influence of tool thread on the mechanical properties of dissimilar al-alloy friction stir spot...

Influence of tool thread on the mechanical properties of dissimilar Al-alloy friction stir spot welds

Ikuta Akihikoa, Yin Yuhuanb and North Thomas Hc

aSchool of Engineering, Kinki University, Hiroshima, Japan; bSchool of Material Science and Engineering, Tianjin University, Tianjin,P.R. China; cDepartment of Materials Science and Engineering, University of Toronto, Toronto, Canada

(Received 17 May 2010; final version received 23 August 2010)

The influence of threaded and wear simulated (half-thread) tools on the mechanical properties of dissimilar Al-alloy frictionstir spot welds is investigated. The failure load properties of dissimilar A5754/A6111 lap joints produced using wearsimulated (half-thread) tools were much poorer in joints made using tool rotational speeds of 1000 and 1500 rpm. Similarfailure load properties were produced using these tools, when the tool rotational speed was increased to 2250 rpm. In jointsmade using a tool rotational speed of 3000 rpm, the failure load properties of joints made using the half-thread tool werehigher than those of joints made using the threaded tool. Similar failure load properties were found in A5052/A6061 lap andbutt joints made using threaded, half-threaded and no thread tools and a tool rotational speed of 1000 rpm. Also, the failureload properties of lap joints made using the different tools corresponded with their bonded widths and Y values. The area ofthe stir zone on the bonded cross section corresponded with the actual bonded region on the fracture surface ofA5052/A6061 butt joints. However, the weld profiles were different from those in A5052/A6061 lap joints. It is, therefore,suggested that the failure load properties of lap joints are similar when the bonded widths and Y values are similar. Inaddition, the thread on the cylindrical probe has negligible influence on the mechanical properties of lap joints. The majorrole played by the thread involves stirring in the vertical direction. Stirring in the horizontal direction is unchanged in jointsmade using the different tools.

Keywords: friction stir spot welding; aluminium alloy; mechanical properties; thread; tool wear

1. Introduction

With the increasing use of aluminium alloys in recent years,

friction stir spot welding (FSSW), a form of friction stir

welding (FSW) has attracted interest in the automotive

industry as a substitute for the previously used resistance

spot welding [1–5]. Studies aimed at applying the technique

to high-tensile strength and other steels is also continuing.

On the other hand, there are currently few reports on the

tools that are essential for joining using both FSW and

FSSW. Possibly, information on tool shapes and tool wear

during welding is of a nature that makes it difficult to make

public as part of individual production technologies but it

will be necessary to obtain basic findings on friction

welding tools for this technique to be used more widely.

In FSW, the probe, regarded as the usual tool, does not

necessarily require a thread and there have been reports

[6,7] that there is almost no effect on the joint properties

even in a tool where the thread wears and is then auto-

optimized, and also a report [8] that a thread is not

necessarily required for materials which readily experi-

ence plastic flow and even with materials which do not

readily exhibit plastic flow, joining may be possible

depending on the welding conditions. In FSSW, however,

it may be impossible or difficult to form a stir zone unless

the probe is threaded [9,10]. These findings indicate that

for FSSW probe thread wear has a major effect on whether

joining can continue without loss of quality. However,

even for FSSW a range of tool shapes have recently been

proposed for the welding of various metals [11–14]. In

future considerations of tool shape, how to address the

problems of whether to use a thread and how it will wear

will also bring questions of increasing importance for

FSW. Against this background, how to carry out welding

of high-tensile strength steel and dissimilar metal welding

between different aluminium alloys and steel and

aluminium alloy, techniques which are not yet established,

will be an important for FSSW in its role as a welding

technique that can replace resistance spot welding in the

automotive industry. In such cases, the questions of what

tool shape to use for badly weldable metals and how to

handle tool wear to maintain joint quality are particularly

important for future applications of FSSW and a policy

will be necessary to answer these.

In this study, different aluminium alloys were lap

welded by FSSW, using a tool with simulated worn probe

thread, with the aim of discovering the effect of the thread

of a standard probe on the mechanical properties of FSSW

joints. After welding, tensile shear tests were carried out

on the joint and the microstructure of the weld was

examined and an examination was made of the effect of

probe thread on the change factors for weldability.

