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J. Mater. Sci. Technol., 2011, 27(7), 647-652.

Friction Stir Welding of Al Alloy Thin Plate

by Rotational Tool without Pin

Liguo Zhang1,3), Shude Ji1), Guohong Luan2), Chunlin Dong2) and Li Fu4)

1) School of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, China2) China FSW Center, Beijing 100024, China3) Shenyang Aircraft Design & Research Institute, Shenyang 110035, China

4) State Key Laboratory for Manufacturing Systems Engineering, Xi an Jiaotong University, Xian 710049, China

[Manuscript received October 22, 2010, in revised form April 11, 2011]

For friction stir welding (FSW), a new idea is put forward in this paper to weld the thin plate of Al alloyby using the rotational tool without pin. The experiments of FSW are carried out by using the tools withinner-concave-flute shoulder, concentric-circles flute shoulder and three-spiral-flute shoulder, respectively.The experimental results show that the grain size in weld nugget zone attained by the tool with three-spiral-flute shoulder is nearly the same while the grain sizes decrease with the decrease of welding velocity. Thedisplacement of material flow in the heat-mechanical affected zone by the tool with three-spiral-flute shoulderis much larger than that by the tool with inner-concave-flute shoulder or concentric-circlesflute shoulder. Theabove-mentioned results are verified by numerical simulation. For the tool with three-spiral-flute shoulder, thetensile strength of FSW joint increases with the decrease of welding velocity while the value of tensile strength

attained by the welding velocity of 20 mm/min and the rotation speed of 1800 r/min is about 398 MPa, whichis 80% more than that of parent mental tensile strength. Those verify that the tool with three-spiral-fluteshoulder can be used to join the thin plate of Al alloy.

KEY WORDS: Friction stir welding; Rotational tool without pin; Shoulder shape; Plastic flow

1. Introduction

As a new solid-state joining technology, frictionstir welding (FSW) owns many advantages, such aslow stress, small distortion, no fusion welding defects,etc

[14]. Because the metal of weldment isnt meltedduring FSW, the material flow in weld of FSW is thekey factor to influence the quality of weld. In order tothoroughly understand the mechanism of FSW, manyresearchers have made lots of scientific work on themetal plastic flow of weld in FSW[57].

Several methods on material flow havebeen published, including the steel ball trac-ing technology[8], the stop-action technology[9],the metallography method[10,11], the marker ma-terial method[12] and the numerical simulation

method[1315]. Guerra et al[11]showed that the mate-

Corresponding author. Ph.D.; Tel.:+86 24 86398270; E-mailaddress: [email protected] (S.D. Ji).

rial on the retreating front side is entrained and filledin the retreating side of the rotational tool. Ke etal.

[12] used the thin copper as maker materials tostudy the material flow in the thickness of weld and

then put forward the sucking-extruding theory. Zhanget al.[14] analyzed the effect of welding parameters onthe flow behavior of metal in weld by FE method.

For FSW, the tunnel defect and the hole defectmay result from the pin of rotational tool when thewelding parameters arent reasonable. Therefore, us-ing the rotational tool without pin can avoid the ap-pearance of the tunnel defects or the hole defects.However, the researches on the tool without pin arerelatively few. Tozakia et al.[16] developed a new toolwithout pin for the friction stir spot welding. DuringFSW, the serious metal flow lies in the contact re-

gion of weldment with rotational tool shoulder[17,18]

.Therefore, the rotational tool without pin may beused to join the thin plate of Al alloy. In this paper,the experiments of FSW of 2024-T3 Al alloy are car-


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648 L.G. Zhang et al.: J. Mater. Sci. Technol., 2011, 27(7), 647752

Fig. 1 Rotational tools used in friction stir welding: (a) inner-concave-flute shoulder; (b) concentric-circles-fluteshoulder; (c) three-spiral-flute shoulder

Table 1 Welding schemes

Specimen No. Shoulder shape Welding velocity/(m/min) Rotation speed/(r/min)

1 inner-concave 50 1800

2 inner-concave 20 1800

3 concentric-circles 50 1800

4 concentric-circles 20 1800

5 three-spiral 50 1800

6 three-spiral 20 1800

ried out by using the rotational tool without pin.The effect of different tool shoulder shapes on themicrostructure and the tensile strength are investi-gated while the shoulder shapes are inner-concave-flute, concentric-circles flute and three-spiral-flute,respectively. The reason why the rotational tool withthe three-spiral-flute shoulder can be used to join thethin plate of Al alloy is illustrated.

