Stress corrosion behavior of friction stir welding joint of 7N01 aluminum alloy
Abstract
The friction stir welding joint of 7N01 aluminum alloy has strong stress corrosion sensitivity due to the inhomogeneity of microstructure. In this paper, the stress corrosion behavior of 14 mm thick plate 7N01-T4 welding joint and base metal was investigated by means of electrochemical impedance spectroscopy, slow strain rate tensile test and electron back scattered diffraction. The results show that the charge transfer resistance of the welding joint is obviously lower than that of the base metal. The lowest impedance at the joint is 698 Ωcm2, but the lowest impedance is 1082 Ωcm2 at the base metal.The lowest Rct value at the weld is only 64.5% of the base metal. The thermal-mechanical affected zone on the rear side is the worst area of the resistance to stress corrosion. There are more deformed grains and larger Schmidt factor close to 0.5 in this area, and the different grain orientation can easily lead to the concentration of internal tensile stress and the formation of crack propagation.
Keywords
1. Introduction
7N01 aluminum alloy is one of the significant lightweight materials, which is widely used in high-speed train profiles [1]. Friction stir welding (FSW) technology has been successfully applied to join various aluminum alloys due to low welding heat input and less welding defects [2]. 7N01 aluminum alloy can be used as the body structure material of high-speed trains due to its high strength ratio and good weldability. Now it has been widely used in China's high-speed railway. However, 7N01 aluminum alloy has some stress corrosion sensitivity due to the high Zn content, the friction stir welded joint also has the same problem. The corrosive environment of these materials is mainly humid atmospheric environment, which may also contain certain chloride ions, such as seaside environment [3,4].
At present, the research focused on mechanical properties of 7xxx aluminum alloy welding joint is systematic, but more attention has been paid on the performance of materials during service [[5], [6], [7]]. Different from the 5xxx aluminum alloy, the 7xxx aluminum alloy has strong stress corrosion sensitivity. Stress corrosion cracking (SCC) of 7xxx aluminum alloy is a low-stress brittle fracture phenomenon caused by tensile stress and specific corrosive media (such as chloride aqueous solution, moist atmosphere), which may lead to the fracture of the material without warning. It has become a great challenge for the service of 7xxx aluminum alloy in the transportation field [8,9].
The electrochemical method is widely used in the study of stress corrosion of aluminum alloys [[10], [11], [12], [13]]. The distribution and size of precipitates at grain boundaries were analyzed by transmission electron microscopy (TEM) to explain the electrochemical mechanism of stress corrosion in 7xxx aluminum alloys [10]. This is due to the different potential between precipitates and base metal, which leads to micro-galvanic corrosion. Kun et al. studied the impedance spectrum of 7A09-T6 aluminum alloy soaked in corrosive media for 15 h–360 h to analyze the corrosion tendency of the material [14]. Shuai Wang et al. found that the discontinuous distribution of grain boundary precipitates in 7N01 aluminum alloy can improve the SCC performance due to avoid the formation of anodic corrosion channels [15]. The FSW joint of aluminum alloy mainly consists of four areas with different microstructure characteristics: base metal (BM), heat affected zone (HAZ), thermal-mechanical affected zone (TMAZ) and nugget zone (NZ). In general, the mechanical properties and corrosion resistance of the welding joint are lower than that of the base metal due to inhomogeneity of microstructure. Zhang Jiayi et al. suggested that cracks in 2297 aluminum alloy tend to propagate along grain boundaries in the orientation of {100}//rolling plane (RP) grains with lower energy, whereas {111}//RP and {110}//RP grain orientations can prevent cracks [16]. Under the influence of welding thermal cycle and plastic flow of material, the microstructure of the FSW joint is different from that of the base metal, which affects the stress corrosion behavior of the welding joint [17]. Bousquet et al. believed that the thermal-mechanical influence zone was the region with the worst corrosion resistance of 2024-T3 aluminum alloy around, while Wadeson et al. also believed that the edge of the thermal-mechanical influence zone of AA7108-T79 was the most susceptible area to corrosion and found that the precipitates η/η ′ (MgZn2) with more negative potential distributed ununiformly around the grain boundary in the TMAZ region [18,19].
