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Experimental and numerical evaluation of micro flexible deep drawing technique using floating ring
Journal of Manufacturing Processes 38 (2019) 556–563
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Experimental and numerical evaluation of micro flexible deep drawing technique using floating ring
T
Ihsan Irthiea University of Anbar, Iraq-Anbar-Ramadi, 55 RAMADI, Ramadi, Iraq
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible tool Floating ring Rubber Micro deep drawing Simulation
A novel micro flexible forming technique utilizing floating ring, as a primary die, is presented in this work. The essential functions of the floating ring part is to overcome the minor wrinkling commonly occurs through flexible forming processes as well as produce micro cups with high accurate dimensions particularly at the shoulder profile. The influence of thickness and surface angle of the floating ring, and blank diameter are investigated. The well-known commercial software Abaqus/Standard is employed to build FE models for micro forming processes. Accordingly, a number of micro deep drawing experiments are conducted to verify the simulation results. To perform the experimental work, a special set up is developed with particular design aspects to satisfy the FE model conditions. The results reveal the feasibility of the floating ring technique adopted in production of micro cylindrical cups with high quality in terms of geometry profile and thickness distribution as well as relatively high aspect ratio.
1. Introduction
results showed that decreasing rubber hardness and/or increasing rubber pad thickness lead to deeper formed channels. Y. Luo L. et al. [12] conducted micro hydro deep drawing experiments to investigate the effects of hydraulic pressure on the wrinkling and the earing of the formed cups. The results revealed that even at the maximum hydraulic pressure of 30 MPa (less than 10% blank yield stress), the wrinkling and earing were not completely overcome. Therefore, ultra-high pressure changed the drawing process in terms of solving the problems of earing and wrinkling. F. Quadrini et al. [13] investigate the influence material properties of flexible die on the forming process of Aluminum sheet metal. The materials of rubber die were silicone rubber (SR), styrene butadiene rubber (SBR) and polyamide 66 (PA). The results revealed that the best forming conditions in terms of reduction of forming force and increase material durability can be provided by using PA material. L. Peng et al. [14] studied the feasibility of fabrication a bipolar plate from thin stainless steel sheet using flexible forming technique. The simulation and experimental results pointed out the capability to fabricate bipolar plate with high quality by using flexible tools. A novel approach in micro deep drawing of using a piezoelectric actuator at various scales is presented by Aminzahed et al. [15]. The study revealed the effects of blank holder pressure on thickness distributions, punch force, and springback. The size effects of blank material are considered through finite element model was developed by Lou et al. [16]. With consideration of the size effects, the results revealed that the surface roughness has significant influence on
Due to the rapid development in different modern technologies which have become essential in our daily life, companies and manufacturers, to stay in competition, are required to provide innovative products and devices fulfill customer requests of miniaturized size, competitive cost and high quality. Consequently, the demand on micro components dramatically increases [1–3] and subsequently the need for accuracy, shape and surface quality becomes higher and higher [4]. The expensive manufacturing processes using conventional tools at micro scale [5] cause utilizing unique techniques, such as flexible tool-assisted micro forming, significantly attractive for wide attention of researchers [6–8]. Various techniques of flexible micro forming have been investigated by many researchers. I. Irthiea et al. [9] proposed a new technique for forming micro SS304 cups using a polyurethane rubber die. The technique adopted render in reducing the overall production costs and improve the quality of the formed cups. Nagarajan et al. [10] present an experimental and numerical study on flexible pad laser shock forming to reveal the effect of flexible pad material and its thickness. The results showed that the hardness and thickness of the flexible pad have significant influence on the deformed crater geometry, thickness distribution across the formed crater and surface hardness at the crater surfaces. The effect of process parameters of micro channel forming using rubber pad technique was studied by C. Jin et al. [11]. The
E-mail address: [email protected]. https://doi.org/10.1016/j.jmapro.2019.01.050 Received 6 April 2018; Received in revised form 29 December 2018; Accepted 28 January 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 38 (2019) 556–563
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springback. Miao et al. [17] presented flexible-die deep drawing process of magnesium sheet using ultrasonic vibration technology. They found that the liquidity and force transmission performance of the granule medium were improved. As well as, all of friction, deformation resistance and punch load were significantly reduced with the augmentation of vibration energy. The so called “size effects” makes the process of traditionally forming cannot be applied directly in micro forming process [18]. However, Designation of micro products is yet considered challenge to researchers and manufacturers as there are not enough information provided on design and manufacture of such parts [3][7]. This work presents a novel flexible tool-assisted micro deep drawing technique by using floating ring for producing micro stainless steel 304 cups. The new issue in this work is to use a floating ring with specific geometric characteristics, as a primary rigid die, in cooperation with polyurethane rubber 50 A pad, as a major flexible die, to complete forming micro cups. Thin stainless steel 304 sheet of 100 μm thickness is cut with different diameters of 8 mm, 9 mm and 10 mm as process workpieces. The key parameters investigated through this work are thickness and surface inclination angle of the floating ring and initial blank diameter. Finite element models are established using the commercial software Abaqus/Standard to simulate the forming process. The numerical models are evaluated through a series of micro deep drawing experiments for which a special set up is developed. The experimental results reveal a good correlation with that obtained by FE simulation in term of thickness distribution, maximum thickness reduction, punch load-displacement relationships and geometry profile of the formed cups.
Fig. 2. Dogbone tensile test specimen.
Fig. 3. (a) LARYEE machine (b) Broken specimen.
2. Characterization of material behaviors Chromium-nickel stainless steel 304, widely used in micro electro mechanical systems, industries of micro-electronics and micro technologies, in form of thin sheets of 100 μm thickness is used as process workpieces. To obtain the mechanical properties of material, tensile tests are performed using a universal tensile machine supported by the LARYEE Company with load capacity of 100 kN and accuracy of +/0.5% (see Fig. 3). The material anisotropy is evaluated through testing three dogbone-shaped specimens cut along rolling, diagonal (45°) and transverse directions by using CNC cutter machine with 2500 rpm spindle velocity and cutting tool of 2 mm diameter as illustrated in Fig. 1. The dimensions of these specimens are obtained from the ASTM E8 [19] standard as shown in Fig. 2. The tensile tests are carried out at velocity of 0.2 mm/s. The stress-strain relationships obtained from the tensile tests are shown in Fig. 4, while the mechanical properties of the SS 304 foil at the different orientations are listed in Table 1. As well as, the mechanical properties of the polyurethane rubber 55 Shore A hardness are presented in Table 2 [7].
Fig. 4. Stress-strain relations for SS 304 foil.
this study to develop FE simulations for micro deep drawing processes. To characterize the deformation behavior of SS 304 foils, an elasticplastic model was adopted. To build this model, 8300 elements with mesh type of 8-node linear brick, reduced integration and hourglass control (C3D8R). The boundary conditions of blank are at Z- axis direction as XSYMM (U1=UR2=UR3 = 0) while at X-axis direction as ZSYMM (U3=UR1=UR2 = 0). The plastic anisotropy is introduced by using Hill's criterion.
