Effect of Friction Stir Processing onGas TungstenArc-Welded and Friction Stir-Welded 5083-H111 Aluminium Alloy Joints

-is paper presents the analysis of the friction stir-processed aluminium alloy 5083-H111 gas tungsten arc-welded and friction stir-welded joints. -e comparative analysis was performed on the processed and unprocessed gas tungsten arc-welded and friction stir-welded joints of similar aluminium alloy 5083-H111. -e results showed a clear distinction between the friction stir processed joints and unprocessed joints. -ere is a good correlation observed between the microstructural results and the tensile results. Ultrafine grain sizes of 4.62 μm and 7.177 μm were observed on the microstructure of the friction stir-processed friction stir-welded and gas tungsten arc-welded joints. -e ultimate tensile strength for friction stir-welded and gas tungsten arc-welded before friction stir processing was 153.75 and 262.083MPa, respectively. -e ultimate tensile strength for friction stir processed friction stir-welded joint was 303.153MPa and gas tungsten arc-welded joints one was 249.917MPa.-emicrohardness values for the unprocessed friction stir-welded and gas tungsten arc-welded joints were both approximately 87HV, while those of the friction stir-processed ones were 86.5 and 86HV, respectively. -e application of friction stir processing transformed the gas tungsten arc morphology from brittle to ductile dimples and reduced the ductile dimple size of the unprocessed friction stirwelded joints from the range of 4.90–38.33 μm to 3.35–15.59 μm.


Introduction
e materials-processing technology is as old as civilization. England started machine automation for forming, shaping, and cutting in the 18th to the 19th century. Since then, materials-processing methods, techniques, and machinery have grown numerously [1]. e material processing was introduced as the method of improving the material properties to suit a specific purpose. e selection of material with specific properties is the key parameter in many manufacturing industries like aircraft industry, automotive industry, and hydrovehicle industry [2]. Aluminium is known to be light in weight and a good corrosion-resistant material. ese features have brought attention towards this material such that it became the best material suitable for building the aircraft structures, ship structures, and automotive components. e other benefit of using aluminium is that its weight lightness contributes to the reduction of power consumption [3]. e 5083-H111 aluminium alloy (AA) from the wrought alloy 5xxx series was selected to be utilized in this paper. Generally, the 5xxx offer outstanding corrosion resistance, making them suitable for marine applications.
e 5083 alloy has the highest strength of the nonheat treatable alloys but is not recommended for use in temperatures exceeding 65°C. e alloy is highly resistant to be attacked by both seawater and industrial chemical environments [4]. Alloy 5083 also retains exceptional strength after welding in comparison to other alloys. Aluminium alloys are produced in different forms which mean different manufacturing or joining methods are required, i.e., welding and riveting. Welding is known to be the most used metal joining method. Welding is also divided into different types, i.e., gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), friction welding (FW), friction stir welding (FSW), and laser welding. e GTAW method has been the preferred method for welding aluminium alloys due to its cleanliness, but recently, the preference has shifted towards FSW. is shift is due to the fact that FSW is clean and does not produce any gas to the atmosphere, hence considered as green technology as well as fusion welding techniques such as arc welding result in several problems, namely, porosity formation, loss of strength, and cracking particularly in high strength Al-alloys [5][6][7][8][9]. Friction stir welding (FSW) is a solid-state welding method, which is invented by the welding institute (TWI) and has been well demonstrated particularly suited for the joining of aluminium alloys [10][11][12][13][14][15][16]. is then suggests that there are numerous progressive developments that are trying to optimize this new technology (FSW). is includes the introduction of friction stir processing (FSP) technology. FSP is another new technique being used to modify the microstructure of metals [17][18][19][20][21]. e FSP technique works similar to FSW but FSP does not join the materials instead it modifies the material's microstructure through the pinned or pinless tool. Due to the frictional heat generated, the material undergoes severe plastic deformation, resulting in significant microstructural changes in the processed zone [22][23][24][25]. FSP generates three distinct microstructural zones in the processed area, namely, the nugget zone (NZ), thermomechanically affected zone (TMAZ), and heat-affected zone (HAZ) [26,27]. e first work on FSP was reported by Mishra et al. [28] where FSP was used in enhancing the strain rate plasticity of 7075 aluminium alloy. Since then, FSP was employed to perform various modifications, and this includes the fabrication of surface composite.
Cast aluminium A206 was subjected to FSP with the purpose of modifying its microstructure. e results showed a significant reduction in grain size on the processed regions compared to the unprocessed regions of the plate. e microhardness for the processed regions was increased compared to the unprocessed regions. e processing of this material was also found to have contributed to the improvement of its tensile strength [29,30]. ere are different types of parameters that are involved in FSP. is includes tool shape and geometry, welding speeds, and rotational speeds. e good combination of these parameters advocates the achievement of a good product. SSM 356 aluminium alloy was used to study the impact of welding and rotational speeds towards the mechanical properties of this material [31]. ese speeds were varied with the purpose of obtaining the optimum combination. e rotational and travelling speeds of 1750 rpm and 160 mm/min, respectively, were found to be optimal values in obtaining an improved result. A notable increase in microhardness and tensile strength of the processed region compared to the unprocessed one was reported. e microstructural analysis also showed homogeneity on the processed region compared to the unprocessed one [32]. e literature has shown some investigations on the influence of FSP on the microstructure and the tensile properties of AL-Si alloy [33]. Golafshani et al. [34] and Saini et al. [35] performed similar studies with the same alloy subjected to FSP. Process parameters used included tool tilted of angle 3°, while the rotational speed and welding speed were fixed at 1400 rpm and 42 mm/min, respectively. ere was a notable increase in ductility on the processed plates compared to the unprocessed one. e ultimate tensile strength (UTS) for the processed plates was found to be higher than those which were unprocessed. Similarly, FSP resulted in fine equiaxed grains improving the tensile properties of the processed alloys [36][37][38].
is paper reports the analysis of the mechanical properties of the processed friction stir-welded (FSWed) and gas tungsten arc-welded (GTAWed) AA5083-H111 joints. e processed joints are analyzed in comparison to the unprocessed welded joints. is study is performed so as to establish whether FSP technique can be used as a postprocessing method in improving the quality of the said welded joints. is topic was carefully chosen after noticing that there is little or no literature available on the friction stir processing of the GTAWed and FSWed joints; most literature studies have been focusing on the friction stir processing of single materials. e outcome of this paper will then give the engineers in the material processing industry an alternative for the enhancing of mechanical properties which will play a huge role in extending life span of welded structures.

