Effect of post-weld heat treatment on mechanical properties and fatigue crack growth rate in welded AA-2024

Journal Pre-proof Effect of post-weld heat treatment on mechanical properties and fatigue crack growth rate in welded AA-2024 Vinay Kumar Yadav, Vidit Gaur, I.V. Singh

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S0921-5093(20)30203-3 https://doi.org/10.1016/j.msea.2020.139116 MSA 139116

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Materials Science & Engineering A

Received date : 19 December 2019 Revised date : 13 February 2020 Accepted date : 15 February 2020 Please cite this article as: V.K. Yadav, V. Gaur and I.V. Singh, Effect of post-weld heat treatment on mechanical properties and fatigue crack growth rate in welded AA-2024, Materials Science & Engineering A (2020), doi: https://doi.org/10.1016/j.msea.2020.139116. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Effect of post-weld heat treatment on mechanical properties and fatigue crack growth rate in welded AA-2024 Vinay Kumar Yadava , Vidit Gaura,∗, I.V. Singha a Department

Abstract

of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

In this study, effect of post weld heat-treatment (PWHT) at two different aged temperatures was investigated on the microstructure and the material properties of the aluminum alloy 2024. The plates of AA2024 were welded using friction stir welding process, followed by PWHT at different aged conditions: 190◦ C for 10 hrs and 200◦ C for 10 hrs. Both tensile and cyclic properties were investigated. PWHT at 200◦ C-10 hours resulted in significant changes in the microstructure and improvement in the mechanical properties of the welded joint. PWHT resulted in re-precipitation of the precipitates, specifically in the thermo-mechanically affected zone (TMAZ) & nugget zone (NZ) but no significant abnormal grain growth was observed in the nugget zone. A significant improvement in the ductility and the hardness of welded joint was observed for PWHT at 200◦ C-10 hours. Long crack growth tests were conducted using sinusoidal loading of 10 Hz frequency and at a stress ratio of 0.1. The PWHT joint at 200◦ C-10 hours resulted in higher threshold stress intensity factor range (∆Kth ) as compared to as-welded joint and the PWHT joint at 190◦ C-10 hours. The observations are explained based on microstructural changes in the FSW joint and are discussed in this work. Keywords: Micro-structure, Fatigue, FSW, PWHT, Aluminum, Fracture

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Aluminum 2xxx and 7xxx series alloys are high strength and heat-treatable alloys, which are mainly used in aerospace industries, structures and heavy-duty vehicles. To fabricate the large structures, metal joining process is quite significant. Many studies have been devoted in past to investigate different metal joining processes specifically for aluminum alloys due to their wide industrial applications. But different challenges are involved due to their high thermal conductivity, high expansion coefficient, high susceptibility to oxidation, etc. For light weight structures, the conventional welding techniques i.e. metal inert gas (MIG) welding and tungsten inert gas (TIG) welding, often result into a typical dendritic microstructure, poor strength and solid-state cracking in the welded joints, thereby reducing the mechanical properties of the joint in these alloys [1]. Among all the techniques used for metal joining, friction-stir welding (FSW) [2] has gained significant attention during past few decades and is used frequently now-a-days. FSW is a solid state welding process in which a rotating tool is plunged into the abutting edges of the plates that are to be joined. The relative motion between the tool and the work-piece leads to intense heat generation due to shear and produces a region with very high plasticity near the plunged area of the tool [3]. Like fusion welding techniques, FSW also has three different weld zones i.e. nugget zone (NZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ). Welding is often associated with various kind of defects in the fusion zone regardless of the welding techniques and its parameters. Like conventional welding processes, FSW also results into the formation of defects. In welded structures, weld toe has often been found to be the preferential location for the crack initiation and specifically from the defects associated with the welding technique. However, many studies have paid attention to the monotonic behavior of the FSW joints for these class of alloys and less attention has been paid to their cyclic properties. Since, most of the real life structures are often subjected to time dependent loading, therefore it is important to study their cyclic properties for the safe and reliable design. For the design of welded structures under cyclic loading, damage tolerance design approach is most commonly used. Several studies have been carried out to investigate the crack growth behavior in the friction stir welded structures

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1. Introduction

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∗ Corresponding

author. Email address: [email protected] (Vidit Gaur)

