- Email: [email protected]
Optimization of initial blank shape for flexible micro deep drawing of square parts
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
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Optimization of initial blank shape for flexible micro deep drawing of square parts Zaid H. Mahmood a,⇑, Ihsan K. Irthiea a, Ahmed K. Ahmed b a b
Department of Mechanical Engineering, College of Engineering, University of Anbar, Iraq Anbar Institute, Mechanical Department, Middle Technical University, Iraq
a r t i c l e
i n f o
Article history: Received 4 July 2019 Received in revised form 4 September 2019 Accepted 29 September 2019 Available online xxxx Keywords: Micro deep drawing Initial blank shape Flexible forming Micro square cups Simulation
a b s t r a c t Micro flexible tool-assisted sheet metal forming is one of the forming techniques that have increasingly attracted extensive attention of researchers. Due to its simplicity, low overall production cost, feasibility of prototyping, this forming process seems appropriate for producing micro components. This work presents an evaluation study on using flexible deep drawing process to produce micro stainless steel 304 square cups. The influence of initial foil thickness and blank shape are investigated in details. An optimization of blank shape is implemented to provide micro square products with net-shape and flange free. The finite element simulations are achieved using the commercial code Abaqus/Standard. The results showed that the initial foil thickness affects the thinning and thickening values of the formed cup wall. As well as, the optimization method is feasible to indicate the optimum initial blank shape for producing free flange- micro square parts. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.
1. Introduction The application of micro components has become more and more extensive in the micro-electro-mechanical system, precision instrument, aerospace, and biomedical devices with the rapid growth of modern industries [1,2]. Usually, the demand for micro components used in such devices increases in a competitive situation. Micro-forming processes have ability to provide such components with mass production [3,4]. However, to reduce the manufacturing cost of forming tools (i.e. punch and die), flexible forming technology can be adopted in which rubber pad is employed instead of punch or die [5,6]. In this technology, there are different process parameters that affect sheet metal forming process such as blank shape, friction coefficients, material properties, holding force, and tool geometry. Between the previous parameters, the blank shape has direct effect on the forming process of square parts. Deep drawing processes are usually followed by trimming the ears formed at flange portion and this action represent of course increase in cost [7]. To overcome this issue, it is crucial to optimize the initial shape of workpieces used in forming
⇑ Corresponding author.
processes [8]. Abbas Vafaeesefat [9] proposed algorithm to predict the initial blank shape of square cup by simulation and experiment. The algorithm is based on iterative finite element simulations. The proposed approach obtained the optimum blank shape and has good agreement with the experiment results. Z. Hu and F. Vollertsen [10] studied the blank shape optimization for micro deep drawing of rectangular parts. This study included simulation and experiment of thin foil of aluminum Al99.5 with 20 mm as thickness and obtained sound parts with flange free. Iseki and Murota [11] investigated the initial blank shape by using numerical approach to find the optimum blank shape that can be drawn to uniform height of the produced part. Gea and Ramamurthy [12] recommended by numerical method the starting design of blank and increasing the drawability of square part. Hu et al. [13] studied the impact of changing planar anisotropic on controlling the value of ear shapes without optimize the initial blank shape. Chen Yang et al. [14] presented finite element simulation and experiments for deep drawing to investigate the effect of blank shape depending on the geometrical resemblance. On the other hand, the blank shape optimization has not been studied yet by using flexible micro deep drawing technique. The current study presents an optimization of initial blank shape to produce micro stainless steel 304 square cups through
E-mail address: [email protected] (Z.H. Mahmood). https://doi.org/10.1016/j.matpr.2019.09.188 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
2
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
Table 1 Mechanical properties of SS 304 foil. Thickness, tb (mm)
Angle to rolling direction (h°)
Young modulus E (GPa)
Yield point ro (MPa)
Ultimate point rult (MPa)
Tensile strength K (MPa)
Poisson’s ratio n
100
0° 45° 90°
194 149 181
291 324 310
540 625 622
1213.1 1298.9 1444.7
0.3708 0.3469 0.4103
Table 2 Mechanical properties of Polyurethane material. Hardness shore A
Young modulus E (GPa)
M-R constant C10 (MPa)
M-R constant C01 (MPa)
Poisson’s ratio (v)
55
2.868
0.382
0.096
0.4997
Fig. 1. Forming system of the proposed technique.
flexible micro deep drawing (FMDD). As well as, the influence of initial foil thickness on final product quality in terms of wall thickness distribution, thinning and thickening values is investigated. The finite element models are built utilizing Abaqus/Standard software. Polyurethane rubber material 55 Shore A in hardness is used for flexible tool. The results showed that the initial foil thickness affects the thinning and thickening values of the formed cup wall. As well as, the optimization method is feasible to indicate the optimum initial blank shape for producing free flange- micro square parts.
