Effect of tool eccentricity on the microstructural and mechanical property of friction stir welded aluminum alloys

L. H. Shah*,1, S. Guo1, S. Walbridge1, A. Gerlich1

1Faculty of Engineering, University of Waterloo, Waterloo, Ontario, Canada.

*Corresponding author: lhahmads@uwaterloo.ca

Abstract

This paper investigates the effect of tool eccentricity on the material flow and mechanical

properties of friction stir welded (FSWed) AA5083 and AA6061 aluminum alloys. Samples were

butt joined parallel to the rolling direction, using a threaded and tapered tool with 3 flats, which

was mounted with and without intentional eccentricity (or coaxial misalignment). The travel speed

and tool rotation speed were 63 mm/min and 1120 rpm, respectively. The weld microstructure

was characterized using optical microscopy and scanning electron microscopy. Mechanical

properties were measured using Vickers hardness indentation and tensile testing. Microstructural

observations show that tool eccentricity is more effective in enhancing the material flow in the

nugget zone for AA6061 alloy. The hardness of the AA6061 sample exhibits increased softening

at the middle region of the stir zone with tool eccentricity, while the stir zone in AA5083 samples

exhibited a negligible difference between the aligned versus eccentric tool. Tensile tests for

AA6061 show similar tensile strength and elongation, while a decrease in strength and elongation

can be observed in AA5083 due to the eccentricity setup. It can be concluded that tool eccentricity

enhances the material flow of the nugget zone and promote further softening of AA6061, but show

negligible impact on the overall mechanical property. On the other hand, AA5083 exhibits a

negligible difference in material flow and hardness profile, although elongation during transverse

tensile testing decreased with tool eccentricity.

Introduction

Friction stir welding (FSW) was invented in The Welding Institute (TWI), United Kingdom

in 1991 (Thomas et al. 1991). Due to the simplicity of the welding operation, since then, the

application of this process has been growing exponentially in various sectors such as aerospace,

ship building, automotive and structural industry (Thomas & Nicholas 1998; Lohwasser & Chen

2010).

The method exploits heat created from friction between the FSW tool and the workpiece

to join materials together by rapid rotation of the tool while traversing along the abutting edges.

Normally, a slight tilt angle towards the travel direction is used to facilitate materials joining.

Advantages of this method includes non-toxicity, low operational cost, and increase in productivity

(Mishra & Ma 2005).

Generally, the tool is securely inserted in a spindle head that rotates about a fixed

rotational axis. However, in some cases, an improper fit up of the tool in the spindle head may

result in the tool rotating in an eccentric manner. In fact, the actual tolerance in tool alignment is

seldom discussed in literature or specifications, as it is assumed variations within typical

machining tolerances will have a negligible role on the resulting weld. However, when an eccentric

alignment occurs, it creates a subtle wobbling motion of the tool during rotation and may affect

the joint quality. To the best of the authors’ knowledge, no study has been done so far to

investigate this issue and the severity of the impact it may have on the joint properties. This study

investigates the effect of tool eccentricity on the microstructural and mechanical properties of

friction stir welds in two common aluminum alloy types used in the industry.

Experimental method

The base metals used in this study are aluminum alloy AA5083-H321 (4.7 wt.% Mg,

0.8 wt.% Mn) and AA6061-T6511 (0.81 wt.% Mg, 0.53 wt.% Si) with 9.1 mm and 9.5 mm

thickness, respectively.

The base metals were welded using a JAFO horizontal universal milling machine in a butt

joint configuration. The FSW tool used is an H13 steel with a 15 mm shoulder diameter, 6 mm pin

diameter as well as 8.9 mm and 9.3 mm pin length for AA5083 and AA6061 sheets, respectively.

The pin geometry is fabricated with a 10º tapered profile, which was threaded and includes three

flats. Through preliminary experiments, the fixed parameters of the welding process are set as

1120 rpm rotational speed, 63 mm/min travel speed, 2.5º tool tilt angle and tool eccentricity of

0.2 mm offset (with the aligned setup being 0 mm offset). The samples were butt joined parallel

to rolling direction.

