Subregional flexible-die control in magnetorheological pressure forming
Graphical Abstract
Introduction
Due to the characteristics of large local size mutation, small wall thickness, and complex shape, complex thin-walled parts have serious uneven deformation characteristics, difficult material flow, and are easy to crack due to excessive thinning during the forming process, which poses a great challenge to the traditional forming process using rigid dies. Flexible-die forming methods, which use liquid, semi-solid and solid media as pressuring-carrying mediums, show certain advantages in manufacturing complex thin-wall components for their characteristics of good filling ability and fluidity, simple die structure, low cost, and good surface quality. Typical flexible-die forming processes, such as hot gas forming, sheet and tube hydroforming, viscous pressure forming (VPF) and rubber pad forming, are playing an increasingly important role in aviation, aerospace, automobile, and other industries.
In the traditional flexible-die forming process, the force transfer characteristic for flexible-die, whether gas, liquid, viscous medium, or rubber, is constant and can not be changed. However, the stress states and flexible-die performance required by different deformation areas of sheet metal in the forming process are different. Due to the unique magnetorheological effect, magnetorheological materials, such as magnetorheological (MR) fluids and magnetorheological (MR) elastomers, have been gradually applied in sheet metal flexible-die forming in recent years. Their properties can be changed by applying different magnetic fields and then their force transfer properties can be adjusted, which provides the possibility of intelligent control during the forming process.
Merklein and Rösel (2010) explored the potential of MR fluids as a flexible-die for hydroforming and characterized the behavior of MR fluid at an appearing leakage. Results showed that the cavity pressure doubled when a magnetic field of 0.27 T is adopted, which shows great potential. On this basis, Rösel and Merklein (2014) enhanced the process window in sheet hydroforming by locally regulating the MR fluid property. Results showed that the MR fluid can be used as the forming and sealing medium meanwhile. However, a local magnetic field is only applied to the flange area during the forming process, and there is no magnetic field in the cavity. The advantages of MR fluid as a flexible-die need to be further explored. Wang et al. (2014) proposed magnetorheological pressure forming (MRPF) and verified that MR fluid’s mechanical property can be changed greatly under a magnetic field in the extrusion test. On this basis, the effect of the magnetic field on the Al1060 sheet’ formability was studied. By using the property-adjustable characteristic of MR fluid, Wang et al. (2020) proposed a new method to change the loading path in the sheet forming process. Experimental and numerical results indicate that the loading path change greatly influences the deformation behavior of sheet metal. In the above studies, a multi-turn coil was used to provide a relatively stable and adjustable magnetic field for the forming area. Liu et al., 2019a, Liu et al., 2019b studied the effect of reverse pre-bulging on magnetic medium-assisted deep drawing of aluminum cylindrical parts. By increasing the pre-bulging height, the specimen’s maximum thinning rate decreases by about 28%, and the final bulging height increases by about 10%. At the same time, the springback is alleviated. Bao et al. (2018) reported the bulging behavior using aluminium alloy double-layer sheets with MR fluid as the flexible-die. With the increase of magnetic flux density or magnetic particle content of MR fluid, the limit bulging height of the inner layer aluminium sheets is greatly improved. The punch is made of DT4 pure iron to enhance the magnetic flux density generated by a coil. However, the adjustment of the magnetic field in the forming process is still overall. Liu et al., 2019a, Liu et al., 2019b adopted MR fluid to provide backpressure for the sheet piercing process. It proved that the quality of blanking fracture is significantly improved by increasing the backpressure. The effect is explained by using a theoretical mechanics model. In recent years, another smart magnetic material, magnetorheological elastomer (MRE), has also been used as a flexible-die in tube forming. Hu et al. (2021) studied the influence of using an MRE forming medium and axial feeding on the forming quality of thin-walled Inconel 718 bellow. The wall thickness uniformity can be improved by applying an external magnetic field. Yang et al. (2022) investigated the impact of using MREs as flexible-die on improving the formability of T-shaped Inconel 718 tubes. A higher branch can be obtained by increasing the magnetic field intensity. But when the magnetic field reaches a certain strength, the height decreased.
In previous studies, the magnetic field is changed before or during the forming process, and the force transfer characteristics of MR materials are changed as a whole. The flexible-die properties for different regions are the same. However, for tailor welded blanks with significant differences in material properties or complex parts with asymmetric structure and unequal depth cavity, the stress and strain states for different areas of parts are different. At present, the flexible-die performance can be changed in time, but it can not be controlled in space. That is, it is still impossible to control the flexible-die property of different areas at the same time.
