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Microstructure and joint properties of ultrasonically brazed Al alloy joints using a Zn–Al hypereutectic filler metal
Materials and Design 47 (2013) 717–724
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and joint properties of ultrasonically brazed Al alloy joints using a Zn–Al hypereutectic filler metal Yong Xiao a, Hongjun Ji a, Mingyu Li a,⇑, Jongmyung Kim b, Hongbae Kim b a b
Shenzhen Key Laboratory of Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China Jeonnam Provincial College, Jeonnam 517802, South Korea
a r t i c l e
i n f o
Article history: Received 21 October 2012 Accepted 2 January 2013 Available online 16 January 2013 Keywords: Ultrasound-assisted brazing Aluminum alloy Hypereutectic filler meal Bonding ratio Microstructure
a b s t r a c t The ultrasound-assisted brazing of 1060 Al alloy using a Zn–14Al hypereutectic filler metal was investigated at different temperatures. The effects of brazing temperature on the bonding ratio, shear strength and microstructure of the joints were studied. Cavities and discontinuous cracks were found in the joint ultrasonically brazed at 410 °C, and the joint showed a low bonding ratio and poor shear strength. Excellent bonding ratios and high shear strength were obtained in the joints ultrasonically brazed at 440 °C and 470 °C. The primary a-Al phase showed a refined spherical shape in the joint ultrasonically brazed at 440 °C, but showed a coarse dendritic structure in the joint ultrasonically brazed at 470 °C and that brazed at 440 °C without ultrasonic vibration. The refined spherical microstructure shown in the joint ultrasonically brazed at 440 °C was attributed to cavitation-aided grain refinement effects. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Al alloy joints with large bonding surfaces and excellent bonding properties (e.g., bonding strength, corrosion resistance and bonding ratio) are essential components in many assemblies, such as heat exchangers and electrical conductors, among others. Recently, Zn–Al hypereutectic alloys have been considered promising filler metals for the brazing of Al alloys. Their advantages include moderate brazing temperature, higher corrosion resistance and superior mechanical properties compared with other filler metals [1–4]. The greatest problem facing the use of hypereutectic filler metals is that dendrites are produced easily under normal brazing conditions due to their large freezing intervals [5]. Such coarse dendritic solidification structures can reduce the corrosion resistance and ductility of Zn–Al alloy [2]. Therefore, suppressing the formation of these coarse dendrites is a key challenge in improving joint properties. It has been reported that if the brazing processes are performed in the semi-solid state of hypereutectic filler metals, the formation of dendritic structures can be avoided [5]. However, conventional brazing methods, which employ fluxes to remove the oxide films on Al faying surfaces, are not suitable in joining Al alloys using semi-solid-state filler metals. This is because the bubbles generated by the reaction of flux are not easily released to the atmosphere and thus become trapped in semi-solid-state slurries. The semi-solidstate brazing of Al alloys with the aid of mechanical stirring [3] ⇑ Corresponding author. Tel.: +86 755 26033463; fax: +86 755 26033504. E-mail address: [email protected] (M. Li). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.004
and mechanical vibration [1] has been successfully carried out to disrupt and remove the oxide films on Al faying surfaces. However, these methods are not suitable for the fabrication of joints with large faying surfaces or complicated shapes. Recently, Nagaoka et al. [4,6] have brazed Al alloys using semi-solid-state Sn–Zn and Zn–Al filler metals by applying ultrasonic vibration to remove the oxide films on Al substrate surfaces. However, the interfacial wetting mechanism in the semi-solid-state ultrasound-assisted brazing process is still unclear. Ultrasound-assisted liquid-state brazing (soldering) has been widely studied in the joining of Al alloys and their composites [7–9]. Previous investigations have revealed that high-intensity ultrasound propagating through liquid filler metals can generate cavitation effects [10,11]. These effects can disrupt the oxide films on Al substrate surfaces and enhance the wettability of filler metals on substrates, thus obviating the need to use flux [10]. In addition, it has been widely reported that applying ultrasonic vibration to a solidifying melt results in a solidified microstructure, reduced grain sizes, improved homogeneity, reduced micro-segregation, etc. [12–14]. The solidification behavior observed during ultrasonic treatment has been extensively studied within the context of light alloy casting [15–18]. However, no previous studies have been reported on the ultrasonic-aided grain refinement of Al alloy joints brazed with Zn–Al hypereutectic filler metals. In the present study, 1060 Al alloy joints were fabricated by an ultrasound-assisted brazing method using a Zn–14Al (wt.%) hypereutectic filler metal. The purpose of this study was to clarify the effects of the brazing temperature on the bonding ratio, shear strength and microstructure of 1060 Al alloy joints. Furthermore,
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the interfacial wetting behavior and the microstructural evolution of the joints under the effects of ultrasonic vibration were investigated.
(a)
2. Experimental procedure 2.1. Materials A set of 1060 Al alloy sheets with a thickness of 3 mm and dimensions of 10 20 mm were used as the substrates. Zn–14Al alloy slices with a thickness of 400 lm and dimensions of 10 7 mm (A) and 10 3 mm (B) were used as the filler metals. The composition and mechanical properties of 1060 Al alloy and Zn–14Al alloy are given in Table 1. The solidus temperature (TS) and liquidus temperature (TL) of the Zn–14Al alloy determined by differential thermal analysis are 379.6 °C and 444.6 °C, respectively.
(b)
2.2. Sample preparation and brazing process All of the substrates and the filler metal slices were mechanically polished and ultrasonically cleaned in ethanol before brazing. Fig. 1 shows schematic diagrams of the ultrasound-assisted fluxless brazing process. The Al substrates were fixed in a lap joint configuration, as shown in Fig. 1a, with an overlapping area of 10 10 mm. Filler metal slices were sandwiched as an interlayer between the Al substrates, as shown in Fig. 1b. An ohmic heating device was used in this work, and the brazing temperature was monitored by a P-type Pt/Rh thermocouple inserted in the brazing seam. During brazing, each sample was first heated to the brazing temperature and held for approximately 30 s; then, ultrasonic vibration with a frequency of 28 kHz and a power of 233 W was applied directly to the top of the sample for 4 s, as shown in Fig. 1b. The as-brazed joint was then cooled in air to room temperature. A constant pressure was applied by the weight of an ultrasonic transducer (500 g) to reduce vibration loss. A series of samples were fabricated by ultrasonically brazing at 410 °C, 440 °C and 470 °C. Furthermore, a control sample was fabricated by brazing under the same processing parameters as the joint ultrasonically brazed at 440 °C without the aid of ultrasonic vibration. It should be noted that, for the joints brazed at 410 °C, the joint gaps were controlled by the semi-solid-state filler metal slurries, as shown in Fig. 1b; meanwhile, for the joints brazed at 440 °C and 470 °C, the joint gaps were confined to 240 lm by artificial spacers, as shown in Fig. 1c, so as not to completely squeeze the liquid phase out of the filler metal. 2.3. Bonding ratio test Scanning acoustic microscopy (SAM, Winsam Vario-3) with an ultrasonic wave frequency of 30 MHz was used to evaluate the joint quality. Before SAM testing, the upper substrate of each as-brazed joint was ground to 0.5 mm thick and polished. The ultrasonic wave emitted by SAM was propagated perpendicularly to this substrate using distilled water as the transmission medium. A C-mode scan (C-scan) based on ultrasonic pulse-echo inspecting technology was used in this study. The two-dimensional (area)
Sonotrode
Al 400µm
Al
Filler Slice (A)
Filler Slice (B)
Heating Device
(c)
Sonotrode
Al Molten Filler Spacer
Al Heating Device
240µm Spacer
Fig. 1. Schematic diagrams of (a) the lap joint configuration and the ultrasoundassisted brazing processes performed at (b) 410 °C, (c) 440 °C and 470 °C.
