Effect of Tool Rotational Speed on the Microstructure and Mechanical

metals

Article Eﬀect of Tool Rotational Speed on the Microstructure and Mechanical Properties of Bobbin Tool Friction Stir Welded 6082-T6 Aluminum Alloy

Yupeng Li 1,2,3,4,*, Daqian Sun 1,2 and Wenbiao Gong 3,4 1 Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, Changchun 130025, China 2 School of Materials Science and Engineering, Jilin University, Changchun 130025, China 3 Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China 4 School of Materials Science and Engineering, Changchun University of Technology, Changchun 130012, China * Correspondence: [email protected]; Tel.: +86-155-2663-9851

Received: 10 July 2019; Accepted: 13 August 2019; Published: 15 August 2019

Abstract: Samples of 6082-T6 aluminum alloy were welded by bobbin tool friction stir welding at different rotational speeds. The thermal cycles, microstructure, microhardness, and tensile properties of the specimens were investigated. The results show that the maximum temperature at the joint increases first and then decreases with increasing rotational speed, and the maximum temperature is 509 ◦C at 1000 r/min. The macromorphology of the cross-section of the joint is rectangular, and an ‘’S” line and gray-white texture can be observed. The stirred zone had much smaller equiaxed recrystallized grains. With increasing welding speed, the average grain size in the stirred zone region decreases. The microhardness distribution of the cross-section of all joints is W-shaped. When the rotational speed increases, the hardness of the heat-affected zone decreases gradually, and the hardness of the stirred zone increases. At 600 r/min, the strength is the lowest. The fracture location is between the stirred zone and the thermomechanically affected zone. When the rotational speed is increased, the fracture location is entirely located in the heat affected zone, and the fracture surface is dimple-like; the strength significantly increases and reaches a maximum at 800 r/min.

Keywords: bobble tool friction stir welding; 6082-T6 aluminum alloy; rotational speed; microstructure; mechanical properties

1. Introduction Friction stir welding (FSW) is a solid-state joining process originally intended for aluminum alloys and developed by The Welding Institute (TWI) in 1991 [1–3]. Because the metal material is not melted during the FSW process, the welding joint has small shrinkage deformation, and the loss of mechanical properties is obviously reduced compared with traditional welding techniques [4–6]. Bobbin tool friction stir welding (BT-FSW) is a variant type of FSW that employs a bottom shoulder to contact the bottom surface of the work piece [7]. There is no need for a backing bar because the BT-FSW allows for less stiffness during the welding process, and closed profiles, such as hollow and complex-shaped structures, can be joined [8]. To date, several studies have been conducted to evaluate the temperature field [9,10], equipment [11], microstructures and mechanical properties [12–15] of bobbin tool friction stir welded joints in aluminum alloys. The microstructure and the mechanical properties are determined by the thermal cycle and the material flow behavior for the FSW [16]. The welding parameters, including the rotational speed and welding speed, have an obvious effect on the thermal cycle and the material

Metals 2019, 9, 894; doi:10.3390/met9080894 www.mdpi.com/journal/metals Metals 2019, 9, 894 2 of 11

flow behavior. Li et al. [17] studied the effects of rotational and welding speeds on the microstructure and mechanical properties of BT-FSW Mg AZ31 and indicated that the average grain size increased as the ratio of the rotational speed to welding speed increased and the rotational and welding speeds had only slight influences on the yield stress and fracture elongation. Wang et al. [18] noted that the tensile strength of the BT-FSW joint welded Al-Li alloy initially increased with rotational speed and then decreased, with the maximal strength efficiency reaching 80%. Liu et al. [19] studied the effect of welding speed on the microstructure and mechanical properties of BT-FSW welded 6061-T6 aluminum alloy and found that the elongation and tensile strength of joints increased with increasing welding speed. Zhang et al. [20] concluded that with increasing welding speed, the joint of the BT-FSW joint welded 2A14-T6 aluminum alloy strength first increased to a peak and then showed a sharp decrease due to the occurrence of void defects. Wang et al. [21] studied the dual-rotation bobbin tool friction stir welding (DBT-FSW) on Al-Li alloy sheets and found that the DBT-FSW has an excellent process stability and can produce the defect-free joints in a wider range of welding parameters. Zhao et al. [22] developed empirical models to describe the relationships between the welding factors, such as welding speed, tool rotation speed, and shoulder pinching gap, and the tensile properties of BT-FSW 2219-T87 aluminum alloy, and the result shows that the models are adequate for predicting and optimizing the BT-FSW factor effects. In summary, although some studies on BT-FSW process parameters have been carried out in recent years, there is little work on the effect of rotational speed on the microstructure and mechanical properties of BT-FSW of 6000 series aluminum alloys, especially in 6082-T6 aluminum alloys. The main purposes of this study are to investigate the application of BT-FSW on 6082-T6 aluminum alloy and to determine the effect of rotational speed on the microstructure and mechanical properties of the obtained joints.

2. Materials and Methods The base metal (BM) used in this study was 6082-T6 aluminum alloy with a thickness of 4 mm in plate form. The size of the test plates used for the welding experiment was 500 mm 150 mm 4 mm. × × The chemical composition and mechanical properties of the BM are presented in Table1.

Table 1. Chemical composition and mechanical properties of 6082-T6 aluminum alloy.

