DEFECT ANALYSIS OF FRICTION STIR WELDED STEEL …Defect Analysis Of Friction Stir Welded Steel Plates On Different Tool Profiles 95 In the weld length, at 25mm cross section as shown

International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 2, Issue- 7, July-2014

Defect Analysis Of Friction Stir Welded Steel Plates On Different Tool Profiles

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DEFECT ANALYSIS OF FRICTION STIR WELDED STEEL PLATES ON DIFFERENT TOOL PROFILES

1A.PRADEEP, 2S.MUTHUKUMARAN

1, 2Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli

Email: [email protected], [email protected]

Abstract: Tool design in Friction stir welding (FSW) of steel is a critical problem and hence experiments were carried on with three combinations of tool profiles having tapered pin, concave and flat shoulders. The study of tool wear resulted in a new finding of Sub shoulder tool and gave reduced tool wear. The tool wear is indirectly being reduced by using a shorter pin length of 2.6mm. Rupture of material surfaces which were in contact created reduction in the tool geometry and defect on welds were obtained. Based on the tool shape corresponding to its weld length, defects were occurred in the weld zone. This study is concentrated on finding the possible defects during welding by using different tool profiles using ultrasonic, radiography and thermography techniques. It was observed from the weld that the flat shoulder gave improper joining defects while the concave shoulder gave reduced defects when compared to flat shoulder, and the Sub shoulder is defect free. Keywords: Tool wear, Ultrasonic scan, Thermography, Radiography. I. INTRODUCTION Friction Stir Welding (FSW) has a non consumable tool with a shoulder and pin [1]. The shape of the shoulder and the pin has its contribution in forming better joint [2-3]. Defects in FSW welds were numerous namely the worm hole, pin hole, tunnel defect, kissing bond, piping defect, groove defect and root defect [4-12]. All these defects occurred due to improper selection of tool geometry and process parameter, as these are the two major elements which guide the FSW processes. The defects can be majorly classified into two, namely visible and non visible defects. Root and surface defects can be visible, which arises due to the insufficient penetration of tool pin provided by the FSW tool. While tunnel defect and kissing bond are non-visible defects, which occurs due to improper joining of metals. Without disturbing the weld piece, there are numerous Non-Destructive Techniques (NDT) to inspect the weld formation. To perform the NDT, the surfaces of the work piece must be clean and flat. In such a case, the flash on the welded plates were removed and held for testing. Ultrasonic testing, liquid penetrant test, radiography and eddy current techniques which are more frequently used in major industries and research labs. The other biggest challenge in FSW is the tool wear and in particular, steels face highest problems [13-14]. PCBN and Tungsten carbide were used as tool material for welding steels [15]. PCBN tools being highly expensive and tungsten alloy tools faces wear problems. When straight pin profiles were used for FSW of aluminium, the wear rate is negligible, but the same profiles when used with steels the tool wear rate is very high. Hence, tapered pin were used for FSW of steels [16]. In addition to this, the work plates were having hole at the insertion point, in order to prevent initial tool wear. The present authors conducted study based on the tool profiles and came out with optimized process parameter. They resulted

with a new optimized tool shape named sub shoulder tool, which gave reduced tool wear [17]. The work piece was tested for each 300mm length of weld. When the tool with flat shoulder produced 500mm length of weld, the tool with concave shoulder produced 900mm length of weld; the sub shoulder tool travelled a highest weld length of 1200mm. Here the defect analysis is performed on the welded plates by using three NDT techniques namely digital thermography, radiography and ultrasonic A, B and C scan. The aim of this study is to quantify the defects which are possible in the welded plates of three different tool profiles by which feasibility of application of the tool shape for reduced tool wear is achieved. II. EXPERIMENTAL PROCEDURE Friction stir butt weld was conducted using IS: 3039, grade II (0.204 wt% C, 0.129 wt% Si, 1.10 wt% Mn, 0.309 wt% Cr) low alloy steel plates, having dimensions of 100x50x3mm. A modified vertical milling machine was used to perform this Friction Stir Welding (FSW) processes. In this investigation Tungsten alloy tool (W-5.7908Ni-3.2318Fe-0.2228Mo-0.1214Co-0.0709O2 wt-%) was used to perform the tool wear study. Shorter pin length of 2.6mm, shoulder diameter of 25mm, and pin diameter of 12mm at the root and 4mm at the tip were maintained as constant. Three tool profiles varying the shoulder shape namely concave, flat and sub shoulders were used for this study is shown in Fig.1. 2° of concavity was used for concave shoulder. The sub shoulder tool has two steps of 1.3mm each, in which the upper step acts as a sub shoulder. One of the steps has the 12mm diameter, which is the upper step connected to the shoulder and lower step with a diameter of 6mm. The tool wear study is carried out for every 300mm length of weld, since 100mm shows negligible difference in