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[4–7]. A general consensus is that the FSW often results in improvement in fatigue lives of the welded joints as compared to the conventional welding techniques [8–12] owing to the micro-structural refinement in the weld zone [13], lower residual stresses [14] and a significant reduction in the welding defects [15, 16]. The parameters used for welding are also known to affect the fatigue crack growth rates in the welded joints. Low heat input during the welding is likely to retard the crack growth rate [17]. The weld heat input also controls the hardness of the welded joint. Fu et al. [18] reported degradation in the hardness value of the weld stir zone due to high heat input. The hardness variation along the weld also influence the crack growth rate in the welded joints [19]: Lower the hardness, higher is the fatigue crack growth rate. In FSW, the rotational speed of the tool is a key parameter in controlling the heat input, the hardness and thus the fatigue crack growth in the welded joint [20–23]. For the same heat input, increase in rotational speed increases the hardness due to less time for thermal exposure [24]. Although, heat generated during FSW process does not result in the melting of the metal but often deteriorates the mechanical properties significantly. Ilman et al. [25] studied that fatigue crack growth rate behavior in FSW Al2024-T3 alloys under thermal transient tension and found that low crack growth rate in the weld nugget zone for lower ∆Ks (stress intensity factor range). Urso et al. [26] have also reported the dependency of crack growth rates on ∆K in different weld zones, and found slow growth of cracks in the weld nugget zone. The presence of residual stresses developed in the welded zone affects the crack propagation and consequently the fatigue lives significantly [4, 7]. Tra et al. [27] investigated the role of tensile residual stresses on the crack propagation rate in the FSW aluminum weld and found higher crack growth rates in the stir zone as compared to that in the base metal. Fatigue crack growth rate are also known to be dependent on the stress ratio i.e. R-ratios (= σmin /σmax ). Moreira et al. [28] and Kim et al. [29] reported higher fatigue crack propagation rates for high stress ratios for both base metal and the dynamically recrystallized zone. On the contrary, Ma et al. [30] also investigated the effect of stress ratios on the crack propagation rates and found that the fatigue crack propagation rate is not much sensitive to the R-ratio in weld nugget zone as compared to that in the base metal. Due to generation of higher temperatures in the stir zone of the weld, nearly 400-500◦ C, thermal cycles are produced across the various zones of the weld, resulting in either coarsening of the material’s inherent precipitates [7] or dissolution of some of the precipitates into the matrix [31]. These precipitates, in general, also play an important role during the fatigue crack propagation in the weld. Sivaraj et al. [32] reported a reduction in the fatigue life of the welded aluminum alloys as compared to the base metal which was attributed to the dissolution of the precipitates in the weld nugget zone. Thus, post-weld heat treatment (PWHT) is generally preferred for such welded components that produces the desired change in the mechanical properties of the weld due to re-precipitation of the strengthened precipitates by aging. Several studies have been done to improve the properties of FSW components using this procedure [20, 33]. Post-weld heat treatment often results in coarsening of grains in thermo-mechanically affected zone but retention of grains in the weld nugget zone. An increase in the annealing temperature enhances the coarsening of the grains. After PWHT, the grain size in the weld zone does not have a dominant influence on the hardness, rather it is the hardened precipitates that drive the strengthening mechanism. The plastic zone size ahead of crack tip in a welded joint is also influenced by PWHT. Aydin et al. [33] observed a significant loss of the ductility in FSW Al2024-T4 joint when PWHT was done at 190◦ C for 10 hours but an improvement in the yield strength, very close to that of the base metal. PWHT results in fine and uniformly distributed hardened precipitates & abnormal grain growth [34, 35] in stir zone which improves the mechanical properties. Costa et al. [36] observed the effect of PWHT under monotonic and cyclic loading and found that FSW joint have better mechanical property in monotonic loading as compared to cyclic. Sometimes, PWHT results in adverse effect on precipitates as it coarsens the precipitates in the nugget zone [37], and thus, reduces the hardness due to over-aging effect. Several studies, as discussed above, have been performed to improve the mechanical property of FSW joints, to understand the effect of microstructure, & process parameter and also the effect of PWHT followed by natural aging and artificial aging on the joint efficiency and the mechanical properties of the welded joints. Some studies also exist on the investigation of crack propagation rate in as-weld condition, but only a handful of data is available on the effect of PWHT in the near-threshold regime of the fatigue crack propagation rate. Thus, this study is aimed to bridge this gap. In this study, the effect of post weld heat-treatment (PWHT) in the threshold regime of the fatigue crack propagation rate in friction stir weld 2024 aluminum alloy has been analyzed at a constant stress ratio (R = 0.1). The effect of aging at two different temperatures: 190◦ C & 200◦ C for 10 hours after PWHT was also analyzed on the microstructure, and cyclic properties of the joint.

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2. Material properties

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3. Experimental Procedures

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3.1. Welding Parameter

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Aluminum 2024 alloy was received in the form of naturally aged rolled plates. The chemical composition of the alloy in as received condition was measured using X-ray photoelectron spectroscopy (XPS) and is shown in Table 1. The measured mechanical properties of as-received material in both longitudinal as well as the transverse directions are reported in Table 2. The as-received rolled plates were cut into several small pieces of size 100 mm x 50 mm x 6 mm, before joining them using friction stir weld (FSW), as described next.

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A vertical milling machine of capacity 15 HP with 2◦ concavity at the shoulder was used for friction stir welding with a backing plate of mild steel. The plates cut from the as-received material were cleaned using acetone and abutted surfaces were machined using a shaper machine. FSW was performed on these plates using a chromium hot work steel cylindrical tool (H13 steel tool) at a rotational speed of 931 rpm and a traverse speed of 70 mm/min. The specifications of FSW tool geometry are given in Table 3. A schematic of friction stir welding is shown in Fig. 1. Following three different cases of welded joints were considered. 1. As-weld (AW) condition without any post weld heat-treatment (W 1) 2. Post weld heat-treatment (PWHT) followed by artificial aging at 190◦ C for 10 hours (W 2) 3. Post weld heat-treatment (PWHT) followed by artificial aging at 200◦ C for 10 hours (W 3)

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The post weld heat-treatment was done at 493◦ C for 2 hours followed by cooling in water. After artificially aging, the samples were retained for one week. An example of sound friction stir weld is shown in Fig. 2.

