Non-destructive Evaluation of Friction Stir Welded Joints ... · strength of the FSW joint for AC4C Al alloy and Steel dissimilar friction stir lap joints [7]. Aluminum alloy welds

Procedia Engineering 86 ( 2014 ) 469 – 475

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1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the Indira Gandhi Centre for Atomic Researchdoi: 10.1016/j.proeng.2014.11.060

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1st International Conference on Structural Integrity, ICONS-2014

Non-Destructive Evaluation of Friction Stir Welded Joints by X-ray Radiography and Infrared Thermography

T. Saravanana,*, B.B. Lahiria, K. Arunmuthua, S. Bagavathiappana, A.S.Sekharb, V.P.M. Pillaib, J.Philipa, B.P.C. Raoa and T. Jayakumara

aMetallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India

bDepartment of Optoelectronics, University of Kerala, Trivandrum-695581, Kerala, India *E-mail ID: tsara@igcar.gov.in

Abstract

Digital X-ray radiography and infrared thermography techniques are used for evaluation of the quality of the friction stir welded aluminum butt joints and aluminum-zinc coated steel dissimilar lap joints. Digital frame integration and gradient operation based image processing techniques are used on the radiography images which ensured 48% increase in the signal-to-noise ratio. The effects of various welding parameters like tool rotation; travel speed etc. on the quality of the weld are studied. A sub-surface tunnel defect along the weld line of a butt-joint is detected using infrared thermography technique and it is observed that the rate of temperature decay is lower for the defect regions. Using lock-in thermography technique, the optimum frequency is determined and the defect depth is quantified at this frequency. Improved visualization and contrast sensitivity are achieved using adaptive single plateau based histogram equalization on the acquired infrared images. © 2014 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research.

Keywords: Friction stir welding, Digital X-ray radiography, Infrared thermography, Lock-in thermography, Image processing

1. Introduction

Weld joints are the origins of structural weakness in maximum cases and must be routinely inspected to ensure structural integrity of the fabricated components. Friction stir welding (FSW) is gaining popularity in engineering industries. FSW is a solid state joining process where simultaneous presence of forging pressure and frictional heating causes the metal pieces to fuse together to form weld joints and the microstructural characteristic remains mostly unchanged. FSW offers a number of advantages like less porosity, shrinkage and distortion, absence of melting and any filler materials resulting in less weld contamination and less number of process variables to control. Friction stir welded joints show superior mechanical properties because of the re-crystallized fine and equi-

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research

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470 T. Saravanan et al. / Procedia Engineering 86 ( 2014 ) 469 – 475

axed grains in the stirred zone. FSW is associated with some unique defects such as kissing bonds, cavity or groove like defects (sub-surface tunnel defects) due to insufficient heat input, voids, porosities, lack of bonding, large masses of flash due to excessive heating and abnormal stirring induced cavities [1, 2]. Non-destructive evaluation (NDE) of FSW joints is essential as defects in weld joints may render a part of the joint unable to meet required merits and their presence may cause premature failure of the joints. Hence, there is a strong need for inspection and monitoring of FSW joints. Farley et al. qualitatively described the importance of NDE in weld inspection by the following relation [3].

Probability of weld failure = probability of flaw occurring × probability of NDE missing the flaw × probability of flaw growing

