Ti dissimilar materials resistance spot welding

Ti dissimilar materials resistance spot welding

Materials & Design 83 (2015) 577–586 Contents lists available at ScienceDirect Materials & Design journal homepage: www.elsevier.com/locate/matdes ...

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Materials & Design 83 (2015) 577–586

Contents lists available at ScienceDirect

Materials & Design journal homepage: www.elsevier.com/locate/matdes

Impact of electromagnetic stirring upon weld quality of Al/Ti dissimilar materials resistance spot welding Y. Li 1, Y. Zhang 1, J. Bi 1, Z. Luo ⇑ School of Materials Science and Engineering, Tianjin University, Tianjin, China Collaborative Innovation Center of Advanced Ship and Deep-Sea Exploration, Shanghai 200240, China

a r t i c l e

i n f o

Article history: Received 14 February 2015 Revised 4 May 2015 Accepted 6 June 2015

Keywords: Resistance spot welding Al/Ti dissimilar materials Electromagnetic stirring Microstructure Mechanical properties

a b s t r a c t The impact of electromagnetic stirring (EMS) upon the microstructure and mechanical properties of the Al/Ti resistance spot weld was investigated. The microstructures in the Al/Ti resistance spot weld joint were characterized by comparing them to those in an Al/Al resistance spot weld joint, and the effects of the welding current, welding time, and electrode force upon the tensile shear properties were also studied. The microstructure in the traditional Al/Ti joint consisted of a partially melted zone, a columnar grain zone, and a transition structure. Under the action of EMS, a fine spheroidal grain structure formed in the Al/Ti joint and, compared with the traditional Al/Ti resistance spot weld, the weld created under the EMS effect exhibited a larger bonding diameter, and higher tensile shear force and energy absorption. The large bonding diameter and fine spheroidal grain structure should account for the superior mechanical performance of the Al/Ti EMS joint. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Resistance spot welding (RSW) is a major joining technique used in the assembly of sheet metal components in the automotive and aerospace industries. However, it has been a challenge for car makers to consistently assure a high weld quality because of the complexity of the RSW process. The nugget diameter is considered to be the most important quality criterion for spot welds, primarily determining the mechanical performance of the welds [1,2], while the microstructure of the spot weld also has an effect upon the mechanical properties of welds. Some researchers have improved weld quality by optimizing the welding parameters, where Marya and Gayden have found that a high current, a long welding time, and a high welding force help to reduce shrinkage voids [3]; Joaquin et al. have indicated that longer hold times help to reduce shrinkage voids in resistance spot welds [4]; and Hernandez et al. have pointed out that a suitable second pulse current can increase the maximum load of resistance spot welds [5]. However, these methods usually lead to an increased consumption of time and energy and accelerate the electrode wear rate. ⇑ Corresponding author at: 25-C-701, School of Materials Science and Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072, China. E-mail addresses: [email protected] (Y. Li), [email protected] (Y. Zhang), [email protected] (J. Bi), [email protected], [email protected] (Z. Luo). 1 Address: 25-C-701, School of Materials Science and Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072, China. http://dx.doi.org/10.1016/j.matdes.2015.06.042 0264-1275/Ó 2015 Elsevier Ltd. All rights reserved.

Electromagnetic stirring (EMS) technology has been proven an effective method to improve the weld quality in RSW. Popov found that a radially oriented, axisymmetric constant magnetic field could remove the porosity in resistance spot welds of 3.5-mm-thick austenitic nickel-free steel and could improve the impact toughness and fatigue life of the welds [6]. Watanabe et al. have proposed that the weld nugget of a spot-welded 301-type stainless steel became larger with an increasing magnetic field applied perpendicular to the welding direction [7]. Shen et al. have found that EMS could refine the grain structure, increase the weld nugget diameter, reduce the risk of shrinkage cavities, and improve the mechanical properties of the joint in RSW of advanced high-strength steel DP590 [8] and DP780 [9,10]. Li et al. have studied the effect of EMS upon the nugget formation of DP980 steel and they found that, under the action of EMS, the nugget symmetry improved and the macrocrystallization direction in the fusion became less obvious [11]. Li et al. have investigated the effect of EMS upon resistance spot welds of 5052 aluminum alloy, and suggested that the effect of EMS was more sensitive to the change of welding time and was insensitive to the change of electrode force [12]. Yao et al. have shown that a magnesium alloy resistance spot weld was more likely to experience pullout failure under the action of EMS [13]. All of the above-mentioned studies focus on RSW of similar materials, but currently, hybrid structures such as Al/Steel, Mg/Steel, or Al/Ti, are widely being used to guarantee performance and cost, and are being developed to use the functionalities of the

