High Temperature Deformation of Friction Stir Processed 7075

8/11/2019 High Temperature Deformation of Friction Stir Processed 7075

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High temperature deformation of friction stir processed 7075

aluminium alloy

P. Cavalierea,*, A. Squillaceb

aDepartment of Ingegneria dellInnovazione, University of Lecce, Via per Arnesano I-73100 Lecce, ItalybDepartment of Materials and Production Engineering Faculty, University of Naples Federico II, Italy

Received 21 January 2005; received in revised form 18 April 2005; accepted 18 April 2005

Abstract

The mechanical and microstructural properties of 7075 aluminium alloy resulting from Friction Stir Processing (FSP), into

sheets of 7 mm thickness, were analysed in the present study. The sheets were processed perpendicularly to the rolling direction;

the tensile mechanical properties were evaluated at room temperature in the transverse and longitudinal directions with respect

to the processing one. Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in

order to analyse the superplastic properties of the recrystallized material and to observe the differences from the parent material

as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was

studied, in the parallel direction, by means of tensile tests in the temperature and strain rate ranges of 150500 8C and 102

104 s1 respectively, electron microscopy (FEGSEM) observations were carried out to investigate more closely the fracture

surfaces of the specimens tested at different temperatures and strain rates.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Friction stir processing; Microstructure evolution; Hot tensile tests; FEGSEM

1. Introduction

Friction Stir Welding (FSW) is being targeted bymodern industries for structurally demanding applica-

tions providing high-performance benefits [1]. FSW

has been shown to strongly decrease severe distortion

and residual stresses compared to the traditional weld-

ing processes[24].The Frictioned zone consists of a

weld nugget, a thermo-mechanically affected zoneand a heat affected zone. The process results in

obtaining a very fine and equiaxed grain structure in

the weld nugget causing a higher mechanical strength

and ductility[57].Jata and Semiatin[8]showed that

the microstructure in the weld nugget zone evolves

through a continuous dynamical recrystallization pro-

cess, the strong grain refinement produced by the

process leads the microstructure to the fine dimen-

1044-5803/$ - see front matterD 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matchar.2005.04.007

* Corresponding author. Tel.: +39 832320324; fax: +39

832320349.

E-mail address: [email protected] (P. Cavaliere).

Materials Characterization 55 (2005) 136142


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The tensile response in the transverse direction of

the FSP 7075 aluminium alloy is shown inFig. 5.The

curve exhibits a classical behaviour; the mechanical

properties,compared to the parent metals, are reported

inTable 1.The material exhibits very good properties

of yield stress, UTS and ductility from a global point-

of-view even if the processed material exhibits a

lower 0.2% proof stress and elongation with respect

to the base metal, the mechanical results are very good

considering the drastic conditions to which the mate-

rial is subjected during the Friction Stir Process, the

elastic modulus results are very different with respect

to the parent material. All the specimens fractured in

the HAZ zones. This demonstrates the classical be-

haviour of this kind of material in which, from a

microstructural point-of-view, the mechanical re-

sponse of the centre is higher with respect to the

parent material and the HAZ because of the large

differences in grain dimensions. In this zone, in fact,

the mean grain equivalent diameter measured from

optical images was close to 4 Am.

The tensile response of the material was also in-vestigated by cutting small samples from the centre of

the samples. Such specimens were tested at room

temperature and higher temperatures up to 300 8C.

The room temperature tensile behaviour of the

material in the nugget zone is shown in Fig. 6. The

strength and ductility of the material increased strong-

ly in the nugget zone with respect to the material in

the transverse direction thanks to the uniform and very

fine structure consequent to the friction stir process.

The behaviour of the material at high temperatures

was studied by performing tensile tests in the temper-

ature range 150300 8C and 103 s1 strain rate. The

true stress vs. true strain response of the material is

shown in Fig. 7. The flow stress decreases with

increasing temperature and the strain to fracture

increases with increasing temperature up to very

high levels at 300 8C and 103 s1.

The very high strain to fracture values reached by

the studied material at high temperatures result in the

0.000 0.005 0.010 0.015 0.020 0.025 0.0300

50

100

150

200

250

300

350

400

7075 FSP 7mm

TrueStress[MPa

]

True Strain

Fig. 5. True stress vs. true strain tensile behaviour of the FSP sheets

perpendicularly to the processing direction.

Table 1

Mechanical properties in transverse direction of the FSP 7075 alloy

compared with the parent material

Material ry (MPa) UTS (MPa) Elongation (%)

7075 T6 524 587 11

7075 T6 FSP 325 (62%) 450 (77%) 7 (64%)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.140

50

100

150

200

250

300

350

400

450

500

7075 FSP Nugget

y=355 MPa

TrueStress[MPa

]

True Strain

Fig. 6. True stress vs. true strain tensile behaviour of the FSP plates

parallel to the processing direction.

0.00 0.25 0.50 0.75 1.000

50

100

150

200

250

300

350

400

450

T=300 C

T=200C

T=150 C

T=25 C7075 FSP 7mm

10-3s

-1

TrueStress

[MPa]

True Strain

Fig. 7. True stress vs. true strain tensile curves at 103 s1 strain

rate at the lower temperatures investigated.

P. Cavaliere, A. Squillace / Materials Characterization 55 (2005) 136142 139


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very attractive superplastic behaviour exhibited by

such alloys after the Friction Stir Process.