Blank holder force (BHF), force loading type, and lubrication are important process parameters in sheet metal forming. The method of subregional regulation has been applied to the control of these parameters. Kinsey et al. (2000) used hydraulic cylinders, which are incorporated into the punch and lower die, to apply clamping forces to the weld line during the forming of tailor-welded blanks (TWBs). Numerical simulation results showed the method could effectively eliminate the fracture in the weld zone and increase the uniformity of strain. Kinsey et al. (2004) proposed a methodology to determine the segmented binder force ratio to address wrinkling concerns in TWBs forming process. In addition, the segmented binder process has also been applied to control material flow in non-TWB sheet metal forming. Wang et al. (2007) used PID closed-loop FEM simulation method to determine the optimal BHF trajectories for segmented binders in the forming of a step rectangle box part. The effectiveness of the proposed approach was verified by experiments using a multipoint variable BHF hydraulic press. Zhang et al. (2020b) reported a radial segmental blank holder technique based on electro-permanent magnet (EPM) technology. The flange wrinkling can be eliminated and the forming limit is greatly improved using the new method. Applying force locally has been proved to change the stress states and flow of the sheet material. Thiruvarudchelvan and Tan (2004) used an annular urethane pad to induce frictional traction for drawing copper conical cups. The punch did not touch the blank during the drawing process, and the sheet was drawn in the die cavity by the local friction force. Phanitwong et al. (2021) proposed a new zoning lubricant technique for forming SUS304 square deep-drawn box parts. The blank-holder and die were divided into four wall zones and four corner zones, and the oleophobic coat was applied only on the four corner zones. In this way, the material flow velocity difference in the flange area was reduced and the formability could be increased by 10%. Xu et al. (2021) reported a new differential lubrication method in push-bending of L-shaped thin-walled tubes. Four zones of optimized tube blank are lubricated with different lubricants. Results showed that the differential lubrication method can relieve wrinkling and broaden the forming process window. The above methods provide enlightenment and ideas for the subregion control in flexible-die forming.
In this paper, an improved method of MRPF based on subregional flexible-die control is proposed, which makes it possible to control the flexible-die performance in time and space. Then subregional magnetic field distribution in the working area is established through experiment and simulation. On this basis, through a series of experiments, the effects of subregional flexible-die control on sheet deformation behavior, including strain distribution, configuration, wall thickness, fracture position, and morphology, are studied, and its influence mechanism is further revealed.
Section snippets
Principle of magnetorheological pressure forming (MRPF) based on subregional flexible-die control
The schematic diagram of the MRPF process based on subregional flexible-die control is shown in Fig. 1. In the traditional MRPF process, the magnetic field in the forming area is usually changed uniformly. Therefore, the mechanical properties of MR fluid are also adjusted as a whole. In this paper, we divide the forming area into the central cylinder and the fringing annular area, corresponding to the magnetic flux intensity of B1 and B2, respectively, and then adjust the force transmission
Establishment of non-uniform magnetic field
The non-uniform magnetic field is established by the combination of coil and iron cores. Fig. 7 shows the comparison of magnetic flux density in the working area with only the lower iron core between experiments and simulation. It shows that the FEM simulation can predict the distribution of magnetic flux density accurately, and the simulated values are in good agreement with the experimental values. As can be seen from Fig. 7a and b, the magnetic field zoning phenomenon is very remarkable in
Conclusions
In this paper, MRPF based on subregional flexible-die control was proposed and its effect on sheet deformation and fracture behavior was studied through systematic experimental investigations. The main conclusions can be drawn as follows:
- (1)
A novel method of MRPF based on subregional flexible-die control was proposed to adjust the stress states of sheet metal by regional flexible-die regulation. The subregional magnetic field is realized by using the combination of the coil and iron core group.
- (2)
CRediT authorship contribution statement
Pengyi Wang: Methodology, Investigation, Writing − original draft, Writing − review & editing, Funding acquisition. Gehui Wan: Investigation, Writing − original draft, Data curation. Jiageng Jin: Investigation, Data curation. Yucong Wang: Investigation, Data curation. Nan Xiang: Visualization, Writing − review & editing, Funding acquisition. Xiaokai Zhao: Visualization, Funding acquisition. Xueni Zhao: Writing − review & editing, Funding acquisition. Binxian Yuan: Methodology, Visualization,
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.
Acknowledgment
The authors are grateful to the National Natural Science Foundation of China (No. 51805309, No. 51905156, No. 51905327), Natural Science Basic Research Program of Shaanxi (Program No. 2019JQ-770), Natural Science Foundation of Shaanxi University of Science and Technology (No. 2017BJ-14), and the Youth Innovation Team of Shaanxi Universities supporting the investigations.
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