Friction stir welding of AA6082-T6 T-joints: process ... - Research

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Abstract: In the paper the authors present the results of a wide range of experiments on T- parts. First, friction stir welding process engineering has been ...
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Friction stir welding of AA6082-T6 T-joints: process engineering and performance measurement L Fratini1*, G Buffa1, L Filice2, and F Gagliardi2 1 Dipartimento di Technologia Meccanica, Universita` di Palermo, Palermo, Italy 2 Dipartimento di Meccanica, Universita` della Calabria, Italy The manuscript was received on 18 March 2005 and was accepted after revision for publication on 28 November 2005. DOI: 10.1243/09544054JEM327

Abstract: In the paper the authors present the results of a wide range of experiments on Tparts. First, friction stir welding process engineering has been developed with the aim of determining the specific process parameters that make up the soundness of the obtained Tparts. Then the performance of the obtained T-joints has been compared with T-joints obtained by metal inert gas welding and extruded T-parts. The parts have been tested utilizing a customized bending test with the aim of highlighting their behaviour both in elastic and plastic fields. Keywords: friction stir welding, T-joints, bending test

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INTRODUCTION

Joining technologies are in a very interesting phase today due to some relevant innovations concerning different aspects. One such innovative process to be considered is that of friction stir welding (FSW). In fact, welded joints are obtained without using any external heat supplier, generating the required temperature increase by means of a revolving pin that follows a proper trajectory partially sunk in the workpiece surface [1–6]. Owing to softening of the worked material, the particular material flow tends to cover again the zone on which the pin acted. In this way, very interesting welding lines may be obtained and, in addition, owing to the intrinsic advantages of this process, a high-performance behaviour of the welded joint is shown. In fact, limited extension of the thermally altered zone produces a suitable crystallographic layout, which causes a better behaviour when axial loads are applied. The mechanics of the FSW process is strongly affected by several parameters. Geometrical parameters have first to be considered with regards to the geometry of the utilized tool (shoulder diameter, pin shape, and *Corresponding author: Department of Mechanical Technology, Production and Management Engineering, University of Palermo, Viale delle Scienze, Palermo 90128, Italy. email: [email protected]

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dimensions) and its positioning during the process (i.e. tool sinking and nuting angle). Furthermore, technological parameters, such as the tool rotating speed and the tool feed rate, have to be properly chosen, since they determine the specific thermal contribution conferred to the joining edges [7–9]. The introduction of such joining technology, due to its performance, simplifies the production scenario of particular shaped profiles. It is well known that in the aircraft and aerospace industries complex profiles and joining of so-called ‘skin and stingers’ are very often utilized for flying structures bodies. In addition, metallurgical joints made by welding are very often utilized in the fabrication industry, as well as in ships, off-shore structures, steel bridges, and pressure vessels. In recent years a few results have been presented that investigate the fatigue resistance of welded T-joints in lightweight alloys, in stainless steels, and also in composite materials [10–14]. In these studies fusion welding processes have been taken into account and the effects of the localized heating and subsequent rapid cooling on residual stresses and distortions in the T-joints have been considered using different approaches [15–17]. It should be observed that in the former studies it was shown that failure of the joints was due to the presence of a wide heat-affected zone, to local metallurgical modifications in the material, and also to discontinuities and defects that occurred during the developed welding process. On the basis

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of its peculiarities, the application of FSW to the development of complex joints and profiles seems very promising in an effort to improve joint effectiveness and performance. In this work, a typical T-shaped aluminum component is used. Of course, this product can be obtained by extrusion but this implies a very high set-up cost due to die acquisition. Using a welding technology the same profile may be obtained starting from flat parts, i.e. the skin and the stinger, but the welding may decrease the quality and the strength of the obtained product. Friction stir welding seems to overcome these problems and does not seem to produce defected parts. In this paper the engineering of the FSW process has been developed with the aim to obtain T-joints. The performance of these T-joints has been compared with T-joints obtained by metal inert gas (MIG) welding and extruded T-parts. In particular, the different parts have been tested utilizing a customized bending test in order to highlight their behaviour both in elastic and plastic fields. 2