Some studies analyzed the performance of corrosion by combining slow strain rate tensile testing and electrochemical method. Qi Xing et al. found that regression and re-aging is an effective heat treatment system to resist SCC, in which the 7003 aluminum alloy samples were soaked in 3.5 wt.% sodium chloride solution and obtained the change of charge transfer resistance after 8 h, 24 h and 48 h respectively [20]. Sunada et al. found the charge transfer resistance of 7075 aluminum alloy was significantly lower than that in the case of no stress through establishing a connection between the testing machine and electrochemical impedance spectroscopy test [18]. By introducing electrochemical impedance measurement into the process of stress corrosion experiments, not only the sensitivity of stress corrosion, but also the evolution of instantaneous corrosion can be obtained under the dynamic action of stress by real-time EIS detection.
However, the study focused on the stress corrosion behavior of 7N01 aluminum alloy FSW joint by analyzing the variation of impedance spectrum data during slow strain rate tensile is limited. In this paper, the stress corrosion behavior of 7N01-T4 aluminum alloy FSW joint and base metal were studied by SSRT testing combining with the electrochemical workstation. Corrosion tendency of the FSW joint and base metal will be revealed by collecting the EIS data at different stages. The correlation between stress corrosion, impedance spectrum and microstructure will be established to reveal the stress corrosion behavior of 7N01 aluminum alloy FSW joint.
2. Methods
2.1. Sample preparation
The 7N01 aluminum alloy plate with a thickness of 14 mm was selected as the base metal which experienced solid solution and T4 treatment. The FSW process was performed under optimized parameters with a pin rotating speed of 400 r/min and 200 mm/min. The welding direction was the same as the rolling direction of the base metal. Table 1 summarized the chemical composition of the 7N01 aluminum alloy material.
Table 1. Composition of the 7N01-T4 aluminum alloy (wt.%).
| Element | Al | Zn | Mg | Cu | Fe | Mn | Si | Ti | Ni | Zr |
|---|---|---|---|---|---|---|---|---|---|---|
| Content | 93.83 | 4.23 | 1.27 | 0.042 | 0.167 | 0.34 | 0.069 | 0.028 | 0.004 | 0.0097 |
2.2. Hardness test
The Vickers hardness was conducted on a DHV-1000 digital hardness tester under a load of 300 g and a holding time of 15 s. Measurements were performed across the weld cross-section perpendicular to the welding direction with a step of 0.5 mm.
2.3. Slow strain rate tensile test (SSRT)
In order to characterize the stress corrosion behavior of the welding joint, the slow strain rate tensile testing was performed with a tensile rate of 2.5 μm/min under the condition of atmospheric environment and 3.5 wt.% NaCl solution respectively. As shown in Fig. 1 (a), 2 mm-thick specimens used for slow strain rate tensile testing were cut from the bottom of the welding joints. During the testing, the sample surface, apart from the welding joint of region A, was isolated using silica gel from the corrosive solution. Here, region A was consisted of the nugget zone, the thermo-mechanical affected zone and a small part of the heat affected zone. As a comparison, stress corrosion testing of the base metal was also performed under the same conditions.
Fig. 12.4. Electrochemical impedance spectroscopy (EIS) test
The impedance measuring device was consisted of a constant temperature tank, a working electrode, an auxiliary electrode, a platinum electrode, a reference electrode Ag/AgCl (3.33 kmol/m3 KCl), a potentiostat, an AC impedance meter and a computer used for controlling the measurement, as shown in Fig. 2. The testing area of the working electrode was set 1 cm2. All potential values were displayed based on Ag/AgCl reference electrode. During EIS measurement, a perturbed AC potential with an amplitude of 10 mV was added to the sample. The analysis frequency range was 0.01 Hz–100,000 Hz and the testing was performed every 30 min. Name the impedance spectrum as the time at which its test begins. The data was fit using ZSimWin software.
Fig. 22.5. Microstructural investigations
(a)
Scanning electrical microscope (SEM)
The SSRT samples were immersed in acetone solution, cleaned by ultrasonic for 3 min and dried with alcohol. The morphology of the fracture was observed by JSM-7800 F ultrahigh resolution thermal field emission scanning electron microscopy.