Materials and Methods
e aluminium alloy 5083-H111 plate with a thickness of 6 mm was used in performing the experiments of this study. Eight rectangular specimens of 530 mm by 70 mm were cut from the supplied aluminium plate. Table 1 presents the chemical composition of the base metal (AA5083-H111). e dimensioned plates were welded together using FSW and GTAW techniques.
Friction stir welding was performed using a semiautomated milling machine. A nonconsumable tool made of high-carbon steel (H13) was used in fabricating the joints. e dimensions of the tool are shown in Figures 1(a) and 1(b). Figure 1(c) shows the image of the tool. e pin was positioned at the centre of the joint line with the purpose of enhancing material and grain refinement [40,41]. A singlepass welding procedure was used to fabricate the joints [22]. e welding parameters used for this study are presented in Table 2. e sample of the weld produced by FSW is shown in Figure 1(d).
e plates used in FSW are similar to the plates used for GTAW.
e only difference was that the edges to be GTAWed had to be cut to a double V-groove as set out in ISO 9692. is was done so as to enhance the strength of the welding joint and also to minimize the weld distortion [42]. e sample of the GTAWed plate is shown in Figure 1(e). It should be noted that a ER5356 filler wire with a 2.4 mm diameter was used. Argon shielding gas was used to protect the weld pool from the dissolution of atmospheric gases [36]. Table 3 presents the GTAW parameters. All the welds produced by GTAW and FSW techniques were later friction stir processed. e processing parameters used for FSP were similar to the ones used for FSW. Figure 2(a) shows the friction stir processing setup for the GTAWed joints, and Figure 2(b) shows the friction stir processing of the FSWed joints. e    single-pass FSP was used in processing the joints. e same tool used for FSW was also used for FSP. e use of FSP tools with pins have been used in previous studies [37,38,[43][44][45].

