Metallurgical Evaluation of Al 6063 Alloy Using Eddy Current Non-Destructive Testing (ECNDT)

1

Metallurgical Evaluation of Al 6063 Alloy Using

Eddy Current Non-Destructive Testing (ECNDT)

Eng. HOZIEFA W. (1)

, Prof. Ahmed Atlam (1)

, Prof. A. Motagaley(1)

, Prof.M. Elshafaaey(2)

(1)

Mining and Pet. Department, Faculty of engineering, Al azhar University (2)

QA/QC Dep. National Center of Nuclear Safety and Radiation Control

ABSTRACT: The paper examines the solution heat treatment of an extruded 6063 aluminum alloy using

eddy current testing (as a NDT tool). The study shows that the strength and fracture resistance

of this metal alloy can be influenced to an appreciable extent by the solution heat treatment

used in this investigation and can be detected by applying the non-destructive techniques by

relating it to the relative electrical conductivity.

AA 6063 alloy was casted using direct shell technique, and then being extruded to obtain high

strength (T4) condition, specimens were Solution Treated (S.T) at 550o C for 3 hours, then

quenched in fresh water, followed by artificial aging for different times and temperatures.

That treatment leads to microstructures evolution and so different states. The material alloy

under investigation record high strength at specified limits which are (120 o

C for 10 h aging

time and 180 o

C for 6 h that called (T6) condition. where after that limit the strength

decreased. Applying ECNDT technique lead to produce a profile look like that profile results

from hardness and tensile results, so characterization of such properties can be carried on

using ECNDT, and that satisfy aim of work.

As all engineering industrial applications subjected to loads that can considered at the elastic

region so fabricated extensometer was manufactured to represent that loads and measure the

relative conductivity; it can be noticed that the relative electrical conductivity being increased

slightly and so we can see that as displacement increases the relative electrical conductivity

increased.

KEYWORDS: Eddy current testing, Fabricated extensometers, Non-Destructive Testing

1. INTRODUCTION A variety of mechanical properties could be changed during alloy working; these changes can

affect the engineering projects, resulting in expensive repairs. One key component for

ensuring such mechanical properties changes is inspection and monitoring for detection and

characterization of the properties. In-service inspection of alloy can be carried out using eddy

current (EC) bobbin coils, which are adequate for the detection of such changes. [1]

Aluminum is a very light metal with a specific weight of 2.7 kg m-3

, about a third that of steel.

Pure, untreated aluminum is a soft metal with insufficient strength for most engineering

applications. Its strength can be adapted to the application required by modifying the

composition of its alloys and by various thermal and mechanical treatments. [2]

Alloys in the 6xxx series contain silicon and magnesium approximately in the proportions

required for the formation of magnesium silicates (Mg2Si), thus making them heat treatable.

Although they are not as strong as most 2xxx and 7xxx alloys, 6xxx series alloys have good

formability, Weldability, machinability, and corrosion resistance with medium strength.

Alloys in this heat treatable group may be formed in the T4 temper (solution heat treated but

not precipitation heat treated) and strengthened after forming to full T6 (solution heat treated

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plus precipitation heat treated). Uses of Al-Mg-Si alloys include architectural applications,

bicycle frames, transportation equipment, bridge railings, and welded structures. [3]

Aluminum alloys are extensively used as structural materials in the nuclear industry such as

in fuel cladding and reactor cores because of their good corrosion resistance and very low

capture cross-section for fast and thermal neutrons. In the past, the choice of a particular alloy

for use as a structural material was based on the measured properties of the un-irradiated

material. [4]

The ability to control the working stresses level in mechanical components and structures is

an important factor in engineering industries. Evaluation and monitoring of the stress state of

these elements is time consuming, because of the conventional techniques involved. [5]

D.E.Esezobor, S. O. Adeosun [6], examined the solution heat treatment of an extruded 6063

aluminum alloy. The study shows that the strength and fracture resistance of this metal alloy

can be influenced to an appreciable extent by the solution heat treatment used in this

investigation.The ultimate tensile strength (UTS) increases as the solution time increases

from 6 to 20 hours for treatment temperature of 90o C. The maximum UTS (198.8MPa and

188.6 MPa) occur at 120o C and 150

o C respectively at the solution holding time of 10 hours.

