Influence of SiC Microparticles and Multi-Pass FSW on Weld Quality of the AA6082 and AA5083 Dissimilar Joints

In the current research, the influence of multi-pass of friction stir welding and SiC microparticles on the tensile strength, microhardness, and %strain of dissimilar reinforced joints of AA6082 and AA5083 was examined. A tool rotating speed of 900 rpm, a transverse speed of 45 mm/min, a tool tilt angle of 2º and 8% volume percentage of SiC microparticles were considered as parameters for multi-pass FSW. The finding demonstrated that increasing the number of FSW passes from one to three augmented the dispersion pattern of SiC microparticles. The grain refinement of multi-pass reinforced joints was achieved by the pinning effect of SiC microparticles and dynamic recrystallization. The three-passes FSWed reinforced joint exhibited the highest tensile strength (247.17 MPa), % strain (13.1%), and microhardness (126.6 HV) due to the higher grain refinement.


Introduction
The fabrication of metal matrix composites employing reinforcing particles (RPs) is a creative technique for augmenting the mechanical characteristics (stiffness, hardness, strength, etc.), corrosion, and wear resistance [1][2][3].The distributed reinforcing particles, via the "pinning effect," inhibit the grain boundaries from expanding during dynamic recrystallization, resulting in equiaxed tiny grains that ameliorate the tribological and mechanical characteristics of welded or processed materials [4][5][6].To enhance tribological and mechanical characteristics, micro/nanoparticles-based aluminum metal matrix (AMM) composites were fabricated through FSP to produce surface and bulk composites [7][8][9].The transformation from traditional materials to lightweight materials is recently developing in almost all industries for fabricating high-strength and lightweight structural components.Aluminum alloys offer exceptional mechanical characteristics like higher strength, light weight, and good corrosion resistance.Therefore, aluminum alloys have attracted great interest for structural applications in almost all industrial domains, including aerospace, marine, and the automobile.Fusion welding methods are inappropriate for welding low melting point materials due to their limitations of high residual stresses, surface cracks, distortion, and porosity [10][11][12].Thus, friction stir welding (FSW) is solidstate in nature and seems to be the most effective method for joining dissimilar aluminum alloys.
Several researchers also employed FSW to produce aluminum metal matrix composites (AMMCs), utilizing various micro-and nano-sized RPs to ameliorate the weld properties of related aluminum alloys.Recent studies have examined the viability of fabricating AMMCs with different reinforcing particles, including AA2124 with SiC particles, AA7005 with Al 2 O 3 particles, and AA2009 with SiC particles [13][14][15].The incorporation of micro/nano particles into the stir zone (SZ) helped the researchers improve the weld properties of numerous metal alloys.Compared to the FSWed samples without SiC particles, improved weld qualities of dissimilar AA7075/AA2024 were produced with different volume percentages (5,8, and 13%) of nano-sized SiC particles.It was found that 5% SiC nanoparticle inclusion provided the best mechanical properties [16].The reinforcement particles of Al 2 O 3 outperformed SiC reinforcement particles and a hybrid of the two in the FSW of the dissimilar AA6101 and AA1350 in terms of mechanical and wear characteristics [17].Incorporation of SiC particles in the weld zone enhanced the tensile strength and microhardness of FSWed joints of AA6061-T6.Higher tensile strength and microhardness were observed with an increasing volume fraction of RPs and number of passes of FSW [18].A higher improvement in the tensile strength, microhardness, and wear characteristics of FSWed joint AA6061 was found compared to the sample without nanoparticles under the same parametric conditions due to the addition of nanosized RPs in the SZ [19].Improvement in hardness for Al 2 O 3 and SiC mixed AA7075 welded samples was observed due to the higher thermal conductivities of the added RPs and the precipitation strengthening mechanism.Whereas intermetallic compound formation was attributed for the decline in tensile strength [20].
The strain-hardenable AA5083 has exceptional corrosion resistance.Additionally, it has superb weldability [18].Consequently, AA5083 is often used not only in the aerospace and automotive industries but also in shipbuilding and armored vehicles [19,20].AA6082 is also frequently utilized in a wide range of automotive and aerospace applications.AA6082 provides an impressive combination of corrosion resistance, strength, and surface characteristics [21].The dissimilar welding of AA6082 and AA5083 has drawn the considerable interest of almost all industrial sectors due to their broad applications and superior weldability.The motivation behind this study is to improve the weld quality of these alloys, which will be tremendously beneficial in a number of industrial domains.Therefore, in the present research, the consequences of multi-pass FSW and SiC microparticles (SiCp) on the weld quality of AA5083 and AA6082 dissimilar joints were investigated.The consequences of multi-pass FSW and SiC particles on weld quality of AA5083 and AA6082 dissimilar joint has not yet been studied, to the author's knowledge.

