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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. ChangDepartment of Mechanical and Electro-Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

bstract

In this study, a hydroforming test machine is designed and developed for tube hydroforming processes. This hydroforming test machine featuresour independent controls of two axial feeding punches, an internal pressure, and a counter punch. Using annealed AA6063-T5 and 6011A aluminum

ubes, experiments of bulge forming and T-shape hydroforming are conducted. Loading paths and thickness distribution of the formed product areiscussed. The branch heights of the formed products with and without the counter punch are compared to manifest the merit of using a counterunch during tube hydroforming. 2007 Elsevier B.V. All rights reserved.

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eywords: Tube hydroforming; Loading path; Counter punch

. Introduction

Nowadays, hydroforming processes have been widelypplied to manufacturing parts in various fields, such as auto-obile, aircraft and aerospace, and ship building industries, due

o the increasing demands for lightweight parts [1–3]. Con-erning the studies on tube and pipe hydroforming processes,okolowski et al. [1] have carried out a series of simulationsnd experiments on tube formability tests. Dohmann and Hartl2] have also undertaken a lot of investigations on tube hydro-orming processes, such as manufacturing axisymmetrical partsnd T-shape parts by expansion and feeding. The present authors4] have developed a model considering sticking friction modeo predict the forming pressure and thickness distribution of theormed parts during expansion in a rectangular die.

Some of the studies concerning hydroforming machinesave been reported. For example, Thiruvarudchelvan et al. [5]esigned a hydraulic bulge-forming machine with axial feedingo carry out the experiments of tube bulge forming. However,LC was used to control the internal hydraulic pressure andxial load in the experiments, so using a personal computerith feedback control was recommended. Rimkus et al. [6] pro-osed a guideline for the design of the load curves during tube

ydroforming. Ahmed and Hashmi [7] also discussed exten-ively the effects of various forming parameters on the internalressure and punch load during bulge forming of tubular com-

∗ Corresponding author. Tel.: +886 7 525 2000x4233; fax: +886 7 525 4299.E-mail address: ymhwang@mail.nsysu.edu.tw (Y.M. Hwang).

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924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.04.087

onents. Mizukoshi et al. [8] carried out experiments of Teetting hydraulic forming of aluminum alloy tubes. The effectsf counter punch on the formability were discussed. Fuchizawat al. [9] carried out experiments of T- and cross-hydroformingf aluminum alloy tubes. The effects of the internal pressure andoad distance on the branch heights and the thickness distribu-ion of the products were systematically discussed. One of theresent authors [10] has presented a finite element model to sim-late the T-shape hydroforming processes with internal pressurend axial feeding.

In this paper, a hydroforming machine for general purposesith internal pressure, two axial feeding punches and a counterunch is designed and developed. Experiments on bulge formingnd T-shape protrusion forming with and without counter punchre carried out to manifest the advantage of using a counterunch during tube hydroforming processes.

. Design and manufacturing of a hydroforming testachine

A hydroforming test machine is designed and manufactured.his test machine consists of three main parts: a platform forupporting the tooling; a hydraulic power system for provid-ng the pressure source of the internal pressure and the feedingunches; and a PC-based control system. This test machine can

perate with up to 70 MPa for the internal pressure and 24 tonnesor the axial force, which is sufficient for hydraulically formingluminum, copper and low carbon steel tubes. Fig. 1 shows achematic diagram of the platform and the tooling set.

244 Y.M. Hwang et al. / Journal of Materials Process

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ig. 1. Schematic diagram of the platform and the tooling set: (a) top view andb) front view.

.1. Platform and tooling set

A platform or foundation is designed to support the threeydraulic cylinders and the tooling set, as shown in Fig. 1. Thislatform made up of carbon steel S35C requires a precision-achined horizontal surface. Three cylinders and the die arexed on the platform. The cylinders are coupled with the feedingunches using a specially designed mechanism to eliminate theending moment between them.

.2. Die set

A die set for T-shape protrusion is designed and manufac-ured. Its dimension and the relationship with the tube andunches (or pushing rods) are shown in Fig. 2. The material

Fig. 2. Relationship between the die, tube and punches.

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ing Technology 192–193 (2007) 243–248

or the die is PDS5 tool steel and has been annealed to a hard-ess of HRC30-33. Using the Tresca yielding criterion, the diean hold an internal pressure of up to 87.9 MPa. The entranceadius of the die is 15 mm. The inner surface of the die is con-tructed by spreading smoothly from the top part to the bottomart of the die. A V-shape is, then, formed at the side part of theie. This kind of die is usually called cross-type die. The initialuter diameter of the tube is 72 mm and that of the protrudedranch is also set as 72 mm. The initial thickness and length ofhe tube are 2.8 mm and about 300 mm, respectively, as shownn Fig. 2.

