CHAPTER (3) RESULTS AND DISCUSSION 3.1. VISUAL INSPECTION

28

CHAPTER (3)

RESULTS AND DISCUSSION

3.1. VISUAL INSPECTION

During visual inspection, excessive lateral flash was observed in most

of the welds, resulting from the outflow of the plasticized material from

underneath of the shoulder. It was previously showed that the large mass of

flash was ejected to the outside due to the softening of the metal by the excess

heat input during the FSW [48]. Figures 3.1, 3.2 and 3.3 show the bead

appearance for different welding conditions. It is clear that increasing the tool

rotational speed and/or reducing the welding speed tends to increase the flash

amount. For example, at a tool rotational speed of 1000 rpm, decreasing the

welding speed from 63 to 20 mm/min increases the flash amount produced

during FSW (See Fig.3.3). Also, at a welding speed of 40 mm/min, increasing

the tool rotational from 800 to 1000 rpm increases the flash amount produced

during FSW (See Fig. 3.3b and Fig. 3.2b). This may be attributed to the high

temperatures reached during FSW at high tool rotational speed and/or low

welding speed.

Comparing the two welds produced using the same travel speed of 40

mm/min and with different rotational speeds of 800 and 1000 rpm. Figure 3.3

shows the joints FS welded at 1000rpm and 40 mm/min, have a surface with

ploughed layers giving a rough surface appearance. Such poor surface quality

is because that lower welding speeds and higher tool rotational speeds

generate high temperatures during FSW. Such high temperatures greatly

soften the upper weld surface. So, the upper weld surface layer tends to attach

to the surface of the tool shoulder and are plucked out of the weld face surface

and transferred with the welding tool. The joints FS welded at 800 rpm and 40

mm/min have a regular surface. This is due to the low temperature generated

compared to 1000 rpm and 40 mm/min. The values of rotation-to-travel speed

29

ratio (R/T) for (1000 rpm and 40 mm/min) and (800 rpm and 40 mm/min)

weld were 25 and 20 respectively. J. Adamowski et al. [48] showed that the

―hotter‖ weld (1230rpm and 115 mm/min) has a rough surface covered with

particles of aluminum giving an abrasive paper-like appearance. Such poor

surface quality is attributable to the fact that at high temperatures generated

during welding, the particles of aluminum tend to attach themselves to the

surface of the shoulder of the tool and are plucked out of the weld face surface

and transferred to another location. Local liquation melting cannot also be

excluded. The other, ―colder‖ weld has a smooth, regular surface.

Another phenomenon was observed that at high rotating speed of the

tool, increasing the welding speed tends to decrease the contact area width of

the tool shoulder. Figure 3.3 shows that at a rotational speed of 1000 rpm,

increasing the welding speed from 20 mm/min to 63 mm/min leads to a

decrease in the contact area width of the tool shoulder from approximately

27.5 mm to 26.5 mm respectively. This may be due to the insufficient

plasticization of the welded material that occurs at higher travel speeds. This

insufficient plasticization of the welded material makes it difficult to form

complete contact area. Decreasing the travel speed leads to sufficient

plasticization and increase the tool contact width.

3.2. X-RAY EXAMINATION.

All welded joints were examined by the X-ray method. Most of

the joints were sound, only three cases were important to discuss as shown in

Fig.3.4. A small tunnel defect of 1 mm width observed in the initial part of the

joint FS welded at a tool rotational of 1000 rpm and welding speed of 63

mm/min. This defect took place over a distance of 10 cm and then

disappeared. J. Adamowski et al. [48] reported a similar type of defect, it is

suggested to be caused by the insufficient plasticization of the welded material

for the applied travel speed.

30

(a)

(b)

(c)

Fig.3.1. Photographs show the bead appearance of the FS welded joints at constant tool

rotational speed of 630 rpm and welding speeds of 63(a), 40 (b) and 20

mm/min(c).

AS A413

RS A319

RS A319

AS A413

AS A413

RS A319

10 mm

10 mm

10 mm

31

(a)

(b)

(c)

Fig.3. 2. Photographs show the bead appearance of the FS welded joints at constant tool


mm/min(c).

