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Experiments on T-shape hydroforming with counter punch
Journal of Materials Processing Technology 192–193 (2007) 243–248
Experiments on T-shape hydroforming with counter punch Y.M. Hwang ∗ , T.C. Lin, W.C. Chang Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
Abstract In this study, a hydroforming test machine is designed and developed for tube hydroforming processes. This hydroforming test machine features four independent controls of two axial feeding punches, an internal pressure, and a counter punch. Using annealed AA6063-T5 and 6011A aluminum tubes, experiments of bulge forming and T-shape hydroforming are conducted. Loading paths and thickness distribution of the formed product are discussed. The branch heights of the formed products with and without the counter punch are compared to manifest the merit of using a counter punch during tube hydroforming. © 2007 Elsevier B.V. All rights reserved. Keywords: Tube hydroforming; Loading path; Counter punch
1. Introduction Nowadays, hydroforming processes have been widely applied to manufacturing parts in various fields, such as automobile, aircraft and aerospace, and ship building industries, due to the increasing demands for lightweight parts [1–3]. Concerning the studies on tube and pipe hydroforming processes, Sokolowski et al. [1] have carried out a series of simulations and experiments on tube formability tests. Dohmann and Hartl [2] have also undertaken a lot of investigations on tube hydroforming processes, such as manufacturing axisymmetrical parts and T-shape parts by expansion and feeding. The present authors [4] have developed a model considering sticking friction mode to predict the forming pressure and thickness distribution of the formed parts during expansion in a rectangular die. Some of the studies concerning hydroforming machines have been reported. For example, Thiruvarudchelvan et al. [5] designed a hydraulic bulge-forming machine with axial feeding to carry out the experiments of tube bulge forming. However, PLC was used to control the internal hydraulic pressure and axial load in the experiments, so using a personal computer with feedback control was recommended. Rimkus et al. [6] proposed a guideline for the design of the load curves during tube hydroforming. Ahmed and Hashmi [7] also discussed extensively the effects of various forming parameters on the internal pressure and punch load during bulge forming of tubular com∗
Corresponding author. Tel.: +886 7 525 2000x4233; fax: +886 7 525 4299. E-mail address: [email protected] (Y.M. Hwang).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.087
ponents. Mizukoshi et al. [8] carried out experiments of Tee fitting hydraulic forming of aluminum alloy tubes. The effects of counter punch on the formability were discussed. Fuchizawa et al. [9] carried out experiments of T- and cross-hydroforming of aluminum alloy tubes. The effects of the internal pressure and load distance on the branch heights and the thickness distribution of the products were systematically discussed. One of the present authors [10] has presented a finite element model to simulate the T-shape hydroforming processes with internal pressure and axial feeding. In this paper, a hydroforming machine for general purposes with internal pressure, two axial feeding punches and a counter punch is designed and developed. Experiments on bulge forming and T-shape protrusion forming with and without counter punch are carried out to manifest the advantage of using a counter punch during tube hydroforming processes. 2. Design and manufacturing of a hydroforming test machine A hydroforming test machine is designed and manufactured. This test machine consists of three main parts: a platform for supporting the tooling; a hydraulic power system for providing the pressure source of the internal pressure and the feeding punches; and a PC-based control system. This test machine can operate with up to 70 MPa for the internal pressure and 24 tonnes for the axial force, which is sufficient for hydraulically forming aluminum, copper and low carbon steel tubes. Fig. 1 shows a schematic diagram of the platform and the tooling set.
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for the die is PDS5 tool steel and has been annealed to a hardness of HRC30-33. Using the Tresca yielding criterion, the die can hold an internal pressure of up to 87.9 MPa. The entrance radius of the die is 15 mm. The inner surface of the die is constructed by spreading smoothly from the top part to the bottom part of the die. A V-shape is, then, formed at the side part of the die. This kind of die is usually called cross-type die. The initial outer diameter of the tube is 72 mm and that of the protruded branch is also set as 72 mm. The initial thickness and length of the tube are 2.8 mm and about 300 mm, respectively, as shown in Fig. 2. 2.3. Pushing punch
Fig. 1. Schematic diagram of the platform and the tooling set: (a) top view and (b) front view.
