Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.14, pp.1359-1377, 2011
jmmce.org Printed in the USA. All rights reserved
1359
Influence of Friction Stir Welding Parameters on Sliding Wear Behavior of
AA6061/0-10 wt.% ZrB
2
in-situ Composite Butt Joints
I. Dinaharan
a*
, N. Murugan
b
a
Department of Mechanical Engineering, Karunya University, Coimbatore – 641114,
Tamil Nadu, India.
b
Department of Mechanical Engineering, Coimbatore Institute of Technology,
Coimbatore – 641014, Tamil Nadu, India.
*
Corresponding author: [e-mail]
ABSTRACT
Over the last decade attempts have been made to fabricate aluminum matrix composites
(AMCs) reinforced with several ceramic particles. Aluminum reinforced with ZrB
2
particles
is one such AMC. The successful application of new kind of AMCs lies in the development of
secondary processes such as machining and joining. Friction stir welding (FSW) is a
relatively new solid state welding which overcomes all the setbacks of fusion welding of
AMCs. An attempt has been made to friction stir weld AA6061/ 0-10 wt. % ZrB
2
in-situ
composites and to develop empirical relationships to predict the sliding wear behavior of butt
joints. Four factors, five levels central composite rotatable design has been used to minimize
the number of experiments. The factors considered are tool rotational speed, welding speed,
axial force and weight percentage of ZrB
2
. The effect of these factors on wear rate (W) and
wear resistance (R) of the welded joints is analyzed and the predicted trends are discussed.
Key words: Metal matrix composite, Friction stir welding, Wear.
1. INTRODUCTION
The performance of soft aluminum alloys is enhanced with the reinforcement of hard ceramic
particles. The resulting material is universally known as aluminum matrix composites
(AMCs). The superior properties of AMCs have created an interest for feasible applications
in several engineering fields including aerospace, automotive, marine and military [1, 2].
Variety of ceramic particles (SiO
2
, TiO
2
, AlN, Si
3
N
4
, TiC, B
4
C, TiB
2
and ZrB
2
) has been
tried in the recent past to fabricate AMCs apart from traditionally used Al
2
O
3
and SiC.

1360 I. Dinaharan, N. Murugan Vol.10, No.14
Adequate development of secondary processes such as cutting, forming, machining and
joining of AMCs has not been fully established which limits its applications.
The presence of ceramic particles reduces the weldability of AMCs. Attempts to join AMCs
using established fusion welding processes resulted in porosity, coarse microstructure,
segregation and decomposition of ceramic particles and formation of brittle intermetallic
compounds. The high temperature attained in fusion welding increases the tendency of
ceramic particles to react with aluminum matrix. Achieving homogeneous distribution of
ceramic particles in the weld zone was found to be difficult. The mechanical properties of the
joints were poor [3-6]. Friction stir welding (FSW) is a promising candidate to join AMCs
without fusion welding defects.
FSW was invented by The Welding Institute (TWI) in 1991. A non consumable rotating tool
under sufficient axial force is plunged into the abetting edges of the plates to be joined and
advanced along the line of the joint. The frictional heat generated by the tool softens the
material and coalescence is achieved at the retreating side of the tool. The temperature rise
during joining is well below the melting point [7]. Though FSW is primarily developed to
join aluminum alloys intense research has been extended to join other alloys such as
magnesium, copper, brass, steel, nickel and titanium [8].
Availability of limited literatures reveals the lack of exploration on all aspects of FSW of
AMCs. Earlier works were focused on FSW of aluminum reinforced with either Al
2
O
3
or SiC
[9-14]. Few works were reported on FSW of aluminum reinforced with B
4
C, TiB
2
, TiC and
Mg
2
Si in the recent past [15-18]. Several researchers carried out FSW using a single set of
parameters while few others attempted to study the influence of process parameters on joint
properties. Prado et al. [9] examined the effect of rotational speed on tool wear of friction stir
welded AA6061/20 vol % Al
2
O
3
and observed that the tool wear was non linear. Shindo et al.
[10] estimated the effect of welding speed on tool wear of friction stir welded A359/20 vol %
SiC and noticed different degree of weld zone hardening. Vijay and Murugan [16] assessed
the effect of different tool pin profiles on microstructural evaluation of friction stir welded
AA6061/10 wt % TiB
2
and found the square pin profile yielding higher tensile strength and
finer grains in the weld zone. Nami et al. [17] analyzed the effect of tool rotational speed on
microstructure and strength of friction stir welded Al/15 wt % Mg
2
Si and recorded different
degree of weld zone hardening. Gopalakrishnan and Murugan [18] developed an empirical
relationship to predict the strength of friction stir welded AA6061/ 3-7 wt % TiC and
described the effect of welding speed, axial force, tool pin profile and weight percentage of
TiC particles.
Among feasible ceramic reinforcements ZrB
2
possesses strong covalent bonding, high
melting point, high strength and hardness, good thermal conductivity and thermal shock
resistance which make it a good promising candidate for extreme environments associated
with aerospace industry [19]. Due to the unique properties of ZrB
2
it has the potential to be
substituted for Al
2
O
3
and SiC [20].

