# Effect of Exfoliation Corrosion on the Mechanical Properties of Friction Stir Spot Welded 2024-T3 AA Joints

Authors: Ramadhan H. Gardi; Abdulkader kadauw; Bertrand Hanschel
Journal: Advances in Materials Science and Engineering
 (2023)

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## Abstract

This study aims to detect the impact of preexfoliation corrosion on the efficiency of 2 mm thick 2024-T3 aluminum alloy friction stir spot welded (FSSW) joints. Five specimens of FSSW joints, welded with 620 r·min−1 rotational speed and a dwell time of 10 seconds, were immersed in an exfoliation corrosion solution for five different times: 10 h, 20 h, 30 h, 40 h, and 50 h. The tensile shear load for the noncorroded joint was 3.5 kN. The results showed that the degradation of mechanical properties increased with increasing exposure time, and the joint efficiency drop reached 62% for specimens exposed for 50 h to the corroded solution. As welded FSSW 2024-T3 AA joints exhibited a nugget debonding failure mode and a mixed-mode tensile-shear fracture, in the corroded specimens, only a shear fracture could be seen. The results were attributed to a reduction in the bonding area between the upper and lower sheets due to corrosion.


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## Body

## 1. Introduction

Friction stir spot welding (FSSW) is a solid-state welding process created to eliminate various fusion welding defects that appear due to the melting and solidification of aluminum alloys, such as solidification and pores. FSSW gained considerable attention in the automotive, marine, and aerospace industries. Compared to resistance spot welding, the FSSW technique is preferred as it eliminates electrode wear, provides better mechanical and corrosion properties of joints, lower residual stress, and short cycle time, and is less energy demanding [1, 2]. In the last decade, an extensive research has been conducted on friction stir spot welding of aluminum alloys. Farhang et al. [3] prepared FSSW joints of AA 2024-T3 sheets with 2 mm thickness using AISI H13 tool steel with 5 mm, 2.2 mm, and 14 mm pin diameter, pin length, and tool shoulder, respectively. They concluded that increasing the rotational tool speed from 800 r·min−1 to 1120 r·min−1 increased the failure load by 52%. They further concluded that increasing dwell time from 2 seconds to 3 seconds during welding increases the failure load by 17%. Moslem Paidar et al. [4] published a work on the effect of FSSW parameters on the fracture mode of 2024-T3 AA spot welded joints and found that low rotational speed and low plunging depth exhibit shear fracture, while the joints welded with high rotational speed and high plunging depth exhibit tension-shear fracture. The same work concluded that the pin geometry influences the hook shape. Lui et al. [5] joined the 3 mm thickness of 2012-T4 aluminum alloy sheet using a round concave shoulder conical threaded pin FSSW tool. They used various rotational speeds of 400, 600, 800, 1000, and 1200 rpm and 5, 10, 20, 30, and 40 mm/min plunging. The experimental results showed that the tensile shear failure load increased with increasing rotational speed and reached its maximum value at 1000 rpm but decreased with increasing plunging rate. Paidar et al. [6] experimentally investigated the effects of rotational tool speed, shoulder penetration depth, and tool pin geometry on the tensile shear stress of 2024-T3 AA friction spot welds. It was observed that at a low shoulder penetration depth of 0.3 mm, the tensile shear load increased with increasing rotational speed, whereas a high penetration depth of 0.7 mm had a bilateral influence on tensile shear load. Also, they found that the strength of a joint made at low rotational speed using a triangular pin was higher than that of the joint made with a cylindrical pin. In a work published by Balamurugan et al. [7] investigated the addition of magnesium powder as a filler to dissimilar friction stir spot-welded AA 5052 –AA 6061 joints. They found that the