(b)
Electron back scattered diffraction (EBSD)
The samples used for EBSD analysis were prepared from the welding joint before and after the SSRT testing as shown in Fig. 3. Before EBSD analysis, the samples were firstly grinded to 3000 # and then electropolished with a voltage of 16 V at the temperature of 277 K. The electrolyte was mixed with perchloric acid and ethanol at a volume fraction of 1:9. The sample surface was observed by JSM-7800 F ultrahigh resolution thermal field emission scanning electron microscope.
Fig. 33. Results and discussions
3.1. Hardness test
In order to efficiently evaluate the mechanical properties of the weld, the hardness test was carried out. The hardness distribution curve of 7N01 aluminum alloy 14 mm thickness plate welded by optimized parameters is shown in Fig. 4. The highest hardness is up to 105 HV in the BM zone and the minimum reaches 81 HV. The hardness distribution is consistent with the data of friction stir welding of 7N01 aluminum alloy tested by other researchers [15]. The hardness value increases in the NZ due to the grain refinement. The higher heat input will cause the precipitates to re-dissolve into the aluminum matrix. The difference of the hardness between the top and bottom of the joint is comparatively large due to the different heat input. There is less re-dissolution of the precipitates at the bottom plate, which hinder the movement of dislocations and improve the hardness.
Fig. 43.2. Slow strain rate tensile test (SSRT) and fracture analysis
Fig. 5 shows the SSRT curves of the base metal (BM) and 7N01 FSW joint tested in air and 3.5 wt.% NaCl solution respectively. Table 2 shows the ultimate tensile strength (UTS), yield strength (YS), elongation fracture (EF) and duration before fracture of BM and joint in the SSRT testing. All the values of the UTS, YS, EF and duration decreased significantly under the condition of 3.5 wt.% NaCl solution. Importantly, the duration before fracture of the joint decreased by 15%, which was almost twice as high as that of the BM by 8.4%. Nevertheless, similar duration for the BM and the welding joint was observed in 3.5 wt.% NaCl solution.
Fig. 5
Table 2. Slow strain rate tensile results.
| Sample | Environment | UTS (MPa) | YS (MPa) | Elongation (%) | Duration (h) |
|---|---|---|---|---|---|
| BM | 3.5wt.%NaCl | 364 | 276 | 26.2 | 28.4 |
| Air | 401 | 291 | 26.5 | 31 | |
| Joint | 3.5wt.%NaCl | 343 | 225 | 27.8 | 27.9 |
| Air | 376 | 257 | 28.9 | 33 |
The sensitivity to the stress corrosion cracking of both the base metal and the welding joint was evaluated by the stress corrosion cracking susceptibility index (Pscc) using the following equation [21]:(1)[Math Processing Error]Pscc=PNaClPair×100%where Pair and PNaCl represent the corresponding parameters obtained through SSRT in air and 3.5 wt.% NaCl solution respectively. The calculated Pscc based on the values of UTS, YS, EF and duration are summarized in Table 3. Overall, the PSCC value of the joint was smaller compared to that of the base metal, indicating a decreased resistance to the SCC.
| Sample | UTSSCC(%) | YSSCC(%) | EFSCC(%) | Duration (%) |
|---|---|---|---|---|
| BM | 90.7% | 94.8% | 98.8% | 91.6% |
| Joint | 91.2% | 87.5% | 96.2% | 83.6% |
The fracture macroscopic morphology of the joint and BM after SSRT in air are shown in Fig. 6. The fractured location near the TMAZ and HAZ. The conclusion is consistent with many reported results [[22], [23], [24]]. Some tearing ridges can be observed in the Fig. 6 (a) and more can be found in the Fig. 6 (b-c). It is inferred that the stress concentration occurs near the TMAZ, indicating the poor plasticity compared to the joint in the region. In addition, there are fewer dimples in the welding joint samples. The second phase particles are found in the dimples, which may be the origin of the cracks, causing the tensile strength to decrease. There are fewer fracture ridges in the base metal and the dimples are small as shown in Fig. 6 (d-f).
Fig. 6Nevertheless, according to Table 2, it can be found that the elongation of the joint was slightly better than that of the base metal. On the one hand, the 7N01-T4 aluminum alloy had been treated by natural aging prior to the welding, which increased the plasticity of the welding joint. On the other hand, TMAZ with poor plastic ductility was only a small part of the welding joint. The equiaxed fine grains of the weld nugget has lower strength but better plastic ductility [25].