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To investigate the microstructural characteristics of the welded joint, small cubical specimen of size 10 mm were cut for microscopic examinations using optical microscope and scanning electron microscope (FE-SEM). Before analyzing samples, the surfaces were sequentially polished in appropriate steps using SiC papers of grit sizes between 200-2000. The surface was then further polished up to 0.3 µm using diamond paste. After polishing, samples were etched using Keller’s solution: 2.5 mL HNO3, 1 mL HF, 1.5 mL HCL and 95 mL distilled water. Energy dispersive X-ray spectroscopy (EDS) equipped with FE-SEM was used for phase analyzing in the weld regions.

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Flat specimens (Fig. 3a) of thickness 6 mm and gage length 25 mm are cut from the welded plates for the three cases i.e. W 1, W 2 and W 3 as stated above. The tensile tests were conducted on these samples using servo hydraulic MTS 810-100kN machine, in compliance with the ASTM E8 standard at a cross-head speed of 4x10−4 /sec at room temperature (24◦ C). All the specimens were cut along the transverse direction of the weld. The micro-hardness of the stir-zone (SZ), the thermo-mechanically affected zone (TMAZ), and the heat-affected zone (HAZ) was measured using Autovick HM 200 Micro-Vickers hardness tester, in compliance with the ASTM E384 standard. A small load of 50g was applied for 10 seconds to measure the hardness value. Hardness was measured along the centerline of the cross-section of the welded joint.

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Standard CT specimens (Fig. 3b) were used to study long crack growth behavior in the FSW joints. The fatigue crack growth tests were performed using servo-controlled hydraulic MTS 810-100kN machine, in compliance with the ASTM E647-15 standard. The alignment of grips and specimen was verified using spirit level after mounting the specimen (Fig. 3c). Crack growth study was conducted for the three cases (W 1, W 2 and W 3) as described in section 3.1. The tests were performed at room temperature (24◦ C) using sinusoidal waveform of frequency 10 Hz and at stress ratio (R) of 0.1. Compliance method was used for monitoring the crack length using the crack opening displacement (COD) gage. ∆K-decreasing test was done to achieve the desired threshold crack growth rate for different weld conditions, followed by the constant load range test i.e. ∆K-increasing. For ∆K-decreasing test, an automated algorithm for continuous exponential decrease was used in accordance with the procedures outlined in ASTM E647

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standard. The threshold stress intensity factor range (∆Kth ) is defined as the ∆K corresponding to the lowest measured crack growth rate in the region 10−9 - 10−10 m/cycle. The fatigue crack propagation rate was analyzed using the Paris law, according to which the stress intensity factor range (∆K) is related to the crack growth rate as [38]:

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da = C ∆Km when∆K > ∆Kth dN Where, the expression for stress-intensity factor range (∆K) for the CT specimen is given as: ∆P √ f (a) B W 2+α (0.886 + 4.64α − 13.32α2 + 14.72α3 − 5.6α4 ) f (a) = (1 − α)3/2 a α= W ∆K =

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where ‘W’ is width of the CT-specimen from load line, ‘B’ is net thickness of specimen, ‘a’ is initial crack length, ‘da/dN’ is crack propagation rate (in m/cycle) and ∆P is applied load range. C & m are the material constants.

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4. Results

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4.1. Microstructure

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Fig. 4a shows the optical microstructure of as-received alloy in the longitudinal, transverse and short transverse planes. The distribution of grain sizes was obtained by fitting the grains to an approximate ellipse. The average ratio of major to minor axis of the grains were 6.25 in longitudinal plane, 2.05 in short transverse plane and 5.8 in longitudinal transverse plane. Average grain size was obtained by taking the square root of the fitted ellipse area. An SEM image of the elongated grains in the rolled direction is shown in Fig. 4b with uniformly dispersed precipitates of an approximate size of 3-5 µm. Fig. 5a shows an optical image of the cross-sectional plane of the weld depicting the various zones in the FSW joint. The observed shape of the fusion region is typically of ‘V’ type, which generally depends upon the process parameters and the tool shape [39]. These zones of the FSW joint have different microstructural characteristics: (i) Heat affected zone (HAZ) (Fig. 5b), which is subjected to thermal gradient, (ii) Shoulder contact zone, near top of abutting surface which increases the zone of deformation at a slight wider depth, (iii) Thermo-mechanically affected zone (TMAZ) (Fig. 5c) characterized by twisted & elongated grains, just after the nugget zone where material does not undergo any shearing process, and (iv) Nugget zone (NZ) (Fig. 5d), where weld tool penetrates into the abutted metal sheet with nearly equal to tool pins diameter. Different zones of the friction stir weld are subjected to different thermal loadings depending upon the tool’s geometry and the weld parameters. Smaller but nearly equi-axed grains were observed in weld nugget zone as compared to heat affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ). The twisted and elongated grains were observed in the TMAZ on the advancing side of the weld while some coarsened grains were found on the retreating side. Fig. 6 shows the SEM image of a region in the nugget zone of FSW joint with precipitates (in white color) accumulated along the grain boundary and an average grain size of ∼ 3-3.5 µm. Weld nugget zone is marked by the clear presence of uniformly distributed and aligned precipitates. The grains in the HAZ region were similar to that in the base metal but coarsened due to frictional heat. The post-weld heat treatment (PWHT) and the subsequent artificial aging at 190◦ C for 10 hours (Fig. 7) and at ◦ 200 C for 10 hours (Fig. 8) had no significant effect on grain size in nugget zone. The transverse cross-section of the welded joint within the sir zone was analyzed by EBSD measurements and the obtained pole figures are shown in Fig. 9 for the planes (100), (110), and (111). The intensities were found nearly equal in all the directions. The distribution of grain orientation remained un-affected by PWHT, which confirms the material’s isotropy. An EDS analysis done on one of the precipitates (as shown in Fig. 10) confirmed the presence of Al-Cu-Mg and Al-Cu in it, which insures the presence of ‘S’ phase “Al2 CuMg” precipitates and ‘α’ phase Al2 Cu precipitates. No significant effect of PWHT on the grain size (3.5-4 µm) in the weld nugget zone was observed i.e. no abnormal grain growth. Similar observations have also been reported in [41].

