Investigation of stress corrosion cracking of friction stir welded 2060 Al-Li alloy

Aluminum lithium 2060 alloy is an ideal structural material for aeronautical components. Friction stir welding (FSW) of two-millimeter-thick 2060 Al-Li alloy plates under a welding speed of 600 mm/min and rotation speed of 400 rpm �ourishes as better joining process. During FSW process, the three zones of weld nugget zone (WNZ), thermally mechanical affected zone (TMAZ) heat affected zone and base metal (BM) on the weld transverse cross section are classi�ed caused by tool stirring and axial force in friction stir welded joints. BM shows that the microstructure is characterized by lamellar structures of the lath-shaped α phase arranged along the rolling direction. The WNZ, with �ne and equiaxed grains, is a typical feature of the recrystallized structures resulted from dynamic recrystallization due to severe plastic deformation and high temperature. The TMAZ, a transition zone surrounding the weld nugget experiences a combined action of friction heating and high bending. The equipment was designed and fabricated to investigate the stress corrosion cracking behavior in simulated environment. Electrochemical evolution of FSW Al-Li 2060 alloy in 3.5wt.% NaCl solution and performed open circuit potential, electrochemical impedance spectroscopy and cyclic polarization in unloaded condition and with applied load of 500, 1000 in speci�c intervals (1, 24, 48) hours. Results show that poor corrosion resistance in unloaded condition and decrease with the speci�c time interval. Results show that corrosion rate in unloaded 102, for 500 g load is 7.5 and for 1000g load is 48.76 in millimeter per year (mm/y).


Introduction
Al-Li alloys have the advantages of low density, excellent resistance to stress corrosion cracking and enhanced mechanical properties such as high strength and good fracture toughness therefore,theyare widely used in aircraft and aerospace structures [1].Compared with the previous second generation of Al-Li alloys, the third generation of Al-Li alloys have displayed superior mechanical behavior in terms of strength and hardness and improved weld ability with lower anisotropy [2].This could be attributed to the chemical composition of third-generation Al-Li alloys.Nevertheless, there are still some issues with Al-Li alloys during welding, such as weld metal porosity and cracks [3].
Studies have shown that the use of appropriate ller materials and preparation of high-quality weld surface are good methods for reducing the weld defects of Al-Li alloys [4].Therefore, the strength of Al-Li weld joints can be greatly improved by choosing appropriate ller materials and effective welding parameters [5].Effective methods for the welding of Al-Li alloys include laser beam welding (LBW) using high-energy beams and friction stir welding (FSW) using solid-phase connections.LBW offers several advantages such as a lower heat application than conventional arc welding and a reduction in porosity and cracks during the welding process [6].Different Al-Li alloys (2060 and 2198) were laser-welded without the addition of ller materials.The effects of welding parameters on the formation of welded joints, microstructure evolution, solute segregation, porosity, and their relationship with the mechanical properties of the joints were studied [7].Research found that reducing welding heat input can effectively prevent grain coarsening, while reducing porosity and thermal cracking tendency, thereby improving joint performance.Investigations of FSW joints of 2098-T8, 2195-T8, 2198-T8, 2199-T8 and 2050-T8 indicated that most of the precipitates dissolved in the NZ and TMAZ while the dissolution and coarsening of precipitates occurred in the HAZ [8].Some studies about Al-Li alloy shows that the loss of Li element as well as hot cracks and pores generated frequently in fusion welded joints can effectively be avoided by using FSW and welds with better mechanical properties can be obtained [9].The FSW technique is considered to have great advantages to weld some metals which are usually considered to be unweldable such as aluminum, magnesium and copper alloys [10].
Studies shows that the in uence of the microscopic morphology of different regions on the mechanical properties of welded joints and found that the micro hardness distribution corresponds to the grain morphology distribution by Liu et al [11].The pitting corrosion possessed the dominated feature in the TMAZ of the AA2024 FSW joint by Bousquet et al [12].In 7039 Al-alloy was initiated by the dissolution of the GBPs and propagated by mechanical processes such as creep.The average micro hardness at the center of the weld was the smallest because the grain size at the center of the weld was the largest reported by Choi et al. [13].In addition, Cu and Mg segregation occurred during the welding process, which reduced the strength of the weld.Bousquet et al [14] who found that the heat affected zone is the most sensitive region with regards to corrosion.
Stress corrosion cracking (SCC) is a degradation or fracture process that occurs in sensitive alloys such as some Al alloys and steels when three conditions must be present at the same time [15].It is di cult to understand the SCC phenomena as it depends on a number of factors, such as corrosive environment, material's composition and tensile stress [16].When a crack initially appears a number of variables such as the microstructure of the material which includes heat treatment, quantity of impurities, the manufacturing processes, this failure happens in potentially sensitive structural alloys and under service circumstances when it might occur unexpectedly and lead to catastrophic collapse [17].Factors affecting stress corrosioncracking of 7xxx series alloys are addressed with a focus on 7075 and 7050 Al alloys since they have a wider range of applications in the aerospace and structural industries [18].
Previously it was reported that 2xxx, 5xxx and 7xxx alloys are susceptible to SCC among the eight series of aluminum alloys.Because of their higher mechanical qualities are compared to 2xxx and 5xxx Al alloys, the 7xxx alloys have specialized structural and aeronautical applications [19].Due to its high strength, ductility, toughness, low density and favorable fatigue qualities 7xxx series is mostly used in the aerospace, military, nuclear and structural components of the building industry.However, SCC resistance is more important in this alloy as SCC has been responsible for many aircraft structure and component failures since 1950's [20].
The current research focus is to explore the microscopic examination and electrochemical behavior of friction stir welded 2060 Al-Li alloy in simulated environment like 3.5% NaCl.Therefore, to investigate the stress corrosion cracking of friction stir welded 2060 aluminum-lithium alloy we design and fabricate the model.With the help of this equipment variable loads can be applied under corrosive environment to analyze the behavior of the tested sample.

