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Optimization of a hydroforming process to realize asymmetrical aeronautical components by FE analysis

F. Capece Minutolo, M. Durante, A. Formisano, A. Langella

Dept. of Materials and Production Engineering, University of Naples Federico II, P.le Tecchio 80, 80125 Naples, Italy

Abstract Hydroforming process is an effective method for manufacturing complicated parts. This process is quite sensitive to the characteristics of the pressure growth laws and the friction between the sheet and the elements of the tooling. In this work a numerical simulation has been carried out using the explicit finite element code LS-DYNA with the purpose to optimize a hydroforming process related to the achievement of an asymmetrical aeronautical component. In particular, the number of the steps of the manufacturing cycle actually used to produce the component has been reduced considering the FEM results. Keywords: Hydroforming, process parameters, FE analysis

1. Introduction Hydroforming allows to overcome some of the limitations of conventional deep drawing, increasing the draw ratio and minimizing the thickness reduction of the formed parts. Among the advantages introduced by hydroforming there are a great flexibility and a remarkable reduction of tooling costs. Hydroforming is a process that makes use of a hydraulic pressure to improve the basic deep drawing process. The fundamental parts of the tool for a hydroforming process include a punch, a blank holder and a pressure chamber with a rubber diaphragm that seals the liquid in the chamber. The draw ratio achievable in hydroforming is high (values of about 3.2 are reported in literature) [1,2], very little thinning occurs and asymmetrical shapes can be drawn. Many failure types can be found in hydroforming, generally divided into fracture and wrinkling [3]. The process is quite sensitive to pressure variation and

friction between the sheet and the tool [4,5]. Figure 1 shows the various outcomes of the process, varying the pressure vs. the punch stroke. A simple theoretical analysis, related to symmetrical cups drawn in hydroforming with constant fluid pressure [6], has furnished the minimum and maximum pressure values for a successful hydroforming. Furthermore, hydroforming process is influenced by the friction interesting the contacts. The studies have pointed out that the punch surface, especially the punch head surface, plays a fundamental role in the process. Owing to an increase in the punch head roughness, the maximum draw ratio raises almost linearly. The surface roughness of the straight side of the punch has no significant influence on the forming process. Therefore, the employment of different local constraints for the sheet, by using different roughnesses in different areas of the punch and an appropriate pressure cycle, will facilitate a successful

forming process. In this work, the evaluation of FEM simulation for the hydroforming process of an aeronautical

Table 1 Hydrodynamic pressure laws imposed to the four-step production cycle. Step 1 Pressure (MPa) 40 45 48 55 35 39 49 20 30 36 40 28 30 35 39 40 Punch displacement (mm) 0 11.6 16.6 23.6 23.6 28.6 38.6 45.6 45.6 52.3 67.3 69.3 69.3 74.3 79.3 84.3

2

3

Fig. 1. Qualitative diagram pressure-punch stroke showing different outcomes of a hydroforming process.

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component generated in different steps has been considered. The optimization of the production cycle has been carried out considering the output data of the FEM program. The parameters examined to optimize the process have been the rolling direction of the sheet and the maximum deformability in every forming step. 2. Production cycle of the aeronautical component The object of this study is an aeronautical component, currently produced at the Avio S.p.A. plant of Pomigliano d'Arco (Naples-Italy); the production cycle is divided into four steps: the first step carries out the symmetrical part of the component; the following three steps, instead, the asymmetrical one. Between the second and the third step and between the third and the fourth step, an annealing process for the elimination of the residual stresses is foreseen. The hydrodynamic pressure laws for every step are shown in Table 1. The sheets, before the forming process, are cut in a particular form, in order to realize the requested complex geometry, but, in this particular manufacturing process, the sheet orientation with respect to rolling direction, hasn't been considered as a process parameter. The used material is a 0.8 mm thick Inconel 718, a Ni-Cr alloy, with the following chemical composition: Ni=50 ÷ 55%; Cr=17 ÷ 21%; Nb=4.75 ÷ 5.5%; Mo=2.8 ÷ 3.3%; Ti=0.65 ÷ 1.15%; Al=0.2 ÷

0.8%; the rest is Fe. The mechanical properties are as follows: elastic modulus E=208 GPa, yield stress Y=425 MPa, anisotropy factors R0=0.33, R45=1.13, R90=1.21 and strain hardening exponent n=0.38. In Fig.2 the photograph shows the aeronautical component to be realized; in this one some points are indicated. The quality of the process, in fact, has been determined measuring the value of the thickness in these points. In particular, the thickness reduction could not be more than 20%.

Fig. 2. The aeronautical component.

3. Reliability of the simulation model A numerical simulation has been carried out using the explicit finite element code LS-DYNA. Default input parameters are generally chosen to give efficient, accurate crash simulation results. These defaults are not necessarily optimal for metal forming simulations, therefore the program has been properly set to simulate the hydroforming process [7]. According to the physical model, all tools have been modelled using a rigid element with a four-node shell. In particular, the blank has been modelled using the four-node quadrilateral, fully integrated shell elements and an elasto-plastic material has been chosen. Run time is decreased using mass scaling and, artificially, a high tool velocity. Both these methods introduce artificial dynamic effects, which must be minimized in an engineering sense. For the steps different from the initial one, the input deck in terms of node, element, initial stress and strain information are imported by the output file of the simulation of the previous step. In fig. 3, the results of

the FE analysis are reported for the different simulated steps. The coincidence between FEM and experimental results in terms of external geometry has been verified. In particular, FEM and experimental results, in terms of thickness, for the characteristic points at the end of the fourth step, are shown in Table 2. These results confirm the reliability of the simulation model.

