- Email: [email protected]
Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints
Accepted Manuscript Title: Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints Authors: Z.L. Ni, F.X. Ye PII: DOI: Reference:
S0924-0136(17)30080-8 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.02.027 PROTEC 15139
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-11-2016 18-2-2017 27-2-2017
Please cite this article as: Ni, Z.L., Ye, F.X., Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.02.027 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 proof before it is published in its final 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.
Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints
Z. L. Ni*, F. X. Ye*
School of Materials Science & Engineering, Tianjin University, Tianjin 300072, China
Corresponding Author: Z. L. Ni, E-mail address: [email protected], Tel: +86 22 27401556, fax: +86 22 27401556;
F. X. Ye, E-mail address: [email protected], Tel: +86 22 27401556, fax: +86 22 27401556.
1
Graphical abstract Clamping pressure
Clamping pressure
Vibration direction
Vibration direction
Al
Interlayer (Al 2219 particle)
Cu Interlayer (Al 2219 particle)
Cu Al/Cu joint with interlayer
Al
Cu/Al joint with interlayer
At a welding time of 0.5s
Abstract Higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer. For the both joints, the hardness perpendicular to the thickness direction of the aluminum sheet has a significant change compared with the original hardness. Peak lap shear tensile strength of Al/Cu joint with interlayer 2
(78.3MPa) is 27.3% higher than that of Cu/Al joint with interlayer (61.5MPa).
Keywords: Welding; Microstructure; Ultrasonic; Interface; Lap configuration.
1. Introduction Dissimilar joining becomes more and more important in industrial applications for their abundant merits, such as excellent product performances and beneficial production economics. Sahin (2010) have reported that joint of aluminum (Al) to copper (Cu) can be applied in certain electric conduction domain, due to their high heat and electrical conductivity, mechanical and corrosion resistance properties. The challenges of dissimilar joint of Al/Cu are their differences in metallurgical and physical properties, resulting in difficulties to obtain sound Al/Cu joint by employing conventional fusion welding processes. Saeid et al. (2010) showed that brittle intermetallic compounds are easily produced during the conventional fusion welding processes, leading to brittleness, cracks and high resistivity in the weld interface. Therefore it is necessary to replace the conventional fusion welding processes with solid state welding processes, such as friction stir spot welding, friction stir welding, electrical resistance spot welding, and ultrasonic metal welding (UMW). Compared with other solid-state welding technology, Bakavos and Prangnell (2010) have suggested that ultrasonic metal welding with advantages of lower energy consumption and higher efficiency is an emerging and evolving welding process. Shakil et al. (2014) showed that UMW can eliminate the common matters in relation to fusion welding, such as brittle intermetallic compounds and distortion. In addition, Janaki Ram et al. (2007) showed that UMW
3
can meet the joining demand of lightweight alloy materials with high diffusivity rates in the melt state, such as Al, magnesium, nickel and copper. In the context, it is demonstrated that UMW is an effective solution to assemble metal sheets. Temperature of the weld interface induced by frictional heat is one of the most critical issues during UMW. Macwan and Chen (2016) demonstrated that UMW involves a high-frequency shear scrubbing between contacting asperities, leading to generate partial heat and then soften the material in the weld interface. Higher temperature and heat promote plastic deformation, as confirmed in reviews by Xu et al. (2000) and Yuichi et al. (2016). Thus local adhesion and microbonds form, and ultimately expand over the entire weld interface, which can hope produce a high bonding strength, as demonstrated in studies by Mirza et al. (2016) and Zhang et al. (2014). De Vries (2004) suggested that the temperature of weld interface could arrive about 40–80% of the material melting point that is rested on the parameters of UMW process. Generally, the maximum temperature is less than the melting point of the metal samples. it is beneficial to generate mechanical bonds between the alloy sheets. Since moderate interface temperature can improve the weldability via lowering the yield strength of materials and controlling the intermetallic compounds. During UMW, intermetallic compound layer generated in the weld interface is poor to the joint strength. For example, Yang et al. (2014) studied the interfacial reaction between copper and Al6061 alloy as a function of welding time. They found that the intermetallic compound layer largely contained CuAl2 and Cu9 Al4, the lap shear load of the joints first increased and then decreased with the increase of welding time, and the failure of the joints mainly generated in the weld interface, which was primarily ascribed to the development of IMC layer at the interface. Zhao et al. (2013) investigated the effects of welding energy on the joint strength, failure behaviour and
4
microstructure of Al–Cu ultrasonic welded joint. They observed that joint strength first increased with welding energy up to its maximum at 1000 J, then decreased. The failure mode changed from interfacial debonding to nugget pullout, and then back to cleavage failure. At a lower energy, the joints were only partly joined by numerous isolated microjoints. At the weld energy of 1000J, a swirl-like structure generated at the weld interface, leading to a mechanical interlocking between the metal workpieces. Whereas, cavity defects and intermetallic compound occurred under an excessively high energy (2000J). The thickness of intermetallic compound layer with main Al4Cu9 phase is 0.5 μm, which was poor to the joint strength. Therefore, it is necessary to eliminate the intermetallic compound layer during UMW. However, both authors did not propose new strategies to resolve the issues and enhance the joint property. Wu et al. (2015) studied the microstructures of the ultrasonic welds between three layers of lithium-ion battery tabs (either Al or Cu) and bus bars. They confirmed that the main mechanism was friction-induced physical bonding in the Al/Cu interfaces, no intermetallic compound layer was found in the weld interface. In view of this, improving the friction-induced physical bonding is a good method for enhancing the strength of UMWed Al/Cu joints. No reports were reported about the improvement of mechanical properties of UMWed joints by enhancing the friction coefficient of the weld interface. Therefore a layer of Al2219 alloy particle added between the Al and Cu sheets to improve the friction coefficient in the faying interfaces. During our study process, lap configuration was a key factor on the microstructure and mechanical properties. As a consequence, it is important to investigate the effect of lap configuration on the interfacial microstructure and the mechanical properties of aluminum to copper joints with Al2219 particle interlayer. UMWed joints of Al to Cu sheets with an Al2219 alloy particle interlayer were successfully
5
completed with different lap configurations. The main objective of this paper is to investigate the effect of lap configuration on the microstructure and mechanical properties of aluminum to copper joints with Al2219 particle interlayer. 2. Experimental procedure C1100 copper (98.7 Hv, wt.% 99.99Cu) sheet with 0.5 mm thick and hardening treatment 1100 aluminum (34.6 Hv, wt.% 99.99 Al) sheet with 1.0 mm thick were utilized for UMW. Al2219 particles and alcoholic were mixed in the weight ratio of one to four. The mixture was coated on the surfaces of C1100 copper at the end of weld side. A SONICS MW-20 welder (Fig.1a.) was utilized for ultrasonic metal welding. The size of the sonotrode tip was 8 × 8 mm2, as seen in Fig.1b. The specimens with 25 mm in width and 100 mm in length were processed. And the relative displacement direction is perpendicular to the rolling direction of the sheets. The optical microstructure of the as received 1100 aluminum and C1100 copper sheets is demonstrated in Fig.2a and b. Fig.2c shows the EBSD Euler map of the original grain structure of aluminum sheet. No asperities or grooves exist near the surface regions, and the elongated 1100 aluminum grain is paralleled to the rolling direction, the average grain size is 4.2μm. SEM images of Al2219 particle is shown in Fig.2d and e. The Al2219 alloy powders (141.5 Hv), fabricated by atomization, are spherical with an average diameter of 8μm, and some asperities and valleys exist in the particle surface. The welding parameters are welding time from 0.3s to 0.6s at 0.1s intervals, with a constant clamping pressure of 50 psi, power setting of 2500 W, with a frequency of 20 kHz. For convenience, the lap joint, which C1100 copper sheet is placed as the top sheet, is called the Cu/Al joint with interlayer, as demonstrated in Fig. 3a. Accordingly, the Al/Cu joint with interlayer refers to the joint
6
which 1100 aluminum sheet is placed as the top sheet, as demonstrated in Fig. 3b. Five specimens were achieved in each welding time level. Two of them were applied to microstructure observation and micro hardness measurements, and the others were utilized for lap shear tensile tests. Specimens of different welds were cut in their center, which was parallel to the welding vibration direction. Microstructure was characterized using optical microscope (OM, OLYMPUS-GX71), scanning electron microscopy (SEM, FEI, Holland), and transmission electron microscope (TEM, JEM-2100) equipped with energy dispersive X-Ray spectroscopy (EDS). A JSM-7001F field-emission scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) system from EDAX-TSL was used for EBSD. Vickers hardness was gauged on the polished cross sections along the thickness direction of the aluminum sheet (Distance from the weld interface of Al/Al219) and cross sections in the middle of the aluminum sheet utilizing a microhardness tester (HXZ-1000, China) with an indentation load of 100g load for 15s. Lap tensile shear tests, calculated by dividing the lap shear load by the nugget area (8×8 mm2), were achieved by employing an AG-100KNA test machine in a constant displacement rate of 1 mm/min in air at room temperature. In the lap shear tensile testing process, restraining shims were employed to minimize the rotation of the joints and maintain the shear loading as long as possible.