display of the data, called a C-scan image, allows for the visualization of disbonded regions as bright areas and well-bonded regions as dark areas. The bonding ratio, the ratio (in percent) of the wellbonded area to the total faying area (10 10 mm) of each joint, was calculated according to the C-scan image. The bonding ratio of each sample was estimated by averaging the results of five trials. 2.4. Shear strength test Shear test samples with dimensions of approximately 4 4 6.24 mm were cut from the as-brazed joints, as shown in
Table 1 Chemical compositions and mechanical properties of 1060 Al alloy and Zn–14Al alloy. Material
1060 Al Zn–14Al
Composition (wt.%) Al
Zn
Cu
Fe
Si
Mg
Mn
Bal. 14.21
0.01 Bal.
0.11 0.219
0.278 0.032
0.079 0.067
0.05 0.02
0.05 –
Tensile strength (MPa)
Elongation (%)
137 194
9 –
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(a)
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of metal-to-metal lap joints. At least five samples were tested under each experimental condition. 4 mm
4 mm
2.5. SEM characterization
6.24 mm
The cross-sections of the joints were polished and etched using a solution of 0.2 vol.% nitric acid in alcohol to reveal their microstructures. The microstructures of the joints were observed using scanning electron microscopy (SEM, Hitachi S-4700).
4 mm
(b)
3. Results and discussion
F
F
Fig. 2. Schematic diagrams of (a) shear test sample and (b) shear test fixture.
Fig. 2a. The shear strength of each joint was evaluated by testing these samples using a specially designed fixture [19], which is schematically shown in Fig. 2b, in an electron tension testing machine (Instron-5569) with a constant cross-head displacement rate of 0.5 mm/min. The test employed is a modification of the standard ASTM: D1002-10, which is used to test the shear strength
3.1. Bonding ratios of the joints Fig. 3a–c shows the C-scan images of joints ultrasonically brazed at 410 °C, 440 °C and 470 °C, respectively. Large disbonded regions can be seen in the joint brazed at 410 °C (Fig. 3a). The gap highlighted by the dashed pane shown in Fig. 3a demonstrates that filler slices A and B shown in Fig. 1a are not bonded together. A magnified view of zone A in Fig. 3a is shown in Fig. 3d. This shows that the well-bonded region in Fig. 3a is not uniform but appears to be a disordered, strip-like distribution. As for the joints brazed at 440 °C and 470 °C (Fig. 3b and c), only small disbonded regions can be seen on the rims of the seams. These disbonded regions are associated with the oxidation of joints when brazed in air.
Fig. 3. C-scan images of the joints ultrasonically brazed at (a) 410 °C, (b) 440 °C, and (c) 470 °C; (d) C-scan image of zone A in (a).
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100
80
80
60
60
40
40
20
20
410
440
o
Shear Strength (MPa)
Bonding Ratio (%)
100
Shear Strength Bonding Ratio
470
Temperature ( C) Fig. 4. Dependence of bonding ratio and shear strength on brazing temperature.
The bonding ratios of the joints brazed at different temperatures are shown in Fig. 4. It can be seen that the joint brazed at 440 °C exhibits the highest bonding ratio. The bonding ratio increases sharply from 54.8% (standard deviation 5.1%) at 410 °C to 98.9% (standard deviation 0.3%) at 440 °C. Then, the value decreases slightly to 97.7% (standard deviation 0.4%) at 470 °C due to the enhanced oxidation of the seam. The standard deviations for samples brazed at 440 °C and 470 °C are both very small, indicating stable joint properties. 3.2. Interfacial properties of the joints Fig. 5a1 shows SEM images of the joint ultrasonically brazed at 410 °C. Some cavities can be found in the joint due to the difference in the shrinkage factor between the liquid and solid phase during solidification [7]. A high-magnification image of the joint brazed at 410 °C is shown in Fig. 5a2. The dark region in the filler metal is the primary a-Al phase, and the bright areas are the Zn–Al eutectic phase. Discontinuous cracks located along the interface between the Al substrate and the filler metal can be seen in the joint. This is consistent with Fig. 3d. The Al substrate in the joint is wetted by the filler metal, indicating that the oxide film on the Al substrate surface is disrupted. However, because filler slices A and B shown in Fig. 3a are not bonded together, it can be inferred that the disruption of the oxide film in the joint brazed at 410 °C was not cavitation-induced in this
study. This is because once the cavitation effect is generated, it will spread throughout the brazing seam [11]. Moreover, because the joint space is controlled by the semi-solid-state filler metal slurry for the joint brazed at 410 °C, the liquid phase in the filler metal can be largely squeezed out by ultrasonic vibration during the brazing process, which can be confirmed by the reduced Zn–Al eutectic phase shown in Fig. 5a2. This can significantly increase the ultrasonic intensity threshold because cavitation effects are difficult to initiate in semi-solid-state alloys with large solid fractions [14]. The disruption of the oxide film in the joint ultrasonically brazed at 410 °C may be explained by a mechanical mechanism [1], wherein the combined effects of compression and friction between the semi-solid-state filler metal slurry and the substrate lead to the local disruption of the oxide film. Once the oxide film is disrupted locally, the liquid Zn–Al alloy can lift the oxide film and wet the substrate [10]. Because there are no cavitation-induced mechanical effects (such as shockwaves, jets or acoustic streaming) [20,21] in the semi-solid-state filler metal, the lifted oxide films cannot be disrupted further but are distributed discontinuously along the brazing interface. Finally, the lifted oxide films that exhibit bad wettability with the semi-solid-state filler metal cause the cracks shown in Fig. 5a2. Fig. 5b1 and c1 shows SEM images of the joints ultrasonically brazed at 440 °C and 470 °C, respectively. A uniform, spherical primary a-Al phase is obtained in the joint brazed at 440 °C. However, the primary a-Al phase shows a coarse dendritic structure in the joint brazed at 470 °C. The microstructural evolution of the primary a-Al phase will be discussed in the next section. High-magnification images of the joints brazed at 440 °C and 470 °C are shown in Fig. 5b2 and c2. It can be seen that, compared with the joint brazed at 410 °C (Fig. 5a1), the joints brazed at 440 °C and 470 °C exhibit improved bonding without any cavities or cracks between the Al substrate and filler metal. The excellent metallurgical bonding between the Al substrate and filler metal in the joints brazed at 440 °C and 470 °C indicates that the oxide films on the Al substrate surfaces were removed completely. It can be seen that the filler metals are liquid at 440 °C and 470 °C, according to the Zn–Al alloy phase diagram [22] shown in Fig. 6 (the calculated mass fraction of liquid phase is approximately 97% at 440 °C). The substrates are separated by artificial spacers. Therefore, it can be deduced that high-intensity cavitation effects are generated in the joints brazed at 440 °C and 470 °C, which lead to the disruption of the oxide films. Moreover, according to the above results, it can be concluded that cavita-
Fig. 5. SEM images of the joints ultrasonically brazed at (a1) 410 °C, (b1) 440 °C, and (c1) 470 °C and their high-magnification images: (a2) 410 °C, (b2) 440 °C, and (c2) 470 °C.
