Influence of Siliconized Zn-Graphene Oxide Complex Nanoparticles on the Microstructure and Mechanical Properties of AA5083: Focus on Gas Metal Arc Welding

Aluminum alloy 5083 has low density, mechanical properties, high corrosion resistance, and high welding capability. Due to high thermal conductivity, specifc heat, and latent heat, as well as the relatively high coefcient of thermal expansion of aluminum, achieving a connection with good mechanical properties is always very important. Tis study aimed to investigate nanoparticles’ surprising and unexpected efects in forming various microstructures, which directly improve the mechanical properties of the welding process as a new and surprising idea. Te efects of siliconized Zn-graphene oxide complex nanoparticles on weld geometry, mechanical properties, and microstructure of AA5083 were investigated. Te gas metal arc welding process welded the samples, and diferent amounts of nanoparticles were investigated. Te results reveal that utilizing nanoparticles can be afected by the weld geometry and the properties of weld metal. 0.25 g of nanoparticles had low face reinforcement, and the bead width and penetration depth dramatically increased. Te presence of nanoparticles inside the molten zone and its efect on grain size improved mechanical properties, according to the Hall–Petch relationship. Te ultimate tensile strength and yield strength of the S 0.25 sample increased by 58.24 and 28.28%, respectively, compared to the welded sample without the presence of nanoparticles. Also, the ductility of this sample during the failure test showed that elongation increased by 36.75%.


Introduction
Aluminum alloys are used in many industrial felds because of their low density, high thermal and electrical conductivity, good corrosion resistance, and good workability [1][2][3][4][5][6][7][8].AA5083, one of the 5xxx series aluminum alloys, is used in shipbuilding due to its mechanical properties and high corrosion resistance.One of the essential features of aluminum 5083 is its high weldability so that it can be welded with most welding processes, such as electric arc [9,10].Although diferent methods are used in welding aluminum alloys, metal inert gas and tungsten inert gas are generally used to join them [10].Gas metal arc welding (GMAW) is a fusion welding process that uses a shielding gas to protect the weld pool [11].GMAW has a very high efciency compared to shielded metal arc welding, and it ofers many advantages, for instance, continuous electrode, high deposition rate, and high speed, as well as without slag.Furthermore, GMAW is one of the most practical processes for welding diferent types of metals and alloys [12][13][14][15], especially for AA5083 [16][17][18][19].Moreover, utilizing nanoparticles is an efective and awe-inspiring option to achieve higher mechanical properties, increased corrosion resistance, more proper microstructures, and more ductility in welding joints.Several studies have been conducted to investigate the mechanical properties and microstructures of the weld metal; for instance, Maurya et al. studied the efect of diferent types of carbonaceous particle reinforcement on the mechanical properties of Al6061.Tey also stated that samples with carbonaceous reinforcement had higher mechanical properties, and graphene had the best performance [20].In another study, Fattahi et al. used graphene/aluminum composite nanoparticles to investigate aluminum joints' mechanical and microstructure properties and reported increased tensile strength and weld microstructure [21].Also, in similar studies, the efect of nanoparticles has been investigated [22][23][24][25].Although the previous research conducted in the feld of welding with the presence of nanoparticles generally proves the usefulness of this method to achieve a better welding joint, it should be noted that the investigation of the use of diferent nanoparticles was proposed as a way to obtain a weld metal with desirable properties due to their inherent properties and compositions and the efects they can have on microstructure and mechanical properties.In this study, as a new and challenging idea, the nanoparticles of the siliconized Zn-graphene oxide complex (functionalized graphene oxide nanoparticles) were used to connect AA5083 plates with the MIG process.Also, in addition to investigating the efects of these nanoparticles on microstructure and mechanical properties, the impact of oxygen in the nanoparticles on the penetration depth of the weld metal was investigated.

Siliconized Zn-Graphene Oxide Complex Nanoparticles
Carbon nanostructures are uniquely positioned in nanoscience and technology due to their high elastic modulus and unique electrical, chemical, and mechanical properties.Tey are used in various felds, such as sensors, energy storage, and composites [26,27].Graphene oxide is a single-layer structure of graphite oxide with oxygenated functional groups such as carbonyl, hydroxyl, and epoxide.It also has a unique structure, high fexibility, and excellent physical and chemical stability [28][29][30].Te properties and chemical composition of graphene oxide nanopowder are shown in Table 1.Reactivity and interactions between nanoparticles and surroundings afect their application signifcantly.One of the unavoidable issues associated with nanoparticles is their inherent instability over long periods, so coating nanoparticles using organic and mineral molecules and modifying surface nanoparticles can increase the potential of using them in various felds and applications.Usually, electrostatic chemical adsorption (ligand addition) and covalent bonding (ligand substitution) are processes used to modify the surface of nanoparticles [31,32].Also, silanization is performed to achieve a more stable complex during the synthesis of nanoparticles (the cover containing hydroxyl groups is coated with silane molecules).Silanization creates a more stable complex by forming a stronger bond on the surface [33,34].In this study, the surface of nanoparticles was functionalized by Zn +2 cations.Also, due to the presence of oxygenated functional groups in graphene oxide, the APTES silane molecule was used for silanization (Figures 1 and 2).

