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Comparison of microstructure, mechanical properties, and corrosion behavior of Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (U-GMA) welded joints of Al–Zn–Mg alloy
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Comparison of microstructure, mechanical properties, and corrosion behavior of Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (UGMA) welded joints of Al–Zn–Mg alloy Weifeng Xiea,b,*, Te Huanga, Chunli Yangb, Chenglei Fanb, Sanbao Linb, Wanghui Xuc a
School of Mechanical Engineering, Northeast Electric Power University, Jilin 132012, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China c Guangdong Provincial Key Laboratory of Advanced Welding Technology, Guangdong Welding Institute (China-Ukraine E. O. Paton Institute of Welding), Guangzhou 510651, China b
ARTICLE INFO
ABSTRACT
Associate editor: S.-J. Na
The microstructural characteristics, mechanical properties, and corrosion behavior were compared for Al–Zn–Mg alloy (thickness: 8 mm) joints obtained via Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (U-GMA) welding. The results reveal that, compared with the GMA welded joint obtained under the same heat input condition, the U-GMA welded joint consists of a larger weld zone (WZ) and narrower heat affected zone (HAZ). The WZ trends of the joints may be summarized as follows: The Fe-rich phases are smaller and considerably fewer in U-GMA welded joint, resulting in higher fracture toughness than that of the GMA welded joint. Ultimate tensile strength (UTS) and elongation determined from tensile testing of the joints are 342 MPa, 7.4 % for U-GMA welded joint and 305.9 MPa, 5.4 % for GMA welded joint. Electrochemical corrosion experiments in a 3.5 wt.% NaCl solution indicate that the corrosion resistance of the WZs in both joints is affected by the distribution of intergranular Fe-rich phases. Each specimen suffered from intergranular corrosion. However, the WZ of the U-GMA welded joint exhibits stronger corrosion resistance than the WZ of the GMA welded joint.
Keywords: Al–Zn–Mg alloy GMA welding U-GMA welding Microstructure Mechanical properties Corrosion behavior
1. Introduction Heat treatable Al–Zn–Mg alloys (7XXX series) with high strength and low density are widely used in the aerospace, high-speed train, automotive, and shipbuilding industries. Marlaud et al. (2010) reported that the overall properties of these alloys are closely correlated with the grain size as well as the occurrence of stable and metastable phases. Starink and Li (2003) reported that the formation of the main phases, such as the η (MgZn2) phase, S (Al2CuMg) phase, and T (AlCuMgZn) phase, is determined by the main hardening elements (i.e., Zn and Mg). Furthermore, Yazdian et al. (2010) and Park and Ardell (1988) showed that minor impurities and additive elements, such as Fe, Si Ti, and Zr, influence the microstructure through the formation of intermetallic compounds. Therefore, Al–Zn–Mg alloys with desirable properties could be obtained by changing the contents of some alloying elements and choosing an appropriate combination of other alloying elements. Arc welding, characterized by advantages such as simple operation, high production efficiency, and high automation capacity, has been
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extensively applied in welding manufacturing. However, the arc welding structure of Al–Zn–Mg alloys suffers from many quality problems, such as low ductility, low toughness, and short fatigue life. Many types of arc welding methods have been employed with the aim of improving the weld quality. Mousavi et al. (2003) investigated that the electromagnetically stir promotes the grain detachment in gas tungsten arc (GTA) welding of AA7020 aluminum alloy, and the electromagnetically stir is considered an important factor for nucleation and grain refinement. Moreover, synchronous rolling and the use of a trailing heat sink can reduce the tensile welding stresses and, hence, hot cracking of 7020 aluminum alloy welds during GTA welding (Ram et al., 2003). Dev et al. (2007) observed that the addition of scandium led to significant grain refinement in the weld zone (WZ), especially for scandium levels greater than the eutectic composition (0.55 wt.%). Balasubramanian et al. (2008) discovered that improved fatigue performance can be obtained for the post-weld aged pulsed-current GTA welded joints of AA7075 aluminum alloy. However, these techniques are paid relatively much attention to grain refinement, and many
Corresponding author at: School of Mechanical Engineering, Northeast Electric Power University, Jilin 132012, China. E-mail address: [email protected] (W. Xie).
https://doi.org/10.1016/j.jmatprotec.2019.116470 Received 9 June 2019; Received in revised form 13 September 2019; Accepted 21 October 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Weifeng Xie, et al., Journal of Materials Processing Tech., https://doi.org/10.1016/j.jmatprotec.2019.116470
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Fig. 1. Schematic of U-GMA welding setup.
problems, such as joint softening, element loss, and corrosion behavior, remain unresolved. Considering the advantages of ultrasonic vibration, such as a mechanical effect, acoustic streaming, and a cavitation effect, many attempts have been made to introduce this treatment into the process of arc welding. Dai (2003) applied ultrasonic vibration to the side surface of a workpiece to be welded during GTA welding of 7075-T7 aluminum alloy. The results revealed that the grain size in the overheated and heat affected zone (HAZ) decreases, owing to this vibration. Cui et al. (2006) observed that the unmixed-zone width of the WZ is reduced and the corrosion resistance is improved by applying ultrasonic vibration to the back of a super-austenitic stainless-steel workpiece. He et al. (2006) coupled an arc-ultrasonic, resulting from excitation of the arc by a highfrequency current, with the ultrasonic treatment generated by the exciting arc. The results revealed that this treatment exerts a grain-refinement effect during the welding of a Ti–6Al–4 V joint. This method is easily executed in arc welding, but can only be used in non-melting electrode welding. Sun et al. (2008) and (2009) reported ultrasonicassisted GTA (U-GTA) welding, where non-contact ultrasonic vibration was imposed along the coaxial direction of the arc, thereby forming an acoustic radiation field in the arc zone. Compared with conventional GTA welding, the U-GTA is compressed, weld penetration increases by more than 100 %, and strength of the joint can be improved by more than 8 %. Fan et al. (2013) and (2017) proposed ultrasonic-assisted gas metal arc (U-GMA) welding, and showed that the droplet size is reduced, and the droplet transfer frequency is significantly increased. That is, the additional ultrasonic wave is helpful for improving the welding stability. Xie et al. (2018) conducted various U-GMA welding experiments, where the effect of an ultrasonic wave on droplet transfer was considered. The results revealed that the acoustic radiation force provides a detaching force to the growing droplet. This force affects the rest of the process, as the droplet separates from the welding wire end and finally enters the weld pool. However, the microstructural characteristics, mechanical properties, and corrosion behavior of GMA welded joints produced from Al–Zn–Mg alloy, especially the joints of materials obtained via high heat input welding, are only partly understood. Many scholars believe that arc welding of Al–Zn–Mg alloy is impossible. Hence, only a few reported studies have focused on the relationship among the microstructure, mechanical properties, and corrosion behavior of GMA and U-GMA welded joints of Al–Zn–Mg alloys. The present work considers the effect of the ultrasonic vibration on the microstructure, mechanical
properties, and corrosion behavior of an Al–Zn–Mg alloy, and provides a reference for the application of ultrasonic vibration during arc welding. Special attention is given to the possible benefits of U-GMA welding, compared with those of GMA welding, resulting from the additional ultrasonic vibration. 2. Materials and methods 2.1. Materials A typical Al–Zn–Mg alloy (7A52 aluminum alloy) with a thickness of 8 mm was selected as the base metal (BM) in this study, which was subjected to hot rolling at 420 °C and solution treatment at 470 °C. The chemical composition (wt.%) of the alloy is given as follows: 4.0–4.8 % Zn, 2.0–2.8 % Mg, 0.05–0.2 % Cu, 0.3 % Fe, 0.15–0.25 % Cr, 0.25 % Si, 0.2–0.5 % Mn, and balance Al. An Al–Mg alloy (ER5356) consisting of (wt.%) 4.5–5.5 % Mg, 0.4 % Fe, 0.25 % Si, 0.1 % Zn, 0.1 % Cu, 0.05–0.2 % Cr, 0.05–0.2 % Mn, and Al (balance) was used as the filler wire (Φ1.2 mm). The rolled sheets of BM were sheared into 200 × 100 mm2 plates for welding. Using an abrasive waterjet cutting machine, single V-groove angles (30°) were cut in the plates with 2-mm root faces for a total 60° included angle between two plates. The porosity of the joints results mainly from the oxidation film. To reduce this porosity prior to welding, all plates were cleaned with 8 % sodium hydroxide solution at 60 °C and subsequently with 30 % nitric acid solution at room temperature. These plates were then rinsed under flowing water and dried in air. In addition, butting surfaces were burnished with a steel brush. Afterward, the plates were tightly fixed into a fixture to prevent root opening. 2.2. U-GMA welding The joints were joined through GMA welding and U-GMA welding using a U-GMA welding setup (see Fig. 1 for a schematic of the setup). The welding gun was fixed, the workpiece moved at a constant speed, and the angle between the axis of the welding gun and the workpiece was 90°. This setup consisted of two parts, i.e., a welding system and an ultrasonic radiation system, consisting of the ultrasonic power supply and ultrasonic transducer. Synchronous mechanical vibration generated by the transducer was amplified by the amplitude transformer and then radiated to surrounding space in the form of ultrasonic waves by the 2
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ultrasonic radiator. The arc was then subjected to ultrasonic energy. The ultrasonic vibration, with an excitation frequency of 19.5 kHz, was generated by the ultrasonic transducer using a concave spherical surface ultrasonic radiator (height: 14 mm). Xie et al. (2016) used a boundary element method to optimize the geometric parameters of the ultrasonic radiator. For the U-GMA welding, the input power of the SY2000 ultrasonic power source was regulated to 2000 W. Without energization of the ultrasonic excitation power supply, the transducer is non-operational, and GMA welding can be realized. The same welding parameters were employed in both welding processes. A KemppiMIG500 welding power supplied with a ProMIG501 wire feeding machine was used during the experiments, and the welding process was performed under direct current electrode positive conditions. Argon (purity: 99.99 %) was used as the shielding gas and the flow rate through the nozzle (height: 11 mm) was 15 L/min. Experiments were performed at a welding speed, wire feed speed, and welding voltage of 0.4 m/min, 11 m/min, and 25.5 V, respectively, the heat input can be achieved as about 700 J/mm in two welding experiments.