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dissimilar materials to the fullest extent [14]. In these hybrid structures, the weld joint can be recognized as a brazed joint comprising a low melting point component (Al or Mg) in the weld joint that gets melted while the high melting point component (Steel or Ti) still retains its solid state during welding. This characterization results in a different temperature history and nugget formation process compared to those exhibited by RSW of similar materials, which makes it necessary to study the effect of EMS upon the weld nugget in RSW of dissimilar materials. Aluminum alloys and titanium alloys are widely used in the transport and aircraft industries because of their high specific strength, excellent resistance to corrosion, and low density. The use of Al/Ti hybrid components can lower the structure weight and reduce energy consumption and costs. However, very little work in the open literature has studied the RSW of titanium and aluminum alloys, though Qiu et al. have investigated the joint characterization of Al/Ti resistance spot welding with a cover plate [15]. This paper studies the effect of EMS upon RSW of an aluminum alloy and titanium, wherein the weld size, tensile–shear properties, and microstructure of the joints with and without EMS are systematically analyzed and compared.

Table 1 Chemical composition of 6061-T6 aluminum alloy (Wt%). Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Al

0.48

0.7

0.18

0.15

0.86

0.05

0.25

0.15

Bal.

Table 2 Chemical composition of TA1 commercial pure titanium (Wt%). Fe

C

N

H

O

Ti

0.2

0.08

0.03

0.010

0.015

Bal.

Table 3 Experimental parameters. Welding current (kA)

Welding time (ms)

Electrode force (kN)

Set 1 8, 10, 12, 14, 16, 18, 20 200 3.6 Set 2 14 100, 150, 200, 250, 300 3.6 Set 3 14 200 1.2, 2.4, 3.6, 4.8, 6.0

2. Materials and methods The material used in this study included a 2.0-mm-thick 6061-T6 aluminum alloy sheet and a 1.0-mm-thick TA1 commercial pure titanium sheet. Tables 1 and 2 list the compositions of the 6061-T6 aluminum alloy and pure titanium, respectively. The TA1 sheets were cleaned with alcohol to remove any surface dirt and then ground with abrasive paper. The aluminum alloy sheets were immersed in 10% NaOH solution for 5 min to remove the surface oxide film. The EMS was produced by an external magnetic field, whose method of generation can be found in Refs. [12,13]. The welding experiments were performed using a 220-kW mediumfrequency, direct current resistance spot welding machine. Two truncated cone electrodes were used whose 8-mm-diameter tip ends were made of a copper alloy of RWMA Class II chrome. The experimental parameters are shown in Table 3. Six sample welds were performed per welding condition including five samples for the tensile–shear test and one sample for metallographic observation. The dimension of each specimen was 100  25 mm2. The tensile–shear tests were performed at a cross-head of 1 mm min 1 with a CSS-44100 material test system. The peak load and the energy absorption at failure were extracted from the load– displacement curve according to the energy calculation standard specified in Ref. [16]. The peak load and energy absorption were evaluated using the average value of five specimens produced under the same welding condition. After welding, the samples were cut along the center of the spot weld nugget in the direction of the width of the sample. The cross-sections of the welds were polished and then etched by Keller’s reagent (1 mL hydrofluoric acid, 1.5 mL hydrochloric acid, 2.5 mL nitric acid, and 95 mL water). Vickers microhardness tests were carried out with a load time of 10 s and a load of 100 g. 3. Results and discussion 3.1. Effect of EMS upon weld nugget morphology and microstructure Fig. 1 shows the macroscopic morphology of the weld nugget produced with and without EMS, where the area surrounded by the red dotted line represents the melted zone (nugget) in the aluminum alloy. It can be seen that the weld nugget produced under the action of EMS (EMS weld) becomes larger and thinner than the

Fig. 1. The typical macroscopic morphology of an Al/Ti weld nugget: (a) without and (b) with electromagnetic stirring.

traditional resistance spot weld. This is consistent with the results from earlier studies [11–13] showing that the EMS drives the molten metal in the nugget to flow clockwise and generates a centrifugal movement, which will promote the growth of the nugget in a radial direction. In this paper, the concept of the ‘‘bonding diameter’’ is used to describe the nugget size rather than the ‘‘nugget diameter’’ typically used in similar materials RSW. This is because, in similar materials RSW, both the upper and lower workpieces are melted during welding and a mixed melted zone forms. However, in an Al/Ti spot weld, only the Al side is melted while the Ti remains solid (Fig. 1), causing a brazed-like joint to form during Al/Ti RSW. Therefore, the concept of ‘‘bonding diameter’’ is used here.