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Table 4 shows the data obtained from the tensile tests in as-received (AR) condition, as-welded condition (W 1), PWHT weld followed by aging at 190◦ C (W 2) and PWHT weld followed by aging at 200◦ C (W 3). The engineering stress vs plastic strain curve for various conditions is shown in Fig. 12a. A decrease in strength was observed post welding of the aluminum plates. The elongation of the friction-stir welded joints was reduced considerably i.e. about 85%.

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Fig. 13a shows that the optimum hardness of welded joint is found at a depth of 2 mm from the top surface of the joint. The observed degradation in the hardness helps in recovery of the precipitates after FSW in the precipitate hardened alloys, as applicable in present case also. After PWHT, no abnormal grain growth was observed, but the re-precipitation of fine and hardened Al-Cu precipitates resulted in uniformly distributed hardness across the weld cross-section. In the stir zone, the PWHT followed by different aging temperatures did not affect much the hardness values of the welded joint. On contrary, a clear and significant effect was observed on the hardness in the TMAZ or the heat affected zone of the welded joint. The hardness of the joint was higher for the aging temperature of 200◦ C as compared to the as-welded joint and the PWHT joint at 190◦ C.

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5.1. Effect of PWHT on microstructure

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In the as-weld condition, twisted elongated grains observed in TMAZ on the advancing side of the weld as compared to that on the retreating side (Fig. 5c and Fig. 5e) is mainly due to high heat of friction developed on the retreating side of the welded plate thereby undergoing intense plastic deformation as compared to the advancing side. Stirring action of the weld tool causes elongation of grains to transform into a narrow region, which is due to high thermal conductivity of the aluminum alloy. This stirring action of the tool produces dynamic re-crystallization of the grains which resulted in not only coarsening of the precipitates but also in removal of some of the hardened precipitates. As shown in Fig. 7 & Fig. 8, after the PWHT, no significant change in the microstructure was observed in various zones of friction stir weld, which confirms that the grains were in stable condition after different aging conditions. The only change observed was in coarsened hardening precipitates. Ilman et al.[25] also reported similar observations for these precipitates in similar class of alloys. Such behavior of hardening precipitates often reduces the strength of welded joint due softening caused by stirring action of rotating tool which generally produces a weld thermal cycle of approximately 400◦ C-500◦ C [31]. After PWHT followed by subsequent artificial aging at 190◦ C for 10 hours & 200◦ C for 10 hours would results in re-precipitation of hardening precipitates in the Al-Cu alloy & distributed uniformly throughout the section thereby leading to an improvement in the strength of the joints by trapping the disloactions [40]. A layered structure was observed at the bottom of the weld due to the rotation of tool, often referred as onion rings (Fig. 11). The formation of these onion rings has been investigated in several studies [42, 43] and is more often

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Long crack growth rate (da/dN) in Al-2024 alloy in as-received (AR) and as-weld condition is plotted against ∆K (stress intensity factor range) for different ∆K tests is shown in Fig. 14a and b. The investigated range of crack growth rate is 10−10 to √10−5 m/cycle. A noticeable change in the slope of the da/dN curve was observed between the ∆K values of 5-7 MPa m. The crack growth data obtained during ∆K-decreasing i.e. the load shedding test, nearly followed the trend of the data obtained during ∆K-increasing test i.e. constant load range test (Fig. 14c). As per the recommendations outlined in ASTM-E647, only the crack growth data obtained during constant load range test was considered for further analysis. To ensure the repeatability of the experimental results, at least two tests were done for each condition for which the crack growth data nearly followed the similar trend. Fig. 16 shows a comparison in the crack growth rates of the welded joints for different conditions stated earlier. The coefficients of Paris equation: ‘C’ and ‘m’ (Eq. 1) for various cases are tabulated in Table 5. A significant difference in the threshold region of the crack growth rates was observed for all the cases while in the Paris regime, the difference was smaller. Crack growth exponents for the welded joint with PWHT followed by aging at 190◦ C (W 2) and at 200◦ C (W 3) condition were very close, which suggests that the crack growth rate was almost similar in the Paris regime but a significant difference was observed in the near-threshold regime i.e. ∆Kth (threshold stress intensity factor range).

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5.2. Effect of PWHT on strength & hardness

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attributed to the successive shearing of thin layers of metal in fusion zone. In the nugget zone, the formation of second phase particles “Al2 CuMg” during re-precipitation leads to formation of bands with different densities [44], resulting in the formation of these rings.

