Rational design of composite interlayer for diffusion bonding of tungsten–steel joints

Accepted Manuscript Rational design of composite interlayer for diffusion bonding of tungsten–steel joints

Yunzhu Ma, Wentan Zhu, Qingshan Cai, Wensheng Liu, Xinkuan Pang, Chaoping Liang PII: DOI: Reference:

S0263-4368(17)30655-8 doi:10.1016/j.ijrmhm.2017.10.002 RMHM 4528

To appear in:

International Journal of Refractory Metals and Hard Materials

Received date: Accepted date:

19 September 2017 4 October 2017

Please cite this article as: Yunzhu Ma, Wentan Zhu, Qingshan Cai, Wensheng Liu, Xinkuan Pang, Chaoping Liang , Rational design of composite interlayer for diffusion bonding of tungsten–steel joints. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Rmhm(2017), doi:10.1016/ j.ijrmhm.2017.10.002

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ACCEPTED MANUSCRIPT Rational Design of Composite Interlayer for Diffusion Bonding of Tungsten–Steel Joints Yunzhu Ma, Wentan Zhu, Qingshan Cai *, Wensheng Liu, Xinkuan Pang, Chaoping Liang

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State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China

ABSTRACT`

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Large residual stresses and brittle intermetallic compounds are two detrimental factors

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in the mechanical reliability of tungsten–steel joints. In this work, we conduct a rational

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design of interlayer structure to tackle these two pitfalls by combining finite element model (FEM) and Hume–Rothery rules. The FEM results show that the selection of

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interlayer materials for tungsten–steel joining depends not only on thermal expansion mismatch, but also on the plastic deformation of interlayer. In practice, it is better to

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relax the residual stress by a soft interlayer with high ductility than to transfer the stress from W by a hard interlayer with a low coefficient of thermal expansion. However, the

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Hume–Rothery rules indicate adding a single interlayer cannot prevent the formation of brittle intermetallic compounds. A composite interlayer design is then proposed in this

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study to minimize the possibility of intermetallic compounds formation in the meantime reduce the residual stress. Using V/Cu and V/Ni as typical examples, we demonstrate that composite interlayer can greatly improve joint performance because of the lower residual stresses and less detrimental intermetallic compounds. Keywords: Intermetallic; Residual stress; Tungsten; Diffusion bonding; Interlayer.

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Corresponding author. Tel.: +86 731 88877825; fax: +86 731 88836476. E–mail address: [email protected], [email protected] (Q.S. Cai). 1

ACCEPTED MANUSCRIPT 1. Introduction Tungsten (W) is a promising refractory material for fusion nuclear application, for its high resistance against sputtering and low tritium retention [1]. To apply W as structural material in current divertor design for demonstration reactor (DEMO), a joint

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to the frame structural materials ferritic–martensitic high chromium steels is urgently

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needed [2]. Among those bonding technology, diffusion bonding is an attractive method

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to join W with steel owing to its tolerable bonding temperature and high–temperature performance. However, the diffusion bonding process of tungsten and steel suffers from

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two serious pitfalls. The first one is the formation of detrimental intermetallic phases.

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As demonstrated by W.W. Basuki and J. Aktaa [3], a thin layer of FeW intermetallic phase and metal carbides formed in the course of directly diffusion bonded W to steel at

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1050 ℃ and these brittle intermetallics cause an early failure of the W/steel bonding.

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The second pitfall is the huge gap in the thermal expansion coefficient between W and steel (CTE, 4.5×10–6 K–1 for W and 12–14 ×10–6 K–1 for steel at room temperature (RT)).

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When temperature drops during cooling from the joining temperature and during

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subsequent service, the bonding area can develop high internal stresses due to the CTE mismatch, which lead to poor joint strength [4, 5]. Adding supplement metallic interlayer which have been proposed by many researchers is a practical way to reduce the residual stress and to minimize the brittle intermetallic compounds formation between tungsten and steel [6–8]. For instance, Basuki and Aktaa [9, 10] reported the bonding of tungsten and EUROFER97 using Nb or V interlayer. Zhong et al. [11] and Wang et al. [12] have developed a laminated

