Ultrasonic Evaluation of TiAl And40 Cr Diffusion Bonding Quality Based on Time-scale Characteristics Extraction

8/13/2019 Ultrasonic Evaluation of TiAl And40 Cr Diffusion Bonding Quality Based on Time-scale Characteristics Extraction

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Ultrasonic evaluation of TiAl and 40Cr diffusion bonding quality based

on time-scale characteristics extraction

Yilin Luan a,n, Tao Sun a, Jicai Feng b, Tie Gang c

a School of Mechatronics Engineering, Harbin Institute of Technology, Harbin, Chinab School of Materials Science & Engineering, Harbin Institute of Technology at Weihai, Weihai, Chinac State Key Lab of Advanced Welding and Jointing, Harbin Institute of Technology, Harbin, China

a r t i c l e i n f o

Article history:Received 25 September 2010

Received in revised form

12 April 2011

Accepted 13 July 2011Available online 27 July 2011

Keywords:

Ultrasonics

Diffusion bonding

Dissimilar materials

Continuous wavelet transform

Time-scale amplitude and phase

a b s t r a c t

To solve the problem of ultrasonic pulse-echo method in the evaluation of kissing bond and unbond inTiAl and 40Cr diffusion bonding, a characteristics extraction algorithm was proposed. The algorithm was

based on continuous wavelet transform to convert ultrasonic TiAl and 40Cr diffusion bonding interface

signals into time-scale domain. The ultrasonic tests were performed by an ultrasonic C-scan imaging

system using a 10 MHz focused transducer. The time-scale amplitude and phase of the interface signals

were calculated and analyzed to distinguish the kissing bond and the unbond from the perfectly

bonded interface. The kissing bond can be detected by the scale-dependent amplitude combined with

phase variation and the unbond can be measured by the opposite phase. The amplitude and phase

characteristics were extracted to reconstruct the amplitude and phase characteristics images for TiAl

and 40Cr diffusion bonding specimens evaluation. The amplitude and phase characteristics images are

effective in the evaluation of bonding quality.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Diffusion bonding has been considered as a potential welding

method and increasingly used in the field of aerospace and

industry, which has many advantages such as high performance,

significant cost and weight savings, and low requirement for the

weldability of materials [1,2]. However, imperfections such as

kissing bond and unbond may occur at the interface due to

improper surface preparation and upset bonding conditions [3].

These defects can degrade bonding strength, especially fracture

toughness and fatigue strength [46]. Thus, it is necessary to

develop non-destructive evaluation of diffusion bonding.

The interfacial imperfections are parallel to the specimen

surface, which is a suitable position for ultrasonic test [7].

A variety of ultrasonic methods have been applied in the evalua-tion of bonding quality, such as pulse-echo method, laser ultra-

sonic system[8], guided waves [911], and nonlinear ultrasonic

measurement [1214]. Pulse-echo method is the most popular

technique among these ultrasonic methods. Palmer et al. [15]

described the application of ultrasonic reflectivity for the char-

acterization of copper diffusion bonds with different bonding

qualities. Ultrasonic reflection coefficients at 10 MHz were

correlated with the ultimate tensile strength. Kato and Abe [16]measured diffusion bondings of steel to titanium plates to obtain

the relationships among bonding strength, state of bonding

interface, and two major components derived from ultrasonic

testing. Considerable progress was made by Greenberg et al. [17]

in developing a real-time system for the monitoring of bonding

process by analyzing the amplitude ratio and attenuation of

acoustic waves. In other efforts, the C-scan images at the bonding

interface were used to calculate the ratio of non-bonded area of

diffusion bonded joints of mild steel, combined with impact tests

for threshold level determination [18]. Similar technique was

applied to field-assisted diffusion bonding joints to assess the

mechanical quality by increasing the ultrasonic frequency up to

20 MHz[19].

In general, the unbonds in similar diffusion bondings arereadily detectable by normal incidence wave since the ultrasonic

wave will be reflected at the defects whereas passing through the

perfectly bonded regions. The bonding quality can be assessed by

the amplitude of the reflected signals. However, the kissing bonds

are only a few micrometers in size, which result in weak

reflection. The bonding joints appear to be flawless under ultra-

sonic inspection [20]. As for the dissimilar diffusion bondings,

some ultrasonic energy is still reflected from the perfectly bonded

interface due to the effect of impedance mismatch between

materials to be bonded[7]. It is difficult to distinguish the defect

signals from the interface signals so that the bonding quality

cannot be assessed by the amplitude of the reflected signals.

