IJAMS EXPERIMENTAL STUDY OF FRICTION STIR WELDED COMMERCIAL ALUMINUM. Atul Suri 1, K. HansRaj 1.

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1 IJAMS EXPERIMENTAL STUDY OF FRICTION STIR WELDED COMMERCIAL ALUMINUM Atul Suri 1, K. HansRaj 1 1 Dayalbagh Educational Institute, Dayalbagh, Agra , INDIA atulsuridei@gmail.com, khansraj@rediffmail.com Abstract: Friction Stir welding (FSW) is increasingly being used on account of certain improved features and faster productivity in fabricating various industrial products. The paper presents the advantages of FSW process for joining commercial Aluminum sheets. A new straight threaded pin tool with flat collar made of hardened steel is considered for FSW of 6.5mm thickness commercial Aluminum plates. Effect of variable tool rpm ranging from 400rpm to 1400rpm at a constant feed rate of 30mm/min is considered for the study. Hardness, strength, toughness and microstructure are evaluated to find out the effect of tool rotational speed. The maximum tensile strength is observed to be 78% of the base metal at 400rpm of single sided FSW joint. The hardness of weld zone (WZ) was found maximum at lower tool rpm in case of FSW. Microstructure on the top surface gets refined with the increase in tool rpm. FSW proved to be an agile manufacturing process for joining commercial aluminum. Keywords: Friction Stir Welding, Hardness, Strength, Toughness, Microstructure. 1. Introduction Aluminum welding still represents a critical operation due to its complexity and the high level of defect that can be produced in the joint. The main problems are related to the properties of aluminum that is high thermal conductivity, high chemical reactivity with oxygen and high hydrogen solubility at high temperature [1]. All these factors can cause the presence of defects on the weld bead. Friction stir welding (FSW) was invented at The Welding Institute (TWI) of the United Kingdom in 1991 as a solid-state joining technique and was initially applied to aluminum alloys [1-7]. In the FSW process, a special tool mounted on a rotating probe travels down through the length of the base metal plates in face-to-face contact. The tool serves three primary functions, that is, heating of the work piece, movement of material to produce the joint, and containment of the hot metal beneath the tool shoulder. Heating is created within the work piece both by friction between the rotating tool pin and shoulder [3] and by severe plastic deformation of the work piece. The probe is slightly shorter than the thickness of the work piece [4]. The FSW joint is created by friction heating as well as severe plastic deformation due to the translation of the tool in the weld material. The stirring of the tool minimizes the risk of having excessive local amounts of inclusions, resulting in a homogenous and void-free weld. Since the amount of heat supplied is smaller than during fusion welding, heat distortions are reduced and thereby the amount of residual stresses. The deformation control is therefore easier. The microstructure in the stir zone is influenced by FSW parameters like tool geometry, tool rpm, shoulder penetration and feed rate [8]. Conventional welding results in the presence of a tenacious oxide layer, high thermal conductivity, high thermal expansion coefficient, solidification shrinkage and, above all, high solubility of hydrogen, and other gases, in the molten state [9]. The objective of this investigation is to determine the mechanical properties of FSW of pure aluminum, and to prove this method as an effective method for joining commercial aluminum. Volume 15 Issue 2 37

