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CHAPTER 5

EFFECT OF MULTIPLE-PASS FSP

5.1 INTRODUCTION

This chapter discusses the effect of number of passes on the

microstructure and mechanical properties of friction stir processed AS7U3G

aluminum alloy. Threaded cylindrical pin profiled tool with concave shoulder

is used to process the alloy. Double pass (with 100% overlapping on the top

of the first pass) and triple pass (with 100% overlapping on the top of the

second pass) FSP experiments were performed. The tensile tests,

metallographic examinations and hardness measurements of the FSPed

samples were carried out as per the section 3.4 mentioned in the third chapter.

5.2 RESULTS

5.2.1 Tensile properties

The longitudinal tensile properties, such as yield strength, tensile

strength, percentage of elongation of the friction stir processed (FSP)

materials were evaluated by testing three specimens in each condition. Data

comparing the tensile behavior of as cast and FSPed materials are presented in

Table 5.1 and Figure 5.1.Significant microstructural refinement, homogeneity

and densification by multiple- FSP in AS7U3G casting resulted in remarkable

improvement in the tensile properties. The tensile strengths of the single pass,

double pass, and triple pass FSPed materials are significantly higher than that

of the as cast alloy. The tensile strength of triple pass FSPed material is 273

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MPa which is 2.25 times higher compared to that of the as cast alloy of 121

MPa. Friction stir processing of this alloy enhanced the elongation from 1.8%

to 10%. Fourth pass has slightly reduced the UTS and the Percentage of

elongation. As friction stir process is a stress induced solid state process,

macro cracks were observed in the fourth pass.

Table 5.1 Mechanical properties of unprocessed and friction stir

processed materials

Condition Yieldstrength(MPa)

Ultimate tensile

strength(MPa)

Elongation in 25mm gauge length (%)

Unprocessed basemetal

109 121 1.8

Single pass FSP 196 218 5

Double pass FSP 232 253 8

Triple pass FSP 268 273 10

Fourth pass 239 258 7.5

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Figure 5.1 Load Vs Displacement diagram of cast and friction stir

processed materials

5.2.2 Microstructure

Scanning electron microscopy examination was carried out to study

the influence of the number of overlapping passes on the microstructure of

FSP region. Figure 5.2 illustrates typical SEM micrographs of as cast base

alloy (a), single pass FSP stir zone (b), double pass FSP stir zone (c) and

triple pass FSP stir zone (d). These SEM micrographs distinctly reveal the

significant favorable effect of friction stir processing on the size, shape, and

distribution of Si particles with increasing number of passes. The stirring

action of the FSP at the nugget zone fragmented the large primary and

secondary phase particles to very fine particles. The stirring action also

compacted the porosities followed by solid state fusion which closed all

voids. However the size of the particles in the nugget zones is different due to

0

1

2

3

4

5

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6Displacement, mm

Load Vs Displacement

1Pass

2Pass

3Pass

4Pass

BM

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the influence of number of passes. The fineness of particles is in the

increasing order with the increasing number of passes. This proves that

overlap of various passes continues to homogenize the microstructure and its

properties. The nugget microstructure of the triple pass FSPed material

consists of very fine eutectic Si particles which are uniformly distributed

throughout the aluminum matrix (Figure 5.2d) than double pass and single

pass FSPed materials.

Figure 5.2 SEM micrographs of as cast alloy (a), single pass FSP stir zone (b), double pass FSP stir zone (c) and triple pass FSP stir zone (d).

Table 5.2 shows that the size of the Si particle in the cast AS7U3G

aluminum alloy is in the range of 1.12 to 23.36 µm, with high aspect ratio of

3.70. Table 5.2 also shows that after single pass of FSP, the average Si

particle size and aspect ratio values were drastically reduced to 1.17± 1.00 µm

and 1.45±1.33, respectively. Further reduction in the average Si particle size

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values to 0.86 µm and0.80 µm were obtained in the case of double pass and

triple pass FSP samples.

Table 5.2 Microstructural characterization (size and aspect ratio of Si

particles) of base metal and FSP zones

ConditionSilicon Particle diameter, µm Aspect Ratio

Min Max Mean Min Max Mean

Base Metal 1.12 23.36 5.55±3.73 1.08 17.50 3.70±2.45

Single pass 0.28 11.89 1.17±1.00 1.02 11.67 1.45±1.33

Doublepass

0.14 9.43 0.86±0.78 1.05 12.22 1.43±1.14

Triple Pass 0.09 4.23 0.80±0.63 1.02 10.12 1.35±1.36

Figure 5.3 shows the energy- dispersive spectra of base metal

and the stir zones of single-pass, double-pass and triple-pass FSPed samples.

