Ti alloy joints

Ti alloy joints

Accepted Manuscript Microstructure and Mechanical Properties of Ultrasonic Spot Welded Al/Ti Alloy Joints S.Q. Wang, V.K. Patel, S.D. Bhole, G.D. Wen,...

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Accepted Manuscript Microstructure and Mechanical Properties of Ultrasonic Spot Welded Al/Ti Alloy Joints S.Q. Wang, V.K. Patel, S.D. Bhole, G.D. Wen, D.L. Chen PII: DOI: Reference:

S0261-3069(15)00199-5 http://dx.doi.org/10.1016/j.matdes.2015.04.023 JMAD 7204

To appear in:

Materials and Design

Received Date: Revised Date: Accepted Date:

20 January 2015 4 March 2015 13 April 2015

Please cite this article as: Wang, S.Q., Patel, V.K., Bhole, S.D., Wen, G.D., Chen, D.L., Microstructure and Mechanical Properties of Ultrasonic Spot Welded Al/Ti Alloy Joints, Materials and Design (2015), doi: http:// dx.doi.org/10.1016/j.matdes.2015.04.023

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Microstructure and Mechanical Properties of Ultrasonic Spot Welded Al/Ti Alloy Joints

S.Q. Wang1,2*, V.K. Patel2, S.D. Bhole2, G.D. Wen3, D.L. Chen2*

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School of Materials Science and Engineering, Xi’an Shiyou University, 18 Dianzier Road, Xi’an, Shaanxi, 710065, PR China

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Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario M5B 2K3, Canada

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Xi’an Research Institute of China Coal Technology&Engineering Group Corp, 1 Jingyeyi Road, Xi’an, Shaanxi,710077, PR China

Abstract

The microstructure, hardness, and tensile properties of solid-state ultrasonic spot welded (USWed) dissimilar joints between Al5754-O and Ti-6Al-4V alloys with or without a pure Al interlayer were studied. Significant difference in the microstructure was observed at the interface of the USWed Al/Ti alloy joints, where the phenomenon of adhesion on each side of the joint with Al interlayer was more obvious than that of the joint without Al interlayer. An asymmetrical hardness profile across the dissimilar joint was observed such that the hardness value increased *

Corresponding author – Tel: +86 29 8838 2607; Email: [email protected] (S.Q. Wang). Tel: (416) 979-5000 ext. 6487; Fax: (416) 979-5265; Email: [email protected] (D.L. Chen).

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gradually from the Al side to the Ti side. With increasing energy input, the lap shear strength of the USWed Al/Ti alloy joint with a 75 µ m thick Al interlayer first increased and then decreased, with the maximum lap shear strength reaching about 206 MPa at a welding energy of 1000 J. No samples failed at the interface when the energy was above 1000 J or the thickness of Al interlayer was more than 75 µ m. The fracture was predominantly characterized by the dimplelike ductile fracture.

Keywords: Titanium alloy; Al alloy; dissimilar welded joints; tensile shear strength; ultrasonic spot welding.

1. Introduction

Due to recent tremendous environmental concerns and ever-increasing global energy demand facing the automotive and aerospace sectors, lightweighting of vehicles is today considered as a salient design tool for improving fuel economy and reducing anthropogenic climate-changing, environment-damaging, costly and human death-causing † emissions [1-6]. This inevitably involves the application of lightweight alloys, e.g., aluminum, magnesium, and titanium alloys, as well as their joining and welding in manufacturing [7-12]. While ultra-lightweight magnesium alloys have increasingly been used in the transportation industry, some issues involving their corrosion resistance, room temperature formability, tension-compression yield asymmetry and



According to Science News entitled “Air pollution kills 7 million people a year” on March 25, 2014 at http://news.sciencemag.org/signal-noise/2014/03/air-pollution-kills-7-million-people-year: “Air pollution isn’t just harming Earth; it’s hurting us, too. Startling new numbers released by the World Health Organization today reveal that one in eight deaths are a result of exposure to air pollution. The data reveal a strong link between the tiny particles that we breathe into our lungs and the illnesses they can lead to, including stroke, heart attack, lung cancer, and chronic obstructive pulmonary disease.”

