Mechanical and Microstructural Characterization of ... · Friction stir welding (FSW) was invented at The Welding Institute (TWI) of the United Kingdom in 1991 as a solid-state joining

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

475-1

Mechanical and Microstructural Characterization of Friction Stir Welded Al-

Si-Mg Alloy

Vikas Sharma1*, Shailendra Singh2, Chaitanya Sharma3, Vikas Upadhyay4

1*Anand Engineering College, Agra, 282007, [email protected] 2 HITM College, Agra, 282007, [email protected] 3 Rustamji Institute of Technology, Gwalior, 475005, [email protected] 4 IMS Engineering College, Ghaziabad, 201009, [email protected]

ABSTRACT

Friction stir welding of Al-Si-Mg alloy was performed in butt joint configuration using varying welding speed and

constant rotary speed. Tensile tests were performed to determine ultimate tensile strength, yield strength and %

elongation. Microstructure was investigated using optical and scanning electron microscope. Micro hardness

measurements across the transverse cross section of FSW joints were carried out to identify variations in micro

hardness in different zones. An optimum combination of welding and rotary speed (120 mm/min and 635 rpm) was

obtained to produce sound and defect free FSW joints that yields maximum mechanical properties. An increase in

welding speed first increased the ultimate tensile strength, and yield strength of FSW joints while % elongation

decreased with increase in welding speed. Welding speed influenced the mechanical properties and mode of fracture

of FSW joints. Keywords: Friction stir welding, Al-Si-Mg alloy (AA6101), Microstructure & Mechanical properties, fracture location

1. Introduction

Friction stir welding (FSW) was invented at The

Welding Institute (TWI) of the United Kingdom in 1991

as a solid-state joining technique and was initially

applied to aluminium alloys (Thomas, 1991). Friction

stir welding is the most significant development in

metal joining in recent decades and considered as a

“green” technology due to its energy efficiency,

environmental friendliness and versatility. In

comparison to conventional welding methods, FSW

consumes less energy, cover gas or flux is not required,

and no harmful emissions are evolved during welding.

In FSW, a non-consumable rotating tool with a specially

designed pin and shoulder is inserted into the abutting

edges and subsequently traversed along the joint line.

Figure 1 illustrates process of friction stir welding.

Friction between the tool and work piece along with

adiabatic heat from metal deformation leads to localized

heating that softens and plasticizes the work piece.

During FSW, the material undergoes intense plastic

deformation due to traverse of rotating pin along the

butting surfaces and after welding, recrystallization

results in significant grain refinement in welded joints

(Amancio et al., 2008). FSW does not involve the use of

filler metal and there is no melting (Misra and Ma,

2005). Aluminium alloys can be joined effectively

without any concern of composition compatibility,

solidification cracking and oxide inclusions issues

associated with fusion welding (Sharma et al., 2012a).

Welding speed governs the maximum temperature

generated during welding and the time length during

which the material is subjected to welding. Also,

dissimilar aluminium alloys and composites can be

joined with equal ease (Moreira et al., 2009;

Somasekharan and Murr, 2004). The degree of

softening and tensile properties are significantly

affected by welding process parameters such as welding

speed and rotational speed (Liu et al., 2003)

Figure 1: Friction Stir welding process (Mishra et

al., 2007)

In FSW, welding parameters such as tool design,

rotation speed and translation speed should be regulated

precisely to control the energy input into the system.

Aval et al., (2011) reported that the conical threaded pin

Mechanical and Microstructural Characterization of Friction Stir Welded Al-Si-Mg Alloy

475-2

with shoulder angle of 2o achieves higher mechanical

properties and the yield strength of AA5086 weld was

found about 12 % greater than that of the base metal.

Sato, 2003 found that the hardness distribution at the

precipitation strengthened type Al alloy weld can be

explained by the density of strengthening precipitation.

