Brazing Ceramics to Titanium Using Amorphous Filler Metal

Transcription

1 Liu &S_Layout 1 10/14/14 3:22 PM Page 66 RZING & SOLERING TOY razing eramics to Titanium Using morphous Filler Metal Y YU HU LIU, JIN ONG HU, ZUO XING GUO, N JIN HEN LI It was demonstrated that microstructure and mechanical properties are significantly influenced by the brazing temperature, heat time, and cooling rate T i-6l-4v is one of the most important titanium alloys and is widely used in aerospace industries. This alloy has excellent fracture toughness and corrosion resistance. lso, it can be readily welded, forged, and machined. On the other hand, ZrO2, one of the most important ceramic materials, has attracted great attention because of its high strength and fracture toughness (Ref. 1). The difficult fabrication of complex-shaped and/or large-sized components limits its widely potential applications because of the brittleness and inflexibility (Refs. 2, 3). pproaches to join ZrO2 with other materials, typically metals and alloys, can overcome this drawback to a great extent, leading to intensive investigations in the past decades (Refs. 4 8). There have been some methods, such as brazing, transient liquid phase bonding, and diffusion bonding, to be proposed for ceramic-metal or ceramic-ceramic joining (Ref. 1). mong the most popular methods is active brazing, particularly using the g-u-ti alloys, which are eutectic or close to eutectic compositions (Refs. 4, 5). Recently, it has been found that amorphous alloys with good glassforming ability could be used as brazing solders (Refs. 9, 10), even replacing the traditional g-u-ti filler alloys, because of the three advantages for the bonding materials: 1) the amorphous alloys can accelerate atomic diffusion and surface reaction during the brazing process at a low brazing temperature, which can reduce 66 WELING JOURNL / NOVEMER 2014 Fig. 1 The XR analyses of brazing joint. Side of ZrO2 ; side of Ti-6l-4V. Fig. 2 ackscattered electron image of the brazing interface at 1148 K for 5 min. residual stress produced in the joint and thus enhance the joint strength; 2) the superior wettability of the amorphous alloy shortens the joint clearances required by traditional atomized powders or paste formulations at the joint for filling; 3) in addition, the amorphous foil with large width and thickness combines with out- standing ductility and flexibility (Refs ). In particular, the Ti-based amorphous filler metals offer a combination of low density, high specific strength, and relatively low cost (Refs ). Zou et al. joined two pieces of ceramic Si3N4 materials using a Ti-Zr-Ni-u amorphous foil as a solder and