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anisotropy have not been fully solved, thus limiting their widespread structural application to a certain extent [13-22]. On the other hand, aluminum and titanium alloys have already found their broad structural applications due to their superior formability, excellent mechanical properties and high corrosion resistance [7,8,23-25]. As high performance, light weight, and cost reduction are major considerations in various industrial sectors, the demand for dissimilar material joints has recently increased [7-9,11,26-28]. In the case of aerospace and automotive industry, the joining of aluminum alloy with titanium alloy could have a wide range of applications in the structural components where high strength and lightweight are preferred [7,8,29]. However, joining Al-to-Ti is challenging since they differ significantly in physical, chemical, and mechanical properties, including crystal structure, melting point, thermal conductivity, and linear coefficient of thermal expansion [30-32]. For example, the melting points of Al alloy and Ti alloy are about 660°C and 1650°C, respectively, with a difference of as large as nearly 1000°C. This huge difference in the melting points between the two metals leads to difficulties in joining them by conventional fusion welding processes. In addition, Al element evaporates at temperatures higher than the melting point of Al alloy, which leads to a severe loss in Al element and also asymmetry in the composition of the weld metal during fusion welding [33,34]. Furthermore, Al alloys always produce coarse grains during welding [28], which deteriorate the mechanical properties. From the Ti-Al phase diagram [35], the intermetallic compounds, namely Ti3Al, TiAl, TiAl2 and TiAl3, could potentially be formed in the fusion zone during welding depending on the composition. Several studies showed that TiAl3 formed more easily than the other intermetallic compounds because of its high kinetic and thermodynamic stability [36-39]. However, TiAl3 is very brittle with a low strength and fracture toughness, and it would become a source of internal microcracking during loading, resulting in a low joint strength [7,8,40]. This

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situation suggests that fusion welding of Ti and Al alloys cannot be practically used. According to Rathod and Kutsuna [41], the dissimilar joints such as steel/Al and Al/Ti alloys, could be attained through a solid/liquid state reaction at the joining interface between two metals, where only the metal with a lower melting temperature is melted. To suppress the formation of excessive intermetallics in the liquid state, it would be better to join Al and Ti alloys via a solidstate welding process which is carried out below the lower melting point.

A number of studies have been reported on the Al/Ti alloy dissimilar joint using solid-state welding, such as friction stir welding (FSW). Fuji [42] investigated the growth behavior of an intermetallic layer in a friction-welded joint between pure Al and pure Ti, and reported that the layer grew from the Al substrate to the Ti substrate, and neither linear nor parabolic time dependence could be used to describe the rate of layer growth. Kim and Fuji [43] studied the dominant factors determining the joint characteristics (strength, ductility, etc.) in friction welds between Al and Ti, and observed that the joint characteristics were dominated mainly by the thickness of the intermetallic compound layer produced at the interface. Aonuma and Nakata [44] examined the weldability of 2024-T3 and 7075-T651 aluminum alloys joined to pure Ti and Ti6Al-4V alloy by FSW, and observed a mixed region of Ti alloy and Al alloy at the joint interface with TiAl3 intermetallic layer, where fractured occurred. Wei et al. [45] joined 1060 aluminum and Ti-6Al-4V titanium alloy in the lap form using FSW, and achieved a joint failure load (1.91 kN) of equivalent to that of 1060 Al base metal. Chen et al. [46] conducted aluminum alloy LF6 and titanium alloy TC1 butt and lap joints with FSW, and evaluated the influence of process parameters on formation of weld surface, cross-section morphology and strength. However, no report has been seen on the dissimilar Al/Ti alloy joints using ultrasonic spot welding (USW),

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which is another solid-state joining process that produces coalescence through a concurrent application of localized high-frequency vibratory energy and slight clamping force [47]. The question remains unknown about the weldability of Al-to-Ti alloy using USW and the effect of the welding energy on the tensile behavior of lap Al/Ti alloy joints with an Al interlayer. The main objective of this work was, therefore, to evaluate the microstructure and mechanical properties of the USWed Al/Ti joints made with varying welding energy levels.

2. Material and Experimental Procedure

A 1.5 mm thick Al5754-O (Al-3.42Mg-0.63Mn-0.23Sc-0.22Zr) Al alloy sheet provided by General Motors Company, and a forged Ti-6Al-4V alloy sheet with a thickness of 1 mm were used for USW. The Al5754-O and Ti-6Al-4V sheets were machined into the specimens with the dimensions 50 mm × 10 mm × 1.5 mm, and 50 mm × 10 mm × 1 mm, respectively. The faying surfaces of the specimen were ground using 120 emery papers and then cleaned using acetone and dried before welding. During welding, pure Al interlayer was placed in-between the workpiece of the Al/Ti alloy sheets. USW was performed using a dual wedge reed SonobondMH2016 HPUSW system at energy inputs from 500 J to 1500 J and welding time from 0.25s to 1s, at a constant power setting of 2000 W, an impedance setting of 8 and a pressure of ~0.4 MPa.