Experiments.
e FSWed, GTAWed, and friction stirprocessed plates were cut for different tests. e waterjet cutting technique was used to cut the specimens, and this cutting technique was chosen because it does not involve heat and does not temper with mechanical properties during the cutting process. e tests performed on these specimens include tensile testing, microhardness, and microstructural analysis. e tensile tests were performed using the Hounsfield tensile testing machine. e ASTM-E8M-04 was used for specimen geometry and tensile testing. e specimen geometry used (dimensions in mm) for tensile testing is shown in Figure 3(a). e tensile test parameters are presented in Table 4. e force versus extension data were logged, and the graphical representation of the logged data is presented under the Results section.
Vicker's microhardness tests were performed using microhardness testing machine with Vicker's scale. e ASTM E384-11 standard was used in performing microhardness testing. e tests were performed along the horizontal section of the welded joint specimen. e interval of 2 mm was used from the centre of the weld to either advancing or retreating side of the specimen. e load of 300 g and a dwell time of 15 s were used. e microstructural analysis was performed using a metallurgical microscope.
Prior to the microstructural analysis, the specimens were cut according to sizes shown in Figure 3(b). e two lines in Figure 3(b) show where the welded area starts and ends. e cut specimens were mounted on Bakelite, hand polished, and etched. e etchant used was the sodium hydroxide (NaOH) made of 2 g of NaOH and 100 ml of distilled water. e etchant was poured on top of the specimen and immersed for 1 min for the preetch and 10 min for full etching.
It should be noted that there is a start location, middle location, and ending location in the friction stir-processed and the welded plates. So the three specimens were produced from the start, middle and the end of either welding or processing (see Figure 4). e specimen that was cut at the beginning of the plate was marked with A, while B symbolized the middle position and C was the end position of the plate. For unprocessed specimen labelling, formats A, B, and C were used while A2, B2, and C2 symbolized friction stir-processed specimens. is presentation was followed in all the tests that were performed. e test results are presented and discussed in the next section.

Results and Discussion
is section gives a detailed discussion of the results that were obtained from different test techniques. e results obtained include macrostructure, microstructure, tensile tests, microhardness, and fracture surface morphology. Figure 5 shows the macrostructure of the unprocessed and friction stir-processed FSWed and GTAWed joints. Figure 5(a)    Advances in Materials Science and Engineering AA5083-H111 BM. e AS is the advancing side and RS is the retreating side in Figures 5(b) to 5(e). Figure 5(b) shows the surface of the unprocessed GTAWed joints. e surface consists of few pores that are appearing on the welded joint with no visible cracks. It is assumed that the distinction between the welded plates and the filler together with the presence of pores on the joint contributes towards the joint weakness [36]. Figure 5(c) shows the friction stir-processed GTAWed joint, where the onion ring structure is present and a small portion of the unprocessed GTAWed structure is also appearing on the edges of the welded joint. ere are no pores and cracks observed from the figure. e macrostructure of the unprocessed FSWed joint is shown in Figure 5(d).

Macrostructure.
e joint shows bonding without defects. Figure 5(e) shows the friction stir-processed FSWed joint macrostructure. e unprocessed FSWed and the friction   Advances in Materials Science and Engineering stir-processed FSWed reveal onion ring microstructure that is also observed on the friction stir-processed GTAW. ere is a normal feature of the banded structure [46]. e visibility of the onion ring feature suggests a good bonding or improved joint quality [47][48][49].