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1361
Lee et al. [21] compared the wear rate of friction stir welded AZ91/10 vol % SiC with parent
composite and observed decrease in wear rate subsequent to FSW. However, he did not
attempt to correlate the effect of FSW parameters on wear rate. This work is an attempt to
develop empirical relationships to predict the sliding wear behavior of friction stir welded
AA6061/0-10 wt % ZrB
2
butt joints and analyze the influence of process parameters on wear
rate and wear resistance of the welded joints. AA6061 has been used as matrix. Experiments
were conducted according to central composite rotatable design. A number of researchers
utilized central composite rotatable design to conduct experiments and developed precise
empirical relationships to predict the influence of process parameters on the responses [18,
22-24].
2. SCHEME OF INVESTIGATION
2.1. Fabrication of AMCs
The AA6061-T6 rods (Ø25 mm) were melted in an electrical furnace using a graphite
crucible. The chemical composition of AA6061 rods is presented in Table1. The weighed
quantities of inorganic salts K
2
ZrF
6
and KBF
4
were added into the molten aluminum to
produce ZrB
2
. The temperature of the melt was maintained at 860
0
C. The melt was stirred
intermittently for 30 minutes. After removing slag the melt was poured into a preheated die.
Castings were obtained with different weight percentage (0, 2.5, 5, 7.5 and 10 %) of ZrB
2
. A
detailed fabrication procedure and formation of in-situ ZrB
2
particles are available elsewhere
[25].
Table 1: Chemical composition of AA6061-T6
Element Mg Si Fe Mn Cu Cr Zn Ni Ti Aluminum
wt.% 0.95 0.54 0.22 0.13 0.17 0.09 0.08 0.02 0.01 Balance
Figure 1: Dimensions of friction stir welding tool.

1362 I. Dinaharan, N. Murugan Vol.10, No.14
2.2. Identification of Process Variables
The predominant FSW process parameters which influence the joint properties are tool
rotational speed (N), welding speed (S) and axial force (F) [7]. The tool pin profile also plays
a significant role on joint properties. A tool made of high carbon high chromium steel with
square pin profile was used in this work [16]. The dimensions of the tool are shown in Fig.1.
The weight percentage of ceramic particles in the composite was reported to influence the
joint properties [15, 18]. Hence, the weight percentage of ZrB
2
particles (C) was also
considered as a factor to understand its effects on sliding wear behavior.
2.3. Finding the Limits of the Process Variables
The FSW window for producing sound welds in AMCs is narrower compared to unreinforced
alloys due to the presence of ceramic particles [14]. A large number of trial welds were
carried out to fix the working ranges of all selected process parameters. Each trial weld was
inspected for smooth bead appearance and cross sectioned to verify the presence of defects
such as pin hole, tunnel and worm hole in the weld zone. The limits of each process
parameter were decided upon yielding defect free welds. The upper limit of a process
parameter was coded as +2 and the lower limit was coded as –2 for the convenience of
recording and processing experimental data. The coded values for intermediate values were
calculated using the following relationship.
X
i
= 2[2X – (X
max
+ X
min
)] / (X
max
– X
min
) (1)
where X
i
is the required coded value of a variable X; X is any value of the variable from X
min
to X
max
; X
min
is the lowest level of the variable; X
max
is the highest level of the variable. The
decided levels of the selected process parameters with their units and notations are given in
Table 2.
Table 2: Friction stir welding parameters and their levels
Levels No. Parameter Notation Unit
-2 -1 0 1 2
1 Rotational speed N rpm 1000 1075 1150 1225 1300
2 Welding speed S mm/min 30 40 50 60 70
3 Axial force F kN 4 5 6 7 8
4 Zirconium boride C wt.% 0 2.5 5 7.5 10
2.4. Developing the Design Matrix
The selected design matrix as shown in Table 3 is a central composite rotatable factorial
design consisting of 31 sets of coded conditions. A detailed description of the design matrix
is available elsewhere [22, 23].

