ORIGINAL ARTICLE

Effect of plunge speeds on hook geometries and mechanicalproperties in friction stir spot welding of A6061-T6 sheets

Xiao Song & Liming Ke & Li Xing & Fencheng Liu &

Chunping Huang

Received: 27 August 2013 /Accepted: 13 January 2014# Springer-Verlag London 2014

Abstract The plunge speed of the tool was divided into twoplunge speeds, including pin- and shoulder-plunging speeds,for a detailed study of the plunging process in friction stir spotwelding of A6061-T6 sheets. The effect of the pin- andshoulder-plunging speeds on hook geometries andmechanicalproperties was investigated. The results showed that theshoulder-plunging speed had an obvious effect on the hookgeometry and tensile shear load, but the pin-plunging speedhad almost no effect. The effective bond width (Weff) andeffective sheet thickness (Teff) used to describe the hookgeometry were important factors for determining the tensileshear load and fracture mode. Two fracture modes were ob-served: tensile/shear mixed fracture and shear fracture. Thelargest tensile shear load was obtained when the joint failed inthe tensile/shear mixed fracture.

Keywords Friction stir spot welding . Pin-plunging speed .

Shoulder-plunging speed . Hook geometry . Fracture . Tensileshear load

AbbreviationsFSW Friction stir weldingFSSW Friction stir spot weldingN Rotary speed (rpm)Vp Pin-plunging speed (mm/min)Vs Shoulder-plunging speed (mm/min)

Vr Retracting speed (mm/min)Weff Effective weld width (mm)Teff Effective sheet thickness (mm)

1 Introduction

Friction stir spot welding (FSSW) is a solid-state joiningtechnology derived from friction stir welding (FSW) and isconsidered as a replacement technology of conventional re-sistance spot welding for manufacturing lighter vehicles inautomotive industry [1, 2]. FSSW has been successfully ap-plied to aluminum [3], magnesium [4], advanced high strengthsteel [5], and polymers [6].

The FSSW tool geometries and welding parameters exertsignificant effect on the hook geometries and mechanicalproperties of the friction stir spot welded joints [7]. TheFSSW tool consists of a shoulder and a pin, and the shoulderand pin geometries were investigated in previous works.Badarinarayan et al. [8] compared three shoulder geometriesand found that the shoulder geometry played a critical role inthe hook geometry and thereby the tensile shear load. Yuanet al. [9] investigated the mechanical properties of the jointsusing a conventional pin tool and an off-center feature tool.Yin et al. [10] concluded that the mechanical property of thejoint made using a three-flat pin with thread was superior tothat of the joint made using a cylindrical pin with thread.Tozaki et al. [11] found that the tensile shear load increasedwith the pin length, and both the thickness of the upper sheetunder the shoulder indentation and the size of the stir zonewere affecting factors. Recently, Tozaki et al. [12] reported anewly developed tool without pin for FSSW. In addition, thewelding parameters, including rotary speed, dwell time,plunge depth, and plunge speed, were also investigated.Shen et al. [13] investigated the effect of the rotary speed

X. Song : L. Ke (*) :C. HuangState Key Laboratory of Solidification Processing, NorthwesternPolytechnical University, Xi’an, People’s Republic of Chinae-mail: liming_ke@126.com

L. Ke : L. Xing : F. LiuNational Defense Key Disciplines Laboratory of Light AlloyProcessing Science and Technology, Nanchang HangkongUniversity, Nanchang, People’s Republic of China

X. Song · L. M. Ke (*) · C. P. Huang

L. M. Ke · L. Xing · F. C. Liu

Int J Adv Manuf TechnolDOI 10.1007/s00170-014-5632-y

and dwell time on the mechanical properties of the joints. Theresults showed that the tensile shear load increased with thetool rotary speed and dwell time, and the rotary speed played acrucial role in determining the strength. Bozzi et al. [14]concluded that the plunge depth must be sufficient to ensurea horizontal hook to guaranty high tensile shear strength. Linet al. [15] studied the effect of the rotary speed, plunge speed,and dwell time on the mechanical properties of the joints, andfound that the effect of the plunge speed on the tensile shearload was unremarkable. Lathabai et al. [16] indicated thatincreasing the plunge depth produced an increase in the tensileshear load, and the plunge speed had little or no effect on thetensile shear load. Yoon et al. [17] also reported that theplunge speed had no remarkable effect on the tensile shearload. On the contrary, Jonckheere et al. [18] found that tensileshear load was affected significantly by the plunge speed.However, it is not clearly known whether the plunge speedaffect the hook geometry and mechanical property.

