Indian Society for Non-Destructive Testing Hyderabad Chapter

Proc. National Seminar on Non-Destructive Evaluation Dec. 7 - 9, 2006, Hyderabad

NDE-2006

Acoustic Emission Response of Ti6al4v Alloy in Different Heat Treatment

Conditions During Tensile Testing

A. Sharma1, M.I. Junaidh

1, K.K. Purushothaman

1, C.P. Kotwal

2, J. Paul

1,

Shalini Tripathi1, B. Pant

1 and A.S. Sankaranarayanan

1

1 Vikram Sarabhai Space Centre (ISRO), Trivandrum-695 022 2Satish Dhavan Space Centre (ISRO), Sriharikota-24

Abstract

Acoustic Emission (AE) Technique is a unique Non Destructive Testing method being used as a global online monitoring tool for detection, location and

characterization of various kinds of active defects. This paper illustrates the attempt

to find the AE response of Ti6Al4V Alloy in different heat treatment conditions

during tensile testing. Ti6Al4V alloy is tested in mill annealed, beta annealed, and

solution treated & aged conditions using specifically designed specimens. The

specimen were tested in a 10 T capacity universal testing machine. The test progress

was monitored using strain gauges and AE sensors and the data was acquired through

a Data Acquisition System (DAS). Various AE parameters were monitored and

analysed in addition to the strain gage data. The pre-yielding phase, yielding phase,

plastic deformation phase and the failure zone were identified and correlated with the

strains. The present study attempts to demonstrate the prediction possibility of

yielding of the alloy with respect to different microstructures well before the failure

of the specimen, which in the turn can generate suitable acceptance criteria for the

evaluation of the hardware in real time.

Keywords: Acoustic emission, Heat treatment, Microstructure, Strain, Titanium

1. Introduction

Ti6Al4V alloy, the workhorse of

titanium alloy family, is extensively used

in aerospace sector owing to its very high

specific strength coupled with excellent

corrosion resistance. This alloy is widely

used in Indian Space Programs both for

launch vehicles and satellites applications,

as high pressure gas bottles and propellant

tanks.

Although ductile materials have been

considered to be low emitters of AE, it is

possible to find the response of Ti6Al4V

alloy during tensile testing. The Ti6Al4V

alloy is used in different heat treatment

conditions for different applications.

Moreover characteristic of a material to

be a good or bad emitter of AE is governed

by the microstructure as well as the

presence of inclusions and second phase

particles [1].

1.1 Ti6Al4V Alloy

Ti6Al4V alloy is heat treatable and

schematic phase diagram of the Ti6Al

alloy with Vanadium addition is shown in

Fig. 1.

Three types of basic microstructures are

feasible in this alloy:

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α - β Processed Structure – results by

processing and heat treatment of material

in two-phase region.

β Processed Structure – results by

processing and heat treatment in single-

phase β region.

STA Structure – results due to solution

treatment and aging.

These microstructures are associated

with different combinations of mechanical

properties.

1.2 Acoustic Emission (AE)

Acoustic Emission is the high

frequency stress wave generated by the

rapid release of the strain energy that

occurs within a material during crack

growth, plastic deformation or phase

transition. AE signal results primarily due

to movement of dislocations

accompanying plastic deformation, grain

boundaries formation, growth of twins,

generation & propagation of cracks,

fracture of brittle inclusions & surface

films, phase transformations (in metals),

fiber breakage & de-lamination (in

composites), micro seismic and seismic

activity (in geological materials) etc.

An AE source causes a dynamic force/

stress field change at the particular

location. The force field change at this

location is propagated as a mechanical

disturbance throughout the structure. The

piezoelectric transducer (sensor) mounted

at a particular location on the structure,

detects the disturbance and produces an

output voltage. This output voltage is

amplified using a pre-amplifier and then

processed using signal processor.

2. Experimental Method

2.1 Technical Approach

The present study is taken up to predict

the possibility of yielding of the Ti6Al4V

alloy with respect to different

microstructures as in normal and different

heat treatment condition well before the

failure of the specimen during the tensile

test by the means of Acoustic Emission

Technique.

2.2 Heat Treatment of Ti6Al4V Alloy

The starting plates were 9 mm thick

two phase rolled Ti6Al4V alloy, before

imparting the following heat treatments:

2.2.1 Mill Annealed (MA)

In this process, Ti6Al4V alloy

specimens were preheated in a furnace to a

temperature of 730°C, soaked for 2 hours,

furnace cooled up to 565°C and

subsequently air-cooled. Microstructure of

MA exhibits equiaxed α in transformed β

matrix.