AS A413

RS A319

RS A319

AS A413

AS A413

RS A319

10 mm

10 mm

10 mm

32

(a)

(b)

(c)

Fig.3. 3. Photographs show the bead appearance of the FS welded joints at constant tool


mm/min(c).

AS A413

RS A319

RS A319

AS A413

AS A413

RS A319

Ploughed layers

Ploughed layers

10 mm

10 mm

10 mm

33

Such condition can take place over the distance of c.a. 50-70mm

(thermal transition zone) until the process reaches its thermodynamic

equilibrium – a situation in which the travel (welding) speed is approximately

equal to the velocity of propagation of the heat wave in the welded material.

3.3. MACROSTRUCTURE OF WELDED JOINTS

Figure 3.5, 3.6 and 3.7 show the macrographs of the cross sections of

the FS welded joints at several tool rotational speeds (typically, 630, 800 and

1000 rpm) and welding speeds of 20, 40 and 63 mm/min. The A413 Al alloy

was fixed at the advancing side while the A319 Al alloy at the retreating side.

From the different etching response of each material, The A319 Al alloy

appeared darker in color than the A413. At the welded zones, dark and light

regions were noticed. The weld zones which appeared to be composed of

different regions (dark and light) of both the alloys were severely plastically

deformed. During FSW, the tool acts as a stirrer extruding the material along

the welding direction. Such complex deformation produces the vortex

structure composed of alternative lamellae of A413 and A319 aluminum

alloys. It is suggested that the A413 Al alloy rich regions occupied the large

fraction in the welded zone.

The welded region is clearly seen in the macrographs shown in Fig. 3.5

to 3.7. At the center of the welded region, the stirred zone (WZ) is located. In

the WZ, both A413 and A319 materials are stirred severely in the vicinity of

the weld centre of the FS welded joint. Under investigated welding

parameters, the obtained joints showed no porosity or other defects in both top

and root weld surface for the applied welding conditions, only a cavity defect

of 1 mm width formed at the advancing side was detected in the joint welded

at 1000 rpm and 63 mm/min as shown in Fig.3.7a.

Fig.3.6. the X-ray radiography and bead appearance.

34

Fig.3.4. The X-ray radiography

Tunnel defect

35

The macrostructure of the FS welded joints shown in Fig. 3.5, 3.6 and

3.7 revealed also that increasing the tool rotation rate from 630 to 1000 rpm at

a constant welding speed or decreasing welding speed from 63 to 20 mm/min,

eliminated the nugget boundary at the retreating side. For example, at a

constant welding speed of 63 mm/min, increasing the tool rotational speed

from 630 rpm to 1000 rpm, eliminated the nugget boundary at the retreating

side. Also, at a constant tool rotational of 630 rpm, decreasing the welding

speed from 63 to 20 mm/min, eliminated the nugget boundary at the retreating

side. This may be due to the insufficient plasticization of the welded material

at lower rotational speed and so, the welded zone is highly deformed.

F.C. Liu et al. [38] reported that during FSW of 6061Al-T651, for a

lower welding speed of 200 mm/min, decreasing the tool rotation rate from

1400 to 900 rpm eliminated the nugget boundary at the retreating side. This

may be due to the fact that the heat generated during FSW is affected by the

combination of welding speed and tool rotational speed. It is important to

mention that the FSW, if well done, can reduce the porosity that can be found

in the both as-cast A319 and A413 alloys. Figure 3.7b shows that inside

unwelded zones in the A413 base Al alloy part, large cavities exist, while at

the welded zone (WZ), the pores were significantly reduced. This is because

of the stirring and extensive plastic deformation action of the FSW tool.

3.4. MICROSTRUCTURE OF WELDED JOINTS

Figure 3.8 shows the microstructure of both alloys, the structure of the

base alloys consists mainly of primary α-Al phase (white regions) and Al-Si

eutectic structure (black regions). In both alloys, coarse Al-Si eutectic

structures were found to be distributed along the boundaries of the α-Al

dendrites. The A319 alloy exhibits a microdendritic cell structure while the

A413 alloy exhibits plates and needles structure (see Fig. 3.8a and Fig. 3.8b).