Sato and Kokawa, (2001) reported that the yield and

ultimate tensile strengths of the as-welded weld were

significantly lower than those of the base material of

aluminium 6063. Liu et al., (2006) found that heat

treatment results in improved tensile strength. Friction

stir welding has been used extensively in the aluminium

superstructures of cruise ships such as the ‘Seven Seas

Navigator’ which contain many kilometres of friction

stir welds, mostly in 6xxx grade extrusions. High speed

aluminium railcars such as the Japanese Shinkanen are

normally built from complex double skin extrusions in

6xxx alloys. Literature review reveals that little work is

reported on 6101 aluminium alloy till date. In this work,

an attempt has been made to find the optimum

combination of welding speed and rotary speed of the

tool to obtain the better mechanical properties of the

FSW welded joint.

2. Experimental work

Medium strength extruded sheets of aluminium

alloy AA6101 of 6 mm thickness, originally in annealed

condition were used for experimental purpose. Electron

Dispersive X-ray (EDAX) analysis was carried out in

order to determine the composition of alloy. The

nominal composition of alloy is given in Table 1 while

Figure 2 shows EDAX. To carry out the FSW

experiment vertical milling machine of (HMT 7 H.P.

and 635 rpm) was used to produce friction stir butt

weld. In present study, die steel tool (pin length: 5.7

mm, pin diameter: 4.7 mm at bottom, shoulder diameter:

15.6 mm) was used to fabricate the FSW joints. The

depth of shoulder plunge was kept 0.2 mm from work

piece.

Table 1 Proportion of elements in AA 6101

Figure 2: Composition of Al alloy tested by EDAX

FSW was conducted to optimize the parameter to

produce defect free welds by keeping constant vertical

plunging force, constant plunge depth and constant

revolution per minute (635 rpm). FSW was performed

using four different welding speeds in the range of 19

mm/min to 190 mm/min. Further, speeds were

categorised as low welding speed i.e. 19 mm/min, 30

mm/min and high welding speed i.e. 120 mm/min and

190 mm/min. The plates welded at different welding

speed are shown in Figure 3. The microstructure was

investigated using optical and scanning electron

microscopy.

Figure 3: Weld samples 19, 30, 120, 190 mm/min

respectively

Mechanical tests were carried out on FSW joints as well

as on base material, in order to determine the

mechanical properties and to correlate them with the

micro and sub-structural characteristics of the joints.

The testing methods used to investigate the mechanical

properties include micro-hardness measurements and

transverse tensile tests. Tensile tests were carried out

using a 25 kN electro-mechanically controlled universal

testing machine (H25 K-S Hounsfield). For statistically

consistent results, three samples from each weld were

prepared.

3. Results and Discussion

3.1 Microstructure

Low magnification optical macrographs of cross

section (perpendicular to welding direction) of friction

stir weld joints developed using different welding

speeds are shown in Figure 4 (a-d)

Figure 4: Macrographs exhibiting the influence of

welding speed on the formation of FSW joints (a) 19

mm/min, (b) 30 mm/min, (c) 120 mm /min and (d)

190 mm/min

a b

c d

Element Weight

(%)

Mn 00.05

Mg 00.55

Al 98.65

Si 00.73

Fe 00.02

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

Guwahati, Assam, India

475-3

It was observed that all the friction stir weld joints were

sound and defect free in investigated range of welding

speed, except the FSW joint developed using welding

speed of 190 mm/min which showed some voids in the

lower part of the WNZ as shown in Figure 5, the same

may be attributed to poor consolidation of the softened

material due to reduced flowability owing to low net

heat input per unit length of weld with increase in

welding speed. At low welding speed (of 19 mm/min

and 30 mm/min) the friction stir weld joints showed

basin or bowl shaped weld nugget zone while at higher

welding speed (of 120 mm/min and 190 mm/min) the

shape of WNZ was trapezoidal. Dimensions of WNZ,

approximately matched with tool dimensions i.e.

shoulder diameter at top (15.6 mm) and pin diameter at

bottom (4.7 mm) of the joints. The overall shape and

size of friction stir weld joints varied with welding

speeds. Width of WNZ decreased from 15.6 mm to 14

mm at top with increase in welding speed from 19 to

190 mm/min. However, the effect of welding speed on

the width of bottom of WNZ showed opposite trend of

variation, which increases from 4.7 mm to 5.6 mm with

the increase in welding speed as evident from

macrograph shown in Figure 4. The variations in

dimensions of WNZ are due to varying material flow

with increasing welding speed (Sharma et al. 2012 b).