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600
L
Temperature ºC
500
400
300
(Al)
470ºC 440ºC 410ºC 83.1
94.0
381ºC
277ºC 77.7
200
(Zn)
Zn-14Al
100
0
50 Al
60
70 80 Weight Percent Zinc
90
100 Zn
Fig. 6. Zn–Al alloy phase diagram [22].
tion-induced interfacial wetting is more efficient than mechanicalinduced interfacial wetting during the ultrasound-assisted brazing of Al alloy.
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The relationship between the brazing temperature and shear strength of the Al alloy joints is shown in Fig. 4. It can be seen that the shear strength of the joints is remarkably improved when the brazing temperature is increased from 410 °C to 440 °C. The shear strength of the joint brazed at 440 °C is 67.8 MPa, which represents a 39.7 MPa improvement compared with that of the joint brazed at 410 °C. However, the shear strength is slightly reduced to 66.6 MPa with a further increase in the brazing temperature to 470 °C. The points of shearing failure in these joints are shown in Fig. 7. It can be seen that shearing failure occurs at the interface between the Al substrate and filler metal in the joint brazed at 410 °C (Fig. 7a) but occurs in the base metals in the joints brazed at 440 °C and 470 °C (Fig. 7b and c). These results indicate that the enhancement in the joints’ shear strength is highly correlated with the bonding ratio of the interface. Meanwhile, the reduced shear strength of the joint brazed at 470 °C may be due to the intensified softening of the Al substrate. 3.3. Microstructure evolution of the joints As mentioned in Section 3.2, excellent bonding properties were obtained in the joints ultrasonically brazed at 440 °C and 470 °C, but their microstructures are different. High-magnification images of the joints ultrasonically brazed at 440 °C and 470 °C are shown
Fig. 7. Shearing failure positions of the joints ultrasonically brazed at (a) 410 °C, (b) 440 °C, and (c) 470 °C.
Fig. 8. High-magnification images of the joints ultrasonically brazed at (a) 440 °C and (b) 470 °C; (c) SEM image of the joint brazed at 440 °C without ultrasonic vibration and (d) high-magnification image in (c).
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in Fig. 8a and b, respectively. The joint brazed at 440 °C consists of a uniform and spherical primary a-Al phase and a Zn–Al eutectic structure. The average diameter of the primary a-Al phase is 26.4 lm (Fig. 8a). As the brazing temperature increases to 470 °C, the a-Al phase forms a nonuniform dendritic structure with an average length of 49.7 lm (Fig. 8b). To examine the formation mechanism of the refined microstructure observed in the joint ultrasonically brazed at 440 °C, a control sample was fabricated. Fig. 8c shows the microstructure of the Al alloy joint brazed under the same processing parameters as the joint ultrasonically brazed at 440 °C without the aid of ultrasonic vibration. It can be seen that the primary a-Al phase shows two different morphologies in the brazing seam: an irregular granular shape on the upper side and a coarse dendritic structure on the lower side. This can be explained as follows. Some primary a-Al particles remain unmelted in the filler metal kept at 440 °C (the calculated mass fraction of solid phase in filler metal kept at 440 °C is approximately 3%). Then, these unmelted a-Al particles float to the upper side of the seam. During the solidification process, the unmelted a-Al particles act as solidification sites and thereby refine the primary a-Al phase located on the upper side of the seam. A high-magnification image of Fig. 8c is shown in Fig. 8d. Clearly, the size of the primary a-Al phase in the joint fabricated without ultrasonic vibration (Fig. 8d) is larger than that in the ultrasonically brazed joint (Fig. 8a), indicating that the primary a-Al phase is refined by ultrasonic vibration.
As discussed in Section 3.2, cavitation effects are generated in joints ultrasonically brazed at 440 °C and 470 °C, through which the oxide films on the Al substrate surfaces are effectively disrupted. It is widely reported that cavitation effects influence the microstructure of solidified alloys [12,23]. Therefore, the morphological evolution and the size variation of the primary a-Al phase shown in Fig. 8 can be explained by the fact that the nucleation and growth of primary a-Al grains are affected by cavitation effects. Two possible mechanisms have been proposed to interpret cavitation-aided grain refinement: (1) cavitation-induced grain multiplication [14,20] and (2) cavitation-aided nucleation [20,24]. The number of grains multiplies because shockwaves and microjets generated from the cavities collapse lead to the fragmentation of solid phases and increase the number of solidification sites [14,20]. For the joint brazed at 440 °C, there are still some unmelted a-Al particles in the molten filler metal. During the ultrasound-assisted brazing process, cavitation-induced shockwaves and microjets can break these particles into smaller ones and thus multiply the number of growth sites of the primary a-Al phase during the solidification of the brazing seam. Cavitation-aided nucleation is governed by two different mechanisms. The first is the adiabatic expansion mechanism [18,20], in which the temperature at the cavity surface drops during the expansion of the ultrasound-induced cavity. As a result, undercooling occurs on the cavity surface and results in the creation of nuclei. The other is the pressure pulse-melting point mechanism
Fig. 9. Grain refinement model of the joint ultrasonically brazed at 440 °C: (a) original joint kept at 440 °C before ultrasonic vibration, (b) at the beginning of ultrasonic vibration, (c) at the end of ultrasonic vibration and (d) at the as-solidified brazing seam.