2.1. Platform and tooling set A platform or foundation is designed to support the three hydraulic cylinders and the tooling set, as shown in Fig. 1. This platform made up of carbon steel S35C requires a precisionmachined horizontal surface. Three cylinders and the die are fixed on the platform. The cylinders are coupled with the feeding punches using a specially designed mechanism to eliminate the bending moment between them.
Two axial pushing punches and one counter punch is designed and manufactured. One oil entry path and one oil exit path is designed in the axial pushing punches, as shown in Fig. 2. The oil entry path is located at the center of the cross-section of the pushing punch. Whereas, the oil exit path is located at the upper part of the pushing punch to let the air flow out easily at the beginning of the hydroforming process. The pushing punches are coupled with the hydraulic cylinders using a special design for releasing the bending moment as the hydraulic cylinders are not completely aligned with the pushing punches. The material for the pushing punches is nickel-chromiummolybdenum-steel SNCM8, the yielding strength of which can reach 880 MPa. The inner diameter and the minimum outer diameter are 6 and 66 mm, respectively. Thus, this pushing punch can resist an internal pressure of up to 70 MPa. A smaller diameter is made at the front end of the pushing punch, as shown in Fig. 2, for oil sealing. The counter punch is just used for applying a force to counterbalance the internal pressure, thus, carbon steel S35C is used for the material. 2.4. Hydraulic power system
2.2. Die set A die set for T-shape protrusion is designed and manufactured. Its dimension and the relationship with the tube and punches (or pushing rods) are shown in Fig. 2. The material
A hydraulic power system made up of an electric motor, a hydraulic pump, an accumulator, filters, an oil tank, control valves, oil pipes, etc., is shown in Fig. 3. The specifications of its parts are given in Table 1. A high-pressure source from Table 1 Specification of the hydraulic system
Fig. 2. Relationship between the die, tube and punches.
No.
Item
Explanations
1.
Hydraulic cylinder
2.
Hydraulic pump
3.
Accumulator
4.
Intensifier booster
5. 6.
Displacement transducer Pressure transducer
Bore size: Ø 125 mm, piston rod: Ø 60 mm, stroke: 150 mm, working range: 0–20 MPa, maximum thrust: 24 tonnes Capacity: 8 cm3 /rev, working range: 0–25 MPa, rotational speed: 500–2000 rpm Capacity: 4 L, allowable pressure: 33 MPa Bore size: Ø 61 mm, capacity: 2000 cm3 , working range: 0–70 MPa Stroke: 300 mm, accuracy: 0.01 mm Working range: 0–40 MPa, accuracy: 0.1 MPa
Y.M. Hwang et al. / Journal of Materials Processing Technology 192–193 (2007) 243–248
245
Fig. 3. Schematic of hydraulic power system and the control loop.
this hydraulic power system is applied to the three hydraulic cylinders, which are used to push the two ends of the tube and counterbalance the protruded branch of the product. 2.5. Pressure intensifier The maximum pressure from the hydraulic power system is only 25 MPa, which is not large enough for the pressure source of the internal pressure during the T-shape hydroforming, thus, a pressure intensifier using a screw-pushing mechanism is designed as denoted by (4) and (5) in Fig. 3. This pressure intensifier can output a pressure of up to 70 MPa. Because this pressure source is independent to that from the hydraulic power system, another merit for using this pressure intensifier is that the control of the internal pressure would not be influenced by the movement of the hydraulic cylinders during hydroforming. 2.6. Computer control system The computer control system consists of a computer, interface cards, control boxes, signal converters, solid-state relays (SSR), three displacement transducers, a pressure transducer, etc. The signals from the pressure transducer and the three position transducers are feed-backed to control simultaneously three hydraulic proportional valves and the pressure intensifier motor, which control the displacements of the three hydraulic cylinders and the internal pressure inside the tube, respectively. The sample rate is 20 times per second. A self-compiled software is used to monitor the signals and display the control status graphically on-line. 3. Testing of the hydroforming machine The dynamic responses of the two axial feeding punches, the counter punch, as well as the internal pressure are tested. The responses of the internal pressure are shown in Fig. 4(a and b). In Fig. 4(a), how fast a constant pressure can be reached is tested for
Fig. 4. Dynamic response of internal pressure: (a) constant pressure tests and (b) increased pressure tests.
