The Influence of Multiple Pass Submerged Friction Stir Processing

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Article The Inﬂuence of Multiple Pass Submerged Friction Stir Processing on the Microstructure and Mechanical Properties of the FSWed AA6082-AA8011 Joints

Sipokazi Mabuwa * and Velaphi Msomi

Mechanical Engineering Department, Cape Peninsula University of Technology, Bellville 7535, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-21-953-8778

Received: 28 September 2020; Accepted: 19 October 2020; Published: 28 October 2020

Abstract: The AA6082–AA8011 friction stir-welded joints were subjected to submerged multiple pass friction stir processing to evaluate the microstructure and mechanical properties of the joints. A maximum of four submerged friction stir processed passes were used in this study. All the specimens were extracted from three diﬀerent joint positions (start, middle and end). The tests conducted included microstructural analysis, tensile tests, hardness and fracture surface morphology of the post-tensile specimens, were performed using a scanning electron microscope (SEM). There was no particular trend in the microstructure and mechanical properties when looking at the specimen positioning in all the passes. The minimum mean grain sizes were reﬁned from 3.54 to 1.49 µm and the standard deviation from 5.43 to 1.87 µm. The ultimate tensile strength was improved from 84.96 to 94.77 MPa. The four-pass SFSPed specimens were found to have more ductile properties compared to the one-pass SFSPed one. The hardness of the stir zones in all the passes was found to be higher compared to the AA8011 base material but lower than the AA6082 one. The maximum stir zone hardness of 75 HV was observed on the one-pass SFSP joints.

Keywords: microstructure; submerged friction stir processing; dynamic recrystallization; tensile properties; hardness; fracture surface morphology

1. Introduction Friction stir processing (FSP) is an advanced microstructural altering process based on the friction stir welding process [1,2]. The grain refinement is a result of the dynamic recrystallization that occurs during the process. The FSP technique does not only refine the microstructural grain size but also modifies the texture of the processed zone [3,4]. FSP has been successfully used in many applications including the fabrication of surface composites of aluminium substrates [5,6], for superplastic high strain rate [7,8], metal matrix composites [9,10] and cast aluminium alloys [11,12]. FSP as an enticing emerging technology continues to grow, however, due to the high temperature the material experiences during FSP, grain growth was found to occur. Submerged friction stir processing (SFSP) came as a solution to minimize grain growth in order to achieve more grain refinement [13]. The SFSP technique works similarly to the normal or rather traditional FSP, the only difference takes place under controlled immersed environments [14,15]. Several studies have investigated the effect of submerged friction stir processing on aluminium alloys. Feng et al. [16] evaluated the impact of submerged friction stir processing on the microstructure of the AA2219–T6 plate. The processing conditions included the traverse speed of 200 mm/min and varying rotational speed. The results showed that the hardness of the stir zone was found to decrease with an increase in rotational speed. The decrease was substantiated to be due to softening of the