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3.2. Effect of EMS upon microstructure Fig. 2 shows the typical microstructure present in an Al/Ti resistance spot weld, where it can be seen that the aluminum alloy melts and a nugget forms in the Al side. To better understand the microstructure of the Al/Ti resistance spot weld, the microstructure of an Al/Al resistance spot weld with and without EMS is also given in Fig. 3. In the traditional resistance spot weld, which is similar to the microstructure in the resistance spot weld of Al/Al RSW, a partially melted zone (PMZ) (Figs. 2(b) and 3(a)) forms at the edge of the nugget, and next to it is the columnar grain zone

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(CGZ) (Figs. 2(b) and 3(b)). However, the equiaxed grain zone (EGZ), which forms in the interior of the nugget in the similar aluminum alloy RSW (Fig. 3(d)), is not found in the interior of the nugget in the Al/Ti resistance spot weld, but instead a type of transition structure (Fig. 2(c)) is formed. It can be seen that the morphology of this transition structure is very similar to that which connects the CGZ and EGZ in the similar aluminum alloy resistance spot weld (Fig. 3(c)). To confirm the relationship between these two types of transition structures, the microhardness of the two weld nuggets are measured and are shown in Fig. 4. It is noted that the microhardness value of the transition

Fig. 2. Microstructure of the Al/Ti resistance spot weld: Images of the traditional weld showing (a) the macroscopic morphology and the acquisition locations for images (b– d) exhibiting (b) the columnar grain zone (CGZ), (c) the transition zone (TZ), and (d) the Al alloy and Ti interface. Images of the EMS weld showing (e) the macroscopic morphology and the acquisition locations for images (f–h) exhibiting (f) the PMZ and CGZ, (g) the spheroidal grain zone (SGZ), and (h) the Al alloy and Ti interface.

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zone in the Al/Ti spot weld is almost the same as that in the Al/Al spot weld (see the comparison in Fig. 4(f)), which suggests that the two transition structures should be the same microstructure. The solidification mode will systematically change from the fusion line toward the centerline from planar to cellular, then to columnar dendritic, and finally to equiaxed dendritic [17]. However, in the case of Al/Ti RSW, the transition from columnar to equiaxed dendrites seems to be interrupted. Fig. 5 shows the schematic cooling curve and temperature gradient of Al/Ti and Al/Al spot welds. It can be seen that the cooling rate in Al/Ti RSW

is lower than that in similar aluminum alloy RSW. This is owing to two reasons. First, in the Al/Ti RSW, the titanium plate acts as a heat barrier and restrains the heat loss and lowers the cooling rate. Second, after the welding current ceased, the high temperature titanium plate acts a secondary heat source to maintain the temperature in aluminum alloy side. This results in the temperature gradient in the Al/Ti liquid nugget remaining at a relatively high level, which will inhibit the formation of equiaxed dendritic. In the EMS resistance spot weld, the PMZ widens in both the Al/Ti weld (Fig. 2(f)) and the Al/Al weld (Fig. 3(e)) because, with

Fig. 3. Microstructure in the Al/Al resistance spot weld: (a–d) Images of the traditional weld showing the (a) partially melted zone (PMZ), (b) the columnar grain zone (CGZ), (c) the transition zone sandwiched between the CGZ and the equiaxed grain zone (EGZ), and (d) the EGZ. (e–h) Images of the EMS weld showing the (e) PMZ, (f) CGZ, (g) transition zone sandwiched between the CGZ and EGZ, and (h) the EGZ.

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Fig. 4. Microhardness of the Al/Ti and Al/Al spot welds: Microhardness of the (a) Al/Ti traditional weld and (c) Al/Ti electromagnetic stirring (EMS) weld across the nugget. Plots of the data points from red dashed boxes in (a) and (c), for the (b) Al/Ti traditional weld showing the transition (red dashed line) from the columnar grain zone (CGZ) to the transition zone (TZ) and for the (d) Al/Ti EMS weld, showing the transition (red dashed line) from the partially melted zone (PMZ) to the spheroidal grain zone (SGZ). (e) Comparison of the microhardness of both Al/Al spot welds, marking the location of the different zones, where EGZ is the equiaxed grain zone. (f) Comparison of the microhardness of the base metal (BM), and various zones of the Al/Ti spot weld with and without EMS and the Al/Al spot weld with and without EMS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the assistance of the EMS, the molten metal with high temperature will be brought to the edge of the growing nugget and will heat up the edge of nugget, which will result in a larger PMZ. The CGZ in the EMS resistance spot weld is narrower than that in the traditional resistance spot weld because, as mentioned in Section 3.1, the molten metal is driven by the EMS to form a centrifugal movement, which will bring the high-temperature molten metal from the nugget center to the nugget edge and thereby lower the temperature gradient in the liquid nugget. Moreover, the EMS will break the growing columnar dendrites during the primary