All the friction stir welded joints showed zigzag line in

WNZs as shown in Figure 4 ( white line in the centre)

and Figure 5

Figure 5: Formation of void and zigzag line defect in

the WNZ of FSW developed using welding speed of

190 mm/min

Thermal and mechanical stresses caused by tool stirring

and axial force during FSW radically transforms the

initial microstructure of the base metal and resulted in

the formation of WNZ, TMAZ and HAZ in the base

metal which are characteristic to friction stir welding

process. Micrographs of the centre of WNZs for all

conditions of welding speed are shown in fig. 6. All the

FSW joints in WNZs invariably showed fine

recrystallized equiaxed grain structure with second

phase strengthening precipitates of Mg2Si as compared

to base metal owing to dynamic recrystallization. Murr,

et al., 1998 also reported similar result that the weld

zone microstructures are dominated by dynamic

recrystallization, smaller, equiaxed grains than the base

metal. The size of α aluminium grains were fine as

compared to the base metal. Micrographs suggests that

size of α aluminium grains decreased with the increase

in welding speed from 19 mm/min to 190 mm/min,

though no attempt was made to measure the size of α

aluminium grains.

Figure 6: Effect of welding speed (a) 19 mm/min, (b)

30 mm/min, (c) 120 mm/min and (d) 190 mm/min on

weld nugget zone microstructure of AA 6101

The variation in welding speed had great influence on

the morphology of second phase strengthening

precipitates (Mg2Si). At low welding speed of 19

mm/min WNZ had coarser secondary β phase

strengthening precipitates while WNZs of other FSW

joints developed using welding speed of 30 mm/min and

120 mm/min showed relatively fine secondary β phase

strengthening precipitates. Moreover, the population of

these strengthening precipitates increased with increase

in welding speed. This variation in the morphology of β

phase strengthening precipitates (Mg2Si) can be

attributed to the different level of solute concentration

available for dissolution into α aluminium matrix. Heat

affected zone is the zone where material is only affected

by thermal transient of the process. Grain structure of

HAZ was similar to base metal except that β phase

strengthening precipitates (Mg2Si) in HAZ were coarser

than the base metal as shown in Figure 7. The welding

speed influenced the size and population of

Void

Zigzag line

Region of sluggish material flow200 µm

α aluminum grains

Mg2Si precipitates

Voids

a b

c d

Mechanical and Microstructural Characterization of Friction Stir Welded Al-Si-Mg Alloy

475-4

strengthening precipitates in HAZ significantly. At low

welding speed (19 and 30 mm/min) β phase

strengthening precipitates (Mg2Si) were few and

somewhat coarser than at higher welding speed (120

and 190 mm/min). The increase in welding speed from

19 mm/min to 120 mm/min decreased the size and

increased the population of β phase strengthening

precipitates in HAZ. Mahoney et al., 1998 reported

similar trend while working with 7075-T651 that HAZ

experience thermal cycle but did not undergo any plastic

deformation but above 2500C results in significant

coursing of the precipitates. This also changed the shape

of second phase strengthening precipitates from

spherical to needle like shape (Figure 7, a-c). The

further increase in welding speed decreased the

population and size of second phase strengthening

precipitates as shown in the Figure 7 d.

Figure 7: Effect of welding speed (a) 19 mm/min, (b)

30 mm/min, (c) 120 mm/min and (d) 190 mm/min on

heat affected zone microstructure of AA 6101

3.2 Microhardness

Figure 8 shows the variation of micro hardness with

various welding speeds. It was observed that

irrespective of welding speed, all friction stir weld joints

had higher average micro hardness of the WNZs (in

range of 51.1 Hv to 54.6 Hv) than the base metal (46.1

Hv). Lee et al., 2003 reported the same trend of increase

in hardness in the weld zone due to presence of fine

equiaxed grains and t re-precipitation of the dissolved

precipitates. The average micro hardness of WNZs first

increased with increase in welding speed from 19

mm/min to 120 mm/min, than decreased with further

increase in welding speed to 190 mm/min. The average

micro hardness of WNZs increased from 51.1 Hv to

54.6 Hv with increase in welding speed from 19

mm/min to 120 mm/min (Figure 9). With further

increase in welding speed from 120 mm/min to 190

mm/min, average micro hardness of WNZs decreased

from 54.6 Hv to 53.8 Hv (Figure 9).

