materials

Article Stress Cracking of Friction Stir-Welded AA-2024 T3 Alloy

Marina Cabrini 1,2,3 , Sara Bocchi 4 , Gianluca D’Urso 4, Claudio Giardini 4 , Sergio Lorenzi 1,2,3,* , Cristian Testa 1,2,3 and Tommaso Pastore 1,2,3

1 Department of Engineering and Applied Sciences, University of Bergamo, 24044 Dalmine (BG), Italy; [email protected] (M.C.); [email protected] (C.T.); [email protected] (T.P.) 2 National Interuniversity Consortium of Materials Science and Technology (INSTM) Research Unit of Bergamo, 24044 Dalmine (BG), Italy 3 Center for Colloid and Surface Science (CSGI) Research Unit of Bergamo, 24044 Dalmine (BG), Italy 4 Information and Production Engineering, Department of Management, University of Bergamo, 24044 Dalmine (BG), Italy; [email protected] (S.B.); [email protected] (G.D.); [email protected] (C.G.) * Correspondence: [email protected]

 Received: 2 May 2020; Accepted: 3 June 2020; Published: 8 June 2020 

Abstract: The paper is devoted to the study of stress corrosion cracking phenomena in friction stir AA-2024 T3 joints. Constant load (CL) cell and slow strain rate (SSR) tests were carried out in aerated NaCl 35 g/L solution. During the tests, open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) were measured in the different zones of the welding. The results evidenced initial practical nobilty of the nugget lower compared to both heat-affected zone and the base metal. This effect can be mainly ascribed to the aluminum matrix depletion in , which precipitates in form of copper-rich second phases. In this zones, no stress corrosion cracking was noticed, but well-evident stress-enhanced intergranular corrosion occurred. This is due to the uneven distribution of platic deformation during the slow strain rate tests. Higher strain values are localized at the heat affected zone, where softening occurs. On the contrary, stress values at the nugget are not sufficient to favor both the initiation and propagation of stress corrosion cracks. In the range of processing parameter studied in this experimental work, the stress corrosion cracking susceptibility of the friction stir welding (FSW)-ed alloy is then similar to that of the base metal.

Keywords: stress corrosion cracking; slow strain rate test; constant load test; friction stir welding (FSW); AA2024

1. Introduction Friction stir welding (FSW) is a solid-state joining process that is gaining a lot of attention especially for high-strength age-hardening aluminum alloys, in which mechanical properties are strictly dependent upon heat treatments at relatively lower temperatures [1–4]. The joint is produced by the friction generated between a rotating pin put directly in contact with the base metal. The temperature of the material increases significantly in the contact zone without melting the material itself [5]. The increase in temperature increases the material deformability, thus allowing the mixing of the two sides of the joint [6]. The thermo-mechanical action causes the material recrystallization that is localized in the central area of the weld, called nugget characterized by outstanding tensile properties [7,8]. The zones close to the nugget, which has the microstructure altered both by the action of the mechanical straining and the thermal effect, is called thermo-mechanically affected zone (TMAZ); after this zone, moving away from the nugget toward the base metal, there is an area that has not

Materials 2020, 13, 2610; doi:10.3390/ma13112610 www.mdpi.com/journal/materials Materials 2020, 13, 2610 2 of 16 undergone mechanical straining, but only heating and therefore it is called heat-affected zone (HAZ). The microstructure and extension of these zone, nugget, TMAZ, and HAZ, depend on the size and shape of the pin, the thickness of the sheet, and the welding parameters, in particular the force exerted on the pin, its rotation speed, and feed rate [9,10]. Depending on the tool rotation side, “advancing side” and “retreating side” are defined [11]. Lot of efforts have been paid to study the effect of the tool rotation on both the extension of these zones and the effects on mechanical properties in function of the process parameters, i.e., speed and feed rate [12–14]. Among all the aluminum alloys, series 2XXX, 6XXX [15], and 7XXX [16,17] are the most affected by the microstructure and mechanical properties modification induced by FSW, because their resulting microstructure is a combination between the original temper and the thermomechanical alterations induced by FSW process. Microstructural modifications are relevant across the welding and both mechanical and corrosion behavior [18] at the nugget, TMAZ, and the HAZ are strictly dependent upon the process parameters. The combined action of applied stress and corrosive environment can also promote stress corrosion cracking (SCC) in susceptible alloys [19–22] and the effect of the microstructural modification induced by FSW on the intrinsic susceptibility of such alloys is yet far to be completely understood. The higher number of works are on the SCC of alloy 7XXX but, in the knowledge of the authors, only few papers reported the SCC behavior of FSW joints in AA-2024. Wang et al. reported SCC insurgence on FSW AA-2024-T3 in the interface between the nugget and the TMAZ in 3.5 wt.% NaCl solution. The stress corrosion cracks were nucleated by pitting. The SCC susceptibility of the FSWed joints increases with increasing travel speed, owing to the increase of the size of second phase particles. A SCC mechanism controlled by the metal anodic dissolution at the OCP and hydrogen accelerated metal embrittlement in the active zones was suggested [23] In a previous paper of the authors, no stress corrosion cracking was observed on four point bent beam specimens after 1500 h of exposure in NaCl 35 g/L, but the intergranular corrosion attack became penetrating in the presence of loading, giving rise to stress-enhanced intergranular corrosion of the alloy [14]. The aim of this work is to study the stress corrosion cracking susceptibility of FSWed AA-2024 T3 aluminum alloy. Electrochemical tests and free corrosion potential measurements were carried out during SCC test to assess corrosion phenomena occurring during exposure.

2. Materials and Methods

2.1. Preparation of the Welds Aluminum sheets (size 200 mm 80 mm 4 mm thickness) were welded by using a tool with × × smooth plane shoulder (16 mm diameter) and pin having a frustum of cone shape (pin maximum and minimum diameters equal to 6 and 4 mm, height equal to 3.8 mm) (Figure1). The rotational speed (S) of the tool was fixed at 1500 rpm and feed rate (F) was 10 mm/min. The set-up of the welding process was reported in previous works by authors [14,18,22,24]. The mechanical properties of the base materials and the welded joints are shown in Table1. For the FWSed specimens, only the ultimate tensile strength (UTS) is reported, because it was not possible to evaluate the strength (YS) owing to the localization of the plastic straining of the specimens in the TMAZ/HAZ zone. The decrease in the mechanical properties at the thermomechanical and heat affected zones is the result of the modification in the microstructure of the alloy, with solubilization and reprecipitation of the straightening second phases, as reported in previous works by authors [10,14,18,22,24]. A good reproducibility was observed in the tensile strength of the FSWed joints, thus confirming the absence of macroscopic defects alongside the weld. Materials 2020, 13, x FOR PEER REVIEW 3 of 16