3. Results and discussion 3.1 Weld appearance and microstructure Fig. 4 shows the typical surface appearances of the UMWed joints utilizing different lap configurations at a welding time of 0.5s. It can be seen that the area of indentation in the bottom sheets is larger than that (8×8 mm2) in the top sheet, owing to the bigger area of anvil compared
7
with the sonotrode tip. The depth of indentation in the top sheet (Al sheet) of Al/Cu joint with interlayer is higher than that (Cu sheet) of Cu/Al joint with interlayer, this is due to the hardness of Al (34.6 Hv) is lower than that of Cu (98.7 Hv). During UMW, the repetitive motion time of slippage and friction in the weld interface are generated by the action of the sonotrode tip exerting the shear force to the material surface by friction. Under the same welding parameters, it depends on the accessibility of the sonotrode knurl penetrating into the material surface. Soften Al sheet is easier to be penetrated by the sonotrode tip. Therefore, the depth of indentation in the top sheet (Al sheet) of Al/Cu joint with interlayer is higher than that (Cu sheet) of Cu/Al joint with interlayer. It can provide a tighter engagement, resulting in more relative slippages in the interface of the top specimen (Al sheet) and the bottom specimen (Cu sheet) as well as longer friction time. As a consequence, the temperature in the weld interface rises, the material is softened. The softening material implies that the yield strength decreases, and then the plastic deformation capacity increases. To observe the interfaces of Al/interlayer, Cu/interlayer, Al2219/Al2219 of the joints with different lap configurations, SEM was conducted. Fig.5 a and b show the microstructure near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; Fig.5 c and d show the microstructure in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; Fig.5e, g and i show the magnified images of box indicated in Fig.5 c, respectively; and Fig.5 f, h and j show the magnified images of box indicated in Fig.5d, respectively. It can be observed from Fig.5a, b, c and d for both joints with different lap configurations that the average thickness of the interlayer is about 30μm, the joining at the center of the weld is better than that in the border areas of the weld, this is due to
8
that during UMW the peak temperature at the center of the weld is higher than that in the border. In comparison with that in Fig.5a and c, it can be seen in Fig.5b and d that there are fewer cavities in the interlayer and fewer unbonding zones in the Cu/interlayer interface; the plastic deformation of copper sheet, in Fig.5b and d, becomes greater in the weld interface. The interfaces of Al/interlayer, Cu/interlayer in Fig.5f and j are better than that in Fig.5e and i, respectively. And the plastic deformation in the Al side is more than that in the Cu side. Compared with the plastic deformation at the Cu side in the interface of Cu/interlayer for the Cu/Al joint, severe plastic deformation at the Cu side in the interface of Cu/interlayer for the Al/Cu joint with interlayer occurs, as indicated between the blue lines in Fig. 5b, d and j, and then Cu and Al2219 particle are complexly intertwined together, which is beneficial to the bonding strength of joint. This is due to that the strength of ultrasonic metal welded joints is a combined effect of interfacial waves, microbonds and mechanical interlocking produced along the weld interface. The size of the precipitates as indicated by the blue arrows in Fig.5h in the interlayer (Al2219) is bigger than that in Fig.5g, and some unbonded areas appear in Fig.5g. From the results mentioned above, it can be concluded that the more friction heat and higher temperature are generated in the weld interface of Al/Cu joint with interlayer compared with that in the weld interface of Cu/Al joint with interlayer. The possible explanation is that, under the same welding parameters for the both joints, the plastic deformation at the Cu side and precipitation size in the Al2219 particle have relationship with temperature in the weld interface, higher temperature promote more severe plastic deformation and bigger precipitation. The reasons for the temperature difference between the weld interfaces of both joints with different lap configurations can be summarized as follows. The repetitive motion time of slippage and friction in the weld interface generated by the action of the sonotrode tip exerting the
9
shear force to the material surface by friction, under the same welding parameters, this depends on the accessibility of the sonotrode knurl penetrating into the material surface. Soften Al sheet as a top specimen is easier to be penetrated by the sonotrode tip and provides a tighter engagement, resulting in less relative slippages in the interface of sonotrode tip and the top specimen (Al sheet) in the Al/Cu joint with interlayer, generating more relative slippages in the interface of the top specimen (Al sheet) and the bottom specimen (Cu sheet) as well as longer friction time. Mutual abrasion of the contact surfaces is a key factor of friction heat and plastic deformation generated. Therefore, the weld temperature in the Al/Cu joint with interlayer is higher than that in the Cu/Al joint with interlayer, the material is softened. The softening material implies that the yield strength decreases, and then the shear plastic deformation capacity increases, which can promote rapid microwelds, leading to the welding and energy dispersion by shear deformation. Clearly, the Al/Cu joint with interlayer can generate a higher temperature and more soften material in the weld interface, then producing a better joint. From the Fig.5a, c, e, g and i, it can be observed cavities and unbonding areas, the poor combined interface areas and presence of voids are the sites of damage. when the load is applied, the sites of damage can occur at the early stage of the loading process, and then accelerate the crack propagation up to result in the final fracture. Therefore, cavities and unbonding areas in the weld interface of Cu/Al joint with interlayer are poor to tensile strength, and the microstructure in the peripheral part is critical to the lap shear tensile strength, since the fracture always initiates at this part. The reason for the better interface of Al/interlayer compared with that of Cu/interlayer, as indicated in Fig.5, can be summarized as follows. Firstly, the hardness of copper is higher than that of aluminum at the same welding temperature below the melting point, continuous mutual abrasion of the contact surfaces and shear deformation are generated in the weld
10
interface, thus the soften aluminum sheet has more deeper scores, combined with the action of the clamping pressure, where the Al2219 particles can penetrate readily. Next the heat generation appears in the mutual abrasion of the contact surfaces and shear deformation, which mostly remain in the pure aluminum side. This is due to that the thermal conductivity (388 W/m·k) of copper is higher than that of aluminum (217 W/m·k) as reported in a review by Fujii et al. (2016), resulting in softening the aluminum side. Therefore Al2219 alloy particles squeeze into the aluminum sheet deeper, instead of copper sheet. To better observe the plastic deformation of the Cu side for both joints with different lap configurations at a welding time of 0.5s, the Cu sheet, which was torn in the lap tensile shear test, was put into the 8% hydrochloric acid till chemical reaction comes to a halt. Then SEM was conducted, as shown in Fig.6. It can be seen that the wear progression for the Al/Cu joint with interlayer is more severe (Fig.6 d and e), and some free of significant friction and wear area is indicated by the arrows in Fig.6b. For both joints with different lap configurations, the wear progression at the center of the weld is more severe than that in the border areas of the weld, indicating that the peak temperature at the center of the weld is higher than that in the border. The phenomena are in agreement with the results mentioned above. It can be concluded that more continuous mutual abrasion of the contact surfaces and shear deformation in the weld interface of Al/Cu joint with interlayer are generated than that of Cu/Al joint with interlayer, resulting in more mechanical interlocking and complex wave-like flow patterns (Fig. 5d and j). This is due to the solid-state deformation as well as the formation and progressive spreading of microjoints, which can hope produce a high bonding strength. Meanwhile, it is suggested that hard Cu as the top sheet is harmful to the relative slippages in the interfaces of Cu/interlayer, Al/interlayer and
11
interlayer/interlayer. The conclusions are consistent with the microstructure demonstrated in Fig.5 and Fig.6. To study the microstructure evolution induced during the welding in the sheets to be welded, OM and EBSD were conducted. Fig.7 shows the OM and EBSD images of Cu side and Al side for both joints with different lap configurations. It can be seen that fine grains and plastic deformation occur in the weld interface of the Cu side (as indicated by the arrows in Fig.7d), however similar results has not been found in Fig.7a and b; For both joints with different lap configurations, the plastic deformation at the center of the weld is more severe than that in the border areas of the weld. The average size for the Cu/Al joint with interlayer near the periphery and in the center is 5.3μm and 6.2 μm (Fig.7g and k), respectively. The average size for the Al/Cu joint with interlayer near the periphery and in the center is 6.3μm and 7.1μm (Fig.7h and l), respectively. For the Al side of both joints, the original microstructure (elongated grains as indicated in Fig.2b and c) vanished, and recrystallized microstructure and grain growth are emerged along the weld interface, as seen in Fig. 7g, h, k and l; and in Fig.7i and j the thicknesses of the decreased elongated aluminum grains (the section above indicated by the red line) are thicker than that in Fig.7e and f. For the Al/Cu joint with interlayer, larger shear deformation and mutual rubbing of the faying surfaces are produced in the weld interface. And then more severe plastic deformation and heat are generated in weld interface, resulting in complex wave-like flow patterns. Therefore, fine grains and plastic deformation occur in the weld interface of Cu side, the elongated aluminum grains decrease, and more recrystallizated microstructure and grain growth occur in the weld interface of Al side (Fig.7g, h, k and l). The microstructure change has relationship with the lap shear tensile strength of the joined materials. The detailed explanation is shown in the section below.