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[13,24], in which the pressure pulse induced by the collapse of a cavity increases the melting point according to the Clapeyron equation. An increase in the melting point indicates increased undercooling and promotes the formation of nuclei. However, the nuclei induced by cavitation effects are unstable and are not able to survive in the superheated melt when the ultrasonic vibration stops [14,16]. Therefore, in the joint ultrasonically brazed at 470 °C, the a-Al nuclei induced by the cavitation effects may be dissolved again when the ultrasonic vibration stops because the ultrasonic processing temperature is approximately 26 °C higher than the liquidus temperature of the filler metal. However, that is not the case for the joint ultrasonically brazed at 440 °C. It has been reported that cavitation and acoustic streaming effects caused by ultrasonic waves can enhance the diffusion of Al away from the substrate and into the brazing seam [9], which can increase the Al content and thus increase the liquidus temperature of the filler metal. At a brazing temperature of 440 °C, as shown in Fig. 6, the liquid filler metal is saturated with Al, and an increase in the liquidus temperature indicates increased undercooling. Therefore, when the joint is isothermally brazed at 440 °C, the ultrasound-aided nucleation process is accompanied by improved undercooling, and thus, the initial a-Al nuclei can survive in the melt. Consequently, due to the effects of cavitation-induced grain multiplication and cavitation-aided nucleation, the number of aAl nuclei surviving in the melt is higher in the joint ultrasonically brazed at 440 °C than that in the joint ultrasonically brazed at 470 °C and that in the joint brazed at 440 °C without ultrasonic vibration. Ultimately, the primary a-Al phase shows a refined spherical shape in the joint ultrasonically brazed at 440 °C and a nonuniform dendritic structure both in the joint ultrasonically brazed at 470 °C and in the joint brazed at 440 °C without ultrasonic vibration. Thus, it can be concluded that, compared with the joints brazed with semi-solid-state filler metal slurries [1,3,4], a refined microstructure can also be obtained in the joints ultrasonically brazed near the liquidus temperature of the filler metal. 3.4. Grain refinement model To further illustrate the formation mechanism of the refined, spherically shaped primary a-Al phase shown in the joint ultrasonically brazed at 440 °C, a model that summarizes the generation of primary a-Al phase is depicted in Fig. 9. The black regions represent the Al substrate, the dark gray regions represent the a-Al phase, and the light-gray regions represent the Zn–Al eutectic phase or the liquid phase. Fig. 9a describes the joint kept at 440 °C before ultrasonic vibration. Some unmelted a-Al particles located on the upper side of the seam can be seen. Fig. 9b shows the joint at the onset of ultrasonic vibration. The oxide film is disrupted, and the Al in the substrate begins to dissolve into the filler metal; the solid a-Al particles are dispersed homogeneously and partially broken, and ultrasound-induced nuclei can be observed. As the ultrasonic vibration continues, a large amount of a-Al nuclei dispersed throughout the entire brazing seam are generated, accompanying the enhanced dissolution of Al in the substrates into the Zn–Al filler metal (Fig. 9c). During the solidification process, these initial nuclei of the primary a-Al phase grow into spherical structures (Fig. 9d). 4. Conclusions In this study, 1060 Al alloy joints were fabricated with an ultrasound-assisted brazing method using a Zn–14Al hypereutectic filler metal. The effects of the brazing temperature on the bonding
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ratio, shear strength and microstructure of the joints were investigated, and the following conclusions were obtained: (1) For the joint ultrasonically brazed at 410 °C, the oxide films on the Al substrate surfaces were disrupted based on a mechanical mechanism. Cavities and discontinuous cracks were found in the joint, resulting in a low bonding ratio and reduced shear strength. (2) For the joints ultrasonically brazed at 440 °C and 470 °C, the oxide films on the Al substrate surfaces were disrupted by ultrasound-induced cavitation effects. Excellent bonding ratios of 98.9% and 97.7% were obtained, respectively. Shearing failure occurred in both base metals. (3) The primary a-Al phase showed a refined spherical shape in the joint ultrasonically brazed at 440 °C and showed a coarse and nonuniform dendritic structure both in the joint ultrasonically brazed at 470 °C and in the joint brazed at 440 °C without ultrasonic vibration. The refined spherical microstructure shown in the joint ultrasonically brazed at 440 °C was attributed to cavitation-aided grain refinement effects.
Acknowledgments This work was supported by the National Science Foundation of China (No. 51005057), the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No. AWPT-M09), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (Grant No. NRF-2012038597). References [1] Shi L, Yan JC, Peng B, Han YF. Deformation behavior of semi-solid Zn–Al alloy filler metal during compression. Mater Sci Eng A 2011;528:7084–92. [2] Yan XQ, Liu SX, Long WM, Huang JL, Zhang LY, Chen Y. The effect of homogenization treatment on microstructure and properties of ZnAl15 solder. Mater Des 2013;45:440–5. [3] Xu HB, Luo QX, Zhou BF, Zeng YL, Du CH. The effect of stirring rate on semisolid stirring brazing of SiCp/A356 composites in air. Mater Des 2012;34:452–8. [4] Nagaoka T, Morisada Y, Fukusumi M, Takemoto T. Selection of soldering temperature for ultrasonic-assisted soldering of 5056 aluminum alloy using Zn–Al system solders. J Mater Process Technol 2011;211:1534–9. [5] Mendez PF, Rice CS, Brown SB. Joining using semisolid metals. Welding Res 2002;9:181–7. [6] Nagaoka T, Morisada Y, Fukusumi M, Takemoto T. Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder. J Mater Process Technol 2009;209:5054–9. [7] Xu ZW, Yan JC, Wu GH, Kong XL, Yang SQ. Interface structure of ultrasonic vibration aided interaction between Zn–Al alloy and Al2O3p/6061Al composite. Compos Sci Technol 2005;65:1959–63. [8] Ding M, Zhang PL, Zhang ZY, Yao S. Direct-soldering 6061 aluminum alloys with ultrasonic coating. Ultrason Sonochem 2010;17:292–7. [9] Li YX, Leng XS, Cheng S, Yan JC. Microstructure design and dissolution behavior between 2024 Al/Sn with the ultrasonic-associated soldering. Mater Des 2012;40:427–32. [10] Xu ZW, Yan JC, Zhang BY, Kong XL, Yang SQ. Behaviors of oxide film at the ultrasonic aided interaction interface of Zn–Al alloy and Al2O3p/6061Al composites in air. Mater Sci Eng A 2006;415(1–2):80–6. [11] Chinnama RK, Fauteuxa C, Neuenschwanderb J, Janczak-Ruscha J. Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidification. Acta Mater 2011;59(4):1474–81. [12] Das A, Kotadia HR. Effect of high-intensity ultrasonic irradiation on the modification of solidification microstructure in a Si-rich hypoeutectic Al–Si alloy. Mater Chem Phys 2011;125(3):853–9. [13] Ramirez A, Qian M, Davis B, Wilks T, StJohn DH. Potency of high-intensity ultrasonic treatment for grain refinement of magnesium alloys. Scr Mater 2008;59:19–22. [14] Atamanenko TV, Eskin DG, Zhang L, Katgerman L. Criteria of grain refinement induced by ultrasonic melt treatment of aluminum alloys containing Zr and Ti. Mater Trans A 2010;41:2056–66. [15] Nie KB, Wang XJ, Wu K, Hu XS, Zheng MY, Xu L. Microstructure and tensile properties of micro-SiC particles reinforced magnesium matrix composites produced by semisolid stirring assisted ultrasonic vibration. Mater Sci Eng A 2011;528:8709–14. [16] Qian M, Ramirez A, Das A, StJohn DH. The effect of solute on ultrasonic grain refinement of magnesium alloys. J Cryst Growth 2010;312:2267–72.
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[17] Aghayani MK, Niroumand B. Effects of ultrasonic treatment on microstructure and tensile strength of AZ91 magnesium alloy. J Alloys Compd 2011;509:114–22. [18] Jian X, Xu H, Meek TT, Han Q. Effect of power ultrasound on solidification of aluminum A356 alloy. Mater Lett 2005;59:190–3. [19] Kavian OC, Tahir IK, Gossett DO. Transient liquid phase diffusion bonding Al6061 using nano-dispersed Ni coatings. Mater Des 2012;33:469–75. [20] Kapustina OA. The physical principles of ultrasonic technology. Moscow: Nauka; 1970.
