Vacuum brazing of TZM alloy and graphite with Ti-35Ni alloy: Microstructure, properties, and mechanism

doi:10.21203/rs.3.rs-2579894/v1

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Vacuum brazing of TZM alloy and graphite with Ti-35Ni alloy: Microstructure, properties, and mechanism

https://doi.org/10.21203/rs.3.rs-2579894/v1

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Reliable TZM/graphite brazed joint was obtained in a vacuum using Ti-35Ni alloy in the present work. The interfacial microstructure, shear strength, and fracture characteristics of the brazed joints were all analyzed. In addition, the formation mechanism of the brazed joint was investigated in detail. The typical interfacial microstructure of the TZM/graphite joint brazed at 1200°C for 10 min was TZM/Ti(s,s)/TiNi+Ti₂Ni/TiC/graphite. The width of the brazing seam increased and then decreased with the elevated temperature, while the thickness of the TiC layer gradually increased. In addition, the amount of Ti₂Ni phase in the brazing seam decreased above 1200°C and eventually disappeared at 1220°C. An optimal shear strength of 14.5 MPa was achieved for the TZM/graphite joint at a brazing temperature of 1300°C for 10 min. At the optimum brazing temperature, the fracture path of the joint started in the TiC layer and then extended to the interior of the graphite substrate.

TZM alloy

Graphite

Brazing

Interfacial microstructure

Mechanical properties

Regarded as the essential component of a fusion reactor, the first wall covers 3/4 of the plasma chamber, which is normally composed of graphite or C/C composites [1, 2]. The first wall is connected with the diverter to form a heat dissipation device [3]. To ensure the normal operation of the nuclear fusion reactor, the heat from the diverter must be transferred in time. TZM alloy (Mo-0.5%Ti-0.08Zr, wt%) with high thermal conductivity, low thermal expansion coefficient, and superior high-temperature mechanical properties is commonly applied in the diverter [4]. In addition, as a non-metallic layered material, graphite possesses a high melting point (3850°C), excellent thermal conductivity, thermal shock resistance, and chemical stability [5, 6]. More importantly, the density of graphite is much smaller than that of molybdenum alloy. Therefore, the reliable joining of TZM alloy and graphite can make full use of the excellent heat dissipation properties and small specific gravity of graphite to exploit the excellent high-temperature mechanical properties of TZM alloy [7, 8].

Research on the joining of graphite and dissimilar materials, such as brazing, diffusion bonding, and mechanical connection has been performed [9–11]. Among these methods, brazing is one of the most common methods for joining graphite to metal due to its simplicity and high joint strength [12]. However, most inactive filler alloys (such as AgCu alloy, and SnAgCu alloy) perform poor wettability on the graphite, resulting in difficulties for the liquid alloy to react with the substrate for forming a reliable joint during the brazing process [5, 13]. To solve the key problem, direct brazing and indirect brazing are widely used. Although the two methods are performed differently, one by adding active elements to the filler alloys and the other by forming a metallization or activation layer on the surface of graphite [6, 13]. However, both require the reaction of active elements (Ti, Zr, Cr, V, etc.) with graphite to form a continuous carbide layer, thus achieving the brazing of graphite and metal.

On the other hand, Ag-based or Ti-based filler alloys are most frequently applied to realizing the brazing of molybdenum alloys and other materials [14–16]. However, Ag-based filler alloys perform poor wettability on the molybdenum alloys, and the addition of a small amount of active elements such as Ti can effectively improve the wettability of filler alloys [15]. Nevertheless, Ag-based filler alloys cannot be applied in environments above 500°C [17]. Ti-based filler alloys, such as Ti-Ni, Ti-Si, Ti-Cr, and Ti-V-Mo alloy, can achieve a good metallurgical connection between molybdenum alloys and other metals or non-metals [18, 19]. Although the types of filler alloys are different, the common characteristic is that the active element Ti in the filler alloys reacts with the molybdenum alloy form Ti-Mo solid solution or Mo(s,s), which realizes the brazing of molybdenum alloy and other materials.

