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Introduction

Aluminum alloys have been widelyused in automobile structures due to theirunique properties such as high specific

weight and strength, positive weldability,greater requirements concerning exhaustemissions, energy consumption, and recy-cling of material (Refs. 1, 2). The use ofaluminum alloys also offers an opportu-nity to reduce vehicle weight, which canalso lead to a reduction of fuel consump-tion and emissions without compromisingperformance, comfort, and safety (Ref. 3).The nonheat-treatable aluminum-magne-sium (Al-Mg) alloys (5000 series) in sheetproduct form are essentially single-phasebinary alloys with moderate to highstrength and toughness. Alloys in this se-

ries possess good welding characteristics,machinability, and corrosion resistance(Refs. 1, 4, 5). Among them, Al 5754 iscommonly employed in the fabrication ofcar body panels because this alloy has ex-ceptional formability characteristics, goodstatic, impact, and fatigue strengths, andhigh resistance to pitting and intercrys-talline corrosion (Refs. 1, 6, 7).

Recently, some fusion and solid-statewelding techniques include laser welding,

electron beam welding, and friction stirspot welding (FSW). Resistance spot

welding in joining Al 5754 is still under in-vestigation to eliminate or minimize thesolidification and hot cracks, porosity de-fects, and loss of alloying elements (Refs.

3, 814).Since its invention in 1991 by The Weld-ing Institute (TWI), UK, the FSW process,

which is a solid-state welding technique, hasgained considerable interest among re-searchers due to its structural advantagessuch as no melting, absence of gas porosityand oxidation, low energy input, low distor-tion, relatively low welding temperature,low cost, and high mechanical properties(Refs. 15, 16). Welding workpieces is per-formed by a nonconsumable rotating toolincluding shoulder and pin. The pin is ro-tated and plunged into the abutting faces of

the workpieces. The plunging of the pincontinues up to contact between the shoul-der and surfaces of the workpieces. The ma-

terial is basically heated by friction betweenthe shoulder and workpiece surfaces, and issimultaneously stirred by the pin. Thus, asoftened and plasticized zone is developedaround the plunged pin and at the interfacebetween the shoulder and workpieces. Thetool is then steadily moved along the weldinterface giving a continuous weld (Ref. 17).

The mechanical, macroscopic, and mi-crostructural features of the friction stir

welded joint are strongly related to FSWparameters. The key parameters aregeometry, rotation speed, travel speed(welding speed), tilt angle, rotation direc-tion, and axial pressure of the tool. Theseparameters have to be chosen properly be-fore the welding process to achieve sound

joints. The tool geometry affects the rateof heat input, plastic material flow, re-quired power, and uniformity of the

welded joint. Increasing the rotationspeed or decreasing travel speeds tends toincrease heat input and peak temperature,and results in more intense stirring andmixing of material. Peak temperature alsoincreases with an increase in the axialpressure. However, extremely high or lowtravel and rotational speeds can adverselyaffect properties. Insufficient and excessheat input and material flow depending onthese FSW parameters can lead to defectslike pinholes, tunnels, cavities, root flaws,and cracks (Refs. 1822).

Several experimental studies havebeen conducted about joining aluminumalloys with FSW, especially concerning2XXX, 6XXX, and 7XXX series (Ref.23). Although there have been some find-ings reported on Al 5083 in 5XXX series(Refs. 2426), current literature indicateslimited research on the FSW of Al 5754(Ref. 27). In addition, no systematic workhas been reported to determine the effectsof FSW parameters on both mechanicaland structural properties for Al 5754.

Jin et al. (Ref. 27) studied the frictionstir welding of cold rolled and aged Alloys

Al 5754 and Al 5182. The authors weldedthe rolled and aged Al 5754 sheets using aconstant FSW parameter, and examinedmicrostructural development and micro-hardness distribution in the weld.

