The document discusses the effect of pin profile geometry on friction stir welded aluminum matrix composites. It finds that a square pin profile produces welds with smaller and finer grains compared to hexagonal and octagonal pin profiles. This leads to improved hardness, tensile strength, and wear resistance due to the relationship between smaller grain size and better mechanical properties. The square pin profile causes less heat input during welding, resulting in less grain growth and better properties compared to the other pin profiles tested. Increasing welding speed and decreasing rotational speed also reduce heat input and improve properties by limiting grain growth during cooling.

1. Effect of Pin Profile on Friction Stir Welded Aluminum Matrix
Composites
Adel Mahmood Hassan*1, Tarek Qasim2, Ahmed Ghaithan3
Department of Industrial Engineering, Jordan University of Science and Technology,
P. O. Box 3030
Irbid 22110, Jordan
1 adel@just.edu.jo; 2 tqqasim@just.edu.jo; 3 amghaithan07@eng.just.edu.jo
Abstract
To clarify the role of pin profile geometry on some properties of friction stir welded of the
considered aluminum matrix composites (Al - 4 wt.% Mg, reinforced with 1 wt.% SiC and
1 wt.% graphite particles) plates of 8 mm thickness were fabricated by compocasting
method then annealed at 400 ˚C for 2 hrs. Tools with different pin profiles (square,
hexagonal and octagonal) were manufactured to be used for FSW of aluminum matrix
composites plates at four different levels of welding (transverse) and rotational speeds. The
effects of these pin profiles on microstructure and some mechanical properties of the
friction stir welded joints were studied. The results show, that the plates welded by square
head pin have better properties compared to the other pin profiles. This pin seems to cause,
better grain refinement and redistribution of SiC and graphite particles in the welded
nugget zone, than the other two types. This has led to better improvement in the
considered mechanical properties. Also these properties were improved by increasing
welding (transverse) speed while increasing the rotational speed has a diverse effect on
them.
Keywords: Aluminum; Composites; Friction; Welding; Compocasting; Properties.
_______________________________________
*Corresponding author
Tel: +962-2-7201000, Ext: 22571
E-mail address: adel@just.edu.jo

2. 1. Introduction
Friction Stir Welding (FSW) was introduced in 1991 by The Welding Institute (TWI) in
Cambridge, England as a solid-state metal joining process [1, 2]. In friction stir welding
process parts to be joined must be tightly clamped to backing plate in order to prevent them
from moving during the welding process. A rotating pin tool is forced down into a hole
along the weld line until shoulder of the tool comes into contact with the parts to be joined.
The rotating tool travels along the joint line direction with a constant welding (traverse)
speed.
During welding process, the material along the joint undergoes intense plastic deformation
due to frictional elevated temperature, resulting in fine and equiaxed recrystallized grains,
which in turns enhances the mechanical properties of the welded joint [3, 4]. The friction
stir weld joint consists of three distinct zones: the nugget zone (NZ) in the middle of the
joint, followed by the thermo-mechanically affected zone (TMAZ) and the third zone is the
heat-affected zone (HAZ). At the NZ, the plastic deformation will produce a recrystallized,
equiaxed, and fine grain microstructure. TMAZ exposes to lower plastic deformation (less
than the nugget zone). Therefore, this zone consists of relatively large grains. The HAZ is
not subjected to any plastic deformation only it is exposed to thermal affect which results
in some modification and coarsening the grains. During the FSW process, because of the
rotation of the profiled pin of the welding tool nearly concentric rings are developed in the
nugget zone, which is called the onion rings structure [5]. The process can be used in many
applications, such as the joining of similar metals, dissimilar metals [6], high-strength
aerospace aluminum alloys and composite materials that have limitations to be welded by
conventional fusion welding processes [7]. More details of the advantages and limitations
of the FSW process can be found in [8].
In the FSW process, the microstructure evolution and the mechanical properties of the
weld joints is influenced by the material flow in the weld zone. The most significant
parameter affects the material flow is the tool geometry [9]. Among other parameters
affecting the material flow are the friction rotational speed and welding (transverse) speed.
All these parameters have a remarkable influence on grain size of the nugget zone
microstructure, which, in turn, will affect the mechanical properties of the weld zone [10].
In general, it can be stated that FSW is a combination of extruding, forging and stirring of
the material [9]. Most of the previous studies in the recent developed field of friction stir
welding have focused on the effect of welding (transverse) speed and rotational speed on
the properties of welded joints [11]. Little work has been done to study the effect of the
welding pin profile tool on properties of friction stir welded joints [12], especially on
composite materials. Accordingly, the present work was concentrated on studying the
effect of pin profile geometry of the welding tools on mechanical properties, utilizing
aluminum matrix composites.
2. Experimental Work
2.1 Materials
Commercial pure aluminum alloyed with 4 wt % Mg as wetting agent reinforced by 1 wt
% SiC and 1 wt % graphite particles were used in fabrication the aluminum matrix
composites plates. Silicon carbide powder having a diameter of 200 µm and a density of
3.21 g/cm3 was chosen as reinforcement particles because it has a high wear resistance. In
1