3. Material models in FEA simulation
f(σ)= F(σ₂₂-σ₃₃)²+G(σ₃₃-σ₁₁)²+H(σ₁₁-σ₂₂)²+2Lσ₂₃²+2Mσ₃₁²+2Nσ₁₂² 3.1. Anisotropy of Stainless steel foil
(1) The material constants F, G, H, L, M and N are obtained from tensile tests in different orientations and σij refers the stress components [20]. These constants are defined as:
The commercial code Abaqus/Standard software was employed in
Fig. 1. Cutting process of dogbone samples. 557
1 1 1 1 F= ⎛⎜ 2 + 2 − 2 ⎞⎟ 2 ⎝ R22 R33 R11 ⎠
(2)
1 1 1 1 G= ⎜⎛ 2 + 2 − 2 ⎞⎟ 2 ⎝ R33 R11 R22 ⎠
(3)
1 1 1 1 H= ⎛⎜ 2 + 2 − 2 ⎞⎟ 2 ⎝ R11 R22 R33 ⎠
(4)
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Table 1 Mechanical properties of SS304 sheet. Thickness (mm)
Rolling direction
E (GPa)
σo (MPa)
σult (MPa)
εult
K (MPa)
n
0.1
0° 45° 90°
94 149 118
291 324 310
540 625 621.84
0.1951 0.2486 0.23202
1213.1 1298.9 1444.7
0.3708 0.3469 0.4103
Table 2 Mechanical properties of Polyurethane rubber materials. Hardness Shore A
M–R constant C10 (MPa)
M–R constant C01 (MPa)
Poisson’s ratio (v)
55
0.382
0.096
0.4999
L=
3 3 3 M= N= 2R₂₃² 2R₁₃² 2R₁₂²
(5)
the factors R11, R22, R33, R12, R13 and R23 are anisotropic yield stress ratios. Due to the stress situation in sheet metal forming processes are characterized by plane stress conditions therefore four anisotropic ratios are considered in this work which are R11, R22, R33 and R12 [19]. In Abaqus software, these factors are expresses using Lankford Factors as r0, r45 and r90 as following equations [21]:
3r₉ₒ(rₒ+1) r₉ₒ(rₒ+1) r₉ₒ(rₒ+1) R12= R33= R22= (2r₄₅+1)(rₒ+r₉ₒ) rₒ+r₉ₒ rₒ(r₉ₒ+1)
Fig. 5. Floating rings design. (a) Flat surface and (b) Oblique surface. Table 4 Geometry parameters.
(6)
where r0, r45 and r90 are width strain to thickness strain ratios of the work-piece material at the rolling, diagonal and transverse directions, respectively. The values of (r) and (R) of SS 304 sheets employed in the current work are calculated using the above equations, as shown in Table 3. 3.2. Hyper-elastic model for rubber pad material To describe the behavior of the polyurethane rubber used for the flexible die, the known Mooney-Rivlin hyperplastic model is adopted in the FE model. It is known that polyurethane rubber is characterized by a nonlinear stress–strain relation for large deformation, as well as it is nearly incompressible. The strain energy form of Mooney–Rivlin is: W = C10 (I1-3)+C01(I2-3)+(J1-1)/D1
Parameters
Value (mm)
Blank diameter (Db) Blank thickness (tb) Punch diameter (Dp) Radius of punch corner (rp) Height of rubber pad (hr) Diameter of rubber pad (Dr) Thickness of flat floating ring (tR) Inclined angle of flat floating ring (Ѳ) Punch/floating ring clearance C (% tb) Diameter of floating ring (DR) Radius of inner corner of floating ring (rR)
8, 9, 10 0.1 4 0.8 10 12 1, 1.25, 1.5 0°, 5°, 10° 10% 13 0.75
Table 5 Coefficients of friction adopted at different interfaces.
(7)
where W is the strain energy per unit of volume, I1 and I2 are the strain invariants, J is volume change. C10 and C01 describe hyperplastic rubber deformation, and D1 indicates the material compressibility [22]. The deformation behavior of rubber material was described by using hyper-elastic model. This part has 13,806 elements with mesh type 8node linear brick, hybrid, constant pressure, reduced integration and hourglass control (C3D8RH).
Interface
Coefficient of friction
Blankholder and blank Floating ring and blank Rubber pad and b Punch and blank Floating ring and rubber pad Adjustment ring and blankholder
0.1 0.05 0.1 0.25 0.05 0.05
components are listed in Table 4. Also, the coefficients of friction adopted at the different contact interfaces in the FE models are presented in Table 5. The forming process is divided into three main steps as illustrated in Fig. 6, through the first one the blank is formed to a depth equal to the floating ring thickness. In the second step, the solid punch moves downward to form the blank in the flexible die. Consequently, the hydrostatic pressure excited in the rubber material causes both floating ring and blank holder to move up through the initial gap.
4. Forming methodology The simulation models of micro forming processes consist of circular blank, rigid punch, blankholder, adjustment ring, floating ring and flexible pad. It can be seen in Fig. 5 that the floating ring is designated with different surface inclination angles to investigate the influence of this parameter on the forming operation. The dimensions of all Table 3 Plastic strain ratios and anisotropic yield stress ratios. Rolling direction
Eng. Length strain
Width strain
Thickness strain
plastic strain ratio (r)
R11
R22
R33
R12
0° 45° 90°
0.2 0.2 0.2
−0.0731 −0.0806 −0.0771
−0.1051 −0.0962 −0.0995
0.714399 0.859553 0.789843
1
1.029077
0.948783
0.996586
558
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Fig. 6. Forming system of the proposed technique.
The undesired issue in flexible deep drawing is the formation of minor wrinkles which occur at the flange portion of the product being formed at a particular stage of the forming stroke (step two in this work) with relatively shallow height which may aggravate with proceeding the process. This issue can be overcome via the proposed technique as the flange portion of the cup being formed is between two solid surfaces (floating ring and adjustment ring). The third step begins just when the blank flange reaches the adjustment ring. In this step, the rubber hydrostatic pressure increases as the punch keep moving down inside the rubber pad until a complete micro cup is formed (see Fig. 6). For the case of oblique ring surface, the same scenario is adopted but at the beginning the blank holder is pushed down to form the flat blank between the inclined surfaces. To conduct micro deep drawing experiments, a special set up is developed with specific design features to meet the requirements of the FE simulations. The experiment set up consists of two main component groups; movable and stationary, in addition to the floating ring. The movable components are punch plate, rigid punch, middle plate, blank holder, holder housing, adjustment ring, three adjustment studs and holding spring. The components of stationary group consist of heavy base, three guides and the supporting ring. The set up base contains inside rubber die container, rubber die and central screw (Fig. 7). Circular workpieces are cut from SS 304 foils using the blanking punch-die set shown in Fig. 8. For blanks of 100 μm thickness with smooth cutting edges, the optimum clearance adopted for the blanking set is 15–20% of initial sheet thickness. The forming experiments were implemented utilizing LARYEE machine with forming velocity of 1 mm/s.
Fig. 8. (a) Blanking tools. (b) Blanks.
Fig. 9. Sound cups obtained from experiment and simulation.
