structure was provided with opposite grooves on the SS and Ti alloy side faces, and the interior of a sealing groove formed by the groove was provided with a thermocouple (Fig. 3a). The thermal cycle curve obtained from SS-Ti alloy interface 2
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 2. Macroscopic feature of the joint: (a) optical image of the cross section of the joint; (b) optical image before corrosion of the SS-Ti alloy interface.
filler metal, and causes its component to deviate from the original component. As the Fig. 5 shows, the interdiffusion of Cu, Zn, Ti and Fe elements occurred at diffusion welding interface (SS-filler metal and filler metal-Ti alloy). At this moment, the dissolution of Ti and Fe into the filler metal occurred under the high concentration gradient, which formed solid-phase reaction layer, and this reaction layer exists only in the smaller region of the SS-Ti alloy interface. As shown in Fig. 4e and f, zone II and zone III were reaction layers formed by element diffusion. Based on Cu-Fe-Zn phase diagram, the microstructure of zone II was defined as β-CuZn + Fe3Zn7. Based on Cu-Ti-Zn phase diagram, the microstructure of zone III was defined as Ti2Zn3 + Ti3Cu4. Therefore, the main microstructures of diffusion weld were β-CuZn + Fe3Zn7, βCuZn and Ti2Zn3 + Ti3Cu4. As shown in Fig. 6, the microhardness distribution in the joint was non-uniform. The average value of Ti alloy was 312 HV, which was slightly below SS. The highest hardness of the fusion weld is located at the weld center, because the temperature of center of the welding pool is the highest during welding, where the concentration of the solute reaches a maximum, and the alloying elements by solid solution strengthened to increase the hardness. Moreover, the hardness of the diffusion weld was very low compared to the fusion weld. The hardness of diffusion weld was low because filler metal was simple metals. Therefore it can relatively deform easily to reduce the residual stresses in the inner of SS-Ti alloy joint. As shown in Fig. 7a, the maximum tensile strength of the joint was about 128 MPa, which was 23% of initial SS (550 MPa). The joint fractured in Ti alloy side of the diffusion weld during tensile tests (Fig. 7b). Fig. 7c shows fracture surface of the joint exhibiting typical brittle characteristics. Moreover, as shown in Fig. 7d, XRD analyses of fracture surface detected Ti2Zn3 and Ti3Cu4 phases. This confirmed the presence of Ti-Zn and Fe-Zn intermetallics at fracture surfaces. It should be noted that there was no Ti-Fe intermetallics in the diffusion weld. Reaction layer at Ti alloy side in diffusion weld became the weak zone of the joint, which led to the failure in the tensile test.
during welding is shown in Fig. 3b. It is suggested that the peak temperatures of diffusion interface (filler metal-Ti alloy and SS-filler metal) were 749 °C and 677 °C, respectively. Hence, the filler metal of SS-Ti alloy interface had a high temperature during welding although it was not subjected to laser radiation. The temperature was high enough to promote atomic interdiffusion. This meets the temperature requirement for diffusion welding. Moreover, the local heating of the Ti alloy side caused uneven volume expansion and thermal stress was produced, which helped to obtain an intimate contact between the SS, Cu-based fillers and Ti alloy surface. The high temperature and the intimate contact at the SS-Ti alloy interface provided favorable conditions for atomic (Cu, Zn, Ti, Fe) interdiffusion. Therefore, a diffusion weld was formed originated from atomic (Cu, Zn, Ti, Fe) interdiffusion at the filler metal-Ti alloy and SS-filler metal interface. The unmelted Ti alloy