Effect of Pin Profile on Friction Stir Welded Aluminum Matrix

Composites

Adel Mahmood Hassan*1, Tarek Qasim

2, Ahmed Ghaithan

3

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

1

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

2

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]:

(b)

3

)/( SDMW ……………………………….…………..…………………….… (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

4

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 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.

85

87

89

91

93

95

97

99

30 35 40 45 50 55 60 65 70

Welding speed (mm/min)

Ava

rag

e R

ockw

ell

ha

rdn

ess (H

RH

)

Square pin

Hexagonal pin

Octagonal pin

Figure 3. - Effect of welding (transverse) speed and pin profile tool geometry on average

Rockwell hardness at rotational speed of 630 rpm.

85

87

89

91

93

95

97

99

500 750 1000 1250 1500

Rotational speed (rpm)

Ava

rag

e R

ockw

ell

ha

rdn

ess (

HR

H)

Square pin

Hexagonal pin

Octagonal pin

Base composite hardness = 88.3

Base composite hardness = 88.3

5

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 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.

0

20

40

60

80

100

120

140

160

180

200

220

30 35 40 45 50 55 60 65 70

Welding speed (mm/min)

Tensi

le s

trength

(M

Pa)

Square pin

Hexagonal pin

Octagonal pin

Figure 5. - Effect of welding (transverse) speed and pin profile tool on the tensile strength

at rotational speed of 630 rpm.

50

70

90

110

130

150

170

190

210

500 750 1000 1250 1500

Rotational speed (rpm)

Te

nsile

str

en

gth

(M

Pa

)

Square pin

Hexagonal pin

Octagonal pin

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.

Base composite tensile strength = 130 MPa

Base composite tensile strength = 130 MPa

6

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

30 35 40 45 50 55 60 65 70

Welding speed (mm/min)

We

ar

rate

(m

m3 /m

)

Square pin

Hexagonal pin

Octagonal pin

Figure 7 - Effect of welding (transverse) speed and pin profile tools on the wear rate at

rotational speed of 630 rpm.

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

500 750 1000 1250 1500

Rotational speed (rpm)

Wear

rate

(m

m3/m

)

Square pin

Hexagonal pin

Octagonal pin

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 cross-

sectional 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

Base composite wear rate

= 0.0027 mm3/m

Base composite wear rate =0.0027 mm3/m

7

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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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.

References

1. Kallee, S.W., Friction stir welding at TWI .The Welding Institute (TWI), Cambridge,

England, 2006.

2. Thomas, W. M. and Dolby, R. E., Friction stir welding developments, Proceedings of

the Sixth International Trends in Welding Research, Materials Park, ASM International,

USA., 2003, 203–211.

3. Jata, V. and Semiatin, S. L., Continuous dynamic recrystallization during friction stir

welding of high strength aluminum alloys, Scripta Materialia, 2000, 43(8), 743–749.

4. Mishra, R.S. and Ma, Z.Y., Friction stir welding and processing, Materials Science and

Engineering, 2005, 50, 1–78.

5. Krishnan, K. N., On the formation of onion rings in friction stir welds, Materials Science

and Engineering, 2002, A327, 246–251.

6. Kwon, Y. J., Shigematsu, I. and Saito, N., Disimilar friction stir welding between

magnesium and alumnum alloys. Materials Letters, 2008, 62, 3827-3829.

7. Storjohann, D., Barabash, O. M., Babu, S. S., David, S. A., Sklad, P. S., Bloom, E. E.,

Fusion and friction stir welding of aluminum–metal–matrix composites, Metallurgical and

Materials Transactions A, 2005, 36, 3237–3247.

8. Nandan, R., DebRoy, T. and Bhadeshia, H. K., Recent advances in friction-stir welding

process, Weldment Structure and Properties, Progress in Materials Science, 2008, 53, 980–

1023.

9. Zhang, Z. and Zhang, H. W., Numerical studies on the effect of transverse speed in

friction stir welding, Materials and Design, 2009, 30, 900–907.

10. Zhao, Y. H., Lin, S. B., Qu, F. X. And Wu, L., Influence of pin geometry on material

flow in friction stir welding process, Materials Science and Technology, 2006, 22, 45-50.

aluminium alloy, Scripta Materialia, 2005, 52, 693–697.

11. Jones, M. J., Heurtier, P., Desrayaud, C., Montheillet, F., Allehaux, D. and Driver, J.H.,

Correlation between microstructure and microhardness in a friction stir welded,2005,

52,693-697.

12. Kumar K., . Kailas Satish V., Srivatsan T. S, The role of tool design in influencing the

mechanism for the formation of friction stir welds in aluminum alloy 7020, Materials and

Manufacturing Processes, 2011, 26, 915-921

13. Suresha, S. and Sridhara B. K., Effect of addition of graphite particulates on the wear

behaviour in aluminum–silicon carbide–graphite composites, Materials and Design, 2010,

31,1804-1812.

14. Hassan, A.M.; Hayajneh, M.; Alrashdan A.; Mayyas, A.T., Prediction of density,

porosity and hardness in aluminum-copper based composite materials using artificial

neural network. J. of Materials Processing Technology, 2009, 209: 894–899.

15. Elangovan K., and Balasubramanian V., Influences of pin profile and rotational

speed of the tool on the formation of friction stir processing zone in AA2219

aluminium alloy. Materials Science and Engineering: A, 2007, 459, 7-18.

9

16. Lakshminarayanan A. K.; Balasubramanian V., K; Elangovan, Effect of welding

processes on tensile properties of AA6061 aluminium alloy joints, 2009, Int. J. Adv.

Manuf. Technology,40,286-296.

17. Shaoyang, Zhang, Fuping. and Wang., Comparison of friction and wear performances

of brake material dry sliding against two aluminum matrix composites reinforced with

different SiC particles, Materials Processing Technology, 2007,182, 122-127.

18. Mahmoud, R. I., Takahashi, M., Shibayanagi, T. and Ikeuchi, K., Effect of friction stir

processing tool probe on fabrication of SiC particle reinforced composite on aluminium

surface, Science and Technology of Welding and Joining, 2009, 5, 214-219.

19. Zeng, W. M., Wu, H. L. and Zhang, J., Effect of tool wear on microstructure,

mechanical properties and acoustic emission of friction stir welded 6061 Al alloy, Acta

Metallurgica Sinica, 2006,19 (1), 9–19.

20. Dehghani K. and Mazinani M., Forming Nanocrystalline Surface Layers in Copper

Using Friction Stir Processing. , Materials and Manufacturing Processes, 2011, 26, 922-

926.

21. Newell, J, Essentials of modern materials science and engineering, John Wiley & Sons,

2009; 58 pp.