[21] Zhang SL, Zhao YT, Cheng XN, Chen G, Dai QX. High-energy ultrasonic field effects on the microstructure and mechanical behaviors of A356 alloy. J Alloys Compd 2009;470:168–72. [22] Murray JL. The Al Zn (aluminum–zinc) system. J Phase Equilib 1983;4:55–73. [23] Gao DM, Li ZJ, Han QY, Zhai QJ. Effect of ultrasonic power on microstructure and mechanical properties of AZ91 alloy. Mater Sci Eng A 2009;502:2–5. [24] Eskin GI. Ultrasonic treatment of light alloy melts. Amsterdam: Gordon and Breach; 1998.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and joint properties of ultrasonically brazed Al alloy joints using a Zn–Al hypereutectic filler metal Yong Xiao a, Hongjun Ji a, Mingyu Li a,⇑, Jongmyung Kim b, Hongbae Kim b a b
Shenzhen Key Laboratory of Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China Jeonnam Provincial College, Jeonnam 517802, South Korea
a r t i c l e
i n f o
Article history: Received 21 October 2012 Accepted 2 January 2013 Available online 16 January 2013 Keywords: Ultrasound-assisted brazing Aluminum alloy Hypereutectic filler meal Bonding ratio Microstructure
a b s t r a c t The ultrasound-assisted brazing of 1060 Al alloy using a Zn–14Al hypereutectic filler metal was investigated at different temperatures. The effects of brazing temperature on the bonding ratio, shear strength and microstructure of the joints were studied. Cavities and discontinuous cracks were found in the joint ultrasonically brazed at 410 °C, and the joint showed a low bonding ratio and poor shear strength. Excellent bonding ratios and high shear strength were obtained in the joints ultrasonically brazed at 440 °C and 470 °C. The primary a-Al phase showed a refined spherical shape in the joint ultrasonically brazed at 440 °C, but showed a coarse dendritic structure in the joint ultrasonically brazed at 470 °C and that brazed at 440 °C without ultrasonic vibration. The refined spherical microstructure shown in the joint ultrasonically brazed at 440 °C was attributed to cavitation-aided grain refinement effects. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Al alloy joints with large bonding surfaces and excellent bonding properties (e.g., bonding strength, corrosion resistance and bonding ratio) are essential components in many assemblies, such as heat exchangers and electrical conductors, among others. Recently, Zn–Al hypereutectic alloys have been considered promising filler metals for the brazing of Al alloys. Their advantages include moderate brazing temperature, higher corrosion resistance and superior mechanical properties compared with other filler metals [1–4]. The greatest problem facing the use of hypereutectic filler metals is that dendrites are produced easily under normal brazing conditions due to their large freezing intervals [5]. Such coarse dendritic solidification structures can reduce the corrosion resistance and ductility of Zn–Al alloy [2]. Therefore, suppressing the formation of these coarse dendrites is a key challenge in improving joint properties. It has been reported that if the brazing processes are performed in the semi-solid state of hypereutectic filler metals, the formation of dendritic structures can be avoided [5]. However, conventional brazing methods, which employ fluxes to remove the oxide films on Al faying surfaces, are not suitable in joining Al alloys using semi-solid-state filler metals. This is because the bubbles generated by the reaction of flux are not easily released to the atmosphere and thus become trapped in semi-solid-state slurries. The semi-solidstate brazing of Al alloys with the aid of mechanical stirring [3] ⇑ Corresponding author. Tel.: +86 755 26033463; fax: +86 755 26033504. E-mail address: [email protected] (M. Li). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.004
and mechanical vibration [1] has been successfully carried out to disrupt and remove the oxide films on Al faying surfaces. However, these methods are not suitable for the fabrication of joints with large faying surfaces or complicated shapes. Recently, Nagaoka et al. [4,6] have brazed Al alloys using semi-solid-state Sn–Zn and Zn–Al filler metals by applying ultrasonic vibration to remove the oxide films on Al substrate surfaces. However, the interfacial wetting mechanism in the semi-solid-state ultrasound-assisted brazing process is still unclear. Ultrasound-assisted liquid-state brazing (soldering) has been widely studied in the joining of Al alloys and their composites [7–9]. Previous investigations have revealed that high-intensity ultrasound propagating through liquid filler metals can generate cavitation effects [10,11]. These effects can disrupt the oxide films on Al substrate surfaces and enhance the wettability of filler metals on substrates, thus obviating the need to use flux [10]. In addition, it has been widely reported that applying ultrasonic vibration to a solidifying melt results in a solidified microstructure, reduced grain sizes, improved homogeneity, reduced micro-segregation, etc. [12–14]. The solidification behavior observed during ultrasonic treatment has been extensively studied within the context of light alloy casting [15–18]. However, no previous studies have been reported on the ultrasonic-aided grain refinement of Al alloy joints brazed with Zn–Al hypereutectic filler metals. In the present study, 1060 Al alloy joints were fabricated by an ultrasound-assisted brazing method using a Zn–14Al (wt.%) hypereutectic filler metal. The purpose of this study was to clarify the effects of the brazing temperature on the bonding ratio, shear strength and microstructure of 1060 Al alloy joints. Furthermore,
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the interfacial wetting behavior and the microstructural evolution of the joints under the effects of ultrasonic vibration were investigated.
(a)
2. Experimental procedure 2.1. Materials A set of 1060 Al alloy sheets with a thickness of 3 mm and dimensions of 10 20 mm were used as the substrates. Zn–14Al alloy slices with a thickness of 400 lm and dimensions of 10 7 mm (A) and 10 3 mm (B) were used as the filler metals. The composition and mechanical properties of 1060 Al alloy and Zn–14Al alloy are given in Table 1. The solidus temperature (TS) and liquidus temperature (TL) of the Zn–14Al alloy determined by differential thermal analysis are 379.6 °C and 444.6 °C, respectively.
(b)
2.2. Sample preparation and brazing process All of the substrates and the filler metal slices were mechanically polished and ultrasonically cleaned in ethanol before brazing. Fig. 1 shows schematic diagrams of the ultrasound-assisted fluxless brazing process. The Al substrates were fixed in a lap joint configuration, as shown in Fig. 1a, with an overlapping area of 10 10 mm. Filler metal slices were sandwiched as an interlayer between the Al substrates, as shown in Fig. 1b. An ohmic heating device was used in this work, and the brazing temperature was monitored by a P-type Pt/Rh thermocouple inserted in the brazing seam. During brazing, each sample was first heated to the brazing temperature and held for approximately 30 s; then, ultrasonic vibration with a frequency of 28 kHz and a power of 233 W was applied directly to the top of the sample for 4 s, as shown in Fig. 1b. The as-brazed joint was then cooled in air to room temperature. A constant pressure was applied by the weight of an ultrasonic transducer (500 g) to reduce vibration loss. A series of samples were fabricated by ultrasonically brazing at 410 °C, 440 °C and 470 °C. Furthermore, a control sample was fabricated by brazing under the same processing parameters as the joint ultrasonically brazed at 440 °C without the aid of ultrasonic vibration. It should be noted that, for the joints brazed at 410 °C, the joint gaps were controlled by the semi-solid-state filler metal slurries, as shown in Fig. 1b; meanwhile, for the joints brazed at 440 °C and 470 °C, the joint gaps were confined to 240 lm by artificial spacers, as shown in Fig. 1c, so as not to completely squeeze the liquid phase out of the filler metal. 2.3. Bonding ratio test Scanning acoustic microscopy (SAM, Winsam Vario-3) with an ultrasonic wave frequency of 30 MHz was used to evaluate the joint quality. Before SAM testing, the upper substrate of each as-brazed joint was ground to 0.5 mm thick and polished. The ultrasonic wave emitted by SAM was propagated perpendicularly to this substrate using distilled water as the transmission medium. A C-mode scan (C-scan) based on ultrasonic pulse-echo inspecting technology was used in this study. The two-dimensional (area)
Sonotrode
Al 400µm
Al
Filler Slice (A)
Filler Slice (B)
Heating Device
(c)
Sonotrode
Al Molten Filler Spacer
Al Heating Device
240µm Spacer
Fig. 1. Schematic diagrams of (a) the lap joint configuration and the ultrasoundassisted brazing processes performed at (b) 410 °C, (c) 440 °C and 470 °C.
