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International Journal of Mechanical Engineering and Technology (IJMET)

Volume 9, Issue 6, June 2018, pp. 667–679, Article ID: IJMET_09_06_076

Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=6

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication Scopus Indexed

EFFECT OF MULTI-PASS FRICTION STIR

PROCESSING ON MECHANICAL PROPERTIES

OF AA6061-T6

Esther Akinlabi, Kayode Oyindamola, Oluwole Olufayo

Department of Mechanical Engineering Science, University of Johannesburg, South Africa

Michael Agarana

Department of Mechanical Engineering Science, University of Johannesburg, South Africa

Department of Mathematics, Covenant University, Nigeria

ABSTRACT

Samples with one through five passes with 100% overlap were produced using

friction stir processing (FSP) technique to study the effect of multi-pass FSP on the

microstructure and mechanical properties of AA6061-T6 alloy. The evolving

microstructure and mechanical properties after each successive FSP pass were

studied in detail. Constant traverse and rotational speeds were used for processing.

The resulting microstructural evolution and the grain sizes after each FSP pass was

seen to be strongly dependent on the processing parameters, the thermal cycle and the

presence of second-phase precipitates. The base material was found to have better

mechanical properties than all processed samples.

Keywords: Aluminum alloy, mechanical properties, microstructure, multi-pass

friction stir processing.

Cite this Article: Esther Akinlabi, Kayode Oyindamola, Oluwole Olufayo and

Michael Agarana, Effect of Multi-Pass Friction Stir Processing on Mechanical

Properties of AA6061-T6, International Journal of Mechanical Engineering and

Technology, 9(6), 2018, pp. 667–679

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=6

1. INTRODUCTION

Friction stir processing (FSP) is a technique developed to modify the properties of a metal

through severe plastic deformation. It entails passing a non-consumable rotating tool

containing a specifically designed pin and shoulder through a metal sheet or plate. FSP is

based on the same approach as friction stir welding (FSW), a solid-state joining technique,

developed by The Welding Institute (TWI) in the United Kingdom in 1991 [1]. Since its

invention, FSP has continually been improved to achieve desired mechanical properties. It has

been effectively applied to a variety of aluminum, magnesium, and copper alloys. Several

studies have shown that FSP is known to improve the mechanical properties of materials. Sun

Effect of Multi-Pass Friction Stir Processing on Mechanical Properties of AA6061-T6


and Apelian [2] reported that there was a high grain refinement and improved mechanical

properties in their study of the microstructural evolution during the single-pass FSP of

aluminum cast alloys. Hashim et al [3] applied a single-pass FSP on 2024-T3 aluminium alloy

and recorded an improvement in hardness and tensile strengths of the material. This was

reported to be as a result of a significant grain refinement of about 77% decrease in grain size.

Furthermore, a study of the effect of single-pass FSP on mechanical properties and the

microstructure of Al-Zn-Mg-Cu alloy by Salman [4] also revealed that FSP result in

significant grain refinement, elimination of casting defects, and an improvement in the

hardness, tensile strength, and ductility of the aluminum alloy. However, some studies

reported deterioration in the mechanical properties after the FSP process. An example is the

reduction in material hardness reported by Gan et al [5] in their study of the evolution of the

microstructure and hardness of rolled pure aluminum after FSP, even though equiaxed and

fully recrystallized grains was achieved. They observed a local material softening in the

friction processed zone (FPZ) and recorded that the decrease in hardness is due to the

dissolution of precipitates during FSP. There was also a similar report by Weglowski [6] on

the reduction in hardness when they applied a single FSP pass on AlSi9Mg aluminum alloy.

Few researchers have applied multi-pass FSP on aluminum materials to study its effect on

microstructural evolution. Chen et al [7] carried out a three-pass FSP with 100% overlap on

Al-5083 aluminum alloy sheet and recorded a refinement in grain size due to recrystallization

in the first pass, but no substantial change in grain size after subsequent overlapping passes.