tensile shear strength of the normal FSSW was 133.88 MPa. They found that adding 4.5 mg of volume fraction magnesium powder as a filler to the joint increased the tensile shear strength to 202.85 MPa, i.e., the joint strength exhibited a significant increase of 34%. Also, they reported that the improvement in joint strength results from Mg precipitates and grain refinement in the weld zone. Paidar and Laali Sarab [8] spot welded 1.6 mm thick 2024-T3 AA with different rotational speeds: 630 r·min−1,1000 r·min−1, and 1600 r·min−1 and penetration depths of 0.3, 0.5, and 0.7 mm. It was observed that increasing tool rotational speed and penetration depth increased the failure load, but the effect of penetration depth was more significant compared to the rotational tool speed. The effect of aging time on the tensile shear force of FSSW AA2024 was experimentally demonstrated by Abass [9]. The author concluded that increasing the aging time increases the tensile shear force. The tensile shear force for the as-welded specimen was 2400 N. Increasing the aging time increased the tensile shear, which reached its maximum value of 3600 N when the aging time was 5 h. A further increase of aging time resulted in the tensile shear strength reduction. Several localized corrosion modes, including exfoliation corrosion, may occur in the 2024-T3 aluminum alloy during operation. FSSW 2024 aluminum alloy joints are more susceptible to exfoliation corrosion than base metal joints since a protective aluminum oxide thin film in the joint is entirely deteriorated during the welding process due to heat and deformation. Several studies concentrated on detecting the relationship between mechanical and metallurgical properties and the corrosion resistance of welded joints. Alexopoulos et al. [10] analyzed the tensile strength of precorroded 2198 Al-Cu-Li and 2024 aluminum alloys for different exposure times to an exfoliation solution. They concluded that the tensile strength and elongation of 2024 AA degraded more significantly compared to 2198 AA. Doveletoglou et al. [11] welded 2024 AA of 3.2 mm thickness with an electron beam welder. The tensile specimens were exposed to an exfoliation corrosion solution for six different exposure times (2 h, 4 h, 8 h, 12 h, 24 h, and 48 h). Tensile tests were carried out, and the results revealed that increasing exposure time decreased the yield stress, and the lowest exfoliation corrosion-induced decrease of 30% could be seen in a specimen exposed for 48 h to an exfoliation corrosive environment. Liu et al. [12] studied 7075 aluminum alloys with a 2 mm thickness. The tensile specimens were immersed in a corrosive exfoliation solution for 48 h. The results revealed a significant drop in strength. Kalemba et al. [13] studied the exfoliation corrosion resistance of friction stir welded AA 7136-T6 friction stir weld joints. It was observed that the exfoliated corrosion resistance in the stir zone was higher than that in the thermomechanical affected zone. Moreover, they found that the distribution of MgZn2 particles had a significant effect on the corrosion behavior. De Sousa Araujo [14] studied the exfoliation corrosion resistance of weld zones in friction stir welded AA 2098-T 351 and compared the results with the base metal. It was revealed that the stir zone exhibited higher exfoliation and corrosion resistance compared to heat-affected zones. In addition, it was also seen that the decreasing temperature in the heat effect zone increased the susceptibility to exfoliation corrosion. The originality of the present paper is that it studies the impact of exfoliation corrosion on friction stir-welded aluminum alloy joints. It is novel, as no work has been reported on the effect of exfoliation corrosion on the mechanical properties of FSSW joints. In this research paper, the tensile shear test was performed for FSSW joints of 2024 aluminum alloy subjected to exfoliation corrosion to study the effect of different exfoliation corrosion exposure times on the specimen tensile shear load.