12
During UMW, as soon as the clamping pressure and tangential force of the sonotrode are carried out, this will cause the shear deformation and mutual rubbing of the faying surfaces. It will lead to heat generation, promoting the temperature increment and materials softening. Then the slight deformation in the weld interface occurs. When the temperature of weld interface is above the recrystallization temperature of Al sheet, dynamic recrystallization occurs in the Al sheets. When the temperature of weld interface is above the precipitation temperature of Al2219, precipitation takes place. As the welding process progresses, the temperature increases, the grain grows as shown in Fig.7, the average size for the Cu/Al joint with interlayer near the periphery and in the center is 5.3μm and 6.2 μm (Fig.7g and k), respectively. The average size for the Al/Cu joint with interlayer near the periphery and in the center is 6.3μm and 7.1μm (Fig.7h and l), respectively. Meanwhile plastic deformation and the size of precipitation increase. 3.2. Microhardness and mechanical property Fig.8 shows the Vickers characteristic microhardness profiles of Al side in the UMWed joints with different lap configurations cross sections along the thickness direction of the aluminum sheet (a) and cross sections in the middle of the aluminum sheet (b). It is observed from Fig.8a that with the increase of distance from the weld interface of Al/Al219 along the thickness direction of the aluminum sheet, the hardness increases till the distance is 600 μm, suggesting that the peak temperature is at the center of the nugget, recrystallization and grain growth take place, and the elongated grain microstructure disappears along the weld interface. In Fig.8b a lower hardness region exists in the center of the weld areas for both welding energies. The hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. It is ascribed to that shear deformation and mutual rubbing of the faying surfaces are generated in the weld interface,
13
and then heat are produced in weld interface, promoting the recrystallization, grain growth and increment of metal temperature. Thus it leads to the decrease of microhardness depending from the Hall-Petch type relationship as reported by Hall (1954). Ji et al. (2014) have suggested that the more completely the recrystallization process occurred, the softer the sheets were. In this work, the temperature in the weld interface of Al/Cu joint with interlayer, from the results mentioned earlier, is higher than that of Cu/Al joint with interlayer, resulting more recrystallization and grain growth took place in the Al side. Therefore, the hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. The results are consistent with the microstructure in Fig. 7. Fig.9 shows the lap shear tensile strength of the UMWed joints with different lap configurations. It can be seen that the lap shear tensile strength first increases, and then decreases as the welding time further increases, the lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer. Higher peak temperature was achieved at a higher level of welding time, leading to a higher extent of softness, better plastic deformability, enhances the welding ability of the Al and Cu alloy. It has again a relation to the formation of numerous microbonds that saturated at the faying surface with increasing time, thus the net bonded areas become more predominant. Therefore, the lap shear tensile strength increases with increasing welding time. If too high welding time was employed, resulting in fatigue damage and cracks in the weld interface, and then the initial plastic deformation become mainly elastic and joining was prevented. Here, more recrystallizated microstructure (see in Fig.7i, j, k and l) appears at the interface, and it is rated as “good” weld. Meanwhile the peak lap shear tensile strength of both UMWed joints with different lap configurations at a welding time of 0.5s is much higher compared with the UMWed Al-Cu joint
14
without interlayer as reported by Balasundaram et al. (2014). In this study, Al2219 particle interlayer is placed in-between the faying surfaces, thus the friction coefficient increases. During UMW, as soon as the clamping pressure and tangential force of the sonotrode are carried out, this will cause the shear deformation and mutual rubbing of the faying surfaces. Panteli et al. (2013) and Xu et al. (2016) confirmed that friction coefficient is critical to obtain sound bonding by removing the surface oxide layers. Hard Al2219 particles (141.5 Hv) are beneficial to the removal of oxide layers of aluminum and copper surfaces combined with the action of the ultrasonic vibrations and clamping pressure, fresh metal surfaces will be exposed to intimate contact. Moreover, it will lead to generate more heat, promoting the temperature increment and materials soften. As a consequence, more severe plastic deformation will occur in the weld interface. Plastic deformation in the weld interface is a key factor to obtain a sound joint in the ultrasonic metal welding. First, it promotes the removal of oxides in the Al and Cu surfaces. The broken oxides are eliminated from the combination zones by the flowing metal, and then transmitted to the weld interface edge. Second, it is beneficial to generation of intimate nascent metal contact, which is a necessary condition for UMW. Atomic forces can begin to take effect till the contact interfaces are brought close enough together. Third, interfacial plastic deformation is critical to produce a weld with satisfactory weld density. Roughness on the surfaces prevents the generation of intimate contact surfaces. Before the welding process, numerous microasperities existed on the contact surfaces of the sheets, and the mutual touch first appears in the asperity surface, leading to a large amount of no-contacted regions remain in the weld interface. Combination in the un-contact zones does not take place until a method of closing these voids produces and makes the surfaces contact perfectly. This is where the interfacial plastic flow plays a key role, promoting the extrusion of materials extruding into the
15
voids. As the welding proceeds, the bonded areas will expand in size and number, resulting in more effective contact areas. In this work, from the results mentioned above, higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer, which is more favorable to obtain a sound joint with relatively high weld density, as indicated in Fig.5b, d, f, h and j. Therefore, lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer. 3.3. Weld interface Considering the maximum lap shear tensile strength of Al/Cu joint with interlayer, to understand the interfaces of Cu/interlayer, Al/interlayer, TEM and EDS was conducted. Fig.10a and c show the interfaces of Al/interlayer and Cu/interlayer, respectively. It is obtained that the interface bonding of Al/interlayer and Cu/interlayer is tightly intimate, and no gaps exist in the interface. In Fig.10b, it is demonstrated that the Al2219 particle grain is very long, indicating that severe plastic deformation takes place in the interlayer during UMW, which is in agreement with the results above. As seen from Fig. 10d, the EDS line scan analysis results show that a sudden change of Al and Cu composition across the border, the diffusion zone width is too small to obtain a conclusion with any degree of confidence, taking into consideration the resolution limit of the EDS technology, suggesting that no intermediate layer might form in the interface. Therefore, the welding mechanisms in this study contain solid-state plastic deformation, mechanical interlocking, as well as the formation and progressive spreading of microwelds, which are in accordance with the previous studies by Chen et al. 2012 and Panteli et al. (2012). 3.4. Fracture morphology To identify effects of lap configuration on the weld interface, fracture morphology at a welding
16
time of 0.5s was carried out. Fig.11a and c show the fracture morphology near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; Fig.11b and d show the fracture morphology in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively. For the Cu/Al joint with interlayer, it can be observed that the fracture surface near the periphery is flat, and the flat fracture surface is replaced with dimples at the center of the weld, indicating that the interface combination strength near the periphery is lower than that at the center of the weld. It is more possibility that the fracture occurs through the cavities and unbonding regions in the peripheral areas (Fig.11a and b). Compared with the Cu/Al joint with interlayer, more dimple fracture occurs in the fracture of the Al/Cu joint with interlayer at the center and in the border areas, where the elongated dimples suggest that the shear fracture via void formation/nucleation, growth and coalescence. Meanwhile it can be seen for both joints with different lap configurations that the dimples at the center of the weld is more than that in the border areas of the weld, and the dimples of Al/Cu joint with interlayer at the center and in the border areas are more than that of Cu/Al joint with interlayer, respectively. Which confirm that the interface combination strength of Cu/interlayer and interlayer/interlayer in Al/Cu joint with interlayer is better than that in Cu/Al joint with interlayer, and the interface combination strength at the center is higher than that near the periphery. Moreover good interface combination can transmit a higher load to the base metal, suggesting that the lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer, and this is well agreement with the lap shear tensile strength shown in Fig.9. 4. Conclusions UMWed joints of Al to Cu sheets with an Al2219 alloy particle interlayer were successfully
17
completed with different lap configurations. The following conclusions could be drawn: (1) Higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer. (2) At the Al side of both joints, the original microstructure (elongated grains) vanished and recrystallized microstructure is emerged along the weld interface. (3) For the both joints, with the increase of distance from the weld interface of Al/Al219 along the thickness direction of the aluminum sheet, the hardness increases till the distance is 600 μm. Meanwhile, the hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. (4) The peak lap shear tensile strength of Al/Cu joint with interlayer at a welding time of 0.5s (78.3MPa) is 27.3% higher than that of Cu/Al joint with interlayer (61.5MPa). Acknowledgments This work was funded by the National Natural Science Foundation of China (No.51375332), Natural Science Foundation of Tianjin (No.16JCYBJC18700).