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ACCEPTED MANUSCRIPT interlayer of Ti for diffusion bonding of tungsten to steel. This kind of interlayer materials are selected due to the CTE is intermediate between W and steel which is beneficial for mitigating the residual stress in the joint. However, the presence of brittle intermetallic phases (FeTi, Fe2Ti, Nb2C and VC, et al) in this type of joint is still

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detrimental for the interlayer/steel interfaces, and deteriorates the overall mechanical

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properties. To overcome this issue, Zhong et al. [8, 13] have tried a soft Ni interlayer as

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inserted material to reduce the residual stress in the meantime prevent the formation of detrimental intermetallics at interlayer/steel interface. The maximum strength of 215

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MPa was obtained for the sample bonded at 900 ℃ for 1 h and the strength decreased

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with increasing bonding temperature or holding time due to the higher volume fraction of Ni4W intermetallic compound at W/interlayer interface. The above results suggest

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that the use of single interlayer is unable to tackle these two pitfalls at the same time,

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and a novel design of interlayer is needed based on a systematic understanding on the degradation of bonding.

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In order to obtain a sound W/steel joint, a rational design of interlayer material is

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crucial to reduce the residual stress and remove detrimental brittle intermetallics. In this work, we first study the distribution of von Mises stress in W–steel diffusion bonded joint by means of FEM. Then an attempt to reduce the residual stress is made by adding interlayer Cu, Ni, Mo, Ta, and V, which yields an optimized interlayer among those candidates. After optimizing those candidates through FEM, we perform a detailed analysis on the possibility of forming solid solution between interlayer and matrixes based on Hume–Rothery rules. By combining FEM results and Hume–Rothery rules, a

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ACCEPTED MANUSCRIPT composite interlayer design is proposed in this study to minimize the possibility of intermetallic compounds formation in the meantime reduce the residual stress. Last but not least, we demonstrate the effectiveness of our design principle by using V/Cu and V/Ni composite interlayers on the diffusion bonding between W and steel.

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2. Experimental procedure

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2.1. Materials and joining procedure

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The base materials used in this study are a high–Cr ferritic steel (Fe–17Cr–0.1C in wt.%) and commercially available W (99.95 wt.% purity). The samples were cut into

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size of Ø25 mm×13 mm. The filler metal used for brazing was a 20 μm thick sheet of

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the Ni–based amorphous alloy (Ni–7Cr–5Si–3B, wt.%). Mo, Ta, V, Ni and Cu foils were used as insert metals, each of which has the purity of 99.95% and thickness of

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0.1~0.5 mm. The surfaces of the materials to be joined were polished and then

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ultrasonically cleaned in acetone. The prepared materials, assembled in the structure of W/interlayers/steel, were mounted in the bonding chamber. The diffusion bonding and

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brazing of the assemblies were carried out at 1050 ℃ for 1h in vacuum (<10–3 Pa).

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2.2. Microstructure evaluation and tensile testing The microstructure evaluation was realized by means of field–emission scanning electron microscopy (FE–SEM) equipped with an energy dispersive spectroscopy (EDS) microanalyzer. Tensile tests were conducted by a tensile testing machine (Instron–3369) at RT with a crosshead speed of 1 mm/min. The interlayer was at the center of the gauge length. An average tensile test value was obtained by testing five specimens. Fracture surface after tensile testing was observed under FE–SEM. More details about the

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ACCEPTED MANUSCRIPT joining and tensile testing procedures were described elsewhere [14, 15]. 2.3. FEM calculations FEM calculations were made to obtain the residual stresses which developed in the bonded sample as it was cooled from the bonding temperature to RT (from 1050 ℃ to

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20 ℃). FEM analysis was using ANSYS program (Ver. 14.5) and the FEM model was

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established for a 2–D axisymmetrical thermal elastic–plastic stress analysis to the joint.

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Due to the axial symmetry of the joint (Fig. 1), an arbitrary meridian plane was selected to analyze the stress in the joint with four–node axisymmetrical elements. The division

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of the finite element mesh was shown in Fig.1. The values of thermal expansion

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coefficient, elastic modulus, Poisson ratio, yield strength of the base materials and various interlayers at different temperatures were obtained from the thermomechanical

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3. Results and discussion

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analysis (TMA) experiments and some references [16, 17].