## 2. Experimental Procedure

### 2.1. Materials

The material used in this study was a 2024-T3 aluminum alloy sheet with a 2 mm thickness. The chemical composition of the base material is shown in Table1. The chemical composition was detected using the spectrometer metal analyzer model SPECTROMAXX (Spectro Company, Germany, 2010). A tensile test was carried out to detect the mechanical properties of base metal according to ASTM standard E8 using the universal testing machine Hualong WAW600. The tests were carried out at the Salahaddin University College of Engineering in Iraq. The mechanical properties of the base material are shown in Table 2.Table 1
Chemical composition of AA 2024-T3 (wt.%).
MaterialCuMgMnSiZnFeCrNiAlAA2024-T34.61.360.650.160.150.170.010.02BalanceTable 2
Mechanical properties of AA 2024-T3.
MaterialTensile strength (MPa)Yield strength (MPa)Elongation (%)Hardness (HB)AA2024-T3469.134515122The tensile shear specimens were machined according to the national standard GB/T 2651-2008/ISO 4136 : 2001 [15] and the American Welding Society standard AWS C1.1 M/C1.1 : 2012 [16]. Two rectangles of 100 × 25 mm with 25 mm overlaps were prepared. A cylindrical tool was selected while performing the FSSW joints. Blici and Yükler [17] conducted the FSSW experiments using different pin geometries: triangular, hexagonal, cylindrical, and square pin geometries. They concluded that the cylindrical pin exhibited the highest joint failure load. The FSSW tool was made of H13 tool steel [3, 17, 18] with a hardness of 55 HRC. The geometrical dimensions of the tool were 2.8 mm pin length, 5 mm pin diameter, and 16 mm shoulder diameter. Figure 1 shows the dimensions of the FSSW tool and specimens.Figure 1
Dimensions of the FSSW tool and specimen.


### 2.2. FSSW Process

The vertical milling machine was used to prepare the FSSW joints. The specimens were clamped in the fixture, as shown in Figure2. Specimens were welded with the following parameters: the rotational speed of the tool was 620 r·min−1, shoulder penetration, and dwell time was 0.8 mm and 10 seconds, respectively, while the penetration rate was 0.18 mm·min−1. The welding parameters were selected based on the literature [5, 16, 19, 20], and the trial experiment was conducted to find out whether a sound joint free of defects could be obtained using the mentioned parameters.Figure 2
Friction stir spot welded setup.


### 2.3. Exfoliation Corrosion Test

To detect the degradation properties of FSSW joints, an exfoliation corrosion test was conducted. The specimens were subjected to exfoliation corrosion solution (EXCO) according to the ASTM G34 standard. For the corrosion solution, the following chemicals were diluted in one liter of distilled water: sodium chloride (4.0 M NaCl), potassium nitrate (0.5 M KNO3), and nitric acid (0.1 M). The solution consisted of 234 g NaCl, 50 g KNO3, and 6.3 ml of concentrated HNO3 in 1 L of water. [21, 22] Before exfoliation corrosion, the FSSW specimens were degreased using soap and acetone and then dried with hot air. FSSW specimens were exposed to a corrosive solution for five different exposure times: 10 h, 20 h, 30 h, 40 h, and 50 h. The corrosive solution temperature was held at 27°C by using a regulated hot plate. The volume of exfoliation corrosion solution applied to the surface area of the specimens was 11 ml/cm2 [21]. Figure 3 shows the FSSW specimen immersed in the EXCO solution. A new corrosive solution was used for each specimen, and the tests were repeated three times. After the specimens were removed and rinsed with water, they were dried with hot air and then subjected to the tensile shear test. A universal testing machine, the Hualong WAW600, was used for the tensile shear test with a load rate of 2 mm·min−1. The joint efficiency, η, was measured in terms of the relative tensile shear load of corroded specimens to the tensile shear load of uncorroded specimens. η was calculated according to the following equation.(1)Efficencyɳ=TensileshearloadofcorrodedFSSWspecimensTensleshearloadofuncorrodedFSSWspecimensASWelded.Figure 3
FSS-welded specimen immersed in the EXCO solution.


### 2.4. Metallographic Analysis

A standard metallographic preparation was used for friction stir spot-welded joints to study the microstructure. The specimens were mechanically prepared with 600, 1000, 1200, and 2000 sandpaper. After that, a polishing was performed using 25μm diamond paste, followed by chemical etching according to ASTM standards [23]. Two etching solutions were used. The first solution consisted of 25 ml HNO3 and 75 ml H2O, whereas the second solution consisted of 0.5 g NaF, 1.0 ml HNO3, 2 ml HCl, and 97 ml H2O. The specimens were immersed for one minute in the first solution and quenched in cold water, and then immersed for 30 seconds in the second solution and washed in warm water. The microstructure examination was carried out on a cross-section of FSSW joints. An optical microscope type Metkon IMM 901 METALLURGICAL MICROSCOPE with computer calibration set was used for an H and V measurement onstage micrometer. After the tensile shear test, the fractured surfaces of the specimens were observed using SEM with CamScan 3200 LV using Caesium TM version 6.1.10 at Kurdistan Institution for Strategic Studies and Scientific Research-Iraq.