crystallization course [10]. These processes will constrain the growth of columnar dendrites and promote the transition from columnar dendrites to equiaxed dendrites. The microstructure in the center of the Al/Ti EMS weld (Fig. 2(g)) is markedly different from that in the nugget zone of the Al/Ti traditional weld (Fig. 2(c)). In the Al/Al traditional weld, the microstructure in the center of nugget is equiaxed dendrites, as shown in Fig. 3(d), while in the Al/Al EMS weld, the equiaxed dendrites (Fig. 3(g)) become finer owing to the strong EMS effect. Comparing the microstructure in the center of the Al/Ti EMS weld with the equiaxed dendrites in

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Fig. 5. Schematic cooling curve and temperature gradient of Al/Ti and Al/Al spot welds.

Fig. 6. Evolution model of the spheroidal grain: (a) Initial dendritic, (b) dendritic growth, (c) rosette, (d) ripened rosette, and (e) spheroid.

the Al/Al EMS weld, it can be seen that the microstructure in the center of the Al/Ti EMS weld is the result of further refinement of the equiaxed dendrites. This microstructure in the Al/Ti EMS weld is close to a spheroidal grain, and this area is therefore called the spheroidal grain zone (SGZ) instead of the EGZ. Flemings showed that the evolution process of the spheroidal grain consists of five stages, comprising the initial dendrite fragment, dendrite growth, ‘‘rosette’’, ripe rosette, and spheroid, as shown in Fig. 6 [18]. Xu et al. have indicated that the proportion of the non-dendritic structure increased with enhanced EMS [19], while Cao et al. have pointed out that the initial dendritic fragments can be attributed to mechanical breakage as a result of the EMS effect [20]. Dendritic root necking and thermal disturbance resulting from the EMS effect also contribute to the formation of initial dendritic fragments that, when brought to the inside of the nugget via EMS, act as new particle nucleation sites that can gradually crystallize into spheroidal grains. Meanwhile, heat

Fig. 7. Impact of the welding current upon the (a) peak load and energy absorption, and the (b) bonding diameter.

transfer and mass transfer processes caused by the forced convection has a strong inhibitory effect upon non-uniform grain growth, which also would lead to the formation of the spheroidal grain.

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As shown in Fig. 4(f), the spheroidal grain in the Al/Ti EMS weld exhibits a relatively high microhardness compared with the equiaxed grain in the Al/Al weld, where the fine grain size of the spheroidal grain should account for its high microhardness. The average grain size is determined according to the ASTM: E112 standard [21], which yields an average spheroidal grain size in the Al/Ti EMS weld of 8.1 lm, which is 40.9% smaller than the average equiaxed grain size in the Al/Al EMS weld (13.7 lm). Fig. 2(d) and (h) show the interface between the aluminum alloy and titanium, where no obvious Al/Ti reaction layer can be observed, making the thickness of any Al/Ti reaction layer that may be present thinner than 1 lm. This extremely thin or absence of a reaction layer is considered to be responsible for the short welding time and fast cooling rate (compared with arc welding) in the Al/Ti RSW. 3.3. Effect of EMS upon tensile–shear properties Fig. 7 shows the impact of the welding current upon the peak load, energy absorption, and bonding diameter. The bonding diameter is measured from the failure surface after the tensile–shear test, according to the AWS D8.9M:2012 test method [22]. In the case of traditional RSW, the peak load, energy absorption, and bonding diameter all increase with the welding current when the welding current is lower than 16 kA. However, when the welding current is in the range 16–18 kA, the average peak load is similar to that when the welding current is 14 kA, but the fluctuation of the peak load is quite large (see the error bars of 16 and 18 kA in Fig. 7(a)). The maximum peak load of 6160 N occurs with a welding current of 16 kA, while the minimum peak load is only 1890 N. The two types of weld joint found in this range are the normal resistance spot weld, as shown in Fig. 1(a), and the brittle resistance spot weld, as shown in Fig. 8. There are many cracks in the brittle joint (Fig. 8(a)), and Fig. 8(b) plots the microhardness values from different zones in the brittle joint, including the mixed zone (MZ) of liquid aluminum and liquid titanium, and the brittle zone (BZ)

