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Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al
Journal of Alloys and Compounds xxx (xxxx) xxx
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al Zhiwu Xu*, Zhengwei Li**, Ben Chai, Jiuchun Yan State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2019 Received in revised form 26 September 2019 Accepted 28 September 2019 Available online xxx
In this work, TA15 joints reinforced in situ with TiAl3 IMCs were successfully fabricated using pure Al filler by ultrasonic brazing. TA15 alloys were first wetted by pure Al with and without ultrasonication and then ultrasonically brazed. The wetting of Al to the TA15 alloy was realized by the formation of a TiAl3 phase at the contacting interface. The complete wetting of the Al filler to the TA15 substrate was achieved at 20 min without ultrasonication. Ultrasonication significantly accelerated the wetting process, and the time required to achieve complete wetting was shortened to 10 s. During ultrasonic brazing, the formation of TiAl3 in the joint mainly depended on the holding time. The content of the TiAl3 phase increased from 35.2% to 77.2% as the holding time was increased from 5 min to 30 min. Joint strength increased with the amount of TiAl3 in the joint. Ultrasonication for 10 s and a holding time of 30 min generated a joint filled with homogeneously distributed TiAl3. The maximum shear strength of this joint was 140.6 MPa, which was 75% higher than that of pure Al. Failure crack propagated through the filler at short holding times and through the filler and TiAl3 phase at prolonged holding times. © 2019 Elsevier B.V. All rights reserved.
Keywords: TA15 alloy TiAl3 Ultrasonic brazing Cavitation Wetting
1. Introduction TA15 alloy is a type of advanced material that provides the properties of high specific strength, excellent corrosion resistance, high temperature performance as well as light weight compared with steels and Ni-based alloys, thus serving as an important structural material in the aerospace, automobile, defense and chemical industries [1]. During the manufacturing of titanium components with complex geometry, joining is a necessary technique and becomes increasingly important for titanium alloys. Nowadays, a variety of joining methods such as fusion welding (e.g. gas tungsten arc welding and laser welding) [2,3], solid state bonding (e.g. diffusion bonding [4] and friction stir welding [5]) and brazing [6e10] have been developed to join titanium alloys. Of these techniques, brazing can provide unique advantages in joining dissimilar materials and many contact points over a large area without severe distortion, as well as in minimizing the thermal impact on the substrates [9]. In the process of brazing titanium alloys, Zrebased, Tiebased [11,12], Cuebased [13], Agebased
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Xu), [email protected] (Z. Li).
[14,15] and Alebased alloys [16] can be used as the filler metals. The application of Al-based filler metal makes it possible to braze titanium alloys at lower temperature and braze Ti/Al dissimilar materials to combine the advantages of titanium and aluminum alloys, which has been attracting more and more interest of the researches [10,16]. As easily estimated from the binary phase diagram, almost all the filler metals now available form intermetallic compounds (IMCs) with titanium in the brazing of titanium alloys [8]. The IMCs have the characteristics of low ductility, high hardness and brittleness, making them easy to be the crack sources during loading. It is found that the category, morphology and distribution of the IMCs have significant influence on the joint quality [7,17e19]. Therefore, various attempts for improving the joint quality by controlling the formation of the IMCs have been proposed. Takemoto et al. [8] investigated the effect of Si content in the Al-based filler metal on the formation of IMCs during brazing of Ti/Ti joint and found that a small addition of Si up to 0.8% in the Al filler metal remarkably reduced the growth rate of TiAl3. Chen et al. [6] reported that TiAl3 IMCs were formed at the joint interface in the case of low-heat input and TiAl3, TiAl, Ti5Si3, and Ti3Al IMCs were observed at high heat input during the laser weldingebrazing of Ti/Al dissimilar alloys using AlSi12 flux-cored wire filler. They also reported that the average tensile strength of the joints with lamella-shaped, cellular-
https://doi.org/10.1016/j.jallcom.2019.152493 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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shaped and clubeshaped morphologies of IMCs was significantly higher than that of the joints with thick and continuous IMCs layer [7]. Ma et al. [16] observed that the joint containing the lamellarelike Ti7Al5Si12 phase showed higher strength than that containing the blockelike TiAl3 phase when brazed the TC4 alloy to Ale4Cue1Mg alloy using Zn-based filler metal. Recently, ultrasonic brazing has been proposed as a promising method to join dissimilar materials (e.g. Al/Ti [16,20], Al/Mg [17,21] and Mg/Ti [22]) and easy-to-be-oxidized materials (e.g. Mg [23e25], Al [26,27] and Ti [28] alloys). This method does not require the use of flux or vacuum environment. It can remove the surface oxides of the substrate and realize the wetting of liquid filler to the substrate in very short time (normally in seconds) by cavitation effect [29,30]. Moreover, ultrasonic generates intensive stirring in the liquid filler, which can remarkably accelerate the mass transfer at the liquid/solid interface as well as in the joint. As a result, the morphology and distribution of IMCs may be modified. With these in mind, we attempted to braze the TA15 alloy using pure Al filler with and without ultrasonication in the present work. The formation of IMCs in the joint was focused on, with the aim to obtain TA15 joint reinforced in situ by dispersed TiAl3 IMCs with the assistance of ultrasonic. The effect of the IMCs on the joint strength under various brazing conditions was also evaluated. 2. Experimental TA15 alloy with dimensions of 15 mm 10 mm 3 mm was used as the substrate in this work. The substrates were polished using #500 emery paper and ultrasonically cleaned in acetone for 10 min. Pure Al was used as the filler metal. The chemical compositions of TA15 alloy are listed in Table 1. The entire experiment was divided into three parts. The first part involved wetting without ultrasonication for the evaluation of the wettability of pure Al to the TA15 substrate. Pure Al was first placed in a self-designed mold and heated to approximately 720 C. The substrate was then immersed into molten Al at holding times of 5, 10, and 20 min. The second part involved ultrasonic-assisted wetting, as illustrated in Fig. 1. This experiment was conducted for the evaluation of the wettability of pure Al to the TA15 substrate with ultrasonication. An ultrasonic vibration system (UPM-U-P1010A01) was used. A frequency of 20 kHz and output power of 1000 W were adopted. The substrate was immersed into molten Al, and ultrasonic vibration was switched on. Ultrasonic vibration time was set to 10 s. Holding times of 5 and 20 min were adopted after ultrasonication. The third part involved brazing, as presented in Fig. 1. Joints were brazed using a specific fixture. This procedure was performed with a 20 min holding time without ultrasonication, followed by 10 s of ultrasoncation and holding times of 5, 10, 15, and 20 min. The parameters used in wetting and brazing experiments are summarized in Table 2. After the wetting and brazing experiments, metallographic samples were cut using an electrical discharge cutting machine, and their microstructures were observed with a scanning electron microscope (SEM, FEI-Quanta 200) equipped with an energy dispersive X-ray spectroscopy (EDS) system and Zeiss SEM with electron back-scattered diffraction (EBSD) detector. IMC content in the joint was calculated by an image analysis software (Image-Pro Plus 6.0), and the size of IMC was examined by a Nano Measurer
Table 1 Chemical composition of TA15 alloy. Element
Al
Mo
V
Zr
N
H
O
Ti
Content
6.64
1.70
2.16
2.09
0.10
0.004
0.01
Bal
(Type 1.2). A nanoindentation instrument (G200) was used to measure the joint’s hardness. The testing distance was 20 mm, testing force was 0.01 N, and dwell time was 10 s. Joint shear strength was tested on an electromechanical material testing machine (Instron-5569) at a speed of 1 mm/min. The geometry of shear strength testing sample and the schematic of the shear strength testing are shown in Fig. 2. The fracture position and surface were observed via SEM. The phases on the fracture surfaces were determined on a D/max-RB X-ray diffraction (XRD) system with Cu Ka radiation. Tube voltage and current were set to 40 kV and 40 mA, respectively. 3. Results 3.1. Microstructure of Al/TA15 interface dipped without ultrasonication Fig. 3 shows the Al/TA15 interface characteristics at different holding times without ultrasonication. Fig. 3a presents the interface characteristic at a holding time of 5 min. The substrate showed limited wetting, featuring the existence of continuous cracks between the Al filler and substrate. The magnification of the Al/TA15 interface showed that some Al atoms penetrated locally into the substrate, forming erosion pits (see Fig. 3b). A gray and continuous layer formed at the Al/TA15 interface inside each erosion pits. Fig. 3c shows the elemental distribution across this layer. The pit region was filled with Al, and the gray layer at the Al/TA15 interface mainly consisted of Al and Ti. According to the variation of Al and Ti, the thickness of the gray layer was approximately 2.5 mm. The EDS results indicated that this layer was composed of 74.2% Al and 25.7% Ti (at.). Thus, we speculate that this layer is TiAl3, which is the IMC that commonly occurs in an Al-Ti reaction system. The TiAl3 formation clearly indicated that the wetting of Al to the TA15 substrate occurred in the pit regions. Fig. 3d shows the interface characteristics at a holding time of 10 min. Additional large erosion pits were observed at the wetting interface. Similarly, the wetting of Al to the TA15 substrate only occurred in the pit regions. The Al/TA15 interface outside of the pit regions remained un-wetted, and showed straight cracks. The Al filler in the pit regions was connected to the bulk Al filler, indicating that partial wetting was obtained under this condition. Fig. 3e shows the magnified image of the interface. The TiAl3 layer was continuous inside the erosion pits. The elemental distribution across the Al/TA15 interface is shown in Fig. 3f. The elemental distribution presented a trend similar to that in Fig. 3c, and the TiAl3 layer was thinner than 5 mm. Complete wetting was obtained when the holding time was increased to 20 min (Fig. 3g). The entire Al/TA15 interface was wavy and totally free of cracks. The TiAl3 layer was thin in most regions but thick in a small region of the wetting interface. Fig. 3h shows a magnified view of the thick TiAl3 layer. The substrate was severely eroded by the Al filler, and the TiAl3 layer was thicker than 20 mm. The elemental distribution across this TiAl3 layer is shown in Fig. 3i. An apparent IMC region was observed where the relative content of the elements remained basically invariable. The above observations demonstrated that even under static wetting condition, a prolonged holding time enables the liquid Al filler to fully wet the TA15 substrate. 3.2. Microstructure of the Al/TA15 interface with ultrasonication Fig. 4a shows the Al/TA15 interface subjected to an ultrasonication time of 10 s without holding. Complete wetting was observed on the flat interface, thereby indicating that the time required for the complete wetting of the substrate was considerably shortened with ultrasonication. A magnified view of the interface is
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 1. Schematic of the wetting and brazing experiments under ultrasonication.