FSW OF T-JOINTS

friction forces. In this way the material mechanical characteristics are decreased locally and the blank material reaches a kind of ‘soft’ state; no melting is observed. Both circumferential metal flow all around the tool pin and close to the tool shoulder contact surface and an upward and downward material flux, mixing the material of the two blanks, are obtained. As the material softens, the tool can be moved along the joint. The tool movement determines heat generation due to both friction forces and material deformation. In order to develop the FSW operations a properly designed clamping fixture was utilized in order to fix the specimens to be welded on a milling machine (Fig. 2). The steel plates composing the fixture were finished at the grinding machine in order to assume a uniform pressure distribution on the fixed specimens. As far as FSW of T-joints is concerned, several parameters need to be considered. Apart from the tool feed rate and the tool rotating speed, which determine the specific thermal contribution [7], and the geometrical characteristics of the tool pin, the die radius (see both Fig. 1 and the section of the experimental device shown in Fig. 3) also needs

FSW of T-joints is obtained by inserting a specially designed rotating pin into the clamped blanks, as shown in Fig. 1, and then moving it along the joint. The pin is inserted at a rather small nuting angle, limiting the contact between the tool shoulder and the blank. As the pin is inserted into the upward sheet (skin), the blank material undergoes a local backward and forward extrusion process in order to penetrate the vertical blank (stinger) and to reach the tool shoulder contact. The tool rotation causes an increase in the material temperature due to

Fig. 2 Sketch of the FSW experimental device

Fig. 1 Positioning of the tool and the two blanks (skin and stinger)

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Fig. 3 Section of the FSW experimental device

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to be considered, since it strongly determines the shape of the welding seams. In particular, for the developed processes, 3 mm thick AA6082-T6 blanks were utilized, while the tool was made from H13 steel quenched at 1020  C, characterized by a 52 HRc hardness. The cylindrical pin had a height of 4 mm and diameter of 4 mm and the tool penetration was equal to 4.2 mm. The matrix radii were equal to 3.5 mm and the tool nuting angle (u) was in the range 1–5 . It should be observed that the latter geometrical parameter determines the effectiveness of the material flow during FSW of T-joints, thus influencing the performances of the obtained parts. Furthermore, it can be shown that too great a tool penetration would cause a shape defect on the skin surface; on the other hand, lack of tool pin penetration would make the joint ineffective since no metal flow between the skin and the stinger would occur. The nuting angle is strictly correlated with the other geometrical parameters as it determines, together with tool sinking, the three-dimensional metal flow during the joining process. The technological parameters utilized were tool rotating speeds (R) ranging from 715 to 1500 r/min and tool feed rates (Vf) ranging from 71 to 200 mm/min. All such process parameter values and ranges were chosen on the basis of literature data [18] and preliminary tests.

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aluminium alloy, which was used for all the tests. The specimens were properly shaped (Fig. 5) to obtain a certain positioning in the equipment and to raise the tangential stress on the welded joint. Figure 6 shows the geometry of the utilized specimens. A simple preliminary numerical simulation was performed in order to calculate the punch stroke that generates the transition from elastic to plastic behaviour (Fig. 7) in an extruded joint assuming constant material characteristics in all the specimens, namely a material yield stress of 280 MPa and a Young modulus of 77 GPa. In particular, three

Fig. 5 The tested specimen

THE EXPERIMENTAL PROCEDURES

In order to test the efficiency of the considered parts the authors developed a complex test to verify several aspects related to the use of this kind of structure in machine element design. To achieve this a three-point bending equipment was developed with the aim of bending a prepared T-shaped element. Figure 4 shows the utilized fixture. The material to be tested was AA6082-T6

Fig. 4 The utilized bending equipment

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Fig. 6 Geometry of the utilized specimens (lengths in mm)

