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Acta Metall. Sin.(Engl. Lett.)Vol.25 No.3 pp225-234 June 2012

Mechanical properties and microstructure analysis of refilling

friction stir welding on 2219 aluminum alloy

Peilin LI 1), Zhongfeng XU 1), Chun YU 1,2), Hao LU 1,2)∗ Junshan YAO 3)

and Guoyu CHEN 3)

1) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China2) Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai 200240, China3) Shanghai Aerospace Equipments Manufacture, Shanghai 200240, ChinaManuscript received 25 November 2011; in revised form 8 May 2012

The 2219 aluminum alloy under refilling friction stir welding (RF-FSW) was investi-gated. The micrographs showed that the bead could be divided into six zones, andthe grain size and shape were greatly different in these zones. According to the mi-crostructure analysis, the weld nugget zone and the shoulder stirring zone consistedof equiaxed grains, while the grains in the heat affected zone were seriously coars-ened. It was obvious that bending deformation occurred in the thermo-mechanicallyaffected zone. According to the microhardness analysis, the lowest hardness of theweld was at the thermo-mechanically affected zone, and the microhardness increasedwith the retraction of the stir-pin. The tensile strength and elongation of the beadwere 70% and 80% of the base metal, respectively. The tensile strength was slightlydifferent for the stable stage and the retraction stage, while the elongation decreasedin the retraction stage. The mechanical properties and microstructure responded todifferent retraction speed were analyzed, and it showed that the elongation decreasedwith increasing retraction speed.KEY WORDS Aluminium alloy; Friction stir welding; Microstructure; Mechan-

ical property

1 Introduction

Friction stir welding (FSW) is a solid-state joining process which can provide a beadwith good quality, such as good mechanical properties and stress corrosion cracking resist-ance [1,2]. The strength of the FSW bead is almost approximate to that of the basemetal [3,4]. However, a hole will form at the end of the bead in the practical welding process.For the girth welding, the hole left by the stir-pin is inevitable when using the classicalFSW process. The stress concentrates at the hole, and seriously affects the strength of thewelded joint [5].

In order to avoid the disadvantage of the hole, the RF-FSW technique and the frictionstir spot welding (FSSW) without a stir-pin technique were developed [6,7]. At the endof the RF-FSW, the stir-head will continue welding and the stir-pin will be retracted asthe stir-shoulder presses the bead, and the base metal around the stir-pin fills the hole

∗Corresponding author. Professor, PhD; Tel: +86 21 34202548, Fax: +86 21 34202548.

E-mail address: shweld@sjtu.edu.cn (Hao LU)

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left by the stir-pin under the pressure. As the hole left by the stir-pin is fully filled, thewelding process is finished. The shape of the stir-pin in the RF-FSW can only be cylindricalbecause the other shape of the stir-pin will lead to a gap between the outer surface of thestir-pin and the inner surface of the hole in the stir-shoulder. The base metal will fillto the gap and it can not form a compact bead, moreover, the stir-pin will be locked inthe stir-shoulder [8]. However, the past researches showed that the cylindrical stir-pin willgenerate many defects, even the bead is lack of fusion [9,10]. Based on the results obtainedby using the cylindrical stir-pin, the process of the RS-FSW should be improved.

The microstructure and mechanical properties of the RF-FSW are barely discussed,and most of the previous researches focus on the classical FSW process. The previousresearches show that there will be voids in the FSW bead [11−13]. Because of the stir-pin shape limit and the retraction of the stir-pin, the voids in the retraction stage of theRF-FSW will be serious.

In this work, the microstructure of the RF-FSW bead was discussed, and it showed thatthe grain sizes were different in the bead. In order to investigate the mechanical propertydifference between the stable stage and the retraction stage, the microhardness test andthe tensile test were applied. The influence of the retraction speed was also discussed.

2 Experimental

In this work, the 2219 aluminum alloy was selected as the base metal sheets in theRF-FSW. The length, width and thickness of the test sheets were 300, 150 and 6 mm,respectively. The composition (wt pct) of the alloy is Si 0.2, Fe 0.3, Cu 6.8, Mn 0.2, Mg0.02, Zn 0.1, V 0.15, Zr 0.1, Ti 0.02 and Al balanced.