While, at 120o C and 10 hrs. the UTS are relatively the same as the as-received specimen,

though the latter exhibits a higher fracture stress. Annealing at 470o C results to lower UTS

value (114.3MPa) and poor fracture resistance (522MPa). [6]

Lifetime extension of components in technical applications is a general task with tremendous

economic benefits. NDT/NDE has developed first attempts for materials characterization

taking into account damage assessment as part of the in service inspection. We have shown in

this work the relation between parameters obtained by eddy currents measurements and

mechanical parameters. The curve representing the impedance or the phase as function of the

elongation or deformation follow a well determined trajectory where the elastic limit and the

start of the plasticity of the material can be detected by the impedance amplitude or the phase

measurement. In case of aluminum the relations between the impedance phase and the load as

function of the elongation has the same shape.

The curve of the phase increases linearly with the load in the elastic regime and therefore it is

possible to determine Young’s modulus. This work shows the ability to determine the

material behavior exposed to external loads in the elastic as well as in the plastic regime by

analysis of eddy current inspection results only. The elastic limit or the start of plasticity can

be detected by the impedance measurement. In the case of aluminum, it is also possible in the

future to evaluate Young’s modulus by the phase analysis. The results are very significant for

the non-destructive mechanical properties determination and useful to be applied for In-

Service Inspection. [7]

2. EXPERIMENTAL PROCEDURES

2.1 Material

The material used in this study is 6063 aluminum alloy with the chemical composition shown in

Table I. The specimens used in this experiment having dimensions of [30*35] and thickness of 8 mm.

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Table I. Chemical Composition of Aluminum Alloy 6063

(Weight Percent) Al 98.9 % Mg 0.457 % Si 0.411 % Fe 0.158 %

Pb 0.0014 % Cu 0.0002 % Cr 0.0008 % Ti 0.012 %

Sn 0.0010 % Mn 0.037 % Ni 0.0037 % Ag 0.0001 %

Sr 0.0001 % V 0.011 % Zn 0.0010 % B 0.0024 %

Be 0.0005 % Bi 0.0010 % Ca 0.0007 % Cd 0.0001 %

Co 0.0010 % Li 0.0001 % Na 0.0006 % P 0.0051 %

2.2 Casting:

The alloy under investigation was direct shell casted, homogenized at 525°± 10° C followed with

formation under extruation to obtain T4 condition.

2.3 Heat Treatment Procedures

2.3.1 Solution Treatment (S.T):

Solution treatment of alloys for at least Three hour (3 h) at 550° C, then quench at fresh water. Proper

solution treating and quenching is essential for the success of this experiment. Measure the hardness of

each specimen immediately after quenching.

2.3.2 Aging Treatments:

Perform the artificial aging treatments on each alloy using the aging temperatures and times which are

suitable. Measure the hardness after each treatment.

The following chart shows heat treatment sequences associate with time and temperature.

Fig.1 Heat Treatment Process Sequence

2.4 Mechanical Testing:

2.4.1 Tensile Test:

Tensile tests were carried to fracture at room temperature using tensile machine of type Tennius Olisen

universal testing machine. The machine has loading range from [0 to 20 ton]. The cross-head speed

(C.H.S) of the machine used in this investigation was (2.5) mm/min. The machine is equipped with a

chart record for the stress-strain curves which is synchronized with the cross-head speed.

2.4.2 Hardness Test:

Using (EQUOTIP2) made in Switzerland, and taking BHN for the heat treated and as received

specimen.

2.5 Microstructure Examination:

- The specimens were grinded and polished using emery papers, starting at 600 till final polishing

stage, followed with suitable etching.

- A licka optical microscope was used to investigate the microstructure evolution

As recieved specimen

Solution Treatment

(S.T)

at 550 °C for 3 hours

Artificial Aging

at 120, 170,180,200,220 °C

for [2, 4,6,8,10 h] for each

4

2.6 Non-Destructive Testing (NDT) Instruments: The measurements of attenuation were carried out using the Elotest B1&B2 “SDM” (Eddy Current

Testing) instrument with bobbin probe. Eddy current test is a method for the inspection of metallic part,

in this technique the probe which is excited with an alternative current, induces eddy current in the part

under inspection.

