Procedia Engineering 81 ( 2014 ) 227 – 232

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1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya Universitydoi: 10.1016/j.proeng.2014.09.155

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11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Investigation on forming precision of flexible rolling process for three-dimensional surface parts of different sheet materials

Daming Wang, Mingzhe Li*, Zhongyi Cai Dieless Forming Technology Center, Jilin University, Changchun, Renmin Street 5988, China

Abstract

Flexible rolling is a novel forming process for three-dimensional surface parts combining the rolling process with multi-point forming technology. By controlling the distribution of the gap between the upper and lower forming rolls in the rolling process, the sheet metal is thinned non-uniformly in thickness direction, and the longitudinal elongation is different along the transverse direction, which makes the sheet metal generate three-dimensional deformation. In this paper, the comparison of the forming precision with different sheet materials is presented by measuring the shape errors of the simulation results, the materials of sheet metal are low carbon steel 08Al sheet and aluminum alloy 2024-O sheet. Torus and saddle surface parts of different sheet materials are simulated and their experimental results are presented. The simulated results are consistent with the experimental results which verify the feasibility of the use of simulation to guide the experiment. The shape errors of simulated results show that the forming precision of the 08Al sheet is better than that of the 2024-O sheet, but all the deviations of the forming parts are less than 2.0 mm excluding the head and tail regions of the forming parts in rolling direction. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology

Keywords: Sheet metal forming; Flexible rolling; Three-dimensional surface; Plastic processing; Forming precision

1. Introduction

Nowadays, three-dimensional surface parts are widely used in the manufacturing of automobiles, airplanes and ships. The demand of market for product variety and product individuation has been growing rapidly, which

* Corresponding author. Tel.: +86-431-85094340; fax: +86-431-85094340. E-mail address: limz@jlu.edu.cn

© 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University

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228 Daming Wang et al. / Procedia Engineering 81 ( 2014 ) 227 – 232

prompts researchers to develop new forming process in order to realize rapid and lower-cost forming. Nakajima (1969) designed complicated dies using small diameter pins. The surface contour was formed by adjusting each pin’s position to produce three-dimensional surface parts. Walczyk and Hardt (1998) introduced a generalized procedure for designing discrete dies. The dies are set to shape and used to stamp benchmark parts. Papazian et al. (2002) investigated a closed-loop shape control system and a reconfigurable tooling for stretch forming of the sheet metal for aircraft. Multi-point forming, named by Li et al. (1992), is a matched-die forming process for three-dimensional surface parts. The reconfigurable dies are comprised by close-packed punch elements and the height of each element could be adjusted automatically by a electric motor to form surface contours.

Continuous manufacturing technology is suitable for producing small-lot or single production because of the low set-up cost. Li et al. (2007) explored a flexible forming process which was composed of a top flexible roll and two bottom flexible rolls as a forming tool. The sheet metal was deformed by adjusting the curvature of each flexible roll and the displacement of the top flexible roll. Shim et al. (2009) developed the line array roll set process which was composed of three rows of lower rolls and three rows of upper rolls, and each row of rolls was composed of multiple independent short rolls. The sheet metal was deformed by adjusting the relative height of each independent short roll. Cai et al. (2012) explored a continuous roll forming process which combined rolling technique with the bendable rolls for manufacturing three-dimensional surface parts. By controlling the roll gap between the upper and lower forming rolls, residual stress makes the sheet metal generate three-dimensional deformation.

2. Flexible rolling process based on two rolls

2.1. Principle of flexible rolling process

The schematic diagram of the flexible rolling process is shown in Fig. 1. The two-roll system is composed of a pair of matched rolls. Each roll could be adjusted and bended with small deflection independently by a series of control points on the roll, and the bendable roll can rotate along its own axis by the restriction of shape-adjusting assembly. At the beginning of the forming process, the curved shape of the forming roll and the non-uniformly distributed gap between two forming rolls are configured by adjusting the relative position of each control point; then driving the forming rolls to rotate around their respectively bent axis, and the sheet metal is fed, the rotation of forming rolls deform and move the sheet metal by the friction between the rolls and the sheet metal. By changing the distribution pattern of the roll gap, three-dimensional surface parts with different shapes could be formed.

Fig. 1. Schematic diagram of flexible rolling process.

2.2. Experimental equipment of flexible rolling process

Based on the principle of flexible rolling process, a small experimental device was designed shown in Fig. 2. The forming rolls are made of high strength material which could bear small bending deformation and the outer diameter of each roll is 5 mm. Each roll is piecewise controlled by 10 control units and the maximum width of sheet metal that could be processed is 300 mm.

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Fig. 2. Experimental equipment.

3. Finite Element model

ABAQUS/explicit is chosen to conduct related numerical simulations. As each roll rotates around its curved axis in the experiment, the bendable roll is divided into many segments to establish the finite element model. The initial blanks are rectangular of 300 mm in length, 120 mm in width and 2 mm in thickness. The sheet blanks are modelled with C3D8R solid elements and the rolls are simulated with analytical rigid bodies. The spin rate of each roll in the simulation is taken as 100 rad/s. The materials of sheet metal used in the numerical simulation are low carbon steel 08Al sheet and aluminum alloy 2024-O sheet, the relevant material parameters are listed in Table 1. The curve of uniaxial stress versus strain was obtained by tensile test shown in Fig. 3. Torus and saddle surfaces are simulated respectively. Fig. 4 shows the finite element model of the torus surface.