In the RF-FSW process, the high speed rotating stir-head drilled to the workpiece atthe starting stage. As the stir-shoulder pressed at the top surface of the sheets, the weldingstarted. The bead length of the stable stage was 100 mm, and the retraction stage wasapplied after the stable stage. The extension length of the stir-pin was 5.4 mm. Accordingto the retraction speed (3, 6, 12, 18, 24, 36 mm/min) of stir-pin, the bead length of theretraction stage was from 30 to 360 mm. The stir-pin was retracted at the retraction stageafter the stable stage. When the stir-pin had been fully retracted and the press broughtby the stir-shoulder had been kept for a while, the welding process was finished. In thiswork, the diameter of the stir-shoulder and the stir-pin were 24 and 8 mm, respectively.The diagram of the RF-FSW is shown in Fig. 1, and the welding speed is 100 mm/min,the rotational speed is 800 rpm.

The metallographic samples and tensile samples of the different stages were preparedafter welding. The metallographic samples were etched by the 1% HF, 1.5% HCl and 2.5%HNO3 mixed solution. The effective length and width of the tensile samples are 35 and6 mm, respectively. The thickness of the samples was 3 mm.

3 Results and Discussion

3.1 Microstructure of the beadFigure 2 shows the cross-sectional structure of the bead. The weld can be divided into

the following six zones: A1, weld nugget zone (WNZ); A2, shoulder stirring zone (SSZ);A3, bottom stirring zone (BSZ); B, thermo-mechanically affected zone (TMAZ); C, heat

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affected zone (HAZ); D, base metal [14].The micrographs of the six zones are

shown in Fig. 3. The WNZ, the SSZ andthe BSZ are located at the center of thebead. The three zones are influenced bythe strong stirring effect, and experience thehigh temperature and seriously plastic defor-mation. The metallurgical structures of theWNZ and the SSZ are dynamically recrys-tallized, and the banded structures changeto fine equiaxed grains. The average grainsize of WNZ and SSZ are approximately17.1 and 16.9 µm, respectively. The BSZis located at the bottom of the stir-pin.Thus, the heat generation is very low, andit is only influenced by the stirring of thestir-pin and the heat conduction form theWNZ. Since the BSZ is mixed with the WNZmechanically, the connection is not verystrong. By the stirring effect, the grains in

Fig. 1 Schematic diagram of RF-FSW

Fig. 2 Microstructure of the RF-FSW joint

the TMAZ are strongly bending deformed, and the average grain size is approximately20.9 µm. The HAZ is at the outside of the TMAZ, and it has not experienced the mechan-ical deformation. On the other hand, the HAZ has been affected by the high temperature,and the grains are coarsened. The average grain size is approximately 42.6 µm.

3.2 Microhardness difference between the different stagesThe microhardness test was applied to investigate the microstructure of the bead. The

following four samples were tested: the stable stage (the penetration was 5.5 mm), the firstretraction stage where the stir-pin retracted for 0.5 mm (the penetration was 5.0 mm),the second retraction stage where the stir-pin retracted for 1.5 mm (the penetration was4.0 mm), the third retraction stage where the stir-pin retraced for 2.5 mm (the penetrationwas 3.0 mm). Three microhardness testing point series were applied on each sample. Theupper line located at the boundary of the SSZ and the WNZ, while the lower line locatedat the boundary of the WNZ and the BSZ. The center line located at the center of thebead. The distance of the test points at the upper line and the lower line was 0.5 mm,while it was 0.25 mm at the center line. As shown in Fig. 4, the AS indicates the advancingside, while the RS indicates the retreating side.

The microhardness distribution of the upper line is shown in Fig. 5a. The boundarybetween the WNZ, TMAZ, HAZ and base metal were approximately 5.0, 7.0 and 13.0 mmfrom the center of the bead, respectively. The highest hardness occurred at the basemetal, and decreased in the HAZ and TMAZ, then become slightly higher in the WNZ.The lowest hardness occurred at the transition zone between the HAZ and the TMAZ. Thelowest microhardness was about 75% of that of the base metal. There was slight differenceon the microhardness between the stable stage and the retraction stage.

