A numerical and experimental study on tubular channel angular pressing (TCAP) process

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1 Journal of Mechanical Science and Technology 00 (2012) 0000~ A numerical and experimental study on tubular channel angular pressing (TCAP) process G. Faraji 1,3*, M.M. Mashhadi 1, A.F. Dizadji 2, M. Hamdi 3 1 Department of Mechanical Engineering, University College of Engineering, University of Tehran, Tehran, , Iran 2 Engineering Science Department, University of Tehran, Tehran, Iran 3 Department of Engineering Design and Manufacture, University of Malaya, Kuala Lumpur, MALAYSIA (Manuscript Received 000 0, 2012; Revised 000 0, 20121; Accepted 000 0, 2012) Abstract Tubular channel angular pressing (TCAP) is a new and novel severe plastic deformation (SPD) technique suitable for fabrication of bulk nanograined tubular materials. There are several parameters in TCAP process. The present paper investigates the effects of curvature angles, deformation ratio (= maximum radius during TCAP/ initial radius) and deformation direction on the plastic deformation behavior, strain homogeneity and required loads in TCAP processing. The results showed that the higher curvature angles (ψ 1 and ψ 3 ) leads to better strain homogeneity while have not any significant effect on the process loads. Also in the second curvature angle of 90 the best condition is achieved from point of view of good strain homogeneity and lower load requiring compared to lower angles. Investigation on deformation direction showed that inlet deformation direction cause tube thinning at the end of process and using inlet case is not recommended. Keywords: Deformation behavior; Load; Nanostructured tube; Strain homogeneity; TCAP Introduction There is considerable current interest in processing metallic samples through procedures involving the imposition of severe plastic deformation (SPD) [1, 2]. The most commonly used SPD methods are equal channel angular pressing (ECAP) [2, 3, 4], high-pressure torsion (HPT) [5], and accumulative roll bonding (ARB) [6]. Though there are other methods with different applications [7-18]. Of these various SPD methods, ECAP is an especially attractive processing technique. Appreciating the outstanding capabilities of ECAP, a noble and effective process named tubular channel angular pressing (TCAP) suitable for processing tubes to very high strains was proposed by the present authors [19]. Faraji et al. [19] was applied TCAP process on an AZ91 magnesium alloy and significant grain refinement was achieved. The principle of TCAP is shown in Fig. 1. The constrained tube between inner and outer dies is pressed by a hollow cylindrical punch into a tubular angular channel. The tube material is pressed to the tubular angular channel in which during one processing cycle as much as three shear events take place. The present paper is dealt with the effects of curvature angles, deformation ratio (= maximum radius/initial radius) and deformation direction on the plastic deformation behavior, * Corresponding author. Tel.: , Fax.: address: ghfaraji@ut.ac.ir Recommended by Editor please leave blank KSME & Springer 2012 strain homogeneity and the required loads in TCAP processing. 2. FEM and Experimental procedures A commercial FEM code Abaqus/Explicit was used to perform the numerical simulation. An axisymmetric model was employed, where the geometrical dimensions and mechanical properties of specimens were the same as those of the experiment, making it possible to compare the simulation results with those obtained experimentally. Axisymmetric four node elements (CAX4R) were used to model the tube. To accommodate the predetermined large strains during simulations, adaptive meshing (automatic remeshing) was employed. The arbitrary Lagrangian Eulerian (ALE) adaptive meshing maintains a high-quality mesh under SPD by allowing the mesh to move independently with respect to the underlying material. The coulomb friction with a friction coefficient of 0.05 [20-22] and penalty method were considered to model the contact between the die and the specimen. The die and the punch were modeled as analytical rigid parts. Experimental alloy properties and process parameters with their values are shown in table 1. The mechanical properties of AZ91 alloy were obtained through a compression test at the TCAP processing temperature of 300ºC and at strain rate of 1x10-5 sec -1 [19].