2.5. Electrochemical corrosion Electrochemical experiments were performed on a CHI660E electrochemistry workstation using a three-electrode system. Saturated calomel electrode (SCE) with a Luggin capillary and platinum was used as the reference electrode and auxiliary electrode, respectively. Additionally, 0.8 cm2 BM and WZ of the two joints were used as the working electrode. All BM and WZs were ground with 2000 grit emery paper and then polished and rinsed with ethanol. The non-tested surfaces were protectively coated with a stop-off gelatin. The potentiodynamic polarization experiments were performed at potentials ranging from -1.2 VSCE to -0.1 VSCE (scan rate: 0.1 mV/s) in a 3.5 wt.% NaCl solution (pH = 6) at (25 ± 1) °C. Additionally, 300-s potentiostatic polarization tests (polarization potential: -0.4 VSCE, amplitude: 5 mV, frequency: 1000 Hz) were conducted for a thorough understanding of the corrosion mechanism. The resulting electrochemical corrosion was then observed via scanning electron microscopy (Quanta 200 FEI SEM), and the corresponding energy spectra were also obtained. 3. Results and discussion
2.3. Microstructural characterization
3.1. Macrographs
After welding, samples for metallographic analysis were sectioned, mounted, and polished, and subsequently etched with Keller’s reagent (2 mL HF + 3 mL HCl+5 mL HNO3+190 mL distilled water). Macrographs showing the cross-section of the joints were obtained using a SZX12 stereoscopic microscope. Additionally, the grain morphology of the WZ, HAZ, and BM was examined via optical microscopy (OM, GX71) and electron backscatter diffraction (EBSD, FEI Nova 400). The diffraction data were collected with HKL Channel 5 software. Specimens for EBSD were prepared via conventional mechanical polishing followed by electrochemical polishing (polishing solution: 20 mL ethylene glycol monobutyl ether+40 mL perchloric acid+140 mL ethanol). Moreover, the chemical compositions of coarse intermetallic compounds and some particles in BM and different microstructural zones of the welded joints were evaluated. This evaluation was performed via backscatter electron imaging on a Quanta 200 field emission gun Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS).
Macrographs of the two joints (see Fig. 3) reveal that full penetration joints are realized. The fusion line is approximately straight in both joints. The width of WZ varies from 17 mm at the top to 0 at the bottom in the GMA welded joint, and from 15 mm to 7 mm in the U-GMA welded joint. For the U-GMA welded joint, the WZ area is substantially larger, and the increase in weld size in the depth direction is more pronounced than that of the GMA welded joint. Simultaneously, the HAZ width is 2.0 mm in the GMA welded joint and only 1.0 mm in the U-GMA welded joint. Compared with the GMA welding, the U-GMA welding creates a larger WZ and a narrower HAZ. During U-GMA welding, the arc and droplet are influenced not only by the axial acoustic radiation force, but also by a radial component of acoustic radiation force. The binding effect of ultrasonic on the arc and droplet has been documented in Xie et al. (2016, 2018). It is the primary reason for the increase in area of U-GMA WZ. Conversely, the dispersion of arc heat and low arc force could not increase the weld depth in the GMA welding. Meanwhile, the ultrasonic vibration should also be considered. The weld pool can be assumed to be a multilayer film, under the action of the axial force, including the arc pressure and the droplet impact force, the film will be forced to vibrate. According to Newton’s second law, the vibration amplitude is proportional to the axial force (Du et al., 2012). For the U-GMA welding, a component of ultrasonic radiation force is added to the axial force, then the rate of melt flow will be faster, and the scouring force acting on the weld pool boundary is also greater than that of the GMA welding. This may also lead to the increase in the WZ area. Because the total heat input is consistent in two welding processes, it means that the heat input per unit volume is reduced during the U-GMA welding. Therefore, the thermal impact of this BM will be reduced. As a result, a narrower HAZ is generated in U-GMA welding.
2.4. Mechanical properties The mechanical properties of the joints were determined via tensile and microhardness tests. An abrasive waterjet cutting machine was used to cut the tensile test coupons from the metal sheets. The tensile specimens comprising the joints were machined to the required dimensions, as shown in Fig. 2. All tests of BM and the joints were performed at ambient temperature, in accordance with ASTM E8M-04 (2006), on an Instron 5569 tensile testing machine. These tests were performed at a strain rate of 1/s, and a gauge length of 50 mm was employed for the extensometer attached to each sample. Each final test result was the average value of three tests. Furthermore, a HXD1000TM microhardness tester was used to measure (load: 200 g, dwell time: 10 s, interval between two points: 0.5 mm) the Vickers hardness along the center of the middle region comprising the cross-section.
3.2. Grain morphology The grain morphology characterizing the WZ, HAZ, and BM of the Fig. 2. Dimensions of flat tensile specimens (unit: mm).
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Fig. 3. Appearance and microstructure of joints welded via GMA and U-GMA welding.
two joints was observed and the grain size is measured via EBSD. As shown in Fig. 4, grains with different orientations are shaded in different colors. The grain size statistics obtained from the HKL CHANNEL5 software are listed to the right of each map. Fig. 4a and b show that the typical solidification structure characterizing each WZ consists of equiaxed grains. Moreover, values of 151.3 μm and 108.6 μm are obtained for the mean grain size of the WZ in the GMA welded joint and U-GMA welded joint, respectively. According to Mi et al. (2015) and Tan et al. (2015), owing to an ultrasonic treatment, the size of the grains in the WZ decreases, speed of the weld-pool heat flow increases, degree of supercooling increases, and nucleation of grains is promoted. From Fig. 4c-e, it can be seen that the grains in both HAZs are significantly elongated along the rolling direction and exhibit a pancake morphology, which is almost identical to that of the BM. Mean grain sizes of 65.0 μm, 45.9 μm, and 46.5 μm are obtained for the BM, HAZ of the GMA welded joint, and HAZ of the U-GMA welded joint, respectively. The size of the grains in both HAZs is very similar, that is, the ultrasonic has little effect on the size of the grains in HAZ, this may mainly because of the dramatic effects of coarse second-phase particles and partially soluble constituent phases distributed along the three orthogonal directions of the rolled plate on the re-crystallization through forming the particle nucleation Srivatsan et al. (2000).
second phases contain many alloying elements, where the alloying elements of these phases are detected via EDS of specified locations as indicated in the figure. To reduce deviations, three tests are performed at each location and the average value of the elemental content is calculated (see Table 1). The treatment temperature of the BM exceeds the dissolution temperature of the second phase, therefore, a small number of insoluble coarse-phase particles are scattered on the aluminum matrix, resulting in a chain-like distribution along the rolling direction. The phase particles with different morphologies were analyzed and then two components, namely Point 1 and Point 2, are found. Combining these results with the atomic percentage reveals that the regular strip precipitates are η (MgZn2) phase dissolved in elements, such as Cu, Fe, and Mn. The fine and irregular precipitates are η phase, as shown in Fig. 5a. Referring to the phase diagram of Al-Mg-Si ternary alloys and the atomic percentage indicates that a large amount of Fe-dissolved β (Mg2Si) phases are precipitated at the grain boundary (GB) in the HAZ of the GMA welded joint (see Fig. 5b). However, from Fig. 5d, it can be found that fewer Fe-dissolved β phase particles occur in the HAZ of the U-GMA welded joint than in the HAZ of the GMA welded joint. Meanwhile, some θ (Al2Cu) phase particles are precipitated in the GB in the HAZ of the U-GMA welded joint, consistent with the EDS result of Point 4. This result from the fact that, compared with the GMA-welding, the heat input per unit volume in the U-GMA welding is lower, resulting in easier precipitation of the θ phase, after all, its precipitation temperature is only 548 °C. Many irregular primary precipitates are intermittently distributed between the dendrites comprising the WZ of the two joints, as presented in Fig. 5c and e. The EDS result of Point 5 shows that the precipitates are an Fe-rich phase. These Fe-rich phases in the UGMA WZ are much finer, and some even present a small spherical shape rather than a coarse rod-like particle in the GMA WZ. In addition, a large number of gray phase particles appear in the U-GMA WZ and the HAZ of GMA welded joint, it should be the T (Al6CuMg4) strengthening
3.3. Alloying-element distribution 3.3.1. Second phases The grain structure has a significant influence on the properties of an aluminum alloy. However, these properties are also controlled by the type, volume fraction, morphology, and distribution of the particle phase including impurity phases, excess phases, and precipitated phases (Vratnica et al., 2010). In the present work, two joint specimens are systematically observed via back scatter electron imaging on a SEM. The SEM images for these second phases are shown in Fig. 5. The 4
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Fig. 4. Grain morphology and grain size measurement of different zones in the joints: WZ in the (a) GMA welded joint and (b) U-GMA welded joint; HAZ in the (c) GMA welded joint and (d) U-GMA welded joint; (e) BM.
