Keywords: micro deep drawing, limit draw ratio, size effect, thickness to grain size ratio, stainless steel ... micro deep drawing tests are shown in Figure 2. For the ...
Effect of Thickness to Grain Size Ratio on Drawability for Micro Deep Drawing of AISI 304 Stainless Steel R.S. Lee1, C.H Chen1, J.T. Gau2 1
2
National Cheng Kung University, Taiwan; Northern Illinois University, USA
Summary Although deep drawing process in the macro scale are well developed, the known concepts cannot be applied to micro deep drawing through directly scale down the specimen and tooling size due to the so-called size effects. Therefore, tensile test and micro deep drawing experiments were conducted to generate the knowledge for the micro deep drawing process. Stainless steel 304 foils with 4 different thicknesses (150µm, 100µm, 50µm and 20µm) were used to investigate the influence of the blank holder forces and T/D ratio (thickness/average grain diameter) on the limit draw ratio (LDR). Four female dies with different die diameters and die shoulder radii and one punch were used for this study. The experimental results show: 1) the required blank holder force increases with increasing of T/D ratio, 2) the limit drawing ratio (LDR) increases with increasing of T/D ratio and increases at a decreasing slope as T/D ratio becomes larger, and 3) the limit drawing ratio (LDR) becomes steady when T/D ratio > 10 for the as-received stainless steel 304 foils. Finally, the authors recommend having at least 10 grains throughout the stainless steel 304 foils thickness for obtaining better formability and steady deep drawing behavior for micro sheet forming. Keywords: micro deep drawing, limit draw ratio, size effect, thickness to grain size ratio, stainless steel
1 Introduction Due to the trend toward miniaturization, the demands for metal micro parts are tremendously increased and widely used in different areas such as consumer electronics, telecommunication devices medical devices, automotive industry etc [1, 2]. Lithographic technologies and micro machining are utilized to produce micro components with high dimensional accuracy, but not cost effective. Besides, the material types for these processes are limited [3]. In comparison with lithographic technologies and micro machining, micro metal forming is the most suitable and cost effective manufacturing process for mass production of micro metal parts and holds promise for numerous new technologies, innovations, and applications. That is the reason the development of the micro sheet metal forming for sheet thickness less than 0.1mm has gained attention recently [4]. Even though metal forming processes in the macro scale are well developed, the known concepts cannot be applied to micro metal forming through directly scale down the specimen and tooling size [5]. Therefore, more metal forming researches in micro scale are necessary to generate the knowledge for micro sheet forming process such as micro deep drawing. In the recent study of Michel et al [6], tensile and hydraulic bulging tests were conducted to study the flow stress curves (stress-strain) of brass in micro scale. Both experiments showed the same trend that is the flow curve decreasing with the decrease of specimen thickness. They also proposed a new model that can model size effects for micro scale metal forming. The influences of size effects on flow stress, formability and springback on aluminum and brass were studied by Gau et al [7]. In their investigation, they found the formability of aluminum decreases with the decrease of the T/D (thickness/ average grain diameter) ratio. Saoteme et al. [8] conducted micro deep drawing study on the steel with thickness less than 0.2mm. They obtained the relationship between the punch diameter and drawability without a large amount of blank holder force. In addition, Vollertsen et al. [3] also conducted micro deep drawing experiments to obtain the limit draw ratio (LDR) with the factors of friction coefficient and the applied pressure. Lee et al [4] investigated the size effects of stainless steel 304 foils on the micro deep drawing of ball retainer and simulated the micro deep drawing process by using LS-DYNA. It was found that the effective stress and effective strain of ball retainer were influenced by thickness/grain size ratio. Furthermore, two-stage cylindrical cup deep drawings were investigated by Manabe et al. [9] through both experiments and simulations to obtain the cup wall
geometry, thickness distribution and surface roughness of the deep draw parts. In their study, the tool roughness was considered while the effects of grain size and thickness were ignored. In this paper, tensile test and micro deep drawing test were conducted for studying the limit draw ratio (LDR) of stainless steel 304 foils in micro scale. The as-received stainless steel 304 foils with four different thicknesses (150µm, 100µm, 50µm and 20µm) were used for this study. The influences of T/D ratio (Thickness to grain size ratio) and blank holder force on LDR were investigated in this study.
2 Material and tooling and experimental method 2.1 Material preparation and grain size measurement Figures 1 shows a specimen for the tensile test while five different size (diameter) specimens for micro deep drawing tests are shown in Figure 2. For the tensile test, NIU water jet was used to cut the specimens of which the dimensions were determined by ASTM E8 standard. As shown in Figure 2, the micro deep drawing specimens were punched out by a set of mini punches with different diameters.
