Effect of tool pin offset on the Mechanical properties of dissimilar materials based on Friction Stir Welding (FSW)

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1 Effect of tool pin offset on the Mechanical properties of dissimilar materials based on Friction Stir Welding (FSW) SATYAVEER SINGH 1 And MOHD MAHMEEN 2 1,2 Assistant Professor Department of Mechanical Engineering IIMT group of institution, Greater Noida Abstract: In this paper an attempt has been made to investigate the effect of tool pin offset on the mechanical properties of dissimilar materials. In this article aluminium alloy 8011 and 99.65%pure copper was friction stir lap welded and their tensile properties were evaluated. On evaluation it was found that optimum tool pin offset for defect free nugget zone was 1mmand tool rotational speed was 1400 rpm and tool feed was found to be 60 mm/min. A cylindrical pin profile is adopted as its geometry had been proven to yield better weld strengths. Keywords: friction Stir welding; dissimilar joints; copper; aluminum. I. INTRODUCTION Friction Stir Welding (FSW) is basically eco-friendly process in which the metal to be welded is not melted during welding, thus the cracking and porosity often associated with fusion welding processes are eliminated. Many emerging applications in power generation, petrochemical, nuclear, aerospace, transportation, and electronics industries lead to the joining of dissimilar materials by different joining methods especially by friction stir welding [1]. Due to different chemical, mechanical and thermal properties of materials, dissimilar materials joining present more challenging problems than similar materials joining [2]. However, when joining dissimilar materials by friction stir welding (FSW), the problems not only arise from a material properties point of view, but also from the possibility of the formation of brittle inter-metallic compound [3]. Friction Stir Welding (FSW) is a solid state joining technique invented and patented by The Welding Institute (TWI) in 1991 for butt and lap welding of ferrous and non ferrous metals. FSW is a continuous process that involves plunging a portion of a specially shaped rotating tool between the butting faces of the joint. The relative motion between the tool and the substrate generates frictional heat that creates a plasticized region around the immersed portion of the tool [4]. Friction stir welding process uses a non-consumable rotating tool consisting of a pin extending below a shoulder that is forced into the adjacent mating edges of the work pieces as illustrated in Fig.1.The rotation of the tool results in the stirring and mixing of material around the rotating pin during the welding process which in turn affect the evolving properties of the weld. There are fewer defects in solidstate welding because the metals do not reach their melting temperatures during the welding process. The major advantage of FSW is that it follows local thermo mechanical metal working process without influencing properties of surrounding areas as observed in other welding process. However, the base metals being joined retain their original properties, and the Heat Affected Zone (HAZ) is small as compared with the fusion welding techniques [5]. The micro structural weld zones of FS weld is shown in All rights Reserved 75

2 International Journal of Modern Trends in Engineering and Research (IJMTER) Schematic diagram of FSW Fig.2.Micro structural weld zones of FS weld Fig.1. Friction stir welding of dissimilar materials such as aluminium to copper in particular need to be fully understood due to their different melting temperatures. The high chemical affinity of both base materials promotes the formation of brittle inter-metallic Al/Cu phases, which still require extensive research [6&7]. Furthermore, aluminium and copper are difficult to weld with conventional welding processes due to their high reflectivity and thermal conductivity. Brittle intermetallic phases develop in the joint zone since copper and aluminium are not very soluble in one another in the solid state. These inter-metallic phases lower the toughness of the weld and lead to cracks during and after the welding [8]. Moreover, aluminium to copper welding is increasingly used in some practical applications such as heat transfer equipments, wiring, electrical and electronics industries, and aesthetical applications. Tungsten carbide tool is used in the research is shown by Fig.3. II. EXPERIMENT The plate size of aluminium and copper was same and having 180 mm length, 45mm width and 3 mm thickness. The welding was carried out using a heavy-duty vertical milling machine adapted for FSW: Bharat Fritz Werner (BFW) with spindle motor capacity 11KW as shown in the Fig.4.The fixture used in the present study was shown in the Fig.5.The welding tool used in this study was made of tungsten carbide and had a shoulder 20mm in diameter and a pin 6mm in diameter and 2.65mm.Welds were made with a clock-wisely rotating pin at a rotation rate of 750 rpm and a constant traverse speed of 100 mm/min. Tool tilt angle of 2 degrees was used in the present study. The chemical compositions and mechanical properties are depicted in Table 1. Welding tool was perpendicular to welding surface and offset towards the Al alloy with pin offset of approximately 1 mm.the tool rotation speed, traverse speed and preheating current are main process parameters in determining the appearance and mechanical properties of FSW joint. The aluminium alloy plate was located on the retreating side and the copper plate on the advancing All rights Reserved 76