2-7 Extensometer (stress application at elastic region)

Some tensile specimens were stretched within the elastic limit with different displacement values. Start

at zero till about 2mm. the corresponding loads were estimated from load- displacement diagram.

The extensometer was calibrated due to the tensile machine used; it found that every two loops are

equal to 0.2543 mm displacement. The extensometer was designed and fabricated for testing purposes;

geometry of fabricated extensometer used is shown in figure 2.

Fig.2 Fabricated extensometer with specimen

3. RESULTS & DISCUSSIONS

3.1. Microstructure Evolution

To assure of structure variation due to heat treating of specimens, microstructure evolution was

examined. Whereas the following scans were taken as shown in Fig. 3

Fig.3 the microstructure evolution for different aged specimens at 6h (aging time)

The microstructures of the specimens show that the structure was changed and that due to formation of

precipitates, this precipitates strength the structure and made it harder.

200° C 220° C

170° C 180° C

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The following states were recorded at same aging time (6 h) and different aging temperatures:

- The precipitates were dissolved by solution treating of samples at 550° C for 3 h.

- The precipitates start to grow with increasing aging temperature where its density increased

at 120° C reaching maximum in segregation and density at 180° C and then decrease with increasing

temperature that refer to that high temperatures after that limit start to dissolve precipitates.

- Precipitates were found on the grain boundaries and that make grains more strength, increasing aging

temperature make such precipitates depilates.

- EDX of such states was carried to show phases and precipitates formed to ensure variation and

determine precipitates nature. Where the following figures show such assay.

3.2. EDX (precipitates analysis)

Fig.4 EDX for S.T specimen

Fig.5 EDX for aged (170- 6h) specimen

Fig.6 EDX for aged (180- 6h) specimen

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Fig.7 EDX for aged (200- 6h) specimen

Fig.8 EDX for aged (220- 6h) specimen

The following states were recorded from EDX:

- The precipitates analysis was taken at magnification of (103 x), that to determine the nature of such

precipitates and its effect.

- Precipitates formed due to artificial aging where (Fe, Ni, Mn, Zn) were found at different percentages

and also noticed from microstructures evolution.

- the precipitates assay was about 22.26 % for S.T specimen, then decrease to be 16.14% at (170-6h)

specimen but increase to be 29.69% for (180-6h) , but again decreased to be 22.16% for (200 -6 h),

continue decreasing to be 10.41% for (220-6h).

That mean the precipitates increased to a specified limit which is (180-6h) and then decrease because

of dissolving some of precipitates that affected by high aging temperatures.

3.3. Mechanical properties

3.3.1 Hardness Test:

The hardness results were obtained and represented the results in Fig. 9 to show the hardness profile

for the different aging conditions.

The as casted specimen seems to give low value; where there is no precipitates formed but by

extruation value increased that because of particles elongation, again the value being lowered that due

to precipitates dissolved by solution treating. Applying the artificial aging lead to a slight increase in

hardness value as the precipitates and the new intermediate phase formed. That increase in hardness

value is at determined limits as a peak formed at 120° C and at 180° C then decrease in hardness value

is again take place as the precipitates start to dissolve.

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Fig.9 The hardness profile for the different aging conditions.

3.3.2 Tensile test:

The tension results for the samples under investigation are shown at Fig.10, where the tension profile

show low UTS for as cast material, and high UTS value for the extruded specimen as result of particles

elongation, UTS decreased gradually by solution treating of samples as the phases dissolved , UTS

increased due to aging treating of samples.

The maximum UTS (188.6MPa and 220.8 MPa) occur at 120° C and 180° C respectively at the

solution holding time of 10hours and 6 hours. While, at 180° C and 6 hrs, the UTS are relatively the

same as the as-received specimen, though the latter exhibits a higher fracture stress. Those results are

typical with D.E.Esezobor, Senior Lecturer, and S. O. Adeosun, Lecturer (from Department of

Metallurgical and Materials Engineering University of Lagos, Akoka- Yaba, Lagos, Nigeria). [6]

Fig.10 The tension profile for the different aging conditions.