2. Test metals and experimental methods

2.1 Test metals

Since a tool with simulated probe thread wear was used,

weld metals in which wear progresses markedly during the

experiment were not desirable for this study. The

aluminium alloys used were A5754-H12 (hereafter:

A5754) and A6111-T4 (hereafter: A6111) since welding

of these is currently being actively carried out [15–17] and

the state of the stir in the upper and lower sheets is easily

observable. Other 5000-grade and 6000-grade aluminium

alloys used were A5052-H34 (hereafter: A5052) and

A6061-T651 (hereafter: A6061). The chemical compo-

sitions of these are shown in Table 1. The dimensions of

the alloys were 100mm long and 25mm wide, with the

q 2013 Taylor & Francis

Welding International, 2013

http://dx.doi.org/10.1080/09507116.2012.753246

Selected from Quarterly Journal of the Japan Welding Society 28(3) 346–354

thickness being 1.6mm for A5754, 1.5mm for A6111,

1.5mm or 3mm for A5052 and 3mm for A6061.

2.2 Friction stir spot welding

The welding conditions for FSSW were a tool rotational

speed varied between 1000, 1500, 2250 and 3000 rpm, a

plunging speed of 2.5mm/s, a dwell time (the time the

position after plunging is maintained) of 1 s and a constant

shoulder plunging depth of 0.5mm. The welding method

was that the upper plate was 5000-grade (A5754 or

A5052) and the lower plate was 6000-grade (A6111 or

A6061), the overlap was 25mm and welding was carried

out at the centre of this.

2.3 FSSW tools

The tools used for welding were H13 tool steel (JIS

SKD61 equivalent), which was machined and then heat-

treated so that the hardness was HRc 46–48. As shown in

Figure 1, the shape of the probes was a standard cylindrical

shape, with a shoulder diameter of 10mm and a probe

length of 2.2mm. In order to examine the effect of wear,

welding was carried out using a tool with a cylindrical

probe that had been M4 threaded (hereafter: threaded tool)

and a tool in which the M4 threaded part has been further

machined to reduce the thread height by half and simulate

wear (hereafter: half-thread tool). In some cases, a tool

with an unthreaded cylindrical probe (hereafter:

unthreaded tool) was used.

2.4 Assessment of the weld

After FSSW, tensile shear fracture tests were carried out

on the joint obtained. The test method was that the as-

welded joint was stretched and the load at which shear

fracture occurred was assessed. The elongation speed was

1mm/min. The joint was cut in the cross-sectional

direction and mirror polished before being etched with

Keller’s reagent (hydrofluoric acid 2ml, hydrochloric acid

3ml, nitric acid 5ml and distilled water 190ml) and the

microstructure examined using an optical microscope.

At this time, in order to quantify the state of the joint, as

shown in Figure 2, the parameters of the distance from the

probe surface to the tip of the hook (an oxide layer at the

interface between the upper plate metal and lower plate

metal) [17,18] (hereafter: Y value) [19], the angle formed

between the hook and the initial interface between the

upper and lower plates (hereafter: curvature) and the

distance between the probe surface to the weld start point,

that is, the total of the stir zone width [20] and deformed

metal width (hereafter: bonded width) were measured. The

Y value and bonded width were measured using an optical

microscope with a magnification of 500-fold. For the Y

value, the position at which the hook fragmented was

taken as the hook tip. If, however, the hook had large

irregularities, the position on the hook baseline at which

the hook broke up was taken as the hook tip as shown in

Figure 2.

Table 1. Chemical compositions of base materials (wt%).

Materials Cu Mg Mn Si Fe Cr Zn Ti Al

A5754-H12 ,0.01 3.01 0.47 ,0.10 – – – – Bal.A6111-T4 0.75 0.75 0.20 0.69 – – – – Bal.A5052-H34 0.01 2.38 0.04 0.08 0.19 0.20 0.01 - Bal.A6061-T651 0.26 1.0 0.05 0.60 0.25 0.16 0.01 0.02 Bal.

M4

2.2

Threaded Half-thread No thread

φ4 φ4φ10

φ3.6

Figure 1. Different tool designs.

Top of hook

Hook

A

Shou

lder

pene

trat

ion

dept

h

Y

v

Upper sheet

Lower sheet

Sheetintersection

t

Rotating tool

Displaced material width

Bonded width

Stir zonewidth

Detail A

Curva

ture

Figure 2. Schematic illustration showing the measurement ofthe stir zone and bonded widths, Y values, shoulder penetrationdepths and the curvatures of the hook regions.