Fig.1: Tensile Test Specimen
Fig. 2: Micro Deep Drawing Specimens
In order to observe the microstructures of the stainless steel foils throughout the thickness, the o
samples were cut along the rolling direction ( 0o ) and transverse direction ( 90 ).Electrolytic etching process was used to reveal austenite grain boundaries and the solution of 60mL HNO3 and 40mL H2O was used. In the electrolytic etching process, the mounted specimens were immersed in the solution with stainless steel cathode, using 1.5 volts DC power for 120 seconds [10]. The pictures of the microstructures shown in Figures 3 were captured by an optical microscope with a fixed CCD camera. Each picture in Figure 3 contains two microstructures of which the one along rolling direction is shown on the top. ASTM E112-Heyn Lineal Intercept Procedure was used to determine the grain size of the specimens. It is obvious that the grain shapes along the rolling direction are longer than those which are along transverse direction. However, the number of grains throughout thickness is almost the same for both directions.
(a) Thickness = 150 µm, T/D ratio = 11.66
(b) Thickness = 100 µm, T/D ratio = 9.96
(c) Thickness = 50 µm, T/D ratio = 6.51 (d) Thickness = 20 µm, T/D ratio = 4.35 Fig. 3: Microstructures of Stainless Steel 304 Foils [1]
2.2 Tooling and experimental procedure MTS Sintech 2/G with 1250 Newton load cells was used to conduct the tensile test and micro deep drawing experiments. Figures 4 and 5 show the setup for the micro deep drawing experiments and the details of the micro die, respectively. As shown in Figure 4, several types of springs (different spring constants) were used to generate different blank holder forces and one wire spring was used to generate kick out force (20 Newton). Table 1 provides the initial blank holder forces at the moment that the punch just touched blank surface. After initial contact, the blank holder forces increased with the increase of punch stroke. The blank holder forces can estimated by adding the initial forces with the product of the total spring constant and the punch travel distance after initial contact.
Fig. 4: The Setup for Micro Deep Drawing Experiment
Fig. 5: Micro Deep Draw Die Components [1] The diameter of punch is 2mm with 0.5mm punch radius and four deep draw dies with different diameters that are 2.33mm, 2.22mm, 2.11mm, 2.044mm with 0.6mm, 0.4mm, 0.2mm and 0.08mm die shoulder radii, respectively, were used for experiments. As shown in Figure 5, the function of the oblique position ring is to locate the blank for deep drawing. For this study, the punch travel speed was set as 0.5mm/sec and no lubrication was used. At least 3 specimens of each T/D ratio (thickness/average grain diameter) were tested. The physical meaning of T/D is the number of grains throughout the foil thickness.
Table 1: Initial Blank Holder Forces No.
1
2
3
4
5
6
7
8
9
10
11
BHF*(N)
2
3.8
7
13
26.4
35.3
50.6
57.2
67.1
75.2
82.4
*BHF: initial blank holder force
3 Results and discussion Figure 6 shows the flow stress curves of the as-received stainless steel 304 foils while Table 2 shows their mechanical properties. The specimens with 150µm thickness have the lowest yield stress and ultimate stress and the highest failure strain in comparison with other foils. By observing Table 2, the yield and ultimate stresses increase as the decrease of thickness and T/D ratio, but the failure
strain decreases. Therefore, the stainless steel 304 foils can be considered as the brittle materials (failure strain < 0.05) when their T/D (thickness/average grain size) ratios are less than 10. That means the stainless steel 304 foils with T/D < 10 may have less formability and drawability.
Table 2: Mechanical Properties of the Stainless Steel 304 foils Thickness (µm)
T/D*
Yield Stress (MPa)
Ultimate Stress (MPa)
Failure Strain
20
4.35
1470.6
1470.6
0.025
50
6.51
1090.4
1301.6
0.041
100
9.96
821.5
1233.9
0.049
150
11.66
730.1
1188.9
0.182
*T/D: material thickness/average grain diameter
500
1600
Thickness = 20µm
1400
Thickness = 50µm
400
Thickness = 100µm Thickness = 150µm
1000
Load (N)
Stress (MPa)
1200
800 Thickness = 20µm
600
Thickness = 50µm
400
Thickness = 100µm
200
Thickness = 150µm
300 200 100 0
0 0
0.05
0.1 Strain
0.15
0.2
Fig. 6: Flow Stress Curves of the Tested Specimens
0
0.5
1 1.5 Punch Stroke (mm)
2
2.5
Fig. 7: Load-Stroke Curves of Drawing Φ3.5mm Blanks
Figure 7 shows the Load-Strokes of drawing 3.5 diameter blanks with different thickness. The thicker material (with larger T/D ratio), the higher peak load. In addition, it can also be observed that the thinner material has the peak load at the shorter punch stroke. Figure 8 shows the top and bottom views of a drawn cup with 50µm initial thickness (T/D=6.51).
1mm
1mm
Fig. 8: 50µm Thickness Cup (Drawing Ratio = 1.75) A proper blank holder force and blank size are crucial in order for obtaining a deeper cup without any split and/or wrinkle. As observed during experiments, the cups fractured on the cup corner areas when the blank holder force and/or the specimen diameter are too large. On the other hand, wrinkles were observed at the rims of the cups at the very early stage when the blank holder forces were too small. Some of the experimental data were plotted in Figure 9. From these plots, the limit drawing ratio (LDR) of stainless steel foils can be obtained.
