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Cu interlayer
Journal Pre-proof Microstructure and mechanical properties of laser welding of Ti6Al4V to Inconel 718 using Nb /Cu interlayer Jing Liu, Huan Liu, Xiao-Long Gao, Haokui Yu
PII:
S0924-0136(19)30440-6
DOI:
https://doi.org/10.1016/j.jmatprotec.2019.116467
Reference:
PROTEC 116467
To appear in:
Journal of Materials Processing Tech.
Received Date:
12 January 2019
Revised Date:
19 September 2019
Accepted Date:
21 October 2019
Please cite this article as: Liu J, Liu H, Gao X-Long, Yu H, Microstructure and mechanical properties of laser welding of Ti6Al4V to Inconel 718 using Nb /Cu interlayer, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116467
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Microstructure and mechanical properties of laser welding of Ti6Al4V to Inconel 718 using Nb /Cu interlayer Jing Liu, Huan Liu, Xiao-Long Gao , Haokui Yu Shannxi Key Laboratory of Advanced Manufacturing and Evalutaion of Robot Key Components，Baoji University of Arts and Sciences, Baoji, 721016, China
Corresponding author at: School of Mechanical Engineering, Baoji University of Arts and Sciences, Baoji, 721016, China. E-mail address:[email protected], Tel: +86-0917-3364295
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Abstract
To improve the quality of the joints between Ti6Al4V and Inconel 718, fiber
laser welding with Nb/Cu multi-interlayer was attempted by laser welding induced
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brazing-welding method. The hybrid joint comprising different metallurgical areas
was created based on fusion welding and brazing welding mechanisms through one
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welding. Ti6Al4V and Nb well bonded by the fusion welding, and the fusion zone was characterized by the formation of (α+β Ti, Nb) solid solutions. Nb and Inconel
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718 were joined by brazing because of the melting of Cu foil, and the Nb/Cu/Inconel 718 interface mainly consisted of the Nb-based, Cu-based, and Ni-based solid solutions. The joint failed in the Nb/Cu/Inconel 718 interface with the maximum
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strength of 260 MPa, exhibiting brittle-like failure behavior due to the formation of voids and microcracks at the Nb/Cu/Inconel 718 interface. The addition of Nb/Cu
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multi-interlayer can help prevent the formation of IMCs and improves the mechanical
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properties of the joint.
Keywords: Dissimilar welding; Ti alloy; Inconel 718; Interlayer; Intermetallic compounds
1 Introduction 1
As demonstrated by Ezugwu et al. (1997), titanium alloys are widely used in the aerospace, petrochemical, and biological medicine industries owing to its high specific strength, good erosion resistance, excellent high-temperature mechanical properties, and biocompatibility. In addition, nickel alloys with excellent mechanical properties and oxidation resistance at elevated temperatures are particularly suitable for manufacturing high-temperature resistant parts (Ramkumara et al., 2017). Therefore, high quality welding of titanium and nickel alloys can combine their respective advantages and increase the design flexibility and the product functionality.
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However, a series of problems have emerged in the welding of Ti and Ni alloys due to differences in their physical and chemical properties. Chatterjee et al. (2006 and 2008) reported that the brittle Ti2Ni and TiNi3 intermetallic compounds (IMCs)
can easily be formed due to the metallurgical incompatibilities of Ti and Ni. Seretsky
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et al. (1976) proved that the crack in the laser-welded titanium to nickel cannot be
eliminated by changing the laser power or by rewelding the samples. Alemná et al.
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(1995) reported the formation of a large number of Ti2Ni and Cr2Ti IMCs when diffusion welding is used for bonding Ti6242 alloy to Inconel 625 alloy.
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Significant attempts have been made to inhibit the formation of TixNiy IMCs during the welding of Ti and Ni alloys. Chen et al. (2011) found that the number of
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the TixNiy brittle phases can be reduced in the laser welded joints of Ti6Al4V and Inconel 718 by the combination of low heat input and laser offset welding. Gao et al. (2018) selected a Cu foil as the interlayer for the gas tungsten arc welding of
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TiBw/Ti6Al4V composites and Inconel 718 alloy and found that the main products from Ti-Ni IMCs changed into the combined of Ti-Cu, Ti-Ni-Cu, and Ti-Ni IMCs in
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the joints. Zhang et al. (2018) studied the electron beam welding of TA15 to GH600 using Cu interlayer. A Cu-based solid solution and Ti-Cu IMCs were formed in the weld, and the tensile strength of the joint was up to 282 MPa. Li et al. (2017) found that the interfacial microstructure mainly comprised continuous Ti2Ni phase for Ti600/Ti-Zr-Ni-Cu/Ni-25a%Si brazed joint at 1213 K. Although the insertion of interlayer can suppress the formation of Ti-Ni IMCs by reducing the amount of Ti and Ni in the weld metal, the Ti-Ni IMCs are still formed in the joint, resulting in poor 2
plasticity of the joint (Zoeram et al., 2014). To completely inhibit the formation of TixNiy brittle phases, the mixing between Ti and Ni must be stopped sufficiently in the molten pool. The Nb interlayer with a higher melting point than the two dissimilar base materials has proven to be successful diffusion barriers between dissimilar materials (Oliveira et al., 2016). Grill et al. (2006) did not found the formation of IMCs in the welding of dissimilar metals Ti and Nb. Torkamany et al. (2014) successfully laser welded Ti6Al4V to Nb without the formation of any IMCs, and the joint failed at the Nb base metal. Gao et al. (2018)
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discovered that the Ti-Ni IMCs in the dissimilar joint between Ti6Al4V and Inconel 718 could be completely eliminated with Nb as an interlayer during the pulsed laser welding; however, the eutectic reaction layer composed of NbNi3 and Nb7Ni6 IMCs formed at the Nb/Inconel 718 interface. The formation of IMCs between Nb and Ni
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still deteriorates the mechanical properties of the dissimilar joints of Ti6Al4V and
Inconel 718. The dissimilar joint of Ti6Al4V and Inconel 718 using Nb interlayer
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failed in the Nb-Ni IMCs layer at the ultimate tensile strength of 145 MPa and exhibited cleavage fracture. The multi-interlayer for the welded joints of Ti and Ni
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may be an ideal solution for preventing the formation of brittle IMCs. Song et al. (2017) confirmed that Cu/Nb multi-interlayer can prevent the formation of IMCs for
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diffusion bonding Ti6Al4V to AISI316L stainless steel, and the joints were broken in the ductile mode. According to the Nb-Cu and Cu-Ni binary phase diagrams, no IMCs formed for Cu/Nb and Ni/Cu dissimilar metals (ASM handbook volume 03, 1992);
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therefore, Cu/Nb multi-interlayer is identified as a potential intermediate structure for bonding Ti alloys to Ni alloys.
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In this study, the effect of Nb/Cu interlayers on the microstructural and
mechanical properties of the pulsed laser welding of Ti6Al4V and Inconel 718 was systemically investigated. 2 Materials and methods 2.1 Materials preparation The base metals are Ti6Al4V and Inconel 718 sheets with the dimensions of 100×50×1.2 mm3. The microstructures of base metals are shown in the Fig.1. The 3
thickness of Nb and Cu foils is 0.8 and 0.08 mm, respectively. Their chemical compositions are listed in Table 1. Prior to welding, the welding surfaces of these sheets and interlayers were mechanically polished by a series of SiC papers to 600 grits and cleaned with acetone in an ultrasonic environment. Table 1 Chemical composition (wt%) of Ti6Al4V, Inconel 718, Nb and Cu Balance 5.5–6.8 3.5-4.5 <0.3 -
0.7-1.15 0.2-0.8 Balance 17-21 4.75-5.5 2.8-3.3 50-55 -
Nb 99.9 -
Cu 99.9
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Ti Al V Fe Cr Nb Mo Ni Cu
Inconel 718
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Ti
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Fig.1 Microstructure of the base metals: (a) Ti6Al4V and (b) Inconel 718 2.2 Laser welding
A high-power (4 kW) fiber laser was used with a transmission fiber of the
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diameter, focal length, and focal spot of 0.2, 150, and 0.2 mm, respectively. The spots were basically circular. The sheets were fixed in a butt weld formation, as shown in
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Fig. 2a.The Nb/Cu multi-interlayer was placed between the base metals. To ensure an intimate contact between welding sheets, the side screws were used on the welding fixture. The laser beam was focused on the Ti/Nb interface to ensure partial melting of niobium, as shown in Fig.2 b. The laser welding parameters are shown in Table 2. Argon was used as the shielding gas to protect welds from air pollution. The shielding gas device is shown in Fig. 3. The flow rates of the front and back shielding gases were 20 and 15 L/min, respectively. 4
Table 2 Welding parameters of LBW Welding parameters Peak power
Pulse duration
Pulse frequency
Focal position
Welding speed
1.5 kW
15 ms
50 Hz
1 mm
1.0 m/min
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LBW
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Fig. 2 Schematic diagrams of welding process(a) welding fixture and (b) laser beam position
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Fig. 3 Shielding gas device (Gao et al.,2019) 2.3 Microstructural characterization
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The metallographic samples of the welded joint were prepared by the standard preparation process. The chemical etching of the metallographic samples is divided into three steps because the joint composed of different materials. Firstly, the
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unmelted Nb was etched with an acid solution of HF: HNO3: H2O = 1:2:2 for 120 s.