2 RZING & SOLERING TOY E achieved the joint with much higher bonding strength than that joined by using a crystalline filler metal (Ref. 19). However, there is no report on the joining of the ZrO 2 /metal assemblies using amorphous filler metals. In this contribution, in view that Ti-6l-4V is one of the most important titanium alloys because of its excellent fracture toughness and corrosion resistance as well as the readily welded, forged, and machined properties for wide applications in aerospace industries, ZrO 2 is brazed to the Ti-6l-4V alloy using a g 53 amorphous solder. The systemical investigation demonstrates that the microstructural evolution and mechanical properties remarkably rely on the brazing conditions, such as temperature and heat time, as well as cooling rate. Joining eramic ZrO 2 and Titanium lloy Ti-6l-4V alloy plate (5.76 l, 4.03 V, 0.28 Fe, 0.06, and Ti, wt-%) and sintered ZrO 2 pieces with the size of mm were used for the joining experiment. The μmthick g 53 amorphous foil with the melting temperature of 1119 K was cut into the size of 15 5 mm for the use of solder. oth the Ti-6l-4V and the ZrO 2 pieces were polished with different sizes of diamond pastes, while the g 53 amorphous foil was gently ground with Si sandpaper. fter cleaning in acetone, ZrO 2 and Ti- 6l-4V plates and g 53 foil were assembled in a sandwich structure. The brazing experiment was performed in a high vacuum of Pa. fter the welding experiment, the specimens were sectioned and then polished for microstructural characterizations by a field emission scanning electron microscope (FESEM) equipped with an energy-dispersive spectrometer (ES). The compounds and alloys at the interfacial region were identified by X-ray diffraction (XR) using ukα radiation. The shear strength of the joint was measured using an MTS tester (MTS-810) at a constant speed of 0.1 mm/min. Effect on the Microstructure of the Joint Figure 1 shows the XR patterns of the brazing joint, demonstrating that there are TiO, Ti 2 O, u 2 O, and g phases in the side of ceramic ZrO 2 and g, uti, uti 2, and α-ti phases in the side of Ti-6l-4V alloy. Figure 2 shows the backscattered electron image of the interface zone of the joint at 1123 K for 5 min. Table 1 shows the chemical composition measured by ES. ecause the first, second, and fifth layers are very thin, their compositions cannot be measured. ombined with Fig. 1 and Table 1, the Ti atoms diffused and reacted with O atoms from ZrO 2, producing TiO and Ti 2 O compounds in the first thin layer (Ref. 20). In the second layer, u 2 O compound formed because of the strong appetency between Ti and u atoms as well as the reaction with ZrO 2 (Ref. 4). It is a single g element in the third layer. The gray and black phases are uti 2 and uti, in the fourth layer, respectively. The Widmanstätten structure in the fifth layer is α-ti. The interface of the joint consists of six layers of different thicknesses, which are ZrO 2 /TiO+ Ti 2 O/u 2 O/g/uTi+uTi 2 /Widmanstätten structure/ti-6l-4v alloy. Figure 3 shows the microstructures of the brazing joints under different brazing temperatures, demonstrating the remarkable changes of the mi- Table 1 hemical ompositions (at. %) of Various Phases Observed in Fig. 4 Region Ti O g u Zr l Phase Fig. 3 Microstructure of the brazing joints in different temperatures K; 1173 K; 1198 K; 1223 K; E 1273 K TiO + Ti 2 O u 2 O g uti uti α Ti NOVEMER 2014 / WELING JOURNL 67

3 RZING & SOLERING TOY crostructures of the joint with increasing temperature. The thickness of the g layer decreases, obviously, and is discontinuous. The thickness of the first layer increases with increasing temperature. s the temperature increases, the evident contrasts in the interface of the joint change to two layers from four layers, because of the enhanced diffusion of the atoms with the increasing temperature. Ti atoms combined with O atoms and formed TiO + Ti 2 O, so that the thicknesses of TiO + Ti 2 O and Widmanstätten structure increases. The heat time effect on the microstructures of the brazing joints at 1173 K is presented in Fig. 4. The thickness of the first layer increases gradually with increasing the time, while the thickness of the second layer decreases from 30 to 8 μm. Figure 5 shows the microstructure changes of brazing the joints with different cooling rates at 1173 K for 10 min. The cracks and the micropores appear in the interface of the ZrO 2 and first layer with increasing of the cooling rate, which depresses the shear strength. Fig. 4 Microstructure of brazing joints for different heat times. 5 min; 10 min; 20 min; 30 min. Effect on the Shear Strength of the Joint t a given heating time of 10 min, the shear strength of the joint as a function of the brazing temperature is plotted in Fig. 6. s shown in this figure, the shear strength increases to 178 MPa as the brazing temperature decreases to 1123 K, reaching the maximum value. With increasing of the temperature, the shear strength of the joint will give rise to a slight depression in shear strength. The fracture of the joint occurs in the second layer near the side of ZrO 2 because of the brittle TiO and Ti 2 O oxides, of which the thickness increases with the increase in brazing temperature, leading to the decrease in shear strength of the joint. The shear strength of the brazing joints is also strongly dependent on the heat times, as shown in Fig. 7, where the shear strength of 178 MPa can be achieved in the heat time of 10 min. While further extending the time to 30 min, the shear strength is almost the same. The fracture occurs at the interface between TiO + Ti 2 O and u 2 O. The shear strength of the Fig. 5 Microstructure of the brazing joints with different cooling rates. 5 K/min; 10 K/min; 20 K/min; 30 K/min. brazing joints rests with the thickness of the u 2 O layer. The thickness of u 2 O decreases with increasing of the heat time, which increases the shear strength. ut a short heat time, such as 5 min, leads to insufficient diffusing of the elements and the residual brazing filler metal. Therefore, the shear strength of the brazing joints is low. Figure 8 shows the shear strength of the brazing joints decreases as the cooling rate increases. The shear strength can reach 178 MPa at a cool- 68 WELING JOURNL / NOVEMER 2014