Material
The material was 2060 Al-Li alloy T8 with the composition illustrated in Table 1.

Microstructure
The study of microstructure and phases of metals typically seen through a metallurgical microscope.The geometric arrangement of grains and the different phases present in metals and alloys are known as the microstructure.Material engineers predict the general properties of materials by observing their microstructures.
Each specimen was mounted in Bakelite powder by using mounting machine (Muller) keeping the working surface of sample exposed.After mounting and grinding all the samples were polished, using ∘ ∘ diamond pastes of grade 0.25, 1, 3, and 7µm on nylon and velvet clotheswere polished using polisher machines (Dace Technologies Nano 2000T).Polished sample of each phase were then etched with Keller solution same as used for macrostructure.The etched samples were rinsed with hot water and then dried by using air blower.The microstructures of samples were observed by using light optical microscope (NOVEX HOLLAND).

Electrochemical test
The sample of Al-Li alloy was ground with SiC carbide emery paper grit size from (120 to 1200).In 50ml beaker a hole was created with the help of drill bit to pass the tested sample.After passing the tested sample thought the hole, Silicon was lled with silicon gun to avoid the leakage of electrolyte from the beaker.To maintain the strength of leakage magic epoxy (epoxy resin and hardener) was also introduced at the bottom of the beaker.Electrochemical measurements were carried out in 3.5 wt.% NaCl which simulates a marine environment.The pH of 3.5 wt.% NaCl was measured before testing between 8.2 ± 0.1.A three-electrode electrochemical cell was used with a silver/silver chloride (Ag/AgCl sat.KCl) as the reference electrode, a graphite rod as the counter electrode, and test specimen was 2060 Al-Li with an exposed area of 6.67 cm 2 .as the working electrode.Open circuit potential, electrochemical impedance spectroscopy, cyclic polarization was examined for dissolution and pitting behavior of each sample by using (1000E, Gamry, USA) as shown in Fig. 2.The value of stress for 500 g load is 405 Nm − 2 for 1000g as 809 Nm − 2 .