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Secondly, the interfacial microstructures and Inconel 718 were etched with a mixture reagent of 5 g CuCl2 + 100 mL HCl + 100 mL C2H5OH for 60s. Thirdly, the Ti6Al4V base metal and fusion zone were etched with 3 mL HF + 5 mL HNO3 + 100 mL H2O for 90 s. The microstructure, elemental distribution, and fracture morphology of the joint were observed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The microstructural characteristics of Nb/Cu/Inconel 718 interface were investigated using a JEOL 2010F TEM/STEM equipped with 5
Quantax 5030 EDS. Since the TEM samples were prepared by conventional ion thinning, the observation position of the transmission sample is random. The phases present in the joint were studied by X-ray diffraction (XRD, Rigaku SmartLab 9) at 45kV and 200 mA, using Cu target. The beam size of X-ray was approximately 5×8 mm2 at 2theta = 20º. The Parallel Beam was used for XRD analysis. Scanning span was 20-90° (2θ) with a speed of 3°per minute. The whole joint, including FZ, Nb/Cu/Inconel 718 reaction layer, and the two base materials, was scanned by X-ray.
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2.4 Mechanical tests Microhardness tests were carried out using a hardness tester with a load, load time, and step size of 200 g, 10 s, and 0.2 mm, respectively. The dimensions of tensile
samples are shown in Fig. 4. The gauge length of the tensile sample is 12.5mm. Three
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samples were tested for their mechanical characteristic using a universal test machine
at a constant drawing speed of 2 mm/min. The fracture behavior of the joint was
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observed by SEM.
Fig.4 Dimensions of tensile samples
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3. Results and discussion
3.1. Macrostructure of the weld The cross section of the joint with Nb/Cu multi-interlayer is shown in Fig. 5. The
sound joint with no cracks or other defects were obtained, indicating that inserting Nb/Cu multi-interlayer was beneficial for the laser welding of Ti6Al4V to Inconel 718. The joint consists of several metallurgical interfaces, including Ti6Al4V/FZ, FZ/Nb, Nb/Cu, Cu foil, and Cu/Inconel 718. The hybrid joint comprising several bonding 6
interfaces were obtained through welding once in this study.
Fig. 5. Cross sections of the laser welded Ti6Al4V/Inconel 718 joint with Nb/Cu multi-interlayer. Fig. 6 shows the mapping distribution of the major elements in the dissimilar
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joint. When Nb/Cu multi-interlayer is inserted betweenTi6Al4V and Inconel 718 base metals, not only the mixture of Ti and Ni in the molten zone stopped by the unmelted
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Nb, but also the mixing of Nb and nickel was significantly suppressed by the Cu foil.
Fig. 6. Elemental distribution of dissimilar joint with Nb/Cu multi-interlayer.
3.2 Microstructures morphologies of the fusion zone Fig. 7 shows the microstructure and the corresponding EDS analysis in the FZ of the dissimilar joint. As shown in Figs. 7a and b, the microstructure of the FZ consists of the dendritic grains and island areas, and IMCs were not observed in the FZ. Fig. 8 shows the phase diagrams of Ti-Nb, Nb-V, and Ti-Nb-Al (ASM handbook volume 03, 7
1992 and Chen et al., 1996). The phase diagrams of Ti-Nb, Nb-V, and Ti-Nb-Al phase diagrams and EDS analysis indicate that IMCs did not form in the Ti6Al4V-Nb FZ. The microstructure of the dendritic grain and island areas is made of (α+β Ti, Nb) solid solutions. The Nb content is higher in the island area than in the dendritic grain, indicating that the two liquid phases of Ti and Nb did not mix completely in the molten pool of joint. The presence of the island regions (region A) indicates that macrosegregation occurs in the FZ of the joint with the Nb/Cu interlayer. Gao et al. (2018) also observed the formation of the island areas with macrosegregation features
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during the laser welding of Ti6Al4V and Nb dissimilar alloys. The macrosegregation phenomenon during the laser welding of dissimilar materials is related to the relative
liquidus temperature of the weld metal and the base materials, highly complex fluid
flow, and high cooling speed (Soysal et al., 2016). Oliveira et al. (2017) proved that
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macrosegregation easily occurs in the laser welding of dissimilar metals. As listed in Table 3, the melting point and heat conductivity of Nb are significantly higher than
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that of Ti alloy. The macrosegregation easily occurs in the FZ under the high cooling
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rate and complex molten pool flow conditions of laser welding.
Fig. 7 Microstructure and the corresponding EDS analysis in the FZ of joint.
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Fig. 8 Phase diagrams: (a) Ti-Nb, (b) Nb-V and (c)Ti-Nb-Al. (ASM handbook volume 03, 1992 and Chen et al., 1996).
a
Specific heata Cp
Thermal conductivitya
(C)
(g/cm3)
(J/(gk))
(W/(mK))
3315 4900 2917 2917
4.43 8.55 8.91 8.96
0.61 0.27 0.44 0.38
6.7 53.7 11.4 393.6
Boiling point
(C)
1655 2469 1260-1336 1084
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Ti6Al4V Nb Inconel 718 Cu
Densitya
Melting point
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Material
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Table 3 Comparison of thermo-physical properties of Ti6Al4V, Niobium, Cu, and Inconel 718 (Torkamany et al., 2014 and Guo et al., 2017)
Measured at 20 C.
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3.2.2 Microstructures morphologies of Nb/Cu/Inconel 718 interface Fig. 9 shows the microstructural characteristics of the Nb/Cu/Inconel 718
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interface. The three different zones visible for Nb/Cu/Inconel 718 interface are as follows: Nb/Cu, Cu, and Cu/Inconel 718 reaction layers. The thicknesses of the Nb/Cu and Cu/Inconel 718 reaction layers were about 15 and 30 µm, respectively. Owing to the high cooling speed of the pulsed laser welding and low diffusivity and solubility of Nb and Cu, the insufficient diffusion and reaction at the Nb/Cu/Inconel 718 interface contributed to the formation of void and cracks at the Nb/Cu and Cu/Inconel 718 reaction layers. The interfaces of Nb/Cu and Cu/Inconel 718 reaction 9
layers are tortuous, indicating that the copper interlayer completely melted. The composition analysis of each location (marked by number 1-9 in Fig. 9) was carried out by EDS, and the test results are listed in Table 4. The EDS analysis result of point 2 in Fig. 9 indicates that the Ni atoms aggregated at the Nb/Cu interface. Fig. 10 shows the Cu/Ni and Cu/Nb binary alloy phase diagrams and Ni-Cu-Nb ternary phase diagram (ASM handbook volume 03, 1992 and Villars et al. 1995). According to the Nb/Cu and Cu/Ni binary phase diagrams, no IMCs generate in the Cu-Nb and Cu-Ni systems. The Cu-Nb system is immiscible in equilibrium state, whereas Cu and Ni
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can completely be dissolved with each other. The Ni atoms of the Inconel 718 dissolved in liquid phase can diffuse across the Cu interlayer to the Nb/Cu interface because of high solubility of Ni in Cu. According to Ni-Cu-Nb ternary phase diagram
(Fig. 10 c), Ni-Nb IMCs would occur in Ni-Cu-Nb ternary system when Nb content is
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high. The content of Cu, Nb and Ni of point 2 in Fig.9 is 48.5, 22.8, and 13.8 at.%, respectively. Combining the EDS results with the Ni-Cu-Nb ternary phase diagram,
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the mixture of Cu-solution, Nb-solution and Ni-Nb IMCs may appear in the Nb/Cu interface. The number of Ni-Nb IMCs formed at the Nb/Cu interface is very few due
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to the relatively low Ni content. From Ni-Cu-Nb ternary phase diagram, it can be observed that the existence of Ni atoms increases the solubility between Nb and Cu.