All the multi-pass friction stir processed (FSPd) samples exhibited reduced hardness, yield

strength and tensile strength in the stir zone (SZ) compared to the base material and no

significant changes with subsequent multiple passes. El-Rayes and El-Danaf [8] studied the

influence of multi-pass FSP on the properties of thick commercial 6082-T651 AA plates.

They carried out one to three passes with 100% overlap and discovered that the single-pass

FSP caused dynamic recrystallization of the stir zone leading to equiaxed grains, but there

was a decrease in the hardness and tensile strength after subsequent multiple passes. The

hardness reduction was reported to be as a result of the SZ softening which accompanied the

increase in the number of passes. This softening was attributed to larger grain size after

subsequent FSP passes. It was also suggested that the reduction in tensile strength was due to

the over aging effect on the previous passes by the subsequent ones. Krishna and

Satyanarayana [9] also reported a reduction in yield strength, tensile strength, hardness, and

elongation with an increase in the number of passes when they carried out three-pass FSP

with 100% overlap on Al6331+SiC composite. There was a reduction in grain sizes and

silicon flakes after subsequent FSP passes. The deterioration in the mechanical properties was

attributed to the precipitate dissolution and the limited re-precipitation by the thermal cycles

of FSP. Yang et al [10] reported contrary results when they studied the effects of four-pass

FSP with 100% on the microstructure and mechanical properties of Al3Ti/A356. There was a

continuous significant reduction in grain size from base material to the fourth pass and the

yield strength, tensile strength, and ductility also improved after every subsequent pass

because of the grain refinement. The variety and inconsistency observed in the results showed

a strong dependency of FSPd materials mechanical properties distinctly on the workpiece

material and FSP processing parameters irrespective of the number of FSP passes.

In the present study, the effects of five-pass FSP on the microstructural evolution and

mechanical properties of AA6061-T6 were investigated. The alloy in its T6 condition is of

great interest because of its tendency to lose some strength in the weld region after

joining/welding - because its solution is heat-treated and artificially aged.

Esther Akinlabi, Kayode Oyindamola, Oluwole Olufayo and Michael Agarana


2. METHODS

The FSW machine used to produce the welds is a custom designed computer controlled 2-

Axis FSW machine with the tool in a horizontal position and the specimen held in vertical

position at the Indian Institute of Science (IISC), Bangalore, India. This machine was

developed with the help of ETA technologies, Bangalore, India. It has the capability to vary

the tool rotational speed, traverse speed and plunge depth during a process. This platform is

shown in Fig.1.

Figure 1 The FSW Platform

The parent material used in this research work was AA6061-T6. The dimension of the test

coupon for each plate was 250 x 210x 6 mm and the length of the welds produced was 200

mm. The chemical composition of the parent material was confirmed, using a spectrometer,

and these were found to conform to the standard AA6061-T6 specifications [11]. Table 1

shows the chemical composition of the parent material.

Table 1 Chemical composition of AA6061-T6 used in this study

Element Weight %

Si 068

Fe 0.49

Cu 0.21

Mn 0.08

Mg 0.84

Cr 0.06

Ni 0.01

Zn 0.07

Ti 0.07

Al 97.40

Ag Balance

The FSP tool used for this research was a High-Density Steel (HDS) tool, with a concave

shoulder diameter of 25mm and a cylindrical pin with tapered pin diameter 6-7mm, pin length

5mm (Fig. 2).



Figure 2 High-Density Steel (HDS) Tool

Among the various tool materials available, HDS was chosen due to its high strength,

hardness, availability and low cost. The tool geometry and parameters were also selected

based on the knowledge from the review of literature. A constant 3° tilting angle was applied

to the tool, and all FSP was performed in position control mode, with a plunge depth of

5.3mm. The plunge depth ensured that the necessary downward pressure is achieved, and the

tool fully penetrates the weld, while the tool tilt angle ensured that the rear of the tool is lower

than the front as reported in the literature review. A constant rotational and transverse speed

of 1600rpm and 40mm/min respectively was used throughout the FSP processes. These were

optimum process parameters earlier obtained for FSP of AA6061 [12]. Samples were

sectioned using a Concord Wire Electric Discharge Machine (EDM) employing a 0.2mm

diameter molybdenum cutting wire.