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The joint efficiency was calculated with respect to as-received material properties using following relation: ηjoint =

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σas-received − σas-weld σas-received

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Where σ is the strength of the material. The joint efficiency (ηjoint ) of welded joints was reduced by approximately 92%. Franchim et al. [13] reported a FSW joint efficiency of about 98.7%, which is near to what has been observed in this study. Other weld parameters like tool geometry, tool rotation speed, tool feed rate and cooling medium also affect the joint efficiency and can explain the observed difference in the values of the weld joint efficiency. The strength of the material is dependent on its grain size: which is inversely proportional to grain size as suggested by Hall-Patch relationship[19]: σgrain boundary = σmatrix + kd−1/2 (4)

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where ‘k’ is constant, ‘d’ is the grain size, & σmatrix is the strength of metal matrix. Using this relationship, it can be interpreted that the weld nugget zone with fine equiaxed grains, is likely to have better strength as compared to other zones of the weld. This decrease in strength and plastic strain is reverted by the post-weld heat-treatment. Aydin et al. [33] also reported an increase in the strength of the welded joint after PWHT but at the cost of its ductility, which was decreased by approximately 33%. This increase in strength after PWHT was attributed to the stable grain size within weld stir zone, re-precipitation of the hardened precipitates and variable grain size in the TMAZ zone. Upon changing the aging condition of welded joint, the tensile strength was reduced marginally at 200◦ C as compared to that at 190 ◦ C but a clear improvement in ductility of the welded joint is observed after aging at 200◦ C. A change in aging temperature does not influence much the yield strength of the joint but a clear difference in other mechanical properties was observed. The fracture in the tensile specimens most often occurred between the nugget zone and the thermo-mechanically affected region of the weld (Fig. 12b). This region had significantly less strengthened precipitates due to coarse grains. A significant increase in the yield strength and the plastic strain after PWHT (as compared to the as-welded joint) was mainly due to the re-precipitation of hardened precipitates and the variation in the grain sizes in TMAZ [41]. High thermal gradient during the welding process leads to over-aging of the precipitates, resulting in softening effect specifically in the TMAZ region and other heat-affected zones. This phenomenon is likely to degrade the hardness of the welded joint and can explain the observed trend (Fig. 13b). After PWHT, although no abnormal grain growth occurred, but the re-precipitation of fine and hardened Al-Cu precipitates resulted in uniformly distributed hardness across the weld section. In the stir zone, the PWHT followed by different aging temperature did not affect much the hardness values of the joint. A clear and significant effect of aging temperature was observed on the hardness in the TMAZ or the heat affected zone of the welded joint. The hardness of the joint was higher for the aging temperature of 200◦ C as compared to the as-welded joint and the PWHT joint at 190◦ C which can be attributed to the hardened precipitates.

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The crack growth rate in the nugget zone is dependent on the welding parameters and the tool geometry. As the Paris regime is independent of the material’s microstructure, thus in all cases similar trend was observed. But √ for smaller ∆Ks (5-7 MPa m), the crack growth rate for as-welded condition (W 1) is more than as-received (AR) condition as well as after the different PWHT conditions. The fractured surfaces of the broken fatigue crack growth specimens were examined using SEM. The chevron lines formed in the as-received condition indicates the ductile failure as shown in Fig. 15a. A quasi-cleavage type fracture mode was predominant for the crack growth in stirred zone. Fatigue striations, which are typical characteristics for steady state fatigue crack growth, were also observed. These striations were most frequently easily observed in asreceived condition as compared to the other conditions which suggest a possible reduction in ductility after PWHT process. Tear ridges [5] were also observed but mostly in the as-weld condition (Fig. 15b). The threshold ∆K for PWHT joint followed by aging at 200◦ C (W 3) is more than the PWHT joint followed by aging at 190◦ C (W 2) (Fig. 16 and Table 5). The PWHT resulted in an improved strength as well as the fatigue life 6

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6. Conclusions

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In this study, influence of post weld heat-treatment followed by artificial aging at two different temperatures (190◦ C and 200◦ C) on the microstructure, mechanical properties and crack propagation rate of friction stir welded joints was investigated experimentally. Based on the observed results, the following major conclusions can be drawn: • No significant grain growth was observed after post-weld heat treatment in the nugget zone i.e. fine equi-axed grains were retained. But post-weld heat treatment causes re-precipitation of coarsened precipitates in thermomechanically affected zone (TMAZ) & nugget zone (NZ). • A loss in strength after friction stir weld was observed due to coarsening of precipitates which was retrieved by PWHT at 200◦ C-10 hours, and plastic strain was increased upto 1.67 times of that of the as-weld condition. • PWHT followed by aging at 200◦ C as compared to that at 190◦ C, decreases the tensile strength marginally but improves the ductility of the joint significantly without affecting the improved yield strength. • PWHT at 190◦ C & 200◦ C resulted in uniform distribution of hardness over the entire weld cross-section. The aged temperature of 200◦ C showed an improvement in the hardness values as compared to that at 190◦ C, specifically in the heat affected zones mainly due to the evolution of the hardened precipitates. • A clear and significant difference in the ∆Kth values of the fatigue crack growth rates was observed for different PWHT conditions. PWHT followed by aging at 200◦ C resulted in an improved value of the ∆Kth as compared to that at 190◦ C , which was closer to that of the as-received condition. Acknowledgments

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resistance probably because of the uniform distribution of hardness across the weld zone and lower resultant residual stresses as compared to the as-welded joint (W 1). Zhang et al. [45] also reported that the PWHT increases residual stresses across weld’s transverse direction as compared to that in the longitudinal direction. From the crack growth behavior observed in this material, it could be deduced that the PWHT is most likely to affect the crack growth rate in near-threshold regimes only and the aging temperature of 200◦ C-10 hours is likely to have beneficial effect on the fatigue resistance as compared to that of 190◦ C-10 hours.