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/ndteint

NDT&E International

0963-8695/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ndteint.2011.07.008

n Corresponding author. Tel./fax: 86045186413115.

E-mail addresses: [email protected] (Y.L. Luan),[email protected] (T. Sun),

[email protected] (J.C. Feng),[email protected] (T. Gang).

NDT&E International 44 (2011) 789796
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In this paper, we focus on the evaluation of TiAl and 40Cr

diffusion bonding quality. A time-scale characteristics extraction

algorithm is proposed to measure TiAl and 40Cr diffusion bonding

interfacial imperfections. The algorithm is based on continuous

wavelet transform to analyze the amplitude and phase variation

of ultrasonic interface signals in the time-scale domain. The

authors shall demonstrate that the defects can be assessed by

the time-scale amplitude and phase characteristics.

2. Theoretical background

Classical boundary condition for ultrasonic wave interaction

with welded or perfectly bonded interface assumes that stress

and displacement across the interface is continuous. When an

ultrasonic wave is normally incident to such an interface, the

reflection coefficient R12 is given by[21]

R12 Z2Z1Z2 Z1

1

whereZ1and Z2 are the acoustic impedances of the materials on

either side of the interface. Note that the reflection coefficient

from the perfectly bonded interface is just a function of the

impedances.

If the bonding is imperfect and the size of imperfections is

considerably smaller than the wavelength of ultrasound, the inter-

face can be modeled by a set of distributed springs. The ultrasonic

wave interaction with such an interface can be described using

spring boundary condition. The reflection coefficient of normal

incidence ultrasonic wave from imperfect interface is given by[22]

R12 Z2Z1 io=knZ1Z2Z2 Z1io=knZ1Z2

2

whereo is the angular frequency of the ultrasonic wave and kn isthe normal interfacial stiffness, which is defined as distributed

spring contacts per unit area. The normal interfacial stiffness varies

from infinity when perfectly bonded is achieved, to zero for an

unbond surface. The normal interfacial stiffness must be much lessthan infinity when kissing bond occurs at the interface.

The reflection coefficient of the imperfect interface is related to

three factors: the acoustic impedances of the materials on either

side of the interface, the ultrasonic frequency, and the normal

interfacial stiffness. The amplitude and phase of the reflection

coefficient of TiAl and 40Cr diffusion bonding interface as shown

in Fig. 1 are calculated to illustrate the relationship among the

reflection coefficient and the three factors. The acoustic impe-

dances of TiAl and 40Cr are 2.73 107 and 4.68 107 Pa s m1,

respectively. As the phase is a periodic function with period p,

the result is only shown between p/2 and p/2. As kn-N,corresponding to the case of perfectly bonded, the amplitude of

the reflection coefficient9R9-(Z2Z1)/(Z2Z1) at all frequencies.The phase of the reflection coefficient tends to zero from the

positive direction, which means the reflected wave and the

incident wave are in-phase. As kn-0, corresponding to the case

of unbond, the amplitude of the reflection coefficient9R9-1 andalso no frequency dependence is observed. There is an exception

to the rule.9R9 tends to (Z2Z1)/(Z2Z1) when fis close to zero.The frequency used in ultrasonic testing is usually greater than2.5 MHz, even greater than 5 MHz. So this tendency has little

effect on the practical ultrasonic testing. The phase of the

reflection coefficient tends to zero from the negative direction,

which is equivalent to F-p. The reflected wave is opposite inphase to the incident wave. As kn is much less than infinity,

corresponding to the case of kissing bond, part of the ultrasonic

energy is reflected from the interface and the amplitude of the

reflection coefficient increases with the frequency. The phase of

the reflection coefficient is the same at low frequencies and

opposite at high frequencies. The phase transition occurs when

F-7p/2.

3. Experimental

3.1. Specimens preparation

TiAl intermetallic compound and 40Cr steel were used in the

study. The specimens were a rectangular shape of 45 mm 30 mm,

and the thicknesses of TiAl and 40Cr were 4.2 and 14.8 mm,

respectively. TiAl specimens were given chemical cleaning by 5%

hydrofluoric acid, then rinsed in water and finally dried in hot

airflow. 40Cr specimens were cleaned using acetone. Six specimens

were then bonded at various temperatures under a constant

pressure of 1.33 103 Pa in a vacuum furnace. The welding

temperatures were 900, 950 and 1000 1C to obtain unbond, kissing

bond, and perfectly bonded joints, respectively. The welding pres-

sure was 15 MPa for 15 min. A TiAl plate without diffusion bondingwas prepared as reference specimen.