As previously described, the active element Ti in the filler alloy reacts not only with the TZM alloy to form a solid solution, but also with graphite to form a stable carbide layer, thus enabling the brazing of the TZM alloy to graphite. Furthermore, nickel-based filler alloys are widely used for brazing under high-temperature conditions because of their high service temperature and excellent mechanical properties [20, 21]. Pure nickel or nickel-based filler alloys have been used for the brazing of TZM alloys and other materials (e.g. Nb-Zr alloys, ZrCp-W) [14, 22]. Ti-61Ni alloy was chosen to braze TZM alloys and ZrCp-W, forming a sound brazed joint and a continuous Ti-Mo solid solution layer on the TZM alloy side [14].

In this research, Ti-35Ni (at%) metal foil was employed as the filler alloy to join TZM alloy and graphite. The microstructure and reaction products of TZM/graphite brazed joints were examined. In addition, the effect of the brazing temperature on the interfacial microstructure and mechanical properties of the joints were investigated. The formation mechanism of the joints was also studied in detail.

The base materials used in this work are TZM alloy (provided by Suzhou Antiport Metal Products Co., Ltd, China) and graphite (supplied by Qingdao Baofeng Graphite Products Co., Ltd., China), where the dimensions of TZM alloy and graphite used for brazing tests were machined into samples with sizes of 4.5 × 4.5 × 5 mm³ and 18 × 18 × 4 mm³. The microstructure of the TZM alloy after corrosion using the prepared corrosion solution (HNO₃:H₂SO₄:H₂O of 5:3:3, vol.%) was shown in Fig. 1(a). Together with the X-ray diffraction (XRD) result (Fig. 1(b)), it could be seen that the alloy is mainly composed of Mo(s,s). The graphite with the purity of over 99.95%, the density of 1.8 g/cm³ and the average shear strength of 21.7 MPa was supplied by Qingdao Baofeng Graphite Products Co., Ltd., China. The microstructure and XRD result of the graphite were shown in Fig. 1(c) and (d).

The Ti-35Ni (Ti: Ni = 65:35, at.%) alloy was prepared by arc melting the mixture of sponge Ti and pure nickel pellets under an argon atmosphere. The melting process was repeated 10 times to ensure the homogeneity of the filler alloy. The filler alloy was then processed into 4.5 mm × 4.5 mm × 500 µm foils for the brazing experiment. The foils were subsequently ground to 200 µm with SiC sandpapers, then ultrasonically cleaned with acetone and dried in air.

Prior to the brazing experiment, the surfaces of the TZM alloys and graphite were ground with SiC sandpapers, then ultrasonically cleaned with acetone for 10 min and dried in air. For the brazing, the Ti-35Ni foil was assembled between the TZM alloy and graphite, and then the brazed assembly was placed in the resistance furnace. The brazing temperature in this work was set to a temperature range of 1100°C to 1400°C, with a heating rate of 10°C/min, a fixed holding time of 10 minutes, and a cooling rate of 5°C/min.

A simultaneous thermal analyzer (DSC 1/1600LF) was used to measure the melting temperature of the filler alloy. The microstructure and composition of the brazed joints and fracture morphology were examined by an SEM (Zeiss, MERLIN Compact) equipped with an energy dispersive spectroscope (EDS, OCTANE PLUS, EDAX). The phase identification of the filler alloy and fracture morphology were further characterized by X-ray diffraction (XRD, DX-2700) with Cu-Kα radiation at a scanning rate of 2°/min in the 2θ range of 20°-80°. The three-dimensional morphology of the fracture morphology was observed by optical microscopy (OM, DSX 510, OLYMPUS). The shear strength of the brazed joints was evaluated by a universal testing machine (Instron 5967) at a constant speed of 0.5mm/min. At least five specimens were tested for each reported value.

3.1 Microstructure characterization of the Ti-35Ni alloy

Figure 2(a) shows the SEM images of the Ti-35Ni alloy, in which the microstructure of the alloy consisted of grey phase A and dark grey phase B. The EDS results (Table 1) show that the atomic ratio of Ti and Ni in grey phase A was close to 1:1. It can be supposed that the grey phase was the TiNi compound as results of the Ti-Ni binary phase diagram[23]. In the dark grey phase B, by contrast, had a high elemental Ti content and an atomic ratio of nearly 2:1 between Ti and Ni, which was presumed to be the Ti₂Ni compound. The XRD analysis was used to further determine the phase compositions of the filler alloy, as shown in Fig. 2(b). The Ti-35Ni alloy consisting of the grey TiNi phase and the dark grey Ti₂Ni phase was determined. Figure 2(d) shows the DSC curve of the Ti-35Ni alloy, which presented that the melting temperature of the alloy was 988°C.