Metallographic samples were cut from the ultrasonic spot welded (USWed) joints, then ground and polished. Microstructures were examined via scanning electron microscopy (SEM) using a JSM-6380LV microscope equipped with Oxford energy dispersive X-ray spectroscopy (EDS) system and 3D fractographic analysis capacity. Microhardness was determined across the weld,

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using a computerized Buehler hardness tester with a load of 100 g and a dwell time of 15 s at an interval of 0.3 mm. To evaluate the mechanical strength of the joints and establish the optimum welding conditions, tensile shear tests of the dissimilar joints were conducted to determine the lap shear failure load using a fully computerized United tensile testing machine with a constant displacement rate of 1 mm/min in air at room temperature. The samples were loaded along the length direction of the joined test coupons. In the tensile shear testing, restraining shims were used to minimize the rotation of the joints and maintain the shear loading as long as possible. The fracture surfaces and the interface of the failed samples were examined via SEM.

3. Results and Discussion

3.1

Joint microstructure

Figs 1 and 2 show the microstructure at the center of the weld nugget of the USWed Al/Ti alloy joints without and with Al interlayer, respectively. A sound joint was obtained under most of the welding conditions because no welding defects were present, such as crack or tunnel type defects. Unlike the case of the joint between Al and Mg [27,28], almost no significant interfacial reaction layer was observed at the center of the weld nugget at both low and high magnifications in the joint with and without Al interlayer, and a clear boundary between Al and Ti alloys was seen from Fig. 1(a), (b) and Fig. 2(a), (b). Similar result was reported on the FSWed dissimilar joints between a 4 mm thick ADC12 cast aluminum alloy sheet and a 2 mm thick commercially pure titanium sheet by Chen and Nakata [36]. However, close examinations on the EDS line scan analysis results shown in Fig. 1(c) and Fig. 2(c) indicated gradual change of Al and Ti

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composition across the border, instead of a sudden change (vertical line), suggesting that a thin transient layer might form at the joining interface. This could be better seen from the fracture surface. Fig. 3 shows the morphology of the matching fracture surfaces on the Al (left) side and Ti (right) side at an energy input of 1000 J without any interlayer. It can be seen that on both Al side and Ti side, there was no significant difference in the center compared to the edge of the nugget zone at low magnification (Fig. 3(a) and (b)). However, on the Al side, a few small white fragments (as indicated by the yellow dashed arrow in Fig. 3(c)), were observed, which are rich in Ti and V elements as seen from the EDS line scan (Fig. 3(e)). On the Ti side, some grey discontinuous “islands” (as indicated by the red solid arrows in Fig. 3(d)) were observed, which mainly contained Al and confirmed by the EDS line scan (Fig. 3(f)).

To further examine the microstructure and phase composition at the interface of the USWed Al/Ti alloy joint with Al interlayer, the interface was separated intentionally using a hand tool, since no joints failed at the interface at a welding energy of 1000 J with a 75 µm thick Al interlayer after the tensile tests. Fig. 4 shows the morphology of matching fracture surfaces for the joint with 75 µm thick pure Al interlayer at an energy input of 1000 J after the tensile test. Compared to the joint without any interlayer (Fig. 3), a significant difference was observed between the center and the edge of the nugget zone. At a lower magnification, especially on the Ti side, a lot of grey Al “islands” (Fig. 4(b)) were present in the center compared to the edge of the nugget zone, which was not observed in the joint without the interlayer. At a higher magnification (Fig. 4(c) and (e)), on the Al side some fragments containing Ti and V elements were observed on the fracture surface of the joint with Al interlayer. Similarly, on the Ti side, a lot of Al “islands” (Fig. 4(d) and (f)) with an irregular shape appeared on the fracture surface of

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the joint with Al interlayer. The phenomenon of adhesion on each side indicated that substantial interdiffusion between Al and Ti occurred within about 0.5 s, and relatively strong bonds were formed between the Al and Ti alloys with Al interlayer. Similar phenomenon was also observed in the FSWed joint between Al and Ti by probe [29]. In addition, the dimples on both Al and Ti sides as shown in Fig. 4(g) and (h) were observed on the fracture interfaces of the joint, which also confirmed that the joint was good.