Metals 2020, 10, 1429; doi:10.3390/met10111429 www.mdpi.com/journal/metals Metals 2020, 10, 1429 2 of 14 processed zone resulting from precipitate hardening. The average grain size was found to be finer than the base material one. The AA6082/AA8011 dissimilar friction stir-welded joint was subjected to SFSP to investigate the changes in the microstructure and mechanical properties of the joints [17]. The results revealed that the average grain size decreased in comparison to that of the friction stir welded and base material one. The tensile strength and the hardness of the submerged processed joints were reported to increase in comparison to that of the friction stir-welded one. Patel et al. [18] proved that FSP active cooling methods in the form of compressed air, water and carbon dioxide (CO2) can enhance the superplastic behaviour of the high strength AA7075 alloy. Amongst the cooling methods used, the CO2 produced the lowest temperature readings due to higher cooling rate, resulting in the prevention of the coarsened grains in the stir zone. This, therefore, resulted in the CO2 having very fine grain sizes compared to other cooling methods. However, all the cooling methods were found to have finer grains compared to the normal friction stir-processed ones. Similar studies where the application of SFSP resulted in finer microstructural grain size were reported [19–22]. A few studies where the SFSP underwent multiple passes were found in the literature. Srivastava et al. [23] investigated the influence of the multiple passes on the microstructure and mechanical properties of the AA5059/SiC surface composites fabricated via submerged friction stir processing. The results revealed an increase from 85 to 159 HV in the hardness of the four-pass SFSPed specimen in comparison to the hardness value of the base material. The ultimate tensile strength (UTS) was also found to increase in the four-pass specimen from 321 MPa of the base material one to a 377 MPa. Additionally, the SFSPed UTS was also higher compared to the that of four-pass FSP under normal conditions, which was 347 MPa. The grain sizes were also noted to be finer on the SFSPed specimens compared to the normal friction stir-processed specimens. Huang et al. [24] subjected the AA5083/TiC to multiple pass SFSP to form bulk aluminium matrix composites. It was reported that the addition of water cooling resulted in a strong suppression of effect on the growth of the recrystal grain, while the addition of the TiC particles boosted recrystallization, owing to the extra dislocations generated at Ti/Al. The formation of the aluminium matrix composites (AMCs) resulted in the microstructural average grain size of about 1 µm. The yield strength and the UTS improved from 78 to 153 MPa in comparison to the base material. The fracture surface morphology of the SFSPed AMCs showed ductile behaviour represented by the well developed small uniform dimples. Similar studies where the multiple pass SFSP resulted in finer grain sizes and improved mechanical properties were reported in the literature [25–27]. It has been noted that most of the available work on the multiple pass SFSP of aluminium alloys is on the surface composites. The available literature also reports on the multiple pass SFSP of the single surface, but there is no searchable work where multiple passes were applied on the welded joints. This study investigates the influence of the multiple pass SFSP on the microstructure and mechanical properties of the friction stir-welded dissimilar aluminium alloys of AA6082-AA8011 joints.

2. Materials and Methods Aluminium alloy AA6082–T651 and AA8011–H14 were used in this study. The chemical compositions of the materials are shown in Table1. The dissimilar aluminium alloys were cut into dimensions of 6 mm 250 mm 55 mm. A total number of 4 pairs were friction stir welded, × × positioning the AA6082 alloy on the advancing side and the AA8011 alloy on the retreating side. The semi-automated Lagun FA.1 milling machine was used for the friction stir welding (FSW) process. The process parameters used for FSW are shown in Table2. A triangular threaded tool made of high-speed steel (HSS) AISI 4140 was used for both FSW. Figure1a shows the friction stir welding setup. Table3 shows the grain size and mechanical properties of the base materials and FSW joint. The friction stir-welded joints were then friction stir processed underwater (submerged) using the same parameters used for FSW. The ﬁrst plate was processed with a one-pass (1-pass), the second one with a two-pass (2-pass), the third one with a three-pass (3-pass) and the fourth one with a four-pass FSP. Figure1b shows SFSP setup, highlighting that the one-pass SFSP and the four-pass (4-pass) SFSP Metals 2020, 10, x FOR PEER REVIEW 3 of 14 shown in Figure 1c were applied to the FSW joint. The same tool and parameters used for friction stir Metals 2020, 10, 1429 3 of 14 welding were also used for friction stir processing. shown in Figure1c were appliedTable to the 1. FSW Chemical joint. compositions The same tool [28,29]. and parameters used for friction stir welding were also usedMg for frictionTi stir processing.Zn Cr Si Mn Fe Cu Al AA8011–H14 0.28 0.016 0.084 0.028 0.52 0.46 0.74 0.13 Bal Table 1. Chemical compositions [28,29]. AA6082–T651 0.6–1.2 - 0.0–0.1 0.0–0.25 0.7–1.3 0.4–1.0 0.0–0.5 0.0–0.1 Bal Mg Ti Zn Cr Si Mn Fe Cu Al AA8011–H14Table 2. Friction 0.28 stir welding 0.016 and 0.084 submerged 0.028 friction 0.52 stir processing 0.46 process 0.74 parameters. 0.13 Bal AA6082–T651 0.6–1.2 - 0.0–0.1 0.0–0.25Tool 0.7–1.3 Pin 0.4–1.0Tool 0.0–0.5 0.0–0.1Tool BalPin Tool Traverse Tool Rotation Tool Tilt Diameter Shoulder Length Speed (mm/min) Speed (rpm) Angle (°) Table 2. Friction stir welding and submerged friction(mm) stir processing process(mm) parameters. (mm) 55 1200 2 7 20 5.8 Tool Traverse Speed Tool Rotation Speed Tool Tilt Angle Tool Pin Diameter Tool Shoulder Tool Pin Length (mm/min) (rpm) ( ) (mm) (mm) (mm) ◦ 55 1200 2 7 20 5.8

