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Author’s Accepted Manuscript Microstructure and mechanical properties of 7075Al alloy joint ultrasonically soldered with Nifoam/Sn composite solder Yong Xiao, Shan Li, Ziqi Wang, Yong Xiao, Zhipeng Song, Yongning Mao, Mingyu Li www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)30664-6 https://doi.org/10.1016/j.msea.2018.05.015 MSA36453
To appear in: Materials Science & Engineering A Received date: 24 December 2017 Revised date: 24 April 2018 Accepted date: 6 May 2018 Cite this article as: Yong Xiao, Shan Li, Ziqi Wang, Yong Xiao, Zhipeng Song, Yongning Mao and Mingyu Li, Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder
Yong Xiaoa*, Shan Lia, Ziqi Wang b, Yong Xiaob*, Zhipeng Songa, Yongning Maoa, Mingyu Lib
a
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan
430070, China b
Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
[email protected] [email protected]
*
Corresponding author. Tel.: +86 13428935574
*
Corresponding author. Tel.: +86 15815518461
Abstract Ultrasound-assisted soldering of 7075 Al alloy was performed using Ni-foam reinforced Sn composite solder. The phase composition, interfacial microstructure and mechanical properties of Al/Ni-Sn/Al joints soldered for different times were investigated. Results showed that, the bonding ratio of joint was increased with increasing ultrasonic soldering time and was stabled at approximately 98 % when the soldering time was longer than 20 s. The Ni-foam in joint was compressed into a strip type and a Ni3Sn4 intermetallic compound (IMC) layer was formed on the Ni skeleton surface, whilst an Al3Ni IMC layer was formed on the Al substrate surface. The Al3Ni phase was dot-distributed in joint soldered for 5 s then formed continuously in joint soldered for 15 s. However, further increasing the soldering time to 30 s resulted in the drastic growth of Al3Ni IMC layer, accompanied with the depletion of Ni3Sn4 phase. The formation and microstructure evolution mechanisms of the Al3Ni IMC layer was discussed. The measured shear strength of joint was first increased then 1
decreased with increasing ultrasonic soldering time, and a highest shear strength of 58.0 MPa was obtained for joint soldered for 20 s.
Keywords: 7075-Al alloy, Ni-Sn composite solder, Ultrasound-assisted soldering, microstructure, interfacial reaction, mechanical properties
1. Introduction Prepared by severe plastic deformation (SPD) technology, ultrafine grained (UFG) 7075 Al alloy exhibits superior properties such as high strength, high hardness, low porosity, light weight and good wear and corrosion resistance [1-3]. However, the high stored energy and driving force for grain growth at high temperatures have been the key factors which have limited its application [4,5]. To retain the fine grained microstructure during welding, the desirable techniques used for joining UFG Al alloy are friction stir welding [6] and low temperature soldering [7]. Unfortunately, friction stir welding is only suitable for well-shaped material, and it is not appropriate when it comes to complex structure or big fusing interfaces. By contrast, low temperature soldering is not only adaptable to the complex workpiece, but also can get reliable bonds. Thus, it is a better choice in some situations to join UFG Al alloys with low temperature soldering process. The common filler metals applied for low temperature soldering Al alloys are Sn-based lead-free solders, including Sn-Zn and Sn-Ag low melting alloys [8-10]. A key problem facing the use of them is that Al alloy joints exhibited lower strength because of the poor mechanical properties of Sn-based alloys. Many methods have been adopted to improve the strength of joint by adding second phase strengthening particles or fibers into base solders [11,12]. However, it seems difficult to remain the uniformity of reinforced phases in solder matrix, since these nano or micro-sized particles are prone to agglomerate and separate during the soldering process. This problem may be solved by employing a high-melting metal foam as the strengthening structure [13,14]. Zhao et al. [13] fabricated Mg-based composite alloy by immersing Ni-foam into Mg melt with pressure-less infiltration method. The unique 2
structure strengthened with three-dimensional continuous network Ni skeletons was expected to display significant improvements in mechanical properties of the Mg matrix. In our recent studies [14], alumina ceramic was bonded with Ni-foam reinforced Sn-based solders, the strength of joint was largely improved by Ni-skeletons and Ni-Sn reactants arose from the metallurgic bonding between Ni-skeletons and Sn melts. Thus, the Ni-foam should be a suitable strengthening material for Sn base solder because of the high strength of Ni alloy and the excellent metallurgic reaction activity between Ni and Sn elements. On the other hand, it is known that the oxide film located on the Al substrate surface is difficult to break at low soldering temperature, strong corrosive fluxes were always employed during soldering, which may induce the corrosion of Al alloy joints [15]. In recent years, ultrasound-assisted fluxless soldering process have been largely used to join Al alloys at low temperature [16-18]. Acoustic cavitations stimulated by ultrasonic waves can create extremely high temperature and pressure on the solid surface, which can strip the oxide film on the Al substrate and generate non-equilibrium reaction environment at liquid/solid interface [19,20]. Li et al. [16] reported that the acoustic cavitations could induce the “super-saturation” dissolving of Al substrate into liquid pure Sn solder; the dissolved Al element may precipitate into α-Al particles and disperse uniformly in the soldering seam, resulting in the strength improvement of joint. Guo et al. [18] reported that a new alumina layer could form at the Sn-Zn/Al interface during the ultrasonic soldering process, which resulted in a strong interfacial bonding between Sn-Zn solders and Al substrate. However, previous studies about ultrasound-assisted soldering of Al alloy were mainly based on the pure Sn or Sn alloy solders. When the Ni-foam was used as the reinforced phase of Sn base solder, the ultrasound-induced interfacial wetting and metallurgic reaction between composite solder and Al substrate may be different. Thus, in this study we attempt to realize the low temperature soldering of 7075 Al alloy using a Ni-foam reinforced Sn composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the microstructure and mechanical properties of Al/Ni-Sn/Al joints were investigated. Furthermore, the ultrasound-correlated interfacial 3
wetting and metallurgical reaction between Ni-foam/Sn composite solder and Al substrate was discussed. This study is beneficial to develop a reliable bonding method for ultrafine-grained Al alloys. 2. Experimental procedures 7075 Al alloy with a dimension of 10×10×3 mm3 was used as base metal. The soldering interlayer used was a self-developed Ni-foam reinforced Sn composite solder slice, which was made by immersing Ni-foam foil into molten Sn base solder at 250 °C for 5 s then pulling out after the Ni-foam was mostly filled with Sn base solder. The purities of Sn base solder and Ni-foam are both 99.99 wt%. The thickness, porosity and average pore diameter of Ni-foam are 0.5 mm, 98 % and 200 μm, respectively. Fig. 1 shows the optic micrograph of Ni-foam used in study. It can be seen that the three-dimensional space of Ni-foam is crisscrossed by Ni skeletons, exhibiting an open-cell porous structure. The as-fabricated Ni-foam/Sn composite solder foil was rolled into a thickness of approximately 0.5 mm and was cut into a size of 10×10 mm2. Prior to soldering, the Al base metal and Ni-foam/Sn composite solder slice were ground and ultrasonically cleaned in anhydrous ethanol to remove the impurities on their surfaces. The ultrasound-assisted soldering process was schematically shown in Fig. 2. The Ni-foam/Sn composite solder slice was sandwiched as an interlayer between two Al substrates, then the Al/Ni-Sn/Al assembly was fixed in a self-made clamping apparatus. An ohmic heating unit was used to melt the Sn based solder and a K-type thermocouple was inserted in the soldering seam to monitor the soldering temperature. A TC4-Ti alloy horn with a weigh of 0.5 kg and a tip size of 8 mm was stretched from an ultrasonic vibration machine (KESON HKD-1008) and was imposed on the upper Al substrate. During soldering, the Al/Ni-Sn/Al assembly was heated to 300 °C and kept stable for several seconds, then the ultrasonic vibration with a power of 180 W was imposed on Al substrate for 1s, 5 s, 10 s, 15 s, 20 s and 30 s, respectively. Ultimately, the Al/Ni-Sn/Al joint was cooled in ambient temperature and the soldering process was finished. The cross-sections of as soldered joints were prepared for metallographic observation 4
