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Effects of ultrasonic irradiation and cooling rate on the solidification microstructure of Sn–3.0Ag–0.5Cu alloy
Journal of Materials Processing Technology 214 (2014) 13–20
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Effects of ultrasonic irradiation and cooling rate on the solidification microstructure of Sn–3.0Ag–0.5Cu alloy Hongjun Ji a,b,∗ , Qiang Wang a , Mingyu Li a,b,∗ , Chunqing Wang b a Shenzhen Key Laboratory of Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, HIT Campus, Shenzhen University Town, Xili, Nanshan, Shenzhen 518055, PR China b State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, 92 Xidazhi Street, Harbin 150001, PR China
a r t i c l e
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Article history: Received 10 April 2013 Received in revised form 15 July 2013 Accepted 17 July 2013 Available online 27 July 2013 Keywords: Sn-based solder alloys Undercooling Solidification Microstructure Ultrasonic EBSD
a b s t r a c t A comparative study on the microstructures of Sn–Ag–Cu alloy ingots grown by ultrasound-assisted solidification was carried out with a specific focus on the limits on the ultrasonic processing depth and time imposed by the cooling rate during the melt solidification. During air-cooling, increasing the ultrasonic power reduced the undercooling temperature and increased the solidification time, leading to ˇ-Sn phase fragmentation from a dendritic shape into a circular equiaxed shape. The grain size was decreased from approximately 300 m to 20 m. When the cooling rate was increased from 4 ◦ C/s in air to 20 ◦ C/s in water, the macro-undercooling temperature was more greatly reduced by an increase in ultrasonic power, but the solidification time seemed to change only slightly because only a limited period for ultrasonic processing was permitted in the melt. Under both cooling rates, the microstructures were inhomogeneous along the processing depth. The functional depth and period for ultrasonic cavitation and acoustic steaming contributed to the differences in the solidification microstructures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The influence of ultrasonic fields on the dynamic solidification of melt metals and alloys has been widely studied. Irradiation with ultrasonic vibrations not only brings acoustic energy into the liquids but also introduces nonlinear effects, such as cavitation and acoustic streaming, which influence the microstructures of the solidified alloys and may improve their properties through grain size refinement, increased microstructure uniformity and degassing. Abramov (1987) introduced the use of high-intensity ultrasound during the solidification of steels and found structural changes arising from the nucleation and dispersion of the growing crystals that were ascribed to cavitation and acoustic flow. Suslick (1989) captured cavities that were enlarging and imploding in liquids; this behavior was associated with tremendous heat and specific chemical conditions. As an alternative to chemical approaches (alloying), such physical means of ultrasonic-assisted solidification have afforded tremendous success in the fields of academic research and practical production. Ultrasonic-assisted solidification has been widely used to fabricate high-performance metallic materials, especially aluminum alloys. Jian et al. (2005) studied the temperature conditions for
∗ Corresponding authors at: HIT Campus, Shenzhen University Town, Xili, Nanshan, Shenzhen 518055, PR China. Tel.: +86 755 26033463; fax: +86 755 26033463. E-mail addresses: [email protected] (H. Ji), [email protected] (M. Li). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.07.013
cavitation-induced heterogeneous nucleation in aluminum alloy A356 melt with ultrasonic vibrations. They verified that globular grains could be obtained as the temperature approached the material’s liquidus temperature under the condition that the cooling rate was high. Liu et al. (2008) certified that fine uniform microstructures in an AZ91 alloy were obtained by irradiating it with ultrasonic vibrations during its nucleation stage, but if the alloy was exposed to ultrasonic vibrations below the liquidus temperature, coarse dendrites would form. Kotadia et al. (2011) found that ternary Al–Sn–Cu alloys were significantly refined by ultrasonic irradiation during solidification, but outside the region of active cavitation, segregation would occur and the refinement was less efficient. Das and Kotadia (2011) investigated the effect of ultrasound irradiation on a high-Si-content Al–Si alloy and found that dendritic growth was suppressed and globular morphology of the ingot was observed for primary ␣-Al grains, but extremely fine and short Si morphology was controlled primarily by the rapid cooling rate rather than the ultrasonic specifications. From these results, it was concluded that cooling rate and ultrasonic irradiation must both be controlled to obtain desirable ingots. Mechanisms for microstructure refinement under ultrasonic irradiation were proposed. Eskin (1994) added insoluble impurities into light alloys to provide active solidification nuclei through the acoustic generation of cavitation bubbles and obtained extremely refined ingots. Abramov et al. (1998) studied the effect of ultrasonic treatment on the microstructures and properties of commercial Al-based alloys and confirmed its positive effects: reduction of
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mean grain size, variation of phase distribution and better material homogeneity and segregation control. Under the influence of ultrasonic vibrations, cavitation producing instantaneous, large pressure and temperature fluctuations resulted in an increase of nucleation (homogeneous and heterogeneous) in the melt. Li et al. (2008) ultrasonically treated an Al–Ti–C alloy to disperse the particles of TiC and TiAl3 uniformly. Acoustic streaming facilitated dendrite fragmentation, particle transportation and an equalized temperature field. These effects refined the grains tremendously. It must be noted that the intensity of the ultrasonic energy is limited, and its functional distance and period in the melt are affected by the cooling rate. Ultrasonic waves are always reflected and absorbed at the solid–liquid interfaces. The fraction of the solid phase in the melt significantly affects the propagation of ultrasonic waves. Kim et al. (2002) studied the effect of cooling rate on the microstructure and tensile properties of Sn–Ag–Cu. A lower rate resulted in larger primary Ag3 Sn platelets and lesser tensile strength. Pereira et al. (2013) examined the effects of the Ag content and cooling rate on the microstructures and mechanical properties of Sn–Ag–Cu alloys. The morphology of the intermetallic compounds (IMCs) Cu6 Sn5 and Ag3 Sn depended on the cooling rate, which affected the dendritic arms and hardness. The solid fraction in the melt is determined by its cooling rate. This important factor has been neglected during ultrasonic-assisted solidification and should be carefully considered, as a high cooling rate is helpful for grain size refinement. In this study, the effects of both ultrasonic intensity and cooling rate on microstructures were systematically investigated based on a widely used lead-free solder alloy.