Material and Experimentation
The materials utilized in this study to develop dissimilar FSWed joints were: AA5083 and AA6082 with dimensions 150 × 40x6 mm as the base and the SiC particles (SiCp) ranging from 50 to 80 µm as reinforcing material.A friction stir welding machine equipped with position controls was employed for multi-pass FSW.The Adjacent surfaces of the base plates were milled to incorporate the SiC powder.To include 8% SiCp, the depth and width of the milled slot were taken as 3 mm and 0.48 mm, respectively as depicted in Fig. 1a.The SiCp was mixed with ethanol, and a thick slurry was made.The milled slots were cleaned with acetone before filling the SiCp slurry.The slurry of SiCp was filled manually in the milled slots.A detailed and systematic explanation of composite development utilizing the groove approach was previously published [22,23].The multi-pass FSW was performed at constant rotating speed, transverse speed, tool tilt angle, and volume fraction of SiCp of 900 rpm, 40 mm/ min, 2 , and 8%, respectively.
The maximum UTS was observed with an increase in TRS and TS, reaches a maximum value then starts decreasing with further increases in TRS and TS.The welded joint at 900 rpm with TS of 45 mm/min [24] has superior UTS.As the TRS increases, the input heat also increases, which in turn results in more mixing and intensive stirring of the base metal.But a further increase in the TRS at 900 rpm, results in a slower cooling and higher temperature in the SZ, which results in microvoids in the lower portion of the SZ thus decreasing the UTS.In this work, the FSW parameters condition and volume fraction of SiCp were chosen based on preliminary trials to obtain defect-free FSWed-reinforced joints.Prior to multi-pass FSW, a pin-less tool with a shoulder diameter of 19.5 mm was employed (Fig. 1b) to close the groove from the top at a constant rotating speed and transverse speed of 900 rpm and 40 mm/min, respectively.The parametric setting of FSW and volume percentage of SiCp were chosen based on machine capability and trail runs.The profiles of the tool's pin significantly impact the flow of material during FSW.The cylindrical threaded-pin tool revealed defect-free FSWed joints with better material flow than the cylindrical pin tool and cylindrical tapered-pin tool [25].Therefore, in the current research, multi-pass FSW was performed utilizing a cylindrical threaded-pin tool made of H13 tool steel.The anticlockwise direction of tool rotation and the same welding direction were employed in each FSW pass.Figure 1b illustrates the dimensional characteristics and pictorial view of the FSW tool.In the case of welding dissimilar materials, the positioning of materials is an essential consideration for defect-free dissimilar joints.Therefore, in the current research, the plates of AA6082 were situated on the advancing side, as suggested by previous studies [26,27].Table 1 illustrates the parametric setting of multi-pass FSW and the reinforcing condition of SiCp.The defect-free FSWed joints after multi-pass FSW are depicted in Fig. 2.
The test sample extraction plan for various tests is cited in Fig. 3.The tensile test samples were fabricated as per the ASTM E8 standard [28].A universal testing machine (Model: UT-04-0100, BISS Pvt. Ltd., India) of a capacity of 100 kN was employed to perform tensile tests.Microhardness indentations were performed under a 100 g load and separated by a 1 mm distance utilizing a Micro-Vickers hardness tester (Model: XHVT1000Z/V, Jinan Victory Inst.Co. Ltd.).To analyze the consequences of SiC particles and multi-pass FSW, samples were crosssectioned and prepared for microstructural evolution.The etching of prepared samples was performed with Keller reagent.Optical microscopy (OM) and field-emissionscanning-electron-microscopy (FESEM) were utilized to evaluate the microstructural behavior of the SZ.OM was performed utilizing a metallurgical inverted microscope (Model: Suxma-met, Conation Technologies), while FESEM analysis was performed using a FESEM (Model: JSM-IT100, Jeol) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (Model: 51-ADD0076, Oxford Instrument).