3.2. Ultrasonic measurement

Ultrasonic tests were performed using ULTRAPAC C-scan

immersion system produced by Physical Acoustic Corporation.

The system consists of an immersion system including a scanning

frame assemble, motorized axis adjusters, an immersion tank, and

a computer with ULTRAWIN software to control test and provide

result display. A broadband focused transducer with central

Fig. 1. Calculated reflection coefficient of TiAl and40

Cr diffusion bonding interface: (a) amplitude of reflection coefficient and (b) phase of reflection coefficient.

Y.L. Luan et al. / NDT&E International 44 (2011) 789796790


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frequency at 10 MHz was used. The ultrasonic wave at normal

incidence from TiAl side was focused on the TiAl and 40Cr

diffusion bonding interface. The C-scan images of the specimens

and the interface A-scan signals were obtained at the same time

with sampling frequency of 100 MHz. The ultrasonic wave was

also focused on the bottom of the reference TiAl plate and the

reference signal was collected from the TiAlair interface, which

had 99.99% amplitude and opposite phase of the incident wave.

Phase inversion was performed on the reference signal.

3.3. Shear test

The central part of each diffusion bonding specimen was cut to

obtain 20 shear test specimens with the dimensions of 4 mm

9 mm 19 mm. A plan view of the shear test specimens cutting

method are illustrated inFig. 2.The area with section lines was not

used for shear test to eliminate the edge effect caused by focused

transducer in the ultrasonic measurement. The numbers were the

serial number of shear test specimens. The shear test specimens were

subjected to shear tests at TiAl and 40Cr diffusion bonding interface in

a universal testing machine, providing information on the shear

strength of 20 areas of the diffusion bonding specimen. The shear

strength tbwas obtained by

tb FbA0

3

whereFbis the loading of final failure and A0is the area of shear test

specimen.

3.4. Metallographic analysis

The cross-sections of TiAl and 40Cr bonding joints were

polished for metallographic analysis. TiAl specimens were lightly

etched with 2% nitric acid. Micrographs of the diffusion bonding

interfaces were obtained by an optical microscope.

4. Time-scale characteristics extraction algorithm

As we know from Section 2, the ultrasonic amplitude and

phase after interacting with the interface are affected by the

bonding quality of the interface. Therefore, the interfacial imper-

fections can be evaluated by the ultrasonic amplitude and phase

characteristics. The time-scale characteristics extraction algo-

rithm is proposed to assess the bonding quality. The algorithm

procedures are as follows:

(1) The continuous wavelet transforms are performed on both

the interface signals and the reference signal according to the

following equation:

Wfa,b

Z 11

ftca,btdt 1ffiffiffiffiffiffiffi

9a9q

Z 11

ftc tb

a

dt

tA0,n 4

whereWf(a,b) is the continuous wavelet transform of function

f(t), f(t) is the interface signal or the reference signal, tis the

time variable, (0,n] is the sampling interval off(t),c(t) is the

basic wavelet, andaandbare referred to as the scale and timeparameter, respectively. The complex morlet wavelet is

employed in the continuous wavelet transform for its simi-

larity to the ultrasonic signal and linear phase. The complex

morlet wavelet is defined as

ct 1ffiffiffiffiffiffiffipfb

p et2=fb e2ipfct 5wherefb and fcare the bandwidth parameter and the central

frequency of the basic wavelet, respectively.

It is necessary to optimize the time-scale resolution and

determine the scale parameter a and the step of the scale s

prior to the continuous wavelet transform. The time-scale

resolution is related to 1=2pfc ffiffiffiffifbp , and therefore, fcis set as1 Hz to adjust fb. The optimal time-scale is obtained when fbequals 0.8. The scale parameter a of the continuous wavelet

transform is determined by the following equation:

a fcfs

fa6

wherefsis the sampling frequency of the ultrasonic measure-

ment and fa is the central frequency of the wavelet corre-

sponding to scale a. As can be seen, the scale is related to fawhenfcand fs are known, which is determined by the useful

bandwidth of the transducer. The useful bandwidth of the

transducer is ranging from 6 to 15 MHz. The scale parameter

is 16.76.7 according to Eq. (6). The scale parameter is

rounded to integers as 176 and the correspondence band-

width is ranging from 5.8 to 16.7 MHz. The step of the scale s

is set as 0.2 considering the computation efficiency.