Metallography
The macrostructure taken from optical microscope various zones of friction stir welded 2060 aluminum alloy samples are analyzes shown in Fig. 3. Showing base metal in left side and weld nugget zone in center, thermally mechanical affected zone (heat affected zone) right side of the weld.The size of the weld cone nugget zone is approximately 2.5 mm.No defects observed in all four zones of friction stir welded 2060 Al-Li alloy [21].
In Fig. 4, the microstructure of BM left was observed to be coarse-grained size of in the range 25 ± 4 m.This microstructure may contain intermetallic compounds of aluminum with copper and other alloying elements in the matrix of pure aluminum as shown [22].Due to non-equilibrium solidi cation, solute segregation is frequent in aluminum alloy welds.Primary α-Al phases with less alloying elements will solidify rst during friction stir welding of Al-Li alloys.Li, Mg, and Cu alloying elements, as well as Si and Fe impurities [23].
The ne grains of the weld nugget zone have larger discrepancies, as shown in Fig. 8.Because the High seed rotating tool contact with the surface of Al-Li alloy which create the heat then cools quickly, the equilibrium solidi cation interval is short, resulting in a thicker thermal boundary layer and ner grains along the fusion line [24].To analyze the grain size and their morphology imageJ software is used The μ grain size of the welded zone in the range 7 ± 2 m.Lithium is a low melting point active element that concentrates easily on the liquid solid interface, increasing the undercooling degree and promoting fresh nucleation.This will stop the grains from growing and help in grain re nement [25].
The grain size of thermally mechanical affected zone in the range 24 ± 4 m.The microstructure of TMAZ was comprised of compounds of aluminum with copper and other alloying elements in the matrix of pure aluminum.Large and small size pores can be observed in the HAZ[26].

Open Circuit Potential
The potential was measured under unloaded and loaded condition to analyze the aqueous corrosion behavior.The open circuit potential was run for the time of 1800s with speci c time intervals of (1, 24 and 48) hours in 3.5 wt.% NaCl with respect to silver/silver chloride reference electrode.The results shows that the value of potential for unloaded condition is -669, -670, -671 (mV vs. Ag/AgCl)respectively.Similarly, the value of open circuit potential for 500g load is -670, -678,-677 (mV vs. Ag/AgCl) in (1, 24 and 48) hours respectively.As the load is applied the stress is produced on the tested sample, due to which they change the potential in more active side.Stress has signi cant effect on the microstructural changes which change the mechanical and electrochemical properties of the material [27].The potential of any metal or alloy depends upon the composition and their corrosive environment.As the results shows negative potential means they required minimum energy or potential to transfer the electrons in the electrolyte and dissolute easily as shown in Fig. 5.The open circuit potential was run with sample period of 0.5 mVs − 1 and area of the tested sample is 3.88cm 2 .The value of the open circuit potential for 1000 g load is -670, -679, -681 (mV vs. Ag/AgCl) in (1, 24 and 48) hours respectively as shown in Fig. 6.The result of the 1000 g load shows more active potential in 3.5% NaCl due to increase in the stress value [28].