using standard polishing techniques. Microstructure and composition analysis were performed by scanning electron microscope (SEM, Hitachi S-4700) equipped with an energy dispersive X-ray spectrometer (EDS). Phase constitutions of the soldering seam was identified by X-ray diffraction (XRD, Rigaku D/max-2500PC, Cu Kα). Optical microscope (VHX-5000, Keyence; Nikon Eclipse LV100 POL) was used to observe the microstructure of Ni-foam. Scanning acoustic microscopy (SAM, Winsam Vario-3) with C-scan mode and testing frequency of 30 MHz was applied to evaluate bonding qualities of joints. During testing, the upper Al substrate was grinded to 1 mm in thickness and polished, then the ultrasonic waves emitted from SAM probe were inducted through water medium into the upper Al substrate of joint. The two-dimension display of the solder seam allows the visualization of unsoldered regions as white color and well-bonded regions as black color. The bonding ratio was defined as the ratio of well-bonded areas to the total faying areas of joint and was calculated by Adobe Photoshop software. Shear strength tests were performed to evaluate the bonding properties of joints, as shown in Fig. 3. Shearing test samples with a dimension of Ø 5×3 mm2 were cut from the as-soldered joint using an electro-discharge wire-cutting machine, as schematically shown in Fig. 3a. The shearing test sample was fixed in a self-made fixture, as schematically shown in Fig. 3b, and the shearing test was performed in a WCW-100 universal testing machine with a crosshead speed of 0.2 mm/min. Five samples were tested to obtain an average value for each parameter. 3. Results and discussion 3.1 Bonding ratio of joint Fig. 4 shows the SAM images of Al/Ni-Sn/Al joints ultrasonically soldered for 1 s, 10 s, 20 s and 30 s, respectively. For the joint soldered for 1 s, as shown in Fig. 4a, only the center and corners of the faying surface are well-bonded. Increasing the ultrasonic soldering time to 10 s, as shown in Fig. 4b, the well-bonded areas are increased sharply, while it seems that some bonding regions are not continuous, since there exist some white regions dispersed in the black faying surface. Further increasing the ultrasonic soldering time to 20 s and 30 s, as shown in Fig. 4c and 4d, no apparent white regions can be found in the joint, demonstrating 5
that Al substrates are completely wetted by Ni-foam/Sn composite solder during the consistent ultrasonic soldering process. It should be noted that the SAM images shown in Fig. 4c and 4d exhibit uneven color, which may be related to the microstructure difference formed in the joints. The calculated bonding ratio of joint ultrasonically soldered for different times is shown in Fig. 5. It can be seen that the bonding ratio increases sharply from 52.55 % to 90.46 % with increasing soldering time from 1 s to 5 s, then the bonding ratio is improved to 99.5 % when the soldering time is increased to 20 s. However, further increasing the soldering time to 30 s induces a slight decrease of bonding ratio to 98 %. This may be caused by the oxidation of liquid solder located at the seam of joint during soldering in atmosphere. 3.2 Microstructure of joint The cross-section image of Ni-foam/Sn composite solder foil is shown in Fig. 6. The gray triangle frames are Ni skeletons, and the white matrix is the pure Sn base solder. The pores in Ni-foam are completely filled with Sn base solder and no cavities are found in the composite solder, as shown in Fig. 6a. Fig. 6b shows the magnified image of Ni skeletons. It can be seen that the Ni skeleton combines well with the Sn base solder, and a thin reaction layer is formed on the Ni skeleton surface. The EDS analysis results in regions marked in Fig. 6b are shown in Table 1. According the EDS results and previous studies [14], it can be surmised that the thin reaction layer formed on Ni skeleton surface is mainly composed of Ni3Sn4 phase. Furthermore, nearly no Ni element was detected in the Sn base solder, demonstrating that the Ni skeleton was not largely dissolved in the Sn matrix. This may be attributed to the diffusion barrier effects of Ni3Sn4 IMC layer [14]. Fig. 7 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 10 s. A typical microstructure of the cross-section of Al/Ni-Sn/Al joint is shown in Fig. 7a. The Ni-foam/Sn composite solder layer is compressed into a thickness of approximately 60 μm under the effects of ultrasonic vibration and pressure. The Ni skeletons show a layer structure and are distributed randomly in the composite solder layer. Fig. 7b shows the magnified image of Ni-foam/Sn composite solder layer. A gray reaction layer is formed on the 6
Ni skeleton surface; moreover, some gaps between Ni skeletons are filled with the reaction phase. Fig. 7c and 7d show the magnified images of upper and lower Al substrates interfaces of Al/Ni-Sn/Al joint. A dark reaction layer is formed on the Al substrate surface, which exhibits point distribution on the Al substrate surface. This convincingly demonstrates that the metallurgic bonding between Ni-foam/Sn composite solder and Al substrate is not continuous in the Al/Ni-Sn/Al joint ultrasonically soldered for 10 s, which is consistent with the SAM results shown in Fig. 4b. The EDS analysis results in regions marked by “1” and “2” are shown in Table 2. The ratio of atomic percentage for Al to Ni is approximately 3 : 1, thus it can be estimated that the dark reaction layers formed on the Al substrate surface and in the Ni-foam are mainly composed of Al3Ni phase with some Sn atoms dissolved. Obviously, the Al element found in the Ni-foam is dissolved from the Al substrates during the ultrasound-assisted soldering process, which induce the creation of Al3Ni IMC layer. A continuous bright layer mixed with black particles is found adjacent to the Al substrate surface, which is inferred to be unreacted Sn solder layer with some α-Al and Ni3Sn4 particles dispersed in it according to the EDS analysis results in region marked by “3”. Fig. 8 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 15 s. The typical cross-section image of the joint is shown in Fig. 8a. It seems that the joint exhibits sound bonding and no cavities are found in the soldering seam. The Ni skeletons exhibit a strip type and some gray and dark interlayers are caught in the Ni skeleton gaps. The magnified image of solder seam is shown in Fig. 8b. It can be seen that some gaps between Ni skeletons are completely filled with the gray reaction layer. According to the EDS analysis results in regions marked by “4”, as shown in Table 2, it can be identified that the reaction layer is mainly composed of Ni3Sn4 phase with some Al atoms dissolved. Some gaps between Ni skeletons are filled with dark phase, the EDS analysis results in regions marked by “5” imply that this dark interlayer is mainly composed of Al3Ni phase. Fig. 8c and 8d show the magnified images of Al substrates interfaces. It can be seen that the dark reaction layers are created continuously on the Al substrates surfaces, and some Sn solder layers are trapped in them. According to the EDS analysis results in regions marked by “6” and “7”, the reaction 7
