International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 4, April 2017, pp. 1967 1976 Article ID: IJCIET_08_04_223 Available online at http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=8&itype=4 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 IAEME Publication Scopus Indexed EVALUATION OF ELECTRO-MECHANICAL PROPERTIES OF FRICTION STIR WELDED AL / CU BIMETALLIC LAP JOINTS Dr. M. Satya Narayana Gupta Professor, Department of Aeronautical Engineering, MLR Institute of technology, Hyderabad, India K. Shiva Shankar Assistant Professor, Department of Aeronautical Engineering, MLR Institute of Technology, Hyderabad, India ABSTRACT Friction stir welding Processes (FSWP) is a sophisticated solid state welding technology to join dissimilar and similar materials. Commercially pure Aluminium and Copper plates were joined using FSWP and the mechanical and electrical properties of bimetallic lap joints have been evaluated by changing the input processes parameters. Number of experiments was performed to obtain the optimum joint properties by adjusting angle of tool tilting, rotational and welding speeds. The experiments were carried out using design of experiments (DoE) to minimize number of experiments. The present paper provides a very valuable information on electromechanical behavior of Al to Cu dissimilar joints used for a critical application i.e. heat sinks, high voltage bus bars and heavy duty earthing strips, etc. It has been observed that high quality welds are obtained at low rotational and welding speeds irrespective of tilting angle. The maximum value of tensile shear strength is 70 MPa. The hardness is uniform throughout the joint. In manufacturing condition the resistance of the joint is negligible. Key words: Friction Stir Welding, Dissimilar Metals, Lap Joints, Mechanical Properties, Electrical Resistance. Cite this Article: Dr. M. Satya Narayana Gupta and K. Shiva Shankar, Evaluation of Electro-Mechanical Properties of Friction Stir Welded Al / Cu Bimetallic Lap Joints. International Journal of Civil Engineering and Technology, 8(4), 2017, pp. 1967-1976 http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=8&itype=4 http://www.iaeme.com/ijciet/index.asp 1967 editor@iaeme.com
Dr. M. Satya Narayana Gupta and K. Shiva Shankar 1. INTRODUCTION Due to the rapid increase in Copper cost, the manufacturers of electronics and electrical equipment manufactures prefer to replace the Cu with Al in many applications [1] like heat shinks, heavy duty busbars, high duty earthing strips, panel boards etc. Different parameters influence the selection of conductor material for heavy duty current bus systems. Generally, Al is taken for one side of the system and Cu for another side. The transition have been made, where the two conductors meets. Originally, bolted joints were used to made transition pieces [2, 3]. Because of difficulty in making electrically reliable and stable bolted joints in between Al and Cu metals, in last decade most of the researchers were focused on making transitions using bonded Cu to Al materials. Welding of Al/Cu is generally utilized as a transition piece as a part of high current transport frameworks to transmit the current. From the perspective of the welding procedure, Al and Cu are incongruent metals to welding since they have a high fondness to each other, at a temperature more than 120 0C and create fragile intermetallics [4, 5] that is mechanically and electrically shaky, in light of the fact that these intermetallic mixes have a nonmetallic covalence band. Hence, an endeavor to weld Al to Cu by routine means through the use of heat energy, to dissolve and fuse, the two metals can bring about a temperamental weld. The regular procedure to achieve this metallic bond has been to plate Al with another metal. This practice includes many stages, is environmentally not friendly and is utilized for only little joints. Explosion welding, friction and friction stir welding have been considered as the qualified welding procedure of these metals because of reliable joint interface and little intermetallic development. Friction stir joining is a lot of versatile, compared to all or any different solid state joining methods, in welding parts rather massive or complex in shape [6]. This welding method is a novel solid state welding method in which the metal that is being welded doesn t melt and recast7. In this joining process, the plates to be welded are firmly clamped in position. FSW tool is a rotating non-consumable tool with specially designed pin and shoulder. This tool penetrates into the two parts to be joined until the tool shoulder touches the top overlap part and moves along the required weld seam as represented in figure1. Figure 1 Schematic representation of single lap joint This method have many merits such as less lead time, great material saving and used to weld different metals and alloys in different thicknesses. Therefore, friction stir welding can minimize the formation of the brittle intermetallic compound at the interface because it is carried under melting temperature, high pressure and shorter process time. http://www.iaeme.com/ijciet/index.asp 1968 editor@iaeme.com
