1

Effect of deformation behavior on the grain size of the 6061 aluminum

alloy joint with alumina by friction welding

Uday M. B.1, Ahmad Fauzi M.N.2, Zuhailawati H.3, Ismail A.B.4

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia 14300 Nibong Tebal, Penang, Malaysia

1 ummb2008@gmail.com, 2 afauzi@eng.usm.my, 3 zuhaila@eng.usm.my, 4 badri@eng.usm.my

Keywords: Friction welding; 6061 Aluminum Alloy; ceramic; deformation; grain size

Abstract

Friction welding is widely used as a mass-production method in various industries. Joining of

ceramic-metal is one of the most essential needs of many industrial applications. Experiments were

performed to observe the deformation characteristics of 6061 aluminum alloy after joining with

alumina using friction-welding process at different rotational speeds and to relate difference in grain

size with differences in deformation behavior. In the present study, electron microscopy and X-ray

diffraction profile analysis were used to study the effect of plastic deformation of interface material

on the grain size of 6061 aluminum alloy when there is a joint with the ceramic. The effect of

rotation speed and the degree of deformation appeared to be high on the 6061 Al alloy than on the

ceramic part. Results showed different deformation mechanisms at different rotational speeds and

confirmed unambiguously the change in crystalline grains as a result of deformation mechanisms.

1. Introduction:

In most engineering fields, the application of ceramics is often restricted by the availability of an

adequate joining technique. In most tribological, thermal, or corrosive applications, ceramic-metal

joints are employed, instead of bulk ceramic parts, owing to the brittle behavior and high machining

costs of ceramic materials. For the joining of ceramics to metals, brazing, active brazing, and

diffusion bonding have been established. As earlier work indicates, friction welding could be an

interesting and cost effective alternative, provided that the strength of friction welded joints reaches

or exceeds the strength of those joints produced by other techniques (1).

Friction welding is a complicated process that involves the interaction of thermal and mechanical

phenomena. In friction welding, the heat for welding is obtained by conversion of mechanical

energy to thermal energy at the interface of the workpieces (2-5). The plastic deformation of the

bonding material, which affects the welding process and the joint quality greatly, is the essence of

friction welding. However, it is difficult to measure the stress, strain, and other field parameters,

thus it is very important to study these areas as well as other parameters of friction welding,

including interface material plastic deformation (6). Frequently, the weld interface appears to

comprise homogenous fine, dynamically recrystallized grains whose size is substantially less than

that in the base materials (7).

Plastic deformation is realized by external forces (8). These forces do work during the

deformation. A major portion of the work done (according to some measurements, 95-98% of the

work done) during plastic deformation is transformed into heat the remaining few percent is stored

in the material, increasing its energy level. The amount of stored energy depends primarily on the

magnitude of deformation, and increases proportionally with it. The magnitude of stored energy

depends to a small extent on the microstructure of the deformed material as well. The same degree

of deformation results in materials having a fine microstructure than in those possessing coarse

grains.

The objective of the present research was to develop a basic understanding of the evolution of

microstructure in the deformation regions and to relate it to the friction welding process variables

i.e. pressure, rotational speed, and time. Such a correlation has not been attempted before perhaps

2

Alumina

because of the difficulty in quantifying the process variables. To overcome such difficulties,

measuring and modeling the local temperature transients during friction welding was utilized, and

an approximate method was employed to estimate the strain and strain rate in the weld interface.

2. Materials and procedures:

The materials used in this investigation consisted of friction-welded bars of alumina ceramic

(99.4Al2O3, 0.33SiO2, 0.11Na2O, 0.024CaO, 0.081Fe2O3, 0.014NiO, wt %) prepared in-house

through slip casting technique, and commercial 6061 aluminum alloy rods (96Al, 0.6Si, 0.95Mg,

0.33Fe, 0.17Cu, 0.066Cr, 0.04Mn, 0.022Ti, 0,014Zn, 0.014Ni, 0.013Ga) which are typically used

for aerospace applications. Tests were conducted on the weld joints, which were produced by

friction welding of 16 mm diameter rods of the sintered alumina and the aluminum alloy. The joints

were prepared on a direct drive friction-welding machine modified from an existing lathe machine

model (APA TUM-35). Friction-bonded alumina/6061 aluminum alloy joints were cross-sectioned

at different locations in the metal side to the bond interfaces by using a low-speed diamond saw.

The cross sectional direction locations of the samples taken for the microstructural analysis are

shown in Fig.1. The sectioned samples were prepared using standard metallographic procedures and

were etched using a sodium hydroxide solution (1%NaOH, 15min time). The microstructure of the

metal alloy was observed using the Field Emission Scanning Electron Microscopy (ZEISS SUPRA

35VP). X-ray diffraction analysis was also performed in order to investigate the effect of plastic

deformation on the grain size of metal side near the welding interface.