3.4 NDT Testing (using ECNDT):

Using the Elotest B1, with specifying following standards, ¥= 0°, ƒ= 1.7 MHZ, ƪ = 16 dB and encoding

the aluminum as standard. Obtaining the following the results that presented at Fig. 11 to show the

conductivity profile for the different aging conditions.

35

45

55

65

75

85

as c

ast

Extr

ud

ed

sp

ec.

ST

BH

12

0-2

h

BH

12

0-4

h

BH

12

0-6

h

BH

12

0-8

h

BH

12

0-1

0h

BH

17

0-2

h

BH

17

0-4

h

BH

17

0-6

h

BH

17

0-8

h

BH

17

0-1

0h

BH

18

0-2

h

BH

18

0-4

h

BH

18

0-6

h

BH

18

0-8

h

BH

18

0-1

0h

BH

20

0-2

h

BH

20

0-4

h

BH

20

0-6

h

BH

20

0-8

h

BH

20

0-1

0h

BH

22

0-2

h

BH

22

0-4

h

BH

22

0-6

h

BH

22

0-8

h

BH

22

0-1

0h

BH

N

aging temp.- time

BHN Poly. (BHN)

100

125

150

175

200

225

250

As

Cas

tex

t. s

pec

.S.

T1

20

-2h

12

0-4

h1

20

-6h

12

0-8

h1

20

-10

h1

70

-2h

17

0-4

h1

70

-6h

17

0-8

h1

70

-10

h1

80

-2h

18

0-4

h1

80

-6h

18

0-8

h1

80

-10

h2

00

-2h

20

0-4

h2

00

-6h

20

0-8

h2

00

-10

h2

20

-2h

22

0-4

h2

20

-6h

22

0-8

h2

20

-10

h

U.T

. (M

Pa)

Aging Time-Temperature

U.T (Mpa) Poly. (U.T (Mpa))

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Fig.11 Electrical conductivity profile for the different aging conditions.

The conductivity is started at zero for the received specimen because it was taken as a datum,

conductivity increased due to increasing of the aging temperature and time, reaching maximum value at

the range of 180° C for 6 h aging time. Slightly decreased after this value, begin to increase again

at 220° C for 6 h. such changes in the conductivity values of the specimens referred to the structure that

changed due to precipitates formed by artificial aging.

Checking the hardness profile and compare it with the obtained conductivity values we see that the two

curves are typical in that at maximum hardness value the highest electrical conductivity was achieved

and that can be seen at fig.12.

The two curves can be noted that are same in the behavior so by animation we can detect that the two

results are same and by calibration of the alloy used in industrial plans to the standard and then using

eddy current techniques, we can obtain the harness profile due to variable times and temperatures

which (in service).As the hardness increased the electrical conductivity increased and as decreased it

was decreased. So that the hardness profile is look like the conductivity profile.

These results can satisfy the aim of the research as we can detect the change of mechanical properties

by using a NDT technique.

Fig.12 The conductivity profile vs. hardness for the different aging conditions.

0

2

4

6

8

10

as c

ast

rolle

d

S.T

12

0-2

H

12

0-4

H

12

0-6

H

12

0-8

H

12

0-1

0H

17

0-2

H

17

0-4

H

17

0-6

H

17

0-8

H

17

0-1

0H

18

0-2

H

18

0-4

H

18

0-6

H

18

0-8

H

18

0-1

0H

20

0-2

H

20

0-4

H

20

0-6

H

20

0-8

H

20

0-1

0H

22

0-2

H

22

0-4

H

22

0-6

H

22

0-8

H

22

0-1

0H

real

tive

ele

ctri

cal c

on

du

ctiv

ity

Aging Temperature - Time

relative elec.conductivity Poly. (relative elec.conductivity)