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tool dimension. It was observed that the tool of flat shoulder travelled 500mm length of weld and found that the tool pin was totally worn out. Concave shoulder travelled a weld length of 900mm and left some impression of tool pin, but the tool is unsuitable for further welding. In case of Concave Sub Shoulder, it travelled a length of 1200mm and left with some impression of tool pin. All the welded samples were tested using NDT techniques. The joint region was subjected to

immersion type C-scan testing. A single probe of 10 MHz is used. 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. Conventional ultrasonic A and B scan testing is done by using an angle probe of 70° and frequency 4MHz. The inspection is done between half skip distance and full skip distance.

Fig.1 Tool Profiles (a) Flat Shoulder (b) Concave shoulder (c) Sub Shoulder

Ir-192 is the source used for radiographic testing with strength of 14 Curie. The size of the source is 2.7 x 3 mm. The source to film distance is 500mm. Single wall single image has been the technique used here. The penetrameter used is of type ASTM 12, and the sensitivity is 2T. Passive thermography has been done on the FSW samples. The samples are heated at 150°C in a furnace, and thermal image is obtained by using the digital thermal camera with an emissivity of 0.70 and sensitivity of 0.1° C. the 3D-dimensional view of image is obtained by using Image j software. The welded samples are tested for ultimate tensile strength using hounsefield tensometer and are prepared according to the ASTM E8 standard.

III. RESULTS AND DISCUSSION An optimum parameter of tool rotational speed 1120rpm, travel feed of 8mm/sec and tool tilt angle of 1° is used for all welds [16]. The observations were made on those welded plates of flat, concave and Sub shoulder tool profiles. Friction stir welded plates are tested for ultimate tensile strength and the values obtained for concave, flat and Sub Shoulder tool profiles are depicted in Table.1. The base metal tensile strength was found to be 442N/mm2. All the welded samples were tested using Non destructive techniques. It was found that similar kind of defects like void, lack of penetration, etc. was seen in the welded plates.

Table.1 Ultimate tensile strength for three tool profiles

Weld length in mm

Ultimate tensile strength N/mm2

(Sub Shoulder tool)


(Concave Shoulder tool)


(Flat Shoulder tool ) 300 480 469 445 600 472 461 423

3.1 Flat shoulder tool The surface of flat shoulder will be completely been in contact with the work piece surface. The frictional contact of this flat surface might produce higher temperature, which makes the material to its plastic material flow quickly. In terms of tool wear for steel plates, this flat surface rubs over the interface surface and results in higher tool wear rate when compared to

other profiles. Defective Weld is formed by using this flat shoulder. Appearance of welded sample is shown in Fig. 2. The surface of the welded sample found to have voids on the upper surface itself. At the bottom of the welded plate, there is a root defect. The marked region opposite to the exit hole shows higher attenuation, which represents the defect in the



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particular region as shown in Fig.3 (a) and this, is the C – scan image of flat shoulder. The defect is a void shown in a circle. The defect is at a depth of 2.8mm which is shown in the A – scan image of Fig.3 (b).

Fig.2 Appearance of flat shoulder tool welded

plate

Fig.3 Flat shoulder tool welded plate (a) C scan (b) A scan (c) B scan (d) Radiography

Fig.4 Thermograph of Flat shoulder welded plate

Fig.5 Appearance of concave shoulder tool welded plate

(a)

(b)

(c) (d)



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In the weld length, at 25mm cross section as shown in Fig.3(c) indicates the defect region with a variation in the signals. The B – scan image shows the defect is at a depth of 2.8mm and size of the defect is 11x5mm. The radiograph reveals that there is void at the marked position as shown in the Fig.3 (d). The voids were seen randomly distributed along the weld length, which is clearly visible in the thermography as shown in the Fig.4. The defects shown in all the NDT technique proves that voids were evenly distributed throughout the weld length. The defects are mainly due to the incomplete joining of metals. Sufficient compaction would have not gained for the weld to be formed. The other important criterion was that a large amount of temperature is evolved during friction stir welding of steels. The tool got worn out at a rapid rate from its initial state itself. The defect which occurred maximum in the welded plates of flat shoulder tool is tunnel defect and voids at greater depth. 3.2 Concave shoulder tool Contact surface of the concave shoulder is not completely in touch with the steel plate, and this avoids minimal tool wear. The pressure for compaction given by the concave shoulder to the work plate is much higher than the flat surface. The plasticized material flow cannot move out of the shoulder compaction, which is under high pressure and the stirring action of the pin moves the material from advancing side to the retreating side. This concave surface creates suitable plastic flow in the gap between the shoulder and work plate, thereby compaction offered by the tool is higher than the flat shoulder. The circumference of the shoulder is chamfered to avoid sharp edges, but during the welding, flash has occurred as shown in Fig.5.