2. Experimental The Sn–3.0Ag–0.5Cu (wt.%, SAC305) solder alloy was melted in an alumina crucible (inner height 25 mm and diameter 20 mm) at 300 ◦ C. To compare the microstructures and properties affected by the fraction of solid phases in the melt (mainly IMCs in SAC305), pure Sn was investigated, and partial results are shown. Except where mentioned below, the default material used was SAC305. Ultrasonic vibrations with 28 kHz frequency and 15 m amplitude were irradiated through a standard horn made of Ti–6Al–4V alloys with a diameter of 3 mm, as schematically shown in Fig. 1a. The horn was immerged to a depth of 2 mm below the melt surface. A K-type NiCr/NiAl thermocouple wire with a diameter of 100 m, covered with duplex-insulated Teflon, was placed near the near middle of the melt just below the submerged ultrasonic horn to record the cooling curves during solidification, as indicated by ‘TC’ in Fig. 1b. To compare the effects of the cooling rates on the ultrasonic processing depth and period, the melt alloy was cooled from 300 ◦ C at two different rates, 4 ◦ C/s (air cooling) and 20 ◦ C/s (water cooling). Once the temperature reached 240 ◦ C, ultrasonic vibrations were applied until the melt alloy solidified. To prevent any differences caused by the presence of the ultrasonic horn and the thermocouple cooling effect, they were immersed in the melt at all times. Three levels of ultrasonic power, 0 W, 67 W and 200 W (corresponding to ultrasonic densities of 0, 0.21 and 0.66 W/mm2 ) were investigated in separate experiments. Each experiment was repeated at least three times to ensure the reproducibility of the obtained data. The solidified samples were sectioned along the central vertical planes, ground, polished, and etched using standard metallographic techniques. The microstructures were analyzed with a VMM200 optical microscope and a HITACHI S-4700 Scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) detector. The scanning acceleration voltage was 15 kV. The
Fig. 1. Schematic of the experimental apparatus. (a) Application of ultrasonic vibrations and (b) zones formed after solidification of the melt alloy. “TC” shows the location of a thermocouple placed to monitor the cooling curves.
step size for the EBSD analysis was 0.5–2 m depending on the grain size of the analyzed samples. The grain size was calculated by both the equivalent circle diameter (ECD) and the linear intercept (LI) from the EBSD data. The misorientation of the grain boundaries was also measured. The hardness was tested on an HVS-1000. During the hardness test, 3 samples from each production condition were selected, and 9 indentations on each sample were carried out; the results were averaged to obtain one data point. 3. Results 3.1. Undercooling and solidification time The ideal liquidus (Tl ) and eutectic (Te ) temperatures for SAC305 alloys are 220 ◦ C and 217 ◦ C, respectively, and solidification may take place at some point between these two temperatures depending on the undercooling temperature, as reported by Zeng and Tu (2002) and Sundelin et al. (2006). The cooling curves for both airand water-cooling were carefully monitored and are presented in Fig. 2. Te represents the eutectic temperature of SAC305 in its equilibrium state. In practice, undercooling must exist to ensure that solidification occurs. For the air-cooled samples, with the increase of ultrasonic power from 0 W to 267 W, the liquidus temperature increased from 214.5 ◦ C to 218.1 ◦ C, indicating that the macro-undercooling temperature for Tl was reduced by 3.6 ◦ C from 5.5 ◦ C to 1.9 ◦ C. The plateau temperature at which the eutectic composition
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ultrasonic energy input and resulted in the microstructure variation of the solidified samples. 3.2. Microstructures of the solidified samples
Fig. 2. Cooling curves of air- and water-cooled samples treated with various levels of ultrasonic power.
started solidification increased from 213.9 ◦ C to 216.1 ◦ C, so the macro-undercooling temperature for Te was reduced by 2.2 ◦ C, from 3.1 ◦ C to 0.9 ◦ C. The solidification time was increased by 10 s. For the water-cooled samples, the cooling rate was so rapid that only one plateau temperature was detected. In comparison with the air-cooled samples that were not treated with ultrasonic vibrations, the liquidus temperature decreased to 211.0 ◦ C. With ultrasonic vibrations at 200 W, it increased to 216.4 ◦ C. The macro-undercooling was reduced by 5.4 ◦ C. The solidification time changed little. The application of ultrasonic vibrations reduced the macro-undercooling temperature and delayed the solidification process to some degree. These phenomena were ascribed to the
Fig. 3 shows the original microstructures of the air- and watercooled samples that were not subjected to ultrasonic vibrations. The dendritic ˇ-Sn phase and fine eutectic networks were clearly apparent. The needle-like Ag3 Sn phase was coarsened and transformed into a flake-like structure as seen in Fig. 3c. In the case of the water-cooled samples, the width of the ˇ-Sn phase cells was much narrower (their diameters were nearly 1/5 to 1/10 of those in the air-cooled samples), and the area fraction of the eutectic phases seemed to be larger. Fig. 3d shows that the tiny needlelike Ag3 Sn phase remained because the rapid cooling resulted in a lower energy and a shorter time for its nucleation and growth. Fig. 4 shows images of the microstructures of the samples in the zones labeled in Fig. 1b. These images were used to investigate the uniformity of the ingots. For the air-cooled samples, when the sample was treated with an ultrasonic power of only 67 W (Fig. 4a), the resultant microstructures were quite similar to those in the corresponding zones in the sample treated by ultrasonic vibrations of 200 W. The exception was the existence of the conventional dendritic ˇ-Sn structures surrounded by fine eutectic phases in zone IV (Fig. 4aIV ). This region seemed not to be influenced by the lower ultrasonic power, suggesting that the processing depth was limited to within a certain distance. Under 200 W ultrasonic vibrations, as seen in Fig. 4b, much larger circular Sn cells totally replaced the conventional dendritic structures seen in Fig. 3a. The sizes of these cells seemed to differ little throughout the whole cross section of the sample, indicating that the microstructures were more uniform.
Fig. 3. Original microstructures of the SAC305 samples produed without ultrasonic vibrations. (a) Air-cooled and (b) water-cooled. (c) and (d) are corresponding magnified images, respectively.
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Fig. 4. Optical micrographs of each zone in the solidified SAC305 samples treated with different procedures. The sub-numbers correspond to the zones shown in Fig. 1b. The magnification of each is indicated by the scale bar in cIV . (a) Air-cooled at 67 W, (b) air-cooled at 200 W, and (c) water-cooled at 200 W.