As mentioned above, the fracture location of the joint in 3.5 wt.% NaCl solution was TMAZ and the fracture morphology was shown in Fig. 7. As shown in Fig. 7 (a-c), the corrosion products covering the surface of the fracture were formed after a large amount of intrusion of the oxide film. A number of cracks can be seen propagating uniformly inward the joint. The fractured ridges proved that the tensile stress concentration was occurred at this location. The cracks were obstructed at some region and finally fractured as the strain continued increasing.
Fig. 7In combination with Table 2, the duration before fracture of SSRT in 3.5 wt.% NaCl solution decreases, indicating that the corrosion products accelerated the rate of propagation of cracks. In the process of plastic flow caused by the stirring needle, the precipitates were broken into small particles and distributed randomly inward the welding joint, which causes the initiation of cracks in the region. As for the base metal soaking in the 3.5 wt.% NaCl, compared to Fig. 7, the base metal fracture showed fewer river-shaped protrusions and more obviously dimples, which meant that the base metal just slightly corroded. To sum up, considering the fact that the most serious intergranular corrosion was found at RS-TMAZ, it is speculated that this region also showed the weakest resistance to SCC.
3.3. Stress corrosion behavior
3.3.1. EIS during the initial stage of corrosion
The open circuit potential and impedance spectrum testing was carried out under the slow strain rate tensile every 1 h. Fig. 8, Fig. 9, Fig. 10, Fig. 11 shows the data of impedance fitted by the software. The equivalent circuit of the curve is shown in Fig. 8 (c-e). Rs is solution resistance, Rf and CPEf are oxide film resistance and capacitance, respectively. Rct and CPEdl denote charge transfer resistance and double layer capacitance at the liquid–solid interface of the material, respectively. Ws denotes Weber impedance. CPEhole and Rhole denote hole capacitance and hole resistance, respectively. L represents inductance. The fitted values are presented in Table 4a, Table 4b, Table 5a, Table 5ba, b. Fig. 8 (a) illustrates the rapid formation of corrosion products when the welding joint was immersed in 3.5 wt.% NaCl solution. From 0.5 h to 1 h, high frequency area shows the semi-circulate while the weber impedance appears at the low frequency area, indicating that the corrosion products have formed and accumulated for 1 h. The similar condition is also observed during 1.5 h–2 h. It can be inferred that the passivation film of aluminum alloy was unstable and ruptured, exposing the substrate with more negative potential leading to the increase of corrosion speed [3]. In the Fig. 8, the blue curve is clearly different from the previous one. This is caused by the deterioration of the corrosion level. From 2.5 h to 3 h, an inductive arc appears at the low frequency area indicating the presence of corrosion pitting [26,27]. The positive ions of metal dissolved in the corrosion pit were difficult to diffuse outward but more negative chloride ions entered the corrosion pit to maintain electrical neutrality. The corrosion products gradually evolved into pitting corrosion, and an inductive arc appeared in the Nyquist diagram [26]. At the low frequency area, since the impedance of the sample was measured under the dynamic conditions, the electrical signal was disturbed by stress, which leads to unstable measurement points. The measurement points that deviate from the fitted curve more and have no physical meaning are not counted [28].
Fig. 8
Fig. 9
Fig. 10
Fig. 11. The trend of R
Table 4a. The electrochemical impedance data of welding joint equivalent circuit elements.