(2) The time-scale ratio of the interface signal and the reference

signal R(a,b) is obtained by

Ra,b Wfa,binterfaceWfa,breference

7

where the subscript interfaceand reference correspond to the

interface signal and the reference signal, respectively.

(3) The time-scale amplitude 9R(a,b)9 and the time-scale phaseF(a,b) are obtained by

Ra,b

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiR2Ra,b R

2I a,b

q 8

Fa,b +Ra,b arctan

RIa,b

RRa,b 9

where the subscript R and I correspond to the real and the

imaginary part of R(a,b), respectively. Concerned only with

the same or opposite of the time-scale phase, 1 is

employed to represent the same phase, and 1 is employed

to represent the opposite phase.

(4) The time-scale amplitude9R(a,bj)9of every time parameter bjis linear fitted along the scale parameter a decreasing direc-

tion to obtain the fitting curve yj according to the following

equation:

yj Aj9Ra,bj9 Kj, j l, l 1=fs,. . .,m 10

where Aj and Kj are the fitting slope and the fitting constant

of the fitting curve yj, respectively, and l and m are the

scope of the time parameter b, respectively. The amplitude

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

17 18 19 20

45

30

4

9

Fig. 2. Plan view of the shear test specimens.

Y.L. Luan et al. / NDT&E International 44 (2011) 789796 791


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characteristicCR is obtained by

CR Xmj l

Aj 11

(5) The phase characteristic CF is calculated by

CF s

vu s

1

mfslf

s1 X

v

i uX

m

j l

Fai,bj

i u,u s,. . .,v, j l, l 1=fs,. . .,m 12

where u and v represent the scope of the scale parameter a,

F(ai,bj) is the time-scale phase of every scale parameter ai and

every time parameter bj. The reason of amplitude and phase

characteristics extraction will be explained in results and discus-

sion section.

5. Results and discussion

Fig. 3 shows the reflected signals of the perfectly bonded

interface, the kissing bond, and the unbond and the correspond-ing shear strengths of the joints are 246.1, 44.7, and 6.7 MPa,

respectively. As can be seen, there are reflected signals not only

from the kissing bond and the unbond but also from the perfectly

bonded interface due to the effect of impedances mismatch (the

reflectivity of the TiAl and 40Cr diffusion bonding interface is

26.3%). Moreover, there is no apparent difference between signals

from the perfectly bonded interface and from the kissing bond. It

is difficult to detect the kissing bond by the signals and the

bonding quality cannot be accessed by the amplitude of the

reflected signals.

Differences are illustrated clearly after the signals are pro-

cessed by the algorithm described in Section 4. Fig. 4 shows

the time-scale amplitude of the signals shown in Fig. 3. The

signal-to-noise ratio of time-scale amplitude is low when time is

less than 0.1 ms and greater than 0.4 ms, so that the signal analysis

was performed at the interval of [0.1ms, 0.4 ms]. Four time

parameters were extracted and the curves of the amplitude

changing with the scale were illustrated on the right side of

the time-scale amplitude. The extracted time parameters were

0.1, 0.2, 0.3, and 0.4 ms. As can be seen from Fig. 4(a), the

amplitude was low and remained roughly constant from 0.1 to0.4 ms at every time parameter for the perfectly bonded interface.

This phenomenon was confirmed by the extracted time para-

meters for the curves of the amplitude changing with the scale

were almost straight lines. As the scale is related to the frequency,

the amplitude does not vary with the frequency. For the kissing

bond as shown inFig. 4(b), the amplitude increased with the scale

decreasing at every time parameter. According to (6), the scale is

in inverse proportion to the central frequency of the wavelet, that

is to say, the amplitude increases with the frequency. The

amplitude was high and did not vary with the scale for the

unbond as shown in Fig. 4(c). The reason of the scale-dependent

amplitude may be explained as follows. The length of the kissing

bond is much smaller than the wavelength of the ultrasonic wave.

The ultrasonic wave of low scale (high frequency) is much more

sensitive to the interfacial imperfection than that of high scale

(low frequency). The lower the scale (the higher the frequency is),

the higher the amplitude after interacting with the defect. Thus,

the scale-dependent amplitude is observed for the kissing bond.