In this paper, digital X-ray radiography (DXRG) and infrared thermography (IRT) techniques are used for defect detection and quality evaluation of two types of friction stir welded joints, viz. aluminum butt joint and aluminum and zinc coated steel dissimilar lap joints [4, 5]. Defect-free butt joints of 3003 Al alloy to mild steel plates with 3 mm thickness were made using FSW process and the effects of welding speed, rotation speed and tool shoulder diameter on the micro-structure and strength of weld was reported [6]. Chen et al have reported that the surface state of the steel (zinc-coated steel, brushed finish steel and mirror finish steel) influences the mechanical strength of the FSW joint for AC4C Al alloy and Steel dissimilar friction stir lap joints [7]. Aluminum alloy welds produced by FSW process has been evaluated by X-ray radiography, conventional ultrasonic NDE procedure and phased array ultrasonic technique for varied welding parameters such as FS tool rotational and traverse speed [8]. Radiography is based on the differential absorption of radiation on its passage through the matter, whereas, in IRT, infrared rays (wavelength lies between 0.75-1000 μm) emitted by an object is detected by an infrared detector and the temperature of the object is measured in a non-contact way from the intensity of the emitted infrared waves. The objective of the present study is to develop a DXRG procedure for dissimilar aluminum and zinc coated steel FSW lap joints and to explore ways for improved signal to noise ratio (SNR) and for enhancing the defect detection limit. This methodology uses digital frame integration for acquisition of data to increase the SNR followed by high-pass filtering to sharpen the image by gradient operation and contrast adjustments to detect micro defects in FSW joints. A sub-surface tunnel defect along the weld line of a friction stir welded butt joint is detected using active IRT techniques. Using lock-in thermography (LI-IRT) technique, the optimum frequency is determined and defect depth is quantified at this frequency. Point operation based image processing techniques are used for enhancing the contrast of the LI-IRT images. Improved visualization and contrast sensitivity is achieved through adaptive single plateau histogram equalization of the acquired LI-IRT images.

2. Materials and Methods

2.1 Materials

Dissimilar lap joints (150 × 100 mm) were fabricated using 2 mm thick aluminum (grade 6061) and zinc coated steel (SS 316) sheet of 1 mm thickness. The lap joints were manufactured using a friction stir welder (Model: RM 1A-0.7) under displacement control mode. The weld joints were produced for a dwell time of 2 s and for two different depths of 2.3 and 2.45 mm, respectively. The dimension of the friction stir welded aluminum (grade 6061) butt joint is 250 × 145 × 3 mm. IRT was performed on this specimen. The specimen surfaces were black painted to enhance the emissivity.

2.2 Experimental Method

DXRG of the weld joints were carried out using a 450kV Balteau constant potential X-ray unit (focal spot size 1.2 mm) and flash scan FS35 Thales flat panel detector was used as the X-ray detector (127 μm pixel pitch). The radiography exposure parameters are given in Table-1. The X-ray images were acquired after an exposure time of 4.2 seconds i.e. 1 frame (frame time) and by integration of a number of single frame of X-ray images. VI3 software was used to integrate the multiple frames of data into one image.

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Table 1 X-ray radiography exposure parameters for FSW joints

Parameters Dissimilar lap joint Aluminum butt joint Voltage (kV) 135 125 Current (mA) 1 1

Exposure time (s) 4.2/105 4.2/105 Number of frames 1/25 (integration) 25 (integration) Source –to-Object

Distance (SOD) (mm) 1000 950

Object –to- Detector Distance (ODD) (mm)

Contact 50

Magnification 1.0 1.05

For conventional active IRT the specimen was heated using one 1 kW halogen lamps and the surface temperature evolution was observed in the transient domain while natural cooling using a FLIR SC 5000 infrared camera (spectral range: 2.0-5.1 μm). Temperature was recorded from the blind side of the specimen. The camera has indium antimonide (InSb) detector with a two dimensional array of 320×256 elements. The detector elements are cooled using Stirling cycle and thermal sensitivity of the camera is better than 25 mK. The infrared camera was positioned at a distance of 700 mm away from the specimens in such a way that the axis of the camera coincides with the axis of the specimen. For LI-IRT, the specimen was heated by sinusoidally modulated heat waves from two 1 kW halogen lamps kept at 300 mm away from the specimen. For generation of sine waves of a single frequency, a programmable frequency generator (HM 8131-2, Hameg) was used. LI-IRT images were acquired using ALTAIR LI software. Several excitation frequencies were used to determine the optimum frequency. As heating of aluminum specimen using optical excitation is difficult due to its high thermal diffusivity, prior to lock-in thermography, appropriate pre-heating was done.