Optical microscopy was conducted using Olympus BX51M and Olympus SZX16 optical

microscopes. Olympus BX51M was used to observe the microstructures, while the Olympus

SZX16 was employed in observation of the samples macrograph. Digital output was captured

and processed using the Olympus Stream Essential software. The TESCAN VEGA3 Scanning

Electron Microscope (SEM) setup was used to study and compare the microstructures

observed in the base material. The Vega TC software was used in acquiring the image on the

SEM. The Vickers microhardness values were measured using the Time Vickers

Microhardness Hester TH713 by Beijing Cap High Technology Co. Ltd according to ASTM

384-16 standard [13]. The transverse tensile tests were performed using the servohydraulic

Instron tensile testing machine model 1195 to obtain the ultimate tensile strength and

elongation. The tensile samples were tested according to ASTM E8M-13 standard [14]. The

tensile properties of the materials were evaluated by testing three specimens in each condition

to quantify the tensile and yield strengths. The percentage of the elongation of the FSPd

samples was evaluated by measuring the final length of the failed specimens to determine the

ductility of the samples.

3. EXPERIMENTS

The FSW machine has an already installed customized program for the control of the FSW

process which provides the interface for inputting the process parameters to create and

execute the FSW/FSP process. 5 multi-pass FSP with 100% overlap were conducted with

increasing FSP passes conducted on different workpiece. The processed samples, in multi-

pass cases, were cooled down to room temperature before successive FSP passes were

conducted. For optical microscopy (OM), Samples of size 25 x 6 x 6 mm at the processed

zone were sectioned, and mounted in polyfast thermoplastic resin with a Struers hot mounting

machine and identified according to the number of FSP passes with an engraving tool. The

samples were then grinded and polished using a Struers polishing machine, and cleaned with

distilled water. The samples were mounted, grinded and polished with the advancing side of

the weld always to the right following the standard metallographic procedures [15]. After this,

samples were chemically etched to reveal microstructure. At the initial stage of the research,

the samples were etched with Keller’s reagent (190ml distilled H2O, 5ml HNO3, 3ml HCL,

and 2ml HF) but the grains were not visible, hence Weck’s reagent (100ml distilled H2O, 4g

KMnO4, 1g NaOH) was used. The samples were observed under the microscope for

microstructural characterization. Microstructures of FSP samples were obtained for the base

material (BM), SZ, thermo-mechanically affected zone (TMAZ), and heat affected zone

(HAZ) for comparison. The grain sizes of the different zones were carried out, according to

the standard test method for determining average grain size: ASTM E112 – 12[16]. For

microhardness, the samples were placed on the testing platform. When the hardness testing



machine is in operation, a pyramid-shaped diamond penetrates the surface of the material with

the set force, generating an indentation into the material. The dimension, D1, and D2, of the

indent are proportional to the depth penetrated by the diamond, and the depth reached is

directly related to the hardness of the material. The hardness profiles were obtained across the

process zones in the FSP samples, to investigate local variations in mechanical properties as a

functional of the experimental variables. The measurements were taken in the as-polished

conditions, across the cross-sections of the process zones with a load of 200g and a dwell time

of 15secs. The indentations were taken at 1.5mm intervals on the sample, with the

indentations manually focused and the hardness measurement digitally displayed. Before

beginning the tensile testing, the computer system connected to the machine was set up by

inputting the necessary information of gauge length and width of the specimen. The computer

system was then prepared to record data and output necessary load-deflection graphs. The test

was conducted at room temperature by gripping the ends of the samples in the tensile test

machine and then loaded until at a constant cross head speed until failure. An extensometer

was used to measure the strain of the samples during the experiment at an extension rate of 5

mm/min and a gauge length of 25 mm, with a maximum load of 100kN. The load-deflection

curve was shown on the computer screen as a visual representation, with the data collected

using the customized Instron Bluehill2 software. The tests were repeated 3 times to check for

consistency within the data to achieve better accuracy.