I. Akihiko et al.2

3. Experimental results and discussion

3.1 Results of tensile shear fracture tests

Figure 3 shows the results of tensile shear fracture tests of

lap joints combining A5754 and A6111, when each of the

tools was used and the tool rotational speed was varied.

First, for the threaded tool, the changes of the tensile shear

strength when the tool rotational speed was varied showed

a low value at 1000 rpm and the highest value, of around

1.8 kN, at 1500 rpm but then fell as the tool rotational

speed increased, with the lowest value at 3000 rpm. For the

half-thread tool tensile shear strength increased linearly

with rotational speed of tool up to 2250 rpm, thereafter

becoming constant, showing the maximum value of

around 1.5 kN at 3000 rpm. The difference in tensile shear

strength between the threaded tool and half-thread tool

was greatest (approximately 1 kN at a slow rotational

speed, whereas the tensile shear strengths were almost the

same (1.2–1.5 kN) at the higher speed. It can thus be

inferred that the tendencies of the threaded tool and half-

thread tool differed and the factors affecting the

mechanical properties of the joints were also different.

3.2 Results of observation of joint structure

Figure 4 shows examples of A5754/A6111 lap joints,

made using the threaded tool and half-thread tool, in which

the greatest tensile shear strengths for each tool was

achieved. In the Figure 1 is the hook tip [20] and 2 the

bonded/unbonded boundary.

First, as shown in Figure 4(a), a concentric circular stir

zone was observed with the threaded tool, even at the

relatively low tool rotational speed of 1500 rpm. Although

not shown in the figure, comparisons at the same tool

rotational speed showed that usually the increase in the stir

zone with a threaded tool is greater than that with a half-

thread tool. This is due to the previously reported [16] fact

that a threaded tool promotes the development of the stir

zone by the actions of the thread. Similarly, the bonded

width, including the hook tip, shown at 1 and 2 is large

even when the tool rotational speed is comparatively slow.

Furthermore, the deformation of the lower plate metal into

the upper plate metal in the heat treatment affected zone is

abrupt and the hook region is highly uneven. The stir zone

development promotion caused by the half-thread tool

shown in Figure 4(b) is weaker than the threaded tool.

Even so, at a tool rotational speed of 3000 rpm, the stir

region becomes larger and the bonded width, shown by 1

and 2, also becomes greater. However, the hook region is

smooth and deformation of the lower plate metal into the

upper plate metal in the heat treatment affected zone

occurs gradually from the unbonded region side rather

than at point 2. Thus, even when the joints have similarly

high strength, the state of the microstructure differs greatly

between the threaded tool and half-thread tool.

Next, Figure 5 shows examples of the results of

examination of the microstructure after tensile shear

fracture tests of lap joints combining A5754 and A6111, in

which there are the lowest and highest tensile shear

strengths for each of the tools. As shown in Figure 5(a), in

a joint with a high tensile shear strength from the threaded

tool, the fracture path is laterally across the heat treatment

affected zone or along the hook deformed into a crank

shape. However, in the joint with the low tensile shear

strength in Figure 5(b), the joined part is large but the

fracture path passes smoothly along the hook with no great

deformation. In the joint with high tensile shear strength

with a half-thread tool shown in Figure 5(c), although one

side of the fracture path is, in part, laterally across the heat

treatment affected zone and mostly passes smoothly along

the hook, the other side, as with the threaded tool, passes

along the hook and is deformed into a crank shape. In the

joint in Figure 5(d), where the tensile shear strength is low,

the fracture path is laterally across a very small heat

treatment affected zone.

From these results of microstructural examinations

before and after fracture, it is easy to estimate the extent to

which the characteristic microstructural morphology of the

joint affects tensile shear strength. Since a comparison of

the joint fracture paths when joints are made with a

threaded tool and half-threaded tool show that these are

different even when the same joint strength is obtained, it

is considered that the following three points are of

particular importance. Since there are fractures along the

2

1 mm

1

(a)

2

1 mm

1

(b)

Figure 4. Profiles of A5754/A6111 lap joints having the highestfailure load properties. In friction stir spot welds made using (a)threaded tool at a rotational speed of 1500 rpm and (b) half-threadtool at a rotational speed of 3000 rpm.