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Figure 1. Schematic representation of the friction stir welding (FSW) and the machining of the specimens.Figure 1. Schematic representation of the friction stir welding (FSW) and the machining of the specimens. TableFigure 1. 1.Mechanical Schematic propertiesrepresentation (yield of strength, the friction YS andstir ultimatewelding tensile(FSW) strength,and the machining UTS) of the of base the Tablematerialsspecimens. 1. Mechanical and the welded properties joint. (yield strength, YS and ultimate tensile strength, UTS) of the base materials and the welded joint. Table 1. Mechanical properties (yieldBase strength, Material YS and ultimate tensile strength, FSW Joint UTS) of the base Base Material FSW Joint materials andAlloy the welded joint. Alloy YS YS UTSUTS Max Strain YSYS UTS UTS [MPa] [MPa] Max Strain[%] [%] [MPa] [MPa] [MPa] [MPa]Base Material [MPa]FSW Joint[MPa] AA 2024–T3AA Alloy2024–T3345 345YS 459UTS459 17 17 ‐ YS - UTS315 315 Max Strain [%] [MPa] [MPa] [MPa] [MPa] 2.2. Constant Constant Load Load Tests TestsAA 2024–T3 345 459 17 ‐ 315 Constant loadload (CL)(CL) tests tests were were carried carried out out on on dog dog bone bone shaped shaped specimens specimens with with 8 mm 8 mm width, width, 4 mm 4 2.2. Constant Load Tests mmthickness, thickness, and 82and mm 82 gage mm length gage length with the with weld the positioned weld positioned at the center at the of center the gage of lengththe gage (Figure length2). (FigureThe specimensConstant 2). The specimensload were (CL) inserted testswere inwere inserted a double carried in compartmenta doubleout on dogcompartment bone cell with shaped cell a window withspecimens a window to expose with to 8 aboutexposemm width, 50% about of 4 50%themm central ofthickness, the areacentral (Figureand area 82 3 mma,b).(Figure gage All 3a,b). the length compartments All with the compartmentsthe weld were positioned filled were with filled aeratedat the with center 35 gaerated/ Lof NaCl the 35 gage solution, g/L lengthNaCl at solution,25(Figure◦C. The 2). at solutionThe 25 °C. specimens The was solution constantly were insertedwas recirculated constantly in a double by recirculated means compartment of a by pump. means cell The with of configurationa apump. window The to ofconfiguration expose the cell about was ofchosen50% the of cell in the orderwas central chosen to monitor area in order(Figure one to side 3a,b).monitor the openAll one the circuit side compartments the potential open circuit (OCP) were potential and filled carry with (OCP) out aerated the and electrochemical carry35 g/L out NaCl the electrochemicalimpedancesolution, at spectroscopy 25 °C. impedance The solution (EIS) spectroscopy test was on constantly the (EIS) other test side recirculated on at the the other same by side time.means at theof a same pump. time. The configuration of the cell was chosen in order to monitor one side the open circuit potential (OCP) and carry out the electrochemical impedance spectroscopy (EIS) test on the other side at the same time.

HAZ/TMAZ TMAZ/HAZ

HAZ/TMAZ TMAZ/HAZ nugget

nugget

Figure 2. FWSed specimen used in constant load (CL) e SSR test; the specimens of the slow strain rate Figure 2. FWSed specimen used in constant load (CL) e SSR test; the specimens of the slow strain rate (SSR) test on base materials have the same dimension without weld. (SSR) test on base materials have the same dimension without weld. TheFigure OCP 2. FWSed monitoring specimen was used done in by constant means load of homemade (CL) e SSR test; three the Ag specimens/AgCl references of the slow electrodes strain rate with The OCP monitoring was done by means of homemade three Ag/AgCl references electrodes E = +(SSR)0.200 test V vs. on NHE base materials (normal hydrogenhave the same electrode). dimension The without results weld. were presented after conversion as the with E = +0.200 V vs. NHE (normal hydrogen electrode). The results were presented after conversion saturate calomel electrode (SCE-E = +0.240 V vs. NHE, Amel Instrument, Milano, Italy). The data as theThe saturate OCP calomelmonitoring electrode was done (SCE by‐E =means +0.240 of V homemade vs. NHE, Amel three Instrument, Ag/AgCl references Milano, Italy).electrodes The were registered by a Ivium CompactStat instrument; the sampling frequency was varied during the datawith were E = +0.200 registered V vs. by NHE a Ivium (normal CompactStat hydrogen instrument;electrode). The the samplingresults were frequency presented was after varied conversion during experiment from one measurement per second in the initial instants and during the loading phases of theas theexperiment saturate calomelfrom one electrode measurement (SCE‐E per = +0.240 second V vs.in theNHE, initial Amel instants Instrument, and during Milano, the Italy). loading The the specimen to a 10-h monitoring, with acquisition of one point per minute every 100 or 200 h, during phasesdata were of the registered specimen by toa Iviuma 10‐h CompactStat monitoring, withinstrument; acquisition the sampling of one point frequency per minute was varied every during100 or maintenance of the constant load of the specimens. The EIS tests were performed using an Ivium 200the h,experiment during maintenance from one ofmeasurement the constant perload second of the specimens.in the initial The instants EIS tests and were during performed the loading using phases of the specimen to a 10‐h monitoring, with acquisition of one point per minute every 100 or 200 h, during maintenance of the constant load of the specimens. The EIS tests were performed using