Figure 5b shows the microhardness distribution of the lower line. The boundary be-

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Fig. 3 Micrographs of the bead: (a) WNZ; (b) SSZ; (c) BSZ; (d) TMAZ; (e) HAZ; (f) base metal

Fig. 4 Location of the microhardness testing point

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-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 1470

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Fig. 5 Microhardness distributions: (a) upper line; (b) lower line; (c) center line

tween the WNZ, TMAZ, HAZ and base metal were approximately 3.0, 3.7 and 9.0 mmfrom the center of the bead, respectively. The trend was similar to that of the upper line,and the lowest microhardness occurred at the transition zone between the HAZ and theTMAZ. The lowest microhardness was about 70% to 80% of that of the base metal. Themicrohardness was different in the different stages, and the largest difference was about15%. The microhardness of the retraction stage is higher than that of the stable stage.

The microhardness distribution of the center line is shown in Fig. 5c. The highesthardness zone was at the top surface of the bead, and decreased along the depth. It wasslightly increased at the transition zone between the SSZ and the WNZ. The least hardnesswas located at the bottom of the bead, and the lowest microhardness was about 80% ofthat at the top surface. For the microhardness at the bottom of the bead, there was:MHst<MHpb1<MHpb2<MHpb3, where MH indicates the microhardness, the subscript stindicates the stable stage, the subscript pb1 to pb3 indicate the three retraction stages.The microhardness increased with the retraction of the stir-pin, because of the pressureof the stir-shoulder increased and the structure became more compact with the increasingpressure.

The microhardness is related to the grain size. Considering the microstructure shownin Fig. 3, the grain size at the HAZ and the TMAZ was largest, and there was precipitatedphase existed at the grain boundary, which could weaken the connection of the grains.Thus, the lowest hardness occurred at the HAZ and TMAZ. Since the stirring effect wasstrong at the top surface, the WNZ and the SSZ were influenced by more heat generation,and the grains were easy to crush and form the fine equiaxed grains. The microstructure

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at the WNZ and the SSZ was almost fine equiaxed grain, and the hardness increased.

3.3 Tensile property and the microstructure difference between the different stagesIn order to analyze the reason of the trend of the microhardness, the tensile property

test and the microstructure analysis were applied. For the girth weld, the bottom of theretraction stage is base metal. The bottom of the retraction stage was unstirred, and theproperties can refer to that of the base metal. As shown in Fig. 6, the tensile samples of theretraction stage were only prepared for thetop zone, and it was assured that the weldedpenetration was deeper than the location ofthe tensile samples. On the other hand, thesheets were penetrated at the stable stage,thus the tensile samples were prepared forboth the top zone and the bottom zone ofthe bead.

Fig. 6 Schematic diagram of the longitudesection in the RF-FSW

Figure 7a shows the fracture location at the top of the stable stage. The fracture wasclosed to the AS, because the AS was the gathering location of the plastic flow. As shownin Fig. 8, the TMAZ was narrow, and it was seriously bending deformed by the stirringeffect of the stir-shoulder. The grains of the TMAZ were partly coarsened, while the grainsof the WNZ were dynamically recrystallized to the fine equiaxed grains. However, the twozones were connected weakly if the heat generation was low, even the defects would form.

Figure 7b shows the fracture location at the bottom of the stable stage, and the fracturewas at the center of the bead. Because of the back tolerance of the FSW, the metal atthe bottom of the sheet was not fully stirred, and the base metal was connected by plasticdeformation. On the other hand, the WNZ and the SSZ was obviously distinguished. Asshown in Fig. 9, the grain size of the two zones was greatly different.