2 0000 G. Faraji et al. / Journal of Mechanical Science and Technology 00 (2011) 0000~0000 The material used in this study was a commercial AZ91 magnesium alloy. Cylindrical tubes of 20 mm in outer diameter, 2.5 mm in thickness, and 40 mm in length were machined from cast ingots. A TCAP die was manufactured from hot worked tool steel and hardened to 55 HRC and is shown in Fig. 2(b). The channel angles φ 1, φ 2, and φ 3 in the TCAP facility were 135º, 90º, and 135º, respectively. The angle of curvature ψ 2 was 90º, and both ψ 1 and ψ 3 were equal to 0º, as in Fig. 2(a). The specimens were subjected to TCAP at 300 ºC with a punch speed of 5 mm/min to minimize increases in the temperature during the deformation. The friction between the specimen and dies was reduced by applying MoS 2 as a lubricant which was applied to all dies and sample surfaces. Initial AZ91 machined cylindrical tubes is placed to the gap between mandrel and tow half dies. This set is placed into a back up cylinder and surrounded by electrical heater element. The tube specimen is extruded into the angular tubular channel by a hollow cylindrical punch using an INTRON 30 ton press and force-displacement diagram was recorded. The temperature increase during forming process was observed to be less than 5 ºC at a pressing speed of 5 mm/min [23]. (a) b a c d 1 3 R1 R2 I 2 III 3 1 II 2 (b) Fig. 2. (a) Process parameters [15] (b) TCAP die picture. (a) (b) (c) Table 1. The physical properties of AZ91 experimental alloy and process parameters. Parameter Value Parameter Value Young s modulus 41 GPa R/R 0 1.2, 1.5, 1.8 (E) Poisson s ration (ν) 0.35 φ 2 90º Density (ρ) 1.78 ψ 1= ψ 3 0º, 17º, 35.5º, 45 º g/cm 3 Thickness (t) 2.5 mm ψ 2 0º, 28º, 67.4º, 90º Deformation direction Inlet and outlet Punch Inner die Tube Outer die Fig. 1. Schematic of TCAP. (e) Fig. 3. (a) -(c) Equivalent plastic strain contour of experimental alloy during TCAP processing with different deformation ratios of 1.2, 1.5 and 1.8 (e) Path plots of equivalent plastic strain through thickness for different deformation ratios of 1.2, 1.5 and Results and discussion 3.1 Effects of deformation ratio Figures 3(a)-3(c) show equivalent plastic strain contours corresponding to three different deformation ratios (R/R 0 ) of 1.2, 1.5, and 1.8. It could be seen that the smaller deformation

3 G. Faraji et al. / Journal of Mechanical Science and Technology 23 (2009) 1261~ ratio leads to higher equivalent plastic strain and lower strain homogeneity. Quantified strain values corresponding to Figs. 3 (a)-3(c) were shown in pass plots of Fig. 3(e). It is clear that an increase in the deformation ratio cause to increase in the equivalent plastic strain. The equivalent plastic strains of (2.45± 0.2), (2.6± 0.35), and (2.85± 0.2) have been achieved after applying one pass TCAP with deformation ratios of 1.2, 1.5, and 1.8, respectively. Hence, strain variation in different deformation ratios of 1.2, 1.5, and 1.8 are 8.1%, 13.4 %, and 7% respectively. It means that strain homogeneity has not identical trend with deformation ratio variation. The best and worst strain homogeneity is achieved in the deformation ratios of 1.8 and 1.5 respectively. Fig. 4 shows the effects of deformation ratio on required load in TCAP process. This figure also shows a comparison between FE and experimental results in deformation ratio of R 2 /R 1 = 1.5. It could be observed that the peak load is almost linearly decreased when deformation ratio is increased. Also, there are a good agreement between FE calculated and experimental loads in the case of R 2 /R 1 = 1.5. Fig. 5 shows the effect of deformation direction on the equivalent strain contour of TCAP processed tube. As observed in this figure, in the case of inlet direction tube thickness thinning is while in the outlet direction case it is not occurred. This phenomena could be attributed the strain state in TCAP. Faraji et al. mentioned that during TCAP there are some peripheral tensile strain between two first consequent shear zones (I, II) and some compression one between two last consequent shear zones (II, III) [20]. This phenomenon is occurred in the case of inlet deformation direction with opposite sequence. In the other word, there are peripheral compression strain between two first consequent shear zones (I, II) and tensile one between two last consequent shear zones (II, III). When the last strains are tensile it causes to decrease the tube thickness in the case of inlet deformation direction. However more explanation about this behaviour is shown in deformation geometry changes of Fig. 6. Figs. 6(a) to 6(e) and Figs. 6(f) to 6(j) show FE predictions of deformation geometry changes during TCAP processing with two deformation directions of outlet and inlet mode respectively. So, using inlet case is not recommended. (a) (b) (c) (d) (e) Inner die Outer die (f) (g) (h) (i) (j) Fig. 4. Effects of deformation ratio on required loads in TCAP process with a comparison between FE and experimental results in deformation ratio of R 2/R 1= 1.5. Fig. 6. FEM predictions of deformation geometry changes during TCAP processing with two channel directions (a)-(e) outlet deformation direction (f)-(j) inlet deformation direction. (a) Tube centerline Fig. 5. Equivalent plastic strain contour during TCAP processing with (a) outlet deformation direction and (b) inlet deformation direction. 