Table 2 FEM and experimental results in terms of thickness for the characteristic points. Point A B C D Experimental (mm) 0.73 0.72 0.73 0.74 FEM (mm) 0.75 0.70 0.70 0.73

Fig. 3. FE analysis for the four-steps cycle.

4. Process optimization The rolling direction and the material anisotropy haven't been considered in manufacturing process. So, the first phase for the optimization has been the determination of the best placement of the sheet with respect to the rolling direction as a function of the material anisotropy [8]. A material model has been used to account for the material anisotropy, setting the anisotropy coefficient for the different directions. So, different simulations have been conducted using the process in four steps, varying the angle between the "nose" of the sheet and the rolling direction. The results point out that basing on FLD curves analysis, the optimal value of the angle is 90° (Fig.4).

Fig. 4. Optimization of sheet cutting.

After the FEM simulation of the four-step process, the simulation of the process in three steps has been carried out. In order to reduce the number of steps from four to three, the depth has been increased for the second and the third step, until the values for which the deformation of the elements coincided with the FLD curves. In particular, a security zone has been created, generating a curve parallel to the FLD limit one, with a lower deformation at least of 20%. So, for every simulation, the parameters have been chosen basing on a comparison between FLD curves [9] (see Figs. 5-7) obtained, respectively, by simulation and by selecting the values that ensure the largest margin with respect to failure.

Figs. 5-7. FLD curves related to three-step simulation cycle.

The pressure cycle values that have been chosen through the FLD analysis are reported in Table 3.

Table 3 Hydrodynamic pressure laws imposed to the three-step production cycle. Step 1 Pressure (MPa) 35 42 45 48 35 37 40 20 34 38 35 30 Punch displacement (mm) 0 9.44 14.16 23.6 23.6 48.8 53.1 57.6 57.6 77.5 80.9 84.3

2

3

The values of the pressure for the different depth have been calculated according to the control curves present in the handbook of the press. Between the first and the second step and between the second and the third step, an annealing process for the elimination of the residual stresses has been carried out. Finally, the production of the part with the set of optimum parameters has been executed. In fig. 8, the FE analysis and the experimental results for the different steps are reported. FEM and experimental results in terms of thickness for the characteristic points at the end of the three steps, are shown in Table 4. The good agreement between the real process and the simulation can be noticed, suggesting the suitability of the numerical model to simulate the hydroforming process.

Fig. 8. FE analysis and experimental results for the three-steps cycle.

Table 4 FEM and experimental results in terms of thickness for the characteristic points. Point A B C D Experimental (mm) 0.73 0.79 0.70 0.70 FEM (mm) 0.76 0.80 0.70 0.70

cycle. References

1] Chabert G. Hydroforming techniques in sheet metal industries. Fifth International Congress on Sheet Metal Work, International Council for Sheet Metal Development, (1976), 18-34. Thiruvarudchelvan S and Wang H. Investigations into the hydraulic-pressure augmented deep drawing process. J. Mater. Process. Technol. 110 (2001), 112-126. Lang L, Danckert J and Nielsen KB. Investigation into hydrodynamic deep drawing assisted by radial pressure. Part I. Experimental observations of the forming process of aluminium alloy. J. Mater. Process. Technol. 148 (2004), 119-131. Tolazzi M, Vahl M and Geiger M. Determination of friction coefficients for the finite element analysis of double sheet hydroforming with a modified cup test, Sixth International ESAFORM conference on Material Forming, Salerno, Italy, (2003), 479-482. Lang LH, Danckert J and Nielsen KB. Analysis of key parameters in sheet hydroforming combined with stretching forming and deep drawing. Proc. Instn. Mech. Engrs. Vol. 218 Part B, J. Engineering Manufacture (2004), 845-856. Thiruvarudchelvan S and Lewis W. A note on hydroforming with constant fluid pressure. J. Mater. Process. Technol. 88 (1999), 51-56. Nielsen KB, Jensen MR and Danckert J. Optimization of sheet metal forming processes by a systematic application of finite element simulations. Second European LS-DYNA User Conference, Gothenburg, Sweden (1999), 3-16. Mielnik EM. Fundamentals of elasticity and plasticity. Metalworking science and engineering (1991), 89-109. McCaandless J and Bahrani AS. Strain paths, limit strains and forming limit diagram. Seventh NAMRC, Berkeley, California (1979), 184-190.

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By comparing the thickness values of experimental tests, it is possible to note that for the three steps process the thickness is less uniform than for the four steps process. This is probably due to higher values of friction coefficient in different contact areas between sheet and punch. However, the variation in thickness is very low and, particularly, it is less than 15 %. 5. Conclusions

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By the use of FE analysis it has been possible to realize a hydroforming process optimization, reducing the production cycle from four to three steps. The obtained results have shown the possibility to determine a preliminary procedure, based on FE analysis, for the definition of the best production cycles to sensitively reduce the throughput times and the material waste, connected with the starting of a new product manufacturing. For the new optimized manufacturing cycle, the thinning turns out higher than the one obtained in four steps cycle. This is probably due to the heavier drawing condition in the single step of the optimized

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