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Effect of external magnetic field on resistance spot welding of aluminum alloy AA6061-T6
Journal of Manufacturing Processes 50 (2020) 456–466
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Effect of external magnetic field on resistance spot welding of aluminum alloy AA6061-T6
T
Ming Huanga,1, Qingxin Zhanga,1, Lin Qia, Lin Denga, Yongbing Lia,b,* a b
Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai, 200240, PR China State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: External magnetic field Resistance spot welding (RSW) Aluminum alloy Electromagnetic stirring (EMS) Microstructure evolution Mechanical performance
An external magnetic field is applied in resistance spot welding (RSW) of 3 mm thick AA 6061-T6 sheets to improve the weld quality. Under the action of the external magnetic field, an additional circumferential flow is generated, resulting in the reduction of cooling rate and the change of solidification mode, and it finally influences the nugget profiles, microstructural evolutions, and mechanical properties. The profile of the magnetically assisted resistance spot welding (MA-RSW) nugget became more regular and freer of defects, e.g., the internal shrinkage defects and eyelash-like hot cracks. Moreover, a double-layered columnar grain zone and a nearly single-structured equiaxed grain zone were finally obtained. The tensile properties of the MA-RSW welds were significantly improved by increasing nugget diameter, eliminating defects, and complicating the crack propagation path. As a result, the application of the external magnetic field could achieve an energy-saving effect.
1. Introduction The use of lightweight materials has been one of the most effective and important methods to alleviate the energy crisis and environmental pollution [1]. Aluminum and its alloys, by virtue of low density, high specific strength, excellent corrosion resistance, and recyclability, have been applied broadly in the aviation, construction, and automobile industries. Thus, the joining of Al alloys is an important subject in the manufacturing and application fields [2]. Resistance spot welding (RSW) is a complicated multivariable coupling process, which in particular has been used widely in the automotive industry, albeit mostly for the joining of steels [2–4]. Among numerous joining processes of Al alloys, the RSW due to its unique advantages, e.g., high efficiency, low cost, high flexibility, and no weight adding, etc. has become one of the most popular joining methods for mass production [5,6]. However, the higher electrical and thermal conductivity, dense and tenacious surface oxide film, and short-lived copper electrodes make the Al alloys more difficult to resistance weld than steels [7]. The occurrence of various defects, e.g., the early expulsion, insufficient nugget size or even incomplete fusion, internal defects, and hot cracking, leads to poor quality, stability, and mechanical performance of the aluminum welds [8–11].
In response to the problem of less generation but more dissipation of Joule heat in Al RSW process, a three times higher welding current but shorter welding time are required to obtain a qualified Al nugget than in steel RSW [12]. Besides, a multi-ring domed (MRD) electrode with sharp curvature radii, developed by General Motors (GM), can reduce the contact resistance by alleviating the adverse impact of the surface oxide film and thus increase the service life of electrodes [13,14]. Our previous work investigates the effects of MRD electrodes on Al RSW and proves that this topography can control the early expulsion, reduce weld penetration, refine grains, create a larger nugget and thus improve weld performance [3]. However, internal defects, e.g., cracks, shrinkage cavities, in the fusion zone (FZ) and liquation cracking issues in the heat-affected zone (HAZ) have not been solved satisfactorily, which would be potential threats to the weld quality. In the past decades, electromagnetic stirring (EMS) has been introduced to improve weld performance, especially in the arc welding process. The non-contact EMS in the weld pool could affect the flow and solidification behaviors of molten metal by creating Lorentz force and thus could influence weld geometry, microstructure, and performance [15–17]. Since 2005, Li et al. have devoted plenty of efforts to study the flow of molten metal in weld nuggets, and based on his theoretical study, a magnetically assisted resistance spot welding (MA-RSW)
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Corresponding author at: Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai, 200240, PR China. E-mail address: [email protected] (Y. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jmapro.2020.01.005 Received 10 December 2019; Received in revised form 1 January 2020; Accepted 4 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 50 (2020) 456–466
M. Huang, et al.
the principle of force resultant, the direction of the resultant force ⎯→ ⎯ would deflect to the direction of Fex , and the deflection angle θ depends ⎯→ ⎯ ⎯→ ⎯ ⎯→ ⎯ ⎯→ ⎯ on the relative sizes of Fex and Fin (θ = arctan Fex / Fin ). Hence, the “centrifugal” resultant force will change the flowing pattern of the molten metal and strengthen the stirring effect in the circumferential direction, therefore, finally influence the solidification process of the ⎯→ ⎯ molten metal [19]. The increase of θ can be realized by increasing Bex , which is a radial magnetic field derived from the mutual repulsion effect of the zygomorphic permanent magnet. As a result, without affecting the welding action, the closer the external permanent magnets ⎯→ ⎯ are, the more significant Bex becomes. Moreover, as shown in Fig. 2(d), the ring-shaped permanent magnets were fixed on the electrode by retractable fixtures, which can move freely along the axial direction and avoid the interference between magnets and tip dresser. The RSW process in this paper was carried out by a FANUC R2000iC robot equipped with a Centerline servo gun and a WTC medium-frequency direct current (MFDC) welding controller. The weld transformer rated capacity was 225 kVA@50% duty cycle, and the overall cooling water rate was 6 L/min. To highlight the effect of the external magnetic field, a simple welding schedule with three stages was used (shown in Fig. 3), i.e., squeezing, welding, and holding. Based on the preliminary optimization results by chisel peeling, the welding current and welding time, as the main parameters in controlling the EMS effect, were set in the ranges of 27 kA–35 kA and 100 ms–400 ms, respectively. Besides, the electrode force, squeezing, and holding times were fixed as 5 kN, 250 ms, and 350 ms, respectively. The electrodes used in the study, whose diameter is 12 mm and curvature radius of its working face is 25 mm, is made of C15000 copper alloy. Besides, five concentric rings with an average ring height of 80 μm were dressed onto the electrode working face by customized tip dresser [3].
Table 1 Chemical composition and mechanical properties of AA6061-T6. Chemical composition (wt-%)
Mechanical properties
Mg
Zn
Cr
Fe
Si
Mn
Al
YS/MPa
UTS/MPa
EL/%
0.9
0.25
0.17
0.63
0.72
0.13
Bal.
283
330
12.5
Note: YS, yield strength; UTS, ultimate strength; EL, elongation.
process is proposed. Being successfully applied to dual-phase steels and stainless steels, the external magnetic field has been proved to be effective in increasing nugget size, refining grains, eliminating defects, and finally improving the mechanical properties [16,18–22]. In the present study, the MA-RSW process together with MRD electrodes is applied to join heat-treatable 6061 Al alloys, which is paramagnetic in nature. The effect of an external magnetic field is systematically studied from the aspects of weld macro morphology, metallurgical and mechanical properties. 2. Materials and experimental procedures 2.1. Materials With the characteristic of a thinner surface oxide film and a higher risk of hot cracking than other series of Al alloys, 3.0 mm-thick AA6061-T6 sheets were selected as a research object in this study to highlight the effect of external magnetic field on the macroscopical and microcosmic characteristics of the RSW nuggets. Its chemical composition and mechanical properties are listed in Table 1. As shown in Fig. 1, the microstructure of the cold-rolled AA6061-T6 sheet contains an α-Al matrix, secondary-precipitated phase AlFeSi along the rolling direction, and dispersed strengthening phase Mg2Si [23,24]. The aluminum sheets were well prepared with the dimensions of 127 mm × 38 mm. A pair of ring-shaped NdFeB permanent magnets were used to generate a stable external magnetic field. The permanent magnet device is 15 mm tall, with outside and inside diameter of Ø32 mm and Ø20 mm, respectively. Besides, the related magnetic properties are given in Table 2.