Fig. 7 Qualitative von Mises stress distribution

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different kinds of specimen were investigated, as reported in the following: (a) extruded T-profile; (b) MIG welded T-shape, using both silica and manganese wires, obtained by industrial procedure; (c) friction stir welded T-shape, by varying the most relevant process parameters, namely the nuting angle, the tool rotation speed, and the tool feed rate. The bending tests were also extended into the plastic zone, even though these structures are always designed in the elastic region, in order to analyse the welding behaviour in the case of extreme deformation. In such a case, the capability of the component to absorb energy may be investigated, thus deriving proper information of the material behaviour when it has undergone destructive action. To study this in more in detail, the equipment was mounted on an Instron 8501 electronically controlled hydraulic machine and the load versus stroke curves were sampled using a data acquisition system. The acquired load versus punch stroke curves give two strategic results: (a) the load requested to reach the elastic–plastic transition when a homogeneous component is used (determined by the numerical simulation); (b) the maximum stroke for which the welding has not collapsed. The former is utilized for the part design, for instance in a finite element (FE) design environment, while the latter is related to the structure toughness. In the next section the results are given, taking into account the two different analyses, namely in the elastic field and in the plastic one. Different specimens from a few joints were crosssectioned perpendicular to the welding direction, for macro and micro observations. Macro observations were used to analyse the material area involved in the process mechanics and eventually macrodefects, while micro observations of the different material zones determined using the developed welding processes were highlighted. In order to obtain such results the specimens were prepared, treated with Keller reagent and observed by a light microscope. 4

observed. These results show the relevant strength of the extruded T-part with respect to the MIG welded one, even if the latter presents a wider plastic field that allows more relevant permanent deformations. The bent extruded profile presents cracks starting from the corners between the skin and the stinger. On the other hand, a detailed observation of the bent MIG welded profile shows that even if two cracks are visible at the front end of the stinger part, the welding seams are largely deformed but do not present any failure. The observed crack starts at the contact surface between the stinger and skin parts and during the bending test reaches the external surface of the joint along the two small edges. Such a crack is due to the development of the MIG welding process in which two lateral seams determine the continuity between the skin and the stinger parts; in this way, in the central part of the joint transverse section, the two jointed parts remain separated. This description is well explained in the next macro image of the MIG welded joint transverse section (Fig. 9). In fact, an imperfect continuity is obtained between the skin and the stinger parts, because the melting baths characterizing the two welding seams do not reach

Fig. 8 Load versus punch stroke curves for extruded and MIG welded profiles

RESULTS AND DISCUSSION

First, the extruded and the MIG welded profiles were considered and tested. In Fig. 8 the full load versus punch stroke curve is shown. No relevant effects on the bending test of the MIG welded T-joints of the utilized wire material were

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Fig. 9 Macro image of the MIG welded joint transverse section

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to cover the full contact interface between the welded parts. A void is then formed at the boundary between the skin and the stinger: the crack initiates from such a void and during the bending test causes failure of the joint. Figure 9 shows the distortions occurring in the T-joint because of the welding process. As far as metallurgical aspects are concerned, Fig. 10 shows the transition between the heat affected zone of the base material, characterized by an evident grain enlargement, and the solidified melting bath (see also Fig. 9). A more detailed observation of the metallic structure characterizing the melting bath shows that long dendrites form in the direction of the thermal gradient occurring during the solidification stage. These results, both in terms of joint strength and from the metallurgical point of view, have been utilized as a comparison in order to investigate the effectiveness of the FSW of T-joints. Three different values of the specific thermal contribution [7] were investigated as follows: (a) low specific thermal contribution (LSTC): R ¼ 715 r/min, Vf ¼ 200 mm/min; (b) medium specific thermal contribution (MSTC): R ¼ 1000 r/min, Vf ¼ 150 mm/min; (c) high specific thermal contribution (HSTC): R ¼ 1500 r/min, Vf ¼ 100 mm/min.

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average results will be presented, neglecting specific cases. In Fig. 11 the load versus punch stroke curve is reported and the elastic fields, up to a stroke of 1.4 mm, are highlighted. The indicated threshold corresponds to the transition from elastic to plastic behaviour of the extruded component. An interesting conclusion may immediately be derived: the friction stir welded and the MIG welded specimen exhibit the same behaviour up to the elastic threshold stroke, even if the FSW process parameters are not optimized. Thus it can be stated that the performance index of the FSW technology reaches almost 100 per cent in this application, as compared to a good-quality MIG welding developed in an industrial environment. Of course, the resistance of the extruded specimen is higher than approximately 80 per cent. On the contrary, if the plastic field is taken into account, the behaviour of FSW becomes worse. In fact, it shows a kind of brittle fracture should be properly taken into account when the produced part is used in a machine. In Fig. 12 this aspect is clearly evident: the ductility of the milled specimen and of the MIG welded one is very large; i.e. large plastic deformations are observed before a definitive collapse is reached. In order to highlight the brittle behaviour of the FSW specimen a macrographic investigation was