Experimental
In the present study, the plates of AA5083 were welded by using the KARA TCK 600P MIG machine (Figure 3) to 20 × 10 × 6 mm thick.SFA/AWS A5.10: ER5183 as a welding wire with a diameter of 1.2 mm was used and developed to provide the highest strengths possible in the as-welded condition of alloy 5083 and other similar high magnesium alloys.Te chemical compositions and mechanical properties of the welding wire and AA5083 plate are reported in Table 2. Pure argon was used as a shielding gas, and the fow rate was kept to 20 liters per minute.Direct current with the electrode positive (DCEP) was used for electrical cleaning, higher deposition rate, and breaking of oxide layers [13].Several experiments were performed to achieve optimal values of the parameters (current was selected at 261.5 amps, arc voltage was set at 24.2 volts, travel speed was determined at 36.2 cm/min, and the deposition rate was established at 73.4 g/min).Nanoparticles scattered when the shielding gas was used, creating a longitudinal groove on the connecting edges to overcome this challenge (Figure 4, the distance from the top edges was 1 mm), and diferent amounts of nanoparticles (0.25, 0.50, and 0.75 g) were placed inside the groove.

Results and Discussion
4.1.Welding Geometry.Te samples were welded, and Figure 5 shows the surface of welded, the weld crosssectional area, and the microstructures of the samples.As the results show, in sample S 0.75 , welded with 0.75 g of nanoparticles, the geometry of the weld containing penetration depth, bead width, and weld height was 7.4 mm, 3 mm, and 9 mm, respectively.Te weld cross-sectional area had inclusion in addition to a lack of fusion in the weld joint.Many spatters accompanied the arc, and a few nanoparticles were agglomerated in the boiling pool, which was observed as cavities in the cross section of the weld metal.Also, there were hot cracks in the cross section of the weld.Te S 0.50 sample that contained 0.5 g of nanoparticles had less spatter, and the surface of the weld was much better than that of S 0.75 .Te weld cross-sectional area in S 0.50 was similar to that in S 0.75 , but its inclusions were smaller.Te geometry of the weld contains penetration depth, bead width, and weld height which were 7.8 mm, 1.9 mm, and 11 mm, respectively.In the S 0.25 sample containing 0.25 g of nanoparticles, the weld had a smooth surface without cutof and inclusion, and the weld's height was lower than in the previous samples.Te weld pool in this sample was more comprehensive, but the penetration depth was much higher.Te weld's penetration depth, bead width, and height were 8.9 mm, 12 mm,    Advances in Materials Science and Engineering and 1.5 mm, respectively.While in the sample without nanoparticles, the value of these parameters was 4.5 mm, 10 mm, and 3 mm, respectively.S 0.25 was something more than the other samples, and the thing that makes this sample unique is its penetration depth; another thing was to be the dramatically smooth surface of the weld.Also, the VT visual inspection and NDT inspections confrmed that this sample was much better.Terefore, S 0.25 was considered a basis for comparison in subsequent experiments.Figure 5 shows the surface of welded, the weld cross-sectional area, and the microstructures of the samples.Spatter is a factor that can cause cavities, insufcient penetration, and disruption in welding cycles.Te spattering of large particles usually occurs due to the low intensity of the current compared to the diameter of the welding wire, or the length of the arc is very high (very high voltage), which causes droplets not to be transferred in a straight axis.Tis type of spraying is prevalent in welding and causes turbulence in the shielding gas fow.But because welding parameters with the same values were used to apply the same conditions during the welding of samples with diferent amounts of nanoparticles, it can be concluded that the cause of less sputtering and also the smooth welding surface in S 0.25 with 0.25 g of nanoparticles are related to the absorption of incoming heat by nanoparticles and its transfer to the bottom area of the weld pool.As a result, the temperature of the weld pool is reduced, and spraying is also reduced.Tis heat transfer process has not been completed entirely in samples with 0.50 g and 0.75 g of nanoparticles.