To identify the phases present, the matching fracture surfaces of the joint were analyzed via Xray diffraction (XRD). Fig. 5 shows the XRD pattern of the joint without any interlayer at the energy of 1000 J, and Al and Ti phases appeared on the Ti side and Al side, respectively, which corresponded well to the microstructure shown in Fig. 3. Fig. 6 shows the XRD pattern of the joint with a 75 µm thick Al interlayer at an energy of 1000 J conducted on both Al and Ti sides, and the peaks of Al, Ti, and TiAl3 phases could be seen from Fig. 6, which corresponded well to the microstructure shown in Fig. 4. It is also seen that the peaks of TiAl3 on the Ti side were more than that on the Al side. Chen et al. [7], Ma et al. [8], Kenevisi and Khoie [32], and Chen and Nakata [36] all observed TiAl3 phase in ultrasonic-assisted brazed, transient liquid phase bonded, and friction stir welded joints between Al and Ti, respectively.

An important factor of welding dissimilar Al and Ti alloys is the formation of intermetallic phases, which was mainly dependent on the thermal cycle, including the heating rate, maximum heating temperature, dwelling time at high temperatures, and cooling rate. As mentioned before, several intermetallic compounds, namely, TiAl3, TiAl, and Ti3Al could be formed. However, XRD results revealed that in this study only TiAl3 formed as shown in Fig. 6. This was related to

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the short time in the order of about 0.5 second during USW where only limited diffusion could occur, thereby only the easy-forming intermetallic compound with a relatively low melting point could be expected. Loo and Rieck [48], Luo and Acoff [49], Wang et al. [50-52] and Rawers and Wrzesinski [53] confirmed that TiAl3 phase was much easier to form than any other titanium aluminide. Indeed, this can clearly be seen from the Ti-Al phase diagram [35] where TiAl3 phase could form easily at a relatively low temperature. Other studies on the dissimilar Al/Ti joining [7,8,36-39,44] also indicated that TiAl3 had the lowest free energy of formation among the compounds of Type-I aluminides including TiAl and Ti3Al. Therefore, it is reasonable that TiAl3 phase forms preferentially in the USWed dissimilar Al/Ti alloy joint.

3.2

Hardness and tensile properties

Fig. 7 shows Vickers microhardness profiles along a diagonal line (dashed line) of the dissimilar USWed Al/Ti alloy joints. It is seen that a typical asymmetrical hardness profile across the Al/Ti alloy joint was obtained. The base metal hardness was observed to be significantly lower in the Al5754-O alloy than in the Ti-6Al-4V alloy. It is seen that the hardness value increased from the Al side to the Ti side gradually within a transitional zone, which was related to the mix of both Al and Ti matrixes during USW. This indicated that interfacial reactions between titanium and aluminum alloys during USW occurred, otherwise a sudden change in the hardness would be expected. On the Al side, the hardness value of the joint with or without Al interlayer was almost the same in different conditions, while on the Ti side the hardness value of the joint with Al interlayer was slightly lower than that of the joint without Al interlayer with a bigger scatter. This was attributed to the diffusion of Al into Ti to form TiAl3 phase having a porous

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characteristic (Fig.3(d) and Fig.4(d)). Such a porous feature of TiAl3 intermetallic compound has also been reported in the diffusion bonding of pure titanium and pure aluminum [48,49]. The hardness distribution observed in this study was in good agreement to that reported in transient liquid phase bonded, friction stir welded, TIG arc welded, transient liquid phase bonded Al/Ti alloy joints [32,44,54,55].

Fig. 8 shows the change of lap shear fracture load as a function of displacement at different energy levels for USWed Al/Ti alloy joints with a 75 µm thick Al interlayer, at a strain rate of 1 mm/min at room temperature. The USWed Al/Ti alloy joint exhibited basically smooth and continuous curves. Similar smooth and continuous stress-strain curves for FSWed Al/Ti alloy joint were also reported by Dressler et al. [26] and Bang et al. [29]. It is seen that the load increased gradually with increasing energy from 500 J to 1000 J, then decreased with further energy increase from 1250 J to 1500 J with the same thickness of Al interlayer. Chen et al. [33], Chen and Nakata et al. [36], Qiu et al. [56], and Lee et al. [57] also investigated the effect of welding parameters on the shear load of the RSWed, FSWed, and Al/Ti alloy joints.

