Defect detection in FSW steel plates by NDT...

Defect detection in FSW steel plates by NDT methods

A.PRADEEP1, G.Sai Krishnan2, G.Suresh3, K.Kanagaraja4

Associate Professor, Department of Mechanical Engineering,

Rajalakshmi Institute of Technology, Chennai, Tamil Nadu, India

[email protected]

ABSTRACT

Friction stir welding is a solid state welding technology used mainly for welding low melting

point metals, such as Al, Mg and its alloys. This is an emerging technology finding its application in

aerospace, naval and automobile fields. At present the research on Friction stir welding on steel is

concentrated because of the major use of steels in industries rather than other metals. Due to its solid state

nature ,friction stir welding produces weldment properties that are better than traditional arc welding

process and are usually free of imperfections. However some imperfections like lack of penetration ,root

imperfection (weak or intermittent welding),tunneling defect may arise. Ultrasonic immersion C-scan

testing and digital radiography have used for inspection of friction stir welded steel plates. The C-scan

images were obtained with the help of a computer controlled immersion ultrasonic C-scan system. The

samples were tested in water tank using a 0.25 inch diameter, focused ultrasonic probe. Scan resolution is

0.1 mm. The ultrasonic immersion testing was done at three different frequencies

15MHz,5MHz,2.25MHz to study the capabilities of ultrasonic testing. Post C-scan image analysis was

carried out to identify the defects. Phased array ultrasonic testing have been used characterization which

gave better results than conventional methods Defects have been identified by the above said non

destructive testing methods and the results have been presented.

Keywords: Ultrasonic C-scan, Friction stir welding, NDT methods.

1. INTRODUCTION

Friction stir welding is a solid state welding technology used for welding low melting point

metals, such as Al, Mg and its alloys. FSW has shown great results for joining metals together without

significantly altering the original properties of the parent metals Material flow and friction heat are the

important factors for the formation of weld. At present the research on Friction stir welding on steel is

concentrated because of the major use of steels in industries rather than other metals. Tool design and

welding parameters contributes a major role for producing a better weld. A.Pradeep[1] in his work has

International Journal of Research

Volume 7, Issue XII, December/2018

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given the review about the basic concepts of Friction Stir Welding on tool design, mode of metal transfer

and process parameters. Tapered threaded pin has been tried instead of cylindrical threaded pin and it has

given better strength and uniform weld throughout the length of the plate. Vertical mixing at high velocity

and tool wear can be minimized through this design. Rotating speed, traverse speed and tilt angle are the

majors factors which were found to influence the weld formation more than the other process parameter

such as plunge depth, force exerted by the tool. When tool tilted at an angle of more than 1° void

formation was observed and this condition occurred for a plate not more than 4mm thickness.

American Bureau of Shipping(ABS)[2] the guide for the approval of friction stir welding in

aluminum has provided the guidelines for the approval of friction stir welding procedures, operators and

the nondestructive testing requirements for production friction stir welding of aluminum. According to

ABS standards 100% visual inspection ,100% ultrasonic testing(UT) and 100% radiographic testing (RT)

are necessary. These attributes include Exit hole uniformity, Flash, Chevron markings, Dimensional

variations in thickness (lack of fill) Misalignment, Cracks, porosity, lack of penetration. Ultrasonic

inspection and radiographic testing should be in accordance with ‘ABS Guide for Nondestructive

Inspection of Hull Welds’. Anish Kumar et al [3] in thePaper titled “Development and applications of c-

scan ultrasonic facility” have described about the various components of the system developed and

details how the c-scan system has been used for various applications such as imaging of weld profiles in

ferritic and austenitic steel weldments, interface characterization of braze joints, explosive welds of

different metals, various cladded specimens and imaging of dummy fuel sub-assembly heads of the Fast

Breeder Test Reactor (FBTR). It was found that the center to center distance between the two heads can

be measured from the C-scan image with an accuracy of ~2% and ~3% by using focused and unfocused

transducers respectively. JFE Technical Report[4]titled “Nondestructive Inspection by Phased Array

Ultrasonic Method for Steel Structures” have used phased array ultrasonic testing for inspection of steel

structures.