.3. Pushing punch

Two axial pushing punches and one counter punch is designednd manufactured. One oil entry path and one oil exit path isesigned in the axial pushing punches, as shown in Fig. 2. Theil entry path is located at the center of the cross-section of theushing punch. Whereas, the oil exit path is located at the upperart of the pushing punch to let the air flow out easily at theeginning of the hydroforming process. The pushing punchesre coupled with the hydraulic cylinders using a special designor releasing the bending moment as the hydraulic cylinders areot completely aligned with the pushing punches.

The material for the pushing punches is nickel-chromium-olybdenum-steel SNCM8, the yielding strength of which can

each 880 MPa. The inner diameter and the minimum outeriameter are 6 and 66 mm, respectively. Thus, this pushing punchan resist an internal pressure of up to 70 MPa. A smaller diam-ter is made at the front end of the pushing punch, as shown inig. 2, for oil sealing. The counter punch is just used for apply-

ng a force to counterbalance the internal pressure, thus, carbonteel S35C is used for the material.

.4. Hydraulic power system

ydraulic pump, an accumulator, filters, an oil tank, controlalves, oil pipes, etc., is shown in Fig. 3. The specificationsf its parts are given in Table 1. A high-pressure source from

able 1pecification of the hydraulic system

o. Item Explanations

. Hydraulic cylinder Bore size: Ø 125 mm, piston rod: Ø60 mm, stroke: 150 mm, working range:0–20 MPa, maximum thrust: 24 tonnes

. Hydraulic pump Capacity: 8 cm3/rev, working range:0–25 MPa, rotational speed:500–2000 rpm

. Accumulator Capacity: 4 L, allowable pressure:33 MPa

. Intensifier booster Bore size: Ø 61 mm, capacity: 2000 cm3,working range: 0–70 MPa

. Displacement transducer Stroke: 300 mm, accuracy: 0.01 mm

. Pressure transducer 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

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Fig. 3. Schematic of hydraulic power system and the control loop.

his hydraulic power system is applied to the three hydraulicylinders, which are used to push the two ends of the tube andounterbalance the protruded branch of the product.

.5. Pressure intensifier

The maximum pressure from the hydraulic power systems only 25 MPa, which is not large enough for the pressureource of the internal pressure during the T-shape hydroforming,hus, a pressure intensifier using a screw-pushing mechanisms designed as denoted by (4) and (5) in Fig. 3. This pressurentensifier can output a pressure of up to 70 MPa. Because thisressure source is independent to that from the hydraulic powerystem, another merit for using this pressure intensifier is thathe control of the internal pressure would not be influenced byhe movement of the hydraulic cylinders during hydroforming.

.6. Computer control system

The computer control system consists of a computer, inter-ace cards, control boxes, signal converters, solid-state relaysSSR), three displacement transducers, a pressure transducer,tc. The signals from the pressure transducer and the three posi-ion transducers are feed-backed to control simultaneously threeydraulic proportional valves and the pressure intensifier motor,hich control the displacements of the three hydraulic cylinders

nd the internal pressure inside the tube, respectively. The sam-le rate is 20 times per second. A self-compiled software is usedo monitor the signals and display the control status graphicallyn-line.

. Testing of the hydroforming machine

The dynamic responses of the two axial feeding punches, theounter punch, as well as the internal pressure are tested. Theesponses of the internal pressure are shown in Fig. 4(a and b). Inig. 4(a), how fast a constant pressure can be reached is tested for

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ig. 4. Dynamic response of internal pressure: (a) constant pressure tests andb) increased pressure tests.

he self-designed pressure intensifier without feedback control.rom the figure, it is known that the increasing rate is slower at

he beginning due to compressibility of oil. In order to increasehe increasing rate at the beginning, a pressure of 1 MPa is presetor the actual forming process. The internal pressure reaches theteady state of 20 MPa after about 10 s. The oscillation or devi-tion is very small at the steady state. In Fig. 4(b), the linear lines the prescribed pressure and the curve with small oscillation ishe actual pressure response with feedback control. Small oscil-ation occurs due to compressibility of oil. The sample rate is0 times per second. From the figure, it is known that the actualressure follows the prescribed pressure curve.

Fig. 5 shows the dynamic response of the left and right axialylinders. The pump pressure is set as 17.5 MPa. Because theil pipe route of the left cylinder is not completely the same ashat of the right cylinder, the let and right cylinders did not moveorward with the same paces, as shown in Fig. 5(a). After adjust-ent of the regulation valves, they move with identical speed

nd pace as shown in Fig. 5(b). The cylinders travel 100 mm

ithin about 12 s. The dynamic response of the counter punch

ylinder is shown in Fig. 6. It is clear that the counter punch canove backward stably with a constant speed. The total stroke is

0 mm.

246 Y.M. Hwang et al. / Journal of Materials Process

Fig. 5. Dynamic response of the right and left axial cylinders: (a) before adjust-ment and (b) after adjustment.

Fig. 6. Dynamic response of the counter punch cylinder.