| Elements | Rs (Ωcm2) | Rf (Ω·cm2) | Q f | R ct (Ω·cm2) | Q dl | W (S-secˆ5/cm2) | ||
|---|---|---|---|---|---|---|---|---|
| Y(F/cm2) | n | Y(F/cm2) | n | |||||
| 0.5–1 h | 1.15 | 2554 | 5.76E-5 | 0.90 | 2513 | 1.34E-3 | 1 | 2.38E7 |
| 1.5–2 h | 1.15 | 2018 | 5.74E-5 | 0.89 | 1453 | 1.37E-3 | 1 | 282.8 |
| 2.5–3 h | 1.16 | 326 | 7.98E-3 | 0.92 | 1397 | 4.49E-5 | 1 | / |
| 3.5–4 h | 1.16 | 238.6 | 2.68E-4 | 1 | 867.5 | 5.56E-5 | 0.82 | / |
| 6.5–7 h | 1.05 | 8.24 | 4.83E-4 | 0.83 | 698.4 | 5.52E-5 | 1 | / |
| 9.5–10 h | 1.07 | 9.04 | 6.54E-4 | 0.80 | 854.1 | 6.48E-5 | 1 | / |
| 12.5–13 h | 1.17 | 2.31 | 2.9E-1 | 1 | 1475 | 8.55E-4 | 0.85 | / |
| 15.5–16 h | 1.18 | 67.84 | 1.04E-3 | 0.99 | 1008 | 7.90E-5 | 0.86 | / |
| 18.5–19 h | 1.17 | 68.06 | 3.15E-3 | 0.77 | 1310 | 7.33E-5 | 0.88 | / |
| 21.5–22 h | 1.15 | 0.83 | 1.70E-4 | 1 | 1205 | 8.82E-5 | 0.92 | / |
| 24.5–25 h | 1.14 | 1.93 | 1.75E-7 | 0.75 | 1432 | 8.89E-5 | 0.87 | / |
| 27.5–28 h | 1.14 | 1.43 | 1.46E-4 | 0.96 | 994.1 | 3.01E-5 | 0.99 | / |
| 28.5–29 h | 1.15 | 1.33 | 9.97E-4 | 1 | 1026 | 4.58E-4 | 0.90 | / |
Table 4b. The other EIS data of welding joint equivalent circuit elements.
| Elements | Chi-squared | Q hole | R hole (Ω·cm2) | L1 | R1 | L2 | R2 | |
|---|---|---|---|---|---|---|---|---|
| Y(F/cm2) | n | |||||||
| 0.5–1 h | 6.68E-3 | / | / | / | / | / | / | / |
| 1.5–2 h | 2.00E-3 | / | / | / | / | / | / | / |
| 2.5–3 h | 2.22E-3 | 2.28E-5 | 1 | 1.90 | 6054 | 216.8 | / | / |
| 3.5–4 h | 4.75E-3 | 3.14E-5 | 0.98 | 11.82 | 6510 | 61.2 | / | / |
| 6.5–7 h | 1.54E-3 | 2.17E-16 | 0.31 | 1.09 | 1520 | 135.8 | / | / |
| 9.5–10 h | 1.63E-3 | 7.06E-15 | 0.65 | 1.08 | 2347 | 231.6 | 2.51E13 | 9.64E11 |
| 12.5–13 h | 1.55E-3 | 1.21E-4 | 0.90 | 0.90 | 234.8 | 2241 | 3984 | 187.6 |
| 15.5–16 h | 1.35E-3 | 4.06E-5 | 1 | 6.27 | 429.9 | 1602 | 1353 | 47.8 |
| 18.5–19 h | 2.02E-3 | 4.05E-5 | 1 | 6.48 | 8795 | 27.62 | 1133 | 3280 |
| 21.5–22 h | 1.28E-3 | 1.18E-4 | 0.93 | 117.7 | 824.7 | 3046 | 3127 | 254.8 |
| 24.5–25 h | 1.16E-3 | 4.31E-5 | 1 | 5.64 | 5264 | 342.4 | 887.5 | 3057 |
| 27.5–28 h | 1.32E-3 | 7.95E-5 | 1 | 159.7 | 4171 | 329.1 | 2.18E-9 | 3.55E4 |
| 28.5–29 h | 2.43E-3 | 2.44E-9 | 0.44 | 1.14 | 1751 | 59.2 | 5.52E-11 | 1.206E11 |
Table 5a. The electrochemical impedance data of base metal equivalent circuit elements.