The process sequence of FSSW consists of three phases[19]: (a) plunging, (b) stirring, and (c) drawing. In the plung-ing process, the rotating tool is slowly plunging into the sheetswith a constant speed until the shoulder reaches the desiredepth [20]. Actually, the tool plunging process can be dividedinto two phases for detail study of the plunging process: pinplunging and shoulder plunging. Figure 1 shows the processsequences of friction stir spot welding. At the beginning, thetool with a given rotary speed (N) is gradually plunging intothe sheets at a given pin-plunging speed (Vp) until the shouldercontacts with the surface of the upper sheet, which is called thepin-plunging phase. Subsequently, the rotating tool continuesplunging into the sheets at a given shoulder-plunging speed

(Vs) until a predetermined plunge depth of the shoulder, whichis called the shoulder-plunging phase. Finally, the tool retractsfrom the sheets at a given retracting speed (Vr) without dwelltime.

In this study, FSSW experiments were conducted over abroad range of pin- and shoulder-plunging speeds. The effectof the pin- and shoulder-plunging speeds on the hook geom-etries, fracture modes, and tensile shear loads wasinvestigated.

2 Experimental

2.1 Base material

A6061-T6 sheets with a thickness of 2 mmwere used to makejoints by FSSW. Table 1 shows the chemical compositions ofA6061-T6 sheet and the tensile strength of A6061-T6 sheet is316 MPa.

2.2 Welding equipment and condition

A numerical control milling machine equipped with a specialfixture for FSSW was used. The tool had a shoulder diameterof 12 mm with a concave. The circular pin with left handthread had a diameter of 4 mm and a length of 2.6 mm. All thespecimens were welded in lap configuration according to thenational standard GB/T 2651-1989 [21]. As shown in Fig. 2,the specimen was made using two 100×25 mm sheets with a25×25 mm overlap area. After cleaning the lap surface andthe surface of the upper sheet by acetone, sheets were weldedat the center of the overlap area.

During all the welding trails, the tool was rotated in theclockwise direction with a rotary speed of 1,550 rpm. Theplunge depth of the shoulder was 0.5 mm.When the shoulder-plunging speed was 6 mm/min, the pin-plunging speedsranged from 20 to 60 mm/min, interval of 10 mm/min.Moreover, when the pin-plunging speed was 60 mm/min,the shoulder-plunging speeds ranged from 2 to 10 mm/min,

Fig. 1 The process sequences of friction stir spot welding. a The pincomes in contact with the surface of the upper sheet. bThe pin stirs in thesheets. cThe shoulder participates in the welding. dThe tool retracts fromthe sheets

Table 1 Chemical compositions of A6061-T6 sheet (wt%)

Si Fe Cu Mn Mg Cr Zn Ti Al

0.66 0.28 0.25 0.09 1.10 0.19 0.06 0.02 Balance

Fig. 2 Schematic diagram of the tensile shear specimen

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interval of 2mm/min. In all cases, no dwell time of the tool wasused, and the retracting speed of the tool was 300 mm/min.

2.3 Evaluation method

For each welding condition, four specimens were produced.Three specimens were used for tensile shear test, and the otherone was used for hook observation. The tensile shear speci-mens were pulled at a rate of 1 mm/min on a WDW-50 MTStesting machine, and the tensile shear load was obtained byaveraging the maximum separation loads of three specimens.The cross-section of the joint was cut along the center line ofthe keyhole with a cutting machine, and the specimens wereprepared in polished and etched condition by Keller’s reagent.The geometries of the hooks were carefully measured using aZeiss optical microscope (OM). The fracture surfaces of thejoints were examined using a SU1510 scanning electron mi-croscope (SEM).

3 Results and discussion

3.1 Hook formation

A characteristic feature of the friction stir spot welded joint inlap configuration is the formation of a geometrical defectoriginating at the interface of the two welded sheets, some-times called as “hook” [22]. Figure 3 shows the macrographsof the friction stir spot welded joint. The surface appearance isshown in Fig. 3a, and a keyhole is left in the center of the jointafter the tool drew out. The cross-sectional macrograph isshown in Fig. 3b, which is cut along the center line of thekeyhole in Fig. 3a. At both sides of the keyhole, two symmet-rical stir zones are formed, and the hook regions are exhibitedbeside the stir zones. Two geometrical features are definedbased on the hook geometry. Effective weld width (Weff) isdefined as the shortest distance from the tip of the hook region(point O) to the keyhole periphery [13]. Moreover, effectivesheet thickness (Teff) is defined as the shortest distance fromthe tip of the hook region to the surface of the upper sheet [8].