Though the kissing bond cannot be detected by the amplitude of

the reflected signals, one can reliably identify the defect using the

scale-dependent amplitude.

As for the kissing bond, the time-scale amplitude 9R(a,bj)9 ofevery time parameter bj increased with the decrease in scale ,

whereas that of the unbond and the perfectly bonded interface

did not vary with the scale. Therefore, the fitting slope Aj of the

fitting curveyjalong the scale decreasing direction was chosen to

0.0-1.0

-0.5

0.0

0.5

1.0

Normalizedamplitude

Time (s)

0.0-1.0

-0.5

0.0

0.5

1.0

Normalizedamplitude

Time (s)

0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5

-1.0

-0.5

0.0

0.5

1.0

Normalized

amplitude

Time (s)

0.0 0.1 0.2 0.3 0.4 0.5

Fig. 3. Ultrasonic signals reflected from TiAl and40

Cr diffusion bonding interface: (a) perfectly bonded interface, (b) kissing bond, and (c) unbond.



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represent the scale-dependent feature of 9R(a,bj)9. The fittingslopes Aj were then summed as the amplitude characteristic CR.

The calculated amplitude characteristics were 6.30, 0.02, and

0.01 for the kissing bond, the unbond, and the perfectly

bonded interface, respectively. The kissing bond was distin-

guished from the unbond and the perfectly bonded interface by

the extracted amplitude characteristics.

A special phenomenon for the unbond was observed in the

experiment as shown in Fig. 5. The time-scale amplitude of theunbond decreased with the scale at every time parameter when

the ultrasonic signal was too high to overflow the oscilloscope. This

was because the signal was distorted by the saturated sampling.

However, this phenomenon has no effect to distinguish the unbond

from the kissing bond and the perfectly bonded interface.

Fig. 6shows the time-scale phase of the signals reflected from

the perfectly bonded interface, the kissing bond, and the unbond.

As can be seen fromFig. 6, the phase was almost the same for the

perfectly bonded interface at the interval of [0.1 ms, 0.4 ms]; the

time-scale phase presented the same at high scale whereas

opposite at low scale for the kissing bond; the time-scale phase

was opposite for the unbond. The opposite phase can be

explained by considering the difference in the acoustic impe-

dances on either side of the interface. The phase is the same after

the ultrasonic wave interacting with the interface in the case of

the acoustic impedance of the top material is less than that of the

bottom material, whereas the phase is opposite in the case of the

acoustic impedance of the top material is greater than that of

the bottom material. The acoustic impedance of the interface

layer is much less than that of the upper materials for the kissing

bond and the unbond, so that the opposite phase occurs. Although

the bonding quality is difficult to be evaluated by the amplitude

of the reflected signal, the kissing bond can be detected by thephase variation, and the unbond could be measured by the

opposite phase. The time-scale phase becomes another useful

tool to assess the kissing bond and the unbond.

The extraordinary feature of the time-scale phase of different

bonding quality was the same and opposite differences. The mean

value of the time-scale phase was able to represent the phase

differences. This is the reason for selecting the mean value as the

phase characteristic. The phase characteristic CR of the kissing

bond predicted to be between 1 and 1 for the time-scale

phaseF(ai,bj) presented the same at high scale whereas opposite

at low scale. The time-scale phase F(ai,bj) for the unbond and the

perfectly bonded interface were opposite and the same, so that

the predicted phase characteristics CR should be 1 and 1.

The calculation result showed that phase characteristics were

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7

0

0.5

1

170.0

0.2

0.4

0.6

0.8

1.0

Amplitude

Scale

t=0.1s

t=0.2s

t=0.3s

t=0.4s

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

70

0.5

1

0.0

0.2

0.4

0.6

0.8

1.0

Amplitude

Scale

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7

0

0.5

1

0.0

0.2

0.4

0.6

0.8

1.0

Amplitude

Scale

16 15 14 13 12 11 10 9 8 7 6

17 16 15 14 13 12 11 10 9 8 7 6

17 16 15 14 13 12 11 10 9 8 7 6

t=0.1s

t=0.2s

t=0.3s

t=0.4s

t=0.1s

t=0.2s

t=0.3s

t=0.4s

Fig. 4. Time-scale amplitude of signals from TiAl and 40Cr diffusion bonding interface: (a) perfectly bonded interface, (b) kissing bond, and (c) unbond.



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0.68, 0.99, and 0.97 for the kissing bond, the unbond, and

the perfectly bonded interface, respectively. The extracted phase

characteristics distinguished the kissing bond and the unbonded

from the perfect bonded interface.