the self-designed pressure intensifier without feedback control. From the figure, it is known that the increasing rate is slower at the beginning due to compressibility of oil. In order to increase the increasing rate at the beginning, a pressure of 1 MPa is preset for the actual forming process. The internal pressure reaches the steady state of 20 MPa after about 10 s. The oscillation or deviation is very small at the steady state. In Fig. 4(b), the linear line is the prescribed pressure and the curve with small oscillation is the actual pressure response with feedback control. Small oscillation occurs due to compressibility of oil. The sample rate is 20 times per second. From the figure, it is known that the actual pressure follows the prescribed pressure curve. Fig. 5 shows the dynamic response of the left and right axial cylinders. The pump pressure is set as 17.5 MPa. Because the oil pipe route of the left cylinder is not completely the same as that of the right cylinder, the let and right cylinders did not move forward with the same paces, as shown in Fig. 5(a). After adjustment of the regulation valves, they move with identical speed and pace as shown in Fig. 5(b). The cylinders travel 100 mm within about 12 s. The dynamic response of the counter punch cylinder is shown in Fig. 6. It is clear that the counter punch can move backward stably with a constant speed. The total stroke is 30 mm.
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4. Experiments on hydroforming processes 4.1. Bulge forming The die set on the platform is changeable. A symmetrical open die with a bulge length of 60 mm is designed and a bulge forming process with axial feeding punches is conducted. Aluminum tubes of 6011A are used and their initial outer diameter and thickness are 51.91 and 1.86 mm, respectively. The loading path used in the forming process and the formed product is shown in Fig. 7(a and b), respectively. In Fig. 7(a), the prescribed internal pressure and the loading feeding are increased in a bi-linear way, according to an adaptive simulation [11]. The maximum internal pressure is 10.5 MPa and the stroke of the axial feeding is 5 mm. The actual loading paths follow completely the prescribed loading path. From Fig. 7(b), it is known that a relatively good product is obtained.
Fig. 5. Dynamic response of the right and left axial cylinders: (a) before adjustment and (b) after adjustment.
Fig. 6. Dynamic response of the counter punch cylinder.
Fig. 7. Loading path and the product by bulge forming: (a) loading path and (b) outlook of the product.