Microstructural Characterization.
Te microstructure of specimens is shown in Figure 6.A microstructural analysis of AA5083 shows that the matrix of this alloy is a fnegrained structure consisting of an alpha-solid solution phase, and particles of solid magnesium solution are distributed in the aluminum matrix.Te aluminum matrix includes three types of sediments: silicon-magnesium with the composition of Mg 2 Si, magnesium-rich sediments with Mg 2 Al 3 composition, and iron-rich deposits with Al 6 (Fe-Mn) composition; also, in addition to Fe, Mn, and Al, some Cr can be possibly to found in it [35].To etch the samples, HBF 4 solution was used [36].After etching, light microscope images were taken.Te matrix contains two Al-Al and Al-Mg interfaces, where the Al-Mg interfaces are observed as holes (Figure 6).
Te FESEM images, elemental analysis, and atomic analysis of specimens are shown in Figure 7. Magnesium tends to develop on grain boundaries; it accumulates in the prone places around the cores of nanoparticles.Te tiny dimensions of nanoparticles in large numbers have played the role of nucleation, and it caused to spread   Advances in Materials Science and Engineering magnesium throughout the weld metal by heterogeneous nucleation.
Figure 8 shows the FESEM images, elemental analysis, and atomic analysis for three points of the base metal (as reference).It contained Mg as the main element, Fe, Mn, and Ti.Also, Figure 8 shows the FESEM images and elemental and atomic analyses of the weld metal in the S 0.00 sample.Te S 0.00 sample contained diferent amounts of Al, Mg, Mn, Fe, and Ti (compounds of the base metal).
Figure 9 shows the elemental and atomic analysis of the points determined in the S 0.25 sample in two areas of the weld metal.Te results showed that Al, Mg, Mn, and Fe related to the base metal contained diferent amounts of C, O, and Zn.Graphene is a two-dimensional single/multilayer of carbon atoms with a compact hexagonal, carbon-flled surface structure [37].So, the nanoparticles in this study can be a source of C, O, and Zn.Also, these elements inside the weld metal confrmed the success of adding nanoparticles to the weld metal.
All samples were welded under the same condition.Sample S 0.00 did not report any weight percentage of oxygen.Terefore, the amount of oxygen shown in the EDS analysis of sample S 0.25 is related to the nanoparticle (graphene oxide) that decomposed and released due to high heat.Oxygen in S 0.25 can cause a concentrated arc and reverse Marangoni fow inside the weld pool [38,39].Full-bodied turns and the change of reverse Marangoni fow lead to more heat, which can signifcantly afect the microstructure of molten and heat-afected areas.As microscopic images of diferent regions of welded samples were shown (Figures 10  and 11), grains in sample S 0.00 were elongated in the heatafected zone.In contrast, in sample S 0.25 , grains were deformed almost spherical and axially.It can be considered that the reversal of the Marangoni fow and the presence of more heat inside the weld pool have caused a more signifcant thermal efect in the region's heat-afected zone and spheroidization of the grains.Furthermore, the penetration depth in S 0.25 is greater than S 0.00 , confrming the presence of the active oxygen element and the change of the Marangoni fow into the weld pool (Figures 10 and 11).Te presence of nanoparticles in the molten zone and the mechanism of heterogeneous nucleation and grain growth have caused sample S 0.25 to have more refned grains than sample S 0.00 .Also, due to the high cooling rate and heterogeneous heat of the GMAW process, grains can change from columnar to elongated, and of course, 0.15% of Ti in electrode compounds can afect grain refnement [40,41].

