Research on Relationships Between Fluid Pressure and Technological Parameters, Shape of Cylindrical Part in Hydro Static Forming

Journal of Science & Technology 127 (2018) 001-005 1 Research on Relationships Between Fluid Pressure and Technological Parameters, Shape of Cylindrical Part in Hydro Static Forming Nguyen Thi Thu*, Nguyen Dac Trung Hanoi University of Science and Technology, No. 1, Dai Co Viet, Hai Ba Trung, Hanoi, Viet Nam Received: September 13, 2017; Accepted: May 25, 2018 Abstract In sheet hydrostatic forming, accuracy of products depends on many technological parameters and shape of die, especially working fluid pressure plays very important role. Determination of this pressure is usually difficult, because it depends on the other parameters, such as blank holder pressure, workpiece material, geometrical shape and tightness of die, so on. This article deals with determination of relationship between forming pressure, blank holder pressure and radius of bottom die using experiments. The results of this study are the basis for further validating the quality and accuracy of product, as well as optimizing hydrostatic forming technology. Keywords: forming pressure, blank holder pressure, sheet hydro static forming. 1. Introduction* In hydrostatic forming, high pressure fluid works as punch (in conventional stamping hard die and punch) and has direct impact on surface of workpiece to deform it following to the die cavity [1]. Hence, the shape of die and working fluid pressures will play an important role for filling of material into small corner and complexity location of the die, while the traditional stamping technology is not able to do this. Therefore, it normally requires many steps for forming of complex parts by using conventional stamping technology, but those parts can be achieved with only one step by using hydrostatic forming. The principle of sheet hydrostatic forming is shown in Fig. 1. Fig 1. Process of sheet hyrostatic forming [2] Sheet hydrostatic forming shows many advantages in comparison to conventional stamping, * Corresponding author: Tel.: (+84) 976512385 Email: thu.nguyenthi@hust.edu.vn such as: enhance surface quality (avoid scratch on surface), decrease elastic deformation, especially suitable for forming complex profiled products[3] [4]. However, this technology has some disadvantages, for example local strong thinning, so that the product thickness it not equal [5]. Therefore, this technology is appropriately applied to manufacture body shells of car [6]. Many developed countries like USA, Germany, Japan, Korea, China are using this technology in the industrial fields such as defense, transportation, aerospace, home appliances ... With development history of more than one hundred years, this technology is concerned both aspects research and application. Research objects are diversity (technological process parameters, effect of friction force, materials, product quality...)[7] In Vietnam, this technology has been researching for more than 20 years, but actually, it has just drawn attention in the last 5 years. Lately, some research has been done on this technology for forming products of sphere, conical and asymmetric shape [8,9]. Meanwhile, cylindrical shape has not been studied thoroughly. Therefore, cylindrical part will be researched to investigate the effect of some technology parameters on product quality by experiment method. Specifically, effect of blank holder pressure on forming pressure and effect of forming pressure on the relative radius at the bottom of product. Finding out the relationship among technological parameters in hydrostatic forming for sheet metal is essential, due to wide application of this techmology into manufacturing, especially thin sheet industry. Journal of Science & Technology 127 (2018) 001-005 2 2. Experiments Since hydrostatic forming is only suitable for products with small depth, a low cylindrical product is chosen for investigation as shown in Fig.2 a. b. Fig 2. Investigated product: a- dimension; b- 3D product Cylindrical product with the thickness of 0.8 mm, material of DC04 steel – a kind of material commonly applied in car production with chemical composition is shown in Table 1, specification in Table 2 is choosen for experimental investigation. Table 1. Chemical composition hóa học of DC04 steel C (%) Mn(%) P(%) S(%) max 0.08 max 0.4 max 0.03 max 0.03 Table 2. Specifications of DC04 steel Mechanical behavior Equivalent quality Rm (Mpa) Re (Mpa) δ (%) Russia-GOST 08kp Japan-JIS SPCE 314-412 210-220 38 With: Rm – Ultimate strength (Mpa) Re – Yield strength (Mpa) Objectives: Experiments are implemented to investigate: - Relationship between blank holder pressure and forming pressure. - Relationship between forming pressure and relative radius at the bottom of product. Experimental devices: The following technological parameters will be determined: - Working fluid pressure in the die Pth = 0÷550 bar. - Blank holder pressure Qch = 0÷150 bar. - Radius at the bottom of die R = 6 mm – expected radius for product. Experiments are implemented in laboratory of Department of Metal Forming, School of Mechanical Engineering, Hanoi University of Science and Technology. After computation and design, the experiment system consists of 4 main modules as shown Fig. 3: - Pump system for supplying high pressure liquid with Pmax = 700 bar [8] - Hydraulic press 125 ton [8] - System for measuring pressure – displacement signals [8] - Die system include die and blank holder as shown in Fig 4. Fig 3. Experiment system for hydrostatic forming a) b) c) Fig. 4 a. Blank holder b. Hydrostatic die c. Die assembly Method of investigation: Using experiment system established to match with the selected product. Statistics are presented in Section 3: Results and discussion Journal of Science & Technology 127 (2018) 001-005 3 3. Results and disscution 3.1 Establishing relationship between blank holder pressure and forming pressure In usual forming, blank holder is used to keep blank stable when blank is drawn into die. Moreover, blank holder is also used to avoid loss of pressure during forming process. Requirement for the product is that, thinning should be less than 20% ( 20%  ), radius of product bottom Rd = 6.00 mm with tolerance +10%. Thinning is calculated by the following formula [10]: ( )0 0*100 /s s s = − Where: s0 – the initial thickness of blank (mm) s – resulting thickness of product (mm) The experimental system is connected to the measuring system through the Dasylab software. Outputs from sensors for blank holder pressure, forming pressure and stroke are demonstrated on the monitor. Thereby, results are collected, and suitable values are then chosen. Based on experiment results, range of suitable blank holder pressure is defined Qch = (75÷125) bar corresponding to range of forming pressure Pth = (350 ÷520) bar. For measuring the radius at bottom: a number of points are measured by optical measuring method, and interpolation is made to obtain the product profile. Results of products meeting the requirement for Rd are shown in Table 3. For thinning ε: Thinning is investigated at the position I-I as shown in Fig. 5. It is recognized by experiments that this is the position having the biggest thinning. Results of products meeting the requirement for thinning are shown in Table 3. Fig. 5. Investigate the thinning at position I-I Table 3. Suitable values of Pth, Rd, ɛ that are defined experimentally along with corresponding values of Qch. Qch (bar) 75 85 95 115 125 Pth (bar) 350 400 460 500 520 Rd (mm) 6.50 6.31 6.14 6.00 6.08 ɛ (%) 17.5 15.34 10.12 8.75 5.78 From Table 3, the relationship between blank holder pressure and forming pressure is shown in Fig.6. Fig. 6. Relationship between blank holder pressure and forming pressure to achieve expected product radius Relationship between blank holder pressure and forming pressure is established. From Fig. 6, there is a tendency that forming pressure increases when blank holder pressure increases. The relation function is interpolated: y = -0.0589x2 + 15.143x - 455.31 (1) with high reliability coefficient: R2 = 0.9916 It is recognized by experimental investigation that, if the blank holder pressure Qch < 75 bar, it is impossible to find out the forming pressure Pth so that the product shape meets the requirement. Radii at bottom are all much greater than Rd = 6mm, as shown in Fig.7a In case of Qch > 125 bar, the blank can hardly be drawn into the die capital because the blank holder pressure is too high, and the great value of blank holder pressure requires a coresponding great value of forming pressure Pth. Consequently, the blank can get too much thinning, even can be cracked as shown in Fig. 7c. Therefore, range of blank holer pressure Qch = (75÷125) bar and range of forming pressure Pth = (350 ÷520) are suitable as shown in Fig. 7b. Journal of Science & Technology 127 (2018) 001-005 4 a. Qch <75 bar b. 75 ≤Qch ≤ 125 bar c. Qch >125 bar Fig. 7. Three specific products after three value domains of Qch are applied a. Qch <75 bar; b. 75 ≤Qch ≤ 125 bar; c. Qch >125 bar 3.2 Establishing relationship between forming pressure Pth and radius at the bottom of product (Rd/so) Relative curve radius is R/so where so: material thickness (mm) Rd: radius at bottom of product (mm) With different kinds of