5. Results It can be observed in Fig. 9 that the physical cup well correlates with that obtained from the numerical model in terms of geometry aspects. The process conditions adopted for these cups are 200 μm initial gap, 4 mm punch diameter with 0.8 mm head radius, 12 mm rubber pad diameter with 10 mm height, 100 μm blank thickness with 10 mm
Fig. 7. Micro deep drawing setup. 559
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Fig. 12. Thickness distributions along rolling direction. Fig. 10. Cups failed by wrinkling and fracture.
diameter, 1 mm floating ring thickness with 0.75 mm shoulder radius, punch/floating ring clearance 20 μm. The interactions of the initial gap with other process parameters have a crucial effect on the forming process. The product may fail by tearing if the gap is less than an optimum value and by wrinkling if it is greater as showing in Fig. 10. The criteria employing for evaluating the product quality depends on maximum thickness reduction and thickness distribution along the rolling and transver directions. Successful physical cup is cut along these two orintations and the wall thickness is measured at selected points at various zones as shown in Fig. 11. Figs. 12 and 13 present comparison in terms of thickness distribution and maximum thinning between experimental and numerical results. It is noticed that the maximum thinning occur at the profile radius region along both rolling and transverse directions. The thinning increases gradually up to the side wall region at which the thickness increases significantly at the upper part. The maximum thickening is observed at the shoulder radius and the thickness then back to decrease slightly towards the flange portion. The maximum thinning observed at the physical cup is 17% along rolling direction, while the simulation results indicate to 14%. On other side, there is no significant variation in maximum thickness reduction along the transfer direction between simulation and experimental results where it is 13.44% for the numerical model and 13% for the physical cup. Fig. 14 presents the punch load-displacement relationship for both simulation and experiments. In general, the curves showed the same trend during the forming process until reach the maximum value of load. In general, the experimental curve is higher than that obtained
Fig. 13. Thickness distributions along transverse direction.
Fig. 14. Punch load-displacement relation.
from simulation which can be attributed to the friction at contact interfaces of the set up components. However, an overlapping can be seen in the figure because the rubber container diameter is relatively greater that of rubber pad and this action allows for the rubber to expand in the radial direction which does not occur in the FE models. The maximum load obtained from simulation is 412.35 N while the experimental value is 396.47 N. The peak observed on the curves is due to the formation of minor wrinkles at the cup flange as much material flows into floating ring. This action may cause stuck the material between punch and floating ring. Therefore, a higher punch force is required for drawing procss. 5.1. Inclination angle of floating ring surface Three models of floating ring with different inclination angles of 0°, 5° and 10° are built. The boundary conditions presented in Tables 4 and 5 are adopted for these models with blank 10 mm in diameter and
Fig. 11. Microscopic photos (a) Base of cup (b1, 2) Profile radius (c1,2) Side wall (d) Shoulder radius (e) Flange portion. 560
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using floating rings with different inclination angles. The curve obtained with flat surface has two notches; the first one is attributed to minor wrinkles at the flange portion which is overcome via utilizing oblique floating ring. Whereas, the second notch which can be observed on the other two curves is due to the initial gap is vanished when the holder reaches the adjustment ring and the rubber pad at this forming stage has no space to extend. Consequently, this action result in an increase in the total load required to complete the forming stroke. Initially, the blank is flat and therefore the first step with inclined ring is to form the blank by the blankholder, which has inclined surface as well, on the floating ring to produce shallow cup. The depth of this step depends on the inclination angle where the depth at angle 5° is 0.22 mm and at angle 10° is 0.45 mm as explained using the shaded circle in Fig. 15. In this figure, it can be observed that the higher curve is that obtained with the flat surface and the maximum punch load required to completely form the product is 412.4 N. This value is significantly reduced utilizing floating ring with oblique surface where it is found that the maximum load of 339.8 N is obtained with 5° surface angle and 316.7 N with 10°. Moreover, when the blankholder touch the adjustment ring the rubber pad will be completely restricted causing the forming loading to increase dramatically as shown at the last part of the curves in Fig. 15.
Fig. 15. Thickness distribution along rolling direction with different angle of oblique surface.
floating ring 1 mm in thickness. The results show that the thickness distributions along rolling and transverse directions have similar trend for the different inclination angles (see Fig. 15). It is found that using flat ring results in maximum reductions in thickness of 17.1% at rolling direction and 13.4% at transverse direction, which is validated experimentally. Whereas at inclination angles of 5° and 10°, the maximum thickness reductions are 15.9% and 15.1% at rolling direction and 12.3% and 11.8% at transverse direction respectively. Therefore, it can conclude that increasing the inclination angle of floating ring causes a reduction in wall thinning of the final product. This action can be interpreted that the inclined surface causes the sheet metal to easily flow into the floating ring orifice and subsequently into the rubber pad. That means that much more material flow into the floating ring orifice leading to reducing the tensile stresses excited in the sheet materials. Consequently, this scenario causes the compressive stresses, excited as a result of the tensile stresses, to decrease leading effectively to reduce the possibility of producing minor wrinkles at the flange portion. In addition, the flange portion of the blank being forming using the proposed technique is basically restricted between two rigid surfaces, the blankholder and floating ring, as well as the holding pressure increases along with forming stroke as the hydrostatic pressure in rubber pad increases. This situation is different of that presented by Ihsan [23] where the blank was allocated between a flexible surface of rubber pad and a rigid one of blankholder and the failure by wrinkling was therefore avoided through reducing the initial gap value which in turn leads to excessive tensile stresses at some critical regions such as cup corner and side wall. These actions provide a crucial contribution in overcome the minor wrinkles. This analytical conclusion is was proved by the results which reveal that the minor wrinkles mostly appear at flange portion are completely eliminated through utilizing floating ring with inclination surface angle of 5° and 10°. Fig. 16 presents the forming load-displacement relation obtained
5.2. Floating ring thickness To investigate the influence of floating ring thickness on the forming process, three floating ring with 1, 1.25 and 1.5 mm in thickness are utilized. Figs. 17 and 18 show the thickness distribution of cup wall at rolling and transverse directions respectively. The thickness variation in the base of cup is clearly homogenous due to the relatively high coefficient of friction at the punch head/blank interface which leads to unremarkable relative movement. The figures indicate that the maximum thinning occurs at the upper part of punch nose corner for all cases. This behaviour can be attributed to the excessive tensile stresses concentrated at this region as it located between the cup bottom restricted between the rubber pad and rigid punch and the flange portion restricted between the blankholder and floating ring. On the other side, the maximum value of thickening increases with floating ring thickness. This finding can be interpreted that as the contact interface of the blank being with the floating ring increases, the area experiences ironing action then increases as well resulting in an increase in the tensile stresses which means an increase in thinning value. It was indicated that the maximum thickening at rolling direction is 30.1%, 26.3 and 23.3% and at transverse direction is 32%, 27.8% and 24.1% for floating rings with 1.5, 1.25 and 1 mm in thickness respectively. The punch load-displacement relationships are depicted in Fig. 19 for the three different cases. The general tendency of the curves is similar. The load increases with displacement until the punch head tough
Fig. 17. Thickness distributions along rolling direction for different FR thickness.
Fig. 16. Load-Displacement relations of different angles of oblique surfaces. 561
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Fig. 18. Thickness distributions along transverse direction for different FR thickness.
Fig. 20. Thickness distributions along rolling direction.
Fig. 19. Load-Displacement relation for different FR thickness.
