Metallurigical and Materials Engineering Department, National Institute of Technology, Tiruchirappalli, India

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1 More Info at Open Access Database Non-Destructive Evaluation Of Aluminum A6061-T6 Welds Produced by Friction Stir spot Welding Y.V.V.M. Mohana Rao 1, V.Kalyanavalli 1, D.Sastikumar 1, S.Muthukumaran 2 and S.Venukumar 2 1 Physics Department, National Institute of Technology, Tiruchirappalli, India 2 Metallurigical and Materials Engineering Department, National Institute of Technology, Tiruchirappalli, India Kalyani.we@gmail.com Keywords: Friction stir spot welding process, Tool hole refilling process, Eddy current testing, Radiography ABSTRACT In automotive industries, resistance spot welding is widely used for welding aluminum materials and very often cracks and pores observed in the welded region due to thermal effects. These defects could be reduced using friction stir spot welding. However, in the friction stir spot welding, presence of tool hole in the spot weld is a disadvantage. In the present work, a refilling process is proposed for refilling the tool hole produced in the spot welds and compared with the conventional refilling process. Aluminum A6061-T6 plates of thickness 2mm at rotational speed of the tool at 900 and 1800 rpm were used. Eddy current inspection and radiography testing were carried out on refilled spot welds for defects detection. Defect free refilling of the tool hole of AA6061-T6 is obtained at a rotational speed of 1800 rpm. Introduction Friction stir welding process is a solid-state welding technology widely used in industries for welding aluminum. The welding process was done by plastic deformation of the base material by the frictional force produced by the tool and shoulder. As an extension of friction stir welding, the friction stir spot welding (FSSW) was developed. FSSW mentioned above can also be called plunge friction stir welding (PFSSW) or conventional friction stir spot welding, which was patented by Mazda in 2003 [1]. However, one of the disadvantages of the conventional FSSW joint is a probe hole left after tool retraction, which inevitably remains at the centre of the weld nugget reducing the joint strength and at which corrosion could take place preferentially because rainwater remains in the hole, where body paint barely reaches the bottom [2]. The refilling technique developed so far sometimes fails to fully refill the plasticized material in the probe hole thus void formation might result in the refilled region. The friction forming process was proposed by one of the present authors [3].This process is suitable for ductile materials and a desired shape or contour can be achieved by generating heat by friction using a suitable external tool called the friction forming tool. Based on the friction forming process, a new friction spot welding process was reported by John Prakash and Muthukumaran known as friction stir spot welding with refilling by friction forming process (RFSSW) [4]. In the present work, the hole in the friction stir spot welds were refilled by conventional and newly proposed refilling process at different tool speed of 900 and 1800 rpm. The refilled spot welds were tested using eddy current testing and radiography for a defect free refilling. Experiment procedure: In the present study, commercially pure aluminum sheets of 3 mm were chosen and the Work piece length, width and overlapping length were100 mm, 25 mm and 50 mm, respectively. The FSW and RFSSW welding were done at a speed of 900 and 1800 rpm. In conventional FSSW

the tool consists of a shoulder of 18 mm diameter and a pin of 3 mm diameter. In RFSSW two types of tools namely, the extrusion tool and the friction forming tool were used.

Fig. 1. Schematic illustration of FSSW-FFP A schematic representation of RFSSW is shown in Fig. 1. This process involves basically two steps namely extrusion and refilling.

The frictional heating at the interface between the tool and work piece enables the softening, deformation, and displacement of work material and creates a bushing projection in the bottom plate

in the second step namely refilling, the bush type material which had come out during first step was plunged back by the friction forming tool as shown in Figs. 1d&1e.