Esther T. Akinlabi [5] in their work used non-destructive testing to check the quality of

dissimilar friction stir welds between 5754 aluminium alloy and C11000 copper. The Friction stir welds

of 5754 aluminium alloy and C11000 copper were produced at different tool rotational speeds and feed

rates. The tool rotational speed was varied between 600 and 1200 rpm while the feed rate was varied

between 50 and 300 mm/min. They have used visual inspection and the x-ray radiographic testing

techniques for inspection of FSW. Anmol.S.Birring[6] in his paper titled “Ultrasonic Phased arrays for

weld testing” tested the ability of phased array ultrasonic testing to detect weld discontinuities in standard



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test specimens used for ultrasonic testing qualification exams. tests were conducted using two phased

array systems. VidyaJoshi etal[7] in their work have used radiography, ultrasonic and phased array

ultrasonic techniques to study about the defects in friction stir welded AA 5083 alloy and compared the

ability of the above said NDT methods. Immersion Ultrasonic C scan testing have been carried out using

a probe with frequency of 10MHz and focal length of 2” with water acting as a couplant. The sample was

scanned using a resolution of 0.2x 0.2 mm .Phased array ultrasonic technique is useful to detect the

randomly located and disoriented defects like crack, porosity etc.In this work 64 elements linear array

probe of frequency 5 MHz have been used in the range of 30°-60° for sectorial scan (S-Scan). An S-scan

image represents a two-dimensional cross-sectional view derived from a series of A-scans that have been

plotted with respect to time delay and refracted angle. The defects found have been compared

byradiography, ultrasonic and phased array ultrasonic techniques.

2. MATERIALS AND METHODS

The parent metal employed in this study is steel. The plates were cut in to required size of 100

mm x 50 mm. Two plates of 100 mm x 50 mm are welded by friction stir welding process to produce a

final dimension of 100 x 100 mm plate. Heavy alloy tungsten carbide tool was employed to fabricate the

joints.

Table 2.1 Material composition

Fe C Si Mn P S Cr Mo Al Cu Ti V Pb

97.

6

0.20

4

0.12

9

1.1

0

0.04

0

0.01

7

0.30

9

0.033

2

0.094

0

0.090

8

0.006

3

0.006

9

0.001

4

Fig.2.1 (a) Schematic diagram of Friction stir welding process (b) Friction stir Welding machine

(a) (b)



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2.1. FRICTION STIR WELDING OF STEEL PLATES

2.1.1. FSW PROCESS DETAILS

In recent years, the research is concentrated on FSW of steels because of the major application

of steels in the industries. When compared to aluminium and its alloys, FSW process performed on steels

are limited. The tool material for welding is a major constraint for welding steels. At present the most

commonly used tool material for steels are polycrystalline cubic boron nitride (PCBN) and tungsten based

tools. Being cost effective and too brittle, PCBN tools are replaced with tungsten alloy tools which have

been used in many early welds. In our work instead of cylindrical pin, tapered pin has been used for

friction stir welding of steel plates.

The rotating tool is plunged along the intersection of two metal plates which are rigidly fixed on a

backing plate .When the upper surface of the plates comes in contact with the shoulder surface the friction

is developed. Plastic deformation of metal occurs at the joint area along the weld direction. This is

influenced by the combined action of shoulder and pin. The pin produces a stirring action at the

intersection region and then produces the transfer of metal from the advancing side to retreating side and

vice – versa. Initially, surface preparation is done before welding to remove oxide layers, scales and rust.

Three FSW samples were prepared for our study at different tool travel speeds as shown in table 4.1(b)

with same rotational speed.

Process parameters

Table.2.2 Operating parameters of FSW

Parameters Sample1 Sample2 Sample3

Tool rotational speed 1120 rpm 1120 rpm 1120 rpm

Shoulder diameter 50 mm 50 mm 50 mm

Shoulder length 30 mm 30 mm 30 mm

Pin length 2.8 mm 2.8 mm 2.8 mm

Travel speed 8 mm/min 15 mm/min 20 mm/min



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2.2. ULTRASONIC IMMERSION TEST

Ultrasonic test systems can take several forms, but the most common for automated test is immersion

testing, as shown in Figure 4.4

Fig.2.2 Ultrasonic Test System

2.2.1 ULTRASONIC IMMERSION TEST EQUIPMENT

The parts of the ultrasonic immersion test equipment and their functions are as follows:

1. Application software - The user interacts with the application software to set up the test and

presentation parameters.