2. Experimental

After the surface of 2024-T3 Al alloy with thethickness of 1.6 mm was simply burnished to wipeoff the oxide layer, the butt joint configuration was jointed by the rotational tool without pin. DuringFSW, three types of tool shoulders were consideredin this paper, namely, the tool with inner-concave-flute shoulder, the tool with concentric-circlesfluteshoulder and the tool with three-spiral-flute shoulder.The structure of rotational tool is shown in Fig. 1.

The six kinds of experimental specimens attainedby different tools or different welding parameters are

discussed, as shown in Table 1.The specimens were observed by scanning electron

microscopy (SEM) after metallographic etching withKellers reagent, while the type of SEM is KYKY-

2008B. The tensile tests were performed to evaluatethe mechanical properties of FSW joints attained byusing different rotational tools. The tensile tests werecarried out at room temperature by using a universaltensile machine with initial strain rate of 103 s1. Inthis paper, the intergranular corrosion test is carriedout for the FSW joint by using the tool with three-spiral-flute shoulder. And the corrosion solution ismade up of 30 g NaCl, 10 ml HCl and 1 L H2O. Thecorrosion time is 6 h while the solution temperatureis about 35.

3. Results and Discussion

3.1 Microstructure

Figure 2 are the macro photos of the welded jointsby using different rotation tools. In Fig. 2, the weldnugget zone, the thermo-mechanically affected zone(TMAZ) and the base metal of welded joint can beclearly distinguished. Moreover, the length of lackof penetration by the inner-concave-flute shoulder ismore than that by the concentric-circles-flute shoul-

der while the lack of penetration doesn

t exist inthe welded joint by the three-spiral-flute shoulder(Fig. 2(c)).

Figure 3 shows the microstructure in weld nugget


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L.G. Zhang et al.: J. Mater. Sci. Technol., 2011, 27(7), 647652 649

Fig. 2 Macro photos of friction stir welding by different

rotational tools: (a) inner-concave-flute shoulder;(b) concentric-circles-flute shoulder; (c) three-spiral-flute shoulder

zone of FSW joint attained by three types of rota-tional tool. The welding velocity (v) is 50 mm/minwhile the rotation speed of tool () is 1800 r/min. Itis seen that the grain size in weld nugget zone is differ-ent for the joints attained by using different rotationaltool. For the tool with three-spiral-flute shoulder, thegrain sizes in weld nugget zone are nearly the same,which verifies that the plastic flow of metal duringFSW is enough. For the tool with inner-concave-flute

shoulder or with concentric-circles-flute shoulder, thegrain sizes are different in different regions of weldnugget zone and the lath-like microstructures simi-lar to microstructures in the parent metal appear insome region of weld nugget zone. The microstructureof parent metal is shown in Fig. 4.

To verify the experimental results of microstruc-ture, the numerical simulation by the software DE-FORM was carried out to investigate the effect of dif-ferent rotational tools without pin on the plastic flowof material in FSW joint.

Figure 5 is the simulation result of plastic flow of

material when the plunging depth of rotational tool is0.14 mm. In the figure, the arrow direction representsthe flow direction of material. It can be seen that theplastic flow of material in the weld and the surround-ing area attained by the tool with three-spiral-fluteshoulder is much better than that by the tool withinner-concave-flute shoulder or with concentric-circles-flute shoulder. Moreover, by analyzing the flow di-rection of material, it is known that the material inthe weld and the surrounding area adequately mixesduring FSW (Fig. 5(c)).

For the pure metal, the relation between the tem-

perature of recrystallization (TR) and the meltingpoint (Tm) is as follows: TR 0.4Tm. Although therecrystallization temperature heightens with the in-crease of the alloys content, the value must be smaller

Fig. 3 Microstructure in weld nugget zone of FSW byusing different rotational tools: (a) inner-concave-flute shoulder; (b) concentric-circles-flute shoul-der; (c) three-spiral-flute shoulder

Fig. 4 Microstructure of parent metal

than the welding temperature of FSW (about 70%of parent metals melting point). Therefore, the dy-namic recrystallization (DRX) must take place duringFSW[19]. In fact, DRX is related with the materials


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Fig. 5 Simulation results of plastic metals flow at theplunging depth of 0.14 mm: (a) tool with inner-concave-flute shoulder; (b) tool with concentric-circles-flute shoulder; (c) tool with three-spiral-flute shoulder

strain and strain rate. With the increase of the strainand the strain rate, the dislocation density of grainrapidly increases, which makes DRX happen moreeasily. During FSW, the more acute the plastic flowof material, the more easily DRX happens [20]. Theplastic flow of metal during FSW also greatly affectsthe dimensions and the sizes of grains. Therefore, thesimulation results (in Fig. 5) explain the experimentalresults of microstructure (in Fig. 3).