Fig. 21. Thickness distributions along transverse direction.
the rubber surface, and thereafter the deformation load will be a function of both sheet resistance and the hydrostatic pressure generated in rubber material. The important observation is that the maximum load decreases with increasing the floating ring thickness. This finding indicates that using thicker floating ring contribute in reducing the duration required for steps 2 and 3 of the forming process (see Fig. 6) in which the product is completely formed inside the flexible die. Therefore, a reduction in the hydrostatic pressure generated in the rubber material was observed as well as in the punch force necessary for complete the forming stroke. The results reveal that the maximum load values 412.4, 367.1, 326 N are obtained with using floating rings of 1, 1.25 and 1.5 mm in thickness respectively. It is clear that each curve can be divided into three regions, the first one is due to punch movement through ring thickness, and therefor as the ring thickness increases this partition of curve is longer. The second region is due to the punch movement when the floating ring and blankholder move up until reach the adjustment ring. Finally, the third region is due to the complete restriction of the rubber pad.
which agree the results obtained by Ihsan [9]. This behaviour can be interpreted that increasing blank diameter implies greater flange portion which leads to an increase in the holding pressure applied by the rubber die. This action in turn results in excessive tensile stresses and consequently an increase in the thinning value. It can be indicated that the maximum thinning at rolling direction is 6.7%, 11.6% and 17% and at transverse direction is 5.6%, 8.1% and 13.4% for blank diameters of 8, 9 and 10 mm respectively. Regarding the punch load-displacement relationships, similar trend of increasing the forming load obtained for punch travel through floating ring thickness is observed. Thereafter, the load increases with higher rate due to the hydrostatic pressure generated in the rubber material until the blankholder reaches the adjustment ring. It is important to realize that as the blank diameter increases the initial gap should be increases as well to obtain sound product. Therefore, the maximum punch load is then a function of both blank diameter and gap value. The maximum forming load obtained with blank diameter of 10 mm and initial gap of 200 μm is 287.3 N, while maximum loads of 340.5 N and 221.9 N are obtained with 9 mm and 8 mm blank diameters respectively. The results proved the feasibility of the proposed technique to produce micro cups with high aspect ratio though just one press. Fig. 22 exhibits simulation and physical cups with aspect ratio of 1.25. The physical cup showed a good correlation with that predicted by simulation in term of geometrical profile and thickness distribution. Also, it can denote that there is a good matching between the two parts in term of the effect of anisotropic behavior of SS304 sheet metal. For this case, the maximum reductions in wall thickness are approximately 18% at rolling direction and 14.7% at transverse direction.
5.3. Blank diameter One of the significant issues in deep drawing technology is how to evaluate the drawing ratio which represents the ratio of the initial blank diameter to the diameter of the formed cup [18]. For this purpose, different initial diameters of 8, 9 and 10 mm are adopted under same conditions. The boundary conditions listed in Tables 4 and 5 are adopted here. An initial gap of 100 μm is adopted with blank diameters 8 and 9 mm while 200 μm with blank diameter of 10 mm to produce cups with similar final profile. The maximum depth can be obtained by applying these boundary conditions with blank diameter 8 mm was 3 mm so that the comparison with other blank diameters will be at this depth. The important observation in Figs. 20 and 21 is the maximum reduction in thickness increases with increasing initial blank diameter
6. Conclusions The current study presents a novel technique for micro deep 562
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[4] Zhang CB, Gong F. Deep drawing of cylindrical cups using polymer powder medium based flexible forming. Int. J. Precise Eng. Manuf.-Green Tech. 2018;5(63). https:// doi.org/10.1007/s40684-018-0007-8. [5] Cooper DR, Rossie KE, Gutowski TG. An environmental and cost analysis of stamping sheet metal parts. J Manuf Sci Eng 2017;139. 041012-1-11, https://doi: 10.1115/1.4034670. [6] Sulaiman S, Leman Z, Baharudin BT, Ariffin MKA, Man S. Process parameters for cylindrical deep drawing components. Adv Mater Process Technol 2016:1143207. https://doi.org/10.1080/2374068X.2016. [7] Peng L, Hu P, Lai X, Mei D, Ni J. Investigation of micro / meso sheet soft punch stamping process – simulation and experiments. Mater Des 2009;30(3):783–90. https://doi.org/10.1016/j.matdes.2008.05.074. [8] Elyasi M, Ghadikolaee HT, Hosseinzadeh M. Fabrication of metallic bipolar plates in PEM fuel cell using semi-stamp rubber forming process. Int J Adv Manuf Technol 2017;92(1-4):765–76. https://doi.org/10.1007/s00170-017-0206-4. [9] Irthiea IK, Green G. Evaluation of micro deep drawing technique using soft diesimulation and experiments. Int J Adv Manuf Technol 2017;89(5-8):2363–74. https://doi.org/10.1007/s00170-016-9167-2. [10] Nagarajan B, Castagne S, Wang Z, Zheng HY, Nadarajan K. Influence of plastic deformation in flexible pad laser shock forming – experimental and numerical analysis. Int J Mater Form 2017;10(1):109–23. https://doi.org/10.1007/s12289015-1264-5. [11] Kyu C, Geun M, Gil C. Effect of rubber forming process parameters on micro-patterning of thin metallic plates. Procedia Eng 2014;81:1439–44. https://doi.org/10. 1016/j.proeng.2014.10.170. [12] Luo L, Wei D, Wang X, Zhou C, Huang O, Xu J, et al. Effects of hydraulic pressure on wrinkling and earing in micro hydro deep drawing of SUS304 circular cups. Int J Adv Manuf Technol 2017;90(1-54):189–97. https://doi.org/10.1007/s00170-0169380-z. [13] Quadrini F, Santo L, Squeo EA. Flexible forming of thin aluminum alloy sheets. Int J Modern Manuf Technol 2010;II(1):79–84. [14] Peng L, Hu P, Lai X, Ni J. Fabrication of metallic bipolar plates for proton exchange membrane fuel cell by flexible forming process-numerical. J Fuel Cell Sci Technol 2010;7:1–9. https://doi.org/10.1115/1.3207870. [15] Aminzahed I, Mashhadi MM, Sereshk M. Investigation of holder pressure and size effects in micro deep drawing of rectangular work pieces driven by piezoelectric actuator. Mater Sci Eng C 2017;71:685–9. https://doi.org/10.1016/j.msec.2016.10. 068. [16] Luo L, Jiang Z, Wei D, Manabe K, Zhao Z, Wu D. Effects of surface roughness on micro deep drawing of circular cups with consideration of size effects. Finite Elem Anal Des 2016;111:46–55. https://doi.org/10.1016/j.finel.2015.11.005. [17] Cao M, Li J, Yuan Y, Zhao C. Flexible die drawing of magnesium alloy sheet by superimposing ultrasonic vibration. Trans. Trans Nonfer Metals Soc China 2017;27:163–71. https://doi.org/10.1016/S1003-6326. (17) 60019-0. [18] Jin CK, Jeong MG, Kang. Effect of process parameters on forming depth of channels in fuel cell bipolar plates fabricated using rubber forming process. Mater Res Innov 2014;18:467–72. https://doi.org/10.1179/1432891714Z.000000000461. [19] American Society for Testing and Materials ASTM. Standard test methods for tension testing of metallic materials. Ala Med 2010:1–27. [20] Le PA, Toussaint F, Arrieux R. Finite element study and sensitive analysis of the deep drawing formability of commercially pure titanium. Int J Mater Form 2009;2:121–9. https://doi.org/10.1007/s12289-009-0398-8. [21] Hai DV, Itoh S, Sakai T, Kamado S, Kojima Y. Experimentally and numerical study on deep drawing process for magnesium alloy sheet at elevated temperatures. Mater Trans 2008;49:1101–6. https://doi.org/10.2320/matertrans.MC200761. [22] Nosrati HG, Gerdooei M, Naghibi MF. Experimental and numerical study on formability in tube bulging: a comparison between hydroforming and rubber pad forming. Mater Manuf Process 2017;32(12):1353–9. https://doi.org/10.1080/ 10426914.2016.1257126. [23] Irthiea Ihsan, Green Graham, Hashim Safa, Kriama Abdulbast. Experimental and numerical investigation on micro deep drawing processofstainlesssteel304foilusing flexible tools. Int J Mach Tools Manuf 2014;76:21–33. https://doi.org/10.1016/j. ijmachtools.2013.09.006.