The energy- dispersive spectra of single-pass FSP zone showed the presence

of all the alloying elements, but the amount of magnesium quantified is very

less. In the stir zones of double-pass and triple-pass samples, magnesium was

not detected by EDS. The heat produced during FSP might have caused the

evaporation of magnesium.

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Figure 5.3 Energy-dispersive spectra of stir zones in (a) Base metal,

(b) single-pass

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Figure 5.3 (Continued) (c) double-pass, and (d) triple-pass FSPed samples

In order to study the composition of the particles in the stir zone of

double-pass FSPed sample, one large particle in the matrix was selected and

subjected to EDS (Figure 5.4). The EDS analysis showed the existence of Al-

Si and CuAl2 phases in the aluminum matrix. The spectra also showed the

absence of magnesium and hence the absence of Mg2Si.

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Figure 5.4 EDS of a marked particle (large particle) in the stir zone of

double-pass FSPed sample

Similarly one of the fine particles in the matrix was selected and

subjected to EDS (Figure 5.5). The spectra showed the existence of Al-Si,

CuAl2 phases and absence of Mg2Si phase.

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Figure 5.5 EDS of a marked particle (fine particle) in the stir zone of

double-pass FSPed sample

5.2.3 Hardness

The effect of number of passes on the distribution of hardness

across FSP region is presented in Figure 5.6. Soft spots found in the casting

due to porosity and aluminum dendrite cores were eliminated by friction stir

processing. The distribution of hardness values also appears narrower in the

stir zone. This is consistent with its more uniform microstructure. The average

hardness values in single pass, double pass, and triple pass FSPed AS7U3G

alloy are lower than that of the as cast alloy. Microhardness values indicate a

softening of the processed material at the friction stir processed zone due to

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inherent nature of the process. With an increase in number of passes, an

increase in the microhardness values was observed in the processed materials.

Figure 5.6 Microhardness profile across the FSP region

5.2.4 Fracture analysis

Figure 5.7a-d displays the fractographs of the unprocessed and the

processed materials. In all conditions studied, the fracture surfaces of the

processed specimens reveal a ductile fracture. It is known that the presence of

flakes promote a tendency towards brittle fracture (Nakata et al 2006).As can

be seen from Figure 5.7 a, that the fracture surface of the base metal reveals

the presence of flakes like structure. In addition, large lamellar structures

were observed containing possibly Mg2Si and Al-Si eutectic.

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Figure 5.7 SEM fractographs of tensile specimens for (a) as cast alloy, (b) single pass FSP, (c) double pass FSP, (d) triple pass FSP samples

Fracture in the processed material is transgranular with medium

to good developed surfaces. Figure 5.7d reveals fine ductile fracture with a

few featureless regions. Fine dimples are a characteristic feature of highly

ductile materials. Consistent with these observations, triple pass specimen

displayed highly ductile behavior before fracture. In contrast, Figure 5.7b, and

c represents a less ductile fracture.

5.3 DISCUSSION

5.3.1 Effect of multiple-pass FSP on tensile properties

From the experimental results (Table 5.1), it is very clear that

multiple-pass FSP resulted in remarkable improvement in the longitudinal

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tensile properties of AS7U3G aluminum alloy. The factors increasing the

tensile strength for the multiple-pass FSPed materials can be as follows: (1)

elimination of casting defects (such as porosity); (2) uniform distribution of

the fine Si particles; (3) grain refinement of aluminum matrix. In all the

conditions studied, the specimens showed a considerable increase in ultimate

tensile strength and ductility values. The triple pass FSPed material displayed

superior tensile properties than the other materials.

Double pass FSP with a 100% overlap produced a better effect on

stir zone microstructure (Ma et al 2006c). Five-pass FSP sample (not 100%

overlap, but the overlap between the passes was one-half of the pin diameter)

in various microstructural regions exhibited strength and ductility values

comparable to those achieved in the single-pass FSP sample (Ma et al

2006b).They also reported that multiple-pass FSP with a 50% overlap is a

feasible route to perform microstructural modification on large-sized

aluminum castings.