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Materials 2020, 13, x FOR PEER REVIEW 4 of 16 CompactStat instrument. Sinusoidal signal with 0.010 V amplitude in the frequencies range between an Ivium CompactStat2 instrument. Sinusoidal signal with 0.010 V amplitude in the frequencies range 10 kHz and 10− Hz was applied. EIS tests were carried out on the CL speciemens just dipped, after 24, between 10 kHz and 10−2 Hz was applied. EIS tests were carried out on the CL speciemens just dipped, 48 (before and after loading the specimen), 480, 504 (before anf after increase the load on the specimen), after 24, 48 (before and after loading the specimen), 480, 504 (before anf after increase the load on the 552, and 650 h (before and after to un-load the specimen). specimen), 552, and 650 h (before and after to un‐load the specimen). The specimens were loaded by means of a tensile testing machine with a double lever arm The specimens were loaded by means of a tensile testing machine with a double lever arm (Figure3c), according to NACE TM 0177-2005 Method A, the loads were imposed by adding weights (Figure 3c), according to NACE TM 0177‐2005 Method A, the loads were imposed by adding weights to the lever, the lever force had previously been calibrated through a calibrated load cell with 10 N to the lever, the lever force had previously been calibrated through a calibrated load cell with 10 N accuracy. The testing procedure is as following. The specimen was initially positioned in the cell, the accuracy. The testing procedure is as following. The specimen was initially positioned in the cell, the testing solution was inserted, and the first EIS spectrum was registered. The second EIS spectrum testing solution was inserted, and the first EIS spectrum was registered. The second EIS spectrum was measured after 24 h, the third after 48 h; after the third EIS measurement, the specimen was was measured after 24 h, the third after 48 h; after the third EIS measurement, the specimen was loaded at 50–55% UTS of the FSWed joints, and the EIS measure immediately repeated. This load loaded at 50–55% UTS of the FSWed joints, and the EIS measure immediately repeated. This load was was chosen because it is practically in the correspondence of the deviation from the linearity of the chosen because it is practically in the correspondence of the deviation from the linearity of the stress– stress–strain curve of the weld; it could be assumed that, under this stress, the less resistance zone of strain curve of the weld; it could be assumed that, under this stress, the less resistance zone of the the gauge length becomes plastically deformed, while the other parts of the specimen remain in elastic gauge length becomes plastically deformed, while the other parts of the specimen remain in elastic straining. The monitoring of OCP and EIS spectrum continued for 504 h, after these the load was straining. The monitoring of OCP and EIS spectrum continued for 504 h, after these the load was further increased in order to achieve a stress value of about the 80% of the ultimate tensile strength of further increased in order to achieve a stress value of about the 80% of the ultimate tensile strength the weld, thus achieving uniform plastic deformation conditions along the gage length of the specimen. of the weld, thus achieving uniform plastic deformation conditions along the gage length of the The load was then maintained for 650 h. specimen. The load was then maintained for 650 h.

(a)

Counter Reference Load specimen

inlet solution inlet solution

outlet outlet solution solution References

(b) EIS Tests OCP monitoring (c) Load

FigureFigure 3. 3. (a()a )Image Image of of the the cell cell used used for for the the CL CL tests. tests. (b (b) Schematic) Schematic illustration illustration of of the the test test apparatus apparatus and and (c(c) )image image of of one one specimen specimen under under the the CL CL testing testing machine. machine.

AtAt the the end end of ofthe the exposure exposure period, period, the specimens the specimens were wereunloaded, unloaded, cleaned, cleaned, and observed and observed under theunder scanning the scanning electron electron microscope microscope (SEM, (SEM, Zeiss Zeiss EVO EVO 40, 40,Oberkochen, Oberkochen, Germany) Germany) with with energy energy dispersivedispersive spectroscopy spectroscopy (EDS) (EDS) Oxford Oxford X X-Act‐Act in in order order to to analyze analyze the the corrosion corrosion products products morphology morphology andand composition. composition. Metallographic Metallographic cross cross sections sections along along the the longitudinal longitudinal direction direction were were also also analyzed. analyzed. TheThe gauge gauge length length was was cut cut from from the the specimen, specimen, then then longitudinally longitudinally sectioned sectioned using using a a metallographic metallographic cuttingcutting machine machine (Metkon (Metkon Servocut, Servocut, Bursa, Bursa, Turkey) Turkey) in in the the central central part. part. The The section section was was embedded embedded in in resinresin and and grinded grinded with with emery emery paper paper and and polished polished with with colloid colloid alumina alumina until until 0.1 0.1 μµmm of of roughness. roughness. TheThe metallographic metallographic section section was was observed observed by by a a Nikon Nikon optical optical microscope microscope and and the the SEM SEM without without metallographicmetallographic attack attack and and after after attack attack with with Keller Keller reagent. reagent.

2.3.2.3. Slow Slow Strain Strain Rate Rate Tests Tests TheThe slow slow strain strain rate rate (SSR) (SSR) tests tests were were carried carried out out by by using using specimens specimens of of the the same same dimensions dimensions as as forfor CL CL tests. tests. The The SSR SSR tests tests were were also also performed performed on on AA AA-2024‐2024 T3 T3 base base material material for for comparison. comparison. The tests were carried out by means of a homemade SSR testing machine with four independent loading stations. The load is applied through an electric motor and reduction gears that can impose displacement rates between 5  10–7 and 5  10–3 mm/s. The calibration of the cells is performed

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The tests were carried out by means of a homemade SSR testing machine with four independent loading stations. The load is applied through an electric motor and reduction gears that can impose displacementMaterials 2020, 13, rates x FOR betweenPEER REVIEW 5 10 7 and 5 10 3 mm/s. The calibration of the cells is performed5 of 16 × − × − independently. Acquisition system (spider 8 HBM Italy, Milano, Italy) read the load and a function of independently. Acquisition system (spider 8 HBM Italy, Milano, Italy) read the load and a function time; the displacement of the tensile grip is evaluated multiplying the displacement rate for the time. of time; the displacement of the tensile grip is evaluated multiplying the displacement rate for the A displacement rate of 8.2 10 4 mm/s, corresponding to 10 6 s 1 average strain rate along the gage time. A displacement rate ×of 8.2−  10−4 mm/s, corresponding− to −10−6 s−1 average strain rate along the length, was chosen. SSR tests were carried out according to the ASTM 129-00 (2013), exception for the gage length, was chosen. SSR tests were carried out according to the ASTM 129‐00 (2013), exception specimen’s geometry that was changed to test all the weld. The cell was like those used for the CL for the specimen’s geometry that was changed to test all the weld. The cell was like those used for tests, with the same configuration of the electrodes, but smaller than that for CL tests, to be settled the CL tests, with the same configuration of the electrodes, but smaller than that for CL tests, to be under the SSR machine, filled with an aerated solution of 35 g/L NaCl. No EIS spectra were obtained settled under the SSR machine, filled with an aerated solution of 35 g/L NaCl. No EIS spectra were during this test because the OCP did not remain constant. At the end of the tests, the specimens were obtained during this test because the OCP did not remain constant. At the end of the tests, the washed in distilled water and rinsed in acetone to permit the observation of the fracture surface under specimens were washed in distilled water and rinsed in acetone to permit the observation of the the SEM. Then the specimens were longitudinally sectioned for the metallographic observation with fracture surface under the SEM. Then the specimens were longitudinally sectioned for the the same procedure illustrated for the CL test. metallographic observation with the same procedure illustrated for the CL test. 2.4. Electrochemical Characterizations of the Different Zones of the Weld 2.4. Electrochemical Characterizations of the Different Zones of the Weld For comparison, EIS and OCP measurements were performed on the separated different zones For comparison, EIS and OCP measurements were performed on the separated different zones of the FSW joints: the nugget, the HAZ/TMAZ (it was not possible to cut the specimens separating of the FSW joints: the nugget, the HAZ/TMAZ (it was not possible to cut the specimens separating these zones), and the base material in unloaded condition. The weld was cut as illustrated in Figure4; these zones), and the base material in unloaded condition. The weld was cut as illustrated in Figure the isolate specimens with the nugget, the TMAZ/HAZ zone, and the base material were connected 4; the isolate specimens with the nugget, the TMAZ/HAZ zone, and the base material were connected with an external wire and then cold mounted with epoxy resin. The exposed surface of the specimen with an external wire and then cold mounted with epoxy resin. The exposed surface of the specimen was 20 20 mm2. It was grounded with emery paper and then polidhed with colloidal alumina up to was 20 ×× 20 mm2 . It was grounded with emery paper and then polidhed with colloidal alumina up 0.3 µm. The specimens were dipped in separate glass cells in aerated 35 g/L NaCl (Carlo Erba, RPA to 0.3 μm. The specimens were dipped in separate glass cells in aerated 35 g/L NaCl (Carlo Erba, RPA grade, Cornaredo, Italy) solution. Standard saturated calomel electrode (SCE E = +0.240 V vs. NHE, grade, Cornaredo, Italy) solution. Standard saturated calomel electrode (SCE E = +0.240 V vs. NHE, Amel Instrument, Milano, Italy) was positioned at the surface of the specimens by using a Luggin Amel Instrument, Milano, Italy) was positioned at the surface of the specimens by using a Luggin capillary to compensate the ohmic drop in the electrolyte. Electrochemical impedance spectroscopy capillary to compensate the ohmic drop in the electrolyte. Electrochemical impedance spectroscopy (EIS) tests were performed with the same procedure used for on the CL test. (EIS) tests were performed with the same procedure used for on the CL test.