Table 1

Element compositions and possible phases of the regions marked in Fig. 2(a) (at. %).
Spot	Ti	Ni	Possible phases
A	50.21	49.79	TiNi
B	65.84	34.16	Ti₂Ni

3.2 Microstructure characterization of the brazed joints

Figure 3 shows the cross-sectional SEM image and elemental mappings of the TZM/graphite brazed joint acquired at 1200°C for 10 min using Ti-35Ni alloy. A homogeneous brazed joint with defect-free was observed where the joint contained three distinct regions based on the morphology shown in Fig. 3(a), marked by region I (reaction layer nearby TZM alloy), region II (brazing seam) and region III (reaction layer adjacent to graphite), respectively. The width of the brazing seam was approximately 75 µm, which was much smaller than the thickness of the original filler alloy foil (200 µm). According to the elemental distribution in Fig. 3, it is indicated that region I was mainly composed of elements Ti and Mo. Region II was the Ti-Ni reaction layer based on Fig. 2. Moreover, region III was primarily composed of elements Ti and C. In addition, a large amount of element Ti was attached around the surface of graphite.

To chacracterize the interfacial products of the brazed joints, the microstructure of three zones was magnified and shown in Fig. 4, and the EDS analysis results were listed in Table 2. It could be found that the brazing seam mainly contained four different phases, which were represented by A, B, C, and D. Phase A contains mainly 53.92% Ti and 38.98% Mo, which could be presumed to be a solid solution Ti(s,s) [18]. Based on the elemental ratios and the microstructural characterization of the filler alloy, it could be inferred that phases B and C were the Ti₂Ni phase and TiNi phase respectively. The grey-black phase D in region III contained mainly 65.51% Ti and 27.69% C, which indicated that phase D was the TiC phase. Similarly, the reaction products within the graphite (marked E and F) were deduced to be TiC and TiNi phases.

Table 2

EDS results of each points tagged in Fig. 4(b)-(e) (at. %).
Spot	Ni	Ti	Mo	C	Zr	Possible phase
A	3.63	53.92	38.98	-	3.47	Ti(s,s)
B	33.49	64.12	1.24	-	1.16	Ti₂Ni
C	43.85	49.21	5.38	-	1.56	TiNi
D	5.14	65.51	0.85	27.69	0.81	TiC
E	3.87	42.32	0.98	51.99	0.83	TiC
F	26.30	28.77	0.79	43.29	0.85	TiNi

Therefore, according to the above analyses, it was concluded that the typical interfacial microstructure of TZM/graphite joint brazed at 1200°C for 10 min was TZM/Ti(s,s)/TiNi + Ti₂Ni/TiC/graphite.

3.3 Microstructure of the TZM/graphite joints with various brazing temperature

Figure 5 shows the interfacial microstructure evolution at different brazing temperatures. All the joints were free of obvious metallurgical defects. It was found that as the brazing temperature raised, the width of the brazing seam showed a trend of first increasing and then decreasing. In addition, above 1200°C, there was a greater reduction in the width of the brazing seam. The reason was that the lower temperature (1100°C) led to a poorer flow of the filler alloy and a weaker reaction with the substrate on both sides. Besides, the elevated temperature led to an increase in the degree of reaction between the liquid alloy and the substrate, a thickening of the reaction layer, and an increase in the width of the brazing seam. The temperature upgrade led to a further addition in the fluidity of the liquid alloy and an increase in the existence time of the liquid alloy during the brazing process. This indicated that the liquid alloy spills out in response to the applied load and that the degree of penetration of the liquid alloy into the porous graphite interior rose. The combined effect of these two factors resulted in a significant reduction in the width of the brazing seam.

