A FEASIBILITY STUDY FOR STATIONARY SHOULDER SELF-REACTING PIN TOOL

WELDING

Toshiyuki SUDA, Yasuhiro SAKAMOTO,

Tomonori MIYAMICHI and Tetsuro SATO

Technology Integrating Department, Rolling Stock Division

Nippon Sharyo, Ltd.

2-20 Honohara, Toyokawa, Aichi, 442-8502, Japan

ABSTRACT

This paper addresses the development of the Stationary Shoulder Self-Reacting Pin

Tool (SSSRPT) and the evaluation of this tool applied to the butt joint of thin aluminum

plates. The newly developed SSSRPT makes the weld bead flat and coplanar with the

workpiece surface. The mechanical properties are better than those of original

Self-Reacting Pin Tool (SRPT) [1] in which the shoulders rotate as its probe.

The essential feature of SSSRPT is that at least one shoulder is free in rotation against

the probe, thus this tool is referred as “stationary shoulder”. Its remaining features are

the same as those of SRPT; installation of the two opposing shoulders that pinch the

workpieces, the positional adaptability of the both shoulders along the thickness

direction, and controllability of arbitrary pinching force, which makes stable contact

despite the misalignment and the gap of the workpieces. So the backing bar is not

required in this process. Butt welding is performed by this tool for 4mm thick heat

treated 6000 series aluminum alloy plates. Appearance observation, macroetch, tensile

and bending test are carried out for evaluating the weld qualities.

Numerical simulation of the welding process is also conducted on the heat input and the

material flow by the finite element method. The sectional shapes of the stir zone of the

simulation result have the good agreement with the macroscopic photos. The size of the

stir zone of SSSRPT is smaller than that of SRPT under the same welding conditions,

and even under the appropriate welding condition of each tool. This result shows the

lower heat input comparing to SRPT. The lower heat input is supposed to serve the

higher weld strength.

1. INTRODUCTION

Self-Reacting Pin Tool (SRPT), that is one of the variations of Friction Stir Welding

(FSW), is adopted to manufacturing of the aluminum-alloy railway carbody structures.

The industrial advantages of SRPT over the conventional FSW are i) the simpler joint

structure, ii) the simpler jig without the backing plate, iii) the more confident joint

performance without the kissing bond. Figure 1 shows the schematic view of typical

railway carbody structure constructed by using aluminum hollow extrusions. The

members for the carbody side structure are joined longitudinally by SRPT; its

appearance is indicated in Figure 2. The outer panels are very flat and the surface

ripples of SRPT weld tracks are removed with finishing. Recently, SRPT has been

contributing to the production of railway carbody because of its superiority in the lower

distortion and the higher qualities, including its beautiful surface appearance, compared

to the arc welding process. Previous development of the tool design and optimization of

the welding parameters [2,3] has achieved the robust and sound welding process.

Stationary Shoulder FSW (SSFSW) has been developed by TWI for joining of lower

thermal conductivity Ti alloys [4]. The tool consists of a rotating pin and a non-rotating

“stationary” shoulder that does not contribute to the heat generation. The stationary

shoulder produces i) balanced weld thermal field ii) smooth welded surfaces iii) reducing

the heat input and the size of the Heat Affected Zone(HAZ). Adopting this stationary

shoulder technique to SRPT is expected to improve the weld qualities. In this study,

three different types of SSSRPT are investigated and developed.

Experimental welding is performed by SSSRPT, and the surface appearance, sectional

features and mechanical properties are evaluated. In addition, numerical simulation of

the welding process is conducted to study on the relation between the material flow and

the shape of the stir zone; difference of the heat input and the result of the welding

qualities are compared.

Figure 1: Schematic view of aluminum-alloy railway carbody structure

Figure 2: Appearance of the side structure (after finishing)

3. TOOL DESIGN & EXPERIMENTAL PROCEDURES

Three different configurations of SSSRPT tested in this study are indicated in Figure 3.

Type A and Type B are one-side stationary shoulder; they have one stationary shoulder

on the top (Type A) or on the bottom (Type B) and the opposite powered shoulder that is

rotated in the same speed with the probe. Type C has double-side stationary shoulders;

it has two stationary shoulders on the top and bottom. The probe is the only part that

contributes to the mechanical work. All stationary shoulders are supported through

thrust bearings; they rotate freely around the spindle. These three type tools are

designed to have the identical probe geometry and the shoulder radius. Workpieces are

4mm thickness of A6N01-T5 extrusion flat plates; they are butt-jointed along the

longitudinal direction. The chemical composition of A6N01 is shown in Table 1.