3.1. Effect of interlayer material on residual stresses

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Fig. 2 (a) shows the contour plots of the von Mises stress distribution for the

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W–steel joint. It can be seen that the maximum residual stress is located adjacent to the joining interface in the W substrate. Accordingly, it can be concluded that the residual stress is much more detrimental for W side than for steel side. To analyze the stress distribution in detail, Fig. 2 (b) presents the stress distribution along the axis of symmetry in the W substrate as a function of the distance from the joining interface. The stress profiles confirm that the residual stress in the W side were maximum in the vicinity of the interface, as also observed by Kalin et al. [6] and Zhong et al. [8], which

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ACCEPTED MANUSCRIPT would be the weakest region against the thermal load or mechanical load. We now turn to the interlayer effect between W and steel joint. In physics there are two kinds of interlayer: (a) a hard interlayer, characterized by Mo, Ta, or V, which has a low CTE but a relatively high strength both at room and high temperature. A decrease of

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residual stress would be expected due to the direct reduction of the mismatch of thermal

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expansion between W and steel; (b) a soft interlayer, characterized by Ni or Cu, which

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has a low strength and good ductility, but a relatively high CTE. A relaxation of the residual stress is predictable as a result the interlayer’s plastic deformation during

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joining operation [18]. In this study, we consider both hard and soft interlayer in order

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to examine their effect on the reduction of residual stress. Table 1 shows the main characteristics of the metal foils and the base materials used in this study.

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Fig. 3 shows the residual stress distribution on the W substrate close to the interface

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for different interlayer. It can be seen that the sequence of the maximum residual stress follows Cu < Ni < Mo < Ta < V < no interlayer. The effect of the soft interlayer is better

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than the hard interlayer in the same thickness of 0.5 mm. This means that under the

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boundary conditions employed for the model construction, the most important characteristic for the interlayer material is ductility. These results suggest that the use of low yield strength and high ductility interlayer is the most effective method of relaxing joint stress. A principle can be discerned from Fig. 3 that, the lower the CTE of the hard interlayer, the smaller the maximum residual stress. A possible mechanism of the reduction of residual stress is that the hard interlayer which has low CTE and high

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ACCEPTED MANUSCRIPT strength compensates the CTE mismatch by transferring some residual stress from W [19]. This leads to a poor overall performance. Fig. 4 shows the residual stress profile across the W/hard interlayer bonding interface. It is observed that, the smaller the gap in CTE, the larger the residual stress in the hard interlayer, which results in a smaller

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residual stress in the W substrate. Correspondingly, the relaxation of the residual stress

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in the W–steel joint by inserting a hard interlayer is based on residual stress transferring

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mechanism.

For a soft interlayer which has a low yield strength and high ductility, it reduces the

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residual stress in the W–steel joint by a creep or yield mechanism during slow cooling

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from joining temperature, as demonstrated by other researchers [18, 20, 21]. Comparing Cu interlayer with Ni interlayer form Fig. 4, it is verified that residual stress acting on

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the W substrate were reduced significantly by using Cu interlayer, which could be

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attributed to the higher ductility of Cu than that of Ni. Fig. 5 shows the relationship between the calculated maximum residual stress in W

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and the tensile strength of W–steel joint produced using various interlayer materials.

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According to the previous tensile results of W–steel brazed joints with different interlayers [14, 22], and the order of the joint strength is Cu > Mo > Ta > no interlayer > V. The calculated value of residual stress is accordance with the tensile test results for all the interlayer materials except V. In V the lowest joint strength is obtained despite it has lower residual stress than pristine W/steel joint. Fig. 2 and 3 indicate that the maximum residual stress is produced in the W substrate near the joining interface, which generally accounts for the failure which occurred during tensile testing (the

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ACCEPTED MANUSCRIPT fracture took place predominantly in W substrate). However, when a V interlayer was used, the fracture occurred at the bonding seam due to the formation of brittle vanadium boride with Ni–V intermetallic compounds [22]. It is considered that the joint strength by using a V interlayer is mostly controlled by the strength of interface between the

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interlayer and the substrate. Thus, the strength of the W–steel joint was strongly

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3.2. Effect of interlayer material on interfacial reaction

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influenced by residual stress as well as the formation of intermetallic compounds.