36

(a)

(b)

(c)

Fig.3.5. Photographs show the macrostructure of A319/A413 joints FS welded at tool

rotational speed of 630 rpm and welding speed 63(a), 40(b) and 20 mm/min(c).

RS A319 AS A413

RS A319 AS A413

RS A319 AS A413

Porosity

WZ

WZ

WZ

37

(a)

(b)

(c)



RS A319 AS A413

RS A319 AS A413

RS A319 AS A413

Porosity

Porosity

WZ

WZ

WZ

38

(a)

(b)

(c)



AS A413

WZ

WZ

WZ

RS A319 AS A413

RS A319

Porosity

RS A319 AS A413

Porosity

Cavity defect

39

Figure 3.9 shows example micrograph of the microstructure at AS

(A413) of the WZ for a sample FS welded at a tool rotational speed of 1000

rpm and a welding speed of 20 mm/ min. Figure 3.9 shows different regions of

the welded joint. It is clear that, beside the base materials (BM), the

macrostructure of the FSW alloy consists mainly of three distinct zones,

typically, (1) fine-grained dynamically recrystallized stirred zone (SZ), (2)

thermo-mechanically affected zone (TMAZ), and (3) heat affected zone

(HAZ). The TMAZ zone experiences both temperature and deformation

during FSW and characterized by a highly deformed structure. The HAZ is

the zone that is believed to be unaffected by any mechanical effects but only

the thermal effects caused by the frictional heat generated by the shoulder and

tool pin rotation. The microstructure of the SZ is very different from that of

the BM. The SZ has much more homogeneous microstructure as compared to

the BM (see Fig. 3.8). The dendrite structure disappeared and finer Si particles

are dispersed over the whole weld zone.

Fig.3.8 Optical micrographs showing the microstructure of (a) A319 base alloy; (b) A413 base alloy.

(a)

(b)

α-Al

Eutectic

α-Al

Eutectic

40

The material within the FSW zones experienced intense stirring and

mixing which resulted in the breakup of the coarse acicular Si particles and

the dendritic structure and gave a homogeneous distribution of the Si particles

throughout the Al alloy matrix. It is important to mention that the SZ

exhibited extra fine and equiaxed α-Al grains; however, it was very difficult to

identify the grain boundaries using the optical microscope.

The TMAZ has a distorted structure which is longitudinally aligned due

to the heat effect and mechanical deformation. The HAZ is affected by only

the thermal effects caused by the frictional heat generated during FSW. So it

has coarser structure than that of the base metal.

Figures (3.10 to 3.18) show the macrostructures and microstructures at

different points at the cross-section of FS welded joints at different tool

rotational and welding speeds. The interesting feature illustrated in comparing

the base metals (see Fig. 3.8) and FSW microstructures is the breakup and/or

redistribution of the primary Al-Si eutectic structure. In the base metals, the

eutectic Si particles are distributed partially at the boundaries of the primary

α-Al phase and formed a eutectic structure. It is clear from the micrographs

presented in Fig. (3.10-3.18) for the SZ of the welded joints that the Si

particles are homogeneously dispersed in the SZ and that the acicular Si

Fig. 3.9. Optical micrograph of microstructure at AS (A413) of the weld Zone showing

SZ,TMAZ,HAZ and BM regions for welding condition ( 1000 rpm – 20 mm/min).

SZ TMAZ BM

HAZ

41

particles have disappeared. Figures (3.10-3.18) show that for all the welding

conditions, TMAZ at AS is clear while TMAZ at RS is unclear. Caizhi Zhou

et al. [22] showed that the TMAZ is unclear at the retreating side, this is due

to the torsion (the rotating motion of the tool) and the circumventing (the

translation motion of the tool) velocity fields having opposite directions on the

retreating side, whereas these velocities have the same direction on the

advancing side. Moreover the second-phase particles in retreating side are

relatively smaller than these advancing in side.

Figure 3.19 shows a schematic illustration for the alignment of the Si

particles at the different weld zones. For example, The Si particles are

horizontally aligned in the top (Fig.3.12a) and bottom (Fig.3.12c) of the SZ,

though they are longitudinally aligned at the TMAZ (Fig.3.12d) on the

advancing side. A similar behavior was reported by Kim et al. [10].