layers formed on Al substrate surface were estimated to be Al3Ni IMC layer. To further identify the phase constitution of reaction layers formed on the Ni skeletons and Al substrates surfaces, XRD analyses were performed on the solder seam and the results are shown in Fig. 9. It can be seen that the peaks of Al3Ni and Ni3Sn4 phases are indexed in the profile, which certainly identifies that the gray and dark reaction layers formed on the Ni skeleton and Al substrate surfaces are composed of Ni3Sn4 and Al3Ni phases, respectively. Fig. 10 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 30 s. It can be seen that the microstructure of joint changed significantly compared with that soldered for 15 s. The cross-section image of the joint is shown in Fig. 10a. The Ni-foam/Sn composite solder layer is largely depleted accompanied with the significant growth of interfacial reaction layer, which was identified to be Al3Ni IMC layer according to the EDS analysis results in region marked by “8”. Fig. 10b shows the magnified image of soldering seam. The gaps between Ni skeletons are completely filled with Ni3Sn4 IMC layer according to the EDS analysis results in region marked by “9”. Moreover, the Ni3Sn4 IMC layer formed on the Ni skeleton surface adjacent to Al3Ni IMC layer is extremely thin, most likely the Ni3Sn4 IMC layer is assimilated by the fast growing Al3Ni IMC layer. The magnified images of Al substrates surfaces are shown in Fig. 10c and 10d. The measured average thickness of Al3Ni IMC layer is increased sharply to approximately 17.2 μm. The discontinuous Sn solder layers trapped among Al3Ni IMC layers (as shown in Fig. 8d) are disappeared completely, accompanied with the formation of some micro cavities in the thick Al3Ni IMC layer. Obviously, these micro cavities are come from the trapped Sn solder layers, which are dissolved into the fast growing Al3Ni IMC layer when the soldering time is prolonged from 15 s to 30 s. The Al substrate surface is rugged and some cracks can be found in the Al3Ni IMC layer, demonstrating that the Al substrate is largely dissolved during the ultrasonic soldering process. 3.3 Interfacial reaction behavior At the initial stage, the Ni-foam contains a certain deformation capacity, which can support and guarantee the thickness of soldering seam during the ultrasonic soldering process 8
[22]. With the application of ultrasonic vibration, acoustic cavities may form in molten Sn solder. The cavitation cavities tend to form and burst at solid/liquid interface, owing to the fact that defects or gaps located on the liquid/solid interface can reduce the nucleation energy of them [23]. The average cavities radius in some molten alloys can reach tens of microns [24-27]. In this study, the average diameter of pores in Ni-foam is approximately 200 μm, as shown in Fig. 1, the space is enough for acoustic cavity to form, grow and collapse in the Ni-foam/Sn composite solder layer before the Ni-foam is compressed into a strip type. The micro-jets and shockwaves generated by the rapid implosion of cavitation cavities can create extremely high temperature and pressure at the liquid/solid interface, which can break the oxide film on the Al substrate surface and thus enhance the wetting and spreading of molten Sn solder on the Al substrate [18,28]. Thus, it can be deduced that the wetting between Ni-foam/Sn composite solder layer and Al substrate is mainly attributed to the acoustic cavitation effects when the ultrasonic soldering time is short. It should be noted that the corners of the faying surface in the Al/Ni-Sn/Al joint ultrasonically soldered for 1 s are preferentially bonded, as shown in Fig. 4a. This may be ascribed to the cutting edge effects since the ultrasonic waves tend to accumulate on the tips of Al substrate, which may induce high intensity acoustic cavitations in the molten Sn solder and thus enhance the wetting of molten Sn solder on these places [21]. However, the Ni-foam was compressed into a stripped type and the spaces between Ni skeletons are mostly filled with Ni3Sn4 and Al3Ni phases with the increase of ultrasonic soldering time. In the soldering seam of Al/Ni-Sn/Al joint ultrasonically soldered for 10 s, as shown in Fig. 7, some regions that filled with Sn-base solder are only several microns. Thus, the acoustic cavities are difficult to form since the shortage of liquid phase. Actually, the well-bonded regions induced by acoustic cavitations are always continuous. Previous studies [20] demonstrated that a dot-connection type was shown in the Al alloy joint ultrasonically soldered with a semi-solid state Zn-14Al hypereutectic filler metal, the wetting between Zn-14Al semi-solid state filler metal layer and Al substrate was mainly ascribed to the mechanical friction between semi-solid state filler metal and Al substrate. This result is 9
consistent with the phenomenon found in the Al/Ni-Sn/Al joint ultrasonic soldered for 10 s, the bonding region exhibits a dot distribution (as shown in Fig. 4b) and discontinuous reaction layer is formed on the Al substrate surface (as shown in Fig. 7). Thus, it can be deduced that the wetting between Ni-foam/Sn composite solder layer and Al substrate in the Al/Ni-Sn/Al joint is mainly ascribed to the mechanical friction effects when the ultrasonic soldering time is longer than 10 s. Note that Al3Ni phase is formed on the Al substrate surface and in the Ni-foam/Sn composite solder layer. It seems impossible to form Al3Ni preferentially in the Al/Ni-Sn/Al joint, since reported literatures [29,30] demonstrated that the Gibbs formation energy of Ni3Sn4 is less than that of Al3Ni at 300 ºC. However, the formation of interfacial reaction phase depends on not only thermos-dynamics, but also reaction kinetics [31]. During heating, stable or metastable intermediate phases may form at the interface of dissimilar metals because of interdiffusion and subsequent phase transformations. Intermixing of reaction elements is a necessary initial process that needs to occur before any phase formation can proceed. Sauvage et al. [32] reported that in cold-rolled Al/Ni multilayers the Al and Ni elements were highly mixed at the Al/Ni interface because of the severe plastic deformation. They measured an onset temperature for the formation of Al3Ni phase in cold-rolled Al/Ni multilayers was much lower than the deposited multilayers, since the Al-Ni mixing was mainly ascribed to thermal diffusion in the deposited Al/Ni multilayers. Thus, the pre-mixing of Al and Ni elements may significantly decrease the activation energy of Al3Ni phase at the Al/Ni interface. As to the Al alloy joints ultrasonically soldered with Ni-foam/Sn composite solder, once the oxide film on the Al substrate surface is locally broken, the microjets and shockwaves created extremely high temperature and pressure can induce the “supersaturated dissolution” of Al substrate into the molten Sn solder [16,33]. Since the solubility limit of Al in pure Sn is less than 1.5 mass percent at 300 °C [16], the dissolved Al element may precipitate as α-Al particles in the molten Sn base solder, as shown in Fig. 7d. At the same time, the Ni3Sn4 IMC formed on the Ni substrate surface may be easily stripped from the reaction layer into the 10