(a)Type A (b)Type B (c)Type C

Figure 3: Tool design variations of SSSRPT

Table 1: The chemical composition of A6N01 [wt %]

Si Fe Cu Mn Mg Cr Zn Ti

0.40-0.9 <0.35 <0.35 <0.50* 0.40-0.8 <0.30* <0.25 <0.10

*Mn+Cr<0.50

SRPT weld tracks

4. RESULTS

4.1 SURFACE QUALITY

Sound joints are obtained for all the three types of SSSRPT.

Figure 4 shows the surface appearance of the weld track for the tool Type A. Figure 4(a)

is the bottom surface processed by the powered shoulder as same as the conventional

SRPT; toe flash and ripples are observed in some degree on this side. On the other

hand, Figure 4(b) shows the top surface processed by the stationary shoulder; no flash

is observed and perfect smooth surface is achieved on this side.

Table 2 shows typical surface roughness of the joint as welded and the base metal. It is

measured along the weld or the extruded direction. The weld track of the bottom surface

has the surface ripples due to the powered shoulder; the surface roughness is Ra 6.98

and it is larger than Ra 0.29 of the base metal. In contrast, the roughness Ra 0.22 of top

surface processed by the stationary shoulder is very small and equivalent to the base

metal. This is classified to the roughness of the fine or the precision finishing with

grinding.

(a) Bottom surface (b) Top surface

Figure 4: Appearances of as welded track (Type A)

Table 2: The surface roughness of as welded SSSRPT seam (in the weld direction)

Surface Ra[m]

Powered shoulder side Fig.4(a) 6.98

Stationary shoulder side Fig.4(b) 0.22

Base metal

(extruded direction) 0.29

5mm 5mm

4.2 SECTIONAL INVESTIGATION

Figure 5(a)-(c) shows the transverse sectional photos of welded joint with SSSRPT of

Type A-C. Type A and Type B have isosceles trapezoidal shape or rather flare shape

Stir Zone (SZ) because of the asymmetry of the upper and the lower shoulders. In

contrast, Type C produces almost symmetric shape SZ about the mid plane. Stir zone’s

sectional area of Type C is about 76% of Type A. Optical microscopics are shown in

Figure 6; these are corresponding to the locations notated p-s in Figure 5(a). p): Base

metal that is far enough away from SZ has distinct grain boundaries. q): Heat Affected

Zone (HAZ) that has experienced thermal cycle has the obscure grain boundaries due

to their higher corrosion tolerance against the base metal region. r): SZ has highly

refined grains. s):Grains that had been bounded on the stationary shoulder are also

refined.

Figure 7 shows the hardness across the welded region of Type A and Type C SSSRPT.

These hardness profiles are gradually decreased from 100-115Hv of the base metal to

65-75Hv of the region where the shoulders and probe have passed through during the

welding process. The minimum hardness is obtained in HAZ. By comparing Type A and

Type C, the width of the softened region of Type C joint is clearly narrowed at the bottom

of the advancing side against that of Type A in spite of the identical shoulder radius on

the bottom surface; this difference suggests that HAZ of Type C joint is narrowed by the

effect of stationary shoulder.

(a) Type A (b) Type B (c) Type C

Figure 5: Cross-section of the A6N01-T5 SSSRPT joints

Figure 6: Microscopic photos of figure 5(a)

p q r

s

p q

r detail of r

Advancing side Retreating side

s

Advancing side Retreating side Advancing side Retreating side

1mm 1mm 1mm

Figure 7: Vickers hardness across the weld region of the A6N01-T5 SSSRPT

4.3 MECHANICAL PROPERTIES

Table 3 shows the comparison of the tensile strength of SSSRPT, SRPT and base metal.

The tensile strength of type A is 226.7MPa (joint efficiency :81.4%), type B is 231.7MPa

(83.2%), type C is 226.0MPa (81.2%). All of them are about 20-25MPa higher than

207.1MPa of SRPT. Fracture of all specimens of SSSRPT occurs in HAZ on the advancing

side. This result suggests that the strength reduction of HAZ is slightly progressed on the

advancing side than that on the retreating side.

Figure 8 shows the appearances of the face and the root bending test specimens of type A

SSSRPT joint. There are no cracks in SZ nor in HAZ. The tip of the oxide film shown in

Figure 5 does not open to be a crack, either.