3 International Journal of Modern Trends in Engineering and Research (IJMTER) Fig.3.Tool pin geometry Fig.4.Friction stir welding set Fig.5.FSW fixture Table.1Chemical Composition of the metals Material Al Cu Mg Si Fe Ni Mn Zn Sn Pb Ti Cr V Al <0.01 Copper During the welding processes, several pin offsets from 0mm to 1.5mm were used. For convenience, in this study, an offset of 0mm denotes the position where the pin just located at the butt line. Micro structural characterization and analysis were carried out using Optical microscope: Mitutoyo, Japan. Tensile tests of 3mm thick specimens having 12mm gauge length were performed according to ASTM E8M standard at room temperature at a crosshead speed of 3mm/min. The tensile test specimens were prepared by Wire EDM and tested on the Tonsometer. Table.2Process Parameters and their levels Parameters Levels 1 Level2 Level 3 Units Tool Pin offset mm Rotational speed Rpm To perform Friction stir welding and testing of dissimilar material welded samples, following machines/equipments were required; Shaper machine for dimensioning the work piece, A heavy-duty vertical milling machine adapted for FSW: Bharat Fritz Werner (BFW),FSW fixture, Wire EDM All rights Reserved 77

4 International Journal of Modern Trends in Engineering and Research (IJMTER) making sample for testing : Steer Corporation, China, Tensometer for Tensile testing: Kudale Instruments Pvt. Ltd. Pune. III. RESULTS AND DISCUSSION The effects of FSW process parameters like tool shoulder diameter, tool pin offset, rotational speed, on ultimate tensile strength is studied. From the graphs, it is found that the tensile strength of FSW joints is higher than the base metal. The four operating parameters considered directly affect the magnitude of frictional heat generated and plastic flow of material. Fig.6. shows the welded plate. 1) Effect of tool shoulder diameter: The tool shoulder diameter is having directly proportional relationship with the heat generation due to friction. If the shoulder diameter is large, then heat generation due to friction is also high due to large contact area and vice versa. In this investigation it has been observed that the larger tool shoulder diameter (20 mm) lead to wider contact area and resulted in wider TMAZ region and HAZ region. 2) The effect of pin offset on the tensile strength of the joints: When the pin offset was larger towards the softer material, only a few Cu pieces with relatively small size were moved from the Cu bulk. Effect of pin offset on the tensile strength is shown in the fig.7. The Cu pieces were harder than the Al matrix, therefore, the large Cu pieces were hard to deform and flow in the Al matrix, and the mixing between the large Cu pieces and the Al matrix would be very difficult. This led to the poor surface bonding and the formation of many voids in the weld bead. Moreover, when the pin offset was smaller, more Al Cu IMCs have been formed because the more Cu pieces were stirred into the nugget zone. Thus, the joining between the Al and Cu became poor due to the brittle nature of the IMCs. Ultimate tensile Strength(MPa) Fig. 6: Welded Plate AS offset values(mm) Fig.7. Effect of pin offset on the tensile strength RS All rights Reserved 78