molten Sn base solder under the effects of acoustic cavitations induced microjets and shockwaves [14,34]. With prolonging ultrasonic soldering time, deformation of Ni-foams occurs and the liquid Sn base solder is largely squeezed out of the solder seam. The α-Al particles and Ni3Sn4 particles dispersed in the residual Sn based solder are gathered together and are highly mixed with each other. The mutual diffusion of Al and Ni elements becomes much easy in α-Al particles gathered regions, which may lower the formation energy of Al3Ni phase [32]. Furthermore, acoustic cavitations can still form in the liquid Sn base solder before the Ni skeletons are completely compressed into a strip type. The extremely high temperature and pressure reaction environment in liquid Sn based solder caused by acoustic cavitations may further enhance the nucleation of Al3Ni phase [35,36]. Thus, Al3Ni phase is initially formed on the Al substrate surface and in the Ni skeleton gaps, as shown Fig. 7c and 7d. Undoubtedly, the initially formed Al3Ni IMC may act as nucleation cites and decrease the formation energy of Al3Ni phase. With the further increase of ultrasonic soldering time, the Ni-foam is compressed into a strip type and the liquid Sn base solder is almost squeezed out of the soldering seam, as shown in Fig. 8, acoustic cavitation is difficult to form in the solder seam. However, the Ni3Sn4 IMC layer formed on the Ni skeleton surface and the Al3Ni IMC layer formed on the Al substrate surface are in direct contact with each other. Previous studies [37] reported that Ni element moves fast in Al grain boundaries and remains there because of practically zero solubility of Ni in Al, forming Al3Ni intermetallic compound. The formation of cracks and pores in Al3Ni IMC layer may enhance the diffusion of Ni element through the Al3Ni IMC layer to the Al substrate. Thus, the Ni element in the Ni3Sn4 IMC layer may diffuse rapidly into Al substrate, resulting in the rapidly growth of Al3Ni IMC layer. With the depletion of Ni3Sn4 IMC layer, the Ni element may diffuse directly from the Ni skeletons to the Al substrate, as shown in Fig. 10c and 10d. Ultimately, a thick Al3Ni IMC layer is formed on the Al substrate surface accompanied with the depletion of Ni-foam/Sn composite solder layer. 3.5 Shear strength of joint Evolution of the bonding ratio and microstructure of joint will surely have an influence 11
on the reliability of joint. To evaluate the effects of ultrasonic soldering time on the mechanical properties of joint, shearing tests were performed. Fig.11 shows the shear strength of Al/Ni-Sn/Al joint ultrasonically soldered for different times. It can be seen that the shear strength of joint increases firstly then decreases with prolonging ultrasonic soldering time. The joint soldered for 20 s exhibited the highest shear strength of 58.0 MPa, which is approximately 28.4 MPa and 6.3 MPa higher than that soldered for 10 s and 30 s, respectively. The fractorgraphs of shearing failed joints were observed but not listed here since the shearing failure place were not uniform for joints fabricated with the same parameter. Results show that the shearing failure mainly happened at the composite solder layer and Al substrate interface for joint soldered for 10 s. This may be ascribed to the discontinuous bonding regions at the Ni-foam/Sn composite solder layer and Al substrate interface. As to the joint soldered for 20 s, the shearing failure randomly happened in the Ni-foam/Sn composite solder layer and at the composite solder/Al3Ni IMC interface. Obviously, the continuous Al3Ni IMC layer formed on the Al substrate surface improves the coalescence of composite solder layer and Al substrate. Furthermore, the thickness of Al3Ni IMC layer formed on the Al substrate surface is thin, and the Al3Ni IMC layer is mainly trapped in the corrosion pits of Al substrate. Thus, the detrimental effects due to the embrittlement in Al3Ni IMC layer cause limit decrease in joint strength [38,39]. On the other hand, when the ultrasonic soldering time is longer than 15 s, the soldering seam of joint is mainly composed of continuous Ni skeletons with Ni3Sn4 and Al3Ni IMCs interspersed between them. It has been reported that a three-dimensional continuous network microstructure of strength phase displayed significant improvements in mechanical properties of metal based composite [40]. Thus, the strip type Ni skeletons along with the Ni3Sn4 and Al3Ni IMCs are most probably behind the strength improvement of joint. While, for joint soldered for 30 s, the shearing failure tended to initiate in the Al3Ni IMC layer then spread to the Al substrate/Al3Ni IMC layer interface. It is known that stress concentrations are easy to accumulate in thick interfacial IMC layers [35,41], which may result in the deterioration of the joint performance. The Al3Ni IMC layers formed 12
on the Al substrate surface is approximately 20 μm in thickness and some cavities are formed in them, thus the joint soldered for 30 s exhibits a decreased shear strength.
Conclusion 7075-Al alloy was successfully soldered using Ni-foam reinforced Sn-based composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the bonding ratio, microstructure and mechanical properties of joints were investigated. Major conclusions are summarized as follows: (1) The bonding ratio of joint was first increased then decreased with prolonging ultrasonic soldering time. The joint ultrasonically soldered for 20 s exhibited the highest bonding ratio of 99.5 %. The interfacial wetting was ascribed to the combined effects of acoustic cavitations and mechanical friction effects. (2) The Ni-foam was gradually compressed into a strip type, and the Sn base solder was largely squeezed out of the solder seam or depleted during the Ni-Sn reacting. The solder seam was mainly composed of Ni skeleton and Ni3Sn4 reaction layer when the soldering time was longer than 15 s. (3) Al3Ni phase was formed on the Al substrate surface, which exhibited a dot-distribution in joint soldered for 10 s, then grew to a continuous layer structure in joint soldered for 15 s. The nucleation of Al3Ni phase was mainly attributed to the mixing of Al-Ni elements and the non-equilibrium reaction environment created by acoustic cavitations. Further increasing the soldering time to 30 s, the Ni element could diffuse rapidly to the Al substrate, resulting the significant growth of Al3Ni IMC layer. (4) The joint ultrasonically soldered for 20 s exhibited the highest shear strength of 58.0 MPa with the shearing failure mainly occurred in the Ni-foam/Sn composite solder layer. Decreasing or increasing the soldering time could both deteriorate the shear strength of joint.
Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 13
51605357), and Nature Science Foundation of Hubei Province (No. 2016CFB335). The authors would like to thank Dr. Meijun Yang for assistance with electron microscopy.
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Fig. 1. Optic micrograph of Ni-foam. Fig. 2. Schematic diagram of the ultrasound-assisted fluxless soldering process. Fig. 3. Schematic diagrams of (a) shearing test sample and (b) shearing test fixture. Fig. 4. SAM images of Al/Ni-Sn/Al joints ultrasonically soldered at 300 ℃ for (a) 1 s, (b) 10 s, (c) 20 s and (d) 30 s. Fig. 5. Relationship between the bonding ratio of Al/Ni-Sn/Al joint and the ultrasonic vibration time. Fig. 6. SEM images of (a) the cross-section of Ni-foam/Sn composite solder foil and (b) magnified image of Ni-skeleton. Fig. 7. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 10 s. Fig. 8. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 15 s. Fig. 9. XRD pattern of the soldering seam in Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 15 s. Fig. 10. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 30 s. Fig. 11. Dependence of shearing strength on soldering time for Al/Ni-Sn/Al joint.
18
Fig. 1. .
Fig. 2. . .
Fig. 3.
19
Fig. 4.
Fig. 5.
Fig. 6.
20
Fig. 7.
Fig. 8.
21
Fig. 9. .
Fig. 10. .
22
Fig. 11..