2. Motion control - The ultrasonic transducer is moved over the appropriate area over the UUT.

3. Communication - The pulser/receiver operation parameters shown in table 4.3, such as pulse energy,

pulse damping, and band pass filtering, are set. The communication path is typically RS232 or USB.

4. Pulser/receiver - This device generates the high-voltage pulse that is required by the ultrasonic

transducer.

5. Ultrasonic transducer - The transducer is pulsed, sending out an ultrasonic wave. The subsequent

echoes generate a voltage in the transducer, which is sent back to the pulser/receiver.



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6. Pulser/receiver - The analog signal from the ultrasonic transducer is amplified and filtered before it is

sent back to the digitizer within the PC.

7. Digitizer - The waveform sent from the pulser/receiver is converted from voltage to bits using an

analog-to-digital converter (ADC).

8. Application software - Data from the digitizer is processed, analyzed, and presented according to the

user-defined parameters.

3. RESULTS AND DISCUSSION

The parent metal employed in this study was carbon steel. The plates were cut in to required size of 100

mm x 50 mm. Two plates of 100 mm x 50 mm are welded by friction stir welding process to produce a

final dimension of 100 x 100 mm plate. Heavy alloy tungsten tool was employed to fabricate the joints.

The joint region was subjected to immersion type C-scan testing. The samples were tested with three

frequencies 15, 5, 2.25 MHz. The scanning length of the test specimen is 50x70 mm in x and y axis. Scan

resolution is 0.1 x 0.1 mm. Sampling rate is 50 MHz After scanning, the images are processed by using

Lab view software.

3.1 Focal length

Focal length: 2 inch (50 mm) for water.

To cover the entire thickness of work piece focal length = 50 – 3.5 x (6000/1500)

= 36 mm.

3.2 Data presentation

The application software used is the labview software. The test parameters and presentation parameters

are set using the labview software. They are

A-scan presentation: A-scan is basically a plot of amplitude versus time (depth), horizontal line indicates

time and vertical represents amplitude of the echo. Flaw size is estimated by comparing the amplitude of

the discontinuity signal with that of a signal from a discontinuity of known size.

B-scan presentation: B-scan display is a plot of time versus distance. Horizontal axis corresponds to time

and vertical axis represents the position of the transducer along the line on the surface of the test piece.

The principal advantage of B-scan presentation is the ability to display cross sectional view of the

component.

C-scan presentation: C-scan display records echoes from the internal portion of the test pieces as a

function of position of the each reflecting surface within an area. Both flaw size and the position of the

test piece within the plan view are recorded.



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Sample1:

Exit

hole position Defect position

Fig. 3.1 (a) Shows the gate settings information for the specimen (b) visual image of Sample1 (c) 15MHz

C scan image of Sample1 (d) 5MHz scan image of Sample1 (e, f) Ascan image of Sample 1 (g) B-Scan

image of Sample1 (h) Digital radiograph of Sample1(i) Linear sectorial scan of defect C Scan image of

sample1

(b)

defect exit hole

(a)

(c)

(d)

defect (f)

(e)

Defect region defect Exit hole

(g)

(h) (i)



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15MHz:

Initial settings like record length ,pulse voltage, pulse repetition rate and gain are done. initial signal

from the base metal region is obtained and gate settings are done. The scan or inspection area ,the scan

resolution are set .after the results are obtained ,post analysis is done by using labview software. The

required information can be obtained. when the focused transducer is used the ultrasonic signals hit a

particular point and the reflected signals are received back. Depending on the amplitude of the received

signals, they are converted into suitable colourscale .violet or blue colour represents higher amplitude and

red colour represents lower amplitude. Exit hole is clearly visible. From that particular region as the

surface is irregular some the signals get deviated as a result only low amplitude has been received by the

transducer hence shown as red colour.the marked region opposite to the exit hole shows higher

attenuation which represents defect in the particular region. The ultrasonic immersion testing is done at

three different frequencies 15,5 and 2.25 MHz for comparison.