3. FE simulation of forming process The software ABAQUS 6.14 was used to simulate the microforming process. The 3-D simulation model consists of blank, rigid punch, blankholder, adjustment ring, floating ring and rubber pad as shown in Fig. 1. The blank was modelled as elastic-plastic material and meshed with an 8-node linear brick, reduced integration, hourglass control (C3D8R). On the other side, the rubber pad was modelled as hyperelastic material and meshed with an 8-node linTable 4 Coefficients of friction.
2. Material characterization The as-received chromium-nickel stainless steel 304 foils (SS 304) with 150 mm 150 mm in area and 100 mm in nominal thickness are used in this study. This alloy is widely employed in various industrial applications such as micro electro mechanical systems, microelectronics and micro technologies. The mechanical behaviors are determined via means of uniaxial tensile tests which were achieved using Instron universal test machine with 50 kN load cell and 0.1 mm/s velocity. In order to define the anisotropic behaviour of SS 304 material, three dogbone specimens were cut for each thickness along rolling (RD), 45° diagonal (DD) and transverse (TD) directions. The cutting processes were carried out according to the ASTM E8 [15] using CNC machine with cutting tool of 2 mm in diameter. The mechanical properties obtained from the tensile tests are listed in Table 1. Polyurethane rubber material with 55 Shore A hardness is used for the soft pad in the current technique. The mechanical properties (hyperelastic coefficients C01 and C10, Young modulus and Poisson’s ratio) of polyurethane materials required in finite element mode to define the deformation behaviour are listed in Table 2 [16,17].
Interface
Coefficient of friction
Blankholder/blank Floating ring/blank Rubber pad/blank Punch/blank Floating ring/rubber pad Adjustment ring/blankholder
0.1 0.05 0.1 0.25 0.05 0.05
Table 3 Geometrical parameters of FMDD model. Parameters
Value (mm)
Blank diameter (Db) Punch section (lp lp) Radius of punch corner (Rp) Radius of punch edge (Re) Height of rubber pad (hr) Section of rubber pad (lr lr) Thickness of floating ring (tr) Section of floating ring (lf lf) Radius of inner corner of floating ring (Ri) Radius of edge corner of floating ring (re) Initial gap distance (q)
5 22 0.4 0.35 5 66 1 6.5 6.5 0.4 0.35 0.1
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 2. Formed square parts out of different blank shapes.
ear brick, constant pressure, hybrid, reduced integration, hourglass control (C3D8RH). The dimensions of all components used in the flexible micro deep drawing processes (FMDD) are listed in Table 2. Besides, the coefficients of friction adopted at the different contact interfaces in the FE models are presented in Tables 3 and 4. The forming process include three main steps (see Fig. 1), in the first step, the blank is pushed into the orifice of floating ring to form a shallow cup with hight just equal the floating ring thickness 1 mm. At the end of this step, the lower face of the blank being formed will just touch rubber pad. In the second step, the punch moves downward to form the shallow-blank in the rubber pad die. Accordingly, the hydrostatic pressure generated in the rubber material causes to move up both the floating ring and blank holder through the initial gap space. The third step begins just when the blank holder touches the adjustment ring. During this step, the forming process is terminated under the effect of punch force and hydrostatic pressure of rubber pad until a whole micro square
cup is formed. The undesired issue of flexible deep drawing, particularly at micro scale, is the miner wrinkles which are small wrinkles occur during forming stroke particularly at the flange portion. Consequently, these miner wrinkles may aggravate during drawing process leading to failure. As the blank flange portion is restrained between two rigid surfaces in the current technique (floating ring and blank holder), this concern is overcome through an appropriate increase in the holding pressure supplied by the rubber pad as a result of the hydrostatic pressure.