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1363
Table 3: Design matrix and experimental results
FSW process
parameters
Trial
Run
N S F C
Wear rate
(x10
-5
mm
3
/m)
Wear
resistance
(m/mm
3
)
Relative
wear
rate
T01 -1 -1 -1 -1 520 192 0.88
T02 [phone] 0.84
T03 -1 [phone] 0.85
T04 +1 [phone] 0.90
T05 -1 -1 [phone] 0.84
T06 +1 -1 [phone] 0.88
T07 -1 +1 [phone] 0.87
T08 +1 +1 [phone] 0.89
T09 -1 -1 -1 +1 373 268 0.91
T10 +1 -1 -1 +1 368 272 0.90
T11 -1 +1 -1 +1 374 267 0.92
T12 +1 +1 -1 +1 382 262 0.94
T13 -1 -1 +1 +1 357 280 0.88
T14 +1 -1 +1 +1 379 264 0.93
T15 -1 +1 +1 +1 352 284 0.86
T16 +1 +1 +1 +1 381 262 0.93
T17 -2 0 0 0 460 217 0.90
T18 [phone] 0.96
T19 0 -2 0 0 481 208 0.94
T20 0 [phone] 0.90
T21 0 0 -2 0 459 218 0.90
T22 0 0 [phone] 0.93
T23 0 0 0 -2 524 191 0.80
T24 0 0 0 +2 265 377 0.77
T25 0 0 0 0 423 236 0.83
T26 0 0 0 0 438 228 0.86
T27 0 0 0 0 418 239 0.82
T28 0 0 0 0 441 227 0.86
T29 0 0 0 0 435 230 0.85
T30 0 0 0 0 411 243 0.81
T31 0 0 0 0 421 238 0.83
2.5. Conducting the Experiments

1364 I. Dinaharan, N. Murugan Vol.10, No.14
Plates of size 100 mm X 50 mm X 6 mm were prepared from the castings. The butt welding
of AA6061- ZrB
2
composites was carried out automatically in an indigenously built FSW
machine (M/s RV Machine Tools, Coimbatore, INDIA). The welding was carried out as per
design matrix at random to eliminate any systematic errors creeping into the system. The tool
was plunged into the abutting surfaces until the shoulder touched the surface of the plates.
The machine table was advanced at the set welding speed after a short dwell period. The
dwell period serves the purpose of generating the required heat to initiate plastic flow of the
material. Typical welded plates of trial run 23 and 24 are shown in Fig.2.
Figure 2: Photograph showing typical friction stir welded plates: a) T23 and b) T24.
2.6. Recording the Response Parameters
Specimens of size 6 mm x 6 mm x 50 mm were extracted from each welded plate. The dry
sliding wear behavior was measured using a pin-on-disc wear apparatus (DUCOM TR20-LE)
at room temperature according to ASTM G99-04 standard. The polished surface of the pin
was slid on a hardened chromium steel disc. The test was carried out at a sliding velocity of
1.5 m/s, normal force of 25 N and sliding distance of 2500 m. The wear parameters were
selected to yield an appreciable steady state wear based on trial experiments. A computer-
aided data acquisition system was used to monitor the loss of height. The volumetric loss was
computed by multiplying the cross section of the test pin with its loss of height. The wear
rate (W) and wear resistance (R) were calculated [26] as follows and given in Table 3.
W (mm
3
/m) = Volumetric loss / Sliding distance (2)
R (m/mm
3
) = 1 / Wear rate (3)
The sliding wear behavior of the parent composite was also measured at the same wear
parameters and presented in Table 4. The relative wear rate was computed as given below
and presented in Table 3.
Relative wear rate = Wear rate of welded composite / Wear rate of parent composite (4)