Figure 1. (a) Friction stir welding process, (b) one-pass submerged friction stir processing (SFSP) and Figure 1. (a) Friction stir welding process, (b) one-pass submerged friction stir processing (SFSP) (c) four-pass SFSP. and (c) four-pass SFSP. Table 3. Grain size and mechanical properties of the base materials and FSW joint [17,30]. Table 3. Grain size and mechanical properties of the base materials and FSW joint [17,30]. Average Grain Size Ultimate Tensile Strength Average Elongation Microhardness Average Grain(µm) Ultimate Tensile(MPa) Average(%) Microhardness(HV)

AA8011–H14Size (µm) 51.57 Strength (MPa) 94.1 Elongation 40.17 (%) (HV) 33.8 AA6082–T651 65.04 308 25.42 92 AA8011–H14FSW AA6082–AA8011 16.7751.57 (stir zone) 94.1 82.9 40.17 17.98 58 33.8 (stir zone) AA6082–T651 65.04 308 25.42 92 FSW AA6082– The friction stir-processed16.77 (stir zone) plates (FSPed) 82.9 were cut for diﬀerent 17.98 tests using 58 waterjet (stir zone) cutting AA8011 technology. The tests conducted were the tensile test, microhardness, microstructural analysis and fatigue test. The Hounsﬁeld 50 K tensile testing machine was used to perform the tensile tests of Metals 2020, 10, x FOR PEER REVIEW 4 of 14

The friction stir-processed plates (FSPed) were cut for different tests using waterjet cutting technology. The tests conducted were the tensile test, microhardness, microstructural analysis and Metals 2020, 10, 1429 4 of 14 fatigue test. The Hounsfield 50 K tensile testing machine was used to perform the tensile tests of the specimens. The tensile specimen geometry and operating parameters were adapted from the ASTM- E8M-04the specimens. standard. The Figure tensile 2a depicts specimen the geometrytensile test and specimen operating used parameters in this study. were The adapted fractured from tensile the specimensASTM-E8M-04 were standard. further analysed Figure2a for depicts the nature the tensile of fracture test specimen and morphology used in this using study. a Tescan The fractured MIRA3 RISEtensile scanning specimens electron were furthermicrosco analysedpe (TESCAN for the Orsay nature Holding, of fracture Kohoutovice, and morphology Czech Republic). using a Tescan The InnovaTestMIRA3 RISE Falcon scanning 500 electron hardness microscope testing (TESCANmachine Orsay(INNOVATEST Holding, Kohoutovice,Europe BV CzechManufacturing, Republic). Maastricht,The InnovaTest The Netherlands) Falcon 500 hardness was used testing to perfor machinem the (INNOVATESTVickers hardness Europe test following BV Manufacturing, the ASTM E384-11Maastricht, standard. The Netherlands) Figure 2b presen wasts used the specimen to perform used the for Vickers microhardness hardness testtesting. following The objective the ASTM 10× andE384-11 objective standard. 20× for Figure specimen2b presents focusing the specimenwere used used during for microhardness setup. A 0.5 kg testing. load and The 1 objective mm interval 10 × wasand objectiveapplied from 20 forthe specimen centre to focusingeither side were of usedthe specimen during setup. (advancing A 0.5 kg and load retreating). and 1 mm A interval single line was × patternapplied was from used the centreto perform to either the sidehardness of the tests. specimen (advancing and retreating). A single line pattern was used to perform the hardness tests.