## 2.1. Materials

The material used in this study was a 2024-T3 aluminum alloy sheet with a 2 mm thickness. The chemical composition of the base material is shown in Table1. The chemical composition was detected using the spectrometer metal analyzer model SPECTROMAXX (Spectro Company, Germany, 2010). A tensile test was carried out to detect the mechanical properties of base metal according to ASTM standard E8 using the universal testing machine Hualong WAW600. The tests were carried out at the Salahaddin University College of Engineering in Iraq. The mechanical properties of the base material are shown in Table 2.Table 1
Chemical composition of AA 2024-T3 (wt.%).
MaterialCuMgMnSiZnFeCrNiAlAA2024-T34.61.360.650.160.150.170.010.02BalanceTable 2
Mechanical properties of AA 2024-T3.
MaterialTensile strength (MPa)Yield strength (MPa)Elongation (%)Hardness (HB)AA2024-T3469.134515122The tensile shear specimens were machined according to the national standard GB/T 2651-2008/ISO 4136 : 2001 [15] and the American Welding Society standard AWS C1.1 M/C1.1 : 2012 [16]. Two rectangles of 100 × 25 mm with 25 mm overlaps were prepared. A cylindrical tool was selected while performing the FSSW joints. Blici and Yükler [17] conducted the FSSW experiments using different pin geometries: triangular, hexagonal, cylindrical, and square pin geometries. They concluded that the cylindrical pin exhibited the highest joint failure load. The FSSW tool was made of H13 tool steel [3, 17, 18] with a hardness of 55 HRC. The geometrical dimensions of the tool were 2.8 mm pin length, 5 mm pin diameter, and 16 mm shoulder diameter. Figure 1 shows the dimensions of the FSSW tool and specimens.Figure 1
Dimensions of the FSSW tool and specimen.


## 2.2. FSSW Process

The vertical milling machine was used to prepare the FSSW joints. The specimens were clamped in the fixture, as shown in Figure2. Specimens were welded with the following parameters: the rotational speed of the tool was 620 r·min−1, shoulder penetration, and dwell time was 0.8 mm and 10 seconds, respectively, while the penetration rate was 0.18 mm·min−1. The welding parameters were selected based on the literature [5, 16, 19, 20], and the trial experiment was conducted to find out whether a sound joint free of defects could be obtained using the mentioned parameters.Figure 2
Friction stir spot welded setup.


## 2.3. Exfoliation Corrosion Test

To detect the degradation properties of FSSW joints, an exfoliation corrosion test was conducted. The specimens were subjected to exfoliation corrosion solution (EXCO) according to the ASTM G34 standard. For the corrosion solution, the following chemicals were diluted in one liter of distilled water: sodium chloride (4.0 M NaCl), potassium nitrate (0.5 M KNO3), and nitric acid (0.1 M). The solution consisted of 234 g NaCl, 50 g KNO3, and 6.3 ml of concentrated HNO3 in 1 L of water. [21, 22] Before exfoliation corrosion, the FSSW specimens were degreased using soap and acetone and then dried with hot air. FSSW specimens were exposed to a corrosive solution for five different exposure times: 10 h, 20 h, 30 h, 40 h, and 50 h. The corrosive solution temperature was held at 27°C by using a regulated hot plate. The volume of exfoliation corrosion solution applied to the surface area of the specimens was 11 ml/cm2 [21]. Figure 3 shows the FSSW specimen immersed in the EXCO solution. A new corrosive solution was used for each specimen, and the tests were repeated three times. After the specimens were removed and rinsed with water, they were dried with hot air and then subjected to the tensile shear test. A universal testing machine, the Hualong WAW600, was used for the tensile shear test with a load rate of 2 mm·min−1. The joint efficiency, η, was measured in terms of the relative tensile shear load of corroded specimens to the tensile shear load of uncorroded specimens. η was calculated according to the following equation.(1)Efficencyɳ=TensileshearloadofcorrodedFSSWspecimensTensleshearloadofuncorrodedFSSWspecimensASWelded.Figure 3
FSS-welded specimen immersed in the EXCO solution.