3.2.
Microstructure. e microstructure of the stir zones of unprocessed and friction stir-processed FSWed and GTAWed joints is presented in Figure 6. Figure 6(a) presents the microstructure of the base metal (AA5083-H111) comprising of the coarse grain structures. Dark dendrites with fine precipitates of Mg 3 Al 2 were noted in the micrograph of the unprocessed GTAWed joint depicted in Figure 6(b), and this is a common phenomenon with filler wire ER 5356 microstructure [50]. e application of FSP on the GTAWed joints resulted in very fine grain structure with distinguished boundary layers in comparison to the unprocessed one (see Figure 6(c)). e unprocessed FSWed micrograph in Figure 6(d) shows the uniform arrangement with fine grains. Ultrafine grains with uniform arrangement is also noticed on friction stir-processed FSW. is arrangement was caused by the dynamic recrystallization (DRX) during the friction stir processing. e same behaviour was noted on the friction stir-processed GTAWed joints which resulted in very fine equiaxed grains on the stir zone [51][52][53].
ree measurements were performed on the stir zones of the welded joints where 1 stands for the beginning, 2 centre, and 3 towards the end of the stir zone in Table 5. Unprocessed GTAWed and FSWed grain sizes were found to be bigger than the friction stir-processed ones. However, the unprocessed joint grain sizes are smaller compared to the base material. e average grain sizes for the unprocessed FSWed joint is 10 μm while an average of 7 μm corresponds with the friction stir-processed FSWed joint. A similar trend is also noticed with the unprocessed and friction stir-processed GTAWed joint. e microstructural results correlate with the tensile properties, hence the ductile fracture. e grain refinement of the friction stir-processed welded joints is in agreement with the Hall-Petch relation which predicts that the grain size decreases with an increase in the UTS. e microstructural smaller grain size resulted in a higher hardness level of the welded region [49,54]. Additional grain  Advances in Materials Science and Engineering measurements were performed on the unprocessed GTAWed joint to determine the size of the porosities in Figure 7(a) and were found to range from 11.24 to 20.46 μm. e average grain size for TMAZ on friction stir-processed GTAWed (Figure 7(b)) was measured to be 8.06 μm with columnar grains ranging from 40.60 to 83 μm and the HAZ    (Figure 7(c)) were 9.36 and 11.56 μm, respectively. e friction stir-processed FSWed average grain size (Figure 7(d)) for TMAZ was 7.53 and 8.39 μm for HAZ. Figure 8 illustrates Vicker's microhardness profiles for unprocessed and friction stirprocessed GTAWed and FSWed joints. ere is a notable decrease in the microhardness value from the centre to either the advancing or retreating side of the stir zone. is behaviour is the same for processed and unprocessed FSWed joint. e processed GTAWed joint exhibits similar behaviour to processed and unprocessed FSWed. is behaviour is a result of the coarsening of the precipitates on the welded region formed during FSW process, as well as the reheating and restirring during FSP [55,56]. e unprocessed GTAWed joint shows an approximately horizontal line to the retreating side with a microhardness value of about 87 HV. Additionally, the microhardness profile for the GTAW is not symmetrical around the welds centreline. ere is a notable decrease from the centre line to the retreating side. is is a very common behaviour when using the ER5356 filler which is most likely to be caused by the nonuniform melt flow field on both sides of the weld centre [57]. Chaurasia et al. [58] also reported a decrease in the microhardness as a result of little higher heat input in advancing than on the retreating side. ere is a decrease in microhardness further away from the centre in both unprocessed and friction stir-processed GTAWed and FSWed joints, which is a result of the joints experiencing less strain and nonuniform grain structures. It was noted that the microhardness values of the processed and unprocessed joints (FSWed and GTAWed) were higher compared to the base metal one. is behaviour of microhardness is influenced by the change in grain sizes and grain distribution [59][60][61]. Table 6 shows the tensile test results for the friction stir-processed and unprocessed GTAWed joints. e notable trend shown in Table 3 is that the ultimate tensile strength (UTS) of the friction stir-processed specimens is higher than the unprocessed one. Moreover, the UTS for specimens cut at the beginning of the plate is lower than the UTS of the specimens cut in the middle and at the end of the plate. It is assumed that this trend emerges from the insufficient heat input occurring at the beginning of the welding or processing. A similar trend was also noted by Çevik [62]. It was also noted that the yield strength and the UTS for all the specimens (processed and unprocessed) were lower than those of the base metal. is is mainly due to the effect of heat input which does have an impact on the mechanical properties of the AA5083-H111 alloy [63]. Additionally, the AA5083-H111 base metal is a work hardened alloy in which the microstructure is highly unstable and has unequiaxed grain structures as seen in Figure 6(a); the application of FSW initiated a recrystallization due to high temperature during the process   Advances in Materials Science and Engineering destroying the work-hardened state and weakening the mechanical properties [61]. en, the re-recrystallization that happens during the FSP application comes and remodifies the microstructure of the alloy strengthening the mechanical properties [25,28,35].