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1365
Table 4: Sliding wear behavior of AA6061/ZrB
2
in-situ composites
ZrB
2
(wt.%)
Wear rate
(x10
-5
mm
3
/m)
Wear resistance
(m/mm
3
)
0 657 152
2.5 594 168
5 510 196
7.5 408 245
10 345 290
2.7. Development of Empirical Relationships
The response functions representing the wear rate and wear resistance of friction stir welded
plates are functions of tool rotational speed (N), welding speed (S), axial force (F) and weight
percentage of ZrB
2
(C) can be expressed as
W = f (N, S, F, C) (5)
R = f (N, S, F, C) (6)
The second order polynomial regression equation used to represent the response surface ‘Y’
for K factors is given by
k k k
Y= b
0
+ ∑ b
i
x
i
+∑ b
ii
x
i2
+∑ b
ij
x
i
x
j
(7)
i=1 i=1 i=1
where b
0
is the average of responses and b
i
, b
ii
and b
ij
are the coefficients which depend on
respective main and interaction effects of the parameters. The values of the coefficients were
estimated using the following expressions [27].
b
0
= 0.142857(∑Y) – 0.035714∑∑ (X
ii
Y) (8)
b
i
= 0.041667 ∑ (X
i
Y) (9)
b
ii
= 0.03125∑ (X
ii
Y) + 0.00372 ∑∑ (X
ii
Y) – 0.035714(∑Y) (10)
b
ij
= 0.0625∑ (X
ii
Y) (11)
The selected polynomial for four factors could be expressed as
W = b
0
+ b
1
(N) + b
2
(S) + b
3
(F) + b
4
(C) + b
11
(N
2
) + b
22
(S
2
) + b
33
(F
2
) + b
44
(C
2
) +
b
12
(NS) + b
13
(NF) + b
14
(NC) + b
23
(SF) + b
24
(SC) + b
34
(FC) (12)
R = b
0
+ b
1
(N) + b
2
(S) + b
3
(F) + b
4
(C) + b
11
(N
2
) + b
22
(S
2
) + b
33
(F
2
) + b
44
(C
2
) +
b
12
(NS) + b
13
(NF) + b
14
(NC) + b
23
(SF) + b
24
(SC) + b
34
(FC) (13)
The coefficients were calculated using the software SYSTAT 12. The empirical relationships
were developed after determining the coefficients. All the coefficients were tested for their
significance at 95% confidence level. The insignificant coefficients were eliminated without

1366 I. Dinaharan, N. Murugan Vol.10, No.14
affecting the accuracy of the empirical relationships using t-test. The significant coefficients
were taken into account to construct the final empirical relationships. The developed final
empirical relationships with FSW parameters in coded form are given below.
W = 426.714 + 6.708N – 0.042S + 0.292F – 69.875C + 10.936N
2
+ 9.186S
2
+ 8.561F
2
–9.439C
2
(14)
R = 234.429 – 3.417 N + 0.25F + 40.75C – 5.857N
2
– 4.982S
2
– 4.732F
2
+ 12.643C
2
(15)
2.8. Checking the Adequacy of the Empirical Relationships
The statistical results of the developed empirical relationships are presented in Table 5. The
predicted empirical relationship values will exactly match with the experimental results if R-
Square value is 1. The higher values of ‘R-Square’ and lower values of standard error (SE)
indicate that the empirical relationships are quite adequate and can be used to predict the
responses (W and R) without appreciable error. The adequacy of the developed empirical
relationships was also tested using the analysis of variance (ANOVA) technique which is
presented in Table 6. The calculated values of F-ratio are greater than the tabulated values at
95% confidence level which means the developed empirical relationships are considered to
be adequate. Further the validity of the empirical relationships is tested by drawing scatter
diagrams as shown in Fig. 3. The experimental values and predicted values from the
empirical relationships are scattered both sides and close to 45
0
line which indicate the
perfect fitness of the developed empirical relationships.
Table 5: Statistical results
Response R-square Adjusted
R-square
Standard
error
W 0.976 0.967 12.002
R 0.976 0.968 7.26
Table 6: ANOVA results
Response Source Sum of
squares
Degrees of
freedom
Mean-
square
F-ratio
(calculated)
F-ratio
(tabulated)
W Regression 129030.468 8 16128.81 111.97 2.40
Residual 3168.887 22 144.04
R Regression 47836.646 8 5979.581 113.45 2.40
Residual 1159.548 22 52.707
2.9. Validation of the Empirical Relationships
Experiments were conducted to confirm the validity of the developed empirical relationships.
Five weld runs were made using different values of tool rotational speed, welding speed and
axial force other that those used in the design matrix and their wear rate and wear resistance
were estimated. The results obtained are shown in Table 7. The error in prediction was

