Study regarding the numerical simulation of the deep drawing process for tailor welded blanks.

Tera, Melania ; Bologa, Octavian ; Oleksik, Valentin 等

1. INTRODUCTION

In order to simplify the manufacturing process, the usage of tailor-welded blanks (TWB) represents an optimal solution due to the fact that we can use one single welded part instead of using several distinct parts which would require a large number of processing stages during manufacturing (Geiger et al., 2008). Tailor-welded blanks allow the combination of different materials and different sheet thicknesses in a single part, thus allowing designers and engineers to meet, local needs of strength or other requirements in the deep-drawn components or the elimination of steps such as trimming operations after the forming process (Ku et al. 2005, Cheng et al., 2007).

TWBs made of steel offer many advantages such as reduction in cost, scrap and weight of the body components, as well as part consolidation and improvements in structural integrity and dimensional consistency. Steel TWBs can be made mostly by means of the laser welding process using C[O.sub.2] and Nd: YAG lasers (Bayraktar et al., 2005, Ozek et al. 2008).

Application examples for this are the lids of pressurised vessels, where the stresses in the area of the part's bottom are relatively high, and where the variant of a part realised of welded blanks.

Starting from this, the current paper aims to present the results of numerical simulations of the deep-drawing process of a circular tailor-welded blank. The study targeted especially the determining of the plastic strain, relative thinning, strain state, forming limit curve and the forces developed in the process.

2. NUMERICAL MODEL

The numerical simulation referred to the deep-drawing of a cylindrical flanged part by means of a cylindrical punch perpendicular on it. For the direct analysis method that was employed, it was necessary to model the die, the punch and the blank holder and to introduce the contacts and frictions between the active elements and the blank. Fig. 1. presents the simulated tools employed in the deep-drawing process simulation. Table 1 presents the general characteristics of these tools. It should be also mentioned here that the clearance between punch and die is 2.2 mm on the diameter and the friction coefficient is 0.1 mm.

[FIGURE 1 OMITTED]

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The blank (fig. 2.) is made of two parts, an annular one and a circular one, butt-welded at the interior.

The material properties of the employed metal sheets are according to the current standard and are presented synthetically in table 2.

3. RESULTS OBTAINED BY MEANS OF SIMULATION

Analysing the data obtained through numerical simulation, a series of results were obtained, of which we present in the following the most important ones. From the analysis of the relative thinning (fig. 3) it can be noticed that in the area of the part's bottom, where the thickness is of 1.5 mm, a very small thinning appears, of just 0.13%. This is due, on the one hand, to the fact the part bottom area is insufficiently stressed during the forming process, and on the other hand to the higher thickness of the disc welded at the inside of the blank.

[TABLE 2 OMITTED]

[FIGURE 3 OMITTED]

In the area of fillet radiuses between the bottom and the vertical wall, a maximal thinning occurs, with a value of approximately 10%. In the flange area, there is a thickening of the material, with a maximal value of 14%. These results are presented graphically in fig. 3.

This shows that the values are similar, especially considering the fact that we are interested in the behaviour of the 1-mm sheet, because the annular blank has a thickness of 1 mm in the filleting area. Therefore no modifications of the forming system are needed when switching from deep-drawing monoblock blanks to deep-drawing the TWB, but the weight of the TWB is significantly lower than that of a monoblock blank made entirely of 1.5 mm steel sheet.

In comparison, for the deep-drawing of monoblock blanks with a thickness of 1 mm and made of the same material, the maximal thinning is of 14% and also occurs in the filleting area, while for blanks of 1.5 mm thickness, the thinning is 10.4 %.

Another result of the numerical simulation is the study of the strain state (fig.4). It can be noticed that the plasticity state is reached in most areas of the part, except for the part bottom.

The same can be seen also from the forming limit curve where it is obvious that on the bottom there exists an area that is insufficiently used from the point of view of formability.

Such an area does not exist in the case of deep drawing a monoblock part of 1.5 mm thickness and also not for the case of the monoblock part of 1 mm thickness (fig. 5). There can be noticed material thickenings on two directions perpendicular to each other, which are due to the material's anisotropy.

[FIGURE 4 OMITTED]

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With regard to the value of the force employed for forming the tailor-welded blank (fig.6) it can be seen that this is almost equal to that necessary for deep-drawing the classical, monoblock blank, thus the force size is not influenced by the usage of a tailor-welded blank.

4. CONCLUSIONS

The results presented in this paper have shown that at the deep drawing of a tailor-welded blank, the main parameters concerning formability (specific strains and thinning) and energy balance are very close to those specific for a monoblock part with the same thickness as the TWB.

This underlines a major advantage of deep drawing TWBs, namely the significant reduction of the finite part's weight, by replacing the monoblock blank with a constant thickness of 1.5 mm (which would provide the required strength) with a TWB that allows using the 1.5 mm sheet only in the most stressed area.

Starting from here, it is sought in future to analyse the possibility of increasing the number of industrial applications of this procedure, and to expand it for a wider range of part types and dimensions.

5. REFERENCES

Bayraktar, E., Isac, N., Arnold, G., (2005). An experimental study on the forming parameters of deep-drawable steel sheets in automotive industry, Journal of Materials Processing Technology, Vol. 162-163, pp. 471-476, Elsevier, ISSN: 0924-0136

Cheng, C. H., Chan, L. C., Chow, C. L. (2007). Weldment properties evaluation and formability study of tailor-welded blanks of different thickness combinations and welding orientation, Journal of Materials Science, Volume 42, Number 15, Springer, ISSN 0022-2461, Netherlands

Geiger, M., Merklein, M., Staud, D., Kaupper, M. (2008). An inverse approach to the numerical design of the process sequence of tailored heat treated blanks, The International Journal of Advanced Manufacturing Technology, Vol. 2, No. 1, Springer, ISSN 0944-6524, Berlin / Heidelberg

Ku, T-W., Kang, B.S., Park, H-J. (2005) Tailored blank design and prediction of weld line movement using the backward tracing scheme of finite element method, The International Journal of Advanced Manufacturing Technology, Volume 25, Numbers 1-2, Springer, ISSN 0268-3768, London

Ozek, C., Bal, M., (2008). The effect of die/blank holder and punch radiuses on limit drawing ratio in angular deep-drawing dies, The International Journal of Advanced Manufacturing Technology, Springer, ISSN 0268-3768, London

Tab. 1. Technical characteristics
of the used forming tools

Deep-drawing depth 20 mm
Die fillet radius 6 mm
Punch fillet radius 3 mm
Die diameter 60 mm