Microstructure evolution and mechanical properties of vacuum brazed ZrO2/Ti–6Al–4V joint utilizing a low-melting-point amorphous filler metal
Introduction
ZrO2 ceramics have numerous applications in the fields of gas turbines, machining tools and aerospace due to their outstanding advantages of high strength, high fracture toughness, high ionic conductivity and excellent chemical stability [[1], [2], [3], [4]]. However, the high brittleness of ZrO2 ceramics seriously makes it hard to manufacture complex shapes and/or large-size parts [5,6]. The hybrid structure obtained by joining ZrO2 ceramics to metals provides a promising method to overcome the above-mentioned shortcomings and expands the scope of applications [[7], [8], [9], [10]]. As is well known to us all, TC4 is a common alloy generally used as the joining substrate of ZrO2 ceramics due to its good comprehensive properties [[11], [12], [13], [14]], and the hybrid structure of TC4/ZrO2 is widely applied to the production of wings, rudders and other products in the aerospace field [[15], [16], [17]]. In addition, the thermal expansion coefficients of TC4 (9.2 × 10−6 °C−1) and ZrO2 (9.0 × 10−6 °C−1) are similar [18], which is also one of the important reasons for choosing these two materials for joining.
In the aspect of the ceramic-metal join, various feasible methods have been proposed such as vacuum brazing, transient liquid phase (TLP) bonding and diffusion bonding (DB) [[19], [20], [21]]. Compared to TLP and DB, the former displays feature of simplicity, low cost, and high welding quality, so it is one of the best methods for joining dissimilar materials [[22], [23], [24], [25], [26], [27]]. And high vacuum degree can effectively inhibit the formation of oxides. Recently, scholars have done some research on the brazing of ZrO2/TC4 by crystalline or amorphous filler. The results suggested that active Ti will be accumulated on the surface of ZrO2, and form a titanium oxide layer through the situ chemical reaction and the thickness of the titanium oxide layer is the main factor affecting the strength of the joint [28]. Dai et al. [29] studied the microstructure and mechanical properties of the Ag–Cu filler metal brazed ZrO2/TC4 joint in detail. They pointed out that a TiO + Cu3Ti3O double-layer structure was formed on the surface of ZrO2 ceramic, and the strength of the joint first increases and then decreases with the increase of the thickness of the TiO + Cu3Ti3O layer. Liu et al. [30] used Ti33Zr17Cu50 amorphous filler metal to successfully join ZrO2 ceramic and TC4 alloy and obtained a high-strength joint. The experimental results suggest that a TiO + TiO2 reaction layer was formed adjacent to the ZrO2 ceramic, and the strength of the joint is also controlled by the thickness of the reaction layer. Unfortunately, the formation and growth mechanism of the reaction layer and the relationship between brazing parameters, joint strength and thickness of the oxide reaction layer is still uncertain, which is need further study.
In this work, we designed a low melting point Ti35Zr25Be30Co10 amorphous filler metal and applied it in the joining of ZrO2 ceramic and TC4 alloy. The melting-point of amorphous Ti35Zr25Be30Co10 filler metal is close to the traditional Ag-based crystalline filler metals [31]. The low melting point can reduce the heat input, thus avoiding the production of thicker intermetallic compounds [[32], [33], [34], [35]]. In addition, compared to crystalline filler metals, amorphous filler metal can accelerate the diffusion of atoms and interfacial reactions during brazing. It is favorable to lower the residual stress generated in the joint, which can dramatically improve the strength of the joint [[36], [37], [38]]. To study the relationship between the brazing parameters, the thickness of the reaction layer and the joint strength, the microstructure and shear strength of the joint was studied under different temperatures and holding time. The formation and growth mechanism of the TiO layer and the relationship between brazing parameters, joint strength and TiO layer thickness was discussed in detail. To analyze the plastic deformation ability of each phase in the joints, a nano-indentation test was carried out on the joint. Furthermore, to study the effect of vacuum degree on the microstructure and shear strength of the joint, brazing experiments were carried out under 10−1Pa, 10−2Pa, and 10−3 Pa atm respectively.
Section snippets
Experimental procedures
The base materials were ZrO2 ceramic (ZrO2-3 mol.% Y2O3) and TC4 alloy block with a dimension of 4 mm × 4 mm × 10 mm and 10 mm × 10 mm × 4 mm, respectively. The preparation of Ti35Zr25Be30Co10 amorphous ribbon with the melting temperature of 750 °C was achieved by rapid solidification in a high-purity argon atmosphere [31].
Fig. 1(a) shows the schematic diagram of the assembled samples. All the surfaces to contacting were polished with SiC sandpaper and cleaned with acetone in an ultrasonic
Microstructure of the ZrO2/TC4 joint
Fig. 2 presents the typical microstructure and corresponding element distribution of ZrO2 and TC4 joints at a brazing parameter of 810 °C/10 min. The joint was composed of five parts: namely, ZrO2 ceramic, TC4 alloy and reaction layer adjacent to ZrO2 ceramic (zone I), internal reaction zone of filler metal (zone II), diffusion layer adjacent to TC4 substrate (zone III). Zone I is a black reaction layer containing O and Ti. Three different phases can be clearly observed in zone Ⅱ, namely, dark
Conclusion
The ZrO2 ceramics and TC4 were successfully brazed with a low-melting-point Ti35Zr25Be30Co10 amorphous filler metal. The main conclusions are outlined as follows:
- (1)
Reliable join is achieved between ZrO2/TC4 were obtained due to the formation of the TiO layer. The typical microstructure of the joint is ZrO2/TiO/Be2Ti + Co2Ti+β-(Ti,Zr)+α-(Ti,Zr)/Widmanstätten structure/TC4.
- (2)
The low vacuum will oxidize the surface of the filler, and oxidation will hinder the melting of the filler and lead to the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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