5MHz:

The above figure represents the c scan image tested at a frequency of 5MHz.the exit hole is visible in

the c scan. There is no defect in the weld zone except a defect at the end.

Conventional Ascan image of sample 1:

The conventional ultrasonic testing is done by using angle probe of 70° and frequency 4MHz.The

inspection is done between half skip distance and full skip distance. the defect signals at exit hole position

and defect position are shown. The defect is at a depth of 2.8mm.

B-Scan:

The sample thickness 3.5mm.The B scan shows cross sectional view of the sample in the indicated

position.Along the thickness of the sample there is no variation in the signal, they are continuous showing

that particular section is regular as shown in left hand side image.The right hand side image shows B scan

at defect position. In the particular cross section, at 25mm position shows variation in the signals, Indicate

the defect. The defect is at a depth of 2.8mm .Size of the defect 11x5mm.

Digital radiograph:

Parameters

Voltage: 110 kv

Current:0.6 mA

Penetrameter:10 to 16 DIN



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Sample2

Fig.3.2.(a) Sample2 (b)15MHz-C Scan image of sample2 (c)5Mhz-C scan image of sample 2 (d)2.25Mhz

C Scan image of Sample2 (e) Lack of root penetration (f) B scan image of Sample2 (g) Exit hole region

(h) Defect (inclusion) (i) Digital radiograph of Sample2 (j) Linear sectorial scan of defect

(b) (a) (c)

Exit hole

f (d)

(e)

Exit hole

Defect(inclusion)

Defect(inclusion) Lack of Penetration continuous

Exit hole defect

(h) (g) (i) (j)



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The digital radiography is obtained by using vidiscodigital radiographic equipment. The voltage 110 kv

and a current of 0.6 mA.based on the thickness of the sample the penetratmeter is chosen as 10 to 16

DIN.the radiograph reveals that at the end position there is no proper joining at the marked position .

Phased array ultrasonic testing:

probe 5 L 64A2

frequency 5MHz

Aperture length 16

probe 64 elements

The probe used for phased array ultrasonic testing is 5L64A2 tested at a frequency 5MHz.The gain is

26.8dB. The depth of defect is 2.76mm.

5MHz:

In the C scan image shown in figure 3.2(c), the green colour represents reflected ultrasonic signals of

higher amplitude indicating soundness of the base metal and weld region.Exit hole is clearly visible. From

that particular region as the surface is irregular some the signals get deviated as a result only low

amplitude has been received by the transducer hence shown as red colour.the marked region opposite to

the exit hole shows higher attenuation which represents defect in the particular region.the defect is found

as inclusion

Lack of root penetration:

The sample2 has lack of root penetration that can be observed in c-scan image fig3.2(e).

B scan image of sample2 :

From the B Scan shown in Fig. 3.2(f) ,we find that the defect is at a depth of 2.60 mm.


Fig. 3.2(g,h) Conventional Ascan image of Sample 2



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Sample3:

Exit hole position Defect (void)

Fig.3.3 (a) Sample3 (b)15MHz-C Scan image of Sample3 (c)5MHz-C Scan image of Sample3

(d)2.25MHz- C Scan image of Sample3 (e,f) Conventional Ascan image of Sample 3 (g) B scan image of

Sample2 (h) Linear sectorial scan of defect (i) Digital radiograph of Sample3.

Defect(voids)

(a) (b)

Surface imperfection Exit hole

(c)

(d) (e) (f)

(g)

(h)

The discontinuity starts

from this location

Discontinuity in

the back wall

(i)

Exit hole Voids



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Phased Array Ultrasonic Testing:

The probe used for phased array ultrasonic testing is 5L64A2 tested at a frequency 5MHz.The gain

is 26.8dB. The depth of defect is 2.50 mm.

Digital radiograph :

The digital radiography asshown in figure 5.15 is obtained by using vidisco digital radiographic

equipment. The voltage 110 kv and a current of 0.6 mA. based on the thickness of the sample the

penetratmeter is chosen as 10 to 16 DIN. The radiograph reveals that at the end position there is a defect

(inclusion).