Fig. 3. Thickness distribution along rolling direction.
Fig. 4. Thickness distribution along transverse direction.
4. Results and discussion 4.1. Blank shape Many advantages can be obtained through optimizing the blank shape in deep drawing processes such as minimizing the forming defects, reduction of the material cost, improvement of the forma-
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
4
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
bility and quality of the formed part. Stainless steel 304 foil with 100 mm in thickness was utilized for the process workpieces. Initially, the blanks were cut with different shapes as shown in Fig. 2. The key pint of this action is to observe how changing the blank shape on the foil material remaining at the flange portion of formed part. The blank shape is designated in a way that the remaining flange area is minimized and the maximum reduction of thickness is reduced. The blank shape resulting in minimum flange material will be taken as the optimum one (see Fig. 2d). Firstly, the blank is cut with circular shape with 5 mm in diameter. However, it was observed that a large amount of the foil material remaining at the flange portion as a waste material as shown in Fig. 2a. In order to minimize the flange material especially at the corner, octagon shaped blank is cut with non-equal sides (see Fig. 2b). The results revealed that the material area at flange portion is remarkably reduced and however additional improvement is still required for the blank shape. The next optimization step is to cut the straight side of the octagon shape with radius equal to that of the initial circular blank i.e. 2.5 mm as seen in Fig. 2c. As a result of the anisotropic behavior of the thin foil, earing phenomenon is observed for the flange portion of the formed part. However, to minimize material waste, the blank shape showed in Fig. 2d with radius of 3.5 mm is adopted. The results revealed that the optimum blank shape in terms of minimum material waste is that presented in Fig. 2d. Furthermore, the thickness distribution of the final cup formed using this blank shape is significantly homogenous more than that obtained from the other blank shapes. Therefore, it adopted for the other blanks of 50 mm and 150 mm. These characteristics evaluated with an aspect ratio equal to 1 for the different blank thicknesses used.
4.2. Blank thickness To investigate the impact of initial blank thickness on the optimized blank shape, three models of FE were built with 50 mm, 100 mm, and 150 mm of initial blank thickness with the same parameters which are adopted in Tables 2 and 3. In general, the three curves have similar views characterized in thinning, thickening and distribution profile. The tendency of maximum thinning increased from 6.72% to 9.66% and then to 10.11% by increasing the blank thickness from 0.05 mm to 0.1 mm and then to 0.15 mm respectively along rolling direction as clarified in Fig. 3. The same increasing blank thicknesses of the previous case are
Fig. 6. Load-Displacement relation for different blank thickness.
reflected in transverse direction which has the tendency to increase the maximum thinning from 6.84% to 8.56% then to 9.24% respectively as it is shown in Fig. 4. While along diagonal 45° to the rolling direction, the maximum thickness reduction increased significantly to be 17.2%, 19.5%, and 22.3% with blank thickness 0.05 mm, 0.1 mm, and 0.15 mm respectively as shown in Fig. 5. On other hand, the earring width is increased by decreasing blank thickness due to the high trend of anisotropic behavior of thin foil. The maximum earring width is 87 mm with blank thickness 0.05 mm and decreased with other thicknesses to be 72.5 mm and 39 mm with 0.1 mm and 0.15 mm respectively. The forming load-displacement relations are shown in Fig. 6 for the three different cases. The overall trend of the curves is the same. The forming load increases with displacement until the punch reaching the upper rubber pad surface, and afterwards, the load behavior will be a function of both sheet resistance and the hydrostatic pressure produced in rubber pad. The significant realizing is that the maximum load decreases with decreasing the blank thickness. The results reveal that the maximum forming load values 41.56, 84.50, 161.68 N are obtained by using a blank thickness of 0.05, 0.1 and 0.15 mm, respectively. Furthermore, these curves show that by decreasing the initial blank thickness, the required initial gap distance that allocated between the adjustment ring and blank holder decreased. 5. Conclusion
Fig. 5. Thickness distribution along diagonal 45° direction.