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Li et al. (2012) proved that the Nb/Cu interface can be strengthened by promoting Cu solution in Nb during the diffusion bonding CP-Ti to AISI 321 with Nb/Cu/Ni multi-interlayer. The presence of appropriate Nb atoms is beneficial for improving the
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strength of Nb/Cu interface. The Nb/Cu reaction layer mainly consisted of Nb-based and Cu-based solid solutions (marked by arrow 1 and 2). The Cu reaction layers
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consisted of Cu-based solid solution (marked by arrows 3 and 5) and the mixture of Cu-based and Ni-based solid solutions (marked by arrow 4). The Cu/Inconel 718 interface consisted of Cu-based and Ni solid solutions (marked by arrows 6–9). The composition distribution characteristic along the yellow line in Fig. 6 was investigated by EDS point analysis, and the space between the points was found as 5 m. The compositional analysis results at the Nb/Cu/Inconel 718 interface are shown in Fig. 8. The Cu atoms diffused not only to the Nb side, but also to Inconel 718. The diffusion 10
distance of the Cu atoms to Inconel 718 is greater than that of Cu atoms to the Nb side. Since the Inconel 718 has a lower melting point, the Inconel 718 near the Nb/Inconel 718 interface may be in the semi-solid state, providing more diffusion paths for the Cu atoms diffusing into Nb (Cahoon et al., 1997). Fig. 11 obviously shows that Cu
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foil prevents the mixing of Nb and Inconel 718.
Fig. 9. Microstructure at the interface in the FZ of Nb-Inconel 718 interface.
Table 4 Chemical composition (at%) of points 1–11 in Fig. 11 Cu
13.83 48.5 86.47 20.92 59.81 28.16 8.44 15.9 1.11
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1 2 3 4 5 6 7 8 9
Atom Percent/%
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Zone
Nb
Ni
81.56 22.80 2.14 5.54 7.92 2.67 3.42 3.37 3.33
1.64 13.80 6.76 39.59 14.73 34.79 45.14 42.16 50.76
Cr 7.46 1.97 16.51 8.41 16.37 19.86 17.75 21.43
Fe 6.16 2.00 14.96 6.81 15.68 17.82 16.65 18.40
11
Mo
Al 0.68 0.42
0.78 0.54 1.12 2.73 1.60 2.43
0.32 1.12 1.53 1.16
Ti
Potential phases
0.78 0.53 0.24 1.70 1.46 1.21 1.47 1.04 1.31
Cu, Nb Cu, Nb Cu Cu, Ni Cu Cu, Ni Ni Cu, Ni Ni
major
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Fig. 10. Phase diagrams: (a) Cu-Ni, (b) Cu-Nb and (c) Ni-Cu-Nb. (ASM handbook volume 03, 1992
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and Villars et al. 1995).
Fig. 11. Result of EDS analysis alone shown by yellow line in Fig. 6.
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Sample for TEM analysis of Nb/Cu/Inconel 718 interface was prepared by ion thinning. Because of the significant difference between the physical and chemical characteristics of copper, niobium, and Inconel 718, only thin foils of Nb/Cu interface formed, while the sample preparation on the Cu/Inconel 718 interface failed. Fig. 12 shows the STEM image and the corresponding EDS map of the Nb/Cu interface. A metallurgical bonding interface formed at the Nb/Cu interface due to the mutual diffusion of copper and Nb atoms. Fig. 13 shows the XRD patterns of the joint. The 12
Nb7Ni6 and NbNi3 IMCs were not observed in the joint with Nb/Cu mutil-interlayer,
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indicating that the addition of Cu foil suppressed the formation of Nb-Ni IMCs.
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Fig. 12. STEM image and corresponding EDS map of Nb/Cu interface.
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Fig. 13. XRD analysis of the joint.
3.3 Mechanical properties of the joint The hardness distribution of the dissimilar joint is shown in Fig. 14. The heat
affected zone of Ti6Al4V side exhibited the highest microhardness, attributing to the formation of the martensite phase in the HAZ during the laser welding of Ti6Al4V (Gao et al., 2013). The microhardness of FZ and Nb/Cu/Inconel 718 interface is 220 and 280 HV, rspectively. Gao et al. (2018) proved that the microhardness of 13
Nb/Inconel 718 interface is higher than 380 HV, when the brittle Nb7Ni6 and NbNi3 IMCs formed at the Nb/Inconel 718 interface during the laser welding of Ti6Al4V alloy to Inconel 718 with Nb interlayer. The addition of Cu foil inhibited the formation of Nb-Ni IMCs, decreasing the hardness of Nb/Cu/Inconel 718 interface.
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The dissimilar joint with the Nb/Cu interlayer possess better mechanical properties.
Fig.14. Microhardness profiles of dissimilar joint.
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Fig. 15 shows the tensile results of the joint. The dissimilar joint with a Nb/Cu multi interlayer have a largest tensile strength of 260 MPa and fractured at the Cu
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reaction layers. The fracture strain of the joint was less than 2.3%, indicating low ductility of the joint. Zhang et al. (2016) found softening appeared in the FZ and HAZ after the copper is welded. As shown in Figs. 5 and 9, the cross-section area of
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the Cu reaction layer is less than the rest of the joint, and welding defects such as voids and cracks form at the Nb/Cu/Inconel 718 reaction layer, resulting in the
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failure of joint appearing at the Cu reaction layer. The tensile samples showed the partial region of the fracture surface of by SEM, as shown in Fig. 16. Compared to
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the joint with Nb interlayer (Gao et at, 2018), the fracture surface of the dissimilar joint with Nb/Cu multi-interlayer shows the some dimples and flat surfaces, indicating that the ductility of joint improved by inserting an Nb/Cu multi-interlayer. The appearance of flat surfaces indicates that the joint is still very brittle despite the presence of some dimples on the fracture surface. Table 5 shows the EDS analysis results of points A–E in Fig. 16. The EDS analyses of the fracture surface of the joint detected Cu-based and Nb solid solutions, indicating that the crack originated from 14
Nb/Cu/Inconel 718 reaction layer. High quality welding of copper and niobium is very difficult, because of the immiscibility of the Cu and Nb systems (Pan et al., 2017). The bonding at the Nb/Cu/Inconel718 interface is poor at high heating and cooling rates, as shown in Fig. 9. Although the formation of brittle IMCs was avoided, high stress concentration easily results at the Nb/Cu/Inconel 718 interface due to the formation of the voids and cracks, resulting in the brittle-like fracture behavior of the joint. Oliveira et al. (2017) proved that the formation of weld defects at the joining interface for dissimilar welding would deteriorate seriously the
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mechanical properties of the joint. The several pores and non-welded regions presented in Fig.9 were responsible for the inferior properties of Ti6Al4V/Inconel
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718 joint and the fracture of joint appearing at the Nb/Cu/Inconel 718 reaction layer.
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Fig. 15. Tensile results of dissimilar joint: (a) tensile curve and (b) Cross sections of fractures of joints
Fig. 16. Fracture surface analysis of the dissimilar joint with a Nb/Cu interlayer. 15
Table 5 Chemical composition (at%) of point A–E in Fig. 16 Zone
Atom percent/%
Potential phases
Cu
Nb
Ni
Cr
A
95.90
2.26
2.25
Cu solution
B
91.74
4.08
2.82
Cu solution
C
60.72
25.01
7.68
D
88.04
7.64
2.35
Cu solution
E
90.47
5.01
3.29
Cu solution
3.22
Cu +Nb solution
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3.37
Fe
4. Discussion
The schematic diagram of the laser welding-induced brazing reaction is shown in
Fig. 17. One of the keys to this method is selecting the appropriate thickness of Nb
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and Cu as the multi-interlayer. When the laser beam is located on the interface between Ti6Al4V and Nb, Nb interlayer with a very high melting point may partially
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melt, and the unmelted Nb would be a good diffusion barrier for Ti6Al4V and Inconel 718. The cross section of the joint with different Nb interlayer is shown in Fig. 18.
