Effect of die channel angle and friction coefficient in ECAP-Conform process for Al-6061 alloy

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1 Effect of die channel angle and friction coefficient in ECAP-Conform process for Al-6061 alloy J. Gholami, M. Sedighi, M. Pourbashiri School of Mechanical Engineering, Iran University of Science and Technology, Tehran , Iran Abstract: Equal-channel angular pressing (ECAP) combined with the Conform process provides a solution for the continuous production of ultrafine-grained materials. In this process a rod with round section leaded to channel with square section. Frictional forces between roller and rod caused that the rod move in the channel. In this study finite element method has been used to study the effects of different parameters on ECAP-Conform process. Al-6061 Wire with initial diameter of 4.1 mm has been used for numerical study of ECAP Conform process in ABAQUS/Explicit software. The effects of three different outlet channel angles (90ᵒ, 100ᵒ and 110ᵒ) and three friction coefficients (0.2, 0.3 and 0.4) has been investigated by FEM. The results shown that by increasing the outlet channel angle from 90ᵒ to 110ᵒ, the average amount of induced plastic strain in cross section of wire and the amount of required force for the process reduced about 40% and 50% respectively. By increasing the coefficient of friction from 0.2 to 0.4, the amount of equivalent plastic strain and required force increased about 8% and 12% respectively. The validation of numerical results has been done by lee s equation. Keywords: SPD; ECAP-Conform; Simulation; Aluminium Introduction Fine grained materials are materials that have unique properties in terms of mechanical and physical and severe plastic deformation methods are generated and the grain size is 1μm or less [1]. One of these methods is equal channel angular pressing (ECAP) [2] that Effective way to create high shear stresses that although there are some limitations for commercialization. These limitations include the 1- limits on the length of the workpiece 2- noncontinuous and 3- the high cost of production. For commercialization the continuous methods should be used In order to produce fine grained materials without longitudinal limitations. To achieve this goal, various SPD processes are proposed such as Equal Channel Angular Drawing (ECAD) [3], Accumulative Roll Bonding (ARB) [4], Equal Channel Angular Rolling (ECAR) [5], Conshearing process [6], Continuous Frictional Angular Extrusion (CFAE) [7], ECAP-Conform [8]. Among these processes, ECAD process isn t able to do several passes because more than 15% reduction in the cross section cannot be created [9]. Other processes that are suitable only for continuous grain refinement of sheets or plates but ECAP-Conform process can be used for different crosssections (sheet, bar, rod and wire) [1]. In this process, different cross-sections are guided to ECAP die using friction force. Fig 1 schematically shows the ECAP- Conform process. As is clear from Fig 1, the most effective parameters of this process is the outlet channel angle ( ). Lee et al. [10] proposed that Ø can be adjusted from 100ᵒ to 140ᵒ for producing ultrafine grains with high angles of misorientation, on the contrary. Nakashima et al. [11] have found that ECAP with Ø=135ᵒ was not capable of producing UFG structure. Through experiments for pure aluminum strip is shown that the optimal value for the in the Con-shearing is equal to 115ᵒ [12]. ECAP process is also commonly used by 90ᵒ. In this study three outlet channel angles of 90ᵒ, 100ᵒ and 110ᵒ has been investigated. Based on the ASM standard, the friction coefficient between the steel and aluminum is about 0.38 to 0.47[13]. So in this paper three friction coefficients of 0.2, 0.3 and 0.4 has been studied. Finally, distribution of equivalent plastic strain in the cross section of wire and the amount of required forces and torques for proposed cases are compared with each other. Finite Element Analysis In this paper, the ECAP-Conform process of wire is simulated by FEM using Abaqus/Explicit. In these simulations, the initial length of wire is equal to 100 mm and the initial wire diameter is reduced from 4.1 mm to 4 mm during the initial state of process. Roller diameter is equal to 200 mm. the wire material is Al-6061 and its true stress-strain curve is shown in Fig 2. Other parameters for FEM simulations are illustrated in Table 1. The C3D8 element type is used for wire. The roller and die are defined as discrete rigid parts. The proper element size for wire and dies are obtained by mesh sensitivity analysis. For studying the effect of output channel angle on the induced plastic strain, three different angles (90ᵒ, 100ᵒ and 110ᵒ) are considered. In these simulations the friction Corresponding author: Tel./Fax: address: mpourbashiri@iust.ac.ir.