Fig. 22. Formed cups with aspect ratio of 1.25.
drawing process of SS 304 foils utilizing a floating ring as a primary rigid die and rubber pad as a main flexible die. FE simulation for the forming process is implemented using the commercial Abaqus/ Standard software. In order to validate the numerical models, a set of drawing experiments are achieved using a special set up developed for this purpose. The key parameters investigated in this work are floating ring thickness, surface inclination angle of floating ring and blank diameter. The results showed that increasing the inclination angle of ring surface leads to a reduction in forming load. It is found that at surface inclination angle of 5° the maximum forming load is 339.8 N which is smaller than that obtained with flat surface by 72.6 N. Increasing the angle to 10° causes this difference to increase by 95.67 N. Also, it is noticed that increasing the floating ring thickness from 1 mm to 1.25 mm results in a reduction in the maximum forming load from 412.4 N to 367.1 N. However, the forming load decreases by 86.31 N when the ring thickness is increased by 0.5 mm. Moreover, it is observed that reducing the initial diameter of blank causes the quality of the formed cups, in terms of maximum thickness reduction, to improve. On the other side, the forming load depends on the initial gap value. The results indicate that increasing the initial gap leads to a reduction in the forming load. The important finding is that adopting floating ring with inclined surface overcomes the minor wrinkling which commonly occurs during flexible deep drawing process at a particular forming stage. Finally, Moreover, this work revealed the feasibility of the proposed technique for producing micro cups with remarkable application flexibility in miniaturization technology with relatively high aspect ratio of 1 and 1.25 through just a single micro forming stage. References [1] Luo L, Jiang Z, Wei D. Influences of micro-friction on surface finish in micro deep drawing of SUS304cups. Wear 2017;374-375:36–45. https://doi.org/10.1016/j. wear.2016.11.043. [2] Zheng W, Wang G, Lin X, Tang B, Huang L, Qing W, et al. The experimental investigation of size effect on micro-cylinder deformation in coining process. Mater Manuf Process 2014;29:687–90. https://doi.org/10.1080/10426914.2014.912306. [3] Wang X, Qiu T, Shen Z, Zhang D, Ma Y, Gu Y, et al. Forming properties of a microscale laser dynamic flexible forming technique. Mater Manuf Process 2016;31(6):745–50. https://doi.org/10.1080/10426914.2014.994749.
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental and numerical evaluation of micro flexible deep drawing technique using floating ring
T
Ihsan Irthiea University of Anbar, Iraq-Anbar-Ramadi, 55 RAMADI, Ramadi, Iraq
A R T I C LE I N FO
A B S T R A C T
Keywords: Flexible tool Floating ring Rubber Micro deep drawing Simulation
A novel micro flexible forming technique utilizing floating ring, as a primary die, is presented in this work. The essential functions of the floating ring part is to overcome the minor wrinkling commonly occurs through flexible forming processes as well as produce micro cups with high accurate dimensions particularly at the shoulder profile. The influence of thickness and surface angle of the floating ring, and blank diameter are investigated. The well-known commercial software Abaqus/Standard is employed to build FE models for micro forming processes. Accordingly, a number of micro deep drawing experiments are conducted to verify the simulation results. To perform the experimental work, a special set up is developed with particular design aspects to satisfy the FE model conditions. The results reveal the feasibility of the floating ring technique adopted in production of micro cylindrical cups with high quality in terms of geometry profile and thickness distribution as well as relatively high aspect ratio.
1. Introduction
results showed that decreasing rubber hardness and/or increasing rubber pad thickness lead to deeper formed channels. Y. Luo L. et al. [12] conducted micro hydro deep drawing experiments to investigate the effects of hydraulic pressure on the wrinkling and the earing of the formed cups. The results revealed that even at the maximum hydraulic pressure of 30 MPa (less than 10% blank yield stress), the wrinkling and earing were not completely overcome. Therefore, ultra-high pressure changed the drawing process in terms of solving the problems of earing and wrinkling. F. Quadrini et al. [13] investigate the influence material properties of flexible die on the forming process of Aluminum sheet metal. The materials of rubber die were silicone rubber (SR), styrene butadiene rubber (SBR) and polyamide 66 (PA). The results revealed that the best forming conditions in terms of reduction of forming force and increase material durability can be provided by using PA material. L. Peng et al. [14] studied the feasibility of fabrication a bipolar plate from thin stainless steel sheet using flexible forming technique. The simulation and experimental results pointed out the capability to fabricate bipolar plate with high quality by using flexible tools. A novel approach in micro deep drawing of using a piezoelectric actuator at various scales is presented by Aminzahed et al. [15]. The study revealed the effects of blank holder pressure on thickness distributions, punch force, and springback. The size effects of blank material are considered through finite element model was developed by Lou et al. [16]. With consideration of the size effects, the results revealed that the surface roughness has significant influence on
Due to the rapid development in different modern technologies which have become essential in our daily life, companies and manufacturers, to stay in competition, are required to provide innovative products and devices fulfill customer requests of miniaturized size, competitive cost and high quality. Consequently, the demand on micro components dramatically increases [1–3] and subsequently the need for accuracy, shape and surface quality becomes higher and higher [4]. The expensive manufacturing processes using conventional tools at micro scale [5] cause utilizing unique techniques, such as flexible tool-assisted micro forming, significantly attractive for wide attention of researchers [6–8]. Various techniques of flexible micro forming have been investigated by many researchers. I. Irthiea et al. [9] proposed a new technique for forming micro SS304 cups using a polyurethane rubber die. The technique adopted render in reducing the overall production costs and improve the quality of the formed cups. Nagarajan et al. [10] present an experimental and numerical study on flexible pad laser shock forming to reveal the effect of flexible pad material and its thickness. The results showed that the hardness and thickness of the flexible pad have significant influence on the deformed crater geometry, thickness distribution across the formed crater and surface hardness at the crater surfaces. The effect of process parameters of micro channel forming using rubber pad technique was studied by C. Jin et al. [11]. The
E-mail address: [email protected]. https://doi.org/10.1016/j.jmapro.2019.01.050 Received 6 April 2018; Received in revised form 29 December 2018; Accepted 28 January 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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springback. Miao et al. [17] presented flexible-die deep drawing process of magnesium sheet using ultrasonic vibration technology. They found that the liquidity and force transmission performance of the granule medium were improved. As well as, all of friction, deformation resistance and punch load were significantly reduced with the augmentation of vibration energy. The so called “size effects” makes the process of traditionally forming cannot be applied directly in micro forming process [18]. However, Designation of micro products is yet considered challenge to researchers and manufacturers as there are not enough information provided on design and manufacture of such parts [3][7]. This work presents a novel flexible tool-assisted micro deep drawing technique by using floating ring for producing micro stainless steel 304 cups. The new issue in this work is to use a floating ring with specific geometric characteristics, as a primary rigid die, in cooperation with polyurethane rubber 50 A pad, as a major flexible die, to complete forming micro cups. Thin stainless steel 304 sheet of 100 μm thickness is cut with different diameters of 8 mm, 9 mm and 10 mm as process workpieces. The key parameters investigated through this work are thickness and surface inclination angle of the floating ring and initial blank diameter. Finite element models are established using the commercial software Abaqus/Standard to simulate the forming process. The numerical models are evaluated through a series of micro deep drawing experiments for which a special set up is developed. The experimental results reveal a good correlation with that obtained by FE simulation in term of thickness distribution, maximum thickness reduction, punch load-displacement relationships and geometry profile of the formed cups.