The further comparison revealed that the interfacial microstructure of the joints remained TZM/Ti(s,s)/TiNi + Ti₂Ni/TiC/graphite at temperatures below 1200°C. In addition, an excessively thick layer of TiNi + Ti₂Ni compound was present in all the brazed joints, which was not conducive to the relief of residual stresses during the cooling of the joints and weakened the joint strength [14, 24]. Above 1200°C, the thickness of region II showed a large degree of reduction. At 1220°C, the Ti₂Ni phase disappeared and the interfacial microstructure of the joint changed to TZM/Ti(s,s)/TiNi/TiC/graphite.

A continuous and dense reaction layer in the joint was a prerequisite for excellent mechanical properties, while an excessively thick or thin compound layer was detrimental to the mechanical properties of the joint. Therefore, it is necessary to investigate the morphology and thickness of region III (TiC layer). A local magnification of the TiC reaction layer at different brazing temperatures (1100°C to 1400°C) was shown in Fig. 6. In all joints, the TiC layer was continuous but of uneven thickness.

In addition, the thickness of TiC layer was closely linked to the shear strength of brazed joint. The formation of TiC layer on the graphite side was conducive to improving the shear strength of the joint, when the thickness of TiC layer was not too thick nor too thin. In Fig. 7(a), the thickness of the TiC layer was gradually increasing as the temperature rose. Below 1200°C, the TiC layer thickness was less than 3 µm, increasing to 1300°C, the thickness reached 6.04 µm. Further to 1350°C, reaching 10.10 µm, at which point the excessive TiC layer thickness would also weaken the mechanical properties of the joint. Therefore, in order to obtain a high-strength joint, it was necessary to investigate the relationship between the thickness of TiC layer and brazing temperature. It is generally believed that the thickness of the reaction layer conforms to the empirical equation [25]:

$$\text{Χ=K}{\text{t}}^{\text{0.5}}$$

$$\text{K=}{\text{K}}_{\text{0}}\text{exp}\left(\text{-Q/RT}\right)$$

Where X (m) is the thickness of reaction layer, K (m²/s) is the reaction rate constant, t (s) is the holding time, K₀ (m²/s) is the pre-exponential factor, Q (kJ/mol) is the activation energy and R is the gas constant which equals 8.314 J/(K·mol). Figure 7(b) shows the linear fitting results of TiC layer thickess at various temperatures, it is calculated that the value of Q and K₀ are about 121.23 kJ/mol and 2.95×10^− 2 m²/s, respectively. Thus, the relationship between X, T and t can be concluded as:

$$\text{X=2.95×}{\text{10}}^{\text{-2}}\text{exp}\left(\text{-121.23×}{\text{10}}^{\text{3}}\text{/8.314T}\right)\text{×}{\text{t}}^{\text{0.5}}$$

where T (K) is the brazing temperature. According to Eq. (3), the relationship between brazing temperature and the thickness of TiC layer is drawn, as shown in Fig. 7(c). It is found that these two results are close, which validating the correctness of Eq. (3). Therefore, the thickness of TiC layer can be controlled by adjusting the brazing temperature, then improving the mechanical properties of the brazed joint.

3.4 Mechanical properties and fracture characterization of the brazed joints

To evaluate the mechanical property of the TZM/graphite brazed joints, the shear strength was measured as shown in Fig. 8. The strength curve shows that the shear strength of the joint increased and then decreased as the temperature raised. Below 1200°C, the shear strength was low, less than 10 MPa. Up to 1300°C, the shear strength reached a maximum of 14.5 MPa. Further increasing the temperature to 1400°C resulted in a decrease in shear strength to 10.7 MPa.

To establish an intrinsic link between the interfacial microstructure and the mechanical properties, the fracture paths of the joints at different temperatures were analyzed, as shown in Fig. 9. Obviously, temperature variations had a large degree of influence on the fracture path of the joint. In conjunction with the fracture morphology and compositional analysis in Fig. 10 and Table 3, it was found that at lower temperatures (1100°C), the fracture location was mainly in the TiC layer and the fracture was relatively straight. Raised to 1300°C, the fracture path changed to one that started in the TiC layer and then extended to the interior of the graphite substrate. Further increasing the temperature to 1400°C, the fracture path did not change significantly and was similar to that at 1300°C.