Figure 4. Scheme of the sampling of the different different areas of the welding and of the base metal for the monitoring of open circuit potential (OCP)(OCP) andand electrochemicalelectrochemical impedanceimpedance spectroscopyspectroscopy (EIS).(EIS). 3. Results and Discussion 3. Results and Discussion Figure5 shows the OCP measurements in aerated NaCl solution at the nugget, the HAZ /TMAZ, Figure 5 shows the OCP measurements in aerated NaCl solution at the nugget, the HAZ/TMAZ, and the base material with regard to exposure time. and the base material with regard to exposure time. Similar values of the OCP were measured on the three different zones just after dipping. The Similar values of the OCP were measured on the three different zones just after dipping. The nugget showed constant OCP values during all the exposure periods. Different behavior was observed nugget showed constant OCP values during all the exposure periods. Different behavior was for the base material and the heat-affected zones (HAZ/TMAZ), which showed an initial increase of observed for the base material and the heat‐affected zones (HAZ/TMAZ), which showed an initial OPC followed by a decrease of about 150 mV. increase of OPC followed by a decrease of about 150 mV. Figure2 shows the EIS spectra for the specimens obtained from the di fferent zones of the weld Figure 2 shows the EIS spectra for the specimens obtained from the different zones of the weld with regard to exposure time. The spectra collected at the nugget are quite different compared to the with regard to exposure time. The spectra collected at the nugget are quite different compared to the base material and TMAZ just after dipping. The Nyquist plot shows two overlapped capacitive loops, base material and TMAZ just after dipping. The Nyquist plot shows two overlapped capacitive loops, and an inductive one at very low frequencies, whereas the base material and the TMAZ presented and an inductive one at very low frequencies, whereas the base material and the TMAZ presented a capacitive loop and diffusive behavior at very low frequencies (Figure 6a). The Bode diagram (Figure 6b) shows the highest value of the impedance modulus of the base materials, indicating a stable passive condition. Conversely, the values of the impedance modulus of the nugget and the TMAZ decrease at the low frequency, thus indicating activation of the metal surface during low frequency

Materials 2020, 13, 2610 6 of 16 a capacitive loop and diffusive behavior at very low frequencies (Figure6a). The Bode diagram (Figure6b) shows the highest value of the impedance modulus of the base materials, indicating a stable passive condition. Conversely, the values of the impedance modulus of the nugget and the TMAZ Materials 2020, 13, x FOR PEER REVIEW 6 of 16 decrease at the low frequency, thus indicating activation of the metal surface during low frequency measurements. All the specimens showed two phase constants, one at intermediate frequency and the measurements. All the specimens showed two phase constants, one at intermediate frequency and other at very low frequencies. the other at very low frequencies.

Figure 5.5. OCPOCP in in function function of of exposure exposure time time of the of dithefferent different zones zones of AA-2024 of AA T3‐2024 FSWed T3 FSWed joint in aeratedjoint in

NaClaerated 0.6 NaCl M at 0.6 23 ◦MC. at 23 °C.

After only 2424 hh ofof immersion,immersion, thethe shapeshape ofof EISEIS spectraspectra significantlysignificantly changedchanged (Figure(Figure6 6c,d).c,d). The The amplitude ofof the the EIS EIS loops loops in in Nyquist Nyquist plot plot decreased decreased significantly significantly for allfor the all dithefferent different zones zones of the of weld. the Diweld.ffusive Diffusive behavior behavior was still was observed still observed for the basefor the metal base and metal the TMAZ, and the while TMAZ, a second while capacitive a second loopcapacitive can be loop distinguished can be distinguished for the nugget—but for the flattened—and nugget—but theflattened—and inductive loop the disappeared. inductive loop All thedisappeared. specimens All showed the specimens only one showed phase constant, only one but phase the frequencyconstant, but range the is frequency lower compared range is tolower just dippedcompared conditions. to just dipped An increase conditions. in phase An increase can be noticed in phase at verycan be low noticed frequency at very for low the basefrequency material for andthe base the TMAZ, material thus and indicating the TMAZ, that thus a second indicating phase that constant a second could phase be present constant at very could low be frequencies. present at 4 2 Thevery impedance low frequencies. modulus The achieved impedance the valuesmodulus of 10achievedΩcm thefor allvalues the zonesof 104 Ω ofcm the2 weld,for all meaning the zones that of activationthe weld, meaning occurred that also activation on base metal occurred and the also TMAZ. on base metal and the TMAZ. After 72 hh ofof immersion,immersion, thethe EISEIS spectraspectra becamebecame practicallypractically overlapped,overlapped, with only a slightslight didifferencefference betweenbetween thethe impedanceimpedance modulus of the nugget with respect to the basebase materialmaterial and thethe TMAZTMAZ (Figure(Figure6 e,f).6e,f). Well Well evident evident di diffusiveffusive behavior behavior can can be be noticed noticed at at very very low low frequencies frequencies because because of theof the presence presence of corrosionof corrosion products products deposit. deposit. These results seem to evidenceevidence an initial higher activity of the nugget with respect to the otherother zones, but thesethese didifferencesfferences becomebecome negligiblenegligible atat longerlonger exposures.exposures. After the tests, all thethe specimensspecimens showedshowed generalgeneral corrosioncorrosion morphologies.morphologies. The behaviorbehavior significantly significantly modifies modifies as loadingas loading is applied. is applied. The corrosion The corrosion potential potential showed showed sudden decreasesudden decrease at the application at the application of monoaxial of monoaxial constant load, constant as reported load, as in reported Figure7. in The Figure e ffect 7. is The attributable effect is toattributable the breakdown to the of breakdown the thick, porous, of the thick, and not porous, adherent and scalenot adherent of aluminum scale oxideof aluminum formed onoxide specimens formed afteron specimens prolonged after exposures, prolonged which exposures, exposes the aluminumwhich exposes matrix the directly aluminum to the aggressivematrix directly environment. to the Theaggressive potential environment. approached theThe steady potential state value—i.e.,approached the the value steady in unloading state value—i.e., conditions—because the value ofin self-healingunloading conditions—because effect of corrosion products of self of‐healing aluminum. effect The of decrease corrosion in theproducts corrosion of potentialaluminum. is more The pronounceddecrease in the when corrosion the specimens potential are is more strained pronounced in the plastic when field. the specimens are strained in the plastic field. Specimens failure was not observed under constant loading for 650‐h exposure. The analysis of the specimens after the tests showed an intergranular attack in the nugget near the zone of beginning of the TMAZ (Figures 8 and 9).

