Deformation behavior in the hydroforming of overlapping tubular blanks

https://doi.org/10.1016/j.ijmachtools.2020.103624Get rights and content

Highlights

  • Circumferential material flow is easier at the overlap than opposite to the overlap.

  • Non-uniform circumferential and axial material flow leads to wrinkling defects at the overlap.

  • Wrinkling defects can be prevented by the normal load achieved by a sub-plate.

  • Forming pressure and wall thinning can be reduced using the overlapping blank.

Abstract

The hydroforming of overlapping blanks (HOB) is a novel method that uses overlapping tubular blanks rather than closed cross-sectional tubes to enhance the forming limit. However, the deformation and instability mechanism of HOB has not been elucidated yet. In this paper, theoretical analysis models were developed for the material flow field, the critical wrinkling stress, and the critical supporting pressure. On the basis of it, the HOB approach was performed experimentally and numerically to validate the proposed models, taking a variable-diameter part as an example. A new self-sealing loading tool made of highly elastic material was developed to realize the sealing. The deformation mechanism in HOB was revealed and the occurrence of material flow along the circumferential direction was demonstrated. In addition, the location and the morphology of potential wrinkles were well predicted. Wrinkling defects were prevented effectively by applying the normal load, which was achieved by a sub-plate covered upon the overlap at the outer layer. The comparison of the required internal pressure and the wall thickness distribution was analyzed in the HOB and the THF processes. It is feasible to reduce the forming pressure and the wall thinning using an overlapping blank.

Introduction

Tube hydroforming (THF) is a forming process in which tubes are formed into complex shapes with a die cavity using the cooperative application of internal pressure and axial compressive loadings [1,2]. The internal pressure is usually supplied by a variety of methods such as pumping hydraulics, a viscous medium, or viscoelastic elements [3,4]. THF has provided a widespread application opportunity because of its weight reduction, improved part quality, excellent material utilization, and low cost [5].

Hydroformability is influenced by material properties, friction conditions, deformation modes, and loading paths. For a given material, axial feeding plays a key role in enhancing the forming limit and avoiding cracks [6]. Axial feeding is usually achieved by axial punches that push a material into an expansion region. The stress state and the strain path change as the axial feeding is applied. Varma et al. reported that axial feeding with a specified flow rate led to a proportional strain path, while axial feeding with a prescribed pressure resulted in a non-proportional strain path [7]. However, excessive axial feeding led to wrinkling defects. Imaninejad et al. suggested that the majority of axial feeding should be applied when a tube material starts to yield under internal pressure to avoid wrinkles [8].

Based on theoretical and experimental research studies, the effects of loading paths (internal pressure versus axial feeding) on wall thickness distributions and forming limits have been widely studied for a variety of hydroformed tubular components [9]. Aue-U-Lan et al. compared two kinds of loading path determinations, self-sealing (SF) and adaptive simulation (AS), in the hydroforming of a variable-diameter part. The experimental and simulation results demonstrated that FE-based loading paths could decrease trial and error and enhance THF capability [10]. In the hydroforming of T-shaped tubes, optimization approaches to thickness uniformity control have been studied such as the artificial network analysis method, the fuzzy load control algorithm, and so on [11,12]. In shear hydro-bending, the relationships of internal pressure and feeding ratio also play an important role in preventing forming defects. Experimental results have shown that the proper feeding ratio, the ratio of axial feeding to the transversal stroke, ranged from 1.0 to 1.2 in the shear hydro-bending of 5A02 aluminum alloys rectangular tubes [13]. Furthermore, Yuan et al. put forward the idea of useful wrinkles that could effectively enlarge the process window [14]. The experimental results from the hydroforming of complex-shaped components have demonstrated that useful wrinkles can improve the forming limit and the wall thickness distribution by increasing the axial feeding [15].

As with the hydroforming of thin-walled tubes, however, there are limitations for the application of axial feeding [16]. Wrinkling defects are prone to occur under axial loading if the diameter-to-thickness ratio (D/t) of a tubular blank is extremely large. The critical strains for wrinkles will drop sharply with the increase of D/t [17]. In addition, it is difficult to push a material from a guiding area to a bulging area if the axis of the part is long [18]. Therefore, Han et al. proposed a novel approach called the hydroforming of overlapping tubular blanks (HOB) [19]. In this approach, overlapping tubular blanks were used instead of closed cross-sectional tubes to enhance the forming limit. The forming limit proved to be enhanced in the hydroforming of a spherical part with an expansion of 60.0% and the wall thickness thinning was reduced. In addition, the required forming pressure can be reduced at the overlap when the blank material and the die cavity were in contact in the HOB process [20]. However, the theoretical study and the wrinkling mechanism of the HOB process were not reported yet.

In this research, theoretical analysis models were developed to study the deformation behavior in the HOB process. A self-sealing elastic deformation body was designed as a loading tool. A variable-diameter part was taken as an example to validate the proposed models through experimental and numerical simulation methods. In addition, the wrinkling mechanism was analyzed. A sound part with an expansion of 31.6% was manufactured under supporting pressure. The comparison of the required internal pressure and the wall thickness distribution was analyzed in the HOB and the THF processes.

Section snippets

Principle of hydroforming of overlapping tubular blanks

An illustration of the hydroforming of the variable-diameter part using overlapping blank shows the principle of HOB. The internal pressure is exerted on the inner surface of the overlapping blank through the self-sealing elastic body. At the same time, a clamping force is applied in order to avoid a separation between the upper die and the lower die, as shown in Fig. 1(a). Fig. 1(b) shows the forming process during different forming stages. The overlapping blank and the elastic body bulge

Analysis of displacement distribution

Due to the geometric configuration of the overlapping blank, the deformation is asymmetric in the HOB, which is different from the hydroforming of closed cross-sectional tubes. The distribution of the displacement components along the axial directions is non-uniform at the cross-sections of the blank. Furthermore, the overlap of the blank expands first and the material can flow along the circumferential direction as the inner pressure increases. A theoretical model was built to elaborate on the

Experimental results

HOB and THF experiments for the test part shown in Fig. 2 were performed. The diameter of the AISI 304 stainless steel tube was 76 mm and the initial wall thickness was 0.5 mm in the THF experiment. The tubular blank was joined by argon-arc welding in advance. Both ends of the tube were free in the axial direction in order to make the material flow from each end towards the bulging area during the hydroforming process. Fig. 8 shows the hydroformed part that was obtained from a closed

Displacement distribution

In order to reveal the low of material flow in HOB, displacement distributions of the test part were calculated according to Eq. (4). Meanwhile, numerical simulation was conducted to obtain the mechanical information based on the experimental parameters. The simulation results exhibited the current profile of the middle cross-section (x = 0.5a) of the deformed overlapping blank under the internal pressure of 0.67py, as shown in Fig. 13. The material flowed along the circumferential direction

Conclusion

This work proposed theoretical analysis models for the material flow field, the critical wrinkling stress, and the critical supporting pressure for avoiding wrinkling defects, which provided a thorough investigation on deformation behavior of the HOB process. A variable-diameter part with an expansion of 31.6% was taken as an example to validate these proposed models. The main findings could be summarized as follows:

  • 1)

    The theoretical analysis revealed the variation of the material flow field

Author statement

Feng Hao: Methodology, Software, Investigation, Formal analysis, Writing - Original Draft, Writing - Review & Editing. Han Cong: Conceptualization, Validation, Resources, Data Curation, Visualization, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundations of China (project numbers: 51775136 and U1937205). The authors would like to take this opportunity to express their sincere appreciation.

References (32)

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