Y.M. Hwang et al. / Journal of Materials Processing Technology 192–193 (2007) 243–248
247
Fig. 9. Outlook of the products for T-shape protrusion forming. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
Fig. 8. Loading paths for T-shape protrusion experiments. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
4.2. T-shape protrusion forming A T-shape protrusion process is conducted to illustrate the controllability of this newly developed hydroforming machine. The counter punch (CP) is used to control the branch height and avoid over-thinning or bursting at the corner of the protruded branch. Two kinds of loading paths are used to control the axial feeding punches, the counter punch and the internal pressure, as shown in Fig. 8(a and b). From the figures, it is known that at the first stage of bulge forming, the counter punch did not move. After an enough contact area is generated between the counter punch and the branch, the counter punch starts to move backward. In this way, the thickness distribution at the top of the branch can be hold until the end of the forming process. In Fig. 8(a), the left and right axial feeding punches travel 62 mm forward, the maximum internal pressure reaches 15 MPa, and the counter punch travels 27 mm backward. Whereas, in Fig. 8(b), the stroke of the axial punches is 56 mm, and the
maximum internal pressure is 18 MPa. At the late stage of the pressurization, the internal pressure is increased dramatically to make the corner radius reach 5 mm. The outlook of the formed products is shown in Fig. 9(a and b). The protruded branch heights for both cases are 42 mm. Because the axial feeding punches move forward too much, and the internal pressure is smaller in case (a), wrinkling occurred at the surface of the product as shown in Fig. 9(a). There are several stops while the counter punch is moving backward, as shown in Fig. 8(a). That is probably one of the reasons for the wrinkling occurring at the surface. The appearance of the product for case (b) is wrinkling-free with the same branch length of case (a). Thus, the loading path of case (b) is a feasible one for obtaining sound products. The thickness distribution for both cases is shown in Fig. 10. It is clear that the thickest part occurs at around the die entrance because the material is accumulated there. The tube at the guiding zone becomes thicker than the initial thickness, because it is subjected a compressive stress and the die and axial punches undergo elastic deformation. The thinnest part is located at the
Fig. 10. Thickness distribution of the formed product. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
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center of the protruded branch. Nevertheless, the thickness ratio is still larger than 0.8. 5. Comparisons between the forming processes with and without counter punch The loading paths with and without a counter punch are shown in Fig. 11(a and b), respectively. For the loading path with a counter punch shown in Fig. 11(a), the axial feeding punches move forward with a constant speed, whereas, the internal pressure is increased with three stages of different slopes. The counter punch begins to move backward after the axial feeding punches travel 10 mm. For the loading path without a counter punch in Fig. 11(b), the internal pressure is increased linearly, and the axial feeding punches move forward with gradually increasing speeds. These loading paths are obtained by adaptive simulation [11]. The stroke of the axial feeding is 50 mm and the maximum internal pressure is 12 MPa for both cases.
Fig. 12. Outlook of the product for T-shape protrusion with and without counter punch: (a) with counter punch and (b) without counter punch.
The products with and without counter punch are shown in Fig. 12(a and b), respectively. It is clear that the product using a counter punch has a higher branch height of 42 mm compared to 34 mm without a counter punch. 6. Conclusions A hydroforming test machine with four independent controls of two axial feeding punches, an internal pressure, and a counter punch was designed and developed. Experiments on bulge forming using aluminum tube 6011A and T-shape protrusion using annealed AA6063-T5 were conducted to test the controllability of this newly developed machine. Two different loading paths and the corresponding thickness distribution of the formed product were discussed. A loading path, which can generate a sound product, was obtained. The branch heights of the formed products with and without a counter punch were also compared to manifest the merit of using a counter punch during tube hydroforming. Acknowledgments The authors would like to extend their thanks to the National Science Council of the Republic of China under Grant No. NSC 93-2212-E110-002. The advice and financial support of NSC are gratefully acknowledged. References
Fig. 11. Loading path for T-shape protrusion with and without counter punch: (a) with counter punch and (b) without counter punch.