2.3. Testing and characterization The weld performance was evaluated by the metallographic and tensile mechanical methods following the AWS D8.2M standard [25]. The metallographic characters (nugget size, indentation, penetration, etc.) were accurately measured from the cross-section of the polished and chemically etched welds by a Leica DFC 295 metallurgical microscope. The microstructural evolution was further studied by a Leica DFC 495 metallurgical microscope and a Tescan Vega 3 XMU scanning electron microscope (SEM). The composition distribution was studied by a Aztec X-MaxN80 energy dispersive spectroscopy (EDS). Besides, the EBSD tests were conducted by the Aztec Nordlys Max3 module of a Tescan Mira3 SEM. The set accelerating voltage and the step size was 20 kV and 0.7 μm, respectively. The tensile-shear testing was carried out by the lap-shear test coupon, with a single weld centered in the overlapping area (38 mm × 38 mm), on a SUNS UTM5000 universal testing machine. The crosshead velocity was 10 mm/min, and two 3 mm-thick AA6061-T6 shim plates were used to keep the force aligned (see Fig. 4). The force-displacement curves were recorded to analyze the diverse patterns of peak loads and corresponding energy absorptions. Three repetitions were conducted for each case. Typical tensile fracture morphologies were observed by a Keyence laser confocal 3D imaging system (VK-X260K) and SEM. Besides, the microhardness distributions were measured with
2.2. Welding process The working principle of the external magnetic field on the RSW process and the actual apparatus are illustrated in Fig. 2. Taking faying surface, for example, refer to Ampere’s right-handed screw rule and Fleming’s left-handed rule, when the welding current is applied (tens of thousands of amperes for aluminum RSW process) a circumferential ⎯→ ⎯ ⎯→ ⎯ induced magnetic field Bin and a centripetal induced magnetic force Fin ⎯→ ⎯ would be simultaneously generated (see Fig. 2(b, c)). And the Fin drives molten metal to flow symmetrically in the four quadrants of the axisymmetrical plane inside the melting zone [19]. ⎯→ ⎯ Based on this, an external radial magnetic field Bex will be created on the faying surface by introducing a pair of repulsive permanent magnets (refer to Fig. 2(a)). As a result, the generated external magnetic ⎯→ ⎯ ⎯→ ⎯ force Fex is perpendicular to the Fin , as shown in Fig. 2(c). According to
Fig. 1. The microstructure of the coldrolled AA6061-T6 sheet and EDS point chemical composition analysis of characteristic phases.
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Table 2 Magnetic properties of the NdFeB permanent magnet [16]. Remanence magnetic induction (Br/T)
Intrinsic coercitive magnetic field (Hci/kA m−1)
Maximum magnetic energy ((BH)max/kJ m−3)
1.41
1158
493
Fig. 2. The working principle of the MA-RSW process and the actual magnetic assisted apparatus. (a) Schematic diagram of magnetic flux and current distribution, (b, c) induced and external magnetic field/force distribution in the faying surface, and (d) the actual experimental apparatus and partially enlarged details.
a Wilson® VH1102 microhardness tester with a step size of 0.2 mm. 3. Results and discussions 3.1. Effect on nugget geometry The evolutions of both RSW and MA-RSW nugget profiles are investigated with the welding current fixed at 33 kA. As shown in Fig. 5, there are many aspects of the similarities and differences. It can be seen that the MRD ring ridges leave their unique trace on the sheet surfaces for all samples, and the indentations gradually increase with the welding time, which is a corollary of the increasingly softened base materials under a constant electrode force. Although the increase in the molten metal volume for both kinds of welds is apparent with the increasing welding time, the introduction of the external magnetic field changes the nugget growth pattern. Refer to Fig. 5, the shape of the MA-RSW nuggets becomes more regular
Fig. 3. The welding schedules.
Fig. 4. Dimensions of the tensile-shear testing specimen. 458
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Fig. 5. The evolution of nugget profiles along with the variation of welding time for both welds. (a–d) The traditional RSW welds, and (e–h) the MA-RSW welds.
colored structure, which is thick in the middle and thin on both ends. And the cross-sectional area of the darker-colored structure has an increasing tendency with the welding time prolonging. Moreover, the external magnetic field can eliminate the internal defects remarkably, refer to Fig. 5(d, h). The above comparison shows that the external magnetic field has many effects on weld growth and solidification. The qualitative analyses are made as follows. As mentioned in Section 2.2, the molten metal in the traditional RSW nugget makes rotational motions in four symmetric cells, which can reach a flow velocity of about 500 mm s−1 [20,26]. At the same time, the external magnetic field could provide an
compared to the traditional RSW ones. Moreover, the nugget size of the MA-RSW at each welding time is always greater than that of the RSW, while the penetration shows a reverse pattern. Taking 100 ms as an example, refer to Fig. 5(a, e), when the external magnetic field is applied, a 34% increment in nugget diameter and a 26% decrement in penetration occur concurrently. It shows that the external magnetic field could make the FZ larger and thinner, without changing any other welding parameters. In addition, as shown in Fig. 5(e–h), double-layered structures are observed in the nugget profiles of the MA-RSW welds. The periphery of the MA-RSW nugget adjacent to the electrodes appears to be a darker459
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Fig. 6. The nugget partitions and detailed EGZ microstructures for both welds (welding current and time are 33 kA and 300 ms, respectively). (a, d) The nugget partitions (the right half), and statistical microstructural analysis (the left half) of the EGZ of RSW and MA-RSW welds, (b, c), and (e, f) the detailed EGZ microstructures of the RSW and MA-RSW welds, respectively. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
Fig. 7. The EDS mapping results of both structures in the EGZ of RSW weld. (a) The secondary electronic image, and (b–e) the distribution of Al, Mg, Si, and Fe elements in the given view.
additional circumferential flow with much higher flow velocity. The relative rapid circumferential flow could constantly scour the weld edge and take the Joule heat from the high-temperature center to the relative cold periphery, resulting in the radial expansion of the FZ. As a result, if the total Joule heat is considered to be a certain value for a parameterfixed welding process, then apply more heat to enlarge the nugget
diameter also means less heat to promote growth in penetration direction. Hence, a larger and thinner nugget is obtained under the action of an external magnetic field. Besides, the molten metal flow could affect the solidification behavior by influencing heat distribution. The solidification mode of the traditional RSW is layered, and thus the shrinkage porosity and cavity 460
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Fig. 8. The EBSD results of both structures in the EGZ of RSW weld. (a) The secondary electronic image, (b) the Euler digraph, and (c) the identification of phase.
Fig. 9. Microstructural evolutions of the periphery regions of both welds (welding current and time is 33 kA and 300 ms, respectively). (a–c) RSW; (d–f) MA-RSW. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
solidify in a simultaneous mode. Hence, a double-layer structure and defect-free MA-RSW joint is finally obtained.
often appear in the nugget center (the last solidification area). However, under the effect of the external magnetic field, the molten metal moves in the circumferential direction and promotes the mixture of high-temperature molten metal in the nugget center and newly melted low-temperature metal around the nugget edge, which significantly reduces the temperature gradient in the nugget and thus reduces the cooling rate [16]. As a result, the periphery of the MA-RSW nugget adjacent to the electrodes where the forced water-cooling conditions exist would solidify firstly, and the rest unsolidified metal would
3.2. Effects on microstructure Taking Fig. 5(c, g) as examples (welding current and time is 33 kA and 300 ms, respectively), from inner to outer, the nugget profiles of both types of welds can be divided into three zones based on grain morphology, i.e., the equiaxed grain zone (EGZ), the columnar grain 461
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Fig. 10. Microhardness profiles of both welds (welding current and time is 33 kA and 300 ms, respectively). (a) RSW, and (b) MA-RSW.
dendrite grains and non-dendrite is finally obtained. While for the MARSW process, the additional circumferential EMS effect could homogenize the temperature and the solute field and reduce the cooling rate, which means adequate time for grain growth [20]. Besides, the strong EMS effect could also break dendrites as new heterogeneous nuclei in the unsolidified liquid phase [30]. Thus, their synergistic effect is the cause of the deformation of a nearly full equiaxed EGZ.
zone (CGZ), and the heat affect zone (HAZ), and their boundaries are marked by dashed lines, as shown in the right half of Fig. 6(a, d). The microstructural evolutions of the center (EGZ) and periphery regions (CGZ and HAZ) are explored in detail, as follows.