Fig.1: The cross sectional view representation of friction-welded alumina and 6061 aluminum alloy showing

the locations cutting surfaces to the interface (a)3mm (b)1mm (c) 0.5mm.

3. Results and Discussion:

3.1 Aluminum alloy characterization before friction welding:

To understand the influence of the friction welding parameters on the microstructural

development, and to correlate it with the deformation zone in this study, the reader must have some

knowledge in 6061 aluminum alloy characterization before the friction welding process. It should

also be noted that the aluminum alloy is affected by the welding process and not by the ceramic

materials. The microstructural characterization was performed for the base metal alloy to quantify

the grain size and shape using image analysis for FESEM. The images (Fig.2) shows that the alloy

consists of average grain size ~12.75µm. The base material 6061 Al alloy is seen to have grains of

equal size and distribution, with the grains tending to be uniform.

Interface

(a)

6061 Al Alloy

(b

)

(c)

3

0

2000

4000

6000

8000

10000

12000

14000

10 30 50 70 90

Inte

nsi

ty (

arb

.unit

)

2-Theta

Fig.2: Microstructure of base 6061 Al Alloy without deformation (a)500X, (b)1000X.

In order to determine the distribution of the grains in the weld metal, the welding sample of 6061

Al alloy was analyzed by X-ray diffraction, using a copper target. The experimental results are

shown in Fig. 3. The relevant parameters corresponding to the diffraction peaks in the diagram have

been computed using X’ Pert High Score Plus software. These computed results were compared

with the relevant parameters of aluminum alloy (reference code 98-010-1523), which have been

deformed by friction welded joint with alumina.

Fig.3: The XRD spectrum and relevant parameters of the base 6061 aluminum alloy.

3.2 Aluminum alloy characterization after friction welding:

Friction welding can be considered as a hot-working process in which a large amount of

deformation is imparted to the workpiece through the rotating dissimilar materials. Such

deformation gives rise to a weld interface, an effected deformation zone (DZ) and unaffected zone

(UZ). Regularly, the weld interface appears to include equaled, fine, dynamically recrystallized

grains whose size is substantially less than that in the base metal (7). In addition, the pressure is

used to bring the atoms of metals and alloys together by plastic deformation to produce welds

without fusion i.e. there is a localized effect on the microstructure. The effect on the microstructure

depends on the temperature that is applied under pressure and a certain speed, which is the reason

for the plastic deformation to take place (9).

The selected measurement boundary is at the positions 3mm, 1mm and 0.5 mm where the original

point (0mm) is located at the interface of the weld as shown in Fig. 4a. Three adjacent samples were

then cut in the transverse section. The selected positions (3mm, 1mm, and 0.5mm) can be seen by

the difference between these samples by means of a scanning electron microscope (FESEM) and X-

ray diffraction profile analysis. The grain size and grain shape are two major criteria used to

determine the deformation zone (10). The average grain size in the effected deformation zone (close

to the boundary) was approximately 2-5μm (Fig.4b) (11). Fig.5 shows a FESEM image of the 6061

aluminum alloy after 3mm distance from the interface. The difference in grain shape and

dimensions between this distance (Fig.5a) and the base metal (Fig.5b) is quite evident. The mean

Formula sum Al4.00

Formula mass/ g/mol 107.9260

Density (calculated)/ g/cm3 2.6830

Space group (No.): F m -3 m (225)

Crystallite Size/ nm (Scherrer method) 42.22

Lattice parameters

a/ Å 4.0573 alpha/ ° 90

b/ Å 4.0573 beta/ ° 90

c/ Å 4.0573 gamma/ ° 90

●113

●222

●022

●002

●111

● Aluminum

(a) (b)

4

value of the grain size as measured by FESEM was between 26.8- 46.84µm in the (3mm) under the

impact of increased rotational speeds in the friction welding machine compared with ~12.75µm in

the base alloy.

Fig. 4 (a) Interface of specimen welded observed under FESEM without etching (b) Grain size of 6061 Al

Alloy in the effected deformation zone with etching (1% NaOH).

The microstructure of the 6061 aluminum alloy with deformation at the position 1mm of the

different rotational speeds is shown in Fig. 6. The grains of the 6061 aluminum alloy are elongated

along the pulled direction. The effect of increasing rotational speed over the friction welding joint is

that both the temperature gradient and axial shortening increased as a result of more mass being

transferred out of the welding interface (11). A through-thickness longitudinal section of the

dissimilar 6061 aluminum alloy/alumina weld at a rotational speed of 1250-2500 rpm is shown in

Figs. 6(a-c). At high rotational speeds, shorter heating upset times are required allowing propagation

of thermal energy along the cross section area of the samples. A greater volume of material is

therefore heated, leading to lower cooling rates and more grains were pulled neatly and evenly

(Fig.6c).

