J. Cent. South Univ. Technol. (2009) 16: 0887-0891

DOI: 10.1007/s11771-009-0147-7

Deformation and defects in hydroforming of 5A06 aluminum alloy dome with

controllable radial pressure

XU Yong-chao(徐永超)¹, LIU Xin(刘 欣) ¹, LIU Xiao-jing(刘晓晶) ², YUAN Shi-jian(苑世剑) ¹

(1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, China)

Abstract: A new process of hydroforming with controllable radial pressure was proposed to overcome difficulties in the forming of low plastic materials and large height-to-diameter ratio workpieces. A typical 5A06 aluminum alloy dome was numerically and experimentally investigated. The reasons for typical defects were analyzed under different radial pressures. Effects of radial pressure on the thickness distribution were discussed and optimal radial pressure was determined. It is shown by numerical simulations and experiment that a cup with a drawing ratio of 2.4 is formed by the new process of hydroforming with controllable radial pressure. It is significantly effective for the forming of low plastic materials and large height-to-diameter ratio workpieces. Two typical thinning points exit along the dome wall. With the radial pressure, thinning is decreased effectively at the two points, the dome forming is achieved and thickness distribution is more uniform.

Key words: aluminum alloy; hydroforming; radial pressure; fracture; thickness

1 Introduction

As an advanced sheet forming process, sheet hydroforming or hydro-mechanical deep drawing has found a wide variety of applications in the field of automobile and aerospace since it was invented[1-3]. Compared with conventional sheet metal deep drawing, advantages of hydroforming include improved forming limit, enhanced surface quality and reduced tool cost[4-5]. Up to now, more than 300 types of workpieces have been manufactured with hydroforming in Sweden, Japan and Germany etc, among which the materials mainly included low carbon steel and stainless steel, and the thickness range of sheet metals was from 0.2 to   3.2 mm[6-8].

In China, hydroforming machines have been developed, which has pushed the development of sheet hydroforming greatly[9-10]. With the increasing importance of protecting environment and saving energy, lightweight materials such as aluminum alloy and magnesium alloy have been applied increasingly[11-13]. 5A06 aluminum alloy is widely applied in the field of aerospace. Unlike low carbon steel, it has a poor formability with small anisotropy parameter r and low hardening index n[14]. 5A06 aluminum alloy dome in aerospace is a typical complex-shaped cup with a hemispherical bottom, and height-to-diameter ratio is about 1.3. With conventional deep drawing, the dome forming needs eight drawing procedures, and annealing is necessary. Moreover, fracture is easy to occur because of insufficient hardening and low load-carrying capacity of material. The production efficiency is low, and the percentage of finished product is 40%-60%, which became a bottleneck to its application. Through conventional hydroforming, it is still difficult to form. Therefore, a new process of hydroforming with controllable radial pressure was proposed for 5A06 aluminum alloy dome. Effects of radial pressure on thickness and defects were explored, and reasonable process parameters were obtained in this work.

2 Principle of hydroforming with con- trollable radial pressure

Fig.1 shows the principle of hydroforming with controllable radial pressure. Based on conventional hydroforming, a controllable radial pressure p_r is imposed to the rim of flange besides the chamber pressure p_c. Because sealing rings exist, the radial pressure p_r and the chamber pressure p_c can be loaded and controlled independently. Thus, the chamber pressure p_c increases the useful friction between the punch and the blank, and the radial pressure p_r aids to push the flange to flow into the chamber when the punch penetrates into the chamber. At the same time, the dual fluid lubrication can be generated, which decreases the friction between the flange and the blank holder, or between the flange and the die. Fluid lubrication and radial pressure are helpful to reducing radial tensile stress when deep drawing, therefore, serious thinning can be eliminated and forming limit can be improved.

Fig.1 Principle of hydroforming with controllable radial pressure

3 Experimental 3.1 Dimensions and material

The dome diameter is 154 mm, and the total height is about 190 mm. The blank diameter is about 370 mm. The material used is 5A06 aluminum alloy sheet metal with 1 mm in thickness. The mechanical properties determined by tensile test are yielding stress σ_s=     160 MPa, strength limit σ_b=340 MPa, elongation δ=18%, anisotropy parameter r=0.85 and hardening index n= 0.23. From the above parameters, it can be seen that the formability is poor. Usually, limiting drawing ratio is about 2.1 (height-to-diameter ratio 1.1) for 5A06 aluminum alloy with conventional hydromechanical deep drawing. According to the dimensions of workpiece, drawing ratio 2.4 (height-to-diameter ratio 1.3) can be calculated approximately. The deformation is larger, and it is difficult to form.

