Effect of anisotropy on the hydroforming of aluminum alloy AA6061-T6 using newly developed method

Transcription

1 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech Effect of anisotropy on the hydroforming of aluminum alloy AA661-T6 using newly developed method Sh. A. Dizaji 1, F. Djavanroodi 2, H. Darendeliler 3 1 Department of Mechanical Engineering, Faculty of Engineering, Middle East Technical University, Ankara, Turkey 2 Department of Mechanical Engineering, Faculty of Engineering, Iran University of Science and Technology, Tehran, Iran 3 Department of Mechanical Engineering, Faculty of Engineering, Middle East Technical University, Ankara, Turkey Abstract Sheet metal forming of light- weight materials like Aluminum and Titanium alloys is one of the most desirable issues lately. There are so many determinative parameters in the hydroforming of these materials. Normal and planar Anisotropy are some of these important parameters which have been investigated in this paper. Thickness distribution of the cup walls have been measured in three directions. It was found that the o respect to rolling direction is prone to fracture in the cups and changes of the punch nose radius cannot change the place of fracture. The experimental results have been compared to finite element simulations and have good agreement with them. Keywords: Sheet metal, hydroforming, anisotropy, thickness distribution, aluminum alloy INTRODUCTION Cup drawing as a simplest process to be investigated theoretically is commonly used method to study the deep drawing parameters by most of the researchers. One category of these processes makes use of hydraulic pressure to improve the basic deep drawing process. These generally increase the draw ratio possible and minimize the thickness variation of the cup formed, in addition to other advantages associated with them [1-2]. Also In the automotive and aerospace industry, there have been continuous demands for the use of lightweight materials. The use of lightweight materials such as aluminum (Al) and magnesium (Mg) can reduce the weight of passenger vehicles by replacing ferrous auto body structures and body panels [3].

2 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech Thus hydroforming process is a good method to form aluminum alloys and has gained more attentions recently. There were so many methods for hydroforming as Zhang et al. [4] have showed some of them. R.Hill [] introduced a quadratic anisotropic yield function to apply anisotropic characteristic of materials to sheet metals. Using a numerical simulation, hydroforming process can be studied and developed systemically, which will be very helpful to the practical application in industry especially metal forming. Abaqus, commercial FE software [6], has been used to model the developed hydroforming model and simulate the process. Hydroforming assisted by floating disk, is a newly developed method that has been carefully investigated by the authors [7]. The related procedure of the new method has been discussed extensively in the referred article and in this article the method will be discussed briefly. MATERIALS AND METHODS The essential tools include a punch, a blank holder, a pressure chamber, a rubber diaphragm, and a floating disk, as shown in (Fig. 1). The diaphragm at the bottom can move up and down due to the pressure of the viscous medium in the chamber; therefore, it makes the disk move up and down. The blank is placed between the blank holder and the floating disk. The blank holding force (BHF) due to the pressure of the chamber and the area of the floating disk can press the blank tightly to the blank holder. As the punch moves down, forming a cup, a control valve regulates the liquid flow, and the blank holding force can consequently be controlled. All of the experiments were carried out using a 2- ton hydraulic double-action press. Figure 2 shows the equipment used in this process [7]. The material used in this work is an Al Mg Si aluminum alloy, AL661-T6, with a thickness of.8 mm. The reason for selecting the AA661 is the wide use of this alloy in the Aerospace industry. Table 1 displays the properties of the material obtained from a uniaxial tensile test based on ASTM E.8 and ASTM E.17. Explicit FE code was used to simulate the process. Because of the symmetric character of the forming, only a quarter of the model was used. All tools were modeled using an analytical rigid,

3 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech Fig. 1 The hydroforming process assisted by a floating disk Fig. 2 Tools and hydraulic press Tab. 1 Properties of the material Al661-T6 Parameters Density (kg/m 3 ) Yielding stress (Mpa) Ultimate tensile stress (Mpa) Strain hardening exponent,(n) Hardening coef., k(mpa) Total elongation (%) Poisson s ratio (ν) Anisotropy factor (r) Rolling direction o o 9 o