acted as a barrier to mixing of the two base materials, which eliminated the formation of brittle Ti-Fe intermetallics in the joint. Additionally, the unmelted Ti alloy was beneficial to relieve and accommodate the thermal stress in the SS-Ti alloy joint, which could help to improve the mechanical properties of the joints. The optical image of the fusion weld is shown in Fig. 4a, and no defects were observed in it. The fusion weld mainly consists of acicular α' martensite. The optical image of the diffusion weld at SS-Ti alloy interface is shown in Fig. 4b. The SEM image of the diffusion weld is shown in Fig. 4c. It can be observed that, the diffusion weld contained three zones marked as I, II and III sorted by their morphologies and colors. Fig. 4d, e and f correspond to the three zones in Fig. 4c, respectively. The compositions of each zone (denoted by letter A-C in Fig. 4) were studied using SEM-EDS. EDS analysis was applied to these zones to measure the compositions of the reaction products and the results are listed in Table 1. Based on the previous analysis, the microstructure of the diffusion weld was mainly composed of Cu-based fillers. The chemical composition of zone I was consistent with the Cubased fillers. Based on the EDS analyses results and Cu-Zn phase diagram, the main microstructure of zone I was defined as β-CuZn phase. When the laser beam was focused near the SS-Ti alloy interface, the element diffusion occurs immediately between the base materials and
Fig. 3. Temperature measurement test of SS-Ti alloy interface during welding: (a) thermocouple distribution; (b) thermal cycle curve of test points. 3
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 4. Microstructures of the joint: (a) optical image of the fusion weld; (b) optical image of the diffusion weld; (c) SEM image of the diffusion weld; (d) refer to the zone I in c; (e) refer to the zone II in c; (f) refer to the zone III in c. Table 1 The chemical composition of each phase (wt.%). Region
Composition% Ti
A B C
35.5
Potential phases
Fe
Cu
Zn
48.2
58.7 37.7 37.1
41.3 14.1 27.4
Ni
Cr
V β-CuZn β-CuZn + Fe3Zn7 Ti2Zn3 + Ti3Cu4
Fig. 6. Vickers microhardness measurements at semi-height of the joint (zero point situated in the center of the joint).
during welding, and the thickness of diffusion weld can reach hundreds of micrometres. The tensile resistance of the joint was determined by diffusion weld. The maximum tensile strength of joint was 128 MPa. 2. In the temperature range of 749–677 °C, the atomic diffusion only existed in the small area of Ti-SS interface and formed a narrow eutectic reaction zone. The diffusion weld consisted of a lamellar structure. The diffusion weld contained three zones marked as I, II and III sorted by their morphologies and colors. A diffusion weld was formed at the SS-Ti alloy interface with the main microstructure of β-CuZn + Fe3Zn7, β-CuZn and Ti2Zn3 + Ti3Cu4.
Fig. 5. EDS line analysis results in the diffusion weld of the joint.
4. Conclusions Declaration of Competing Interest
1. With a laser beam offset of 1.5 mm for Ti alloy side of the joint, the unmelted Ti alloy was selected as an barrier to avoid mixing of the SS and Ti alloy. Due to the presence of unmelted Ti alloys, the formation of Ti-Fe intermetallic compounds was not found in the joint, which improved the mechanical properties of the joints. A great amount of atomic diffusion occurs at the SS-Ti alloy interface
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
4
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 7. Tensile test results of the joint: (a) tensile test curve; (b) fracture location; (c) SEM image of fracture surface; (d) XRD analysis results of fracture surface.