3. Results and Discussion

Figures 1 (a & b) show the radiographic images obtained with 1 and 25 frames exposure, respectively for a typical FSW joint made at a rotational speed of 500 min-1 and travel speed of 50 mm min-1 for a plunger depth of 2.30 mm. From Fig.1a, on the weld joint location, the normalized SNR was measured as 114, whereas, after integration (Fig. 1b) SNR increased to 169. The radiographic image in Fig. 1a is not acceptable, as the ASTM E 2737 accepts the images with SNR > 130. As an image quality indication ASTM 2-B wire penetrameter was pasted on the source side of the specimen as per the standards. Using the modified digital radiography procedure, the 63 μm thick IQI was clearly seen. Figure 2a shows the radiographic image of a FSW joint for the welding parameters of 1000 min-1 rotational speed and 50 mm min-1 travel speed and penetration depth of 2.45 mm. From Fig. 2a, it is observed that the heat energy produced is optimal which resulted in a sound (defect-free) FSW joint. These welding parameters resulted in reduction of thermo-mechanically affected zone (TMAZ) thickness in the FSW plate which facilitated the improved contrast of TMA zone in the radiography image. Figure 2b shows the radiography image of a FSW joint with a tool rotational speed of 1500 min-1 under identical welding and X-ray exposure parameters to that of Fig. 2a. At the same travel speed of 50 mm min-1, due to higher thermal gradient at higher tool rotational speed of 1500 min-1, small cavities were present in the weld as indicated in Fig. 2b. At a constant travel speed of 50 mm min-1, with increase in rpm, energy input increases from 88.9 kJ mm-1 to 223.8 kJ mm-1, the Z force decreases from 6 kN to 3.5 kN and the torque decreases from 15 Nm to 11.2 Nm. Thus, the material was more softened with increasing energy input, forming a more sticking condition at 1500 rpm and 50 mm min-1. Under these conditions material reaches to a state of abnormal stirring with a very low downward force and torque which may lead to the formation of cavity.

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Fig. 1 Digital X-ray radiography image of a FSW joint with rotational speed of 500 min-1 and travel speed of 50 mm min-1 for a plunger depth of 2.30 mm (a) exposure performed at 1 frame (b) exposure with 25 frames integration.

Fig. 2 Radiography image of the FSW joint with a travel speed of 50 mm min-1 for a plunger depth of 2.45 mm with weld tool tip rotational speed of (a) 1000 min-1 and (b) 1500 min-1.

Fig. 3 X-ray radiography image of the FSW joint with rotational speed of 500 min-1 and travel speed of 50 mm min-1

(a) plunger depth of 2.30 mm (b) plunger depth 2.45mm

Figure 3a shows the radiography image of the FSW lap joint for the welding parameters of 500 min-1 rotational speed and 50 mm min-1 travel speed for a penetration depth of 2.30 mm. It can be observed from Fig. 3a that the voids and porosities are present. The formation of defects was due to poor material flow-ability as the frictional heat input produced is less in the case of 2.30 mm penetration depth conditions. Figure 3b shows the radiography image of FSW joint for the welding parameters of 500 min-1 rotational speed and 50 mm min-1 travel speed for a penetration depth of 2.45 mm. From Fig. 3b, it is observed that a lack of bonding type defect is present as indicated by arrows at the root side. For the welding parameters of 500 min-1 rotational speed and 50 mm min-1 travel speed, the heat energy produced was insufficient to plasticize the weld zone. Thus, a cold weld was formed at the weld joint.

(a) (b)

(b) (a)

(a) (b)

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Figure 4a shows the temperature decaaluminum butt weld. It can be seen from Fig. 4compared to the defect-free region. This is attrfilled with air as probability of contamination several times lower than that of aluminum and hThe temperature decay data was modeled with athe defect and defect-free regions are found totemperature of the defective region indeed decathermal diffusion length [μ; μ= (2 / ) 0.5, whfrequency] must be of the order of defect depthcontrast by a thermal wave of a fixed frequenaluminum specimen and the optimum frequencydetermined. Figure 4b shows the variation of phregions) with excitation frequency. As can be sattains a maximum at 11.1 Hz and thereafter, doptimum frequency, defect depth (1.8μ) was est