2 Atul Suri, K. Hans Raj The friction- stir- welded zone implies a region changed in microstructure by mechanical stirring action of the rotating tool during the FSW. Figure 4 shows the FS- Welded plate produced at 800rpm. Table 1: Chemical Composition of Base Metal Element s Si Mg Mn Cu Fe Zn Ni Cr Ti Weight Figure 1: Principle of FSW operation Tang et al. [10] states that the temperature raised during FSW operation is ranges from 70%-90% of the melting temperature of the work piece material. Colegrove et al. [11] states that due to the increase in the temperature lesser than the melting temperature, the welding defects and large distortion commonly associated with the fusion welding are minimized. Table 2: FSW tool specifications Tool Material Tempered steel Shoulder diameter 17mm Shoulder chamfer 1mm all around Pin diameter 5mm Pin Length 4.5mm Pin Threads 24TPI, BSW, LH 2. Experimental Procedure In this study the specimen are made by using commercial Al plates. The dimensions of the specimen are 90mm X 14mm x 6.5 mm. Nominal composition of Al plates is shown in table 1. The welds are developed in square butt joint configuration. The steel tool used for FSW is shown in figure 2. A straight threaded pin tool is considered for the application. Tool Shoulder diameter, pin diameter and pin length are taken as 17mm, 5mm, and 4.5mm respectively. A 24TPI left hand threads were produced on the tool pin. Table 2, represents the details of the tool. The welds are developed at different rpm i.e. 400, 600, 800, 1200 and 1400rpm on a constant feed rate of 30mm/min. tool is kept perpendicular to the welding surface during the process. Figure 3, represents the dimensions of the specimens used for tensile testing. FSW is performed using vertical milling machine. FS- Tool was held in vertical spindle which rotates at different rpm and remains stationary on its position. Welding takes place by moving the plates under the rotating tool at the constant feed rate of 30mm/min. First the pin is inserted on the outer end of abutting plates and then the plates/ fixture is moved in forward direction. Tool pin is ejected at the end of FSW. Figure 2: FSW tool used for study Figure 3: Specimen dimensions in millimeter Volume 15 Issue 2 38

3 Experimental Study of Friction Stir Welded Commercial Aluminum FSW was performed at five different rpm ranging from 400rpm to 1400rpm and the specimen are produced for doing tensile testing. Five point average hardness is considered on the WZ along the weld line for FSW specimen. Microstructural observations are made using optical microscope and the effect of tool rpm on grain size is observed. The specimens are tensile tested on a computerized UTM to observe the mechanical behavior at different rpm. Tensile tests are performed at a constant cross head speed of 5mm/min. represents a good plastic behavior of FS- Welded specimen. Figure 6, shows a 90 0 bent specimen. Figure 4: FS- Welded plates produced at 800rpm 3. Experimental Results Figure 5: Surface appearance of weld Zone produced at different rpm and constant feed rate of 30mm/min. FSW was performed at different tool rpm using straight threaded pin tool and the output parameters like surface appearance, microstructure, weld zone hardness, strength, elongation etc; are compared to find out the most suitable parameter for producing optimal mechanical properties. 3.1 Surface Appearance Figure 5, shows the surface appearance of FS welded plates at tool rotation speeds of 400,600, 800, 1200 and 1400rpm. Larger defects like excessive burrs and valleys were produced on 400rpm which keep reducing at increasing rpm. A good surface was produced on 1200 rpm which further starts diminishing at 1400rpm as excessive melting on top surface causing instability of the tool shoulder on the top surface resulting in poor surface finish. 3.2 Plastic Behavior Plastic behavior of the specimen is observed by bending the specimen at the weld zone by giving impact hammer blows. Up to of bending no crack is observed on the weld surface, which Figure 6: 90 0 bent specimen 3.3 Microstructure Figure 7, shows the microstructure of the Weld Zone (WZ) produced at different tool rpm. It clearly represents that with the increasing rpm the grain size is reducing. At 400rpm the grain size is observed to be 3.2µm which reduced to 0.95 µm at 1200rpm. With further increase to 1400rpm a slight increase to 1.0 µm is observed due to excessive heating in the weld nugget. Figure 8, depicts the comparison of grain size on top surface produced at different rpm of FSW. It clearly represents that grain size keeps reducing with the increasing rpm. A minute increase in Volume 15 Issue 2 39

4 Atul Suri, K. Hans Raj grain size is observed at 1400rpm which is caused due to the excessive heat in the weld nugget. 400rpm 600rpm Figure 9: Tensile tested specimen 800rpm 1200rpm 1400rpm Figure 7: Microstructure of FS-Weld Zone 3.5 Strength Figure 10, showing the comparison of tensile strength and yield strength for specimen produced at different rpm. These values are to reduce with the increased rpm. In FSW, due to friction and SPD action caused by traverse of tool and the material become semi solid and forms a solid state joint with defect free fine microstructure joint. At higher rpm due to excessive melting some internal defects are introduced causing weaker joints. Figure 8: Grain size at weld zone corresponding to tool 3.4 Tensile test Figure 9, shows the tensile tested specimen of Al produced at different tool rotation and constant feed rate of 30mm/min. Specimens are tested on autograph computerized UTM with gradually increasing load and parameters like Tensile strength (Mpa), yield strength (Mpa), percentage elongation are recorded. It was observed that FSW specimen breaks at the weaker WZ without considerable deformation. Figure10: Strength characteristics corresponding to tool rpm 3.6 Hardness Brinell hardness was checked for both uniform and FSW specimen. Portable and automatic hardness testing machine supplied by Chennai METCO is used for the purpose. Figure 11, represents the hardness of FSW weld zone produced at different rpm. For uniform specimen, hardness is checked at nine different points along the length of the specimen where as for FSW specimen, five point hardness is checked on the Volume 15 Issue 2 40