From Figs. 7 (a-c), it can be seen that the microstructure of the metal alloy began to change grains

shape after 0.5mm position at the different rotational speeds. Hence, this analysis indicates that

restoration processes such as dynamic recrystallization were operative during friction welding of

6061 Al alloy especially with the decreased grain size in this distance (0.5mm). The XRD data did

show (Table1) that changes in metal alloy crystallite size, density and lattice parameters occurred

over the test distances at different rotational speeds. The data obtained from the results show more

change, indicating the crystallite size was influenced significantly during the friction welding

experiments. These XRD results Figs. (8-10) were compared to the base metal measurements to

determine the differences of the XRD for crystallite size evaluation using Scherrer method. The

results also showed the location density in the metal alloy after friction welded using XRD analysis.

Compared to the initial location density in the base metal alloy (2.6830g/cm3), which could be

found in the different test distances (3mm,1mm,0.5mm), a slight increase of the location density

was obvious up to 2.6830 g/cm3 at different rotational speeds during the friction welding. The

location density and lattice parameters of the grain structure can significantly influence the strain-

hardening behavior associated with location movements during deformation in friction welding.

Therefore, this variables observation of transient microstructure is important in understanding the

complicated friction welding process, which includes the plastic deformation and dynamic

recrystallization at changed rotational speeds.

Alumina 6061 Al Alloy (a) 6061 Al Alloy (b)

5

Fig.5: Microstructure of the 6061 Al Alloy with (a) deformation at the position 3mm compared with (b) base

metal alloy.

Fig. 6: Microstructure of the 6061 Al Alloy with deformation at the position 1mm for rotational speed (a)

1250rpm, (b) 1800rpm, (c) 2500rpm.

Table 1: XRD results on deformed of 6061Al-Alloys with pure alumina joint at different rotational speed

Fig.7: Microstructure of the 6061 Al Alloy with deformation at the position 0.5mm at rotational speed (a)

1250rpm, (b) 1800rpm, (c) 2500rpm.

Relevant parameters Distance at the 3mm Distance at the 1mm Distance at the 0.5mm

Rotational speeds [rpm] 1250 1800 2500 1250 1800 2500 1250 1800 2500

Crystal system Cubic Cubic Cubic Cubic Cubic Cubic Cubic Cubic Cubic

Density (calculated) [g/cm3] 2.6926 2.6954 2.6928 2.6923 2.6919 2.6956 2.6939 2.6962 2.6914

Crystallite Size [nm] 72.88 51.25 45.96 128.28 143.63 50.14 173.35 93.02 166.17

Lattice parameters(a, b, c)[Å] 4.0524 4.051 4.052 4.0526 4.0528 4.051 4.0517 4.0506 4.0530

14.3µm

13.7µm

14.0µm

12.8µm (b)

Pa1= 46.84µm

(a)

(a) (b)

(a) (b) (c)

(c)

6

Fig. 8: XRD profiles for 6061 aluminum alloy deformation (3mm distance) with ceramic joint at different

rotational speeds.

Fig.9: XRD profiles for 6061 aluminum alloy deformation (1mm distance) with ceramic joint at different

rotational speeds.

Fig.10: XRD profiles for 6061 aluminum alloy deformation (0.5 mm distance) with ceramic joint at different

rotational speeds

35 45 55 65 75 85

Inte

nsi

ty (

arb

.un

it)

2-Theta (Degree)

●Aluminum

Without deformation

1250 rpm deformation

1800 rpm deformation

2500 rpm deformation ●111

●002

●022

●113

●222

35 45 55 65 75 85

Inte

nsi

ty (

arb

-unit

)

2-Theta (Degree)

Without deformation

1250 rpm deformation

1800 rpm deformation

2500 rpm deformation

●Aluminum

●111

●002

●022 ●113

●222

35 45 55 65 75 85

Inte

nsi

ty (

arb-u

nit

)

2-Theta (Degree)

●Aluminum

2500 rpm deformation

1800 rpm deformation

1250 rpm deformation

without deformation

●111

●002

●022 ●113 ●222

7

Conclusion

Aluminium alloy 6061 was suffered a significant deformation when friction welded joint with

alumina, relative to the base metal alloy. The microstructure after joint could be divided into two

regions: an effected deformation zone (DZ) and unaffected zone (UZ). The grains were fine and

equaled dynamically recrystallized near the weld interface and elongated after the affected

deformation zone region. The effect of increasing rotational speed over the friction-welding joint

was that both the temperature gradient and the axial shortening increased as a result of more mass

being transferred out of the welding interface. The XRD data did show that changes in metal alloy

crystallite size, density and lattice parameters occurred over the test distances at different rotational

speeds. The data obtained from the results show more change, indicating the crystallite size was

influenced significantly during the friction welding experiments. The XRD variables observation of

transient microstructure is important in understanding the complicated friction welding process,

which includes the plastic deformation and dynamic recrystallization at changed rotational speeds.

Acknowledgements

This work is supported by the USM-RU-PGRS grant No. 8042035. The authors would also like to

acknowledge the support provided by the Universiti Sains Malaysia Fellowship scheme.

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