Kulekci et al. has investigated the ef-fects of the tool pin diameter and tool ro-

Effects of FSW Parameters on JointProperties of AlMg3 Alloy

The tool rotation speed, tool tilt angle, and tool rotation direction were evaluatedfor friction stir welded aluminum Alloy Al 5754

BY Z. BARLAS AND U. OZSARAC

KEYWORDS

Friction Stir WeldingAlMg3 Aluminum AlloyFSW ParametersJoint Properties

Z. BARLAS and U. OZSARAC (ozsarac@sakarya.edu.tr)are with Sakarya University, Fac-ulty of Technology, Department of Metallurgical

and Materials Engineer ing, Esentepe Campus,Sakarya, Turkey.

ABSTRACT

The purpose of the present study was to determine the effects of friction stir weld-ing (FSW) parameters, which are the tool rotation speed, tool tilt angle, and tool rota-tion direction, on the macrostructure and microstructure, plus mechanical propertiesof butt joint AlMg3 aluminum Alloy (Al 5754) sheets. The macroscopic and mi-crostructure examinations and tensile test results indicated that the joint properties

were significantly affected by FSW parameters. A sound and defect-free weld was

achieved with a tool rotation speed of 1100 rev/min and tool tilt angle of 2 deg, whenthe tool was rotated counterclockwise. The maximum tensile strength of the joint fab-ricated with FSW parameters was 217 MPa, which is 14% lower than that of the Al 5754base metal. In this weld, closer to a symmetrical microhardness distribution was meas-ured, and hardness values of the weld nugget zone slightly increased and reached about82 HV. A softened heat-affected zone was not detected by the microhardness testing.

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diameter (root), 2 mm in minor diameter(tip), and 3 mm in length. The pin has aright-hand 0.9-mm pitch thread and athread depth of 0.6 mm. All FSW trials weremade in a square butt joint configuration,and the pin axis was adjusted to the weld in-terface of the butt joint. To produce the re-quired preheating and softening of the ma-terial, the tool was held for a dwell time of15 s when the tool plunged 2.9 mm into theupper surface of the butted sheets, and thentraversed along the weld interface. All weld-ing trials were performed perpendicular tothe rolling direction of the sheets. No an-other process such as filing or milling wasperformed on the surface and root sides ofthe FSW trials after the weld processes toremove weld flashes, tool marks, and oxidelayers. As a result, the weld trials were usedin the as-welded condition for subsequentexaminations and tests.

Cross-sectional samples were prepared

using standard metal-lographic methods formacroscopic and mi-crostructural exami-nations of the weldzones. A solution con-sisted of 25 mLmethanol, 25 mL HCl,25 mL HNO3, and 1drop HF was used toetch the weld zones.The weld zones werecharacterized using aNikon Eclipse L150Aoptical microscope(OM) equipped withimage analysis soft-

ware (Clemex VisionLite 5.0) and a JEOLJSM 6060LV scanning

electron microscope (SEM) equippedwith an energ-dispersive X-ray spec-troscopy (EDS) apparatus. Transversetensile tests were carried out to evaluatethe effect of variable parameters on the

joint efficiency of the FSW trials. All testswere performed on an Instron 3367 test-ing machine at a constant crosshead speeddisplacement rate of 2.5 mm/min. Threetensile test samples in the as-welded con-dition were tested for each FSW parame-ter, and Al 5754 base metal and averagetest values are presented in the study. Thetensile test samples were machined ac-cording to EN 895 specification by a CNCmilling machine.

Figure 2 shows the tensile test samplegeometry with dimensions. The fracturesurfaces of the failed tensile test samples

were examined by SEM to determine thegoverning fracture mechanism. Vickers

microhardness (HV) testing was con-ducted on polished specimens at near mid-thickness across the weld zone applying aload of 100 g and a dwell time of 10 s, and

was spaced at intervals of 0.5 mm in eachtesting line using a Future-Tech FM 700hardness tester.