3.2.1. Central microstructure As shown in Fig. 6(b, e), there are two types of morphologies in the EGZ of the traditional RSW weld, and almost only the light-colored one is observed in the MA-RSW weld. The statistical results in the left half of Fig. 6(a, d) have shown that this phenomenon is not accidental. The dark-colored structure (filled in blue in Fig. 6(a, d)) spreads all over the EGZ of the RSW weld with a proportion of 29%, but it appears under a belt-like distribution in the vicinity of the faying surface of MA-RSW weld with a proportion of 6%. The partially enlarged details in Fig. 6(c) indicates that the morphology in dark color is a block-shaped structure, while the one in light color is an equiaxed grain structure., the blockshaped structure is only detected in the dendrite clearance of the equiaxed grains of the MA-RSW weld, refer to Fig. 6(f), which is identified as non-dendritic structure [27,28]. The EDS mapping results of both structures in the EGZ of RSW weld are shown in Fig. 7, and it reveals the distributions of the main adding elements (Mg, Si, and Fe). For the equiaxed grain structure, the Mg, Si, and Fe elements mainly concentrate on the grain boundaries, while the Al element shows a reverse pattern. Besides, silicon of the simple substance phase is also detected. As for the non-dendrite structure, the elements are more abundant (poorer for Al element) and well-distributed than in the equiaxed grain structure, but the grain boundary enrichment is no longer obvious. Further EBSD phase analysis results in Fig. 8 reveal that both of them could be identified as the α-Al phase. Compared to the secondary-phase identification in the base metal (see Fig. 1), refer to the EDS mapping results (see Fig. 7), the Mg2Si phase and AlFeSi phase still exist, but they are apparent coarser so that they are no longer the ones with a dispersed strengthening effect. Besides, the atomic ratio of Mg: Si is 1.73: 1 in Mg2Si, but 1.25: 1 for AA6061-T6 (refer to Table 1), so the surplus silicon exists in the form of the simple substance. Thus, the phase composition of both structures could be α-Al + Mg2Si + AlFeSi + Si, but less severe element segregation appears in the non-dendrite. The large temperature gradient and significant constitutional supercooling of the molten metal in the RSW process are conducive to the dendritic growth, and finally, the columnar and equiaxed dendrites are formed. Meanwhile, the relatively rapid cooling rate in RSW nugget leads to inadequate heat and mass transfer, and thus inhibits the heterogeneous growth, and the non-dendrite structure is eventually formed [26,29]. Hence, a hybrid structure composed of equiaxed
3.2.2. Peripheral microstructure The peripheral microstructure of both welds right under the electrode is further investigated, and the areas marked by dotted red frames in Fig. 6(a, d) are observed. As shown in Fig. 9(a), typical columnar crystal structures are observed in the periphery of the RSW weld, extending 600 μm inward in the thickness direction, and below that, a fine-equiaxed crystal region is found. The columnar crystals prefer to grow along the direction with the fastest heat dissipation rate, i.e., the most negative temperature gradient direction, and thus the preferential direction of crystal growth in the rapid-cooling RSW process is perpendicular to the fusion line (the black dashed line). However, with the introduction of the external magnet filed, this pattern has been changed. Refer to Fig. 9(d), the CGZ changes into a double-layered structure with a noticeable contrast difference (named as zone I and zone II), and a yellow dashed line is drawn to distinguish them. The consistency of grain orientation (refer to Fig. 9(d)) and the further enlarged images (shown as Fig. 9(e, f)) additionally tell that both structures are columnar, but the amount of secondary-precipitated phase in the outer layer is a far cry from that of the inner layer. Besides, as shown in Fig. 6(a, d), the zone I is thick under the electrode working surface and thin in the notch positions, while zone II shows an opposite pattern. The occurrence of the double-layered structured CGZ should be attributed to the synergistic effect of temperature field variation and solute element concentration. For zone I, which is adjacent to the forced water cooling conditions, there is not enough time for the secondary phase precipitation. While the joint action of a relatively slower solidification rate (simultaneous solidification mode) and higher concentrations of the solute elements (solidification segregation of the zone I) plays a promoting role in the microstructural formation of zone II. Besides, for MA-RSW welds, the crystal growth direction is no longer perpendicular to the fusion line; instead, it inclines approximately at a 45-degree angle (refer to Fig. 9(d)). Furthermore, the deflection of the grain growth direction presents an axial and mirror symmetry in the nugget about the z-axis and y-axis (refer to the base coordinate system in Fig. 2(a)), respectively. These characters fully reflect the heat distribution change that happened in the MA-RSW nugget formation. 462
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Fig. 11. EDS linear scanning results from HAZ to EGZ along typical routes of both welds (welding current and time is 33 kA and 300 ms, respectively). (a, d) The navigation for EDS of RSW and MA-RSW welds, (b, c) EDS results of RSW, and (e, f) of MA-RSW welds.
CGZ/HAZ. The distribution of microhardness exhibits a similar pattern in both types of welds, i.e., the microhardness value appears to the highest in the base metal (BM) while it declines to the lowest at the center of EGZ. However, there are some subtle but important differences: (1) the microhardness distribution in the MA-RSW shows an obvious multi-layer phenomenon, and the zone I is one of the softest regions; (2) the width of HAZ in diameter direction of the MA-RSW weld is slightly smaller than that of the RSW weld despite having a bigger nugget size. The microhardness decrease in the solid-state HAZ regions is caused by the disappearance of the work hardening effect, the redistribution of the principal alloying elements Mg and Si during the welding heat cycle, and the loss of T6 solution heat treatment due to welding heat. Besides, the subsequent grain growth aggravates the HAZ softening degree during thermal cycling. The casting-state structure and coarse strengthening phases in the remelted FZ directly result in the overall dramatical decrease of microhardness, and the local microhardness
In addition, an eyelash-like crack is observed outside of the HAZ of the RSW weld. Refer to the oval red dashed in the lower right corner of Fig. 6(a), and this kind of crack frequently appears for the RSW welds (refer to Fig. 5(a–d)). Based on our previous study [3], these cracks are believed to be caused by the significantly increased thermomechanical loading along the tangent of isotherms of the HAZ during the nugget growth. However, the forming tendency of these cracks is greatly reduced when the external magnetic field is applied (refer to Fig. 5(e–h)). It indicates that the EMS could homogenize the temperature distribution and reduce the thermal stress in the weld. Therefore, the formation of the eyelash-like crack could be effectively avoided. 3.3. Effect on mechanical properties 3.3.1. Microhardness distribution The microhardness profiles of both welds are shown in Fig. 10, and auxiliary lines are used to distinguish the boundaries of EGZ/CGZ and 463
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Fig. 12. Mechanical performance comparison of the traditional RSW and the MA-RSW welds with increasing welding current (welding time is 300 ms). (a) The peak load of lap-shear testing and the corresponding nugget diameters, and (b) the total energy absorption at peak load for both welds.
RSW despite having a bigger nugget size.
variation should be attributed to the differences in solidification structure. Hence, to further explore the microhardness variation in the view of strengthening phase, a series of EDS linear scanning results from HAZ to EGZ along typical routes (scanning length is 2.4 cm) are also provided in Fig. 11. As shown in Fig. 11(a, d), the typical routes are part of the short axis and long axis in the first quadrant of both the elliptical nuggets, and all of them cross HAZ, CGZ, and EGZ. The linear scanning results are integrated into the corresponding secondary electronic images, and the trace elements (i.e., Mg, Si, and Fe) have the same y-coordinate, but the abundant Al element is described by interrupted coordinate axis with suitable limit, refer to Fig. 11(b–c, e–f). Besides, the corresponding zones of the long-range linear scanning results are also differentiated. Some interesting phenomena can be found by observation and comparison: (1) There is an apparent positive correlation between Mg and Si contents, and the Al content exists an apparent negative correlation with that of other elements. Refer to Figs. 1 and 7, and it once again shows evidence of the Mg2Si and AlFeSi phases, which can be identified as strengthening phases. (2) For RSW welds, the distribution density of the strengthening phases experiences a successive decline from the HAZ to EGZ. While for MA-RSW welds, the contents of elements are more uniform, and one notable exception is the zone I in Fig. 11(e) where the number of strengthening phases has a minimum. As to RSW welds, the grain size of equiaxed grains near the columnar to equiaxed transition (CET) is relatively small and uniform, and thus its microhardness is high [30]. Besides, the local microhardness of equiaxed and non-dendrite structure are 62.96 ± 0.78 HV and 66.51 ± 0.63 HV, respectively, thus the hybrid structure of equiaxed and non-dendrite structure brings apparent microhardness fluctuation. Furthermore, the solidification defects in the final solidified region would further reduce the microhardness. Refer to Figs. 9(e) and 11 (e), the zone I of the MA-RSW weld is the first solidified region, and the lacking of the strengthening phases directly leads to the worst microhardness. As discussed in Section 3.2.1, the MA-RSW welds (except for zone I) would experience a simultaneous solidification under the EMS effect, so that the relatively uniform temperature field promotes the growth of inner equiaxed dendrite. Hence, a decreasing inward stratified microhardness distribution and a relatively uniform chemical contents distribution are finally obtained. In addition, the EMS effect reduces the cooling rate, and thus the extension of solidification time might increase the heat dissipation proportions by other means (e.g., electrodes and air) and result in relatively less heat conduction through the BM. Hence, the width of HAZ in the diameter direction of the MA-RSW weld is slightly smaller than
3.3.2. Lap-shear performance The nugget diameter, the peak load of lap-shear testing, and corresponding energy absorption at peak load for both welds under increasing welding currents (27–35 kA, welding time is fixed at 300 ms) are investigated. The variation of nugget diameter is presented by lines and symbols, while the lap-shear performance is shown with a histogram chart. Moreover, the absolute increment and relative percentage between both welds at each current level are also provided. Refer to Fig. 12, the nugget diameter and lap-shear performances of both welds increase with the increasing current and reach their peak values at 33 kA, but quickly fall in 35 kA due to the expulsion during the welding process. The external magnetic field plays a remarkable role in improving weldability and manufacturing processes from the following three aspects. Firstly, it can facilitate the emergence of larger nugget diameter and better lap-shear performance. Due to the comparable increase in the nugget diameter of RSW welds and dimensional limit from the working surface of electrodes, the nugget diameter enlargement effect is more pronounced at lower welding currents, and accordingly, the performance improvement is more significant at lower currents resulting from the larger nugget size increment. More specifically, for traditional RSW welds under 27 kA and 33 kA, once the external magnetic field is introduced, the nugget size increments are 1.89 mm (33.9%) and 1.04 mm (13.0%), and the lap-shear peak load and energy absorption increments are 1.21 kN (41.7%), 2.09 J (59.6%) and 1.53 kN (19.3%), 4.12 J (34.2%). Secondly, it can enlarge the acceptable range of welding current. According to AWS D8.2 M [25], the minimum requirements of acceptable weld size, e.g., 7 mm, and lap-shear strength, e.g., 6.56 kN, of the test coupons in this work are also provided in Fig. 12(a). As shown, the acceptable range of welding current for traditional RSW is around 32–34 kA in terms of nugget size or the lap-shear strength. However, the introduction of the external magnetic field directly expands the acceptable range of welding current to nearly fully examined range (27–34.5 kA) in terms of nugget size, and 31–34 kA in terms of lapshear strength. Thirdly, it can achieve a significant energy-saving effect. Refer to Fig. 12(b), the energy absorptions of MA-RSW welds in 27 kA and 29 kA are superior to that of traditional RSW welds in 29 kA and 31 kA, respectively. Furthermore, it can be further inferred that the energy absorptions of MA-RSW welds in 32 kA would be higher than that in 33 kA for RSW welds. Hence, by introducing the external magnetic field, 464
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Fig. 13. Fractograph of fractured lap-shear test coupons of both welds (welding current and time is 33 kA and 300 ms, respectively). (a, b) and (c, d) The laser confocal 3D scanning images of the two sides of the traditional RSW and the MA-RSW welds, respectively, and (e–j) the partially enlarged details of both welds by SEM.