Table 2 Parameters in wetting and brazing experiments. Experiments
Ultrasonication time (s)
Holding time (min)
Temperature ( C)
Wetting without ultrasonication
0 0 0
5 10 20
720 720 720
Wetting with ultrasonication
10 10 10
0 5 20
720 720 720
20
720
0 5 10 20 30
720 720 720 720 720
Brazing without ultrasonication Brazing with ultrasonication
10 10 10 10 10
Power (W)
Frequency (kHz)
1000 1000 1000
20 20 20
1000 1000 1000 1000 1000
20 20 20 20 20
5 mm was found along the wetting interface because TiAl3 had sufficient time to grow. The elemental distribution in Fig. 4f revealed an evident IMC layer with invariable relative content of elements. Fig. 4g shows the interface at an ultrasonication time of 10 s and holding time of 20 min. An even IMC layer thicker than 50 mm was formed, but this layer consisted of small TiAl3 particles in contrast to the layers observed in Fig. 4b and e. The elements in Fig. 4i presented an uneven distribution in the TiAl3 region. 3.3. IMCs at the Al/TA15 interface
Fig. 2. Geometry of shear strength testing sample (a) and schematic of the shear strength testing (b).
shown in Fig. 4b. A continuous TiAl3 layer with even thickness (<5 mm) was observed. Fig. 4c reveals the elemental variation across the wetting interface. A flat wetting interface was formed by ultrasonic vibrations on the substrate surface, which removed the oxide layer. Fig. 4d shows the interface characteristic at an ultrasonication time of 10 s and a holding time of 5 min. A flat interface similar to that in Fig. 4a was observed. A TiAl3 layer thicker than
For the identification of the intermetallic compounds formed at the wetting interface, the Al filler covering the substrate was removed by corrosion. The wetted samples with and without ultrasonication were immersed into 500 ml of dilute 20% HNO3 for approximately 60 min and then ultrasonically cleaned in acetone. SEM was used to observe the exposed IMCs. Fig. 5a reveals that blocky TiAl3 IMCs with irregular morphologies were formed. The EDS results in Fig. 5b also show that the IMC was most likely TiAl3. 3.4. Microstructure of TA15 joints brazed with and without ultrasonication We brazed TA15 alloys under different conditions on the basis of the results in Figs. 3 and 4. Fig. 6a shows the joint cross section at a holding time of 20 min without ultrasonication. The TA15 alloys
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 3. Interface characteristics at the holding time of 5 min (a), magnified view (b), and EDS results (c). Interface characteristics at the holding time of 10 min (d), magnified view (e), and EDS results (f). Interface characteristics at the holding time of 20 min (g), magnified view (h), and EDS results (i).
were completely wetted, similar to that observed in Fig. 3g. The entire joint was filled with three layers of particulate TiAl3. The two layers adjacent to the substrates were composed of small and scattered TiAl3 particles. The layer at the joint center contained large and decreasingly dense TiAl3 particles. Fig. 6b shows a magnified view of the joint center. An apparent boundary existed between the layer that contained large TiAl3 particles and the layer that contained small ones. The EDS results in Fig. 6d show that the framed region contained a high content of O (3.2 at.%), thus indicating that an oxide layer was entrapped between the layers of TiAl3. Fig. 6c shows the joint cross section at an ultrasonication time of 10 s without holding. The Al/TA15 interface was relatively flat, and several blocky IMCs formed at the joint center. The TiAl3 content in the joint was vol. 1.5%. The average diameter of the TiAl3 phases was 9.2 mm. Fig. 6d shows the interface at an ultrasonication time of 10 s. A TiAl3 layer with a thickness smaller than 5 mm was observed. Fig. 6e depicts the joint cross section morphology at an ultrasonication time of 10 s and holding time of 5 min. Remarkably, numerous blocky TiAl3 particles formed at the joint center when a
holding process was used. The volume fraction of TiAl3 reached 35.2% in the joint. The average diameter of the TiAl3 phases was 14.1 mm, which is larger than that shown in Fig. 6c. Such large TiAl3 phase was formed because of sufficient Al content at the joint center. Furthermore, the Al filler fully wetted the substrate, and the TiAl3 layer at the wetting interface was thicker than that presented in Fig. 3c. Fig. 6f presents a magnified view of the TiAl3 at the joint center. Small cracks were observed inside the large TiAl3 particles and may have resulted from the great mismatching of hardness and ductility between TiAl3 and Al during cooling. Fig. 6g shows the joint cross section at an ultrasonication time of 10 s and holding time of 10 min. In contrast to the layer in Fig. 6e, a layer containing tiny TiAl3 particles formed adjacent to the substrate interfaces and TiAl3 particles at the joint center decreased in diameter (average: 12.0 mm in Fig. 6h) with increased holding time. The volume fraction of TiAl3 in the joint increased, reaching 56.3%. Fig. 6i shows the cross section of the joint at an ultrasonication time of 10 s and holding time of 20 min. The layers containing tiny TiAl3 particles were significantly enlarged and only a thin layer of large blocky TiAl3 particles (in Fig. 6j) remained at the joint center. The content
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 4. Interface characteristics at an ultrasonication time of 10 s (a), magnified view (b), and EDS result (c). Interface characteristic characteristics at 10 s of ultrasonication þ 5 min of holding (d), magnified view (e), and EDS result (f). Interface characteristics at ultrasonication þ 20 min of holding (g), magnified view (h), and EDS results (i).
Fig. 5. Morphology of TiAl3 (a) and EDS results (b).