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The thickness of Nb is too less to effectively prevent the mixing of the two parent materials in the molten pool, resulting in unsuccessful welding of Ti6Al4V and
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Inconel 718. When the Nb interlayer is too thick, numerous voids and cracks formed at the Nb/Cu/Inconel 718 interface due to the low interface temperature, consequently
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deteriorating the mechanical property of the joint. The cross section and microstructure of the joint with different Cu interlayer are shown in Figs. 19 and 20,
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respectively. Many Ni atoms can diffuse to the Cu/Nb interface due to thin Cu layer, resulting in the formation of Nb-Ni and Nb-Fe IMCs layer, as shown in Fig.19 and Table 6. The formation of many Nb-Ni and Nb-Fe IMCs would embrittle the Nb-Cu interface and lead to the decline of joint strength (Li et al., 2012). Many cracks appeared at the Cu/Nb and Cu/Inconel 718 interfaces with the increasing of the thickness of Cu foil, as shown in Fig.20. The thick copper foil makes it more difficult for Nb atoms to diffuse through it to Nb/Cu interface and reduces the temperature of 16
Cu/Inconel 718 interface. The disappearance of Ni atom aggregation suppresses Nb solution in the Cu, leading to the formation of cracks at the Nb/Cu interface. The low temperature of Cu/Inconel 718 interface is responsible for the formation of cracks. Another key to this method is to ensure that the interface temperature of Nb/Inconel 718 is higher than the melting point of Cu (1084 C). Under appropriate laser welding parameters, a large amount of heat will be transferred by the unmelted Nb from (Ti, Nb) molten pool to the interface of the Nb/Cu/Inconel 718 interface, because of high thermal conductivity of Nb. For Nb/Cu/Inconel 718 interface, Cu began to melt when
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the interface temperature exceeded its melting point. The Nb and Inconel 718 were joined by brazing with dissimilar metals forming a Nb/Cu interlayer. The Ti6Al4V and Nb would be joined by the FZ, whereas the brazing welding would be responsible
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for bonding Nb to Inconel 718.
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Fig. 17. Sketch schematic diagrams of laser welding-induced brazing reaction.
Fig. 18 Cross section of joint with different Nb interlayer: (a) 0.4 mm and (b) 1.0 mm. 17
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Fig. 19 Cross section of joint with Cu foils of thickness 0.03mm Table 6 Chemical composition (at%) of point A–C in Fig. 19
Atom percent/%
Potential phases
Ni
Cr
Fe
A
15.35
38.46
17.54
16.48
B
17.71
37.29
20.44
C
1.16
5.58
11.2
Cu
12.17
Nb-Ni +Nb-Fe IMCs
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Nb
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Zone
7.65
Nb-Ni +Nb-Fe IMCs
3.04
79.02
Cu solution
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16.91
Fig. 20 Cross section of joint with Cu foils of thickness 0.15mm
4.3 Microstructure evolution of Nb/Cu/Inconel 718 interface Fig. 21 shows the microstructure evolution of Nb/Cu/Inconel 718 interface. As the temperature of Nb/Cu interface is higher than the melting point of Cu (1084 C), the Cu liquid phase forms at the Nb/Cu/Inconel 718 interface, as shown in Fig. 21a. 18
Subsequently, the Cu liquid phase wets the surfaces of solid Nb and Inconel 718, and the interdiffusion of elements occurs at the Nb/Cu and Cu/ Inconel 718 interfaces. The Niobium and Inconel 718 dissolve in the Cu liquid phase as a result of diffusion at the solid Nb-liquid Cu and solid Inconel718-liquid Cu interfaces. Accompanied by the dissolution of solid Nb and Inconel 718, the Cu atoms in the liquid phase would diffuse into Nb interlayer and Inconel 718, as shown in Fig. 21b. Brazing at the Nb/Cu/Inconel 718 interface followed a diffusion and dissolution mechanism. A comparison of the thickness of Nb/Cu and Cu/Inconel 718 reaction layer (Fig. 9)
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indicates less thickness of the Nb/Cu reaction layer than that of Nb/Inconel 718, because the liquid Cu dissolves a much more amount of the Inconel 718 and Cu atoms easily diffuse into Inconel718. The melting point of Inconel 718 is less different from
that of Cu, and the Inconel 718 base metal melted near the Cu/Inconel 718 interface
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and may be in the semi-solid state. In contrast, the melting point of Nb is much higher
than the melting point of Cu, and Nb still may be in the solid state. Pan et al. (2017)
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demonstrated successful welding of Cu and Nb in the temperature range between 92% and 98% of the melting point of Cu, because the Cu in the semi-solid state can
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provide more diffusion paths. The diffusion quantity and distance of the Cu atom to the Nb atom is smaller than that of the Cu atom to Inconel 718.
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Fig. 21c shows the microstructure evolution of Ti/Nb/Cu/Inconel 718 during the cooling process. During cooling, Cu-based solid solutions primarily precipitate from the liquid phase. The Nb- and Ni-based solid solutions form by the diffusion of the Cu
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atoms in the liquid phase to Nb and Inconel 718. The elements Nb, Ni, Cr, and Fe dissolved in the liquid Cu diffuse with difficulty to the opposite side, because of the
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high cooling rate of the pulsed laser welding and thick Cu interlayer. Therefore, no intermetallic compounds formed at the Nb/Cu/Inconel 718 interface. The phase transformation of liquid formation and solidification for the Nb/Cu/Inconel 718 interface can be summarized as follows: With the heating stage:
Cu 1084 ℃ L 19
diffusion L Nb Inconel 718 L dissolution
With the cooling stage: L →Cu (congruent reaction) L →Cu+Ni (congruent reaction), L →Cu + Nb (congruent reaction), Two metallurgical bonding zones based on the fusion welding and brazing mechanism were simultaneously identified in the Ti6Al4V/Inconel 718 with Nb/Cu
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interlayer welded. One of the zones was also (Ti, Nb) zone formed by the fusion
welding mechanism, and the other zones were the bonding layer formed at the Nb-Cu
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interface and Nb-Inconel 718 interface attributed to the brazing mechanism.
Fig. 21. Schematic of the formation of reaction layer for Nb/Cu/Inconel 718 interface: (a) liquid phase
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formed by heat conduction, (b) interaction between Cu liquid and base metal and (c) final solidification microstructure of the brazing seam.
5. Conclusions
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1. Laser welding induced brazing-welding method was used for creating hybrids with the fusion zone and brazing reaction zone separated by the unmelted Nb. 2. Fusion zone comprising Ti-based and Nb-based solutions formed at the Ti/Nb interface, and the brazing layer with the main microstructures of Nb-based, Cu-based, and Ni-based solid solution formed between Nb and Inconel 718, because of the melting of Cu foil. 3. The strength of the joint with Nb/Cu multi-interlayer exhibited the largest tensile 20
strength of 260 MPa, whereas the joint facture in brittle-like failure. 4. The Nb/Cu multi-interlayer could help prevent the formation of IMCs, and voids occurred at the Nb/Cu/Inconel718 reaction layer, because of the immiscibility of Cu and Nb and high heating and cooling rates of laser welding, which can be the origin of the brittle fracture of the joint. Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant Nos. 51705005 and 51905006), Scientific Research Plan Projects of Shaanxi
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Education Department (Grant No. 17JK0050), Natural Science Foundation of Shaanxi Province (Grant No. 2018JQ5204), Scientific Research Plan Projects of Baoji (Grant No. 2017JH2-10), and Fundamental Research Funds for Baoji University of Arts and
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Sciences University of China (Grant No. ZK 16045).
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Int. J. Adv. Manuf. Tech. 94, 937–3947. Gao X.L., Liu H., Liu J., Yu H.K., 2019. Laser welding of Ti6Al4V to Cu using a niobium interlayer. J. Mater. Process. Tech. 270, 293-305. Gao Y.N., Huang L.J., An Q., Bao Y., Li X.T., Zhang J., Geng L., 2018. Microstructure evolution and mechanical properties of titanium matrix composites and Ni-based superalloy joints with Cu interlayer. J. Alloy. Compd. 764,665-673. Grill R., Gnadenberger A., 2006. Niobium as mint metal: production properties processing. Int. J. Refract Metal Hard Mater. 24, 275-282.