The material flow behavior will be different for cast alloys and

wrought alloys due to large differences in ductility. Usually cast alloys will

have lower ductility compared to wrought alloys due to the presence of

porosity, inclusions and higher amount of silicon. This will reduce the

ductility of cast alloys and subsequently affect the material flow behavior

under the action of rotating FSP tool. All the processed specimens invariably

showed considerable increase in ductility compared to the base metal. The

elongation of triple pass FSPed sample is 10%, which is 5.5 times higher than

that of the as cast unprocessed alloy. This could have been due to grain

refinement in the stir zone. The ultimate tensile strength and ductility

generally improve as porosity levels and the microstructure scale decrease. In

the present set of experiments increasing the FSP pass resulted in enhanced

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mechanical properties, due to the reduced Si particle size and porosity level,

and increased dissolution of CuAl2 and Mg2Si.

5.3.2 Effect of multiple-pass FSP on microstructure

Multiple-pass FSP resulted in significant refinement in the

microstructure of the alloy. Virtually all traces of dendritic solidification

microstructure were eliminated throughout the stir zone. With increasing

number of FSP passes, the refinement of Si particles increased. Double pass

FSP produced a pronounced effect in refining the stir zone microstructure and

the break-up of the Si particles is further intensified in the triple pass FSP.

Triple pass FSP produced a stupendous effect on microstructural refinement,

homogeneity and densification of Si particles. The reduction in the average

size of Si particles after single pass FSP of AS7U3G alloy is 79%, which

further refined to 84.6% and 85.6% after double pass and triple pass FSP

respectively.

In the case of hypoeutectic A356 aluminum alloy, Ma et al (2006b)

have reported about 86% reduction in the average size of Si particles after one

pass FSP. They have also reported that the 50% overlapped multi-pass FSP

did not influence the size, aspect ratio and distribution of Si particles. Rao et

al (2009) reported that double pass FSP with 100% overlapping on the top of

the first pass itself had a pronounced effect on size, shape, and distribution of

Si particles. The reduction in the average size of the Si particles after single

pass FSP is 98%., which further refined to 99% after second pass.

Similarly, Nakata et al (2006) produced a fine grain structure of 2 –

3 µm in ADC12 die casting alloy via multi-pass FSP. For AA2219 Al alloy,

single-pass FSP resulted in an average grain size of 6.2 µm, but in the

subsequent passes (two-pass, three-pass) the average grain size showed a

marginal increase (Surekha et al 2008). Nascimento et al (2009) reported

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that for AA7022- T6 alloy, single-pass FSP reduced the grain size from 160

µm to an average grain size of 7.1 µm and this remained constant

independently of the number of passes and overlap ratios tested. The present

results indicate that, with increase in number of passes, a decrease in size of

Al-Si eutectic particles (densification of particles is in increasing order) is

obtained. This proves that 100% overlap of various passes continues to

homogenize the microstructure in the stir zone. In comparison, for the sample

processed with three passes with 100% overlapping, a more homogeneous

processed area was obtained with average Si particle size of 800 nm.

5.3.3 Effect of multiple-pass FSP on microhardness

It was found that the hardness in the FSPed region increased with

an increase in number of passes. The observed increase in microhardness

values can be attributed to the reduced grain sizes. By the Hall-Petch

relationship, Hv= Ho+kH d -1/2, where Ho and kH are appropriate constants.

Because Hv is proportional to d -1/2, the finer the grain size is, the higher the

hardness value is. The effect of frictional heat on microstructure during single

pass FSP results in generation of dynamic recrystallized grains having low

dislocation density (Santella et al 2005 and Karthikeyan et al 2009), reduction

of dislocation density having greater effect on softening than the hardening

effect of increased grain-boundary area, through dynamic recrystalization.

The subsequent increase in hardness upon two pass FSP and three pass FSP

could be due to the significant increase in dislocation density and frequency

of sub micron silicon particles in the stir zone.

Rao et al (2009) reported that the average hardness values in both

single pass FSP and two pass FSP friction stir processed Al-30Si alloy were

lower than that of the base metal. Surekha et al (2008) studied the effect of

multiple-pass FSP on AA2219 aluminum alloy and reported that the hardness

in the nugget region increased with increase in number of passes. They also

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reported that in all the studied conditions, the nugget showed a lower hardness

compared to the base material. The microhardness behavior of friction stir

processed AS7U3G aluminum alloy is consistent with this behavior pattern.

5.4 CONCLUSION

In this investigation, cast AS7U3G (Al-Si-Mg (Cu)) aluminum

alloy was friction stir processed (FSP) with multiple passes (100% overlap)

and the following important conclusions are derived;

(i) Triple-pass (3P) FSP produced a stupendous effect on

microstructural refinement, homogeneity and densification of

Si particles.

(ii) It was found that the hardness in the friction stir processed

region increased with increase in number of passes.

(iii) The tensile strength of triple pass friction stir processed

material is 273 MPa which is 2.25 times higher compared to

that of the as cast alloy.