## 2.4. Metallographic Analysis

A standard metallographic preparation was used for friction stir spot-welded joints to study the microstructure. The specimens were mechanically prepared with 600, 1000, 1200, and 2000 sandpaper. After that, a polishing was performed using 25μm diamond paste, followed by chemical etching according to ASTM standards [23]. Two etching solutions were used. The first solution consisted of 25 ml HNO3 and 75 ml H2O, whereas the second solution consisted of 0.5 g NaF, 1.0 ml HNO3, 2 ml HCl, and 97 ml H2O. The specimens were immersed for one minute in the first solution and quenched in cold water, and then immersed for 30 seconds in the second solution and washed in warm water. The microstructure examination was carried out on a cross-section of FSSW joints. An optical microscope type Metkon IMM 901 METALLURGICAL MICROSCOPE with computer calibration set was used for an H and V measurement onstage micrometer. After the tensile shear test, the fractured surfaces of the specimens were observed using SEM with CamScan 3200 LV using Caesium TM version 6.1.10 at Kurdistan Institution for Strategic Studies and Scientific Research-Iraq.


## 3. Results and Discussion

Figure4 shows the variation of temperature with time during the FSSW process. With increasing tool penetration, the temperature increased gradually and reached 339°C during the first 20 seconds of the plunging stage (16.95°C.s−1 heating rate). The temperature increased continuously during the dwell time stage from 20 s to 30 s but at a lower rate of 4.8°C.s−1 compared to the tool penetration interval, and reached its maximum value of 387°C. Goushegir et al. [24] stated that during the fabrication of an FSS welding joint of 2024-T3 aluminum alloy, the average peak temperature was between 350 and 400°C. This is due to friction heat and the thermomechanical effect between the tool and the 2024-T3 aluminum alloy plates. After 10 seconds of dwell time, the tool was drawn out and the temperature decreased nearly to a uniform cooling rate of 9.67°C.s−1. The joint’s temperature profile directly impacts the microstructural properties of the joint including grain size, grain shape, number, and distribution of precipitated particles in different zones of the joint. As a result, the mechanical properties of the joint change [25].Figure 4
Temperature-time profile during the FSSW process.Figure5 shows the macrograph of FSSW joints of as-welded and as-corroded specimens. The corroded specimens were immersed in the corroded solution for five different exposure times: 10 h, 20 h, 30 h, 40 h, and 50 h. The figure shows a slight corrosion attack in the stir zone, which can be attributed to finely equiaxed grain and the dissolution of precipitates. This observation coincides with that ofRalston et al. [26], who concluded that the tendency of high purity aluminum to corrosion in NaCl decreases with decreasing grain size. In contrast, a progressive exfoliation corrosion can be seen in the thermomechanical affected zone (TMAZ), and the degree of exfoliation corrosion increases with increasing immersion time. Figure 5 shows that the lowest corrosion attack in the TMAZ is observed in specimens immersed for 10 h and could be defined as a small number and small size of pits. Increasing the immersion times to 20 h increases the degree of exfoliation corrosion due to increasing pits density. Increasing the exfoliation time further increases the susceptibility to exfoliation corrosion. An advanced exfoliation corrosion degree occurs in the specimen immersed for the most prolonged period of 50 h. It could be defined as an ED according to ASTM standards, and the plates lost nearly 25% of their thickness due to exfoliation corrosion.Figure 5
Photograph of corroded 2024-T3 AA FSSW joint subjected to corroded solution for the different immersing periods.Figure6(a) shows the macroscopic crosssection of the 2024-T3 aluminum alloy friction stir spot-welded joints. The crosssection consists of four zones: the stir zone (SZ), the thermomechanical affected zone (TMAZ), the heat affected zone (HAZ), and the base metal (BM). As a result of tool penetration into the upper plate, a hook has been formed, separating the fully bonded region (FBR) and the partially bonded region between sheets. The width of the fully bonded region and the height of the hook were 585.8 and 156 21 micrometers, respectively. Figure 6(b) depicts the base metal microstructure. A magnified view of region A, marked in macroscopic cross-section, consists of elongated grain parallel to the rolling direction and particles of different sizes spread over the entire structure. Figures 6(c)–6(e) display the microstructure of the stir zone adjacent to the FSSW tool around the keyhole and tool shoulders and are magnified views of regions B, C, and D, respectively. In the stir zone, severe plastic deformation occurred due to high temperatures and dynamic recrystallization during the welding process. The microstructure of the stir zone consists of refined grains and second-phase particles expected to be Al2CuMg precipitates distributed uniformly [27]. Figure 6(f) displays the microstructure of TMAZ, which is located between SZ and HAZ, and magnified views of region E. The grains in this zone are elongated and oriented in the direction of the material flow. The grain size in this region is larger than that of the stir zone and smaller than that of HAZ. As a result of thermomechanical action and tool penetration, burrs appear all around the tool, and shoulder grain distortions occur. Figure 6(g) is a magnified view of region F marked in the macroscopic cross-section. In HAZ, as a result of heat generation, the precipitated particles were coarsened, as shown in Figure 6(h) (the magnified view of region G).Figure 6