The aspect ratio of Ag3 Sn and the volume fraction of the eutectic phase clearly varied. In zones I (Fig. 4bI ) and II (Fig. 4bII ), primary needle-like Ag3 Sn and Cu6 Sn5 fragments were distributed both in and around the Sn-rich phase (cells). In zones III (Fig. 4bIII ) and IV (Fig. 4bIV ), the number of primary needle-like Ag3 Sn fragments decreased considerably, with a morphology like bent dashed lines. The eutectic phase existed with morphologies similar to river deltas and concentric circles. In the midst of those concentric circles, one or several Sn phase cells were usually found. As for the water-cooled samples treated with 200 W ultrasonic vibrations (Fig. 4c), their microstructures were quite different. In zones I (Fig. 4cI ) and II (Fig. 4cII ), very little dendritic ˇ-Sn was found, and the Sn phase appeared as discontinuous cobblestones. The number of these Sn cobblestones was lesser in zone I. Spherical eutectic phases surrounding Sn-rich cells were identified, as in the insert image shown in Fig. 4cI . The central circular phase was a ˇSn cell with eutectic phases around it. The detailed structures of such Sn cells are depicted in Fig. 5. The eutectic phase in zone II had a larger volume fraction than that in zone I. In zone III (Fig. 4cIII ), the ˇ-Sn phase changed from a dendrite structure to an equiaxed
structure, and the widths of both the first and second dendritic arms were larger than those in zone IV (Fig. 4cIV ). In zone IV, the structure was the same as in the conventional case illustrated in Fig. 3b. When no ultrasonic vibrations were applied, the grain size was larger than 300 m measured by both the ECD and the LI (Fig. 5a and b). With ultrasonic vibrations, one single Sn-rich cell shown in Fig. 4 consisted of several much smaller Sn grains with diameters of approximately 20 m measured by the ECD and 23 m measured by the LI (Fig. 5c and d). The misorientation of their grain boundaries was measured along the black line indicated in Fig. 5d. As shown by the red profile in Fig. 6, the misorientation among these grains in one Sn-rich cell was greater than 40◦ . Fig. 7 shows the polarized optical microstructures of the aircooled SAC305 and pure Sn ingots treated with 200 W ultrasonic vibrations. At the same power density, the grain refinement for SAC305 was deeper than that for pure Sn. The grain size was approximately 70 m measured by the ECD and 75 m by the LI in the former case, but it was approximately 160 m measured by the ECD and 155 m by the LI in the latter. The composition of the ingot or the exact precipitated/constituent phase may have
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Fig. 5. SEM micrographs and the corresponding EBSD inverse pole figure (IPF) of the air-cooled samples. (a and b) SAC305 produced without ultrasonci treatment, (c and d) SAC305 treated with 200 W ultrasonic vibrations, and (e) pure Sn produced without ultrasonic treatment.
considerable influence on the refinement mechanism, aside from the effects of any additional elements. 3.3. Mechanical properties and storage test The grain size and IMC morphologies of the solidifying ingots were changed by exposure to field of ultrasonic vibrations. Microstructural changes affect the mechanical properties of the solidified ingots. Fig. 8 shows the hardness profiles as a function of ultrasonic power for both SAC305 and pure Sn ingots, respectively, that were solidified under air-cooling. Opposite trends appear in the two cases. For pure Sn, the hardness increased from 7.7 HV
in the sample that was not treated with ultrasonic vibrations to 8.4 HV in the sample exposed to 133 W, after which it reached a plateau. This effect can probably be attributed to the grain refinement when the ultrasonic power was lower. Above 133 W, the effects of increased ultrasonic vibrations on the grain refinement were on longer significant, resulting in little variation in hardness. For SAC305, the hardness decreased with the increase of ultrasonic power. The effect of the fine-grain strengthening was minimal because it was irrelevant in the second-phase dispersion strengthening (mainly due to the segregation of Ag3 Sn from the Sn-rich phase) according to Miyazawa and Ariga (1999). As seen in Fig. 9, the IMCs, including Cu6 Sn5 and Ag3 Sn, precipitated mainly
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Fig. 6. Misorientation profile of grains measured along the dark line in Fig. 5d.
along the grain/cell boundaries of the Sn-rich phases. The leakage of the IMCs from the inner grains softened the ingot. These refined microstructures can be helpful for preventing electromigration and for obtaining an anisotropic grain (making the whole joint more isotropic), resulting in a more stable microstructure. The images shown in Fig. 10 verified that the microstructures changed little when the samples were observed in situ after being
Fig. 7. Polarized optical images of the samples treated with 200 W ultrasonic vibrations during air cooling. (a) Pure Sn and (b) SAC305.
Fig. 8. Hardness as a function of ultrasonic power for SAC305 and pure Sn solidified under air cooling.
stored at 150 ◦ C for 216 h. Both the Sn cells, marked by dashed circles, and the grains, marked by numbers, underwent little evolution. The application of ultrasound to a solidifying solder joint may be beneficial because any unwanted oxide inter-layer might be disrupted and the grains may be refined.
Fig. 9. Segregation of the IMCs along the grain/cell boundaries of the SAC305 in zone II. (a) Treated with 133 W and (b) treated with 200 W ultrasonic vibrations during air cooling.
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provide a unique means for driving chemical reactions under extreme conditions. Homogeneous nucleation may occur. However, if those homogeneously formed nuclei were surrounded by overheating melt, they would re-melt. Heterogeneous nucleation occurs much more easily when cavitation takes place near solid/liquid interfaces, which can be attributed to the grain refinement. Based on the model of cavitation nuclei in the form of hydrophobic solid particles provided by Frenkel and Harvey in the literature written by Eskin (1998), at the defects in IMCs, cavitation-induced nuclei can remain stable at temperatures above equilibrium and survive to initiate further nucleation and growth in the subsequent cooling process. Due to higher local undercooling, solidification can happen at higher temperatures than the conventional case, and macro-undercooling decreases as a consequence, as shown in Fig. 2. This result is in accordance with a study by Ramirez et al. (2008). The energy injection into the ingot by ultrasonic vibrations was another reason that the solidification time increased. Acoustic streaming could have equalized both the temperature and concentration fields, contributing to the time increase. Based on the Sn–Ag–Cu phase diagram, the solidification sequence can be written as: L → L + ˇ-Sn
(2)
L + ˇ-Sn → L + ˇ-Sn + Ag3 Sn
(3)
L + ˇ-Sn + Ag3 Sn → ˇ-Sn + Ag3 Sn + E(ˇ-Sn + Ag3 Sn + Cu6 Sn5 ) (4)
Fig. 10. Microstructures of a Sn cell in the SAC305 sample treated with 200 W ultrasonic vibrations during air cooling. (a) As-obtained and (b) in situ after being stored at 150 ◦ C for 216 h.