The schematic of the tool eccentricity setup compared to the aligned (conventional) setup

is shown in Figure 1. The aligned tool holder uses a collet to clamp the material evenly throughout

the circumference of the tool shoulder to ensure proper fit up (Figure 1(a)). In contrast, the tool

eccentricity setup uses a side locking set-screw in the holder to fix the tool in place. Hence, it is

possible to have the desired eccentricity by adjusting the clamping so that the tool is fixed on an

offset as shown in Figure 1(b). Tool eccentricity was measured using a dial gauge before the FSW

process for accuracy. To differentiate the samples according to the tool setup, the samples will

be designated 5083A and 6061A for the aligned setup, while the samples fabricated using the

tool eccentricity setup will be designated as 5083E and 6061E, respectively.

Figure 1. Schematic of (a) aligned tool setup, and (b) tool eccentricity setup.

To enhance observation on the material flow at the abutting surface, a 0.1 mm thin pure

Al foil was inserted between the faying surface of the base material as shown in Figure 2.

Markers/tracers have been used in previous FSW studies as effective means to study the flow in

the nugget region (Leitão et al. 2008; Guerra et al. 2003; Seidel & Reynolds 2001; Dickerson et

al. 2003). Pure Al foil is used in this study since it has similar main composition as the workpiece

but gives good contrast with its surroundings.

Figure 2. Schematic of the FSW setup with thin pure Al foil insert.

For microstructural observation, samples were cold mounted and ground with silicon

carbide abrasive papers of grit sizes 240, 320, 400 and 600. Next, polishing was done using 9 µm

and 3 µm diamond paste before finishing with 0.1 µm colloidal silica. Finally, etching was done

using Keller’s reagent (190ml H2O, 5ml HNO3, 3ml HCl and 2ml HF) for 120 s to give an overall

feature of the sample. Optical microscope and scanning electron microscopy was used to observe

the microstructure of joints. Mechanical property characterization was conducted using Vickers

hardness tests and quasi-static tensile tests. Hardness was done using 0.2 kgf load and 10 s

dwell time, while the traverse tensile test was done on dog-bone coupons in accordance to ASTM

E8M-04 using quasi-static loading of 1 mm/min.

Results and discussion

Figure 3 shows the top view of the as-welded samples of AA5083 and 6061. Good weld

bead with minor surface defects can be observed for all specimen. Particularly, the AA5083

samples show minimal toe flash and surface galling.

Figure 3. Top view of as-welded samples (a) 5083A, (b) 5083E, (c) 6061A and (d) 6061E.

Figure 4 shows the cross-sectional macrograph of specimens etched with Keller’s reagent.

The etching reveals better contrast for AA6061 compared to AA5083 samples. For all specimen,

a clear delineation between the weld region and the base metal can be observed at the advancing

side, while a diffused boundary can be seen at the retreating side. This is a common observation

in cross-sectional macrograph of FSWs (James & Mahoney 1999). The AA6061 specimens show

no observable defects, while the AA5083 specimens show a subsurface void in both aligned and

eccentric samples which had a length of approximately 1 mm and 0.4 mm, respectively.

The AA5083 specimens only show a faint indication of Al foil line near the top region. On

the other hand, a clear distinction can be observed between the Al foil and the surroundings in

AA6061 samples, where the foil exhibits formation of a tortuous line about the periphery of the

weld center and inclined to the advancing side at the crown of the weld. Near the root of the weld

of AA6061 specimens, however, there were no observable foil lines due to the extension of the

white bands from the advancing side of the nugget zone towards the retreating side. Furthermore,

this ‘no foil zone’ is further expanded upwards in the 6061E sample as a consequence to the

expansion of the white bands.

Figure 4. Cross sectional macrograph of (a) 5083A, (b) 5083E, (c) 6061A and (d) 6061E

samples.

Figure 5. Al foil finely dispersed throughout the nugget zone center of (a) 5083A and (b) 5083E

sample.