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The authors are thankful to the Ministry of Human Resource Department (MHRD), Government of India for funding this project.

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References

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[1] C. Rhodes, M. Mahoney, W. Bingel, R. Spurling, C. Bampton, Effects of friction stir welding on microstructure of 7075 aluminum, Scripta materialia 36 (1).

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[2] W. Thomas, E. Nicholas, J. Needham, M. Murch, P. Temple-Smith, C. Dawes, Friction stir butt welding, international patent appl. n. pct/gb92/02203 and gb patent appl. n. 9125978.8, US Patent (5,460,317). [3] W. M. Thomas, Friction stir welding and related friction process characteristics, in: Proc. 7th Int. Conf. on Joints in aluminium–INALCO, Vol. 98, 1998, pp. 157–174. [4] R. Galatolo, A. Lanciotti, Fatigue crack propagation in residual stress fields of welded plates, International Journal of Fatigue 19 (1) (1997) 43–49. [5] Q. Dai, Z. Liang, G. Chen, L. Meng, Q. Shi, Explore the mechanism of high fatigue crack propagation rate in fine microstructure of friction stir welded aluminum alloy, Materials Science and Engineering: A 580 (2013) 184–190. [6] K. Jata, K. Sankaran, J. Ruschau, Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-t7451, Metallurgical and materials transactions A 31 (9) (2000) 2181–2192.

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[8] S. Malarvizhi, V. Balasubramanian, Fatigue crack growth resistance of gas tungsten arc, electron beam and friction stir welded joints of aa2219 aluminium alloy, Materials & Design 32 (3) (2011) 1205–1214.

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[7] G. Bussu, P. Irving, The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024-t351 aluminium joints, International Journal of Fatigue 25 (1) (2003) 77–88.

[9] M. Wu, C. Wu, S. Gao, Effect of ultrasonic vibration on fatigue performance of aa 2024-t3 friction stir weld joints, Journal of Manufacturing Processes 29 (2017) 85–95. [10] M. Ericsson, R. Sandstr¨om, Influence of welding speed on the fatigue of friction stir welds, and comparison with mig and tig, International Journal of Fatigue 25 (12) (2003) 1379–1387. [11] P. Moreira, M. De Figueiredo, P. De Castro, Fatigue behaviour of fsw and mig weldments for two aluminium alloys, Theoretical and applied fracture mechanics 48 (2) (2007) 169–177. [12] X. Wang, K. Wang, Y. Shen, K. Hu, Comparison of fatigue property between friction stir and tig welds, Journal of University of Science and Technology Beijing, Mineral, Metallurgy, Material 15 (3) (2008) 280–284. [13] A. S. Franchim, F. F. Fernandez, D. N. Travessa, Microstructural aspects and mechanical properties of friction stir welded aa2024-t3 aluminium alloy sheet, Materials & Design 32 (10) (2011) 4684–4688. [14] J. Ouyang, R. Kovacevic, Material flow and microstructure in the friction stir butt welds of the same and dissimilar aluminum alloys, Journal of Materials Engineering and Performance 11 (1) (2002) 51–63. [15] W. Thomas, E. Nicholas, Friction stir welding for the transportation industries, Materials & design 18 (4-6) (1997) 269–273. [16] A. C. Munoz, G. R¨uckert, B. Huneau, X. Sauvage, S. Marya, Comparison of tig welded and friction stir welded al–4.5 mg–0.26 sc alloy, Journal of materials processing technology 197 (1-3) (2008) 337–343. [17] D. G. Moghadam, K. Farhangdoost, Influence of welding parameters on fracture toughness and fatigue crack growth rate in friction stir welded nugget of 2024-t351 aluminum alloy joints, Transactions of Nonferrous Metals Society of China 26 (10) (2016) 2567–2585. [18] R.-d. Fu, J.-f. Zhang, Y.-j. Li, J. Kang, H.-j. Liu, F.-c. Zhang, Effect of welding heat input and post-welding natural aging on hardness of stir zone for friction stir-welded 2024-t3 aluminum alloy thin-sheet, Materials Science and Engineering: A 559 (2013) 319–324.

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[19] L. Fratini, S. Pasta, A. P. Reynolds, Fatigue crack growth in 2024-t351 friction stir welded joints: Longitudinal residual stress and microstructural effects, International Journal of Fatigue 31 (3) (2009) 495–500. [20] Y. Chen, H. Ding, J. Z. Li, J. Zhao, M. J. Fu, X. H. Li, Effect of welding heat input and post-welded heat treatment on hardness of stir zone for friction stir-welded 2024-t3 aluminum alloy.