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1367
calculated as [(experimental value – empirical relationship value) / empirical relationship
value] X 100. It is found from the table that the error is within ± 7% which confirms the
accuracy of the developed empirical relationships.
Figure 3: Scatter diagram for the developed empirical relationships: (a) Wear rate and (b)
Wear resistance.
Table 7: Results of conformity experiments
Wear rate (x10
-5
mm
3
/m) Wear resistance (m/mm
3
)
FSW process
parameters
Trial
Run
N S F C
Experi
mental
Predicted Error
(%)
Experi
mental Predicted Error (%)
1 1.25 -1.50 -1.25 -2 610.43 587.92 3.83 163.82 171.16 -4.29
2 0.75 -0.25 0.5 -1 525.78 501.20 4.90 190.19 199.10 -4.47
3 0.25 -0.75 1.5 0 425.05 453.97 -6.37 235.27 220.13 6.88
4 -0.5 0.75 0.75 1 378.56 356.95 6.05 264.16 282.79 -6.59
5 -1.5 -1.25 -0.5 2 270.35 280.15 -3.50 369.89 349.36 5.88
2.10. Wear Surface Morphology
Wear surface of selected specimens were observed using scanning electron microscope
(JEOL-JSM-6390).
3. RESULTS AND DISCUSSIONS
The developed empirical relationships do not have any interaction terms (NS, NF, NC, SF,
SC and FC).This reveals that the FSW parameters independently influence the sliding wear
behavior of the welded composites over the entire region studied in this work. Compared to
fusion welding the FSW parameters affect the joining process thermally as wells as

1368 I. Dinaharan, N. Murugan Vol.10, No.14
mechanically. Each parameter independently contributes to frictional heat generation in
addition to stirring, extruding and forging the plasticized material.
The effects of process parameters such as tool rotational speed, welding speed, axial force
and weight percentage of ZrB
2
on sliding wear behavior of friction stir welded AA6061-ZrB
2
in-situ composites are evaluated using the developed empirical relationships. The trends
obtained for each process parameter are represented in Figs.4-7. The possible causes for the
effects of different process parameters on sliding wear behavior are elaborated as follows.
3.1. Effect of Tool Rotational Speed
Fig.4 shows the sliding wear behavior of friction stir welded AA6061-ZrB
2
in-situ
composites as a function of tool rotational speed. The wear rate decreases as tool rotational
speed increases and reaches minimum at 1125 rpm. Further increase in tool rotational speed
leads to increased wear rate. The wear resistance follows an inverse trend of wear rate as
estimated.
Figure 4: Effect of tool rotational speed on sliding wear behavior.
The tool rotation generates frictional heat as well as stirring and mixing of material around
the tool pin. Optimum stirring and sufficient heat generation is required to produce sound
joints with fine recrystallized grains. When this condition is achieved during welding the
joints produced will exhibit highest wear resistance. Increase in frictional heat generation is
observed with increase in tool rotational speed. Lower heat input condition prevails at lower
tool rotational speeds (1000 rpm and 1075 rpm) which are also associated with lack of
stirring. The net result is poor consolidation of material which leads to poor wear resistance
at lower tool rotational speeds.
Higher tool rotational speeds (1225 rpm and 1300 rpm) lead to higher heat generation than
required and release excessive stirred materials. Excessive stirring causes irregular flow of
plasticized material. Micro level voids appear at higher tool rotational speeds. The frictional
heat generated during welding affects the grain size [28]. Coarsening of grains takes place at
higher tool rotational speeds which leads to poor wear resistance. Further the temperature
distribution is influenced by tool rotational speed which may contribute to this trend.