3.3

Effect of Al interlayer thickness and welding energy on lap shear strength

Fig. 9 shows the effect of Al interlayer thickness on the lap shear strength of USWed Al/Ti alloy joint at a constant welding energy of 1000 J, where the lap shear strength is defined as the failure load divided by the nugget area under the weld tip of 5 mm × 8 mm. It is seen that the lap shear strength was fairly low (about 75 MPa) when the thickness of Al interlayer was about 25 µm. However, the lap shear strength reached about 200 MPa when the thickness of Al interlayer

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reached about 75 µm, and it remained nearly constant with a further increase in the interlayer thickness up to ~500 µm. Kim and Fuji [43] reported that the dominant factor determining the joint mechanical characteristic in friction welds between Al and Ti was the thickness of the TiAl3 intermetallic compound layer produced at the interface, and that the critical thickness of the intermetallic compound layer was about 5 µm. The joint characteristics were deteriorated as the thickness of the intermetallic compound layer increased. In the present experiment, the TiAl3 phase occurred in the extremely narrow region at the interface (Figs 1 and 2) and did not exceed the critical value of 5 µm. Therefore, the dissimilar USWed Al/Ti alloy joints showed a fairly high lap shear strength (Fig.9).

Fig. 10 shows the lap shear strength of the USWed Al/Ti alloy joint with a 75 µm thick Al interlayer as a function of energy inputs. As the welding energy increased, the lap shear strength first increased, and then decreased. As seen from Fig. 10, the maximum lap shear strength was about 206 MPa at a welding energy of 1000 J. A summary on the lap shear strength of the joints between Al and Ti alloys in different welding conditions is given in Table 1. It is seen that the present USWed Al/Ti alloy joint exhibited a higher lap shear strength than the transient liquid phase bonded, liquid state diffusion bonded, friction stir welded Al/Ti alloy joints [30,31,44,46,55], which suggested that sound welded joints can be made using USW with proper parameters in the present study.

The joining mechanism in the USW could be described as follows. When the welding starts, the heat is generated by the ultrasonic vibration at the interface, and the temperature increases rapidly [47], and could be high enough to melt the pure Al interlayer (placed at the center in-

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between the two sheets) and even potentially melt Al alloy sheet in the vicinity of interface. However, it would be difficult to melt the Ti-6Al-4V alloy at the same temperature. At this temperature, the solid Ti-6Al-4V alloy is seen to dissolve into the liquid-like Al as shown in Fig. 4 to form a reaction layer. The reaction layer first develops from isolated areas or islands, and these weld islands then increase in size and spread across the interface as the welding progresses until they merge into a continuous layer [58]. In addition, the heat generated by ultrasonic vibration raises the temperature to initiate the exothermic reaction 3Al+Ti→TiAl3+Q [36]. The reaction heat release further increases the temperature [36], which can promote the reaction to achieve a sound joint. Furthermore, Lee et al. [59] noted that the interdiffusion coefficient increases gradually with increasing Al content in β-titanium at a temperature range from 1050 to 1550°C. Similarly, using pure Al would accelerate the diffusion between Al alloy and Ti alloy. Therefore, when the thickness of pure Al interlayer is 75 µm thick or greater, the reaction layer will be almost continuously distributed at the interface between the two sheets and is very stable as shown in Fig. 4, suggesting that Al and Ti alloys were comprehensively joined.

3.4

Failure location and fractography

Fig. 11(a) and (b) show the failure location of the dissimilar USWed Al/Ti alloy joints made with different thicknesses of Al interlayer or different welding energies after the lap shear tensile tests. When the thickness of Al interlayer was about 25 µm or the welding energy was lower than 750 J, the interfacial failure occurred in the joints, as shown by the top samples in Fig. 11(a) and (b). However, when the thickness of Al interlayer was equal to or more than 75 µm, or the welding energy was equal to or more than 1000 J, failure occurred in the heat-affected zone (HAZ) on the