The size of the Si particles was investigated in detail from top to middle

and bottom of the stir zone by image processing software. For example of

the variation of the size of the Si particles at top, middle and bottom of the SZ.

Figures 3.11a, 3.11b and 3.11c show that at rotational speed of 1000 rpm and

welding speed of 40 mm/min, the average size of the Si particles slightly

larger at the top of the SZ than at the middle and the bottom of the SZ

respectively. This is because that The frictional heat is generated at the top

surface of the weld zone and then transferred to the middle and the bottom of

the WZ, so the heat generated decreases as moving toward the bottom of the

weld zone. In addition that the middle and bottom of the weld zone are

surrounded by the two base metals as well as the baking plate which act as a

heat sink, so at the top of the WZ, there is a crystal grain growth progress

during the recrystallization owing to the slow cooling rate. Y.G. Kim et al.

[10] studied the distribution of Si particles in the stir zone of FSW joints of the

ADC12 alloy and it has been found that the size of the Si particles in the

bottom is smaller than that in the top or the middle.

42

Fig

.3.1

0. T

he

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

1000 r

pm

and 6

3 m

m/m

in, (a

) to

p,

(b

) m

iddle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

and (

f) c

avit

y d

efec

t.

43

Fig

.3.1

1. T

he

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

1000 r

pm

and 4

0 m

m/m

in, (a

) to

p,

(b

) m

iddle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

44

Fig

.3.1

2. T

he

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

10

00 r

pm

and 4

0 m

m/m

in, (a

) to

p,

(b

) m

iddle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

45

Fig

.3.1

3. T

he

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

800

rpm

and 6

3 m

m/m

in, (a

) to

p,

(b)

mid

dle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

46

Fig

.3.1

4. T

he

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

800

rpm

and 4

0 m

m/m

in, (a

) to

p,

(b)

mid

dle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

47

Fig

.3.1

5 T

he

mac

ro-

and m

icro

stru

cture

fo

r th

e jo

int

FS

wel

ded

at

80

0 r

pm

and 2

0 m

m/m

in, (a

) to

p,

(b

) m

iddle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

48

Fig

.3.1

6.

The

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

630

rpm

and 6

3 m

m/m

in, (a

) to

p,

(b)

mid

dle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

49

Fig

.3.1

7.

The

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

630

rpm

and 4

0 m

m/m

in, (a

) to

p,

(b)

mid

dle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

50

Fig

.3.1

8.

The

mac

ro-

and

mic

rost

ruct

ure

fo

r th

e jo

int

FS

wel

ded

at

630 r

pm

and 2

0 m

m/m

in, (a

) to

p,

(b)

mid

dle

, (c

) bott

om

of

SZ

, (d

) ad

van

cing s

ide

,(e)

the

retr

eati

ng s

ide

of

the

WZ

.

51

Shusheng Di et al. [51] explained that the top side of weld fully

contacted with the welding shoulder in the welding progress and thus

generated much more heat through the rotating tool shoulder. On the other

hand the bottom side directly contacted with the back plate which acted as a

heat sink. The rapid cooling rate resulted in the faster recrystallization

procedure on the bottom side of weld. Figure 3.20 shows that for all welded

joints, it was found that the size of the Si particles in the bottom is smaller

than that in the top or the middle. Figure 3.21 shows the variation of Si

particles size at the middle of the SZ with FSW process parameters. The Si

particles average size was measured by using image processing software

called SoftCast on 100 particles for each condition.

Horizontally aligned Si particles at the top of the SZ

Longitudinally aligned Si particles at TMAZ on the AS

Horizontally aligned Si particles at the bottom of the SZ

Fig.3.19. schematic illustration of the alignment of the Si particles in different weld zones.

Top

Middle

Bottom

52

Figure 3.21 shows the variation of Si particles size with the tool

rotational speeds at several welding speeds. The results revealed that, the

average size of the Si particles increases with increasing the tool rotational

speed and/or decreasing the welding speed. For example, at a constant

welding speed of 63 mm/min, increasing the tool rotational from 630 to 1000

rpm increases the the average size of the Si particles from ≈ 7.35 to ≈ 7.61

μm, also at constant tool rotational of 630 rpm, decreasing the welding speed

from 63 to 20 mm/min increases the Si particles average size from ≈ 7.35 to

≈ 7.62 μm. the variation of Si particles size with welding paramters was

reported by many researches [10,13,15]. For example, Kim et al. [10]

investigated the effect of the welding speed and the rotation speed on the

Fig.3.20. Variation of Si particles average size in top, middle and bottom of the SZ at

different welding speed and at tool rotational of 630 (a), 800 (b) and 1000 rpm (c).

(C)

Tool rotational speed of 1000 rpm

(b)


(a)


53

microstructure in the stir zone by measuring the Si particle distribution in the

ADC12 alloy. The results showed that the stir zone has fine recrystallized

grains without a dendritic structure, and eutectic Si phases are uniformly

dispersed in the stir zone. The size of the Si particles decreases with

increasing welding speed.

Kim et al. [10] showed also that the size of the Si particles in the

retreating side is almost the same as that on the advancing side in the SZ.

Based on their results, they considered that the size of the Si particles should

be affected by the temperature. They suggested that Si particles will be finer

and more granular due to the solid collisions at the lower temperature as a

result of the more difficult plastic deformation. They showed also that the

directions of the Si particles in the stir zone were different for each region.

The Si particles are horizontally aligned in the top and bottom, though they

are longitudinally aligned on the retreating and advancing sides.

The size of the Si particles in the SZ is significantly influenced by both

tool rotational and welding speeds. The combination between the tool

rotational and welding speed is very important and plays an important role in

determining the size of Si particles.

It can be concluded that the average size of the Si particles increases

with increasing the tool rotational and the reduction of welding speeds.

Increasing the tool rotational speed and/or reducing the welding speed raises

the heat input during the FSW which affects the size of the Si particles. The

size of the Si particles should increase with increasing the heat input.

54

Figure 3.22 shows the average size of Si particles and the standard

deviation of the Si particle size in the SZ with increasing welding speed from

20 to 63 mm/min at constant tool rotational of 800 rpm (a) and with increasing

the tool rotational from 630 to 800 rpm at a constant welding speed of 40

mm/min (b).

Figure 3.22a shows that at a constant tool rotational speed of 800 rpm,

the average size of the Si particles slightly decreases from 7.7 to 7.42 μm with

increasing welding speed from 20 to 63 mm/ min. The standard deviation of

the Si particle size also increases from 2.04 to 2.48 as welding speed

increases. Figure 3.22b also shows that at constant welding speed of 40

mm/min, The average size of the Si particles slightly increases from 7.42 to

7.73 μm with increasing tool rotational from 630 to 1000 rpm.

It has been found that the standard deviation of the Si particle size

decreases from 2.56 to 2.08 as tool rotational increases from 630 to 1000 rpm

at constant welding speed of 40 mm/min. Based on the above result, the Si

particles average size in cases of lower welding speed or higher tool rotational

are distributed homogeneously in the SZ rather than at higher welding speed

or lower tool rotational. W.B. Lee et al. [11] show that the eutectic Si particles

7.20

7.30

7.40

7.50

7.60

7.70

7.80

7.90

8.00

600 700 800 900 1000 1100

Tool rotating speed [rpm]

Av

era

ge

Si

pa

rtic

les

size

[μ

m]

63 mm/min

40 mm/min

20 mm/min

Fig.3.21. Variation of the Si particles average size in the middle of the SZ with welding parameters.

55

in cases of lower welding speed are distributed more finely and

homogeneously in the SZ rather than at higher welding speed. Few

investigators reported the effect of the tool rotational speed on the distribution

of the Si particle size in the SZ. This behavior may be as a result that at lower

rotating speed or higher welding speed, the material reaches low working

temperature and so, the microstructure consists of a mixture of coarse and

broken particles which produces a non homogenous microstructure [37].

56

(a)

(b

)

7.2

0

7.4

0

7.6

0

7.8

0

8.0

0

20

30

35

40

50

63

70

weld

ing

sp

eed

[m

m/m

in]]

Average Si particles size [μm]

2.0

0

3.0

0

4.0

0

standard deviation

Avera

ge S

i part

icle

s si

ze

standard

devia

tion

63

0

8

00

10

00

20

4

0

6

3

7.2

0

7.4

0

7.6

0

7.8

0

8.0

0

63

07

00

80

09

00

10

00

11

00

Ro

tati

on

al

speed

[rp

m]

Average Si particles size [μm]

234

standard deviation

Av

era

ge S

i pa

rtic

les

size

sta

nda

rd d

ev

iati

on

63

0

8

00

10

00

630

80

0

100

0

Fig

. 3.2

2. T

he

aver

age

size

and s

tand

ard d

evia

tion o

f th

e S

i par

ticl

es w

ith

var

ious

wel

din

g c

ondit

ions,

(a

) w

ith i

ncr

easi

ng w

eldin

g s

pee

d f

rom

20 t

o 6

3 m

m/m

in a

t co

nst

ant

tool

rota

tional

of

800 r

pm

and

(b

) w

ith i

ncr

easi

ng t

he

tool

rota

tional

fro

m 6

30 t

o 8

00 r

pm

at

const

ant

wel

din

g s

pee

d o

f 4

0 m

m/m

in(b

).

57

3.5. SEM EXAMINATIONS OF THE WELDED JOINTS

Figures (3.23 to 3.31) show SEM micrographs at the middle of the SZ

at the cross-section of FS welded joints at different tool rotational and welding

speeds. The grain structures were only amenable through SEM analysis.

A grain refinement occurs in the stirred zone due to the stirring action of the

FSW tool. The microstructure of the SZ appears as dynamically recrystallized

fine grains. the original coarse primary Al grains and large plate-like eutectic

silicon in both base materials have been transformed to fine grains and small

silicon particles in the SZ.

Figure 3.32 shows the variation of the average grain size with the tool

rotational speeds at several welding speeds. The results revealed that, the

average grain size increases with increasing the tool rotational speed and/or

decreasing the welding speed. For example, at a constant welding speed of 63

mm/min, increasing the tool rotational from 630 to 1000 rpm increases the the

average grain size from ≈ 4.6 to ≈ 10.3 μm, also at constant tool rotational of

630 rpm, decreasing the welding speed from 63 to 20 mm/min increases the

average grain size from ≈ 4.6 to ≈ 6 μm.

SEM micrograph of the irregular shaped region at the welded zone is

shown in Fig. 3.33a for a specimen welded at a tool rotational speed of 1000

rpm and welding speed of 63 mm/min. The Al, Si, Cu and Fe distributions in

these regions were analyzed by EDS (see Fig. 3.33b to 3.33e). These results

show that mixture structure involves Al, Si, Cu and Fe. The materials in the

weld zone have undergone the co-action of high temperature action and severe

plastic deformation during FSW.

Figure 3.34 shows a SEM micrograph identifying the intermetallic phases

with corresponding reference EDS spectra in the SZ of the FS joints welded at

tool rotational speed of 630 rpm and a welding speed of 63 mm/min. The EDS

shows that the light gray particles are Al2Cu while the dark gray particles are

Si (eutectic).

58

Both tool rotation speed and welding speed exert a significant effect on

the heat generated during FSW. The combination between the tool rotational

and welding speed is very important and plays an important role in

determining the size of Si particles and grain size.

Figure 3.35 shows the variation of the average size of Si particles with the

ratio of tool rotational speed to welding speed (R/W). The results show that

the average size of Si particles tends to increase with increasing the ratio of

R/W. Figure 3.36 shows the variation of the average grain size with the ratio

of tool rotational speed to welding speed (R/W). the results show that the

average grain size increases with increasing the ratio of R/W. increasing R/W

increases the heat input and consequently increases the average size of Si

particles and also the average grain size.

Both tool rotation speed and welding speed exert a significant effect on the

heat generated during FSW. The combination between the tool rotational and

welding speed is very important and plays an important role in determining

the size of Si particles and grain size. T. Hashimoto et al. [52] reported that

the peak temperature in the weld zone increases with increasing x/m ratio in

2024Al–T6, 5083Al–O and 7075Al–T6.

59

Fig.3.23. SEM micrograph for the joint FS welded at 1000 rpm and 63 mm/min.

Average grain size = 10.3 µm

60


Average grain size = 11 µm

61



62



63



64



65



66



67



68

4.00

6.00

8.00

10.00

12.00

600 700 800 900 1000 1100

Av

era

ge

gra

in si

ze [

μm

]

Tool rotating speed [rpm]

63 mm/min

40 mm/min

20 mm/min

Fig.3.32. Variation of the average grain size in the middle of the SZ with welding parameters.

69

Fig.3.33. Qualitative EDS maps of Al (b), Si(c), Cu(d) and Fe(e) distribution in the two different colored

regions indicated at (a) by A and B. in each EDS map, points with higher concentration are white

colored. The specimen welded at tool rotation of 1000 rpm and welding speed of 63 mm/min.

(b) (c)

(d) (e)

(a)

A

B

70

Al2Cu

Si

Si Al2Cu

Fig.3.34. SEM micrograph showing the intermetallic phases with corresponding reference EDS spectra in the SZ

of the FS joints welded at tool rotational speed of 630 rpm and welding speed of 63 mm/min.

71

Fig.3.35. Variation of the average size of the Si particles in the middle of the SZ with the ratio of

rotational speed to travel speed (R/W).

Fig.3.36. Variation of the average grain size in the middle of the SZ with the ratio of rotational

speed to travel speed (R/W).

R/W

(a)

R/W

(a)

72

3.6. HARDNESS OF FRICTION STIR WELDED JOINTS.

The Vickers hardness profiles of the FSW joints were measured in the

cross sections at the middle of the SZ in order to evaluate the material

behavior as a function of the different welding parameters. Plots of the

Vickers hardness (HV) versus distance from the weld centre are shown in

Fig.3.35. The hardness of A319 base metal approximately varies from 72 to

88 HV while the hardness of A413 base metal exhibits lower values which

approximately vary from 70 to 82 HV. The wide scattering in the hardness

values of the base metal of both alloys is due to the non uniform distribution

of the Si particles inside the Al matrix since it is well known that the hardness

of Al-Si depends on the measured point of hardness indenter [11,12].

Figure 3.37 shows that for the most of the welded joints, the hardness of

the SZ is higher than the wide range of A413 base metal hardness but it is

approximately located within the wide range of A319 base metal hardness.

However, the hardness of the SZ shows more uniform values than that of the

two base alloys due to finer and uniformly dispersed Si particles. Based on the

microstructure of the two base alloys shown in Fig.3.8, it is clear that the

A413 base alloy contains very coarse plate structure which is stirred into

uniformly distributed coarse Si particles which couldn't contribute to raising

SZ hardness, while the A319 base alloy contains a dendritic cell structure

which is stirred into uniformly distributed finer Si particles which highly

contributed to raising SZ hardness, so, it is logical to have the shown SZ

hardness profile.

A similar behavior was observed by N. A. Rodrguez e [14] who showed

that the A319 FS welded zone is observed to be roughly 15% harder than the

corresponding base metal. The A413 FS welded zone, by contrast, exhibits a

nearly constant hardness from the base metal through the FS welded zone.

The hardness increase in the SZ observed in the A319 FSW is due to a

73

noticeably finer inclusion microstructure, i.e. a considerably closer spacing

between inclusions than in the A413 FSW.

Figure 3.38 shows the hardness profiles along the centerline of the

cross-section, (a) with different welding speeds at a constant tool rotational

speed of 1000 rpm and (b) with different tool rotational speed at a constant

welding speed of 20 mm/min. Figure 3.38a shows that at constant tool

rotational speed of 1000 rpm, the hardness of the WZ at 20 mm/min is

uniformly distributed and shows less variation than the hardness of the WZ at

40 and 63 mm/min which shows a wide scattering. A similar behavior was

observed in FSW of A356 by W.B. Lee et al. [11]. Figure 3.38b shows that at

constant welding speed of 20 mm/min, the hardness of the WZ at tool

rotational of 1000 rpm, is uniformly distributed and shows less variation than

the hardness of the WZ at 800 rpm and 630 rpm which shows a wide

scattering.

Few investigators [11-16] reported the effect of the tool rotational

speed on the hardness distribution in the SZ. All the welded joints showed

the same behavior indicated in fig.3.38. This behavior may be as a result that

at lower rotating speed or higher welding speed, the material reaches low

working temperature and so, the microstructure consists of a mixture of coarse

and broken particles which produces non homogenous microstructure [47].

This non homogenous microstructure produces non- uniformly distributed

hardness in the WZ. These results are in agreement with Fig. 3.36.

Fig.3.39 shows the variation of the average hardness at the center of the

SZ with different welding parameters. For all the welded joints, the average

hardness of the WZ increases with increasing the welding speed and/or

reducing the tool rotational speed. For example, at 20 mm/min welding speed,

increasing the tool rotational from 630 to 1000 rpm reduces the average

hardness of the WZ from 82 to 75 VHN. Also, at a tool rotational of 630 rpm,

increasing the welding speed from 20 to 63 mm/min increases the average

74

hardness from 82 to 89 VHN. The maximum hardness is approximately 89

VHN at welding conditions of 630 rpm and 63 mm/min while the minimum

hardness is approximately 75 VHN at welding conditions of 1000 rpm and 20

mm/min. Many studies [11,13] reported that hardness increase in the WZ is

due to deleting casting defects, a fine dispersion of Si particles and the grain

refinement.

75

Fig

. 3.3

7. T

he

har

dnes

s p

rofi

les

along t

he

cente

rlin

e of

the

cross

-sec

tion w

ith d

iffe

rent

wel

din

g

p

aram

eter

s an

d t

he

har

dnes

s ra

nge

of

both

A319 b

ase

met

al (

rig

ht

side)

and A

413 b

ase

met

al

(l

eft

side)

in t

he

das

h l

ines

.

.

76

(b)

Fig

. 3.3

8. T

he

har

dnes

s p

rofi

les

along t

he

cente

rlin

e of

the

cross

-sec

tion, (a

) w

ith d

iffe

rent

wel

din

g s

pee

ds

at

a c

onst

ant

tool

rota

tional

of

1000 r

pm

and (

b)

wit

h d

iffe

rent

tool

rota

tional

at

a co

nst

ant

wel

din

g

sp

eed o

f 20 m

m/m

in.

(a)

Wel

din

g s

pee

d o

f 20 m

m/m

in.

77

Fig. 3.39. Variation of the average hardness at the center of the WZ with tool rotational speeds at

different welding speeds.

78

3.7. TENSILE TEST OF FRICTION STIR WELDED JOINTS.

The configuration and size of the transverse tensile specimens are

illustrated in Fig.2.8. The FSW samples were machined by a wire cut m/c and

tested without any surface modification after welding. The typical rough

surface shown is induced by the tool of FSW. All of the tensile specimens

fractured far from the weld zone at the A413 base alloy side (i.e. AS). The

tensile properties of the A319 and A413 base alloys are listed in Table 3.1.

The hardness test revealed that the SZs exhibit higher hardness than that

of A413 base alloy and it is approximately equal to that of A319 base alloy. It

can be concluded that the good performance exhibited by the FS welded joints

may be attributed to the higher hardness of the SZs compared to the A413

base alloy. Due to coarse Si plate structure and larger porosity defect, it is

clear from Table 3.1 that A413 base alloy exhibited lower ultimate tensile and

yield strengths as compared with A319 base alloy. This explains why the

tensile specimens fractured at A413 alloy side.

Table 3.1 Tensile properties of A319 and A413 base alloys*

*UTS: ultimate tensile strength; YS: yield strength.

Figure 3.40 shows an example for fractured FS welded tensile

specimen.

Alloys UTS [Mpa] YS [Mpa] %elongation

A319 198.35 158.7 2.72

A413 150.7 120.56 1.37

Fig. 3.40. Fractured FS welded tensile specimen. Notice that the fracture took place at

the A413 (advancing side). This specimen is FS welded at 1000 rpm and 63 mm/min.

Fracture A413 (AS) A319 (RS)

CHAPTER (3) RESULTS AND DISCUSSION 3.1. VISUAL INSPECTION

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