Welding conditions (A5754/A6111)Shoulder penetration depth: 0.5 mmPlunging speed: 2.5 mm/sDwell time: 1 s

4

3

2

1

0

Tool rotational speed (rpm)1000 2000 3000

ThreadedHalf-thread

Failu

re lo

ad (

kN)

Figure 3. Relation between failure load properties and toolrotational speed in A5754/A6111 lap joints.

Welding International 3

hook with relatively little deformation, the size of the

region where the bond is in the hook, even partially, is

considered to be important. Next, since there are fractures

along the hook with relatively great deformation, the size

of the region in front of the hook tip where it is considered

that the upper and lower plates are completely bonded is

also considered to be important. Furthermore, since there

are cases unlike these in which the fracture passes laterally

across the heat treatment affected zone, whether the

fracture path is linear or complex is important. From these,

taken together, it is clear that the size of the bonded area is

an important factor for a joint to achieve great strength. In

all joints, the bonded width becomes larger as the stir zone

becomes larger but the growth rate of the bonded width is

not as marked as the growth rate of the stir zone and it is,

therefore, thought that in actuality it is the bonded width

not the stir zone that affects the mechanical characteristics

of the joint. Furthermore, the interface strength of the hook

is low and it becomes the propagation route for cracks, in

some cases, the part in front of the hook tip appearing to

handle the load when the deformation is relatively great.

This is where the route has a microstructure morphology in

which crack propagation is difficult, possibly formed in

relation to the amount of fractures crossing the heat

treatment affected zone and stir zone, so that cracks in the

hook, where the interface strength should be small, do not

propagate. When considering the parameters for the state

of joints shown in Figure 2, the following are thought to be

important: (1) the bonded width, the largest region in

which there is actual bonding, even if partial, (2) the region

where the load is handled at the part in front of the hook

tip, that is, the Y value, the region where the upper and

lower plates in front of the hook are completely bonded

and (3) a microstructural morphology that makes it

difficult for cracks to propagate along a path, that is, the

curvature considered to affect whether the fracture path is

linear or complex.

3.3 The effect of microstructural morphology on themechanical properties of the joint

The above results have indicated the possibility that

microstructural morphology affects the mechanical proper-

ties of joints. Accordingly, an attempt wasmade to quantify

the microstructural state of a lap joint combining A5754

and A6111 in terms of bonded width, Y value and curvature

and the results are shown in Figures 6–8.

It is clear from the results shown in Figure 6 that,

although there were slight differences between a threaded

tool and a half-thread tool, the bonded width showed a

tendency to increase, to a maximum of 1500mm, as the

rotational speed increased. With threaded tools, the

increase in bonded width as the rotational speed increased

was slightly greater at lower rotational speeds than with

half-thread tools. On the other hand, the increase in bonded

widthwith an increase in rotational speed is slightly smaller

than with half-thread tools. Next, it is clear from the results

shown in Figure 7 that the Y value for threaded and half-

thread tools, are very different. With threaded tools, the Y

value decreases as the rotational speed increases, but with

half-thread tools, the Y value increases. At a high tool

(a)

(b)

(c)

(d)

1 mm

1 mm

1 mm

1 mm

Figure 5. Failure modes in A5754/A6111 lap joints having (a)the highest failure load properties when using the threaded tooland a rotational speed of 1500 rpm; (b) the lowest failure loadproperties when using the threaded tool and a rotational speed of3000 rpm; (c) the highest failure load properties when using thehalf-thread tool and a rotational speed of 3000 rpm; (d) the lowestfailure load properties when using the half-thread tool and arotational speed of 1000 rpm.

2000Welding conditions (A5754/A6111)

Shoulder penetration depth: 0.5 mmPlunging speed: 2.5 mm/sDwell time: 1 s

1500

Bon

ded

wid

th (

µm)

1000

500

01000 2000 3000

Tool rotational speed (rpm)

ThreadedHalf-thread

Figure 6. Relation between bonded width and tool rotationalspeed in A5754/A6111 lap joints.

I. Akihiko et al.4

rotational speed, the Y value was 200–300mm for both

threaded and half-thread tools but at a low rotational speed

there was a marked difference in Y values: approximately

535mm for a threaded tool and approximately 175mm for a

half-thread tool. It is also clear from the results shown in

Figure 8 that for both threaded and half-thread tools a

curvature of 1258 at a low rotational speed increases to 1558

at a high rotational speed.