Table 1 Elements and phase constitutions of regions marked in Fig. 6. Element compositions（at.%）
Positions
Phase constitutions
Sn
Ni
1
0.08
99.92
Ni
2
58.47
41.53
Ni3Sn4
3
0.12
99.88
Sn
4
0.09
99.91
Sn
Table 2 Elements and phase constitutions of regions marked in Fig. 7, Fig. 8 and Fig. 10. Positions
Element compositions（at.%）
Phase constitutions
Al
Sn
Ni
1
71.69
4.98
23.33
Al3Ni
2
66.55
9.00
24.45
Al3Ni
3
10.52
79.65
9.83
Sn
4
5.31
52.48
42.21
Ni3Sn4
5
65.11
8.73
26.17
Al3Ni
6
70.11
7.68
22.21
Al3Ni
7
71.46
4.41
24.13
Al3Ni
8
72.13
3.82
24.05
Al3Ni
9
5.80
51.98
42.23
Ni3Sn4
23
PII: DOI: Reference:
S0921-5093(18)30664-6 https://doi.org/10.1016/j.msea.2018.05.015 MSA36453
To appear in: Materials Science & Engineering A Received date: 24 December 2017 Revised date: 24 April 2018 Accepted date: 6 May 2018 Cite this article as: Yong Xiao, Shan Li, Ziqi Wang, Yong Xiao, Zhipeng Song, Yongning Mao and Mingyu Li, Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of 7075-Al alloy joint ultrasonically soldered with Ni-foam/Sn composite solder
Yong Xiaoa*, Shan Lia, Ziqi Wang b, Yong Xiaob*, Zhipeng Songa, Yongning Maoa, Mingyu Lib
a
School of Materials Science and Engineering, Wuhan University of Technology, Wuhan
430070, China b
Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
[email protected] [email protected]
*
Corresponding author. Tel.: +86 13428935574
*
Corresponding author. Tel.: +86 15815518461
Abstract Ultrasound-assisted soldering of 7075 Al alloy was performed using Ni-foam reinforced Sn composite solder. The phase composition, interfacial microstructure and mechanical properties of Al/Ni-Sn/Al joints soldered for different times were investigated. Results showed that, the bonding ratio of joint was increased with increasing ultrasonic soldering time and was stabled at approximately 98 % when the soldering time was longer than 20 s. The Ni-foam in joint was compressed into a strip type and a Ni3Sn4 intermetallic compound (IMC) layer was formed on the Ni skeleton surface, whilst an Al3Ni IMC layer was formed on the Al substrate surface. The Al3Ni phase was dot-distributed in joint soldered for 5 s then formed continuously in joint soldered for 15 s. However, further increasing the soldering time to 30 s resulted in the drastic growth of Al3Ni IMC layer, accompanied with the depletion of Ni3Sn4 phase. The formation and microstructure evolution mechanisms of the Al3Ni IMC layer was discussed. The measured shear strength of joint was first increased then 1
decreased with increasing ultrasonic soldering time, and a highest shear strength of 58.0 MPa was obtained for joint soldered for 20 s.
Keywords: 7075-Al alloy, Ni-Sn composite solder, Ultrasound-assisted soldering, microstructure, interfacial reaction, mechanical properties
1. Introduction Prepared by severe plastic deformation (SPD) technology, ultrafine grained (UFG) 7075 Al alloy exhibits superior properties such as high strength, high hardness, low porosity, light weight and good wear and corrosion resistance [1-3]. However, the high stored energy and driving force for grain growth at high temperatures have been the key factors which have limited its application [4,5]. To retain the fine grained microstructure during welding, the desirable techniques used for joining UFG Al alloy are friction stir welding [6] and low temperature soldering [7]. Unfortunately, friction stir welding is only suitable for well-shaped material, and it is not appropriate when it comes to complex structure or big fusing interfaces. By contrast, low temperature soldering is not only adaptable to the complex workpiece, but also can get reliable bonds. Thus, it is a better choice in some situations to join UFG Al alloys with low temperature soldering process. The common filler metals applied for low temperature soldering Al alloys are Sn-based lead-free solders, including Sn-Zn and Sn-Ag low melting alloys [8-10]. A key problem facing the use of them is that Al alloy joints exhibited lower strength because of the poor mechanical properties of Sn-based alloys. Many methods have been adopted to improve the strength of joint by adding second phase strengthening particles or fibers into base solders [11,12]. However, it seems difficult to remain the uniformity of reinforced phases in solder matrix, since these nano or micro-sized particles are prone to agglomerate and separate during the soldering process. This problem may be solved by employing a high-melting metal foam as the strengthening structure [13,14]. Zhao et al. [13] fabricated Mg-based composite alloy by immersing Ni-foam into Mg melt with pressure-less infiltration method. The unique 2
structure strengthened with three-dimensional continuous network Ni skeletons was expected to display significant improvements in mechanical properties of the Mg matrix. In our recent studies [14], alumina ceramic was bonded with Ni-foam reinforced Sn-based solders, the strength of joint was largely improved by Ni-skeletons and Ni-Sn reactants arose from the metallurgic bonding between Ni-skeletons and Sn melts. Thus, the Ni-foam should be a suitable strengthening material for Sn base solder because of the high strength of Ni alloy and the excellent metallurgic reaction activity between Ni and Sn elements. On the other hand, it is known that the oxide film located on the Al substrate surface is difficult to break at low soldering temperature, strong corrosive fluxes were always employed during soldering, which may induce the corrosion of Al alloy joints [15]. In recent years, ultrasound-assisted fluxless soldering process have been largely used to join Al alloys at low temperature [16-18]. Acoustic cavitations stimulated by ultrasonic waves can create extremely high temperature and pressure on the solid surface, which can strip the oxide film on the Al substrate and generate non-equilibrium reaction environment at liquid/solid interface [19,20]. Li et al. [16] reported that the acoustic cavitations could induce the “super-saturation” dissolving of Al substrate into liquid pure Sn solder; the dissolved Al element may precipitate into α-Al particles and disperse uniformly in the soldering seam, resulting in the strength improvement of joint. Guo et al. [18] reported that a new alumina layer could form at the Sn-Zn/Al interface during the ultrasonic soldering process, which resulted in a strong interfacial bonding between Sn-Zn solders and Al substrate. However, previous studies about ultrasound-assisted soldering of Al alloy were mainly based on the pure Sn or Sn alloy solders. When the Ni-foam was used as the reinforced phase of Sn base solder, the ultrasound-induced interfacial wetting and metallurgic reaction between composite solder and Al substrate may be different. Thus, in this study we attempt to realize the low temperature soldering of 7075 Al alloy using a Ni-foam reinforced Sn composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the microstructure and mechanical properties of Al/Ni-Sn/Al joints were investigated. Furthermore, the ultrasound-correlated interfacial 3