Results and Discussion

Figures 36 show the weld surface ap-pearances and macrostructures, includingthe root side of the FSW trials, respec-tively. A sound and continuous weld wasnot made for the welding trial in700/2/ccw, and the trial failed Fig. 3G.The lowest tool rotation speed (700rev/min) when the tool was rotated in ccwcaused incomplete joining, creating verypoor surface quality and a cavity or tunnel-like weld defect. It might be the case thatthe metal was pushed into the bottom ofthe plates by the right-hand threaded pin,but abnormal stirring of the metal and rel-atively insufficient heat input did not allowmixing of this metal in the weld zone.

Jayaraman et al. reported similar diffi-culties for joining cast aluminum Alloy

A319 by FSW (Ref. 22). A large mass ofweld flash occurred on the retreating side(RS) in Mode II welding trials, becausethe leading edge of the shoulder removedthe material from the front of the rotatingtool when the tool axis was perpendicularto the weld interface during the weldingprocess (Fig. 3DF). The weld surfaces ofother FSW trials were clean, and notice-able weld defects were observed. Further-more, the surface roughness of the trials

was relatively improved with increasingthe tool rotation speed. Macroscopic in-

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Fig. 4 Macrostructure and root side appearances of the FSW trials inMode I. A and B 700/2/cw; C and D 900/2/cw; E and F 1100/2/cw.

Fig. 5 Macrostructure and root side appearances of the FSW trials inMode II. A and B 700/0/cw; C and D 900/0/cw; E and F 1100/0/cw.

Fig. 6 Macrostructure and root side appearances of the FSW trials inMode III. A and B 900/2/ccw; C and D 1100/2/ccw.

A

A

A

B

B

B

C

C

C

D

D

D

F

F

E

E

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spections show that the applied FSW pa-rameters have considerable effects on theproperties of the stirred zone structure.

First, theweld nugget shapes

were affected by thetool tilt angle and toolrotation direction.The FSW trials inMode I exhibitedbasin-shaped nuggetzone formation that

widens to both the ad-vancing side (AS) andRS near the uppersurface (Fig. 4). Al-though Mode II weldsare generally similarto Mode I welds, these

weld trials show a nar-rower basin-shaped

weld nugget with changing of the tool tiltangle Fig. 5. In addition to the weld tri-als in Modes I and II, a symmetrical weld

nugget shape was seen. The nugget shapesof trials in Mode III were noticeably

changed with changing the tool rotationdirection. These weld trials show an ellip-tical nugget shape that slightly extendedtoward the upper surface on the AS, aspresented in Fig. 6. As a result, changingthe tool tilt angle and tool rotation direc-tion with the same tool geometry resultedin different weld nugget zone shapes in Al5754 butt joints.

Second, the tool tilt angle, tool rotationdirection, and tool rotation speed have aneffect on the defect formation and weldpenetration depth. The cavity defects

were formed underneath the pin for theweld trials in Modes I and II. The cavitydefect was only formed on the RS forMode I, whereas it was formed on both theRS and AS for Mode II. Nandan et al.(Ref. 20) reported that the defects tend to

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Fig. 8 Microhardness distribution in the weld zone for 1100/2/ccw weld.

Fig. 7 Microstructural evolution in the friction stir butt welded joints at900/2/ccw and 1100/2/ccw. A Al 5754 BM; B HAZ for 900/2/ccw; C

HAZ for 1100/2/ccw; D TMAZ for 900/2/ccw; E TMAZ for1100/2/ccw; F center of WNZ for 900/2/ccw; G center of WNZ for1100/2/ccw.

Table 2 Tensile Test Results of Al 5754 BM and the FSW Trials

Test Specimen Tensile Strength (MPa) Yield Strength (MPa) Elongation (%) Tensile Strength Performance (%) Failure Location

Al 5754 253 170 12.3

700/2/cw 160 26 3 63

900/2/cw 162 53 3.3 64

1100/2/cw 165 57 3.5 65

700/0/cw 118 37 1.8 47

900/0/cw 141 46 2.6 56

1100/0/cw 145 46 4.2 57

900/2/ccw 207 42 5.5 82

1100/2/ccw 217 74 10.1 86

Note: cw = clockwise and ccw = counterclockwise.