| Elements | Rs (Ωcm2) | Rf (Ω·cm2) | Q f | R ct (Ω·cm2) | Q dl | W (S-secˆ5/cm2) | ||
|---|---|---|---|---|---|---|---|---|
| Y(F/cm2) | n | Y(F/cm2) | n | |||||
| 0.5–1 h | 0.61 | 0.24 | 7.48E-5 | 0.90 | 1923 | 5.47E-8 | 0.3 | 3.72E-3 |
| 1.5–2 h | 0.99 | 1.74 | 1.43E-4 | 0.87 | 1825 | 1.08E-2 | 1 | 1.59E-2 |
| 2.5–3 h | 0.74 | 6.91 | 2.67E-2 | 1 | 1752 | 2.70E-5 | 0.94 | / |
| 3.5–4 h | 0.56 | 3.07 | 8.49E-4 | 0.77 | 1616 | 1.04E-5 | 0.93 | / |
| 6.5–7 h | 0.64 | 0.30 | 1.18E-4 | 0.97 | 1466 | 5.77E-5 | 0.86 | / |
| 9.5–10 h | 0.74 | 2.05 | 1.37E-3 | 0.72 | 1564 | 6.97E-4 | 0.92 | / |
| 12.5–13 h | 1.30 | 2.52E5 | 1.95E-2 | 0.40 | 1082 | 1.69E-5 | 1 | / |
| 15.5–16 h | 0.56 | 10.15 | 1.75E-3 | 0.67 | 1319 | 9.126E-5 | 1 | / |
| 18.5–19 h | 0.41 | 14.2 | 1.703E-3 | 0.67 | 1413 | 9.817E-5 | 1 | / |
| 21.5–22 h | 0.52 | 0.89 | 6.60E-4 | 0.82 | 2374 | 1.41E-14 | 0.55 | / |
| 24.5–25 h | 0.48 | 0.68 | 1.67E-3 | 0.68 | 1372 | 1.47E-4 | 1 | / |
| 27.5–28 h | 0.51 | 13.01 | 1.19E-4 | 0.76 | 1510 | 4.10E-5 | 1 | / |
| 28.5–29 h | 0.48 | 10.91 | 1.68E-3 | 0.69 | 1304 | 1.69E-13 | 0.23 | / |
Table 5b. The other EIS data of base metal equivalent circuit elements.
| Elements | Chi-squared | Q hole | R hole (Ω·cm2) | L1 | R1 | L2 | R2 | |
|---|---|---|---|---|---|---|---|---|
| Y(F/cm2) | n | |||||||
| 0.5–1 h | 2.01E-3 | / | / | / | / | / | / | / |
| 1.5–2 h | 2.02E-3 | / | / | / | / | / | / | / |
| 2.5–3 h | 4.56E-3 | 9.04E-5 | 0.89 | 15.12 | 2.239E4 | 0.5538 | 1.256E6 | 170.7 |
| 3.5–4 h | 4.97E-3 | 3.47E-23 | 0.04 | 0.04 | 2.209E4 | 913.2 | 0.09532 | 4.428E23 |
| 6.5–7 h | 5.32E-3 | 5.50E-5 | 0.97 | 16.77 | 1158 | 3235 | 3896 | 2659 |
| 9.5–10 h | 3.23E-3 | 1.12E-4 | 0.93 | 15.38 | 5009 | 378.7 | 924 | 3274 |
| 12.5–13 h | 1.22E-3 | 7.75E-5 | 1 | 140.2 | 4255 | 81.05 | 1.24E-19 | 1.362E5 |
| 15.5–16 h | 1.44E-3 | 1.37E-8 | 0.80 | 0.02 | 4159 | 274.5 | / | / |
| 18.5–19 h | 3.10E-3 | 9.22E-14 | 0.29 | 0.46 | 5749 | 479.5 | 9.99E-21 | 7.30E13 |
| 21.5–22 h | 2.58E-3 | 1.37E-4 | 0.92 | 9.95 | 6286 | 435.4 | 692.2 | 2606 |
| 24.5–25 h | 3.76E-3 | 1.23E-4 | 1 | 0.01 | 2895 | 0.035 | 1199 | 2386 |
| 27.5–28 h | 5.78E-3 | 6.4E-5 | 1 | 1.90 | 7110 | 599.1 | / | / |
| 28.5–29 h | 6.78E-3 | 1.04E-4 | 1 | 1.03 | 4639 | 449.7 | / | / |
3.3.2. EIS during the intermediate stage of corrosion
Fig. 9(a) shows the smallest radius of the Nyquist loop is a red curve during 6.5 h–7 h and it will hardly change due to the serious corrosion. The equivalent circuit are also shown in Fig. 9(e-g). The corrosion product continued to thicken and gradually covered the corrosion holes resulting in the slow rate of corrosion under the SSRT. It is necessary to connect an additional hole resistance (R hole) in the equivalent circuit to accurately describe the actual electrode process. Corresponding to the fracture of Fig. 7, it can be found that the fracture was covered with corrosion product after being immersed in 3.5 wt.% NaCl solution for 27 h.