material, different working conditions, the relationships shown by graph and function are different. Here, with this experiment conditions and boundary conditions, a specific relationship between forming pressure and radius at the bottom of product are defined. Based on experiments as mentions in 3.1, different values of blank holder pressure are investigated to determine corresponding product radius: Qch = 75, 85, 95, 115, 125 (bar) Fig. 8. Dependence of relative curve radius (Rd/so) on working pressure at different values of blank holder pressure (experimental and tend curves) Fig. 8 illustrates dependence of relative radius (Rd/so) on forming pressure at different values blank holder pressure. Each value of blank holder pressure Qch is shown by 2 curves: experimental and trend curves. The trend of curves is similar, which means that the greater forming pressure, the smaller relative product radius. However, the graph shows that the curves are not homothetic. That means at each value of blank holder pressure, the law of forming pressure acting to form relative curve radius is different. Through experiment, it can be seen that blank holder pressure has important effect on forming of product radius. As the higher blank holder pressure, the better hermetical the die is kept, thus, forming pressure can be higher, forming smaller product radius. However, when blank holder pressure is too high, it is so difficult for material to fill into die, preventing the formation, causing thinning on the free part of material. Hence, although forming pressure can be obtained at high value (over 520 bar), it is almost impossible for the radius Rd to reach a small value as expected. In case blank holder pressure Qch = 95 bar, effect of forming pressure Pth on relative radius is shown in Fig. 9. Fig. 9. Dependence of relative product radius on forming pressure at the value of blank holder pressure Qch = 95 bar For the case Qch = 95 bar (Fig. 9), the relation function is interpolated: y = 4.10-5x2 - 0.0466x + 21.136 (2) with high reliability coefficient: R² = 0.9737 4. Conclusion Based on experiments, it is able to determine blank holder pressure depending on forming pressure in hydrostatic forming for low cylindrical product through relation function (1). Compared to Journal of Science & Technology 127 (2018) 001-005 5 conventional stamping, blank holder pressure in hydrostatic forming has the mission to keep working pressure stable and prevents leakage. Therefore, in terms of value, blank holder pressure in hydrostatic forming is bigger than in conventional stamping. The relationship between forming pressure Pth and relative radius at bottom of product Rd/so can be defined experimentally and demonstrated through relation function (2). At different values of blank holder pressure, the relationship is different, but the general trend is that the higher forming pressure, the smaller the relative radius. This trend is proved to be useful in product design and selection of forming equipments. Furthermore, other relationships such as effect of forming pressure on thinning, effect of forming pressure on relative height of product, so on, also need to be investigated to develop the research orientation. References [1]. Nghệ, Phạm Văn (2006), Công nghệ dập thủy tĩnh, NXB Bách Khoa HN. [2]. M. Kleiner and W. Homberg, “New 100,000kN Press for Sheet Metal Hydroforming,” Hydroforming of Tubes, Extrusions and Sheet Metals, ed. K. Siegert, Vol. 2 (2001), pp. 351 - 362. [3]. M. Geiger, J. Duflou, H.J.J. Kals, B. Shirvani and U.P. Singh (2005), Tube and Sheet Hydroforming- Advances in Material Modeling, Tooling and Process Simulation, p1-12 [4]. Zhang, S. H., et al. (2004), "Recent developments in sheet hydroforming technology", Journal of Materials Processing Technology. 151(1-3), pp. 237-241 [5]. Altan, T. and Tekkaya, A.E. (2012), Sheet metal forming process and applications, ASM International. [6]. Omar, Mohammed A. (2011), The Automotive Body Manufacturing Systems and Processes, John Wiley & Sons. [7]. Tolazzi, M. (2010), "Hydroforming applications in automotive: a review", International Journal of Material Forming. 3(1), pp. 307-310. [8]. Lê Trung Kiên, Đinh Văn Duy, Phạm Văn Nghệ, “Thiết kế, chế tạo hệ thống thiết bị dập thủy tĩnh chi tiết dạng tấm có hình dạng phức tạp”, Tạp chí Cơ khí – Tổng hội cơ khí Việt Nam, ISSN 0866 – 7056, Số 1+2, tháng 1+2/2013, trang 81-86. [9]. Đề tài 01C-01/07-2008-2 (2008), Nghiên cứu, thiết kế công nghệ dập thủy cơ để chế tạo các sản phẩm công nghiệp dạng 3 lớp kim loại có chiều dày và vật liệu khác nhau. [10]. [10] Nguyễn Mậu Đằng, Công nghệ tạo hình kim loại tấm, NXB Khoa học và kỹ thuật, 2008