18
References Bakavos, D., Prangnell, P.B., 2010. Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet. Mater. Scie. Eng. A. 527, 6320-6334. Balasundaram, R., Patel, V.K., Bhole, S.D., Chen, D.L., 2014. Effect of zinc interlayer on ultrasonic spot welded aluminum-to-copper joints. Mater. Scie. Eng. A. 607, 277-286. Chen, Y., Bakavos, D., Gholinia, A., Prangnell, P., 2012. HAZ development and accelerated post-weld natural ageing in ultrasonic spot welding aluminium 6111-T4 automotive sheet. Acta Mater. 60(6), 2816-2828. De Vries, E., (Dissertation) 2004. Mechanics and mechanisms of ultrasonic metal welding. The Ohio State University. Fujii, H.T., Goto,Y., Sato, Y.S., Kokawa, H., 2016. Microstructure and lap shear strength of the weld interface in ultrasonic welding of Al alloy to stainless steel. Scripta Mater. 116, 135–138. Hall, E. O., 1954. Variation of hardness of metals with grain size. Nature 173(4411), 948-949. Janaki Ram, G.D., Robinson, C., Yang, Y., Stucker, B.E., 2007. Use of ultrasonic consolidation for fabrication of multi-material structures. Rapid Prototyping J. 13 (4), 226–235. Ji, H., Wang, J., Li, M., 2014. Evolution of the bulk microstructure in 1100 aluminum builds fabricated by ultrasonic metal welding. J. Mater. Process. Technol. 214(2), 175-182. Macwan, A., Chen, D.L., 2016. Ultrasonic spot welding of rare-earth containing ZEK100 magnesium alloy to 5754 aluminum alloy. Mater. Scie. Eng. A. 666, 139-148. Mirza, F.A., Macwan, A., Bhole, S. D., Chen, D.L., Chen, X.G., 2016. Effect of welding energy on microstructure and strength of ultrasonic spot welded dissimilar joints of aluminum to steel
19
sheets. Mater. Scie. Eng. A. 668, 73-85. Panteli, A., Robson, J.D., Brough, I., Prangnell, P.B., 2012. The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium. Mater. Scie. Eng. A. 556, 31-42. Panteli, A., Robson, J.D., Chen, Y.C., Prangnell, P. B., 2013. The effectiveness of surface coatings on preventing interfacial reaction during ultrasonic welding of aluminum to magnesium. Metall. Mater. Trans. A. 44(13), 5773-5781. Saeid, T., Abdollah-Zadeh, A., Sazgari, B., 2010. Weldability and mechanical properties of dissimilar aluminum–copper lap joints made by friction stir welding. J. Alloy. Compd. 490(1-2), 652-655. Sahin, M., 2010. Joining of aluminium and copper materials with friction welding. Int. J. Adv. Manuf. Tech. 49(5), 527-534. Shakil, M., Tariq, N.H., Ahmad, M., Choudhary, M. A., Akhter, J. I., Babu, S. S., 2014. Effect of ultrasonic welding parameters on microstructure and mechanical properties of dissimilar joints. Mater. Design 55(6), 263-273. Wu, X., Liu, T., Cai, W., 2015. Microstructure, welding mechanism, and failure of Al/Cu ultrasonic welds. J. Manuf. Process. 20, 321-331. Xu, L., Wang, L., Chen, Y.C., Robson, J.D., Prangnell, P. B., 2016. Effect of interfacial reaction on the mechanical performance of steel to aluminum dissimilar ultrasonic spot welds. Metall. Mater. Trans. A. 47(1), 1-13. Xu, W., Liu, J., Luan, G., Dong, C., 2000. Microstructure and mechanical properties of friction stir welded joints in 2219-T6 aluminum alloy. Mater. Design 30(9), 3460-3467.
20
Xue, P., Ni, D.R., Wang, D., Xiao, B.L., Ma, Z.Y., 2011. Effect of friction stir welding parameters on the microstructure and mechanical properties of the dissimilar Al–Cu joints. Mater. Scie. Eng. A. 528, 4683–4689. Yang, J.W., Cao, B., He, X.C., Luo, H. S., 2014. Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding. Sci. Technol. Weld. Joining 19(6), 500-504. Yuichi, H., Chihiro, I., Yoshihito, K., 2016. Microstructure evolution and mechanical properties of extruded Mg96Zn2Y2 alloy joints with ultrasonic spot welding. Mater. Scie. Eng. A. 651, 925-934. Zhang, C.Y., Chen, D.L., Luo, A.A., 2014. Joining 5754 automotive aluminum alloy 2-mm-thick sheets using ultrasonic spot welding. Weld. J. 93(4), 131S-138S. Zhao, Y.Y., Li, D., Zhang, Y.S., 2013. Effect of welding energy on interface zone of Al–Cu ultrasonic welded joint. Sci. Technol. Weld. Joining 18(4): 354-360.
21
a
Pressurizing bar
Frame
Smart control power box
Transducer
Anvil
Sonotrode tip
Die
b
Fig.1 (a) A SONICS MW-20 welder employed for UMW; (b) detailed knurl pattern of sonotrode tip (left) and its dimension (right).
22
Fig.2 (a) and (b) showing the optical microstructure of original aluminum and copper sheets, respectively; (c) EBSD Euler map of the original grain structure of aluminum sheet; (d) and (e) SEM observations of the Al2219 particle.
23
b a
Clamping pressure
Clamping pressure
Vibration direction
Al
Cu
Vibration direction Interlayer (Al 2219 particle)
Interlayer Cu (Al 2219 Al particle) Fig.3 Joints configuration in this study: (a) Cu/Al joint with interlayer and (b) Al/Cu joint with interlayer.
24
Fig.4 Typical surface appearances of the UMWed joints utilizing different lap configurations at a welding time of 0.5s: (a) and (b) showing the top and bottom sheets of Al/Cu joint with interlayer, respectively; (c) and (d) showing the top and bottom sheets of Cu/Al joint with interlayer, respectively.
25
26
Al2Cu
Fig.5 (a) and (b) showing the microstructure near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; (c) and (d) showing the microstructure in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; (e), (g) and (i) showing the magnified images of box indicated in (c); and (f), (h) and (j) showing the magnified images of box indicated in (d).
27
Fig.6 Plastic deformation of Cu side for both joints with different lap configurations at a welding time of 0.5s: (a) original state of Cu sheet; (b) and (d) showing the plastic deformation near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively;
28
(c) and (e) showing the plastic deformation in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively.
29
30
Fig.7 OM and EBSD images of Cu side and Al side for both joints with different lap configurations at a welding time of 0.5s: (a) and (c) showing the OM images of Cu side near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (b) and (d)
31
showing the OM images of Cu side in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (e) and (i) showing the OM images of Al side near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (f) and (j) showing the OM images of Al side in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (g), (h), (k) and (l) the grain images (EBSD) taken in the areas close to the Al/Al2219 interface corresponding to (e), (f), (i) and (j), respectively.
32
a
b
Fig.8 Vickers characteristic microhardness profiles of Al side in the UMWed joints with different lap configurations at a welding time of 0.5s; (a) vertical in the weld center; (b) horizontal.
33
Fig.9 Lap shear tensile strength of the UMWed joints with different lap configurations.
34
Fig.10 TEM and EDS analysis of the Al/Cu joint with interlayer at a welding time of 0.5s: (a) interface of Al/interlayer; (b) interlayer; (c) interface of Cu/interlayer; (d) EDS result as indicated in (c).
35
Fig.11 Fracture morphology of Al side of the joints with different lap configurations at a welding time of 0.5s: (a) and (c) showing the fracture morphology near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (b) and (d) showing the fracture morphology in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively.
36
S0924-0136(17)30080-8 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.02.027 PROTEC 15139
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
29-11-2016 18-2-2017 27-2-2017
Please cite this article as: Ni, Z.L., Ye, F.X., Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.02.027 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 proof before it is published in its final 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.