The control of intermetallic layers is important for improving interfacial strength in

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processing techniques concerned with the interfacial reaction between W and steel.

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Elements of chosen interlayer materials should form solid solution or eutectic phase with that of substrate materials as possible in order to avoid the formation of brittle

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intermetallic compounds.

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For the binary alloy systems, according to Hume–Rothery rules [23], when the atomic radiuses difference between the two elements is less than 8%, they can form

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continuous solid solution, but when the difference is larger than 15%, the size factor

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prevents the formation of solid solution. Meanwhile, a continuous solid solution might be formed between two metal elements when their electronegativities difference is less than 0.2. A limited solid solution might be formed when their electronegativities difference is between 0.2 and 0.4, and an intermetallic compound might be formed when their electronegativities difference is over 0.4. Based on Hume–Rothery rules, we construct a Darken–Gurry map [23] for W and Fe, as shown in Fig. 6. The formula of the interlayer selection for W–steel joining is as

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ACCEPTED MANUSCRIPT the following expression: 2 （x  Rx） / 0.0064Rx  ( y  ey ) 2 / 0.04  1

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2 （x  Rx） / 0.0225Rx  ( y  e y ) 2 / 0.16  1

(2)

2

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where Rx and e y are the atomic radius and electronegativities of the material being

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bonded, respectively. In Fig. 6, we used the main elements W and Fe of the base

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materials as the center to draw an ellipse that reflects the mutual solubility domain. The

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metal elements which locate at a small ellipse obtained by Eq. (1) can form continuous solid solutions with W or Fe. The metal elements which sit between the small ellipse

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from Eq. (1) and the large elliptic from Eq. (2) can form limited solid solutions with W

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or Fe. As can be seen from the diagram, the metal elements Pt, Mo and Au, etc. have good solubility in W, and Ni, Cu, Cr, V and Be, etc. can dissolve in Fe. Unfortunately, it

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is unable to find a metal element can have a good mutual solubility in both W and Fe, as

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there is no element lies in the overlap of the two ellipses. It should be mentioned here that the metal element must be the non–carbide forming element. Otherwise, hard and

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brittle carbides will form and deteriorate the overall performance of bonding.

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Thus, a single metal interlayer (Ni [8,13], Nb[9], Ti [11] or V [10], etc.), which reacts with one or both of the base metals and forms intermetallic phases or other brittle phases, is not appropriate to be chosen as the interlayer between W and steel. In this paper, we propose composite interlayer design that adding two or more interlayer materials for diffusion bonding W to steel. As mentioned in Sec. 3.1, the hard and soft interlayers have different effects on the bonding. Our strategy is that the hard interlayer is put on W side, while soft one on steel site. Consequently, the hard metal (Pt, Ta, Mo,

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ACCEPTED MANUSCRIPT V, Nb or Ti) with a low CTE can form continuous solid solution with W, the soft metal (Ni or Cu) with a better ductility bonded with steel by eliminating the formation of detrimental carbides. Combining hard and soft interlayers can thus inherit the merits of individual interlayer but also be able to circumvent the obstacle in forming solid

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solution with both substrates.

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3.3. Interlayer design for W–steel joining

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Based on the design principle elaborated above, V/Ni and V/Cu composite interlayers are added between W and steel in present study. The bonding strength and

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fracture feature of W–steel joints with composites interlayers are presented in Table 2,

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as well as the W–steel joints with single interlayer reported in the literatures. It is clear that the W/steel joints with V/Ni and V/Cu composite interlayers display a much higher

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to our best knowledge.

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strength than those with only one interlayer. Actually it is the highest bonding strength

The interfacial microstructures of the W–steel joints with V/Ni and V/Cu composite

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interlayers are shown in Fig. 7. Neither intermetallic compounds nor brittle carbides are

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discernable at both the W/interlayer and interlayer/steel interfaces. On the other hand, the hard/soft interlayer interfaces were well–bonded, particularly the V/Cu interface, there is free from intermetallic compounds in the diffusion zones. This suggests that the design of a hard/soft composite interlayer between W and steel prevent the formation of the hard and brittle intermetallic compounds, which is also beneficial for the bonding strength of W–steel joint. The mixed fracture mode was observed by using the composite interlayer (Fig. 8), which suggests that the W/steel joint have both an