Fig. 13(a–d). As for traditional RSW, the fracture is relatively flat, and its zenith is situated 600 μm-high above the reference plane. The height and position of the zenith confirm with the central softening zone in Fig. 10(a). Besides, an obvious shrinkage defect zone is also observed in the center. Hence, the crack propagation path begins in the faying surface and gradually propagates to the central softening zone, and the widespread defects would undoubtedly accelerate the crack propagation velocity. As a result, all these ultimately lead to the weak lap-shear strengths and poor energy absorptions of the traditional RSW welds. The partially enlarged details in Fig. 13(e–g), which have quasi-cleavage characteristics, supporting an inferior ductility property. By contrast with the traditional RSW weld, the considerable height difference has become a very prominent phenomenon in the fractograph of the MA-RSW weld. Refer to Fig. 13(c), the height of the
the welding current could be reduced by at least 1 kA under the premise of guaranteeing the mechanical performance. It would help to save energy and lower manufacturing costs for carmakers. 3.3.3. Fractographic analysis Due to the relatively high stiffness of the 3.0 mm + 3.0 mm AA6061-T6 combination and apparent softening behavior in the fusion zone, the fracture mode of the mentioned lap-shear test coupons for both welds in the acceptance welding current range is interfacial. However, there are some subtle but essential differences between them. Taking both welds in 33 kA and 300 ms, for example, the fractograph of the fractured lap-shear test coupons are shown in Fig. 13. To reveal the fracture behaviors of both welds, the fractographic height information is obtained by laser confocal 3D scanning technology, and the scanning results of the two sides of a lap-shear test coupon are shown in 465
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fracture is mostly 800–1000 μm above the reference plane, and it is 1500 μm for the highest position (i.e., the central location), while a deep hollow is found in the opposite side as shown in Fig. 13(d), indicating that the hollow is the trace of the pull-out metal. Refer to the previous description in Fig. 10(b), the distance between the softest zone I and the faying surface matches the height difference well, and thus, the crack propagation path of MA-RSW welds becomes very interesting. It can be inferred from the height information that the crack initially propagates along with the range of zone I, and then switches to the central softening zone in the vicinity of the faying surface. The longer propagation path always means increased plastic deformation and energy absorption. The partially enlarged details in Fig. 13(h–j) are good interpretations of that.
References [1] Jiang DX, Zhang QX, Zhao M, Xia HB, Wang ST, Li YB. Effects of welds distribution and high-low temperature humidity alternating aging on sealing performance of weld-bonded stainless steel structures. J Manuf Process 2019;48:77–85. [2] Mathers G. The welding of aluminium and its alloys. Boca Raton: CRC Press; 2002. [3] Deng L, Li YB, Carlson BE, Sigler DR. Effects of electrode surface topography on aluminum resistance spot welding. Weld J 2018;97:120–32. [4] Komatsu Y, Ban K, Ito T, Muraoka Y, Yahaba T, Yasunaga K, et al. Application of all aluminium automotive body for Honda NSX. SAE technical paper series Feb. 25, 1991. SAE International; 1991. [5] Zhang HY, Senkara J. Resistance welding: fundamentals and applications. 2nd ed Boca Raton: CRC Press; 2011. [6] Lei HY, Li YB, Carlson BE. Cold metal transfer spot welding of 1 mm thick AA6061T6. J Manuf Process 2017;28:209–19. [7] Manladan SM, Yusof F, Ramesh S, Fadzil M, Luo Z, Ao S. A review on resistance spot welding of aluminum alloys. Int J Adv Manuf Technol 2016;90:605–34. [8] Senkara J, Zhang HY. Cracking in spot welding aluminum alloy AA5754. Weld J 2000;79:194–201. [9] Zhang HY, Senkara J, Wu X. Suppressing cracking in resistance welding AA5754 by mechanical means. J Manuf Sci Eng 2002;124:79–85. [10] Du HM, Bi J, Zhang Y, Wu YD, Li Y, Luo Z. The role of the partial melting zone in the nugget growth process of unequal-thickness dissimilar aluminum alloy 2219/5A06 resistance spot welding. J Manuf Process 2019;45:304–11. [11] Xia YJ, Su ZW, Li YB, Zhou L, Shen Y. Online quantitative evaluation of expulsion in resistance spot welding. J Manuf Process 2019;46:34–43. [12] Mazur W, Kyriakopoulos A, Bott N, West D. Use of modified electrode caps for surface quality welds in resistance spot welding. J Manuf Process 2016;22:60–73. [13] Sigler DR, Carlson BE, Janiak P. Aluminum resistance spot welding electrode degradation. AWS Sheet Metal Welding Conference XV. 2012. [14] Sigler DR, Carlson BE, Janiak P. A preliminary assessment of GM’s multi-ring domed (MRD) electrode for alternately welding aluminum-to-aluminum and steelto-steel stack-ups. AWS Sheet Metal Welding Conference XVI. 2014. [15] Luo J, Luo Q, Lin YH, Xue J. A new approach for fluid flow model in gas tungsten arc weld pool using longitudinal electromagnetic control. Weld J 2003;82:202–6. [16] Li YB, Zhang QX, Qi L, David SA. Improving austenitic stainless steel resistance spot weld quality using external magnetic field. Sci Technol Weld Join 2018;23:619–27. [17] Li YB, Lin ZQ, Chen GL, Wang YS, Xi SY. Study on moving GTA weld pool in an externally applied longitudinal magnetic field with experimental and finite element methods. Modell Simul Mater Sci Eng 2002;10:781–98. [18] Li YB, Li YT, Shen Q, Lin ZQ. Magnetically assisted resistance spot welding of dualphase steel. Weld J 2013;92:124–32. [19] Li YB, Shen Q, Lin ZQ, Hu SJ. Quality improvement in resistance spot weld of advanced high strength steel using external magnetic field. Sci Technol Weld Join 2013;16:465–9. [20] Li YB, Li DL, Lin ZQ, David SA, Feng Z, Tang W. Review: magnetically assisted resistance spot welding. Sci Technol Weld Join 2016;21:59–74. [21] Shen Q, Li YB, Lin ZQ, Chen GL. Effect of external constant magnetic field on weld nugget of resistance spot welded dual-phase steel DP590. IEEE Trans Magn 2011;47:4116–9. [22] Shen Q, Li YB, Lin ZQ, Chen GL. Impact of external magnetic field on weld quality of resistance spot welding. J Manuf Sci Eng 2011;133:051001. [23] Zhan XH, Chen JC, Liu JJ, Wei YH, Zhou JJ, Meng Y. Microstructure and magnesium burning loss behavior of AA6061 electron beam welding joints. Mater Des 2016;99:449–58. [24] Hirose A, Todaka H, Yamaoka H, Kurosawa N, Kobayashi KF. Quantitative evaluation of softened regions in weld heat-affected zones of 6061-T6 aluminum alloycharacterizing of the laser beam welding process. Metall Mater Trans A 1999;30:2115–20. [25] AWS D8.2M:2017. Specification for automotive weld quality-resistance spot welding of aluminum. American Welding Society; 2017. [26] Li YB, Wei ZY, Li YT, Shen Q, Lin ZQ. Effects of cone angle of truncated electrode on heat and mass transfer in resistance spot welding. Int J Heat Mass Transf 2013;65:400–8. [27] Li Y, Luo Z, Yan FY, Duan R, Yao Q. Effect of external magnetic field on resistance spot welds of aluminum alloy. Mater Des 2014;56:1025–33. [28] Wu M, Fjeld A, Ludwig A. Modelling mixed columnar-equiaxed solidification with melt convection and grain sedimentation-part I: model description. Comput Mater Sci 2010;50:32–42. [29] Wei PS, Wu TH. Electrode geometry effects on microstructure determined by heat transfer and solidification rate during resistance spot welding. Int J Heat Mass Transf 2014;79:408–16. [30] Cantor B, O’Reilly K. Solidification and casting. Boca Raton: Taylor & Francis; 2002.