2 the tool consists of a shoulder of 18 mm diameter and a pin of 3 mm diameter. In RFSSW two types of tools namely, the extrusion tool and the friction forming tool were used. The extrusion tool was similar to the conventional FSSW tool but the pin length was greater. In the modified refilling process, the extrusion tool used has convex edge to do the refilling process. Fig. 1. Schematic illustration of FSSW-FFP A schematic representation of RFSSW is shown in Fig. 1. This process involves basically two steps namely extrusion and refilling. The first step involves forcing a rotating tool through a sheet metal work piece. The frictional heating at the interface between the tool and work piece enables the softening, deformation, and displacement of work material and creates a bushing projection in the bottom plate leaving behind a probe hole in the top plate, as shown in Fig. 1c. This was followed by reversal of the remaining two plates for refilling operation.in the second step namely refilling, the bush type material which had come out during first step was plunged back by the friction forming tool as shown in Figs. 1d&1e. [5] Tool hole (a) Sample with tool hole Tool hole refilled (b) sample with tool hole refilled Fig. 2 Macrocopic view of samples In the modified refilling process, the conventionally FSSW spot welds are taken. A plate in the size of the tool hole is placed on the hole. On rotation of the tool the friction produced enables the heating between the tool and filler plate which fills the tool hole in the work piece. The refilling process and the welding parameters are given in Table 1. The images of the sample are given in Figure 2.The non-destructive studies of the refilled spot welds were done by eddy current testing and radiography.

Table 1: Process parameters of the refilling process Process Sample Rotational Speed Pin length Plunge depth Conventional FSSW process Conventional

The eddy current testing to detect flaws in the welded area was done by conventional eddy current system (Olympus Model: Norteh 500 series) for these

3 shows the eddy current results of the friction stir spot welded aluminum samples. The Fig.

hole at the center of the weld as indicated. The Fig.

3 Table 1: Process parameters of the refilling process Process Sample Rotational Speed Pin length Plunge depth Conventional FSSW process Conventional refilling process Modified Refilling process Results and Discussion. The eddy current testing to detect flaws in the welded area was done by conventional eddy current system (Olympus Model: Norteh 500 series) for these studies and 700 Hz frequency probe was used. The Fig. 3 shows the eddy current results of the friction stir spot welded aluminum samples. The Fig.3 (a) and 3 (b) are the eddy current results of the conventional FSW samples, and the samples are defect free sample except for the presence of the tool hole at the center of the weld as indicated. The Fig.3 (c) and 3 (d) are the results for the conventional RFSSW for the speed of 800 and 1200 rpm, which show the presence of defect in the refilled spot welds. The Fig.3 (e) and 3(f) are the eddy current results of the modified RFSSW for the speed of 800 and 1200 rpm. The Fig.3 (f) suggest a defect-free refilling process. (a)ect of Conventional FSSW at 900rpm (b) ECT of Conventional FSSW at 1800rpm (c) ECT of RFSSW at 900rpm (d) ECT of RFSSW at1800rpm

(e) ECT of modified-rfssw at 900rpm (f) ECT of modified-rfssw at 1800rpm Fig.

The RT was carried out on the AA6061-T6 FSSW samples along the parallel direction of the localized joint. The Fig.

4 (a) and 4 (b) are the image of conventional FSSW for the speed of 900rpm and 1800rpm,and show a defect free weld with tool hole in the

4 (e) ECT of modified-rfssw at 900rpm (f) ECT of modified-rfssw at 1800rpm Fig. 3: Eddy current results of the samples 1-6 The radiographic analysis was done using x-rays and the model used for the experiment is RPD 2OOnp, Make: SYNTEZNDT,Russia. In case of light metals like aluminum and its alloys X- ray analysis is done compared to other radiographic methods. The RT was carried out on the AA6061-T6 FSSW samples along the parallel direction of the localized joint. The Fig. 4 show the x-ray radiography images of friction stir spot welded samples. The Fig.4 (a) and 4 (b) are the image of conventional FSSW for the speed of 900rpm and 1800rpm,and show a defect free weld with tool hole in the center. The Fig.4 (c) and (d) shows the radiographic image of conventional RFSSW, which show the presence of lack of penetration in the refilled weld zone. The Fig.4 (e) and 4 (f) are the images of modified RFSSW at speed of 900 and 1800 rpm. For the speed of 1800 in the modified refilling process we can see a defect free refilling process. (a) RT of Conventional FSSW at 900rpm (b) RT of Conventional FSSW at 1800rpm

(c) RT of RFSSW at 900rpm (d) RT of RFSSW at 1800rpm (e) RT of modified-rfssw at 900rpm (f) RT of modified-rfssw at 1800rpm Fig.

spot welding of AA6061- T6 was done by conventional and modified refilling process.