The base metal has the smallest impedance radius in the 12 h curve shown in Fig. 9 (b). This is corresponded to lowest value of charge transfer resistance (Rct). In the following time, the value of Rct even tended to increase. Nevertheless, it is not because the corrosion resistance of the sample is restored. It may be concerned with other additional resistance. On the one hand, it comes from the corrosion holes. A new interface formed on the original liquid–solid interface of the working electrode and is influenced by the hole resistance. On the other hand, it may come from corrosive media, that is the intrusion of 3.5 wt.% NaCl solution, which is influenced by liquid resistance.
The impedance fitting curve after 15 h is shown in Fig. 9 (c-d). The overlap of the curves is very high and the value of Rct is maintained at a relatively stable and low level. From the tensile curve in Fig. 5, it can be seen that the sample at this stage has been plastically deformed and the corrosion continues to deepen with the stress, but the rate of corrosion decreases. According to the data in Table 5a, Table 5ba, b and Fig. 11, it can be found that the dark blue curve in Fig. 9 (d) corresponds to a larger value of Rct. This may be related to the lower value of the double layer capacitance, when the accumulation pattern of corrosion products leads to an increase in the charge transfer resistance. The capacitance value of the double layer CPEdl were evaluated by equation proposed by G.J. Brug and the capacitance value of the passivation film Cf were evaluated by equation proposed by C. H. Hsu and F. Mansfeld [29,30]:(2)[Math Processing Error]Q−1=Cdl1−α[RΩ−1+Rct−1]α(3)[Math Processing Error]C=Y0(ω′′m)n−1where ω′′ = m 2πfm, and fm is the frequency at which Z″ is the maximum value.
3.3.3. EIS during the last stage of corrosion
According to the results in Table 2, the duration of the welding joint is about 27.9 h and the base metal is about 28.4 h. The impedance ring radius increased near the duration of the sample fracture as shown in Fig. 9. As shown in Fig. 10 and Fig. 11, the charge transfer resistance increases slightly in the later stages of specimen stretching. Because the crack penetrates deep into the interior of the substrate near the fracture, the new substrate and surface are continuously exposed to form a new liquid–solid interface leading to an increase in resistance.
Rct represents the resistance to the charge transfer between passive film or oxide film and the aluminum alloy matrix, which can be used for evaluating the corrosion resistance of the material [31,32]. The purpose of this paper to study Rct is to find the appropriate resistance to charge transfer as a way to evaluate the evolution of corrosion resistance and stress corrosion tendency of materials and welded joint. It is found in Fig. 11 that the overall trend of Rct can be divided into two parts. In the first part, the Rct of the samples decreased rapidly due to the breaking passivation film and the appearance of corrosion product. The lowest value of welding joint is 698 Ωcm2 at the 6 h, and the lowest value of base metal is 1082 Ωcm2 at the 12 h. It can be judged that the corrosion resistance of welding joint is worse than base metal.
In the second part, the Rct tended to increase due to the passivation film was gradually covered by the oxide film with a lot of holes formed by the corrosion product, which causes the resistance of the charge transfer to increase, but this did not mean that the corrosion resistance of aluminum alloy had become better. In addition, as plastic deformation occurs and the rate of crack expansion increases, there is a constant exposure of new substrate surfaces so that the corrosion medium continues to enter the substrate. This process may affect the increase of Rct.
3.4. Microstructure analysis of welding joint by EBSD
3.4.1. Grain orientation of welded joints
Fig. 12 shows the grain boundary of different areas of the welding joint. The reverse pole diagrams are listed below and the selected location is as marked in Fig. 3. It can be seen that the distribution of the grains orientation in the welding joint is very complicated. The grain size of nuclear zone was small due to dynamic recrystallization. The grains of the base metal area were severely elongated after rolling and the grain size of the HAZ and the TMAZ were also larger than the nugget zone. The obvious difference of grain size affected the resistance to the stress corrosion. The propagation of crack was parallel to the stress direction but was influenced by grain boundaries and texture. The grain orientation and texture strength of each area can be observed through the reverse pole diagram, as shown in Fig. 12. The grain orientation of the HAZ is similar to that of the base metal but is quite different from the nugget zone. The grain orientation in the nugget zone is similar to AS-TMAZ, but is very different from the RS-TMAZ.