display of the data, called a C-scan image, allows for the visualization of disbonded regions as bright areas and well-bonded regions as dark areas. The bonding ratio, the ratio (in percent) of the wellbonded area to the total faying area (10 10 mm) of each joint, was calculated according to the C-scan image. The bonding ratio of each sample was estimated by averaging the results of five trials. 2.4. Shear strength test Shear test samples with dimensions of approximately 4 4 6.24 mm were cut from the as-brazed joints, as shown in
Table 1 Chemical compositions and mechanical properties of 1060 Al alloy and Zn–14Al alloy. Material
1060 Al Zn–14Al
Composition (wt.%) Al
Zn
Cu
Fe
Si
Mg
Mn
Bal. 14.21
0.01 Bal.
0.11 0.219
0.278 0.032
0.079 0.067
0.05 0.02
0.05 –
Tensile strength (MPa)
Elongation (%)
137 194
9 –
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(a)
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of metal-to-metal lap joints. At least five samples were tested under each experimental condition. 4 mm
4 mm
2.5. SEM characterization
6.24 mm
The cross-sections of the joints were polished and etched using a solution of 0.2 vol.% nitric acid in alcohol to reveal their microstructures. The microstructures of the joints were observed using scanning electron microscopy (SEM, Hitachi S-4700).
4 mm
(b)
3. Results and discussion
F
F
Fig. 2. Schematic diagrams of (a) shear test sample and (b) shear test fixture.
Fig. 2a. The shear strength of each joint was evaluated by testing these samples using a specially designed fixture [19], which is schematically shown in Fig. 2b, in an electron tension testing machine (Instron-5569) with a constant cross-head displacement rate of 0.5 mm/min. The test employed is a modification of the standard ASTM: D1002-10, which is used to test the shear strength
3.1. Bonding ratios of the joints Fig. 3a–c shows the C-scan images of joints ultrasonically brazed at 410 °C, 440 °C and 470 °C, respectively. Large disbonded regions can be seen in the joint brazed at 410 °C (Fig. 3a). The gap highlighted by the dashed pane shown in Fig. 3a demonstrates that filler slices A and B shown in Fig. 1a are not bonded together. A magnified view of zone A in Fig. 3a is shown in Fig. 3d. This shows that the well-bonded region in Fig. 3a is not uniform but appears to be a disordered, strip-like distribution. As for the joints brazed at 440 °C and 470 °C (Fig. 3b and c), only small disbonded regions can be seen on the rims of the seams. These disbonded regions are associated with the oxidation of joints when brazed in air.
Fig. 3. C-scan images of the joints ultrasonically brazed at (a) 410 °C, (b) 440 °C, and (c) 470 °C; (d) C-scan image of zone A in (a).
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100
80
80
60
60
40
40
20
20
410
440
o
Shear Strength (MPa)
Bonding Ratio (%)
100
Shear Strength Bonding Ratio
470
Temperature ( C) Fig. 4. Dependence of bonding ratio and shear strength on brazing temperature.
The bonding ratios of the joints brazed at different temperatures are shown in Fig. 4. It can be seen that the joint brazed at 440 °C exhibits the highest bonding ratio. The bonding ratio increases sharply from 54.8% (standard deviation 5.1%) at 410 °C to 98.9% (standard deviation 0.3%) at 440 °C. Then, the value decreases slightly to 97.7% (standard deviation 0.4%) at 470 °C due to the enhanced oxidation of the seam. The standard deviations for samples brazed at 440 °C and 470 °C are both very small, indicating stable joint properties. 3.2. Interfacial properties of the joints Fig. 5a1 shows SEM images of the joint ultrasonically brazed at 410 °C. Some cavities can be found in the joint due to the difference in the shrinkage factor between the liquid and solid phase during solidification [7]. A high-magnification image of the joint brazed at 410 °C is shown in Fig. 5a2. The dark region in the filler metal is the primary a-Al phase, and the bright areas are the Zn–Al eutectic phase. Discontinuous cracks located along the interface between the Al substrate and the filler metal can be seen in the joint. This is consistent with Fig. 3d. The Al substrate in the joint is wetted by the filler metal, indicating that the oxide film on the Al substrate surface is disrupted. However, because filler slices A and B shown in Fig. 3a are not bonded together, it can be inferred that the disruption of the oxide film in the joint brazed at 410 °C was not cavitation-induced in this
study. This is because once the cavitation effect is generated, it will spread throughout the brazing seam [11]. Moreover, because the joint space is controlled by the semi-solid-state filler metal slurry for the joint brazed at 410 °C, the liquid phase in the filler metal can be largely squeezed out by ultrasonic vibration during the brazing process, which can be confirmed by the reduced Zn–Al eutectic phase shown in Fig. 5a2. This can significantly increase the ultrasonic intensity threshold because cavitation effects are difficult to initiate in semi-solid-state alloys with large solid fractions [14]. The disruption of the oxide film in the joint ultrasonically brazed at 410 °C may be explained by a mechanical mechanism [1], wherein the combined effects of compression and friction between the semi-solid-state filler metal slurry and the substrate lead to the local disruption of the oxide film. Once the oxide film is disrupted locally, the liquid Zn–Al alloy can lift the oxide film and wet the substrate [10]. Because there are no cavitation-induced mechanical effects (such as shockwaves, jets or acoustic streaming) [20,21] in the semi-solid-state filler metal, the lifted oxide films cannot be disrupted further but are distributed discontinuously along the brazing interface. Finally, the lifted oxide films that exhibit bad wettability with the semi-solid-state filler metal cause the cracks shown in Fig. 5a2. Fig. 5b1 and c1 shows SEM images of the joints ultrasonically brazed at 440 °C and 470 °C, respectively. A uniform, spherical primary a-Al phase is obtained in the joint brazed at 440 °C. However, the primary a-Al phase shows a coarse dendritic structure in the joint brazed at 470 °C. The microstructural evolution of the primary a-Al phase will be discussed in the next section. High-magnification images of the joints brazed at 440 °C and 470 °C are shown in Fig. 5b2 and c2. It can be seen that, compared with the joint brazed at 410 °C (Fig. 5a1), the joints brazed at 440 °C and 470 °C exhibit improved bonding without any cavities or cracks between the Al substrate and filler metal. The excellent metallurgical bonding between the Al substrate and filler metal in the joints brazed at 440 °C and 470 °C indicates that the oxide films on the Al substrate surfaces were removed completely. It can be seen that the filler metals are liquid at 440 °C and 470 °C, according to the Zn–Al alloy phase diagram [22] shown in Fig. 6 (the calculated mass fraction of liquid phase is approximately 97% at 440 °C). The substrates are separated by artificial spacers. Therefore, it can be deduced that high-intensity cavitation effects are generated in the joints brazed at 440 °C and 470 °C, which lead to the disruption of the oxide films. Moreover, according to the above results, it can be concluded that cavita-
Fig. 5. SEM images of the joints ultrasonically brazed at (a1) 410 °C, (b1) 440 °C, and (c1) 470 °C and their high-magnification images: (a2) 410 °C, (b2) 440 °C, and (c2) 470 °C.