Evaluation of Electro-Mechanical Properties of Friction Stir Welded Al / Cu Bimetallic Lap Joints The main aim of this paper is to test the weldability of Cu/Al lap joints fabricated by FSW and to evaluate the electro-mechanical properties of the weld by changing input parameters such as rotational speed of tool, speed of welding and the tilting angle. 2. EXPERIMENTAL PROCEDURE The pure Copper and Aluminium plates of size 150mm x 100mm x 3mm were used to fabricate FSW Cu/Al lap joints. Tables 1a and 1b represents the mechanical and chemical properties of the parent materials. The relative positions of Cu and Al plates for FSW are shown in figure2. In this work, single pass welding processes used for fabrication of lap joints. High speed tool M2 of hardness Rc62 has been used for welding. The taper pin tool geometry and material chemical composition is given in table 2a and 2b. The friction stir welding machine is shown in figure4. This machine has an arrangement for adjustment of angle of tilt, welding speed, rotating speed of tool, frictional time and speed of plunging. Base metal Table 1a Parent metal mechanical properties Mechanical properties Y.S (MPa) T.S (MPa) %Elongation T.S.S(M.Pa) (0.6xT.S) Pure Aluminium 95 119 16.7 71.4 Pure Copper 108 200 52.62 100 Y.S: yield strength; T.S: tensile strength; T.S.S: tensile shear strength Table 1b Parent metal chemical composition (Wt %) Base metal Chemical compositions (Weight %) Pure Al Si Mn Cr Ti Cu Zn Mg Fe Aluminium 99.64 0.18 0.04 0.001 0.002 0.012 0.001 0.001 0.1 Pure Cupper Cu Ag Sn Sb Pb As Zn Bi Fe 99.98 0.002 0.001 0.001 0.001 0.002 0.002 0.001 0.002 Table 2a Tool Geometry Si. No. Tool Geometry Dimension (mm) 1 Shoulder Diameter 15 2 Maximum Diameter of pin 5 3 Minimum Diameter of pin 3.5 4 Length of pin 5.7 Table 2b Tool Material Chemical compositions (in Wt %) Tool material Chemical compositions (Weight %) HSS(M2) C Si Mn Cr Mo W V 0.8 0.4 0.4 4.15 5 6.1 1.9 http://www.iaeme.com/ijciet/index.asp 1969 editor@iaeme.com
Dr. M. Satya Narayana Gupta and K. Shiva Shankar Figure 2. Relative Position of Copper and Aluminium Plates Figure 3 Non-Consumable Rotating Tool Figure 4 FSW Machine The three welding process parameters chosen are tool rotational speed, traverse speed and tilting angle, which are generally considered to be the variables controlling quality in FSW. The other parameters- plunging speed, tool sinking and frictional heating time are maintained constant at appropriate levels. Trial experiments are conducted to fix the working range of the input process variables. The range of feasible processes variables has been taken in such a way that, to keep the welds free from noticeable outer defects. The important factors that affect the strength of the welds and their working limits are represented in table3. Due to the wide range of factors, it has been decided to use only 3 factors, two levels, full factorial Design matrix to optimize the experimental conditions. Table 4 shows the 8 (23) sets of coded conditions used to form the design matrix. For convenience of recording and processing experimental data, upper and lower levels of the factors have been coded as +1 and -1 respectively. http://www.iaeme.com/ijciet/index.asp 1970 editor@iaeme.com
Evaluation of Electro-Mechanical Properties of Friction Stir Welded Al / Cu Bimetallic Lap Joints S.No 1 2 3 Parameter Rotational speed Welding speed Tool tilting angle Table 3 Important factors and their levels. Factor Designation N V A Unit Rpm mm/min Degrees Design Levels -1 (lower) +1(upper) Constant parameters: plunging speed is 10 mm/min, tool sinking is 5.8mm and frictional heating time is 10sec. 1500 30 0 2500 80 3 Experiment number Table 4 Design matrix and experimental results Factors N V A Tensile shear strength(mpa) Fracture location Weld bead L 1 _ 70 Al base metal L 2 + 41 Interface L 3 _ + _ 55 Interface L 4 + + _ 52 Interface L 5 + 68 Al base metal L 6 + _ + 46 Interface L 7 _ + + 30 Interface L 8 + + + 40 Interface As specified by the design matrix, eight lap joints have been fabricated. The welded joints are sliced using a shear cutting machine and then machined to the required size as shown in figure 5. The tensile shear specimens are prepared as per the ASTM D-1002 standard to evaluate the tensile shear strength of the welded joints. Tensile shear strength of the FSW lap joints is evaluated by conducting shear test on universal testing machine (UTM). The consistency of the results has been checked by repeating the experiment L1 four times. The standard deviation and variance were calculated for these four experiments as 1.515 and 2.296 respectively. This value is much less than the standard deviation of eight treatment combinations i.e 13.88 http://www.iaeme.com/ijciet/index.asp 1971 editor@iaeme.com