4. RESULTS

The average grain size in the base material was found to be 6.69 µm while the average size of

precipitates (Mg2Si) present was found to be 6.57 µm using the linear intercept method. The

microstructures of the various zones of the processed plates in this work are illustrated in Fig.

3 showing each zone and the various number of FSPd samples. It was observed that

microstructures of the FSPd regions are different from that of the base material. The FSPd

zones exhibited a much more distinct spherical grain morphology compared to the base

material. The BM had yield strength of ~311 MPa, ultimate tensile strength (UTS) of ~338

MPa and a ductility of 15%. It was clear that all the FSPd samples show a reduction in the

UTS and yield strength compared to BM samples. The samples average UTS and yield

strengths of all the FSPd samples are between the ranges of 170-200 MPa and 165-190 MPa

respectively. However, there was no significant change in the elongation percentages of the

processed samples compared to the BM. There was a range of 1-2% reduction in the

elongation percentage from the single-pass sample to the four-pass sample, while the five-

pass sample exhibit the same elongation percentage as the BM showing that FSP does not

have a significant effect on the ductility of the material using the processing parameters

employed in this study. A graphic representation of these properties is presented in Fig. 4.

Fig. 5 shows a graphic representation of the mean hardness values of all the processed

samples. The highest hardness values were seen in the BM, similar to the report by Al-

Fadhalah et al [17]. The microhardness profiles of the BM and the FSPd samples are

presented in Fig. 6 to fully understand the variation of the hardness values across the samples.



Figure 3 OM micrographs of FSPd samples (a) SZ (b) TMAZ (c) HAZ

Figure 4 Tensile Properties

Figure 5 Average Hardness Values



Figure 6 Microhardness Profile

5. DISCUSSION

The SZ of the single-pass sample consists of nearly equiaxed grains as a result of dynamic

recrystallization. Grain growth and a complete dissolution of the second-phase precipitate into

the matrix are also clearly visible in the micrograph. The complete dissolution of the second-

phase precipitates is attributed to the inability of the identified precipitates to withstand high

temperatures. El-Rayes and El-Danaf [8] reported that the precipitates are not temperature

resistant and rapidly dissolve when exposed to high temperatures resulting from FSP. Woo et

al [18] also reported that the frictional heating resulting from FSP causes the dissolution of

the precipitates. During FSP, the SZ experiences intense plastic deformation and thermal

exposure with peak temperatures almost up to the melting point of the alloy [7]. The size of

the grains in the TMAZ and HAZ in the single-pass samples seems to appear similar to the

grain size in the SZ. However, there are differences in grain size across the different zones.

The result shows almost 70% increase in the grain size of the SZ after a single FSP pass,

with 23% and 17% increases in the TMAZ and HAZ respectively, showing that the TMAZ

and HAZ of the base material have experienced lower strains and strain rates as well as lower

peak temperatures compared to the SZ. It is noteworthy that the consequential grain growth in

the SZ is defined by the factors impacting on the nucleation and growth of the dynamic

recrystallization. Mishra and Ma [19] reported that the FSP parameters, tool geometry,

material chemistry, workpiece temperature, vertical pressure, and active cooling significantly

impact on the size of the grains in the SZ. The grain growth observed in this sample can be

classified as abnormal grain growth (AGG), as the resulting microstructure is dominated by a

few very large grains which usually results from a subset of grains growing at a high rate at

the expense of their neighbours. AGG occurs when there is an inhibition in the normal growth

of the matrix grains, and when the temperature is high enough to allow a few special grains to

overcome the inhibiting force and to grow disproportionately. All two-pass sample

microstructural zones show a refinement in grain size. This could be attributed to dynamic

recrystallization after the second pass. A grain refinement of 25%, 11%, and 14% decrease in

grain size in the SZ, TMAZ, and HAZ respectively compared to the single-pass sample is

recorded. This dynamic recrystallization is usually attributed to the plastic deformation and

the high temperatures from FSP [8]. It is noteworthy that in the micrograph of the two-pass

sample SZ, it can be observed that there is a competition between the dynamic

recrystallization and concurrent recovery. Dynamic recrystallization also occurred in the

three-pass sample resulting in further refinement in the grain sizes. The various

microstructural zones show a 34%, 27%, and 18% reduction in size in the SZ, TMAZ, and