The changes that occurred in the fracture path of the joint at different temperatures corresponded to the changes in the shear strength in Fig. 9. At lower temperatures, the reaction between the liquid alloy and the substrate on both sides was not sufficient, leading to a thin thickness of the reaction layer formed (2.12 µm). As a result, the bond between the TiC layer and the graphite base was weak and became the weak link in the joint, thus the fracture location was mainly in the TiC layer. The thickness of the TiC layer increased as the temperature rose, strengthening the bond with the graphite substrate and increasing the strength of the joint, thus resulting in a change in the fracture path. However, excessively high brazing temperatures could intensify the degree of reaction between the liquid alloy and the substrate, leading to the formation of an excessively thick reaction layer. In addition, excessive temperature variation in the brazing process could generate large amounts of brazing residual stresses. In the combined action of these two factors, the shear strength of the joint decreased.

Table 3

EDS chemical compositions and possible phases of each spot in Fig. 10 (at.%).
Spot	Ni	Ti	Mo	C	Zr	Possible phase
A	0.13	1.14	0.07	98.59	0.08	Graphite
B	3.27	54.87	0.20	41.53	0.13	TiC
C	0.06	0.50	0.03	99.74	0.04	Graphite
D	0.30	56.47	0.44	42.62	0.18	TiC

3.5 Formation mechanism of brazed joints

From the previous results, it could be deduced that the formation of the TiC reaction layer induced the formation of an intimate joint between the filler alloy and the graphite. Therefore, an in-depth analysis of the thermodynamic principles of TiC reaction layer formation is necessary in order to investigate the formation mechanism of the joint. The Gibbs free energy of the reaction was calculated with the simplified approximate Eq. (4):

$$\text{∆}{\text{G}}_{\text{T}}^{\text{Θ}}\text{=∆}{\text{H}}_{\text{298}}^{\text{Θ}}\text{-T∆}{\text{Φ}}_{\text{T}}^{{\prime }}$$

The interfacial reaction layer in brazed joints was formed mainly by the reaction of the active element Ti with the substrate. The standard Gibbs free energy for each reaction formula was as follows:

$$\text{Ti(l)+C(s)=TiC(s)}$$

$$\text{∆G=-186.606+0.01322T }\left(\text{kJ/mol}\right)$$

$$\text{Ti(l)+Ni(l)=TiNi(s)}$$

$$\text{∆G=-55.585+0.015962T }\left(\text{kJ/mol}\right)$$

$$\text{2Ti(l)+Ni(l)=}{\text{Ti}}_{\text{2}}\text{Ni(s)}$$

$$\text{∆G=-49.120+0.017208T }\left(\text{k}\text{J}\text{/mol}\right)$$

A plot of the Gibbs free energy versus temperature for reaction equations (5)-(7) was shown in Fig. 11. It was clear that the Gibbs free energy for all three reaction equations was negative in the temperature interval (1100°C to 1400°C). This indicated that all three reaction equations were likely to proceed spontaneously during the brazing process, which was confirmed by the interfacial microstructure of the joints in the previous section. Furthermore, a comparison of the Gibbs free energy values showed that the Gibbs free energy of the TiC phase was much smaller than that of the TiNi and Ti₂Ni phases in the temperature range 1100°C to 1400°C. This indicated that the active element Ti in the liquid alloy reacted more readily with the graphite substrate during the brazing process. This was confirmed by the fact that the element Ti was enriched on the graphite side in Fig. 3. In addition, the Gibbs free energy of TiNi was lower than that of Ti₂Ni, indicating that the residual liquid phase preferentially formed the TiNi phase rather than the Ti₂Ni phase during cooling. This was the main reason why the Ti₂Ni phase did not form in the joint at temperatures above 1220°C.