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Li X.P., Wang H.Q., Wang T., Zhang B.G., Yu T., Li R.S., 2017. Microstructural
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Oliveira J.P., Zeng Z., Andr C., Braz Fernandesc F.M., Miranda R.M., Ramirez A.J., Omori T., Zhou N., 2017. Dissimilar laser welding of superelastic NiTi and CuAlMn
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306-322. Seretsky J., Ryba E.R., 1976. Laser Welding of Dissimilar Metals: Titanium to Nickel. Weld. J. 55, 208-211. Song T.F., Jiang X.S., Shao Z.Y., Fang Y.J., Mo D.F., Zhu D.G., Zhu M.H., 2017. Microstructure and mechanical properties of vacuum diffusion bonded joints between Ti-6Al-4V titanium alloy and AISI316L stainless steel using Cu/Nb multi-interlayer. Vacuum 145, 68-76. Soysal T., Kou S., Tat D., Pasang T., 2016. Macrosegregation in dissimilar-metal
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24
PII:
S0924-0136(19)30440-6
DOI:
https://doi.org/10.1016/j.jmatprotec.2019.116467
Reference:
PROTEC 116467
To appear in:
Journal of Materials Processing Tech.
Received Date:
12 January 2019
Revised Date:
19 September 2019
Accepted Date:
21 October 2019
Please cite this article as: Liu J, Liu H, Gao X-Long, Yu H, Microstructure and mechanical properties of laser welding of Ti6Al4V to Inconel 718 using Nb /Cu interlayer, Journal of Materials Processing Tech. (2019), doi: https://doi.org/10.1016/j.jmatprotec.2019.116467
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Microstructure and mechanical properties of laser welding of Ti6Al4V to Inconel 718 using Nb /Cu interlayer Jing Liu, Huan Liu, Xiao-Long Gao , Haokui Yu Shannxi Key Laboratory of Advanced Manufacturing and Evalutaion of Robot Key Components，Baoji University of Arts and Sciences, Baoji, 721016, China
Corresponding author at: School of Mechanical Engineering, Baoji University of Arts and Sciences, Baoji, 721016, China. E-mail address:[email protected], Tel: +86-0917-3364295
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Abstract
To improve the quality of the joints between Ti6Al4V and Inconel 718, fiber
laser welding with Nb/Cu multi-interlayer was attempted by laser welding induced
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brazing-welding method. The hybrid joint comprising different metallurgical areas
was created based on fusion welding and brazing welding mechanisms through one
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welding. Ti6Al4V and Nb well bonded by the fusion welding, and the fusion zone was characterized by the formation of (α+β Ti, Nb) solid solutions. Nb and Inconel
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718 were joined by brazing because of the melting of Cu foil, and the Nb/Cu/Inconel 718 interface mainly consisted of the Nb-based, Cu-based, and Ni-based solid solutions. The joint failed in the Nb/Cu/Inconel 718 interface with the maximum
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strength of 260 MPa, exhibiting brittle-like failure behavior due to the formation of voids and microcracks at the Nb/Cu/Inconel 718 interface. The addition of Nb/Cu
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multi-interlayer can help prevent the formation of IMCs and improves the mechanical
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properties of the joint.
Keywords: Dissimilar welding; Ti alloy; Inconel 718; Interlayer; Intermetallic compounds
1 Introduction 1
As demonstrated by Ezugwu et al. (1997), titanium alloys are widely used in the aerospace, petrochemical, and biological medicine industries owing to its high specific strength, good erosion resistance, excellent high-temperature mechanical properties, and biocompatibility. In addition, nickel alloys with excellent mechanical properties and oxidation resistance at elevated temperatures are particularly suitable for manufacturing high-temperature resistant parts (Ramkumara et al., 2017). Therefore, high quality welding of titanium and nickel alloys can combine their respective advantages and increase the design flexibility and the product functionality.
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However, a series of problems have emerged in the welding of Ti and Ni alloys due to differences in their physical and chemical properties. Chatterjee et al. (2006 and 2008) reported that the brittle Ti2Ni and TiNi3 intermetallic compounds (IMCs)
can easily be formed due to the metallurgical incompatibilities of Ti and Ni. Seretsky
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et al. (1976) proved that the crack in the laser-welded titanium to nickel cannot be
eliminated by changing the laser power or by rewelding the samples. Alemná et al.
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(1995) reported the formation of a large number of Ti2Ni and Cr2Ti IMCs when diffusion welding is used for bonding Ti6242 alloy to Inconel 625 alloy.
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Significant attempts have been made to inhibit the formation of TixNiy IMCs during the welding of Ti and Ni alloys. Chen et al. (2011) found that the number of
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the TixNiy brittle phases can be reduced in the laser welded joints of Ti6Al4V and Inconel 718 by the combination of low heat input and laser offset welding. Gao et al. (2018) selected a Cu foil as the interlayer for the gas tungsten arc welding of
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TiBw/Ti6Al4V composites and Inconel 718 alloy and found that the main products from Ti-Ni IMCs changed into the combined of Ti-Cu, Ti-Ni-Cu, and Ti-Ni IMCs in
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the joints. Zhang et al. (2018) studied the electron beam welding of TA15 to GH600 using Cu interlayer. A Cu-based solid solution and Ti-Cu IMCs were formed in the weld, and the tensile strength of the joint was up to 282 MPa. Li et al. (2017) found that the interfacial microstructure mainly comprised continuous Ti2Ni phase for Ti600/Ti-Zr-Ni-Cu/Ni-25a%Si brazed joint at 1213 K. Although the insertion of interlayer can suppress the formation of Ti-Ni IMCs by reducing the amount of Ti and Ni in the weld metal, the Ti-Ni IMCs are still formed in the joint, resulting in poor 2
plasticity of the joint (Zoeram et al., 2014). To completely inhibit the formation of TixNiy brittle phases, the mixing between Ti and Ni must be stopped sufficiently in the molten pool. The Nb interlayer with a higher melting point than the two dissimilar base materials has proven to be successful diffusion barriers between dissimilar materials (Oliveira et al., 2016). Grill et al. (2006) did not found the formation of IMCs in the welding of dissimilar metals Ti and Nb. Torkamany et al. (2014) successfully laser welded Ti6Al4V to Nb without the formation of any IMCs, and the joint failed at the Nb base metal. Gao et al. (2018)
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discovered that the Ti-Ni IMCs in the dissimilar joint between Ti6Al4V and Inconel 718 could be completely eliminated with Nb as an interlayer during the pulsed laser welding; however, the eutectic reaction layer composed of NbNi3 and Nb7Ni6 IMCs formed at the Nb/Inconel 718 interface. The formation of IMCs between Nb and Ni
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still deteriorates the mechanical properties of the dissimilar joints of Ti6Al4V and
Inconel 718. The dissimilar joint of Ti6Al4V and Inconel 718 using Nb interlayer
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failed in the Nb-Ni IMCs layer at the ultimate tensile strength of 145 MPa and exhibited cleavage fracture. The multi-interlayer for the welded joints of Ti and Ni
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may be an ideal solution for preventing the formation of brittle IMCs. Song et al. (2017) confirmed that Cu/Nb multi-interlayer can prevent the formation of IMCs for
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diffusion bonding Ti6Al4V to AISI316L stainless steel, and the joints were broken in the ductile mode. According to the Nb-Cu and Cu-Ni binary phase diagrams, no IMCs formed for Cu/Nb and Ni/Cu dissimilar metals (ASM handbook volume 03, 1992);
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therefore, Cu/Nb multi-interlayer is identified as a potential intermediate structure for bonding Ti alloys to Ni alloys.