Table 3: Results of tensile strength of SSSRPT as welded joint

Specimen Tensile strength [MPa] Joint Efficiency[%]

SSSRPT (TypeA) 226.7 81.4

SSSRPT (TypeB) 231.7 83.2

SSSRPT (TypeC) 226.0 81.2

SRPT [2] 207.1 74.4

Base Metal (A6N01-T5) 278.5 100.0

Top

Center

Bottom

Top

0.5mm

Bottom 0.5mm

Center

Measurement location

(a)Face bend(stationary shoulder side) (b)Root bend (powered shoulder side)

Figure 8: The appearances of face and root bending test specimens for Type A

5. DISCUSSION

The Finite element simulation for the FSW process is performed to investigate the

material flow and the temperature distribution. The numerical model and the material

model are the same as those described in Reference [2,3]. In Figure 9, the transverse

sectional views of the shear strain rate and the temperature obtained by the numerical

simulation are compared between SSSRPT(Type A) and SRPT under the appropriate

welding parameters for each tool configuration. The temperature of the characteristic

points shown in Figure 9(c) and (f) is shown in Table 4. The input energy applied to the

unit butt area of the joint obtained by the numerical simulation is shown in Table 5. The

stir zone of SRPT is constricted shape; its width is narrowest at the mid plane, it is

getting wider toward the upper and the lower shoulders (Figure 9(a)). This constricted

shape can be attributed to the distribution of the rotating shear force and the frictional

heat input caused by the probe and the shoulders. The isoline of the shear strain rate on

the transverse section has the similar shape (Figure 9(b)). The temperature on

transverse section also has the similar shape (Figure 9(c)).

For SSSRPT Type A, in which the upper shoulder has no contribution to the mechanical

work, the stir zone is flare-shaped (Figure 9(d)). The isolines of the shear strain rate and

the temperature have the similar shape (Figure 9(e) and (f)). The temperature adjacent

to the tool probe (Point 2 and 3) is almost the same for SRPT and SSSRPT. It is

understood that the adequate temperature adjacent to the tool probe is essential for the

joining process to maintain the stable material transfer around the tool. In contrast, the

temperature on Point 1 and 4 are lower for SSSRPT than that of SRPT. Butt joint can be

successfully joined with the mechanical work done by the probe solely; the rotational

force and the frictional heat supplied by the probe can be sufficient to plasticize and to

transfer the workpiece material. The mechanical work done by the shoulders is spread

to the extent of its radius where the circumferential speed is the maximum; this heat

could not be essential. Thus SSSRPT is an effective tool to supply the lower and the

minimal necessary heat input. It is understood that the stronger tensile strength of

SSSRPT compared to SRPT (Table 3) is related to the lower heat input.

Figure 9: The shear strain rate and temperature due to the numerical simulation

Table 4: The temperature of the characteristic points in Figure 9(e) and (f)

Temperature [deg C.]

Point 1 2 3 4

SRPT 399.7 543.6 465.3 366.0

SSSRPT 353.4 531.7 476.0 350.1

Table 5: The input energy obtained by the numerical simulation

Tool type Energy density [MJ/m2]

SRPT 32.2

SSSRPT 31.2

400degC. 500degC.

1 2 3 4

(e) Temperature field of SRPT (f) Temperature field of SSSRPT

(c) The shear strain rate of SRPT (d) The shear strain rate of SSSRPT

(a) Transverse section of SRPT (b) Transverse section of SSSRPT

25/sec

1 2 3 4

400degC. 500degC.

6. CONCLUSION

This feasibility study is summarized as follows.

・ Sound weld of 6xxx aluminum-alloy butt joint is obtained by all the three types of

SSSRPT developed.

・ Surface of SSSRPT weld track is very smooth and the surface roughness is Ra0.22.

It is similar to the base metal surface.

・ SSSRPT effectively produces the weld with the lower and the minimal necessary

heat input. It restricts the region of the plasticized material closely around the probe.

It keeps the temperature of HAZ lower. As a result, the input energy density can be

made lower than that of SRPT for the same thickness.

・ The width of HAZ processed by the stationary shoulder can be narrower than that of

SRPT.

・ Joint efficiency of SSSRPT is 81-83%; it is higher than that of SRPT, 74%.

7. FUTURE WORKS

Nippon Sharyo is progressing SSSRPT process for practical applications. Fatigue

strength of the weld, the tolerance of the gap and the misalignment, the reduction of the

tool cost and welding speed are investigated.

REFERENCES

[1] G. Sylva, R. Edwards and T. Sassa, A Feasibility Study for Self Reacting Pin Tool

Welding Of Thin Section Aluminum, Proceedings of the 5th International Symposium on

Friction Stir Welding 2004

[2] D. Otsuka and Y. Sakai, Self Reacting Pin Tool Application for Railway Car Body

Assembly, Proceedings of the 7th International Symposium on Friction Stir Welding

2008

[3] T. Sato and T. Suda, Finite Element Analysis of Friction Stir Welding Affected by

Heat Conduction through the Welding Jig, INALCO2010

[4] M J Russell and C Blignault, Recent Developments in Friction Stir Welding of Ti

Alloys, Proceedings of the 6th International Symposium on Friction Stir Welding 2006