3. addition, graphite particles having a density of 2.1 g/cm3 were used as second
reinforcement particles to improve the machinability and wear resistance of the considered
composite, graphite acts as a lubricating agent [13].
2.2 Processing the Plates
The processing of the composite plates (100 mm X 75 mm X 8 mm) used in the present
study was manufactured by compocasting method. More details about this method can be
found in [14]. All plates produced were annealed at 400°C for a period of 2 hrs, before
they were butt welded by friction stir welding process. Prior to welding the annealed plates
properties were tested and recorded for comparative reasons. The annealed plates before
welding have a tensile strength of 130 MPa and Rockwell hardness of 88.3 HRH.
2.3 Welding Tool Fabrication
Tools with square, hexagonal, and octagonal pin profiles were fabricated from 0.4% C
plain carbon steel using conventional milling process. The choice of these pin profiles
has two folds. Firstly for comparative reasons with previous studies utilized similar
pin profiles. [15] Secondly, these pin profiles have similar geometry i.e. sharp corners
with differing boundary area during rotation. The steel were oil hardened to reach a
hardness of 63 HRC. The schematic diagram for the square head pin tool is shown in Fig.1.
The hexagonal and octagonal head tools are identical in their design to the square head
tool.
Figure 1.-Square head pin friction stir welding tool
2.4 Welding Procedure
The fabricated and annealed plates were butt welded by FSW process using a conventional
milling machine. The plates were clamped firmly to a specially designed fixture, Fig.4,
which was mounted and fixed tightly on the milling machine. For each pin profile tool,
four welding (transverse) speeds of 35, 45, 55, 65 mm/min and four rotational speeds 630,
800, 1000, 1250 rpm were utilized in the present study. The choice of these speeds fall
around the optimum process parameters for FSW for similar parent metal base (i.e.
Aluminum) described in the literature [16].
2.5 Metallurgical and Mechanical Tests
Microstructure analysis of the weld joints was carried out using an optical microscope, the
specimens were etched with Keller’s reagent (1 mL HF, 1.5 mL HCL, 2.5 mL HNO3,
95 mL distilled water) solution. Rockwell hardness was conducted using a universal
hardness testing machine. Tensile test specimens were prepared by CNC milling machine
so that the welded joint was latterly in the center of the specimen.
The wear tests were carried out at a normal load of 50 N and rotational speed of 100 rpm
using a pin-on-disk type test machine at dry conditions. Wear specimen with 25 mm length
and 4 mm in diameter pin was prepared from the center of the nugget zone (NZ) of the
weld joint. The wear rate can be calculated using equation 1 [17]:
2
(b)