3.2 Effects of deformation direction (b) Fig. 7. Effects of curvature angles of ψ 1= ψ 3 on equivalent plastic strain path plots through thickness (R2/R1=1.5, ψ 2=90 ). 3.3 Effects of curvature angles Figure 7 shows the equivalent plastic strain pass plot of TCAP processed tube in different curvature angles (ψ 1 = ψ 3 ) of 0, 17, 35.5 and 45. As observed an increase in the curvature angle leads to an increase in the equivalent strain in the

4 0000 G. Faraji et al. / Journal of Mechanical Science and Technology 00 (2011) 0000~0000 inner surface regions of the tube. But, it has not significant effect on strain in the outer surface regions. It means that the strain homogeneity is increased when the curvature angles of ψ 1 = ψ 3 is increased. Fig. 8 shows the effect of curvature angles of ψ 1 = ψ 3 on the TCAP required loads in which R 2 /R 1 =1.5 and ψ 2 =90. From this figure it is seen that the curvature angles of ψ 1 = ψ 3 has not significant effect on the required load. It could be concluded from Figs. 7 and 8 that in the processing by TCAP selecting the higher curvature angles ψ 1 = ψ 3 could enhance the homogeneity while any significant effect on process loads. Figure 9(a)-9(d) show the equivalent plastic strain contour during TCAP processing with second curvature angles of ψ 2 = 0, 28, 67.4 and 90 respectively. From this figure it is seen that increase in the second curvature angle cause to decrease in the level of strain. It is also was seen in the conventional ECAP process [24, 25]. Quantitative values of equivalent plastic strain through thickness is shown in path plots of Fig. 9 (e) correspond to curvature angles of ψ 2 = 0, 28, 67.4 and 90. It is observed that the higher and lower strains are occurred in the inner and outer surface of the processed tube respectively and the strain homogeneity is increased when the second curvature angle is increased up to Increasing the second curvature angle to 90 cause to change the trend and higher and lower strains takes place in the outer and inner surface of the tube. Fig. 10 shows the effect of second curvature angle on the required loads in the TCAP process in which the curvatures ψ 1 = ψ 3 =0 and the deformation ratio R 2 /R 1 =1.2. From this figure it is observed that the required load is decreased by increasing the second curvature angle. Considering the strain homogeneity and required load from Figs. 9 and 10 it could be concluded that the second curvature angle of 90 is a best choice. (a) (b) (c) (d) (e) Fig. 9. (a) -(d) Equivalent plastic strain contour during TCAP processing with second curvature angles of ψ2= 0, 28, 67.4 and 90 (e) Path plots of equivalent plastic strain through thickness for curvature angles of ψ2= 0, 28, 67.4 and 90. Fig. 10. Effects of second curvature angle on the required loads in TCAP process (ψ1= ψ3=0, R2/R1 =1.2). Fig. 8. Effects of curvature angles of ψ 1= ψ 3 on the TCAP required loads (R 2/R 1=1.5, ψ 2=90 ). 3.4 Comparison between FE and experimental results An important feature of TCAP process is that a square element in the unprocessed tube appears unchanged after processing [19]. It is also takes place in the parallel ECAP proposed by Raab [26]. This cause to the tail parts appears to be symmetric and the waste material is decreased. This phenomenon is studied experimentally by physical modeling by using a plasticine tube. Fig. 11 shows the effect of TCAP on the deformation of an element on a plasticine tube. As shown