(a) Optical macro cross-sectional image of FSSW 2024-T3 aluminum alloy joint; (b), (c), (d), (e), (f), (g), and (h) magnified view of regionA, B, C, D, E, F, and G, respectively.Figures7 and 8 show the tensile shear load vs. extension curves of the as-welded and corroded specimens of FSSW AA 2024-T3 joints, respectively.Figure 7
Load vs. extension curve for as-welded FSSW 2024-T3 AA.Figure 8
Load vs. extension curve for corroded FSSW 2024-T3 AA joints for five different exposure times.The tensile shear load value and elongation to fracture of as-welded FSSW 2024-T3 joints are 3.5 kN and 2.2 mm, respectively. Figure9 shows the tensile shear load for five FSSW 2024-T3 aluminum alloy joints exposed for five different exposure times to an exfoliation and corrosion solution. The results are compared with uncorroded (as-welded) specimens. The tensile shear load of uncorroded (as welded) specimens is 3.5 kN. After 10 h in immersion in the corroded solution, the tensile shear load was reduced to 3.1 kN. The degradation rate of the tensile shear load was 0.4 kN·h−1, which means that during 10 h of exposure time, the efficiency of the joint was reduced by 11%. When the exfoliation corrosion exposure time increased to 20 h, the tensile shear load was significantly reduced. It reached 2.2 kN, and this reduction in tensile shear load resulted in the loss of nearly 37% of the joint’s efficiency. The degradation rate of tensile shear load at this stage reached its highest value of 0.09 kN·h−1. The corrosion of FSSW 2024-T3 aluminum alloy joints is mainly attributed to the hydrogen embrittlement [28] and precipitated particles Al2Cu (ѳ phase) and Al2CuMg (S phases). The ѳ phase acts as a cathode, while the S phase acts as an anode with respect to the aluminum matrix. The S phase dissolves because of galvanic coupling between the S phase particles and the surrounding matrix. At the same time, the ѳ phase promotes the dissolution of the copper-depleted zone around the particle. Because of hydrogen embrittlement [26] and the preferential corrosion of a phase, pits are created in the joints. They act as crack initiation sites in addition to reduce the contact area between the upper and lower sheets. Boag et al. [29] and Xiao et al. [30] reported a dissolution of the cathodic intermetallic phase due to pitting corrosion on these particles themselves. Increasing the exposure time to 30 h, the tensile shear load showed a continuous reduction down to 1.8 kN (0.04 kN/h.), i.e., 48% of joint efficiency was lost due to the impact of exfoliation corrosion. The tensile shear load dropped to its minimum value of 1.3 kN when FSSW 2024-T3 aluminum alloy joint was exposed for 50 h to the corroded solution. This causes a loss of nearly 62% of joint efficiency with a degradation rate of 0.04% in the tensile shear load. Compared to the earlier stage, the reduction rate of tensile shear load at this stage may be related to the formation of corrosion product which acts as a barrier between the corroded solution and the surface of the joint [22].Figure 9
Tensile shear stress of FSSW AA 2024-T3 exposed to exfoliation corrosion solution at different times.Figure10 shows the elongation to fracture for five FSSW 2024-T3 aluminum alloy specimens that were exposed to an exfoliation corrosion solutionfor five different exposure times. The results are compared with uncorroded (as-welded) specimens. The as-welded specimens had the highest elongation value (2.2 mm). At the same time, after only 10 h in corroded solution, the 2024-T3 FSSW joint elongation dramatically dropped to 1.2 mm, i.e., it represented only 54% of the uncorroded specimen’s elongation. By increasing the exposure time to 20 h, the elongation decreased further and reached only 0.9 mm. The increase in exposure time led to a further elongation drop, but it was observed at a lower degradation rate compared to earlier stages. It finally reached its minimum value of 0.66 mm for specimens exposed to 50 h of the corroded solution, which represented a nearly 67% drop in elongation compared to as-welded specimens. This result could be attributed to increasing hydrogen production with increasing exfoliation time and absorption of hydrogen by the joint surface [31, 32].Figure 10
Elongation to fracture of FSSW AA2024-T3 exposed for different times.The SEM images of the fracture surfaces of as-welded specimens and specimens subjected for 50 h to exfoliation corrosion solution are compared in Figure11. As-welded specimens exhibit the nugget debonding failure mode, and a mixed tensile-shear fracture occurs. The corresponding bonding area between the upper and lower sheets was 32.47 mm2, and the fracture surface of the specimens consisted of fine dimples. The corroded specimen’s nugget pull-out fracture mode can be seen, and only a shear fracture occurs. As such, the smooth, fractured surface can be observed. As a result of corrosion, the bonding area between an upper and lower sheet of the corroded specimens subjected to 50 h of exfoliation corrosion was reduced to 8.63 mm2. This result confirms the reduction of tensile shear stress in corroded specimens.Figure 11
SEM of the fracture surface of the tensile shear specimens (a) fracture surface of the lower sheet of as-welded specimens; (b)-(c) magnified views of pointsA and B, respectively; (d) fracture surface of the lower sheet of corroded specimens for 50 h; (d)-(e) magnified views of points C and D, respectively.
(a)(b)(c)(d)(e)(f)