For the tool with three-spiral-flute shoulder, theeffect of welding velocity on microstructure in weld

nugget zone of FSW joint is shown in Fig. 6.It is known from Fig. 6 that the grain size de-

creases with the decrease of welding velocity. DuringFSW, the revolutions of rotational tool in a second

Fig. 6 Effect of welding velocity on microstructure inweld nugget zone of FSW by rotational toolwith the three-spiral-flute shoulder: (a) v=20mm/min and =1800 r/min; (b) v=50 mm/minand =1800 r/min

increases with the decrease of welding velocity, whichcan result in the increase of welding temperature, im-prove the plastic flow of metal and make the size ofgrain less[21,22].

The microstructure near the heat-mechanical af-fected zone is shown in Fig. 7.

For the tool with three-spiral-flute shoulder, thedisplacement of metal material in the heat-mechanicalaffected zone is relatively large while the transfer di-rection is from the bottom of specimen to the top(Fig. 7(c)). For the tool with inner-concave-fluteshoulder or with concentric-circles -flute shoulder, thematerial in the heat-mechanical affected zone doesnt

almost transfer (Fig. 7(a) and (b)). During FSW,the rotational tool doesnt directly touch the mater-ial in heat-mechanical affected zone, so the materialtransfer only results from the material flow in weldnugget zone. Therefore, the results in Fig.5 showsthat material flow during FSW using the tool withthree-spiral-flute shoulder is much better than thatusing the other two types of tools.

3.2 Tensile strength

The experimental results of tensile strength are

shown in Table 2. The tensile strength of 2024-T3at room temperature is 485 MPa. When the weldingparameters are the same, the tensile strength of FSW joint attained by tool with three-spiral-flute shoulder


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Table 2 Results of tensile experiment of FSW joints

Specimen No. Rotational tool shape Welding velocity/(mm/min) Tensile strength/MPa

2 inner-concave 20 80.7

4 concentric-circles 20 265.5

5 three-spiral 50 151.2

6 three-spiral 20 398.7

Fig. 7 Microstructure near boundary between the weldnugget zone and heat-mechanics affected zonewhen =50 mm/min and =1800 r/min: (a)inner-concave-flute shoulder; (b) concentric-circles-flute shoulder; (c) three-spiral-flute shoul-der

is the highest while the tensile strength of joint at-tained by with inner-concave-flute shoulder is the low-est. For the tool with three-spiral-flute shoulder, thetensile strength of joint increases with the decreaseof welding velocity and the value of tensile strength

can reaches more than 80 percent of parent metal

stensile strength. Integrating the experimental resultof microstructure, it is known that the thin plate ofAl alloy can be well jointed by FSW using the no-pin

Fig. 8 Experimental result of intergranular corrosion ofFSW joint attained by tool with three-spiral-fluteshoulder

tool with three-spiral-flute shoulder.

3.3 Intergranular corrosion

The microstructure of specimen 6 after corrosion is

shown in Fig. 8. Thereinto, the declining black regionin the figure is the marker for easily distinguishing be-tween the weld nugget zone and the heat-mechanicalaffected zone. It is known from the figure that no ob-vious grain boundary appears in the weld and the sur-rounding area after corrosion, which verifies that theintergranular corrosion phenomenon doesnt happensin FSW joint attained by the tool with three-spiral-flute shoulder. Moreover, the corrosion resistance ofheat-mechanical affected zone is lower than that ofweld nugget zone, which results from the difference ofgrain sizes (Fig. 6 and Fig. 7(c)).

4. Conclusions

(1) The microstructure of FSW joint attainedby using the rotational tools without pin is inves-tigated and the shoulders of tool are inner-concave-flute, concentric -circles-flute and three-spiral-flute,respectively. The results show that the degree of uni-formity of grains in weld nugget zone attained by thetool with three-spiral-flute shoulder is much betterthan that by the tool with inner-concave-flute shoul-der or concentric-circles-flute shoulder. For the toolwith three-spiral-flute shoulder, the material in the

heat-mechanically affected zone undergoes relativelybig plastic transfer while transfer direction is from thebottom of plate to the top.

(2) By using the no-pin rotational tool with three-


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spiral-flute shoulder, 2024-T3 Al alloy plates of 1.6mm in thickness can be well jointed. At the weldingvelocity of 20 mm/min and the rotation speed of 1800r/min, the corrosion resistance of weld nugget zoneis very excellent while the tensile strength of weldedjoint is 80% more than that of parent mental tensile

strength.

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