4. Conclusions An external magnetic field was applied in the resistance spot welding (RSW) of two identical 3.0 mm-thick AA6061-T6 sheets. The nugget profiles, microstructural evolutions, microhardness profiles and lap-shear performance of the RSW welds with/without the external magnetic field (i.e., MA-RSW and RSW) were systematically investigated, and the following conclusions could be drawn: (1) The external magnetic field could provide electromagnetic stirring (EMS) in the circumferential direction. By constantly scouring the weld edge and accelerating the molten metal flow, larger and thinner, more regularly shaped, and defect-free MA-RSW joints are finally achieved. (2) The EMS effect could homogenize the temperature distribution, reduce the temperature gradient, and change the solidification mode from layered mode to simultaneous mode. As a result, the peripheral columnar grain zone and central hybrid structured equiaxed grain zone evolve into a double-layered structure and a nearly single structured equiaxed grain zone, respectively. (3) With the external magnetic field, a declining inward stratified microhardness distribution and a uniform chemical contents distribution are finally obtained in the MA-RSW welds. Besides, the zone I of the MA-RSW welds becomes an apparent softening zone due to the deficiency of strengthening phases. (4) MA-RSW can significantly improve the peak load and the energy absorption ability of the welds by increasing nugget diameter, eliminating defects and reducing thermal stress, and increasing the crack propagation path and could also enlarge the acceptable range of welding current and achieve an energy-saving effect. Conflict of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China [grant numbers U1564204, U1764251, and 51805323].
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Effect of external magnetic field on resistance spot welding of aluminum alloy AA6061-T6
T
Ming Huanga,1, Qingxin Zhanga,1, Lin Qia, Lin Denga, Yongbing Lia,b,* a b
Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai, 200240, PR China State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: External magnetic field Resistance spot welding (RSW) Aluminum alloy Electromagnetic stirring (EMS) Microstructure evolution Mechanical performance
An external magnetic field is applied in resistance spot welding (RSW) of 3 mm thick AA 6061-T6 sheets to improve the weld quality. Under the action of the external magnetic field, an additional circumferential flow is generated, resulting in the reduction of cooling rate and the change of solidification mode, and it finally influences the nugget profiles, microstructural evolutions, and mechanical properties. The profile of the magnetically assisted resistance spot welding (MA-RSW) nugget became more regular and freer of defects, e.g., the internal shrinkage defects and eyelash-like hot cracks. Moreover, a double-layered columnar grain zone and a nearly single-structured equiaxed grain zone were finally obtained. The tensile properties of the MA-RSW welds were significantly improved by increasing nugget diameter, eliminating defects, and complicating the crack propagation path. As a result, the application of the external magnetic field could achieve an energy-saving effect.
1. Introduction The use of lightweight materials has been one of the most effective and important methods to alleviate the energy crisis and environmental pollution [1]. Aluminum and its alloys, by virtue of low density, high specific strength, excellent corrosion resistance, and recyclability, have been applied broadly in the aviation, construction, and automobile industries. Thus, the joining of Al alloys is an important subject in the manufacturing and application fields [2]. Resistance spot welding (RSW) is a complicated multivariable coupling process, which in particular has been used widely in the automotive industry, albeit mostly for the joining of steels [2–4]. Among numerous joining processes of Al alloys, the RSW due to its unique advantages, e.g., high efficiency, low cost, high flexibility, and no weight adding, etc. has become one of the most popular joining methods for mass production [5,6]. However, the higher electrical and thermal conductivity, dense and tenacious surface oxide film, and short-lived copper electrodes make the Al alloys more difficult to resistance weld than steels [7]. The occurrence of various defects, e.g., the early expulsion, insufficient nugget size or even incomplete fusion, internal defects, and hot cracking, leads to poor quality, stability, and mechanical performance of the aluminum welds [8–11].
In response to the problem of less generation but more dissipation of Joule heat in Al RSW process, a three times higher welding current but shorter welding time are required to obtain a qualified Al nugget than in steel RSW [12]. Besides, a multi-ring domed (MRD) electrode with sharp curvature radii, developed by General Motors (GM), can reduce the contact resistance by alleviating the adverse impact of the surface oxide film and thus increase the service life of electrodes [13,14]. Our previous work investigates the effects of MRD electrodes on Al RSW and proves that this topography can control the early expulsion, reduce weld penetration, refine grains, create a larger nugget and thus improve weld performance [3]. However, internal defects, e.g., cracks, shrinkage cavities, in the fusion zone (FZ) and liquation cracking issues in the heat-affected zone (HAZ) have not been solved satisfactorily, which would be potential threats to the weld quality. In the past decades, electromagnetic stirring (EMS) has been introduced to improve weld performance, especially in the arc welding process. The non-contact EMS in the weld pool could affect the flow and solidification behaviors of molten metal by creating Lorentz force and thus could influence weld geometry, microstructure, and performance [15–17]. Since 2005, Li et al. have devoted plenty of efforts to study the flow of molten metal in weld nuggets, and based on his theoretical study, a magnetically assisted resistance spot welding (MA-RSW)
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Corresponding author at: Shanghai Key Laboratory of Digital Manufacture for Thin-walled Structures, Shanghai Jiao Tong University, Shanghai, 200240, PR China. E-mail address: [email protected] (Y. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jmapro.2020.01.005 Received 10 December 2019; Received in revised form 1 January 2020; Accepted 4 January 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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the principle of force resultant, the direction of the resultant force ⎯→ ⎯ would deflect to the direction of Fex , and the deflection angle θ depends ⎯→ ⎯ ⎯→ ⎯ ⎯→ ⎯ ⎯→ ⎯ on the relative sizes of Fex and Fin (θ = arctan Fex / Fin ). Hence, the “centrifugal” resultant force will change the flowing pattern of the molten metal and strengthen the stirring effect in the circumferential direction, therefore, finally influence the solidification process of the ⎯→ ⎯ molten metal [19]. The increase of θ can be realized by increasing Bex , which is a radial magnetic field derived from the mutual repulsion effect of the zygomorphic permanent magnet. As a result, without affecting the welding action, the closer the external permanent magnets ⎯→ ⎯ are, the more significant Bex becomes. Moreover, as shown in Fig. 2(d), the ring-shaped permanent magnets were fixed on the electrode by retractable fixtures, which can move freely along the axial direction and avoid the interference between magnets and tip dresser. The RSW process in this paper was carried out by a FANUC R2000iC robot equipped with a Centerline servo gun and a WTC medium-frequency direct current (MFDC) welding controller. The weld transformer rated capacity was 225 kVA@50% duty cycle, and the overall cooling water rate was 6 L/min. To highlight the effect of the external magnetic field, a simple welding schedule with three stages was used (shown in Fig. 3), i.e., squeezing, welding, and holding. Based on the preliminary optimization results by chisel peeling, the welding current and welding time, as the main parameters in controlling the EMS effect, were set in the ranges of 27 kA–35 kA and 100 ms–400 ms, respectively. Besides, the electrode force, squeezing, and holding times were fixed as 5 kN, 250 ms, and 350 ms, respectively. The electrodes used in the study, whose diameter is 12 mm and curvature radius of its working face is 25 mm, is made of C15000 copper alloy. Besides, five concentric rings with an average ring height of 80 μm were dressed onto the electrode working face by customized tip dresser [3].