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In this study, the effect of Nb/Cu interlayers on the microstructural and
mechanical properties of the pulsed laser welding of Ti6Al4V and Inconel 718 was systemically investigated. 2 Materials and methods 2.1 Materials preparation The base metals are Ti6Al4V and Inconel 718 sheets with the dimensions of 100×50×1.2 mm3. The microstructures of base metals are shown in the Fig.1. The 3
thickness of Nb and Cu foils is 0.8 and 0.08 mm, respectively. Their chemical compositions are listed in Table 1. Prior to welding, the welding surfaces of these sheets and interlayers were mechanically polished by a series of SiC papers to 600 grits and cleaned with acetone in an ultrasonic environment. Table 1 Chemical composition (wt%) of Ti6Al4V, Inconel 718, Nb and Cu Balance 5.5–6.8 3.5-4.5 <0.3 -
0.7-1.15 0.2-0.8 Balance 17-21 4.75-5.5 2.8-3.3 50-55 -
Nb 99.9 -
Cu 99.9
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Ti Al V Fe Cr Nb Mo Ni Cu
Inconel 718
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Ti
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Fig.1 Microstructure of the base metals: (a) Ti6Al4V and (b) Inconel 718 2.2 Laser welding
A high-power (4 kW) fiber laser was used with a transmission fiber of the
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diameter, focal length, and focal spot of 0.2, 150, and 0.2 mm, respectively. The spots were basically circular. The sheets were fixed in a butt weld formation, as shown in
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Fig. 2a.The Nb/Cu multi-interlayer was placed between the base metals. To ensure an intimate contact between welding sheets, the side screws were used on the welding fixture. The laser beam was focused on the Ti/Nb interface to ensure partial melting of niobium, as shown in Fig.2 b. The laser welding parameters are shown in Table 2. Argon was used as the shielding gas to protect welds from air pollution. The shielding gas device is shown in Fig. 3. The flow rates of the front and back shielding gases were 20 and 15 L/min, respectively. 4
Table 2 Welding parameters of LBW Welding parameters Peak power
Pulse duration
Pulse frequency
Focal position
Welding speed
1.5 kW
15 ms
50 Hz
1 mm
1.0 m/min
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LBW
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Fig. 2 Schematic diagrams of welding process(a) welding fixture and (b) laser beam position
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Fig. 3 Shielding gas device (Gao et al.,2019) 2.3 Microstructural characterization
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The metallographic samples of the welded joint were prepared by the standard preparation process. The chemical etching of the metallographic samples is divided into three steps because the joint composed of different materials. Firstly, the
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unmelted Nb was etched with an acid solution of HF: HNO3: H2O = 1:2:2 for 120 s.
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Secondly, the interfacial microstructures and Inconel 718 were etched with a mixture reagent of 5 g CuCl2 + 100 mL HCl + 100 mL C2H5OH for 60s. Thirdly, the Ti6Al4V base metal and fusion zone were etched with 3 mL HF + 5 mL HNO3 + 100 mL H2O for 90 s. The microstructure, elemental distribution, and fracture morphology of the joint were observed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The microstructural characteristics of Nb/Cu/Inconel 718 interface were investigated using a JEOL 2010F TEM/STEM equipped with 5
Quantax 5030 EDS. Since the TEM samples were prepared by conventional ion thinning, the observation position of the transmission sample is random. The phases present in the joint were studied by X-ray diffraction (XRD, Rigaku SmartLab 9) at 45kV and 200 mA, using Cu target. The beam size of X-ray was approximately 5×8 mm2 at 2theta = 20º. The Parallel Beam was used for XRD analysis. Scanning span was 20-90° (2θ) with a speed of 3°per minute. The whole joint, including FZ, Nb/Cu/Inconel 718 reaction layer, and the two base materials, was scanned by X-ray.
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2.4 Mechanical tests Microhardness tests were carried out using a hardness tester with a load, load time, and step size of 200 g, 10 s, and 0.2 mm, respectively. The dimensions of tensile
samples are shown in Fig. 4. The gauge length of the tensile sample is 12.5mm. Three
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samples were tested for their mechanical characteristic using a universal test machine
at a constant drawing speed of 2 mm/min. The fracture behavior of the joint was
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observed by SEM.
Fig.4 Dimensions of tensile samples
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3. Results and discussion
3.1. Macrostructure of the weld The cross section of the joint with Nb/Cu multi-interlayer is shown in Fig. 5. The
sound joint with no cracks or other defects were obtained, indicating that inserting Nb/Cu multi-interlayer was beneficial for the laser welding of Ti6Al4V to Inconel 718. The joint consists of several metallurgical interfaces, including Ti6Al4V/FZ, FZ/Nb, Nb/Cu, Cu foil, and Cu/Inconel 718. The hybrid joint comprising several bonding 6
interfaces were obtained through welding once in this study.
Fig. 5. Cross sections of the laser welded Ti6Al4V/Inconel 718 joint with Nb/Cu multi-interlayer. Fig. 6 shows the mapping distribution of the major elements in the dissimilar
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joint. When Nb/Cu multi-interlayer is inserted betweenTi6Al4V and Inconel 718 base metals, not only the mixture of Ti and Ni in the molten zone stopped by the unmelted
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Nb, but also the mixing of Nb and nickel was significantly suppressed by the Cu foil.
Fig. 6. Elemental distribution of dissimilar joint with Nb/Cu multi-interlayer.
3.2 Microstructures morphologies of the fusion zone Fig. 7 shows the microstructure and the corresponding EDS analysis in the FZ of the dissimilar joint. As shown in Figs. 7a and b, the microstructure of the FZ consists of the dendritic grains and island areas, and IMCs were not observed in the FZ. Fig. 8 shows the phase diagrams of Ti-Nb, Nb-V, and Ti-Nb-Al (ASM handbook volume 03, 7
1992 and Chen et al., 1996). The phase diagrams of Ti-Nb, Nb-V, and Ti-Nb-Al phase diagrams and EDS analysis indicate that IMCs did not form in the Ti6Al4V-Nb FZ. The microstructure of the dendritic grain and island areas is made of (α+β Ti, Nb) solid solutions. The Nb content is higher in the island area than in the dendritic grain, indicating that the two liquid phases of Ti and Nb did not mix completely in the molten pool of joint. The presence of the island regions (region A) indicates that macrosegregation occurs in the FZ of the joint with the Nb/Cu interlayer. Gao et al. (2018) also observed the formation of the island areas with macrosegregation features
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during the laser welding of Ti6Al4V and Nb dissimilar alloys. The macrosegregation phenomenon during the laser welding of dissimilar materials is related to the relative
liquidus temperature of the weld metal and the base materials, highly complex fluid
flow, and high cooling speed (Soysal et al., 2016). Oliveira et al. (2017) proved that
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macrosegregation easily occurs in the laser welding of dissimilar metals. As listed in Table 3, the melting point and heat conductivity of Nb are significantly higher than
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that of Ti alloy. The macrosegregation easily occurs in the FZ under the high cooling
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rate and complex molten pool flow conditions of laser welding.
Fig. 7 Microstructure and the corresponding EDS analysis in the FZ of joint.
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Fig. 8 Phase diagrams: (a) Ti-Nb, (b) Nb-V and (c)Ti-Nb-Al. (ASM handbook volume 03, 1992 and Chen et al., 1996).
a
Specific heata Cp
Thermal conductivitya
(C)
(g/cm3)
(J/(gk))
(W/(mK))
3315 4900 2917 2917
4.43 8.55 8.91 8.96
0.61 0.27 0.44 0.38
6.7 53.7 11.4 393.6
Boiling point
(C)
1655 2469 1260-1336 1084
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Ti6Al4V Nb Inconel 718 Cu
Densitya
Melting point
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Table 3 Comparison of thermo-physical properties of Ti6Al4V, Niobium, Cu, and Inconel 718 (Torkamany et al., 2014 and Guo et al., 2017)
Measured at 20 C.
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3.2.2 Microstructures morphologies of Nb/Cu/Inconel 718 interface Fig. 9 shows the microstructural characteristics of the Nb/Cu/Inconel 718
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interface. The three different zones visible for Nb/Cu/Inconel 718 interface are as follows: Nb/Cu, Cu, and Cu/Inconel 718 reaction layers. The thicknesses of the Nb/Cu and Cu/Inconel 718 reaction layers were about 15 and 30 µm, respectively. Owing to the high cooling speed of the pulsed laser welding and low diffusivity and solubility of Nb and Cu, the insufficient diffusion and reaction at the Nb/Cu/Inconel 718 interface contributed to the formation of void and cracks at the Nb/Cu and Cu/Inconel 718 reaction layers. The interfaces of Nb/Cu and Cu/Inconel 718 reaction 9
layers are tortuous, indicating that the copper interlayer completely melted. The composition analysis of each location (marked by number 1-9 in Fig. 9) was carried out by EDS, and the test results are listed in Table 4. The EDS analysis result of point 2 in Fig. 9 indicates that the Ni atoms aggregated at the Nb/Cu interface. Fig. 10 shows the Cu/Ni and Cu/Nb binary alloy phase diagrams and Ni-Cu-Nb ternary phase diagram (ASM handbook volume 03, 1992 and Villars et al. 1995). According to the Nb/Cu and Cu/Ni binary phase diagrams, no IMCs generate in the Cu-Nb and Cu-Ni systems. The Cu-Nb system is immiscible in equilibrium state, whereas Cu and Ni
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can completely be dissolved with each other. The Ni atoms of the Inconel 718 dissolved in liquid phase can diffuse across the Cu interlayer to the Nb/Cu interface because of high solubility of Ni in Cu. According to Ni-Cu-Nb ternary phase diagram
(Fig. 10 c), Ni-Nb IMCs would occur in Ni-Cu-Nb ternary system when Nb content is
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high. The content of Cu, Nb and Ni of point 2 in Fig.9 is 48.5, 22.8, and 13.8 at.%, respectively. Combining the EDS results with the Ni-Cu-Nb ternary phase diagram,
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the mixture of Cu-solution, Nb-solution and Ni-Nb IMCs may appear in the Nb/Cu interface. The number of Ni-Nb IMCs formed at the Nb/Cu interface is very few due
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to the relatively low Ni content. From Ni-Cu-Nb ternary phase diagram, it can be observed that the existence of Ni atoms increases the solubility between Nb and Cu.