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Al side close to the edge of weld nugget, which was the weakest position across the welded joint due to the deformation. In this case no samples failed at the interface, indicating that the strength of weld nugget was higher than that of the HAZ on the Al side due to the presence of reaction layer by the formation of TiAl3 phase (Figs 4 and 6). Furthermore, the difference in hardness between the Al and Ti alloys significantly affects metal deformation during USW. Al alloy is softer than Ti alloy as shown in Fig. 7, and is easily deformed plastically during USW with the harder metal (Ti) undergoing less deformation. Therefore, when the thickness of Al interlayer was more than 75 µm or the energy was more than 1000 J, sound joints were achieved and they failed in the HAZ on the Al side. Fig. 11(c) shows a typical image of the fracture surface of a joint made at a welding energy of 1000 J with an Al interlayer thickness of 75 µm. The crack initiated from the intersection of the weld tip edge with the HAZ (as indicated by a red dashed arrow in Fig.11(a)) due to the presence of stress concentration and low hardness on the Al side (Fig.7). Then the crack propagated in the HAZ, which was predominantly characterized by the dimple-like ductile failure (Fig.11(d)).

4. Conclusions

A sound joint of USWed Al/Ti alloy sheets with an Al interlayer was successfully achieved and the relevant microstructure, tensile properties, and fracture mode of the joints were evaluated. The significant microstructural change was observed at the interface of the USWed Al/Ti alloy joints with or without Al interlayer, where the phenomenon of adhesion on each side of the joints with an Al interlayer was more obvious than that of the joints without Al interlayer. A characteristic asymmetrical microhardness profile across the dissimilar joint was observed and

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the hardness value increased gradually from Al side to Ti side. A lap shear strength of 206 MPa was achieved at a welding energy of 1000 J with a 75 µm Al interlayer. With further increasing thickness of Al interlayer, the lap shear strength of the USWed Al/Ti alloy joints remained almost the same at a constant energy of 1000 J. No samples were observed to fail at the interface as long as proper welding parameters were used. The fracture was predominantly characterized by the dimple-like ductile failure.

Acknowledgements

The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support, and Northwestern Polytechnical University (NWPU), Xi’an, China for providing test materials. One of the authors (D.L. Chen) is also grateful for the financial support by the Premier’s Research Excellence Award (PREA), NSERC-Discovery Accelerator Supplement (DAS) Award, Automotive Partnership Canada (APC), Canada Foundation for Innovation (CFI), and Ryerson Research Chair (RRC) program. The authors would also like to thank Messrs. Q. Li, A. Machin, J. Amankrah and R. Churaman for easy access to the laboratory facilities of Ryerson University and their assistance in the experiments.

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Table Captions

Table 1 Comparison of the lap shear strength of the joints between Al and Ti alloys.

Figure Captions

Fig. 1 Microstructure of a dissimilar USWed Al/Ti alloy joint without interlayer at a welding energy of 1000 J in the middle of weld nugget, (a) overall view of the cross section, (b) magnified area of the dashed box in (a), and (c) EDS line scan across the welded joint as indicated by the dashed line in (b). Fig. 2 The microstructure of a dissimilar USWed Al/Ti alloy joint with a 75 µm thick pure Al interlayer at a welding energy of 1000 J in the middle of weld nugget, (a) overall view of the cross section, (b) magnified area of the dashed box in (a), and (c) EDS line scan across welded joint as indicated by dashed line in (b). Fig. 3 Interface of a dissimilar USWed Al/Ti alloy joint without interlayer at a welding energy of 1000 J in the middle of weld nugget, showing (a) and (b) the interface of the welded joint on the Al (left) and Ti (right) sides, respectively, (c) and (d) magnified area of the

21

dashed boxes in (a) and (b), respectively, and (e) and (f) EDS line scan across the interface as indicated by the dashed lines, respectively. Fig. 4 Interface of a dissimilar USWed Al/Ti alloy joint with 75 µm Al interlayer at a welding energy of 1000 J in the middle of weld nugget, showing (a) and (b) the interface of the welded joint on the Al (left) and Ti (right) sides, respectively, (c) and (g), (d) and (h) magnified area of the dashed box in (a) and (b), (e) and (f) EDS line scan as indicated by the dashed lines. Fig. 5 XRD patterns of a dissimilar USWed Al/Ti alloy joint without interlayers at a welding energy of 1000 J, (a) on the Al side, and (b) on the Ti side. Fig. 6 XRD patterns of a dissimilar USWed Al/Ti alloy joint with 75 µm thick pure Al interlayers at a welding energy of 1000 J, (a) on the Al side, and (b) on the Ti side. Fig. 7 Hardness distribution of the dissimilar USWed Al/Ti alloy joints with or without pure Al at different welding energy levels. Fig. 8 Load vs. displacement curves for the dissimilar USWed Al/Ti alloy joints made at different energy levels. Fig. 9 Lap shear strength of the dissimilar USWed Al/Ti alloy joint made at a welding energy of 1000 J as a function of Al interlayer thickness. Fig. 10 Lap shear strength of the dissimilar USWed Al/Ti alloy joint with a 75 µm thick pure Al interlayer as a function of welding energy. Fig. 11 Failure location and fracture surface morphology of the dissimilar USWed Al/Ti alloy joint, (a) and (b) an overall view of failure location at different thicknesses of pure Al interlayer and different welding energy levels, respectively, (c) fracture surface in the HAZ of Al side with a 75 µm thick pure Al interlayer at a welding energy of 1000 J (as

