Advanced Materials Manufacturing & Characterization. Two dimensional complex shape analyses during low pressure tube hydroforming

Transcription

1 Advanced Materials Manufacturing & Characterization Vol 6 Issue 2 (2016) Advanced Materials Manufacturing & Characterization journal home page: Two dimensional complex shape analyses during low pressure tube hydroforming 1* C. Nikhare, 2 M. Weiss and 2 P D. Hodgson 1Mechanical Engineering Department, The Pennsylvania State University, The Behrend College, Erie, PA Institute for Technology Research and Innovation, Deakin University, Waurn Ponds 3217, Australia Abstract Hollow products are increasingly employed for automotive parts to reduce vehicle weight. Hydroforming technology is widely used for forming complicated or complex components from tube blanks. Complex shape can be easily formed with the help of high pressure. However high pressure equipments and high tonnage press are the resistance to product cost benefits. Whereas low pressure tube hydroforming can also achieve some of the complex shape with minimum pressure requirements and lower die closing force. This work represents the forming of two dimensional different complex shapes by using the low pressure tube hydroforming. The die filling conditions were critically studied. It is determined that for the complex shape with low pressure tube hydroforming slightly lesser perimeter length is required which allows for weight reduction. Stress and thickness distribution of the part during tube crushing were analysed. Keywords: Advanced High Strength Steels; Hollow products; Tube hydroforming; Complex shapes; Low pressure. Introduction Lightweight design is a key solution for the modern manufacturing technology, especially in the automotive industry [1]. The automotive industry is facing ongoing pressures to reduce weight while maintaining crash and other performance characteristics. For stamped components there are many challenges in developing complex, light weight components from high strength materials compared to alternative production processes hydroforming is becoming increasingly used, because it can be applied to the manufacturing of complex structural components in a single body with high overall stiffness [2, 3]. Tube hydroforming has been successfully developed to manufacture the components for automotive vehicles [4], where it is also called a soft-punch forming technology [5]. Corresponing author: C. Nikhare address: cpn10@psu.edu Doi: Publications. All rights reserved. Apart from the many advantages, main drawback of hydroforming is that it generally requires high capacity mechanical presses and high pressurization systems. There are three pressurization systems which are used commonly for tube hydroformed products: 1) High Pressure hydroforming (P = 83 to 414MPa) 2) Multi-pressure hydroforming (P = 69 to 173MPa) and 3) Low pressure hydroforming (P<83MPa) [6]. Higher pressures allow the hydroformed section length of line to be expanded up to the limit allowed by the material s ductility. During multi-pressure hydroforming tube is preformed and then expanded to a desired shape by pressurizing the fluid. In Low pressure hydroforming the hydroformed section length of line stays almost the same as the circumference of the blank tube [6]. At present most work focuses on the high pressure forming due to its ability to produce more complex shapes. Several studies on the production of complex shapes [7-9] in high pressure tube hydroforming have revealed difficulties in the designing of the tube diameter and/or thickness leading to a product without thinning and bursting. Axial material feed has been used to reduce the thinning in critical corners [5] or bend sections [10] in high pressure hydroforming. Pre-formed shapes have also been applied to improve the formability [11]. Axial punch stroke designs with respect to applied pressure [12] for different complex shape are quite difficult and required clear understanding of the process. To achieve weight reduction the first generation of advanced high strength steel (AHSS), such as dual phase and transformed induced plasticity steels have been developed. The advantages of these steels are high strength with good formability. The minimum measured yield strength for these steels is around 450MPa. However, it has been observed [13, 14] that pressure required to deform the mild steel simple shapes from 37