2 Stress(MPa) coefficient between the roller and wire is considered to be constant and is proposed to be 0.4. Correct definition of friction in FEM simulations is so important because the friction force is the main factor that drive the wire into the die channel. The rotational speed of roller for FEM Simulations is 0.05 rad / s and it is constant during the process. plastic strain in the bottom of outlet channel is greater than its upper portion. Also Fig 3 shows that the amount of induced plastic strain for outlet channel angle of 90ᵒ is greater than 100ᵒ and 110ᵒ. Comparison of average amount of equivalent plastic strain for described three cases is shown in Fig 4. Table 1 FEM simulation parameters Rotational velocity, ω(rad s) Friction coefficient, (μ)(wire and roller) Friction coefficient, (μ)(wire and die) Material Young s modulus (GPa) Poisson s ratio, ε Density, ρ (kg/m3) Die channel angle ( ) , 0.4, Al , 100, 110 (a) Fig. 1. Schematic view of the ECAP-Conform process The ECAP-Conform process is performed at low speed relative to other SPD processes. Because the slipping of wire on the roller surface should be avoid and as a result the friction force could be applied to the wire completely. For studying the effect of friction coefficient on the induced plastic strain and reaction forces and torques, three different friction coefficient (0.3, 0.4 and 0.5) are considered. In these simulations, the output channel angle of die is considered to be constant and equal to 110ᵒ. Other simulation parameters are as before. (b) strain Fig. 2. True Stress strain curve of Al-6061[14] Results and Discussions 1. Effect of the outlet channel angle Outlet channel angle is the principle parameter in ECAP- Conform process. Fig 3 shows the distribution of plastic strain in the cross-section of wire for different outlet channel angles. It can be seen that the amount of equivalent (c) Fig. 3. Distribution of plastic strain in the cross-section of wire for different output channel angles. a) Ø=90ᵒ, b) Ø=100ᵒ, c) Ø=110 This conclusion could be considered based on the analytical relationship Presented by Lee and colleagues [10] too. This relation that predict effective plastic strain in the outlet channel is as follow: ε e = 2N 3 K2 cot φ 2 (1)

3 Maximum Reaction Force (KN) Maximum Reaction Torque (N.m) Equivalent Plastic Strain Where K is the ratio of changing the diameter of the input state to output state, N is number of passes and Ø is the angle of the outlet channel. With replacement angles of 90, 100 and 110 in the equation (1) approximate value of induced strain is obtained equal to 1.15, 0.97 and 0.81 respectively. Comparison of analytical and numerical results are presented in Figure 4. The results are in good agreement. Comparison of the forces and torques acting on the axis of rotation for outlet channel angles of 90, 100 and 110 is shown in Fig 5. The maximum amount of power and torque required for ECAP-Conform process happens at angle of 90 degree, and then the amount of force and torque required for the process is reduced with increasing the outlet channel angle. Because for the angle of 90 degree, the wire is perpendicular to the die surface and more force must be exerted by the roller so that the wire could be passed through the channel FEM 1.42 Analytical Die channel angle Ø(ᵒ) 2. Effect of friction coefficient Figure 6 shows the distribution of equivalent plastic strain for the outlet angle of 110 and three friction coefficient of 0.2, 0.3 and 0.4. The results show that the friction coefficient of 0.5 induced more plastic strain in crosssection of wire. So if the friction coefficient and therefor the amount of frictional forces increase, the amount of induced plastic strain will be greater. The analytical and simulation results of induced plastic strain for different friction coefficient is shown in Fig 7. As is shown Analytical and simulation results are in good agreement. According to equation (1), plastic strain for angle of 110 and 1 pass is calculated equal to There is a little difference between the analytical and simulations results. This difference is caused due to lack of consideration of the frictional conditions in equation (1). Also the amount of power and torque needed for the process are investigated for different frictional conditions. µ=0.2 µ=0.3 Fig. 4. Comparing the analytical and finite element results for equivalent plastic strain (PEEQ) and different outlet channel angle 90ᵒ, 100ᵒ, 110ᵒ. MRF MRT µ= Die Channel Angle Ø (ᵒ) Fig. 5. Comparing the forces and torques acting on the axis of rotation for angles of 90ᵒ, 100ᵒ and 110. Fig. 6. Distribution of equivalent plastic strain in the output angle of 110 and friction coefficient of 0.2, 0.3 and 0.4. According to Fig 8 it can be seen that by increasing the coefficient of friction, the amount of required force and torque are increased. Further friction coefficient causes the