Fig. 7 Fracture location of the tensile samples: (a) top of the stable stage; (b) bottom of the stablestage; (c) retraction stage

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at the retraction stage, and the fracturewas at the RS. During the retraction stage,the pressure of the stir-shoulder was in-creased, thus the heat generation was alsoincreased, and the metal on the AS wasfully stirred. The grains at the RS wereobviously grown. As shown in Fig. 10, thegrain size of the retraction stage was largerthan that at the stable stage. The aver-age grain size of the stable stage and re-traction stage are 24.8 µm and 33.4 µm,respectively. The grain boundary was ob-vious, and some precipitated phase existed.The precipitated phase gathered and grew,

Fig. 8 Micrograph at the top of the stablestage

then aggravated the non-uniform of the microstructure. The plasticity was deteriorated.The tensile test was applied by Zwick tensile test machine, and the tensile rate was

1 mm/min. The tensile curves are shown in Fig. 11. The tensile test is repeated for several

Fig. 9 Micrographs at the bottom of the stable stage: (a) the bottom of the bead; (b) boundary ofthe WNZ and the BSZ

Fig. 10 Micrograph comparison of the different stages: (a) stable stage; (b) retraction stage

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0 1 2 3 4 5 6 7 8 9 10 110

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Fig. 11 Tensile curves of different stages: (a) base metal; (b) stable stage; (c) retraction stage

times, and each curve in Fig. 11 shows the single test result. The corresponding averagedata are listed in Table 1.

Table 1 Tensile strength and the elongation of different stages

Stage Tensile strength/MPa Elongation/%

Base metal 478.4 8.63Stable stage (top) 336.5 8.54Stable stage (bottom) 327.9 7.50Retraction stage 330.5 6.93

The tensile strengths of the different stages were slightly different, and they were lowerthan that of the base metal. The tensile strength of the bead was about 70% of the basemetal. However, the elongations were in great difference. The elongation at the stablestage (top), the stable stage (bottom) and the retraction stage were 1.0%, 13.1% and19.7% decreased compared with that in the base metal, respectively. It showed that thegrains at the retraction stage grew and reduced the plasticity because of the excessive heatgeneration. As shown in Fig. 12, the voids were formed. In the stable stage, most defectsoccurred at the bottom of the bead, especially at the boundary between the TMAZ, WNZand the BSZ on the AS.

The mechanical properties at different retraction speeds are shown in Fig. 13. Theretraction speed had little influence on the tensile strength, and the average tensile strength

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Fig. 12 Boundary between the TMAZ, WNZand the BSZ at the AS

0 5 10 15 20 25 30 35 400

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Fig. 13 Mechanical properties in different re-traction speed

was about 330 MPa. However, the elongation was strongly influenced by the retractionspeed, and the elongation trended to decrease with increasing retraction speed. The maxi-mal elongation was 8.1% at a retraction speed of 6 mm/min. Compared with the elongationof the base metal, the 6 mm/min retraction speed led to a good bead quality.

4 Conclusions

(1) The bead can be divided to six zones. The grains were fine at the weld nugget zoneand the shoulder stirring zone, while they were coarse at the thermo-mechanically affectedzone and the heat affected zone. The mechanical properties are inverse proportion to thesize of the grains.

(2) The results of microhardness testing showed that for the upper line in the bead, thelowest microhardness occurred at the boundary between the thermo-mechanically affectedzone and the heat affected zone, and it was approximately 75% of that of the base metal,while for the lower line, it was approximately from 70% to 80% of that of the base metal.For the center line, the lowest microhardness was approximately 80% of that at the topsurface, and the microhardness increased with the retraction of the stir-pin.

(3) The fracture of the top zone in the stable stage was fractured near the advancingside, while the fracture of the bottom zone in the stable stage was fractured at the centerof the bead, and the fracture in the retraction stage was fractured near the retreating side.The tensile strength was slightly different in the different stages. However, the elongationwas in great difference. The retraction speed has little influence on the tensile strength, butthe elongation trended to decrease with increasing retraction speed. The best retractionspeed was 6 mm/min.

(4) The analysis of the microstructures, microhardness and tensile tests showed thatmost defects occurred at the bottom of the bead, especially at the boundary between thethermo-mechanically affected zone, weld nugget zone and the bottom stirring zone at theadvancing side in the stable stage. The grains at the retraction stage grew larger andreduced the plasticity of the metal because of the excessive heat generation.

Acknowledgements—This work was supported by the National Natural Science Foundation of China(Nos. 50975176, 51035004 and 51105251).

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