5 Experimental Study of Friction Stir Welded Commercial Aluminum WZ along the weld line. About 50% hardness increase took place at the weld zone produced at 400rpm. Figure 11: Hardness corresponding to tool rpm. Table 3: Comparison of mechanical properties of the specimen produced at different rpm of FSW Speci men Strength (MPa) Elongation Brinel l Hardn ess Grain Size RPM Tensi Yield % HB (µm) le Discussion In the current study the Butt welded specimen of commercial Aluminum are produced using the FSW process. The influences of the tool rotation speed on surface appearance, microstructure and tensile properties of the butt weld are experimentally investigated for straight threaded tool. The following results are observed. The surface with minimum accumulation of material in the tool advancing side appeared at 1200rpm. Surface appearance and the accumulation of material on the advancing side keeps increasing with the decreasing rpm. The feed was kept constant at 30mm/min. At 1400rpm the surface finish starts deteriorating due to excessive melting of the base metal in the weld nugget. Brinell hardness on the weld zone (WZ) is measured at five different points along the weld line and it is observed that the hardness of the WZ at 400rpm was found to be 50% higher than the virgin material. The hardness keeps reducing with the increasing rpm. At 1400rpm only 10-15% increase in the hardness is reported. Grain size on the top surface keeps reducing with the stirring action of the tool shoulder. A remarkable reduction in the grain size of about 600% took place at 1200rpm in comparison to virgin material. At 1400rpm and a low feed rate of 30mm/min the excessive melting in the abutting surface causing grain enlargement. Slight increase in grain size took place at 1400rpm in comparison to 1200rpm. At very high rpm i.e and 1400rpm more surface defects were observed resulting in poor mechanical strength at the weld zone. During the tensile testing the butt welded FSW specimen are fractured in the WZ at the early stage of plastic deformation i.e. about after 2mm of elongation. The tensile strength of 326MPa is observed for the FSW specimen made with the tool rotation speed of 400rpm, which is 78% of the virgin specimen. Both the tensile and yield strengths are observed to reduce with the increase rpm, maintaining the constant feed rate of 30mm/min. Strength is minimum at 1400rpm. The maximum strain of the butt welded FSW specimen is about 3.7% at 400rpm which is only 20% of the base metal. 5. Summary A brief review of the Friction Stir Welding (FSW) process was discussed mentioning the advantages over conventional welding process. Microstructure of the nugget zone is nonhomogeneous because of the various geometrical features of the tool, complex movement of material around pin and variation of rpm, strain rate and temperature. Fine grain microstructure produced after FSW possess with excellent Volume 15 Issue 2 41