4. W  M /( D  S ) ……………………………….…………..…………………….… (1)
Where, W: Wear rate expressed in (cm3/m), M: Mass loss during wear in (g), S: Sliding
distance in (m), and D: Density of the respective composite in (g/cm3), which is equal to
2.67 g/cm3, as determined by the rule of mixture method.
(a)
(b)
(c)
Figure 2. - Comparison between the effect of tool pin profiles geometry on FSW nugget
zone microstructure (a) Square, (b) Hexagonal and (c) Octagonal head pin tools at
rotational speed of 630 rpm and welding transverse speed of 65 mm/min.
Magnification 500X
3

5. 3. Results
3.1 Microstructure Analysis
A comparative photographs of the effects of pin profile geometry on the microstructure
of the friction stir welded nugget zone of the three different tool profiles at a rotational
speed of 630 rpm and a welding transverse speed of 65 mm/min are shown in Fig.2. The
square pin profile tool produces weld joints with small and fine grains relative to the other
considered profiled tools as shown in Fig.2. The microstructure of the friction stir weld
joint is affected by the pin profile tool type, and the mechanical properties are expected to
be changed relevance to the micro structural changes [18].
3.2 Hardness
Avarage Rockwell hardness (HRH)
Figures.3 and 4 show that the square head tool has the highest effect on hardness values at
the same welding transverse speed and rotational speed, than the hardness obtained by
other profiled tools.
Square pin
Hexagonal pin
Octagonal pin
99
97
95
93
91
89
Base composite hardness = 88.3
87
85
30
35
40
45
50
55
60
65
70
Welding speed (mm/min)
Avarage Rockwell hardness (HRH)
Figure 3. - Effect of welding (transverse) speed and pin profile tool geometry on average
Rockwell hardness at rotational speed of 630 rpm.
Square pin
Hexagonal pin
Octagonal pin
99
97
95
93
91
89
87
85
500
Base composite hardness = 88.3
750
1000
Rotational speed (rpm)
4
1250
1500

6. Figure 4. - Effect of rotational speed and pin profile tools on average Rockwell hardness at
a welding (transverse) speed of 65 mm/min.
3.3 Tensile Strength
Tensile strength (MPa)
Figures 5 and 6 indicate that the used of the square pin profiled pin has given the highest
values for both welding (transverse) speed and rotational speed. Again, the highest
improvement of the tensile strength was encountered with square profile pin.
Square pin
Hexagonal pin
Octagonal pin
220
200
180
160
140
120
100
Base composite tensile strength = 130 MPa
80
60
40
20
0
30
35
40
45
50
55
60
65
70
Welding speed (mm/min)
Figure 5. - Effect of welding (transverse) speed and pin profile tool on the tensile strength
at rotational speed of 630 rpm.
Square pin
Hexagonal pin
Octagonal pin
210
Tensile strength (MPa)
190
170
150
130
110
Base composite tensile strength = 130 MPa
90
70
50
500
750
1000
1250
1500
Rotational speed (rpm)
Figure 6 - Effect of rotational speed and pin profile tool on the tensile strength at welding
(transverse) speed of 65 mm/min.
3.4 Wear Resistance
Results are obtained for both welding transverse speed and rotational speed as shown in
Fig.7 and Fig.8 respectively, where the wear resistance in both figures is higher for the
square profiled pin than the other types of profiled pins.
5