wetting and metallurgical reaction between Ni-foam/Sn composite solder and Al substrate was discussed. This study is beneficial to develop a reliable bonding method for ultrafine-grained Al alloys. 2. Experimental procedures 7075 Al alloy with a dimension of 10×10×3 mm3 was used as base metal. The soldering interlayer used was a self-developed Ni-foam reinforced Sn composite solder slice, which was made by immersing Ni-foam foil into molten Sn base solder at 250 °C for 5 s then pulling out after the Ni-foam was mostly filled with Sn base solder. The purities of Sn base solder and Ni-foam are both 99.99 wt%. The thickness, porosity and average pore diameter of Ni-foam are 0.5 mm, 98 % and 200 μm, respectively. Fig. 1 shows the optic micrograph of Ni-foam used in study. It can be seen that the three-dimensional space of Ni-foam is crisscrossed by Ni skeletons, exhibiting an open-cell porous structure. The as-fabricated Ni-foam/Sn composite solder foil was rolled into a thickness of approximately 0.5 mm and was cut into a size of 10×10 mm2. Prior to soldering, the Al base metal and Ni-foam/Sn composite solder slice were ground and ultrasonically cleaned in anhydrous ethanol to remove the impurities on their surfaces. The ultrasound-assisted soldering process was schematically shown in Fig. 2. The Ni-foam/Sn composite solder slice was sandwiched as an interlayer between two Al substrates, then the Al/Ni-Sn/Al assembly was fixed in a self-made clamping apparatus. An ohmic heating unit was used to melt the Sn based solder and a K-type thermocouple was inserted in the soldering seam to monitor the soldering temperature. A TC4-Ti alloy horn with a weigh of 0.5 kg and a tip size of 8 mm was stretched from an ultrasonic vibration machine (KESON HKD-1008) and was imposed on the upper Al substrate. During soldering, the Al/Ni-Sn/Al assembly was heated to 300 °C and kept stable for several seconds, then the ultrasonic vibration with a power of 180 W was imposed on Al substrate for 1s, 5 s, 10 s, 15 s, 20 s and 30 s, respectively. Ultimately, the Al/Ni-Sn/Al joint was cooled in ambient temperature and the soldering process was finished. The cross-sections of as soldered joints were prepared for metallographic observation 4
using standard polishing techniques. Microstructure and composition analysis were performed by scanning electron microscope (SEM, Hitachi S-4700) equipped with an energy dispersive X-ray spectrometer (EDS). Phase constitutions of the soldering seam was identified by X-ray diffraction (XRD, Rigaku D/max-2500PC, Cu Kα). Optical microscope (VHX-5000, Keyence; Nikon Eclipse LV100 POL) was used to observe the microstructure of Ni-foam. Scanning acoustic microscopy (SAM, Winsam Vario-3) with C-scan mode and testing frequency of 30 MHz was applied to evaluate bonding qualities of joints. During testing, the upper Al substrate was grinded to 1 mm in thickness and polished, then the ultrasonic waves emitted from SAM probe were inducted through water medium into the upper Al substrate of joint. The two-dimension display of the solder seam allows the visualization of unsoldered regions as white color and well-bonded regions as black color. The bonding ratio was defined as the ratio of well-bonded areas to the total faying areas of joint and was calculated by Adobe Photoshop software. Shear strength tests were performed to evaluate the bonding properties of joints, as shown in Fig. 3. Shearing test samples with a dimension of Ø 5×3 mm2 were cut from the as-soldered joint using an electro-discharge wire-cutting machine, as schematically shown in Fig. 3a. The shearing test sample was fixed in a self-made fixture, as schematically shown in Fig. 3b, and the shearing test was performed in a WCW-100 universal testing machine with a crosshead speed of 0.2 mm/min. Five samples were tested to obtain an average value for each parameter. 3. Results and discussion 3.1 Bonding ratio of joint Fig. 4 shows the SAM images of Al/Ni-Sn/Al joints ultrasonically soldered for 1 s, 10 s, 20 s and 30 s, respectively. For the joint soldered for 1 s, as shown in Fig. 4a, only the center and corners of the faying surface are well-bonded. Increasing the ultrasonic soldering time to 10 s, as shown in Fig. 4b, the well-bonded areas are increased sharply, while it seems that some bonding regions are not continuous, since there exist some white regions dispersed in the black faying surface. Further increasing the ultrasonic soldering time to 20 s and 30 s, as shown in Fig. 4c and 4d, no apparent white regions can be found in the joint, demonstrating 5
that Al substrates are completely wetted by Ni-foam/Sn composite solder during the consistent ultrasonic soldering process. It should be noted that the SAM images shown in Fig. 4c and 4d exhibit uneven color, which may be related to the microstructure difference formed in the joints. The calculated bonding ratio of joint ultrasonically soldered for different times is shown in Fig. 5. It can be seen that the bonding ratio increases sharply from 52.55 % to 90.46 % with increasing soldering time from 1 s to 5 s, then the bonding ratio is improved to 99.5 % when the soldering time is increased to 20 s. However, further increasing the soldering time to 30 s induces a slight decrease of bonding ratio to 98 %. This may be caused by the oxidation of liquid solder located at the seam of joint during soldering in atmosphere. 3.2 Microstructure of joint The cross-section image of Ni-foam/Sn composite solder foil is shown in Fig. 6. The gray triangle frames are Ni skeletons, and the white matrix is the pure Sn base solder. The pores in Ni-foam are completely filled with Sn base solder and no cavities are found in the composite solder, as shown in Fig. 6a. Fig. 6b shows the magnified image of Ni skeletons. It can be seen that the Ni skeleton combines well with the Sn base solder, and a thin reaction layer is formed on the Ni skeleton surface. The EDS analysis results in regions marked in Fig. 6b are shown in Table 1. According the EDS results and previous studies [14], it can be surmised that the thin reaction layer formed on Ni skeleton surface is mainly composed of Ni3Sn4 phase. Furthermore, nearly no Ni element was detected in the Sn base solder, demonstrating that the Ni skeleton was not largely dissolved in the Sn matrix. This may be attributed to the diffusion barrier effects of Ni3Sn4 IMC layer [14]. Fig. 7 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 10 s. A typical microstructure of the cross-section of Al/Ni-Sn/Al joint is shown in Fig. 7a. The Ni-foam/Sn composite solder layer is compressed into a thickness of approximately 60 μm under the effects of ultrasonic vibration and pressure. The Ni skeletons show a layer structure and are distributed randomly in the composite solder layer. Fig. 7b shows the magnified image of Ni-foam/Sn composite solder layer. A gray reaction layer is formed on the 6
Ni skeleton surface; moreover, some gaps between Ni skeletons are filled with the reaction phase. Fig. 7c and 7d show the magnified images of upper and lower Al substrates interfaces of Al/Ni-Sn/Al joint. A dark reaction layer is formed on the Al substrate surface, which exhibits point distribution on the Al substrate surface. This convincingly demonstrates that the metallurgic bonding between Ni-foam/Sn composite solder and Al substrate is not continuous in the Al/Ni-Sn/Al joint ultrasonically soldered for 10 s, which is consistent with the SAM results shown in Fig. 4b. The EDS analysis results in regions marked by “1” and “2” are shown in Table 2. The ratio of atomic percentage for Al to Ni is approximately 3 : 1, thus it can be estimated that the dark reaction layers formed on the Al substrate surface and in the Ni-foam are mainly composed of Al3Ni phase with some Sn atoms dissolved. Obviously, the Al element found in the Ni-foam is dissolved from the Al substrates during the ultrasound-assisted soldering process, which induce the creation of Al3Ni IMC layer. A continuous bright layer mixed with black particles is found adjacent to the Al substrate surface, which is inferred to be unreacted Sn solder layer with some α-Al and Ni3Sn4 particles dispersed in it according to the EDS analysis results in region marked by “3”. Fig. 8 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 15 s. The typical cross-section image of the joint is shown in Fig. 8a. It seems that the joint exhibits sound bonding and no cavities are found in the soldering seam. The Ni skeletons exhibit a strip type and some gray and dark interlayers are caught in the Ni skeleton gaps. The magnified image of solder seam is shown in Fig. 8b. It can be seen that some gaps between Ni skeletons are completely filled with the gray reaction layer. According to the EDS analysis results in regions marked by “4”, as shown in Table 2, it can be identified that the reaction layer is mainly composed of Ni3Sn4 phase with some Al atoms dissolved. Some gaps between Ni skeletons are filled with dark phase, the EDS analysis results in regions marked by “5” imply that this dark interlayer is mainly composed of Al3Ni phase. Fig. 8c and 8d show the magnified images of Al substrates interfaces. It can be seen that the dark reaction layers are created continuously on the Al substrates surfaces, and some Sn solder layers are trapped in them. According to the EDS analysis results in regions marked by “6” and “7”, the reaction 7