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600
L
Temperature ºC
500
400
300
(Al)
470ºC 440ºC 410ºC 83.1
94.0
381ºC
277ºC 77.7
200
(Zn)
Zn-14Al
100
0
50 Al
60
70 80 Weight Percent Zinc
90
100 Zn
Fig. 6. Zn–Al alloy phase diagram [22].
tion-induced interfacial wetting is more efficient than mechanicalinduced interfacial wetting during the ultrasound-assisted brazing of Al alloy.
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The relationship between the brazing temperature and shear strength of the Al alloy joints is shown in Fig. 4. It can be seen that the shear strength of the joints is remarkably improved when the brazing temperature is increased from 410 °C to 440 °C. The shear strength of the joint brazed at 440 °C is 67.8 MPa, which represents a 39.7 MPa improvement compared with that of the joint brazed at 410 °C. However, the shear strength is slightly reduced to 66.6 MPa with a further increase in the brazing temperature to 470 °C. The points of shearing failure in these joints are shown in Fig. 7. It can be seen that shearing failure occurs at the interface between the Al substrate and filler metal in the joint brazed at 410 °C (Fig. 7a) but occurs in the base metals in the joints brazed at 440 °C and 470 °C (Fig. 7b and c). These results indicate that the enhancement in the joints’ shear strength is highly correlated with the bonding ratio of the interface. Meanwhile, the reduced shear strength of the joint brazed at 470 °C may be due to the intensified softening of the Al substrate. 3.3. Microstructure evolution of the joints As mentioned in Section 3.2, excellent bonding properties were obtained in the joints ultrasonically brazed at 440 °C and 470 °C, but their microstructures are different. High-magnification images of the joints ultrasonically brazed at 440 °C and 470 °C are shown
Fig. 7. Shearing failure positions of the joints ultrasonically brazed at (a) 410 °C, (b) 440 °C, and (c) 470 °C.
Fig. 8. High-magnification images of the joints ultrasonically brazed at (a) 440 °C and (b) 470 °C; (c) SEM image of the joint brazed at 440 °C without ultrasonic vibration and (d) high-magnification image in (c).
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in Fig. 8a and b, respectively. The joint brazed at 440 °C consists of a uniform and spherical primary a-Al phase and a Zn–Al eutectic structure. The average diameter of the primary a-Al phase is 26.4 lm (Fig. 8a). As the brazing temperature increases to 470 °C, the a-Al phase forms a nonuniform dendritic structure with an average length of 49.7 lm (Fig. 8b). To examine the formation mechanism of the refined microstructure observed in the joint ultrasonically brazed at 440 °C, a control sample was fabricated. Fig. 8c shows the microstructure of the Al alloy joint brazed under the same processing parameters as the joint ultrasonically brazed at 440 °C without the aid of ultrasonic vibration. It can be seen that the primary a-Al phase shows two different morphologies in the brazing seam: an irregular granular shape on the upper side and a coarse dendritic structure on the lower side. This can be explained as follows. Some primary a-Al particles remain unmelted in the filler metal kept at 440 °C (the calculated mass fraction of solid phase in filler metal kept at 440 °C is approximately 3%). Then, these unmelted a-Al particles float to the upper side of the seam. During the solidification process, the unmelted a-Al particles act as solidification sites and thereby refine the primary a-Al phase located on the upper side of the seam. A high-magnification image of Fig. 8c is shown in Fig. 8d. Clearly, the size of the primary a-Al phase in the joint fabricated without ultrasonic vibration (Fig. 8d) is larger than that in the ultrasonically brazed joint (Fig. 8a), indicating that the primary a-Al phase is refined by ultrasonic vibration.
As discussed in Section 3.2, cavitation effects are generated in joints ultrasonically brazed at 440 °C and 470 °C, through which the oxide films on the Al substrate surfaces are effectively disrupted. It is widely reported that cavitation effects influence the microstructure of solidified alloys [12,23]. Therefore, the morphological evolution and the size variation of the primary a-Al phase shown in Fig. 8 can be explained by the fact that the nucleation and growth of primary a-Al grains are affected by cavitation effects. Two possible mechanisms have been proposed to interpret cavitation-aided grain refinement: (1) cavitation-induced grain multiplication [14,20] and (2) cavitation-aided nucleation [20,24]. The number of grains multiplies because shockwaves and microjets generated from the cavities collapse lead to the fragmentation of solid phases and increase the number of solidification sites [14,20]. For the joint brazed at 440 °C, there are still some unmelted a-Al particles in the molten filler metal. During the ultrasound-assisted brazing process, cavitation-induced shockwaves and microjets can break these particles into smaller ones and thus multiply the number of growth sites of the primary a-Al phase during the solidification of the brazing seam. Cavitation-aided nucleation is governed by two different mechanisms. The first is the adiabatic expansion mechanism [18,20], in which the temperature at the cavity surface drops during the expansion of the ultrasound-induced cavity. As a result, undercooling occurs on the cavity surface and results in the creation of nuclei. The other is the pressure pulse-melting point mechanism
Fig. 9. Grain refinement model of the joint ultrasonically brazed at 440 °C: (a) original joint kept at 440 °C before ultrasonic vibration, (b) at the beginning of ultrasonic vibration, (c) at the end of ultrasonic vibration and (d) at the as-solidified brazing seam.