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Fig. 5. SEM images of different zones: (a) BM; (b) HAZ and (c) WZ in the GMA welded joint; (d) HAZ and (e) WZ in the U-GMA welded joint.
phase as proved by Rokhlin et al. (2004). Zhao et al. (2009) demonstrated that the velocity of acoustic flow can reach 10-103 times that of fluid thermal convection, and this flow
can reduce the temperature gradient and improve the uniformity of the temperature distribution. It may be helpful for increasing the uniformity of chemical composition by accelerating the diffusion of alloy 6
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Table 1 Composition of the second phases in the two WZs, HAZs, and BM.
Point Point Point Point Point Point Point Point Point Point Point Point
1 1 2 2 3 3 4 4 5 5 6 6
in in in in in in in in in in in in
BM (wt.%) BM (at.%) BM (wt.%) BM (at.%) HAZ of GMA (wt.%) HAZ of GMA (at.%) HAZ of U-GMA (wt.%) HAZ of U- GMA (at.%) WZ of GMA (wt.%) WZ of GMA (at.%) WZ of U- GMA (wt.%) WZ of U- GMA (at.%)
Mg
Zn
Fe
Cu
Mn
Cr
Si
Al
14.57 20.64 2.96 3.41 9.59 14.04 1.65 2.59 8.19 10.89 9.48 11.58
16.73 8.82 5.74 2.46 2.40 1.19 4.99 2.90 11.05 5.47 9.44 4.29
20.48 12.00 0.39 0.20 19.68 11.41 4.11 2.23 7.27 4.21 0.24 0.13
6.53 3.36 1.15 0.51 2.67 1.36 45.74 27.38 7.98 4.06 7.50 3.51
1.36 0.81 0.41 0.21 2.90 1.71 0.61 0.31 2.20 1.29 0.24 0.13
/ / / / 4.02 2.50 / / 3.06 1.90 1.03 0.60
0.23 0.23 / / 4.54 5.24 0.88 0.91 0.29 0.28 0.14 0.14
40.10 54.14 89.35 93.21 54.20 62.54 42.01 63.68 59.96 71.91 71.93 79.62
possibly to an increase in the mass percentage of Fe element in WZ. This, in turn, indirectly promotes the formation of the Fe-rich phase. Compared with GMA welding, U-GMA welding is characterized by a more constrained arc, as reported by Xie et al. (2018). However, the experimental result reported in that paper could be attributed to other factors (additional to the fact that, compared with the former, the latter produces a more constrained arc and significantly reduces weld spatter). The total welding heat input is the same in both processes. Therefore, the larger WZ obtained in U-GMA welding (see Fig. 3) means that the heat input per unit volume is lower than that of GMA welding. Additionally, the wire feeding speeds are constant in both experiments. The same speeds are employed for the melting volume of the filler wire used in the two weld pools. Most of the WZ metal in U-GMA welded joint originates from BM and, hence, compared with that added during GMA welding, more Zn element is gradually added to the weld pool during U-GMA welding. According to Darcy’s law, if the velocity of ultrasonic streaming determined by the input power of the ultrasonic wave is sufficiently large, the duration of direct arc action on the elements may be reduced. Consequently, the elemental gasification loss may also be effectively mitigated.
Fig. 6. Microhardness profiles of the joints obtained via different welding methods.
elements in the two-phase zone. In addition, Dai (2003) described that the cooling rate and cooling time can be significantly improved by the ultrasonic waves for the welding of the aluminum alloy 7075-T6. So it can also be considered that the ultrasonic wave could bring the excess heat, or reduce the time of overheating, then the fine particle phases do not have enough time or enough power to grow into the coarse impurity phases. The ultrasonic improves the solidification process for the U-GMA welding. Those factors may explain why the content of the coarse Fe-rich phases, including the Fe-rich phase in WZ and the Fedissolved β phase in HAZ, are reduced under the action of ultrasonic vibration.
3.4. Mechanical properties 3.4.1. Microhardness profile The results of the microhardness tests performed on the joints obtained via GMA and U-GMA welding are plotted in Fig. 6. The average microhardness of the two HAZs and the two WZs is lower than that of the BM. Consistent with the results shown in Fig. 3, the HAZ width of the U-GMA welded joint is considerably narrower than that of the GMA welded joint. The hardness of the WZ corresponding to the U-GMA and GMA welded joint can be classified into two groups, i.e., HV105 to HV115 and HV90 to HV100. The hardness of the BM ranges from HV140 to HV160 and, hence, the hardness of the WZs is 70 % and 65 %, respectively, that of the BM. Moreover, the variation in the hardness values approximately reflects the precipitation and dissolution of the second-phase particles during welding. For the GMA WZ, the coarse Fe-rich phases may reduce the lattice distortion of the aluminum crystal and be favorable for dislocation movement. This leads to a reduction in the hardness and an increase in the width of the softening zone. In contrast, the dissolution of the coarse Fe-rich phases and the reduction of the number density for the coarse Fe-rich phases in the U-GMA WZ increase the lattice distortion of the crystal. For the HAZ, the hardening effect of the θ phase occurring in the U-GMA welded joint is possible to be negligible. A different trend is observed for both HAZs where more T-phase particles are relatively uniformly distributed inside the grain interior for the GMA HAZ (see Fig. 5b). Compared with the U-GMA HAZ, the T phase in the GMA HAZ is more likely to cause the higher hardness.
3.3.2. Elemental gasification loss For the two types of WZs shown in Fig. 5c and e, the main elements, Mg and Zn, in the alloy were identified by means of EDS surface scanning. The Mg and Zn contents, respectively, of the WZs are summarized as follows: GMA WZs: 3.31 wt.% and 1.74 wt.%, U-GMA WZs: 2.94 wt.% and 3.91 wt.%. The primary metallic elements of the Al–Zn–Mg aluminum alloy are Al and Zn, while the filler wire contains mainly Al and Mg. Assuming that the initial content of alloying elements is constant, the reduction in the content of some elements results mainly from the effect of strong gasification loss. Furthermore, the boiling points of Zn and Mg (907 °C and 1107 °C, respectively) are considerably lower than the boiling points of Al and Fe (2467 °C and 2750 °C, respectively). The gasification loss of Al and Fe elements is therefore relatively small. The gasification loss of Zn and Mg elements occurs primarily under the welding heat source. The experimental results suggest that, compared with the U-GMA welding, the GMA welding yielded a greater gasification loss of Zn element, leading 7
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Fig. 7. UTS, yield strength, and elongation of the BM and both joints.
3.4.2. Tensile strength Tensile tests at room temperature were performed on BM and both joints, and the result is shown in Fig. 7. Both joints broke in the WZ during testing. The ultimate tensile strength (UTS), yield strength and elongation of the BM were 525 MPa, 471 MPa and 13.2 %, respectively. The UTS, yield strength and elongation of 344.2 MPa, 298 MPa and 7.4 % as-welded condition were obtained in U-GMA welded joint. The GMA welded joint showed the lower tensile strength (305.9 MPa), yield strength (277.6 MPa) and elongation (5.4 %). Therefore, the UTS, yield strength, and elongation of the U-GMA welded joint are 13 %, 7 %, and 37 %, respectively, higher than those of the GMA welded joint. The fracture surfaces of the two joints are observed in scanning electron micrographs (magnification: 4000×, see Fig. 8). The occurrence of many dimples and tearing ridges in the surfaces indicates that, in both cases, fracture occurs mainly via microvoid coalescence. However, compared with that of the GMA welded joint, the fracture surface of the U-GMA welded joint is covered with deeper and larger dimples. Many broken particles, with EDS-determined chemical compositions (wt.%) of 2.27Zn–3.44Mg–3.56Cu–14.98Fe–0.80Cr–0.32Si–1.44Mn–(bal.)Al (point in Fig. 8a) and 3.44Zn–4.38Mg–2.19Cu–11.77Fe–0.21Cr–0.45Si–2.78Mn–(bal.)Al (point in Fig. 8b), are observed at the bottom of dimples in both surfaces. This chemical composition is basically the same as that of the Fe-rich phase in the WZ (see Table 1). Those phase particles are probably Fe-rich phases distributed between dendrites. Furthermore, the decrease in the plasticity of the GMA welded joints results mainly from an increase in the content of the coarse Fe-rich phase. The Fe-rich phase is a brittle particle, which is incompatible with the aluminum matrix and is easily broken or separated from the matrix,
Fig. 9. Potentiodynamic polarization curves of BM and the two WZs.
