Met. Mater. Int., Vol. 15, No. 6 (2009), pp. 1027~1031

doi: 10.1007/s12540-009-1027-2 Published 26 December 2009

Investigation of the Material Flow and Texture Evolution in Friction-Stir Welded Aluminum Alloy

Suk Hoon Kang1, Heung Nam Han

1,*, Kyu Hwan Oh

1, Jae-Hyung Cho

2,

Chang Gil Lee2, and Sung-Joon Kim

2

1Department of Material Science and Engineering, Seoul National University,

San 56-1 Shillim-dong, Gwanak-gu, Seoul 151-744, Korea2Korea Institute of Materials Science,

66 Sangnam-dong, Changwon-si, Gyeongnam 641-010, Korea

(received date: 15 February 2009 / accepted date: 3 June 2009)

The material flow and crystallographic orientation in aluminum alloy sheets joined by friction stir welding(FSW) were investigated by electron back scattered diffraction (EBSD). The microstructure and microtextureof the material near the stir zone was found to be influenced by the rotational behavior of the tool pin.It was found that, during FSW, the forward movement of the tool pin resulted in loose contact betweenthe tool pin and the receding material at the advancing side. This material behavior inside the joined aluminumplates was also observed by an X-ray micrograph by inlaying a gold marker into the plates. As the advancingspeed of the tool increases at a given rotation speed, the loose contact region widens. As the microtextureof the material near the stir zone is very close to the simple shear texture on the basis of the frame ofthe tool pin in the normal and tangent directions, the amount of incompletely rotated material due to theloose contact could be estimated from the tilt angle of the shear texture in the pole figure around the key hole.

Keywords: friction-stir welding (FSW), microstructure, EBSD, shear texture, X-ray micrograph

1. INTRODUCTION

Friction-stir welding (FSW) is a method that enables solid

phase joining. It involves the use of a rotating non-consum-

able tool to introduce frictional and quasi-adiabatic heat and

severe plastic flow locally [1-5]. It is well known that the

microstructure and microtexture of an FSWed material vary

from position to position due to the complex flow that will

have occurred in the material during FSW. The microstruc-

ture of an FSWed material can be divided into three parts:

the weld zone (WZ), the thermo-mechanically affected zone

(TMAZ), and the parent material (PM) [6]. Equiaxed fine

grains are formed in the WZ. The average size of these

grains is nearly 3 to 5 times smaller than that of the grains in

the PM. The severe viscous material flow in the WZ gives

rise to plastic deformation, even in the TMAZ, which is

adjacent to the WZ. As a consequence of this severe plastic

deformation, a dislocation cell structure can be expected in

the TMAZ [6-8], where the cells remain as subgrains inside

the grains. Finally, many low-angle boundaries form in the

TMAZ [7,8].

In addition to the microstructural differences, the micro-

textures vary throughout the thickness and across the width

of the welded joint [9-13]. The crystallographic orientation

of the material after FSW is directly affected by the rotation

of the FSW tool. The FSW tool consists of a shoulder and a

pin, which form the basis of two different shear deformation

modes. The tool shoulder results in frictional shear on the

top surface between the tool and the joined metal. The tool

pin is plunged into the joint line between two plates that are

butted together and then traverses along the joint line while

stirring. Therefore, the texture resulting from the FSW pro-

cess is affected by a combination of these two shear modes.

Recently, numerous studies of direct flow visualization

[14-18] and numerical flow simulations [19-22] have been

performed with the goal of understanding the behavior of

materials during FSW. This behavior is very complicated

because the threaded tool rotates and moves forward simul-

taneously. In this study, the material flow that occurs during

FSW was investigated by observing the microstructure and

microtexture of the material surrounding the tool pin. This

material flow inside the sample was also confirmed by an X-

ray micrograph in which a gold marker was inlaid into the*Corresponding author: hnhan@snu.ac.kr

©KIM and Springer

1028 Suk Hoon Kang et al.

sample. The rotation of the material was estimated by char-

acterizing the shear texture in the pole figure.

2. EXPERIMENTAL PROCEDURE

4 mm thick Al6061-T651 sheets were subjected to one-

pass FSW with a tool rotation speed of 2000 rpm. Two

different advancing speeds of 200 mm/min and 500 mm/min

were used. The material flow inside the aluminum plates

was observed in X-ray micrographs that were obtained from

a marker inlay technique. A gold wire with a thickness of

1.35 mm was used as the marker because it has a higher

elongation and heavier molecular weight than aluminum.