Results and Discussion
Figure 4 reveals a different zone in the FSW, including the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ).FSW tool induces a heating and complicated plastic deformation in the surrounding material, depending on the materials joining and conditions.The grain size at the SZ was observed to be very fine compared to the TMAZ and HAZ [28][29][30].The interface between the TMAZ and HAZ was the softest zone where the cracks initiate and lead to the fracture of the specimens.The grain structure, FESEM photo-image, and elemental composition of AA5083 and AA6082 obtained using EDS are cited in Fig. 5 (a-d).The mean grain size of AA5083 and AA6082 was found to be ~ 35 and ~ 39 μm, respectively.The grain size was analyzed by ImageJ software.The FESEM photo-image and elemental composition obtained using EDS of SiCp are cited in Fig. 5(e, f).

Microstructural characterization
To examine the influences of multi-pass FSW and SiC particles on the weld quality of AA6082 and AA5083 dissimilar joints, an investigation associated with the distribution of SiCp and grain structure of SZ was carried out.Figure 6 depicts the FESEM photo-image of the SZ of one-pass FSWed unreinforced joint and one, two, and three passes FSWed reinforced joints.The defect-free joining of AA6082 and AA5083 with the proper intermixing was observed in the one-pass FSWed unreinforced joint, as depicted in Fig. 6a. Figure 6 (b-d) depicts the FESEM photo-image of the SZ of one, two, and three-passes FSWed reinforced joints and confirms the consequences of the increasing FSW passes on the dispersion pattern of particles.The clustering of SiCp along with poor bonding with AMM was noticed in the SZ of the one-pass FSWed reinforced joint as delineated in Fig. 6b.This demonstrates that one FSW pass was not producing sufficient strain to prevent clustering of SiCp [31].The clustering of SiCp was found to be absent as the FSW passes enhanced from one to two, but the dispersion pattern of SiCp was observed as nonuniform in the two-passes FSWed reinforced joint, as cited in Fig. 6c [32].The augmented dispersion pattern of SiCp may be attributed to the repeated strain developed by the tool's stirring action, resulting in disjoining of clustering of SiCp as the FSW passes enhanced from one to two.Further improved dispersion of SiCp was found after implementing three passes of FSW.Therefore, the uniform dispersion of SiCp was observed after implementing three FSW passes, as cited in Fig. 6d.The absence of SiCp clustering revealed the excellent bonding between SiCp and the surrounding AMM [33,34].The uniformity of SiCp in the SZ of three-passes FSWed reinforced joints was confirmed by the EDS mapping analysis.It is evident from the Si elemental mapping (Fig. 6e) that the dispersion pattern of SiCp is uniform after three passes of FSW.Referring to Fig. 7 (a-c), The EDS analysis of FSWed unreinforced joints and reinforced joints after one and three FSW passes also confirms the incorporation of SiCp in FSWed reinforced joints.Considering the EDS analysis finding, the silicon percentage in the one-pass FSWed unreinforced joint was 0.56%.In contrast, the silicon percentage in the one-pass and three-passes FSWed reinforced joints was 6.98% and 7.66%, respectively.The carbon content in the one-pass and three-passes reinforced FSWed joints was 2.02% and 2.83%, respectively.
Figure 8 depicts the grain structure of the SZ of FSWed unreinforced joint and various reinforced joints.The stretched grain structure of AA6082 and AA5083 transformed into equiaxed and fined grains due to DRX [35].The grain size of the SZ is mainly influenced by the temperature effect, DRX, and RPs in the AMM.The dominating factors among these three phenomena determine the grain size.The temperature effect arises due to increased heat input at lower transverse speeds and higher rotating speeds, which induce grain growth and coarsening of grains [36].DRX occurs at high temperature owing to severe plastic deformation resulting in the conversion of the low-angle boundaries to the high-angle boundaries and nucleates the new grains at developmental regions, which diminishes the grain size.The RPs in the SZ serve as grain boundary barriers, inhibiting grain development via the Zener-pinning effect [36].Consequently, at constant rotating speed (900 rpm) and transverse speed (45 mm/min), the phenomena of DRX and the influence of SiCp dominated in diminishing the grain size.Whereas, due to the absence of SiCp in FSWed unreinforced joint, only the DRX phenomenon dominated in diminishing the grain size.Thus, higher grain refinement was achieved in the three-passes FSW reinforced joints due to the DRX and the effect of uniformly distributed SiCp [37].