3.2 Loading path

The dome is a kind of complex-shaped workpieces. Due to the hemispherical bottom, achieving a completely formed workpiece depends on an appropriate loading path of chamber pressure p_c for a given material and workpiece at first. On the basis of appropriate chamber pressure p_c, effects of radial pressure p_r on thickness can be determined.

Different from a cup with a flat bottom, for the dome with a hemispherical bottom, there is a larger suspended region of blank between the punch and the blank holder, and a smaller contact region between the punch and the blank in the initial stage, as shown in  Fig.2. With the punch penetrating into the chamber,

Fig.2 Illustration of dome hydroforming

curvature radius r₀ decreases gradually in the suspended region. The above deformation behavior demands an appropriate chamber pressure p_c with deformation. On one side, the chamber pressure cannot be too high, in order to avoid the serious thinning due to a large radial stress σ₁ in the suspended region. On the other side, the chamber pressure cannot be too low, in order to establish an effective friction (μp_c) between the punch and the blank. According to the theoretical analysis and a few simulations, an appropriate loading path of chamber punch stroke is about 50 mm, the hemisphere is almost formed, and the chamber pressure maintains 20 MPa, as shown in Fig.3(a). Effects of radial pressure on the forming can be analyzed after the appropriate chamber pressure is determined. The radial pressure aids to push the rim of flange to flow into the chamber. Loading paths of radial pressure were designed, as shown in Fig.3(b). The radial pressures are 5, 10, 20, 30, 40, 50, 60 and 65 MPa, respectively.

3.3 FEM model

The pre/post software DYNAFORM was used to establish model for hydroforming with radial pressure. The finite element solver was LS-DYNA. In order to save calculation time, only quarter of the blank and the tool components were simulated, as shown in Fig.4. Belytschko-Tsay shell elements were used for the blank, and the tool was treated as rigid bodies. The fixed gap between the blank holder and the die was 1.2 mm.

In simulation, the work hardening law of material was σ=Kεⁿ. Strength coefficient K=549 and work hardening exponent n=0.23. The artificial speed of punch was 1 m/s. Friction was modeled between the blank and the tool interfaces using the Coulomb assumption. The friction coefficient was assumed to be 0.05 due to the fluid lubrication between the blank and the die, 0.12 between the blank and the punch, and 0.05 between the

Fig.3 Loading paths for hydroforming: (a) Chamber pressure;  (b) Radial pressure

Fig.4 Finite element model for hydroforming

blank and the blank holder.

The pressure loading of hydroforming with radial pressure was dynamic. Because the chamber pressure and the radial pressure could not be applied to the elements automatically through the pre-processor of the finite element software, the keyword LOAD_MASK was used to load the chamber pressure, which was supplied by LS-DYNA. As to the radial pressure, because shell elements had no thickness, the radial pressure could not be applied to the rim of elements. So, equivalent nodal forces were imposed to the nodes of the blank rim instead. During the forming, the equivalent node force was reduced gradually when the diameter of flange decreased.

4 Typical defects in hydroforming with radial pressure

For the cup with a hemispherical bottom, the deformation behavior of blank was firstly analyzed in the chamber pressure loading path with a radial pressure of 30 MPa. Fig.5 shows the deformation process. The chamber pressure was about 8 MPa when the punch travel was 27 mm in the initial stage, as shown in Fig.5(a). In this time, because of the hemispherical shape, the small chamber pressure and contact region resulted in an insufficient friction force between the punch and the blank, the flange of blank did not almost flow into the chamber although a radial pressure of 30 MPa was applied. This illustrates that the main deformation mode of blank is stretching in the initial stage, and thinning cannot be avoided in this time. The chamber pressure would be 20 MPa when the punch travel was 50 mm in the middle stage, as shown in Fig.5(b). In this stage, the suspended region decreased, the contact region increased, and the chamber pressure got higher. The flange of blank started to flow into the chamber. The deformation mode of blank changed into deep drawing from stretching. The chamber pressure maintained 20 MPa after the middle stage, and the flange transferred to the straight wall of workpiece in the final stage, as shown in Fig.5(c).