Figure 9 shows curvature of typical traced upper and

lower plate interfaces of joints made using a threaded tool

and a half-thread tool. If these are taken together with

Figure 6–8, and the differences in tensile shear strength of

the joints made by the different tools are taken into

consideration, the fact that a threaded tool shows a high

tensile shear strength at a low rotational speed, as shown in

Figure 9(a), is evidently due to the Y value being largewhile

the bondedwidth is comparatively sufficient.As also shown

in the low rotational speed case of Figure 9(b), the half-

thread tool achieves a comparatively sufficient bonded

width at low rotational speed but the tensile shear strength is

reduced, evidently due to the Y value being low. It is also

thought that, when the rotational speed increases, as shown

in the high rotational speed case in Figure 9(b), with a half-

thread tool, both the bondedwidth and Y value increase and,

as a result, the tensile shear strength increases. The reason

that, although the bonded width increases with an increase

in rotational speedwith a threaded tool, as shown in the high

rotational speed case in Figure 9(a), theY value decreases as

the rotational speed increases remains obscure but it is

thought that the tensile shear strength decreases due to the Y

value decreasing. On the other hand, as seen in the high tool

rotational speed case in Figure 9(a),(b), curvature increases

when rotational speed increases with both threaded and

half-thread tools and there is a tendency for this increase to

be somewhat greater with threaded tools than with half-

thread tools. Based on these findings, it is thought that

tensile shear strength may be inadequate, even when the

bonded width and Y value are comparatively large, because

the interface between upper and lower plates, which will be

the fracture path, is linear.

3.4 Examination of other aluminium alloys

It is clear from the results so far that in a joint made by

joining A5254 and A6111 using a half-thread tool,

simulating wear to the thread of a probe, a tensile shear

strength similar to that achieved with a threaded tool is

obtained when a comparatively high rotational speed is

used. It also became clear that the tensile shear strength of

the joint depended on the shape and dimensions of the joint

microstructure but it is not clear whether this is applicable

to other metals.

The reason for this is that, even when the same kind of

aluminium alloy is dealt with, there are several varying

reports [21–23] on effects on the tensile shear strength of

the joint. Accordingly, verification experiments were

carried out on other aluminium alloys using the same tools.

At this time, 1.5-mm-thick A5052 was used for the upper

plate and 3-mm-thick A6061 for the lower plate, with the

other conditions being the same as those for the

A5754/A6111 lap-welded joints.

Figure 10 shows the results of microstructural

examination of the A5052/A6061 lap-welded joint.

Here, Figure 10(a)–(c) show joints made using threaded,

half-thread and, for reference, no-thread tools, each at a

rotational speed of 1000 rpm. It is clear from a

comparison of Figures 10 and 4 that the stir zone reduces

as the thread of the probe is reduced. In particular, as

stated above, it is clear that the shape of the heat treatment

affected zone varies according to the tool used. However,

the bonded widths, Y values and curvatures, which are as

stated above thought to have a large effect on the tensile

shear strength, measured as in Figure 10(a)–(c), showed

that the bonded widths were 1302, 1303 and 1294mm,

the Y values were 575, 586 and 465mm and curvatures

were 72, 120 and 908.

Next, the tensile shear strengths of the lap joints shown

in Figure 10 were found. Furthermore, in order to examine

the stir state of the bonded part in the horizontal direction,

something not considered hitherto, A5052/A6061 but

joints were prepared by FSSW under the same conditions

and the fracture load was found. 3-mm-thick A5052 and


1000

800

600

400Y v

alue

(µm

)

200

0

ThreadedHalf-thread

Tool rotational speed (rpm)1000 2000 3000

Figure 7. Relation between Y values and tool rotational speed inA5754/A6111 lap joints.

150

100

50

0


ThreadedHalf-thread

Tool rotational speed (rpm)

1000 2000 3000

Cur

vatu

re, °

Figure 8. Relation between hook region curvature and toolrotational speed in A5754/A6111 lap joints.


A6061 were used and it was performed at the centre of the

butt line of the two metals. The welding conditions were

the same as those shown in Figure 10. The results are

shown in Figure 11. The tensile shear strengths of lap

joints made using threaded, half-thread and no-thread tools

were 2.30, 2.23 and 2.14 kN; these values are very similar.