The strength of the composite increases with the incorporation of RPs as represented by Chen et al. [38].The strengthening mechanism by Orowan-Ashby demonstrated the yield strength with respect to volume percentage of the RPs as expressed in Eq. ( 1) [39].
where G m represents the "matrix material's shear modulus", b represents the "burgers vector", r represents the "radius of RPs", and λ represents the "mean interparticle spacing".The where v p represents the "volume percentage of the RPs" and d p represents the "diameter of the RPs".The Eqs. ( 1) and ( 2) revealed that yield strength is directly proportional to the volume percentage of the RPs.Moreover, it is well known that the inclusion of RPs reduces grain size due to the pinning effect.Grain size reduction improves the joint's mechanical properties.
The coarse grain structure of AA6082 and AA5083 exhibits a mean grain size of 39 and 35 μm, respectively.An abrupt reduction in the grain size was observed after implementing one-pass FSW owing to DRX [40].Therefore, a mean grain size of 19.4 μm (Fig. 8a) was found in the onepass FSWed unreinforced joint, as delineated in Fig. 8a.The grain size of the one-pass FSWed reinforced joint was found to be smaller (12.6 μm) than that of the one-pass FSWed unreinforced joint, as cited in Figs.8a and b.This may be ascribed to the presence of SiCp in the SZ, as the same rotating speed and transverse speed were used for unreinforced and reinforced joints.As the FSW passes proceeded from one to three, it was observed that the grain size of SZ was further diminished.The further reduction in the grain size may be ascribed to the dispersion pattern of SiCp [41].The more uniformly distributed RPs decreased the grain size by providing more obstacles to grain boundaries and restricting grain growth by the pinning effect [42].When dislocations begin to accumulate during plastic deformation owing to RPs, the particles-induced nucleation-based DRX is feasible [43,44].The presence of more uniformly dispersed SiCp enhances the pinning effect, which results in higher grain refinement.Consequently, the grain sizes of the two-passes and three-passes FSWed reinforced joints were found to be 7.2 (Fig. 8c) and 3.4 µm (Fig. 8d), respectively.Thus, the grain structure of multi-pass FSWed reinforced joints evident that equiaxed and fine grains were developed owing to the dominance of DRX and the pinning effect of uniformly dispersed SiCp [44]. Figure 9a-b reveals the dark and bright field high magnification TEM images of single pass and 3rd pass FSW from the individual dispersed SiC reinforcement particles and precipitates with the base metal.Some coarse Mg-rich precipitates were observed beside the SiC reinforcement particles.As the number of passes increases, the uniform dispersion of SiC particles and 2nd phase precipitates occur within the grain boundaries.It was perceived that the grain growth was restrained by the SiC particles and 2nd phase particles due to the pinning effect resulting in finer grain size and grain refinement [45,46].The dislocation precipitations around the SiC particles occurred due to incompatibilities in the coefficient of thermal expansion between the aluminum and SiC particles.Many sub-grains were found revealing the existence of dynamic recovery [47].According to the precipitation mechanism and dislocation strengthening, the movement of dislocation in the grains occurs through large plastic deformation.The dislocation passes the precipitation phases when the precipitation phase is improved.At small precipitates, the dislocation wounds off the precipitates and increase the interface area which leads to enhance in the interface energy [48].The interface structure plays a significant role in determining the overall mechanical and physical properties of an AMM.The processed zone with particles depends on the bonding quality at the reinforcement matrix interface and fracture mechanism.