The gold inlay and the aluminum matrix could easily be

distinguished in the X-ray micrograph after FSW.

For the electron back scattered diffraction (EBSD) analysis,

each FSWed sheet was cut into two parts along the plane

direction, i.e., top and bottom parts. The top part would have

experienced shear deformation due to both the tool pin and

the shoulder, while the bottom part would have deformed

mainly due to the action of the tool pin. In this study, the

bottom part was used for analyzing the texture in order to

minimize the shear effect that resulted from the tool shoulder

on the sample. Electrolytic polishing of the samples was

carried out prior to the EBSD analysis. An HR-EBSD

system (Jeol JSM 6500F (SEM) with an Oxford Inca System

(EBSD)) was used for the microstructure and microtexture

analyses of the FSWed samples. In the analysis of the micro-

texture, hundreds of grains within 50 µm of the end of the

trace of the tool pin (key hole) were investigated in order to

construct one pole figure. The specimen coordinate system

of the pole figure was transformed into a coordinate system

in the tool pin normal (TN) and tangent direction (TD).

3. RESULTS AND DISCUSSION

Optical microscope images of the surfaces of the plates

after FSW are shown in Fig. 1(a). The furrows on the sur-

faces can clearly be seen in the magnified images. The space

between the furrows indicates the amount of the material

flow, which was transferred from the leading to the trailing

sides during each revolution [23,24]. The advancing tool

leaves an empty space at the loose contact region between

the tool and the receding material on the advancing side, as

shown in Fig. 1(b). This space is then filled by the material

that is transferred from the leading side. As observed in Fig.

1(a), the widths of the furrows vary according to the

advancing speed. For example, the results suggest that at a

rotational speed of 2000 rpm and an advancing speed of

200 mm/min, the tool advances 0.1 mm per rotation. In con-

trast, at the same rotational speed but with an advancing

speed of 500 mm/min, the tool advances 0.25 mm per rota-

tion. This suggests that as the advancing speed of the tool

increases at a given tool rotation speed, the looser contact

region between the tool and the sample widens.

In Fig. 2, cross-sections of the FSWed samples are displayed.

Distinguishing the parent material (PM), the thermo-mechani-

cally affected zone (TMAZ), and the weld zone (WZ) using

a conventional chemical etching method was attempted, as

shown in Fig. 2(a). The boundary between the TMAZ and

the PM, however, is not clearly visible in this image. As

mentioned previously, it is known that dislocation cell struc-

tures due to the plastic deformation and many low-angle

boundaries are observed in the TMAZ [7,8]. In this study,

EBSD observation of the subgrain boundaries with low mis-

orientation angles (3° to 15°) was adopted to distinguish the

TMAZ and the PM. Figures 2(b) and (c) show maps of the

subgrain boundary after the FSW with both the tool shoulder

and pin, and only the tool shoulder, respectively. These maps

enabled the measurement of the size of the TMAZ in both

cases. Figure 2(c) shows that the depth of the TMAZ, which

Fig. 1. (a) Optical micrographs of FSWed aluminum surface. (b)Schematic drawing of loose contact between tool and receding mate-rial during FSW.

Fig. 2. Cross section of FSWed sample: (a) Optical micrograph, andEBSD maps of subgrain boundary with low angle (3-15°) after FSWswith (b) both tool shoulder and pin and (c) only the tool shoulder.

An Investigation of the Material Flow and Texture Evolution in Friction-Stir Welded Aluminum Alloy 1029

was plastically affected by only the tool shoulder, was lim-

ited to a maximum of 1.8 mm at a given condition. There-

fore, only the effect of the tool pin on the material flow could

be analyzed by removing the upper half of the sample.

Figure 3 displays an orientation map of the sample

observed at the circumference of the trace end. The circle-

shaped inner space indicates the key hole. Directly adjacent

to the key hole, there are small recrystallized grains. The

width of the recrystallized region in the advancing side is

nearly three times larger than that in the retreating side. This

is related to the fact that a loose contact area exists between

the tool and the receding material, as shown in Fig. 1(b).

This is generated at the advancing side. The shear-deformed

material flow that is transferred from the leading to the trail-

ing side is mainly stored in the loose contact space. On the

other hand, the width of the recrystallized region in the

retreating side is narrow because the tool has tight contact

with the joining material. This phenomenon has previously

been simulated using a finite element method, in which

tighter and narrower material streamlines were expected in

the retreating side [25]. Those findings also explain why the

TMAZ shapes shown in Fig. 2 are asymmetric between the

advancing and retreating sides.