thereby forming holes under relatively low stresses. This phase serves as a shortcut for crack growth and a crack source to some extent. In addition, Liu et al. (2012) showed the platelet-like Fe-rich phase can effectively be suppressed in 206 Al-Cu cast alloys by controlling the alloy chemistry. Conversely, under the condition that some alloying elements are not added enough by filling the wire, the increase in the content of the Fe-rich phase may also hinder the formation of other precipitates, which are beneficial to the plasticity. 3.5. Corrosion behavior During the welding process, an uneven element distribution and a metallographic structure are easily formed in the WZ, owing to the influence of the welding heat input. This distribution and structure lead to a sharp decrease in the corrosion resistance of WZ. The corrosion behavior of the WZs is typically evaluated via electrochemical testing. Fig. 9 shows typical polarization curves of the BM and both WZs for different welding methods tested in a 3.5 wt.% NaCl solution. The materials are all characterized by a classical passive zone. The current associated with this zone is practically independent of the applied potential, up to the pitting potential that leads to formation of the passive film, which protects the aluminum alloy from corrosion. Prior to pitting corrosion, the passivation potential range of BM is ∼200 mV, and the BM exhibits the highest corrosion resistance of the three samples. Using the Tafel extrapolation method, the corrosion potential (Ecorr) and corrosion current density (Icorr) values are determined from the
Fig. 8. SEM images showing the fracture surface of the (a) GMA welded joint and (b) U-GMA welded joint. 8
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the surface, then intergranular cracks are gradually formed and grow larger, i.e., intergranular corrosion occurs. As the corrosion occurs, some loose corrosion products are formed on both surfaces. Compared with that of the U-GMA WZ, the corrosion degree of the GMA WZ is more severe, with many small pits gradually accumulating into large patches of very deep pits. Fig. 11 shows the EDS element mappings in two WZs after polarization tests. Al and Mg are the main elements lost in corrosion pits after polarization, and the corrosion products are mainly oxides and chlorides of aluminum. For Al–Zn–Mg alloys, intergranular corrosion, which may be affected by the second phases and the structure of GBs, is mainly controlled by electrochemical corrosion of the GBs. The morphology of the attack confirms that the coarse Fe-rich phases play a key role in creating pitting pits in the WZ surfaces of this study by acting as nucleation sites for pitting, and accelerate the loss of materials during pitting. For some areas, local dissolution is almost initiated, while near the location where the iron is distributed, the loss of the aluminum and magnesium is more serious than other locations. Higher electrochemical potential for the Fe-rich phases than the aluminum matrix have been exhibited by Andreatta et al. (2004). Hence, due to the existence of a rather strong galvanic coupling between Fe-rich phases and matrix, it can be inferred the presence of a higher density of the Fe-rich phases in the GMA WZ (see Fig. 5c) has significant effect on the cathodic reaction: The greater the number of Fe-rich phases, the greater the cathodic reactivity. On the other hand, GBs contain more segregation of solute elements
Table 2 Electrochemical parameters obtained from I/E Tafel slope analyses of the plots shown in Fig. 9.
Icorr (A/cm2) Ecorr (VSCE)
BM
WZ in GMA joint
WZ in U-GMA joint
4.426 × 10−6 −0.759
1.761 × 10−5 −0.852
1.395 × 10−5 −0.733
respective polarization curves. The mean values of Ecorr and Icorr obtained from three independent measurements are given in Table 2. The largest Icorr (17.61 μA/cm2) and the smallest (4.426 μA/cm2) Icorr values are obtained for the WZ of the GMA welded joint and the BM, respectively (see Table 2). The corrosion rate increases with increasing corrosion current density and, hence, the largest rate and the smallest rate are obtained for the GMA weld in the 3.5 % NaCl solution and the BM, respectively. The Ecorr value is the largest for the WZ of the U-GMA welded joint (-0.733 VSCE) and smallest for the WZ of the GMA welded joint (-0.852 VSCE). The susceptibility to corrosion (i.e., corrosion tendency) will increase with decreasing corrosion potential. The zones may be listed as follows, WZ of the GMA welded joint, BM, and WZ of the U-GMA welded joint, i.e., in descending order (strong to weak) of corrosion tendency in the 3.5 % NaCl solution. SEM images of the specimens after potentiostatic polarization tests are shown in Fig. 10. It can be seen both WZs suffer from intergranular corrosion. The corrosion for both WZs begins with pitting corrosion on
Fig. 10. Morphology of the attack after potentiodynamic polarization in WZ of the (a) GMA welded joint and (b) U-GMA welded joint. 9
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Fig. 11. EDS element mapping in the WZ of the (a) GMA welded joint and (b) U-GMA welded joint.
than the grain interiors, and are dissolved more quickly than the grain interiors, so the intergranular cracks can form widely and deeply. Although the GB area per grain volume for the U-GMA WZ is higher
than for the GMA WZ, it does not seem to show more severe intergranular corrosion. In addition to the low content of the Fe-rich phase, Zn-element loss attenuated and high density dislocation could play an 10
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important role to improve the corrosion resistance. However, up to now, those influence mechanisms on the corrosion resistance are not still clear. Thus, further studies on the effect of Zn element and dislocations on corrosion resistance in the surface of the WZ of the U-GMA welded joint would be reported in the future.
of AA7075 aluminium alloy. Electrochim. Acta 49 (17-18), 2851–2862. Balasubramanian, V., Ravisankar, V., Madhusudhan Reddy, G., 2008. Influences of pulsed current welding and post weld aging treatment on fatigue crack growth behaviour of AA7075 aluminium alloy joints. Int. J. Fatigue 30 (3), 405–416. Cui, Y., Xu, C.L., Han, Q., 2006. Effect of ultrasonic vibration on unmixed zone formation. Scr. Mater. 55 (11), 975–978. Dai, W.L., 2003. Effects of high-intensity ultrasonic-wave emission on the weldability of aluminum alloy 7075-T6. Mater. Lett. 57 (16-17), 2447–2454. Dev, S., Stuart, A.A., Kumaar, R.R.D., Murty, B.S., Rao, K.P., 2007. Effect of scandium additions on microstructure and mechanical properties of Al–Zn–Mg alloy welds. Mater. Sci. Eng. A-Struct. 467 (1-2), 132–138. Du, G.H., Zhu, Z.M., Gong, X.F., 2012. Acoustic Foundation, third ed. Nanjing University Press, Nanjing, pp. 60–75. Fan, C.L., Yang, C.L., Lin, S.B., Fan, Y.Y., 2013. Arc characteristics of ultrasonic waveassisted GMAW. Weld. J. 92 (12), 375s–380s. Fan, C., Xie, W., Yang, C., Lin, S., Fan, Y., 2017. Process stability of ultrasonic-waveassisted gas metal arc welding. Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 48 (10), 4615–4621. He, L.B., Wu, M.S., Li, L.M., Hao, H.W., 2006. Ultrasonic generation by exciting electric arc: a tool for grain refinement in welding process. Appl. Phys. Lett. 89 (13), 131504. Liu, K., Cao, X., Chen, X.G., 2012. Effect of Mn, Si, and cooling rate on the formation of iron-rich intermetallics in 206 Al-Cu cast alloys. Metall. Mater. Trans. B-Proc. Metall. Mater. Proc. Sci. 43 (5), 1231–1240. Marlaud, T., Deschamps, A., Bley, F., Lefebvre, W., Baroux, B., 2010. Evolution of precipitate microstructures during the retrogression and re-ageing heat treatment of an Al–Zn–Mg–Cu alloy. Acta Mater. 58 (14), 4814–4826. Mi, J.W., Tan, D.Y., Lee, T.L., 2015. In situ synchrotron x-ray study of ultrasound cavitation and its effect on solidification microstructures. Metall. Mater. Trans. B-Proc. Metall. Mater. Proc. Sci. 46 (4), 1615–1619. Mousavi, M.G., Hermans, M.J.M., Richardson, I.U., Den Ouden, G., 2003. Grain refinement due to grain detachment in electromagnetically stirred AA7020 welds. Sci. Technol. Weld. Join. 8 (4), 309–312. Park, J.K., Ardell, A.J., 1988. Precipitate microstructure of peak-aged 7075 Al. Scr. Mater. 22 (7), 1115–1119. Ram, G.D.J., Mitra, T.K., Shankar, V., Sundaresand, S., 2003. Microstructural refinement through inoculation of type 7020 Al–Zn–Mg alloy welds and its effect on hot cracking and tensile properties. J. Mater. Process. Tech. 142 (1), 174–181. Rokhlin, L.L., Dobatkina, T.V., Bochvar, N.R., Lysova, E.V., 2004. Investigation of phase equilibria in alloys of the Al–Zn–Mg–Cu–Zr–Sc system. J. Alloys. Compd. 367 (1-2), 10–16. Srivatsan, T.S., Anand, S., Sriram, S., Vasudevan, V.K., 2000. The high-cycle fatigue and fracture behavior of aluminium alloy 7055. Mater. Sci. Eng. A. 281 (1-2), 292–304. Starink, M.J., Li, X.M., 2003. A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys. Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 34 (4), 899–911. Sun, Q.J., Lin, S.B., Yang, C.L., Fan, C.L., Zhao, G.Q., 2008. The arc characteristic of ultrasonic assisted TIG welding. China. Weld 17 (4), 52–57. Sun, Q.J., Lin, S.B., Yang, C.L., Zhao, G.Q., 2009. Penetration increase of AISI 304 using ultrasonic assisted tungsten inert gas welding. Sci. Technol. Weld. Join. 14 (8), 765–767. Tan, D., Lee, T.L., Khong, J.C., Connolley, T., Fezzaa, K., Mi, J., 2015. High-speed synchrotron x-ray imaging studies of the ultrasound shockwave and enhanced flow during metal solidification processes. Metall. Mater. Trans. A-Phys. Metall. Mater. Sci. 46 (7), 2851–2861. Vratnica, M., Pluvinage, G., Jodin, P., Cvijović, Z., Rakin, M., Burzić, Z., 2010. Influence of notch radius and microstructure on the fracture behavior of Al–Zn–Mg–Cu alloys of different purity. Mater. Des. 31 (4), 1790–1798. Xie, W., Fan, C., Yang, C., Lin, S., 2016. Effect of acoustic field parameters on arc acoustic binding during ultrasonic wave-assisted arc welding. Ultrason. Sonochem. 29, 476–484. Xie, W.F., Fan, C.L., Yang, C.L., Lin, S.B., 2018. Experimental investigation on acoustic control droplet transfer in ultrasonic-wave-assisted gas metal arc welding. Metall. Mater. Trans. B-Proc. Metall. Mater. Proc. Sci. 49 (1), 274–281. Yazdian, N., Karimzadeh, F., Tavoosi, M., 2010. Microstructural evolution of nanostructure 7075 aluminum alloy during isothermal annealing. J. Alloys. Compd. 493 (12), 137–141. Zhao, J., Wu, S., Wan, L., Chen, Q., An, P., 2009. Evolution of microstructure of semisolid metal slurry in ultrasound field. Acta Metall. Sin. 45 (3), 314–319.