Dr. M. Satya Narayana Gupta and K. Shiva Shankar Figure 5 Schematic Diagram of Tensile Specimen The test specimens for measurement of electrical resistance are cut from the friction stir welded Al /Cu bimetallic lap joints and had a dimension of 3mm x 20mm x 170mm. The contact resistance across interface of friction stir welded Al/Cu was measured using the high-precision nano-voltmeter (KEITHLEY 2182) and DC current source (KEITHLEY 6220) with a resolution of 10 nano - ohms. Figure 6 shows the simplified schematic of the assembly used to measure resistance of Al/Cu lap joints. Figure 7 shows the resistance test arrangement during measuring the resistance of Al/Cu lap joints. By measuring the potential between the two ends of the specimen, the specimen resistance was calculated from Ohm s law (V=IR). The specimen resistance was determined by averaging 10 resistance measurements made across this interface after selected time interval. Figure 6. Simplified Schematic Representation of The Setup Used to Measure Resistance Of Al/Cu Bimetallic Lap Joints Figure 7 Resistance Test Arrangement http://www.iaeme.com/ijciet/index.asp 1972 editor@iaeme.com
Evaluation of Electro-Mechanical Properties of Friction Stir Welded Al / Cu Bimetallic Lap Joints 3. RESULTS AND DISCUSSION 3.1 Mechanical Properties 3.1.1. Tensile Shear Strength The tensile shear strength of friction stir welded lap joints of aluminium and copper has been experimentally determined using UTM. The tensile shear test results weld bead and fracture location at various treatment combinations are given in table 4. The test results indicated that the high-quality welds are observed at low rotational speed and welding speed irrespective of tilting angle. The maximum value of tensile shear strength obtained was 70MPa. It is about 100% of base metal Aluminium and 70% of base metal Copper. The maximum tensile shear strength obtained was much more than the maximum shear strength obtained by the previous studies [8]. As can be seen in table 4, there are some conditions in which failure has been occurred in the base metal, sufficiently far from the jointed area as shown in figure 8 and therefore, it is possible to achieve the optimum shear tensile strength of joint by adjusting the rotational speed, welding speed. On other hand, most of the specimens failed on the advancing side because formation of intermetallic compounds is likely to occur in advancing side because of higher amount of temperature and strain in this side. High rotational and welding speeds decreases the shear loads because the higher rotational speed increases the amount of intermetallic components and lack of vertical flow of material at high welding speeds. (a) (b) Figure 8 Failure locations at a) N =1500 rpm, V=30 mm/min and A = 0 0 b) N =1500 rpm, V=30 mm/min and A = 3 0 3.1.2. Surface Hardness In order to get experimental indications of the temperature distribution during the friction stir welding processes and in particular of the effects of the heat fluxes on the mechanical characteristics of base metal, micro-hardness tests have been measured along the transverse section of the joints using a load of 500 g. The results from the hardness measurements are shown in Figure 9. The vertical line represents the position of the tool pin during welding. The hardness varied between 36 and 44 HV0.5 and between 62 to 69 HV0.5 for Aluminium and copper plates respectively. The minimum hardness in Aluminium and copper was found in the http://www.iaeme.com/ijciet/index.asp 1973 editor@iaeme.com
Dr. M. Satya Narayana Gupta and K. Shiva Shankar heat affected zone (HAZ) of both the advancing and retreating side. This shows the local softening of the material in HAZ, because of the thermal action of the welding processes. The hardness of the nugget was higher than that of HAZ and the base metal. This may be attributed to the formation of brittle and hard intermetallic compound and very fine recrystallized grains in the nugget zone and grain growth in HAZ[9,10]. Figure 9 Micro hardness distribution for the joint produced by N=1500rpm and v = 80 mm/min The hardness in longitudinal direction also performed near interface and top of the plates in order to know uniformity of the joint. The measurement results in longitudinal direction are shown in the figure10 and figure11. The hardness results indicate that the hardness has been uniform throughout the joint. This indicates that the joint is uniform from the beginning to the end of the joint. At some point s high hardness (120 and 168) has been observed this is due to the inter-metallic compound Al3Cu9. Figure10 Hardness distribution of longitudinal section in Al plate near interface and top of the plate http://www.iaeme.com/ijciet/index.asp 1974 editor@iaeme.com