80
Good Parts
Blank Holder Force (N)
Blank Holder Force (N)
40
Broken parts
30
Wrinkled parts
20 10
Broken parts
60
1.6
1.7
1.8
1.9
20 0
2
Drawing Ratio
1.5
1.7 1.9 Drawing Ratio
(a) 20µm (T/D=4.35)
(b) 50µm (T/D=6.51)
2.1
90 Blank Holder Force (N)
80 Blank Holder Force (N)
Wrinkled parts
40
0 1.5
Good parts
60 40 20
Good parts Broken parts
75 60 45 30 Good parts
15
Broken parts Wrinlked parts
Wrinkled parts
0
0 1.7
1.8
1.9 2 Drawing Ratio
2.1
2.2
1.7
1.8
1.9 2 Drawing Ratio
2.1
2.2
(c) 100µm (T/D=9.96) (d) 150µm (T/D=11.66) Fig. 9: Drawing Ratio in Different Thickness and Blank Holder Force
80
Limit Drawing Ratio
Max Blank Holder Force (N)
Sufficient blank holder force is required for resisting wrinkling. However, excessive blank holder force may result in fracture. For Higher T/D ratio, it needs higher blank holder force. The max blank holder force at limit drawing ratio for different T/D ratios are shown in Figure 10. Limit drawing ratio versus thickness to grain size ratio is shown in Figure 11. The limit drawing ratio becomes steady (around 2) when T/D ratio >10 and decreases with the decrease of T/D ratio when T/D ratio < 10.
60 40 20 0 4
6
8 T/D
10
12
Fig. 10: Blank Holder Force versus T/D Ratio
2.1 2 1.9 1.8 1.7 4
6
8 T/D
10
12
Fig. 11: Limit Drawing Ratio versus T/D Ratio
4 Conclusions The as-received stainless steel 304 foils with four different thicknesses (150µm, 100µm, 50µm and 20µm) were used for the tensile test and micro deep drawing experiments. The influence of the T/D ratios and the bank holder forces on the micro deep drawing were obtained and listed as follows. • The T/D ratio decreases with the decrease of thickness. • Ultimate stress increases with decreasing of T/D ratio while ultimate strain decreases with the decrease of T/D ratio.
• • •
The maximum blank holder force increases with increasing of T/D ratio. The limit draw ratio (LDR) increases with increasing of T/D ratio and increases at a decreasing slope as T/D ratio becomes larger. Limit drawing ratio (LDR) becomes steady (around 2) when T/D ratio > 10 for the as-received stainless steel 304 foils.
The limit drawing ratio decreases with decreasing of T/D ratio when T/D ratio < 10 while it becomes steady (around 2) when T/D ratio > 10. Therefore, it is recommend that at least 10 grains throughout the stainless steel 304 foils thickness (T/D>10) for better formability and steady deep drawing behavior.
Acknowledgements The authors would like to thank National Science Council (NSC) of Taiwan sponsors Mr. Chen to conduct micro forming research at NIU with Dr. Gau. In addition, the authors also would like to thank Metal Industries Research & Development Centre (MIRDC) in Taiwan for providing micro tooling for this experimental study. Part of this research is sponsored by National Science Council of Taiwan under grant No. NSC 96-2221-E-006-273
References [1] Chen, C.H., Gau, J.T., Lee, R.S., 2008, “Tensile and micro bending stretch bending experiments for studying stainless steel 304 foil for micro sheet forming”, Submitted to 2008 International Manufacturing Science And Engineering Conference. [2] Geiger, M., Kleiner, M., Eckstein, R., Tiesler, N. and Engel, U., 2001, “Microforming”, Keynote Paper, Annals of the CIRP, 50-2, pp. 445-462 [3] Vollertsen, F., Hu, Z., Schulze Niehoff H. and Theiler, C., 2004, “State of the art in micro forming and investigations in micro deep drawing”, Journal of Materials Processing Technology, Vol. 151, Issues 1-3, pp. 35–44. [4] Lee, R.S., Chen, C.H., Song, Y.F., 2007, “Investigation of Micro Sheet Metal Forming of Ball Retainer using Finite Element Analysis”, Proceeding of the 35th International MATADOR Conference, pp. 101-104. [5] Vollertsen, F., 2001, “Metal Forming: Microparts”, Encyclopedia of Materials, Science and Technology [6] Michel, J.F., Picart, P., 2003, “Size Effect on the Constitutive Behavior for Brass in Sheet Metal Forming”, Journal of Materials Processing Technology, Vol.141, pp. 439-446. [7] Gau, J., Principe, C. and Wang, J., 2007, “An Experimental Study on Size Effects on Flow Stress and Formability of Aluminum and Brass for Microforming,” Journal of Material Processing Technology, Vol. 184, pp. 42-46. [8] Saotome Y, Yasuda K, Kaga H, 2001, “Microdeep Drawability of Very Thin Sheet Steel”, Journal of Materials Processing Technology, Vol. 113, pp. 641-647. [9]Manabe K, Shimizu T, Koyama H, 2007, “Evaluation of Milli-Scale Cylindrical Cup in Two-Stage Deep Drawing Process”, Journal of Materials Processing Technology, Vol. 187-188, 12, pp. 245-249. [10]Metallography and Microstructure, ASM Handbook, 1992, 9, ASM International, pp. 534-535