[1] T. Sokolowski, K. Gerke, M. Ahmetoglu, T. Altan, J. Mater. Proc. Technol. 98 (2000) 34–40. [2] F. Dohmann, Ch. Hartl, J. Mater. Proc. Technol. 60 (1996) 669–676. [3] S. Fuchizawa, M. Narazaki, J. Jpn. Soc. Technol. Plast. 30 (1989) 520. [4] Y.M. Hwang, W.C. Chen, Int. J. Plast. 21 (2005) 1815–1833. [5] S. Thiruvarudchelvan, G.L. Seet, H.E. Ang, J. Mater. Proc. Technol. 57 (1995) 182–188. [6] W. Rimkus, H. Bauer, M.J.A. Mihsein, J. Mater. Proc. Technol. 108 (2000) 97–105. [7] M. Ahmed, M.S.J. Hashmi, J. Mater. Proc. Technol. 64 (1997) 9–23. [8] H. Mizukoshi, H. Okada, H. Wakabayashi, Proceedings of the 49th Japanese Joint Conference Technology of Plasticity (1998) 323– 324. [9] S. Fuchizawa, T. Takano, A. Shirayori, M. Narazaki, Proceedings of the 51st Japanese Joint Conference Technology of Plasticity, 2000, pp. 357– 358. [10] Y.M. Hwang, Y.K. Lin, Key Eng. Mater. 233–236 (2003) 317–322. [11] B.H. Chen, Adaptive simulation of tube hydroforming processes with counter pressure, Master Thesis, National Sun Yat-Sen University, Kaohsiung, Taiwan, 2005 (in Chinese).
Experiments on T-shape hydroforming with counter punch Y.M. Hwang ∗ , T.C. Lin, W.C. Chang Department of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
Abstract In this study, a hydroforming test machine is designed and developed for tube hydroforming processes. This hydroforming test machine features four independent controls of two axial feeding punches, an internal pressure, and a counter punch. Using annealed AA6063-T5 and 6011A aluminum tubes, experiments of bulge forming and T-shape hydroforming are conducted. Loading paths and thickness distribution of the formed product are discussed. The branch heights of the formed products with and without the counter punch are compared to manifest the merit of using a counter punch during tube hydroforming. © 2007 Elsevier B.V. All rights reserved. Keywords: Tube hydroforming; Loading path; Counter punch
1. Introduction Nowadays, hydroforming processes have been widely applied to manufacturing parts in various fields, such as automobile, aircraft and aerospace, and ship building industries, due to the increasing demands for lightweight parts [1–3]. Concerning the studies on tube and pipe hydroforming processes, Sokolowski et al. [1] have carried out a series of simulations and experiments on tube formability tests. Dohmann and Hartl [2] have also undertaken a lot of investigations on tube hydroforming processes, such as manufacturing axisymmetrical parts and T-shape parts by expansion and feeding. The present authors [4] have developed a model considering sticking friction mode to predict the forming pressure and thickness distribution of the formed parts during expansion in a rectangular die. Some of the studies concerning hydroforming machines have been reported. For example, Thiruvarudchelvan et al. [5] designed a hydraulic bulge-forming machine with axial feeding to carry out the experiments of tube bulge forming. However, PLC was used to control the internal hydraulic pressure and axial load in the experiments, so using a personal computer with feedback control was recommended. Rimkus et al. [6] proposed a guideline for the design of the load curves during tube hydroforming. Ahmed and Hashmi [7] also discussed extensively the effects of various forming parameters on the internal pressure and punch load during bulge forming of tubular com∗
Corresponding author. Tel.: +886 7 525 2000x4233; fax: +886 7 525 4299. E-mail address: [email protected] (Y.M. Hwang).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.087
ponents. Mizukoshi et al. [8] carried out experiments of Tee fitting hydraulic forming of aluminum alloy tubes. The effects of counter punch on the formability were discussed. Fuchizawa et al. [9] carried out experiments of T- and cross-hydroforming of aluminum alloy tubes. The effects of the internal pressure and load distance on the branch heights and the thickness distribution of the products were systematically discussed. One of the present authors [10] has presented a finite element model to simulate the T-shape hydroforming processes with internal pressure and axial feeding. In this paper, a hydroforming machine for general purposes with internal pressure, two axial feeding punches and a counter punch is designed and developed. Experiments on bulge forming and T-shape protrusion forming with and without counter punch are carried out to manifest the advantage of using a counter punch during tube hydroforming processes. 2. Design and manufacturing of a hydroforming test machine A hydroforming test machine is designed and manufactured. This test machine consists of three main parts: a platform for supporting the tooling; a hydraulic power system for providing the pressure source of the internal pressure and the feeding punches; and a PC-based control system. This test machine can operate with up to 70 MPa for the internal pressure and 24 tonnes for the axial force, which is sufficient for hydraulically forming aluminum, copper and low carbon steel tubes. Fig. 1 shows a schematic diagram of the platform and the tooling set.