Fig. 2. Dogbone tensile test specimen.
Fig. 3. (a) LARYEE machine (b) Broken specimen.
2. Characterization of material behaviors Chromium-nickel stainless steel 304, widely used in micro electro mechanical systems, industries of micro-electronics and micro technologies, in form of thin sheets of 100 μm thickness is used as process workpieces. To obtain the mechanical properties of material, tensile tests are performed using a universal tensile machine supported by the LARYEE Company with load capacity of 100 kN and accuracy of +/0.5% (see Fig. 3). The material anisotropy is evaluated through testing three dogbone-shaped specimens cut along rolling, diagonal (45°) and transverse directions by using CNC cutter machine with 2500 rpm spindle velocity and cutting tool of 2 mm diameter as illustrated in Fig. 1. The dimensions of these specimens are obtained from the ASTM E8 [19] standard as shown in Fig. 2. The tensile tests are carried out at velocity of 0.2 mm/s. The stress-strain relationships obtained from the tensile tests are shown in Fig. 4, while the mechanical properties of the SS 304 foil at the different orientations are listed in Table 1. As well as, the mechanical properties of the polyurethane rubber 55 Shore A hardness are presented in Table 2 [7].
Fig. 4. Stress-strain relations for SS 304 foil.
this study to develop FE simulations for micro deep drawing processes. To characterize the deformation behavior of SS 304 foils, an elasticplastic model was adopted. To build this model, 8300 elements with mesh type of 8-node linear brick, reduced integration and hourglass control (C3D8R). The boundary conditions of blank are at Z- axis direction as XSYMM (U1=UR2=UR3 = 0) while at X-axis direction as ZSYMM (U3=UR1=UR2 = 0). The plastic anisotropy is introduced by using Hill's criterion.
3. Material models in FEA simulation
f(σ)= F(σ₂₂-σ₃₃)²+G(σ₃₃-σ₁₁)²+H(σ₁₁-σ₂₂)²+2Lσ₂₃²+2Mσ₃₁²+2Nσ₁₂² 3.1. Anisotropy of Stainless steel foil
(1) The material constants F, G, H, L, M and N are obtained from tensile tests in different orientations and σij refers the stress components [20]. These constants are defined as:
The commercial code Abaqus/Standard software was employed in
Fig. 1. Cutting process of dogbone samples. 557
1 1 1 1 F= ⎛⎜ 2 + 2 − 2 ⎞⎟ 2 ⎝ R22 R33 R11 ⎠
(2)
1 1 1 1 G= ⎜⎛ 2 + 2 − 2 ⎞⎟ 2 ⎝ R33 R11 R22 ⎠
(3)
1 1 1 1 H= ⎛⎜ 2 + 2 − 2 ⎞⎟ 2 ⎝ R11 R22 R33 ⎠
(4)
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Table 1 Mechanical properties of SS304 sheet. Thickness (mm)
Rolling direction
E (GPa)
σo (MPa)
σult (MPa)
εult
K (MPa)
n
0.1
0° 45° 90°
94 149 118
291 324 310
540 625 621.84
0.1951 0.2486 0.23202
1213.1 1298.9 1444.7
0.3708 0.3469 0.4103
Table 2 Mechanical properties of Polyurethane rubber materials. Hardness Shore A
M–R constant C10 (MPa)
M–R constant C01 (MPa)
Poisson’s ratio (v)
55
0.382
0.096
0.4999
L=
3 3 3 M= N= 2R₂₃² 2R₁₃² 2R₁₂²
(5)
the factors R11, R22, R33, R12, R13 and R23 are anisotropic yield stress ratios. Due to the stress situation in sheet metal forming processes are characterized by plane stress conditions therefore four anisotropic ratios are considered in this work which are R11, R22, R33 and R12 [19]. In Abaqus software, these factors are expresses using Lankford Factors as r0, r45 and r90 as following equations [21]:
3r₉ₒ(rₒ+1) r₉ₒ(rₒ+1) r₉ₒ(rₒ+1) R12= R33= R22= (2r₄₅+1)(rₒ+r₉ₒ) rₒ+r₉ₒ rₒ(r₉ₒ+1)
Fig. 5. Floating rings design. (a) Flat surface and (b) Oblique surface. Table 4 Geometry parameters.
(6)
where r0, r45 and r90 are width strain to thickness strain ratios of the work-piece material at the rolling, diagonal and transverse directions, respectively. The values of (r) and (R) of SS 304 sheets employed in the current work are calculated using the above equations, as shown in Table 3. 3.2. Hyper-elastic model for rubber pad material To describe the behavior of the polyurethane rubber used for the flexible die, the known Mooney-Rivlin hyperplastic model is adopted in the FE model. It is known that polyurethane rubber is characterized by a nonlinear stress–strain relation for large deformation, as well as it is nearly incompressible. The strain energy form of Mooney–Rivlin is: W = C10 (I1-3)+C01(I2-3)+(J1-1)/D1
Parameters
Value (mm)
Blank diameter (Db) Blank thickness (tb) Punch diameter (Dp) Radius of punch corner (rp) Height of rubber pad (hr) Diameter of rubber pad (Dr) Thickness of flat floating ring (tR) Inclined angle of flat floating ring (Ѳ) Punch/floating ring clearance C (% tb) Diameter of floating ring (DR) Radius of inner corner of floating ring (rR)
8, 9, 10 0.1 4 0.8 10 12 1, 1.25, 1.5 0°, 5°, 10° 10% 13 0.75
Table 5 Coefficients of friction adopted at different interfaces.
(7)
where W is the strain energy per unit of volume, I1 and I2 are the strain invariants, J is volume change. C10 and C01 describe hyperplastic rubber deformation, and D1 indicates the material compressibility [22]. The deformation behavior of rubber material was described by using hyper-elastic model. This part has 13,806 elements with mesh type 8node linear brick, hybrid, constant pressure, reduced integration and hourglass control (C3D8RH).
Interface
Coefficient of friction
Blankholder and blank Floating ring and blank Rubber pad and b Punch and blank Floating ring and rubber pad Adjustment ring and blankholder
0.1 0.05 0.1 0.25 0.05 0.05
components are listed in Table 4. Also, the coefficients of friction adopted at the different contact interfaces in the FE models are presented in Table 5. The forming process is divided into three main steps as illustrated in Fig. 6, through the first one the blank is formed to a depth equal to the floating ring thickness. In the second step, the solid punch moves downward to form the blank in the flexible die. Consequently, the hydrostatic pressure excited in the rubber material causes both floating ring and blank holder to move up through the initial gap.
4. Forming methodology The simulation models of micro forming processes consist of circular blank, rigid punch, blankholder, adjustment ring, floating ring and flexible pad. It can be seen in Fig. 5 that the floating ring is designated with different surface inclination angles to investigate the influence of this parameter on the forming operation. The dimensions of all Table 3 Plastic strain ratios and anisotropic yield stress ratios. Rolling direction
Eng. Length strain
Width strain
Thickness strain
plastic strain ratio (r)
R11
R22
R33
R12
0° 45° 90°
0.2 0.2 0.2
−0.0731 −0.0806 −0.0771
−0.1051 −0.0962 −0.0995
0.714399 0.859553 0.789843
1
1.029077
0.948783
0.996586
558
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Fig. 6. Forming system of the proposed technique.