5 G. Faraji et al. / Journal of Mechanical Science and Technology 23 (2009) 1261~ in this figure the square element appears same as initial state at the end of TCAP. This is also takes place in the FE result shown in Fig. 11 (c) and a square element appears unchanged at the end of process. So there are a very good agreement between FE and experiment. A comparison between the peak loads predicted by FE and resulted from experiment (Fig. 4(e)) showed that there is also a good agreement with a little difference about 5%. (a) Punch Tube appearance before deformation Tube appearance after deformation Inner die Tube element before deformation Tube element after deformation (b) Fig. 11. (a) and (b) Effect of TCAP on the deformation of plasticine 4. Conclusions tube elements (c) FE predicted element deformation. The effects of curvature angles, deformation ratio (= maximum radius during TCAP/ initial radius) and deformation direction on the plastic deformation behavior, strain homogeneity and the required loads in TCAP processing were investigated using finite element (FE) method with experimental verifications. The results showed that the higher first and third curvature angles could enhance the homogeneity while these parameters have not any significant effect on the process load. An increase in the first and third curvature angles leads to an increase in the equivalent strain in the inner surface regions of the tube. But, it has not significant effect on strain in the outer surface regions. The strain homogeneity is increased when the second curvature angle is increased up to Increasing the second curvature angle to 90 cause to change the trend and higher and lower strains takes place in the outer and inner surface of the tube. From this figure it is observed that the required load is decreased by increasing the second curvature angle. The second curvature angle of 90 prepare the best condition from good strain homogeneity and lower load point of view compared to lower angles. Results on deformation direction showed that inlet deformation direction cause tube thinning at the end of process and using inlet case is not recommended. It is because of the sequence of tensile and compression peripheral strains which takes place in TCAP process. Also, when the smaller deformation ratio, the higher equivalent plastic strain and the worse strain homogeneity. Acknowledgment (c) The authors would like to acknowledge the financial support of the University of Tehran for this research under grant number References [1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Bulk nanostructured materials from severe plastic deformation, Progress in Materials Science, 45 (2000) [2] R.Z. Valiev, T.G. 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6 0000 G. Faraji et al. / Journal of Mechanical Science and Technology 00 (2011) 0000~0000 ( 2010) [15] B. Tolaminejad, K. Dehghani, Microstructural characterization and mechanical properties of nanostructured AA1070 aluminum after equal channel angular extrusion, Materials & Design, 34 (2012) [16] G. Faraji, H. Jafarzadeh, Accumulative Torsion Back (ATB) Processing as a New Plastic Deformation Technique, Materials and Manufacturing Processes, 27 (2012) [17] X. Han, L. Hua, Comparison between cold rotary forging and conventional forging, Journal of Mechanical Science and Technology, 23 (2009) [18] S.M. Byon, C.H. Moon, Y. Lee, Strain gradient plasticity based finite element analysis of ultra-fine wire drawing process, Journal of Mechanical Science and Technology, 23(2009) [19] G. Faraji, M.M. Mashhadi, H.S. Kim, Tubular channel angular pressing (TCAP) as a novel severe plastic deformation method for cylindrical tubes, Materials Letters, 65 (2011) [20] G. Faraji, M.M. Mashhadi, H.S. Kim, Deformation Behavior in Tubular Channel Angular Pressing (TCAP) Using Triangular and Semicircular Channels, Materials Transactions, 53 (2012) [21] A.V. Nagasekhar, S.C. Yoon, Y. Tick-Hon, H.S. Kim, An experimental verification of the finite element modelling of equal channel angular pressing, Computational Materials Science, 46 (2009) [22] G. Faraji, M. M. Mashhadi, S-H. Joo, H.S. Kim, The role of channel angle on plastic deformation behavior in tubular channel angular pressing (TCAP), Reviews on Advanced Materials Science, (2012) In press. [23] G. Faraji, H. Jafarzadeh, H.J. Jeong, M.M. Mashhadi, H. S. Kim, Numerical and experimental investigation of the deformation behavior during the accumulative back extrusion of an AZ91 magnesium alloy, Materials & Design, 35 (2012) [24] H.S. Kim, Finite element analysis of deformation behaviour of metals during equal channel multi-angular pressing, Materials Science and Engineering A, 328 (2002) [25] H.S. Kim, M.H. Seo, S.I. Hong, On the die corner gap formation in equal channel angular pressing, Materials Science and Engineering A, 291 (2000) [26] G.I. Raab, Plastic flow at equal channel angular processing in parallel channels, Materials Science and Engineering A, 410 (2005) Ghader Faraji is PhD candidate in the University of Tehran in the field of Mechanical and Manufacturing Engineering. He was selected as a best researcher PhD candidate of the University of Tehran in He has published more than 27 ISI journal papers, 3 Iranian Journal papers and more than 18 conference papers until now. He has also 7 Iranian patents. Main research fields are Metal Forming, Severe Plastic Deformation and producing ultra fine grain materials.