Acknowledgements
[8] Yan Zhang, Daqian Sun, Gu Xiaoyan, Int. J. Adv. Manuf. Technol. 90 (2017) 953–961. [9] M.I.S. Ismail, Y. Okamoto, A. Okada, Y. Uno, K. Ueoka, Int. J. Precis. Eng. Manuf. 13 (2012) 321–329. [10] G. Casalino, P. Guglielmi, V.D. Lorusso, M. Mortello, J. Mater. Process. Technol. 242 (2017) 49–59. [11] J.P. Oliveira, N. Schell, N. Zhou, L. Wood, O. Benafan, Mater. Des. 162 (2019) 229–234. [12] Yan Zhang, Daqian Sun, Xiaoyan Gu, Int. J. Adv. Manuf. Technol. 94 (2018) 1073–1085. [13] Yan Zhang, Daqian Sun, Gu. Xiaoyan, Mater. Lett. 185 (2016) 152–155. [14] Shailesh N. Pandya, Jyoti Menghani, Mater. Today: Proc. 5 (2018) 26974–26980. [15] A. Shamsolhodaei, J.P. Oliveira, N. Schell, E. Maawad, B. Panton, Y.N. Zhou, Intermetallics 116 (2020) 106656. [16] W. Chen, A. Paul, M. Pal, Mater. Des. 30 (2009) 245–251. [17] K. Amit, Y. Duck, C. Darek, Opt. Laser Eng. 51 (2013) 1143–1152. [18] Giuseppe Casalino, Paola Le, Metals 7 (2017) 1–17. [19] Yan Zhang, Daqian Sun, Gu Xiaoyan, et al., J. Mater. Eng. Perf. 28 (2019) 6092–6101. [20] Yan Zhang, Yankun Chen, Jianping Zhou, et al., J. Mater. Res. Technol. 9 (1) (2020) 465–477.
This research was mainly supported by Postdoctoral Science Fund of China (2019M663861). This research also was supported by Natural Science Foundation of China (51765063). References [1] Yan Zhang, Daqian Sun, Gu. Xiaoyan, J. Mater. Eng. Perform. 28 (2019) 6092–6101. [2] J.P. Oliveira, B. Panton, Z. Zeng, Acta Mater. 105 (2016) 9–15. [3] Gang Li, Lu. Xiaofeng, Xiaolei Zhu, Yupeng Guo, Jufeng Song, Opt. Laser Technol. 117 (2019) 215–226. [4] I. Tomashchuk, P. Sallamand, H. Andrzejewski, D. Grevey, Intermetallics 19 (2011) 1466–1473. [5] C. Shuhai, Z. Mingxin, H. Jihua, Mater. Des. 53 (2014) 504–511. [6] Y. Zhang, D.Q. Sun, X.Y. Gu, Y.J. Liu, Int. J. Adv. Manuf. Technol. 90 (2017) 953–961. [7] S.A. Mousavi, P.F. Sartangi, Mater. Des. 30 (2009) 459–468.
5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Characterization of laser beam offset welding of titanium to steel with 38Zn61Cu alloy filler
T
⁎
Yan Zhanga, , YanKun Chena, JianPing Zhoua, RuiLei Xueb, DaQian Sunb, HongMei Lib a b
School of Mechanical Engineering, Xinjiang University, Wulumuqi 830000, China Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, Changchun 130022, China
H I GH L IG H T S
possibility of laser welding-diffusion bonding for Ti alloy-SS joint with Cu-base filler metal was studied. • The joint based on two different joining mechanisms. • AThecomposite Ti alloy acted as a barrier to mixing of the base materials. • The unmelted • unmelted Ti alloy and filler metal prevented the formation of brittle Ti-Fe intermetallics in the joint.
A R T I C LE I N FO
A B S T R A C T
Keywords: TC4 Ti alloy 304 austenitic stainless steel Laser welding Diffusion welding Microstructure Filler metal
Laser welding of TC4 Titanium (Ti) alloy to 304 austenitic stainless steel (SS) has been applied using 38Zn-61Cu alloy as filler metal. A new welding process for SS-Ti alloy joint was introduced on the basis of the controlling the formation of Ti-Fe intermetallics in the joint. One pass welding involving creation of a joint with one fusion weld and one diffusion weld separated by remaining unmelted Ti alloy. When laser beam on the Ti alloy side was 1.5 mm, Ti alloy would not be completely melted in joint. Through heat conduction of unmelted Ti alloy, the atomic diffusion occurred at the SS-Ti alloy interface. A diffusion weld was formed at the SS-Ti alloy interface with the main microstructure of β-CuZn + Fe3Zn7, β-CuZn and Ti2Zn3 + Ti3Cu4. The joint fractured at the diffusion weld with the maximum tensile strength of 128 MPa.