2.25MHz:

In the C scan image as shown in figure3.3(d),the blue colour represents reflected ultrasonic signals of

higher amplitude indicating soundness of the base metal .Exit hole is clearly visible. From that particular

region as the surface is irregular some the signals get deviated as a result only low amplitude has been

received by the transducer hence shown as red colour. The marked region shows higher attenuation which

represents defect in the particular region.the defect are found as surface breaking voids.


The conventional ultrasonic testing is done by using angle probe of 70° and frequency 4MHz.The

inspection is done between half skip distance and full skip distance. the defect signals at exit hole position

and defect position are shown. The defect is at a depth of 2.80mm.

Phased Array Ultrasonic Testing:

The probe used for phased array ultrasonic testing is 5L64A2 tested at a frequency 5MHz.The gain is

26.8dB. The depth of defect is 2.76mm.length of the defect 12mm.the defects have been indentifiedas

voids.

CONCLUSIONS:

The conclusions drawn from the present work are as follows:

1.The defects such as lack of penetration, surface breaking voids and flaws have been identified by

ultrasonic c- scan immersion testing method and phased array ultrasonic method .

2.The welds prepared at lower travel speeds have less defects compared to welds at higher travel speeds



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3.phased array Ultrasonic testing removes inherent ‘human error’ factor of other ultrasonic test systems,

enabling highly repeatable, consistent monitoring of material

4. The results of the analyses are provided as a permanent image, as well as a digital automated analysis,

which is more efficient than conventional methods.

Results:

REFERENCES

A. Pradeep,(2012), A Review on Friction Stir Welding of Steel , International Journal of

Engineering Research and Development, Volume 3, Issue 11 (September 2012), Pages 75-91.

4. American Bureau of Shipping(ABS), (2011), The guide for the approval of friction stir welding in

aluminum, Pages 1-18.

5. Anish Kumar, K. V. Rajkumar, P. Palanichamy, T. Jayakumar, R. Chellapandian, K.V.

Kasiviswanathan and Baldev Raj, (2006),Development and applications of C-scan ultrasonic

facility ,BARC newsletter.

6. JFE Technical Report, (2012),Nondestructive Inspection by Phased Array Ultrasonic Method for

Steel Structures, No. 17.

7. Esther T. AkinlabiMember, IAENG, Adrian C. S. Levy and Stephen A. AkinlabiMember,

IAENG,(2012), Non-Destructive Testing of Dissimilar Friction Stir Welds, Proceedings of the

World Congress on Engineering ,Volume III WCE 2012, London, U.K.

8. Anmol.S. Birring, (2008),Ultrasonic Phased arrays for weld testing,American society of Non

destructive Testing, volume 66, Pages 282-284.

sample Travel speed Defects C scan PAUT

Sample1 8 mm/min Improper joining 2.80mm 2.76mm

Sample2 15 mm/min inclusion 2.60mm 2.50mm

Sample3 20 mm/min voids

2.80mm 2.80mm



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9. Vidya Joshi, (2011),Study of defects in friction stir welded AA 5083 by radiography, ultrasonic

and phased array ultrasonic technique, Proceedings of the National Seminar & Exhibition on

Non-Destructive Evaluation 2011.

10. Raj Mazumder, N. R. Bandyopadhyay and S. PalitSagar, “Detection and Quantification of

Lack of Penetration in Al-Al Friction-Stir Welded Plates using Phased Array Ultrasonic

Technique” Journal of Nondestructive Testing & Evaluation Vol. 10,Issue 4,2012, pages

47-52.

11. R.J. Hudgell, G.M. Worrall, J. Ford, B.S. Gray (1989) Ultrasonic characterization and inspection

of austenitic welds , International Journal of Pressure Vessels and Piping, Volume 39, Issue 4,

Pages 247-263.

12. A J Leonard, (2003), Flaws in friction stir welds,4th international symposium on friction stir

welding.

13. Ramesh Singh, (2012),Ultrasonic testing, Applied Welding Engineering, Pages 293-304.

14. X. Maldague, (2001),Theory and Practice of Infrared Technology for Non Destructive Testing,

John-Wiley & Sons, pages 684-686,.

15. P. O. Moore ed., X. Maldague technical ed., NDT Handbook on Infrared technology, ASNT

Handbook Series, ASNT Press (American Society for NonDestructive Testing) Press, 732 p,

2001.



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