22

indicated by the red dashed arrow in (a)), and (d) appearance of magnified area of the dashed box in (c).

23

Table 1 Comparison of the lap shear strength of the joints between Al and Ti alloys.

Materials/Joint

Interlayer/filler

Welding technique

Al5754-O/ Ti6Al4V

Pure Al Sn-4Ag-3.5Bi film

Lap shear strength, MPa 206

This study

36

[30]

19.5

[55]

84

[31]

201

[44]

Ref.

Al7075/Ti6Al4V

Cu

Al1050/Ti

Al-10-Si-1Mg

7075 Al/Ti

-

Ultrasonic spot welding Transient liquid phase bonded Transient liquid phase bonding Liquid state diffusion bonding Friction stir welding

2024/Ti6Al4V

-

Friction stir welding

156

[44]

7075/Ti6Al4V

-

Friction stir welding

55

[44]

LF6/TC1

-

Friction stir welding

131

[46]

Al7075/Ti6Al4V

Wang et al.,

24

(a)

(b) Al alloy Ti-6Al-4V

Al alloy Ti-6Al-4V

350

(c)

Intensity, Counts

Al Mg Ti V

Al

300 250 200

Ti

150 100

Interface

50

V

Mg

0 0

10

20 30 40 Distance, µm

50

60

Fig. 1 Microstructure of a dissimilar USWed Al/Ti alloy joint without interlayer at a welding energy of 1000 J in the middle of weld nugget, (a) overall view of the cross section, (b) magnified area of the dashed box in (a), and (c) EDS line scan across the welded joint as indicated by the dashed line in (b).

Wang et al.,

25

(a)

(b)

Al alloy Ti-6Al-4V

Al alloy

Ti-6Al-4V

500

(c)

Al Mg Ti V

Al

Intensity, Counts

400 300 200

Ti

100 V

Mg 0 0

10

20 30 40 Distance, µm Fig. 2 The microstructure of a dissimilar USWed Al/Ti alloy joint with a 75 μm thick pure Al interlayer at a welding energy of 1000 J in the middle of weld nugget, (a) overall view of the cross section, (b) magnified area of the dashed box in (a), and (c) EDS line scan across welded joint as indicated by dashed line in (b).

Wang et al.,

26

(a)

(b)

Al alloy

(d)

(c)

1200

3000

(e) Al

Intensity, Counts

Intensity, Counts

Ti-6Al-4V

900 Al Mg Ti V

600 Ti 300 V

(f)

Al Mg Ti V

2000

Al Ti

1000 Mg

Mg

0

V

0 0

20

40 60 80 Distance , µm

100

0

10 20 30 40 50 60 70 Distance , µm

Fig. 3 Interface of a dissimilar USWed Al/Ti alloy joint without interlayer at a welding energy of 1000 J in the middle of weld nugget, showing (a) and (b) the interface of the welded joint on the Al (left) and Ti (right) sides, respectively, (c) and (d) magnified area of the dashed boxes in (a) and (b), respectively, and (e) and (f) EDS line scan across the interface as indicated by the dashed lines, respectively.

Wang et al.,

27

(a)

(b)

Al alloy

Ti-6Al-4V

c g

d h

(c)

(d)

Al Mg Ti V

(e)

Intensity, Counts

Al 600

800

(f)

Al Mg Ti V

Al

Intensity, Counts

800

600

400

400

Ti

200

Ti

200

Mg

Mg

V

0

V

0 0

10 20 30 40 50 60 70 Distance , µm

Wang et al.,

0

28

20

40 60 80 Distance , µm

100

(g)

(h)

Ti-6Al-4V

Al alloy

Fig. 4 Interface of a dissimilar USWed Al/Ti alloy joint with 75 µm Al interlayer at a welding energy of 1000 J in the middle of weld nugget, showing (a) and (b) the interface of the welded joint on the Al (left) and Ti (right) sides, respectively, (c) and (g), (d) and (h) magnified area of the dashed box in (a) and (b), (e) and (f) EDS line scan as indicated by the dashed lines.