Effect of lap configuration on the microstructure and mechanical properties of dissimilar ultrasonic metal welded copper-aluminum joints
Z. L. Ni*, F. X. Ye*
School of Materials Science & Engineering, Tianjin University, Tianjin 300072, China
Corresponding Author: Z. L. Ni, E-mail address: [email protected], Tel: +86 22 27401556, fax: +86 22 27401556;
F. X. Ye, E-mail address: [email protected], Tel: +86 22 27401556, fax: +86 22 27401556.
1
Graphical abstract Clamping pressure
Clamping pressure
Vibration direction
Vibration direction
Al
Interlayer (Al 2219 particle)
Cu Interlayer (Al 2219 particle)
Cu Al/Cu joint with interlayer
Al
Cu/Al joint with interlayer
At a welding time of 0.5s
Abstract Higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer. For the both joints, the hardness perpendicular to the thickness direction of the aluminum sheet has a significant change compared with the original hardness. Peak lap shear tensile strength of Al/Cu joint with interlayer 2
(78.3MPa) is 27.3% higher than that of Cu/Al joint with interlayer (61.5MPa).
Keywords: Welding; Microstructure; Ultrasonic; Interface; Lap configuration.
1. Introduction Dissimilar joining becomes more and more important in industrial applications for their abundant merits, such as excellent product performances and beneficial production economics. Sahin (2010) have reported that joint of aluminum (Al) to copper (Cu) can be applied in certain electric conduction domain, due to their high heat and electrical conductivity, mechanical and corrosion resistance properties. The challenges of dissimilar joint of Al/Cu are their differences in metallurgical and physical properties, resulting in difficulties to obtain sound Al/Cu joint by employing conventional fusion welding processes. Saeid et al. (2010) showed that brittle intermetallic compounds are easily produced during the conventional fusion welding processes, leading to brittleness, cracks and high resistivity in the weld interface. Therefore it is necessary to replace the conventional fusion welding processes with solid state welding processes, such as friction stir spot welding, friction stir welding, electrical resistance spot welding, and ultrasonic metal welding (UMW). Compared with other solid-state welding technology, Bakavos and Prangnell (2010) have suggested that ultrasonic metal welding with advantages of lower energy consumption and higher efficiency is an emerging and evolving welding process. Shakil et al. (2014) showed that UMW can eliminate the common matters in relation to fusion welding, such as brittle intermetallic compounds and distortion. In addition, Janaki Ram et al. (2007) showed that UMW
3
can meet the joining demand of lightweight alloy materials with high diffusivity rates in the melt state, such as Al, magnesium, nickel and copper. In the context, it is demonstrated that UMW is an effective solution to assemble metal sheets. Temperature of the weld interface induced by frictional heat is one of the most critical issues during UMW. Macwan and Chen (2016) demonstrated that UMW involves a high-frequency shear scrubbing between contacting asperities, leading to generate partial heat and then soften the material in the weld interface. Higher temperature and heat promote plastic deformation, as confirmed in reviews by Xu et al. (2000) and Yuichi et al. (2016). Thus local adhesion and microbonds form, and ultimately expand over the entire weld interface, which can hope produce a high bonding strength, as demonstrated in studies by Mirza et al. (2016) and Zhang et al. (2014). De Vries (2004) suggested that the temperature of weld interface could arrive about 40–80% of the material melting point that is rested on the parameters of UMW process. Generally, the maximum temperature is less than the melting point of the metal samples. it is beneficial to generate mechanical bonds between the alloy sheets. Since moderate interface temperature can improve the weldability via lowering the yield strength of materials and controlling the intermetallic compounds. During UMW, intermetallic compound layer generated in the weld interface is poor to the joint strength. For example, Yang et al. (2014) studied the interfacial reaction between copper and Al6061 alloy as a function of welding time. They found that the intermetallic compound layer largely contained CuAl2 and Cu9 Al4, the lap shear load of the joints first increased and then decreased with the increase of welding time, and the failure of the joints mainly generated in the weld interface, which was primarily ascribed to the development of IMC layer at the interface. Zhao et al. (2013) investigated the effects of welding energy on the joint strength, failure behaviour and
4
microstructure of Al–Cu ultrasonic welded joint. They observed that joint strength first increased with welding energy up to its maximum at 1000 J, then decreased. The failure mode changed from interfacial debonding to nugget pullout, and then back to cleavage failure. At a lower energy, the joints were only partly joined by numerous isolated microjoints. At the weld energy of 1000J, a swirl-like structure generated at the weld interface, leading to a mechanical interlocking between the metal workpieces. Whereas, cavity defects and intermetallic compound occurred under an excessively high energy (2000J). The thickness of intermetallic compound layer with main Al4Cu9 phase is 0.5 μm, which was poor to the joint strength. Therefore, it is necessary to eliminate the intermetallic compound layer during UMW. However, both authors did not propose new strategies to resolve the issues and enhance the joint property. Wu et al. (2015) studied the microstructures of the ultrasonic welds between three layers of lithium-ion battery tabs (either Al or Cu) and bus bars. They confirmed that the main mechanism was friction-induced physical bonding in the Al/Cu interfaces, no intermetallic compound layer was found in the weld interface. In view of this, improving the friction-induced physical bonding is a good method for enhancing the strength of UMWed Al/Cu joints. No reports were reported about the improvement of mechanical properties of UMWed joints by enhancing the friction coefficient of the weld interface. Therefore a layer of Al2219 alloy particle added between the Al and Cu sheets to improve the friction coefficient in the faying interfaces. During our study process, lap configuration was a key factor on the microstructure and mechanical properties. As a consequence, it is important to investigate the effect of lap configuration on the interfacial microstructure and the mechanical properties of aluminum to copper joints with Al2219 particle interlayer. UMWed joints of Al to Cu sheets with an Al2219 alloy particle interlayer were successfully
5
completed with different lap configurations. The main objective of this paper is to investigate the effect of lap configuration on the microstructure and mechanical properties of aluminum to copper joints with Al2219 particle interlayer. 2. Experimental procedure C1100 copper (98.7 Hv, wt.% 99.99Cu) sheet with 0.5 mm thick and hardening treatment 1100 aluminum (34.6 Hv, wt.% 99.99 Al) sheet with 1.0 mm thick were utilized for UMW. Al2219 particles and alcoholic were mixed in the weight ratio of one to four. The mixture was coated on the surfaces of C1100 copper at the end of weld side. A SONICS MW-20 welder (Fig.1a.) was utilized for ultrasonic metal welding. The size of the sonotrode tip was 8 × 8 mm2, as seen in Fig.1b. The specimens with 25 mm in width and 100 mm in length were processed. And the relative displacement direction is perpendicular to the rolling direction of the sheets. The optical microstructure of the as received 1100 aluminum and C1100 copper sheets is demonstrated in Fig.2a and b. Fig.2c shows the EBSD Euler map of the original grain structure of aluminum sheet. No asperities or grooves exist near the surface regions, and the elongated 1100 aluminum grain is paralleled to the rolling direction, the average grain size is 4.2μm. SEM images of Al2219 particle is shown in Fig.2d and e. The Al2219 alloy powders (141.5 Hv), fabricated by atomization, are spherical with an average diameter of 8μm, and some asperities and valleys exist in the particle surface. The welding parameters are welding time from 0.3s to 0.6s at 0.1s intervals, with a constant clamping pressure of 50 psi, power setting of 2500 W, with a frequency of 20 kHz. For convenience, the lap joint, which C1100 copper sheet is placed as the top sheet, is called the Cu/Al joint with interlayer, as demonstrated in Fig. 3a. Accordingly, the Al/Cu joint with interlayer refers to the joint
6
which 1100 aluminum sheet is placed as the top sheet, as demonstrated in Fig. 3b. Five specimens were achieved in each welding time level. Two of them were applied to microstructure observation and micro hardness measurements, and the others were utilized for lap shear tensile tests. Specimens of different welds were cut in their center, which was parallel to the welding vibration direction. Microstructure was characterized using optical microscope (OM, OLYMPUS-GX71), scanning electron microscopy (SEM, FEI, Holland), and transmission electron microscope (TEM, JEM-2100) equipped with energy dispersive X-Ray spectroscopy (EDS). A JSM-7001F field-emission scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) system from EDAX-TSL was used for EBSD. Vickers hardness was gauged on the polished cross sections along the thickness direction of the aluminum sheet (Distance from the weld interface of Al/Al219) and cross sections in the middle of the aluminum sheet utilizing a microhardness tester (HXZ-1000, China) with an indentation load of 100g load for 15s. Lap tensile shear tests, calculated by dividing the lap shear load by the nugget area (8×8 mm2), were achieved by employing an AG-100KNA test machine in a constant displacement rate of 1 mm/min in air at room temperature. In the lap shear tensile testing process, restraining shims were employed to minimize the rotation of the joints and maintain the shear loading as long as possible.
