Available online at www.sciencedirect.com
ScienceDirect Procedia Manufacturing 2 (2015) 230 – 235
2nd International Materials, Industrial, and Manufacturing Engineering Conference, MIMEC2015, 4-6 February 2015, Bali Indonesia
Strain distribution equal channel angular pressing of magnesium alloy at 90° and 120° corner angles M. H. M. Samsudina, D. Kurniawanb and Fethma M. Nora* a
Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Malaysia b Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Malaysia
Abstract Severe plastic deformation (SPD) is capable of producing metals with ultrafine grained microstructure. Equal channel angular pressing (ECAP) is a type of SPD process and is the focus of this study. The process was simulated using finite element analysis at different channel angles of 90° and 120°. The input for material properties, loads, velocities, boundary conditions and contacts were assigned to the finite element models to simulate the process. The strain distribution value were be obtained from the finite element analysis to determine the effect of the channel angle to the magnesium alloys AZ80 sample. The result shows that when the channel angle was 120°, the strain was lower but the concentrated stress at the corner region of the channel also lowers compared to when the channel angle was 90°. At any channel angle, presence of corner radius lowers the strain and the stress. © 2015 by Elsevier B.V.byThis is an open © 2015 Published The Authors. Published Elsevier B.V. access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the Scientific Committee of MIMEC2015. Selection and Peer-review under responsibility of the Scientific Committee of MIMEC2015 Keywords: Finite element, Biomaterials, Severe plastic deformation, ECAP, Magnesium Alloys
1. Introduction Magnesium alloys was introduced as an orthopedic biomaterials in 1907, when plate of pure Mg alloys with gold-plated steel was used to secure fractured bone on the lower leg [1]. Magnesium alloys is one of alternative biomaterials that is light weight and have stiffness closer to the bone but low ductility at low temperature [2,3]. To improve the mechanical properties of the magnesium alloys is by reducing the grain size without sacrificing the ductility of the material. * Corresponding author. E-mail address:
[email protected]
2351-9789 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the Scientific Committee of MIMEC2015 doi:10.1016/j.promfg.2015.07.040
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Russian scientist has first developed severe plastic deformation (SPD) [3] that is a method of manufacturing bulk specimens to have ultrafine grain size without showing any significant change in the bulk solid dimension. This method can increase the mechanical properties and super plastic behavior of the material. There are various kinds of SPD methods studied to produce grain size reduction, like Equal Channel Angular Pressing (ECAP)[4], High Pressure Torsion (HPT) [5], Accumulative Roll Bonding (ARB) [6], Tubular Channel Angular Pressing (TCAP) [7], among others. Among the various SPD methods, ECAP is the method of interest in this study. This ECAP method which was invented by Segal et al. [8] can induce large shear plastic deformation by pressing the workpiece material through a die that contains two intersection channels that have predetermined angles while maintaining the original cross-sectional of the material. It was calculated that when the channel angle exceeds 90°, strain distribution will decrease but the concentrated stress point at the intersection of the channel will also decrease compared with when channel angle of 90°. Finite element analysis (FEA) is used for plastic deformation process to analyze the deformation of the workpiece with nonlinear conditions of material properties, loading, and boundary condition to compare and analyze the optimum process condition for the material [9]. In this present study, FEA was used to analyze the strain distribution behavior by ECAP at 90° and 120° die corner angle with different channel intersection radii. The equivalent plastic strains for all channel intersection angles are to be analyzed and discussed. 2. Specimen and Die Geometry The workpiece material in this study is magnesium alloy AZ80. The material properties refer to reported previous study [10], as shown in Table 1 and Fig. 1. The dimension of the specimen was 8 mm x 8 mm x 50 mm. The die dimension and types of die designs that used for this analysis is shown in Fig. 2, with different types of channel intersection angles. For angle of 90˚, the outer and inner corner radii were 8 mm and 4 mm, respectively. Then for angle 120˚, the radii were 11 mm for outer and 7 mm for inner. The designation of channel angle and inner and outer radii in Fig. 2 (a) to (f) is used on the subsequent figures showing the results (Figs. 4 to 6). Table 1. Elastic properties of magnesium alloy AZ80 [9].