4 RZING & SOLERING TOY Fig. 6 The shear strength of the brazing joints in different brazing temperatures. Fig. 7 The shear strength for the brazing joints for different heat times. Fig. 8 The shear strength of the brazing joints with different cooling rates. ing rate of 5 K/min. When the cooling rate is increased to 30 K/min, the shear strength decreases to 136 MPa due to fracture occurring at the interface between TiO + Ti 2 O and u 2 O. low cooling rate, such as 5 K/min, can offer abundant time for the diffusion and the reaction of the atoms in the filler metal and the base metal, reducing the thermal mismatch and the residual stress at the interface between the ceramic and metal. In contrast, elevating the cooling rate gives rise to the increase of the fraction of brittle u 2 O phase with long strip and the presence of a high residual stress at the interfaces, which depresses the shear strength. Therefore, the optimal technical parameters make it possible to achieve the brazing of ZrO 2 and Ti-6l- 4V alloy using g 53 amorphous brazing filler metal with the maximum shear strength of 178 MPa, which is higher than that of zirconia/stainless steel with g-u filler metal (90 MPa) (Ref. 21), and that of ZrO 2 and Ti-6l-4V alloy using 7 Zr 28 u 14 Ni 11 amorphous brazing filler metal (108 MPa) (Ref. 22). onclusions eramic ZrO 2 and metallic Ti-6l- 4V alloy have been successfully brazed using amorphous g 53 filler foil. It is found that the shear strength decreases with the increase of the brazing temperature, the heat time, and cooling rate because of the concomitant changes of the microstructures and components at the interface of the brazing seam. The maximum shear strength of 178 MPa can be achieved based on the optimal technical parameters: the brazing temperature of 1123 K, the heat time of 10 min, and the cooling rate of 5 K/min. WJ cknowledgments This work was supported by 2012 Open Foundation of the Key Lab of utomobile Materials, Jilin University, from Natural Scientific asic Research Fund for Platform and ase onstruction (Grant No ), the State Key Laboratory of dvanced Welding and Joining, Harbin Institute of Technology (No. WPT-Z03), and epartment of Science & Technology of Jilin Province (Grant No ). References 1. Hanson, W.., Ironside, K. I., and Fernie, J ctive metal brazing of zirconia. cta Mater. 48: Hao, H., Wang, Y., Jin, Z., and Wang, X Joining of zirconia ceramic to stainless steel and to itself using g 57 u 38 Ti 5 filler metal. J. m. eram. Soc. 78: Wang, X. H., and Zhou, Y Layered machinable and electrically conductive Ti(2)l and Ti(3)l(2) ceramics: review. J. Mater. Sci. Technol. 26: Muolo, M. L., Ferrera, E., Morbelli, L., and Passerone, Wetting, spreading, and joining in the alumina-zirconia-inconel 738 system. Scripta Mater. 50: Smorygo, O., Kim, J. S., Kim, M.., and Eom, T. G Evolution of the interlayer microstructure and the fracture modes of the zirconia/u-g-ti filler/ti active brazing joints. Mater. Lett. 61: urov,. V., Kostjuk,.., Shevchenko,. V., and Naidich, Y. V Joining of zirconia to metal with u-ga-ti NOVEMER 2014 / WELING JOURNL 69

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