The undesired issue in flexible deep drawing is the formation of minor wrinkles which occur at the flange portion of the product being formed at a particular stage of the forming stroke (step two in this work) with relatively shallow height which may aggravate with proceeding the process. This issue can be overcome via the proposed technique as the flange portion of the cup being formed is between two solid surfaces (floating ring and adjustment ring). The third step begins just when the blank flange reaches the adjustment ring. In this step, the rubber hydrostatic pressure increases as the punch keep moving down inside the rubber pad until a complete micro cup is formed (see Fig. 6). For the case of oblique ring surface, the same scenario is adopted but at the beginning the blank holder is pushed down to form the flat blank between the inclined surfaces. To conduct micro deep drawing experiments, a special set up is developed with specific design features to meet the requirements of the FE simulations. The experiment set up consists of two main component groups; movable and stationary, in addition to the floating ring. The movable components are punch plate, rigid punch, middle plate, blank holder, holder housing, adjustment ring, three adjustment studs and holding spring. The components of stationary group consist of heavy base, three guides and the supporting ring. The set up base contains inside rubber die container, rubber die and central screw (Fig. 7). Circular workpieces are cut from SS 304 foils using the blanking punch-die set shown in Fig. 8. For blanks of 100 μm thickness with smooth cutting edges, the optimum clearance adopted for the blanking set is 15–20% of initial sheet thickness. The forming experiments were implemented utilizing LARYEE machine with forming velocity of 1 mm/s.
Fig. 8. (a) Blanking tools. (b) Blanks.
Fig. 9. Sound cups obtained from experiment and simulation.
5. Results It can be observed in Fig. 9 that the physical cup well correlates with that obtained from the numerical model in terms of geometry aspects. The process conditions adopted for these cups are 200 μm initial gap, 4 mm punch diameter with 0.8 mm head radius, 12 mm rubber pad diameter with 10 mm height, 100 μm blank thickness with 10 mm
Fig. 7. Micro deep drawing setup. 559
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Fig. 12. Thickness distributions along rolling direction. Fig. 10. Cups failed by wrinkling and fracture.
diameter, 1 mm floating ring thickness with 0.75 mm shoulder radius, punch/floating ring clearance 20 μm. The interactions of the initial gap with other process parameters have a crucial effect on the forming process. The product may fail by tearing if the gap is less than an optimum value and by wrinkling if it is greater as showing in Fig. 10. The criteria employing for evaluating the product quality depends on maximum thickness reduction and thickness distribution along the rolling and transver directions. Successful physical cup is cut along these two orintations and the wall thickness is measured at selected points at various zones as shown in Fig. 11. Figs. 12 and 13 present comparison in terms of thickness distribution and maximum thinning between experimental and numerical results. It is noticed that the maximum thinning occur at the profile radius region along both rolling and transverse directions. The thinning increases gradually up to the side wall region at which the thickness increases significantly at the upper part. The maximum thickening is observed at the shoulder radius and the thickness then back to decrease slightly towards the flange portion. The maximum thinning observed at the physical cup is 17% along rolling direction, while the simulation results indicate to 14%. On other side, there is no significant variation in maximum thickness reduction along the transfer direction between simulation and experimental results where it is 13.44% for the numerical model and 13% for the physical cup. Fig. 14 presents the punch load-displacement relationship for both simulation and experiments. In general, the curves showed the same trend during the forming process until reach the maximum value of load. In general, the experimental curve is higher than that obtained
Fig. 13. Thickness distributions along transverse direction.
Fig. 14. Punch load-displacement relation.
from simulation which can be attributed to the friction at contact interfaces of the set up components. However, an overlapping can be seen in the figure because the rubber container diameter is relatively greater that of rubber pad and this action allows for the rubber to expand in the radial direction which does not occur in the FE models. The maximum load obtained from simulation is 412.35 N while the experimental value is 396.47 N. The peak observed on the curves is due to the formation of minor wrinkles at the cup flange as much material flows into floating ring. This action may cause stuck the material between punch and floating ring. Therefore, a higher punch force is required for drawing procss. 5.1. Inclination angle of floating ring surface Three models of floating ring with different inclination angles of 0°, 5° and 10° are built. The boundary conditions presented in Tables 4 and 5 are adopted for these models with blank 10 mm in diameter and
Fig. 11. Microscopic photos (a) Base of cup (b1, 2) Profile radius (c1,2) Side wall (d) Shoulder radius (e) Flange portion. 560
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using floating rings with different inclination angles. The curve obtained with flat surface has two notches; the first one is attributed to minor wrinkles at the flange portion which is overcome via utilizing oblique floating ring. Whereas, the second notch which can be observed on the other two curves is due to the initial gap is vanished when the holder reaches the adjustment ring and the rubber pad at this forming stage has no space to extend. Consequently, this action result in an increase in the total load required to complete the forming stroke. Initially, the blank is flat and therefore the first step with inclined ring is to form the blank by the blankholder, which has inclined surface as well, on the floating ring to produce shallow cup. The depth of this step depends on the inclination angle where the depth at angle 5° is 0.22 mm and at angle 10° is 0.45 mm as explained using the shaded circle in Fig. 15. In this figure, it can be observed that the higher curve is that obtained with the flat surface and the maximum punch load required to completely form the product is 412.4 N. This value is significantly reduced utilizing floating ring with oblique surface where it is found that the maximum load of 339.8 N is obtained with 5° surface angle and 316.7 N with 10°. Moreover, when the blankholder touch the adjustment ring the rubber pad will be completely restricted causing the forming loading to increase dramatically as shown at the last part of the curves in Fig. 15.
Fig. 15. Thickness distribution along rolling direction with different angle of oblique surface.
floating ring 1 mm in thickness. The results show that the thickness distributions along rolling and transverse directions have similar trend for the different inclination angles (see Fig. 15). It is found that using flat ring results in maximum reductions in thickness of 17.1% at rolling direction and 13.4% at transverse direction, which is validated experimentally. Whereas at inclination angles of 5° and 10°, the maximum thickness reductions are 15.9% and 15.1% at rolling direction and 12.3% and 11.8% at transverse direction respectively. Therefore, it can conclude that increasing the inclination angle of floating ring causes a reduction in wall thinning of the final product. This action can be interpreted that the inclined surface causes the sheet metal to easily flow into the floating ring orifice and subsequently into the rubber pad. That means that much more material flow into the floating ring orifice leading to reducing the tensile stresses excited in the sheet materials. Consequently, this scenario causes the compressive stresses, excited as a result of the tensile stresses, to decrease leading effectively to reduce the possibility of producing minor wrinkles at the flange portion. In addition, the flange portion of the blank being forming using the proposed technique is basically restricted between two rigid surfaces, the blankholder and floating ring, as well as the holding pressure increases along with forming stroke as the hydrostatic pressure in rubber pad increases. This situation is different of that presented by Ihsan [23] where the blank was allocated between a flexible surface of rubber pad and a rigid one of blankholder and the failure by wrinkling was therefore avoided through reducing the initial gap value which in turn leads to excessive tensile stresses at some critical regions such as cup corner and side wall. These actions provide a crucial contribution in overcome the minor wrinkles. This analytical conclusion is was proved by the results which reveal that the minor wrinkles mostly appear at flange portion are completely eliminated through utilizing floating ring with inclination surface angle of 5° and 10°. Fig. 16 presents the forming load-displacement relation obtained
5.2. Floating ring thickness To investigate the influence of floating ring thickness on the forming process, three floating ring with 1, 1.25 and 1.5 mm in thickness are utilized. Figs. 17 and 18 show the thickness distribution of cup wall at rolling and transverse directions respectively. The thickness variation in the base of cup is clearly homogenous due to the relatively high coefficient of friction at the punch head/blank interface which leads to unremarkable relative movement. The figures indicate that the maximum thinning occurs at the upper part of punch nose corner for all cases. This behaviour can be attributed to the excessive tensile stresses concentrated at this region as it located between the cup bottom restricted between the rubber pad and rigid punch and the flange portion restricted between the blankholder and floating ring. On the other side, the maximum value of thickening increases with floating ring thickness. This finding can be interpreted that as the contact interface of the blank being with the floating ring increases, the area experiences ironing action then increases as well resulting in an increase in the tensile stresses which means an increase in thinning value. It was indicated that the maximum thickening at rolling direction is 30.1%, 26.3 and 23.3% and at transverse direction is 32%, 27.8% and 24.1% for floating rings with 1.5, 1.25 and 1 mm in thickness respectively. The punch load-displacement relationships are depicted in Fig. 19 for the three different cases. The general tendency of the curves is similar. The load increases with displacement until the punch head tough
Fig. 17. Thickness distributions along rolling direction for different FR thickness.
