Study Of Defects In Friction Stir Welded AA 5083 By Radiography ...

Vidya Joshi et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

STUDY OF DEFECTS IN FRICTION STIR WELDED AA 5083 BY RADIOGRAPHY, ULTRASONIC AND PHASED ARRAY ULTRASONIC TECHNIQUE

Vidya Joshi1, Krishnan Balasubramaniam1 and Raghu V. Prakash 2

1 Center for Non Destructive Evaluation and Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India.

2 Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India.

ABSTRACT. Friction Stir welding (FSW) is very effective technique mainly to weld the Aluminum alloy which are very difficult to weld by conventional welding method due to occurrence of defects. Even though FSW has some defects, heat input plays a very important role in occurrences of these defects. Heat input during the welding is dependent on the many factors like welding parameter (Rotational Speed and transverse speed), tool design (pin geometry, pin diameter, shoulder diameter etc.) and axial force. In the phased array ultrasonic technique, the excitation of piezo composite element can generate the focused beam with possibility of modifying beam parameters such as angle, focal distance and focal spot size. Thus this technique is useful to detect the randomly located and disoriented defects like crack, porosity etc. Certain variations in welding parameter generate the defects in the weld due to improper heat input. In this paper, the defects in FSW is studied by radiography, conventional ultrasonic and phased array ultrasonic technique and then compared.

Keywords: Friction Stir Welding, Radiography, Ultrasonic, Phased Array Ultrasonic technique.

INTRODUCTION Aluminum (Al) alloy is a light weight material and has many industrial application. But they are not easily weldable by conventional fusion welding techniques because the quality of the welded joint is deteriorated due to the presence of porosity, hot cracking and distortion [3]. To overcome this, The Welding Institute (TWI), United Kingdom in December 1991, has invented Friction Stir Welding (FSW) as a solid-state joining technique, and it was initially applied to aluminum alloys [1]. In the FSW process, a non-consumable tool with a specially designed pin and shoulder is used which is rotated as well as translated throughout the welding. This causes the generation of heat to plasticize the material and movement of plasticized material to join the material [2]. Dissimilar alloy material can also be joined with the FSW process [4, 5]. It is a new and promising welding process which welds the material below its melting temperature and it has shown superior features such as excellent joint performance, mechanical properties and low energy consumption. However many defects like porosity, kissing bong, solid inclusion and

NDE 2011, December 8-10, 2011

linear crack cavity or groove like defects, large mass of flash out are reported due to improper heat input during the process [7, 8]. It reduces the quality of the product and impacts the manufacturing cost. Thus is it very much required to find the defects in FSW by NDE. NDE plays a crucial role in ensuring cost effective operation, safety of use and reliability of a wide range of industrial components especially on aerospace, power generation, automotive, railway and petrochemical applications. Non-destructive testing (NDT) is based on techniques that rely on the application of physical principles to determine the characteristics of materials and to detect and assess flaws or harmful defects without change of the usefulness or serviceability of materials [9]. There is a broad range of NDT methods based on different physical principles like ultrasonic, eddy currents evaluation, X-radiography, magnetic particles inspection and dye penetrant application [11, 12, 13]. In this paper, X- radiography and ultrasonic methods are used to study the defects in FSW and compared. Radiography using X-rays is used for imaging the welds which helps to detect and locate the defects. In X-ray image of welds, variation in intensity is caused by in homogeneity of the welds [10]. In conventional ultrasonic and phased array ultrasonic technique, Ultrasonic waves interact with interface boundaries, grain interstices, pores, inclusions, cracks, etc. and gather substantial information about the details of the geometry and physical properties of the insonified medium. EXPERIMENTAL PROCEDURE A. FSW SAMPLE PREPARATION

The base material used in this study is AA 5083 aluminum alloy with thickness of 6.1 mm whose chemical composition is listed in the Table 1. Samples were butt welded using FSW machine along the rolling direction. The FSW process is schematically shown is the Fig. 1. A FSW tool made of HDS H13 material, with cylindrical taper profile having pin diameter 6 mm and 10° taper, pin length 5.72 mm, and shoulder diameter of 20 mm was chosen for welding.

Table 1. Chemical Composition

Si Fe Cu Mn Mg Cr Zn Ti Al

0.08 0.15 0.03 0.7 4.4 0.06 0.03 0.06 Remainder

The welding parameter like rotational and transverse speed, plunge depth, tool tilt, initial heating time and tool down speed are important input during the welding process. Rotational speed and transverse speed used are 300rpm and 60 mm/min. The plunge depth used during the welding process is 6.05mm and the tool tilt used is 20 towards the advancing side of weld. Initial heating time and tool down feed are 8 sec and 10 mm/min respectively. The FSW tool is shown in Fig. 2

Pin

Shoulder

Vidya Joshi et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Fig.1 Schematic of FSW [2] Fig.2 Tool for FSW

B. DIGITAL RADIOGRAPHY

Radiography was carried out using the digital radiography system (Make: Vidisco LTD). The X-ray was generated from the source with 100 KV of voltage, 0.5 mA of and the exposure time used for getting the image was 5 s. C. IMMERSION ULTRASONIC TESTING

Immersion Ultrasonic C scan testing was carried out using a probe with frequency of 10MHz, focal length of 2” and water acting as a couplant. The sample was scanned using a resolution of 0.2mm along the joining as well as across the joining. The experimental set up was as shown in the Fig 3. it also shows the view of immersion tank where probe is scanning the test sample. The first signal amplitude from the ultrasonic test method shows the front wall echo whereas the second signal amplitude shows the back wall echo. Electronic gates used to extract selected echoes from the entire return signal. Detection and localization of defect is done by the A, B and C scan. D. PHASED ARRAY ULTRASONIC TESTING

In the phased array ultrasonic technique, the excitation of piezocomposite element can generate the focused beam with possibility of modifying beam parameters such as angle, focal distance and focal spot size. Thus this technique is useful to detect the randomly located and disoriented defects like crack, porosity etc. A single crystal probe, with the limited movement and beam angle has a high probability of missing misoriented crack, or cracks located away from the beam axis. In this work 64 elements linear array probe of frequency 5 MHz was 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 horizontal axis corresponds to test piece width, and the vertical axis to depth. Fig 4 shows the phased array system used for the experiments (Model: OMNISCAN, make: RD Tech, Canada).