These tests were repeated utilizing a nuting angle of u ¼ 1.5 , 3 , and 4.5 respectively, in order to highlight the effect of such geometrical parameters on the FSW of T-joint process mechanics. Each of the considered tests was repeated three times and average results were considered in order to take into account the process variability. First of all, the results for u ¼ 1.5 are presented; no effects of the specific thermal contribution to the FSW effectiveness have been observed and very similar results have been obtained. Thus only Fig. 11

Fig. 10

Transition between the heat affected zone of the base material and the solidified melting bath

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Fig. 12

Load versus punch stroke (u ¼ 1.5 )

Full load versus punch stroke curve (u ¼ 1.5 )

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developed with the aim of verifying the effectiveness of the FSW process and of the utilized process parameters. Direct observation of the FSW T-joint section indicates that an obvious tunnel defect is observed (Fig. 13) due to a wrong and insufficient material flow during the joining process. In the macrograph, traces of oxide films on the starting blanks are present as white bands in the joint section. It should be observed that the presence of such a void in the T-joint section strongly reduces the joint resistant section and, what is more, determines a brittle fracture of the part since the fracture starts from the tunnel border due to the stress concentration, causing the resistant section to reach the limit condition rapidly. When the results with u ¼ 3.0 are investigated, limited effects of the specific thermal contribution on the FSW effectiveness have been observed and similar results have been obtained, as shown in Fig. 14. The best performances in terms of joint strength and ductility are given in the MSTC process conditions. In Fig. 15 the FSW-MSTC load versus punch stroke curve is compared with the former MIG and extruded results. Again, the MIG joint shows very large ductility with respect to the FSW one; it should

be observed that there is strong improvement in the performance of FSW joints for the various values of the nuting angle. A macrographic investigation of the FSW joint was developed and a direct observation of the FSW Tjoint section indicated an improvement in the material flow during the joining process. In particular, no tunnel defect is observed (Fig. 16). Finally, the performance of the developed T-joint with a nuting angle u equal to 4.5 has been investigated. Again, as for u ¼ 1.5 , no influence of the process conditions, namely of the specific thermal contribution, was observed on the joint performance. In particular, no tunnel defects were found (Fig. 17) and very similar load versus punch stroke curves were obtained during the bending tests. In this way the average results shown in Fig. 18 were found. A strong decrease in the FSW T-joint performance is found for large values of the nuting angle, even if no tunnel defects are observed. In this way, the nuting angle provides very important parameters in the process mechanics of FSW of T-joints (Fig. 19). The best performance joints have been obtained for u ¼ 3 , but a deeper investigation is being

Fig. 15 Fig. 13

The tunnel defect in the FSW T-joint (u ¼ 1.5 )

Fig. 14

Full load versus punch stroke curves (u ¼ 3.0 )

Fig. 16

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Full load versus punch stroke curves (u ¼ 3.0 )

The FSW T-joint section (u ¼ 3.0 , MSTC process conditions)

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Fig. 17

The FSW T-joint section (u ¼ 4.5 )

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namely extruded ones and T-joints obtained by MIG welding and FSW. A customized bending test was developed in order to test the efficiency of the parts both in the elastic field and in the plastic one. The performances of the joints have been compared both in the elastic and in the plastic fields. In particular, the influence of the most relevant process parameters on FSW has been investigated. The definitive result of the developed experiments is that the nuting angle is the most relevant process parameter used to optimize the T-joint performances. A deeper investigation should be developed in order to highlight the definitive reasons for such behaviour.

ACKNOWLEDGEMENTS This work was carried out using MIUR (Italian Ministry for University and Scientific Research) funds.

REFERENCES

Fig. 18

Fig. 19

Full load versus punch stroke curves (u ¼ 4.5 )

Load versus punch stroke curves for FSW joints at various nuting angles

developed through both experiments and numerical simulations in order to highlight the effects of such geometrical parameters on the process mechanics of the joining technique when T-joints are considered. 5

CONCLUSIONS

In the paper the results of a wide experimental campaign on T-parts are presented: three different kinds of component have been taken into account,

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