4. Conclusions The microstructure, mechanical properties, and corrosion behavior of Al–Zn–Mg alloy welded joints obtained via GMA and U-GMA welding processes are investigated. The major findings are summarized as follows: (1) The WZ and HAZ of the U-GMA welded joint are larger and narrower, respectively, than those of the GMA welded joint, although the same welding heat input is employed in both welding processes. The WZs of the GMA and U-GMA welded joints consist of equiaxed grains with mean sizes of 151.3 μm and 108.6 μm, respectively. (2) For the two WZs, irregular Fe-rich phases are intermittently distributed between dendrites, although the precipitates in the U-GMA welded joint are smaller and considerably fewer than those in the GMA welded joint. Moreover, the gasification loss of Zn element from the U-GMA welded joint is significantly lower than that of the GMA welded joint. (3) The UTS of U-GMA and GMA welded joints is 344.2 MPa and 305.9 MPa, and the corresponding elongation values are 7.4 % and 5.4 %, respectively. Analysis of the broken particles at the bottom of dimples reveals that the alloying-element content is almost the same as that of the Fe-rich phases of the WZ. Furthermore, the results confirm that the decrease in the fracture toughness of the GMA welded joints results mainly from an increase in the content of the coarse Fe-rich phase. (4) The electrochemical corrosion resistance of the WZ in two joints is affected by the distribution of intergranular Fe-rich phases and all specimens suffer from intergranular corrosion. However, the WZ of the U-GMA welded joint exhibits stronger corrosion resistance than the WZ of the GMA welded joint. Acknowledgments This work was supported by the National Natural Science Foundation of China (No.51705072), the Science Foundation for the Excellent Youth Scholars of the Science and Technology Department of Jilin Province (No. 20190103037JH), China, the Research Program on Science and Technology of the 13th Five-Year Plan of the Education Department of Jilin Province (No. JJKH20180428KJ), China, the Project of Guangdong Provincial Key Laboratory (No. 2017B030314048), China and the Guangzhou Science and Technology Program key projects (No.2017A07070102 6), China. References Andreatta, F., Terryn, H., De Wit, J.H.W., 2004. Corrosion behaviour of different tempers
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Comparison of microstructure, mechanical properties, and corrosion behavior of Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (UGMA) welded joints of Al–Zn–Mg alloy Weifeng Xiea,b,*, Te Huanga, Chunli Yangb, Chenglei Fanb, Sanbao Linb, Wanghui Xuc a
School of Mechanical Engineering, Northeast Electric Power University, Jilin 132012, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China c Guangdong Provincial Key Laboratory of Advanced Welding Technology, Guangdong Welding Institute (China-Ukraine E. O. Paton Institute of Welding), Guangzhou 510651, China b
ARTICLE INFO
ABSTRACT
Associate editor: S.-J. Na
The microstructural characteristics, mechanical properties, and corrosion behavior were compared for Al–Zn–Mg alloy (thickness: 8 mm) joints obtained via Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (U-GMA) welding. The results reveal that, compared with the GMA welded joint obtained under the same heat input condition, the U-GMA welded joint consists of a larger weld zone (WZ) and narrower heat affected zone (HAZ). The WZ trends of the joints may be summarized as follows: The Fe-rich phases are smaller and considerably fewer in U-GMA welded joint, resulting in higher fracture toughness than that of the GMA welded joint. Ultimate tensile strength (UTS) and elongation determined from tensile testing of the joints are 342 MPa, 7.4 % for U-GMA welded joint and 305.9 MPa, 5.4 % for GMA welded joint. Electrochemical corrosion experiments in a 3.5 wt.% NaCl solution indicate that the corrosion resistance of the WZs in both joints is affected by the distribution of intergranular Fe-rich phases. Each specimen suffered from intergranular corrosion. However, the WZ of the U-GMA welded joint exhibits stronger corrosion resistance than the WZ of the GMA welded joint.
Keywords: Al–Zn–Mg alloy GMA welding U-GMA welding Microstructure Mechanical properties Corrosion behavior
1. Introduction Heat treatable Al–Zn–Mg alloys (7XXX series) with high strength and low density are widely used in the aerospace, high-speed train, automotive, and shipbuilding industries. Marlaud et al. (2010) reported that the overall properties of these alloys are closely correlated with the grain size as well as the occurrence of stable and metastable phases. Starink and Li (2003) reported that the formation of the main phases, such as the η (MgZn2) phase, S (Al2CuMg) phase, and T (AlCuMgZn) phase, is determined by the main hardening elements (i.e., Zn and Mg). Furthermore, Yazdian et al. (2010) and Park and Ardell (1988) showed that minor impurities and additive elements, such as Fe, Si Ti, and Zr, influence the microstructure through the formation of intermetallic compounds. Therefore, Al–Zn–Mg alloys with desirable properties could be obtained by changing the contents of some alloying elements and choosing an appropriate combination of other alloying elements. Arc welding, characterized by advantages such as simple operation, high production efficiency, and high automation capacity, has been
⁎
extensively applied in welding manufacturing. However, the arc welding structure of Al–Zn–Mg alloys suffers from many quality problems, such as low ductility, low toughness, and short fatigue life. Many types of arc welding methods have been employed with the aim of improving the weld quality. Mousavi et al. (2003) investigated that the electromagnetically stir promotes the grain detachment in gas tungsten arc (GTA) welding of AA7020 aluminum alloy, and the electromagnetically stir is considered an important factor for nucleation and grain refinement. Moreover, synchronous rolling and the use of a trailing heat sink can reduce the tensile welding stresses and, hence, hot cracking of 7020 aluminum alloy welds during GTA welding (Ram et al., 2003). Dev et al. (2007) observed that the addition of scandium led to significant grain refinement in the weld zone (WZ), especially for scandium levels greater than the eutectic composition (0.55 wt.%). Balasubramanian et al. (2008) discovered that improved fatigue performance can be obtained for the post-weld aged pulsed-current GTA welded joints of AA7075 aluminum alloy. However, these techniques are paid relatively much attention to grain refinement, and many
Corresponding author at: School of Mechanical Engineering, Northeast Electric Power University, Jilin 132012, China. E-mail address: [email protected] (W. Xie).
https://doi.org/10.1016/j.jmatprotec.2019.116470 Received 9 June 2019; Received in revised form 13 September 2019; Accepted 21 October 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Weifeng Xie, et al., Journal of Materials Processing Tech., https://doi.org/10.1016/j.jmatprotec.2019.116470
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Fig. 1. Schematic of U-GMA welding setup.