Figure 2. (a) Tensile specimen, (b) hardness and microstructure specimen, (c) specimen positioning Figure 2. (a) Tensile specimen, (b) hardness and microstructure specimen, (c) specimen positioning diagram. diagram. The microstructural analysis was performed using the Motic AE2000 metallurgical microscope (MoticThe Europe microstructural S.L.U., Barcelona, analysis Spain). was performed The specimens using werethe Motic mounted, AE2000 ground, metallurgical polished microscope and etched (Moticusingthe Europe modified S.L.U., Keller’s Barcelona, and Weck’sSpain). agents.The spec Theimens modified were mounted, Keller’s reagent ground, chemical polished composition and etched using the modified Keller’s and Weck’s agents. The modified Keller’s reagent chemical composition was 10 mL nitric acid (HNO3), 1.5 mL hydrochloric acid (HCl), 1.0 mL hydrofluoric acid (HF) and was 10 mL nitric acid (HNO3), 1.5 mL hydrochloric acid (HCl), 1.0 mL hydrofluoric acid (HF) and 87.5 mL distilled water (H2O) and the Weck’s reagents composition was 1 g sodium hydroxide (NaOH), 4 g potassium permanganate (KMnO4) and 100 mL distilled water (H2O). The ASTM E112-12 standard was used to determine average grain size through the use of ImageJ software. Figure2c shows the specimen positioning used in this study where “S” represents specimens extracted from the start, Metals 2020, 10, x FOR PEER REVIEW 5 of 14

87.5 mL distilled water (H2O) and the Weck’s reagents composition was 1 g sodium hydroxide (NaOH), 4g potassium permanganate (KMnO4) and 100 mL distilled water (H2O). The ASTM E112- 12Metals standard2020, 10 was, 1429 used to determine average grain size through the use of ImageJ software. Figure5 2c of 14 shows the specimen positioning used in this study where “S” represents specimens extracted from the start, “M” is used for specimens extracted from the middle, and “E” for specimens extracted towards“M” is used the forend specimens of the processed extracted area. from the middle, and “E” for specimens extracted towards the end of the processed area. 3. Results and Discussions 3. Results and Discussions

3.1.3.1. Microstructural Microstructural Analysis Analysis Figure 3 shows the microstructural grain sizes of the multiple passes of submerged friction stir- Figure3 shows the microstructural grain sizes of the multiple passes of submerged friction processed FSWed joints. Table 4 presents the grain sizes measured concerning Figure 3. The 1-pass stir-processed FSWed joints. Table4 presents the grain sizes measured concerning Figure3. The 1-pass friction stir-processed FSWed joints had a mean grain size range of 13.17 to 16.51 µm, minimum grain friction stir-processed FSWed joints had a mean grain size range of 13.17 to 16.51 µm, minimum grain size range of 3.29 to 3.63 µm and a standard deviation range of 4.32 to 6.42 µm. Similar results were size range of 3.29 to 3.63 µm and a standard deviation range of 4.32 to 6.42 µm. Similar results were reported in the literature [17]. The 2-pass friction stir-processed FSWed joints had a mean grain size reported in the literature [17]. The 2-pass friction stir-processed FSWed joints had a mean grain size range of 7.03 to 7.51 µm, standard deviation range of 2.12 to 3.56 µm and minimum grain size of 3.04 range of 7.03 to 7.51 µm, standard deviation range of 2.12 to 3.56 µm and minimum grain size of 3.04 to to 4.12 µm. The second pass had a mean grain size range of 5.61 to 6.36 µm, a standard deviation 4.12 µm. The second pass had a mean grain size range of 5.61 to 6.36 µm, a standard deviation range of range of 2.03 to 2.61 µm and minimum grain size range of 2.54 to 3.36 µm. The fourth pass had a 2.03 to 2.61 µm and minimum grain size range of 2.54 to 3.36 µm. The fourth pass had a mean grain mean grain size range of 5.03 to 5.38 µm, a standard deviation range of 1.65 to 2.18 µm and a size range of 5.03 to 5.38 µm, a standard deviation range of 1.65 to 2.18 µm and a minimum grain minimum grain size ranges of 1.20 to 1.86 µm. The minimum grain size, mean grain size and the size ranges of 1.20 to 1.86 µm. The minimum grain size, mean grain size and the standard deviation standard deviation decreased with an increase in the number of SFSP passes. The mechanism behind thedecreased grain refinement with an increase is based in on the the number high plastic of SFSP deformation passes. The and mechanism repeated dynamic behind the re-crystalization grain reﬁnement thatis based occurred on the during high plasticthe SFSP deformation process. Furthermore, and repeated the dynamic post-grain re-crystalization growth during that the occurred SFSP is duringalso preventedthe SFSP process. by the removal Furthermore, of excess the post-grainfrictional heat growth as a duringresult of the rapid SFSP water is also cooling. prevented by the removal of excess frictional heat as a result of rapid water cooling.

Figure 3. Cont.