Table 1 Chemical composition and mechanical properties of AA6061-T6. Chemical composition (wt-%)
Mechanical properties
Mg
Zn
Cr
Fe
Si
Mn
Al
YS/MPa
UTS/MPa
EL/%
0.9
0.25
0.17
0.63
0.72
0.13
Bal.
283
330
12.5
Note: YS, yield strength; UTS, ultimate strength; EL, elongation.
process is proposed. Being successfully applied to dual-phase steels and stainless steels, the external magnetic field has been proved to be effective in increasing nugget size, refining grains, eliminating defects, and finally improving the mechanical properties [16,18–22]. In the present study, the MA-RSW process together with MRD electrodes is applied to join heat-treatable 6061 Al alloys, which is paramagnetic in nature. The effect of an external magnetic field is systematically studied from the aspects of weld macro morphology, metallurgical and mechanical properties. 2. Materials and experimental procedures 2.1. Materials With the characteristic of a thinner surface oxide film and a higher risk of hot cracking than other series of Al alloys, 3.0 mm-thick AA6061-T6 sheets were selected as a research object in this study to highlight the effect of external magnetic field on the macroscopical and microcosmic characteristics of the RSW nuggets. Its chemical composition and mechanical properties are listed in Table 1. As shown in Fig. 1, the microstructure of the cold-rolled AA6061-T6 sheet contains an α-Al matrix, secondary-precipitated phase AlFeSi along the rolling direction, and dispersed strengthening phase Mg2Si [23,24]. The aluminum sheets were well prepared with the dimensions of 127 mm × 38 mm. A pair of ring-shaped NdFeB permanent magnets were used to generate a stable external magnetic field. The permanent magnet device is 15 mm tall, with outside and inside diameter of Ø32 mm and Ø20 mm, respectively. Besides, the related magnetic properties are given in Table 2.
2.3. Testing and characterization The weld performance was evaluated by the metallographic and tensile mechanical methods following the AWS D8.2M standard [25]. The metallographic characters (nugget size, indentation, penetration, etc.) were accurately measured from the cross-section of the polished and chemically etched welds by a Leica DFC 295 metallurgical microscope. The microstructural evolution was further studied by a Leica DFC 495 metallurgical microscope and a Tescan Vega 3 XMU scanning electron microscope (SEM). The composition distribution was studied by a Aztec X-MaxN80 energy dispersive spectroscopy (EDS). Besides, the EBSD tests were conducted by the Aztec Nordlys Max3 module of a Tescan Mira3 SEM. The set accelerating voltage and the step size was 20 kV and 0.7 μm, respectively. The tensile-shear testing was carried out by the lap-shear test coupon, with a single weld centered in the overlapping area (38 mm × 38 mm), on a SUNS UTM5000 universal testing machine. The crosshead velocity was 10 mm/min, and two 3 mm-thick AA6061-T6 shim plates were used to keep the force aligned (see Fig. 4). The force-displacement curves were recorded to analyze the diverse patterns of peak loads and corresponding energy absorptions. Three repetitions were conducted for each case. Typical tensile fracture morphologies were observed by a Keyence laser confocal 3D imaging system (VK-X260K) and SEM. Besides, the microhardness distributions were measured with
2.2. Welding process The working principle of the external magnetic field on the RSW process and the actual apparatus are illustrated in Fig. 2. Taking faying surface, for example, refer to Ampere’s right-handed screw rule and Fleming’s left-handed rule, when the welding current is applied (tens of thousands of amperes for aluminum RSW process) a circumferential ⎯→ ⎯ ⎯→ ⎯ induced magnetic field Bin and a centripetal induced magnetic force Fin ⎯→ ⎯ would be simultaneously generated (see Fig. 2(b, c)). And the Fin drives molten metal to flow symmetrically in the four quadrants of the axisymmetrical plane inside the melting zone [19]. ⎯→ ⎯ Based on this, an external radial magnetic field Bex will be created on the faying surface by introducing a pair of repulsive permanent magnets (refer to Fig. 2(a)). As a result, the generated external magnetic ⎯→ ⎯ ⎯→ ⎯ force Fex is perpendicular to the Fin , as shown in Fig. 2(c). According to
Fig. 1. The microstructure of the coldrolled AA6061-T6 sheet and EDS point chemical composition analysis of characteristic phases.
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Table 2 Magnetic properties of the NdFeB permanent magnet [16]. Remanence magnetic induction (Br/T)
Intrinsic coercitive magnetic field (Hci/kA m−1)
Maximum magnetic energy ((BH)max/kJ m−3)
1.41
1158
493
Fig. 2. The working principle of the MA-RSW process and the actual magnetic assisted apparatus. (a) Schematic diagram of magnetic flux and current distribution, (b, c) induced and external magnetic field/force distribution in the faying surface, and (d) the actual experimental apparatus and partially enlarged details.
a Wilson® VH1102 microhardness tester with a step size of 0.2 mm. 3. Results and discussions 3.1. Effect on nugget geometry The evolutions of both RSW and MA-RSW nugget profiles are investigated with the welding current fixed at 33 kA. As shown in Fig. 5, there are many aspects of the similarities and differences. It can be seen that the MRD ring ridges leave their unique trace on the sheet surfaces for all samples, and the indentations gradually increase with the welding time, which is a corollary of the increasingly softened base materials under a constant electrode force. Although the increase in the molten metal volume for both kinds of welds is apparent with the increasing welding time, the introduction of the external magnetic field changes the nugget growth pattern. Refer to Fig. 5, the shape of the MA-RSW nuggets becomes more regular
Fig. 3. The welding schedules.
Fig. 4. Dimensions of the tensile-shear testing specimen. 458
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Fig. 5. The evolution of nugget profiles along with the variation of welding time for both welds. (a–d) The traditional RSW welds, and (e–h) the MA-RSW welds.
colored structure, which is thick in the middle and thin on both ends. And the cross-sectional area of the darker-colored structure has an increasing tendency with the welding time prolonging. Moreover, the external magnetic field can eliminate the internal defects remarkably, refer to Fig. 5(d, h). The above comparison shows that the external magnetic field has many effects on weld growth and solidification. The qualitative analyses are made as follows. As mentioned in Section 2.2, the molten metal in the traditional RSW nugget makes rotational motions in four symmetric cells, which can reach a flow velocity of about 500 mm s−1 [20,26]. At the same time, the external magnetic field could provide an
compared to the traditional RSW ones. Moreover, the nugget size of the MA-RSW at each welding time is always greater than that of the RSW, while the penetration shows a reverse pattern. Taking 100 ms as an example, refer to Fig. 5(a, e), when the external magnetic field is applied, a 34% increment in nugget diameter and a 26% decrement in penetration occur concurrently. It shows that the external magnetic field could make the FZ larger and thinner, without changing any other welding parameters. In addition, as shown in Fig. 5(e–h), double-layered structures are observed in the nugget profiles of the MA-RSW welds. The periphery of the MA-RSW nugget adjacent to the electrodes appears to be a darker459
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Fig. 6. The nugget partitions and detailed EGZ microstructures for both welds (welding current and time are 33 kA and 300 ms, respectively). (a, d) The nugget partitions (the right half), and statistical microstructural analysis (the left half) of the EGZ of RSW and MA-RSW welds, (b, c), and (e, f) the detailed EGZ microstructures of the RSW and MA-RSW welds, respectively. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
Fig. 7. The EDS mapping results of both structures in the EGZ of RSW weld. (a) The secondary electronic image, and (b–e) the distribution of Al, Mg, Si, and Fe elements in the given view.
additional circumferential flow with much higher flow velocity. The relative rapid circumferential flow could constantly scour the weld edge and take the Joule heat from the high-temperature center to the relative cold periphery, resulting in the radial expansion of the FZ. As a result, if the total Joule heat is considered to be a certain value for a parameterfixed welding process, then apply more heat to enlarge the nugget
diameter also means less heat to promote growth in penetration direction. Hence, a larger and thinner nugget is obtained under the action of an external magnetic field. Besides, the molten metal flow could affect the solidification behavior by influencing heat distribution. The solidification mode of the traditional RSW is layered, and thus the shrinkage porosity and cavity 460
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Fig. 8. The EBSD results of both structures in the EGZ of RSW weld. (a) The secondary electronic image, (b) the Euler digraph, and (c) the identification of phase.