The Non-Destructive Evaluation of the samples was done by eddy current testing and radiographic testing.

refilling process. References: [1]. T. Iwashita, U.S. Patent S6601751B2. (2003) [2]. Y. Uematsu, K. Tokaji, Y.Tozaki, T.

5 (c) RT of RFSSW at 900rpm (d) RT of RFSSW at 1800rpm (e) RT of modified-rfssw at 900rpm (f) RT of modified-rfssw at 1800rpm Fig. 4: Radiography images of the samples 1-6 Conclusions: In the present study, the refilling of the tool hole in the friction stir spot welding of AA6061- T6 was done by conventional and modified refilling process. The refilling process was done at tool speeds of 900rpm and 1800 rpm in both methods. The Non-Destructive Evaluation of the samples was done by eddy current testing and radiographic testing. Defect free refilling of the tool hole in AA6061-T6 is obtained by using tool rotational speed at 1800 rpm by modified refilling process. References: [1]. T. Iwashita, U.S. Patent S B2. (2003) [2]. Y. Uematsu, K. Tokaji, Y.Tozaki, T. Kurita, S. Murata, Effect of re-filling probe hole on tensile failure and fatigue behavior of friction stir spot welded joints in Al Mg Si alloy. International Journal for Fatigue. 30 (2008) [3]. S. Muthukumaran. Indian patent (2010)

6 [4]. S. John Prakash, S. Muthukumaran, Refilling Probe Hole of Friction Spot Joints by Friction Forming, Material Manufacturing Process. 26 (2011) [5]. S. Venukumar, S. Muthukumaran, Y. Swaroop, Microstructure and Mechanical Properties of Refilled Friction Stir Spot Welding of Commercial Pure Aluminium, Materials Science Forum. 765 (2013) [6] Philip B Prangnell and Dimitrios Bakavos, Novel Approaches to Friction Spot Welding of Thin Aluminum Automotive Sheet Materials Science Forum, (2010) [7] S Satonaka, C Iwamoto, R Qui & T Fujioka, Trends and new applications of spot welding for aluminum alloy sheets, Welding International (11) / Selected from Journal of Light Metal Welding & Construction (2) 41-48; Reference JL/06/2/41; Translation [8] Chiang Mai, Chee Fai Tan, and Mohamad R. Said, Effect of Hardness Test on Precipitation Hardening Aluminum Alloy 6061-T, Journal of Science. 36(3) ( 2009 ) : [9] Yoshiaki Yamamoto, Hiizu Ochi, Takeshi Sawai, Hiroshi Yamaguchi and Koichi Ogawa, Fatigue Strength of Friction-Welded 6061 Aluminum Alloy Joints Materials Transactions. 48 (2007) [10] Telmo Santos, Pedro Vilaca and Luisa Quintino, Developments in NDT for Detecting Imperfections in Friction Stir Spot Welds in Aluminum Alloys, Technical University of Lisbon, Portugal. [11] Okan Okay Kocak, Defect Assessment of Spot welds by NDI, MS thesis (September 2003) from The Middle East Technical University, Turkey,21-50, [12] Triyono, Jamasri, M.N.Ilman, R.Soekrisno and Sutiarso, Assessment of Nugget Size of Spot Weld using Neutron Radiography, Triyono, et al / Atom Indonesia 37 (2011) [13] Waldo J. Perez Regalado, Andriy M. Chertov, Roman Gr. Maev, Valdir Furlanetto, Integration of the Ultrasonic Real-Time Spot Weld Monitoring System, 5th Pan American Conference for NDT, 2-6 October 2011, Cancun, Mexico

Comparison of Resistance Spot Welding and Refill Friction Stir Welding of Al 7075 Sheets. Jeff Hou

Comparison of Resistance Spot Welding and Refill Friction Stir Welding of Al 7075 Sheets Jeff Hou Z.Shen, Y. Chen, N. Zhou, M. Worswick, A. Gerlich, K. Chan, N. Scotchmer 1 Friction Stir Welding (FSW)