Normally, to improve the formability leading to

superplastic properties for aluminium alloys belong-ing to the 7XXX series is complex involving overa-

ging and multiple warm rolling phases with re-heating

after each step and final forming at 104 s1 strain

rate. The widespread use of superplastic forming of

aluminium alloys is limited by the low optimum strain

rate for superplasticity. The possibility of obtaining a

material with a very fine and uniform grain structure

which could be directly formed in superplastic condi-

tions at lower strain rates results in a very useful tool

for the modern aerospace and automotive industries.

Considering that the material when tested in warmforming conditions exhibited high ductility, when the

deformation required for most forming operations is

lower (200%) the results are attractive also for normal

forging operations in which the high strength re-

quired is obtained thanks to the very fine starting

microstructure.

The results of hot tensile tests at different tempera-

tures and different strain rates are shown in Fig. 8.

There is no steady state exhibited by the curves for all

the temperatures and strain rates investigated; with

increasing temperature, the peak stress is shifted to

higher strains.

The flow stress as a function of the different strain

rates investigated in the temperature range 400500

8C is shown in Fig. 9, a sigmoidal relationship was

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.000

10

20

30

40

50

60

70

80

90

100

475 C

425 C

500 C

450 C

400 C

350 C

7075 FSP 7mm

10-3s

-1

TrueStress[M

Pa]

True Strain

0.0 0.5 1.0 1.5 2.0 2.50

5

10

15

20

25

30

35

40

5*10-4s-1

5*10-2s

-1

10-4s

-1

10-3s

-1

10-2s

-1

TrueStress[MPa]

True Strain

AA7075 FSP 500 C

(a)

(b)

Fig. 8. True stress vs. true strain tensile curves at 103 s1 strain

rate (a) and 500 8C (b) at the higher temperatures investigated.

1E-4 1E-3 0.01 0.1

10

100

AA7075 FSP Flow Stress

400 C

425 C450 C475 C500 C

FlowStress[MPa

]

Strain Rate [s-1]

Fig. 9. Flow stress behaviour as a function of strain rate calculated

in the superplastic regime exhibited by the FSP material.

1E-4 1E-3 0.01 0.1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

400 C

425 C450 C475 C500 C

StrainRateSe

nsitivity[m]

Strain Rate [s-1]

Fig. 10. Strain rate sensitivity (m) variation as a function of strain

rate calculated in the superplastic regime exhibited by the FSP

material.

P. Cavaliere, A. Squillace / Materials Characterization 55 (2005) 136142140


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observed between stress and strain rate at high testing

temperatures. The strain rate sensitivity variation,

obtained by the hot tensile values at a true strain=1,

is shown in Fig. 10, the m value increases upon

increasing the strain rate from 104 s1 to 103

s1 and then decreases for the higher strain rates

investigated for all the temperatures chosen in thepresent study. The higher measured value (m =0.75)

was observed for a strain rate of 5*104 s1 at 500

8C; over the ranges 450500 8C and 5*104 s1 to

5*103 s1 temperature and strain rate respectively,

the Friction Stir Processed material exhibits superplas-

tic properties.

In the understanding of the microstructural effects

on fracture properties some FEGSEM observations

were performed on the fractured surfaces of the tested

samples.

The fracture surface of the FSP 7075 alloy tested intension in the longitudinal direction was covered with

a broad population of microscopic voids of different

Fig. 11. Fracture surface of the material tested perpendicularly to the

travelling direction showing voids due to the FS process.

Fig. 12. Microscopic voids observed in the specimen tested at 300

8C and 103 s1.

Fig. 13. Fracture surfaces of the specimens tested at 400 8C (a) and

500 8C (b) at 5*104 s1 strain rate.

P. Cavaliere, A. Squillace / Materials Characterization 55 (2005) 136142 141


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sizes and shapes (Fig. 11), at room temperature the

material showed some ductility with fracture, the

observations performed by employing FEGSEM

revealed local ductile mechanisms.Two different types of dimples were observed,

those immediately close to the voids and those asso-

ciated with the second phase coarse particles and

precipitates which were much smaller and shallower.

At the higher temperatures investigated, many dif-

ferent behaviours were observed in the material; at 300

8C many microscopic voids were observed on the

tensile fractured surfaces (Fig. 12a) around such

voids a broad population of very fine dimples were

recognized (Fig. 12b) for all the strain rates investigat-

ed. The fractographic analyses of the tensile testedsurfaces at higher temperatures (400 8C) revealed a

bimodal size distribution of dimples (Fig. 13a) and

the presence of some smooth zones, by increasing the

test temperature (500 8C) the size distribution of ductile

dimples became more uniform on all the observed

surfaces with a smaller and shallow aspect (Fig. 13b).

4. Conclusions

The 7075 aluminium alloy were successfully Fric-

tion Stir Processed in 7 mm thick sheets. The resultingmicrostructure was widely investigated by optical mi-

croscopy showing the grain structure differences

resulting by the process. The mechanical properties

of the material were evaluated by tensile tests in both

the longitudinal and transverse directions with respect

to the processing one resulting in retaining the high

mechanical properties with respect to the parent ma-

terial. The potential superplastic properties of the fric-

tion stirred material were evaluated by hot tensile tests

at different strain rates obtaining high values of the

strain rate sensitivity at the higher strain rates investi-gated in the present study. The fracture surfaces of the

specimens tested at different conditions of tempera-

tures and strain rates were extensively investigated by

using a FEGSEM microscope revealing the defects

resulting from the Friction process and the microscop-

ic mechanisms taking place during hot deformation.

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High Temperature Deformation of Friction Stir Processed 7075

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