1. Introduction The aerospace and nuclear industries have strong demand for Titanium (Ti) alloy and stainless steel (SS) applications due to the potential advantages of Ti alloys in terms of material cost savings, weight reduction, design flexibility and complexity, and improved product functionality [1,2]. As reported by Gang Li et al. [3], the welding processes and their effects on materials were also subjected to tremendous research efforts to understand the connection and interaction between processing, microstructure and mechanical properties. However, the joint between Ti alloy and SS is facing great difficulties due to the poor metallurgical compatibility between Ti alloy and SS, and the serious mismatch of physical and mechanical properties [4]. The research of TIG welding between pure Ti and pure Fe showed that the continuous distribution of TiFe and TiFe2 was the fundamental cause of joint embrittlement. The hardness of weld metal was within the range of HV740-HV1324 [5]. The presence of these brittle phases reduced the
⁎
strength and ductility of the joint. In this case, complex intermetallic compounds were formed between Ti, Fe, Cr and Ni, making the weld more brittle [6]. Therefore, the resulting weld was very brittle and easy to crack. Due to the physical properties of the Ti alloy-SS joint, such as the significant difference in thermal expansion coefficient and melting point, the joint is excessively deformed and residual stress, are clamped each other. Current research indicates that conventional fusion welding would result in the formation of a thick and brittle intermetallic compound layer and the accumulation of residual stress at the joint. Due to the metallurgical incompatibility of Ti alloy and SS, the direct fusion welding method is not suitable for the welding of Ti alloy and SS. At present, indirect joining is usually achieved by adding intermediate layers (such as Cu, Ni or Ag) to prevent atom diffusion between Ti and Fe, Cr or Ni [7]. The tensile strength of pulsed laser welding dissimilar joint of TC4 Ti alloy to 301L SS using Cu as an interlayer reached 340 MPa, as found by XiaoYan Gu et al. [8]. Using Cu as interlayer can greatly reduce the brittleness of the weld. As their studies
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.optlastec.2020.106195 Received 8 December 2019; Received in revised form 23 February 2020; Accepted 29 February 2020 0030-3992/ © 2020 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
suggest, the amount of brittle Ti-Fe intermetallics decreased in the joint. Yet brittle Cu-Ti intermetallics, such as CuTi2 were formed. Though these Cu-Ti intermetallics were found less brittle than the Ti-Fe intermetallics, they still adversely impacted the ductility of the joint. Since the intermediate layer formed an intermetallic phase with Ti and Fe, the strength of this weld depended on the brittleness of TixMy and FexMy (M-metal of the intermediate layer) relative to TixFey and the spatial distribution of intermetallic compounds in the joint [9]. Therefore, as long as the intermediate layer was completely melted, the Ti and Fe elements reacted in the molten pool to produce a Ti-Fe intermetallic compound during the welding process. According to the study of G. Casalino et al., the feasibility of using a fiber laser to perform a dissimilar metal joining was explored. AZ31B magnesium and 316 stainless steel were autogenously joined in butt configuration [10]. The metallurgy of fusion zone indicated the effectiveness of phases coalescence, without mixing at liquid states. As reported by Gang Li et al. [3], the feasibility of fiber laser butt joining AZ31B alloy to 304 stainless steels with copper foil is investigated. The microstructure and mechanical properties of AZ31B/304SS butt joints during laser welding are studied. J.P Oliveira et al. [11] studied the laser welding of a precipitation strengthened Ni-rich NiTiHf high temperature shape memory alloy is reported for the first time. By one pass welding, Yan zhang et al. [12–13] studied pulsed laser welding of SUS 301L SS to TC4 Ti alloy via pure Nb interlayer. The joint was formed with one welding zone and a reaction layer separated by residual unmelted Nb interlayer. The unmelted Nb interlayer served as a diffusion barrier between Ti and Fe