Electrochemical impedance spectroscopy
EIS scan of friction stir welded 2060 Al-Li alloy under unloaded and loaded condition give the Nyquist and bode plot in 3.5% NaCl with speci c time of interval (1, 24 and 48) hours.
Polarization resistance refers to the resistance of anode to oxidation when potential is applied externally directed at the dissolution in the presence of electrolyte.
Results show that the resistance of dissolution increases with the passage of time which indicates that the concentration of ions in the electrolyte is reduced because they are consumed or used in the chemical reaction as shown in Fig. 6 for unloaded condition.Aluminum-lithium alloy surface interact with the electrolyte.In bode plot the phase angle increase as the time interval increases [29].
In this study, the evolution of the electrochemical behavior of each individual region was monitored by EIS using the mini cell.Data dispersion was seen in the lower frequency region.Consequently, only the After immersion of time, the shape of EIS spectra signi cantly changed.The amplitude of the EIS loops in Nyquist plot of 500g applied load increase signi cantly for all the different zones of the weld.Diffusive behavior was still observed for the welded 2060 Al-Li alloy as shown in Fig. 7.All the specimens showed only one phase constant, but the frequency range is lower compared to just dip conditions [30].An increase in phase can be noticed at very low frequency for the base material and the TMAZ, thus indicating that a second phase constant could be present at very low frequencies.After 48 h of immersion, the EIS spectra became practically overlapped, with only a slight difference between the impedance modulus of the welded sample of 1000g load as shown in Fig. 8. Well evident diffusive behavior can be noticed at very low frequencies because of the presence of corrosion products deposit [31].These results seem to evidence an initial higher activity of the nugget with respect to the other zones, but these differences become negligible at longer exposures.After the tests, all the specimens showed general corrosion morphologies [32].The behavior signi cantly modi es as loading is applied.The corrosion potential showed sudden decrease at the application of monoaxial constant load.The effect is attributable to the breakdown of the thick, porous, and not adherent scale of aluminum oxide formed on specimens after prolonged exposures, which exposes the aluminum matrix directly to the aggressive environment [4].

Cyclic Polarization
The cyclic polarization curves presented the active/passive behavior of friction stir welded 2060 aluminum alloy for unloaded and loaded condition are plotted as shown in Fig. 9.To obtain the values of anodic polarization slope (β a ), cathodic polarization slope (β c ), corrosion current density (I corr ), corrosion potential (E corr ), corrosion rate, and chi-squared values, the cyclic polarization curves were Tafel tted using Echem Analyst software as tabulated in Table 2 [33].The cyclic polarization scan runs after 48 hrs.To analyze the pitting behavior of Al-li 2060 alloy in 3.5% NaCl.It was observed that the friction stir welded 2060 aluminum alloy under 500 g exhibited the lowest corrosion rate of 6.847mm/y in 3.5 wt.% NaCl solution [34].
Because of equiaxed ne grain-sized microstructure of weld nugget zone.The attack of NaCl was relatively stronger at WNZ. While, in unloaded condition the tested sample show highest corrosion rate as 112.6 mm/y.Similarly, in 1000g load the cyclic polarization show the moderate corrosion behavior as compared to unloaded and 500g load as 38.58 mm/y [35].

Conclusions
The following are the conclusion of the research work higher and medium frequencies are presented.The Bode graph of unloaded sample shown in Fig. 8 in 1 hr, 24hr, 48 hr.

Figures Figure 1 Friction 2
Figures

Table 1
Chemical composition of the 2060 Al-Li alloy (wt.%) After cleaning with ethanol solution, the sample sheets (2 millimetres thick, 120 millimetres in length and 3 millimetres height) were subjected to the friction stir welding.The friction stir welding parameters were adjusted as tool rotation of 400 rpm and transverse speed of 600 mm/min with the tilt angle of 2 as schematic diagram shown in Fig. 1.
The tool materials employed for Friction Stir Welding of high-softening temperature alloys should display exceptional characteristics when subjected to temperature surpassing 900 C. The two main categories of materials that have showcased signi cant achievements are polycrystalline cubic boron nitride (PCBN) and refractory metals.The tool was rotated in clockwise direction resulting in fusion of Al-Li 2060 alloy plates without melting.The change in welding parameters result in alters the properties of nished product.There forms two distinct sides during friction stir welding of 2060 Al-Li alloy as Advancing sideand Retreating side.2.2.Metallography2.2.1.Macro etchingTo analyze the macrostructureof welded plate macro-etching in Keller's solution (950 mL Distilled water, 25 mL HNO 3 (70%), 15 mL HCL (30%), 10mL HF (50%) was done.The representative samples of Al-Li 2060 alloy have three zones as base metal, weld nugget zone and thermal mechanical affected zone (heat affected zone).After the samples were ground by rough grinding with emery sand papers from grit sizes 100 to 4000.The test objects were cleaned with ethanol solution to analyze the different zones with the help of optical microscope at 10X.

Table 2
Parameters of Ta e tting on cyclic polarization curve of unloaded and loaded condition.