4. Discussion Ultrasonic cavitation and acoustic streaming are the two main effects of treating alloys with ultrasonic vibrations during solidification, and they are regarded as the mechanisms by which such treatment can refine microstructures of the solidified alloys. In liquids, acoustic cavitation generates a large number of tiny bubbles that undergo formation, growth and collapse, providing sonochemical effects. Great localized pressure, intense local heating and very short life-times during cavitation are produced by the collapse of these bubbles, according to the Clausius–Clapeyron equation: Tm =
Tm P V H
(1)
The great increase of localized P and V provided by ultrasonic cavitation increases the localized melting point in the vicinity of cavitation bubbles, which is equivalent to raising the localized undercooling. Flint and Suslick (1991) studied the cavitation temperature during the implosive collapse of the bubbles (the transient hot spots) produced by ultrasonic irradiation of liquids. By capturing sonoluminescence spectra from silicone oil, they determined that the effective cavitation temperature was approximately 5000 K. Suslick et al. (1999) reported that the collapse of bubbles in liquids resulted in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into the heating of the contents of the bubbles. Such high local temperatures and pressures combined with extraordinarily rapid cooling
Moon et al. (2000) and Kang et al. (2003) reported that Ag3 Sn plates nucleated and grew with minimal undercooling, while the ˇ-Sn required much higher undercooling to initiate nucleation. The energies for Cu6 Sn5 and Ag3 Sn coarsening were approximately 82 kJ/mol and 38 kJ/mol, respectively, which were both lower than that of ˇ-Sn, reported to be almost 107 kJ/mol by Allen et al. (2011). IMCs were much easier to form. These Cu6 Sn5 and Ag3 Sn IMCs can be broken by the huge impulse forces caused by cavitation when the liquid streams rushed toward the solid faces at high speeds. These IMCs with surface defects significantly contributed to the Sn phase nucleation, resulting in the grain refinement in a single Sn cell shown in Fig. 5. Any ˇ-Sn dendritic arms that were formed would be fragmented, and circular Sn cells were produced, similar to the reports by Campbell (1981) and Ochoa et al. (2003). Relatively far from the ultrasonic horn, the cavitation intensity decreased because the acoustic streaming became dominant, and this effect symmetrized both the temperature and concentration gradients. The rest of the liquid alloy solidified under equilibriumlike conditions, as shown in Fig. 4aIII , bIII and bIV . The rapid cooling rates left only a very short time for the ultrasonic processing. Beyond the ultrasonic horn, the microstructures were the same as in the conventional case (Fig. 4cIV ). The acoustic streaming helped to change ˇ-Sn phases from a dendritic structure to an equiaxed structure and allowed the Sn cells to grow much larger by equalizing the temperature and concentration gradients (Fig. 4cIII and cII ). The cavitation influences were confined within a much smaller region, where cobblestone-shaped Sn grains developed (Fig. 4cI ). Because parts of the eutectic were pushed from the periphery to the top middle and solidified after the solidification of the primary ˇ-Sn, these cobblestone-shaped grains were surrounded by eutectic. Acoustic streaming ensured the symmetrical nucleation and growth of the eutectic phases (around the Sn cells, as shown in the inset image in Fig. 4cI ). The high cavitation intensity and rapid cooling rate suppressed the overgrowth of intermetallics.
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5. Conclusions (a) The ultrasonic processing depth and period were limited by the cooling rate. The undercooling temperature was reduced significantly. (b) Ultrasonic vibrations can refine ˇ-Sn grains from larger than 300 m to 20 m. The grain refinement for SAC305 was deeper than that for pure Sn. Inhomogeneous nucleation at the defects on IMCs was probably the cause of this result. Intermetallics were greatly suppressed, but they may segregate out of the grain interiors. (c) The segregation of these IMCs was tremendous in the SAC305 ingots and mainly distributed along the Sn phase boundaries, resulting in some decrease of the alloy’s hardness. (d) Simultaneously optimizing the ultrasonic power and the cooling rate may both refine the grains and homogenize the microstructures of a solidified alloy. Acknowledgements The work was supported by the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No. AWPT-M09), the HIT.NSFIR.2010135, the Natural Science Foundation of China (NSFC 51005057), the SRF for RoCS, SEM, and the projects funded by the Shenzhen government No. CXB201105100075A and No. KQC201105300005A. References Abramov, O.V., 1987. Action of high intensity ultrasound on solidifying metal. Ultrasonics 25, 73–82. Abramov, V., Abramov, O., Bulgakov, V., Sommer, F., 1998. Solidification of aluminium alloys under ultrasonic irradiation using water-cooled resonator. Materials Letters 37, 27–34. Allen, S.L., Notis, M.R., Chromik, R.R., et al., 2011. Microstructural evolution in leadfree solder alloys: Part II. Directionally solidified Sn–Ag–Cu, Sn–Cu and Sn–Ag. Journal of Materials Research 19, 1425–1431. Campbell, J., 1981. Effect of vibration during solidification. International Metals Reviews 26, 71–108. Das, A., Kotadia, H.R., 2011. Effect of high-intensity ultrasonic irradiation on the modification of solidification microstructure in a Si-rich hypoeutectic Al–Si alloy. Materials Chemistry and Physics 125, 853–859.