HAZ respectively compared to the two-pass sample. Platelet-shaped re-precipitation of the

second-phase precipitates is clearly visible in the HAZ of the sample. The precipitates were

more relatively homogenous in the HAZ with an average size of 9.76 µm, which is much



larger than those found in the base material. This limited re-precipitation could be attributed

to the thermal cycle [9]. A reduction in temperature because of reduced friction during the

third pass FSP is assumed to have favoured the re-precipitation. A similar two stage

mechanism involving full dissolution of the precipitates followed by re-precipitation was

reported during FSW on 6056 aluminum alloy by Cabibbo et al [20].

The four-pass sample shows grain growth in all the microstructural zones. There was a

47%, 32%, and 18% increase in grain sizes in the SZ, TMAZ, and HAZ respectively. There

was a complete dissolution of the second-phase precipitates observed in the three-pass

sample. The behavior in the transition from the three-pass sample to the four-pass sample is

similar to the transition of the base material to the single-pass sample with respect to second-

phase precipitates dissolution and increase in grain size. The four-pass sample also exhibited

nearly equiaxed distinct spherical grains in all the FPZ zones like the first sample. However,

in this case, it is believed that normal grain growth (NGG) has occurred, as the grain growth

seems to have occurred in a uniform manner. A significant coarsening and growth of the

grains is seen in all the five-pass sample FPZ zones; the SZ seems to be coarser than the

TMAZ and HAZ. This coarsening could be attributed to the additional/accumulated thermal

cycles which the plate has experienced. Sinhmar et al [21] stated a similar reason for grain

coarsening after multiple FSP passes. It is apparent that the various mechanisms acting at

different stages of the microstructure evolution after every successive multiple FSP pass are

related to the strain, strain rate, and thermal cycle which the material undergoes at each stage.

The significant high strength in the base material is attributed to the presence of the

second-phase precipitates. Shankar et al [22] reported that precipitation treatable aluminum

alloys such as Al6061-T6 which is peak aged (T6 temper) have an optimum distribution of

precipitates that ensures the greatest strength of the material. A similar high strength is

recorded in a study by Ravikumar et al [23] on the characterization of the mechanical

properties of Al6061-T6 after FSW. The yield strength is seen to decrease uniformly as the

UTS decreases. A significant reduction in the tensile properties after the first FSP pass could

be attributed to the dissolution of the precipitates resulting in softening which occurred in the

SZ, and a reduction in pre-existing dislocations. Al-Fadhalah et al [17] reported that age-

hardened aluminum alloys depend strongly on precipitate size and distributions rather than on

grain size. This is displayed in the tensile values of the second and third passes, where there

was a reduction in the grain size, and a reduction in the UTS and yield strength, against the

expected increment in the values according to Hall-Petch relation [24]. It could be concluded

that these reductions of UTS and yield strength are due to the overaging effect which the

subsequent passes cause to the previous one [8]. The re-precipitation seen in the three-pass

sample is ~1.5 times bigger than the precipitates in the base material. It is noteworthy that the

size of the precipitates has a major influence on how the precipitates influence the mechanical

properties of materials. There was no significant change in the UTS and yield strength after

the fourth FSP pass, but a sharp increase in these properties was observed after the fifth FSP

pass. This could be attributed to the completely homogenous FPZ microstructure obtained

after the pass.