2 .2.2 Beta Annealing (BA)

In this process, mill annealed

specimens were loaded in a furnace

preheated to 870ºC and then heated to

1020ºC. At this temperature specimen

were soaked for 1 hour and subsequently

air-cooled. Microstructure of BA exhibits

fully transformed β structure.

2.2.3 Solution Treating & Aging (STA)

In this process, mill annealed specimens

were loaded in a furnace preheated to

800ºC and then heated to 950ºC.

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Table 1: Instrumentation Details

Specifications of Universal Testing Machine

Make Lloyed Instruments, U. K.

Model LR100K

Capacity 10 Tonnes

Grips Friction / Pin

AE Instrumentation Specifications

Acoustic Sensor Type R 15 – 150 KHz Resonant (single ended)

Pre amplifier PAC – 1220 AST with Filter (20 – 1200 KHz)

Data acquisition system MISTRAS, Physical Acoustic Corporation

Frequency 20 – 400 KHz

Sampling Rate 4 MHz

Threshold 30 dB

Strain Gauge Specifications

Type 1 Type 2

Type KFG – 2 – 350 – C1 – 23 FLA – 6 – 11

Temperature compensation for ALUMINUM ALUMINUM

Gauge length 2 mm 6 mm

Gauge resistance 349.4 ± 0.6 Ω 120 ± 0.3 Ω

Gauge factor 2.15 ± 1.0% 2.12 ± 1.0%

Table 2: Loading Cycle

Stage Load (Tonnes) Cross Head Speed (mm / min)

Load Hold Time (sec)

1 0.50 0.5 60

2 1.00 0.5 60

3 0.50 1.0 30

4 1.00 0.5 60

5 1.50 0.5 60

6 2.00 0.5 60

7 0.50 1.0 30

8 2.00 1.0 60

9 2.25 0.5 60

10 2.50 0.5 60

11 0.50 1.0 30

12 2.50 1.0 60

13 2.75 0.5 60

14 3.00 0.5 60

15 Up to Failure 0.5 -

Table 3: Mechanical Properties of Ti6Al4V Alloys

Ultimate Tensile Stress (N/mm

2)

Sl. No.

Specimen Type

1 2

Hardness* (Rc)

1 Mill Annealed (MA) 967.3 1042.8 27.25

2 Beta Annealed (BA) 930.2 991.8 30.75

3 Solution Treated & Aged (STA) 1143.3 1128.1 34.75

* Average of 4 values

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Fig. 1: Phase Diagram of Ti6Al alloy with Vanadium Addition

Fig. 2: Microstructures of Ti6Al4V Alloy in Different Heat Treatment Conditions

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Fig. 3: Tensile Test Specimen

Fig. 4: Specimen with Strain Gage, AE Sensor with Pre-Amplifier

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Fig. 5: AE Performance of Mill Annealed Ti6Al4V Specimen during Tensile Test

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Fig. 6: Load & Extension of Mill Annealed Ti6Al4V Specimen during Tensile Test

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Fig. 7: AE Performance of Beta Annealed Ti6Al4V Specimen during Tensile Test

Acoustic Emission Response of Ti6al4v Alloy

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Fig. 8: Load & Extension of Beta Annealed Ti6Al4V Specimen during Tensile Test

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Fig. 9: AE Performance of Solution Treated & Aged Ti6Al4V Specimen during Tensile Test

Acoustic Emission Response of Ti6al4v Alloy

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Fig. 10: Load & Extension of Solution Treated & Aged Ti6Al4V Specimen during Tensile Test

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Fig. 11: Stress & Strain Curves for Ti6Al4V Alloys in Different Heat Treatment Conditions

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Fig. 12: Effect of Heat Treatment on Ti6Al4V Alloys

At this temperature specimen were soaked

for 1 hour. After soaking, the specimens

were quenched in water. Subsequently

these specimens were aged at 480oC for 4

hours and then air-cooled. Microstructure

of STA exhibits fine precipitation of α

phase in equi-axed α plus transformedβ.

The microstructures of Ti6Al4V alloys

are shown in Fig. 2.

2.3 Specimen Preparation

Six number of Ti6Al4V Alloy 7mm

thick plates were used as starting material.