Chemical Composition (wt%) Mechanical Properties Tensile Yield Si Fe Cu Mn Mg Cr Zn Ti Al Elongation Strength Strength 0.97 0.50 0.10 0.7 1.02 0.25 0.20 0.10 Bal 323 MPa 272 MPa 13%

The test plates were butt-welded by the BT-FSW method with a special FSW Machine (FSW-LM-AM16, AVIC Beijing FSW Technology Co., Ltd., Beijing, China). The welding tool consisted of two shoulders with 16 mm diameters and a smooth cylindrical pin 6 mm in diameter and 3.8 mm in length, as shown in Figure1; the shoulders and the pin are all made of steel H13. During welding, the welding tool, with a tilting angle of 0◦, was rotated in the clockwise direction; the welding speed was 500 mm/min constantly, and the rotational speeds of the tool were 600 r/min, 800 r/min, 1000 r/min and 1200 r/min. The thermal cycle curves of diﬀerent positions on both sides of the weld were measured by a Digital Recorder (RX4008BJL, Hangzhou Excon Technology Co., Ltd., Hangzhou, China) and several thermocouples. The thermocouples were K-type armored thermocouples with a diameter of 1 mm, and the installation method is shown in Figure2. Samples for microstructure analysis were cut perpendicular to the welding direction, mechanically ground and polished, etched with a reagent (20 g NaOH and 100 mL H2O), and examined by Optical Microscopy (DMI3000M, Leica Microsystems Inc., Wetzlar, Germany). Electron Backscattered Diﬀraction (EBSD, Tescan Vega3, Tescan Orsay Holding, Metals 2019, 9, 894 3 of 11

Metals 2019, 9, x FOR PEER REVIEW 3 of 11 Metals 2019, 9, x FOR PEER REVIEW 3 of 11 MetalsBrno, 2019 Czech, 9, x Republic)FOR PEER REVIEW was also used to study the microstructure. The fracture surface was observed3 of 11 The microhardness was measured along the cross-section of the joint using an FM-800 by Scanning Electron Microscopy (SEM, JSM-5600, JEOL Ltd., Tokyo, Japan). MicrohardnessThe microhardness (Future-Tech was Corp. measured Kanagawa, along Kawaki, the cross Japan-section) instrument of the with joint a usingtest load an of FM 100-800 g The microhardness was measured along the crosscross-section-section of the joint using an FM-800FM-800 andMicrohardness an indentation (Future time-Tech of 15Corp. s. MeasurementsKanagawa, Kawaki, were Japan performed) instrument pointwise with at a thetest centerload of of 100 the g MMicrohardnessicrohardness (Future-Tech(Future-Tech Corp. Corp. Kanagawa, Kanagawa, Kawaki, Kawaki, Japan) Japan instrument) instrument with with a test a test load load of 100 of g100 and g thicknessand an indentation at 0.5 mm intervals. time of 15Tensile s. Measurements samples were were machined performed from the pointwise middle parts at the of center the weld of , the as andan indentation an indentation time of time 15 s. of Measurements 15 s. Measurements were performed were performed pointwise pointwise at the center at the of centerthe thickness of the shownthickness in Figureat 0.5 mm 3, with intervals. a gauge Tensile 50 mm samples long and were 12.5 machined mm wide fromaccording the middle to ASTM parts E8/E8M of the- 13aweld [2, 3as], thicknessat 0.5 mm at intervals. 0.5 mm intervals. Tensile samples Tensile were samples machined were machined from the middlefrom the parts middle of the parts weld, of the as shown weld, as in andshown tensile in Figure tests 3for, with each a conditiongauge 50 mm were long performed and 12.5 in mm triplicate. wide according The tensile to ASTM properties E8/E8M were-13a tested [23], shownFigure3 in, with Figure a gauge 3, with 50 a mm gauge long 50 and mm 12.5 long mm and wide 12.5 according mm wide to according ASTM E8 to/E8M-13a ASTM E8/E8M [23], and-13a tensile [23], withand tensilea WDW tests-200 for Tensile each Tconditionesting M achinewere performed (Changchun in Kexintriplicate. Test The Instrument tensile properties Co., Ltd., Chawerengchun, tested andtests tensile for each tests condition for each werecondition performed were performed in triplicate. in triplicate. The tensile The propertiestensile properties were tested were withtested a China)with a WDWat room-200 temperature Tensile Testing and atM aachine testing (Changchun speed of 1 mm/min.Kexin Test Instrument Co., Ltd., Changchun, WDW-200with a WDW Tensile-200 T Testingensile T Machineesting M (Changchunachine (Changchun Kexin Test Kexin Instrument Test Instrument Co., Ltd., Co., Changchun, Ltd., Cha China)ngchun, at China) at room temperature and at a testing speed of 1 mm/min. China)room temperature at room temperature and at a testing and at speed a testing of 1 speed mm/min. of 1 mm/min.

Figure 1. Schematic illustration of the bobbin tool friction stir welding ( BT-FSW) process. Figure 1. Schematic illustration of the bobbin tool friction stir welding (BT-FSW) process. Figure 1. SchematicSchematic illustration illustration of of the the b bobbinobbin tool tool friction stir welding ( (BT-FSW)BT-FSW) process.

Figure 2. Schematic illustration of the test method for the thermal cycle curve. Figure 2.2. Schematic illustration of the test method for the thermal cyclecycle curve.curve. Figure 2. Schematic illustration of the test method for the thermal cycle curve.