Wang et al.,

29

120

(a) Al alloy

Intensity, counts

100

Al Ti

80 60 40 20 0 20 800

30

(b)

40 50 60 70 80 Diffraction angle 2θ, ° Ti-6Al-4V

90 Al

Intensity, counts

Ti

600

400

200

0 20

30

40 50 60 70 80 Diffraction angle 2θ, °

90

Fig. 5 XRD patterns of a dissimilar USWed Al/Ti alloy joint without interlayers at a welding energy of 1000 J, (a) on the Al side, and (b) on the Ti side.

Wang et al.,

30

1200

(a)

Al alloy

Al

Intensity, counts

Ti

900

TiAl3

600

300

0 20 1200

30

40 50 60 70 80 Diffraction angle 2θ, °

(b) Ti-6Al-4V

90

Al

Intensity, counts

Ti

900

TiAl3

600

300

0 20

30

40 50 60 70 80 Diffraction angle 2θ, °

90

Fig. 6 XRD patterns of a dissimilar USWed Al/Ti alloy joint with 75 μm thick pure Al interlayers at a welding energy of 1000 J, (a) on the Al side, and (b) on the Ti side.

Wang et al.,

31

400

Joint-1000J-without Al interlayer

Hardness, HV

Joint-2000J-without Al interlayer

Ti-6Al-4V

Joint-1000J-with Al interlayer

300

Joint-2000J-with Al interlayer

200 Al alloy

100

Al alloy Ti-6Al-4V

0 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 Distance from weld centre, mm Fig. 7 Hardness distribution of the dissimilar USWed Al/Ti alloy joints with or without pure Al at different welding energy levels.

Lap shear fracture load, N

4000

3000

2000

Ti/Al joint-500J Ti/Al joint-750J Ti/Al joint-1000J Ti/Al joint-1250J

1000

Ti/Al joint-1500J

0 0

2

4 6 Distance, mm

8

10

Fig. 8 Load vs. displacement curves for the dissimilar USWed Al/Ti alloy joints made at different energy levels. Wang et al.,

32

Lap shear stress, MPa

300 250 200 150 100 50 0 0

100

200

300

400

500

600

Interlayers thickness, µm Fig. 9 Lap shear strength of the dissimilar USWed Al/Ti alloy joint made at a welding energy of 1000 J as a function of Al interlayer thickness.

Lap shear stress, MPa

300 250 200 150 100 50 0 0

500

1000

1500

2000

Welding energy, J Fig. 10 Lap shear strength of the dissimilar USWed Al/Ti alloy joint with a 75 μm thick pure Al interlayer as a function of welding energy.

Wang et al.,

33

(a)

Al alloy

Al alloy

75µm

500 J

125µm

750 J

175µm

1000 J

275µm

1500 J

525µm

(c)

(b)

Ti-6Al-4V

25µm

20 mm

Ti-6Al-4V

20 mm (d)

Fig. 11 Failure location and fracture surface morphology of the dissimilar USWed Al/Ti alloy joint, (a) and (b) an overall view of failure location at different thicknesses of pure Al interlayer and different welding energy levels, respectively, (c) fracture surface in the HAZ of Al side with a 75 µm thick pure Al interlayer at a welding energy of 1000 J (as indicated by the red dashed arrow in (a)), and (d) appearance of magnified area of the dashed box in (c).

Wang et al.,

34

Ultrasonic Spot Welded Al/Ti Alloy Joints

At different welding energy with same Al interlayer thickness

500J

1000J

1500J

At different Al interlayer thickness with same welding energy

25um

Microstructure and phase composition

75um

225um

Mechanical properties

Hardness

Shear strength

The joining mechanism of USWed Al/Ti Alloy Joints

Research flow chart

Fractography

Research highlights    

USWed dissimilar joints between Al5754-O and Ti-6Al-4V alloys were studied. Significant difference in the microstructure was observed at interface of joints. An asymmetrical hardness profile across the dissimilar joint was observed. A sound joint was obtained.

40