Influence on tensile properties
In Fig. 10, the average tensile characteristics of the base materials and the various FSWed joints were presented.In the case of unreinforced joints, grain size is the key variable influencing the mechanical characteristics of the FSWed joint.However, the size, quantity, dispersion pattern of RPs, and bonding strength between RPs and AMM Fig. 9 TEM images, a single pass FSP with SiC, b three pass FSP with SiC also influence the mechanical characteristics of the FSWed reinforced joints [49].It can be seen from Fig. 10 that the tensile strength of the FSWed unreinforced and reinforced joints is less than that of the base materials.In contrast, the one-pass FSWed reinforced joint exhibits higher tensile strength than that of the unreinforced joint.This is ascribed to the existence of SiCp, which inhibits dislocation boundary movement and prevents grain development, resulting in reduced grain size [50].The bonding between AMM and RPs, grain size, and dislocation density all significantly influenced the tensile strength of the FSWed reinforced joint [51].The improved dispersion of SiCp was achieved by the increase in FSW passes.More uniform dispersion of RPs developed more barriers to grain development and further diminished grain size [16].The tensile strength is inversely correlated with grain size, as stated by the Hall-Petch equation [52].The grain size of FSWed reinforced joints was observed to be smaller than that of the base materials due to the pinning effect of SiCp and DRX [53].Consequently, the three-passes FSWed reinforced joint exhibited a higher tensile strength of 247.17 MPa.Whereas, the one-pass FSWed unreinforced joint exhibited a minimum tensile strength of 206.83 MPa.These observations are consistent with those of Jamalian et al. [54].The %strain of one-pass FSWed unreinforced joint and reinforced joint was found to be higher than AA6082 but lower than AA5083.The %strain of the one-pass FSWed reinforced joint was observed lower than that of unreinforced joints and both base alloys.But the %strain of two-and three-passes FSWed reinforced joints was 12.2% and 13.1%, respectively, which is higher than the base materials.
Figure 11(a-d) depicts the fractography of the fractured tensile samples of various FSWed joints.The fracture occurred towards the AA6082 side at HAZ, where the hardness value was observed as minimal [55].The dimple fracture, symptomatic of the ductile approach to failure, was noticed in all tensile samples.Under the tensile stress, the test samples' edges formed a shear plane with a cup-cone shape.In the FSWed reinforced joints, the ductile fracture with honeycomb dimples was identified (Fig. 11).The smaller dimple size was observed in the one-pass FSWed reinforced joint (Fig. 11b) compared to the one-pass FSWed unreinforced joint.From Fig. 11 (c, d), it can be noticed that the dimple size was further decreased as the FSW passes were enhanced from one to three.The diminished grain size was found in the three-passes FSWed reinforced joint via the pinning effect of uniformly dispersed SiCp.The fractography of the raptured surface (Fig. 11d) also revealed fine and equiaxed dimples developed by the micro-voids' coalescence [56,57].The ruptured zone was found at the HAZ toward AA6082, confirmed by hardness variation, which led to augmented %strain.Thus, it can be concluded that the embedded SiCp ameliorated the tensile characteristics of the FSWed joints.This may be correlated to the enhanced grain refinement by DRX and the pinning effect of uniformly dispersed SiCp and excellent bonding between the SiCp and the AMM, resulting in higher resistance to fracture [58].