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Li et al. (2012) proved that the Nb/Cu interface can be strengthened by promoting Cu solution in Nb during the diffusion bonding CP-Ti to AISI 321 with Nb/Cu/Ni multi-interlayer. The presence of appropriate Nb atoms is beneficial for improving the
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strength of Nb/Cu interface. The Nb/Cu reaction layer mainly consisted of Nb-based and Cu-based solid solutions (marked by arrow 1 and 2). The Cu reaction layers
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consisted of Cu-based solid solution (marked by arrows 3 and 5) and the mixture of Cu-based and Ni-based solid solutions (marked by arrow 4). The Cu/Inconel 718 interface consisted of Cu-based and Ni solid solutions (marked by arrows 6–9). The composition distribution characteristic along the yellow line in Fig. 6 was investigated by EDS point analysis, and the space between the points was found as 5 m. The compositional analysis results at the Nb/Cu/Inconel 718 interface are shown in Fig. 8. The Cu atoms diffused not only to the Nb side, but also to Inconel 718. The diffusion 10
distance of the Cu atoms to Inconel 718 is greater than that of Cu atoms to the Nb side. Since the Inconel 718 has a lower melting point, the Inconel 718 near the Nb/Inconel 718 interface may be in the semi-solid state, providing more diffusion paths for the Cu atoms diffusing into Nb (Cahoon et al., 1997). Fig. 11 obviously shows that Cu
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foil prevents the mixing of Nb and Inconel 718.
Fig. 9. Microstructure at the interface in the FZ of Nb-Inconel 718 interface.
Table 4 Chemical composition (at%) of points 1–11 in Fig. 11 Cu
13.83 48.5 86.47 20.92 59.81 28.16 8.44 15.9 1.11
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1 2 3 4 5 6 7 8 9
Atom Percent/%
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Zone
Nb
Ni
81.56 22.80 2.14 5.54 7.92 2.67 3.42 3.37 3.33
1.64 13.80 6.76 39.59 14.73 34.79 45.14 42.16 50.76
Cr 7.46 1.97 16.51 8.41 16.37 19.86 17.75 21.43
Fe 6.16 2.00 14.96 6.81 15.68 17.82 16.65 18.40
11
Mo
Al 0.68 0.42
0.78 0.54 1.12 2.73 1.60 2.43
0.32 1.12 1.53 1.16
Ti
Potential phases
0.78 0.53 0.24 1.70 1.46 1.21 1.47 1.04 1.31
Cu, Nb Cu, Nb Cu Cu, Ni Cu Cu, Ni Ni Cu, Ni Ni
major
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Fig. 10. Phase diagrams: (a) Cu-Ni, (b) Cu-Nb and (c) Ni-Cu-Nb. (ASM handbook volume 03, 1992
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and Villars et al. 1995).
Fig. 11. Result of EDS analysis alone shown by yellow line in Fig. 6.
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Sample for TEM analysis of Nb/Cu/Inconel 718 interface was prepared by ion thinning. Because of the significant difference between the physical and chemical characteristics of copper, niobium, and Inconel 718, only thin foils of Nb/Cu interface formed, while the sample preparation on the Cu/Inconel 718 interface failed. Fig. 12 shows the STEM image and the corresponding EDS map of the Nb/Cu interface. A metallurgical bonding interface formed at the Nb/Cu interface due to the mutual diffusion of copper and Nb atoms. Fig. 13 shows the XRD patterns of the joint. The 12
Nb7Ni6 and NbNi3 IMCs were not observed in the joint with Nb/Cu mutil-interlayer,
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indicating that the addition of Cu foil suppressed the formation of Nb-Ni IMCs.
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Fig. 12. STEM image and corresponding EDS map of Nb/Cu interface.
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Fig. 13. XRD analysis of the joint.
3.3 Mechanical properties of the joint The hardness distribution of the dissimilar joint is shown in Fig. 14. The heat
affected zone of Ti6Al4V side exhibited the highest microhardness, attributing to the formation of the martensite phase in the HAZ during the laser welding of Ti6Al4V (Gao et al., 2013). The microhardness of FZ and Nb/Cu/Inconel 718 interface is 220 and 280 HV, rspectively. Gao et al. (2018) proved that the microhardness of 13
Nb/Inconel 718 interface is higher than 380 HV, when the brittle Nb7Ni6 and NbNi3 IMCs formed at the Nb/Inconel 718 interface during the laser welding of Ti6Al4V alloy to Inconel 718 with Nb interlayer. The addition of Cu foil inhibited the formation of Nb-Ni IMCs, decreasing the hardness of Nb/Cu/Inconel 718 interface.
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The dissimilar joint with the Nb/Cu interlayer possess better mechanical properties.
Fig.14. Microhardness profiles of dissimilar joint.
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Fig. 15 shows the tensile results of the joint. The dissimilar joint with a Nb/Cu multi interlayer have a largest tensile strength of 260 MPa and fractured at the Cu
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reaction layers. The fracture strain of the joint was less than 2.3%, indicating low ductility of the joint. Zhang et al. (2016) found softening appeared in the FZ and HAZ after the copper is welded. As shown in Figs. 5 and 9, the cross-section area of
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the Cu reaction layer is less than the rest of the joint, and welding defects such as voids and cracks form at the Nb/Cu/Inconel 718 reaction layer, resulting in the
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failure of joint appearing at the Cu reaction layer. The tensile samples showed the partial region of the fracture surface of by SEM, as shown in Fig. 16. Compared to
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the joint with Nb interlayer (Gao et at, 2018), the fracture surface of the dissimilar joint with Nb/Cu multi-interlayer shows the some dimples and flat surfaces, indicating that the ductility of joint improved by inserting an Nb/Cu multi-interlayer. The appearance of flat surfaces indicates that the joint is still very brittle despite the presence of some dimples on the fracture surface. Table 5 shows the EDS analysis results of points A–E in Fig. 16. The EDS analyses of the fracture surface of the joint detected Cu-based and Nb solid solutions, indicating that the crack originated from 14
Nb/Cu/Inconel 718 reaction layer. High quality welding of copper and niobium is very difficult, because of the immiscibility of the Cu and Nb systems (Pan et al., 2017). The bonding at the Nb/Cu/Inconel718 interface is poor at high heating and cooling rates, as shown in Fig. 9. Although the formation of brittle IMCs was avoided, high stress concentration easily results at the Nb/Cu/Inconel 718 interface due to the formation of the voids and cracks, resulting in the brittle-like fracture behavior of the joint. Oliveira et al. (2017) proved that the formation of weld defects at the joining interface for dissimilar welding would deteriorate seriously the
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mechanical properties of the joint. The several pores and non-welded regions presented in Fig.9 were responsible for the inferior properties of Ti6Al4V/Inconel
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718 joint and the fracture of joint appearing at the Nb/Cu/Inconel 718 reaction layer.