Fig. 9. Microstructural evolutions of the periphery regions of both welds (welding current and time is 33 kA and 300 ms, respectively). (a–c) RSW; (d–f) MA-RSW. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
solidify in a simultaneous mode. Hence, a double-layer structure and defect-free MA-RSW joint is finally obtained.
often appear in the nugget center (the last solidification area). However, under the effect of the external magnetic field, the molten metal moves in the circumferential direction and promotes the mixture of high-temperature molten metal in the nugget center and newly melted low-temperature metal around the nugget edge, which significantly reduces the temperature gradient in the nugget and thus reduces the cooling rate [16]. As a result, the periphery of the MA-RSW nugget adjacent to the electrodes where the forced water-cooling conditions exist would solidify firstly, and the rest unsolidified metal would
3.2. Effects on microstructure Taking Fig. 5(c, g) as examples (welding current and time is 33 kA and 300 ms, respectively), from inner to outer, the nugget profiles of both types of welds can be divided into three zones based on grain morphology, i.e., the equiaxed grain zone (EGZ), the columnar grain 461
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Fig. 10. Microhardness profiles of both welds (welding current and time is 33 kA and 300 ms, respectively). (a) RSW, and (b) MA-RSW.
dendrite grains and non-dendrite is finally obtained. While for the MARSW process, the additional circumferential EMS effect could homogenize the temperature and the solute field and reduce the cooling rate, which means adequate time for grain growth [20]. Besides, the strong EMS effect could also break dendrites as new heterogeneous nuclei in the unsolidified liquid phase [30]. Thus, their synergistic effect is the cause of the deformation of a nearly full equiaxed EGZ.
zone (CGZ), and the heat affect zone (HAZ), and their boundaries are marked by dashed lines, as shown in the right half of Fig. 6(a, d). The microstructural evolutions of the center (EGZ) and periphery regions (CGZ and HAZ) are explored in detail, as follows.
3.2.1. Central microstructure As shown in Fig. 6(b, e), there are two types of morphologies in the EGZ of the traditional RSW weld, and almost only the light-colored one is observed in the MA-RSW weld. The statistical results in the left half of Fig. 6(a, d) have shown that this phenomenon is not accidental. The dark-colored structure (filled in blue in Fig. 6(a, d)) spreads all over the EGZ of the RSW weld with a proportion of 29%, but it appears under a belt-like distribution in the vicinity of the faying surface of MA-RSW weld with a proportion of 6%. The partially enlarged details in Fig. 6(c) indicates that the morphology in dark color is a block-shaped structure, while the one in light color is an equiaxed grain structure., the blockshaped structure is only detected in the dendrite clearance of the equiaxed grains of the MA-RSW weld, refer to Fig. 6(f), which is identified as non-dendritic structure [27,28]. The EDS mapping results of both structures in the EGZ of RSW weld are shown in Fig. 7, and it reveals the distributions of the main adding elements (Mg, Si, and Fe). For the equiaxed grain structure, the Mg, Si, and Fe elements mainly concentrate on the grain boundaries, while the Al element shows a reverse pattern. Besides, silicon of the simple substance phase is also detected. As for the non-dendrite structure, the elements are more abundant (poorer for Al element) and well-distributed than in the equiaxed grain structure, but the grain boundary enrichment is no longer obvious. Further EBSD phase analysis results in Fig. 8 reveal that both of them could be identified as the α-Al phase. Compared to the secondary-phase identification in the base metal (see Fig. 1), refer to the EDS mapping results (see Fig. 7), the Mg2Si phase and AlFeSi phase still exist, but they are apparent coarser so that they are no longer the ones with a dispersed strengthening effect. Besides, the atomic ratio of Mg: Si is 1.73: 1 in Mg2Si, but 1.25: 1 for AA6061-T6 (refer to Table 1), so the surplus silicon exists in the form of the simple substance. Thus, the phase composition of both structures could be α-Al + Mg2Si + AlFeSi + Si, but less severe element segregation appears in the non-dendrite. The large temperature gradient and significant constitutional supercooling of the molten metal in the RSW process are conducive to the dendritic growth, and finally, the columnar and equiaxed dendrites are formed. Meanwhile, the relatively rapid cooling rate in RSW nugget leads to inadequate heat and mass transfer, and thus inhibits the heterogeneous growth, and the non-dendrite structure is eventually formed [26,29]. Hence, a hybrid structure composed of equiaxed
3.2.2. Peripheral microstructure The peripheral microstructure of both welds right under the electrode is further investigated, and the areas marked by dotted red frames in Fig. 6(a, d) are observed. As shown in Fig. 9(a), typical columnar crystal structures are observed in the periphery of the RSW weld, extending 600 μm inward in the thickness direction, and below that, a fine-equiaxed crystal region is found. The columnar crystals prefer to grow along the direction with the fastest heat dissipation rate, i.e., the most negative temperature gradient direction, and thus the preferential direction of crystal growth in the rapid-cooling RSW process is perpendicular to the fusion line (the black dashed line). However, with the introduction of the external magnet filed, this pattern has been changed. Refer to Fig. 9(d), the CGZ changes into a double-layered structure with a noticeable contrast difference (named as zone I and zone II), and a yellow dashed line is drawn to distinguish them. The consistency of grain orientation (refer to Fig. 9(d)) and the further enlarged images (shown as Fig. 9(e, f)) additionally tell that both structures are columnar, but the amount of secondary-precipitated phase in the outer layer is a far cry from that of the inner layer. Besides, as shown in Fig. 6(a, d), the zone I is thick under the electrode working surface and thin in the notch positions, while zone II shows an opposite pattern. The occurrence of the double-layered structured CGZ should be attributed to the synergistic effect of temperature field variation and solute element concentration. For zone I, which is adjacent to the forced water cooling conditions, there is not enough time for the secondary phase precipitation. While the joint action of a relatively slower solidification rate (simultaneous solidification mode) and higher concentrations of the solute elements (solidification segregation of the zone I) plays a promoting role in the microstructural formation of zone II. Besides, for MA-RSW welds, the crystal growth direction is no longer perpendicular to the fusion line; instead, it inclines approximately at a 45-degree angle (refer to Fig. 9(d)). Furthermore, the deflection of the grain growth direction presents an axial and mirror symmetry in the nugget about the z-axis and y-axis (refer to the base coordinate system in Fig. 2(a)), respectively. These characters fully reflect the heat distribution change that happened in the MA-RSW nugget formation. 462
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Fig. 11. EDS linear scanning results from HAZ to EGZ along typical routes of both welds (welding current and time is 33 kA and 300 ms, respectively). (a, d) The navigation for EDS of RSW and MA-RSW welds, (b, c) EDS results of RSW, and (e, f) of MA-RSW welds.
CGZ/HAZ. The distribution of microhardness exhibits a similar pattern in both types of welds, i.e., the microhardness value appears to the highest in the base metal (BM) while it declines to the lowest at the center of EGZ. However, there are some subtle but important differences: (1) the microhardness distribution in the MA-RSW shows an obvious multi-layer phenomenon, and the zone I is one of the softest regions; (2) the width of HAZ in diameter direction of the MA-RSW weld is slightly smaller than that of the RSW weld despite having a bigger nugget size. The microhardness decrease in the solid-state HAZ regions is caused by the disappearance of the work hardening effect, the redistribution of the principal alloying elements Mg and Si during the welding heat cycle, and the loss of T6 solution heat treatment due to welding heat. Besides, the subsequent grain growth aggravates the HAZ softening degree during thermal cycling. The casting-state structure and coarse strengthening phases in the remelted FZ directly result in the overall dramatical decrease of microhardness, and the local microhardness
In addition, an eyelash-like crack is observed outside of the HAZ of the RSW weld. Refer to the oval red dashed in the lower right corner of Fig. 6(a), and this kind of crack frequently appears for the RSW welds (refer to Fig. 5(a–d)). Based on our previous study [3], these cracks are believed to be caused by the significantly increased thermomechanical loading along the tangent of isotherms of the HAZ during the nugget growth. However, the forming tendency of these cracks is greatly reduced when the external magnetic field is applied (refer to Fig. 5(e–h)). It indicates that the EMS could homogenize the temperature distribution and reduce the thermal stress in the weld. Therefore, the formation of the eyelash-like crack could be effectively avoided. 3.3. Effect on mechanical properties 3.3.1. Microhardness distribution The microhardness profiles of both welds are shown in Fig. 10, and auxiliary lines are used to distinguish the boundaries of EGZ/CGZ and 463
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Fig. 12. Mechanical performance comparison of the traditional RSW and the MA-RSW welds with increasing welding current (welding time is 300 ms). (a) The peak load of lap-shear testing and the corresponding nugget diameters, and (b) the total energy absorption at peak load for both welds.
