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: [email protected]
©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.
REFERENCES
1. C. J. Dawes, Weld. Met. Fabrication 63, 13 (1995).
2. H. Schmidt, J. Hattel, and J. Wert, Model. Simul. Mater.
Sci. Eng. 12, 143 (2004).
3. M. M. Attallah and H. G. Salem, Mater. Sci. Eng. A 391, 51
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.
An Investigation of the Material Flow and Texture Evolution in Friction-Stir Welded Aluminum Alloy 1031
(2005).
4. M. Peel, A. Steuwer, M. Preuss, and P. J. Withers, Acta mater.
51, 4791 (2003).
5. Y. S. Sato, Y. Kurihara, S. H. C. Park, H. Kokawa, and N.
Tsuji, Scripta mater. 50, 57 (2004).
6. S. H. Kang, H.-S. Chung, H. N. Han, K. H. Oh, C. G. Lee,
and S.-J. Kim, Scripta mater. 57, 17 (2007).
7. S. H. Kang, W. H. Bang, J.-H. Cho, H. N. Han, K. H. Oh,
C. G. Lee, and S.-J. Kim, Mater. Sci. Forum. 495-497, 901
(2005).
8. S. J. Hong, Y. H. Jang, Y. I. Jeong, T. J. Lee, C. G. Lee, S. J.
Kim, and S. S. Kim, J. Kor. Inst. Met. & Mater. 45, 90 (2007).
9. Y. S. Sato, H. Kokawa, K. Ikeda, M. Enomoto, S. Jogan,
and T. Hashimoto, Metall. Mater. Trans. A 32, 941 (2001).
10. D. P. Field, T. W. Nelson, Y. Hovanski, and K. V. Jata, Met-
all. Mater. Trans. A 32, 2869 (2001).
11. R. W. Fonda, J. F. Bingert, and K. J. Colligan, Scripta mater.
51, 243 (2004).
12. R. W. Fonda and J. F. Bingert, Scripta mater. 57, 1052
(2007).
13. P. B. Prangnell and C. P. Heason, Acta mater. 53, 3179 (2005).
14. T. U. Seidel and A. P. Reynolds, Metall. Mater. Trans. A
32, 2879 (2001).
15. K. Colligan, Weld. J. 65, 229 (1999).
16. J. A. Schneider and A. C. Nunes Jr., Metall. Trans. B 35,
777 (2004).
17. B. C. Liechty and B. W. Webb, J. Mater. Process. Technol.
184, 240 (2007).
18. H. N. B. Schmidt, T. L. Dickerson, and J. H. Hattel, Acta
mater. 54, 1199 (2006).
19. H. Schmidt and J. Hattel, Modell. Simul. Mater. Sci. Eng.
13, 77 (2005).
20. P. Ulysse, Int. J. Mach. Tools Manuf. 42, 1549 (2002).
21. P. A. Colegrove and H. R. Shercliff, Sci. Technol. Weld.
Joining 9, 345 (2004).
22. R. Nandan, G. G. Roy, and T. DebRoy, Metall. Mater.
Trans. A 37, 1247 (2006).
23. Y. H. Zhao, S. B. Lin, S. B. F. X. Qu, and L. Wu, Mater.
Sci. Technol. 22, 45 (2006).
24. Z. W. Chen, T. Pasang, and Y. Qi, Mater. Sci Eng. A 474,
312 (2008).
25. J.-H. Cho, S. H. Kang, H. N. Han, and K. H. Oh, Met.
Mater. Int. 14, 247 (2008).
26. G. R. Canova, U. F. Kocks, and J. J. Jonas, Acta metall. 32,
211 (1984).
27. F. Montheillet, M. Cohen, and J. J. Jonas, Acta metall. 32,
2077 (1984).
28. F. Montheillet, P. Gilormini, and J. J. Jonas, Acta metall.
33, 705 (1985).
29. L. S. Toth, P. Gilormini, and J. J. Jonas, Acta metall. 36,
3077 (1988).