Concave shoulder tool gave good welds till 600mm length of welding. The tool wear was rapid after 600mm length of weld. After 900mm length of weld, the bonding between the plates was mechanical and not metallurgical bonding. The plates can be easily separated just applying little force on hands. Rarely defects were seen on the welded plates of concave shoulder and hence less surface voids were found on the welded plates after 600mm. The shape of tool shoulder has greater importance in forming a better weld. Thus when compared to flat shoulder the concave shoulder tool gave higher length of weld. An interface lining can be seen at the bottom side of the plate. This lining is not the root defect which is as formed in the flat shoulder. After grinding the surface for sample preparation, this line is not visible. The welded plate under ultrasonic testing shows a void over the plate as shown in C – scan image of Fig.6 (a). A defect is located at 2.60mm depth as shown in the A scan image of Fig.6 (b). The discontinuity of the frequency and the location of the void are clearly shown in the B – scan image of Fig.6(c). But this defect is not distributed throughout the sample. The occurrence of defect in the weld plate is also higher in the flat shoulder tool than the concave shoulder tool. When the welded plate is tested for radiography, the defects were seen at the edges as shown in Fig.6 (d). The initial plunge and final removal of tool impression is seen as a defect in Fig.7. This thermograph image shows that there is defect at the edges of welded plate and no voids were found as such found in the flat shoulder tool.

(b) (a)



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Fig.6 Concave shoulder tool welded plate (a) C scan (b) A scan (c) B scan (d) Radiography

Fig.7 Thermograph of concave shoulder welded

plate 3.3 Sub shoulder tool During the experiments conducted on concave shoulders, a step like formation was naturally occurred and this was developed as a new tool design tool and has been named as sub shoulder. It was found that the tool could produce 1200mm length of weld, which was the highest weld length among the other tools that could travel. The welded plate by sub shoulder tool is shown in Fig.8 and found with defect free. Concentric circles can be seen over the plate surface. This is the metal flow because of first mode of metal transfer, as shown in Fig.9 (a) and roughness in surface is clearly seen in A-scan image of Fig.9 (b). However, the welded plates are found to befree from defects along the thickness of the weld, and this is assured from the B-scan image as shown in Fig.9(c). Ultrasonic images show that there is no defect along the weld length. Exit hole is visible on the radiograph as shown in Fig.9 (d). Except the initial plunge impression of tool

and exit hole, there is no irregularity or voids, found on the thermograph image as shown in Fig.10.

Fig.8 Appearance of Sub shoulder welded plate

Fig.10 Appearance of Sub shoulder welded plate

(c) (d)

(d)



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Fig.9 Sub shoulder tool welded plate (a) C scan (b) A scan (c) B scan (d) Radiography

From the results of NDT techniques, Sub shoulder tool is found to be free from defects. Flash on welded plates cannot be considered as defect, if it is in allowable limits and this does not affect the weld formation. All the welded plates of flat shoulder were found with tunnel defect and surface irregularities. Lack of material flow and poor tool design are the reasons for such defect. Among the tool profiles, flat shoulder tool is found to give maximum number of defects. Conclusion: Frictions stir welding is performed on the IS: 3039 steel plates with three different tool profiles and the welded samples were tested with NDT techniques. The following are the defects that were observed from the welded plates of Flat, concave and Sub shoulder.

1. The welded plates by the flat shoulder are found with numerous voids along the length of the weld and root defects were also observed.

2. The welded plates are found to be free from defect by using a sub shoulder tool.

3. Proper combination of tool design and process parameter could make welds free from defects.

From the parent study, it is clear that Sub shoulder tool could serve as a better tool for welding of steels. REFERENCE

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DEFECT ANALYSIS OF FRICTION STIR WELDED STEEL …Defect Analysis Of Friction Stir Welded Steel Plates On Different Tool Profiles 95 In the weld length, at 25mm cross section as shown