Based on the above results and discussion, a schematic diagram of the formation process of the TZM/Ti-35Ni/graphite brazed joint was drawn, as shown in Fig. 12. The formation process of the joint could be divided into five stages as follows:

Initial stage: as shown in Fig. 12(a), the filler alloy was not melted and there was only a relatively weak solid phase diffusion between the filler alloy and the substrate.
Melting of filler alloy: when the temperature exceeded the melting point of the filler alloy, the filler alloy melted to form a liquid phase. Compared to the element Ni, the element Ti had a stronger affinity to the elements Mo and C. Therefore, the Ti atoms in the filler alloy were gradually enriched towards the base on both sides. Due to the higher affinity between element Ti and C than element Mo, Ti atoms tended to diffuse more towards the graphite substrate. At the same time, the Mo atoms in the TZM alloy would gradually dissolve into the liquid alloy and enrich on one side of the TZM alloy, as shown in Fig. 12(b).
Formation of reaction layers: the Ti atoms enriched on the TZM alloy side reacted with the Mo atoms to form a Ti-Mo solid solution, as shown in Fig. 12(c). Since the melting point of this solid solution was much higher than the brazing temperature, the Ti-Mo solid solution precipitated directly from the liquid phase to the solid phase. Additionally, the Ti atoms enriched on the graphite side reacted with the graphite substrate to form a TiC reaction layer.
Liquid phase homogenization: as shown in Fig. 12(d), the temperature continued to rise and then held, at which point the brazing seam was still dominated by the liquid alloy. Along with the continuous reaction between the Ti atoms in the liquid alloy and the substrate, the amount of Ti-Mo solid solutions gradually increased and joined together to form a Ti(s,s) layer, the thickness of which was also gradually increasing. At the same time, the thickness of the TiC reaction layer also increased with the continuous reaction.
Residual liquid phase precipitation: during subsequent cooling, the residual liquid phase in the brazing seam gradually solidified and precipitated to form the Ti₂Ni and TiNi phases, as shown in Fig. 12(e). In particular, with the preferential formation of the TiNi phase, only the TiNi phase was formed in region II when the residual liquid phase in the brazing seam was not sufficient to form the Ti₂Ni phase.

The TZM alloy and graphite were successfully brazed with Ti-35Ni alloy. The effects of the brazing temperature on the interfacial microstructure and mechanical properties were investigated. Primary conclusions were summarized as follows.

The typical interfacial microstructure of the TZM/graphite joint was: TZM/Ti(s,s)/TiNi + Ti₂Ni/TiC/graphite. During the brazing process, the element Ti in the liquid alloy reacted with the graphite to form a continuous but uneven-thickness TiC layer. At the same time, element Ti also reacted with the TZM alloy to form Ti(s,s). During cooling, the residual liquid phase of the brazing seam solidified to precipitate the Ti₂Ni and TiNi phases.

With the rising in temperature, the width of the brazing seam increased and then decreased, while the thickness of the TiC layer gradually increased. Above 1200°C, the amount of Ti₂Ni phase in the brazing seam gradually decreased. Eventually, the Ti₂Ni phase disappeared from the brazing seam at 1220°C.

The shear strength of the brazed joint increased and then decreased with the temperature increases, reaching a maximum value of 14.5 MPa at 1300°C for 10 min. At lower temperatures, the joint fractures were located in the TiC layer. At higher temperatures, the joint started to fracture in the TiC layer and then extended to the interior of the graphite substrate.

Author contribution

Yanyu Song, Haitao Zhu, and Duo Liu conceived the idea. Material preparation, data collection, and analysis were performed by Haitao Zhu, Kehan Zhao, Naibin Chen, and Shengpeng Hu. Yanyu Song and Haitao Zhu wrote the manuscripts. Duo Liu and Xiaoguo Song supervised the project. All authors read and approved the final manuscript.

Funding

The project is supported by National Natural Science Foundation of China (grant Nos. 52275318 and 52175307), and the Taishan Scholars Foundation of Shandong Province (NO. tsqn201812128).

Conflict of interest

The authors declare no competing interests.

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Vacuum brazing of TZM alloy and graphite with Ti-35Ni alloy: Microstructure, properties, and mechanism

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Abstract

Figures

1. Introduction

2. Experimental

3. Results and Discussion

3.1 Microstructure characterization of the Ti-35Ni alloy

3.2 Microstructure characterization of the brazed joints

3.3 Microstructure of the TZM/graphite joints with various brazing temperature

3.4 Mechanical properties and fracture characterization of the brazed joints

3.5 Formation mechanism of brazed joints

4. Conclusion

Declarations

References

Additional Declarations

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