Out of these two numbers were mill

annealed, two were beta annealed and

remaining two were solution treated and

aged. All these plates were machined to

the configuration as shown in the Fig. 3.

The deciding factors in finalizing the

configuration of specimen were: thickness

(to limit max load of failure within the

capacity of the machine), bonding of strain

gauge at center (failure must be ensured at

center), diameter of AE sensor (width of

the second step), and use of pin grips for

testing machine.

2.4 Instrumentation Details

The details of tensile testing machine,

AE instrumentation and strain gauges used

for this study are given in Table 1.

The specimen along with the AE

sensor, strain gauge and preamplifier is

shown in Fig. 4.

2.5 Loading Cycle

The loading cycle as followed during

the tensile testing is given in Table 2. This

loading cycle facilitates the recording of

AE response at various load levels. The

load was raised to the next load level and

brings back to the previous load level to

observe the Kaiser Effect [2]. The load

was hold for some time to observe the

Roll-Over Effect.

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3. Results and Discussions

Total six specimens were tested,

designated as Mill Annealed (MA 1 & MA

2), Beat Annealed (BA 1 & BA 2) and

Solution Treated & aged (STA 1 & STA

2).

AE performance of all the 6 specimens

is given in the form of graphs. For each

specimen, six different AE parameters

Amplitude (dB), Duration (ms), Event,

Count, Energy, and RMS (V) are plotted

against the time. Refer Fig. 5, 7 & 9 for

these graphs.

Loading details are given in the form of

graphs of Load & Extension, Load & Time

and Extension & Time. Refer Fig. 6, 8 &

10 for these graphs.

Strain Gauge values were recorded at

different times. For Stress & Strain

Curves, Refer Fig. 11.

For the first specimen (MA – 1),

friction grips were used. But considerable

noise level (95 dB) was observed. Friction

grips were changed to pin grips for

subsequent testing.

During yielding of the specimens de-

bonding of strain gauge (2 mm gauge

length) occurred due to the bending

present in the specimen. Hence no strain

reading was recorded beyond the yielding.

To overcome this, one of the specimens

(STA – 2) was instrumented with longer (6

mm gauge length) strain gauge. In this

case de-bond of strain gauge was not

observed and the strain gauge readings

were obtained beyond yielding.

All specimens except STA2 registered

low level (<50 dB) AE up to 2T load. Both

STA specimens registered few AE (<65

dB) up to 2 T due to straightening of the

slight bend in these specimens due to heat

treatment.

For every specimen, during 1 minute

hold at each load step, ‘Roll Over’ was

observed, confirming absence of active

flaw in the specimen. Refer Fig. 5.

For every specimen, during repeat

loading negligible AE activity was

observed proving ‘Kaiser Effect’ Refer

Fig. 5.

For every specimen, during elastic

loading very low AE activity was

observed.

For every specimen, AE activity

increased considerably during

commencement of yielding. Max. 100 dB

amplitude signals were observed (Refer

Fig. 5, circled area).

For every specimen, during plastic

deformation decrease in AE activity was

observed.

For every specimen, failure of

specimens was also registered by AE

(Refer Fig. 5, rectangle area).

The results obtained on Ti6Al4V alloy

samples in different heat treatment

conditions are given in Table 3.

Effect of heat treatment was noticed as

can be seen from all the graphs (Refer Fig.

5 – 10). Number of AE hits increased

considerably from MA to BA and to STA

(Refer Fig. 12) and also STA has higher

UTS as compared to BA & MA. This

phenomenon can be correlated to confirm

the heat treatment condition of the alloy).

4. Conclusion

The study promises the prediction of

yielding in Ti6Al4V alloys. However for

developing a well established model, more

specimen level studies are to be carried

out. One of the important inferences made

by this study is that all the phases during

the loading, i.e., elastic loading, yielding,

plastic loading and failure of the

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specimens were clearly identifiable.

Yielding in these alloys can be correlated

with the sudden increase in AE signals.

Also effect of heat treatment can be

correlated with the number of hits. From

the present study it can be hinted that as the alloy get strong and harder, the

corresponding AE activity also increases.

With further testing it is possible to

establish a well-proved relation.

6. References

1. Ronnie K. Miller. Acoustic Emission

Testing (Non Destructive Testing

Handbook, Volume 5). American Society

for Nondestructive Testing, 1987; 13.

2. James R. Mathews. Acoustic Emission,

Non Destructive Testing Monographs and

Tracts. Gordon & Breach Science

Publishers, 1983; 2.