2 conventional high pressure hydroforming is already higher, then it is expected that it will be much higher with these new AHSS steels. It is known that many companies are considering the hydroforming of these steels but the publications in the literature are limited and generally focus more on applications. Several investigations have been performed to reduce the pressure required to hydroform mild steel. The concept of a sequenced forming pressure is proposed to reduce the thickness variation of the product and the forming pressure [15]. The applied internal pressure and die closing force needed in the crushing processes combined with preforming and hydroforming are only about less than 10% and almost 50%, respectively, of those in the expansion test [13, 14, 16-18]. Thus to deform the AHSS steels by hydroforming, low pressure tube hydroforming appears to be a promising option. The plastic flow pattern, thickness distribution of the tube and forming pressure required have been studied on simple square and triangular shapes; however forming of different complex shapes during low pressure tube hydroforming has not yet been performed. Also the design of tube diameter and thickness for the complex shapes are still areas requiring more work. In this paper, five different two-dimensional complex shapes during low pressure tube hydroforming on TRIP steel have been studied. As the required pressure is very low, constant pressure was applied to form the desired complex shape in which the pressure remains constant throughout the process until the tube forms completely. The corner filling condition were analysed during the process. The die closing force required to form the tube was predicted along with the thickness distribution. Table 1 Mechanical properties of TRIP steel 2.2 Methodology The low pressure hydroforming of complex shapes was numerically investigated using Abaqus/Explicit. Thereby the die filling conditions were analysed for each case with respect to the defined pressure Low Pressure Tube Hydroforming (LPTH) In low pressure tube hydroforming, a tube is formed into the desired shape using low fluid pressure. For this, the tube is placed between the upper and lower die. During forming the lower die is fixed; while the upper die moves down and forms the pressurized tube into the desired shape. Thereby the applied pressure is kept constant throughout the process. A schematic view of the low pressure hydroforming process is shown in Figure 2. In this process, the hydroformed section length of line stays approximately the same as the circumference of the undeformed tube. Thus the perimeter of outer un-deformed tube must be same as that of the inner perimeter of the die (equation 1). Perimeter of Final Product = Perimeter of Initial tube for LPTH (ЛxD) (1) 2. Material and Methodology 2.1 Material The forming of TRIP 780 with an overall thickness of 1mm was investigated in this study. The true stress-strain curve used for the numerical investigation is shown in Figure 1 while the mechanical properties can be found in Table 1. Figure 2 Preform tube and start of Low Pressure Tube Hydroforming (LPTH) Figure 1 True stress-strain curve determined in tensile tests for TRIP steel Complex shapes The tool filling conditions in the low pressure hydroforming of five complex shapes (Figure 3) was analysed in this study. Geometry 1 was the simplest shape. Geometry 2 and 3 were the unique complex shape while remaining three were the replica of other three shapes. Thus the overall analyses of five complex shapes were done. For this the three different die shapes, shown 38

3 in Figure 4 and 5, were combined appropriately. The part shapes had an overall width and height of 48 and 16mm and all die corner radii were 6mm. The die was considered as a rigid body while the tube was deformable (Figure 6). CPE4R 4-node bilinear plane strain quadrilateral, reduced integration, hourglass control elements were used. The approximate element size for the tube was 0.5mm with a curvature control of 0.1 and there were two layers of elements in the thickness direction. The interaction between die and the tube was surface to surface tangential contact and a coefficient of friction of 0.1. Radial expansion and compression of the tube was allowed. Internal pressure of the tube and the upper die movement were applied simultaneously and proportionally. Constant pressure was applied throughout the process. The calculated results, for all end conditions considered, are symmetrical to the mid-section of the tube. Figure 3 Different Complex shapes Figure 4 Upper dies used to form complex shape Figure 5 Lower dies used to form complex shape As in low pressure hydroforming, the circular perimeter of the initial tube needs to be equal to the perimeter of the 2D section perpendicular to the thickness of the final part. Thus by perimeter equivalence (equation 1), the calculated outer diameter of undeformed tube for different complex shapes was bigger than 48mm (width of the die), thus preforming was required to fit the tube in the lower die before crushing. Therefore in the numerical model the tube is first pre-formed and then pressurized and formed into the final shape (Figure 6) Numerical Modelling In this study ABAQUS/Explicit was used and the forming process was simulated with a 2D-model approach. Thereby any variation in wall thickness and material properties around the circumference of the tube was neglected. The wall thickness of the tube was taken to be 1mm and true stress-strain data (Figure 1) was used to simulate the model. Figure 6 Tube crushing model 3 Results and discussion 3.1 Complex shapes Forming results of five different complex shapes formed during low pressure tube hydroforming are shown in Figure As can be seen in figures, the die are not fill completely with the calculated diameter for equivalent die perimeter (Dcal) and giving excess material which affects the dimensionality and results in un-filled regions for all investigated final shapes. Complex shape 2, 3 and 6 are having 12 corners and most complex geometries in all five cases. However by using slightly reduced diameters improved die filling can be achieved. In further study the tube diameter giving the best possible filling conditions is considered as optimum diameter (Dopt) for each individual case. Figure 7 Complex shape 2 Dcal = Calculated diameter for equivalent die perimeter Dopt = Optimum diameter 39