to restrain the formation of Ti-Fe intermetallics. The mechanical performance of the joint was determined by the reaction layer at the Nb-SS interface with a tensile strength of 370 MPa. Yet Fe-Nb intermetallics were formed in the reaction layer, which decreased the mechanical property of the joints. As reported by Shailesh N et al. [14], the effects of the heat source position offset on mechanical properties of dissimilar joints where in experimental investigations have been carried out. A theoretical approach to decide optimum heat source position offset is presented here and validated with published literature results for dissimilar Al to Cu welded joints. In terms of mechanical properties, both the laser offset welding and the use of an Ni interlayer, were seen to improve the tensile strength of the dissimilar joints (above 400 MPa) compared to the centerline welding condition (around 200 MPa). Hence, LOW was confirmed to be an effective method to laser weld the NiTi/stainless steels [15]. In fact, butt welding of Ti alloy and SS is technically convenient. Moreover, laser welding as an efficient and flexible non-contact welding technology, has made major achievements in the connection of refractory materials and dissimilar materials [16]. Laser welding was particularly suitable for welding of materials with high thermal diffusivity and conductivity, crack-sensitive, different melting points [17]. Therefore, the laser is selected as a welding heat source for Ti alloy and stainless steel. As reported by Giuseppe Casalino [18], the laser offset selection is fundamental to control the heat propagation and mixing. The base metals liquid-state mixing of the Ti alloy-SS joint should be avoided during the welding process. Based on the above analysis, the welding mechanism of fusion welding and diffusion welding are combined to avoid melting and liquid mixing of the base metal during welding. The laser was used as the welding heat source, and the Cu-based solder was used as the intermediate layer material. The two welding mechanisms (fusion welding and diffusion welding) are combined to avoid melting and liquid mixing of the Ti alloy and SS during welding, and the advantages of the two welding methods are complementary. Based on the above analysis, this paper proposed to use laser welding and diffusion welding to develop a welding process to ensure partial melting of Ti alloy and avoid mixing of Ti alloy and SS. The melted Ti alloy formed a fusion weld, and a diffusion weld was formed at the interface of the unmelted Ti alloy and SS. A special kind of joint was obtained by this method, and since the unmelted Ti alloy acted as a diffusion barrier, the Ti-Fe intermetallic compound in the joint could be
Fig. 1. Schematic diagram of the welding process.
completely avoided. The relationship between joint structures, mechanical properties and fracture modes was discussed in detail. 2. Experimental The base materials used are 1 mm plates of 304 stainless steel (68 wt % Fe, 19.5 wt% Cr and 9.25 wt% Ni), TC4 Ti alloy (88 wt% Ti, 6.06 wt % Al and 4.03 wt% V). The specimens for butt welding experiments were machined into 100 mm × 80 mm × 1 mm plates. The filler metal used was 0.2 mm plate of (melting point 820 °C) Cu-base filler metal (61.2 wt% Cu, 37.2 wt% Zn, 0.28 wt% Si and 0.89 wt% Sn). Before welding, the specimens were mechanically and chemically cleaned. The gap between the edges of the Ti alloy and SS was very important to adequate heat transfer and prevent porosity formation. The specimens are clamped each other tightly in order to get the minimum gap formation between the edges. CW laser was used with average power of 1.20 kW, wavelength of 1080 nm and beam spot diameter of 0.1 mm. A schematic diagram of the welding procedure is shown in Fig. 1. In order to ensure that SS was not completely melted, the laser beam was focused on the SS plate 1.5 mm away from the Ti alloy interface. The welding parameters were: laser beam power of 480 W, defocusing distance of +5 mm, welding speed of 650 mm/min. Argon gas with the purity of 99.99% was applied as a shielding gas with total flow of 20 L/ min at top of the joint. The cross sections of joints were polished and etched in the reagent with 2 ml concentrated HNO3 and 6 ml concentrated HF. The microstructure of joints were studied by optical microscopy (Scope Axio ZEISS), scanning electron microscope SEM (S-3400) with fast energy dispersion spectrum EDS analyzerand and selected area XRD (X’Pert3 Powder) analysis. Vickers microhardness tests for the weld carried out with a 10 s load time and a 200 g load. Tensile strength of the joints was measured by using universal testing machine (MTS Insight 10 kN) with cross head speed of 2 mm/min. WRN-191 K sheathed thermocouple (measuring range −250 to 1350 °C) as the temperature sensor. 