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. Experiments on hydroforming processes

.1. Bulge forming

The die set on the platform is changeable. A symmetrical openie with a bulge length of 60 mm is designed and a bulge formingrocess with axial feeding punches is conducted. Aluminumubes of 6011A are used and their initial outer diameter andhickness are 51.91 and 1.86 mm, respectively. The loading pathsed in the forming process and the formed product is shown inig. 7(a and b), respectively. In Fig. 7(a), the prescribed internalressure and the loading feeding are increased in a bi-linearay, according to an adaptive simulation [11]. The maximum

nternal pressure is 10.5 MPa and the stroke of the axial feeding ismm. The actual loading paths follow completely the prescribed

oading path. From Fig. 7(b), it is known that a relatively goodroduct is obtained.

ig. 7. Loading path and the product by bulge forming: (a) loading path and (b)utlook of the product.

Y.M. Hwang et al. / Journal of Materials Processing Technology 192–193 (2007) 243–248 247

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Ibing zone becomes thicker than the initial thickness, because itis subjected a compressive stress and the die and axial punchesundergo elastic deformation. The thinnest part is located at the

ig. 8. Loading paths for T-shape protrusion experiments. (a) AF = 62 mm,

imax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.

.2. T-shape protrusion forming

A T-shape protrusion process is conducted to illustrate theontrollability of this newly developed hydroforming machine.he counter punch (CP) is used to control the branch heightnd avoid over-thinning or bursting at the corner of the pro-ruded branch. Two kinds of loading paths are used to control thexial feeding punches, the counter punch and the internal pres-ure, as shown in Fig. 8(a and b). From the figures, it is knownhat at the first stage of bulge forming, the counter punch didot move. After an enough contact area is generated betweenhe counter punch and the branch, the counter punch starts to

ove backward. In this way, the thickness distribution at theop of the branch can be hold until the end of the formingrocess.

In Fig. 8(a), the left and right axial feeding punches travel2 mm forward, the maximum internal pressure reaches 15 MPa,nd the counter punch travels 27 mm backward. Whereas, inig. 8(b), the stroke of the axial punches is 56 mm, and the

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ig. 9. Outlook of the products for T-shape protrusion forming. (a) AF = 62 mm,

imax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.

aximum internal pressure is 18 MPa. At the late stage of theressurization, the internal pressure is increased dramatically toake the corner radius reach 5 mm.The outlook of the formed products is shown in Fig. 9(a

nd b). The protruded branch heights for both cases are 42 mm.ecause the axial feeding punches move forward too much, and

he internal pressure is smaller in case (a), wrinkling occurredt the surface of the product as shown in Fig. 9(a). There areeveral stops while the counter punch is moving backward, ashown in Fig. 8(a). That is probably one of the reasons for therinkling occurring at the surface. The appearance of the prod-ct for case (b) is wrinkling-free with the same branch length ofase (a). Thus, the loading path of case (b) is a feasible one forbtaining sound products.

The thickness distribution for both cases is shown in Fig. 10.t is clear that the thickest part occurs at around the die entranceecause the material is accumulated there. The tube at the guid-

ig. 10. Thickness distribution of the formed product. (a) AF = 62 mm,

imax = 15 MPa. (b) AF = 56 mm, Pimax = 18 MPa.

2 ocessing Technology 192–193 (2007) 243–248

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enter of the protruded branch. Nevertheless, the thickness ratios still larger than 0.8.

. Comparisons between the forming processes withnd without counter punch

The loading paths with and without a counter punch arehown in Fig. 11(a and b), respectively. For the loading pathith a counter punch shown in Fig. 11(a), the axial feedingunches move forward with a constant speed, whereas, the inter-al pressure is increased with three stages of different slopes. Theounter punch begins to move backward after the axial feedingunches travel 10 mm. For the loading path without a counterunch in Fig. 11(b), the internal pressure is increased linearly,nd the axial feeding punches move forward with gradually

ncreasing speeds. These loading paths are obtained by adap-ive simulation [11]. The stroke of the axial feeding is 50 mmnd the maximum internal pressure is 12 MPa for both cases.

ig. 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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ig. 12. Outlook of the product for T-shape protrusion with and without counterunch: (a) with counter punch and (b) without counter punch.

The products with and without counter punch are shown inig. 12(a and b), respectively. It is clear that the product usingcounter punch has a higher branch height of 42 mm compared

o 34 mm without a counter punch.

. Conclusions

A hydroforming test machine with four independent con-rols of two axial feeding punches, an internal pressure, andcounter punch was designed and developed. Experiments on

ulge forming using aluminum tube 6011A and T-shape pro-rusion using annealed AA6063-T5 were conducted to test theontrollability of this newly developed machine. Two differentoading paths and the corresponding thickness distribution ofhe formed product were discussed. A loading path, which canenerate a sound product, was obtained. The branch heights ofhe formed products with and without a counter punch were alsoompared to manifest the merit of using a counter punch duringube hydroforming.

cknowledgments

The authors would like to extend their thanks to the Nationalcience Council of the Republic of China under Grant No. NSC3-2212-E110-002. The advice and financial support of NSCre gratefully acknowledged.

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