4 Dcal is calculated by equation 1. Л * Dcal = Die inner perimeter The required profile A ; profile of original calculated diameter (Dcal) B and profile of optimum diameter (Dopt) C is shown in Figures The optimum diameter perimeter is less than the required perimeter to fill the shape and thereby giving the reduction in weight. Figure 11 Complex shape 6 Figure 8 Complex shape 3 Figure 12 Required optimum and calculated diameter for different complex shape Figure 9 Complex shape 4 Table 2 Optimum diameter and reduced perimeter for five different complex shapes Figure 10 Complex shape 5 Figure 12 shows the tube diameter required to for the individual complex part while simulating with Dcal and Dopt. With Dopt, all investigated shapes fill the die with negligible imperfection. The Dopt line equation is based on the five considered complex geometries. This equation is not a general optimum equation. It is clear that more complex shape required better understanding of important design parameters during low pressure tube hydroforming. As can be observed in Table 2, the Dopt with reduced perimeter with respect to the Dcal for different complex 40

5 shapes indicates the weight reduction for each part. The die closing force required for the most complex shape, i.e. part 3 is more than the other investigated complex shapes (Table 3). This indicates that in low pressure hydroforming the die closing force depends on the part complexity. Table 3 Required preform and die closing force for optimum condition for five different complex shapes Figure compares the thinning for the complex shape for Dcal and Dopt. Thinning occurs in both cases as the fluid slightly restricts the material from feeding from section B to section D. As the punch travels down, the material is stretched to fill the gap. Thus, with the Dcal, (the tube outer perimeter is same as the perimeter of the die) the material gone excess and ended with an unfilled regions. However with the Dopt, (the tube outer perimeter is less than the perimeter of the die) the die is filled completely by thinning the material. Figure 14 Thickness distribution of complex shape 3 Figure 13 Thickness distribution of complex shape 2 Figure 15 Thickness distribution of complex shape 4 41

6 Figure 17 Thickness distribution of complex shape 6 Table 4 shows that part complexity (i.e. more corner radius) is independent to the reduced perimeter. Thus part complexity factor didn t affect much on the weight reduction, as part with 12 corners; the reduced perimeters are different. Thereby it is clear that weight reduction depends on the part geometry design. As in part 3, the punch is pushing the material inside during the forming and had the maximum surface contact than any other part, while at the same time the fluid force is resisting the material to feed in, might be the reason to get more reduced perimeter i.e. more reduction in weight. Table 4 Number of part corners with reduced perimeter for optimum condition for five different complex shapes Figure 16 Thickness distribution of complex shape 5 With Dcal, the material is thickening in section A as the punch is giving the compression load, but in section B, C and D, the material thins and stretches due to punch force and fluid pressure, shows excess material inside the shape with an unfilled regions. Conclusion The low pressure hydroforming of five different part shapes was numerically investigated and the die filling conditions were studied. It is shown that for complex shapes with low pressure tube hydroforming slightly lesser perimeter length is required to fill the die completely which also allows for weight reduction. The forming and design conditions for the given complex shapes might not be the optimum for any other generalised complex shapes during low pressure tube hydroforming. Thickness distribution of optimum diameter for the part during low pressure tube hydroforming were analysed and compared with calculated diameter from equivalent die perimeter. It is observed that thinning occurred in the part and made the reason for weight reduction. It is also observed that weight reduction in complex part during low pressure tube hydroforming only depends on the part geometry. Thus overall conclusion is that friction of the die wall tends to cause thinning in the tube (although thickening is possible in isolated cases). Therefore the appropriate starting tube size may be different from that calculated assuming a constant perimeter length. 42