Material Magnesium Alloys
Young’s Modulus [MPa] 7089
Poisson’s Ratio 0.35
Fig. 1. Stress-strain curve of Magnesium Alloys AZ80 at various strain rates [9]
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90˚ angle
(a)
(b)
(c)
(e)
(f)
120˚ angle
(d)
Fig. 2. Die geometry and channel intersection angle design for 90˚ and 120˚. The designation (a) to (f) is used for Figs. 4-6.
3. Finite Element Analysis The plastic deformation behavior of the specimen during ECAP process was controlled by ram displacement speed that is 1 mm/s. The ram displacement speed was placed on the top surface of the specimen that moves down forward along Y axis as shown at Fig. 3. The specimen was assumed as solid, deformable material. Element type of this analysis was C3D8R. The number of elements of the workpiece was 3200 and number of nodes was 4133. Workpiece boundary condition was constrained at Z axis and moveable at X and Y axis. The interaction between outside surface of the workpiece and inside surface of the die was surface to surface and contact properties were frictionless and hard contact. The step procedure in this simulation was dynamic, explicit and period time was 30 s. The die was modeled as a rigid body. The boundary condition of the die was constrained. 4. Results and Discussion The strain contour on the workpiece is shown in Fig. 4. The maximum strain contour occurs at the bottom surface of the workpiece under ECAP with 90˚ channel angle with no specified inner or outer radii (Fig. 4a). As outer radius was introduced (Fig. 4b), the strain gets lower, and it gets even lower when inner radius was featured (Fig. 4c). Similar trend happens when the channel angle was 120˚, only at lower strains compared to when the channel angle was 90˚.
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Fig. 3. Boundary condition of the die and the specimen
The pattern of strain changes along the workpiece at its bottom, middle, and top portions as it passes through the die is depicted in Fig. 5. The scale of x-axis in Fig. 5 is normalized towards the length of the workpiece, with the corner of the channel was situated at 0.5 of the scale. The start of strain was at the radius. For workpiece passing a die without specified corner radius (which is (a) and (d)), the strain started later but the magnitude is higher than for workpieces passing dies with corner radii. This is especially the case at the bottom region of the workpieces. It should be noted that the material properties assigned were only elastic ones, while for ECAP, one can expect the strain already reaches the plastic region of the workpieces. Nonetheless, the simulation with only elastic properties can readily indicate how the strain will be when the magnesium alloy workpiece passes through ECAP die with the features as mentioned. In terms of stress distribution (Fig. 6), it can be observed that the maximum stress occurs within the corner region of the channel. Presence of outer radius lowers the maximum stress and widens the stress distribution. Presence of additional inner radius lowers the maximum stress and spreads the stress distribution even further. These happen on both 90˚ and 120˚ channel angles. The maximum stress that occurs on 90˚ channel angle is higher than on 120˚ channel angle for any corner radius.
90° angle
(a)
(b)
120° angle
(c)
(d)
Fig. 4. Equivalent Plastic Strain (PEEQ) Contour Pattern
(e)
(f)
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 5. Path A-B (initial to final points on the workpiece, normalized scale) vs. Equivalent Plastic Strain (PEEQ)
90° angle
(a)
(b)
120° angle
(c)
(d)
(e)
(f)
Fig. 6. Stress Von Misses Contour Pattern
5. Conclusion Based on the finite element analysis on ECAP on magnesium alloy AZ80 at multiple channel angles (90˚ and 120˚) and corner radii (none, outer, and outer and inner) done in this present study, the results show that when the channel angle was 90°, the strain was higher but this is associated with higher stress at the corner region of the channel compared to when the channel angle was 120°. At both channel angles, presence of corner radius lowers the strain and the stress (in magnitude and distribution).
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Acknowledgements Financial supports from the Ministry of Education, Malaysia and Univesiti Tun Hussein Onn Malaysia through Fundamental Research Grant Scheme and DEFINE Resources are gratefully acknowledged. Reference [1]
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