0

15

30

45

60

75

as c

ast

rolle

d

S.T

12

0-2

H

12

0-4

H

12

0-6

H

12

0-8

H

12

0-1

0H

17

0-2

H

17

0-4

H

17

0-6

H

17

0-8

H

17

0-1

0H

18

0-2

H

18

0-4

H

18

0-6

H

18

0-8

H

18

0-1

0H

20

0-2

H

20

0-4

H

20

0-6

H

20

0-8

H

20

0-1

0H

22

0-2

H

22

0-4

H

22

0-6

H

22

0-8

H

22

0-1

0Hre

alti

ve e

lec.

co

nd

. -

-- B

HN

Aging Temperature - time

relative elec.conductivity BHN Poly. (BHN)

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3.5. Extensometer (stress application in elastic region)

Using the fabricated extensometer to detect change in relative electrical conductivity due to change in

displacement due to yield stress, Using the Elotest B2, with specifying following standards:

¥= 0°, ƒ= 104 KHZ, ƪ = 38 dB, and encoding the aluminum as standard, the following results were

obtained. Fig.13, show that the relative electrical conductivity being increased slightly and so we can

see that as displacement increases the relative electrical conductivity increase and that refer to particles

elongation due to tension that make conductivity more active.

Fig.13 Extensometer displacement vs. relative elec. conductivity

That can be applied in the field by applying eddy current testing at the beginning of the project and

obtaining the result (which is considered as a default), periodically in service inspection should be

carried on and so any alteration can be recorded, relating this alteration with the displacement can give

a good imagination of displacement happen.

4. CONCLUSIONS

- This work has studied the material characteristics (taking AA 6063) using a NDT technique

(which is ECNDT).

- To obtain the highest mechanical properties of AA 6063, artificial aging is carried on to a

specified limit which is (aging temperature of 120° C at 10h and 180° C and at 6 h). Where

after that limit precipitates start to dissolve and values being decreased.

- Measuring relative conductivity of the alloy under investigation lead to obtain a profile which

like that represent the hardness and tensile profiles, and satisfy aim of work.

- As all engineering industrial applications subjected to loads that can considered at the elastic

region so fabricated extensometer was manufactured to represent that loads and measure the

relative conductivity; it can be noticed that the relative electrical conductivity being increased

slightly and so we can see that as displacement increases the relative electrical conductivity

increased.

- That can be applied in the field by applying eddy current testing at the beginning of the project

and obtaining the result (which is considered as a default), periodically in service inspection

should be carried on and so any alteration can be recorded, relating this alteration with the

displacement can give a good imagination of displacement happen.

- Detecting the mechanical behavior of the alloy by using eddy current testing is the important

step as it gives the chance for detection of mechanical properties during in service.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.2 0.4 0.6 0.8 1 1.2 1.4

real

tive

ele

c. c

on

du

ctiv

ity

displacement (mm)

realtive elec. Conductivity Linear (realtive elec. Conductivity)

11

REFERENCES

[1] L.Obrutsky, B.Lepine,J.Lu, R.Cassidy and J. Carter, “Eddy current technology for heat exchanger

and steam generator tube inspection", published by Atomic Energy of Canada Limited, Chalk River

Laboratories, Chalk River, Ontario, Canada, p.1, 2004.

[2] S. H. Avner, “Introduction to Physical Metallurgy", published by Mc Craw-hill Inc. NY, p.361,

364, 1964.

[3] “Properties and selection nonferrous alloys and special- purpose material", ASM handbook,

formerly tenth edition, Vol.2, p. 3-15, 32, 37-40, 44-57, 1991.

[4] R. T. King and A. Jostsons, Material Trans. A 6a, p.863, 1975.

[5] B. Raj, V. Moorthy,T. Jayakumar and K. B. S. Rao,”Assessment of Microstructures and

Mechanical behavior of Metallic Materials Through Non Destructive Characterization”, International

Materials Reviews ,Vol. 48 ,N°5, October 2003.

[6] D.E.Esezobor, S. O. Adeosun. “Improvement on the Strength of 6063 Aluminum Alloy by Means

of Solution Heat Treatment”, Material Science and Technology (MS&T), p.645-647, 2006.

[7] M. Zergoug, G.Kamel, N.Boucherou, “Mechanical Stress Analysis by Eddy Current Method”,

ECNDT, December 2006.and the Journal of American Science, Vol. 4, p.4, 2008.