3. Results and discussion The optical microscopy image of the cross section of the joint is shown in Fig. 2a. The joint can fall into three parts: the fusion weld formed at the Ti alloy side, unmelted Ti alloy and the diffusion weld formed at the SS-Ti alloy interface. The fusion weld did not form Ti-Fe intermetallics due to the presence of unmelted Ti alloy. The average width of fusion weld, unmelted Ti alloy and diffusion weld was 2.1 mm, 0.37 mm and 0.16 mm, respectively. The unmelted part of Ti alloy acted as a heat sink absorbing a significant amount of energy from the welding pool and transferring it to the SS side [19,20]. Because the microstructure of the fusion weld is quite different from that of the diffusion weld, the diffusion weld becomes black after etching. Fig. 2b presents the optical image before corrosion of the diffusion weld. It does not present such defects as pores and macro-cracks. To simplify the test and make the test more precise, this paper studied the heat cycle test from the side slot of the test plate. The structure was provided with opposite grooves on the SS and Ti alloy side faces, and the interior of a sealing groove formed by the groove was provided with a thermocouple (Fig. 3a). The thermal cycle curve obtained from SS-Ti alloy interface 2
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 2. Macroscopic feature of the joint: (a) optical image of the cross section of the joint; (b) optical image before corrosion of the SS-Ti alloy interface.
filler metal, and causes its component to deviate from the original component. As the Fig. 5 shows, the interdiffusion of Cu, Zn, Ti and Fe elements occurred at diffusion welding interface (SS-filler metal and filler metal-Ti alloy). At this moment, the dissolution of Ti and Fe into the filler metal occurred under the high concentration gradient, which formed solid-phase reaction layer, and this reaction layer exists only in the smaller region of the SS-Ti alloy interface. As shown in Fig. 4e and f, zone II and zone III were reaction layers formed by element diffusion. Based on Cu-Fe-Zn phase diagram, the microstructure of zone II was defined as β-CuZn + Fe3Zn7. Based on Cu-Ti-Zn phase diagram, the microstructure of zone III was defined as Ti2Zn3 + Ti3Cu4. Therefore, the main microstructures of diffusion weld were β-CuZn + Fe3Zn7, βCuZn and Ti2Zn3 + Ti3Cu4. As shown in Fig. 6, the microhardness distribution in the joint was non-uniform. The average value of Ti alloy was 312 HV, which was slightly below SS. The highest hardness of the fusion weld is located at the weld center, because the temperature of center of the welding pool is the highest during welding, where the concentration of the solute reaches a maximum, and the alloying elements by solid solution strengthened to increase the hardness. Moreover, the hardness of the diffusion weld was very low compared to the fusion weld. The hardness of diffusion weld was low because filler metal was simple metals. Therefore it can relatively deform easily to reduce the residual stresses in the inner of SS-Ti alloy joint. As shown in Fig. 7a, the maximum tensile strength of the joint was about 128 MPa, which was 23% of initial SS (550 MPa). The joint fractured in Ti alloy side of the diffusion weld during tensile tests (Fig. 7b). Fig. 7c shows fracture surface of the joint exhibiting typical brittle characteristics. Moreover, as shown in Fig. 7d, XRD analyses of fracture surface detected Ti2Zn3 and Ti3Cu4 phases. This confirmed the presence of Ti-Zn and Fe-Zn intermetallics at fracture surfaces. It should be noted that there was no Ti-Fe intermetallics in the diffusion weld. Reaction layer at Ti alloy side in diffusion weld became the weak zone of the joint, which led to the failure in the tensile test.