A

B C

D

F

E

G

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occur on the AS due to sudden mi-crostructural transition between the weldnugget zone and thermomechanically af-fected zone in contrast to the presentstudy. The authors consider that the FSWparameters such as tool geometry play animportant role on the defect formation lo-cation. The root flaw can be defined as anunstirred region observed in all FSW trialsin Modes I, II, and 900/2/ccw in Mode III,as can be seen in Figs. 4, 5, and 6B. How-ever, the root flaws in the FSW trials inModes I and II are clearer than 900/2/ccw

in Mode III. Insufficient stirring and heatinput depending on low tool rotationspeed and unsuitable tool tilt angle areprobably the main reasons for the rootflaw occurrence.

The literature highlights that when theother FSW parameters are constant, an in-crease in tool rotation speed causes highertemperature due to higher friction heat andresults in intense stirring and mixing of ma-terial (Ref. 18). The formations of cavityand root flaws indicate that the heat input,stirring rate, and downward force generatedby the tool were not adequate to plastic de-

formation and material flow for the FSWparameters used in Modes I and II.

Finally, the results of this study indi-cate that the weld penetration depth wasslightly improved through increasing thetool rotation speed because of more effi-cient heat input and stirring, as presentedin the macrostructures in Mode III Fig.6. Overall macroscopic inspections showthat a defect-free weld having high pene-tration depth can be successfully con-ducted with a tool rotation speed of 1100rev/min and tool tilt angle of 2 deg, whenthe tool was rotated counterclockwise(1100/2/ccw).

Four different microstructural zoneswere distinctively revealed in the weldzone of all FSW trials by OM examina-tions. These zones are as follows: 1) unaf-fected base metal (BM); 2) heat-affectedzone (HAZ); 3) thermomechanically af-fected zone (TMAZ); and 4) weld nuggetzone (WNZ) (also known as stirred zone).

Unlike the microstructural results of aprevious study (Ref. 27), HAZs were ob-served on both sides, and there were dis-tinct boundaries between the other zones.Figure 7 shows the typical microstructuralzones of the welding trials for 900/2/ccw

and 1100/2/ccw. Al 5754 BM exhibited thecharacteristic cold-rolled structure with

grains elongated in the rolling direction Fig. 7A. While the HAZs consisted of thecoarsened grains (Fig. 7B, C) compared

with the BM due to increasing the weldtemperature, the TMAZs displayed thedeformed and elongated grain structurearound the WNZs (Fig. 7D, E) because ofboth insufficient plastic deformation andrising weld temperature in this region. TheWNZs were characterized by fine andequiaxed recrystallized grains (Fig. 7F, G)due to intense plastic deformation andthermal exposure during the FSW

process. However, the HAZ and WNZ in1100/2/ccw have more coarse grains thanthat of 900/2/ccw. For example, based onimage analysis results, the average grainsize in the WNZ for 900/2/ccw was about7.6 m while it was about 10.2 m for1100/2/ccw. The increasing of the grainsize in the HAZ and WNZ might be at-tributed to a longer thermal cycle due tohigher heat input for the tool rotationspeed of 1100 rev/min than that at 900rev/min.

Figure 8 shows the hardness profilealong the centerline on a cross section of

defect-free 1100/2/ccw weld. Almost asymmetric hardness distribution occurred

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Fig. 9 Tensile fracture surface of the HAZ for 1100/2/ccw. A SEM image; BD EDS analysis results taken from the points 1, 2, and 3, respectively.

AB

CD

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across the weld zone. A similar hardnesstrend was observed between the BM andHAZ, and it was found that the averagehardness values of the BM and HAZ wereapproximately 75 and 74 HV, respectively.

As the results of this study indicate, thereis no high degree variation in hardness be-tween the BM and HAZ at both sides ofthe weld. Although the HAZs, which werecharacterized by a coarse grain size, wereidentified by the optical examinations, theBM and softened HAZ were not clearlyseparated via the microhardness testing,similar to the test results in a previously re-ported study by Jin et al. (Ref. 27). An in-creasing hardness trend was observedafter a transition from the TMAZ toWNZ, and the WNZ exhibited an M-like hardness distribution across the weld.The highest hardness value (~82 HV) forthe weld zone was measured in the WNZ,and the average hardness of this zone wasabout 78 HV because of the grain refine-ment and possible modest work hardening(Ref. 32).