Yin et al. [23], Badarinarayan et al. [8], and Jonckheereet al. [18] similarly found that the hook regions were curvedupwards. Yin et al. [23] considered that the upward displace-ment of the lower sheet material resulted in upward hook

region. The driving force of the upward lower sheet wasprovided by the shoulder penetration into the surface of theupper sheet and the pin penetration into the lower sheet duringthe dwell period. As reported by Zhang et al. [24], the hookregion went upwards and then immediately went downwardstowards the lower sheet, and no lower sheet material under-neath the hook region flowed upwards into the upper sheet.The difference in the hook geometries in these studies isrelated to different mechanisms of the material flow, whichis attributed to different tool geometries used in their works.

The formation of the hook in this work is related to thematerial flow in the stir zone affected by the pin with left handthread. According to Yang et al. [25], the stir zone was formedby the cooperation of three material transport processes: (1)the upward motion of the lower sheet material and the incor-poration of the upper and lower sheet materials; (2) the down-ward spiral motion of the incorporated materials along the pin;and (3) the release of the incorporated materials from the pin.By analyzing the material flow in the sheet thickness direc-tion, a model of hook formation was proposed by the author inprevious work [26], and Fig. 4 shows the schematic diagramof the hook formation. The incorporated materials of the upperand lower sheets moved downwards via the thread, and re-leased from the bottom of the pin, and moved outwards andupwards before moving back towards the root of the pin anddownwards again. Since more and more material from thelower sheet moved upwards and incorporated with the uppersheet material, the stir zone became larger and larger. Thematerials presented outside the stir zone were extruded by thestir zone and thenmoved upward. As a result, the hookmovedupwards and outwards from the axis of the keyhole along withthe materials presented outside the stir zone.

Fig. 3 Macrographs of thefriction stir spot welded joint(Vp, 60 mm/min and Vs,6 mm/min). aSurface appearance.bCross-sectional macrograph

Fig. 4 Schematic diagram of the hook formation

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3.2 Hook geometries

Figure 5 shows the cross-sectional macrographs of the jointsat different pin-plunging speeds. Only one half of the joint isgiven as shown for the similar characteristics existed at bothsides of the keyhole. The pin-plunging speeds in Fig. 5a–e are20, 30, 40, 50, and 60mm/min, respectively. The hook regionsare all curved upwards and outwards from the axis of thekeyhole. It can be seen that the pin-plunging speed has noobvious effect on the size of the stir zone. Moreover, the Weff

and Teff are relatively unchanged with increasing the pin-plunging speed from 20 to 60 mm/min.

Figure 6 shows the cross-sectional macrographs of thejoints at different shoulder-plunging speeds. As shown inFig. 6a and b, the hooks in the joints at the shoulder-plunging speeds of 2 and 4 mm/min are curved upwards andtowards the keyhole periphery. In contrast, the hooks in thejoints at the shoulder-plunging speeds of 6, 8, and 10 mm/minare curved upwards and outwards from the axis of the key-hole, as shown in Fig. 6c–e. It can be seen that the size of thestir zone becomes smaller, the Weff decreases while the Teffincreases with increasing the shoulder-plunging speed from 2to 10 mm/min.

Badarinarayan et al. [8] observed that the hook was veryclose to the keyhole when only the pin was in contact with thesheet. Once the shoulder came in contact, the hook geometrywas pushed outwards from the axis of the keyhole because thematerial flow had significantly increased. In other words, theshoulder, rather than the pin, is the main factor for determiningthe hook geometry. When the pin-plunging speed increases,there is no obvious material flow changing, resulting in

relatively unchanged size of the stir zone, Weff and Teff.When the shoulder-plunging speed increases, the stir time ofthe tool in the sheets becomes shorter, and the material flowbecomes less obvious. As a result, the size of the stir zonebecomes smaller, and the hook geometry changes with the stirzone, which leads to a decrease in the Weff and an increase inthe Teff. When the shoulder-plunging speed is low, it takeslonger time for the tool to stir. The material presented outsidethe stir zone flows intensely so that the tip of the hook movestowards the keyhole. Thus, the hooks in the joints at theshoulder-plunging speeds of 2 and 4 mm/min are curvedtowards the keyhole.