The current work presents an evaluation study on utilizing flexible deep drawing technique to produce micro square cups from stainless steel 304 foils. The process parameters investigated are initial foil thickness and blanks shape. The study includes an optimization for the initial blank shape in order to minimize the material waste due to earing phenomenon at the flange portion of formed parts. The commercial Abaqus/standard software is used to simulate the forming process. The results proved the feasibility of using flexible micro deep drawing to form net-shape micro square parts. The action of blank shape optimization resulted in free flange-successful square cups, which contribute remarkably in reducing the overall production cost. As well as, the forming technique and optimization action resulted in good product quality in terms of thickness distribution, thinning and thickening values. The other finding is that reducing the initial blank thickness, the maximum thickness reduction improved and the forming punch load value decreased too.
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
References [1] U.E.M. Geiger, M. Kleiner, R. Eckstein, N. Tiesler, Microforming, CIRP Ann. Manuf. Technol. 50 (2) (2001) 445–462. [2] L. Peng, P. Hu, X. Lai, D. Mei, J. Ni, Investigation of micro/meso sheet soft punch stamping process – simulation and experiments, Mater. Des. 30 (3) (2009) 783–790. [3] I.K. Irthiea, G. Green, Evaluation of micro deep drawing technique using soft die-simulation and experiments, Int. J. Adv. Manuf. Technol. 89 (2016) 2363– 2374. [4] A.R. Razalia, Y. Qin, A review on micro-manufacturing, micro-forming and their key issues, Procedia Eng. 53 (2013) 665–672. [5] M. Ramezani, Z.M. Ripin, R. Ahmad, Computer aided modelling of friction in rubber-pad forming process, J. Mater. Process. Technol. 209 (2009) 4925–4934. [6] L. Belhassen, S. Koubaa, M. Wali, F. Dammak, Numerical prediction of springback and ductile damage in rubber-pad forming process of aluminum sheet metal, Int. J. Mech. Sci. 117 (2016) 218–226. [7] R.F. Shyu, T.S. Yang, The design of blank initial shape in the near net-shape deep drawing of square cup, J. Mech. Sci. Technol. 21 (2007) 1585–1592. [8] S. Kitayama, M. Saikyo, K. Kawamoto, K. Yamamichi, Multi-objective optimization of blank shape for deep drawing with variable blank holder
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force via sequential approximate optimization, Struct. Multidiscip. Optim. 52 (5) (2015) 1001–1012. Abbas Vafaeesefat, Finite element simulation for blank shape optimization in sheet metal forming, Mater. Manuf. Process. 26 (1) (2011) 37–41. Z. Hu, F. Vollertsen, Optimisation of the blank shape for micro deep drawing of rectangular parts, AIP Conf. Proc. 1353 (1) (2011) 523–528. H. Iseki, T. Murota, On the determination of the optimum blank shape of nonaxisymmetric drawn cup by finite element method, Bull. JSME 29 (249) (1986) 249–254. H. Gea, R. Ramamurthy, Blank design optimization on deep drawing of square shells, Trans IIE 30 (10) (1998) 913–921. P. Hu, Y. Liu, J. Wang, Numerical study of the flange earing of deep drawing sheets with stronger anisotropy, Int. J. Mech. Sci. 43 (2000) 279–296. C. Yang, P. Li, L. Fan, Blank shape design for sheet metal forming based on geometrical resemblance, Procedia Eng. 81 (2014) 1487–1492. American Society for Testing and Materials ASTM. Standard Test Methods for Tension Testing of Metallic Materials. Am. Assoc. State 2010; 1–27. Yanxiong Liu, Lin Hua, Fabrication of metallic bipolar plate for proton exchange membrane fuel cells by rubber pad forming, J. Power Sources 195 (2010) 3529–3535. https://vdocuments.mx/ansys-users-grouphyperelastic-materials.html.
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Optimization of initial blank shape for flexible micro deep drawing of square parts Zaid H. Mahmood a,⇑, Ihsan K. Irthiea a, Ahmed K. Ahmed b a b
Department of Mechanical Engineering, College of Engineering, University of Anbar, Iraq Anbar Institute, Mechanical Department, Middle Technical University, Iraq
a r t i c l e
i n f o
Article history: Received 4 July 2019 Received in revised form 4 September 2019 Accepted 29 September 2019 Available online xxxx Keywords: Micro deep drawing Initial blank shape Flexible forming Micro square cups Simulation
a b s t r a c t Micro flexible tool-assisted sheet metal forming is one of the forming techniques that have increasingly attracted extensive attention of researchers. Due to its simplicity, low overall production cost, feasibility of prototyping, this forming process seems appropriate for producing micro components. This work presents an evaluation study on using flexible deep drawing process to produce micro stainless steel 304 square cups. The influence of initial foil thickness and blank shape are investigated in details. An optimization of blank shape is implemented to provide micro square products with net-shape and flange free. The finite element simulations are achieved using the commercial code Abaqus/Standard. The results showed that the initial foil thickness affects the thinning and thickening values of the formed cup wall. As well as, the optimization method is feasible to indicate the optimum initial blank shape for producing free flange- micro square parts. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.