Figure 3. Schematic illustration of the tensile sample. Figure 3. Schematic illustration of the tensile sample . Figure 3. Schematic illustration of the tensile sample. 3. Results Figure 3. Schematic illustration of the tensile sample. 3. Results 3. Results 3.3.1. Results Welding Thermal Cycles 3.1. Welding Thermal Cycles 3.1. WeldingIn the BT-FSW Thermal process, Cycles the friction heat produced by friction between the BT-FSW tool and the workpiece3.1. WeldingIn the andBT Thermal-FSW the deformation process,Cycles the heatfriction produced heat produced by severe by plastic friction deformation between the in BT the-FSW zone tool around and the In the BT-FSW process, the friction heat produced by friction between the BT-FSW tool and the stirringworkpieceIn the pin andBT will-FSW causethe deformation process, the temperature the heatfriction inproduced the heat stirring produced by zonesevere andby plastic friction its vicinity deformation between to rise the rapidly. in BT the-FSW zone The tool distributionaround and the workpiece and the deformation heat produced by severe plastic deformation in the zone around the workpiecestirring pin and will the cause deformation the temperature heat produced in the by stirring severe plastic zone and deformation its vicinity in the to risezone rapidly. around Thethe stirring pin will cause the temperature in the stirring zone and its vicinity to rise rapidly. The stirringdistribution pin of will the cause temperature the temperature field around in thethe stirring zone zone not and only its vicinityaffects the to plastic rise rapidly. flow of Thethe distribution of the temperature field around the stirring zone not only affects the plastic flow of the distribution of the temperature field around the stirring zone not only affects the plastic flow of the

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Metals 2019, 9, x FOR PEER REVIEW 4 of 11 of the temperature field around the stirring zone not only affects the plastic flow of the material but materialalso directly but aalsoffects directly the microstructure affects the microstructure of different regions of different of the regions welded of joint, the suchwelded as thejoint, grain such size, as precipitationthe grain size coarsening, precipitation and dissolution coarsening of and the second dissolution phase. of Furthermore, the second the phase. mechanical Furthermore, properties the mechanicaland welding properties quality of theand joints welding are a ffqualityected. Figureof the 4joints shows are the affected. e ffect of Figure different 4 rotationalshows the speeds effect onof differentthe recorded rotational welding speeds thermal on the cycles. recorded As the welding rotational thermal speed cycles. increases As the from rotational 600 r/min speed to 1000 increases r/min, fromthe peak 600 temperaturer/min to 1000 of r/min, the retreating the peak temperature side increases of from the retreating 463 ◦C to side 509 ◦increasesC. When from the speed 463 °C reached to 509 1200°C. When r/min, the the speed maximum reached temperature 1200 r/min, fell the back maximum to 491.5 temperature◦C. fell back to 491.5 °C. The heat heat production production of of FSW FSW is relatedis related to theto rotationalthe rotational speed speed of the of tool, the yield tool, shear yield stress, shear friction stress, frictioncoefficient, coefficient, contact contact pressure pressure between between the tool the and tool the and matrix, the matrix, etc. For etc. the For aluminum the aluminum alloy, alloy, the yield the yieldstrength strength decreases decreases with increasing with increasing temperature, temperature, so the yieldso the shear yield stress, shear contactstress, contact pressure pressure and friction and frictioncoefficient coefficient will decrease will decrease with increasing with increasing temperature. temperature. Therefore, Therefore, when the rotational when the speed rotational of the speed tool is ofincreased, the tool althoughis increased, the although heat production the heat may production be increased may and be increased the temperature and the will temperature rise, the yield will shear rise, thestress, yield contact shear pressure stress, and contact friction pressure coefficient and will friction decrease coefficient with increasing will decrease temperature, with which increasing will temperature,lead to a decrease which in will the actuallead to heat a decrease production. in the Because actual heat of this, production. FSW can keepBecause the baseof this, metal FSW in can the keepsolid the state base for metal welding. in the solid state for welding.

Figure 4. WeldingWelding thermal thermal cycle cycle curves at didifferentﬀerent rotationalrotational speeds. speeds. RS: RS: retreating retreating side; AS: AS: advancing side.

It can also be seen in Figure 44 that that thethe maximummaximum temperaturetemperature ofof eacheach rotationrotation atat thethe retreatingretreating side (RS) of thethe weldedwelded jointjoint isis higherhigher thanthan the the advancing advancing side side (AS), (AS), and and the the temperature temperature di differencefference is isapproximately approximately 20 20◦C. °C. In In the the BT-FSW, BT-FSW, with with the the rotation rotation of of the the tool, tool, thethe shouldershoulder andand thethe pin not only produce heat by friction but also drive the plastic metals on the upper and lower surfaces of the plate and the weld nugget area to rotate from the AS of the tool to the RS. At the same time, heat transfer occurs when the plastic metals transfer. In addition, in the stir zone (SZ) and thermomechanically affectedaffected zone (TMAZ) of the friction stir welding joint, the welded metal undergoes severe plastic defdeformation,ormation, resultingresulting inin plastic plastic deformation deformation heat. heat. The The deformation deformation and and strain strain rate rate of theof the metal metal on theon theRS areRS are higher higher than than those those on theon the AS, AS, and and the the heat heat of plastic of plastic deformation deformation produced produced by theby the metal metal on theon theRS isRS higher is higher than than that that on theon AS,the AS, so the so temperaturethe temperature onthe on RSthe isRS higher is higher than than that that on the on AS.the AS. 3.2. Microstructure of the Joints 3.2. Microstructure of the Joints The macromorphology of the joints after welding is shown in Figure5. The surfaces of the joint The macromorphology of the joints after welding is shown in Figure 5. The surfaces of the joint have no disfigurement, the weld ripple is arranged regularly, and there are a few flying edges on both have no disfigurement, the weld ripple is arranged regularly, and there are a few flying edges on sides of the weld. Comparing the weld ripple with different rotating speeds, the distance between the both sides of the weld. Comparing the weld ripple with different rotating speeds, the distance ripples is shortened when the rotating speed is increased. Measurements of ripple spacing values are between the ripples is shortened when the rotating speed is increased. Measurements of ripple shown in Table2. The ripple spacing corresponds to the ratio of the forward speed and rotational speed spacing values are shown in Table 2. The ripple spacing corresponds to the ratio of the forward speed of the tool, which is approximately equal to the forward distance of the tool in each circle of rotation. and rotational speed of the tool, which is approximately equal to the forward distance of the tool in each circle of rotation.

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Figure 5. Macromorphology of the weld surface at different rotational speeds . Figure 5. MacromorphologyMacromorphology of the weld surface at different different rotational speeds.speeds. Table 2. The distance of the weld ripple at different rotational speeds. Table 2. The distance of the weld ripple at different rotational speeds. Table 2. The distance of the weld ripple at different rotational speedsThe D. istance Per Welding Speed Rotational Speed Distance of Ripple Welding Speed Rotational Speed Distance of Ripple The Distance Per Revolution No.No. RevolutionThe Distance of the Per Tool Welding(mm/min)(mm Speed/min) Rotational(r/min)(r Speed/min) Distance(mm) of(mm) R ipple of the Tool (mm) No. Revolution(mm) of the Tool (mm/min) (r/min) (mm) 1 1500 500600 6000.82 0.82 (mm)0.833 0.833 2 2500 500800 8000.61 0.61 0.625 0.625 1 3500 500600 10000.82 0.5 0.833 0.5 23 4500 5001000800 12000.610.5 0.41 0.625 0.4170.5 34 500 10001200 0.410.5 0.4170.5 4 500 1200 0.41 0.417 Low magnificationmagnification optical macrographs of the cross cross-section-section of BT BT-FSW-FSW joints as the rotational speed varied from 600 to 1200 r/min are shown in Figure6. With respect to conventional FSW, all the speedLow varied magnification from 600 to optical 1200 r/min macrographs are shown of in the Figure cross 6-.section With respect of BT- FSWto conventional joints as the FSW, rotational all the joints exhibit four distinctive zones between the AS and the RS, i.e., the BM, heat-affected zone (HAZ), speedjoints exhibitvaried fourfrom distinctive 600 to 1200 zones r/min between are shown the inAS Figure and the 6. RS,With i.e., respect the BM, to heatconventional-affected zoneFSW, (HAZ), all the thermomechanically affected zone (TMAZ), and stir zone (SZ), as depicted in Figure6a. jointsthermomechanically exhibit four distinctive affected zones zone (TMAZ)between, the and AS stir and zone the (SZ), RS, i.e., as depictedthe BM, heat in Figure-affected 6a. zone (HAZ), As shown in Figure6, the black line at the center of the SZ is the “S” line. According thermomechanicallyAs shown in Figure affected 6, the zone black (TMAZ) line at ,the and center stir zone of the (SZ), SZ asis thedepicted “S” line. in FigureAccording 6a. to Sato and to Sato and coworkers, in the as-welded condition, the “S” line is composed of broken oxide coworkers,As shown in the in Figureas-welded 6, the condition, black line the at “S”the centerline is ofcomposed the SZ is of the broken “S” line. oxide According particles, to and Sato these and particles, and these broken oxide particles disperse along the fine equiaxed grain boundaries [24]. coworkers,broken oxide in the particles as-welded disperse condition, along the the “S” fine line equiaxed is composed grain of boundaries broken oxide [24 ].particles, Zhang etand al. these [25] Zhang et al. [25] concluded that the “S” line originated from the oxide film on the initial butting brokenconcluded oxide that particles the ‘‘S’’ disperse line originated along the from fine the equiaxed oxide filmgrain on boundaries the initial [2 butting4]. Zhang surfaces et al. and[25] surfaces and experienced only the thermomechanical deformation process without a change in the concludedexperienced that only the the ‘‘S’’ thermomechanical line originated from deformation the oxide process film on without the initial a change butting in surfacesthe chemical and chemical composition. The morphologies of the segregation bands and the “S” line were dependent on experiencedcomposition. onlyThe morphologies the thermomechanical of the segregati deformationon bands process and the without “S” line a changewere dependent in the chemical on the the welding parameters. composition.welding parameters. The morphologies of the segregation bands and the “S” line were dependent on the welding parameters.

Figure 6. 6. CrossCross-sections-sections of ofBT BT-FSW-FSW joints joints welded welded for different for different rotational rotational speeds: speeds: (a) 600 (r/min,a) 600 (b r/)min, 800 (Figurer/min,b) 800 6. (rc/ min,)Cross 1000 (-csections ) r/min 1000 r / andofmin BT ( and-dFSW) 1200 (d joints) 1200 r/min. welded r/min. BM: for BM: base different base metal; metal; rotational HAZ: HAZ: speeds:heat heat-a-affected ff(aected) 600 zone;r/min, zone; TMAZ:(b) 800 thermomechanicallyr/min, (c) 1000 r/min aaffectedff ected and ( zone;zone;d) 1200 SZ:SZ: stir r/min.stir zone.zone. BM: base metal; HAZ: heat-affected zone; TMAZ: thermomechanically affected zone; SZ: stir zone. Figure 7a shows the “S” line schematic diagram of welded joints at different rotational speeds. FromFigure the diagram, 7a shows it canthe “S”be seen line thatschematic the S- linediagram presents of welded an opposite joints “C” at different shape, opening rotational on speeds. the AS From the diagram, it can be seen that the S-line presents an opposite “C” shape, opening on the AS

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MetalsFigure 2019, 9, 7xa FOR shows PEER the REVIEW “S”line schematic diagram of welded joints at di ﬀerent rotational speeds.6 of 11 From the diagram, it can be seen that the S-line presents an opposite “C” shape, opening on the AS side sideand closingand closing on the on RS the side. RS Withside. theWith increase the increase of the rotationalof the rotational speed, thespeed, “S” the line “S” gradually line gradually extends extendsto both sides,to both and sides, the and height the becomesheight becomes narrower. narrower. In BT-FSW, In BT the-FSW, plastic the plastic metal locatedmetal located in front in of front the oftool the will tool be will driven be driven by the by shoulders the shoulders and pin and and pin transferred and transferred to the to gap the formed gap formed by the by advance the advance of the oftool the behind tool behind the stirring the stirring needle, needle, thus forming thus forming a weld, a asweld shown, as shown in Figure in Figure7b. When 7b. theWhen rotation the rotation speed speedis 600 ris/min, 600 r/min, the distance the distance per revolution per revolution of the toolof the is 0.833tool is mm, 0.833 the mm, clearance the clearance formed formed by the toolby the of toolthe agitatorof the agitator is large, is and large, the and molding the molding metal of metal the butt of the surface butt cansurface be transferred can be transferred to a longer to distance.a longer Thedistance. distance The per distance revolution per revolution of the tool of decreases the tool to decreases 0.417 mm to when 0.417 themm rotating when the speed rotating increases speed to 1200increases r/min, to so 1200 the plastic r/min, metal so the is plastic transferred metal closer, is transferred and butting closer, surfaces and are butting only surfaces transferred areto only the transferredside of the stirring to the side needle. of the stirring needle. Series of light and dark stripes can be observed in the SZ, as shown in Figure6 6.. InIn frictionfriction stirstir welding, high temperature plastic metals are stirred, extruded and formed under the action of upper and lower shoulders and pins. The The original original oxide oxide on on the surface and side of the specimen is stirred and broken, broken, and and other other plastic plastic metals metals are are mixed mixed and and transferred transferred to the to the back back of the of thepin; pin;eventually, eventually, the brightthe bright and and dark dark stripes stripes in in the the weld weld are are formed. formed. When When the the stirring stirring speed speed increases, increases, the the stirring intensity increases, increases, and and the the texture texture becomes becomes more more obvious. obvious. At At the the same same time, time, due due to tothe the increase increase of stirringof stirring speed, speed, the the heat heat input input of of joints joints increases, increases, and and more more Al Al matrix matrix is is oxidized oxidized rapidly during welding, which makes the texture more pronounced.pronounced.

Figure 7. Schematic diagram of: (a) the “S” line morphology of the section at different rotational speeds, Figure 7. Schematic diagram of: (a) the “S” line morphology of the section at different rotational (b) the flow process of plastic metal. speeds, (b) the flow process of plastic metal. Figure8 shows the microstructure of the grain morphologies of the di fferent zones in the joint Figure 8 shows the microstructure of the grain morphologies of the different zones in the joint at 1200 r/min. Compared with the elongated coarse and fine grained areas of the BM, as shown in at 1200 r/min. Compared with the elongated coarse and fine grained areas of the BM, as shown in Figure8a, the SZ, as shown in Figure8d, has much smaller equiaxed recrystallized grains. Under Figure 8a, the SZ, as shown in Figure 8d, has much smaller equiaxed recrystallized grains. Under the the action of the pin, the SZ produced a large plastic deformation and formed new grain boundaries action of the pin, the SZ produced a large plastic deformation and formed new grain boundaries and and a higher density of dislocations. Recrystallization occurred at the same time and was formed of a higher density of dislocations. Recrystallization occurred at the same time and was formed of uniform fine equiaxed crystals. Because of the stirring shear effect of the mechanical rotation of the uniform fine equiaxed crystals. Because of the stirring shear effect of the mechanical rotation of the shoulder and the pin, the grains of the TMAZ, as shown in Figure8c,e, were transformed to bending shoulder and the pin, the grains of the TMAZ, as shown in Figure 8c,e, were transformed to bending and elongated grains. Due to the effect of thermal cycling, some grains recrystallized and have a and elongated grains. Due to the effect of thermal cycling, some grains recrystallized and have a more more uniform grain size distribution on the basis of the rolling morphologies of the BM in the HAZ, uniform grain size distribution on the basis of the rolling morphologies of the BM in the HAZ, as as shown in Figure8b,f, and the average grain size of HAZ has increased. shown in Figure 8b,f, and the average grain size of HAZ has increased.

Metals 2019, 9, x FOR PEER REVIEW 6 of 11 side and closing on the RS side. With the increase of the rotational speed, the “S” line gradually extends to both sides, and the height becomes narrower. In BT-FSW, the plastic metal located in front of the tool will be driven by the shoulders and pin and transferred to the gap formed by the advance of the tool behind the stirring needle, thus forming a weld, as shown in Figure 7b. When the rotation speed is 600 r/min, the distance per revolution of the tool is 0.833 mm, the clearance formed by the tool of the agitator is large, and the molding metal of the butt surface can be transferred to a longer distance. The distance per revolution of the tool decreases to 0.417 mm when the rotating speed increases to 1200 r/min, so the plastic metal is transferred closer, and butting surfaces are only transferred to the side of the stirring needle. Series of light and dark stripes can be observed in the SZ, as shown in Figure 6. In friction stir welding, high temperature plastic metals are stirred, extruded and formed under the action of upper and lower shoulders and pins. The original oxide on the surface and side of the specimen is stirred and broken, and other plastic metals are mixed and transferred to the back of the pin; eventually, the bright and dark stripes in the weld are formed. When the stirring speed increases, the stirring intensity increases, and the texture becomes more obvious. At the same time, due to the increase of stirring speed, the heat input of joints increases, and more Al matrix is oxidized rapidly during welding, which makes the texture more pronounced.

Figure 7. Schematic diagram of: (a) the “S” line morphology of the section at different rotational speeds, (b) the flow process of plastic metal.

Figure 8 shows the microstructure of the grain morphologies of the different zones in the joint at 1200 r/min. Compared with the elongated coarse and fine grained areas of the BM, as shown in Figure 8a, the SZ, as shown in Figure 8d, has much smaller equiaxed recrystallized grains. Under the action of the pin, the SZ produced a large plastic deformation and formed new grain boundaries and a higher density of dislocations. Recrystallization occurred at the same time and was formed of uniform fine equiaxed crystals. Because of the stirring shear effect of the mechanical rotation of the shoulder and the pin, the grains of the TMAZ, as shown in Figure 8c,e, were transformed to bending and elongated grains. Due to the effect of thermal cycling, some grains recrystallized and have a more uniformMetals 2019 grain, 9, 894 size distribution on the basis of the rolling morphologies of the BM in the HAZ7 of, as 11 shown in Figure 8b,f, and the average grain size of HAZ has increased.

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Figure 8.8. MicrostructuresMicrostructures viewed viewed with with an an optical optical microscope microscope of theof the joint joint at 1200 at 1200 r/min: r/min: (a) BM, (a) (bBM,) HAZ (b) onHAZ AS, on (c )AS, TMAZ (c) TMAZ on AS, on (d AS,) SZ, (d (e) )SZ, TMAZ (e) TMAZ on RS on and RS (f )and HAZ (f) onHAZ RS. on RS.

Figure9 9 shows shows the the EBSD EBSD micrographs micrographs of of SZ SZ at diat ffdifferenterent rotational rotational speeds. speeds. It canIt can be seenbe seen from from the micrographsthe micrographs that that the rotationalthe rotational speed speed increases increases from from 600 r/600min r/min to 1200 to r1200/min, r/min, and the and average the average grain sizegrain in size SZ decreasesin SZ decreases from 9.7fromµm 9.7 to 7.1μmµ tom, 7.1 as μm shown, as shown in Table in3. Table It is generally 3. It is generally believed believed that during that FSW,during the FSW, microstructures the microstructures in the SZ will in the undergo SZ will complex undergo evolution complex processes, evolution such processes, as discontinuous such as dynamicdiscontinuous recrystallization, dynamic recrystallization, dynamic recovery dynamic and grain recovery growth, and continuous grain growth, dynamic continuous recrystallization, dynamic andrecrystallization, grain coarsening and [ grain26]. When coarsening the metal [26]. is When stirred the directly metal by is the stirred pin, directly severe plastic by the deformation pin, severe willplastic occur, deformation and a large will number occur, and of dislocations a large number will of form. dislocations The stress will imbalance form. The instress the high-densityimbalance in dislocationthe high-density region dislocation results in the region local results migration in the of local grain migration boundaries of andgrain protrusions. boundaries These and protrusions. protrusions areThese cut protrusions and broken are in subsequent cut and broken deformation, in subsequent resulting deformation, in a large numberresulting of in fine a large nuclei. number These of nuclei fine graduallynuclei. These coarsen nuclei and gradually grow under coarsen the and action grow of under friction the heat action and of eventually friction heat form and equiaxed eventually crystals. form Inequiaxed this experiment, crystals. In the this temperature experiment, di thefference temperature caused bydifference the change caused of rotating by the change speed is of not rotating large, butspeed the is degree not large, of deformation but the degree and dynamicof deformation recrystallization and dynamic caused recrystallization by rotating speed caused is greater, by rotating so the grainspeed sizeis greater, decreases so the with grain the increasesize decreases of rotating with speed.the increase of rotating speed.

Figure 9. 9. ElectronElectron backscattered backscattered diffraction diﬀraction ( (EBSD)EBSD) micrographs micrographs of of the SZ obtained at didifferentﬀerent rotational speeds:speeds: (a) 600 rr/min,/min, (b) 800 rr/min,/min, (cc)) 10001000 rr/min/min andand ((dd)) 12001200 rr/min./min.

Table 3. Grain size in SZ at different rotational speeds. Rotation Speed (r/min) Average Grain Size/μm 600 9.1 800 8.5 1000 7.4 1200 7.1

3.3. Mechanical Properties of the Joints Figure 10 shows the variation in the microhardness along the mid-thickness of the joints at different rotational speeds. It can be seen from the diagram that with respect to the BT-FSW joints, all the microhardness distribution profiles exhibit a W-shape. The minimum hardness appeared in the HAZ and was HV 68–74, and it rose to HV 80–94 at the center of the SZ. The hardness of the BM was highest, with an average value of HV 120.

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Table 3. Grain size in SZ at diﬀerent rotational speeds.

Rotation Speed (r/min) Average Grain Size/µm 600 9.1 800 8.5 1000 7.4 1200 7.1

3.3. Mechanical Properties of the Joints Figure 10 shows the variation in the microhardness along the mid-thickness of the joints at diﬀerent rotational speeds. It can be seen from the diagram that with respect to the BT-FSW joints, all the microhardness distribution proﬁles exhibit a W-shape. The minimum hardness appeared in the HAZ and was HV 68–74, and it rose to HV 80–94 at the center of the SZ. The hardness of the BM was highest,Metals 2019 with, 9, x FOR an average PEER REVIEW value of HV 120. 8 of 11

Figure 10. Microhardness distribution of the joints welded at didifferentﬀerent rotationalrotational speeds.speeds.

With the the rotational rotational speed speed increasing increasing from from 600 r 600/min r/min to 1200 to r / 1200min, the r/min, hardness the hardness of the SZ of increased. the SZ Whenincreased. the speed When increases, the speed the increases average, grain the average size in SZ grain decreases, size in and SZ the decreases, dislocation and density the dislocation increases, whichdensity leads increases, to an increasewhich leads in microhardness. to an increase Within microhardnes the increases. of With rotational the increase speed, of the rotational hardness speed, of the HAZthe hardness adjacent of to the the HAZ TMAZ adjacent tends to to the decrease. TMAZ tends According to decrease. to [13,27 According], this is mainly to [13,2 attributed7], this is mainly to the dissolutionattributed to and the precipitationdissolution and of theprecipitation second phase of the and second the change phase and of the the grain change size of of the the grain aluminum size of matrixthe alumi in thenum microstructure matrix in the microstructure of the zone. With of the increasezone. With of rotationalthe increase speed, of rotational the heat inputspeed, and the peakheat temperatureinput and peak increase temperature obviously, increase which obviously, promotes thewhich dissolution promotes of the the dissolutionβ” phase and of the the β″ precipitation phase and andthe precipitation growth of the andβ0 phase. growth Moreover, of the β′ the phase. grain Moreover, size of the aluminumthe grain size matrix of isthe coarser, aluminum which matrix leads tois acoarser, reduction which in the leads hardness to a reduction of the HAZ. in the hardness of the HAZ. The tensile properties of the joints welded at different diﬀerent rotational speeds are shown in Figure 1111.. TheThe graphgraph showsshows thatthat thethe lowestlowest tensiletensile strengthstrength atat 600600 rr/mi/minn isis 197.7197.7 MPa.MPa. TheThe maximummaximum tensiletensile strength is 262.7 262.7 MPa MPa at at a arotational rotational speed speed of of800 800 r/min. r/min. The The elongation elongation reached reached a maximum a maximum at 1000 at 1000r/min, r/min, which which was was8.3%. 8.3%. Figure Figure 12 shows12 shows the the typical typical fracture fracture location location of of joints joints produced produced at at different diﬀerent rotational speeds.speeds. TheThe fracture fracture locations locations of of 600 600 r/min r/min were were found found at the at border the border of the of SZ the and SZ the and TMAZ the ofTMAZ AS and of AS that and of 800that r /ofmin 800 to r/min 1200 rto/min 1200 in r/min the HAZ in the of HAZ AS. of AS.

Figure 11. Tensile properties of the joints at different rotational speeds.

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Figure 10. Microhardness distribution of the joints welded at different rotational speeds. Figure 10. Microhardness distribution of the joints welded at different rotational speeds. With the rotational speed increasing from 600 r/min to 1200 r/min, the hardness of the SZ increased.With When the rotational the speed speed increases increasing, the average from 600 grain r/min size to in 1200 SZ r/min,decreases, the andhardness the dislocation of the SZ densityincreased. increases, When thewhich speed leads increases to an increase, the average in microhardnes grain sizes. inWith SZ the decreases, increase andof rotational the dislocation speed, thedensity hardness increases, of the which HAZ leadsadjacent to an to increasethe TMAZ in tendsmicrohardnes to decrease.s. With According the increase to [1 3of,2 rotational7], this is mainly speed, attributedthe hardness to theof the dissolution HAZ adjacent and precipitation to the TMAZ of tends the second to decrease. phase Accordingand the change to [13 of,2 7the], t hisgrain is mainlysize of theattributed aluminum to the matrix dissolution in the microstructure and precipitation of the of thezone. second With phasethe increase and the of change rotational of the speed, grain the size heat of inputthe alumi andnum peak matrix temperature in the microstructure increase obviously, of the which zone. Withpromotes the increase the dissolution of rotational of the speed, β″ phase the heatand theinput precipitation and peak temperature and growth increase of the β′obviously, phase. Moreover, which promotes the grai then size dissolution of the aluminum of the β″ phase matrix and is coarser,the precipitation which leads and to growth a reduction of the in β′the phase. hardness Moreover, of the HAZ. the grai n size of the aluminum matrix is coarser,The which tensile leads properties to a reduction of the joints in the welded hardness at different of the HAZ. rotational speeds are shown in Figure 11. The graphThe tensile shows properties that the lowestof the joints tensile welded strength at differentat 600 r/mi rotationaln is 197.7 speeds MPa. are The shown maximum in Figure tensile 11. strengthThe graph is 262.7shows MPa that at the a rotational lowest tensile speed strength of 800 r/min. at 600 The r/mi elongationn is 197.7 reachedMPa. The a maximummaximum at tensile 1000 r/min,strength which is 262.7 was MPa 8.3%. at Figurea rotational 12 shows speed the of typical 800 r/min. fracture The elongationlocation of reachedjoints produced a maximum at different at 1000 rotationalr/min, which speeds. was 8.3%.The fracture Figure 1locations2 shows theof 600 typical r/min fracture were foundlocation at ofthe joints border produced of the SZat differentand the rotational speeds. The fracture locations of 600 r/min were found at the border of the SZ and the MetalsTMAZ2019 of, AS9, 894 and that of 800 r/min to 1200 r/min in the HAZ of AS. 9 of 11 TMAZ of AS and that of 800 r/min to 1200 r/min in the HAZ of AS.

Figure 11. Tensile properties of the joints at different rotational speeds. Figure 11. Tensile properties of the joints at didifferentﬀerent rotationalrotational speeds.speeds.

Metals 2019, 9, x FOR PEER REVIEW 9 of 11

Figure 12. TypicalTypical fracture location of joints at didifferentﬀerent rotationrotation speeds:speeds: (a)) 600600 rr/min;/min; ((bb)) 800800 rr/min./min.