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Fig. 15. Tensile results of dissimilar joint: (a) tensile curve and (b) Cross sections of fractures of joints
Fig. 16. Fracture surface analysis of the dissimilar joint with a Nb/Cu interlayer. 15
Table 5 Chemical composition (at%) of point A–E in Fig. 16 Zone
Atom percent/%
Potential phases
Cu
Nb
Ni
Cr
A
95.90
2.26
2.25
Cu solution
B
91.74
4.08
2.82
Cu solution
C
60.72
25.01
7.68
D
88.04
7.64
2.35
Cu solution
E
90.47
5.01
3.29
Cu solution
3.22
Cu +Nb solution
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3.37
Fe
4. Discussion
The schematic diagram of the laser welding-induced brazing reaction is shown in
Fig. 17. One of the keys to this method is selecting the appropriate thickness of Nb
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and Cu as the multi-interlayer. When the laser beam is located on the interface between Ti6Al4V and Nb, Nb interlayer with a very high melting point may partially
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melt, and the unmelted Nb would be a good diffusion barrier for Ti6Al4V and Inconel 718. The cross section of the joint with different Nb interlayer is shown in Fig. 18.
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The thickness of Nb is too less to effectively prevent the mixing of the two parent materials in the molten pool, resulting in unsuccessful welding of Ti6Al4V and
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Inconel 718. When the Nb interlayer is too thick, numerous voids and cracks formed at the Nb/Cu/Inconel 718 interface due to the low interface temperature, consequently
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deteriorating the mechanical property of the joint. The cross section and microstructure of the joint with different Cu interlayer are shown in Figs. 19 and 20,
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respectively. Many Ni atoms can diffuse to the Cu/Nb interface due to thin Cu layer, resulting in the formation of Nb-Ni and Nb-Fe IMCs layer, as shown in Fig.19 and Table 6. The formation of many Nb-Ni and Nb-Fe IMCs would embrittle the Nb-Cu interface and lead to the decline of joint strength (Li et al., 2012). Many cracks appeared at the Cu/Nb and Cu/Inconel 718 interfaces with the increasing of the thickness of Cu foil, as shown in Fig.20. The thick copper foil makes it more difficult for Nb atoms to diffuse through it to Nb/Cu interface and reduces the temperature of 16
Cu/Inconel 718 interface. The disappearance of Ni atom aggregation suppresses Nb solution in the Cu, leading to the formation of cracks at the Nb/Cu interface. The low temperature of Cu/Inconel 718 interface is responsible for the formation of cracks. Another key to this method is to ensure that the interface temperature of Nb/Inconel 718 is higher than the melting point of Cu (1084 C). Under appropriate laser welding parameters, a large amount of heat will be transferred by the unmelted Nb from (Ti, Nb) molten pool to the interface of the Nb/Cu/Inconel 718 interface, because of high thermal conductivity of Nb. For Nb/Cu/Inconel 718 interface, Cu began to melt when
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the interface temperature exceeded its melting point. The Nb and Inconel 718 were joined by brazing with dissimilar metals forming a Nb/Cu interlayer. The Ti6Al4V and Nb would be joined by the FZ, whereas the brazing welding would be responsible
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for bonding Nb to Inconel 718.
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Fig. 17. Sketch schematic diagrams of laser welding-induced brazing reaction.
Fig. 18 Cross section of joint with different Nb interlayer: (a) 0.4 mm and (b) 1.0 mm. 17
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Fig. 19 Cross section of joint with Cu foils of thickness 0.03mm Table 6 Chemical composition (at%) of point A–C in Fig. 19
Atom percent/%
Potential phases
Ni
Cr
Fe
A
15.35
38.46
17.54
16.48
B
17.71
37.29
20.44
C
1.16
5.58
11.2
Cu
12.17
Nb-Ni +Nb-Fe IMCs
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Nb
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Zone
7.65
Nb-Ni +Nb-Fe IMCs
3.04
79.02
Cu solution
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16.91
Fig. 20 Cross section of joint with Cu foils of thickness 0.15mm
4.3 Microstructure evolution of Nb/Cu/Inconel 718 interface Fig. 21 shows the microstructure evolution of Nb/Cu/Inconel 718 interface. As the temperature of Nb/Cu interface is higher than the melting point of Cu (1084 C), the Cu liquid phase forms at the Nb/Cu/Inconel 718 interface, as shown in Fig. 21a. 18
Subsequently, the Cu liquid phase wets the surfaces of solid Nb and Inconel 718, and the interdiffusion of elements occurs at the Nb/Cu and Cu/ Inconel 718 interfaces. The Niobium and Inconel 718 dissolve in the Cu liquid phase as a result of diffusion at the solid Nb-liquid Cu and solid Inconel718-liquid Cu interfaces. Accompanied by the dissolution of solid Nb and Inconel 718, the Cu atoms in the liquid phase would diffuse into Nb interlayer and Inconel 718, as shown in Fig. 21b. Brazing at the Nb/Cu/Inconel 718 interface followed a diffusion and dissolution mechanism. A comparison of the thickness of Nb/Cu and Cu/Inconel 718 reaction layer (Fig. 9)
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indicates less thickness of the Nb/Cu reaction layer than that of Nb/Inconel 718, because the liquid Cu dissolves a much more amount of the Inconel 718 and Cu atoms easily diffuse into Inconel718. The melting point of Inconel 718 is less different from
that of Cu, and the Inconel 718 base metal melted near the Cu/Inconel 718 interface
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and may be in the semi-solid state. In contrast, the melting point of Nb is much higher
than the melting point of Cu, and Nb still may be in the solid state. Pan et al. (2017)
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demonstrated successful welding of Cu and Nb in the temperature range between 92% and 98% of the melting point of Cu, because the Cu in the semi-solid state can
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provide more diffusion paths. The diffusion quantity and distance of the Cu atom to the Nb atom is smaller than that of the Cu atom to Inconel 718.
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Fig. 21c shows the microstructure evolution of Ti/Nb/Cu/Inconel 718 during the cooling process. During cooling, Cu-based solid solutions primarily precipitate from the liquid phase. The Nb- and Ni-based solid solutions form by the diffusion of the Cu
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atoms in the liquid phase to Nb and Inconel 718. The elements Nb, Ni, Cr, and Fe dissolved in the liquid Cu diffuse with difficulty to the opposite side, because of the
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high cooling rate of the pulsed laser welding and thick Cu interlayer. Therefore, no intermetallic compounds formed at the Nb/Cu/Inconel 718 interface. The phase transformation of liquid formation and solidification for the Nb/Cu/Inconel 718 interface can be summarized as follows: With the heating stage:
Cu 1084 ℃ L 19
diffusion L Nb Inconel 718 L dissolution
With the cooling stage: L →Cu (congruent reaction) L →Cu+Ni (congruent reaction), L →Cu + Nb (congruent reaction), Two metallurgical bonding zones based on the fusion welding and brazing mechanism were simultaneously identified in the Ti6Al4V/Inconel 718 with Nb/Cu
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interlayer welded. One of the zones was also (Ti, Nb) zone formed by the fusion
welding mechanism, and the other zones were the bonding layer formed at the Nb-Cu
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interface and Nb-Inconel 718 interface attributed to the brazing mechanism.
Fig. 21. Schematic of the formation of reaction layer for Nb/Cu/Inconel 718 interface: (a) liquid phase
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formed by heat conduction, (b) interaction between Cu liquid and base metal and (c) final solidification microstructure of the brazing seam.
5. Conclusions
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1. Laser welding induced brazing-welding method was used for creating hybrids with the fusion zone and brazing reaction zone separated by the unmelted Nb. 2. Fusion zone comprising Ti-based and Nb-based solutions formed at the Ti/Nb interface, and the brazing layer with the main microstructures of Nb-based, Cu-based, and Ni-based solid solution formed between Nb and Inconel 718, because of the melting of Cu foil. 3. The strength of the joint with Nb/Cu multi-interlayer exhibited the largest tensile 20
strength of 260 MPa, whereas the joint facture in brittle-like failure. 4. The Nb/Cu multi-interlayer could help prevent the formation of IMCs, and voids occurred at the Nb/Cu/Inconel718 reaction layer, because of the immiscibility of Cu and Nb and high heating and cooling rates of laser welding, which can be the origin of the brittle fracture of the joint. Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant Nos. 51705005 and 51905006), Scientific Research Plan Projects of Shaanxi
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Education Department (Grant No. 17JK0050), Natural Science Foundation of Shaanxi Province (Grant No. 2018JQ5204), Scientific Research Plan Projects of Baoji (Grant No. 2017JH2-10), and Fundamental Research Funds for Baoji University of Arts and
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Sciences University of China (Grant No. ZK 16045).
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