1. Introduction The application of micro components has become more and more extensive in the micro-electro-mechanical system, precision instrument, aerospace, and biomedical devices with the rapid growth of modern industries [1,2]. Usually, the demand for micro components used in such devices increases in a competitive situation. Micro-forming processes have ability to provide such components with mass production [3,4]. However, to reduce the manufacturing cost of forming tools (i.e. punch and die), flexible forming technology can be adopted in which rubber pad is employed instead of punch or die [5,6]. In this technology, there are different process parameters that affect sheet metal forming process such as blank shape, friction coefficients, material properties, holding force, and tool geometry. Between the previous parameters, the blank shape has direct effect on the forming process of square parts. Deep drawing processes are usually followed by trimming the ears formed at flange portion and this action represent of course increase in cost [7]. To overcome this issue, it is crucial to optimize the initial shape of workpieces used in forming
⇑ Corresponding author.
processes [8]. Abbas Vafaeesefat [9] proposed algorithm to predict the initial blank shape of square cup by simulation and experiment. The algorithm is based on iterative finite element simulations. The proposed approach obtained the optimum blank shape and has good agreement with the experiment results. Z. Hu and F. Vollertsen [10] studied the blank shape optimization for micro deep drawing of rectangular parts. This study included simulation and experiment of thin foil of aluminum Al99.5 with 20 mm as thickness and obtained sound parts with flange free. Iseki and Murota [11] investigated the initial blank shape by using numerical approach to find the optimum blank shape that can be drawn to uniform height of the produced part. Gea and Ramamurthy [12] recommended by numerical method the starting design of blank and increasing the drawability of square part. Hu et al. [13] studied the impact of changing planar anisotropic on controlling the value of ear shapes without optimize the initial blank shape. Chen Yang et al. [14] presented finite element simulation and experiments for deep drawing to investigate the effect of blank shape depending on the geometrical resemblance. On the other hand, the blank shape optimization has not been studied yet by using flexible micro deep drawing technique. The current study presents an optimization of initial blank shape to produce micro stainless steel 304 square cups through
E-mail address: [email protected] (Z.H. Mahmood). https://doi.org/10.1016/j.matpr.2019.09.188 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 2nd International Conference on Materials Engineering & Science.
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
2
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
Table 1 Mechanical properties of SS 304 foil. Thickness, tb (mm)
Angle to rolling direction (h°)
Young modulus E (GPa)
Yield point ro (MPa)
Ultimate point rult (MPa)
Tensile strength K (MPa)
Poisson’s ratio n
100
0° 45° 90°
194 149 181
291 324 310
540 625 622
1213.1 1298.9 1444.7
0.3708 0.3469 0.4103
Table 2 Mechanical properties of Polyurethane material. Hardness shore A
Young modulus E (GPa)
M-R constant C10 (MPa)
M-R constant C01 (MPa)
Poisson’s ratio (v)
55
2.868
0.382
0.096
0.4997
Fig. 1. Forming system of the proposed technique.
flexible micro deep drawing (FMDD). As well as, the influence of initial foil thickness on final product quality in terms of wall thickness distribution, thinning and thickening values is investigated. The finite element models are built utilizing Abaqus/Standard software. Polyurethane rubber material 55 Shore A in hardness is used for flexible tool. The results showed that the initial foil thickness affects the thinning and thickening values of the formed cup wall. As well as, the optimization method is feasible to indicate the optimum initial blank shape for producing free flange- micro square parts.