3. Results and discussion 3.1 Weld appearance and microstructure Fig. 4 shows the typical surface appearances of the UMWed joints utilizing different lap configurations at a welding time of 0.5s. It can be seen that the area of indentation in the bottom sheets is larger than that (8×8 mm2) in the top sheet, owing to the bigger area of anvil compared
7
with the sonotrode tip. The depth of indentation in the top sheet (Al sheet) of Al/Cu joint with interlayer is higher than that (Cu sheet) of Cu/Al joint with interlayer, this is due to the hardness of Al (34.6 Hv) is lower than that of Cu (98.7 Hv). During UMW, the repetitive motion time of slippage and friction in the weld interface are generated by the action of the sonotrode tip exerting the shear force to the material surface by friction. Under the same welding parameters, it depends on the accessibility of the sonotrode knurl penetrating into the material surface. Soften Al sheet is easier to be penetrated by the sonotrode tip. Therefore, the depth of indentation in the top sheet (Al sheet) of Al/Cu joint with interlayer is higher than that (Cu sheet) of Cu/Al joint with interlayer. It can provide a tighter engagement, resulting in more relative slippages in the interface of the top specimen (Al sheet) and the bottom specimen (Cu sheet) as well as longer friction time. As a consequence, the temperature in the weld interface rises, the material is softened. The softening material implies that the yield strength decreases, and then the plastic deformation capacity increases. To observe the interfaces of Al/interlayer, Cu/interlayer, Al2219/Al2219 of the joints with different lap configurations, SEM was conducted. Fig.5 a and b show the microstructure near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; Fig.5 c and d show the microstructure in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; Fig.5e, g and i show the magnified images of box indicated in Fig.5 c, respectively; and Fig.5 f, h and j show the magnified images of box indicated in Fig.5d, respectively. It can be observed from Fig.5a, b, c and d for both joints with different lap configurations that the average thickness of the interlayer is about 30μm, the joining at the center of the weld is better than that in the border areas of the weld, this is due to
8
that during UMW the peak temperature at the center of the weld is higher than that in the border. In comparison with that in Fig.5a and c, it can be seen in Fig.5b and d that there are fewer cavities in the interlayer and fewer unbonding zones in the Cu/interlayer interface; the plastic deformation of copper sheet, in Fig.5b and d, becomes greater in the weld interface. The interfaces of Al/interlayer, Cu/interlayer in Fig.5f and j are better than that in Fig.5e and i, respectively. And the plastic deformation in the Al side is more than that in the Cu side. Compared with the plastic deformation at the Cu side in the interface of Cu/interlayer for the Cu/Al joint, severe plastic deformation at the Cu side in the interface of Cu/interlayer for the Al/Cu joint with interlayer occurs, as indicated between the blue lines in Fig. 5b, d and j, and then Cu and Al2219 particle are complexly intertwined together, which is beneficial to the bonding strength of joint. This is due to that the strength of ultrasonic metal welded joints is a combined effect of interfacial waves, microbonds and mechanical interlocking produced along the weld interface. The size of the precipitates as indicated by the blue arrows in Fig.5h in the interlayer (Al2219) is bigger than that in Fig.5g, and some unbonded areas appear in Fig.5g. From the results mentioned above, it can be concluded that the more friction heat and higher temperature are generated in the weld interface of Al/Cu joint with interlayer compared with that in the weld interface of Cu/Al joint with interlayer. The possible explanation is that, under the same welding parameters for the both joints, the plastic deformation at the Cu side and precipitation size in the Al2219 particle have relationship with temperature in the weld interface, higher temperature promote more severe plastic deformation and bigger precipitation. The reasons for the temperature difference between the weld interfaces of both joints with different lap configurations can be summarized as follows. The repetitive motion time of slippage and friction in the weld interface generated by the action of the sonotrode tip exerting the
9
shear force to the material surface by friction, under the same welding parameters, this depends on the accessibility of the sonotrode knurl penetrating into the material surface. Soften Al sheet as a top specimen is easier to be penetrated by the sonotrode tip and provides a tighter engagement, resulting in less relative slippages in the interface of sonotrode tip and the top specimen (Al sheet) in the Al/Cu joint with interlayer, generating more relative slippages in the interface of the top specimen (Al sheet) and the bottom specimen (Cu sheet) as well as longer friction time. Mutual abrasion of the contact surfaces is a key factor of friction heat and plastic deformation generated. Therefore, the weld temperature in the Al/Cu joint with interlayer is higher than that in the Cu/Al joint with interlayer, the material is softened. The softening material implies that the yield strength decreases, and then the shear plastic deformation capacity increases, which can promote rapid microwelds, leading to the welding and energy dispersion by shear deformation. Clearly, the Al/Cu joint with interlayer can generate a higher temperature and more soften material in the weld interface, then producing a better joint. From the Fig.5a, c, e, g and i, it can be observed cavities and unbonding areas, the poor combined interface areas and presence of voids are the sites of damage. when the load is applied, the sites of damage can occur at the early stage of the loading process, and then accelerate the crack propagation up to result in the final fracture. Therefore, cavities and unbonding areas in the weld interface of Cu/Al joint with interlayer are poor to tensile strength, and the microstructure in the peripheral part is critical to the lap shear tensile strength, since the fracture always initiates at this part. The reason for the better interface of Al/interlayer compared with that of Cu/interlayer, as indicated in Fig.5, can be summarized as follows. Firstly, the hardness of copper is higher than that of aluminum at the same welding temperature below the melting point, continuous mutual abrasion of the contact surfaces and shear deformation are generated in the weld
10
interface, thus the soften aluminum sheet has more deeper scores, combined with the action of the clamping pressure, where the Al2219 particles can penetrate readily. Next the heat generation appears in the mutual abrasion of the contact surfaces and shear deformation, which mostly remain in the pure aluminum side. This is due to that the thermal conductivity (388 W/m·k) of copper is higher than that of aluminum (217 W/m·k) as reported in a review by Fujii et al. (2016), resulting in softening the aluminum side. Therefore Al2219 alloy particles squeeze into the aluminum sheet deeper, instead of copper sheet. To better observe the plastic deformation of the Cu side for both joints with different lap configurations at a welding time of 0.5s, the Cu sheet, which was torn in the lap tensile shear test, was put into the 8% hydrochloric acid till chemical reaction comes to a halt. Then SEM was conducted, as shown in Fig.6. It can be seen that the wear progression for the Al/Cu joint with interlayer is more severe (Fig.6 d and e), and some free of significant friction and wear area is indicated by the arrows in Fig.6b. For both joints with different lap configurations, the wear progression at the center of the weld is more severe than that in the border areas of the weld, indicating that the peak temperature at the center of the weld is higher than that in the border. The phenomena are in agreement with the results mentioned above. It can be concluded that more continuous mutual abrasion of the contact surfaces and shear deformation in the weld interface of Al/Cu joint with interlayer are generated than that of Cu/Al joint with interlayer, resulting in more mechanical interlocking and complex wave-like flow patterns (Fig. 5d and j). This is due to the solid-state deformation as well as the formation and progressive spreading of microjoints, which can hope produce a high bonding strength. Meanwhile, it is suggested that hard Cu as the top sheet is harmful to the relative slippages in the interfaces of Cu/interlayer, Al/interlayer and
11
interlayer/interlayer. The conclusions are consistent with the microstructure demonstrated in Fig.5 and Fig.6. To study the microstructure evolution induced during the welding in the sheets to be welded, OM and EBSD were conducted. Fig.7 shows the OM and EBSD images of Cu side and Al side for both joints with different lap configurations. It can be seen that fine grains and plastic deformation occur in the weld interface of the Cu side (as indicated by the arrows in Fig.7d), however similar results has not been found in Fig.7a and b; For both joints with different lap configurations, the plastic deformation at the center of the weld is more severe than that in the border areas of the weld. The average size for the Cu/Al joint with interlayer near the periphery and in the center is 5.3μm and 6.2 μm (Fig.7g and k), respectively. The average size for the Al/Cu joint with interlayer near the periphery and in the center is 6.3μm and 7.1μm (Fig.7h and l), respectively. For the Al side of both joints, the original microstructure (elongated grains as indicated in Fig.2b and c) vanished, and recrystallized microstructure and grain growth are emerged along the weld interface, as seen in Fig. 7g, h, k and l; and in Fig.7i and j the thicknesses of the decreased elongated aluminum grains (the section above indicated by the red line) are thicker than that in Fig.7e and f. For the Al/Cu joint with interlayer, larger shear deformation and mutual rubbing of the faying surfaces are produced in the weld interface. And then more severe plastic deformation and heat are generated in weld interface, resulting in complex wave-like flow patterns. Therefore, fine grains and plastic deformation occur in the weld interface of Cu side, the elongated aluminum grains decrease, and more recrystallizated microstructure and grain growth occur in the weld interface of Al side (Fig.7g, h, k and l). The microstructure change has relationship with the lap shear tensile strength of the joined materials. The detailed explanation is shown in the section below.