Eskin, G.I., 1994. Influence of cavitation treatment of melts on the processes of nucleation and growth of crystals during solidification of ingots and casting from light alloys. Ultrasonics Sonochemistry 1, 59–63. Eskin, G.I., 1998. Ultrasonic Treatment of Light Alloy Melts. Gordon and Breach, Amsterdam. Flint, E.B., Suslick, K.S., 1991. The temperature of cavitation. Science 253, 1397–1399. Jian, X., Xu, H., Meek, T.T., Han, Q., 2005. Effect of power ultrasound on solidification of aluminum A356 alloy. Materials Letters 59, 190–193. Kim, K.S., Huh, S.H., Suganuma, K., 2002. Effects of cooling rate on microstructure and tensile properties of Sn–Ag–Cu alloys. Materials Science and Engineering A 333, 106–114. Kang, S.K., Choi, W.K., et al., 2003. Ag3 Sn plate formation in the solidification of near-ternary eutectic Sn–Ag–Cu. Research Summary Lead-Free Solder, 61–65. Kotadia, H.R., Das, A., Doernberg, E., Schmid-Fetzer, R., 2011. A comparative study of ternary Al–Sn–Cu immiscible alloys prepared by conventional casting and casting under high-intensity ultrasonic irradiation. Materials Chemistry and Physics 131, 241–249. Liu, X., Osawa, Y., Takamori, S., Mukai, T., 2008. Grain refinement of AZ91 alloy by introducing ultrasonic vibration during solidification. Materials Letters 62, 2872–2875. Li, Y.L., Feng, H.K., Cao, F.R., Chen, Y.B., Gong, L.Y., 2008. Effect of high density ultrasonic on the microstructure and refining property of Al–5Ti–0.25C grain refiner alloy. Materials Science and Engineering A 487, 518–523. Miyazawa, Y., Ariga, T., 1999. Microstructural change and hardness of lead free solder alloys. Proceedings First International Symposium on Environmentally Conscious Design and Inverse Manufacturing, 616–691. Moon, K.W., Boettinger, W.J., Kattner, U.R., 2000. Experimental and thermodynamic assessment of Sn–Ag–Cu solder alloys. Journal of Electronic Materials 29, 1122–1136. Ochoa, F., Williams, J.J., Chawla, N., 2003. Effects of cooling rate on the microstructure and tensile behavior of a Sn–3.5wt.%Ag solder. Journal of Electronic Materials 32, 1414–1420. Pereira, P.D., Spinelli, J.E., Garcia, A., 2013. Combined effects of Ag content and cooling rate on microstructure and mechanical behavior of Sn–Ag–Cu. Materials & Design 45, 377–383. Ramirez, A., Qian, M., Davis, B., Wilks, T., StJohn, D.H., 2008. Potency of high-intensity ultrasonic treatment for grain refinement of magnesium alloys. Scripta Materialia 59, 19–22. Suslick, K.S., 1989. The chemical effects of ultrasound. Scientific American 260, 80–88. Suslick, K.S., Didenko, Y., Fang, M.M., Hyeon, T., et al., 1999. Acoustic cavitation and its chemical consequences. Philosophical Transactions of the Royal Society London Series A 357, 335–353. Sundelin, J.J., Nurmi, S.T., Lepistö, T.K., Ristolainen, E.O., 2006. Mechanical and microstructural properties of SnAgCu solder joints. Materials Science and Engineering A 420, 55–62. Zeng, K., Tu, K.N., 2002. Six cases of reliability study of Pb-free solder joints in electronic packaging technology. Materials Science and Engineering R 38, 55–105.
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Effects of ultrasonic irradiation and cooling rate on the solidification microstructure of Sn–3.0Ag–0.5Cu alloy Hongjun Ji a,b,∗ , Qiang Wang a , Mingyu Li a,b,∗ , Chunqing Wang b a Shenzhen Key Laboratory of Advanced Materials, Shenzhen Graduate School, Harbin Institute of Technology, HIT Campus, Shenzhen University Town, Xili, Nanshan, Shenzhen 518055, PR China b State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, 92 Xidazhi Street, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 10 April 2013 Received in revised form 15 July 2013 Accepted 17 July 2013 Available online 27 July 2013 Keywords: Sn-based solder alloys Undercooling Solidification Microstructure Ultrasonic EBSD
a b s t r a c t A comparative study on the microstructures of Sn–Ag–Cu alloy ingots grown by ultrasound-assisted solidification was carried out with a specific focus on the limits on the ultrasonic processing depth and time imposed by the cooling rate during the melt solidification. During air-cooling, increasing the ultrasonic power reduced the undercooling temperature and increased the solidification time, leading to ˇ-Sn phase fragmentation from a dendritic shape into a circular equiaxed shape. The grain size was decreased from approximately 300 m to 20 m. When the cooling rate was increased from 4 ◦ C/s in air to 20 ◦ C/s in water, the macro-undercooling temperature was more greatly reduced by an increase in ultrasonic power, but the solidification time seemed to change only slightly because only a limited period for ultrasonic processing was permitted in the melt. Under both cooling rates, the microstructures were inhomogeneous along the processing depth. The functional depth and period for ultrasonic cavitation and acoustic steaming contributed to the differences in the solidification microstructures. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The influence of ultrasonic fields on the dynamic solidification of melt metals and alloys has been widely studied. Irradiation with ultrasonic vibrations not only brings acoustic energy into the liquids but also introduces nonlinear effects, such as cavitation and acoustic streaming, which influence the microstructures of the solidified alloys and may improve their properties through grain size refinement, increased microstructure uniformity and degassing. Abramov (1987) introduced the use of high-intensity ultrasound during the solidification of steels and found structural changes arising from the nucleation and dispersion of the growing crystals that were ascribed to cavitation and acoustic flow. Suslick (1989) captured cavities that were enlarging and imploding in liquids; this behavior was associated with tremendous heat and specific chemical conditions. As an alternative to chemical approaches (alloying), such physical means of ultrasonic-assisted solidification have afforded tremendous success in the fields of academic research and practical production. Ultrasonic-assisted solidification has been widely used to fabricate high-performance metallic materials, especially aluminum alloys. Jian et al. (2005) studied the temperature conditions for
∗ Corresponding authors at: HIT Campus, Shenzhen University Town, Xili, Nanshan, Shenzhen 518055, PR China. Tel.: +86 755 26033463; fax: +86 755 26033463. E-mail addresses: [email protected] (H. Ji), [email protected] (M. Li). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.07.013
cavitation-induced heterogeneous nucleation in aluminum alloy A356 melt with ultrasonic vibrations. They verified that globular grains could be obtained as the temperature approached the material’s liquidus temperature under the condition that the cooling rate was high. Liu et al. (2008) certified that fine uniform microstructures in an AZ91 alloy were obtained by irradiating it with ultrasonic vibrations during its nucleation stage, but if the alloy was exposed to ultrasonic vibrations below the liquidus temperature, coarse dendrites would form. Kotadia et al. (2011) found that ternary Al–Sn–Cu alloys were significantly refined by ultrasonic irradiation during solidification, but outside the region of active cavitation, segregation would occur and the refinement was less efficient. Das and Kotadia (2011) investigated the effect of ultrasound irradiation on a high-Si-content Al–Si alloy and found that dendritic growth was suppressed and globular morphology of the ingot was observed for primary ␣-Al grains, but extremely fine and short Si morphology was controlled primarily by the rapid cooling rate rather than the ultrasonic specifications. From these results, it was concluded that cooling rate and ultrasonic irradiation must both be controlled to obtain desirable ingots. Mechanisms for microstructure refinement under ultrasonic irradiation were proposed. Eskin (1994) added insoluble impurities into light alloys to provide active solidification nuclei through the acoustic generation of cavitation bubbles and obtained extremely refined ingots. Abramov et al. (1998) studied the effect of ultrasonic treatment on the microstructures and properties of commercial Al-based alloys and confirmed its positive effects: reduction of
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mean grain size, variation of phase distribution and better material homogeneity and segregation control. Under the influence of ultrasonic vibrations, cavitation producing instantaneous, large pressure and temperature fluctuations resulted in an increase of nucleation (homogeneous and heterogeneous) in the melt. Li et al. (2008) ultrasonically treated an Al–Ti–C alloy to disperse the particles of TiC and TiAl3 uniformly. Acoustic streaming facilitated dendrite fragmentation, particle transportation and an equalized temperature field. These effects refined the grains tremendously. It must be noted that the intensity of the ultrasonic energy is limited, and its functional distance and period in the melt are affected by the cooling rate. Ultrasonic waves are always reflected and absorbed at the solid–liquid interfaces. The fraction of the solid phase in the melt significantly affects the propagation of ultrasonic waves. Kim et al. (2002) studied the effect of cooling rate on the microstructure and tensile properties of Sn–Ag–Cu. A lower rate resulted in larger primary Ag3 Sn platelets and lesser tensile strength. Pereira et al. (2013) examined the effects of the Ag content and cooling rate on the microstructures and mechanical properties of Sn–Ag–Cu alloys. The morphology of the intermetallic compounds (IMCs) Cu6 Sn5 and Ag3 Sn depended on the cooling rate, which affected the dendritic arms and hardness. The solid fraction in the melt is determined by its cooling rate. This important factor has been neglected during ultrasonic-assisted solidification and should be carefully considered, as a high cooling rate is helpful for grain size refinement. In this study, the effects of both ultrasonic intensity and cooling rate on microstructures were systematically investigated based on a widely used lead-free solder alloy.