The fracture loads of butt joints made using threaded, half-

thread and no-thread tools were 1.98, 1.95 and 2.01 kN;

these values also are very similar.

Figure 12 shows typical results of microstructural

examination and examination of the fracture surface after

tensile tests of theA5052 side of a butt joint. In Figure 12(a),

which shows the results of microstructural examinations, a

stir zone boundary (shown by 1) and an unbonded boundary

(shown by 2) are observed in both joints. Figure 12(b)

shows the results of examination of the fracture surface,

however on examination of the interior of the stir zone

boundary shown by 1 is observed a fracture surface,

whereas the interior of the unbonded boundary (shown by

2) has a metallic lustre and cannot be considered to be a

fracture surface. It is thought that, due to the rotational

direction of the tool, the A6061 becomes the heat treatment

affected zone and is deformed to and sunk into the A5052

side by the tool rotation. It is estimated, therefore, that it

contains a large unbounded area and does not essentially

make a contribution to the strength of the butt joint.

Accordingly, the actual bonded surface area (effective

cross-sectional surface area) was found. To calculate the

bonded area, the fracture surface was photographed by

optical microscope at 10-fold magnification and the image

was analysed by image analysis software. Judgment on the

1 mm 1 mm 1 mm

(a) (b) (c)

Figure 10. Profiles of A5052/A6061 joints made using a tool rotational speed of 1000 rpm and a dwell time of 1 s. (a) threaded tool; (b)half-thread tool; (c) no thread tool.

4

3

2

1

0Threaded Half-thread No thread

Lap joint


Butt joint

Failu

re lo

ad (

kN)

Figure 11. Failure load properties of A5052/A6061 lap and buttjoints made using threaded, half-thread and no thread tools. In allcases, the tool rotational speed is 1000 rpm and the dwell time is1 s.

2

2

1

1

Keyholeperiphery

Sheet intersection

400 µm

491 µm

1020 µm

292 µm

1482 µmSheet intersection

133˚

170˚

Tool rotational speed: 1000 rpm


2

2

1

1

Keyholeperiphery

Sheet intersection

Sheet intersection

124˚

153˚



(a)

(b)400 µm

566 µm

105 µm

264 µm

1452 µm

Figure 9. Schematic illustrations showing the different hookregions in A5754/A6111 lap joints made using tool rotationalspeeds of 1000 and 3000 rpm. (a) A joint made using the threadedtool; (b) A joint made using the half-thread tool.

I. Akihiko et al.6

fracture surface was that the region that was grey in colour

and thought to be a ductile fracture surface was the fracture

surface and the lustrousmetallic part that was deformed and

sunk (into the opposite surface) was not included in this.

Figure 13, a schematic illustration of this relationship,

shows that the bonded surface areas in butt joints madewith

the different tools were almost the same value: 5.12mm2

for the threaded tool, 5.77mm2 for the half-thread tool and

5.42mm2 for the non-thread tool, and the tensile strength of

joints made by threaded, half-thread and no-thread tool

tensile strengths were also almost the same: 378, 328 and

369Mpa. Other alloys different from A5052 and A6061,

were also examined but when A6061 plates were butt

welded in the same way using threaded, half-thread and no-

thread tool, the fracture loads were 1.67, 1.69 and 1.83 kN

and the bonded surfaces areas were 6.93, 6.76 and

7.74mm2. These results were almost the same as those

for different alloys and it was evident that the combinations

of metals had almost no effect.

These results indicate the high probability that, in

particular, the bonded width and Y value have a great effect

on the mechanical properties of joints. This does not seem

to fit with the reported [18] cases in which curvature

increases stepwise depending on the metal used but is

considered to be a particularly important factor for

aluminium alloys that form similar joint microstructures.

Accordingly, it is probable that, if bonded width and Y

value can be controlled by welding conditions etc., joints

with good mechanical properties can be obtained whatever

the state of tool probe wear. Also, the operation of the

probe thread is principally vertical stirring, it probably has

little relationship with horizontal stirring. Because of this,

when a standard cylindrical probe is used for FSSW, where

vertical stirring is important, the effect of thread wear is

very small if the welding conditions are examined closely

and it is only in cases in which it is impossible to control

the bonded width and Y values that thread wear has any

effect. This agrees well with the series of reports [8,9,24]

that for FSSWa thread is necessary to create a stir zone but

the presence or absence of a thread has almost no effect on

the strength of lap welds.

If all the above findings are considered together, it is

evident that the effects of probe thread are that, when the

thread is worn, the efficiency with which a stir zone is

formed is poorer thanwhen the thread is sound.On the other

hand, although it is considered probable that the stir zone

Tool Failure load(kN)

Tensile strength(MPa)

Actual bonded region(mm2)

Actual bondedregion (effectivecross section)

Apparent bondedregion (apparent

cross section)

ThreadedHalf-threadNo thread

1.98 / 5.12 386=

/2.01 5.42 370=/1.95 5.77 339=

Figure 13. Schematic illustrations showing the actual bondedregion in a failed A5052/A6061 butt joint and the tensile strengthproperties of dissimilar butt joints made using the threaded, half-thread and no thread tools.

A5052

A6061

A5052A-A cross section

Tensile direction

Tensile direction

A5052

A5052A-A cross section

A6061

A ARotationaldirection

Threaded

Threaded

Half-thread

Half-thread

1 mm

1 mm

1

2

2

1

1 mm

1 mm

2

2

1

1

A ARotationaldirection

(a)

(b)

Figure 12. (a) Profiles showing the A5052 side of the A5052/A6061 butt joint; (b) fracture surface showing the A5052 side of theA5052/A6061 butt joint.


may be increased in size by increasing the tool rotational

speed and lengthening the dwell time, since as a result of

this, joint strengths converge whether the thread is worn or

sound, it is evident that thread wear essentially has a limited

effect. Furthermore even when tools with different degrees

of wear used are used, as in this study, it is probable that the

plastic flow model is the type in which the bonded width is

increased by the growth of a stir zone formed by the

operation of the thread. However, since the bonded width

does not increase markedly with growth of a stir zone and

the mechanical properties of the joint are such that the

bonded width, Y value and curvature affect and determine

each other, as when the bonded width increases by the

growth of a stir zone and the curvature of that portion also

increases, it is evident that, overall, the mechanical

properties of the joint do not deteriorate when the

operations of the thread become poorer. It is also evident

from this that the effects of the thread are limited.

Furthermore, since the stir zones resulting from the

horizontal stirring by each of the tools are approximately

the same, it is evident that, in terms of the three-dimensional

flow of the metal, the state of the thread has almost no effect

on the volume of the region in which plastic flow is caused,

again indicating the limited effect of thread wear.

4. Conclusions

In this study, different aluminium alloys were lap welded

with the aim of discovering the effect of standard probe

thread on the mechanical properties of FSSW joints and

the following conclusions were reached as a result of an

examination of the effect of the probe head on the change

factors for weldability.

(1) In an A5754/A6111 lap joint prepared using a

threaded tool, the tensile shear strength was the

high value of approximately 1.8 kN at a tool

rotational speed of 1500 rpm and this reduced to

approximately 1.2 kN at 3000 rpm. On the other

hand, in an A5754/A6111 lap joint prepared using

a half-thread tool, the tensile shear strength

increased with an increase in tool rotational

speed to reach the highest value of approximately

1.5 kN at 3000 rpm.

(2) In A5754/A6111 lap joints prepared using a

threaded tool and a half-thread tool, the bonded

width and curvature increased with an increase in

tool rotational speed and the Y value increased

with an increase in rotational speed of the half-

thread tool, but showed a tendency to reduce with

an increase in rotational speed of the threaded

tool.

(3) The tensile shear strengths of A5052/A6061

joints prepared at a tool rotational speed of

1000 rpm with threaded tool, half-thread tool and

no-thread tool were similar, 2.1–2.3 kN in each

case, and the bonded widths (approximately

1300mm) and Y values (465–586mm) were also

similar. This indicates that, if the size of these can

be made approximately similar, it is possible to

achieve similar values for the mechanical proper-

ties of the joints, whatever the thread wear.

(4) It is evident from the results for A5052/A6061 lap

joints and butt joints prepared at a tool rotational

speed of 1000 rpm that the actions of the thread of

a cylindrical probe are principally stirring in the

vertical direction, and that the stir zones made by

the horizontal stirring of the different tools are

approximately the same and have almost no

effect on the mechanical properties of the joint.

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