Fig. 16. Load-Displacement relations of different angles of oblique surfaces. 561
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Fig. 18. Thickness distributions along transverse direction for different FR thickness.
Fig. 20. Thickness distributions along rolling direction.
Fig. 19. Load-Displacement relation for different FR thickness.
Fig. 21. Thickness distributions along transverse direction.
the rubber surface, and thereafter the deformation load will be a function of both sheet resistance and the hydrostatic pressure generated in rubber material. The important observation is that the maximum load decreases with increasing the floating ring thickness. This finding indicates that using thicker floating ring contribute in reducing the duration required for steps 2 and 3 of the forming process (see Fig. 6) in which the product is completely formed inside the flexible die. Therefore, a reduction in the hydrostatic pressure generated in the rubber material was observed as well as in the punch force necessary for complete the forming stroke. The results reveal that the maximum load values 412.4, 367.1, 326 N are obtained with using floating rings of 1, 1.25 and 1.5 mm in thickness respectively. It is clear that each curve can be divided into three regions, the first one is due to punch movement through ring thickness, and therefor as the ring thickness increases this partition of curve is longer. The second region is due to the punch movement when the floating ring and blankholder move up until reach the adjustment ring. Finally, the third region is due to the complete restriction of the rubber pad.
which agree the results obtained by Ihsan [9]. This behaviour can be interpreted that increasing blank diameter implies greater flange portion which leads to an increase in the holding pressure applied by the rubber die. This action in turn results in excessive tensile stresses and consequently an increase in the thinning value. It can be indicated that the maximum thinning at rolling direction is 6.7%, 11.6% and 17% and at transverse direction is 5.6%, 8.1% and 13.4% for blank diameters of 8, 9 and 10 mm respectively. Regarding the punch load-displacement relationships, similar trend of increasing the forming load obtained for punch travel through floating ring thickness is observed. Thereafter, the load increases with higher rate due to the hydrostatic pressure generated in the rubber material until the blankholder reaches the adjustment ring. It is important to realize that as the blank diameter increases the initial gap should be increases as well to obtain sound product. Therefore, the maximum punch load is then a function of both blank diameter and gap value. The maximum forming load obtained with blank diameter of 10 mm and initial gap of 200 μm is 287.3 N, while maximum loads of 340.5 N and 221.9 N are obtained with 9 mm and 8 mm blank diameters respectively. The results proved the feasibility of the proposed technique to produce micro cups with high aspect ratio though just one press. Fig. 22 exhibits simulation and physical cups with aspect ratio of 1.25. The physical cup showed a good correlation with that predicted by simulation in term of geometrical profile and thickness distribution. Also, it can denote that there is a good matching between the two parts in term of the effect of anisotropic behavior of SS304 sheet metal. For this case, the maximum reductions in wall thickness are approximately 18% at rolling direction and 14.7% at transverse direction.
5.3. Blank diameter One of the significant issues in deep drawing technology is how to evaluate the drawing ratio which represents the ratio of the initial blank diameter to the diameter of the formed cup [18]. For this purpose, different initial diameters of 8, 9 and 10 mm are adopted under same conditions. The boundary conditions listed in Tables 4 and 5 are adopted here. An initial gap of 100 μm is adopted with blank diameters 8 and 9 mm while 200 μm with blank diameter of 10 mm to produce cups with similar final profile. The maximum depth can be obtained by applying these boundary conditions with blank diameter 8 mm was 3 mm so that the comparison with other blank diameters will be at this depth. The important observation in Figs. 20 and 21 is the maximum reduction in thickness increases with increasing initial blank diameter
6. Conclusions The current study presents a novel technique for micro deep 562
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Fig. 22. Formed cups with aspect ratio of 1.25.
drawing process of SS 304 foils utilizing a floating ring as a primary rigid die and rubber pad as a main flexible die. FE simulation for the forming process is implemented using the commercial Abaqus/ Standard software. In order to validate the numerical models, a set of drawing experiments are achieved using a special set up developed for this purpose. The key parameters investigated in this work are floating ring thickness, surface inclination angle of floating ring and blank diameter. The results showed that increasing the inclination angle of ring surface leads to a reduction in forming load. It is found that at surface inclination angle of 5° the maximum forming load is 339.8 N which is smaller than that obtained with flat surface by 72.6 N. Increasing the angle to 10° causes this difference to increase by 95.67 N. Also, it is noticed that increasing the floating ring thickness from 1 mm to 1.25 mm results in a reduction in the maximum forming load from 412.4 N to 367.1 N. However, the forming load decreases by 86.31 N when the ring thickness is increased by 0.5 mm. Moreover, it is observed that reducing the initial diameter of blank causes the quality of the formed cups, in terms of maximum thickness reduction, to improve. On the other side, the forming load depends on the initial gap value. The results indicate that increasing the initial gap leads to a reduction in the forming load. The important finding is that adopting floating ring with inclined surface overcomes the minor wrinkling which commonly occurs during flexible deep drawing process at a particular forming stage. Finally, Moreover, this work revealed the feasibility of the proposed technique for producing micro cups with remarkable application flexibility in miniaturization technology with relatively high aspect ratio of 1 and 1.25 through just a single micro forming stage. References [1] Luo L, Jiang Z, Wei D. Influences of micro-friction on surface finish in micro deep drawing of SUS304cups. Wear 2017;374-375:36–45. https://doi.org/10.1016/j. wear.2016.11.043. [2] Zheng W, Wang G, Lin X, Tang B, Huang L, Qing W, et al. The experimental investigation of size effect on micro-cylinder deformation in coining process. Mater Manuf Process 2014;29:687–90. https://doi.org/10.1080/10426914.2014.912306. [3] Wang X, Qiu T, Shen Z, Zhang D, Ma Y, Gu Y, et al. Forming properties of a microscale laser dynamic flexible forming technique. Mater Manuf Process 2016;31(6):745–50. https://doi.org/10.1080/10426914.2014.994749.
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