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where intermetallic compounds (IMCs) form. It can be seen that the BZ possesses the maximum hardness. Fig. 8(d) shows the EDS (energy dispersive spectral) analysis of the brittle zone where, according to the weight ratio and atomic ratio of the Al and Ti elements, the most possible composition in the BZ is TiAl3, TiAl2, or Ti5Al11, or a mixture thereof. In the welding current range of 16–18 kA, there is sufficient heat input to melt titanium, which will facilitate the diffusion and mixture of Al and Ti atoms that, in turn, will promote the formation of Al/Ti IMCs. When the welding current reaches 20 kA, the heat input is too high and Al/Ti IMCs form in all of the joints, causing the peak load to decrease accordingly. Additionally, all of the joints break completely during the tensile–shear test (Fig. 7(b)), making it impossible to measure the bonding diameter. Compared with the traditional resistance spot weld, the average peak load of the EMS weld joint increases in strength by 141.1%, 19.7%, 12.4%, 20.2%, 26.7%, and 29.0% for welding currents of 6, 8, 10, 12, 14, and 16 kA, respectively. It should be noted that the weld quality becomes stable at the welding current of 16 kA, which is a welding current where the weld quality is rather unstable in traditional resistance spot welds. This increased weld quality is because the EMS drives the liquid aluminum rotation and accelerates the heat conduction in the radial direction, causing the melting of the titanium to be restricted and the welding lobe to be broadened. The energy absorption, which represents the ductility of the RSW joint, is also higher than that of traditional RSW joint, which could be primarily attributed to the increased nugget diameter and refined grain size. In addition, the weld made at 12 kA with EMS can attain the same strength as the weld at 14 kA without EMS, which means that, with the help of EMS, power consumption and electrode wear can be reduced. A similar phenomenon is also observed in previous related work [9,12]. Fig. 9 shows the impact of the welding time upon the peak load, energy absorption, and bonding diameter. Compared with the traditional resistance spot weld, the average peak loads of the EMS welds are seen to increase by 38.7%, 38.8%, 26.7%, 47.8%, and

Fig. 8. Characterization of a joint possessing brittle intermetallic compounds: (a) Macroscopic morphology showing the brittle zone (BZ) and mixed zone (MZ). (b) Microhardness of the BZ and MZ, as well as the Al and Ti. (c) Amplification of Region 1 in (b), (d) energy dispersive spectral analysis of Region 2 in (c).

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Fig. 9. Impact of the welding time upon the (a) peak load and energy absorption, and the (b) bonding diameter of Al/Ti resistance spot welds with and without electromagnetic stirring (EMS).

24.6% at a welding time of 100, 150, 200, 250, and 300 ms, respectively, while the average energy absorptions increase by 108.5%, 174.8%, 163.4%, 245.3%, and 32.3%, respectively. The welding time has only a small effect upon the tensile–shear properties and the bonding diameter of traditional Al/Ti welds (Fig. 9 dotted lines) whereas it has a larger effect upon the EMS weld, especially at a welding time of 250 ms. This difference is because an appropriately prolonged welding time will enhance the stirring effect of the EMS and result in a large bonding diameter and a strong peak load. However, a too-prolonged welding time (300 ms) will cause intense splash and deep electrode indentation, which will lead to the decline of the bonding diameter and peak load. The impact of the electrode force upon the peak load, energy absorption, and bonding diameter is presented in Fig. 10, where it can be seen that the electrode force has little effect upon the tensile shear load-bearing capacity, failure energy absorption, and bonding diameter. This can be attributed to the fact that the bulk resistance of titanium accounts for the majority of the heat generation (the resistivity of pure titanium is about 0.5 lX m while the resistivity of AA6061 is about 0.04 lX m at room temperature [23]), so the electrode force naturally has little effect upon the bulk resistance of titanium, and therefore has little effect upon the joint performances. Compared with the traditional resistance spot weld, the average peak loads of the EMS weld joint increase by 38.3%, 34.6%, 26.7%, 34.1%, and 32.8% for an electrode force of 1.2, 2.4, 3.6, 4.8, and 6.0 kN, respectively, while the average energy absorptions increase by 200.4%, 130.5%, 163.4%, 183.8.3%, and 94.1%, respectively.