4 Maximum Reaction Force (KN) Maximum Reaction Torque (N.m) Equivalent Plastic Strain increase of friction force between the wire and the roller. Therefore more torque is needed to allow the wires to be guided into the channel. It should be noted that in reality the friction between the wire and the roller should increase as much as possible, so more plastic strain could be induced in workpiece. More plastic strain caused more grain refinement and increased the strength of workpiece Fig. 7. Comparing the analytical and finite element results for plastic strain equivalent (PEEQ) and friction coefficients of 0.2, 0.3, and FEM Analytical Friction Coefficient (µ) MRF Friction Coefficient µ MRT Fig. 8. Comparing the forces and torques acting on the axis of rotation for friction coefficients of µ=0.2, 0.3, 0.4 Conclusion In this paper, ECAP-Conform process has been studied by FEM simulation. Plastic strain distribution in the crosssection of the wire, the power and torque required for different cases was examined and the following results were obtained: 1. With increasing angle of the outlet channel from 90ᵒ to 110ᵒ, the average amount of plastic strain in the wire cross-section is reduced from 1.42 to The amount of power and torque needed to carry out the process is also reduced about 50%. 2. The amount of plastic strain on the surface of the wire is in contact with the bottom of the channel, is more than the rest of the wire cross-section and it could be mentioned as a disadvantage. 3. By increasing the coefficient of friction, the amount of plastic strain in the cross section of the wire is increased 0.76 to 0.83 and the amount of required force and torque for the process increased about 12%. References [1] W. Wei, W. Zhang, K.X. Wei, Y. Zhong, G. Cheng, J. Hu, Finite element analysis of deformation behavior in continuous ECAP process, Materials Science and Engineering: A 516 (2009) [2] R. Valiev, Y. Estrin, Z. Horita, T. Langdon, M. Zechetbauer, Y. Zhu, Producing Bulk Ultrafine-Grained Materials by Severe Plastic Deformation, JOM 58 (2006) [3] U. Chakkingal, A.B. Suriadi, P.F. Thomson, Microstructure development during equal channel angular drawing of Al at room temperature, Scripta Materialia 39 (1998) [4] Y. Saito, N. Tsuji, H. Utsunomiya, T. Sakai, R.G. Hong, Ultra-fine grained bulk aluminum produced by accumulative roll-bonding (ARB) process, Scripta Materialia 39 (1998) [5] J.-C. Lee, H.-K. Seok, J.-H. Han, Y.-H. Chung, Controlling the textures of the metal strips via the continuous confined strip shearing(c2s2) process, Materials Research Bulletin 36 (2001) [6] Y. Saito, H.Utsunomiya, H. Suzuki, T. Sakai, Improvement in the r-value of aluminum strip by a continuous shear deformation process, Scripta Materialia 42 (2000) [7] Y. Huang, P.B. Prangnell, Continuous frictional angular extrusion and its application in the production of ultrafine-grained sheet metals, Scripta Materialia 56 (2007) [8] G.J. Raab, R.Z. Valiev, T.C. Lowe, Y.T. Zhu, Continuous processing of ultrafine grained Al by ECAP Conform, Materials Science and Engineering: A 382 (2004) [9] J. Alkorta, M. Rombouts, J. De Messemaeker, L. Froyen, J. Gil Sevillano, On the impossibility of multipass equal-channel angular drawing, Scripta Materialia 47 (2002) [10] J.C. Lee, H.K. Seok, J.Y. Suh, Microstructural

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