DEEP DRAWING PROCESS SIMULATION OF TAILOR WELDED BLANKS OF DIFFERENT MATERIALS IN TOOL WITH ELASTIC BLANKHOLDER

Abstract

The current trend in the vehicle construction related to weight reduction requires the application of high-strength steels and tailor welded blanks (TWB) for particular body parts. However, the application of TWB brings possible complications in forming process. The differences of mechanical properties of the individual parts of TWB result in non-constant material flow and consequently a negative movement of the weld interface. One of the ways of elimination of this negative effect is to choose a suitable blankholder system with optimal distribution of blankholder forces. Finite element methods (FEM) based simulation has an irreplaceable role in the study of formability of TWB whereby we can determine the values and points of application of the blankholder forces. In this contribution the results of FEM simulation computed by LSDYNA code are presented. The simulations have focused on ways of minimisation of the weld interface movement of TWB in the tool with the elastic blankholder. [PUBLICATION ABSTRACT]

Headnote

Abstract: The current trend in the vehicle construction related to weight reduction requires the application of high-strength steels and tailor welded blanks (TWB) for particular body parts. However, the application of TWB brings possible complications in forming process. The differences of mechanical properties of the individual parts of TWB result in non-constant material flow and consequently a negative movement of the weld interface. One of the ways of elimination of this negative effect is to choose a suitable blankholder system with optimal distribution of blankholder forces. Finite element methods (FEM) based simulation has an irreplaceable role in the study of formability of TWB whereby we can determine the values and points of application of the blankholder forces. In this contribution the results of FEM simulation computed by LSDYNA code are presented. The simulations have focused on ways of minimisation of the weld interface movement of TWB in the tool with the elastic blankholder.

Keywords: vehicle construction, high-strength steels, TWB, FEM

1. INTRODUCTION

The use of tailor welded blanks (TWB) of advanced steels is becoming more popular in the automotive industry because of their advantage of saving weight and costs by combinig different thickeness or/and material properties. Material suppliers are claiming that demands for TWB are increasing. Nowadays the amount of parts of body structure made of TWB is between 15 to 20%. This number has potential to grow until 30% in next 5 years [1].

The use of TWBs is associated with many advantages as the ease of manufacturability and assembling steps reduction. On the other hand, their following treatment by forming procedures brings several problems of decreased formability and ductility of weld interface, non-uniform material flow that results in weld line movement towards thicker or "stronger" side of the part.

At the Institute of technologies and materials we are studying the problem of the optimisation of deep drawing process of such tailored blanks. To observe the behaviour of TWB while forming process of complex shaped parts would not be practical. For that reason a simple rectangular box (Figure 2) was chosen as an experimental forming shape so that any non-uniformity of material flow could be easily observed and evaluated.

In practice, several methods are used to stabilize or minimise the weld line movement. In our field of research we are focused on methods using non-uniform blankholder force that occurs when the load is applied on eight pressure pins and an elastic blankholder plate.

2. EXPERIMENTAL TOOL AND SIMULATION

A special experimental tooling was designed and installed on hydraulic double-acting press (Figure 3.a). The configuration of the device is so-called "inverted - single-action drawing with a draw cushion". The simulation model represents classical non-inverted case (Figure 3.b) - "exploded" for better comprehensibility. Eight pressure pins, transfer the blankholder force from the draw cushion onto the blankholder. [4] The blankholder plate deforms elasticly during the process. This concept allows an elastic adaptation of plate when using a different thickness TWB and by means of high adjustable pressure pins it enables to set a convenient blankholder force distribution [5]. This set up is controlled by elongation or shortening of the pressure pins equipped with piezoelectric load cells.

To study the behaviour of TWB while forming process, many experiments have to be done. The majority of experiments on TWB fomability is done virtually by means of FEM simulation software. There is more specialized sheet metal forming simulation softwares. Using specialized sheet metal forming FEM software for deep drawing simulation of TWB in the tool with elastic blankholder plate is limited or even impossible because of automatic rigid material for all tool parts including blankholder. This problem was solved by editing the tool material to elastic one directly in the exported LS-Dyna code before it was computed by solver. Section shell property of the blankholder, shell thickness, had to be edited as well.

For better understanding the behaviour of TWB with different material properties we choose materials of same thickness 1 mm and of significantly different true stress - true strain characteristic (Figure 4).

Advanced High Strength Steel - AHSS (red) is of three times higher values of true stress than the mild steel (blue).

In the case of blankholder force that is uniformly distributed around the flange of TWB (Figure 5.a), at the end of deep-drawing process the resultant weld line displacement on the box bottom (blue line) is about +3 mm in X direction (Figure 6.a,b) to the side of the material with higher strength characteristics (red one).

In order to minimise the weld line movement we repeated the simulation with non-uniform blankholder force distribution by applying additional load on the leftpin (Figure 5.b). When the load applied on the leftpin achieved value of 26 kN, the weld line movement represented by the nodal displacement in X direction on the box bottom was practically zero (Figure 6. c). Greater load causes even negative displacement of the weld line and excessive and unfavourable stretch of flange on the side of the "weaker" material (blue).

3. CONCLUSION

The simulation is in accordance with predicted material flow of TWB during the process. We managed to simulate elastic behaviour of the blankholder plate (Fig. 7) and its influence on TWB's forming process. By means of adjusting the appropriate non-uniform force distribution, the weld line on the bottom of drawn part was prevented from moving. These results are supposed to provide trustworthy input parameters for planned experiments of real deep-drawing process.

References

REFERENCES

[1.] MONACO, A., SINKE, J., BENEDICTUS, R.: Experimental and numerical analysis of a beam made of adhesively bonded tailor - made blanks. International Journal of Advanced Manufacturing Technology 2009; 44 : 766 - 80.l

[2.] www.worldautosteel.org

[3.] ZITNANSKÝ, P.: Simulácia tvárnenia súciastok z plosných polotovarov novej generácie, Dizertacná práca, Ústav technológií a materiálov SjF STU Bratislava, 2009.

[4.] CEKAN, P.: Computer Simulation of Deep Drawing Processes, Slovak University of Technology, Bratislava, May 2005.

[5.] ZITNANSKÝ, P., KOSTKA, P., SCHREK, A.: Deep Drawing of Combined Tailored Blank from Highstrength Steel, Hutnické listy - Metallurgical Journal, vol. LXIII, 2010, (61-64)