Fig. 3. Uniaxial stress-strain curves of the sheet metals.

Fig. 4. Finite element model of torus surface.

Table 1. Mechanical properties of sheet metals used in simulations and experiments.

Material Young’s modulus (GPa) Yield stress (MPa) Poisson’s ratio Density (kg/m3) 08Al 210 135 0.31 7800 2024-O 40.6 75.5 0.33 2720

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4. Results of numerical simulation and experiment

4.1. Analysis of the forming results

Fig. 5 shows the simulated result, experimental result and plastic strain distribution of three-dimensional surface parts of different sheet materials. Figs. 5(a) and (c) shows three-dimensional surface parts of low carbon steel 08Al sheet, and Figs. 5(b) and (d) shows three-dimensional surface parts of aluminum alloy 2024-O sheet. From the simulated and experimental results, it is observed that the simulated results are consistent with the experimental results which verify the feasibility of the use of simulation to guide the experiment. The non-uniform roll gap leads to the non-uniform plastic strain distributions of the forming parts in the transverse direction, and the plastic strain distributions of the forming parts are substantially continuous in the longitudinal direction which indicates the forming process is stable.

Fig. 5. Simulated results, experimental results and plastic strain distribution of three-dimensional surface parts of different sheet materials. (a) Torus part of low carbon steel 08Al sheet, (b) torus part of aluminum alloy 2024-O sheet, (c) saddle part of low carbon steel 08Al sheet and (d) saddle part of aluminum alloy 2024-O sheet.

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4.2. Analysis of shape errors of simulated results

The forming accuracy of the forming part is investigated by analyzing the simulation result. The transverse and longitudinal target curvature radius is RT = 300 mm and RL = 330 mm for the spherical surface parts, and RT = 300 mm and RL = 320 mm for the saddle surface parts. Three-dimensional point-cloud of the simulated results could be obtained from the software of ABAQUS. The software of Geomagic Qualify is chosen to obtain the shape error of forming surface by comparing the measured data with the target surface. Fig. 6 displays the distributions of normal errors of simulated results. Fig. 6(a) and (c) are forming parts of low carbon steel 08Al sheet, and Fig. 6(b) and (d) are forming parts of aluminum alloy 2024-O sheet. The shape errors of forming parts are relatively small in the middle region and relatively large errors in the head and tail regions of the simulated results. Fig. 7 displays the distributions of normal errors of the forming parts in the transverse and longitudinal directions, the measured locations are the transverse and longitudinal centerlines of the forming parts. It is observed that the normal errors of the forming parts of low carbon steel 08Al sheet are better than that of the forming parts of aluminum alloy 2024-O sheet excluding the head and tail region of the forming parts. The reason is that the fluidity of low carbon steel 08Al sheet is better than that of aluminum alloy 2024-O sheet. On the whole, all the deviations of the forming parts are less than 2 mm excluding the head and tail regions of the deformed sheet in rolling direction. The measured results demonstrate that a three-dimensional part can be obtained by means of flexible rolling process.

Fig. 6. Shape errors of the forming parts of different sheet materials. (a) Torus part of low carbon steel 08Al sheet, (b) torus part of aluminum alloy 2024-O sheet, (c) saddle part of low carbon steel 08Al sheet and (d) saddle part of aluminum alloy 2024-O sheet.

Fig. 7. Shape errors of the transverse and longitudinal centerlines of the forming parts. (a) Torus part of low carbon steel 08Al sheet in the transverse direction, (b) torus part of low carbon steel 08Al sheet in the longitudinal direction, (c) saddle part of aluminum alloy 2024-O sheet in the transverse direction and (d) saddle part of aluminum alloy 2024-O sheet in the longitudinal direction.

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5. Conclusions

Flexible rolling is a novel forming process for three-dimensional surface parts combining the rolling process with multi-point adjusting technology. Based on the investigation, the following conclusions can be drawn:

1. Simulated results of torus and saddle surfaces of different sheet materials are obtained. The simulated results are well agreed with the experimental results. The non-uniform roll gap leads to the non-uniform plastic strain distribution of the forming surfaces in the transverse direction and the plastic strain distribution of the forming surfaces is substantially continuous in the longitudinal direction which indicates the forming process is stable.

2. The comparison of the forming precision with different sheet materials is presented by measuring the shape errors of the simulation results, The shape errors of simulated results show that the forming precision of the 08Al sheet is better than that of the 2024-O sheet, but all the deviations of the forming parts are less than 2.0 mm excluding the head and tail regions of the forming parts in rolling direction.

Acknowledgements

The authors would like to acknowledge the financial support provided by National Natural Science Foundation of China (No. 51275202) and the computer hardware support provided by High Performance Computing Center of Jilin University, China.

References

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