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Figure 3. Multiple pass submerged friction stir-processed FSWed micrographs: 1-pass (a) start, Figure 3. Multiple pass submerged friction stir-processed FSWed micrographs: 1-pass (a) start, (b) (b) middle, (c) end; 2-pass (d) start, (e) middle, (f) end; 3-pass (g) start, (h) middle, (i) end; 4-pass middle, (c) end; 2-pass (d) start, (e) middle, (f) end; 3-pass (g) start, (h) middle, (i) end; 4-pass (j) start, (j) start, (k) middle, (l) end. (k) middle, (l) end. Table 4. Measured grain sizes. Table 4. Measured grain sizes. No. of SFSP Specimen Mean Grain Size Standard Deviation Minimum Mean Grain Size No. Passesof SFSP PositionSpecimen (µMeanm) Grain (µStandardm) Minimum(µm) Mean Passes PositionS 15.69Size (µm) Deviation 6.42 (µm) Grain 3.63 Size (µm) 1-pass MS 16.5115.69 5.52 6.42 3.74 3.63 1-pass EM 13.1716.51 4.32 5.52 3.29 3.74 SE 7.5113.17 3.764.32 4.04 3.29 2-pass MS 7.037.51 3.13 3.76 3.04 4.04 2-pass EM 7.367.03 3.56 3.13 4.12 3.04 SE 5.617.36 2.12 3.56 3.26 4.12 3-pass MS 6.035.61 2.03 2.12 2.54 3.26 3-pass EM 6.366.03 2.61 2.03 3.36 2.54 SE 5.156.36 1.65 2.61 1.20 3.36 4-pass MS 5.035.15 1.78 1.65 1.86 1.20 E 5.38 2.18 1.42 4-pass M 5.03 1.78 1.86 E 5.38 2.18 1.42 The microstructural grains during the SFSP process reduced the grain growth and the migration The microstructural grains during the SFSP process reduced the grain growth and the migration rate of grain boundaries, which then led to fine equiaxed grain structure in the stir zone of the rate of grain boundaries, which then led to fine equiaxed grain structure in the stir zone of the joint joint [31–33]. Additionally, Lou et al. [34] proved that during the multi-pass SFSP, the processed surface [31–33]. Additionally, Lou et al. [34] proved that during the multi-pass SFSP, the processed surface undergoes an enhanced cooling rate caused by water, resulting in significantly refined grain sizes. undergoes an enhanced cooling rate caused by water, resulting in significantly refined grain sizes. Regarding the specimen positioning, the measured grain sizes had no specific trend. The average grain Regarding the specimen positioning, the measured grain sizes had no specific trend. The average sizes are depicted in Figure4. The average minimum grain sizes for the 1-pass SFSPed joint was 3.56 µm, grain sizes are depicted in Figure 4. The average minimum grain sizes for the 1-pass SFSPed joint was 3.53 µm for the 2-pass, 2.25 µm for the 3-pass and 1.49 µm for the 4-pass joint. The fourth SFSPed 3.56 µm, 3.53 µm for the 2-pass, 2.25 µm for the 3-pass and 1.49 µm for the 4-pass joint. The fourth

MetalsMetals 20202020,, 1010,, x 1429 FOR PEER REVIEW 77 of of 14 14 Metals 2020, 10, x FOR PEER REVIEW 7 of 14 SFSPed pass resulted in a very fine homogeneous grain structure. Similar work including the SFSPedpass resulted pass inresulted a very ﬁnein a homogeneousvery fine homogeneous grain structure. grain Similar structure. work Similar including work the including application the of application of multiple pass SFSP was reported in the literature [34–36]. applicationmultiple pass of SFSPmultiple was pass reported SFSP inwas the reported literature in [ 34the– 36literature]. [34–36]. 16 16 16 15.12 Mean grain size 16 15.12 Mean grain size Minimum grain size 14 Minimum grain size 14 14 Standard deviation 14 12 Standard deviation 12 12 12 10 10 10 10 8 8 8 7.26 8

7.26 6 5.96 6 6 5.43 5.96 5.18 6 5.43 5.18 4 3.56 3.53 3.73 4 4 3.56 3.53 3.73 3.07 4 2.25 3.07 2 2.25 1.87 2 2 1.49 1.87 2 (µm) Standard deviation 1.49 (µm) Standard deviation Mean, Minimum grain size (µm) Minimum Mean,