12
During UMW, as soon as the clamping pressure and tangential force of the sonotrode are carried out, this will cause the shear deformation and mutual rubbing of the faying surfaces. It will lead to heat generation, promoting the temperature increment and materials softening. Then the slight deformation in the weld interface occurs. When the temperature of weld interface is above the recrystallization temperature of Al sheet, dynamic recrystallization occurs in the Al sheets. When the temperature of weld interface is above the precipitation temperature of Al2219, precipitation takes place. As the welding process progresses, the temperature increases, the grain grows as shown in Fig.7, the average size for the Cu/Al joint with interlayer near the periphery and in the center is 5.3μm and 6.2 μm (Fig.7g and k), respectively. The average size for the Al/Cu joint with interlayer near the periphery and in the center is 6.3μm and 7.1μm (Fig.7h and l), respectively. Meanwhile plastic deformation and the size of precipitation increase. 3.2. Microhardness and mechanical property Fig.8 shows the Vickers characteristic microhardness profiles of Al side in the UMWed joints with different lap configurations cross sections along the thickness direction of the aluminum sheet (a) and cross sections in the middle of the aluminum sheet (b). It is observed from Fig.8a that with the increase of distance from the weld interface of Al/Al219 along the thickness direction of the aluminum sheet, the hardness increases till the distance is 600 μm, suggesting that the peak temperature is at the center of the nugget, recrystallization and grain growth take place, and the elongated grain microstructure disappears along the weld interface. In Fig.8b a lower hardness region exists in the center of the weld areas for both welding energies. The hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. It is ascribed to that shear deformation and mutual rubbing of the faying surfaces are generated in the weld interface,
13
and then heat are produced in weld interface, promoting the recrystallization, grain growth and increment of metal temperature. Thus it leads to the decrease of microhardness depending from the Hall-Petch type relationship as reported by Hall (1954). Ji et al. (2014) have suggested that the more completely the recrystallization process occurred, the softer the sheets were. In this work, the temperature in the weld interface of Al/Cu joint with interlayer, from the results mentioned earlier, is higher than that of Cu/Al joint with interlayer, resulting more recrystallization and grain growth took place in the Al side. Therefore, the hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. The results are consistent with the microstructure in Fig. 7. Fig.9 shows the lap shear tensile strength of the UMWed joints with different lap configurations. It can be seen that the lap shear tensile strength first increases, and then decreases as the welding time further increases, the lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer. Higher peak temperature was achieved at a higher level of welding time, leading to a higher extent of softness, better plastic deformability, enhances the welding ability of the Al and Cu alloy. It has again a relation to the formation of numerous microbonds that saturated at the faying surface with increasing time, thus the net bonded areas become more predominant. Therefore, the lap shear tensile strength increases with increasing welding time. If too high welding time was employed, resulting in fatigue damage and cracks in the weld interface, and then the initial plastic deformation become mainly elastic and joining was prevented. Here, more recrystallizated microstructure (see in Fig.7i, j, k and l) appears at the interface, and it is rated as “good” weld. Meanwhile the peak lap shear tensile strength of both UMWed joints with different lap configurations at a welding time of 0.5s is much higher compared with the UMWed Al-Cu joint
14
without interlayer as reported by Balasundaram et al. (2014). In this study, Al2219 particle interlayer is placed in-between the faying surfaces, thus the friction coefficient increases. During UMW, as soon as the clamping pressure and tangential force of the sonotrode are carried out, this will cause the shear deformation and mutual rubbing of the faying surfaces. Panteli et al. (2013) and Xu et al. (2016) confirmed that friction coefficient is critical to obtain sound bonding by removing the surface oxide layers. Hard Al2219 particles (141.5 Hv) are beneficial to the removal of oxide layers of aluminum and copper surfaces combined with the action of the ultrasonic vibrations and clamping pressure, fresh metal surfaces will be exposed to intimate contact. Moreover, it will lead to generate more heat, promoting the temperature increment and materials soften. As a consequence, more severe plastic deformation will occur in the weld interface. Plastic deformation in the weld interface is a key factor to obtain a sound joint in the ultrasonic metal welding. First, it promotes the removal of oxides in the Al and Cu surfaces. The broken oxides are eliminated from the combination zones by the flowing metal, and then transmitted to the weld interface edge. Second, it is beneficial to generation of intimate nascent metal contact, which is a necessary condition for UMW. Atomic forces can begin to take effect till the contact interfaces are brought close enough together. Third, interfacial plastic deformation is critical to produce a weld with satisfactory weld density. Roughness on the surfaces prevents the generation of intimate contact surfaces. Before the welding process, numerous microasperities existed on the contact surfaces of the sheets, and the mutual touch first appears in the asperity surface, leading to a large amount of no-contacted regions remain in the weld interface. Combination in the un-contact zones does not take place until a method of closing these voids produces and makes the surfaces contact perfectly. This is where the interfacial plastic flow plays a key role, promoting the extrusion of materials extruding into the
15
voids. As the welding proceeds, the bonded areas will expand in size and number, resulting in more effective contact areas. In this work, from the results mentioned above, higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer, which is more favorable to obtain a sound joint with relatively high weld density, as indicated in Fig.5b, d, f, h and j. Therefore, lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer. 3.3. Weld interface Considering the maximum lap shear tensile strength of Al/Cu joint with interlayer, to understand the interfaces of Cu/interlayer, Al/interlayer, TEM and EDS was conducted. Fig.10a and c show the interfaces of Al/interlayer and Cu/interlayer, respectively. It is obtained that the interface bonding of Al/interlayer and Cu/interlayer is tightly intimate, and no gaps exist in the interface. In Fig.10b, it is demonstrated that the Al2219 particle grain is very long, indicating that severe plastic deformation takes place in the interlayer during UMW, which is in agreement with the results above. As seen from Fig. 10d, the EDS line scan analysis results show that a sudden change of Al and Cu composition across the border, the diffusion zone width is too small to obtain a conclusion with any degree of confidence, taking into consideration the resolution limit of the EDS technology, suggesting that no intermediate layer might form in the interface. Therefore, the welding mechanisms in this study contain solid-state plastic deformation, mechanical interlocking, as well as the formation and progressive spreading of microwelds, which are in accordance with the previous studies by Chen et al. 2012 and Panteli et al. (2012). 3.4. Fracture morphology To identify effects of lap configuration on the weld interface, fracture morphology at a welding
16
time of 0.5s was carried out. Fig.11a and c show the fracture morphology near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; Fig.11b and d show the fracture morphology in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively. For the Cu/Al joint with interlayer, it can be observed that the fracture surface near the periphery is flat, and the flat fracture surface is replaced with dimples at the center of the weld, indicating that the interface combination strength near the periphery is lower than that at the center of the weld. It is more possibility that the fracture occurs through the cavities and unbonding regions in the peripheral areas (Fig.11a and b). Compared with the Cu/Al joint with interlayer, more dimple fracture occurs in the fracture of the Al/Cu joint with interlayer at the center and in the border areas, where the elongated dimples suggest that the shear fracture via void formation/nucleation, growth and coalescence. Meanwhile it can be seen for both joints with different lap configurations that the dimples at the center of the weld is more than that in the border areas of the weld, and the dimples of Al/Cu joint with interlayer at the center and in the border areas are more than that of Cu/Al joint with interlayer, respectively. Which confirm that the interface combination strength of Cu/interlayer and interlayer/interlayer in Al/Cu joint with interlayer is better than that in Cu/Al joint with interlayer, and the interface combination strength at the center is higher than that near the periphery. Moreover good interface combination can transmit a higher load to the base metal, suggesting that the lap shear tensile strength of Al/Cu joint with interlayer is higher than that of Cu/Al joint with interlayer, and this is well agreement with the lap shear tensile strength shown in Fig.9. 4. Conclusions UMWed joints of Al to Cu sheets with an Al2219 alloy particle interlayer were successfully
17
completed with different lap configurations. The following conclusions could be drawn: (1) Higher temperature and more severe plastic deformations took place in the weld interface of Al/Cu joint with interlayer compared with that of Cu/Al joint with interlayer. (2) At the Al side of both joints, the original microstructure (elongated grains) vanished and recrystallized microstructure is emerged along the weld interface. (3) For the both joints, with the increase of distance from the weld interface of Al/Al219 along the thickness direction of the aluminum sheet, the hardness increases till the distance is 600 μm. Meanwhile, the hardness of Al/Cu joint with interlayer is lower than that of Cu/Al joint with interlayer in the same distance. (4) The peak lap shear tensile strength of Al/Cu joint with interlayer at a welding time of 0.5s (78.3MPa) is 27.3% higher than that of Cu/Al joint with interlayer (61.5MPa). Acknowledgments This work was funded by the National Natural Science Foundation of China (No.51375332), Natural Science Foundation of Tianjin (No.16JCYBJC18700).