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Specimens failure was not observed under constant loading for 650-h exposure. The analysis of the specimens after the tests showed an intergranular attack in the nugget near the zone of beginning of the TMAZ (Figures8 and9). The EIS spectra collected on CL specimens showed certain different behavior just after dipping and after 24 h (Figure 10a,b). The spectra mainly approach that of the nugget (compare Figure 10a,b with Figure6a,b). It can then be stated that EIS tests carried out during CL tests are mainly a ffected by the most active zone of the joint—i.e., the nugget. The specimen just after dipping presents two overlapped capacitive loops and an inductive one (Figure 10a)—typical of non-stationary OCP values during early exposures EIS tests (Figure 10b). After 24 h, the Nyquist diagram presented a well-evident diffusive behavior (compare Figure 10c,d to Figure6c,d). The increase in the load does not modify the EIS spectra, that are overlapped to the condition without applied load after 24-h exposure (Figure 10c–f), thus confirming that the corrosion process at longer exposures is only limited by the presence of the pseudo-passive film by corrosion products that are able to heal the cracks promoted by loading. Materials 2020, 13, x FOR PEER REVIEW 7 of 16

(a) (b)

(c) (d)

(e) (f)

FigureFigure 6. EIS6. EIS spectra spectra (a (),a), (c (),c), and and ( e(e)) Nyquist Nyquist andand ((b), (d), and ( (ee)) Bode Bode representation, representation, of of the the separate separate diffdifferenterent zones zones of of the the weld; weld; ( a()a) and and ( b(b)) just just dipped;dipped; (c) and ( (dd)) after after 24 24 h; h; (e ()e )and and (f ()f after) after 72 72 h hof of

immersionimmersion in in aerated aerated NaCl NaCl 0.6 0.6 M M solution. solution.

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FigureFigure 7. OCP7. OCP measurements measurements during during 720 720 h h exposure exposure ofof CL specimen of of the the FSWed FSWed AA AA-2024‐2024 T3 T3 alloy alloy in in aerated NaCl 0.6 M at 23 °C. aeratedFigure NaCl 7. OCP 0.6 measurements M at 23 ◦C. during 720 h exposure of CL specimen of the FSWed AA-2024 T3 alloy inFigure aerated 7. NaClOCP measurements0.6 M at 23 °C. during 720 h exposure of CL specimen of the FSWed AA‐2024 T3 alloy in aerated NaCl 0.6 M at 23 °C.

HAZ TMAZ nugget TMAZ HAZ

HAZ TMAZ nugget TMAZ HAZ HAZ TMAZ nugget TMAZ HAZ

Figure 8. Corrosion morphology of FSWed AA‐2024 T3 alloy after CL test in aerated 35 g/L NaCl.

Figure 8. Corrosion morphology of FSWed AA-2024 T3 alloy after CL test in aerated 35 g/L NaCl. FigureFigure 8. Corrosion 8. Corrosion morphology morphology of of FSWed FSWed AA-2024 AA‐2024 T3 alloy after after CL CL test test in in aerated aerated 35 35 g/L g /NaCl.L NaCl.

(a) (b)

Figure 9. Metallographic section of Figure 6: (a) Macro image of the nugget with several localized attacks; (b) close‐up of( athe(a) ) red zone in (a). (b)

FigureFigureFigure 9. 9.Metallographic 9. Metallographic Metallographic sectionsection section ofof of FigureFigure Figure6 6:(6:: ((aaa))) Macro MacroMacro imageimage image ofof of thethe the nugget nugget with with several several localizedlocalized localized attacks; (b) close-up of the red zone in (a). attacks;attacks; (b) ( close-upb) close‐up of of the the red red zone zone in in (a ().a).

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The EIS spectra collected on CL specimens showed certain different behavior just after dipping and after 24 h (Figure 10a,b). The spectra mainly approach that of the nugget (compare Figure 10a,b with Figure 6a,b). It can then be stated that EIS tests carried out during CL tests are mainly affected by the most active zone of the joint—i.e., the nugget. The specimen just after dipping presents two overlapped capacitive loops and an inductive one (Figure 10a)—typical of non‐stationary OCP values during early exposures EIS tests (Figure 10b). After 24 h, the Nyquist diagram presented a well‐ evident diffusive behavior (compare Figure 10c,d to Figure 6c,d). The increase in the load does not modify the EIS spectra, that are overlapped to the condition without applied load after 24‐h exposure (Figure 10c–f), thus confirming that the corrosion process at longer exposures is only limited by the Materialspresence2020 of, 13the, 2610 pseudo‐passive film by corrosion products that are able to heal the cracks promoted9 of 16 by loading.

(a) (b)

(c) (d)

(e) (f)

FigureFigure 10.10. EISEIS spectraspectra ((aa),), ((cc),), andand ((ee)) NyquistNyquist representation,representation, ((bb),), ((dd),), andand ((ee)) BodeBode representationrepresentation ofof thethe constantconstant loadload specimenspecimen withoutwithout loadload justjust dippeddipped andand afterafter 2424 hh ((aa,,bb),), afterafter 4848 hh withoutwithout load,load, andand afterafter 175175 MPa, MPa, and and after after 480 480 h h loaded loaded at at 175 175 MPa MPa (a, d(a), andd) and after after 480 480 h at h 175 at MPa175 MPa and 650and h 650 at 268 h at MPa 268 (MPae,f) in (e aerated,f) in aerated NaCl NaCl 0.6 M 0.6 solution. M solution.