Mean, Minimum grain size (µm) Minimum Mean, 0 0 0 1-pass 2-pass 3-pass 4-pass 0 1-pass 2-pass 3-pass 4-pass

Figure 4. Multiple pass submerged friction stir-processed FSWed average grain sizes. FigureFigure 4. 4. MultipleMultiple pass pass submerged submerged friction friction stir-processed stir-processed FSWed FSWed average grain sizes. 3.2.3.2. Tensile Tensile Properties Properties 3.2. Tensile Properties FigureFigure 55 depicts the the tensile tensile stress stress and and strain strain cu curvesrves for for the the multiple multiple submerged submerged friction friction stir- Figure 5 depicts the tensile stress and strain curves for the multiple submerged friction stir- processedstir-processed FSWed FSWed joints. joints. It is eviden It is evidentt that the that UTS the increased UTS increased with an with increase an increase in the innumber the number of SFSP of processed FSWed joints. It is evident that the UTS increased with an increase in the number of SFSP passes.SFSP passes. The tensile The tensile strain strainalso followed also followed the same the trend same except trend exceptfor the for2-pass the 2-passSFSP, which SFSP, whichhad lower had passes. The tensile strain also followed the same trend except for the 2-pass SFSP, which had lower strainlower compared strain compared to the rest to theof the rest passes. of the This passes. was due This to was the duejoint to having the joint a tunnel having defect. a tunnel Looking defect. at strain compared to the rest of the passes. This was due to the joint having a tunnel defect. Looking at specimenLooking atpositioning specimen positioningas depicted asin depictedFigure 6a, in ther Figuree was6a, no there particular was no trend particular noticed trend for noticedthe UTS for in specimen positioning as depicted in Figure 6a, there was no particular trend noticed for the UTS in allthe the UTS passes, in all but the for passes, the percentage but for the elongation, percentage the elongation, 2-pass SFSP the one 2-pass increased SFSP onetowards increased the specimen towards all the passes, but for the percentage elongation, the 2-pass SFSP one increased towards the specimen extractedthe specimen toward extracted the end toward of the the joint. end Figure of the 6b joint. shows Figure the6 baverage shows theUTS average and percentage UTS and elongation. percentage extracted toward the end of the joint. Figure 6b shows the average UTS and percentage elongation. Bothelongation. the average Both theUTS average and the UTS percentage and the elongation percentage increased elongation as increased the number as the of numberpasses increased. of passes Both the average UTS and the percentage elongation increased as the number of passes increased. However,increased. the However, percentage the percentageelongation of elongation the 2-pass of strain the 2-pass and percentage strain and elongation percentage was elongation found to was be However, the percentage elongation of the 2-pass strain and percentage elongation was found to be lowerfound compared to be lower to the compared rest of the to thepasses rest due of the to the passes tunnel due defect to the that tunnel was defectobserved. that The was increase observed. in lower compared to the rest of the passes due to the tunnel defect that was observed. The increase in theThe tensile increase strength in the tensileand percentage strength andelongation percentage corre elongationlates with correlatesthe microstructural with the microstructural grain sizes. Similar grain the tensile strength and percentage elongation correlates with the microstructural grain sizes. Similar studiessizes. Similar where the studies application where theof submerged application multiple of submerged pass SFSP multiple increased pass SFSPUTS were increased reported UTS in were the studies where the application of submerged multiple pass SFSP increased UTS were reported in the literaturereported in[23,35–37]. the literature [23,35–37]. literature [23,35–37].

Figure 5. Cont.

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Figure 5. Multiple pass SFSP stress–strain curves, (a) 1-pass, (b) 2-pass, (c) 3-pass and (d) 4-pass. FigureFigure 5.5. Multiple passpass SFSPSFSP stress–strainstress–strain curves,curves, ((aa)) 1-pass,1-pass, ((bb)) 2-pass,2-pass, ((cc)) 3-pass3-pass andand ((dd)) 4-pass.4-pass.

(a) (a)

(b)

Figure 6. ((aa)) Multiple Multiple pass pass SFSP SFSP tensile tensile stre strengthngth(b and and) percentage percentage elongation, ( b) average multiple pass SFSPFigure tensile 6. (a) strengthMultiple andan passd percentage SFSP tensile elongation. strength and percentage elongation, (b) average multiple pass

SFSP tensile strength and percentage elongation.