Fig.3 Ultrasonic Test – Experimental set up

RESULTS AND DISCUSSION

Defect 1

Fig 5 Experimental Specimen a) Before Milling b) After Milling

NDE 2011, December 8-10, 2011

Experimental set up

RESULTS AND DISCUSSION

Defect 1

Defect 2

a) Before Milling Fig 6 Radiography Image a) Before Milling b) After Milling

Fig 4 OMNISCAN; Phased Array Ultrasonic Testing (PAUT) System

a) Before Milling

OMNISCAN; Phased Array Ultrasonic

Vidya Joshi et al. : Proceedings of the National Seminar & Exhibition on Non

Fig 7 Immersion Ultrasonic Testing

The experimental sample used for the NDE 5. the samples were studied twice. first,was removed by milling from top and bottom of the plate to avoid irregularitiessignal and studied. The fig. 6 shows the radiography image of the samplemilling process. Fig 7 shows the A, B and C scan of the sample by immersion ultrasonic testing method. by both the method it is clear that sample is defect free. sample was studied with contact ultrasonic method by using theand 20 MHz frequency. It is found that the amplitude of signal got reduced at defect location with 20 MHz frequency probe while it remain unchanged with other two. The amplitude reduction at defect location shows that the energy abreflection of signal from the back wall. Figultrasonic method with 10 MHz, 15 MHz and 20 MHz frequency probe at defect 1, defect 2 and defect free location. The defect 1 and defect 2 was possiultrasonic technique, the images are shown in the Fig. 9phased array ultrasonic method due to its beam focusing and steering ability.

A Scan by Contact Ultrasonic method:

: Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

Immersion Ultrasonic Testing Image a) A Scan b) C Scan c) B Scan

The experimental sample used for the NDE investigation was as shown in the ed twice. first, after welding process, then thin layer of material from top and bottom of the plate to avoid irregularities

signal and studied. The fig. 6 shows the radiography image of the sample before and after milling process. Fig 7 shows the A, B and C scan of the sample by immersion ultrasonic testing method. by both the method it is clear that sample is defect free. but when same sample was studied with contact ultrasonic method by using the probe of 10 MHz, 15 MHz and 20 MHz frequency. It is found that the amplitude of signal got reduced at defect location with 20 MHz frequency probe while it remain unchanged with other two. The amplitude reduction at defect location shows that the energy absorption at defect and reflection of signal from the back wall. Fig. 8 shows the A scan obtained by contact

10 MHz, 15 MHz and 20 MHz frequency probe at defect 1, defect 2 and defect free location. The defect 1 and defect 2 was possible to detect by phased array ultrasonic technique, the images are shown in the Fig. 9. The defects were detected by phased array ultrasonic method due to its beam focusing and steering ability.

A Scan by Contact Ultrasonic method:

Destructive Evaluation

was as shown in the fig. n layer of material

from top and bottom of the plate to avoid irregularities in the before and after

milling process. Fig 7 shows the A, B and C scan of the sample by immersion ultrasonic but when same

probe of 10 MHz, 15 MHz and 20 MHz frequency. It is found that the amplitude of signal got reduced at defect location with 20 MHz frequency probe while it remain unchanged with other two. The

sorption at defect and 8 shows the A scan obtained by contact

10 MHz, 15 MHz and 20 MHz frequency probe at defect 1, defect ble to detect by phased array he defects were detected by

Fig 8 A Scan by Contact ultrasonic method by 10 MHz, 15 MHz and 20 MHz frequency

Fig 9 OMNISCAN; Phased Array Ultrasonic Testing (PAUT)

CONCLUSION

� The investigation of the defects in FSW is very much essential to obtain product quality.

� The ultrasonic technique to detect defect is better than radiography technique.� Defect detection by contact ultrasonic technique is due to back wall refection but

not by the scattering from the defect � Phased array ultrasonic technique

beam focusing and steering ability.

NDE 2011, December 8-10, 2011

A Scan by Contact ultrasonic method by 10 MHz, 15 MHz and 20 MHz frequency

OMNISCAN; Phased Array Ultrasonic Testing (PAUT) Image

The investigation of the defects in FSW is very much essential to obtain product

ultrasonic technique to detect defect is better than radiography technique.Defect detection by contact ultrasonic technique is due to back wall refection but not by the scattering from the defect hased array ultrasonic technique gives advantage over other method due to its

beam focusing and steering ability.

A Scan by Contact ultrasonic method by 10 MHz, 15 MHz and 20 MHz frequency

The investigation of the defects in FSW is very much essential to obtain product

ultrasonic technique to detect defect is better than radiography technique. Defect detection by contact ultrasonic technique is due to back wall refection but

her method due to its

Vidya Joshi et al. : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

ACKNOWLEDGEMENT The authors are very grateful to Prof. Satish Kailash, Department of Mechanical Engineering, Indian Institute of Science for availing the facility of Friction Stir welding to carry out this research. REFERENCES 1. W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Templesmith, C.J.

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