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ACCEPTED MANUSCRIPT excellent interfacial bonding strength and a favorable stress distribution in the bonded zone. In addition, Fig. 9 shows the contour plots of the von Mises stress distribution for the W/V/Ni/steel and W/V/Cu/steel joints. Even though the maximum residual stress

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still located adjacent to the joining interface in the W substrate (Fig. 9), similar to that of

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no interlayer and single interlayer (Fig. 2 and 3), the residual stress acting on the W

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substrate is greatly reduced by using composite interlayer (Fig. 10). This can be attributed to the dual–effect of hard and soft interlayers, which yields a win–win

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situation on the bonding interfaces. The hard interlayer (V) could transfer some residual

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stresses from W and the soft interlayer (Ni or Cu) can relieve the residual stress by plastic deformation when cooling from the bonding temperature. This suggests that the

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design of a hard/soft composite interlayer between W and steel can effectively reduce

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the residual stress, thus enhancing the joint properties. From the above results, we can conclude that our design principle on composite

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interlayers can enhance the diffusion bonding of W and steel and circumvent the two

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pitfalls, large residual stresses and brittle intermetallic compounds on the bonding interfaces. The design principle is reiterated here (1) a layer of hard interlayer with a low CTE to transfer some residual stress from W, (2) a soft interlayer with good ductility to relax residual stress, (3) a strong interfacial bonding of interlayers with the substrate materials and (4) composite interlayers reduce or to avoid the formation of brittle intermetallic phases. 4. Conclusions

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ACCEPTED MANUSCRIPT In this work, we propose a hard/soft composite interlayer for diffusion bonding W to steel. Such composite interlayer is designed to prevent the formation of the hard and brittle intermetallic compounds and to reduce the residual stress in the joint. Based on the design principle, we have successfully bonding W and steel using V/Cu and V/Ni

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composite interlayers. The bonding strength of these two W–steel joints shows much

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higher value that those with only one interlayer. The hard/soft composite interlayer

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design not only inherit the merits of individual interlayer but also be able to circumvent these two pitfalls, large residual stresses and brittle intermetallic compounds. The

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rationale in the design of composite interlayer not only be proven effective in W–steel

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Acknowledgment

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huge gap in physical properties.

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joints, but also bring new insights to the general joints between two components with

This work was supported by the National Natural Science Foundation of China,

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China Postdoctoral Science Foundation and Postdoctoral research funding plan in

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Hunan Province and Central South University.

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ACCEPTED MANUSCRIPT using a titanium interlayer, J. Alloys Compd. 489 (2010) 545–551. [12] J.C. Wang, W. Wang, R. Wei, X. Wang, Z. Sun, C.Y. Xie, Q. Li, G.N. Luo, Effect of Ti interlayer on the bonding quality of W and steel HIP joint, J. Nucl. Mater. 485

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ACCEPTED MANUSCRIPT [20] A.P. Xian, Z.Y. Si, Residual stress in a soft-buffer-inserted metal/ceramic joint, J. Am. Ceram. Soc. 73 (2010) 3462–3465. [21] Z.Y. Wang, G. Wang, M.N. Li, J.H. Lin, Q. Ma, A.T. Zhang, Z.X. Zhong, J.L. Qi, J.C. Feng, Three-dimensional graphene-reinforced Cu foam interlayer for brazing

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Schematic representation of the model geometry resulted from the geometric simplifications (a) and the mesh configuration (b) used in the FEM calculation. Fig. 2. (a) Distribution of residual stress in W/steel joint (left) and magnification of near

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joining interface (right). (b) Residual stress profile in W substrate as a function of

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distance from the joining interface.

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Fig. 3. Residual stress distribution on the W substrate close to the bonding interface for different interlayer.

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Fig. 4. Residual stress profile across the bonding interface of W/hard interlayer.

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Fig. 5. Relationship between calculated maximum residual stress in W and tensile strength of W–steel brazed joint produced using various interlayer materials.

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insert between W and steel.

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Fig. 6. The distribution of atomic radius and electronegativity for interlayer material

Fig. 7. SEM micrograph of the interface of W–steel joints diffusion bonded at 1050 ℃

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for 1 h with V/Ni and V/Cu composite interlayer materials.