4 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech and the materials were modeled using C3D8R (an 8- node linear brick with reduced integration) elements for modeling anisotropic effects in sheets. Anisotropy options calculated according to ASTM-E17 and r, r and r 9 were used to calculate F, G, H, N, L and M parameters which are material constants in Hill48 yield function []. Hill s potential function is an extension from the Mises function and can be expressed as, f ( ) G F( ) ( ) H( ) 2L 2M 2L (1) Where σ ij denote the stress components, these constants can be expressed in terms of six yield stress ratios R 11, R 22, R 33, R 12, R 13 and R 23 according to equations (2-). In sheet metal forming, anisotropic material data is defined in terms of ratios of width strain to thickness strain commonly. The stress ratios can then be defined as equation (6). Then these calculated ratios can be interred to software directly to simulate anisotropic material based on Hill criteria in ABAQUS. Due to these anisotropic parameters and also planner anisotropy effect, four ears will be created in the cups (Fig. ) F ( ), 2 R R R G ( ), 2 R R R H ( ), 2 R R R L, M, N 2R 2R 2R (2) (3) (4) () R R R R R ( r 1) , 22, r ( r9 1) r ( r 1) 3 r ( r 1), R (2 1)( ) r9 r r r9 r r (6)

5 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech RESULTS AND DISCUSSION Three types of punch (Fig. 3) were used to study the effect of anisotropy on the thickness distribution of the cups. In the Fig. 4, the place of fracture in the deep drawn cups has been shown schematically. The region encircled in the illustrated figure is where normally fracture mode takes place in the cups especially in the first stages of the forming. The successfully drawn cups and their simulations have been illustrated in the Fig.. Three directions have been marked on the specimens and shown in Fig. 6. (a) (b) (c) Fig. 3 Three types of punch used in the test Fig. 4 schematic of expected fracture place in the drawn cup

6 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech Punch type (a) Punch type (b) Punch type (c) Fig. Successfully drawn and simulated cups Fig. 6 Three directions of anisotropy on the cups Thickness distributions of the specimens have been shown in Fig. 7. The presented arrows in the simulated diagrams show exactly where necking (Fig. 4) takes place in the cup s wall just before

7 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech the fracture at the early stages of the forming process. These places are very prone to fail because the thickness strain is higher than the other places in the wall Punch (a) Punch (b) Punch (c) Fig. 7 Thickness Distributions, Left: Simulation, Right: Experimental The other important thing which can be understood from the diagrams in the Fig 7, is that the thickness strain at o from the rolling direction is higher than the thickness strain at the other two

8 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech directions ( o and 9 o ) and if fracture takes place in the cup at the early stages of forming, it probably will be in o direction (Fig. 8). Furthermore, paying attention to shape of the diagrams in Fig. 7 will lead us to this fact that changing nose radius of the punch or shape of it, hardly affects the place of necking which is where the punch profile starts to be shaped or just after it was shaped. Fig. 8 Fracture at early stages of the forming process CONCLUSION Effect of anisotropy on the hydroforming of AA661-T6 was investigated and its role on the creation of ears and different thickness distributions was presented experimentally and numerically by diagrams and figures and good results were obtained. 1. The weak regions of the cup which are prone to fail at the early stages of the forming were detected. 2. The direction, in which the ears come to be created ( o ), is where the fracture will be started in the cup wall firstly. 3. Punch nose radius or shape has no effect on the place of necking in the cup wall at the early stages of the forming process.

9 th International Mechanical Engineering Forum Prague 212, 2-22 February 212, Prague Czech 4. Simulation results had good agreement with the experimental ones and proved that costly experimental tests can successfully be replaced with FEM models. REFERENCES 1. YOSSEFIN S., TIROSH T.: On the permissible fluid-pressure path in hydroforming deep drawing processes-analysis of failures and experiments. Trans. ASME J. Eng. 1988, 11: TIROSH T., YOSSEFIN S., Eshel R., Betzer A.A.: Hydroforming process for uniform wall thickness products. Trans. ASME., 1977, 99: CARPENTER J.A.: The Freedom CAR Challenge and Steel, American Iron and Steel Institute. Great Designs in Steel Seminar, Livonia, MI, 24, pp ZHANG S.H., DANCKERT J. Development of hydro-mechanical deep drawing. Int. J. Mech. Sci.,1998, 4: HILL R.: The Mathematical Theory of Plasticity, Oxford University Press, London, ABAQUS Explicit User s Manual, Version 6.1, Hibbit, Karlsson & Sorensen, Inc., Pawtucket, R.I., DJAVANROODI F., ABBASNEJAD D. Sh., NEZAMI E. H.: Deep Drawing of Aluminum Alloys Using a Novel Hydroforming Tooling, Mat. and Manuf. Proc., 211, 26: Corresponding author: Shahram Abbasnejad Dizaji, Ph.D., Department of mechanical engineering, Faculty of Engineering, Middle East Technical University, 631, Ankara, Turkey, Tel: (9) Abbasnegzad@gmail.com