These results are in agreement with those obtained in previous works [14,22,24]. It was observed that the OCP did not change in four point bent beam (4PBB) specimens in un-loaded or loaded conditions at the 80% of the ultimate tensile strength of the weld (Figure 11). The specimens were loaded before the immersion. On the contrary, differences between the EIS spectra of un-loaded and loaded specimens were observed only at very early exposures. This was mainly attributed to the presence of corrosion products scale formed on the surface of the specimens, that hindered the EIS signal response. On 4PBB specimens, the corrosion process was mainly localized at the nugget, with wide and shallow deep morphology on the un-loaded specimens and stress-enhanced intergranular morphology on loaded specimens [22]. The effect of the applied load on intergranular corrosion initiation and propagation in alloy AA-2424 T3 was first highlighted by Zhang and Frankel [25,26]. Materials 2020, 13, x FOR PEER REVIEW 10 of 16

These results are in agreement with those obtained in previous works [14,22,24]. It was observed that the OCP did not change in four point bent beam (4PBB) specimens in un‐loaded or loaded conditions at the 80% of the ultimate tensile strength of the weld (Figure 11). The specimens were loaded before the immersion. On the contrary, differences between the EIS spectra of un‐loaded and loaded specimens were observed only at very early exposures. This was mainly attributed to the presence of corrosion products scale formed on the surface of the specimens, that hindered the EIS signal response. On 4PBB specimens, the corrosion process was mainly localized at the nugget, with wide and shallow deep morphology on the un‐loaded specimens and stress‐enhanced intergranular Materials 2020, 13, 2610 10 of 16 morphology on loaded specimens [22]. The effect of the applied load on intergranular corrosion initiation and propagation in alloy AA‐2424 T3 was first highlighted by Zhang and Frankel [25,26]. However,However, the the entire entire surface surface of theof specimensthe specimens is covered is covered with a with continuous a continuous but porous but layer porous of corrosion layer of productscorrosion as products the exposure as the time exposure increases. time Thisincreases. is well This evident is well in theevident EIS spectra in the EIS that spectra are practically that are overlappedpractically asoverlapped they are mainly as they governed are mainly by di ffgovernedusive phenomena by diffusive occurring phenomena at the corrosion occurring products at the scalecorrosion [22]. products scale [22].

FigureFigure 11. 11.E Effectffect of of the the time time of of immersion immersion on on the the OCP OCP of un-loadedof un‐loaded and and loaded loaded 4PBB 4PBB specimens specimens of the of FWSedthe FWSed alloy alloy AA 2024-T3AA 2024 in‐T3 aerated in aerated NaCl NaCl 0.6 M 0.6 at M 23 at◦C 23 (data °C (data obtained obtained by [22 by]). [22]).

TheThe OCP OCP ofof thethe FSWedFSWed AA-2024 AA‐2024 T3 T3 alloy alloy under under slow slow strain strain rate rate conditions,conditions, (Figure(Figure 12 12)) modifiesmodifies inin functionfunction ofof thethe applied applied monotonic monotonic tensiletensile stress.stress. AA slightslight decreasedecrease inin thethe valuesvalues cancan bebe noticednoticed asas thethe strainstrain valuesvalues laylay inin thethe elasticelastic field.field. AA sharpsharp decreasedecrease waswas noticed—i.e.,noticed—i.e., 100–250 mV—at thethe YS. ExceedingExceeding thethe YS,YS, thethe OCPOCP increasesincreases upup toto thethe valuesvalues recordedrecorded inin thethe elasticelastic field,field, untiluntil thethe rupture.rupture. TheThe specimen specimen breaking breaking did did not not occur occur at at the the nugget nugget that that is characterized is characterized by higherby higher tensile tensile strength, strength, but inbut the in heat the heat affected affected zone. zone. Because Because of this of this consideration, consideration, it can it can be derived be derived that that the the attack attack observed observed at theat the nugget nugget (Figure (Figure 13 )13) did did not not reach reach su sufficientfficient depth depth to to promote promote the the specimen specimen rupture, rupture, as as the the plastic plastic deformationdeformation mainlymainly taketake placeplace inin thethe softenedsoftened zones,zones, typicaltypical ofof thethe HAZHAZ/TMAZ./TMAZ. Consequently,Consequently, thethe eeffectffect of of FSW FSW on on the the alloy alloy microstructure microstructure and and on on the the mechanical mechanical properties properties significantly significantly affect affect also also the Materialsstressthe 2020 stress corrosion, 13, corrosionx FOR cracking PEER cracking REVIEW susceptibility susceptibility of the of alloy. the alloy. 11 of 16 SSR tests performed on commercial AA‐2024 T3 alloy showed a completely different behavior compared to FSWed. The OCP values are stable at more noble values compared to FSWed, without the occurrence of stress corrosion cracks (Figure 14). For both the weld and the commercial AA‐2024 T3 alloy, the time‐to‐failure is longer in NaCl solution compared to air. All the tests were twice repeated with similar results. It could be hypothesized that active corrosion in NaCl solution is stimulated at the dislocations, thus generating vacancies able to accelerate the intrinsic mobility and thus the plastic deformation, as already evidenced by other authors that named this phenomenon as anodic attenuation of strain hardening [27,28].

FigureFigure 12. Stress 12. Stress vs. strain vs. strain and and OCP OCP FSWed FSWed AA AA 2024 2024-T3‐T3 alloy duringduring the the SSR SSR test test in in aerated aerated NaCl NaCl 0.6 0.6 M solutionM solution at 23 at °C. 23 ◦C.

Figure 13. (a) Image of a FSWed specimen after the SSR test in NaCl 35 g/L; (b) close‐up of the lateral surface in the correspondence of the fracture surface; (c) close‐up of the dimples in the fully ductile fracture surface; (d) and (e) SCC microcracks in the nugget of the specimen of the FWSed AA 2024‐ T3.

Figure 14. Stress vs. strain and OCP of the base and FSWed AA‐2024 T3 alloy during the SSR test in aerated NaCl 0.6 M solution at 23 °C.

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Materials 2020, 13, x FOR PEER REVIEW 11 of 16

FigureMaterials Figure12.2020 Stress ,12.13, Stress 2610 vs. strain vs. strain and and OCP OCP FSWed FSWed AA AA 2024 2024‐T3 alloyalloy during during the the SSR SSR test testin aerated in aerated NaCl NaCl0.611 of 16 0.6 M solutionM solution at 23 °C.at 23 °C.