Tensile Tests.
Amongst the tensile properties is the joint efficiency which is a numerical value, which represents a percentage, expressed as the ratio of the strength of a riveted, welded, or brazed joint to the strength of the base material [25]. e joint efficiency was determined by dividing the UTS of the weld by that of the base material. e friction stir-processed GTAWed joint showed maximum joint efficiency of about 71.4% while the unprocessed one was found to be 44%. ere is also a notable improvement on the percentage elongation of the friction stir-processed joints compared to the unprocessed ones. is suggests that the ductility of the material has improved. e specimen C2 had highest percentage elongation compared to all specimens (base metal, unprocessed and other friction stir-processed specimens). Figure 9 shows the engineering stress and strain curves of the unprocessed and friction stir-processed GTAWed joints in correlation to Table 6. A similar impact of FSP on tensile properties of materials was also reported in the literature [24,47,64]. Table 7 and Figure 10 shows the tensile test results for the friction stir-processed and unprocessed FSWed joints. e trend noted with unprocessed and friction stir-processed GTAWed joint is also noticed with unprocessed and friction stir-processed FSWed results. e only difference is that the UTS and joint efficiency of the unprocessed FSWed joint were higher than those of the unprocessed GTAWed joint. Liu et al. [65] and Ceschini [66] obtained very similar behaviour of the tensile properties. e UTS of 303.153 MPa and joint efficiency of 86.60% for the friction stir-processed FSWed joint shows a clear increment along the joint while the unprocessed shows some fluctuations. e main reason for the increase in tensile properties of the friction stirprocessed FSWed joints is that the joints were re-reheated and re-recrystallized using the same parameters used for welding to enhance the mechanical properties and refine grain sizes [44,45,67]. A similar behaviour was also reported on the study by Salman et al. [20]. Figure 11 reveals the SEM micrograph of fractured surface for the unprocessed and the friction stir-processed GTAWed and FSWed joint specimens. Figure 11(a) shows the fractured surface for the base  Advances in Materials Science and Engineering 9
e unprocessed GTAWed joint shown in Figure 11(b) reveals some rough fracture surface with some notable voids and cleavage facets which correlated with the microstructure results. is kind of observation suggests that the GTAWed joint had a brittle failure. Cleavage facets, dimples, and matrix cracks were noted on the surface of the other three fracture surfaces (see Figure 11(c)-11(e)). is then suggests a ductile failure of the unprocessed FSWed and  friction stir-processed GTAWed and FSWed joints. e dimple sizes measured were ranging from 10.64 to 22.18 μm for the unprocessed GTAWed fracture surface, while for the friction stir-processed GTAWed joints, the dimples ranged from 1.66 to 17.72 μm.
e unprocessed FSWed joint dimples were found to be about 4.90 to 17.72 μm, and the friction stir-processed GTAWed joint ones were found to be ranging from 3.33 to 15.59 μm. e same ductile behaviour was obtained where the dimple of the fracture is obvious without the phenomenon of intergranular fracture [35,[68][69][70].
e friction stir-processed GTAWed and FSWed morphology showed a lot of dimples due to recrystallization and reheating of the joint which softened the material.

Conclusions
e analysis of the friction stir-processed and unprocessed GTAWed and FSWed joints were successfully fabricated and analyzed in this study. e main conclusions can be summarized as follows: (i) e unprocessed GTAWed joint shows very low mechanical properties compared to the friction stir- processed GTAWed joint.
e mechanical properties of the GTAWed joint improved after it has been friction stir processed. Similar behaviour is also observed with friction stir-processed and unprocessed FSWed joints. Maximum tensile properties obtained were found on the friction stirprocessed FSWed specimen C. e best results were a UTS of 303.153 MPa, yield strength of 118.611 MPa, percentage elongation of 24%, and a joint efficiency of 86.6%. (ii) e application of FSP also improved the ductility of the welded joints, with friction stir-processed FSWed joint being more ductile in comparison to the friction stir-processed GTAWed ones. e ductility is correlated by the surface morphology results where the friction stir-processed FSWed results revealed best reduced dimple sizes. (iii) e macroscopic and microscopic results have shown that FSP technique can be used as a weld joint enhancement technique. e reheating and restirring in the stir zone resulted in significant improvement of the mechanical and microstructural properties of the processed joints. e distinctive grain size refinement occurred in both friction stir-processed FSWed and GTAWed joints. However, the friction stir-processed FSWed joints had the greatest refinement due to the severe rereheating, re-restirring, and re-recrystallization experienced during FSP. (iv) e microhardness of the unprocessed and friction stir-processed joints was marginally affected by the friction stir processing technique giving maximum microhardness of 87 HV.
Data Availability e authors would like to confirm that the data generated during the study is available and can be accessed on request.

Conflicts of Interest
e authors declare that there are no conflicts of interest that may arise from this work.