Fig. 4 (a) Temperature decay as a function of ti(b) Phase contrast as a function

Figure 5a shows the phase image of thdefect can be clearly seen from the image. Figurof the defect can be distinctly identified. Figuradiography and lock-in thermography, respecsuitable pixel calibration within an error margin59.00 (60.00) and 0.53 mm (0.55 mm), respecdeveloped for improving the contrast sensitivitymost widely used point operation technique bbackground, blurriness of edges and reduced SNinfrared thermography) with large background aimplemented [9]. In single plateau histogramcontrolled using a plateau threshold value whichautomatic determination of this threshold value Once through and recursive adaptive single plaresults for the aluminum specimen. The variashown in Fig. 6e and it can be seen that once thand 10%, respectively compared to the conve

ay as a function of time for the defect and defect-free re4a that the rate of temperature decay is slower in the de

ributed to the fact that the defect region is devoid of aluis very low in friction stir welding. Thermal conductiv

hence, the temperature of the defect region decreases at a an exponential equation. The time constants of temperaturo be 0.00646 and 0.00671 s, respectively, which confirays at a slower rate. For quantification of defects using here is the thermal diffusivity and is the angulars. Therefore, defects of a particular depth are visible with

ncy. A range of excitation frequencies (9-13 Hz) were ty, where the defect can be demarcated with maximum co

hase contrast (i.e. phase difference between the defect andseen initially the phase contrast increases with excitationdecreases to a very low value. From the thermal migratiotimated as 2.79 mm.

ime for the defect and defect-free region of the aluminumn of excitation frquency for the aluminum butt joint.

e aluminum specimen acquired at 11.1 Hz excitation freqre 5b shows a 2D colour map of the phase image, where ture 5c shows the magnified view of the defect from ditively. The length and width of the defects were deter

n of ±3.6%. Estimated (and actual) length and width of thctively. Point operations based image enhancement techny of the infrared images. Conventional histogram equalizbut suffers from serious drawbacks like over equaliza

NR. For non-Gaussian histogram images (which are very and small target, single plateau histogram equalization tecm equalization technique, the equalization of the bach is in general image dependent. An adaptive algorithm wand gray levels greater than this are truncated to this thres

ateau histogram equalization was implemented and Fig. 6ation of intensity contrast with the image processing tehrough and recursive technique enhances the intensity conentional histogram equalization technique. This clearly

egion of the efect region

uminum and ity of air is slower rate. re decay for rms that the LI-IRT, the r excitation h maximum tried for the ontrast, was

d defect-free n frequency, on length at

m butt joint.

quency. The the presence igital X-ray mined after

he defect are niques were zation is the ation of the common in

chnique was ckground is was used for shold value. 6 shows the echniques is ntrast by 80 shows that

474 T. Saravanan et al. / Procedia Engineering 86 ( 2014 ) 469 – 475

plateau histogram equalization based processing of infrared thermography enhances the contrast and enables visualization of defects. This technique is highly suitable for defect detection in weld joints using IRT.

Fig. 5 (a) Phase image of the aluminum butt joint specimen at 11.1 Hz lock-in frequency. (b) 2D colour map of the phase image. (c) Magnified view of the defect region from XRG and LI-IRT images.

Fig. 6 (a) Original phase image, (b) conventional histogram equalized image, (c) once through single plateau histogram equalized image, (d) recursive single plateau histogram equalized image and (e) Comparison of intensity contrast achieved by various image processing algorithms.

4. Conclusion

A digital X-ray radiography procedure is developed to detect micro-pores and voids in aluminum-zinc coated steel dissimilar friction stir weld lap joints. The studies confirmed that by using this procedure, signal to noise ratio increases by 48%. This methodology has been successfully used to study the effect of welding parameters like rotational speed, travel speed and penetration depth. A sub-surface tunnel defect along the weld line was detected using infrared thermography and it was observed that the rate of temperature decay was slower for the defect region. Using lock-in thermography, defect depth was quantified from the thermal diffusion length. Compared to conventional histogram equalization, intensity contrast was increased by 80% using single plateau histogram equalization of the acquired lock-in thermography images.

5. Acknowledgement

The authors thank Professor T. K. Pal and Mr. Hrishikesh Das, Jadavpur University, India for fruitful discussions and technical help.

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6. References

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