## 4. Conclusions

In this study, 2024-T3 aluminum alloy sheets of 2 mm thickness were FSSW using a cylindrical pin with the tool rotational speed 620 r·min−1 and dwell time 10 s. The microstructure and mechanical properties of joints were investigated. FSSW joints were exposed to an exfoliation corrosion solution for five different exposure times (10 h, 20 h, 30 h, 40 h, and 50 h). The effect of exfoliation corrosion exposure time on tensile shear load and elongation of FSSW joints was investigated. The results were compared to the as-welded joint. The following conclusions can be outlined:(1)
The tensile shear load and elongation obtained in the AA 2024-T3 FSSW joint were 3.5 kN and 2.2 mm, respectively.(2)
The microstructural examination of FSSW 2024-T3 AA joints revealed that the stir zone had fine precipitates and fine grain size, while the thermomechanically affected zone exhibited coarse precipitates and distorted grains.(3)
With increasing exfoliation corrosion exposure time, the joint efficiency of 2024-T3 AA decreased.(4)
11% of joint efficiency decreased during the first 10 h exposure time.(5)
The maximum degradation in joint efficiency of nearly 62% was observed at the highest exposure time of 50 h.(6)
When a FSSW AA 2024-T3 joint is immersed for 50 h in a corroded solution, elongation drops by nearly 70% compared to as-welded specimens.(7)
As-welded FSSW 2024-T3 AA joints exhibit a nugget debonding failure mode, and mixed tensile-shear fracture occurs. In the corroded specimens, only a shear fracture can be seen.



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*Source: 9629740-2023-01-24.xml*