18
References Bakavos, D., Prangnell, P.B., 2010. Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet. Mater. Scie. Eng. A. 527, 6320-6334. Balasundaram, R., Patel, V.K., Bhole, S.D., Chen, D.L., 2014. Effect of zinc interlayer on ultrasonic spot welded aluminum-to-copper joints. Mater. Scie. Eng. A. 607, 277-286. Chen, Y., Bakavos, D., Gholinia, A., Prangnell, P., 2012. HAZ development and accelerated post-weld natural ageing in ultrasonic spot welding aluminium 6111-T4 automotive sheet. Acta Mater. 60(6), 2816-2828. De Vries, E., (Dissertation) 2004. Mechanics and mechanisms of ultrasonic metal welding. The Ohio State University. Fujii, H.T., Goto,Y., Sato, Y.S., Kokawa, H., 2016. Microstructure and lap shear strength of the weld interface in ultrasonic welding of Al alloy to stainless steel. Scripta Mater. 116, 135–138. Hall, E. O., 1954. Variation of hardness of metals with grain size. Nature 173(4411), 948-949. Janaki Ram, G.D., Robinson, C., Yang, Y., Stucker, B.E., 2007. Use of ultrasonic consolidation for fabrication of multi-material structures. Rapid Prototyping J. 13 (4), 226–235. Ji, H., Wang, J., Li, M., 2014. Evolution of the bulk microstructure in 1100 aluminum builds fabricated by ultrasonic metal welding. J. Mater. Process. Technol. 214(2), 175-182. Macwan, A., Chen, D.L., 2016. Ultrasonic spot welding of rare-earth containing ZEK100 magnesium alloy to 5754 aluminum alloy. Mater. Scie. Eng. A. 666, 139-148. Mirza, F.A., Macwan, A., Bhole, S. D., Chen, D.L., Chen, X.G., 2016. Effect of welding energy on microstructure and strength of ultrasonic spot welded dissimilar joints of aluminum to steel
19
sheets. Mater. Scie. Eng. A. 668, 73-85. Panteli, A., Robson, J.D., Brough, I., Prangnell, P.B., 2012. The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium. Mater. Scie. Eng. A. 556, 31-42. Panteli, A., Robson, J.D., Chen, Y.C., Prangnell, P. B., 2013. The effectiveness of surface coatings on preventing interfacial reaction during ultrasonic welding of aluminum to magnesium. Metall. Mater. Trans. A. 44(13), 5773-5781. Saeid, T., Abdollah-Zadeh, A., Sazgari, B., 2010. Weldability and mechanical properties of dissimilar aluminum–copper lap joints made by friction stir welding. J. Alloy. Compd. 490(1-2), 652-655. Sahin, M., 2010. Joining of aluminium and copper materials with friction welding. Int. J. Adv. Manuf. Tech. 49(5), 527-534. Shakil, M., Tariq, N.H., Ahmad, M., Choudhary, M. A., Akhter, J. I., Babu, S. S., 2014. Effect of ultrasonic welding parameters on microstructure and mechanical properties of dissimilar joints. Mater. Design 55(6), 263-273. Wu, X., Liu, T., Cai, W., 2015. Microstructure, welding mechanism, and failure of Al/Cu ultrasonic welds. J. Manuf. Process. 20, 321-331. Xu, L., Wang, L., Chen, Y.C., Robson, J.D., Prangnell, P. B., 2016. Effect of interfacial reaction on the mechanical performance of steel to aluminum dissimilar ultrasonic spot welds. Metall. Mater. Trans. A. 47(1), 1-13. Xu, W., Liu, J., Luan, G., Dong, C., 2000. Microstructure and mechanical properties of friction stir welded joints in 2219-T6 aluminum alloy. Mater. Design 30(9), 3460-3467.
20
Xue, P., Ni, D.R., Wang, D., Xiao, B.L., Ma, Z.Y., 2011. Effect of friction stir welding parameters on the microstructure and mechanical properties of the dissimilar Al–Cu joints. Mater. Scie. Eng. A. 528, 4683–4689. Yang, J.W., Cao, B., He, X.C., Luo, H. S., 2014. Microstructure evolution and mechanical properties of Cu–Al joints by ultrasonic welding. Sci. Technol. Weld. Joining 19(6), 500-504. Yuichi, H., Chihiro, I., Yoshihito, K., 2016. Microstructure evolution and mechanical properties of extruded Mg96Zn2Y2 alloy joints with ultrasonic spot welding. Mater. Scie. Eng. A. 651, 925-934. Zhang, C.Y., Chen, D.L., Luo, A.A., 2014. Joining 5754 automotive aluminum alloy 2-mm-thick sheets using ultrasonic spot welding. Weld. J. 93(4), 131S-138S. Zhao, Y.Y., Li, D., Zhang, Y.S., 2013. Effect of welding energy on interface zone of Al–Cu ultrasonic welded joint. Sci. Technol. Weld. Joining 18(4): 354-360.
21
a
Pressurizing bar
Frame
Smart control power box
Transducer
Anvil
Sonotrode tip
Die
b
Fig.1 (a) A SONICS MW-20 welder employed for UMW; (b) detailed knurl pattern of sonotrode tip (left) and its dimension (right).
22
Fig.2 (a) and (b) showing the optical microstructure of original aluminum and copper sheets, respectively; (c) EBSD Euler map of the original grain structure of aluminum sheet; (d) and (e) SEM observations of the Al2219 particle.
23
b a
Clamping pressure
Clamping pressure
Vibration direction
Al
Cu
Vibration direction Interlayer (Al 2219 particle)
Interlayer Cu (Al 2219 Al particle) Fig.3 Joints configuration in this study: (a) Cu/Al joint with interlayer and (b) Al/Cu joint with interlayer.
24
Fig.4 Typical surface appearances of the UMWed joints utilizing different lap configurations at a welding time of 0.5s: (a) and (b) showing the top and bottom sheets of Al/Cu joint with interlayer, respectively; (c) and (d) showing the top and bottom sheets of Cu/Al joint with interlayer, respectively.
25
26
Al2Cu
Fig.5 (a) and (b) showing the microstructure near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; (c) and (d) showing the microstructure in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer at a welding time of 0.5s, respectively; (e), (g) and (i) showing the magnified images of box indicated in (c); and (f), (h) and (j) showing the magnified images of box indicated in (d).
27
Fig.6 Plastic deformation of Cu side for both joints with different lap configurations at a welding time of 0.5s: (a) original state of Cu sheet; (b) and (d) showing the plastic deformation near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively;
28
(c) and (e) showing the plastic deformation in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively.
29
30
Fig.7 OM and EBSD images of Cu side and Al side for both joints with different lap configurations at a welding time of 0.5s: (a) and (c) showing the OM images of Cu side near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (b) and (d)
31
showing the OM images of Cu side in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (e) and (i) showing the OM images of Al side near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (f) and (j) showing the OM images of Al side in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (g), (h), (k) and (l) the grain images (EBSD) taken in the areas close to the Al/Al2219 interface corresponding to (e), (f), (i) and (j), respectively.
32
a
b
Fig.8 Vickers characteristic microhardness profiles of Al side in the UMWed joints with different lap configurations at a welding time of 0.5s; (a) vertical in the weld center; (b) horizontal.
33
Fig.9 Lap shear tensile strength of the UMWed joints with different lap configurations.
34
Fig.10 TEM and EDS analysis of the Al/Cu joint with interlayer at a welding time of 0.5s: (a) interface of Al/interlayer; (b) interlayer; (c) interface of Cu/interlayer; (d) EDS result as indicated in (c).
35
Fig.11 Fracture morphology of Al side of the joints with different lap configurations at a welding time of 0.5s: (a) and (c) showing the fracture morphology near the periphery of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively; (b) and (d) showing the fracture morphology in the center of the Cu/Al joint with interlayer and Al/Cu joint with interlayer, respectively.
36