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Fig. 8. Representative fractured surfaces after tensile test on the W sides for the W–steel joints diffusion bonded at 1050 ℃ for 1 h with V/Ni and V/Cu composite interlayer materials.

Fig. 9. Contour plots of the von Mises stress distribution for the W–steel joints with the 0.2V/0.1Ni and 0.2V/0.1Cu interlayers, respectively. Fig. 10. Residual stress profile in W substrate as a function of distance from the joining interface for the W–steel joints with the V/Ni and V/Cu interlayers.

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ACCEPTED MANUSCRIPT Table Captions Table 1 Physical and mechanical properties of the metal foils and the base materials at room temperature. Table 2 Joint strength of diffusion bonded W and steel at RT, using different interlayer

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materials.

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(b)

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Fig. 1. Schematic representation of the model geometry resulted from the geometric

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simplifications (a) and the mesh configuration (b) used in the FEM calculation.

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(a)

interface

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800

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Residual stress (MPa)

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W matrix

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(b)

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Distance from the bonding interface (mm)

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Fig. 2. (a) Distribution of residual stress in W/steel joint (left) and magnification of near

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joining interface (right). (b) Residual stress profile in W substrate as a function of

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distance from the joining interface.

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Fig. 3. Residual stress distribution on the W substrate close to the bonding interface for

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different interlayer.

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Fig. 4. Residual stress profile across the bonding interface of W/hard interlayer.

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Fig. 5. Relationship between calculated maximum residual stress in W and tensile

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strength of W–steel brazed joint produced using various interlayer materials.

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Fig. 6. The distribution of atomic radius and electronegativity for interlayer material

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insert between W and steel.

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(b)

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(a) (a)

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Fig. 7. SEM micrograph of the interface of W–steel joints diffusion bonded at 1050 ℃

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for 1 h with V/Ni and V/Cu composite interlayer materials.

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(b)

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(a)

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Fig. 8. Representative fractured surfaces after tensile test on the W sides for the W–steel

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joints diffusion bonded at 1050 ℃ for 1 h with V/Ni (a) and V/Cu (b) composite

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interlayer materials.

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(b)

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(a)

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Fig. 9. Contour plots of the von Mises stress distribution for the W–steel joints with the

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0.2V/0.1Ni (a) and 0.2V/0.1Cu (b) interlayers, respectively.

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(b)

Fig. 10. Residual stress profile in W substrate as a function of distance from the joining

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interface for the W–steel joints with the V/Ni and V/Cu interlayers.

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ACCEPTED MANUSCRIPT Table 1 Physical and mechanical properties of the metal foils and the base materials at room temperature.

–1

coefficient (10 /℃ )

(GPa)

W

4.2

398

Mo

5.1

275

Ta

6.6

186

V

8.3

128

steel

12.6

Ni

13.3

Cu

16.5

strength (MPa) 600–900

ratio 0.31 0.29

460

0.31

380

0.36

206

520

0.25

208

148

0.31

78

0.35

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Poisson

435

125

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Yield

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–6

Elastic modulus

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Material

Thermal expansion

ACCEPTED MANUSCRIPT Table 2 Joint strength of diffusion bonded W and steel at RT, using different interlayer materials. Interlayer

Joint strength

material

(MPa)

V

215

Nb

272

Nb/steel interface

Ti

113

Ti/steel diffusion zone

[11]

Ni

215

W/Ni interface

[8, 13]

Fracture position

Reference

Brittle vanadium carbide layer in the

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region adjacent to the V/steel interface

[10] [9]

Predominantly in W substrate, and some V/Ni

362

in V–W diffusion zone, V interlayer and

402

Predominantly in W substrate, and some in both W/V interface and V interlayer

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V/Cu

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Ni–V intermetallics layer

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Our study

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Highlights

 A hard/soft composite barrier interlayer was designed for diffusion bonding of W to

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steel.

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 The relaxation of the residual stress by inserting a hard interlayer is based on

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residual stress transferring mechanism.

 The bonding strength of W–steel joints with composite interlayer shows much

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higher value that those with only one interlayer.

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 The mixed fracture mode was observed by using the composite interlayer.

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