Investigation of weld defects in dissimilar friction stir welding of ...

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friction stir welding of aluminium to brass by radiography. A. Esmaeili*. 1. , M. K. Besharati Givi. 1 and H. R. Zareie Rajani. 2. The present study was focused on ...
Investigation of weld defects in dissimilar friction stir welding of aluminium to brass by radiography A. Esmaeili*1, M. K. Besharati Givi1 and H. R. Zareie Rajani2 The present study was focused on detecting weld defects in dissimilar joints made by friction stir welding. Radiography test and optical microscopy were used to evaluate the main defects. Coarse and continuous fragments of brass in aluminium matrix, tunnelling and void defects in vicinity of fragments were the main observed defects. Keywords: Defects, Radiography, Dissimilar friction stir welding

Introduction Since invention of friction stir welding (FSW) in TWI,1 many attempts have been made to evaluate the capability of FSW in joining similar and dissimilar materials. The majority of these efforts are dedicated to joining the similar materials, specifically aluminium alloys.2–7 One of the most comprehensive reviews on FSW has been carried out by Nandan et al.8 Through the mentioned study, the principles of FSW including heat generation, materials flow, welding defects and residual stresses have been investigated. Also, joining of various metals such as copper, titanium and steel alloys through FSW is discussed in their review. Furthermore, C ¸ am9 has studied the possibility of applying FSW method to join the industrially engaging alloys such as Mg, Cu and steel alloys. According to the outcomes of his research, FSW is an appropriate solid state technique to join a wide range of materials. Since brass is one of the utilised base plates in current study, it is also helpful to have a short review on previous researches about joining of copper alloys. Joining of pure copper has been investigated by Lee et al.10 and Xie et al.11 The former one has reported a defect free joint at the rotation and traverse speeds of 1250 rev min21 and 61 mm min21 respectively, and the latter one has mainly focused on the role of rotation speed in grain size and mechanical properties of nugget zone. According to the both reports, at a traverse speed of 50 mm min21, the safe range of rotation speed which can lead to a defect free weld in pure copper is 400–800 rev min21. However, so far only few researches about joining of brass through FSW have been reported.12–15 Park has probed the mechanical and microstructural properties of 60Cu–40Zn alloy,12 where a wide range of rotation and welding speeds (1000– 1500 rev min21 and 500–2000 mm min21 respectively)

1 School of Mechanical Engineering, University College of Engineering, University of Tehran, Tehran, Iran 2 School of Engineering, The University of British Columbia, 3333 University Way, Kelowna, BC V1V 1V7, Canada

*Corresponding author, email [email protected]

can result in a defect free joint. Also, C ¸ am et al.13,14 have obtained a sound joint of brass at rotation speed of 1600 rev min21 while using traverse speed of 225 mm min21. Furthermore, Meran has fabricated a defect free joint through FSW using rotation and traverse speed of 2050 rev min21 and 112 mm min21 respectively.15 Though all of mentioned reports are valuable attempts to explain the influential parameters in weldability of similar alloys through FSW, only few investigations have extended this effort to dissimilar materials.16–23 In previous studies, the authors have discussed the weldability of Al 1050 to brass by FSW in butt position.22–23 Results suggested that tensile strength of the weld is severely sensitive to formation of weld defects, especially the coarse and continuous fragments of brass scattered in aluminium matrix called fragment defects. Also, it was observed that fragment defects are accompanied by occurrence of other major defects including tunnelling and void defects.22 Based on outcomes of previous investigations,22,23 adjusting the welding parameters to optimum values can hinder the occurrence of fragment defects, and consequently enhances the mechanical properties of dissimilar joint. As mentioned above, existence of fragment defects can be interpreted as an evidence for occurrence of other defects (tunnelling and void defect), showing the importance of finding this imperfection through inspection process. Although radiography can be one of the best methods to easily reveal the fragment defects, no study has been reported on applying the radiography test (RT) to detect the fragment defects in dissimilar joints fabricated by FSW. This study aims to reveal the existence of fragment defects via RT to ensure soundness of the joint. Since fragments of harder material mainly lie underneath of the joints; therefore, one of the best methods to detect them is RT. Also, RT can contribute to observing the flow of material within the weld, and consequently making sure of a proper material flow. Furthermore, the RT images can show a relatively large area of weld, unlike the destructive tests such as tensile test in which only small selected regions can be investigated.