7 Acknowledgements This research was supported by Deakin University and AUTOCRC. The authors gratefully extend their gratitude to Professor John L Duncan from Auckland University, New Zealand. Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper [16] Nikhare C, Weiss M, Hodgson PD. Numerical investigation of high and low pressure tube hydroforming. Proc. of Numisheet, Switzerland, September 2008; [17] Nikhare C, Weiss M, Hodgson PD. Experimental and Numerical investigation of low pressure tube hydroforming on stainless steel. Steel Research International 2008; [18] Nikhare C, Weiss M, Hodgson PD. FEA comparison of high and low pressure tube hydroforming of TRIP steel. Computational Materials Science 2009; 47: References [1] Kleiner M, Geiger M, Klaus A. Manufacturing of Lightweight Components by Metal Forming. Annals of the ClRP 2003; 52/2: [2] Schuler. SCHULER Metal Forming Handbook. Germany: Springer-Verlag Berlin Heidelberg; [3] Lucke HC, Hartl C, Abbey T. Hydroforming. Journal of Materials Processing Technology 2001; 115: [4] Kim TJ, Yang DY, Han SS. Numerical modelling of multi-stage sheet pair hydroforming process. Journal of Materials Processing Technology 2004; 151: [5] Dohmann F, Hartl C. Tube hydroforming research and practical application. Journal of Materials Processing Technology 1997; 71: [6] Singh H. Fundamentals of Hydroforming. Michigan: Association for Forming and Fabricating Technologies of the Society of Manufacturing Engineers; [7] Merklein M, Geiger M, Celeghini M. Combined tube and double sheet hydroforming for the manufacturing of complex parts. CIRP Annals - Manufacturing Technology 2005; 54(1): [8] Kang BH, Lee MY, Shon SM, Moon YH. Forming Various Shapes of Tubular Bellows Using a Single Step Hydroforming Process. Journal of Materials Processing Technology 2007; 194: 1-6. [9] Yuan SJ, Liu G, Huang XR, Wang XS, Xie WC, Wang ZR. Hydroforming of typical hollow components. Journal of Materials Processing Technology 2004; 151: [10] Yuan SJ, Han C, Wang XS. Hydroforming of automotive structural components with rectangular sections. International Journal of Machine Tools and Manufacture 2006; 46: [11] Song WJ, Heo SC, Kim J, Kang BS. Investigation on preformed shape design to improve formability in tube hydroforming process using FEM. Journal of Materials Processing Technology 2006; 177: [12] Hama T, Ohkubo T, Kurisu K, Fujimoto H, Takuda H. Formability of tube hydroforming under various loading paths. Journal of Materials Processing Technology 2006; 177: [13] Hwang YM, Altan T. Finite element analyses of tube hydroforming processes in a rectangular die. Finite Elements in Analysis and Design 2003; 39: [14] Hwang YM, Altan T. FE simulation of the crushing of circular tubes in triangular cross-sections. Journal of Materials Processing Technology 2002; : [15] Mason M. Tube hydroforming using sequenced forming pressures. Proc. of the International Seminar on Report Status and Trend of Tube Hydroforming, Tokyo, Japan;