3. FE simulation of forming process The software ABAQUS 6.14 was used to simulate the microforming process. The 3-D simulation model consists of blank, rigid punch, blankholder, adjustment ring, floating ring and rubber pad as shown in Fig. 1. The blank was modelled as elastic-plastic material and meshed with an 8-node linear brick, reduced integration, hourglass control (C3D8R). On the other side, the rubber pad was modelled as hyperelastic material and meshed with an 8-node linTable 4 Coefficients of friction.
2. Material characterization The as-received chromium-nickel stainless steel 304 foils (SS 304) with 150 mm 150 mm in area and 100 mm in nominal thickness are used in this study. This alloy is widely employed in various industrial applications such as micro electro mechanical systems, microelectronics and micro technologies. The mechanical behaviors are determined via means of uniaxial tensile tests which were achieved using Instron universal test machine with 50 kN load cell and 0.1 mm/s velocity. In order to define the anisotropic behaviour of SS 304 material, three dogbone specimens were cut for each thickness along rolling (RD), 45° diagonal (DD) and transverse (TD) directions. The cutting processes were carried out according to the ASTM E8 [15] using CNC machine with cutting tool of 2 mm in diameter. The mechanical properties obtained from the tensile tests are listed in Table 1. Polyurethane rubber material with 55 Shore A hardness is used for the soft pad in the current technique. The mechanical properties (hyperelastic coefficients C01 and C10, Young modulus and Poisson’s ratio) of polyurethane materials required in finite element mode to define the deformation behaviour are listed in Table 2 [16,17].
Interface
Coefficient of friction
Blankholder/blank Floating ring/blank Rubber pad/blank Punch/blank Floating ring/rubber pad Adjustment ring/blankholder
0.1 0.05 0.1 0.25 0.05 0.05
Table 3 Geometrical parameters of FMDD model. Parameters
Value (mm)
Blank diameter (Db) Punch section (lp lp) Radius of punch corner (Rp) Radius of punch edge (Re) Height of rubber pad (hr) Section of rubber pad (lr lr) Thickness of floating ring (tr) Section of floating ring (lf lf) Radius of inner corner of floating ring (Ri) Radius of edge corner of floating ring (re) Initial gap distance (q)
5 22 0.4 0.35 5 66 1 6.5 6.5 0.4 0.35 0.1
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 2. Formed square parts out of different blank shapes.
ear brick, constant pressure, hybrid, reduced integration, hourglass control (C3D8RH). The dimensions of all components used in the flexible micro deep drawing processes (FMDD) are listed in Table 2. Besides, the coefficients of friction adopted at the different contact interfaces in the FE models are presented in Tables 3 and 4. The forming process include three main steps (see Fig. 1), in the first step, the blank is pushed into the orifice of floating ring to form a shallow cup with hight just equal the floating ring thickness 1 mm. At the end of this step, the lower face of the blank being formed will just touch rubber pad. In the second step, the punch moves downward to form the shallow-blank in the rubber pad die. Accordingly, the hydrostatic pressure generated in the rubber material causes to move up both the floating ring and blank holder through the initial gap space. The third step begins just when the blank holder touches the adjustment ring. During this step, the forming process is terminated under the effect of punch force and hydrostatic pressure of rubber pad until a whole micro square
cup is formed. The undesired issue of flexible deep drawing, particularly at micro scale, is the miner wrinkles which are small wrinkles occur during forming stroke particularly at the flange portion. Consequently, these miner wrinkles may aggravate during drawing process leading to failure. As the blank flange portion is restrained between two rigid surfaces in the current technique (floating ring and blank holder), this concern is overcome through an appropriate increase in the holding pressure supplied by the rubber pad as a result of the hydrostatic pressure.
Fig. 3. Thickness distribution along rolling direction.
Fig. 4. Thickness distribution along transverse direction.