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 6. Microstructure of the joints. Joint cross section at a holding time of 20 min without ultrasonication (a) and microstructure at the joint center (b). Joint cross section at an ultrasonication time of 10 s (c) and TiAl3 layer at the filler/substrate interface (d). Joint cross section at an ultrasonication time of 10 s and a holding time of 5 min (e) and IMC at the joint center (f). Joint cross section at an ultrasonication time of 10 s and holding time of 10 min (g) and IMC at the joint center (h). Joint cross section at an ultrasonication time of 10 s and holding time of 20 min (i) and IMC at the joint center (j). Joint cross section at an ultrasonication time of 10 s and a holding time of 30 min (k) and IMC at the joint center (l).
of TiAl3 increased to 70.7% in this sample, and the average diameter of the large TiAl3 particles was approximately 11.7 mm. As the holding time was further increased to 30 min (Fig. 6k and l), the joint was filled with uniform and tiny TiAl3 particles, whose content reached 77.2%. The laminar structure of TiAl3 was absent. The TiAl3 particles in this samples were small (<11.5 mm) and exhibited a near spherical-shaped morphology without cracks (Fig. 6l). The results presented in Fig. 6 indicate that ultrasonication is effective in eliminating the oxide layer in the joint and holding time plays a major role on the formation of the TiAl3 phase. A TA15 composite joint reinforced with different volume fractions of TiAl3 particles can be successfully fabricated with ultrasonication and at different holding times. Fig. 7 presents the EBSD analysis of the microstructure inside joints brazed at different conditions. Fig. 7a shows the microstructure of the joint brazed at an ultrasonication time of 10 s and a holding time of 5 min. Only several TiAl3 grains without evident distribution characteristics were observed. The phase distribution inside this region is shown in Fig. 7b. TiAl3 phases, Al and Ti are indicated in yellow, red, and blue, respectively. The EBSD results revealed a composition of 73.8% Al, 2.7% Ti, and 23.5% TiAl3, as shown in Fig. 7b. Tiny Ti particles were eroded from the substrate due to intense cavitation during brazing. Increased amounts of small TiAl3 phases with sizes of approximately 5.8 mm were observed inside the joint brazed at an ultrasonication time of 10 s and holding time of 10 min (Fig. 7d). In this sample, the TiAl3 phases had uniform dimensions and distributions. The EBSD phase distributions in Fig. 7e show a composition of 55.6% Al, 1.4% Ti and
43.0% TiAl3. The phase ratio in Fig. 7 corresponds well with the results shown in Fig. 6. Fig. 7c and f demonstrate that the growth of the TiAl3 phases had no preferential orientation. Thus, the TiAl3 phases were homogeneously formed and distributed in the Al matrix during brazing. Such TiAl3 phases may serve as excellent reinforcements and produce a joint with improved and isotropic properties [31]. 3.5. Mechanical properties of the brazed joints Fig. 8 shows the nanoindentation hardness and the loaddisplacement curves of the tested points. The hardness and modulus of TA15 alloy were 7.428 and 192.737 GPa, respectively (Fig. 8a). The hardness and modulus of TiAl3 were considerably higher than those of the TA15 alloy (13.551 and 270.883 GPa, respectively). This result corresponds to the high hardness of TiAl3. Of all the tested points, pure Al, namely, the matrix of the joint seam, showed the lowest hardness of only 3.034 GPa. Fig. 8a indicates that pure Al has a higher modulus than TA15 alloy but a lower modulus than TiAl3 IMC. The load-displacement curves of the tested points are shown in Fig. 8b. The matrix of the joint (pure Al) exhibited the highest displacement among the tested points. Fig. 9 shows the shear strength of the brazed joints. The joint brazed with a holding time of 20 min without ultrasonication had a shear strength of 74.5 MPa. A short ultrasonication time of 10 s resulted in a shear strength of 66.4 MPa. The shear strength of the joint evidently increased at prolonged holding times. A high joint strength of 121.7 MPa was obtained at a holding time of 20 min
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 7. EBSD analysis of the joint cross section at an ultrasonication time of 10 s and a holding time of 5 min: (a) microstructure of the grain distribution, (b) distribution of phases, and (c) IPF map along the z direction. Joint cross section at an ultrasonication time of 10 s and holding time of 10 min: (d) microstructure of the grain distribution, (e) distribution of phases, and (f) IPF map along the z direction.
Fig. 8. Nanoindentation hardness (a) and load-displacement curves (b).
Fig. 9. Shear strength of the brazed joints.
with ultrasonication for 10 s. At 30 min holding time, the joint exhibited a maximum shear strength of 140.6 MPa, which was 75% higher than that of the pure Al filler. In the joints brazed with an ultrasonication time of 10 s and different holding times, a prolonged holding time resulted in smaller TiAl3 phases in the joint
centers (Fig. 6), leading to a high joint shear strength. Figs. 6 and 9 illustrate that reinforcing with small and dispersed IMC particles is an effective way to increase joint strength. The fracture of the ultrasonically brazed joint was investigated. The fracture positions of the joints are shown in Fig. 10. The fracture
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 10. Fracture positions at an ultrasonication time of 10 s (a), an ultrasonication time of 10 s and a holding time of 10 min (b), an ultrasonication time of 10 s and a holding time of 20 min (c), and an ultrasonication time of 10 s and a holding time of 30 min (d), A magnified view of the crack propagation path (e).
travelled across the center of the joint brazed at an ultrasonication time of 10 s (Fig. 10a). In this case, limited TiAl3 particles were involved in the joint, and the joint presented shear strength that is similar to that of pure Al. As the joint center was the last area to solidify, it may have the largest grains and may have been the softest area. Thus, the fracture tended to initiate and propagate along this area. A similar fracture morphology was observed in the joint brazed at an ultrasonication time of 10 s and holding times of 10 and 20 min (Fig. 10b and c). In these two cases, the laminar structures of TiAl3 were present in the joint. The joint center contained the softest matrix and the least content of TiAl3 reinforcing particles. It was inevitably the weakest zone in testing. The crack propagation did not reveal any preferential direction when the joint was reinforced with uniform TiAl3 particles (Fig. 10d). The scattered distribution of TiAl3 particles facilitated the improvement of joint strength. These observations partly respond to the results shown in Fig. 9. A magnified crack propagation path of the joint filled with TiAl3 particles is shown in Fig. 10e. The main crack that caused the failure of the joint propagated through the Al filler, and secondary cracks propagated through the IMC and the Al/IMC interfaces. Many tiny cracks were observed at the Al/IMC interfaces and inside the brittle IMC. Theoretically, an Al/IMC interface serves as the crack initiation point because this location tends to form stress concentration during loading due to the mismatch of deformation between Al and IMC [7]. Once crack occurs in the joint, it preferentially propagates through the Al filler, given that Al has far lower yield strength than the IMC. Fig. 11 shows the fracture morphology of the joints. The fracture morphology of the joint brazed at an ultrasonication time of 10 s is presented in Fig. 11a. Evident shear bands could be observed. The EDS results show that the fracture surface mainly consisted of pure Al, thereby implying that crack propagated through the Al filler. Similar shear bands were observed on the fracture morphology of the joint brazed at an ultrasonication time of 10 s and holding time of 10 min (Fig. 11b). Fig. 11c and d shows the fracture morphology of the joints brazed at ultrasonication time of 10 s and holding times of 20 and 30 min, respectively. Shear bands were absent on the
fracture surface of the two joints. Granular TiAl3 particles and small pits were observed on the fracture surfaces, and additional TiAl3 particles were observed on the fracture surface of the joint brazed at a holding time of 30 min (Fig. 11d). The results in Figs. 11 and 6 show that the joint ductility was evidently reduced when numerous TiAl3 phases were present in the joint. The XRD patterns determined from the fracture surfaces are depicted in Fig. 12. The patterns revealed that only one type of IMC, TiAl3, was formed in the joint. The peak of TiAl3 greatly increased with holding time. Al and TiAl3 were present in all joints, confirming that the cracks mostly propagated through the joints. 4. Discussion 4.1. Joint formation without ultrasonication Fig. 3 shows that the TA15 substrate readily interacted with pure Al filler at 720 C even without ultrasonication. Thuillard et al. [32] reported that Al atoms can diffuse through surface oxide when the oxide layer on a Ti substrate is thin. In the present work, the TA15 substrate was subjected to polishing and ultrasonic cleaning before it was dipped into the molten Al filler. The surface oxide of the substrate may be very thin. Moreover, cracks likely occurred in the oxide layer during heating due to the expansion coefficient mismatch between the oxide layer and substrate. Therefore, Al atoms diffused through the cracks to wet the TA15 substrate, reacting with Ti to form TiAl3 in situ. The newly formed TiAl3 phase could lift the oxide layer (Fig. 13a). This process is called undermining and has been reported in other wetting system [33]. As the undermining continued, Al filler could fully wet the substrate under the oxide layer. Furthermore, the oxide layer became “trapped” in the Al filler after the wetting process. When two specimens were laminated to form a joint, the “sandwich structure” of the Al filler formed in the joint. Thus, the joint was divided into three layers by the surface oxide of each substrate (Fig. 13b). During the holding process, Ti dissolved into the Al filler to form TiAl3. Ti atoms diffused through the cracks of oxide layers to joint
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 11. Fracture morphology of the joints at an ultrasonication time of 10 s and (a) without holding time, and with (b) holding time of 10 min, (c) 20 min, and (d) 30 min.