The absence of the foil line in most of the through thickness of AA5083 samples and the

‘no foil zone’ region in AA6061 samples are in fact indication of good material flow in the weld

region. Microscopic observations of these ‘no foil zone’ regions using higher magnification

revealed that the foils were sheared and displaced throughout the weld region. As an example,

Figure 5 shows montages of the 5083A and 5083E welds with the Al foil (bright curves) being

finely dispersed in the nugget zone center. While both AA5083 samples show a similar dispersed

Al foil pattern in the weld nugget, the upward extension of the ‘no foil zone’ in 6061E implies that

tool eccentricity enhances the material flow of AA6061.

Figure 6 presents the hardness profile for the top, middle, and bottom cross-sectional

welded regions of AA5083 and AA6061 specimens. The top and bottom region measurements

are taken approximately 1 mm from the sample’s horizontal surface. The advancing side is to the

left of each image, while the retreating side is to the right of each image. The dashed black line is

an approximation of the weld nugget boundary for both cases. For the AA5083 samples, the

overall hardness results show minimal differences between aligned and eccentric sample. On the

other hand, the weld produced with tool eccentricity for AA6061 samples indicates softening

particularly at the middle region of the cross-section, where the observable white bands expanded

towards the center of the weld nugget (Figure 4(d)).

Figure 6. Hardness profile of the top, middle and bottom cross-sectional welded region for

(a) AA5083 samples and (b) AA6061 samples.

5083 alloys are non-heat treatable aluminum alloys, which means that heat treatment

processes do not cause significant changes in the mechanical characteristics of the material. In

contrast, properties of heat treatable aluminum alloys such as AA6061 alloys can be heavily

influenced by heat treatment processes such as during the welding process. This is due to

dissolution of strengthening precipitates due to the heat produced during the welding process

(Sato et al. 1999). Further softening of the region in the eccentricity setup may suggest an

increase in heat input possibly due to increase in material shearing activity. The higher heat input

consequently allows grain growth which softens the lower region (Gerlich et al. 2007).

The average stress-strain curve of the quasi-static tensile tests for all specimen are shown

in Figure 7. All tests were repeated three times and the plotted curves are representatives of each

group. All AA5083 specimens failed at the nugget zone. On the other hand, all 6061A samples

fractured at the heat affected zone (HAZ) of the advancing side. One of the 6061E samples failed

at the same region, while the two other samples failed at the HAZ of retreating side. Failure at the

HAZ region is a common observation in heat-treatable aluminum alloys due to the grain growth

and softening of the HAZ compared to the weld nugget (Mathers 2002).

It can be seen from Figure 7 that there is minimal change in the curve pattern between

6061A and 6061E. On the contrary, a decrease in elongation can be seen in 5083E (5%) sample

compared to 5083A (7%). Furthermore, as presented in Figure 8, 5083E also exhibit a slight

decrease in ultimate tensile strength (UTS), while the 6061E sample shows negligible change as

compared to 6061A. The reason for the decrease in UTS and elongation in AA5083 due to

eccentric setup is currently unclear and further investigation is needed to clarify this phenomenon.

Figure 7. Stress-strain curve for all traverse tensile specimens.

Figure 8. The ultimate tensile strength (UTS) for all specimens with base metals UTS as

reference.

Conclusion

The study of tool eccentricity effects on the material flow and mechanical property of

friction stir welded AA5083 and AA6061 aluminum alloy was successfully conducted. It can be

summarized by the following points:

- Microstructural observations show that tool eccentricity enhances the material flow in the

nugget zone of AA6061 aluminum alloys.

- The Vickers hardness in AA6061 welds exhibit softening at the middle region of the cross

section due to tool eccentricity, while the microhardness of AA5083 welds were not

sensitive to tool eccentricity.

- Tensile tests for AA6061 samples show similar tensile strength and strain behavior, while

AA5083 samples show a slight tensile strength decrease and lower elongation with

eccentricity setup.

- It can be concluded that tool eccentricity enhances the material flow of the nugget zone

and promotes further softening of heat-treatable aluminum alloys, but show negligible

impact on the overall mechanical properties. On the other hand, AA5083 samples show

negligible difference in material flow and hardness profile, but exhibit a slight decrease in

elongation and tensile strength.

Acknowledgements

The authors would like to thank Universiti Malaysia Pahang and Ministry of Higher

Education Malaysia for research funding. Support from the National Sciences and Engineering

Research Council of Canada is also greatly appreciated.

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