2. Experimental The Sn–3.0Ag–0.5Cu (wt.%, SAC305) solder alloy was melted in an alumina crucible (inner height 25 mm and diameter 20 mm) at 300 ◦ C. To compare the microstructures and properties affected by the fraction of solid phases in the melt (mainly IMCs in SAC305), pure Sn was investigated, and partial results are shown. Except where mentioned below, the default material used was SAC305. Ultrasonic vibrations with 28 kHz frequency and 15 m amplitude were irradiated through a standard horn made of Ti–6Al–4V alloys with a diameter of 3 mm, as schematically shown in Fig. 1a. The horn was immerged to a depth of 2 mm below the melt surface. A K-type NiCr/NiAl thermocouple wire with a diameter of 100 m, covered with duplex-insulated Teflon, was placed near the near middle of the melt just below the submerged ultrasonic horn to record the cooling curves during solidification, as indicated by ‘TC’ in Fig. 1b. To compare the effects of the cooling rates on the ultrasonic processing depth and period, the melt alloy was cooled from 300 ◦ C at two different rates, 4 ◦ C/s (air cooling) and 20 ◦ C/s (water cooling). Once the temperature reached 240 ◦ C, ultrasonic vibrations were applied until the melt alloy solidified. To prevent any differences caused by the presence of the ultrasonic horn and the thermocouple cooling effect, they were immersed in the melt at all times. Three levels of ultrasonic power, 0 W, 67 W and 200 W (corresponding to ultrasonic densities of 0, 0.21 and 0.66 W/mm2 ) were investigated in separate experiments. Each experiment was repeated at least three times to ensure the reproducibility of the obtained data. The solidified samples were sectioned along the central vertical planes, ground, polished, and etched using standard metallographic techniques. The microstructures were analyzed with a VMM200 optical microscope and a HITACHI S-4700 Scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) detector. The scanning acceleration voltage was 15 kV. The
Fig. 1. Schematic of the experimental apparatus. (a) Application of ultrasonic vibrations and (b) zones formed after solidification of the melt alloy. “TC” shows the location of a thermocouple placed to monitor the cooling curves.
step size for the EBSD analysis was 0.5–2 m depending on the grain size of the analyzed samples. The grain size was calculated by both the equivalent circle diameter (ECD) and the linear intercept (LI) from the EBSD data. The misorientation of the grain boundaries was also measured. The hardness was tested on an HVS-1000. During the hardness test, 3 samples from each production condition were selected, and 9 indentations on each sample were carried out; the results were averaged to obtain one data point. 3. Results 3.1. Undercooling and solidification time The ideal liquidus (Tl ) and eutectic (Te ) temperatures for SAC305 alloys are 220 ◦ C and 217 ◦ C, respectively, and solidification may take place at some point between these two temperatures depending on the undercooling temperature, as reported by Zeng and Tu (2002) and Sundelin et al. (2006). The cooling curves for both airand water-cooling were carefully monitored and are presented in Fig. 2. Te represents the eutectic temperature of SAC305 in its equilibrium state. In practice, undercooling must exist to ensure that solidification occurs. For the air-cooled samples, with the increase of ultrasonic power from 0 W to 267 W, the liquidus temperature increased from 214.5 ◦ C to 218.1 ◦ C, indicating that the macro-undercooling temperature for Tl was reduced by 3.6 ◦ C from 5.5 ◦ C to 1.9 ◦ C. The plateau temperature at which the eutectic composition
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ultrasonic energy input and resulted in the microstructure variation of the solidified samples. 3.2. Microstructures of the solidified samples
Fig. 2. Cooling curves of air- and water-cooled samples treated with various levels of ultrasonic power.
started solidification increased from 213.9 ◦ C to 216.1 ◦ C, so the macro-undercooling temperature for Te was reduced by 2.2 ◦ C, from 3.1 ◦ C to 0.9 ◦ C. The solidification time was increased by 10 s. For the water-cooled samples, the cooling rate was so rapid that only one plateau temperature was detected. In comparison with the air-cooled samples that were not treated with ultrasonic vibrations, the liquidus temperature decreased to 211.0 ◦ C. With ultrasonic vibrations at 200 W, it increased to 216.4 ◦ C. The macro-undercooling was reduced by 5.4 ◦ C. The solidification time changed little. The application of ultrasonic vibrations reduced the macro-undercooling temperature and delayed the solidification process to some degree. These phenomena were ascribed to the
Fig. 3 shows the original microstructures of the air- and watercooled samples that were not subjected to ultrasonic vibrations. The dendritic ˇ-Sn phase and fine eutectic networks were clearly apparent. The needle-like Ag3 Sn phase was coarsened and transformed into a flake-like structure as seen in Fig. 3c. In the case of the water-cooled samples, the width of the ˇ-Sn phase cells was much narrower (their diameters were nearly 1/5 to 1/10 of those in the air-cooled samples), and the area fraction of the eutectic phases seemed to be larger. Fig. 3d shows that the tiny needlelike Ag3 Sn phase remained because the rapid cooling resulted in a lower energy and a shorter time for its nucleation and growth. Fig. 4 shows images of the microstructures of the samples in the zones labeled in Fig. 1b. These images were used to investigate the uniformity of the ingots. For the air-cooled samples, when the sample was treated with an ultrasonic power of only 67 W (Fig. 4a), the resultant microstructures were quite similar to those in the corresponding zones in the sample treated by ultrasonic vibrations of 200 W. The exception was the existence of the conventional dendritic ˇ-Sn structures surrounded by fine eutectic phases in zone IV (Fig. 4aIV ). This region seemed not to be influenced by the lower ultrasonic power, suggesting that the processing depth was limited to within a certain distance. Under 200 W ultrasonic vibrations, as seen in Fig. 4b, much larger circular Sn cells totally replaced the conventional dendritic structures seen in Fig. 3a. The sizes of these cells seemed to differ little throughout the whole cross section of the sample, indicating that the microstructures were more uniform.