centers due to the excellent diffusion ability of Ti [34]. Consequently, in situ TiAl3 phases were observed at the joint center, as shown in Fig. 13b. The Al filler liquid in the joint was gradually depleted by its reaction with Ti to form a TiAl3 phase. Compared with the layers adjacent to the substrate, the Al filler at the joint center was consumed slowly. The Al filler was maintained in liquid for a long time, and TiAl3 phases could considerably grow, as illustrated in Fig. 13b. 4.2. Joint formation with ultrasonication
Fig. 12. XRD patterns of fracture surfaces brazed under different conditions.
Fig. 14 shows the formation and growth of the TiAl3 phase during ultrasonic brazing process. Cavitation occurred inside the liquid Al filler when ultrasonic vibration was applied. The collapse of cavitation bubbles produced acoustic streaming and microjets, which were fundamentally responsible for the removal of the oxide layer (Fig. 14a). After the removal of the oxide layer, fresh Ti and Al atoms came into contact with each other, thereby forming a thin in situ TiAl3 layer at the Al/TA15 interface (Fig. 14b). Mirjalili et al. [35] reported that the activation energy for the high-temperature bulk diffusion-controlled growth of TiAl3 was 296.2 kJ/mol. Such energy
Fig. 13. Newly formed TiAl3 IMC that lifts the oxide layer (a), and the completely lifted oxide layer (b).
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 14. Formation and growth of TiAl3 inside joints: removal of oxide layer via ultrasonic cavitation (a), formation of TiAl3 after ultrasoncation for 10 s (b), thickening of the TiAl3 layer with short holding time (c), and entire joint filled with TiAl3 at prolonged holding time (d).
was easily realized in the present work with the application of ultrasonication and high temperature. However, the diffusion of Ti into the Al filler was limited because of the short ultrasonication time. Therefore, rare TiAl3 phases (Fig. 14b) were formed at the joint center. During the holding process, Al atoms diffused through the TiAl3 layer into the TA15 substrate, and new TiAl3 phases formed at the TiAl3/TA15 interface [36]. At increased time, the TiAl3/TA15 interface became rough (Fig. 3h), and the TiAl3 layer thickened (Fig. 14c). Ti atoms also dissolved and diffused into the Al filler, and in situ TiAl3 phases formed at the joint center. Apparently, the formation of the TiAl3 phases in the joint was a diffusion-based process. However, the formation of TiAl3 at the wetting interface depended on the diffusion of Al into the TA15 substrate, and such formation inside the Al filler depended on the dissolution and diffusion of Ti from the substrate. The diffusion of Ti was more efficient in the latter case than in the former case as it occurred in the liquid, which increased the growth rate of TiAl3 in the Al filler. This condition was one of the reasons that the TiAl3 phases in the liquid filler were large. At the holding temperature, the Al filler retained its liquid form. The diffusion of Al into the substrate prevailed and the formation of TiAl3 phases at the Al/TA15 interfaces was dominant. As a result, the two TiAl3 layers adjacent to the substrate gradually enlarged and occupied the entire joint at the end (Fig. 14d). A brazed joint reinforced with numerous in situ TiAl3 phases was therefore obtained.
phase (DsLoad ), (2) the geometrical dislocations to accommodate mismatches in the coefficients of thermal expansion (CET) between pure Al matrix and TiAl3 phase (DsCTE ); (3) Orowan strengthening (DsOrowan ); and (4) Hall-Petch effect due to grain refinement (DsHP ). The reinforcing effect can be expressed as follows:
4.3. Reinforcing effect of the TiAl3 phase
where Vp is the volume fraction of the TiAl3 phase inside the joint, smo is the yield strength of pure Al, dp is the average diameter of TiAl3 phase, Gm is the shear modulus of pure Al, b is the Burgers vector of pure Al, Da is the difference in CET between pure Al and TiAl3 phase, DT is the difference between the processing and the
As previously discussed, TiAl3 phases provide reinforcing effects to brazed joints. Several factors can contribute to this effect, as discussed by Goh et al. [37]: (1) the load-bearing effects of the TiAl3
Ds ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 s2Load þ Ds2CTE þ Ds2Orowan þ ðDsHP Þ
(1)
DsLoad , DsCTE , DsOrowan , DsHP are respectively expressed as Eqs. (2)e(5) [38e41]:
DsLoad ¼ 0:5Vp smo pffiffiffi DsCTE ¼ 3bGm b
DsOrowan ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12Vp DaDT bdp
d 0:13Gm b 2 3 ln p 2b 1 6 1 3 7 7 dp 6 4 2Vp 15
DsHP ¼ ky d1=2 m
(2)
(3)
(4)
(5)
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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test temperatures, b and ky are constants, and dm is the grain size of pure Al. The value of Vp at an ultrasonication time of 10 s and holding time of 30 min was 77.2% (Fig. 6l). The yield strength of pure Al was approximately 30 MPa. Therefore, the calculated value of DsLoad was approximately 11.6 MPa. The value of b was assumed to be 1.25 [40]. The value of dp was approximately 11.5 mm, as shown in Fig. 6l. The shear modulus of pure Al is approximately 27 GPa DT was 700 K in this work. The respective CETs of pure Al and TiAl3 were 23 106/K and 19.5 106/K [42]. The Burgers vector of pure Al was assumed to be 0.1 nm in this work. Therefore, the calculated value of DsCTE was approximately 25.0 MPa. This result shows that the dislocation due to CTE mismatch between the pure Al and the TiAl3 phase has a remarkable effect in improving the strength of the joint. The calculated value of DsOrowan was approximately 3 MPa, which indicates that Orowan strengthening plays a relatively negligible role in improving joint strength. The shear strength of the joint brazed at 10 s ultrasonication time and 30 min holding time was 140.6 MPa, whereas the shear strength of pure Al was 80 MPa. Based on this theory, the value of DsHP was approximately 53.9 MPa. This result shows that DsHP and DsCTE have remarkable strengthening effects, similar to the findings of a previous study [42]. 5. Conclusions 1. Al atoms could diffuse through the crack in the surface oxide layer of the TA15 alloy to wet the substrate without ultrasonication. However, the diffusion of Al atoms is a slow process. The complete wetting was attained after a holding time of 20 min. The brazed joint was divided into three layers by the surface oxide of each substrate and showed shear strength of 74.5 MPa. 2. Cavitation in the liquid filler caused by ultrasonication can quickly remove the oxide layer on the TA15 substrate. A short ultrasonication time of 10 s realized the complete wetting of liquid pure Al to the TA15 substrate. 3. TiAl3 IMCs were developed at the Al/TA15 interface and inside the joint during ultrasonic brazing. The formation and distribution of the TiAl3 phase were mainly determined by the holding time. 4. An ultrasonication time of 10 s and holding time of 30 min formed a joint filled with homogeneously distributed TiAl3 phases, which exerted a reinforcing effect on the brazed joint. A maximum strength of 140.6 MPa, which was 75% higher than the strength of pure Al, was obtained under this condition. 5. Hall-Petch effect due to grain refinement (DsHP ) played the most important role on the reinforcing effect of TiAl3 phase, followed by the coefficients of thermal expansion (CET) between pure Al matrix and TiAl3 phase (DsCTE ). Declaration of competing interest No potential conflict of interest was reported by the authors. Acknowledgement This project was supported by the National Natural Science Foundation of China (No. 51574099, 51435004). References [1] M. Zhan, Q. Wang, D. Han, H. Yang, Geometric precision and microstructure evolution of TA15 alloy by hot shear spinning, T. Nonferr. Metal. Soc. 23 (6) (2013) 1617e1627.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al Zhiwu Xu*, Zhengwei Li**, Ben Chai, Jiuchun Yan State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2019 Received in revised form 26 September 2019 Accepted 28 September 2019 Available online xxx
In this work, TA15 joints reinforced in situ with TiAl3 IMCs were successfully fabricated using pure Al filler by ultrasonic brazing. TA15 alloys were first wetted by pure Al with and without ultrasonication and then ultrasonically brazed. The wetting of Al to the TA15 alloy was realized by the formation of a TiAl3 phase at the contacting interface. The complete wetting of the Al filler to the TA15 substrate was achieved at 20 min without ultrasonication. Ultrasonication significantly accelerated the wetting process, and the time required to achieve complete wetting was shortened to 10 s. During ultrasonic brazing, the formation of TiAl3 in the joint mainly depended on the holding time. The content of the TiAl3 phase increased from 35.2% to 77.2% as the holding time was increased from 5 min to 30 min. Joint strength increased with the amount of TiAl3 in the joint. Ultrasonication for 10 s and a holding time of 30 min generated a joint filled with homogeneously distributed TiAl3. The maximum shear strength of this joint was 140.6 MPa, which was 75% higher than that of pure Al. Failure crack propagated through the filler at short holding times and through the filler and TiAl3 phase at prolonged holding times. © 2019 Elsevier B.V. All rights reserved.