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FigureFigure7 7depicts depicts the the SEM SEM fracture fracture surface surface morphologiesmorphologies ofof thethe multiplemultiple passpass SFSPed tensile specimens.specimens. All All the the specimens specimens showed showed ductile ductile failure failur withe with di ﬀdifferenterent dimple dimple arrangements. arrangements. The The four-pass four- showedpass showed ﬁner equiaxed finer equiaxed dimples dimples compared compared to the other to the SFSP other passes. SFSP The passes. SEM The morphologies SEM morphologies correlated withcorrelated the grain with sizes the of grain the respective sizes of SFSPthe respective passes. The SFSP morphologies passes. The were morphologies also in agreement were withalso thein percentageagreement elongationwith the percentage results of elongati the specimenson results [38 –of40 the]. specimens [38–40].

FigureFigure 7. 7.SEM SEM morphologies:morphologies: 1-pass ( a)) start, start, ( (bb)) middle, middle, ( (cc)) end; end; 2-pass 2-pass (d (d) )start, start, (e (e) )middle, middle, (f ()f )end; end; 3-pass3-pass ( g(g)) start, start, ( (hh)) middle,middle, ((ii)) end;end; 4-pass4-pass ((jj)) start,start, ((k) middle, ( l) end.

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Metals 2020, 10, x FOR PEER REVIEW 10 of 14 3.3. Hardness 3.3. Hardness Figure8 shows the hardness profiles for the multiple pass SFSPed joints. Studying the figure, Figure 8 shows the hardness profiles for the multiple pass SFSPed joints. Studying the figure, the specimen extracted in the middle of the joint had higher hardness compared to the specimens the specimen extracted in the middle of the joint had higher hardness compared to the specimens extractedextracted towards towards the the start start and and the the end end of theof the joints. joints. Similar Similar observations observations were were noted noted in in the the literature literature [17 ]. The[17]. hardness The hardness of the heat-aof the ffheat-affectedected zone (HAZ)zone (HAZ) on the on AA6082 the AA6082 side side in 1-pass, in 1-pass, 3-pass 3-pass and and 4-pass 4-pass was foundwas to found be higher to be comparedhigher compared to the thermo-mechanical to the thermo-mechanical affected affected zone (TMAZ) zone (TMAZ) and stir and zone, stir while zone, the 2-passwhile TMAZ the 2-pass hardness TMAZ was hardness higher compared was higher to compared the HAZ andto the stir HAZ zone. and However, stir zone. TMAZ, However, HAZ TMAZ, and stir zoneHAZ harnesses and stir were zone lower harnesses in all were the passes lower comparedin all the passes to the compared base material to the of base AA6082. material This of is AA6082. due to the AA6082This is being due to precipitate the AA6082 hardened being precipitate alloy [17, 41harden–43].ed The alloy hardness [17,41–43]. profiles The forhardness all the profiles passes declinedfor all towardsthe passes the stirdeclined zone towards and further the stir to thezone AA8011 and further side. to However,the AA8011 the side. degree However, of declining the degree diff ofered accordingdeclining to differed the number according of SFSP to the passes. number Additionally, of SFSP passes. the Additionally, hardness of the hardness TMAZ and of the HAZ TMAZ of the AA8011and HAZ side of in the all theAA8011 passes side was in foundall the topasses be lower was found compared to be tolower the stircompared zone. Theto the stir stir zone zone. hardness The wasstir found zone to hardness be higher was compared found to thebe basehigher material compared AA8011 to the hardness. base material Additionally, AA8011 looking hardness. at the hardnessAdditionally, of the stirlooking zone at to the the hardness AA8011 of base the stir material zone to of the the AA8011 3-pass base SFSP material for the of specimen the 3-pass extracted SFSP towardsfor the the specimen end of theextracted joint, ittowards can be the seen end that of itthe was join notablyt, it can lower be seen due that to it the was defects notably on lower the position due to the defects on the position as a result of being cut too close to the SFSP tool exit hole. This is due as a result of being cut too close to the SFSP tool exit hole. This is due to the submerged conditions to the submerged conditions preventing the material softening and coarsening of the grains [17,23– preventing the material softening and coarsening of the grains [17,23–25]. The 1-pass hardness of the 25]. The 1-pass hardness of the stir zone maximum hardness was found to be 75 HV, 65 HV for the 2- stir zone maximum hardness was found to be 75 HV, 65 HV for the 2-pass, 65 HV for the 3-pass and pass, 65 HV for the 3-pass and 67 HV for the 4-pass specimen. 67 HV for the 4-pass specimen.