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for the die is PDS5 tool steel and has been annealed to a hardness of HRC30-33. Using the Tresca yielding criterion, the die can hold an internal pressure of up to 87.9 MPa. The entrance radius of the die is 15 mm. The inner surface of the die is constructed by spreading smoothly from the top part to the bottom part of the die. A V-shape is, then, formed at the side part of the die. This kind of die is usually called cross-type die. The initial outer diameter of the tube is 72 mm and that of the protruded branch is also set as 72 mm. The initial thickness and length of the tube are 2.8 mm and about 300 mm, respectively, as shown in Fig. 2. 2.3. Pushing punch
Fig. 1. Schematic diagram of the platform and the tooling set: (a) top view and (b) front view.
2.1. Platform and tooling set A platform or foundation is designed to support the three hydraulic cylinders and the tooling set, as shown in Fig. 1. This platform made up of carbon steel S35C requires a precisionmachined horizontal surface. Three cylinders and the die are fixed on the platform. The cylinders are coupled with the feeding punches using a specially designed mechanism to eliminate the bending moment between them.
Two axial pushing punches and one counter punch is designed and manufactured. One oil entry path and one oil exit path is designed in the axial pushing punches, as shown in Fig. 2. The oil entry path is located at the center of the cross-section of the pushing punch. Whereas, the oil exit path is located at the upper part of the pushing punch to let the air flow out easily at the beginning of the hydroforming process. The pushing punches are coupled with the hydraulic cylinders using a special design for releasing the bending moment as the hydraulic cylinders are not completely aligned with the pushing punches. The material for the pushing punches is nickel-chromiummolybdenum-steel SNCM8, the yielding strength of which can reach 880 MPa. The inner diameter and the minimum outer diameter are 6 and 66 mm, respectively. Thus, this pushing punch can resist an internal pressure of up to 70 MPa. A smaller diameter is made at the front end of the pushing punch, as shown in Fig. 2, for oil sealing. The counter punch is just used for applying a force to counterbalance the internal pressure, thus, carbon steel S35C is used for the material. 2.4. Hydraulic power system
2.2. Die set A die set for T-shape protrusion is designed and manufactured. Its dimension and the relationship with the tube and punches (or pushing rods) are shown in Fig. 2. The material
A hydraulic power system made up of an electric motor, a hydraulic pump, an accumulator, filters, an oil tank, control valves, oil pipes, etc., is shown in Fig. 3. The specifications of its parts are given in Table 1. A high-pressure source from Table 1 Specification of the hydraulic system
Fig. 2. Relationship between the die, tube and punches.
No.
Item
Explanations
1.
Hydraulic cylinder
2.
Hydraulic pump
3.
Accumulator
4.
Intensifier booster
5. 6.
Displacement transducer Pressure transducer
Bore size: Ø 125 mm, piston rod: Ø 60 mm, stroke: 150 mm, working range: 0–20 MPa, maximum thrust: 24 tonnes Capacity: 8 cm3 /rev, working range: 0–25 MPa, rotational speed: 500–2000 rpm Capacity: 4 L, allowable pressure: 33 MPa Bore size: Ø 61 mm, capacity: 2000 cm3 , working range: 0–70 MPa Stroke: 300 mm, accuracy: 0.01 mm Working range: 0–40 MPa, accuracy: 0.1 MPa
Y.M. Hwang et al. / Journal of Materials Processing Technology 192–193 (2007) 243–248
245
Fig. 3. Schematic of hydraulic power system and the control loop.