Keywords: TA15 alloy TiAl3 Ultrasonic brazing Cavitation Wetting
1. Introduction TA15 alloy is a type of advanced material that provides the properties of high specific strength, excellent corrosion resistance, high temperature performance as well as light weight compared with steels and Ni-based alloys, thus serving as an important structural material in the aerospace, automobile, defense and chemical industries [1]. During the manufacturing of titanium components with complex geometry, joining is a necessary technique and becomes increasingly important for titanium alloys. Nowadays, a variety of joining methods such as fusion welding (e.g. gas tungsten arc welding and laser welding) [2,3], solid state bonding (e.g. diffusion bonding [4] and friction stir welding [5]) and brazing [6e10] have been developed to join titanium alloys. Of these techniques, brazing can provide unique advantages in joining dissimilar materials and many contact points over a large area without severe distortion, as well as in minimizing the thermal impact on the substrates [9]. In the process of brazing titanium alloys, Zrebased, Tiebased [11,12], Cuebased [13], Agebased
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Xu), [email protected] (Z. Li).
[14,15] and Alebased alloys [16] can be used as the filler metals. The application of Al-based filler metal makes it possible to braze titanium alloys at lower temperature and braze Ti/Al dissimilar materials to combine the advantages of titanium and aluminum alloys, which has been attracting more and more interest of the researches [10,16]. As easily estimated from the binary phase diagram, almost all the filler metals now available form intermetallic compounds (IMCs) with titanium in the brazing of titanium alloys [8]. The IMCs have the characteristics of low ductility, high hardness and brittleness, making them easy to be the crack sources during loading. It is found that the category, morphology and distribution of the IMCs have significant influence on the joint quality [7,17e19]. Therefore, various attempts for improving the joint quality by controlling the formation of the IMCs have been proposed. Takemoto et al. [8] investigated the effect of Si content in the Al-based filler metal on the formation of IMCs during brazing of Ti/Ti joint and found that a small addition of Si up to 0.8% in the Al filler metal remarkably reduced the growth rate of TiAl3. Chen et al. [6] reported that TiAl3 IMCs were formed at the joint interface in the case of low-heat input and TiAl3, TiAl, Ti5Si3, and Ti3Al IMCs were observed at high heat input during the laser weldingebrazing of Ti/Al dissimilar alloys using AlSi12 flux-cored wire filler. They also reported that the average tensile strength of the joints with lamella-shaped, cellular-
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Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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shaped and clubeshaped morphologies of IMCs was significantly higher than that of the joints with thick and continuous IMCs layer [7]. Ma et al. [16] observed that the joint containing the lamellarelike Ti7Al5Si12 phase showed higher strength than that containing the blockelike TiAl3 phase when brazed the TC4 alloy to Ale4Cue1Mg alloy using Zn-based filler metal. Recently, ultrasonic brazing has been proposed as a promising method to join dissimilar materials (e.g. Al/Ti [16,20], Al/Mg [17,21] and Mg/Ti [22]) and easy-to-be-oxidized materials (e.g. Mg [23e25], Al [26,27] and Ti [28] alloys). This method does not require the use of flux or vacuum environment. It can remove the surface oxides of the substrate and realize the wetting of liquid filler to the substrate in very short time (normally in seconds) by cavitation effect [29,30]. Moreover, ultrasonic generates intensive stirring in the liquid filler, which can remarkably accelerate the mass transfer at the liquid/solid interface as well as in the joint. As a result, the morphology and distribution of IMCs may be modified. With these in mind, we attempted to braze the TA15 alloy using pure Al filler with and without ultrasonication in the present work. The formation of IMCs in the joint was focused on, with the aim to obtain TA15 joint reinforced in situ by dispersed TiAl3 IMCs with the assistance of ultrasonic. The effect of the IMCs on the joint strength under various brazing conditions was also evaluated. 2. Experimental TA15 alloy with dimensions of 15 mm 10 mm 3 mm was used as the substrate in this work. The substrates were polished using #500 emery paper and ultrasonically cleaned in acetone for 10 min. Pure Al was used as the filler metal. The chemical compositions of TA15 alloy are listed in Table 1. The entire experiment was divided into three parts. The first part involved wetting without ultrasonication for the evaluation of the wettability of pure Al to the TA15 substrate. Pure Al was first placed in a self-designed mold and heated to approximately 720 C. The substrate was then immersed into molten Al at holding times of 5, 10, and 20 min. The second part involved ultrasonic-assisted wetting, as illustrated in Fig. 1. This experiment was conducted for the evaluation of the wettability of pure Al to the TA15 substrate with ultrasonication. An ultrasonic vibration system (UPM-U-P1010A01) was used. A frequency of 20 kHz and output power of 1000 W were adopted. The substrate was immersed into molten Al, and ultrasonic vibration was switched on. Ultrasonic vibration time was set to 10 s. Holding times of 5 and 20 min were adopted after ultrasonication. The third part involved brazing, as presented in Fig. 1. Joints were brazed using a specific fixture. This procedure was performed with a 20 min holding time without ultrasonication, followed by 10 s of ultrasoncation and holding times of 5, 10, 15, and 20 min. The parameters used in wetting and brazing experiments are summarized in Table 2. After the wetting and brazing experiments, metallographic samples were cut using an electrical discharge cutting machine, and their microstructures were observed with a scanning electron microscope (SEM, FEI-Quanta 200) equipped with an energy dispersive X-ray spectroscopy (EDS) system and Zeiss SEM with electron back-scattered diffraction (EBSD) detector. IMC content in the joint was calculated by an image analysis software (Image-Pro Plus 6.0), and the size of IMC was examined by a Nano Measurer
Table 1 Chemical composition of TA15 alloy. Element
Al
Mo
V
Zr
N
H
O
Ti
Content
6.64
1.70
2.16
2.09
0.10
0.004
0.01
Bal
(Type 1.2). A nanoindentation instrument (G200) was used to measure the joint’s hardness. The testing distance was 20 mm, testing force was 0.01 N, and dwell time was 10 s. Joint shear strength was tested on an electromechanical material testing machine (Instron-5569) at a speed of 1 mm/min. The geometry of shear strength testing sample and the schematic of the shear strength testing are shown in Fig. 2. The fracture position and surface were observed via SEM. The phases on the fracture surfaces were determined on a D/max-RB X-ray diffraction (XRD) system with Cu Ka radiation. Tube voltage and current were set to 40 kV and 40 mA, respectively. 3. Results 3.1. Microstructure of Al/TA15 interface dipped without ultrasonication Fig. 3 shows the Al/TA15 interface characteristics at different holding times without ultrasonication. Fig. 3a presents the interface characteristic at a holding time of 5 min. The substrate showed limited wetting, featuring the existence of continuous cracks between the Al filler and substrate. The magnification of the Al/TA15 interface showed that some Al atoms penetrated locally into the substrate, forming erosion pits (see Fig. 3b). A gray and continuous layer formed at the Al/TA15 interface inside each erosion pits. Fig. 3c shows the elemental distribution across this layer. The pit region was filled with Al, and the gray layer at the Al/TA15 interface mainly consisted of Al and Ti. According to the variation of Al and Ti, the thickness of the gray layer was approximately 2.5 mm. The EDS results indicated that this layer was composed of 74.2% Al and 25.7% Ti (at.). Thus, we speculate that this layer is TiAl3, which is the IMC that commonly occurs in an Al-Ti reaction system. The TiAl3 formation clearly indicated that the wetting of Al to the TA15 substrate occurred in the pit regions. Fig. 3d shows the interface characteristics at a holding time of 10 min. Additional large erosion pits were observed at the wetting interface. Similarly, the wetting of Al to the TA15 substrate only occurred in the pit regions. The Al/TA15 interface outside of the pit regions remained un-wetted, and showed straight cracks. The Al filler in the pit regions was connected to the bulk Al filler, indicating that partial wetting was obtained under this condition. Fig. 3e shows the magnified image of the interface. The TiAl3 layer was continuous inside the erosion pits. The elemental distribution across the Al/TA15 interface is shown in Fig. 3f. The elemental distribution presented a trend similar to that in Fig. 3c, and the TiAl3 layer was thinner than 5 mm. Complete wetting was obtained when the holding time was increased to 20 min (Fig. 3g). The entire Al/TA15 interface was wavy and totally free of cracks. The TiAl3 layer was thin in most regions but thick in a small region of the wetting interface. Fig. 3h shows a magnified view of the thick TiAl3 layer. The substrate was severely eroded by the Al filler, and the TiAl3 layer was thicker than 20 mm. The elemental distribution across this TiAl3 layer is shown in Fig. 3i. An apparent IMC region was observed where the relative content of the elements remained basically invariable. The above observations demonstrated that even under static wetting condition, a prolonged holding time enables the liquid Al filler to fully wet the TA15 substrate. 3.2. Microstructure of the Al/TA15 interface with ultrasonication Fig. 4a shows the Al/TA15 interface subjected to an ultrasonication time of 10 s without holding. Complete wetting was observed on the flat interface, thereby indicating that the time required for the complete wetting of the substrate was considerably shortened with ultrasonication. A magnified view of the interface is
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 1. Schematic of the wetting and brazing experiments under ultrasonication.