4. Results and discussion 4.1. Blank shape Many advantages can be obtained through optimizing the blank shape in deep drawing processes such as minimizing the forming defects, reduction of the material cost, improvement of the forma-
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
4
Z.H. Mahmood et al. / Materials Today: Proceedings xxx (xxxx) xxx
bility and quality of the formed part. Stainless steel 304 foil with 100 mm in thickness was utilized for the process workpieces. Initially, the blanks were cut with different shapes as shown in Fig. 2. The key pint of this action is to observe how changing the blank shape on the foil material remaining at the flange portion of formed part. The blank shape is designated in a way that the remaining flange area is minimized and the maximum reduction of thickness is reduced. The blank shape resulting in minimum flange material will be taken as the optimum one (see Fig. 2d). Firstly, the blank is cut with circular shape with 5 mm in diameter. However, it was observed that a large amount of the foil material remaining at the flange portion as a waste material as shown in Fig. 2a. In order to minimize the flange material especially at the corner, octagon shaped blank is cut with non-equal sides (see Fig. 2b). The results revealed that the material area at flange portion is remarkably reduced and however additional improvement is still required for the blank shape. The next optimization step is to cut the straight side of the octagon shape with radius equal to that of the initial circular blank i.e. 2.5 mm as seen in Fig. 2c. As a result of the anisotropic behavior of the thin foil, earing phenomenon is observed for the flange portion of the formed part. However, to minimize material waste, the blank shape showed in Fig. 2d with radius of 3.5 mm is adopted. The results revealed that the optimum blank shape in terms of minimum material waste is that presented in Fig. 2d. Furthermore, the thickness distribution of the final cup formed using this blank shape is significantly homogenous more than that obtained from the other blank shapes. Therefore, it adopted for the other blanks of 50 mm and 150 mm. These characteristics evaluated with an aspect ratio equal to 1 for the different blank thicknesses used.
4.2. Blank thickness To investigate the impact of initial blank thickness on the optimized blank shape, three models of FE were built with 50 mm, 100 mm, and 150 mm of initial blank thickness with the same parameters which are adopted in Tables 2 and 3. In general, the three curves have similar views characterized in thinning, thickening and distribution profile. The tendency of maximum thinning increased from 6.72% to 9.66% and then to 10.11% by increasing the blank thickness from 0.05 mm to 0.1 mm and then to 0.15 mm respectively along rolling direction as clarified in Fig. 3. The same increasing blank thicknesses of the previous case are
Fig. 6. Load-Displacement relation for different blank thickness.
reflected in transverse direction which has the tendency to increase the maximum thinning from 6.84% to 8.56% then to 9.24% respectively as it is shown in Fig. 4. While along diagonal 45° to the rolling direction, the maximum thickness reduction increased significantly to be 17.2%, 19.5%, and 22.3% with blank thickness 0.05 mm, 0.1 mm, and 0.15 mm respectively as shown in Fig. 5. On other hand, the earring width is increased by decreasing blank thickness due to the high trend of anisotropic behavior of thin foil. The maximum earring width is 87 mm with blank thickness 0.05 mm and decreased with other thicknesses to be 72.5 mm and 39 mm with 0.1 mm and 0.15 mm respectively. The forming load-displacement relations are shown in Fig. 6 for the three different cases. The overall trend of the curves is the same. The forming load increases with displacement until the punch reaching the upper rubber pad surface, and afterwards, the load behavior will be a function of both sheet resistance and the hydrostatic pressure produced in rubber pad. The significant realizing is that the maximum load decreases with decreasing the blank thickness. The results reveal that the maximum forming load values 41.56, 84.50, 161.68 N are obtained by using a blank thickness of 0.05, 0.1 and 0.15 mm, respectively. Furthermore, these curves show that by decreasing the initial blank thickness, the required initial gap distance that allocated between the adjustment ring and blank holder decreased. 5. Conclusion
Fig. 5. Thickness distribution along diagonal 45° direction.
The current work presents an evaluation study on utilizing flexible deep drawing technique to produce micro square cups from stainless steel 304 foils. The process parameters investigated are initial foil thickness and blanks shape. The study includes an optimization for the initial blank shape in order to minimize the material waste due to earing phenomenon at the flange portion of formed parts. The commercial Abaqus/standard software is used to simulate the forming process. The results proved the feasibility of using flexible micro deep drawing to form net-shape micro square parts. The action of blank shape optimization resulted in free flange-successful square cups, which contribute remarkably in reducing the overall production cost. As well as, the forming technique and optimization action resulted in good product quality in terms of thickness distribution, thinning and thickening values. The other finding is that reducing the initial blank thickness, the maximum thickness reduction improved and the forming punch load value decreased too.
Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188
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Please cite this article as: Z. H. Mahmood, I. K. Irthiea and A. K. Ahmed, Optimization of initial blank shape for flexible micro deep drawing of square parts, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.09.188