layers formed on Al substrate surface were estimated to be Al3Ni IMC layer. To further identify the phase constitution of reaction layers formed on the Ni skeletons and Al substrates surfaces, XRD analyses were performed on the solder seam and the results are shown in Fig. 9. It can be seen that the peaks of Al3Ni and Ni3Sn4 phases are indexed in the profile, which certainly identifies that the gray and dark reaction layers formed on the Ni skeleton and Al substrate surfaces are composed of Ni3Sn4 and Al3Ni phases, respectively. Fig. 10 shows the SEM images of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ºC for 30 s. It can be seen that the microstructure of joint changed significantly compared with that soldered for 15 s. The cross-section image of the joint is shown in Fig. 10a. The Ni-foam/Sn composite solder layer is largely depleted accompanied with the significant growth of interfacial reaction layer, which was identified to be Al3Ni IMC layer according to the EDS analysis results in region marked by “8”. Fig. 10b shows the magnified image of soldering seam. The gaps between Ni skeletons are completely filled with Ni3Sn4 IMC layer according to the EDS analysis results in region marked by “9”. Moreover, the Ni3Sn4 IMC layer formed on the Ni skeleton surface adjacent to Al3Ni IMC layer is extremely thin, most likely the Ni3Sn4 IMC layer is assimilated by the fast growing Al3Ni IMC layer. The magnified images of Al substrates surfaces are shown in Fig. 10c and 10d. The measured average thickness of Al3Ni IMC layer is increased sharply to approximately 17.2 μm. The discontinuous Sn solder layers trapped among Al3Ni IMC layers (as shown in Fig. 8d) are disappeared completely, accompanied with the formation of some micro cavities in the thick Al3Ni IMC layer. Obviously, these micro cavities are come from the trapped Sn solder layers, which are dissolved into the fast growing Al3Ni IMC layer when the soldering time is prolonged from 15 s to 30 s. The Al substrate surface is rugged and some cracks can be found in the Al3Ni IMC layer, demonstrating that the Al substrate is largely dissolved during the ultrasonic soldering process. 3.3 Interfacial reaction behavior At the initial stage, the Ni-foam contains a certain deformation capacity, which can support and guarantee the thickness of soldering seam during the ultrasonic soldering process 8
[22]. With the application of ultrasonic vibration, acoustic cavities may form in molten Sn solder. The cavitation cavities tend to form and burst at solid/liquid interface, owing to the fact that defects or gaps located on the liquid/solid interface can reduce the nucleation energy of them [23]. The average cavities radius in some molten alloys can reach tens of microns [24-27]. In this study, the average diameter of pores in Ni-foam is approximately 200 μm, as shown in Fig. 1, the space is enough for acoustic cavity to form, grow and collapse in the Ni-foam/Sn composite solder layer before the Ni-foam is compressed into a strip type. The micro-jets and shockwaves generated by the rapid implosion of cavitation cavities can create extremely high temperature and pressure at the liquid/solid interface, which can break the oxide film on the Al substrate surface and thus enhance the wetting and spreading of molten Sn solder on the Al substrate [18,28]. Thus, it can be deduced that the wetting between Ni-foam/Sn composite solder layer and Al substrate is mainly attributed to the acoustic cavitation effects when the ultrasonic soldering time is short. It should be noted that the corners of the faying surface in the Al/Ni-Sn/Al joint ultrasonically soldered for 1 s are preferentially bonded, as shown in Fig. 4a. This may be ascribed to the cutting edge effects since the ultrasonic waves tend to accumulate on the tips of Al substrate, which may induce high intensity acoustic cavitations in the molten Sn solder and thus enhance the wetting of molten Sn solder on these places [21]. However, the Ni-foam was compressed into a stripped type and the spaces between Ni skeletons are mostly filled with Ni3Sn4 and Al3Ni phases with the increase of ultrasonic soldering time. In the soldering seam of Al/Ni-Sn/Al joint ultrasonically soldered for 10 s, as shown in Fig. 7, some regions that filled with Sn-base solder are only several microns. Thus, the acoustic cavities are difficult to form since the shortage of liquid phase. Actually, the well-bonded regions induced by acoustic cavitations are always continuous. Previous studies [20] demonstrated that a dot-connection type was shown in the Al alloy joint ultrasonically soldered with a semi-solid state Zn-14Al hypereutectic filler metal, the wetting between Zn-14Al semi-solid state filler metal layer and Al substrate was mainly ascribed to the mechanical friction between semi-solid state filler metal and Al substrate. This result is 9
consistent with the phenomenon found in the Al/Ni-Sn/Al joint ultrasonic soldered for 10 s, the bonding region exhibits a dot distribution (as shown in Fig. 4b) and discontinuous reaction layer is formed on the Al substrate surface (as shown in Fig. 7). Thus, it can be deduced that the wetting between Ni-foam/Sn composite solder layer and Al substrate in the Al/Ni-Sn/Al joint is mainly ascribed to the mechanical friction effects when the ultrasonic soldering time is longer than 10 s. Note that Al3Ni phase is formed on the Al substrate surface and in the Ni-foam/Sn composite solder layer. It seems impossible to form Al3Ni preferentially in the Al/Ni-Sn/Al joint, since reported literatures [29,30] demonstrated that the Gibbs formation energy of Ni3Sn4 is less than that of Al3Ni at 300 ºC. However, the formation of interfacial reaction phase depends on not only thermos-dynamics, but also reaction kinetics [31]. During heating, stable or metastable intermediate phases may form at the interface of dissimilar metals because of interdiffusion and subsequent phase transformations. Intermixing of reaction elements is a necessary initial process that needs to occur before any phase formation can proceed. Sauvage et al. [32] reported that in cold-rolled Al/Ni multilayers the Al and Ni elements were highly mixed at the Al/Ni interface because of the severe plastic deformation. They measured an onset temperature for the formation of Al3Ni phase in cold-rolled Al/Ni multilayers was much lower than the deposited multilayers, since the Al-Ni mixing was mainly ascribed to thermal diffusion in the deposited Al/Ni multilayers. Thus, the pre-mixing of Al and Ni elements may significantly decrease the activation energy of Al3Ni phase at the Al/Ni interface. As to the Al alloy joints ultrasonically soldered with Ni-foam/Sn composite solder, once the oxide film on the Al substrate surface is locally broken, the microjets and shockwaves created extremely high temperature and pressure can induce the “supersaturated dissolution” of Al substrate into the molten Sn solder [16,33]. Since the solubility limit of Al in pure Sn is less than 1.5 mass percent at 300 °C [16], the dissolved Al element may precipitate as α-Al particles in the molten Sn base solder, as shown in Fig. 7d. At the same time, the Ni3Sn4 IMC formed on the Ni substrate surface may be easily stripped from the reaction layer into the 10