Table 2 Parameters in wetting and brazing experiments. Experiments
Ultrasonication time (s)
Holding time (min)
Temperature ( C)
Wetting without ultrasonication
0 0 0
5 10 20
720 720 720
Wetting with ultrasonication
10 10 10
0 5 20
720 720 720
20
720
0 5 10 20 30
720 720 720 720 720
Brazing without ultrasonication Brazing with ultrasonication
10 10 10 10 10
Power (W)
Frequency (kHz)
1000 1000 1000
20 20 20
1000 1000 1000 1000 1000
20 20 20 20 20
5 mm was found along the wetting interface because TiAl3 had sufficient time to grow. The elemental distribution in Fig. 4f revealed an evident IMC layer with invariable relative content of elements. Fig. 4g shows the interface at an ultrasonication time of 10 s and holding time of 20 min. An even IMC layer thicker than 50 mm was formed, but this layer consisted of small TiAl3 particles in contrast to the layers observed in Fig. 4b and e. The elements in Fig. 4i presented an uneven distribution in the TiAl3 region. 3.3. IMCs at the Al/TA15 interface
Fig. 2. Geometry of shear strength testing sample (a) and schematic of the shear strength testing (b).
shown in Fig. 4b. A continuous TiAl3 layer with even thickness (<5 mm) was observed. Fig. 4c reveals the elemental variation across the wetting interface. A flat wetting interface was formed by ultrasonic vibrations on the substrate surface, which removed the oxide layer. Fig. 4d shows the interface characteristic at an ultrasonication time of 10 s and a holding time of 5 min. A flat interface similar to that in Fig. 4a was observed. A TiAl3 layer thicker than
For the identification of the intermetallic compounds formed at the wetting interface, the Al filler covering the substrate was removed by corrosion. The wetted samples with and without ultrasonication were immersed into 500 ml of dilute 20% HNO3 for approximately 60 min and then ultrasonically cleaned in acetone. SEM was used to observe the exposed IMCs. Fig. 5a reveals that blocky TiAl3 IMCs with irregular morphologies were formed. The EDS results in Fig. 5b also show that the IMC was most likely TiAl3. 3.4. Microstructure of TA15 joints brazed with and without ultrasonication We brazed TA15 alloys under different conditions on the basis of the results in Figs. 3 and 4. Fig. 6a shows the joint cross section at a holding time of 20 min without ultrasonication. The TA15 alloys
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 3. Interface characteristics at the holding time of 5 min (a), magnified view (b), and EDS results (c). Interface characteristics at the holding time of 10 min (d), magnified view (e), and EDS results (f). Interface characteristics at the holding time of 20 min (g), magnified view (h), and EDS results (i).
were completely wetted, similar to that observed in Fig. 3g. The entire joint was filled with three layers of particulate TiAl3. The two layers adjacent to the substrates were composed of small and scattered TiAl3 particles. The layer at the joint center contained large and decreasingly dense TiAl3 particles. Fig. 6b shows a magnified view of the joint center. An apparent boundary existed between the layer that contained large TiAl3 particles and the layer that contained small ones. The EDS results in Fig. 6d show that the framed region contained a high content of O (3.2 at.%), thus indicating that an oxide layer was entrapped between the layers of TiAl3. Fig. 6c shows the joint cross section at an ultrasonication time of 10 s without holding. The Al/TA15 interface was relatively flat, and several blocky IMCs formed at the joint center. The TiAl3 content in the joint was vol. 1.5%. The average diameter of the TiAl3 phases was 9.2 mm. Fig. 6d shows the interface at an ultrasonication time of 10 s. A TiAl3 layer with a thickness smaller than 5 mm was observed. Fig. 6e depicts the joint cross section morphology at an ultrasonication time of 10 s and holding time of 5 min. Remarkably, numerous blocky TiAl3 particles formed at the joint center when a
holding process was used. The volume fraction of TiAl3 reached 35.2% in the joint. The average diameter of the TiAl3 phases was 14.1 mm, which is larger than that shown in Fig. 6c. Such large TiAl3 phase was formed because of sufficient Al content at the joint center. Furthermore, the Al filler fully wetted the substrate, and the TiAl3 layer at the wetting interface was thicker than that presented in Fig. 3c. Fig. 6f presents a magnified view of the TiAl3 at the joint center. Small cracks were observed inside the large TiAl3 particles and may have resulted from the great mismatching of hardness and ductility between TiAl3 and Al during cooling. Fig. 6g shows the joint cross section at an ultrasonication time of 10 s and holding time of 10 min. In contrast to the layer in Fig. 6e, a layer containing tiny TiAl3 particles formed adjacent to the substrate interfaces and TiAl3 particles at the joint center decreased in diameter (average: 12.0 mm in Fig. 6h) with increased holding time. The volume fraction of TiAl3 in the joint increased, reaching 56.3%. Fig. 6i shows the cross section of the joint at an ultrasonication time of 10 s and holding time of 20 min. The layers containing tiny TiAl3 particles were significantly enlarged and only a thin layer of large blocky TiAl3 particles (in Fig. 6j) remained at the joint center. The content
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 4. Interface characteristics at an ultrasonication time of 10 s (a), magnified view (b), and EDS result (c). Interface characteristic characteristics at 10 s of ultrasonication þ 5 min of holding (d), magnified view (e), and EDS result (f). Interface characteristics at ultrasonication þ 20 min of holding (g), magnified view (h), and EDS results (i).
Fig. 5. Morphology of TiAl3 (a) and EDS results (b).