Figure 13. (a) Image of a FSWed specimen after the SSR test in NaCl 35 gg/L;/L; (b)) close-upclose‐up of the lateral surface in the correspondencecorrespondence of the fracture surface; (c) close-upclose‐up of thethe dimples inin thethe fullyfully ductileductile fracture surface;surface; ((dd)) and and ( e(e)) SCC SCC microcracks microcracks in in the the nugget nugget of of the the specimen specimen of of the the FWSed FWSed AA AA 2024-T3. 2024‐ Figure T3.13. (a) Image of a FSWed specimen after the SSR test in NaCl 35 g/L; (b) close‐up of the lateral surfaceSSR in the tests correspondence performed on commercialof the fracture AA-2024 surface; T3 ( alloyc) close showed‐up of a the completely dimples diinff theerent fully behavior ductile fracturecompared surface; to FSWed. (d) and The (e OCP) SCC values microcracks are stable in at the more nugget noble of values the specimen compared of to the FSWed, FWSed without AA 2024 the ‐ occurrence of stress corrosion cracks (Figure 14). T3.

Figure 14. Stress vs. strain and OCP of the base and FSWed AA‐2024 T3 alloy during the SSR test in aerated NaCl 0.6 M solution at 23 °C.

Figure Figure14. Stress 14. Stress vs. strain vs. strain and and OCP OCP of ofthe the base base and and FSWed AA-2024 AA‐2024 T3 T3 alloy alloy during during the SSR the test SSR in test in aeratedaerated NaCl NaCl0.6 M 0.6 solution M solution at 23 at 23°C.◦ C. For both the weld and the commercial AA-2024 T3 alloy, the time-to-failure is longer in NaCl solution compared to air. All the tests were twice repeated with similar results. It could be hypothesized that active corrosion in NaCl solution is stimulated at the dislocations, thus generating vacancies able to accelerate the intrinsic mobility and thus the plastic deformation, as already evidenced by other authors that named this phenomenon as anodic attenuation of strain hardening [27,28]. FSW modify the microstructure of the alloy and its electrochemical and stress corrosion cracking behavior, consequently. In the base material, both macro-precipitates and nanoprecipitates are present Materials 2020, 13, x FOR PEER REVIEW 12 of 16

Materials 2020, 13, 2610 12 of 16 FSW modify the microstructure of the alloy and its electrochemical and stress corrosion cracking behavior, consequently. In the base material, both macro‐precipitates and nanoprecipitates are present(Figure (Figure 15)[9,11 15)]. [9,11]. The last The ones last are ones produced are produced because because of solution of solution annealing annealing and and natural natural aging. aging. The Themacro-precipitates macro‐precipitates are mainlyare mainly produced produced by primary by primary solidification solidification processes processes and are notand solubilized are not solubilizedduring heat during treatment heat [ 29treatment]. They are [29]. considered They are to considered be responsible to be of responsible the short-term of electrochemicalthe short‐term electrochemicalbehavior of the behavior alloy as it of is the well-known alloy as it that is well the‐ interfaceknown that between the interface these particles between and these the particles matrix is andparticularly the matrix active is particularly because of active its diff becauseerent practical of its different nobility practical [30]. nobility [30].

FigureFigure 15. 15. MicrostructureMicrostructure of of the the AA AA 2024 2024-T3‐T3 alloy alloy (Keller (Keller attack). attack).

The main second phases of AA-2024 T3 alloy are S phases (Al2CuMg) and Al-Cu-Mn-Fe [30,31]. The main second phases of AA‐2024 T3 alloy are S phases (Al2CuMg) and Al‐Cu‐Mn‐Fe [30,31]. Generally, the formers are anodic with respect to the aluminum matrix because of the presence of Generally, the formers are anodic with respect to the aluminum matrix because of the presence of , while the latter, consisting of more noble elements such as copper, , and , magnesium, while the latter, consisting of more noble elements such as copper, iron, and manganese, are cathodic. S phase is particularly active and higher amount of this phase is generally present. It are cathodic. S phase is particularly active and higher amount of this phase is generally present. It quickly reacts with the solution just after dipping, thus leading to the dissolution of magnesium. As a quickly reacts with the solution just after dipping, thus leading to the dissolution of magnesium. As result, the alloy surface becomes richer in copper and the corrosion potential tends to increase. At a result, the alloy surface becomes richer in copper and the corrosion potential tends to increase. At longer exposures, corrosion initiation occurs in some areas, then propagate in the aluminum matrix. longer exposures, corrosion initiation occurs in some areas, then propagate in the aluminum matrix. Thick layer of insulating corrosion products then forms on the metal surface, which hinder the corrosion Thick layer of insulating corrosion products then forms on the metal surface, which hinder the process as a result of the settlement of pseudo-passivation conditions. EIS spectra well evidence corrosion process as a result of the settlement of pseudo‐passivation conditions. EIS spectra well the formation of the corrosion products layer as a diffusive behavior at very low frequencies that is evidence the formation of the corrosion products layer as a diffusive behavior at very low frequencies compatible with the hypothesized mechanism. The rate-determining step of the corrosion process is that is compatible with the hypothesized mechanism. The rate‐determining step of the corrosion then mainly related to the diffusion processes inside the porous non-conductive scale. The phase peaks process is then mainly related to the diffusion processes inside the porous non‐conductive scale. The at low frequencies can be related to the dissolution of the intermetallics at metal/particle interface [32]. phase peaks at low frequencies can be related to the dissolution of the intermetallics at metal/particle During the welding process, recrystallization induced by the thermomechanical action occurs interface [32]. mainly at the nugget. In these zones, dissolution of nanometric S phase precipitates occurs, which can During the welding process, recrystallization induced by the thermomechanical action occurs re-precipitate in form of micrometric particles, clearly detectable at the scanning electron microscope mainly at the nugget. In these zones, dissolution of nanometric S phase precipitates occurs, which (Figure 16). These precipitates are mainly located at the recrystallized grain boundaries. The presence can re‐precipitate in form of micrometric particles, clearly detectable at the scanning electron of copper in the supersaturated solid solution in the aluminum matrix gives rise to initially nobler microscope (Figure 16). These precipitates are mainly located at the recrystallized grain boundaries. OCP values compared to the nugget and HAZ. The enriching in copper because of the preferential The presence of copper in the supersaturated solid solution in the aluminum matrix gives rise to dissolution of magnesium in S-phase, is responsible for the initial increasing of the OCP, but the rapid initially nobler OCP values compared to the nugget and HAZ. The enriching in copper because of activation of corrosion phenomena at precipitate-free zones causes the decrease in the OCP after a few the preferential dissolution of magnesium in S‐phase, is responsible for the initial increasing of the hours of immersion. OCP, but the rapid activation of corrosion phenomena at precipitate‐free zones causes the decrease The anodic dissolution, catalyzed also by the presence of precipitates along the grain boundaries, in the OCP after a few hours of immersion. is considered to be the main SCC mechanism in high strength 2xxx (Al–Cu–Mg) series alloys [33]. The galvanic interaction between the particles and the surrounding matrix causes the preferential dissolution of the anodic particles or the dissolution of the adjacent matrix surrounding nobler particles. Evolution of hydrogen always occurs in actively growing localized corrosion sites and cracks. The absorption of hydrogen into metals leads to the possibility of crack propagation by hydrogen embrittlement (HE) [33–44]

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Figure 16. SEM imageimage ofof the the nugget nugget without without metallographic metallographic attack: attack: the the bright bright zones zones are are the macrothe macro and andmicrometric micrometric precipitates precipitates of S-phase. of S‐phase.

