Optimization and Validation of Process Parameters in Friction Stir Welding on AA 6061 Aluminum Alloy Using Gray Relational Analysis

Friction stir welding, a solid state joining technique, is widely being used for joining Aluminum alloys for aerospace, marine, automotive and many other applications of commercial importance. Friction stir welding (FSW) can produce better mechanical properties in the weld zone compared to other conventional welding techniques. FSW trials were carried out using a vertical milling machine on AA 6061 alloy. The main objective of this article is to find the optimum operating conditions for butt joint made of aluminum alloy AA6061. Four major controllable factors each at four levels, namely, rotational speed, welding speed, tool pin length, offset distance are considered for the present study. The uncontrollable factors include ultimate tensile strength, percentage of elongation and hardness which can be converted to signal-to-noise ratios. The gray based taguchi method which is a multiple response process is used to optimize the factors. A gray relational grade obtained from gray relational analysis is used as the multiple performance characteristic. The resulting optimum process parameters are rotational speed at 800 rpm, welding speed at 10 mm/min, pin tool length at 5.7mm and offset distance 0.4mm for the best multiple performance characteristics. Further a three dimensional solid model has been developed using Ansys parametric development language code(APDL) for validation of the experimental results. The results of the simulation are in good agreement with that of experimental results. Keywords— Aluminum; Ansys; Friction Stir welding; Gray based Taguchi; Optimization; 1.0. INTRODUCTION FSW is a novel material joining technique invented by Thomas et al.The welding institute, TWI) in 1991[1]. Material subjected to FSW does not melt and recast and hence the resultant weldment offers advantages over conventional arc weldments, such as better mechanical properties at weld zone and fewer weld defects. In recent years FSW has become one of the most important solid state joining process, and it consumes considerably less energy. No shielding gas or flux is used, thereby making the process environmental friendly.In FSW, as in Elongovan et al. conducted experiments using a non consumable rotating tool with a specially designed pin and shoulder[2]. The heat is generated between the wear resistant welding tool and the material of the work pieces. The heat causes the latter to soften without reaching the melting point and allows traveling of the tool along the welding line. Comparing the velocity of the tool and the time required for the pieces to reach softening temperature, the optimal tool velocity has been provided by Chien et al.[3]. The experimental and numerical evaluation of friction stir welds of AA 6061-T6 aluminum alloy was studied by Prasanna, P. et al.[4]. Finite element modeling for maximum temperature in friction stir welding and its validation has been proved by Prasanna,P.et all [5]. The percentage of the generated heat from the tool shoulder or the tool pin was investigated by Song and Kovacevic [6]. The Taguchi technique for the experimental data analysis is a common method used in conventional welding as in Jayaraman.M et al. [7]. The graybased Taguchi technique for experimental data analysis has been applied to conventional welding by Tarng et al. [8]. As per the available literature, this method has not been applied to FSW. Peel et al. [9]. Chi-hui chien,et all [10] have studied mechanical properties like ultimate tensile stress and percentage of elongation as a function of four parameters like tool rotation speed, transverse, tool tilt angle with respect to the work piece surface and pin tool length. Previous researchers have focused on the different parameters of friction stir welding process like rotational speed, welding speed, tool pin length, tool tilt angle, with cylindrical tool on AA 5083 alloy. In the present work, the butt joint made on AA 6061 alloy by using hexagonal tool profile by considering the controllable factors like rotational speed, welding speed, tool pin length and tool pin offset distance from axis of the shoulder , to show the best multiple performance characteristics by using Gray Relational Analysis(GRA) according to Lin et al [11]. The maximum temperature created by FSW process P.Prasanna, Dr.Ch.Penchalayya, Dr.D. Anandamohana Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera. com Vol. 3, Issue 1, January -February 2013, pp.1471-1481 1472 | P a g e ranges from 70% to 90% of the melting temperature of the work piece material, as measured by the tang et al [12] and col grove et al [13], so that welding defects and large distortion commonly associated with fusion welding are minimized or avoided. The objective of this article is to study the effects of rotational speed, welding speed (velocity of the tool), pin tool length, tool pin offset distance on the ultimate tensile strength, percentage of elongation and hardness. The four four-level controllable variables are assigned to the L16 orthogonal array. The values of uncontrollable variables are converted to signal-to-noise (S/N) ratio performance measures. Taguchi parameter design can optimize the performance characteristics through the setting of process parameters and can reduce the sensitivity of the system performance to sources of variation. The multiple-response process of robustness, the gray-based Taguchi method as in Lin[11]. The statistical method ANOVA is used to interpret the experimental data. Further, the maximum temperate is tested through simulation at optimized parameters using Ansys. The validity of the proposed simulation model is checked with the existing literature as in the tang et al [12] and col grove et al [13]. 2.0. Experimental Details: (a). Friction Stir welding with a rotating tool (b). Conventional milling (c). Hexagonal tool profile machine (d). Fabricated Joints (e). Tensile test machine (f). Hardness test machine Fig.1. Experimental details The specimens of the size of 200mmx100mmx6mm were machined from AA6061 aluminum alloy plates. The two plates of AA6061 aluminum alloy were Friction stir welded in the butt configuration by using conventional vertical milling machine. The two plates were placed side by side and clamped firmly to prevent the abutting joint faces from being forced apart. The FSW procedure was based on the TWI procedure described in the patent by Thomas et al.(1991). The experimental set up is shown in Fig.1(a-c). The welding direction of aluminum alloy was along the line of the joint. The rotation of the tool resulted in stirring and mixing of material around the rotating pin and the linear movement of the tool moved the stirred material from the front to the back of the pin and finished the welding process. The insertion depth of the pin into the work pieces was associated with the pin height (length). The tool shoulder contacting the work piece surface depends on the insertion depth of the pin, which results in generation of welds with inner channel, surface groove, excessive flash, and providing small tool pin off set distance from the center of the shoulder will give more heat between the shoulder surface and work piece material. Totally, 16 FSW joints were produced as shown in fig 1(d-e). Tensile tests were carried based on ASTM standard. The FSW joint plates were sawed into the dimension 200x20mm. The tensile tests were carried out by universal testing machine to find maximum loading and percentage of elongation. Percentage of elongation is defined as ratio of deformation to original length of 50mm. Hardness tests were carried out on Rockwell hardness machine at a force of 60kgf as shown in fig1(f). Properties of aluminum alloy AA 6061 is given in the table. P.Prasanna, Dr.Ch.Penchalayya, Dr.D. Anandamohana Rao / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera. com Vol. 3, Issue 1, January -February 2013, pp.1471-1481 1473 | P a g e Table1:% of chemical composition AA 6061 –T6 alloy Mg Si Fe Cu Zn Ti Mn Cr others Al 0.8 1.2 0.40.8 0.7 0.150. 4 0.25 0.15 0.15 0.040. 35 0.05 98.7 Table2. Mechanical and physical properties of AA 6061 –T6 alloy Young's modulus Tensile strength Ultimate tensile strength (G Pa) (M Pa) (M Pa)