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 6. Microstructure of the joints. Joint cross section at a holding time of 20 min without ultrasonication (a) and microstructure at the joint center (b). Joint cross section at an ultrasonication time of 10 s (c) and TiAl3 layer at the filler/substrate interface (d). Joint cross section at an ultrasonication time of 10 s and a holding time of 5 min (e) and IMC at the joint center (f). Joint cross section at an ultrasonication time of 10 s and holding time of 10 min (g) and IMC at the joint center (h). Joint cross section at an ultrasonication time of 10 s and holding time of 20 min (i) and IMC at the joint center (j). Joint cross section at an ultrasonication time of 10 s and a holding time of 30 min (k) and IMC at the joint center (l).
of TiAl3 increased to 70.7% in this sample, and the average diameter of the large TiAl3 particles was approximately 11.7 mm. As the holding time was further increased to 30 min (Fig. 6k and l), the joint was filled with uniform and tiny TiAl3 particles, whose content reached 77.2%. The laminar structure of TiAl3 was absent. The TiAl3 particles in this samples were small (<11.5 mm) and exhibited a near spherical-shaped morphology without cracks (Fig. 6l). The results presented in Fig. 6 indicate that ultrasonication is effective in eliminating the oxide layer in the joint and holding time plays a major role on the formation of the TiAl3 phase. A TA15 composite joint reinforced with different volume fractions of TiAl3 particles can be successfully fabricated with ultrasonication and at different holding times. Fig. 7 presents the EBSD analysis of the microstructure inside joints brazed at different conditions. Fig. 7a shows the microstructure of the joint brazed at an ultrasonication time of 10 s and a holding time of 5 min. Only several TiAl3 grains without evident distribution characteristics were observed. The phase distribution inside this region is shown in Fig. 7b. TiAl3 phases, Al and Ti are indicated in yellow, red, and blue, respectively. The EBSD results revealed a composition of 73.8% Al, 2.7% Ti, and 23.5% TiAl3, as shown in Fig. 7b. Tiny Ti particles were eroded from the substrate due to intense cavitation during brazing. Increased amounts of small TiAl3 phases with sizes of approximately 5.8 mm were observed inside the joint brazed at an ultrasonication time of 10 s and holding time of 10 min (Fig. 7d). In this sample, the TiAl3 phases had uniform dimensions and distributions. The EBSD phase distributions in Fig. 7e show a composition of 55.6% Al, 1.4% Ti and
43.0% TiAl3. The phase ratio in Fig. 7 corresponds well with the results shown in Fig. 6. Fig. 7c and f demonstrate that the growth of the TiAl3 phases had no preferential orientation. Thus, the TiAl3 phases were homogeneously formed and distributed in the Al matrix during brazing. Such TiAl3 phases may serve as excellent reinforcements and produce a joint with improved and isotropic properties [31]. 3.5. Mechanical properties of the brazed joints Fig. 8 shows the nanoindentation hardness and the loaddisplacement curves of the tested points. The hardness and modulus of TA15 alloy were 7.428 and 192.737 GPa, respectively (Fig. 8a). The hardness and modulus of TiAl3 were considerably higher than those of the TA15 alloy (13.551 and 270.883 GPa, respectively). This result corresponds to the high hardness of TiAl3. Of all the tested points, pure Al, namely, the matrix of the joint seam, showed the lowest hardness of only 3.034 GPa. Fig. 8a indicates that pure Al has a higher modulus than TA15 alloy but a lower modulus than TiAl3 IMC. The load-displacement curves of the tested points are shown in Fig. 8b. The matrix of the joint (pure Al) exhibited the highest displacement among the tested points. Fig. 9 shows the shear strength of the brazed joints. The joint brazed with a holding time of 20 min without ultrasonication had a shear strength of 74.5 MPa. A short ultrasonication time of 10 s resulted in a shear strength of 66.4 MPa. The shear strength of the joint evidently increased at prolonged holding times. A high joint strength of 121.7 MPa was obtained at a holding time of 20 min
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 7. EBSD analysis of the joint cross section at an ultrasonication time of 10 s and a holding time of 5 min: (a) microstructure of the grain distribution, (b) distribution of phases, and (c) IPF map along the z direction. Joint cross section at an ultrasonication time of 10 s and holding time of 10 min: (d) microstructure of the grain distribution, (e) distribution of phases, and (f) IPF map along the z direction.
Fig. 8. Nanoindentation hardness (a) and load-displacement curves (b).
Fig. 9. Shear strength of the brazed joints.
with ultrasonication for 10 s. At 30 min holding time, the joint exhibited a maximum shear strength of 140.6 MPa, which was 75% higher than that of the pure Al filler. In the joints brazed with an ultrasonication time of 10 s and different holding times, a prolonged holding time resulted in smaller TiAl3 phases in the joint
centers (Fig. 6), leading to a high joint shear strength. Figs. 6 and 9 illustrate that reinforcing with small and dispersed IMC particles is an effective way to increase joint strength. The fracture of the ultrasonically brazed joint was investigated. The fracture positions of the joints are shown in Fig. 10. The fracture
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 10. Fracture positions at an ultrasonication time of 10 s (a), an ultrasonication time of 10 s and a holding time of 10 min (b), an ultrasonication time of 10 s and a holding time of 20 min (c), and an ultrasonication time of 10 s and a holding time of 30 min (d), A magnified view of the crack propagation path (e).
travelled across the center of the joint brazed at an ultrasonication time of 10 s (Fig. 10a). In this case, limited TiAl3 particles were involved in the joint, and the joint presented shear strength that is similar to that of pure Al. As the joint center was the last area to solidify, it may have the largest grains and may have been the softest area. Thus, the fracture tended to initiate and propagate along this area. A similar fracture morphology was observed in the joint brazed at an ultrasonication time of 10 s and holding times of 10 and 20 min (Fig. 10b and c). In these two cases, the laminar structures of TiAl3 were present in the joint. The joint center contained the softest matrix and the least content of TiAl3 reinforcing particles. It was inevitably the weakest zone in testing. The crack propagation did not reveal any preferential direction when the joint was reinforced with uniform TiAl3 particles (Fig. 10d). The scattered distribution of TiAl3 particles facilitated the improvement of joint strength. These observations partly respond to the results shown in Fig. 9. A magnified crack propagation path of the joint filled with TiAl3 particles is shown in Fig. 10e. The main crack that caused the failure of the joint propagated through the Al filler, and secondary cracks propagated through the IMC and the Al/IMC interfaces. Many tiny cracks were observed at the Al/IMC interfaces and inside the brittle IMC. Theoretically, an Al/IMC interface serves as the crack initiation point because this location tends to form stress concentration during loading due to the mismatch of deformation between Al and IMC [7]. Once crack occurs in the joint, it preferentially propagates through the Al filler, given that Al has far lower yield strength than the IMC. Fig. 11 shows the fracture morphology of the joints. The fracture morphology of the joint brazed at an ultrasonication time of 10 s is presented in Fig. 11a. Evident shear bands could be observed. The EDS results show that the fracture surface mainly consisted of pure Al, thereby implying that crack propagated through the Al filler. Similar shear bands were observed on the fracture morphology of the joint brazed at an ultrasonication time of 10 s and holding time of 10 min (Fig. 11b). Fig. 11c and d shows the fracture morphology of the joints brazed at ultrasonication time of 10 s and holding times of 20 and 30 min, respectively. Shear bands were absent on the
fracture surface of the two joints. Granular TiAl3 particles and small pits were observed on the fracture surfaces, and additional TiAl3 particles were observed on the fracture surface of the joint brazed at a holding time of 30 min (Fig. 11d). The results in Figs. 11 and 6 show that the joint ductility was evidently reduced when numerous TiAl3 phases were present in the joint. The XRD patterns determined from the fracture surfaces are depicted in Fig. 12. The patterns revealed that only one type of IMC, TiAl3, was formed in the joint. The peak of TiAl3 greatly increased with holding time. Al and TiAl3 were present in all joints, confirming that the cracks mostly propagated through the joints. 4. Discussion 4.1. Joint formation without ultrasonication Fig. 3 shows that the TA15 substrate readily interacted with pure Al filler at 720 C even without ultrasonication. Thuillard et al. [32] reported that Al atoms can diffuse through surface oxide when the oxide layer on a Ti substrate is thin. In the present work, the TA15 substrate was subjected to polishing and ultrasonic cleaning before it was dipped into the molten Al filler. The surface oxide of the substrate may be very thin. Moreover, cracks likely occurred in the oxide layer during heating due to the expansion coefficient mismatch between the oxide layer and substrate. Therefore, Al atoms diffused through the cracks to wet the TA15 substrate, reacting with Ti to form TiAl3 in situ. The newly formed TiAl3 phase could lift the oxide layer (Fig. 13a). This process is called undermining and has been reported in other wetting system [33]. As the undermining continued, Al filler could fully wet the substrate under the oxide layer. Furthermore, the oxide layer became “trapped” in the Al filler after the wetting process. When two specimens were laminated to form a joint, the “sandwich structure” of the Al filler formed in the joint. Thus, the joint was divided into three layers by the surface oxide of each substrate (Fig. 13b). During the holding process, Ti dissolved into the Al filler to form TiAl3. Ti atoms diffused through the cracks of oxide layers to joint
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 11. Fracture morphology of the joints at an ultrasonication time of 10 s and (a) without holding time, and with (b) holding time of 10 min, (c) 20 min, and (d) 30 min.
centers due to the excellent diffusion ability of Ti [34]. Consequently, in situ TiAl3 phases were observed at the joint center, as shown in Fig. 13b. The Al filler liquid in the joint was gradually depleted by its reaction with Ti to form a TiAl3 phase. Compared with the layers adjacent to the substrate, the Al filler at the joint center was consumed slowly. The Al filler was maintained in liquid for a long time, and TiAl3 phases could considerably grow, as illustrated in Fig. 13b. 4.2. Joint formation with ultrasonication
Fig. 12. XRD patterns of fracture surfaces brazed under different conditions.