5 International Journal of Modern Trends in Engineering and Research (IJMTER) During the FSW process, the materials were transported from the retreating side to the advancing side behind the pin where the weld was formed [16]. The hardness of the Cu is larger than that of the Al, and the pin stirred mainly in the Al base metal during FSW, so the material flow occurred mainly in the soft Al base metal. If the stronger material was fixed at the retreating side, the hard material was difficult to transfer to the advancing side because the hard material hardly flew. In this case, a large volume defect would form and the excessive soft material would be extruded out from the nugget zone. However, when the softer material was fixed at the retreating side, the soft material was transported to the advancing side easily, and the material flow cycle in the nugget zone was performed normally. 3) The effect of rotational speed on the tensile strength of the joints: Fewer cracks were seen under a lower rotation rate of 600 rpm but at 950 rpm and 1200 rpm sound weld was not achieved. This might be due to the formation of inter-metallic compounds under the enhanced reaction between Al and Cu. Under the rotation speed of 1200 rpm many macro cracks were observed. When the rotation of the tool pin was high, large pieces of Cu would be detached from the bulk and get distributed in the bottom and the retreating side of the nugget zone. When the rotation speed was fixed to 750 rpm small pieces of Cu would be scratched off from the bulk and thus at a certain portion of the weld zone seem to have mixed properly. The increase in rotational speed from 750 to 1200 rpm resulted in a harsher material flow. 4) Tensile testing outcomes: Tensile tests of 3mm thick specimens having 12mm gauge length were performed according to ASTM E8M.Properties that are directly measured via tensile test are ultimate tensile strength, maximum elongation and reduction in area. Fig.8 shows tensile testing specimen. Mechanical properties of the base metal were shown in table 3. Fig.8.TensileTesting specimen Fig.9. fracture location at various offsets. The fracture location position in the tensile specimens was shown in the fig.9. In the current study, tool pin offset was varied between 0 to1.5 mm. Ultimate tensile was found to be 105MPa at zero offset. The tensile strength of the joint was 139MPa at a pin offset of 0.5mm in Al side. Maximum strength was obtained at a pin offset of 1mm towards Al side. The strength of the joint was decreased at a pin offset of 1.5mm.when pin is offsetting in copper side then maximum tensile strength was obtained at a pin offset of 0.5 mm and minimum tensile strength was obtained at a pin offset of 1.5mm. Stress - strain curves of welded specimens welded with 750rpm and 100 mm/min are shown in All rights Reserved 79

6 International Journal of Modern Trends in Engineering and Research (IJMTER) Load In kgf Displacement in mm Load In kgf 0 Displacement in mm Fig.10. Stress - strain curves of welded specimens welded with 750rpm and 100 mm/min at (a) 0.5 mm tool pin offset, (b) 1mm tool pin offset. IV. Conclusion Sound defect-free joint could be obtained only when the hard Cu plate was fixed at the advancing side. A large volume defect was observed when the soft Al plate was fixed at the advancing side. This is attributed that the hard Cu bulk material was hard to transport to the advancing side during FSW. Sound defect-free joints weree obtained under the pin offsets of 1mm to the Al matrix, and a good metallurgical bonding between the Cu bulk/pieces and Al matrix was achieved. However, defects formed easily at smaller pin offsets due to the hard mixing between the large Cu pieces and Al matrix. Poor tensile properties were obtained at the very large pin offsets and/or low rotation rates due to the insufficient reaction between the Cu bulk pieces and Al matrix. Sufficient reaction were achieved in the FSW Al Cu joints produced at higher rotation rates and proper pin offset of 1mm towards Al side, resulting in the good tensile properties REFERENCES [1] Rai, R., De, A., Bhadeshia, H. K. D. H. and DebRoy, T. (2011), Review: friction stir welding tools, Science and Technology of Welding and Joining, Vol. 16(4), pp [2] Kimapong, K. and Watanabe, T. (2004), Friction stir welding of aluminum alloy to steel, Welding Journal, Vol. 83(10), pp [3] Xue, P., Xiao, B. P., Ni, D. R. and Ma, Z. Y. (2010), Enhanced mechanical properties of friction stir welded dissimilar Al Cu joint by intermetallic compounds. Journal of Materials Science and Engineering: B, Vol. 527, pp [4] W.M. Thomas, E.D. Nicholas, J.C. Needham, M.G. Murch, P. Temple-Smith, C.J. Dawes, Friction stir Butt Welding. International Patent No. PCT/GB92/02203, 1991, GB patent application No [5] A. O Brien, C. Guzman (eds.) American Welding Society, Welding Handbook Welding Processes, part 2 volume 3 Ninth Edition Miami: American Welding Society; 2007 [6] B.S. Yilba, A.Z. Sahin, N. Kahraman, A.Z. Al-Garni, Friction stir welding of St-Al and Al-Cu materials J. Mater. Process. Technol., 1995, 49, [7] J. Ouyang, E. Yarrapareddy, R. Kovacevic, Microstractural evolution in the friction stir welded 6061 aluminium alloy ( T6- temper condition) to copper, J. Mater. Process. Technol., 2006, 172, [8] T. A. Mai, A C. Spowage, Characterisation of dissimilar joints in laser welding of steel kovar, copper steel and copper aluminium. Materials Science and Engineering A, 2004, 374(1 2): 224 All rights Reserved 80