Evaluation of Electro-Mechanical Properties of Friction Stir Welded Al / Cu Bimetallic Lap Joints Figure11 Hardness distribution of longitudinal section in Cu plate near interface and top of the plate 3.2. Electrical Resistance of the Joint Electrical resistance of the friction stir welded Al/Cu bimetallic joints, fabricated at optimum conditions, was measured using the nano-ohm meter. Table 5 presents the resistance measurement for 3mm x 20mm x 100mm specimens of the pure Al and Cu. In service, the electricity path is through the thickness of the Al/Cu joint. The electrical resistance through the thickness of joint is the sum of resistances of Al and Cu R joint = R Al + R Cu Where R joint = Resistance of Al/Cu joint R Al = Resistance of Al R Cu = Resistance of Cu Table 6 presents the calculated resistance of Al/Cu bimetallic test specimens (3mm x 20mm x 170mm) compared with the experimentally measured resistance of the stranded Al/Cu bimetallic test specimens. The data in table 7 shows that the measured resistance of the Al/Cu joints is approximately equal to the sum of the resistances of Al and Cu. This indicates that the bond zone resistance is essentially negligible as in manufactured condition. The measured resistance is slightly more than the calculated resistance. This is because of intermetallic formation in nugget zone [11]. Table 5 Measured Resistance of Al and Cu (each 3mm x 20mm x 100mm) S. No Material Electrical Resistance (µω) 1 Aluminium 27.08 2 Copper 41.08 Table 6 Measured Vs Calculated Resistance Resistance in Micro-Ohm s Si. No Joint Calculated Bond Zone Measured Resistance Resistance Resistance 1 L 1 59.55 58.14 1.41 2 L 5 59.85 58.14 1.71 http://www.iaeme.com/ijciet/index.asp 1975 editor@iaeme.com
Dr. M. Satya Narayana Gupta and K. Shiva Shankar 4. CONCLUSIONS Sound lap joints of Al/Cu have been successfully obtained by friction stir welding technology. Joint fabricated at low rotational speed and low welding speed yielded superior tensile shear strength irrespective of tilting angle and the failure was in base metal Aluminum. The maximum value of tensile shear strength was 70 MPa. It is about 100% of base metal aluminium and 70% of base metal Copper. The minimum hardness in Al and Cu was found in the heat affected zone (HAZ) of both the advancing and retreating side. The joint is uniform from the beginning to the end of the joint. The joint resistance is essentially negligible as in manufactured condition 5. ACKNOWLEDGEMENTS The authors would like to acknowledge the help by M/S DMRL, Hyderabad and M/S BHEL R&D, Hyderabad for giving permission to conduct experiments on friction stir welding machine and to measure joint resistance respectively. REFERENCES [1] P.K. Rohatgi and C.Weiss, Technology forecasting for commodity projections: A case study on the effect of substitution by Aluminium on the future demand for Copper, Technology forecasting and social change volume 11, 1977, 25-46. [2] William F.Olashaw, Aluminum- copper electrical joint, United States patent, June 1, 1980. [3] William E. Veerkamp, Copper-to-Aluminum transitions in high DC bus system, IEEE transactions on industry applications 1997, Vol.33, No 4. [4] Behcet Gulenc, Investigation of interface properties and weld ability of Aluminum and copper plates by explosive welding method, Materials and design Volume 29, 2008, 29, 75-278. [5] Bekir S. Yilbas, Ahmet Z. Shin, Friction welding of St-Al and Al-Cu materials, Journal of material processing technology, Volume 45, 1995, 49, 431-443. [6] L. Fratini, G.Buffa, R. Shivpuri Improving friction stir welding of blanks of different thicknesses, Material Science and Engineering, 2007, A 459, 209-215. [7] R.S. Mishra, Z.Y. Ma Friction stir welding and processing, Materials Science and Engineering, Volume 50, 2005, 1-78. [8] A. Abdollah-Zadeh, T. Saeid, B.Sazgari, Microstructure and mechanical properties of friction stir welded aluminium/copper lap joints Alloys and Compounds volume 460, 2008, 535-538. [9] C.J. Hang, C.Q. Wang, M. Mayer, Y.H. Tian, Y. Zhou, H.H. Wang Growth behavior of Cu/Al intermetallic compounds and cracks in copper ball bonds during isothermal aging, Microelectronics Reliability, Valume 48, 2008, 416 424. [10] Arun Nishchal Guleria and Sandeep Salhotra, Effects of Silica Fume (Micro Silica or Nano Silica) On Mechanical Properties of Concrete: A Review. International Journal of Civil Engineering and Technology, 7(4), 2016, pp.345 357. [11] P. Xue, D.R. Ni, D. Wang, B.L. Xiao, Z.Y. Ma, Effect of friction stir welding parameters on the microstructure and mechanical properties of the dissimilar Al Cu joints, Materials Science and Engineering volume A 528, 2011, 4683 4689. [12] Won-Bae Lee, Kuek-saeng, Seung-Boo Jung, Effects of intermetallic compound on the electrical and mechanical properties of friction welded Cu/Al bimetallic joints during annealing, Journal of alloys and compounds volume 390, 2005, 212-219. http://www.iaeme.com/ijciet/index.asp 1976 editor@iaeme.com