TheIn this anodic way, dissolution, FSW modifies catalyzed the microstructure also by the presence of AA 2024-T3 of precipitates alloy, thus along making the grain the nugget boundaries, more issusceptible considered to to SCC be thanthe main the base SCC material. mechanism However, in high the strength HAZ and 2xxx TMAZ (Al–Cu–Mg) are characterized series alloys by lower [33]. Thetensile galvanic strength interaction compared between to both the the particles nugget andand thethe basesurrounding metal. The matrix application causes the of loadpreferential causes dissolutionthe occurrence of the of microcracksanodic particles at the or nugget the dissolution which do notof the propagate adjacent as matrix the fracture surrounding is localized nobler in particles.zones with Evolution lower tensile of hydrogen strength, always i.e., plastic occurs deformation in actively is significantlygrowing localized shifted corrosion in the HAZ sites/TMAZ. and cracks.Therefore, The the absorption strain at the of nugget hydrogen is limited into andmetals pseudo-passive leads to the filmpossibility can be restored of crack in propagation such conditions, by hydrogenas demonstrated embrittlement by the increases (HE) [33–44] in the OCP. InSimilar this way, effect FSW was modifies observed the on microstructure the same alloy byof AA Ferri 2024 et al.‐T3 on alloy, 4PBB thus loaded making in elastic the nugget field during more susceptiblepotentiodynamic to SCC tests than [45 the]. base This wasmaterial. attributed However, to dissolution the HAZ mechanismand TMAZ are assisted characterized by the mechanical by lower tensilestress, accordingstrength compared to the chemomechanical to both the nugget theory and of the Gutman base metal. [46]. TheThe weakening application of of the load eff causesect as load the occurrencelevels approach of microcracks the YS was at ascribed the nugget to a which barrier do action not propagate of corrosion as the products fracture film is localized hindering in the zones exit withof dislocations lower tensile from strength, the surface i.e., and plastic thus deformation inhibiting the is creation significantly of fresh shifted surfaces in [the45, 47HAZ/TMAZ.]. The stress Therefore,enhanced intergranularthe strain at corrosionthe nugget at nuggetis limited is inhibited and pseudo as the‐passive specimens film arecan plastically be restored strained in such in conditions,TMAM/HAZ as zones.demonstrated In this way, by the the increases SCC phenomena in the OCP. cannot be observed, and the specimen fails in a ductileSimilar way ineffect the was TMAZ observed/HAZ, similarlyon the same to thealloy test by in Ferri air. et al. on 4PBB loaded in elastic field during potentiodynamicIn other words, tests the [45]. decreasing This was of attributed tensile properties to dissolution in TMAZ mechanism/HAZ caused assisted a deformation by the mechanical in these stress,zones, according counteracting to the the chemomechanical hypothetically higher theory stress of Gutman enhanced [46]. intergranular The weakening corrosion of the susceptibilityeffect as load levelsof the approach nugget. the YS was ascribed to a barrier action of corrosion products film hindering the exit of dislocationsA similar SCCfrom behavior the surface for and joint thus and baseinhibiting metal the of FSWedcreation AA2219-T87 of fresh surfaces through [45,47]. slow The strain stress rate enhancedtests (SSR) intergranular is reported by corrosion Paglia andat nugget Buchheit is inhibited [48]. On as the the contrary, specimens intergranular are plastically fracture strained in the in TMAM/HAZinterface between zones. the In nuggets this way, and the TMAZ SCC ofphenomena welded joints cannot tested be in observed, 0.6 M NaCl and solution the specimen is reported fails byin aWang ductile et al. way [23 in]. Thethe TMAZ/HAZ, last authors reported similarly that to the teststest in in air. air showed ductile fracture in the same zone. ThisIn di ffothererent words, behavior the decreasing could be attributed of tensile toproperties the different in TMAZ/HAZ welding parameters. caused a deformation The same in authors these zones,reported counteracting that the stress the corrosion hypothetically cracking higher (SCC) stress susceptibility enhanced intergranular of the FSW 2024-T4 corrosion aluminum susceptibility alloy ofjoint the increased nugget. with feed rate due to the increase of the second phases size. A similar SCC behavior for joint and base metal of FSWed AA2219‐T87 through slow strain rate tests4. Conclusions (SSR) is reported by Paglia and Buchheit [48]. On the contrary, intergranular fracture in the interfaceThe between paper analyzed the nuggets stress and corrosion TMAZ crackingof welded phenomena joints tested in in friction 0.6 M NaCl stir welding solution AA-2024 is reported T3 byjoints. Wang Constant et al. [23]. load The (CL) last cell authors and slow reported strain that rate the (SSR) tests tests in air were showed carried ductile out in fracture aerated 35in the g/L same NaCl zone.solution. This During different the behavior tests, open could circuit be potential attributed and to electrochemical the different welding impedance parameters. spectroscopy The same were authorsmeasured reported in the dithatfferent the stress zones corrosion of the welding. cracking (SCC) susceptibility of the FSW 2024‐T4 aluminum alloy Thejoint FSW increased process with significantly feed rate modifiesdue to the the increase microstructure of the second of the phases alloy, and size. therefore, its corrosion and stress corrosion cracking behavior. 4. Conclusions The paper analyzed stress corrosion cracking phenomena in friction stir welding AA‐2024 T3 joints. Constant load (CL) cell and slow strain rate (SSR) tests were carried out in aerated 35 g/L NaCl

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The applied load enhances the intergranular corrosion at the nugget of the AA 2024-T3 alloy owing to the presence of micrometric copper-rich precipitates at the border of the recrystallized grains. In this zones, no stress corrosion cracking was noticed, but well-evident stress-enhanced intergranular corrosion occurred. Moreover, the applied strain is preferentially localized at HAZ/TMAZ zones which are characterized by lower tensile strength compared to the nugget. Higher strain values are then localized at the heat-affected zones, where softening occurs. Because of these contrasting effects, it is possible to state that the stress corrosion cracking susceptibility of the FSW-ed alloy is then similar to that of the base metal in the range of processing parameter studied in this experimental work.

Author Contributions: Methodology, S.L., C.T. and S.B.; investigation, S.L., C.T. and S.B.; data curation, G.D. and M.C.; all the authors contributed to writing the original draft of the paper. Writing—review and editing T.P., C.G. and M.C.; supervision, T.P. and C.G.; project administration, T.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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