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1369
3.2. Effect of Welding Speed
Fig.5 shows the sliding wear behavior of friction stir welded AA6061-ZrB
2
in-situ
composites as a function of welding speed. The wear rate decreases as welding speed
increases and reaches minimum at 50 mm/min. Further increase in welding speed leads to
increased wear rate. The wear resistance follows an inverse trend of wear rate as estimated.
The rotating tool stirs the material as discussed earlier. The welding speed prompts the
translation of tool which in turn pushes the stirred material from front to the back of the tool
pin and completes the welding. The rubbing of tool shoulder and pin with the work piece
generates frictional heat. The welding speed determines the exposure time of this frictional
heat per unit length of weld and subsequently affects the grain growth [28]. Optimum
exposure time and translation of stirred material will lead to good consolidation of material
with fine grains. Joints experience such condition during welding will exhibit higher wear
resistance.
Figure 5: Effect of welding speed on sliding wear behavior.
3.3. Effect of Axial Force
Fig.6 shows the sliding wear behavior of friction stir welded AA6061-ZrB
2
in-situ
composites as a function of axial force. The wear rate decreases as axial force increases and
reaches minimum at 6 kN. Further increase in axial force leads to increased wear rate. The
wear resistance follows an inverse trend of wear rate as estimated.
Bonding occurs in FSW when a pair of surfaces is brought in the vicinity of inter atomic
forces. Adequate axial force exceeding the flow stress of material is required to make defect
free joints. Axial force propels the plasticized material in the weld zone to complete the
extrusion process. Axial force is also responsible for the plunge depth of the pin [30].

1370 I. Dinaharan, N. Murugan Vol.10, No.14
Figure 6: Effect of axial force on sliding wear behavior.
Frictional heat generated between the tool shoulder and the surface of the plate to be welded
is dependent upon the coefficient of friction which is decided by the axial force. Optimum
frictional heat coupled with sufficient extrusion of plasticized material is required to produce
sound joints. When this condition is encountered during welding the joint will yield higher
wear resistance. When axial force increases frictional heat generation also increases. Lower
heat is generated at lower axial forces (4 kN and 5 kN) as well as cause improper
consolidation of material. Micro voids appear at lower axial forces which leads to poor wear
resistance at lower axial forces. Higher heat is generated exceeding the desired level at higher
axial forces (7 kN and 8 kN). The plunge depth of the tool into the welded plate is higher at
higher axial forces. Further the flash level increases with increased axial force. Increased
flash level causes local thinning of welded plate leading to poor wear resistance at higher
axial forces.
3.4. Effect of ZrB
2
Particles
Fig.7 shows the sliding wear behavior of friction stir welded AA6061-ZrB
2
in-situ
composites as a function of weight percentage of ZrB
2
particles. The wear rate decreases with
increase in ZrB
2
content while the wear resistance follows an inverse trend as estimated. The
joints fabricated at N = 1150 rpm, S = 50 mm/min and F = 6 kN show higher wear resistance.
Good consolidation of material with optimum stirring and adequate heat generation may be
taking place at this combination of parameters.
It is evident from Table 4 that addition of in-situ formed ZrB
2
particles improved the wear
resistance of AA6061. This can be attributed to the hardness imparted by the in-situ formed
ZrB
2
particles due to the creation of strain fields around the particles during solidification.
The increase in wear resistance of welded joints with increased ZrB
2
particles is due to
increased presence of ZrB
2
particles in the weld zone. This leads to a conclusion that FSW
resulted in homogeneous distribution of ZrB
2
particles irrespective of weight percentage
studied in this work.

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1371
Figure 7: Effect of weight percentage of ZrB
2
on sliding wear behavior.
The inherent characteristic of applying FSW to ceramic particulate reinforced metal matrix
composites is the ability to provide identical distribution of ceramic particles in the weld zone
to that of parent composite. The weight percentage of ZrB
2
particles was considered as a
process parameter to substantiate this statement. Figs.8-9 reveal the microstructure of parent
composite and weld zone of joints which have different weight percentage of ZrB
2
particles.
The specimens were prepared as per standard metallographic procedure and color etched with
1 g NaOH, 4 g KMnO
4
in 100 ml distilled water. ZrB
2
particles appear as white and circular
in shape. The reaction of K
2
ZrF
6
with KBF
4
produced ZrB
2
particles at varying sizes. The
uniform distribution of ZrB
2
particles in the welded joints irrespective of weight percentage is
explicit.
3.5. Effect of FSW
The effect of FSW on the developed AMCs is indicated by the relative wear rate. Table 3
shows the relative wear rate of all the welded composites which is observed to be less than
one. The wear resistance of the composites improved subsequent to FSW which is described
as follows. FSW closes the presence of micro porosities in the cast composite. The grain size
of aluminum in the weld zone is reduced by dynamic recrystallization. It is evident from
Figs.8-9 that FSW resulted in fragmentation of ZrB
2
particles. The weld zone is filled with
more particles homogeneously dispersed compared to parent composite. Consequently the
dislocation density in the weld zone increases. The age hardening and softening
characteristics of cast composite is different to that of wrought/heat treated composite. The
above factors contribute to hardening of weld zone. The degree of hardening is dictated by
the process parameters [10, 17] which results in improved wear resistance of the AMCs.