In addition, other welding parameters like the rotary speed,dwell time, and plunge depth also have significant effect onthe Weff and Teff. Pathak et al. [20] reported that the Weff

increased with the rotary speed. Babu et al. [27] concludedthat the Weff increased with the rotary speed up to 1,500 rpm,and further increase in the rotary speed did not improve theWeff. Shen et al. [13] found that the largestWeff was obtained atthe highest rotary speed and with the longest dwell time. Yinet al. [23] found that the Teff decreased with increasing thedwell time. According to Badarinarayan et al. [8], the size ofthe stir zone became larger with increasing the plunge depth,and the hook presented outside the stir zone was pushedoutwards from the axis of the keyhole, resulting in an increasein the Weff.

3.3 Fractures

Figure 7 shows the macrographs of the fractured joints atdifferent pin-plunging speeds. The white arrows in the figures

Fig. 5 Cross-sectional macrographs of the joints at different pin-plunging speeds: a 20 mm/min, b 30 mm/min, c 40 mm/min, d 50 mm/min, and e60 mm/min

Fig. 6 Cross-sectional macrographs of the joints at different shoulder-plunging speeds. a 2 mm/min. b 4 mm/min. c 6 mm/min. d 8 mm/min. e10 mm/min

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indicate the loading direction. The upper figures show theupper sheets viewed from the bottom, and the lower figuresshow the lower sheets viewed from the top. All the joints failin tensile/shear mixed fracture, which is a way similar to thepullout of a button. It can be seen that there is almost no effecton the fracture mode with increasing the pin-plunging speedsfrom 20 to 60 mm/min.

Figure 8 shows the macrographs of the fractured joints atdifferent shoulder-plunging speeds. Two different fracturemodes are observed under the tensile shear loading: tensile/shear mixed fracture and shear fracture. Similar fracturemodes were also observed by Tozaki et al. [11] and Lathabaiet al. [16]. The tensile/shear mixed fractures are shown inFig. 8a–c, and the diameter of the button left in the lowersheet becomes smaller with increasing the shoulder-plungingspeed from 2 to 6 mm/min. The shear fractures are shown inFig. 8d and e, and the diameter of the bonded zone becomessmaller with increasing the shoulder-plunging speed from 8 to10 mm/min. Association with the hook geometries in Fig. 6, itcan be found that the diameter of the button and bonded zoneare related to the Weff.

Jonckheere et al. [18] proposed that a triangular cavityopened in the hook region at the loading side of the uppersheet during the tensile shear loading, and the crack wouldinitiate at the upper corner or side corner of the triangular,which might lead to different fracture modes. Figure 9 showsthe cross-sectional macrographs of the fractured joints, and theblack arrows in the figures indicate the loading direction.When the shoulder-plunging speeds are 2, 4, and 6 mm/min,the joints have a large Weff but a small Teff. As shown inFig. 9a, the crack initiated from the upper corner of thetriangular (the white dashed line) spreads through the uppersheet vertically due to a small Teff, resulting in the tensilefracture. Then, the crack propagates along the circumferentialdirection of the shoulder indentation. Finally, the fracturetakes place between the upper sheet and the loading side ofthe lower sheet, resulting in the shear fracture. Figure 10a andb shows the magnifications of the regions A and B in Fig. 9a,and the fracture surfaces reveal elongated dimples, indicatingthe tensile failure in ductile mode at the loading side of theupper sheet and the shear failure in ductile mode at the loadingside of the lower sheet. When the shoulder-plunging speeds

Fig. 7 Macrographs of the fractured joints at different pin-plunging speeds: a20 mm/min, b30 mm/min, c40 mm/min, d50 mm/min, and e60 mm/min

Fig. 8 Macrographs of the fractured joints at different shoulder-plunging speeds: a2 mm/min, b4 mm/min, c6 mm/min, d8 mm/min, and e10 mm/min

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are 8 and 10 mm/min, the joints have a large Teff but a smallWeff. As shown in Fig. 9b, the crack initiated from the uppercorner does not reach the top of the upper sheet because of alarge Teff, while the crack initiated from the side corner reachesthe keyhole in the horizontal direction because of a small Teff.Figure 10c and d shows the magnifications of the regions Cand D in Fig. 9b, and it can be seen that the joint fractures inthe shear failure in ductile mode.