100 BM HAZ TMAZ Stir zone TMAZ HAZ BM

90 1-Pass S 1-Pass M 80 1-Pass E

Hardness HV0.5 40

30 AA6082 AA8011 ------20 ------16-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 Distance from the centre (mm) (a) 110 BM HAZ TMAZ Stir zone TMAZ HAZ BM 100 90 2-Pass S 80 2-Pass M 70 2-Pass E 60 50

Hardness HV0.5 40 30 AA6082 AA8011 ------20 ------16-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 Distance from the centre (mm) (b)

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110 BM HAZ TMAZ Stir zone TMAZ HAZ BM 100 3-Pass S 90 3-Pass M 80 3-Pass E 70 60 50 Hardness HV0.5 40 30 AA6082 AA8011 ------20 ------16-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16

Distance from the centre (mm) (c)

100 BM HAZ TMAZ Stir zone TMAZ HAZ BM 90 4-Pass S 80 4-Pass M 70 4-Pass E

Hardness HV0.5 40

30 AA6082 AA8011 ------20 ------16-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16

Distance from the centre (mm) (d)

FigureFigure 8. 8.SFSP SFSP Hardness Hardness proﬁles,profiles, ((a)) 1-pass, ( (bb)) 2-pass, 2-pass, (c ()c )3-pass 3-pass and and (d ()d 4-pass.) 4-pass.

4. Conclusions4. Conclusions TheThe eﬀ effectsects of of multiple multiple passes passes ofof thethe submergedsubmerged friction stir-processed stir-processed friction friction stir-welded stir-welded AA6082-AA8011AA6082-AA8011 dissimilar dissimilar joints joints on on the the microstructuremicrostructure and and mechanical mechanical properties properties were were successfully successfully investigated.investigated. Based Based on on the the results results obtained, obtained, thethe following conclusions conclusions can can be be drawn: drawn:

1. 1. ThereThere was was no no particular particular trendtrend inin the microstructure and and mechanical mechanical positioning positioning when when looking looking at the specimen positioning in all the passes. The mean grain sizes were refined from 15.12 to at the specimen positioning in all the passes. The mean grain sizes were reﬁned from 15.12 to 5.42 µm. The minimum mean grain sizes were refined from 3.54 to 1.49 µm and the standard 5.42 µm. The minimum mean grain sizes were reﬁned from 3.54 to 1.49 µm and the standard deviation from 5.43 to 1.87 µm. deviation from 5.43 to 1.87 µm. 2. 2. TheThe ultimate ultimate tensile tensile strength was was improved improved from from 84.96 84.96 to 94.77 to 94.77 MPa. MPa. The four-pass The four-pass SFSPed SFSPed specimensspecimens were were found found to to have have more more ductile ductile properties properties compared compared to to the the one-pass one-pass SFSPed SFSPed specimen. specimen. The percentage elongation of the joints was improved from 18.13% to 24.13%. The The percentage elongation of the joints was improved from 18.13% to 24.13%. The fracture surface fracture surface morphologies revealed a ductile failure mode. morphologies revealed a ductile failure mode. 3. The hardness of the stir zones in all the passes was found to be higher compared to the AA8011

base material but lower than the AA6082 one. The maximum stir zone hardness of 75 HV was observed on the one-pass SFSP joints. Metals 2020, 10, 1429 12 of 14

Author Contributions: Conceptualization and validation, V.M. and S.M.; methodology, formal analysis, investigation, data curation, writing—original draft preparation, writing—review and editing and visualization, S.M.; software, resources, project administration, supervision and funding acquisition, V.M.; Both authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Research Foundation Thuthuka Grant, grant number 118046 and the APC was funded by Cape Peninsula University of Technology. Acknowledgments: The authors would like to acknowledge the Cape Peninsula University of Technology mechanical engineering workshop staff, and especially S. Petersen for her assistance in specimen preparation. Special appreciation also goes to Miranda Waldron from the University of Cape Town for assistance with scanning electron microscopy tests. Conflicts of Interest: The authors declare no conflict of interest.

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