Fig. 10. Impact of the electrode force upon the (a) peak load and energy absorption, and the (b) bonding diameter of Al/Ti resistance spot welds with and without electromagnetic stirring (EMS).

Fig. 11. Relationship of the peak load and the bonding diameter of the Al/Ti resistance spot welds with and without electromagnetic stirring (EMS).

In the RSW of similar materials, the nugget diameter is acknowledged as the major quality criterion [2], so in the Al/Ti dissimilar materials RSW, the bonding diameter should be used as the major quality criterion. The relationship between the bonding diameter and the peak load is summarized in Fig. 11, where it can be seen that the overall tendency of the peak load is to increase

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with the bonding diameter. The points in Fig. 11 exhibiting extreme scatter represent the brittle weld joint, which has a large bonding diameter but a low peak load. For the welds created with EMS, the data points shift to higher peak loads and larger bonding diameter compared to welds without EMS. The data, excluding the highly scattered points, are fitted by linear fitting, and the fitted line of the RSW with EMS is above that of the RSW without EMS, meaning that the peak load of the EMS weld is higher than that of the traditional weld even when their bonding diameters are equal. This confirms that the finer grain structure in the EMS weld strengthens the weld and improves the weld quality.

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3.4. Comparison with aluminum alloy RSW Fig. 12 shows the comparison between the Al/Al and Al/Ti RSW. The aluminum alloy sheet thickness used in the Al/Al RSW is 2 mm, while the welding current used for the Al/Al RSW varies from 16 to 22 kA and the welding time and electrode force are fixed at 200 ms and 3600 N, respectively. In the traditional RSW, the peak load of the Al/Ti joint is higher than the Al/Al joint, whereas its energy absorption is lower than the Al/Al joint (Fig. 12(a) and (b), where the maximum average peak load and energy absorption values are marked on the figures).

Fig. 12. Comparison between Al/Al and Al/Ti resistance spot welds: Peak loads and energy absorption of welds created (a and b) without and (b and c) with electromagnetic stirring. (e) Load–displacement curves of the welds. (f) Bonding and nugget diameters of the welds.

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Under the action of EMS, the energy absorption of the Al/Ti joint is enhanced to such a degree that the maximum average energy absorption of the Al/Ti joint is commensurate with the Al/Al joint (3.40 and 3.30 J, as shown in Fig. 12(c) and (d)). The load–displacement curves of the Al/Ti weld shown in Fig. 12(e) indicate that the Al/Ti weld has certain ductility [2]. The EMS process improves the mechanical properties of the Al/Ti joint, which can be attributed to two aspects. First, the Al/Ti EMS weld has a larger bonding diameter than the Al/Al weld, because the high heat conductivity of the aluminum alloy limits the growth of the nugget size in Al/Al RSW. In Al/Ti RSW, however, the titanium has a large resistivity and a low heat conductivity and acts as a heat barrier to prevent heat loss from the titanium side. As a result, the Al/Ti joint has a larger bonding diameter even at a lower welding current. Second, the microstructure-hardness characteristics of the spot weld joints play an important role in their mechanical properties [24]. As discussed above, the finer spheroidal grain structure in the Al/Ti EMS weld, which has higher hardness, should also contribute to its superior mechanical properties. Under the action of EMS, the Al/Ti joint displays an improved performance compared to the Al/Al joint. The magnetically assisted Al/Ti RSW has a potential prospect in industrial applications.

4. Conclusions This paper investigates the effect of electromagnetic stirring upon the weld quality of Al/Ti dissimilar materials resistance spot welding, and the following conclusions can be drawn. (1) The electromagnetic stirring caused by an external magnetic field promotes the rotation of the molten metal, which results in the enlargement of the bonding diameter of the Al/Ti joint. (2) The microstructures in the traditional Al/Ti RSW joint are composed of a columnar grain zone and a transition structure. Under the action of EMS, the microstructures in the Al/Ti joint are refined and a type of non-dendritic structure/spheroid grain is formed, wherein the spheroid grain has a finer size and higher microhardness than the equiaxed grain in the Al/Al RSW joint. (3) Under the action of EMS, the Al/Ti joint exhibits an improved performance compared with the Al/Al joint. The larger bonding diameter and finer microstructure of the Al/Ti EMS weld contributes to its superior mechanical properties.

Acknowledgment The authors gratefully acknowledge the support of the National Natural Science Foundation of China under Grant Nos. 51405334 and 51275342.

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