ß 2012 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 4 May 2012; accepted 22 May 2012 DOI 10.1179/1362171812Y.0000000044

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2 Formation of tunnelling defects in vicinity of coarse fragment of brass in aluminium matrix in cross-section views 1 Effect of offset on tensile strength

Experimental The materials used in this study were aluminium 1050 and CuZn30 plates with 3 mm thickness. All of the welds were fabricated by following parameters: rotational speed of 450 rev min21, welding speed of 8 mm min21, depth of sinking pin of 0?25 mm and a tilt angle of 1?5u. These parameters were kept constant, while the offset value was varying between 0 and z1?6 mm. In measurement of offset, the absolute value of offset equals the distance between interface and centre of the tool. Also, the tool movement towards the aluminium side has been assumed as positive value. Advance and retreating sides were brass and aluminium, respectively. Tool was made of hot working alloy steel (1?2344), and tool shoulder diameter and height of the tapered slotted pin were 15 mm, 18u and 2?85 mm respectively. Also, specimens were analysed using the traditional X-ray (X-ray parameters are defined in Table 1). Regarding the atomic weight of the elements, it is important to know that in RT images brass and aluminium are seen light and dark respectively.

Results and discussion Before starting to investigate the RT images, it is essential to introduce two main parameters affecting the quality of the welds. The first parameter is rotational speed which directly influences the heat generation in welding.8,22,23 The second parameter is offset value that strongly affects the formation of coarse and continuous fragments of brass.22,23 Since the required heat for plasticising the aluminium and brass are not equal, the only way leading to a sound joint is selecting the suitable parameters to reach an appropriate flow of both materials. Figure 1 shows the effect of offset on tensile strength. It can be seen that low offset values strongly spoil the

mechanical properties. By increasing the offset to optimum value a sound joint will be reached. However, keeping the offset value constant is almost difficult, relative to other welding parameters such as rotational speed and welding speed which can be controlled readily. As Fig. 2 depicts the cross-sectional view of the joint, several course fragments of brass accompanied by nearby tunnelling defects are detectable in aluminium matrix. However, no defect can be observed in vicinity of smaller particles of brass (dotted arrow in Fig. 2a). Formation of tunnelling defects near the coarse fragments is arisen from disturbance of material flow during welding process. Comparison of required rotational speeds for plasticisation of brass and aluminium can contribute to a better understanding of fragment defects formation. As mentioned through the introduction, the required rotational speed for joining the similar plates of brass (.1000 rev min21) is significantly higher than what is needed for welding the brass plate to aluminium (450 rev min21).12–15 Consequently, it can be concluded that utilised rotational speed in joining of aluminium to brass is not high enough to plasticise the brass as well as aluminium. Such a lack of plasticisation contributes to formation of coarse brass particles. The only way that can remove the coarse fragments is using a proper offset value to crumble the coarse particles of brass.22,23 The crumbling phenomenon develops a metal matrix composite through the nugget zone where fine brass particles act as a reinforcing component in aluminium matrix, and consequently detrimental effect of fragment defects disappears. Figure 3 illustrates the radiography images of two welds fabricated at low offset values (left and right samples are welded at offset values of 0?5 and 0 mm respectively). According to Fig. 3a, c, d and f, upper and lower surfaces of both welds are free of defects, showing an apparent sound joint in visual inspection. However, the radiography images in Fig. 3b and e revealing

Table 1 X-ray parameters Source strength/kV

SFD/cm

Ug

Experimental time/min

Film type

IQI

Density

Sensitivity/%

170

120

0?25

0?1

Kodak AA400

10–16 AL

2?5

2

Note: SFD: source-film distance is the distance between the focal spot of an X-ray tube and the film; Ug: geometric unsharpness; IQI: Image quality indicator.