Influence on microhardness
The microhardness distribution in the weld zone of several FSWed joints of AA5083 and AA6082 was evaluated using a Vickers microhardness tester.The asymmetrical microhardness variation was noted in the weld zone of FSWed joints due to the irregular plastic flow on the retreating and advancing sides [59].The mean microhardness of the base materials (AA6082 and AA5083) and at the center of the weld zone of FSWed joints is cited in Fig. 12a.The one-pass FSWed unreinforced joint exhibited a microhardness of 93.2 HV, which is lower than that of AA5083 but higher than AA6082.According to the microhardness findings, the one-pass FSWed unreinforced joint exhibits the lowest hardness compared to the multi-pass FSWed reinforced joints.The microhardness was enhanced by incorporating SiCp and implementing one, two, and three passes of FSW.Therefore, the microhardness of one, two, and three passes of FSWed joints was observed to be 105.8,113.1, and 126.6.4HV, respectively.The threepasses FSWed reinforced joint has the maximum microhardness of 126.6 HV among all the FSWed joints.The distribution of microhardness in the weld zone of the various FSWed joints is cited in Fig. 12b.The SZ indicates the higher microhardness owing to the smaller grain size caused by the pinning effect of SiCp and DRX compared to the TMAZ and HAZ.Lower hardness in the HAZ was observed to be caused by coarsening of strengthening precipitates, dissolution of existing clusters, and formation and coarsening of the stable phase.The processes of precipitation transformations occurring within the HAZ cannot be explicitly formulated due to susceptibility to many factors [60,61].In contrast, the microhardness in the HAZ was noticed less owing to coarsening of grains at high temperature and over-aging.The microhardness of the FSWed reinforced joints was also augmented owing to the hard nature of SiCp.The three-passes FSWed reinforced joint revealed the highest microhardness due to better grain refinement than other FSWed joints.

Conclusions
In the current work, dissimilar FSWed reinforced joints of AA5083 and AA6082 were developed using 8% volume fraction of SiCp.The impact of SiC microparticles along with multi-pass FSW on the weld quality of AA5083 and AA6082 dissimilar FSWed joints was investigated.The following are the main findings from the obtained results: • In one-pass FSWed unreinforced joint, only DRX was involved in the grain refining.In contrast, the improved grain refinement in the multi-pass FSWed reinforced joints was accomplished by the pinning effect of SiCp and DRX.• The improved dispersion of SiCp was found as the FSW passes enhanced from one to three.The uniformly dispersed SiCp was observed in the three-passes FSWed reinforced joint.• The multi-pass FSWed reinforced joints revealed a higher tensile strength than the unreinforced joint.Furthermore, the three-passes FSWed reinforced joint exhibited a higher tensile strength of 247.17 MPa.
• The %strain of one-pass FSWed unreinforced joint and reinforced joint was found to be higher than AA6082 but lower than AA5083.The %strain of one-pass FSWed reinforced joint was found to be lower than that of unreinforced joints and both base alloys.But the %strain of two and three-passes FSWed reinforced 2, and 21.4%, respectively, which is higher than that of both the base materials.• The one-pass FSWed unreinforced joint exhibited a microhardness of 93.2 HV, which is lower than that of AA5083 but higher than AA5083.The microhardness was enhanced by incorporating SiCp and implementing one, two, and three passes of Therefore, the microhardness of one, two, and three passes of FSWed joints was observed to be 105.8,113.1, and 129.6 HV, respectively.

Fig. 1
Fig. 1 (a) Steps of multi-pass FSWed reinforced joints; (b) Dimensions of FSW tools used

Fig. 5 a
Fig. 5 a-d Grain structure, FESEM photo-image and EDS elemental configuration of base materials; a, b AA6082, c, d AA5083; e FESEM photoimage of SiCp, f EDS elemental configuration of SiCp

Fig. 6 aFig. 7 a
Fig. 6 a FESEM photo-images of one-pass FSWed unreinforced joint; (b-d) FESEM photo-images of FSWed reinforced joint with; b one-pass, c Two-passes, d Three-passes; e Elemental (EDS) mapping of three-passes FSWed reinforced Joint

Fig. 12 a
Fig. 12 a-b Microhardness of various reinforced FSWed joints (a) Mean microhardness at the weld center; b Microhardness distribution in the welded zone

Table 1
Experimental condition of various FSWed joints Fig. 2 Various FSWed plates of AA5083 and AA6082