Fig. 3. Original microstructures of the SAC305 samples produed without ultrasonic vibrations. (a) Air-cooled and (b) water-cooled. (c) and (d) are corresponding magnified images, respectively.
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Fig. 4. Optical micrographs of each zone in the solidified SAC305 samples treated with different procedures. The sub-numbers correspond to the zones shown in Fig. 1b. The magnification of each is indicated by the scale bar in cIV . (a) Air-cooled at 67 W, (b) air-cooled at 200 W, and (c) water-cooled at 200 W.
The aspect ratio of Ag3 Sn and the volume fraction of the eutectic phase clearly varied. In zones I (Fig. 4bI ) and II (Fig. 4bII ), primary needle-like Ag3 Sn and Cu6 Sn5 fragments were distributed both in and around the Sn-rich phase (cells). In zones III (Fig. 4bIII ) and IV (Fig. 4bIV ), the number of primary needle-like Ag3 Sn fragments decreased considerably, with a morphology like bent dashed lines. The eutectic phase existed with morphologies similar to river deltas and concentric circles. In the midst of those concentric circles, one or several Sn phase cells were usually found. As for the water-cooled samples treated with 200 W ultrasonic vibrations (Fig. 4c), their microstructures were quite different. In zones I (Fig. 4cI ) and II (Fig. 4cII ), very little dendritic ˇ-Sn was found, and the Sn phase appeared as discontinuous cobblestones. The number of these Sn cobblestones was lesser in zone I. Spherical eutectic phases surrounding Sn-rich cells were identified, as in the insert image shown in Fig. 4cI . The central circular phase was a ˇSn cell with eutectic phases around it. The detailed structures of such Sn cells are depicted in Fig. 5. The eutectic phase in zone II had a larger volume fraction than that in zone I. In zone III (Fig. 4cIII ), the ˇ-Sn phase changed from a dendrite structure to an equiaxed
structure, and the widths of both the first and second dendritic arms were larger than those in zone IV (Fig. 4cIV ). In zone IV, the structure was the same as in the conventional case illustrated in Fig. 3b. When no ultrasonic vibrations were applied, the grain size was larger than 300 m measured by both the ECD and the LI (Fig. 5a and b). With ultrasonic vibrations, one single Sn-rich cell shown in Fig. 4 consisted of several much smaller Sn grains with diameters of approximately 20 m measured by the ECD and 23 m measured by the LI (Fig. 5c and d). The misorientation of their grain boundaries was measured along the black line indicated in Fig. 5d. As shown by the red profile in Fig. 6, the misorientation among these grains in one Sn-rich cell was greater than 40◦ . Fig. 7 shows the polarized optical microstructures of the aircooled SAC305 and pure Sn ingots treated with 200 W ultrasonic vibrations. At the same power density, the grain refinement for SAC305 was deeper than that for pure Sn. The grain size was approximately 70 m measured by the ECD and 75 m by the LI in the former case, but it was approximately 160 m measured by the ECD and 155 m by the LI in the latter. The composition of the ingot or the exact precipitated/constituent phase may have
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Fig. 5. SEM micrographs and the corresponding EBSD inverse pole figure (IPF) of the air-cooled samples. (a and b) SAC305 produced without ultrasonci treatment, (c and d) SAC305 treated with 200 W ultrasonic vibrations, and (e) pure Sn produced without ultrasonic treatment.
considerable influence on the refinement mechanism, aside from the effects of any additional elements. 3.3. Mechanical properties and storage test The grain size and IMC morphologies of the solidifying ingots were changed by exposure to field of ultrasonic vibrations. Microstructural changes affect the mechanical properties of the solidified ingots. Fig. 8 shows the hardness profiles as a function of ultrasonic power for both SAC305 and pure Sn ingots, respectively, that were solidified under air-cooling. Opposite trends appear in the two cases. For pure Sn, the hardness increased from 7.7 HV
in the sample that was not treated with ultrasonic vibrations to 8.4 HV in the sample exposed to 133 W, after which it reached a plateau. This effect can probably be attributed to the grain refinement when the ultrasonic power was lower. Above 133 W, the effects of increased ultrasonic vibrations on the grain refinement were on longer significant, resulting in little variation in hardness. For SAC305, the hardness decreased with the increase of ultrasonic power. The effect of the fine-grain strengthening was minimal because it was irrelevant in the second-phase dispersion strengthening (mainly due to the segregation of Ag3 Sn from the Sn-rich phase) according to Miyazawa and Ariga (1999). As seen in Fig. 9, the IMCs, including Cu6 Sn5 and Ag3 Sn, precipitated mainly
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Fig. 6. Misorientation profile of grains measured along the dark line in Fig. 5d.
along the grain/cell boundaries of the Sn-rich phases. The leakage of the IMCs from the inner grains softened the ingot. These refined microstructures can be helpful for preventing electromigration and for obtaining an anisotropic grain (making the whole joint more isotropic), resulting in a more stable microstructure. The images shown in Fig. 10 verified that the microstructures changed little when the samples were observed in situ after being
Fig. 7. Polarized optical images of the samples treated with 200 W ultrasonic vibrations during air cooling. (a) Pure Sn and (b) SAC305.
Fig. 8. Hardness as a function of ultrasonic power for SAC305 and pure Sn solidified under air cooling.
stored at 150 ◦ C for 216 h. Both the Sn cells, marked by dashed circles, and the grains, marked by numbers, underwent little evolution. The application of ultrasound to a solidifying solder joint may be beneficial because any unwanted oxide inter-layer might be disrupted and the grains may be refined.
Fig. 9. Segregation of the IMCs along the grain/cell boundaries of the SAC305 in zone II. (a) Treated with 133 W and (b) treated with 200 W ultrasonic vibrations during air cooling.