Due to a large deformation, the workpiece was difficult to be formed without radial pressure. However,

Fig.5 Deformation process of dome hydroforming: (a) Initial stage; (b) Middle stage; (c) Final stage

when a small radial pressure of 5 MPa, or smaller than 10 MPa was applied, fracture occurred at the bottom of the hemisphere. The main reason was that, for the enlarged blank, due to the small radial pressure, although the useful friction force existed in the contact region between the punch and the blank, the larger tensile stress could not still be eliminated partly at the bottom. The deformation mode was completely stretching, and serious thinning caused the occurrence of fracture. Fig.6 shows the results of both simulation and experiment under a radial pressure of 5 MPa.

When a large radial pressure 65 MPa was applied, the tensile stress could be eliminated partly, and load-carrying ability of blank was improved at the bottom of hemisphere. So the hemisphere could be formed in the middle stage without fracture. However, due to a radial pressure 65 MPa, the average pressure was larger between the blank holder and the blank, and a caused unhelpful friction force was increased between the die and the blank. It was difficult for the flange to flow into the chamber. So, the fracture occurred near the die profile when the tensile stress reached the strength limit. Fig.7 shows the results of both simulation and experiment under a radial pressure of 65 MPa.

5 Thickness distribution

Fig.8 shows the photo of a formed workpiece and the thickness distribution obtained by numerical simulation and experiment under a radial pressure of 30 MPa. In Fig.8(b), horizontal coordinate describes the linear distance to the centerline, and vertical coordinate describes the thickness at different positions. From Fig.8, there are two lower thinning points in the thickness distribution curve. The first lower thinning point is at the bottom of the hemisphere, and the thickness is about 0.84 mm. The second lower thinning point is near the straight wall, and the thickness is about 0.89 mm. The results also show that it is easy to fracture at the two lower thinning points, as analyzed in the above defects. It can be seen from the experimental results that, the average thickness is 0.86 mm at the two points, and the experiment is in a good agreement with the simulation.

Fig.9 shows the effect of radial pressure on the average thickness of two lower thinning points. Judging from the average thickness, thinning ratio decreases gradually when the radial pressure increases in the range from 10 to 40 MPa, but thinning ratio increases when the radial pressure increases in the range from 40 to 60 MPa. The optimal radial pressure is 40 MPa, and the average thickness is 0.88 mm at two typical thinning points. The results demonstrate that radial pressure has positive and negative effects on the thinning, which depends upon the useful pushing force on the rim of flange and the friction force between the blank and the die. When the positive effect is more than the negative one, the flowing resistant

Fig.6 Fracture type under radial pressure of 5 MPa: (a) Simulation result; (b) Experimental result

Fig.7 Fracture type under radial pressure of 65 MPa: (a) Simulation result; (b) Experimental result

Fig.8 Photo of hydroformed workpiece (a) and thickness distribution (b)

Fig.9 Effect of radial pressure on average thickness

is decreased in the flange, the radial tensile stress is reduced and serious thinning is suppressed. On the contrary, the radial tensile stress is increased, and serious thinning or fracture occurs.

6 Conclusions

(1) A cup with a drawing ratio of 2.4 is formed by the new process of hydroforming with controllable radial pressure. It is significantly effective for the forming of low plastic materials and large height-to-diameter ratio workpiece.

(2) For the dome hydroforming with radial pressure, the main deformation is stretching in the initial stage, and changes into deep drawing from stretching in the middle stage. Thinning occurs, and two lower points are respectively located at the bottom of hemisphere and the straight wall.

(3) Fracture occurs at the bottom of hemisphere when the radial pressure is below 10 MPa, and at the straight wall when the radial pressure is above 60 MPa.

(4) Thinning ratio decreases gradually when the radial pressure increases in the range from 10 to 40 MPa, but thinning ratio increases when the radial pressure increases in the range from 40 to 60 MPa. The optimal radial pressure is 40 MPa, and the average thickness of 0.88 mm can be achieved.

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(Edited by YANG You-ping)

Foundation item: Projects(50525516, 50875062 ) supported by the National Natural Science Foundation of China

Received date: 2009-03-22; Accepted date: 2009-06-31

Corresponding author: XU Yong-chao, Associate professor, PhD; Tel: +86-451-86417917; E-mail: yongchaoxu@hit.edu.cn