problems, such as joint softening, element loss, and corrosion behavior, remain unresolved. Considering the advantages of ultrasonic vibration, such as a mechanical effect, acoustic streaming, and a cavitation effect, many attempts have been made to introduce this treatment into the process of arc welding. Dai (2003) applied ultrasonic vibration to the side surface of a workpiece to be welded during GTA welding of 7075-T7 aluminum alloy. The results revealed that the grain size in the overheated and heat affected zone (HAZ) decreases, owing to this vibration. Cui et al. (2006) observed that the unmixed-zone width of the WZ is reduced and the corrosion resistance is improved by applying ultrasonic vibration to the back of a super-austenitic stainless-steel workpiece. He et al. (2006) coupled an arc-ultrasonic, resulting from excitation of the arc by a highfrequency current, with the ultrasonic treatment generated by the exciting arc. The results revealed that this treatment exerts a grain-refinement effect during the welding of a Ti–6Al–4 V joint. This method is easily executed in arc welding, but can only be used in non-melting electrode welding. Sun et al. (2008) and (2009) reported ultrasonicassisted GTA (U-GTA) welding, where non-contact ultrasonic vibration was imposed along the coaxial direction of the arc, thereby forming an acoustic radiation field in the arc zone. Compared with conventional GTA welding, the U-GTA is compressed, weld penetration increases by more than 100 %, and strength of the joint can be improved by more than 8 %. Fan et al. (2013) and (2017) proposed ultrasonic-assisted gas metal arc (U-GMA) welding, and showed that the droplet size is reduced, and the droplet transfer frequency is significantly increased. That is, the additional ultrasonic wave is helpful for improving the welding stability. Xie et al. (2018) conducted various U-GMA welding experiments, where the effect of an ultrasonic wave on droplet transfer was considered. The results revealed that the acoustic radiation force provides a detaching force to the growing droplet. This force affects the rest of the process, as the droplet separates from the welding wire end and finally enters the weld pool. However, the microstructural characteristics, mechanical properties, and corrosion behavior of GMA welded joints produced from Al–Zn–Mg alloy, especially the joints of materials obtained via high heat input welding, are only partly understood. Many scholars believe that arc welding of Al–Zn–Mg alloy is impossible. Hence, only a few reported studies have focused on the relationship among the microstructure, mechanical properties, and corrosion behavior of GMA and U-GMA welded joints of Al–Zn–Mg alloys. The present work considers the effect of the ultrasonic vibration on the microstructure, mechanical
properties, and corrosion behavior of an Al–Zn–Mg alloy, and provides a reference for the application of ultrasonic vibration during arc welding. Special attention is given to the possible benefits of U-GMA welding, compared with those of GMA welding, resulting from the additional ultrasonic vibration. 2. Materials and methods 2.1. Materials A typical Al–Zn–Mg alloy (7A52 aluminum alloy) with a thickness of 8 mm was selected as the base metal (BM) in this study, which was subjected to hot rolling at 420 °C and solution treatment at 470 °C. The chemical composition (wt.%) of the alloy is given as follows: 4.0–4.8 % Zn, 2.0–2.8 % Mg, 0.05–0.2 % Cu, 0.3 % Fe, 0.15–0.25 % Cr, 0.25 % Si, 0.2–0.5 % Mn, and balance Al. An Al–Mg alloy (ER5356) consisting of (wt.%) 4.5–5.5 % Mg, 0.4 % Fe, 0.25 % Si, 0.1 % Zn, 0.1 % Cu, 0.05–0.2 % Cr, 0.05–0.2 % Mn, and Al (balance) was used as the filler wire (Φ1.2 mm). The rolled sheets of BM were sheared into 200 × 100 mm2 plates for welding. Using an abrasive waterjet cutting machine, single V-groove angles (30°) were cut in the plates with 2-mm root faces for a total 60° included angle between two plates. The porosity of the joints results mainly from the oxidation film. To reduce this porosity prior to welding, all plates were cleaned with 8 % sodium hydroxide solution at 60 °C and subsequently with 30 % nitric acid solution at room temperature. These plates were then rinsed under flowing water and dried in air. In addition, butting surfaces were burnished with a steel brush. Afterward, the plates were tightly fixed into a fixture to prevent root opening. 2.2. U-GMA welding The joints were joined through GMA welding and U-GMA welding using a U-GMA welding setup (see Fig. 1 for a schematic of the setup). The welding gun was fixed, the workpiece moved at a constant speed, and the angle between the axis of the welding gun and the workpiece was 90°. This setup consisted of two parts, i.e., a welding system and an ultrasonic radiation system, consisting of the ultrasonic power supply and ultrasonic transducer. Synchronous mechanical vibration generated by the transducer was amplified by the amplitude transformer and then radiated to surrounding space in the form of ultrasonic waves by the 2
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ultrasonic radiator. The arc was then subjected to ultrasonic energy. The ultrasonic vibration, with an excitation frequency of 19.5 kHz, was generated by the ultrasonic transducer using a concave spherical surface ultrasonic radiator (height: 14 mm). Xie et al. (2016) used a boundary element method to optimize the geometric parameters of the ultrasonic radiator. For the U-GMA welding, the input power of the SY2000 ultrasonic power source was regulated to 2000 W. Without energization of the ultrasonic excitation power supply, the transducer is non-operational, and GMA welding can be realized. The same welding parameters were employed in both welding processes. A KemppiMIG500 welding power supplied with a ProMIG501 wire feeding machine was used during the experiments, and the welding process was performed under direct current electrode positive conditions. Argon (purity: 99.99 %) was used as the shielding gas and the flow rate through the nozzle (height: 11 mm) was 15 L/min. Experiments were performed at a welding speed, wire feed speed, and welding voltage of 0.4 m/min, 11 m/min, and 25.5 V, respectively, the heat input can be achieved as about 700 J/mm in two welding experiments.
2.5. Electrochemical corrosion Electrochemical experiments were performed on a CHI660E electrochemistry workstation using a three-electrode system. Saturated calomel electrode (SCE) with a Luggin capillary and platinum was used as the reference electrode and auxiliary electrode, respectively. Additionally, 0.8 cm2 BM and WZ of the two joints were used as the working electrode. All BM and WZs were ground with 2000 grit emery paper and then polished and rinsed with ethanol. The non-tested surfaces were protectively coated with a stop-off gelatin. The potentiodynamic polarization experiments were performed at potentials ranging from -1.2 VSCE to -0.1 VSCE (scan rate: 0.1 mV/s) in a 3.5 wt.% NaCl solution (pH = 6) at (25 ± 1) °C. Additionally, 300-s potentiostatic polarization tests (polarization potential: -0.4 VSCE, amplitude: 5 mV, frequency: 1000 Hz) were conducted for a thorough understanding of the corrosion mechanism. The resulting electrochemical corrosion was then observed via scanning electron microscopy (Quanta 200 FEI SEM), and the corresponding energy spectra were also obtained. 3. Results and discussion
2.3. Microstructural characterization
3.1. Macrographs
After welding, samples for metallographic analysis were sectioned, mounted, and polished, and subsequently etched with Keller’s reagent (2 mL HF + 3 mL HCl+5 mL HNO3+190 mL distilled water). Macrographs showing the cross-section of the joints were obtained using a SZX12 stereoscopic microscope. Additionally, the grain morphology of the WZ, HAZ, and BM was examined via optical microscopy (OM, GX71) and electron backscatter diffraction (EBSD, FEI Nova 400). The diffraction data were collected with HKL Channel 5 software. Specimens for EBSD were prepared via conventional mechanical polishing followed by electrochemical polishing (polishing solution: 20 mL ethylene glycol monobutyl ether+40 mL perchloric acid+140 mL ethanol). Moreover, the chemical compositions of coarse intermetallic compounds and some particles in BM and different microstructural zones of the welded joints were evaluated. This evaluation was performed via backscatter electron imaging on a Quanta 200 field emission gun Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS).
Macrographs of the two joints (see Fig. 3) reveal that full penetration joints are realized. The fusion line is approximately straight in both joints. The width of WZ varies from 17 mm at the top to 0 at the bottom in the GMA welded joint, and from 15 mm to 7 mm in the U-GMA welded joint. For the U-GMA welded joint, the WZ area is substantially larger, and the increase in weld size in the depth direction is more pronounced than that of the GMA welded joint. Simultaneously, the HAZ width is 2.0 mm in the GMA welded joint and only 1.0 mm in the U-GMA welded joint. Compared with the GMA welding, the U-GMA welding creates a larger WZ and a narrower HAZ. During U-GMA welding, the arc and droplet are influenced not only by the axial acoustic radiation force, but also by a radial component of acoustic radiation force. The binding effect of ultrasonic on the arc and droplet has been documented in Xie et al. (2016, 2018). It is the primary reason for the increase in area of U-GMA WZ. Conversely, the dispersion of arc heat and low arc force could not increase the weld depth in the GMA welding. Meanwhile, the ultrasonic vibration should also be considered. The weld pool can be assumed to be a multilayer film, under the action of the axial force, including the arc pressure and the droplet impact force, the film will be forced to vibrate. According to Newton’s second law, the vibration amplitude is proportional to the axial force (Du et al., 2012). For the U-GMA welding, a component of ultrasonic radiation force is added to the axial force, then the rate of melt flow will be faster, and the scouring force acting on the weld pool boundary is also greater than that of the GMA welding. This may also lead to the increase in the WZ area. Because the total heat input is consistent in two welding processes, it means that the heat input per unit volume is reduced during the U-GMA welding. Therefore, the thermal impact of this BM will be reduced. As a result, a narrower HAZ is generated in U-GMA welding.
2.4. Mechanical properties The mechanical properties of the joints were determined via tensile and microhardness tests. An abrasive waterjet cutting machine was used to cut the tensile test coupons from the metal sheets. The tensile specimens comprising the joints were machined to the required dimensions, as shown in Fig. 2. All tests of BM and the joints were performed at ambient temperature, in accordance with ASTM E8M-04 (2006), on an Instron 5569 tensile testing machine. These tests were performed at a strain rate of 1/s, and a gauge length of 50 mm was employed for the extensometer attached to each sample. Each final test result was the average value of three tests. Furthermore, a HXD1000TM microhardness tester was used to measure (load: 200 g, dwell time: 10 s, interval between two points: 0.5 mm) the Vickers hardness along the center of the middle region comprising the cross-section.
3.2. Grain morphology The grain morphology characterizing the WZ, HAZ, and BM of the Fig. 2. Dimensions of flat tensile specimens (unit: mm).