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provide a unique means for driving chemical reactions under extreme conditions. Homogeneous nucleation may occur. However, if those homogeneously formed nuclei were surrounded by overheating melt, they would re-melt. Heterogeneous nucleation occurs much more easily when cavitation takes place near solid/liquid interfaces, which can be attributed to the grain refinement. Based on the model of cavitation nuclei in the form of hydrophobic solid particles provided by Frenkel and Harvey in the literature written by Eskin (1998), at the defects in IMCs, cavitation-induced nuclei can remain stable at temperatures above equilibrium and survive to initiate further nucleation and growth in the subsequent cooling process. Due to higher local undercooling, solidification can happen at higher temperatures than the conventional case, and macro-undercooling decreases as a consequence, as shown in Fig. 2. This result is in accordance with a study by Ramirez et al. (2008). The energy injection into the ingot by ultrasonic vibrations was another reason that the solidification time increased. Acoustic streaming could have equalized both the temperature and concentration fields, contributing to the time increase. Based on the Sn–Ag–Cu phase diagram, the solidification sequence can be written as: L → L + ˇ-Sn
(2)
L + ˇ-Sn → L + ˇ-Sn + Ag3 Sn
(3)
L + ˇ-Sn + Ag3 Sn → ˇ-Sn + Ag3 Sn + E(ˇ-Sn + Ag3 Sn + Cu6 Sn5 ) (4)
Fig. 10. Microstructures of a Sn cell in the SAC305 sample treated with 200 W ultrasonic vibrations during air cooling. (a) As-obtained and (b) in situ after being stored at 150 ◦ C for 216 h.
4. Discussion Ultrasonic cavitation and acoustic streaming are the two main effects of treating alloys with ultrasonic vibrations during solidification, and they are regarded as the mechanisms by which such treatment can refine microstructures of the solidified alloys. In liquids, acoustic cavitation generates a large number of tiny bubbles that undergo formation, growth and collapse, providing sonochemical effects. Great localized pressure, intense local heating and very short life-times during cavitation are produced by the collapse of these bubbles, according to the Clausius–Clapeyron equation: Tm =
Tm P V H
(1)
The great increase of localized P and V provided by ultrasonic cavitation increases the localized melting point in the vicinity of cavitation bubbles, which is equivalent to raising the localized undercooling. Flint and Suslick (1991) studied the cavitation temperature during the implosive collapse of the bubbles (the transient hot spots) produced by ultrasonic irradiation of liquids. By capturing sonoluminescence spectra from silicone oil, they determined that the effective cavitation temperature was approximately 5000 K. Suslick et al. (1999) reported that the collapse of bubbles in liquids resulted in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into the heating of the contents of the bubbles. Such high local temperatures and pressures combined with extraordinarily rapid cooling
Moon et al. (2000) and Kang et al. (2003) reported that Ag3 Sn plates nucleated and grew with minimal undercooling, while the ˇ-Sn required much higher undercooling to initiate nucleation. The energies for Cu6 Sn5 and Ag3 Sn coarsening were approximately 82 kJ/mol and 38 kJ/mol, respectively, which were both lower than that of ˇ-Sn, reported to be almost 107 kJ/mol by Allen et al. (2011). IMCs were much easier to form. These Cu6 Sn5 and Ag3 Sn IMCs can be broken by the huge impulse forces caused by cavitation when the liquid streams rushed toward the solid faces at high speeds. These IMCs with surface defects significantly contributed to the Sn phase nucleation, resulting in the grain refinement in a single Sn cell shown in Fig. 5. Any ˇ-Sn dendritic arms that were formed would be fragmented, and circular Sn cells were produced, similar to the reports by Campbell (1981) and Ochoa et al. (2003). Relatively far from the ultrasonic horn, the cavitation intensity decreased because the acoustic streaming became dominant, and this effect symmetrized both the temperature and concentration gradients. The rest of the liquid alloy solidified under equilibriumlike conditions, as shown in Fig. 4aIII , bIII and bIV . The rapid cooling rates left only a very short time for the ultrasonic processing. Beyond the ultrasonic horn, the microstructures were the same as in the conventional case (Fig. 4cIV ). The acoustic streaming helped to change ˇ-Sn phases from a dendritic structure to an equiaxed structure and allowed the Sn cells to grow much larger by equalizing the temperature and concentration gradients (Fig. 4cIII and cII ). The cavitation influences were confined within a much smaller region, where cobblestone-shaped Sn grains developed (Fig. 4cI ). Because parts of the eutectic were pushed from the periphery to the top middle and solidified after the solidification of the primary ˇ-Sn, these cobblestone-shaped grains were surrounded by eutectic. Acoustic streaming ensured the symmetrical nucleation and growth of the eutectic phases (around the Sn cells, as shown in the inset image in Fig. 4cI ). The high cavitation intensity and rapid cooling rate suppressed the overgrowth of intermetallics.
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5. Conclusions (a) The ultrasonic processing depth and period were limited by the cooling rate. The undercooling temperature was reduced significantly. (b) Ultrasonic vibrations can refine ˇ-Sn grains from larger than 300 m to 20 m. The grain refinement for SAC305 was deeper than that for pure Sn. Inhomogeneous nucleation at the defects on IMCs was probably the cause of this result. Intermetallics were greatly suppressed, but they may segregate out of the grain interiors. (c) The segregation of these IMCs was tremendous in the SAC305 ingots and mainly distributed along the Sn phase boundaries, resulting in some decrease of the alloy’s hardness. (d) Simultaneously optimizing the ultrasonic power and the cooling rate may both refine the grains and homogenize the microstructures of a solidified alloy. Acknowledgements The work was supported by the State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology (No. AWPT-M09), the HIT.NSFIR.2010135, the Natural Science Foundation of China (NSFC 51005057), the SRF for RoCS, SEM, and the projects funded by the Shenzhen government No. CXB201105100075A and No. KQC201105300005A. References Abramov, O.V., 1987. Action of high intensity ultrasound on solidifying metal. Ultrasonics 25, 73–82. Abramov, V., Abramov, O., Bulgakov, V., Sommer, F., 1998. Solidification of aluminium alloys under ultrasonic irradiation using water-cooled resonator. Materials Letters 37, 27–34. Allen, S.L., Notis, M.R., Chromik, R.R., et al., 2011. Microstructural evolution in leadfree solder alloys: Part II. Directionally solidified Sn–Ag–Cu, Sn–Cu and Sn–Ag. Journal of Materials Research 19, 1425–1431. Campbell, J., 1981. Effect of vibration during solidification. International Metals Reviews 26, 71–108. Das, A., Kotadia, H.R., 2011. Effect of high-intensity ultrasonic irradiation on the modification of solidification microstructure in a Si-rich hypoeutectic Al–Si alloy. Materials Chemistry and Physics 125, 853–859.
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