during welding is shown in Fig. 3b. It is suggested that the peak temperatures of diffusion interface (filler metal-Ti alloy and SS-filler metal) were 749 °C and 677 °C, respectively. Hence, the filler metal of SS-Ti alloy interface had a high temperature during welding although it was not subjected to laser radiation. The temperature was high enough to promote atomic interdiffusion. This meets the temperature requirement for diffusion welding. Moreover, the local heating of the Ti alloy side caused uneven volume expansion and thermal stress was produced, which helped to obtain an intimate contact between the SS, Cu-based fillers and Ti alloy surface. The high temperature and the intimate contact at the SS-Ti alloy interface provided favorable conditions for atomic (Cu, Zn, Ti, Fe) interdiffusion. Therefore, a diffusion weld was formed originated from atomic (Cu, Zn, Ti, Fe) interdiffusion at the filler metal-Ti alloy and SS-filler metal interface. The unmelted Ti alloy acted as a barrier to mixing of the two base materials, which eliminated the formation of brittle Ti-Fe intermetallics in the joint. Additionally, the unmelted Ti alloy was beneficial to relieve and accommodate the thermal stress in the SS-Ti alloy joint, which could help to improve the mechanical properties of the joints. The optical image of the fusion weld is shown in Fig. 4a, and no defects were observed in it. The fusion weld mainly consists of acicular α' martensite. The optical image of the diffusion weld at SS-Ti alloy interface is shown in Fig. 4b. The SEM image of the diffusion weld is shown in Fig. 4c. It can be observed that, the diffusion weld contained three zones marked as I, II and III sorted by their morphologies and colors. Fig. 4d, e and f correspond to the three zones in Fig. 4c, respectively. The compositions of each zone (denoted by letter A-C in Fig. 4) were studied using SEM-EDS. EDS analysis was applied to these zones to measure the compositions of the reaction products and the results are listed in Table 1. Based on the previous analysis, the microstructure of the diffusion weld was mainly composed of Cu-based fillers. The chemical composition of zone I was consistent with the Cubased fillers. Based on the EDS analyses results and Cu-Zn phase diagram, the main microstructure of zone I was defined as β-CuZn phase. When the laser beam was focused near the SS-Ti alloy interface, the element diffusion occurs immediately between the base materials and
Fig. 3. Temperature measurement test of SS-Ti alloy interface during welding: (a) thermocouple distribution; (b) thermal cycle curve of test points. 3
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 4. Microstructures of the joint: (a) optical image of the fusion weld; (b) optical image of the diffusion weld; (c) SEM image of the diffusion weld; (d) refer to the zone I in c; (e) refer to the zone II in c; (f) refer to the zone III in c. Table 1 The chemical composition of each phase (wt.%). Region
Composition% Ti
A B C
35.5
Potential phases
Fe
Cu
Zn
48.2
58.7 37.7 37.1
41.3 14.1 27.4
Ni
Cr
V β-CuZn β-CuZn + Fe3Zn7 Ti2Zn3 + Ti3Cu4
Fig. 6. Vickers microhardness measurements at semi-height of the joint (zero point situated in the center of the joint).
during welding, and the thickness of diffusion weld can reach hundreds of micrometres. The tensile resistance of the joint was determined by diffusion weld. The maximum tensile strength of joint was 128 MPa. 2. In the temperature range of 749–677 °C, the atomic diffusion only existed in the small area of Ti-SS interface and formed a narrow eutectic reaction zone. The diffusion weld consisted of a lamellar structure. The diffusion weld contained three zones marked as I, II and III sorted by their morphologies and colors. A diffusion weld was formed at the SS-Ti alloy interface with the main microstructure of β-CuZn + Fe3Zn7, β-CuZn and Ti2Zn3 + Ti3Cu4.