3.4 Tensile shear loads

Figure 11 shows the tensile shear loads at different pin-plunging speeds. The difference in the tensile shear loads israther small with increasing the pin-plunging speed from 20 to

60 mm/min. Badarinarayan et al. [22], Babu et al. [27], andBozzi et al. [14] reported that the hook geometry had signif-icant effect on the tensile shear load of the joint. As shown inFig. 7, the joints at different pin-plunging speeds fail in thetensile/shear mixed fracture. According to Badarinarayanet al. [8], the Teff offered the resistance against the externalloading in the vertical direction, so the Teff was very significantfor the tensile shear load of the joint that failed in the tensile/shear mixed fracture. The small difference in the tensile shearloads at different pin-plunging speeds is due to the relativelyunchanged Teff.

Figure 12 shows the tensile shear loads at differentshoulder-plunging speeds. Firstly, the tensile shear load in-creases with increasing the shoulder-plunging speed from 2 to

Fig. 9 Cross-sectionalmacrographs of the fracturedjoints. a The tensile/shear mixedfracture (Vp, 60 mm/min and Vs,6 mm/min). b The shear fracture(Vp, 60 mm/min and Vs,8 mm/min)

Fig. 10 SEM fractographs ofdifferent regions in Fig. 9: aregion A, b region B, c region C,and d region D

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6 mm/min. The largest tensile shear load is obtained at theshoulder-plunging speed of 6 mm/min, which is about5,366 N. Furthermore, the tensile shear load decreases withthe increasing shoulder-plunging speed of more than 8 mm/min. Pathak et al. [20] reported that the tensile shear loadincreased with the plunge depth, which was attributed to thelarger bonded zone. Shen et al. [13] found that there was adirect correlation between the Weff and tensile shear load, andthe presence of a larger Weff resulted in a stronger joint withincreasing the tool rotary speed. Similar result is obtained thatwith decreasing the shoulder-plunging speed from 10 to6 mm/min, the tensile shear load increases with the Weff.However, further increase in the Weff does not significantlybenefit the tensile shear load but leads to a significant decreasein the tensile shear load when the shoulder-plunging speeddecreases from 6 to 2 mm/min. It is noted that the fracturemode shifts from the shear fracture to the tensile/shear mixedfracture with decreasing the shoulder-plunging speed from 8to 6 mm/min, as shown in Fig. 8c and d. It can be found thatthe tensile shear load decreases with the Teff when theshoulder-plunging speed decreases from 6 to 2 mm/min. Inother words, the Teff becomes the predominant factor fordetermining the tensile shear load when the shoulder-

plunging speeds is lower than 6 mm/min. Similar result wasobtained by Badarinarayan et al. [8] that the tensile shear loaddecreased with the Teff when the joints produced by threeshoulder profiles were compared. In addition, Yin et al. [4]found that when the hook regions were curved towards thekeyhole periphery, the Weff had a major effect on the tensileshear load. The hooks in Fig. 6a and b are curved towards theperiphery of the keyhole, but the Teff, rather than the Weff, isactually the major effect on the tensile shear load.

Babu et al. [27] proposed that a good basis for processoptimization was “maximize bond width and minimize hookheight.”Current results suggest that it is necessary to make theWeff and Teff as large as possible to achieve good joint withhigh tensile shear load. In addition, there is an optimalshoulder-plunging speed, which is beneficial to the tensileshear load. Because the pin-plunging speed has no obviouseffect on the tensile shear load, a large pin-plunging speed isan approach to reduce the welding time.

4 Conclusions

1. The effective weld width increased and the effective weldwidth decreased with increasing the shoulder-plungingspeed from 2 to 10 mm/min. In contrast, increasing thepin-plunging speed from 20 to 60 mm/min had almost noeffect on the hook geometries.

2. The fracture mode was governed mainly by the hookgeometry. The joint with a small effective sheet thicknessfailed in tensile/shear mixed fracture, while the joint witha small effective weld width failed in shear fracture.

3. The shoulder-plunging speed had an obvious effect on thetensile shear load because of different hook geometries.On the contrary, the pin-plunging speed had no obviouseffect on the tensile shear load due to relative unchangedhook geometries. The largest tensile shear load was ob-tained when the joint failed in the tensile/shear mixedfracture.

Acknowledgments This research was financially supported by Nation-al Natural Science Foundation of China (no. 51364037, 51265043, and51201087), the Landed Plan of Science and Technology in Colleges andUniversities of Jiangxi Province (KJLD12074 and 13055), and the Foun-dation of the State Key Laboratory of Solidification Processing in NWPU(no. SKLSP201306).

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Fig. 11 Tensile shear loads at different pin-plunging speeds

Fig. 12 Tensile shear loads at different shoulder-plunging speeds

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