Fig. 14 shows the formation and growth of the TiAl3 phase during ultrasonic brazing process. Cavitation occurred inside the liquid Al filler when ultrasonic vibration was applied. The collapse of cavitation bubbles produced acoustic streaming and microjets, which were fundamentally responsible for the removal of the oxide layer (Fig. 14a). After the removal of the oxide layer, fresh Ti and Al atoms came into contact with each other, thereby forming a thin in situ TiAl3 layer at the Al/TA15 interface (Fig. 14b). Mirjalili et al. [35] reported that the activation energy for the high-temperature bulk diffusion-controlled growth of TiAl3 was 296.2 kJ/mol. Such energy
Fig. 13. Newly formed TiAl3 IMC that lifts the oxide layer (a), and the completely lifted oxide layer (b).
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Fig. 14. Formation and growth of TiAl3 inside joints: removal of oxide layer via ultrasonic cavitation (a), formation of TiAl3 after ultrasoncation for 10 s (b), thickening of the TiAl3 layer with short holding time (c), and entire joint filled with TiAl3 at prolonged holding time (d).
was easily realized in the present work with the application of ultrasonication and high temperature. However, the diffusion of Ti into the Al filler was limited because of the short ultrasonication time. Therefore, rare TiAl3 phases (Fig. 14b) were formed at the joint center. During the holding process, Al atoms diffused through the TiAl3 layer into the TA15 substrate, and new TiAl3 phases formed at the TiAl3/TA15 interface [36]. At increased time, the TiAl3/TA15 interface became rough (Fig. 3h), and the TiAl3 layer thickened (Fig. 14c). Ti atoms also dissolved and diffused into the Al filler, and in situ TiAl3 phases formed at the joint center. Apparently, the formation of the TiAl3 phases in the joint was a diffusion-based process. However, the formation of TiAl3 at the wetting interface depended on the diffusion of Al into the TA15 substrate, and such formation inside the Al filler depended on the dissolution and diffusion of Ti from the substrate. The diffusion of Ti was more efficient in the latter case than in the former case as it occurred in the liquid, which increased the growth rate of TiAl3 in the Al filler. This condition was one of the reasons that the TiAl3 phases in the liquid filler were large. At the holding temperature, the Al filler retained its liquid form. The diffusion of Al into the substrate prevailed and the formation of TiAl3 phases at the Al/TA15 interfaces was dominant. As a result, the two TiAl3 layers adjacent to the substrate gradually enlarged and occupied the entire joint at the end (Fig. 14d). A brazed joint reinforced with numerous in situ TiAl3 phases was therefore obtained.
phase (DsLoad ), (2) the geometrical dislocations to accommodate mismatches in the coefficients of thermal expansion (CET) between pure Al matrix and TiAl3 phase (DsCTE ); (3) Orowan strengthening (DsOrowan ); and (4) Hall-Petch effect due to grain refinement (DsHP ). The reinforcing effect can be expressed as follows:
4.3. Reinforcing effect of the TiAl3 phase
where Vp is the volume fraction of the TiAl3 phase inside the joint, smo is the yield strength of pure Al, dp is the average diameter of TiAl3 phase, Gm is the shear modulus of pure Al, b is the Burgers vector of pure Al, Da is the difference in CET between pure Al and TiAl3 phase, DT is the difference between the processing and the
As previously discussed, TiAl3 phases provide reinforcing effects to brazed joints. Several factors can contribute to this effect, as discussed by Goh et al. [37]: (1) the load-bearing effects of the TiAl3
Ds ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 s2Load þ Ds2CTE þ Ds2Orowan þ ðDsHP Þ
(1)
DsLoad , DsCTE , DsOrowan , DsHP are respectively expressed as Eqs. (2)e(5) [38e41]:
DsLoad ¼ 0:5Vp smo pffiffiffi DsCTE ¼ 3bGm b
DsOrowan ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12Vp DaDT bdp
d 0:13Gm b 2 3 ln p 2b 1 6 1 3 7 7 dp 6 4 2Vp 15
DsHP ¼ ky d1=2 m
(2)
(3)
(4)
(5)
Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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test temperatures, b and ky are constants, and dm is the grain size of pure Al. The value of Vp at an ultrasonication time of 10 s and holding time of 30 min was 77.2% (Fig. 6l). The yield strength of pure Al was approximately 30 MPa. Therefore, the calculated value of DsLoad was approximately 11.6 MPa. The value of b was assumed to be 1.25 [40]. The value of dp was approximately 11.5 mm, as shown in Fig. 6l. The shear modulus of pure Al is approximately 27 GPa DT was 700 K in this work. The respective CETs of pure Al and TiAl3 were 23 106/K and 19.5 106/K [42]. The Burgers vector of pure Al was assumed to be 0.1 nm in this work. Therefore, the calculated value of DsCTE was approximately 25.0 MPa. This result shows that the dislocation due to CTE mismatch between the pure Al and the TiAl3 phase has a remarkable effect in improving the strength of the joint. The calculated value of DsOrowan was approximately 3 MPa, which indicates that Orowan strengthening plays a relatively negligible role in improving joint strength. The shear strength of the joint brazed at 10 s ultrasonication time and 30 min holding time was 140.6 MPa, whereas the shear strength of pure Al was 80 MPa. Based on this theory, the value of DsHP was approximately 53.9 MPa. This result shows that DsHP and DsCTE have remarkable strengthening effects, similar to the findings of a previous study [42]. 5. Conclusions 1. Al atoms could diffuse through the crack in the surface oxide layer of the TA15 alloy to wet the substrate without ultrasonication. However, the diffusion of Al atoms is a slow process. The complete wetting was attained after a holding time of 20 min. The brazed joint was divided into three layers by the surface oxide of each substrate and showed shear strength of 74.5 MPa. 2. Cavitation in the liquid filler caused by ultrasonication can quickly remove the oxide layer on the TA15 substrate. A short ultrasonication time of 10 s realized the complete wetting of liquid pure Al to the TA15 substrate. 3. TiAl3 IMCs were developed at the Al/TA15 interface and inside the joint during ultrasonic brazing. The formation and distribution of the TiAl3 phase were mainly determined by the holding time. 4. An ultrasonication time of 10 s and holding time of 30 min formed a joint filled with homogeneously distributed TiAl3 phases, which exerted a reinforcing effect on the brazed joint. A maximum strength of 140.6 MPa, which was 75% higher than the strength of pure Al, was obtained under this condition. 5. Hall-Petch effect due to grain refinement (DsHP ) played the most important role on the reinforcing effect of TiAl3 phase, followed by the coefficients of thermal expansion (CET) between pure Al matrix and TiAl3 phase (DsCTE ). Declaration of competing interest No potential conflict of interest was reported by the authors. Acknowledgement This project was supported by the National Natural Science Foundation of China (No. 51574099, 51435004). References [1] M. Zhan, Q. Wang, D. Han, H. Yang, Geometric precision and microstructure evolution of TA15 alloy by hot shear spinning, T. Nonferr. Metal. Soc. 23 (6) (2013) 1617e1627.
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Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493
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Please cite this article as: Z. Xu et al., Formation of TiAl3 and its reinforcing effect in TA15 alloy joint ultrasonically brazed with pure Al, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152493