Fig. 5. EDS line analysis results in the diffusion weld of the joint.
4. Conclusions Declaration of Competing Interest
1. With a laser beam offset of 1.5 mm for Ti alloy side of the joint, the unmelted Ti alloy was selected as an barrier to avoid mixing of the SS and Ti alloy. Due to the presence of unmelted Ti alloys, the formation of Ti-Fe intermetallic compounds was not found in the joint, which improved the mechanical properties of the joints. A great amount of atomic diffusion occurs at the SS-Ti alloy interface
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
4
Optics and Laser Technology 127 (2020) 106195
Y. Zhang, et al.
Fig. 7. Tensile test results of the joint: (a) tensile test curve; (b) fracture location; (c) SEM image of fracture surface; (d) XRD analysis results of fracture surface.
Acknowledgements
[8] Yan Zhang, Daqian Sun, Gu Xiaoyan, Int. J. Adv. Manuf. Technol. 90 (2017) 953–961. [9] M.I.S. Ismail, Y. Okamoto, A. Okada, Y. Uno, K. Ueoka, Int. J. Precis. Eng. Manuf. 13 (2012) 321–329. [10] G. Casalino, P. Guglielmi, V.D. Lorusso, M. Mortello, J. Mater. Process. Technol. 242 (2017) 49–59. [11] J.P. Oliveira, N. Schell, N. Zhou, L. Wood, O. Benafan, Mater. Des. 162 (2019) 229–234. [12] Yan Zhang, Daqian Sun, Xiaoyan Gu, Int. J. Adv. Manuf. Technol. 94 (2018) 1073–1085. [13] Yan Zhang, Daqian Sun, Gu. Xiaoyan, Mater. Lett. 185 (2016) 152–155. [14] Shailesh N. Pandya, Jyoti Menghani, Mater. Today: Proc. 5 (2018) 26974–26980. [15] A. Shamsolhodaei, J.P. Oliveira, N. Schell, E. Maawad, B. Panton, Y.N. Zhou, Intermetallics 116 (2020) 106656. [16] W. Chen, A. Paul, M. Pal, Mater. Des. 30 (2009) 245–251. [17] K. Amit, Y. Duck, C. Darek, Opt. Laser Eng. 51 (2013) 1143–1152. [18] Giuseppe Casalino, Paola Le, Metals 7 (2017) 1–17. [19] Yan Zhang, Daqian Sun, Gu Xiaoyan, et al., J. Mater. Eng. Perf. 28 (2019) 6092–6101. [20] Yan Zhang, Yankun Chen, Jianping Zhou, et al., J. Mater. Res. Technol. 9 (1) (2020) 465–477.
This research was mainly supported by Postdoctoral Science Fund of China (2019M663861). This research also was supported by Natural Science Foundation of China (51765063). References [1] Yan Zhang, Daqian Sun, Gu. Xiaoyan, J. Mater. Eng. Perform. 28 (2019) 6092–6101. [2] J.P. Oliveira, B. Panton, Z. Zeng, Acta Mater. 105 (2016) 9–15. [3] Gang Li, Lu. Xiaofeng, Xiaolei Zhu, Yupeng Guo, Jufeng Song, Opt. Laser Technol. 117 (2019) 215–226. [4] I. Tomashchuk, P. Sallamand, H. Andrzejewski, D. Grevey, Intermetallics 19 (2011) 1466–1473. [5] C. Shuhai, Z. Mingxin, H. Jihua, Mater. Des. 53 (2014) 504–511. [6] Y. Zhang, D.Q. Sun, X.Y. Gu, Y.J. Liu, Int. J. Adv. Manuf. Technol. 90 (2017) 953–961. [7] S.A. Mousavi, P.F. Sartangi, Mater. Des. 30 (2009) 459–468.
5