Jou

294

[21] B. Safarbali, M. Shamanian, A. Eslami, Effect of post-weld heat treatment on joint properties of dissimilar friction stir welded 2024-t4 and 7075-t6 aluminum alloys, Transactions of Nonferrous Metals Society of China 28 (7) (2018) 1287–1297. [22] R. K. R. Singh, R. Prasad, S. Pandey, S. K. Sharma, Effect of cooling environment and welding speed on fatigue properties of friction stir welded al-mg-cr alloy, International Journal of Fatigue 127 (2019) 551–563. [23] A. Alavi Nia, A. Shirazi, An investigation into the effect of welding parameters on fatigue crack growth rate and fracture toughness in friction stir welded copper sheets, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications 232 (3) (2018) 191–203. [24] S. Ren, Z. Ma, L. Chen, Effect of welding parameters on tensile properties and fracture behavior of friction stir welded al–mg–si alloy, Scripta Materialia 56 (1) (2007) 69–72. [25] M. Ilman, P. Iswanto, et al., Fatigue crack growth rate behaviour of friction-stir aluminium alloy aa2024-t3 welds under transient thermal tensioning, Materials & Design 50 (2013) 235–243. 8

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[26] G. DUrso, C. Giardini, S. Lorenzi, T. Pastore, Fatigue crack growth in the welding nugget of fsw joints of a 6060 aluminum alloy, Journal of Materials Processing Technology 214 (10) (2014) 2075–2084. [27] T. H. Tra, M. Okazaki, K. Suzuki, Fatigue crack propagation behavior in friction stir welding of aa6063-t5: Roles of residual stress and microstructure, International Journal of Fatigue 43 (2012) 23–29.

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[28] P. Moreira, A. De Jesus, A. Ribeiro, P. De Castro, Fatigue crack growth in friction stir welds of 6082-t6 and 6061-t6 aluminium alloys: A comparison, Theoretical and Applied Fracture Mechanics 50 (2) (2008) 81–91. [29] S. Kim, C. G. Lee, S.-J. Kim, Fatigue crack propagation behavior of friction stir welded 5083-h32 and 6061-t651 aluminum alloys, Materials Science and Engineering: A 478 (1-2) (2008) 56–64. [30] Y. E. Ma, Z. Zhao, B. Liu, W. Li, Mechanical properties and fatigue crack growth rates in friction stir welded nugget of 2198-t8 al–li alloy joints, Materials Science and Engineering: A 569 (2013) 41–47. [31] R. S. Mishra, Z. Ma, Friction stir welding and processing, Materials science and engineering: R: reports 50 (1-2) (2005) 1–78. [32] P. Sivaraj, D. Kanagarajan, V. Balasubramanian, Fatigue crack growth behaviour of friction stir welded aa7075t651 aluminium alloy joints, Transactions of nonferrous metals society of china 24 (8) (2014) 2459–2467. [33] H. Aydın, A. Bayram, I. Durgun, The effect of post-weld heat treatment on the mechanical properties of 2024-t4 friction stir-welded joints, Materials & Design (1980-2015) 31 (5) (2010) 2568–2577. [34] D. Muruganandam, D. Raguraman, L. Kumaraswamidhas, Effect of post-welding heat treatment on mechanical properties of butt fsw joints in high strength aluminium alloys. [35] M. M. Moradi, H. J. Aval, R. Jamaati, Effect of pre and post welding heat treatment in sic-fortified dissimilar aa6061-aa2024 fsw butt joint, Journal of Manufacturing Processes 30 (2017) 97–105. [36] M. Costa, C. Leit˜ao, D. Rodrigues, Influence of post-welding heat-treatment on the monotonic and fatigue strength of 6082-t6 friction stir lap welds, Journal of Materials Processing Technology 250 (2017) 289–296. [37] A. Sullivan, J. Robson, Microstructural properties of friction stir welded and post-weld heat-treated 7449 aluminium alloy thick plate, Materials Science and Engineering: A 478 (1-2) (2008) 351–360.

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[38] P. Kumar, K. Prashant, Elements of fracture mechanics, Tata McGraw-Hill Education, 2009.

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[39] P. Cavaliere, A. De Santis, F. Panella, A. Squillace, Effect of welding parameters on mechanical and microstructural properties of dissimilar aa6082–aa2024 joints produced by friction stir welding, Materials & Design 30 (3) (2009) 609–616. [40] Y. L. Zhao, Z. Q. Yang, Z. Zhang, G. Su, X. Ma, Double-peak age strengthening of cold-worked 2024 aluminum alloy, Acta Materialia 61 (5) (2013) 1624–1638.

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[41] Z. Hu, S. Yuan, X. Wang, G. Liu, Y. Huang, Effect of post-weld heat treatment on the microstructure and plastic deformation behavior of friction stir welded 2024, Materials & Design 32 (10) (2011) 5055–5060. [42] A. Dmitriev, E. Kolubaev, A. Nikonov, V. Rubtsov, S. Psakhie, Study patterns of microstructure formation during friction stir welding, in: Proceedings of XLII International Summer School–Conference Advanced Problems in Mechanics, St. Petersburg, Russia, 2014. [43] K. Krishnan, On the formation of onion rings in friction stir welds, Materials science and engineering: A 327 (2) (2002) 246–251. [44] A. Norman, I. Brough, P. B. Prangnell, High resolution ebsd analysis of the grain structure in an aa2024 friction stir weld, in: Materials Science Forum, Vol. 331, Trans Tech Publ, 2000, pp. 1713–1718. [45] Z. Zhang, P. Ge, G. Zhao, Numerical studies of post weld heat treatment on residual stresses in welded impeller, International Journal of Pressure Vessels and Piping 153 (2017) 1–14.

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Cu 4.31

Mg 1.38

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Table 1: Chemical composition (% by weight) of AA2024. Fe 0.31

Si 0.16

Mn 0.59

Cr 0.03

Zn 0.27

Al Remaining

Table 2: Mechanical properties of AA2024-T3 in as received condition. Direction Longitudinal Transverse

Tensile strength 450 MPa 412 MPa

Yield strength 343 MPa 248 MPa

% Elongation 23 26

Hardness 138 135

Table 3: Specification of FSW tool geometry.