In order to observe the material flow inside the joined alu-

minum plates, a marker inlay technique was adopted, as

shown in Fig. 4(a). Before the FSW procedure, one of the

aluminum plates was carved by a grinder. Subsequently, a

gold wire was inlayed 2 mm below the plate surface, as shown

in Fig. 4(b). After FSW, the flow of the gold inlay could be

observed using X-ray micrographic imaging. Figure 4(c)

shows an X-ray micrograph of the FSWed aluminum plate

with the gold marker. The figure shows that the gold marker

is elongated around the tool pin and that its flow ends at the

advancing side. This observation confirms that the material

flow at the retreating side is faster than that at the advancing

side. This tendency coincides with the result of Fig. 3.

As mentioned in the introductory section, a combination

of two different deformation modes controls the texture

resulting from the FSW process. A drawing of two different

shear deformation modes is displayed in Fig. 5(a). The two

TNs are orthogonal and have a common TD. Each interface

between the TMAZ and the WZ is tilted by approximately

10° from the two shear planes along the TD. This is caused

by the decrease of the stirring effect of tool shoulder with the

depth along the thickness direction of the sample. In Figs.

Fig. 3. EBSD orientation map around circumference of tool pin nearend of trace.

Fig. 4. (a) Schematic diagram of marker inlay technique. (b) Aluminumplate which is inlayed with 1.35 mm thick gold wire. (c) X-ray micro-graph of FSWed aluminum plate with gold marker.

Fig. 5. (a) Drawing for two different shear modes. (b) (111) pole figure measured near tool pin. (c) (111) pole figure measured near tool shoulder.(d) Ideal (111) pole figure obtained under simple shear condition.

1030 Suk Hoon Kang et al.

5(b) and (c), the (111) pole figures for each mode based on

the TN and TD frame are compared. Figure 5(d) shows the

ideal texture obtained under the simple shear condition. The

positions of the peak in the ideal pole figure are consistent

with the {111}<110> texture. The development of FCC shear

textures, described and modeled by Canova et al. [26],

Montheillet et al. [27,28], and Toth et al. [29], generally fol-

lows from the continuous lattice rotations favoring the align-

ment of the {111} slip plane with the shear plane and/or the

alignment of the <110> slip direction with the shear direc-

tion. Here, the interfaces between the TMAZ and the WZ are

tilted by nearly 10° from the ideal shear planes, as shown in

Fig. 5(a). This results in a pole rotation of about 10°, as

observed in Figs. 5(b) and (c).

Figure 6 shows the (111) pole figures developed along the

circumference of the key hole after the FSW. Each pole fig-

ure corresponds to the evolved texture at each rotating angle.

All pole figures were based on the TN and TD frame. It can

be observed that the pole figures corresponding to the 210°

and 240° positions do not display a typical shear texture.

This is caused by the insufficient rotation of the materials

due to the loose contact in these regions. This insufficient

rotation effect increases as the tool advancing speed increases,

as shown in Figs. 6(a) and (b). For rotational and advancing

speeds of 2000 rpm and 200 mm/min, respectively, the max-

imum insufficient rotation angle is approximately 45°. At

2000 rpm and 500 mm/min, the maximum insufficient rota-

tion angle is close to 60°.

4. CONCLUSION

This study found that, during FSW, the forward movement

of the tool pin results in loose contact between the tool pin

and the receding material on the advancing side. From an X-

ray micrograph of an inlaid gold marker into two joined

plates, it was confirmed that the material flow on the retreat-

ing side is faster than that on the advancing side. The amount

of incompletely rotated material due to this loose contact

could be estimated from the tilt angle of the shear texture in

the pole figure around the key hole. The insufficient rotation

effect on the evolution of the texture increased as the tool

advancing speed increased.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and

Engineering Foundation (KOSEF) grant funded by the

Korea government (MOST) (R0A-2007-000-10014-0) and

supported by the Korea Institute of Industrial Technology

(KITECH) in part.

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Fig. 6. (111) pole figures along circumference of key hole after FSW under conditions of (a) 2000 rpm-200 mm/min and (b) 2000 rpm-500 mm/min, respectively. Each pole figure corresponds to evolved texture at each rotating angle.

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