Mechanical Properties and Fractography Analysis.
In this study, the tensile test was used to investigate the mechanical properties of samples.Also, its results were used to determine the behavior of the material, such as the range of elastic and plastic, elongation, ultimate tensile strength (UTS), and yield strength (YS) in samples [42].According to the ASTM-E8-subsize standard, samples were tested by using a SANTAM STM-600 traction machine under the ISO/IEC170258M standard at 22 °C and 27% humidity (capacity was 10 tons, and speed was 1 mm/min).Te results showed that the S 0.25 sample broke under 8.68 KN of force in the weld zone (the weld cross-sectional area was 34.85 mm 2 ), and the S 0.00 sample, which did not use nanoparticles, broke under a force of 3.79 KN. Figure 12 shows the stress-strain diagram, and Figure 13 shows the values of UTS and YS in MPa.Te UTS and YS in the S 0.25 sample were 58.84% and 28.24%, respectively, which were higher than those of the S 0.00 sample.Since the tensile and yield strength represent the material's strength, the weld strength in S 0.25 has increased compared to the weld sample without nanoparticles.Te UTS and YS in the base metal (no thermal efects and no sediments at the grain boundaries) were 295.61MPa and 218.94 MPa, respectively.Defects such as porosity in the weld metal can afect the fnal strength of the samples.Also, local changes in temperature cause changes in the rate of dissolution and redeposition (especially β-shaped needle deposits, which are one of the most infuential factors in strengthening aluminum alloys).Generally, four mechanisms of strain hardening, solute hardening, precipitation hardening, and grain size hardening are used to strengthen aluminum alloys.Terefore, the presence of the Mg element with the solute-hardening mechanism can cause a high strength of AA5083 [8,43,44].Te results of metallography and SEM images of the cross section of the sample welded with 0.25 g of nanoparticles (S 0.25 ) and comparing it with the sample welded without nanoparticles (S 0.00 ) showed that the presence of nanoparticles in the molten zone and the mechanism of heterogeneous nucleation and grain growth as well as creating suitable places for magnesium deposition have caused magnesium to be distributed on the entire surface of the weld metal.So the presence of nanoparticles inside the molten zone and its efect on grain size improved mechanical properties, according to the Hall-Petch relationship [45].Te elongation parameters and cross-sectional area show the ductility of materials, so the elongation rate was measured to compare the ductility of samples during the failure test.Te relative elongation rate of S 0.25 (with a thickness and width of 6.01 × 5.80 mm 2 ) was 16%, and also, the relative elongation rate of S 0.00 (with a thickness and width of 6.03 × 5.57 mm 2 ) was 6.5%.A comparison of the results showed that ductility in S 0.25 is higher, and the elongation rate in this sample was 36.75% and higher than in S 0.00 .Generally, the decrease in the percentage of elongation in S 0.00 can occur due to a wide variety of reasons, such as porosity and hot cracks.Also, the reduced load-bearing surface will result in premature failure.In 6 Advances in Materials Science and Engineering welding of AA5083 by the GMAW process, due to the soft nature of the aluminum alloy, the fracture surface in the weld metal has dimples with smooth oval edges, and the size and depth of the holes created in the fracture section indicate the degree of fracture softness [16].Figure 14 shows the scanning electron microscopy images and the weight percentage of the elements present on the broken surface of the S 0.25 and S 0.00 samples.Examination and comparison of the amplitude on the fracture surface showed that in S 0.25 , dimples indicate the ductile fracture mechanism.At the same time, S 0.00 had more profound dimple growth, and the failure surface mainly consisted of interconnected cavities.Advances in Materials Science and Engineering

Conclusions
In this study, sheets of AA5083 with 6 mm thickness were welded by gas metal arc welding under the protection of argon gas (MIG process).Te efect of functionalized graphene oxide the weld geometry, microstructure, and mechanical properties of AA5083.Te visible results were as follows: (i) Utilizing nanoparticles signifcantly afected the weld geometry, especially the penetration depth.Face reinforcement was low in the S 0.25 sample containing 0.25 g of nanoparticles, and the weld pool in this sample was vaster.Te penetration depth was much higher than in the previous samples.(ii) Te oxygen in nanoparticles inverted the direction of Marangoni fow in the welding pool.In addition, the presence of nanoparticles has caused the arc's focus, so these mechanisms led to an increased penetration depth in the S 0.25 sample.(iii) Nanoparticles as centers of growth, as well as creating suitable places for magnesium deposition, caused magnesium to spread over the entire surface of the weld metal.So, according to the Hall-Petch relationship, grain refnement improved mechanical properties.Te tensile test showed that the ultimate tensile strength and yield strength increased by 28.24% and 58.24%, respectively, compared to the welded sample without nanoparticles.(iv) A comparison of the ductility of samples during the failure test showed that the rate of elongation in the S 0.25 sample was 36.75% and higher than in the S 0.00 sample.

Figure 2 :
Figure 2: Comparison of XRD spectra of graphene oxide and nanoparticles after functionalization with Zn +2 and FESEM image of nanoparticles.

Figure 3 :
Figure 3: Experimental setup of the MIG welding process.

Figure 4 :Figure 5 :
Figure 4: Workpiece sample prepared for the welding process and groove geometry.

Figure 7 :
Figure 7: SEM images of welded samples for S 0.25 and S 0.00 .

Figure 8 :
Figure 8: FESEM images, weight percent, and atomic percentage at specifed points in the base metal (AA 5083) and welded sample without nanoparticles (S 0.00 ).

Figure 9 :
Figure 9: FESEM images, weight percent, and atomic percentage at specifed points in the welded sample with 2.5 g of nanoparticles (S 0.25 ), areas A and B.

Figure 10 :
Figure 10: Microstructure of the welded sample with 0.0 g of nanoparticles (S 0.00 ).

Fusion zone-area 2 Figure 13 :Figure 12 :
Figure 13: Result of mechanical properties for samples.

Figure 14 :
Figure 14: Comparison between the failure weld cross-sectional area in samples S 0.25 and S 0.00 .

Table 2 :
Chemical composition (wt%) and mechanical properties of the base matal and welding wire.
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