1372 I. Dinaharan, N. Murugan Vol.10, No.14
Figure 8: Photomicrographs of base composites containing ZrB
2
: a) 0% (T23); b) 2.5%
(T01); c) 5% (T29); d) 7.5% (T15) and e) 10% (T24).
3.6. Wear Surface Morphology
Fig.10 reveals the SEM micrographs of wear surface of selected specimens with different
weight percentage of ZrB
2
particles. A change in wear mode is observed with increase in
weight percentage of ZrB
2
particles which can be attributed to the homogeneous distribution
of ZrB
2
particles in the weld zone as a result of FSW. The wear mode changes from adhesion
(Fig.10a-b) to abrasive wear (Fig.10c-e). Welded matrix alloy and composite containing 2.5
weight percentage of ZrB
2
particles exhibit adhesion wear mode. The frictional heat increases

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1373
wear surface temperature which causes plastic deformation and dislocation in the inner
surface of the composites.
Figure 9: Photomicrographs of Weld zone of AMCs containing ZrB
2
: a) 0% (T23); b) 2.5%
(T01); c) 5% (T29); d) 7.5% (T15) and e) 10% (T24).
The congestion of dislocation results in stress concentration and initiation of cracks. Welded
composites containing weight percentage of ZrB
2
particles above 2.5 exhibits abrasive wear
mode. The abrasive wear is the result of ZrB
2
particles on the wear surface and the abrasive
dusts between two surfaces. ZrB
2
particles bear the load initially. As sliding wear proceeds

1374 I. Dinaharan, N. Murugan Vol.10, No.14
the frictional heat softens the surface layer. The difference in thermal expansion coefficient
between the matrix and the ZrB
2
particles creates the interface stress. When the interface
stress exceeds the bond strength the particles are pulled off. The pulled off particles begin to
act as wear particles in the sliding wear course.
Figure 10: SEM micrograph of wear surface of specimens with: a) 0 wt.% ZrB
2
(T23); b) 2.5
wt.% ZrB
2
(T02); c) 5.0 wt.% ZrB
2
(T30); d) 7.5 wt.% ZrB
2
(T11) and e) 10 wt.% ZrB
2
(T24).

Vol.10, No.14 Influence of Friction Stir Welding Parameters 1375
4. CONCLUSIONS
The following conclusions are derived from the present work:
• Empirical relationships incorporating the welding parameters are developed to predict
the sliding wear behavior of AA6061/0-10 wt.% ZrB
2
in-situ composite butt welded
joints.
• The process parameters independently influence the sliding wear behavior over the
entire range of parameters studied.
• The joints fabricated at N = 1150 rpm, S = 50 mm/min and F = 6 kN yields highest
wear resistance.
• A homogeneous distribution of ZrB
2
particles in the welded joints irrespective of
weight percentage is observed.
• FSW enhances the wear resistance of the developed AMCs
• A change in the wear mode from adhesion wear to abrasive wear with the increase in
weight percentage of ZrB
2
particles is observed.
ACKNOWLEDGEMENT
The authors are grateful to the Management and Department of Mechanical Engineering,
Coimbatore Institute of Technology, Coimbatore, India for extending the facilities to carry
out this investigation. The authors acknowledge the financial support rendered by All India
Council for Technical Education, Govt. of India. The corresponding author acknowledges the
INSPIRE fellowship awarded by Department of Science and Technology, Govt. of India.
Authors are also thankful to Mr.S.J.Vijay, Mr.K.Kalaiselvan, Mr.B.Ashok Kumar,
Mr.A.Raja, Mr. A. Samson Ratnakumar, Mr. S.Vijaya Ganesh, Mr. Palanisamy and Mr.
Mahalingam for their assistance.
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