Figure 8: Effect of welding speed on micro hardness

variations across friction stir weld joints

The influence of welding speed on average micro

hardness of HAZs was similar to that of WNZs. The

average micro hardness of HAZs was also greater than

the base metal that first increased with increase in

welding speed from 19 mm/min to 120 mm/min, than

decreases with further increase in welding speed to 190

mm/min.

Figure 9: Variation of weld nugget micro-hardness

with welding speed

The average micro hardness of HAZs increased from

50.3 Hv to 53.2 Hv with increase in welding speed from

19 mm/min to 120 mm/min (Figure 9). With further,

increase in welding speed from 120 mm/min to 190

mm/min, average micro hardness of HAZs decreased to

51.3 Hv from 53.2 Hv. The location of minimum micro

hardness was at the end of HAZ or the base metal for

c d

ba

Mg2Si precipitates

-15 -10 -5 0 5 10 15

36

38

40

42

44

46

48

50

52

54

56

58

60

62

64

66

Mic

rohard

ness (H

v)

Distance from weld centre (mm)

WS 19 mm/min

WS 30 mm/min

WS 120 mm/min

WS 190 mm/min

0 30 60 90 120 150 180 210

40

42

44

46

48

50

52

54

56

Mic

rohard

ness (H

v)

Welding speed (mm/min)

Average WNZ hardness

Average HAZ hardness

Average minimum hardness

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

Guwahati, Assam, India

friction stir weld joints developed using various

speeds. The increase in micro hardness of FSW joints in

WNZ, TMAZ and HAZ region is because of favourable

re-precipitation of strengthening precipitate owing to

better natural aging response Fuller et al.

the similar impact of natural aging.

3.3 Tensile properties

Average tensile properties of friction stir weld

joints of AA 6101 aluminium alloy, produced using

different welding speed are presented in T

Strength of all the friction stir welded joints of AA6101

is lower than that of base metal. The percentage (%)

elongation of friction stir weld joints was higher than

the base metal except the joints developed using

welding speed of 190 mm/min. It is evident from

2 that yield strength and ultimate tensile strength of all

the friction stir weld joints of AA6101

that of base metal. The percentage (%) elongation of

friction stir weld joints was higher than the base metal

except the joints developed using welding speed of 190

mm/min. Figure 10 shows the effect of welding speed

on ultimate tensile strength, yield strength, % elongation

of friction stir weld joints.

Figure 10: Effect of welding speed on ultimate tensile

strength, yield strength and % elongation of FSW

joints

At constant rotary speed of 635 rpm

welding speed from 19 mm/min to 120 mm/min,

increased the ultimate tensile strength

from 133.7 MPa to 149.2 MPa and yield strength

94.2 MPa to 115.9 MPa. With further

welding speed from 120 mm/min to 190 mm/min,

ultimate tensile strength and yield strength of FSW

joints decreased from 149.2 MPa to

115.9 MPa to 110.2 MPa respectively.

elongation of the FSW joints was found to decrease

linearly from 22.5 % to 12.4 % with the increase in

welding speed from 19 mm/min to 190 mm/min.

0 30 60 90 120

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

Ultim

ate

/ Y

iled

str

en

gth

(M

Pa

), E

lon

ga

tio

n (

%)

Welding speed (mm/min)

Ultimate tensile strength

Yield strength

% Elongation

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

d joints developed using various welding

ro hardness of FSW joints in

WNZ, TMAZ and HAZ region is because of favourable

precipitation of strengthening precipitate owing to

et al., 2010 found

ge tensile properties of friction stir weld

joints of AA 6101 aluminium alloy, produced using

welding speed are presented in Table 2.

joints of AA6101

is lower than that of base metal. The percentage (%)

longation of friction stir weld joints was higher than

the base metal except the joints developed using

evident from Table

tensile strength of all

the friction stir weld joints of AA6101 are lower than

that of base metal. The percentage (%) elongation of

friction stir weld joints was higher than the base metal

except the joints developed using welding speed of 190

shows the effect of welding speed

ength, yield strength, % elongation

: Effect of welding speed on ultimate tensile

strength, yield strength and % elongation of FSW

rpm, increase in

to 120 mm/min,

of welded joints

yield strength from

With further increase in

welding speed from 120 mm/min to 190 mm/min,

ultimate tensile strength and yield strength of FSW

MPa to 128.6 MPa and

110.2 MPa respectively. The %

elongation of the FSW joints was found to decrease

early from 22.5 % to 12.4 % with the increase in

welding speed from 19 mm/min to 190 mm/min.

Table 2: Variations in tensile properties of friction

stir weld joints of AA6101 aluminium alloy with

welding speed

W.S.

(mm/min)

UTS

(MPa)

Y. S.

(MPa)

BM 158.2 118.1

A: WS 19 133.7 94.2

B: WS 30 138.8 81.3

C: WS 120 149.2 115.9

D: WS 190 128.6 110.2

The highest ultimate tensile strength and yield strength

of FSW joints developed using welding speed of 120

mm/min were 149.2 MPa and 115.9 MPa respectively.

However, this FSW joint showed lower % elongation

than other FSW joints developed using low welding

speed of 19 and 30 mm/min.

3.4 Fracture location

All the joints were fractured on retreating side

except joint developed using welding speed of 19

mm/min confirming that advancing side is stronger than

other side. Table 3 enlists effect of welding speed on

fracture properties of FSW joints of AA6101

Table 3: Effect of FSW welding speed on fracture

properties of friction stir weld joints of

Wel

din

g S

pee

d

Min

imu

m

Har

dn

ess

(HV

) Fracture

Location

&

distance

from weld

centre

WS 19

mm/m

41.9 AS BM,

13 mm

WS 30

mm/m

44.4 RS HAZ,

10 mm

WS 120

mm/m

48.6 RS HAZ,

7 mm

WS 190

mm/m

47.8 RS WNZ,

3 mm

Similar trends were also reported

2012b. The friction stir weld joints developed using low

150 180 210

Welding speed (mm/min)

Ultimate tensile strength

Yield strength

% Elongation

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT

475-5

Variations in tensile properties of friction

aluminium alloy with

a)

Elongation

(%)

118.1 15.5

22.5

20.9

115.9 17.6

110.2 12.4

The highest ultimate tensile strength and yield strength

developed using welding speed of 120

mm/min were 149.2 MPa and 115.9 MPa respectively.

this FSW joint showed lower % elongation

than other FSW joints developed using low welding

All the joints were fractured on retreating side

oint developed using welding speed of 19

mm/min confirming that advancing side is stronger than

enlists effect of welding speed on

joints of AA6101.

Effect of FSW welding speed on fracture

of friction stir weld joints of AA6101

Photograph of

Fractured

Specimen

by Sharma e et al.,

The friction stir weld joints developed using low

Mechanical and Microstructural Characterization of Friction Stir Welded Al-Si-Mg Alloy

475-6

welding speed (19 and 30 mm/min) showed minimum

micro hardness in HAZ while in case of FSW joints

developed using high welding speed (190 mm/min) it

was found in WNZ. Variation in welding speed from 19

mm/min to 120 mm/min at constant rotary speed

resulted in the change of fracture location from

advancing side base metal to retreating side HAZ.

Further increase in welding speed to 190 mm/min

changed the fracture location from retreating side HAZ

to WNZ towards the retreating side. Thus, increase in

welding speed shifts minimum hardness region from

base metal to WNZ, which in turn controls the fracture

locations.

4 Conclusions

Welding speed was found to have a significant effect on

macrostructure, microstructure, tensile properties and

fracture locations. Increase in welding speed decreased

the width of WNZ and resulted in voids in FSW joints.

Increase in welding speed first increased the average

micro hardness of the WNZ, which then decreased, with

further increases in the same. An increase in welding

speed first increased the ultimate tensile strength, and

yield strength of FSW joints while % elongation

decreased with increase in welding speed. An increase

in welding speed shifts fracture locations from BM to

WNZ. The careful selection of FSW process parameters

can avoid the formation of void; maximize mechanical

properties by minimizing softening, change fracture

location and mode of fracture of FSW joints of AA6101

aluminium alloy.

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