RSW despite having a bigger nugget size.
variation should be attributed to the differences in solidification structure. Hence, to further explore the microhardness variation in the view of strengthening phase, a series of EDS linear scanning results from HAZ to EGZ along typical routes (scanning length is 2.4 cm) are also provided in Fig. 11. As shown in Fig. 11(a, d), the typical routes are part of the short axis and long axis in the first quadrant of both the elliptical nuggets, and all of them cross HAZ, CGZ, and EGZ. The linear scanning results are integrated into the corresponding secondary electronic images, and the trace elements (i.e., Mg, Si, and Fe) have the same y-coordinate, but the abundant Al element is described by interrupted coordinate axis with suitable limit, refer to Fig. 11(b–c, e–f). Besides, the corresponding zones of the long-range linear scanning results are also differentiated. Some interesting phenomena can be found by observation and comparison: (1) There is an apparent positive correlation between Mg and Si contents, and the Al content exists an apparent negative correlation with that of other elements. Refer to Figs. 1 and 7, and it once again shows evidence of the Mg2Si and AlFeSi phases, which can be identified as strengthening phases. (2) For RSW welds, the distribution density of the strengthening phases experiences a successive decline from the HAZ to EGZ. While for MA-RSW welds, the contents of elements are more uniform, and one notable exception is the zone I in Fig. 11(e) where the number of strengthening phases has a minimum. As to RSW welds, the grain size of equiaxed grains near the columnar to equiaxed transition (CET) is relatively small and uniform, and thus its microhardness is high [30]. Besides, the local microhardness of equiaxed and non-dendrite structure are 62.96 ± 0.78 HV and 66.51 ± 0.63 HV, respectively, thus the hybrid structure of equiaxed and non-dendrite structure brings apparent microhardness fluctuation. Furthermore, the solidification defects in the final solidified region would further reduce the microhardness. Refer to Figs. 9(e) and 11 (e), the zone I of the MA-RSW weld is the first solidified region, and the lacking of the strengthening phases directly leads to the worst microhardness. As discussed in Section 3.2.1, the MA-RSW welds (except for zone I) would experience a simultaneous solidification under the EMS effect, so that the relatively uniform temperature field promotes the growth of inner equiaxed dendrite. Hence, a decreasing inward stratified microhardness distribution and a relatively uniform chemical contents distribution are finally obtained. In addition, the EMS effect reduces the cooling rate, and thus the extension of solidification time might increase the heat dissipation proportions by other means (e.g., electrodes and air) and result in relatively less heat conduction through the BM. Hence, the width of HAZ in the diameter direction of the MA-RSW weld is slightly smaller than
3.3.2. Lap-shear performance The nugget diameter, the peak load of lap-shear testing, and corresponding energy absorption at peak load for both welds under increasing welding currents (27–35 kA, welding time is fixed at 300 ms) are investigated. The variation of nugget diameter is presented by lines and symbols, while the lap-shear performance is shown with a histogram chart. Moreover, the absolute increment and relative percentage between both welds at each current level are also provided. Refer to Fig. 12, the nugget diameter and lap-shear performances of both welds increase with the increasing current and reach their peak values at 33 kA, but quickly fall in 35 kA due to the expulsion during the welding process. The external magnetic field plays a remarkable role in improving weldability and manufacturing processes from the following three aspects. Firstly, it can facilitate the emergence of larger nugget diameter and better lap-shear performance. Due to the comparable increase in the nugget diameter of RSW welds and dimensional limit from the working surface of electrodes, the nugget diameter enlargement effect is more pronounced at lower welding currents, and accordingly, the performance improvement is more significant at lower currents resulting from the larger nugget size increment. More specifically, for traditional RSW welds under 27 kA and 33 kA, once the external magnetic field is introduced, the nugget size increments are 1.89 mm (33.9%) and 1.04 mm (13.0%), and the lap-shear peak load and energy absorption increments are 1.21 kN (41.7%), 2.09 J (59.6%) and 1.53 kN (19.3%), 4.12 J (34.2%). Secondly, it can enlarge the acceptable range of welding current. According to AWS D8.2 M [25], the minimum requirements of acceptable weld size, e.g., 7 mm, and lap-shear strength, e.g., 6.56 kN, of the test coupons in this work are also provided in Fig. 12(a). As shown, the acceptable range of welding current for traditional RSW is around 32–34 kA in terms of nugget size or the lap-shear strength. However, the introduction of the external magnetic field directly expands the acceptable range of welding current to nearly fully examined range (27–34.5 kA) in terms of nugget size, and 31–34 kA in terms of lapshear strength. Thirdly, it can achieve a significant energy-saving effect. Refer to Fig. 12(b), the energy absorptions of MA-RSW welds in 27 kA and 29 kA are superior to that of traditional RSW welds in 29 kA and 31 kA, respectively. Furthermore, it can be further inferred that the energy absorptions of MA-RSW welds in 32 kA would be higher than that in 33 kA for RSW welds. Hence, by introducing the external magnetic field, 464
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Fig. 13. Fractograph of fractured lap-shear test coupons of both welds (welding current and time is 33 kA and 300 ms, respectively). (a, b) and (c, d) The laser confocal 3D scanning images of the two sides of the traditional RSW and the MA-RSW welds, respectively, and (e–j) the partially enlarged details of both welds by SEM.
Fig. 13(a–d). As for traditional RSW, the fracture is relatively flat, and its zenith is situated 600 μm-high above the reference plane. The height and position of the zenith confirm with the central softening zone in Fig. 10(a). Besides, an obvious shrinkage defect zone is also observed in the center. Hence, the crack propagation path begins in the faying surface and gradually propagates to the central softening zone, and the widespread defects would undoubtedly accelerate the crack propagation velocity. As a result, all these ultimately lead to the weak lap-shear strengths and poor energy absorptions of the traditional RSW welds. The partially enlarged details in Fig. 13(e–g), which have quasi-cleavage characteristics, supporting an inferior ductility property. By contrast with the traditional RSW weld, the considerable height difference has become a very prominent phenomenon in the fractograph of the MA-RSW weld. Refer to Fig. 13(c), the height of the
the welding current could be reduced by at least 1 kA under the premise of guaranteeing the mechanical performance. It would help to save energy and lower manufacturing costs for carmakers. 3.3.3. Fractographic analysis Due to the relatively high stiffness of the 3.0 mm + 3.0 mm AA6061-T6 combination and apparent softening behavior in the fusion zone, the fracture mode of the mentioned lap-shear test coupons for both welds in the acceptance welding current range is interfacial. However, there are some subtle but essential differences between them. Taking both welds in 33 kA and 300 ms, for example, the fractograph of the fractured lap-shear test coupons are shown in Fig. 13. To reveal the fracture behaviors of both welds, the fractographic height information is obtained by laser confocal 3D scanning technology, and the scanning results of the two sides of a lap-shear test coupon are shown in 465
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fracture is mostly 800–1000 μm above the reference plane, and it is 1500 μm for the highest position (i.e., the central location), while a deep hollow is found in the opposite side as shown in Fig. 13(d), indicating that the hollow is the trace of the pull-out metal. Refer to the previous description in Fig. 10(b), the distance between the softest zone I and the faying surface matches the height difference well, and thus, the crack propagation path of MA-RSW welds becomes very interesting. It can be inferred from the height information that the crack initially propagates along with the range of zone I, and then switches to the central softening zone in the vicinity of the faying surface. The longer propagation path always means increased plastic deformation and energy absorption. The partially enlarged details in Fig. 13(h–j) are good interpretations of that.
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4. Conclusions An external magnetic field was applied in the resistance spot welding (RSW) of two identical 3.0 mm-thick AA6061-T6 sheets. The nugget profiles, microstructural evolutions, microhardness profiles and lap-shear performance of the RSW welds with/without the external magnetic field (i.e., MA-RSW and RSW) were systematically investigated, and the following conclusions could be drawn: (1) The external magnetic field could provide electromagnetic stirring (EMS) in the circumferential direction. By constantly scouring the weld edge and accelerating the molten metal flow, larger and thinner, more regularly shaped, and defect-free MA-RSW joints are finally achieved. (2) The EMS effect could homogenize the temperature distribution, reduce the temperature gradient, and change the solidification mode from layered mode to simultaneous mode. As a result, the peripheral columnar grain zone and central hybrid structured equiaxed grain zone evolve into a double-layered structure and a nearly single structured equiaxed grain zone, respectively. (3) With the external magnetic field, a declining inward stratified microhardness distribution and a uniform chemical contents distribution are finally obtained in the MA-RSW welds. Besides, the zone I of the MA-RSW welds becomes an apparent softening zone due to the deficiency of strengthening phases. (4) MA-RSW can significantly improve the peak load and the energy absorption ability of the welds by increasing nugget diameter, eliminating defects and reducing thermal stress, and increasing the crack propagation path and could also enlarge the acceptable range of welding current and achieve an energy-saving effect. Conflict of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The authors would like to acknowledge the National Natural Science Foundation of China [grant numbers U1564204, U1764251, and 51805323].
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