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Shoulder Diameter (mm) Pin Diameter (mm) Pin Length (mm)

18.0 6.0 5.8

Table 4: Mechanical properties of FSW joint under various conditions. Tensile strength 412 MPa 309 MPa 388 MPa 382 MPa

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Specimen As-received As-weld PWHT at 190◦ C-10 hrs PWHT at 200◦ C-10 hrs

Yield strength 248 MPa 229 MPa 283 MPa 286 MPa

% Elongation 26 4.07 7.39 10.99

Fracture Location Nugget/TMAZ Nugget/TMAZ Nugget/TMAZ

Table 5: Paris law coefficients under various condition with their respective ∆Kth . Condition As-received As-weld PWHT at 190◦ C-10 hrs PWHT at 200◦ C-10 hrs

m 3.2289 2.9267 3.4993 3.4957

C 6.50E-11 2.34E-10 4.95E-11 4.69E-11

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√ ∆Kth (MPa m) 7.5 5.3 5.1 6

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Fig. 1: Schematic diagram of friction stir weld (FSW).

Advancing side

Retreating side 10 mm

Fig. 2: An illustration of FSW at 931rpm rotational speed and feed 70 mm/min.

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𝜙6

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(a) Tensile test specimen

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(b) Standard CT-specimen

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COD

CT-specimen

12 (c) Schematic arrangement of COD with CT-specimen

Fig. 3: Geometry of specimen (all dimensions are in mm).

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rientation of grain size after etching in as-received condition (a), Grain size distribution in as-received condition: (b)

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ge along rolling direction in as-received condition

Short Transverse

Precipitates

Grains

40μm

(a) Grain size distribution in as-received condition

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Fig. 4: Microstructural analysis of as-received aluminum(b) alloy.

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(a)

(b) SEM image along the rolled direction

Fig. 5: Micro-structure of the weld (a) cross-sectional plane, (b) heat-affected zone, (c) thermo-mechanically affected zone on retreating side, (d) nugget zone, (e) thermo-mechanically affected zone on advancing side side, in as-weld condition

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Grains

10μm

(a)

(b)

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Fig. 6: (a) SEM image of weld nugget zone, and (b) grain size distribution in nugget zone, in as-weld condition

Fig. 7: (a) Micro-structure, (b) distribution of precipitates and (c) grains in weld nugget zone after PWHT at 190◦ C-10 hours.

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Fig. 8: (a) Micro-structure, (b) distribution of precipitates and (c) grains in weld nugget zone after PWHT at 200◦ C-10 hours.

(a) As-welded without PWHT

(b) After PWHT at 200◦ C-10 hours

Fig. 9: Pole figure displaying crystallographic texture in weld cross-sectional plane. 15

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Fig. 10: EDS analysis of precipitates in as-weld (W 1) condition.

Fig. 11: Onion rings formation in the root of weld.

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4 0 0

3 0 0

2 0 0

A sA sP o s P o s

0 0 .0 0

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1 0 0

R e c ie v e d W e ld t w e ld h e a t tre a tm e n t_ 1 9 0 0C fo r 1 0 h o u rs t w e ld h e a t tre a tm e n t_ 2 0 0 0C fo r 1 0 h o u rs

0 .0 5

0 .1 0

0 .1 5

0 .2 0

0 .2 5

0 .3 0

P la s tic s tra in

(a) Plot of engineering stress vs the plastic strain for various con- (b) Fractured specimen during tensile tests (ASdition Advancing side, RS- Retreating side).

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Fig. 12: Tensile tests of the welded joints.

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(a) In-depth hardness variation for PWHT at 200◦ C-10 hrs

A s -w e ld P W H T a t 1 9 0 0_ 1 0 h o u rs P W H T a t 2 0 0 0_ 1 0 h o u rs

S tir Z o n e

V i c k e r 's H a r d n e s s " H M V "

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1 6 0

1 5 0

1 4 0

1 3 0

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1 2 0

1 1 0

A d v a n c in g S id e

-1 5

-1 0

R e tre a tin g S id e -5

D is ta n c e fro m

0

5

1 0

1 5

w e ld c e n tre lin e " m m "

(b) hardness distribution in various weld zones at depth of 2 mm

Fig. 13: Micro-Hardness distribution across the cross-section of welded joints.

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(a) ∆K-decreasing & increasing curve in as-received condition on CT-specimen

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(b) ∆K-decreasing & increasing curve in as-weld condition on CT-specimen

(c) da/dN vs ∆K curve for ∆K-decreasing & constant load-range for as-weld condition.

Fig. 14: Crack Propagation rate (da/dN) vs stress intensity factor range (∆K) for various conditions. 19

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Fig. 15: SEM image of fractured surfaces in : (a) as-received condition, and (b) as-weld condition.

Fig. 16: Crack growth behavior in welded joints for different heat-treatment condition.

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*Credit Author Statement

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CRediT author statement Vinay Kumar Yadav

Vidit Gaur

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Conceptualization, Methodology, Software, Investigation, Validation, Writing – Original Draft, Visualization

Conceptualization, Methodology, Software, Investigation, Validation, Writing – Original Draft, Visualization, Supervision, Project Administration, Writing – Review & Editing

I.V.Singh

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Conceptualization, Supervision, Validation, Funding Acquisition, Project Administration, Writing – Review & Editing

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Declaration of interests

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: