The Joint Properties of 5754 Aluminium Alloy by Friction Stir Spot Welding

Journal of Manufacturing and Materials Processing

Article The Joint Properties of 5754 Aluminium Alloy by Friction Stir Spot Welding

Orhan Dedeo˘glu 1,* and Hande Güler Özgül 2

1 Department of Automotive Engineering, Graduate School of Natural and Applied Sciences, Uludag University, 16059 Gorukle-Bursa, Turkey 2 Department of Automotive Engineering, Faculty of Engineering, Uludag University, 16059 Gorukle-Bursa, Turkey; [email protected] * Correspondence: [email protected]; Tel.: +90-507-162-34-23

Received: 20 December 2018; Accepted: 15 January 2019; Published: 16 January 2019

Abstract: In this study, AA5754 sheet samples joined by using a friction stir spot welding method using diﬀerent rotational speed parameters has been analyzed experimentally. During the experiments, rotational speed of the tool was changed while other process parameters and the tool geometry were kept constant. The eﬀect of tool rotational speed on the hardness values, macrostructure, and tensile properties of joints has been investigated and the results of the experiments show the best tensile shear strength and hardness values are obtained for the tool rotation speed of 1850 rpm. According to the macroscopic investigation, all of the fractured samples failed by nugget pullout and the fractured samples have only a single type fracture pattern.

Keywords: aluminum alloys; friction stir spot welding; macrostructure; mechanical properties

1. Introduction Weight reduction without affecting the safety performance is a great challenge in the manufacturing of automotive in order to improve fuel economy and reduce emissions. It has been reported that fuel devouring can be decreased by 5.5% for each 10% decline in vehicle weight and a 0.45 kg decline in the weight of a car would decline carbon dioxide emissions by 9.07 kg over the life of the vehicle. An automobile consists of outer panels and a platform, which is typically made of steel and contains the drive system, engine system, and exhaust system. The weight of the platform is around 70% of the total weight of an automobile [1]. Steel has been applied widely in the automotive industry because of its wide range of desirable properties, ease of processing, availability, and recyclability. However, lightweight materials like aluminum have obvious advantages over steel with comparable properties but nearly three times lower density, a resistance of high corrosion, and a high degree of usage reaching 85–95%. Aluminum alloys are promising candidates for replacing equivalent steel assemblies and the use of them in the manufacturing of automotive is increasing recently [1]. For replacing steel with aluminum in the structure of automobiles, it is necessary to explore joining methods that can be used efficiently. Current panel welding techniques used to join steels involve resistance spot welding and self-piercing rivets. However, these welding techniques cannot be applied easily to aluminum alloy, because of its physical properties, particularly surface oxide film. Friction stir spot welding is a derivative of friction stir joining, which was developed by TWI (Abington, UK) in 1991 as a solid-state method for aluminum alloy joining. This novel joining mechanism is advantageous for producing aluminum joints without contamination, blowholes, porosity, and cracks [1]. This study is focused on aluminum alloy 5754, which is the most common aluminum series utilized by the automotive industry. There are many studies on friction stir spot welding of aluminum alloys.

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Er [2] studied the optimum welding limits for the friction stir spot welding of AA5005 sheet samples by comparing the mechanical properties of friction stir spot welding and the resistance spot welding method. He found that the samples which are joined by friction stir spot welding have better mechanical properties compared to the samples that are joined with resistance spot weld. Kulekci and Er [3], studied the effect of dwell time, tool rotational speed, and tool plunge depth parameters of friction stir spot welding using AA5005 sheet samples. They concluded that the optimum welding parameter ranges for; tool dwell time between 5 s and 10 s, tool rotational speed between 1500 rpm and 2000 rpm, and tool plunge depth between 2.2 mm and 2.6 mm. In addition, it is found that tool plunge depth is the most important parameter that influences welding performance. Kacar et al. [4] studied friction stir spot welding of AA5754 sheet samples using copper interlayer. It was found that the tensile shear load carrying capacities of AA5754 (on the top)—Cu (on the middle)—AA5754 (on the bottom) material welded by using copper interlayer was greater than the welded AA5754 (on the top)—AA5754 (on the bottom) material with the same welding limits without using the copper interlayer. Bilici et al. [5] searched friction stir spot welding of AA5754-H22 and AA2024-T3 sheet samples by using the Taguchi method. As a result, two types of fracture type; cross-nugget and nugget fracture occurred after the tensile shear test. Piccini and Svoboda [6] studied friction stir spot welding of AA5052-H32 and low carbon steel sheet samples and in addition to the mechanical properties they studied the effect of tool geometry and depth limits of the process. Jeon et al. [7] explored the mechanical properties of AA5052-H32 and AA6061-T6 sheet samples welded by friction stir spot welding process. The value of the highest tensile shear strength was obtained for the AA5052 (on the top)—AA5052 (on the bottom) sample combination. The value of the lowest tensile shear strength was obtained for the AA6061 (on the bottom)—AA6061 (on the top) sample combination. Due to its potential in light-weigthing applications of automotive and other relevant industries, friction stir spot welding can be spread in many industries. For this purpose, three different experimental configurations were created with AA5754 sheet samples (thickness of 1 mm). These samples were welded by friction stir spot welding with three different unstudied tool rotational speeds and most common tool geometry. Then, tensile shear strength test, microhardness test, and macrostructure analysis were performed on these samples.

2. Experimental Procedure

2.1. Material and Its Properties In this study, the aluminum alloy 5754 sheets of 1 mm thickness were produced in accordance with the American Society for Testing and Materials (ASTM) B209-10 standard. The chemical composition of the aluminum alloy 5754 is given in Table1 and the mechanical properties are given in Table2, according to ASTM B209-10, BS EN 485-2:2016, respectively. The test samples were prepared in 45 mm 105 mm 1 mm dimensions according to BS EN ISO 14273:2016 (Figure1a,b). × × Table 1. Chemical composition of the AA5754.

Cu Si Fe Cr Mg Ti Mn Zn Other Elements Al Chemical 0.1 0.4 0.4 0.3 2.6–3.6 0.15 0.5 0.2 0.15 94.2–95.2 Composition (%)

Table 2. Mechanical properties of the AA5754.

Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) Hardness (HBV) 80 190–240 14 52 J. J.Manuf. Manuf. Mater. Mater. Process. Process. 2019 2019, 3, ,3 x, xFOR FOR PEER PEER REVIEW REVIEW 3 3of of 11 11

TableTable 2. 2. Mechanical Mechanical properties properties of of the the AA5754. AA5754.

YieldYield Strength Strength TensileTensile Strength Strength ElongationElongation HardnessHardness (MPa) (MPa) (%) (HBV) J. Manuf. Mater. Process. 2019(MPa), 3, 8 (MPa) (%) (HBV) 3 of 11 8080 190–240190–240 1414 5252

(a()a ) (b(b) ) Figure 1. Dimensions (a) and image (b) of test sample. FigureFigure 1. Dimensions1. Dimensions (a) ( anda) and image image (b )(b of) testof test sample. sample.

2.2.2.2. Friction Friction Friction Stir Stir Spot Spot Welding Welding Application Application FrictionFriction stir stirstir spotspot weldingwelding welding involvesinvolves involves simply simply simply plunging plunging plunging the the rotatingthe rotating rotating tool intotool tool theinto into workpiece, the the workpiece, workpiece, holding holdingholdingit there it forit there there a certain for for a acertain dwellcertain timedwell dwell and time time then and and retracting then then re retractingtracting it (Figure it it (Figure 2(Figure). The 2). process2). The The process process is usually is is usually usually used for used used lap forforjoints lap lap joints where joints where thewhere materials the the materials materials to be joined to to be be joined are joined stacked are are stacked stacked on top on of on onetop top of another of one one another another with a with backing with a abacking backing plate or plate anvilplate ororunderneath anvil anvil underneath underneath to support to to support support the downforce. the the downforce. downforce. The rotatingThe The rotating rotating tool penetratestool tool penetrates penetrates the topthe the surface,top top surface, surface, causing causing causing heat heatheatby frictionby by friction friction and and softeningand softening softening the the material the material material adjacent adjacent adjacent to theto to the tool.the tool. tool. This This This material material material ﬂows flows flows plastically plastically plastically under under under the thethecompressive compressive compressive and and and rotational rotational rotational forces, forces, forces, thus thus thus when when when the the the tool tool tool is is removed,is removed, removed, the the the materials materials materialshave have have joinedjoined joined withwith with a a asolid-phasesolid-phase solid-phase bondbond bond inin in the the the weld weld weld region. region. region. Successful Successful Successful welding welding welding depends depends depends on on theon the the tool tool tool plunge plunge plunge depth, depth, depth, the the toolthe toolplungetool plunge plunge rate, rate, rate, the the tool the tool rotationaltool rotational rotational speed, speed, speed, the th tool the etool dwelltool dwell dwell time, time, time, and and theand the tool the tool geometrytool geometry geometry [8]. [8]. [8].

FigureFigure 2. 2. Schematic SchematicSchematic drawing drawing drawing of of ofthe the the friction friction friction stir stirstir spot spot spot welding welding welding application application application (A: (A: Plunging, (A: Plunging, Plunging, B: B: Stirring, Stirring, B: Stirring, C: C: Retracting)Retracting)C: Retracting) [8]. [8]. [ 8].

TheThe samples samples prepared prepared prepared for for for the the the experiment experiment experiment were were were posi posi positionedtionedtioned on on ontop toptop of of each ofeach each other other other (overlap (overlap (overlap area area area45 45 mm45 mm× 45 mm)45 mm) as shown as shown in inFigure Figure 3a3a,b. and The Figure test samples3b. The weretest samples placed in were the welding placed ﬁxturein the (Figurewelding4) mm × 45× mm) as shown in Figure 3a and Figure 3b. The test samples were placed in the welding fixturefixturein order (Figure (Figure to ﬁx 4) them4) in in order order during to to fix operationfix them them during during and theoperation operation friction and and stir the spotthe friction friction welding stir stir processspot spot welding welding was applied process process towas was the appliedappliedcenter ofto to thethe the overlapcenter center of areaof the the as overlap overlap shown area inarea Figure as as shown shown3b. During in in Figure Figure the experiments,3b. 3b. During During the the a universalexperiments, experiments, milling a auniversal universal machine millingwasmilling used machine machine for the was was friction used used stirfor for spotthe the friction weldingfriction stir stir process. spot spot welding welding process. process.

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(a()a ) (b(b) )

FigureFigure 3. 3.(a )(a The ) The isometric isometric view view forfor weldingwelding position of of the the samples. samples. ( b( (b)b )The) The The dimension dimension of of ofthe the top top viewview and and the the side side view view for for the the weldingwe weldinglding positionposition of the samples. samples.

Figure 4. The welding fixture. Figure 4. The welding fixture. ﬁxture. 2.3. Friction Stir Spot Welding Tool 2.3. Friction Friction Stir Stir Spot Welding Tool The X153CrMoV12 (1.2379) tool steel produced according to ISO 4957:2018 standard, was used The X153CrMoV12 (1.2379)(1.2379) tool tool steel steel produced produced according according to to ISO ISO 4957:2018 4957:2018 standard, standard, was was used used for for the friction stir spot welding process. The chemical composition of tool steel is given in Table 3. forthe the friction friction stir stir spot spot welding welding process. process. The The chemical chemical composition composition of tool of tool steel steel is given is given in Table in Table3. 3. Table 3. Chemical composition of tool steel. Table 3. Table 3. Chemical composition of tool steel. MnMn Si Si V VCr Cr C Mo C Mo Chemical Mn Si V Cr C Mo Chemical Composition (%) 0.2–0.6 0.1–0.6 0.7–1.0 11–13 1.45–1.6 0.7–1 CompositionChemical 0.2–0.6 0.1–0.6 0.7–1.0 11–13 1.45–1.6 0.7–1 Composition(%) 0.2–0.6 0.1–0.6 0.7–1.0 11–13 1.45–1.6 0.7–1 The geometry of(%) the tool is circular pin and this type of geometry has been used by many researchersThe geometry in the literature of the fortool AA5754 is circular material pin and [2, 3this,9,10 type]. Dimensions of geometry and has image been ofused the circularby many pin areresearchers givenThe ingeometry Figure in the5 a,b.literatureof the In addition,tool for isAA5754 circular the pin material ispin without and [2,3,9,10]. this a thread. type Dimensions of Heat geometry treatment and image has was been of applied the used circular toby the pinmany tool researcherssteel.are Thegiven hardness in Figurethe literature of 5a,b. the tool In for addition, steel AA5754 after the thematerial pin heat is without treatment [2,3,9,10]. a thread. reached Dimensions Heat 52-54 treatment and Hardness image was of of Rockwellapplied the circular to (HRC). the pin aretool given steel. in TheFigure hardness 5a,b. Inof theaddition, tool steel the after pin theis without heat treatment a thread. reached Heat 52-54treatment Hardness was appliedof Rockwell to the tool(HRC). steel. The hardness of the tool steel after the heat treatment reached 52-54 Hardness of Rockwell (HRC).

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(a) (b) Figure 5. Dimensions (a) and image (b) of the tool used for friction stir spot welding experiments. Figure 5. Dimensions (a) and image (b) of the tool used for friction stir spot welding experiments.

2.4. Friction Friction Stir Stir Spot Spot Welding Experiment Limits Experiments of the friction stir spot welding were carried out for three different different tool rotational speeds of 1350, 1850, and 2530, respectively. Accordin Accordingg to the literature data, these tool rotational speeds were not utilized for the friction stir spot welding process of the AA5754 material. In addition to thethe variable variable tool tool rotational rotational speed speed the other the processother process parameters; parameters; plunge depth, plunge shoulder depth, penetration, shoulder penetration,tilt angle, and tilt dwell angle, time and were dwell kept time constant. were kept Details constant. of the constant Details parametersof the constant are givenparameters in Table are4. givenThe in Table constant 4. and variable parameters which have been used during the experiments are given in TableThe4 constant and the and experimental variable parameters configuration which details have are been given used in during Table5 the. For experiments each experimental are given inconfiguration, Table 4 and fivethe samplesexperimental were configuration produced, and deta fourils of are five given samples in Table were 5. utilized For each for experimental tensile shear configuration,strength test. The five other samples sample were was produced, used for and the microhardnessfour of five samples measurement. were utilized for tensile shear strength test. The other sample was used for the microhardness measurement. Table 4. Process parameters. Table 4. Process parameters. Dimension Value ToolDimension rotational speed 1350, 1850,Value 2530 rpm Tool rotationalPlunge depth speed 1350, 1.61850, mm 2530 rpm ShoulderPlunge depth penetration 0.1 1.6 mm mm Tilt angle 0 Shoulder penetration 0.1◦ mm Dwell time 6 s Tilt angle 0° Dwell time 6 s Table 5. Experimental configuration of the friction stir spot welding process. Table 5. Experimental configuration of the friction stir spot welding process. Test Case Test Code Tool Rotational Speed (rpm)

Test Case1 Test P1350 Code Tool Rotational 1350 Speed (rpm) 2 P1850 1850 3 P2530 2530 1 P1350 1350 2 P1850 1850 2.5. Microhardness Testing 3 P2530 2530 Microhardness testing was performed on the weld sections in order to establish local yield stresses. 2.5.In order Microhardness to prepare Testing the samples for the microhardness test, samples were cut from the middle of the weld center (weld cross section). Samples were mounted and polished using standard metallographic techniquesMicrohardness with sand testing papers, was while performed paying particularon the weld attention sections to keepin order the topto andestablish bottom local surfaces yield stresses.in parallel. In order The microhardnessto prepare the testsamples measurements for the microhardness were carried test, out withsamples Metkon were MH-3 cut from Vickers the middleMicrohardness of the weld Test center Machine. (weld Tests cross were section). conducted Samples under were the parametermounted and which polished are at ausing load standard of 100 gr, metallographic techniques with sand papers, while paying particular attention to keep the top and bottom surfaces in parallel. The microhardness test measurements were carried out with Metkon

J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 6 of 11 J. Manuf. Mater. Process. 2019, 3, 8 6 of 11 MH-3 Vickers Microhardness Test Machine. Tests were conducted under the parameter which are at a load of 100 gr, duration of 10 s, and a spacing of 1 mm. Microhardness measurements of the joint materialduration were of 10 performed s, and a spacing along ofthe 1 cross-section mm. Microhardness centerline measurements of the top sheet of the left joint side material weld area were as performedshown in Figure along 6. the cross-section centerline of the top sheet left side weld area as shown in Figure6.

Figure 6. TheThe microhardness microhardness measuremen measurementt position of the sample.

2.6. Tensile Shear Strength Test The tensile shear strength test was performed on four of the five five samples for each experimental configurationconfiguration to determine the tensile shear load.load. The universal testing machine load capacity of 250 kN was utilized to perform the experiments and tests were carried out using a crosshead travel velocity of 5 mmmm/min./min. 2.7. Macrostructure Analysis 2.7. Macrostructure Analysis The macrostructure of fracture surfaces was examined visually. The macroscopic analysis was The macrostructure of fracture surfaces was examined visually. The macroscopic analysis was performed based on qualitative observation visual examination, and the macrostructure of the welding performed based on qualitative observation visual examination, and the macrostructure of the fracture surfaces was investigated. The examination was carried out for all the samples which were welding fracture surfaces was investigated. The examination was carried out for all the samples used in the tensile test. which were used in the tensile test. 3. Experimental Results 3. Experimental Results 3.1. Tensile Properties of Joints 3.1. Tensile Properties of Joints During the experiments, the tensile shear load of the joints was obtained, and the average values of theDuring tensile the test loadsexperiments, were calculated the tensile for allshear experimental load of the configurations. joints was obtained, The graphical and presentationthe average valuesof the averageof the tensile tensile test shear loads load were values calculated is shown for in Figure all experimental7. configurations. The graphical presentationThe average of the tensile average shear tensile load valuesshear load of experimental values is shown configurations in Figure are7. 1.1725, 1.35 and 1.1733 kN for 1350,The 1850,average and tensile 2530 rpm,shear respectively. load values of experimental configurations are 1.1725, 1.35 and 1.1733 kN forThe 1350, diff erence1850, and between 2530 rpm, the tensile respectively. shear load value obtained at 1350 and 2530 rpm, was found to be quiteThe low. difference The maximum between tensile the tensile shear shear load wasload measured value obtained as 1.35 at kN 1350 at 1850 and rpm 2530 and rpm, the was minimum found totensile be quite shear low. load The was maximum measured tensile as 1.17 shear kN at load 1350 was rpm. measured The tensile as shear1.35 kN load at was1850 augmented rpm and the by 15.38%minimum when tensile the tool shear rotation load speedwas measured was increased as 1.17 from kN 1350 at rpm1350 to rpm. 1850 The rpm. tensile On the shear other load hand, was the augmentedtensile shear by load 15.38% was decreased when the bytool 13.33%, rotation while spee thed toolwas rotationincreased speed from was 1350 increased rpm to from1850 1850rpm. rpm On theto 2530 other rpm. hand, The the obtained tensile resultsshear load show was that decrease the toold rotational by 13.33%, speed while was the an tool important rotation parameter speed was in increasedlap shear strength.from 1850 rpm to 2530 rpm. The obtained results show that the tool rotational speed was an important parameter in lap shear strength.

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Figure 7. Shear load for the friction stir spot welding via welding tool at different tool rotational Figurespeed. 7.7. ShearShearload load for for the the friction friction stir stir spot spot welding welding via weldingvia welding tool attool di ffaterent different tool rotational tool rotational speed. speed. The tensile shear load was increased for all the experiments, as the tool rotation speed was increasedThe tensile from 1350 shear to load 18501850 was rpm.rpm. increased According According for to to all Güle Güler ther experiments,[11] [11] explained as that the antool increase rotation in speed the tensile was shearincreased loadload from maymay be1350 be explained explainedto 1850 withrpm. with theAccording the microstructural microstructural to Güler changes [11] changes explained due todue the that to heat thean generation increaseheat generation in at the a stirringtensile at a location.stirringshear load location. On may the otherbeOn explainedthe hand, other the hand,with tensile thethe shear microstructuraltensile load sh wasear decreasedload changes was decreased when due theto the toolwhen heat rotational the generation tool speedrotational wasat a increasedspeedstirring was location. from increased 1850 On to thefrom 2530 other rpm.1850 hand, Thisto 2530 the situation tensilerpm. canThis shear be situation explained load was can bydecreased morebe explai frictional whenned bythe energy more tool generation rotationalfrictional dueenergyspeed to thewasgeneration increment increased due of from toolto the rotational1850 increment to 2530 speed ofrpm. tool and This rotational the fluiditysituation speed of can the and plasticizedbe theexplai fluidityned material by of morethe in theplasticized frictional stirring zonematerialenergy being generation in increased,the stirring due as zone to supported the being increment increased, by reference of tool as supported [rotational12]. byspeed reference and the [12]. fluidity of the plasticized material in the stirring zone being increased, as supported by reference [12]. 3.2. Hardness Values of Joints 3.2. HardnessThe Vickers Values hardness of Joints values were obtained for the three different different rotational speeds of the joints weldedThe by Vickers the welding hardness tool values are shownshown were ininobtained FigureFigure8 8.for. the three different rotational speeds of the joints welded by the welding tool are shown in Figure 8.

Figure 8.8. TheThe Vickers Vickers hardness hardness values values of frictionof friction stir spotstir weldingspot welding via the via welding the welding tool at threetool diatﬀ threeerent rotationaldifferentFigure 8. rotational speeds.The Vickers speeds. hardness values of friction stir spot welding via the welding tool at three different rotational speeds. The hardness values for the friction stir spot welding via the welding tool are shown in FigureThe 8. Maximumhardness values and minimum for the frictionhardness stir values spot werewelding obtained via the as 92.2welding and 71.6tool HVare atshown the tool in Figure 8. Maximum and minimum hardness values were obtained as 92.2 and 71.6 HV at the tool

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The hardness values for the friction stir spot welding via the welding tool are shown in Figure8. Maximum and minimum hardness values were obtained as 92.2 and 71.6 HV at the tool rotational speeds of 1850 and 1350 rpm, respectively. Minimum and maximum hardness values of all experimental conﬁgurations are presented in Table6.

Table 6. Maximum and minimum hardness values for diﬀerent experimental conﬁgurations.

Case Tool Rotational Speed Min. Hardness (HV) Max. Hardness (HV) 1 1350 71.6 85.6 2 1850 75.4 92.2 3 2530 73.0 86.7

In the keyhole area of the joints, greater hardness values were obtained. Regarding this situation, Güler [13] explained that the plastic deformation in the welding process causes strain hardening, which is the main reason of the hardness increase. During the welding process, fine grains that consisted with the dynamic recrystallizations are the reason for this situation. According to Figure8, from the weld center to the base metal hardness profile, it is firstly reduced and then a gradual increment occurs, and the trend of the hardness profile is in good agreement with the existing literature data [6,14–16]. As a result of the tensile shear test, it is found that test groups with maximum strength values have maximum hardness values. The plastic deformation generates an increment in the hardness and strength and Güler [13] reported that both of them are raised together, contingent upon dynamic recrystallization.

3.3. Macrostructure of Joints In the literature, generally, three failure modes are observed after the shear tests for friction stir spot welding. These are, mode 1—shear failure: The failure occurs along the joint line of betwixt the sheets (sheet thickness> 2 mm); mode 2—mixed cleavage failure: The diminish stamina failure begins by cleavage, pursuing the oxide debris describing the surfaces betwixt the sheets; and mode 3—nugget pullout: The joint’s interface doesn’t fail, and the tears circumference the shoulder edge on the upper sheet contact surface, and are separated and connected to the welding nugget on the lower sheet. This failure mode is also stimulated by thinning of the top sheet on the side of the tool shoulder, depending on the tool shoulder plunge process [13]. In this study, macrostructures of fractured surfaces of the samples were investigated for all experimental configurations. All the test samples were investigated based on qualitative visual inspection. The failure modes of all fractured welding joints are shown in Figure9. In addition, upper sheet top view, upper sheet back view, lower sheet top view, and the average shear load values are given in Figure9. In all fractures, failure mode 3 was observed as a single type of fracture pattern. A single fracture pattern; failure mode 3 was observed for all experimental configurations and the increase in the tool speed from 1350 to 2530 rpm did not affect the failure pattern. A nugget area around the tool pin length was formed by plasticized material blending into each other during the friction stir spot welding. The nugget area is given in Figure 10a,b. The nugget area contains materials of lower and upper sheets due to the physical phenomenon of the process. During the tensile shear test, the fracture occurs at the outer contour of the welding area and proceeds around the welded section, and it keeps proceeding until the lower and upper sheets are separated from each other. It was observed that the fracture propagation occurs parallel according to the upper sheet surface. Therefore, this failure type is named as a nugget pullout failure and our findings are supported by the existing literature data. J. Manuf. Mater. Process. 2019, 3, 8 9 of 11 J. Manuf. Mater. Process. 2019, 3, x FOR PEER REVIEW 9 of 11

Average Upper Sheet Upper Sheet Lower Sheet Shear Type of Top View Back View Top View Load Failure Test Code Test

Test Group Test (kN)

P1350 - 1 - P1350 Mag = X4 Mag = X4 Mag = X4

C P1350 - 2 - P1350 Mag = X4 Mag = X4 Mag = X4 1.1725 P1350

P1350 - 3 - P1350 Mag = X4 Mag = X4 Mag = X4

P1350 - 4 - P1350 Mag = X4 Mag = X4 Mag = X4

P1850 - 1 - P1850 Mag = X4 Mag = X4 Mag = X4

P1850 - 2 - P1850 Mag = X4 Mag = X4 Mag = X4 1.3525 P1850

P1850 - 3 - P1850 Mag = X4 Mag = X4 Mag = X4

P1850 - 4 - P1850 Mag = X4 Mag = X4 Mag = X4

P2530 - 1 - P2530 Mag = X4 Mag = X4 Mag = X4

P2530 - 2 - P2530 Mag = X4 Mag = X4 Mag = X4 1.1733 P2530

P2530 - 3 - P2530 Mag = X4 Mag = X4 Mag = X4

P2530 - 4 - P2530 Mag = X4 Mag = X4 Mag = X4 FigureFigure 9. 9.Failure Failure modes modesof of brokenbroken testtest samples. (C: (C: Mode Mode 3; 3; nugget nugget pullout pullout failure). failure).

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(a) (b) Figure 10. (a) The cross-section view of the friction stir spot welding and (b) top view of the nugget area and the Figure 10. (a) The cross-section view of the friction stir spot welding and (b) top view of the nugget stirring zone [12]. area and the stirring zone [12].

4.4. Conclusions Conclusions InIn this this work, work, the the e effectsffects ofof thethe tooltool rotationalrotational speed and and the the tool tool geometry geometry parameters parameters on on the the mechanicalmechanical properties properties of theof frictionthe friction stir spotstir weldingspot welding of the AA5754of the aluminumAA5754 aluminum alloy were alloy investigated were ininvestigated terms of hardness in terms valuesof hardness and fracturevalues and morphologies. fracture morphologies. The tool The geometry tool geometry is a circular is a circular pin and pin and rotational tool speed is taken as 1350, 1850, and 2530 rpm for experimental configurations. rotational tool speed is taken as 1350, 1850, and 2530 rpm for experimental configurations. The following conclusions were made: The following conclusions were made: 1. Increasing rotational tool speed from 1350 to 1850 rpm caused an increase in the tensile shear 1. Increasingload and harness rotational values tool speedbut they from are 1350 diminished to 1850 rpmwhen caused the tool an speed increase is increased in the tensile to 2350 shear rpm. load andIt is harness concluded values that but the they plastic are diminisheddeformation whenin the the welding tool speed process isincreased causes strain to 2350 hardening, rpm. It is concludedwhich is the that main the reason plastic for deformation the hardness in theincrease welding. During process the welding causes strain process, hardening, fine grains which that is theconsisted main reason with dynamic for the hardness recrystallizations increase. are During the reason the welding for this process, situation. fine grains that consisted 2. withIt isdynamic found that recrystallizations experimental configurations are the reason forwith this the situation. maximum strength values have the 2. Itmaximum is found that hardness experimental values. configurations The best value with of thetensile maximum shear strength strength and values hardness have the are maximum taken hardnessplace at the values. tool rotation The best speed value of of 1850 tensile rpm. shear strength and hardness are taken place at the tool 3. rotationA single speed fracture of 1850pattern; rpm. failure mode 3 was observed for all experimental configurations and 3. Athe single increase fracture in the pattern; tool speed failure from mode 1350 3to was 2530 observed rpm did fornot allaffect experimental the failure pattern. configurations and Funding:the increase This research in the was tool funded speed by from Bursa 1350 Uludag to 2530University. rpm did not affect the failure pattern.

Author Contributions: H.G.Ö. designed the experiment, O.D. and H.G.Ö performed the tensile tests. O.D. Authorperformed Contributions: the hardnessH.G.Ö. tests and designed the macrostructure the experiment, of O.D.the joints. and H.G.Ö.O.D. wrote performed and H.G.Ö the tensilesupervised tests. the O.D. performedmanuscript. the hardness tests and the macrostructure of the joints. O.D. wrote and H.G.Ö. supervised the manuscript. Acknowledgments: This study was supported by Uludag University, Department of Scientific Research Funding:Projects This(Project research No: KUAP(M)-2013/53). was funded by Bursa The Uludag authors University. would like to thank the department for their valuable Acknowledgments:support. This study was supported by Uludag University, Department of Scientific Research Projects (Project No: KUAP(M)-2013/53). The authors would like to thank the department for their valuable support. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Wei, Y. Friction Stir Spot Welding of Aluminum Alloys. Master’s Dissertation, Missouri University of 1. Wei,Science Y. Friction and Technology, Stir Spot Welding Rolla, MO, of Aluminum USA, 2008. Alloys. Master’s Dissertation, Missouri University of Science 2. andEr, Technology,O. Elektrik Rolla,Direnç MO, ve USA,Sürtünme 2008. Karıştırma Nokta Kaynaklı Alüminyum Alaşımı Bağlantıların 2. Er,Mekanik O. Elektrik Özelliklerinin Direnç ve Sürtünmeİncelenmesi. Karı¸stırmaNokta Master’s Dissertation, Kaynaklı Mersin Alüminyum University—Institute Ala¸sımıBa˘glantılarınMekanik of Science and ÖzelliklerininTechnology, Mersin,Incelenmesi.˙ Turkey, Master’s 2010. Dissertation, Mersin University—Institute of Science and Technology, 3. Mersin,Külekçi, Turkey, M.K.; 2010.Er, O. Sürtünme Kariştirma Nokta Kaynakli EN AW—5005 (Al Mg1) Alüminyum Alaşımı 3. Külekçi,İçin Optimum M.K.; Er, Kaynak O. Sürtünme Parametre Kari¸stirmaNokta Seviyelerinin Belirlenmesi. Kaynakli ENJ. Fac. AW—5005 Eng. Arch. (Al Gazi Mg1) Univ. Alüminyum 2012, 27, 537–545. Ala¸sımı 4. Için˙ Kaçar, Optimum R.; Emre, Kaynak H.E.; Parametre Demir, H.; Seviyelerinin Gündüz, S. Belirlenmesi.Al-Cu-Al MalzemeJ. Fac. Eng.Çiftinin Arch. Sürtünme Gazi Univ. Kari2012ştirma, 27 ,Nokta 537–545. 4. Kaçar,Kaynak R.; Emre,Kabiliyeti. H.E.; J. Demir,Fac. Eng. H.; Arch. Gündüz, Gazi Univ. S. Al-Cu-Al 2011, 26 Malzeme, 349–357. Çiftinin Sürtünme Kari¸stirmaNokta Kaynak 5. Bilici, M.K.; Bakır, B.; Bozkurt, Y.; Çalış, İ. Sürtünme karıştırma nokta kaynak tekniği ile birleştirilen farklı Kabiliyeti. J. Fac. Eng. Arch. Gazi Univ. 2011, 26, 349–357.

J. Manuf. Mater. Process. 2019, 3, 8 11 of 11

5. Bilici, M.K.; Bakır, B.; Bozkurt, Y.; Çalı¸s, I.˙ Sürtünme karı¸stırmanokta kaynak tekni˘giile birle¸stirilenfarklı alüminyum levhaların taguchi analizi. Pamukkale Univ. J. Eng. Sci. 2016, 22, 17–23. [CrossRef] 6. Piccini, J.M.; Svoboda, H.G. Tool geometry optimization in friction stir spot welding of Al-steel joints. J. Manuf. Process. 2017, 26, 142–154. [CrossRef] 7. Jeon, C.S.; Hong, S.T.; Kwon, Y.J.; Cho, H.H.; Han, H.N. Material properties of friction stir spot welded joints of dissimilar aluminum alloys. Trans. Nonferrous Met. Soc. China 2012, 22, 605–613. [CrossRef] 8. Panteli, A. Friction Joining of Aluminium-to-Magnesium for Lightweight Automotive Applications. Ph.D. Dissertation, University of Manchester—Faculty of Engineering and Physical Sciences, Manchester, UK, 2012. 9. Kahraman, B. Otomotiv Endüstrisinde Kullanılan 5754 Alüminyum Ala¸sımıSacların Direnç Nokta Kayna˘gı (RSW) ve Sürtünme Karı¸stırmaNokta Kayna˘gı(FSSW) Yöntemleri ile Birle¸stirilmesi.Master’s Dissertation, Kocaeli University—Institute of Science and Technology, Kocaeli, Turkey, 2009. 10. Bozkurt, Y.; Bilici, M.K. Application of Taguchi approach to optimize of FSSW parameters on joint properties of dissimilar AA2024-T3 and AA5754-H22 aluminum alloys. Mater. Des. 2013, 51, 513–521. [CrossRef] 11. Güler, H. Influence of the Tool Geometry and Process Parameters on the Static Strength and Hardness of Friction-Stir Spot-Welded Aluminium-Alloy Sheets. Mater. Technol. 2015, 49, 457–460. [CrossRef] 12. Ojo, O.O. 2219 Alüminyum Ala¸sımınınSürtünme Karı¸stırmaNokta Kaynaklı Ba˘glantılarınınÖzellikleri ve Deneysel Tasarımla Optimizasyonu. Ph.D. Dissertation, Kocaeli University—Institute of Science and Technology, Kocaeli, Turkey, 2016. 13. Güler, H. The Mechanical Behavior of Friction-Stir Spot Welded Aluminum Alloys. JOM 2014, 66, 2156–2160. [CrossRef] 14. Piccini, J.M.; Svoboda, H.G. Effect of the Tool Penetration Depth in Friction Stir Spot Welding (FSSW) of Dissimilar Aluminum Alloys. Procedia Mater. Sci. 2015, 8, 868–877. [CrossRef] 15. Zhang, Z.; Yang, X.; Zhang, J.; Zhou, G.; Xu, X.; Zou, B. Effect of welding parameters on microstructure and mechanical properties of friction stir spot welded 5052 aluminum alloy. Mater. Des. 2011, 32, 4461–4470. [CrossRef] 16. Wang, D.-A.; Lee, S.-C. Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061-T6 sheets. J. Mater. Process. Technol. 2007, 186, 291–297. [CrossRef]

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