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Fig. 3. Appearance and microstructure of joints welded via GMA and U-GMA welding.
two joints was observed and the grain size is measured via EBSD. As shown in Fig. 4, grains with different orientations are shaded in different colors. The grain size statistics obtained from the HKL CHANNEL5 software are listed to the right of each map. Fig. 4a and b show that the typical solidification structure characterizing each WZ consists of equiaxed grains. Moreover, values of 151.3 μm and 108.6 μm are obtained for the mean grain size of the WZ in the GMA welded joint and U-GMA welded joint, respectively. According to Mi et al. (2015) and Tan et al. (2015), owing to an ultrasonic treatment, the size of the grains in the WZ decreases, speed of the weld-pool heat flow increases, degree of supercooling increases, and nucleation of grains is promoted. From Fig. 4c-e, it can be seen that the grains in both HAZs are significantly elongated along the rolling direction and exhibit a pancake morphology, which is almost identical to that of the BM. Mean grain sizes of 65.0 μm, 45.9 μm, and 46.5 μm are obtained for the BM, HAZ of the GMA welded joint, and HAZ of the U-GMA welded joint, respectively. The size of the grains in both HAZs is very similar, that is, the ultrasonic has little effect on the size of the grains in HAZ, this may mainly because of the dramatic effects of coarse second-phase particles and partially soluble constituent phases distributed along the three orthogonal directions of the rolled plate on the re-crystallization through forming the particle nucleation Srivatsan et al. (2000).
second phases contain many alloying elements, where the alloying elements of these phases are detected via EDS of specified locations as indicated in the figure. To reduce deviations, three tests are performed at each location and the average value of the elemental content is calculated (see Table 1). The treatment temperature of the BM exceeds the dissolution temperature of the second phase, therefore, a small number of insoluble coarse-phase particles are scattered on the aluminum matrix, resulting in a chain-like distribution along the rolling direction. The phase particles with different morphologies were analyzed and then two components, namely Point 1 and Point 2, are found. Combining these results with the atomic percentage reveals that the regular strip precipitates are η (MgZn2) phase dissolved in elements, such as Cu, Fe, and Mn. The fine and irregular precipitates are η phase, as shown in Fig. 5a. Referring to the phase diagram of Al-Mg-Si ternary alloys and the atomic percentage indicates that a large amount of Fe-dissolved β (Mg2Si) phases are precipitated at the grain boundary (GB) in the HAZ of the GMA welded joint (see Fig. 5b). However, from Fig. 5d, it can be found that fewer Fe-dissolved β phase particles occur in the HAZ of the U-GMA welded joint than in the HAZ of the GMA welded joint. Meanwhile, some θ (Al2Cu) phase particles are precipitated in the GB in the HAZ of the U-GMA welded joint, consistent with the EDS result of Point 4. This result from the fact that, compared with the GMA-welding, the heat input per unit volume in the U-GMA welding is lower, resulting in easier precipitation of the θ phase, after all, its precipitation temperature is only 548 °C. Many irregular primary precipitates are intermittently distributed between the dendrites comprising the WZ of the two joints, as presented in Fig. 5c and e. The EDS result of Point 5 shows that the precipitates are an Fe-rich phase. These Fe-rich phases in the UGMA WZ are much finer, and some even present a small spherical shape rather than a coarse rod-like particle in the GMA WZ. In addition, a large number of gray phase particles appear in the U-GMA WZ and the HAZ of GMA welded joint, it should be the T (Al6CuMg4) strengthening
3.3. Alloying-element distribution 3.3.1. Second phases The grain structure has a significant influence on the properties of an aluminum alloy. However, these properties are also controlled by the type, volume fraction, morphology, and distribution of the particle phase including impurity phases, excess phases, and precipitated phases (Vratnica et al., 2010). In the present work, two joint specimens are systematically observed via back scatter electron imaging on a SEM. The SEM images for these second phases are shown in Fig. 5. The 4
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Fig. 4. Grain morphology and grain size measurement of different zones in the joints: WZ in the (a) GMA welded joint and (b) U-GMA welded joint; HAZ in the (c) GMA welded joint and (d) U-GMA welded joint; (e) BM.
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Fig. 5. SEM images of different zones: (a) BM; (b) HAZ and (c) WZ in the GMA welded joint; (d) HAZ and (e) WZ in the U-GMA welded joint.
phase as proved by Rokhlin et al. (2004). Zhao et al. (2009) demonstrated that the velocity of acoustic flow can reach 10-103 times that of fluid thermal convection, and this flow
can reduce the temperature gradient and improve the uniformity of the temperature distribution. It may be helpful for increasing the uniformity of chemical composition by accelerating the diffusion of alloy 6
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Table 1 Composition of the second phases in the two WZs, HAZs, and BM.
Point Point Point Point Point Point Point Point Point Point Point Point
1 1 2 2 3 3 4 4 5 5 6 6
in in in in in in in in in in in in
BM (wt.%) BM (at.%) BM (wt.%) BM (at.%) HAZ of GMA (wt.%) HAZ of GMA (at.%) HAZ of U-GMA (wt.%) HAZ of U- GMA (at.%) WZ of GMA (wt.%) WZ of GMA (at.%) WZ of U- GMA (wt.%) WZ of U- GMA (at.%)
Mg
Zn
Fe
Cu
Mn
Cr
Si
Al
14.57 20.64 2.96 3.41 9.59 14.04 1.65 2.59 8.19 10.89 9.48 11.58
16.73 8.82 5.74 2.46 2.40 1.19 4.99 2.90 11.05 5.47 9.44 4.29
20.48 12.00 0.39 0.20 19.68 11.41 4.11 2.23 7.27 4.21 0.24 0.13
6.53 3.36 1.15 0.51 2.67 1.36 45.74 27.38 7.98 4.06 7.50 3.51
1.36 0.81 0.41 0.21 2.90 1.71 0.61 0.31 2.20 1.29 0.24 0.13
/ / / / 4.02 2.50 / / 3.06 1.90 1.03 0.60
0.23 0.23 / / 4.54 5.24 0.88 0.91 0.29 0.28 0.14 0.14
40.10 54.14 89.35 93.21 54.20 62.54 42.01 63.68 59.96 71.91 71.93 79.62
possibly to an increase in the mass percentage of Fe element in WZ. This, in turn, indirectly promotes the formation of the Fe-rich phase. Compared with GMA welding, U-GMA welding is characterized by a more constrained arc, as reported by Xie et al. (2018). However, the experimental result reported in that paper could be attributed to other factors (additional to the fact that, compared with the former, the latter produces a more constrained arc and significantly reduces weld spatter). The total welding heat input is the same in both processes. Therefore, the larger WZ obtained in U-GMA welding (see Fig. 3) means that the heat input per unit volume is lower than that of GMA welding. Additionally, the wire feeding speeds are constant in both experiments. The same speeds are employed for the melting volume of the filler wire used in the two weld pools. Most of the WZ metal in U-GMA welded joint originates from BM and, hence, compared with that added during GMA welding, more Zn element is gradually added to the weld pool during U-GMA welding. According to Darcy’s law, if the velocity of ultrasonic streaming determined by the input power of the ultrasonic wave is sufficiently large, the duration of direct arc action on the elements may be reduced. Consequently, the elemental gasification loss may also be effectively mitigated.
Fig. 6. Microhardness profiles of the joints obtained via different welding methods.
elements in the two-phase zone. In addition, Dai (2003) described that the cooling rate and cooling time can be significantly improved by the ultrasonic waves for the welding of the aluminum alloy 7075-T6. So it can also be considered that the ultrasonic wave could bring the excess heat, or reduce the time of overheating, then the fine particle phases do not have enough time or enough power to grow into the coarse impurity phases. The ultrasonic improves the solidification process for the U-GMA welding. Those factors may explain why the content of the coarse Fe-rich phases, including the Fe-rich phase in WZ and the Fedissolved β phase in HAZ, are reduced under the action of ultrasonic vibration.
3.4. Mechanical properties 3.4.1. Microhardness profile The results of the microhardness tests performed on the joints obtained via GMA and U-GMA welding are plotted in Fig. 6. The average microhardness of the two HAZs and the two WZs is lower than that of the BM. Consistent with the results shown in Fig. 3, the HAZ width of the U-GMA welded joint is considerably narrower than that of the GMA welded joint. The hardness of the WZ corresponding to the U-GMA and GMA welded joint can be classified into two groups, i.e., HV105 to HV115 and HV90 to HV100. The hardness of the BM ranges from HV140 to HV160 and, hence, the hardness of the WZs is 70 % and 65 %, respectively, that of the BM. Moreover, the variation in the hardness values approximately reflects the precipitation and dissolution of the second-phase particles during welding. For the GMA WZ, the coarse Fe-rich phases may reduce the lattice distortion of the aluminum crystal and be favorable for dislocation movement. This leads to a reduction in the hardness and an increase in the width of the softening zone. In contrast, the dissolution of the coarse Fe-rich phases and the reduction of the number density for the coarse Fe-rich phases in the U-GMA WZ increase the lattice distortion of the crystal. For the HAZ, the hardening effect of the θ phase occurring in the U-GMA welded joint is possible to be negligible. A different trend is observed for both HAZs where more T-phase particles are relatively uniformly distributed inside the grain interior for the GMA HAZ (see Fig. 5b). Compared with the U-GMA HAZ, the T phase in the GMA HAZ is more likely to cause the higher hardness.
3.3.2. Elemental gasification loss For the two types of WZs shown in Fig. 5c and e, the main elements, Mg and Zn, in the alloy were identified by means of EDS surface scanning. The Mg and Zn contents, respectively, of the WZs are summarized as follows: GMA WZs: 3.31 wt.% and 1.74 wt.%, U-GMA WZs: 2.94 wt.% and 3.91 wt.%. The primary metallic elements of the Al–Zn–Mg aluminum alloy are Al and Zn, while the filler wire contains mainly Al and Mg. Assuming that the initial content of alloying elements is constant, the reduction in the content of some elements results mainly from the effect of strong gasification loss. Furthermore, the boiling points of Zn and Mg (907 °C and 1107 °C, respectively) are considerably lower than the boiling points of Al and Fe (2467 °C and 2750 °C, respectively). The gasification loss of Al and Fe elements is therefore relatively small. The gasification loss of Zn and Mg elements occurs primarily under the welding heat source. The experimental results suggest that, compared with the U-GMA welding, the GMA welding yielded a greater gasification loss of Zn element, leading 7
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Fig. 7. UTS, yield strength, and elongation of the BM and both joints.