6 Atul Suri, K. Hans Raj mechanical properties. Butt welded specimens are produced at five different tool rpm. Microscopic observations are made to see the effect the tool rpm on grain structure. Hardness of the WZ is inspected for all the specimens considering five points along the weld length. Tensile test is conducted on computerized UTM at a constant crosshead speed of 5mm/min. results are tabulated and analyzed and it is concluded that FSW on pure aluminum alloy plates is agile and feasible process that produces good welds with higher mechanical strength. 6. Acknowledgement We gratefully acknowledge the inspiration and guidance provided by Most Revered Chairman of Advisory Committee on Education, Dayalbagh. 7. References 1. Garware, M., Kridli, G.T., and Mallick, P.K, Tensile and Fatigue Behavior of Friction-Stir Welded Tailor-Welded Blank of Aluminum Alloy 5754,Journal of Materials Engineering and Performance, 19(8) Sahin, M., Joining of Aluminium and Copper materials with Friction,International Journal of Advanced Manufacturing Technologies, Tansel, I. N., Demetgul, M., Okuyucu, H, and Yapici, A., Optimization of friction stir welding of aluminium alloy using genetically optimized nural network, International Journal of Advanced Manufacturing Technologies, Prasanna, P., Rao, B.S., and Rao, K.M, Finite Element Modeling for Maximum temperature in friction stir welding and its validation, International Journal of Advanced Manufacturing Technologies, Shigematsu, I., Kwon, Y.J., and Satio, N,2009. Dissimilar Friction Stir Welding for Tailor- Welded Blanks of Aluminum and Magnesium Alloys,Material Transactions, 50(1) Thomas, W. M., Nicholas, E.D., Needham, M.G., and Dawes, C.J,G.B. Patent , Dec Dawes, C., Thomas. W,1995,TWI Bull., Hans Raj, K., Sharma, Rahul Swarup., Singh, Preetam., Experimental Studies of Friction Stir Welding Process, International Journal of Engineering Studies, 2 (3) Frigaard,G., Midling,O.T,2001. "A Process Model for Friction Stir Welding of age hardening aluminium alloys", Metallurgical and Materials Transactions, 32A(5) Chao, Y.J., Qi. X., and Tang, W, Heat Transfer in Friction Stir Welding- Experimental and Numerical Studies, Journal ASME Journal of Manufacturing Science Engineering, Colegrove. P., Painter, M., Graham, D., and Miller, T, Dimensional flow and thermal modelling of the friction stir weldingprocess,proceedings of the Second International Symposium on Friction Stir Welding, June 26-28, Gothenburg. Biography Atul Suri is working as Instructor- Machine Shop in Technical College, Dayalbagh Educational Institute, Agra (India) since He has a degree in Mechanical Engineering from the Institution of Engineers, Kolkata (India), M. Tech in Engineering Systems with gold medal from Dayalbagh Educatioal Institute, Agra (India). He has 10 publications in international conferences and journals. He is a Life Member of Systems Society of India, Volume 15 Issue 2 42

7 Experimental Study of Friction Stir Welded Commercial Aluminum International Society of Agile Manufacturing (ISAM), corporate member of Institution of Engineers (Kolkata). His research interests include Experimental study of Friction Stir Welding, Friction Stir Processing, CAD/CAM and FE simulation of Metal Forming Processes. Dr. Kandikonda Hans Raj was born on 31 st July 63. He has received B.Sc. Engineering in Mechanical Engineering from Agra University, M. Tech. in Mechanical Engineering from I.I.T. Roorkee and Ph.D. from Dayalbagh Educational Institute. Presently he is a Professor in Mechanical Engineering Department at Dayalbagh Educational Institute, Dayalbagh, Agra. He is actively involved in research and teaching since His research interest includes Intelligent and Agile Manufacturing, Metal Forming Process Modeling and Optimization, Structural Design and Optimization, Finite Element Analysis, Soft Computing Applications in Manufacturing and Quantum Evolutionary Optimization. He has taught several courses to graduate and undergraduate students. He has 126 publications to his credit in journals and conferences. He is a research consultant to ADRDE (DRDO), India. He has successfully completed eleven research projects granted by Government of India. He has been a visiting scientist to CEMEF Laboratory, France, University of Kiel, Germany, Mathematics and Computer Science Department, University of Maryland, MD, Rensselaer Polytechnic Institute, NY, MIT, Boston, U.S.A. He has been awarded prestigious Production Engineering Division Medal for 1999 & 2000 and most coveted Institution medal for the year 2001 of Institution of Engineers (India) for his research papers. He has also been awarded Academic Excellence Award 2011 by Aerial Delivery Research and Development (ADRDE), DRDO, Agra for his long standing academic and research support to ADRDE. He is a life member of Indian Society for Mechanical Engineers, Indian Society for Technical Education, Indian Society for Continuing Engineering Education, Institution of Engineers (India), Systems Society of India and. He is also a Fellow of Institution of Engineers, India (FIE), Aeronautical Society of India(FAeSI) and International Society of Productivity Enhancement (FISPE). He is an Associate Editor of International Journal of Agile Manufacturing (IJAM) and Chief Editor of International Journal of Advanced Manufacturing Systems (IJAMS). Volume 15 Issue 2 43