this hydraulic power system is applied to the three hydraulic cylinders, which are used to push the two ends of the tube and counterbalance the protruded branch of the product. 2.5. Pressure intensifier The maximum pressure from the hydraulic power system is only 25 MPa, which is not large enough for the pressure source of the internal pressure during the T-shape hydroforming, thus, a pressure intensifier using a screw-pushing mechanism is designed as denoted by (4) and (5) in Fig. 3. This pressure intensifier can output a pressure of up to 70 MPa. Because this pressure source is independent to that from the hydraulic power system, another merit for using this pressure intensifier is that the control of the internal pressure would not be influenced by the movement of the hydraulic cylinders during hydroforming. 2.6. Computer control system The computer control system consists of a computer, interface cards, control boxes, signal converters, solid-state relays (SSR), three displacement transducers, a pressure transducer, etc. The signals from the pressure transducer and the three position transducers are feed-backed to control simultaneously three hydraulic proportional valves and the pressure intensifier motor, which control the displacements of the three hydraulic cylinders and the internal pressure inside the tube, respectively. The sample rate is 20 times per second. A self-compiled software is used to monitor the signals and display the control status graphically on-line. 3. Testing of the hydroforming machine The dynamic responses of the two axial feeding punches, the counter punch, as well as the internal pressure are tested. The responses of the internal pressure are shown in Fig. 4(a and b). In Fig. 4(a), how fast a constant pressure can be reached is tested for
Fig. 4. Dynamic response of internal pressure: (a) constant pressure tests and (b) increased pressure tests.
the self-designed pressure intensifier without feedback control. From the figure, it is known that the increasing rate is slower at the beginning due to compressibility of oil. In order to increase the increasing rate at the beginning, a pressure of 1 MPa is preset for the actual forming process. The internal pressure reaches the steady state of 20 MPa after about 10 s. The oscillation or deviation is very small at the steady state. In Fig. 4(b), the linear line is the prescribed pressure and the curve with small oscillation is the actual pressure response with feedback control. Small oscillation occurs due to compressibility of oil. The sample rate is 20 times per second. From the figure, it is known that the actual pressure follows the prescribed pressure curve. Fig. 5 shows the dynamic response of the left and right axial cylinders. The pump pressure is set as 17.5 MPa. Because the oil pipe route of the left cylinder is not completely the same as that of the right cylinder, the let and right cylinders did not move forward with the same paces, as shown in Fig. 5(a). After adjustment of the regulation valves, they move with identical speed and pace as shown in Fig. 5(b). The cylinders travel 100 mm within about 12 s. The dynamic response of the counter punch cylinder is shown in Fig. 6. It is clear that the counter punch can move backward stably with a constant speed. The total stroke is 30 mm.
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4. Experiments on hydroforming processes 4.1. Bulge forming The die set on the platform is changeable. A symmetrical open die with a bulge length of 60 mm is designed and a bulge forming process with axial feeding punches is conducted. Aluminum tubes of 6011A are used and their initial outer diameter and thickness are 51.91 and 1.86 mm, respectively. The loading path used in the forming process and the formed product is shown in Fig. 7(a and b), respectively. In Fig. 7(a), the prescribed internal pressure and the loading feeding are increased in a bi-linear way, according to an adaptive simulation [11]. The maximum internal pressure is 10.5 MPa and the stroke of the axial feeding is 5 mm. The actual loading paths follow completely the prescribed loading path. From Fig. 7(b), it is known that a relatively good product is obtained.
Fig. 5. Dynamic response of the right and left axial cylinders: (a) before adjustment and (b) after adjustment.
Fig. 6. Dynamic response of the counter punch cylinder.
Fig. 7. Loading path and the product by bulge forming: (a) loading path and (b) outlook of the product.