The reduction in the hardness of the FSPd samples compared to the BM can be attributed

to softening from the complete dissolution of the second-phase precipitates. The BM has an

average hardness of ~99 ± 7.4 HV reaching a peak hardness value of 107 HV. This is

relatively high when compared to the 35% drop in hardness value to ~64 ± 2.6 HV after the

first FSP pass. The single-pass sample shows a relatively uniform microhardness distribution

with less scatter. An increase in the softening area in the sample after the second pass led to a

reduction in the microhardness value to an average value of ~59 ± 3.1 HV. The two-pass

sample show a distribution of hardness values lower in all regions when compared to the



hardness distribution of the single-pass sample. However, both samples’ peak hardness values

were observed to be on the AS. Jiang and Kovacecis [25] reported that this behavior was as a

result of the substantial fluctuation related to the heterogeneous constitution of the nugget.

Yadav and Buari [26] claimed that hardness variations may result from different

microstructural features at different locations in the FPZ. They explain that the flow of

material from the RS to the AS during FSW/FSP gives rise to gradients in temperature, strain

and strain rate in the stir zone.

There was no significant change in the average hardness values of the three-pass sample

compared with the two-pass sample. An average hardness value of ~59 ± 6.3 HV was

recorded. However, a significant peak hardness up to 60 HV is observed towards the AS

which could be attributed to the re-precipitation observed in the three-pass sample HAZ. Even

though the re-precipitation has a negligible effect on the tensile properties, this shows that it

has an effect on the hardness properties of the material. From this observation, it could be said

that the negligible effect of re-precipitation on the tensile properties of a material is not an

indication that the precipitates will not influence the hardness properties of the material. A

significant observation in the behavior of the microhardness values is its increase as the

samples near a homogenous FPZ. A 4.4% increase in the average hardness value is observed

in the four-pass sample, which further increased by an 8.8% increase in the five-pass sample.

The four-pass and five-pass samples have an average hardness value of ~61 ± 3.7 HV and ~67

± 8.7 HV respectively. The average hardness value, as well as the distribution of the hardness

values in the fifth pass as shown in Fig. 5 and 6 respectively can be seen to be higher than all

the values obtained from the preceding numbers of FSP passes. However, this is still far

below the hardness values obtained from the BM. From the observations, it could be said that

material flow and mixture have a strong effect on the hardness of the material making the

microstructural homogeneity of the FPZ a very important factor in determining the hardness

of a material. The increase in hardness after the fourth and fifth FSP passes could be

attributed to the re-precipitation of the second-phase strengthening precipitates.

6. CONCLUSIONS

FSP has been performed on 6061-T6 aluminum alloy by applying one through five 100%

overlapping passes with the main emphasis on the effects of the multi-pass on the evolving

microstructure and mechanical properties of the material. The BM has more improved

mechanical properties when compared to all FSPd samples, irrespective of the number of FSP

passes. This is attributed to the presence of unaltered second-phase precipitates and T6

condition (heat-treated and artificially aged).

The microstructural evolution and the resulting grain sizes are strongly dependent on the

processing parameters, the thermal cycle, and the presence of second-phase precipitates in the

matrix. Single-pass FSP led to a non-homogenous FPZ, decreased strength, hardness and

ductility, and abnormal grain growth while multi-pass FSP led to a completely homogenous

FPZ after the fifth FSP pass, attributed to accumulative plastic strain. This indicates that the

number of FSP passes have a significant effect on the homogeneity of FPZ in a material. The

homogenous FPZ led to improvements in previously reduced mechanical properties

(hardness, and tensile strength, and ductility) of AA6061-T6.

The thermal cycle has more influence on the mechanical properties than the grain size, the

increase in the number of FSP passes accumulates more heat which leads to a complete

dissolution of hardening second-phase precipitates. This dissolution impairs the mechanical

properties. An increase in the grain sizes is also observed after dissolution of the second-phase

precipitates (as observed after the first and fourth FSP passes).



ACKNOWLEDGEMENT

The authors would like to express their sincere appreciation to the Department of Mechanical

Engineering - Indian Institute of Science - India for supporting this work. The main author

will also like to acknowledge University of Johannesburg for the Global Excellence Stature

(GES) scholarship award. This work was supported by the University of Johannesburg

Research Committee (URC).

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effect of multi-pass friction stir processing on ... · the samples were etched with keller’s...

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