A summary of the transverse tensiletest results for the FSW trials and Al 5754BM are presented in Table 2. The resultshighlight that the tensile properties of the

welds were influenced by the tool rotationspeed, tool tilt angle, and tool rotation di-rection. The maximum tensile strength

values of all welds varied from 118 to 217MPa and slightly increased with increasingthe tool rotation speed due to efficientstirring and good bonding around the rootside of the joints.

This study also highlights that the ten-sile strengths could be affected by the weldsurface quality depending on the tool ro-tation speed because the surface rough-ness was improved with increasing the toolrotation speed. The low tensile strength

values have been determined in the weldtrials having a tool tilt angle of 0 deg dueto the cavities and root flaws. Also, thetensile properties can be improved withchanging the tool rotation direction. Thebest tensile properties were obtained atthe highest tool rotation speed when thetool was rotated in ccw (1100/2/ccw). Atensile strength of 217 MPa was observedin the 1100/2/ccw weld. This value is about86% that of the Al 5754 BM. Also, per-centage elongation of this joint was ap-proximately 10.1%, which is close to theBM and much higher, about 25 timesthan that of the other FSW trials. Asshown in Table 2, all welds fractured in theWNZ, except the 1100/2/ccw weld. Themain reasons for fracture in the WNZ areincomplete weld penetration depth and/orpresence of the unstirred root zone,namely the root flaw. Therefore, it is esti-mated that the initial crack occurred in theunstirred zone and then propagated to theupper region, resulting in failure. How-ever, 1100/2/ccw weld fractured in the

HAZ on the RS. As mentioned previously,this joint exhibited high weld penetrationdepth and sufficient stirring around theroot zone. In addition, no weld defect wasdetected in the microstructural inspec-tions. Hence, the 1100/2/ccw weld showedthe highest tensile test properties. How-ever, it should be noted that the other

welds had incomplete penetration on theroot side of the joints, so the tensile data

were affected. There is no clear relationbetween hardness and fracture location(RS), because the weld shows a similarhardness distribution in the HAZ at eachside.

Figure 9 shows the SEM micropho-tographs and EDS analysis results of thetensile fracture surface of the 1100/2/ccw

weld. The presence of dimple patterns in-dicates that ductile fracture took place inthe HAZ of the joint during the tensile test(Fig. 9A). However, the presence of Fe-rich intermetallic particles (Fig. 9C, D), inaddition to the base metal (Fig. 9B) in thefracture zone of the weld was revealed byEDS analysis. It is estimated that the duc-tility of the weld was influenced by theseconstituent particles (Ref. 33), thus result-ing in the decreasing ductility for the1100/2/ccw weld (Table 2).

Conclusions

In this study, the authors evaluated theeffects of the tool rotation speed, tool tiltangle, and tool rotation direction on thefriction stir welded aluminum Alloy Al5754. Even if the same tool geometry wasused, these FSW parameters have notice-able effects on the features of Al 5754

joints. These features can be defined asthe defect formation such as cavity androot flaw, weld penetration depth, andtensile test performance. The best results

were obtained at a tool rotation speed of1100 rev/min, tool tilt angle of 2 deg whenthe tool was rotated in ccw. This jointshows the tensile strength performanceabout of 86% as considered the basemetal. Macroscopic and microstructuralexaminations show that this joint does notpresent any obvious weld defect. Four mi-crostructural zones were identified in the

joints. These are unaffected base metal,heat-affected zone having coarse grains,thermomechanically affected zone con-sisting of plastically deformed and elon-gated grains, and recrystallized weldnugget zone. The microhardness profiledid not display a softened heat-affectedzone when considered the base metal. Thehighest hardness value of approximately82 HV was measured in the WNZ in this

weld.

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