molten Sn base solder under the effects of acoustic cavitations induced microjets and shockwaves [14,34]. With prolonging ultrasonic soldering time, deformation of Ni-foams occurs and the liquid Sn base solder is largely squeezed out of the solder seam. The α-Al particles and Ni3Sn4 particles dispersed in the residual Sn based solder are gathered together and are highly mixed with each other. The mutual diffusion of Al and Ni elements becomes much easy in α-Al particles gathered regions, which may lower the formation energy of Al3Ni phase [32]. Furthermore, acoustic cavitations can still form in the liquid Sn base solder before the Ni skeletons are completely compressed into a strip type. The extremely high temperature and pressure reaction environment in liquid Sn based solder caused by acoustic cavitations may further enhance the nucleation of Al3Ni phase [35,36]. Thus, Al3Ni phase is initially formed on the Al substrate surface and in the Ni skeleton gaps, as shown Fig. 7c and 7d. Undoubtedly, the initially formed Al3Ni IMC may act as nucleation cites and decrease the formation energy of Al3Ni phase. With the further increase of ultrasonic soldering time, the Ni-foam is compressed into a strip type and the liquid Sn base solder is almost squeezed out of the soldering seam, as shown in Fig. 8, acoustic cavitation is difficult to form in the solder seam. However, the Ni3Sn4 IMC layer formed on the Ni skeleton surface and the Al3Ni IMC layer formed on the Al substrate surface are in direct contact with each other. Previous studies [37] reported that Ni element moves fast in Al grain boundaries and remains there because of practically zero solubility of Ni in Al, forming Al3Ni intermetallic compound. The formation of cracks and pores in Al3Ni IMC layer may enhance the diffusion of Ni element through the Al3Ni IMC layer to the Al substrate. Thus, the Ni element in the Ni3Sn4 IMC layer may diffuse rapidly into Al substrate, resulting in the rapidly growth of Al3Ni IMC layer. With the depletion of Ni3Sn4 IMC layer, the Ni element may diffuse directly from the Ni skeletons to the Al substrate, as shown in Fig. 10c and 10d. Ultimately, a thick Al3Ni IMC layer is formed on the Al substrate surface accompanied with the depletion of Ni-foam/Sn composite solder layer. 3.5 Shear strength of joint Evolution of the bonding ratio and microstructure of joint will surely have an influence 11
on the reliability of joint. To evaluate the effects of ultrasonic soldering time on the mechanical properties of joint, shearing tests were performed. Fig.11 shows the shear strength of Al/Ni-Sn/Al joint ultrasonically soldered for different times. It can be seen that the shear strength of joint increases firstly then decreases with prolonging ultrasonic soldering time. The joint soldered for 20 s exhibited the highest shear strength of 58.0 MPa, which is approximately 28.4 MPa and 6.3 MPa higher than that soldered for 10 s and 30 s, respectively. The fractorgraphs of shearing failed joints were observed but not listed here since the shearing failure place were not uniform for joints fabricated with the same parameter. Results show that the shearing failure mainly happened at the composite solder layer and Al substrate interface for joint soldered for 10 s. This may be ascribed to the discontinuous bonding regions at the Ni-foam/Sn composite solder layer and Al substrate interface. As to the joint soldered for 20 s, the shearing failure randomly happened in the Ni-foam/Sn composite solder layer and at the composite solder/Al3Ni IMC interface. Obviously, the continuous Al3Ni IMC layer formed on the Al substrate surface improves the coalescence of composite solder layer and Al substrate. Furthermore, the thickness of Al3Ni IMC layer formed on the Al substrate surface is thin, and the Al3Ni IMC layer is mainly trapped in the corrosion pits of Al substrate. Thus, the detrimental effects due to the embrittlement in Al3Ni IMC layer cause limit decrease in joint strength [38,39]. On the other hand, when the ultrasonic soldering time is longer than 15 s, the soldering seam of joint is mainly composed of continuous Ni skeletons with Ni3Sn4 and Al3Ni IMCs interspersed between them. It has been reported that a three-dimensional continuous network microstructure of strength phase displayed significant improvements in mechanical properties of metal based composite [40]. Thus, the strip type Ni skeletons along with the Ni3Sn4 and Al3Ni IMCs are most probably behind the strength improvement of joint. While, for joint soldered for 30 s, the shearing failure tended to initiate in the Al3Ni IMC layer then spread to the Al substrate/Al3Ni IMC layer interface. It is known that stress concentrations are easy to accumulate in thick interfacial IMC layers [35,41], which may result in the deterioration of the joint performance. The Al3Ni IMC layers formed 12
on the Al substrate surface is approximately 20 μm in thickness and some cavities are formed in them, thus the joint soldered for 30 s exhibits a decreased shear strength.
Conclusion 7075-Al alloy was successfully soldered using Ni-foam reinforced Sn-based composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the bonding ratio, microstructure and mechanical properties of joints were investigated. Major conclusions are summarized as follows: (1) The bonding ratio of joint was first increased then decreased with prolonging ultrasonic soldering time. The joint ultrasonically soldered for 20 s exhibited the highest bonding ratio of 99.5 %. The interfacial wetting was ascribed to the combined effects of acoustic cavitations and mechanical friction effects. (2) The Ni-foam was gradually compressed into a strip type, and the Sn base solder was largely squeezed out of the solder seam or depleted during the Ni-Sn reacting. The solder seam was mainly composed of Ni skeleton and Ni3Sn4 reaction layer when the soldering time was longer than 15 s. (3) Al3Ni phase was formed on the Al substrate surface, which exhibited a dot-distribution in joint soldered for 10 s, then grew to a continuous layer structure in joint soldered for 15 s. The nucleation of Al3Ni phase was mainly attributed to the mixing of Al-Ni elements and the non-equilibrium reaction environment created by acoustic cavitations. Further increasing the soldering time to 30 s, the Ni element could diffuse rapidly to the Al substrate, resulting the significant growth of Al3Ni IMC layer. (4) The joint ultrasonically soldered for 20 s exhibited the highest shear strength of 58.0 MPa with the shearing failure mainly occurred in the Ni-foam/Sn composite solder layer. Decreasing or increasing the soldering time could both deteriorate the shear strength of joint.
Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 13
51605357), and Nature Science Foundation of Hubei Province (No. 2016CFB335). The authors would like to thank Dr. Meijun Yang for assistance with electron microscopy.
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Fig. 1. Optic micrograph of Ni-foam. Fig. 2. Schematic diagram of the ultrasound-assisted fluxless soldering process. Fig. 3. Schematic diagrams of (a) shearing test sample and (b) shearing test fixture. Fig. 4. SAM images of Al/Ni-Sn/Al joints ultrasonically soldered at 300 ℃ for (a) 1 s, (b) 10 s, (c) 20 s and (d) 30 s. Fig. 5. Relationship between the bonding ratio of Al/Ni-Sn/Al joint and the ultrasonic vibration time. Fig. 6. SEM images of (a) the cross-section of Ni-foam/Sn composite solder foil and (b) magnified image of Ni-skeleton. Fig. 7. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 10 s. Fig. 8. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 15 s. Fig. 9. XRD pattern of the soldering seam in Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 15 s. Fig. 10. SEM images of (a) the cross-section, (b) the filler metal layer, (c) the upper substrate interface and (d) the lower substrate interface of Al/Ni-Sn/Al joint ultrasonically soldered at 300 ℃ for 30 s. Fig. 11. Dependence of shearing strength on soldering time for Al/Ni-Sn/Al joint.
18
Fig. 1. .
Fig. 2. . .
Fig. 3.
19
Fig. 4.
Fig. 5.
Fig. 6.
20
Fig. 7.
Fig. 8.
21
Fig. 9. .
Fig. 10. .
22
Fig. 11..
Table 1 Elements and phase constitutions of regions marked in Fig. 6. Element compositions（at.%）
Positions
Phase constitutions
Sn
Ni
1
0.08
99.92
Ni
2
58.47
41.53
Ni3Sn4
3
0.12
99.88
Sn
4
0.09
99.91
Sn
Table 2 Elements and phase constitutions of regions marked in Fig. 7, Fig. 8 and Fig. 10. Positions
Element compositions（at.%）
Phase constitutions
Al
Sn
Ni
1
71.69
4.98
23.33
Al3Ni
2
66.55
9.00
24.45
Al3Ni
3
10.52
79.65
9.83
Sn
4
5.31
52.48
42.21
Ni3Sn4
5
65.11
8.73
26.17
Al3Ni
6
70.11
7.68
22.21
Al3Ni
7
71.46
4.41
24.13
Al3Ni
8
72.13
3.82
24.05
Al3Ni
9
5.80
51.98
42.23
Ni3Sn4
23