The characteristics extraction was performed on the diffusion

bonding specimens. The amplitude and phase characteristics

images were reconstructed according to the position of the ultra-

sonic C-scan images using color indicating their values. Results of

one specimen are shown in Fig. 7. Fig. 7(a)(d) are the ultrasonicC-scan image, the shear strength of 20 areas of the specimen, the

amplitude characteristics image, and the phase characteristics

image, respectively. The lines on Fig. 7(b) were manually added

according to the dimension of the shear test specimens. The

elliptical regions A, B, and C were discussed. The amplitude of the

C-scan image is approximately 40% in region A, which is close to

region C. However, the shear strength of region C is 173.1 MPa,

whereas that of region A is averagely 19.5 MPa. There must be some

imperfections on the interface and their characteristics match those

of the kissing bonds. However, it is too difficult to identify the

kissing bonds by the ultrasonic C-scan image. Differences are

illustrated clearly in the amplitude and phase characteristics

images. The amplitude characteristics approach to 5 and the phase

characteristics are approximately 0.7 in region A. The amplitude

and phase characteristics of region C are approximately 0.02 and

0.98. The amplitude of the ultrasonic C-scan image in region B is

too high to overflow the oscilloscope and the shear strength is zero.

These defects belong to the unbond. The amplitude characteristics

are less than zero due to signal saturation sampling and the phase

characteristics are approximately 1.

The microstructures of the regions A and B are shown in Fig. 8.

Small areas with the length of a few micrometers in which

diffusion process is inhibited can be seen at the kissing bondinterface; and a narrow long gap with the width of approximately

89 mm is located at the unbond interface. The amplitude and

phase characteristics images are effective to assess the kissing

bonds and the unbonds in the TiAl and 40Cr diffusion bonding

specimens. Analogous signal analyses were performed on five

other specimens, and the same results were obtained.

6. Conclusions

Ultrasonic interface signals of the TiAl and 40Cr diffusion

bondings are transformed in the time-scale domain to analyze

the time-scale amplitude and phase and extract characteristics for

the bonding quality assessment. The algorithm proposed by the

0.0-1.0

-0.5

0.0

0.5

1.0

No

rmalized

amplitude

Time (s) Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7

0

0.5

1

0.1 0.2 0.3 0.4 0.5

Fig. 5. Saturated sampling signal from unbond interface and corresponding time-scale amplitude: (a) saturated sampling signal and (b) time-scale amplitude.

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7-1

-0.5

0

0.5

1

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7-1

-0.5

0

0.5

1

Time (s)

Scale

0 0.1 0.2 0.3 0.4 0.5

17

15

13

11

9

7

-1

-0.5

0

0.5

1

Fig. 6. Time-scale phase of signals from TiAl and 40Cr diffusion bonding interface: (a) perfectly bonded interface, (b) kissing bond, and (c) unbond.



7/8

paper differs from conventional ultrasonic evaluation for it

utilizes the scale-dependent amplitude instead of the amplitude

of the reflected signal. Another difference of the algorithm is the

application of the phase information. From the above study we

arrive at the conclusion that the kissing bond can be detected by

the scale-dependent amplitude combined with phase variation

and the unbond can be measured by the opposite phase. The

kissing bonds and the unbonds exist not only in diffusion

bondings but also in other solid-state welding methods, such as

high-frequency induction brazing and friction welding. The algo-

rithm shall be applied in other solid-state welding methods toanalyze its universaliability in our further study.

Acknowledgment

The authors are grateful to all the members of non-destructive

testing research team of the State Key Lab of Advanced Welding

Production Technology in the Harbin Institute of Technology for

their help.

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aluminum alloy and steel. Res Nondestr Eval 2006;17:22339.

-10

-5

0

5

10

-1

-0.5

0

0.5

1

[%]

100

50

0

Fig. 7. Characteristics extraction of TiAl and 40Cr diffusion bonding specimen: (a) Ultrasonic C-scan image, (b) shear strength of specimen, (c) reconstructed image of

amplitude characteristics, and (d) reconstructed image of phase characteristics.

Fig. 8. Microstructures of TiAl and 40Cr diffusion bonding specimen: (a) kissing bond and (b) unbond.



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Ultrasonic Evaluation of TiAl And40 Cr Diffusion Bonding Quality Based on Time-scale Characteristics Extraction