The fracture surfaces surfaces of of all all specimens specimens were were analyzed analyzed by by scanning scanning electron electron microscopy microscopy (SEM), (SEM), as asshown shown in in Figure Figure 13 13. When. When the the rotating rotating speed speed is is 600 600 r/min, r/min, the the heat heat input input is is low, low, thethe dynamic dynamic recrystallization in in the the SZ SZ region region is low is and low the and grain the size grain is large. size In is addition, large. In the addition, relative displacement the relative ofdisplacement the tool is the of largest the tool in is each the rotatinglargest in circle, each and rotating the extrusion circle, and force the of extrusion the tool on force the metalof the transferredtool on the bymetal stirring transferred decreases, by stirring which resultsdecreases, in the which fracture results of thein the weak fracture area betweenof the weak SZ andarea TMAZ.between Fracture SZ and morphology,TMAZ. Fracture as shown morphology in Figure, as 13showna, shows in Fig thature this 13a type, shows of fracture that this presents type of fracture a large number presents of a smalllarge andnumber shallow of small dimples and withshallow layered dimples distribution, with layered which distribution, belongs to which a ductile belongs fracture. to a ductile fracture.

Figure 13. TypicalTypical facture surfaces of joints at didifferentﬀerent rotation speeds:speeds: (a)) 600600 rr/min,/min, ((bb)) 800800 rr/min./min.

When thethe rotationrotation speed speed increases increases to to more more than than 800 800 r/min, r/min, the the SZ SZ is stirredis stirred suffi sufficiently,ciently, and and the finethe grainsfine grains formed formed by dynamic by dynamic recrystallization recrystallization improve improve the strength the strength of this zone. of this However, zone. However, with increasing with heatincreasing input, heat the strengthenedinput, the strengthened phase dissolves, phase dissolves, and the grain and the size grain increases size increases in the HAZ. in the As HAZ. a result, As thea result, strength the strength and hardness and hardness of the region of the are region reduced. are reduced. When the When load isthe applied load is and applied reaches and the reaches yield strength,the yield the strength, HAZ will the HAZ yield willand deformyield and first. deform It may first. be Itbecause may be of because the diff erent of the work different hardening work exponentshardening ofexponents the HAZ of under the HAZ stress under that the stress joint that breaks the injoint the breaks HAZ onin the AS duringHAZ on tension. AS during The tension. fracture morphologyThe fracture ofmorphology Figure 13b of shows Figure that 13 theb shows fracture that of the HAZ fracture is mainly of HAZ dimple is mainly fractures, dimple with largefractures, and shallowwith large dimples and shallow and white dimples AlFeMnSi and white second-phase AlFeMnSi particles second- atphase the bottom.particles at the bottom.

4. Conclusion In this paper, 6082-T6 aluminum alloy was successfully welded by BT-FSW using different rotation speeds. The typical microstructures and mechanical properties of every joint were investigated. The main conclusions can be summarized as follows. 1. The maximum temperature at the joint increases first and then decreases with increasing rotational speed, reaching a maximum temperature of 509 °C at 1000 r/min. The maximum temperature at the RS of the joints is higher than the AS, and the temperature difference is approximately 20 °C. 2. The macromorphology of the cross-section of the joint is rectangular, and an “S” line and gray- white texture can be observed. The “S” line is an opposite “C” shape, and the opening is on the AS side. With increasing welding speed, the “S” line is elongated and narrowed. 3. In the welded joint, the SZ has much smaller equiaxed recrystallized grains, and the TMAZ are elongated to form bending and elongated grains. In the HAZ, some grains are recrystallized and have a better grain size uniformity based on the rolling morphologies of BM. With increasing welding speed, the average grain size in the SZ region decreases from 9.1 micron to 7.1 micron. 4. The joints present a W-shaped hardness profile along their cross-section. The minimum hardness appears in the HAZ and is HV 68–74, rising to HV 80–94 at the center of the SZ. When the rotational speed increases from 600 r/min to 1200 r/min, the hardness of HAZ decreases gradually, and the hardness of SZ increases.

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4. Conclusions In this paper, 6082-T6 aluminum alloy was successfully welded by BT-FSW using different rotation speeds. The typical microstructures and mechanical properties of every joint were investigated. The main conclusions can be summarized as follows. 1. The maximum temperature at the joint increases first and then decreases with increasing rotational speed, reaching a maximum temperature of 509 ◦C at 1000 r/min. The maximum temperature at the RS of the joints is higher than the AS, and the temperature difference is approximately 20 ◦C. 2. The macromorphology of the cross-section of the joint is rectangular, and an “S” line and gray-white texture can be observed. The “S” line is an opposite “C” shape, and the opening is on the AS side. With increasing welding speed, the “S” line is elongated and narrowed. 3. In the welded joint, the SZ has much smaller equiaxed recrystallized grains, and the TMAZ are elongated to form bending and elongated grains. In the HAZ, some grains are recrystallized and have a better grain size uniformity based on the rolling morphologies of BM. With increasing welding speed, the average grain size in the SZ region decreases from 9.1 micron to 7.1 micron. 4. The joints present a W-shaped hardness profile along their cross-section. The minimum hardness appears in the HAZ and is HV 68–74, rising to HV 80–94 at the center of the SZ. When the rotational speed increases from 600 r/min to 1200 r/min, the hardness of HAZ decreases gradually, and the hardness of SZ increases. 5. When the rotational speed is 600 r/min, the strength is the lowest. The fracture location between SZ and TMAZ. When the rotational speed is increased, the fracture location is all in HAZ, and the fracture surface is dimple-like. The ultimate tensile strength significantly increases and reaches a maximum (262.7 MPa) at 800 r/min; the engineering strain is 7.9%.

Author Contributions: Conceptualization, W.G. and Y.L.; methodology, Y.L.; investigation, Y.L.; writing—original draft preparation, Y.L.; writing—review and editing, Y.L. and W.G.; supervision, D.S. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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