Fig. 12After the fracture, the grains of AS-TMAZ and RS-TMAZ are elongated and the degree of grain deformation in RS-TMAZ is greater. The grain size is larger than that of AS-TMAZ. The crack could propagate along the grain boundaries of the severely deformed grains, which probably accelerated the fracture of the material. Different grain orientations around the grain boundary are more likely to form dislocation packing, resulting in stress concentration, which is conducive to crack propagation [33].
3.4.2. Schmitt factor statistics of the welding joint
Schmitt factor (SF) is the ratio of shear stress to principal stress after orthogonal decomposition. The maximum shear stress of slip system can be calculated according to external force load and SF [25]. The axial tensile stress was set and the sliding system was {111} <101>. Fig. 13 shows the SF distribution of AS-HAZ, AS-TMAZ, NZ, RS-TMAZ, RS-HAZ and the base metal. The columnar statistical results of Schmitt factors are also shown in Fig. 13. The order of average SF is RS-TMAZ>AS-TMAZ>NZ>RS-HAZ> AS-HAZ. In addition, the proportion of SF greater than 0.4 in AS-TMAZ and RS-TMAZ regions is higher, and the shear stress in each region of the welded joint is greater, so the crack is easier to propagate in this region [34].
Fig. 133.4.3. Grain morphology and size of the welding joint
The proportions of different types of grains in the welding joint are shown in Fig. 14. The percentage of recrystallized grains in nugget zone is 72.2%, which is dominated by fine equiaxed grains. The latest research of Peng et al. showed that the surrounding metal plasticized and fully flowed in nuclear of FSW due to the large amount of heat generated by the stirring needle and the workpiece [35]. The proportion of dynamic recrystallization is the highest.
Fig. 14From the perspective of grain size, the grain boundaries of a large number of fine equiaxed grains hinder the development of cracks. The proportion of recrystallized grains decreases and the proportion of deformed grains increases in the TMAZ and HAZ. By EBSD analysis, it is found that the grains in the TMAZ Fig. 12 (g-h) of the slow strain rate tensile fracture are seriously deformed and most of them consist of deformed grains. Therefore, the stress concentration after tensile deformation cannot be eliminated, which makes the TMAZ become an area with poor resistance to the stress corrosion.
4. Conclusion
To sum up, this paper studies the stress corrosion behavior of 7N01 aluminum alloy friction stir welded joints. The tendency of the resistance to stress corrosion is analyzed through the evolution of the Rct at different time stages under the slow stress. The corrosion tendency of difference area of the joint was observed by EBSD.
(1)
The tensile strength, yield strength, elongation and duration before fracture of the welding joint decrease by 8.8%, 12.5%, 3.8% and 16.4% under the 3.5 wt.% NaCl solution compared to the performance in air.
(2)
The corrosion product has been accumulating in the welding joint from the 0 h–3 h. The value of welding joint Rct decreased from the peak value of 2513 Ωcm2 to 698 Ωcm2 at the 6 h and the resistance to stress corrosion decreased significantly. The value of base metal Rct decreased from the peak value of 1903 Ωcm2 to 1082 Ωcm2 at the 12 h. According to the trend of Rct, the welded joint and base metal have been severely corroded in 6 h and 12 h respectively.
(3)
Nugget zone has fine equiaxed grains, the average Schmitt factor is smaller than that of HAZ, and the grain orientation is close to that of base metal. HAZ and TMAZ have more deformed grains and larger average Schmidt factor, which can easily cause tensile stress concentration and promote crack propagation. It corresponds to the poor resistance to stress corrosion. The order of the resistance to stress corrosion of the welding joint is as follows: RS-TMAZ < AS-TMAZ < HAZ < BM < NZ.
Funding
The authors gratefully acknowledge the financial support from the project funded by National Natural Science Foundation of China [grant number 51975331] and Key Research and Development Plan of Shandong province [grant number 2020CXGC010206].
Declaration of Competing Interest
We have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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