7. Square pin
Hexagonal pin
Octagonal pin
Wear rate (mm 3/m)
0.003
0.0025
Base composite wear rate
= 0.0027 mm3/m
0.002
0.0015
0.001
0.0005
0
30
35
40
45
50
55
60
65
70
W elding speed (mm/min)
Figure 7 - Effect of welding (transverse) speed and pin profile tools on the wear rate at
rotational speed of 630 rpm.
Square pin
Hexagonal pin
Octagonal pin
Wear rate (mm3/m)
0.003
0.0025
Base composite wear rate =0.0027 mm3/m
0.002
0.0015
0.001
0.0005
0
500
750
1000
1250
1500
Rotational speed (rpm)
Figure 8 - Effect of rotational speed and pin profile tools on the wear rate at welding
(transverse) speed of 65 mm/min.
4. Discussion
Microstructure evolution in the friction stir weld joints were resulted from the intensive
plastic deformation which causes grain refinement in the weld zone. In addition to that
there is a breaking up and uniform redistributions of the SiC and graphite particles within
the NZ. due. Pin profile geometry plays an important role in material flow at the weld zone
[19]. In general, the pin stirs the material to make a complete joint. The material flow due
to the action of the rotating tool will lead, in turn, to an improvement in the mechanical
properties, such as hardness, tensile strength and wear resistance (See Figs. 3-8).
The higher improvement in the above mentioned mechanical properties is encountered by
using square pin profile geometry, since the square head pin tool has the smallest crosssectional area followed by hexagonal head, then the octagonal head for the same circle
diameter, in which these profiles are drawn. So that the frictional heat during the welding
tool rotation of this smaller cross-sectional area of the square head pin will cause less heat
input in the weld zone. This has a significant importance in terms of properties such as
fatigue, wear and even corrosion [20]. The highest frictional heat input will be caused by
6

8. the octagonal head pin. Accordingly, the microstructure of the nugget zone welded by the
square head tool will have fine grains, because less frictional heat is encountered by this
type of profiled tool, and when it is cooled by the surrounding air, there will not be enough
time for the grains to grow, in contrast to the other two types of pins. Larger grain size will
be found in the nugget zone welded by the octagonal head pin, as more frictional heat input
will be developed, since, there is more time for the grain to cool to room temperature. This
argument, also, can be applied to the hexagonal head pin, where the grains of the nugget
zone are larger than those obtained by the square head pin, but smaller than those obtained
by the octagonal head pin, as its cross-sectional area is intermediate between the square
and the octagonal head pins. According to Hall-Petch relationship [21], it can be stated that
the smaller the grain size is, the improvement in the hardness, tensile strength and wear
resistance will be better,
The above discussion can be considered true with other welding parameters i.e. welding
speed and rotational speed, as the nugget zone will have smaller grain size, when the
welding speed is increased at a constant rotational speed, as there will be smaller frictional
heat input encountered within the weld causing a small grains to be formed and an
improvement in the considered properties, Fig.3, Fig.5 and Fig.7. But the increase in the
rotational speed at constant welding speed causes more frictional heat to form within the
nugget zone, and rather a long time will be taken by the material to cool to room
temperature, so that the grains will have time to grow. So that a relatively large grains will
be formed causing a reduction in the values of hardness, tensile strength and wear
resistance, Fig. 4, Fig.6 and Fig.8.
5. Conclusions
The microstructure of the friction stir weld joint has great affect on the considered
mechanical properties, i.e. hardness, tensile strength and wear resistance, as the reduction
of the grain size will cause an improvement in them, according to Hall–Petch relationship.
In addition, the heat input caused by frictional forces is lower in the square head pin rather
than the other two profiles of the welding tools, so that less growth in the grain of the
nugget zone structure will occur during the cooling to room temperature. This means that
the square head pin have more influence on the considered mechanical properties.
It is important to note that smaller heat input developed in the nugget zone, when there is
an increase in the welding speed and / or a decrease in the rotational speed. So that, less
time will be required to cool the nugget zone to room temperature, causing its structure to
develop smaller grain size, which in turn increase the considered mechanical properties.
The implications of the current study go beyond showing the ability of friction stir welding
method to join successfully aluminum matrix composites, but, also, studying of the process
parameters and understanding the effect of pin profile on the joints welded by FSW are of
importance to many industrial applications.
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9. Acknowledgement
This work was supported by a grant from the Deanship of Scientific Research at Jordan
University of Science and Technology (Grant No. 2010/195). The authors also would like
to acknowledge all members of the Industrial Engineering Department workshops and
laboratories for their help in using the machines and other available facilities.
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