Y.M. Hwang et al. / Journal of Materials Processing Technology 192–193 (2007) 243–248
247
Fig. 9. Outlook of the products for T-shape protrusion forming. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
Fig. 8. Loading paths for T-shape protrusion experiments. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
4.2. T-shape protrusion forming A T-shape protrusion process is conducted to illustrate the controllability of this newly developed hydroforming machine. The counter punch (CP) is used to control the branch height and avoid over-thinning or bursting at the corner of the protruded branch. Two kinds of loading paths are used to control the axial feeding punches, the counter punch and the internal pressure, as shown in Fig. 8(a and b). From the figures, it is known that at the first stage of bulge forming, the counter punch did not move. After an enough contact area is generated between the counter punch and the branch, the counter punch starts to move backward. In this way, the thickness distribution at the top of the branch can be hold until the end of the forming process. In Fig. 8(a), the left and right axial feeding punches travel 62 mm forward, the maximum internal pressure reaches 15 MPa, and the counter punch travels 27 mm backward. Whereas, in Fig. 8(b), the stroke of the axial punches is 56 mm, and the
maximum internal pressure is 18 MPa. At the late stage of the pressurization, the internal pressure is increased dramatically to make the corner radius reach 5 mm. The outlook of the formed products is shown in Fig. 9(a and b). The protruded branch heights for both cases are 42 mm. Because the axial feeding punches move forward too much, and the internal pressure is smaller in case (a), wrinkling occurred at the surface of the product as shown in Fig. 9(a). There are several stops while the counter punch is moving backward, as shown in Fig. 8(a). That is probably one of the reasons for the wrinkling occurring at the surface. The appearance of the product for case (b) is wrinkling-free with the same branch length of case (a). Thus, the loading path of case (b) is a feasible one for obtaining sound products. The thickness distribution for both cases is shown in Fig. 10. It is clear that the thickest part occurs at around the die entrance because the material is accumulated there. The tube at the guiding zone becomes thicker than the initial thickness, because it is subjected a compressive stress and the die and axial punches undergo elastic deformation. The thinnest part is located at the
Fig. 10. Thickness distribution of the formed product. (a) AF = 62 mm, Pimax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.
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center of the protruded branch. Nevertheless, the thickness ratio is still larger than 0.8. 5. Comparisons between the forming processes with and without counter punch The loading paths with and without a counter punch are shown in Fig. 11(a and b), respectively. For the loading path with a counter punch shown in Fig. 11(a), the axial feeding punches move forward with a constant speed, whereas, the internal pressure is increased with three stages of different slopes. The counter punch begins to move backward after the axial feeding punches travel 10 mm. For the loading path without a counter punch in Fig. 11(b), the internal pressure is increased linearly, and the axial feeding punches move forward with gradually increasing speeds. These loading paths are obtained by adaptive simulation [11]. The stroke of the axial feeding is 50 mm and the maximum internal pressure is 12 MPa for both cases.
Fig. 12. Outlook of the product for T-shape protrusion with and without counter punch: (a) with counter punch and (b) without counter punch.
The products with and without counter punch are shown in Fig. 12(a and b), respectively. It is clear that the product using a counter punch has a higher branch height of 42 mm compared to 34 mm without a counter punch. 6. Conclusions A hydroforming test machine with four independent controls of two axial feeding punches, an internal pressure, and a counter punch was designed and developed. Experiments on bulge forming using aluminum tube 6011A and T-shape protrusion using annealed AA6063-T5 were conducted to test the controllability of this newly developed machine. Two different loading paths and the corresponding thickness distribution of the formed product were discussed. A loading path, which can generate a sound product, was obtained. The branch heights of the formed products with and without a counter punch were also compared to manifest the merit of using a counter punch during tube hydroforming. Acknowledgments The authors would like to extend their thanks to the National Science Council of the Republic of China under Grant No. NSC 93-2212-E110-002. The advice and financial support of NSC are gratefully acknowledged. References
Fig. 11. Loading path for T-shape protrusion with and without counter punch: (a) with counter punch and (b) without counter punch.
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