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a The surface view; b the Radiographic image and; c the root view of the weld fabricated by the offset value of 0.5 mm. Also, parts d, e and f respectively illustrate the surface view, the radiographic image and the root view of the weld fabricated at the offset value of 0 3 Formation of the continuous layer of brass in the aluminum matrix

a formation of tunnelling defect in vicinity of coarse fragments in offset 1 mm; b observation of void around continuous layer of brass in offset 0 mm (Ref. 15) 4 Lower appearances after machining rood surface

continuous and coarse brass fragments scattered in aluminium matrix bring the visual interpretation to question. As it was discussed previously, development of coarse particles increases the probability of defects formation which ultimately aggravates the mechanical properties. As a result, the inner parts of welds must be observed to make sure if fragment defects are accompanied by other destroying defects or not, and assess the

negative evaluation of weld quality provided by RT test. Machining the root surface of the joints, and going through the inner sections of welds rejects the visual inspection and gives a credit to the RT inspection. As it is shown in Fig. 4b, the machined surface of the joint fabricated in offset value of 0 mm demonstrates some defects such as voids close to the continuous layers of brass. It must be noted that although tunnelling and void defects are low density areas and are expected to be projected as dark spots on radiography film (according to principles of RT), the close coarse fragments of brass cover them during exposure. Therefore, RT cannot reveal the existence of these defects close to the fragment defects. Figure 5 shows RT images of other joints, including RT images of tensile specimens fabricated at offset values of 0, 0?5 and 1 mm. All tensile specimens shown in Fig. 5 are expected to fail due to existence of fragment defects in aluminium stir zone. Figure 6 shows macroscopic and RT images of a sound joint obtained in offset value of 1?6 mm. Lower

5 Radiography test images of other specimens fabricated in offset values 0 up to 1 mm

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a upper appearance; b RT image; c lower appearance; d lower appearance after machining 6 Sound joint obtained in offset 1?6 mm

and upper surfaces of the joint have no fragment defects in visual inspection. Radiography test image of the joint is shown in Fig. 6b which approves the results of visual inspection. It shows that by increasing the offset value (Figs. 3–5) up to an optimum value (shifting tool centre from the interface toward the aluminium side) in Fig. 6, coarse and continuous particles of brass will be replaced by fine particles in aluminium. As Fig. 6b indicates, no coarse fragment of brass is detectable in aluminium side, and RT images predict a sound joint which shows satisfactory mechanical properties. To make sure of this positive prediction, the tensile test was carried out on optimum joint (blank spaces in Fig. 6b are due to cutting out the tensile test specimen). The outcomes showing the maximum tensile strength for this joint22,23 approve the reliability of RT images. Furthermore, Fig. 6d compares the lower face of the weld with machined root of the joint. This comparison demonstrates that no defect is detectable in inner parts of weld, and verifies accuracy of RT results. By comparing the Fig. 3b and e with Figs. 6b and 7a, two main differences are distinguishable. The first difference of these two RT images is replacement of coarse fragments by fine brass particles by bringing the offset to its optimum value. Distribution of fine particles besides removal of coarse fragments develops a proper composite structure (Fig. 7b) in the stir zone, improving the mechanical properties of the joint.22,23 Another major difference between RT images of the sound joint (Figs. 6b and 7a) and the failed specimens (Fig. 3b and e) is existence of a light shadow composing of white points in aluminium matrix of the sound joint (Fig. 6b). This vague area can be attributed to accumulation of fine particles of brass in the stir zone of aluminium (composite structure). On the other hand, RT images of failed specimens fabricated at low offset values show bright and sharp projections of fragments of brass in aluminium matrix (Fig. 3b and e).

a RT image; b SEM image (white points are brass particles) 7 Composite structure of sound joint

Conclusions Radiography test was successfully carried out to certify the Al/brass joints fabricated by FSW. The results suggest the ability of RT to reveal the internal defects. The primary detected defect was distribution of coarse and continuous fragments of brass in aluminium matrix called fragment defects. According to the outcomes, the fragment defects are accompanied by formation of other defects including tunnelling defects and voids in vicinity of them. Although tunnelling and void defects are not directly observable through RT images, this study indicates that occurrence of coarse and continuous fragments of brass in RT images can be interpreted as a reliable evidence for formation of other detrimental defects. Moreover, RT image of the sound joint shows a vague structure in the stir zone which can be ascribed to fine crumbled particles of brass. As results suggest, the RT can be used as a reliable approach to detect any hidden internal defect and prevent subsequent failure of joints fabricated by FSW.

Acknowledgement Authors like to appreciate Nicole Marjorie Kurtz who has played an important role in completion of this study.

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