3.4.2. Tensile strength Tensile tests at room temperature were performed on BM and both joints, and the result is shown in Fig. 7. Both joints broke in the WZ during testing. The ultimate tensile strength (UTS), yield strength and elongation of the BM were 525 MPa, 471 MPa and 13.2 %, respectively. The UTS, yield strength and elongation of 344.2 MPa, 298 MPa and 7.4 % as-welded condition were obtained in U-GMA welded joint. The GMA welded joint showed the lower tensile strength (305.9 MPa), yield strength (277.6 MPa) and elongation (5.4 %). Therefore, the UTS, yield strength, and elongation of the U-GMA welded joint are 13 %, 7 %, and 37 %, respectively, higher than those of the GMA welded joint. The fracture surfaces of the two joints are observed in scanning electron micrographs (magnification: 4000×, see Fig. 8). The occurrence of many dimples and tearing ridges in the surfaces indicates that, in both cases, fracture occurs mainly via microvoid coalescence. However, compared with that of the GMA welded joint, the fracture surface of the U-GMA welded joint is covered with deeper and larger dimples. Many broken particles, with EDS-determined chemical compositions (wt.%) of 2.27Zn–3.44Mg–3.56Cu–14.98Fe–0.80Cr–0.32Si–1.44Mn–(bal.)Al (point in Fig. 8a) and 3.44Zn–4.38Mg–2.19Cu–11.77Fe–0.21Cr–0.45Si–2.78Mn–(bal.)Al (point in Fig. 8b), are observed at the bottom of dimples in both surfaces. This chemical composition is basically the same as that of the Fe-rich phase in the WZ (see Table 1). Those phase particles are probably Fe-rich phases distributed between dendrites. Furthermore, the decrease in the plasticity of the GMA welded joints results mainly from an increase in the content of the coarse Fe-rich phase. The Fe-rich phase is a brittle particle, which is incompatible with the aluminum matrix and is easily broken or separated from the matrix,
Fig. 9. Potentiodynamic polarization curves of BM and the two WZs.
thereby forming holes under relatively low stresses. This phase serves as a shortcut for crack growth and a crack source to some extent. In addition, Liu et al. (2012) showed the platelet-like Fe-rich phase can effectively be suppressed in 206 Al-Cu cast alloys by controlling the alloy chemistry. Conversely, under the condition that some alloying elements are not added enough by filling the wire, the increase in the content of the Fe-rich phase may also hinder the formation of other precipitates, which are beneficial to the plasticity. 3.5. Corrosion behavior During the welding process, an uneven element distribution and a metallographic structure are easily formed in the WZ, owing to the influence of the welding heat input. This distribution and structure lead to a sharp decrease in the corrosion resistance of WZ. The corrosion behavior of the WZs is typically evaluated via electrochemical testing. Fig. 9 shows typical polarization curves of the BM and both WZs for different welding methods tested in a 3.5 wt.% NaCl solution. The materials are all characterized by a classical passive zone. The current associated with this zone is practically independent of the applied potential, up to the pitting potential that leads to formation of the passive film, which protects the aluminum alloy from corrosion. Prior to pitting corrosion, the passivation potential range of BM is ∼200 mV, and the BM exhibits the highest corrosion resistance of the three samples. Using the Tafel extrapolation method, the corrosion potential (Ecorr) and corrosion current density (Icorr) values are determined from the
Fig. 8. SEM images showing the fracture surface of the (a) GMA welded joint and (b) U-GMA welded joint. 8
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the surface, then intergranular cracks are gradually formed and grow larger, i.e., intergranular corrosion occurs. As the corrosion occurs, some loose corrosion products are formed on both surfaces. Compared with that of the U-GMA WZ, the corrosion degree of the GMA WZ is more severe, with many small pits gradually accumulating into large patches of very deep pits. Fig. 11 shows the EDS element mappings in two WZs after polarization tests. Al and Mg are the main elements lost in corrosion pits after polarization, and the corrosion products are mainly oxides and chlorides of aluminum. For Al–Zn–Mg alloys, intergranular corrosion, which may be affected by the second phases and the structure of GBs, is mainly controlled by electrochemical corrosion of the GBs. The morphology of the attack confirms that the coarse Fe-rich phases play a key role in creating pitting pits in the WZ surfaces of this study by acting as nucleation sites for pitting, and accelerate the loss of materials during pitting. For some areas, local dissolution is almost initiated, while near the location where the iron is distributed, the loss of the aluminum and magnesium is more serious than other locations. Higher electrochemical potential for the Fe-rich phases than the aluminum matrix have been exhibited by Andreatta et al. (2004). Hence, due to the existence of a rather strong galvanic coupling between Fe-rich phases and matrix, it can be inferred the presence of a higher density of the Fe-rich phases in the GMA WZ (see Fig. 5c) has significant effect on the cathodic reaction: The greater the number of Fe-rich phases, the greater the cathodic reactivity. On the other hand, GBs contain more segregation of solute elements
Table 2 Electrochemical parameters obtained from I/E Tafel slope analyses of the plots shown in Fig. 9.
Icorr (A/cm2) Ecorr (VSCE)
BM
WZ in GMA joint
WZ in U-GMA joint
4.426 × 10−6 −0.759
1.761 × 10−5 −0.852
1.395 × 10−5 −0.733
respective polarization curves. The mean values of Ecorr and Icorr obtained from three independent measurements are given in Table 2. The largest Icorr (17.61 μA/cm2) and the smallest (4.426 μA/cm2) Icorr values are obtained for the WZ of the GMA welded joint and the BM, respectively (see Table 2). The corrosion rate increases with increasing corrosion current density and, hence, the largest rate and the smallest rate are obtained for the GMA weld in the 3.5 % NaCl solution and the BM, respectively. The Ecorr value is the largest for the WZ of the U-GMA welded joint (-0.733 VSCE) and smallest for the WZ of the GMA welded joint (-0.852 VSCE). The susceptibility to corrosion (i.e., corrosion tendency) will increase with decreasing corrosion potential. The zones may be listed as follows, WZ of the GMA welded joint, BM, and WZ of the U-GMA welded joint, i.e., in descending order (strong to weak) of corrosion tendency in the 3.5 % NaCl solution. SEM images of the specimens after potentiostatic polarization tests are shown in Fig. 10. It can be seen both WZs suffer from intergranular corrosion. The corrosion for both WZs begins with pitting corrosion on
Fig. 10. Morphology of the attack after potentiodynamic polarization in WZ of the (a) GMA welded joint and (b) U-GMA welded joint. 9
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Fig. 11. EDS element mapping in the WZ of the (a) GMA welded joint and (b) U-GMA welded joint.
than the grain interiors, and are dissolved more quickly than the grain interiors, so the intergranular cracks can form widely and deeply. Although the GB area per grain volume for the U-GMA WZ is higher
than for the GMA WZ, it does not seem to show more severe intergranular corrosion. In addition to the low content of the Fe-rich phase, Zn-element loss attenuated and high density dislocation could play an 10
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important role to improve the corrosion resistance. However, up to now, those influence mechanisms on the corrosion resistance are not still clear. Thus, further studies on the effect of Zn element and dislocations on corrosion resistance in the surface of the WZ of the U-GMA welded joint would be reported in the future.
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4. Conclusions The microstructure, mechanical properties, and corrosion behavior of Al–Zn–Mg alloy welded joints obtained via GMA and U-GMA welding processes are investigated. The major findings are summarized as follows: (1) The WZ and HAZ of the U-GMA welded joint are larger and narrower, respectively, than those of the GMA welded joint, although the same welding heat input is employed in both welding processes. The WZs of the GMA and U-GMA welded joints consist of equiaxed grains with mean sizes of 151.3 μm and 108.6 μm, respectively. (2) For the two WZs, irregular Fe-rich phases are intermittently distributed between dendrites, although the precipitates in the U-GMA welded joint are smaller and considerably fewer than those in the GMA welded joint. Moreover, the gasification loss of Zn element from the U-GMA welded joint is significantly lower than that of the GMA welded joint. (3) The UTS of U-GMA and GMA welded joints is 344.2 MPa and 305.9 MPa, and the corresponding elongation values are 7.4 % and 5.4 %, respectively. Analysis of the broken particles at the bottom of dimples reveals that the alloying-element content is almost the same as that of the Fe-rich phases of the WZ. Furthermore, the results confirm that the decrease in the fracture toughness of the GMA welded joints results mainly from an increase in the content of the coarse Fe-rich phase. (4) The electrochemical corrosion resistance of the WZ in two joints is affected by the distribution of intergranular Fe-rich phases and all specimens suffer from intergranular corrosion. However, the WZ of the U-GMA welded joint exhibits stronger corrosion resistance than the WZ of the GMA welded joint. Acknowledgments This work was supported by the National Natural Science Foundation of China (No.51705072), the Science Foundation for the Excellent Youth Scholars of the Science and Technology Department of Jilin Province (No. 20190103037JH), China, the Research Program on Science and Technology of the 13th Five-Year Plan of the Education Department of Jilin Province (No. JJKH20180428KJ), China, the Project of Guangdong Provincial Key Laboratory (No. 2017B030314048), China and the Guangzhou Science and Technology Program key projects (No.2017A07070102 6), China. References Andreatta, F., Terryn, H., De Wit, J.H.W., 2004. Corrosion behaviour of different tempers
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