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[13,24], in which the pressure pulse induced by the collapse of a cavity increases the melting point according to the Clapeyron equation. An increase in the melting point indicates increased undercooling and promotes the formation of nuclei. However, the nuclei induced by cavitation effects are unstable and are not able to survive in the superheated melt when the ultrasonic vibration stops [14,16]. Therefore, in the joint ultrasonically brazed at 470 °C, the a-Al nuclei induced by the cavitation effects may be dissolved again when the ultrasonic vibration stops because the ultrasonic processing temperature is approximately 26 °C higher than the liquidus temperature of the filler metal. However, that is not the case for the joint ultrasonically brazed at 440 °C. It has been reported that cavitation and acoustic streaming effects caused by ultrasonic waves can enhance the diffusion of Al away from the substrate and into the brazing seam [9], which can increase the Al content and thus increase the liquidus temperature of the filler metal. At a brazing temperature of 440 °C, as shown in Fig. 6, the liquid filler metal is saturated with Al, and an increase in the liquidus temperature indicates increased undercooling. Therefore, when the joint is isothermally brazed at 440 °C, the ultrasound-aided nucleation process is accompanied by improved undercooling, and thus, the initial a-Al nuclei can survive in the melt. Consequently, due to the effects of cavitation-induced grain multiplication and cavitation-aided nucleation, the number of aAl nuclei surviving in the melt is higher in the joint ultrasonically brazed at 440 °C than that in the joint ultrasonically brazed at 470 °C and that in the joint brazed at 440 °C without ultrasonic vibration. Ultimately, the primary a-Al phase shows a refined spherical shape in the joint ultrasonically brazed at 440 °C and a nonuniform dendritic structure both in the joint ultrasonically brazed at 470 °C and in the joint brazed at 440 °C without ultrasonic vibration. Thus, it can be concluded that, compared with the joints brazed with semi-solid-state filler metal slurries [1,3,4], a refined microstructure can also be obtained in the joints ultrasonically brazed near the liquidus temperature of the filler metal. 3.4. Grain refinement model To further illustrate the formation mechanism of the refined, spherically shaped primary a-Al phase shown in the joint ultrasonically brazed at 440 °C, a model that summarizes the generation of primary a-Al phase is depicted in Fig. 9. The black regions represent the Al substrate, the dark gray regions represent the a-Al phase, and the light-gray regions represent the Zn–Al eutectic phase or the liquid phase. Fig. 9a describes the joint kept at 440 °C before ultrasonic vibration. Some unmelted a-Al particles located on the upper side of the seam can be seen. Fig. 9b shows the joint at the onset of ultrasonic vibration. The oxide film is disrupted, and the Al in the substrate begins to dissolve into the filler metal; the solid a-Al particles are dispersed homogeneously and partially broken, and ultrasound-induced nuclei can be observed. As the ultrasonic vibration continues, a large amount of a-Al nuclei dispersed throughout the entire brazing seam are generated, accompanying the enhanced dissolution of Al in the substrates into the Zn–Al filler metal (Fig. 9c). During the solidification process, these initial nuclei of the primary a-Al phase grow into spherical structures (Fig. 9d). 4. Conclusions In this study, 1060 Al alloy joints were fabricated with an ultrasound-assisted brazing method using a Zn–14Al hypereutectic filler metal. The effects of the brazing temperature on the bonding
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ratio, shear strength and microstructure of the joints were investigated, and the following conclusions were obtained: (1) For the joint ultrasonically brazed at 410 °C, the oxide films on the Al substrate surfaces were disrupted based on a mechanical mechanism. Cavities and discontinuous cracks were found in the joint, resulting in a low bonding ratio and reduced shear strength. (2) For the joints ultrasonically brazed at 440 °C and 470 °C, the oxide films on the Al substrate surfaces were disrupted by ultrasound-induced cavitation effects. Excellent bonding ratios of 98.9% and 97.7% were obtained, respectively. Shearing failure occurred in both base metals. (3) The primary a-Al phase showed a refined spherical shape in the joint ultrasonically brazed at 440 °C and showed a coarse and nonuniform dendritic structure both in the joint ultrasonically brazed at 470 °C and in the joint brazed at 440 °C without ultrasonic vibration. The refined spherical microstructure shown in the joint ultrasonically brazed at 440 °C was attributed to cavitation-aided grain refinement effects.
Acknowledgments This work was supported by the National Science Foundation of China (No. 51005057), the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No. AWPT-M09), and the National Research Foundation of Korea (NRF) grant funded by the Korea government (Grant No. NRF-2012038597). References [1] Shi L, Yan JC, Peng B, Han YF. Deformation behavior of semi-solid Zn–Al alloy filler metal during compression. Mater Sci Eng A 2011;528:7084–92. [2] Yan XQ, Liu SX, Long WM, Huang JL, Zhang LY, Chen Y. The effect of homogenization treatment on microstructure and properties of ZnAl15 solder. Mater Des 2013;45:440–5. [3] Xu HB, Luo QX, Zhou BF, Zeng YL, Du CH. The effect of stirring rate on semisolid stirring brazing of SiCp/A356 composites in air. Mater Des 2012;34:452–8. [4] Nagaoka T, Morisada Y, Fukusumi M, Takemoto T. Selection of soldering temperature for ultrasonic-assisted soldering of 5056 aluminum alloy using Zn–Al system solders. J Mater Process Technol 2011;211:1534–9. [5] Mendez PF, Rice CS, Brown SB. Joining using semisolid metals. Welding Res 2002;9:181–7. [6] Nagaoka T, Morisada Y, Fukusumi M, Takemoto T. Joint strength of aluminum ultrasonic soldered under liquidus temperature of Sn–Zn hypereutectic solder. J Mater Process Technol 2009;209:5054–9. [7] Xu ZW, Yan JC, Wu GH, Kong XL, Yang SQ. Interface structure of ultrasonic vibration aided interaction between Zn–Al alloy and Al2O3p/6061Al composite. Compos Sci Technol 2005;65:1959–63. [8] Ding M, Zhang PL, Zhang ZY, Yao S. Direct-soldering 6061 aluminum alloys with ultrasonic coating. Ultrason Sonochem 2010;17:292–7. [9] Li YX, Leng XS, Cheng S, Yan JC. Microstructure design and dissolution behavior between 2024 Al/Sn with the ultrasonic-associated soldering. Mater Des 2012;40:427–32. [10] Xu ZW, Yan JC, Zhang BY, Kong XL, Yang SQ. Behaviors of oxide film at the ultrasonic aided interaction interface of Zn–Al alloy and Al2O3p/6061Al composites in air. Mater Sci Eng A 2006;415(1–2):80–6. [11] Chinnama RK, Fauteuxa C, Neuenschwanderb J, Janczak-Ruscha J. Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidification. Acta Mater 2011;59(4):1474–81. [12] Das A, Kotadia HR. Effect of high-intensity ultrasonic irradiation on the modification of solidification microstructure in a Si-rich hypoeutectic Al–Si alloy. Mater Chem Phys 2011;125(3):853–9. [13] Ramirez A, Qian M, Davis B, Wilks T, StJohn DH. Potency of high-intensity ultrasonic treatment for grain refinement of magnesium alloys. Scr Mater 2008;59:19–22. [14] Atamanenko TV, Eskin DG, Zhang L, Katgerman L. Criteria of grain refinement induced by ultrasonic melt treatment of aluminum alloys containing Zr and Ti. Mater Trans A 2010;41:2056–66. [15] Nie KB, Wang XJ, Wu K, Hu XS, Zheng MY, Xu L. Microstructure and tensile properties of micro-SiC particles reinforced magnesium matrix composites produced by semisolid stirring assisted ultrasonic vibration. Mater Sci Eng A 2011;528:8709–14. [16] Qian M, Ramirez A, Das A, StJohn DH. The effect of solute on ultrasonic grain refinement of magnesium alloys. J Cryst Growth 2010;312:2267–72.
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[17] Aghayani MK, Niroumand B. Effects of ultrasonic treatment on microstructure and tensile strength of AZ91 magnesium alloy. J Alloys Compd 2011;509:114–22. [18] Jian X, Xu H, Meek TT, Han Q. Effect of power ultrasound on solidification of aluminum A356 alloy. Mater Lett 2005;59:190–3. [19] Kavian OC, Tahir IK, Gossett DO. Transient liquid phase diffusion bonding Al6061 using nano-dispersed Ni coatings. Mater Des 2012;33:469–75. [20] Kapustina OA. The physical principles of ultrasonic technology. Moscow: Nauka; 1970.
[21] Zhang SL, Zhao YT, Cheng XN, Chen G, Dai QX. High-energy ultrasonic field effects on the microstructure and mechanical behaviors of A356 alloy. J Alloys Compd 2009;470:168–72. [22] Murray JL. The Al Zn (aluminum–zinc) system. J Phase Equilib 1983;4:55–73. [23] Gao DM, Li ZJ, Han QY, Zhai QJ. Effect of ultrasonic power on microstructure and mechanical properties of AZ91 alloy. Mater Sci Eng A 2009;502:2–5. [24] Eskin GI. Ultrasonic treatment of light alloy melts. Amsterdam: Gordon and Breach; 1998.