Liquid-bulge-forming as a flexible production method

Journal of

Materials Processing Technology ELSEVIER

Liquid-Bulge-Forming

J. Mater. Process. Technol. 45 (1994) 377-382

as a Flexible Production Method

Fritz Dohmann and Christoph Hartl Laboratorium mr Umformende Fertigungsverfahren, Universit~t-GH Paderborn, Paderborn, Germany This paper represents the structure of a flexible tool system for internal high-pressure metal forming, together with the forming possibilities that it opens up and looks into the results of studies that shows the applicability of the system. tube ends and introduce axial compressive stresses into the tube wall and also serve to transport tube material into the forming zone

1. INTRODUCTION

Internal high-pressure forming is the general term used for processes that involve the forming of tubular workpieces with the support of active media /1/. The internal high-pressure forming processes employed today take in the operations of expanding, displacement and calibration (Fig. 1)/1/. All these processes are based on the same forming principle, i.e. the alignment of a tube to the inside surface of a surrounding profile tool, under the simultaneous action of external mechanical loading and a hydrostatic internal pressure. E×PANSON

CAUBRAT ON

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Fw2 Figure 1. Process types The forming principle requires the following forces to act: -

the closing forces Fs with which the tool is closed

-

the forming forces Fu, which act mainly on the

-

the die forces Fw, which can act perpendicular to the tube wall, although also on elements

branching off the workpiece. The internal pressure Pi is generated in a pressure intensifier which is generally connected to the inside of the tube by a hole in the male die. The forming result is determined by the combined action of the forming forces, the die forces and the internal pressure/2/. These, together with the starting parameters of the tube, are designated the process parameters. They have to be applied selectively as a function of the forming result required/3/. The fields of application for internal highpressure forming include the production of lightweight components for drive and chassis engineering, exhaust system engineering and fitting production/4/. The product range in these fields is subject to the general trend that is currently prevailing in product design, i.e. to a reduction in the volume of identical-geometry parts being produced in favour of a broader range of variants. Hence, flexible process application is a key issue when it comes to internal high-pressure forming too. 2. APPROACHES TO G R E A T E R

FLEXIBILITY There are two different approaches that can be adopted in order to make the process more flexible:

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378

influencing the forming result through the way in which the process parameters are controlled

-

dividing up part of the profile die into segments, which can then be driven separately. This would seem to be a practicable approach, since the loads on the die are comparatively low for a large number of process applications.

-

The present paper shows how the latter approach can be implemented in a flexible die system. This is illustrated in Fig. 2 for the process type "expanding". The profile die is made up of six profile die segments, each t00 mm in height, which can be moved 30 mm in the radial direction by means of hydraulic drives, with a supporting force of 100 kN. A forming force of 1000 kN can be applied, and the pressure intensifier will pernut internal pressures of up to 2000 bar to be achieved+ The die closing force is 1000 kN. This makes it possible to form thin-walled tubes in steel or aluminium with an outside diameter of up to 80 mm. PROCESS

1 EXPANSION

TYPES

2 DtSPLACEMEN T 5 REDUCT'ON

KOMBINATION OF 1, 2 AND 5

supported regions develop between the segments when the tube is expanded, and these are not allowed to exceed a critical value. By swivelling the segments, it is possible to bring these regions into contact with the die again for the ncxi forming operation. 3.

ATTAINABLE

The workpiece shapes thal can be produced are shown in Fig. 3. These cross-sections can be attained through the directional, radial movement of the segments, coordinated with the movements of the male die and the internal pressure control, which is proportional to this. This then gives rise to process variants involving expansion, reduction and displacement operations on the tube wall. B~ combining the principles referred to, it is possible to produce more complex workpiece shapes as well During the expansion process, the lube wall is pressed against the radially adjustable die segments+ Round, oval, square and polygonal cross-sections then result as a function of the particular position of the segments in the expansion region and the segment shape. These cross-sections are not necessarily concentric to the longitudinal axis of the starting tube. It is also possible to achieve partial expansion of the tube, with different cross-sections in the expanded parts.

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Figure 2. The flexible die system Figure 3. Sample workpieces In addition to this, the die with the adjustable segments can be swivelled around the central ~nis of the tube. This movement is necessary because non-

During displacement, a section of tube is displaced in parallel to its starting position through

379

a segment movement perpendicular to the longitudinal axis of the tube. The cross-section of the tube can be either increased through the action of the internal pressure, or reduced through the radial movement of the segments. The shape of the die segments and the forming displacement that they permit makes it possible to achieve stepped and curved workpiece geometries in the longitudinal direction. The displacement of these workpieces will then permit a transition to tube bending under internal pressure.

finite element method in order to establish when the tube becomes pushed inwards. These are set out for different ratios of expansion diameter to starting diameter, dl/do, and for different loads exerted by F u and Pi. It is seen that the critical value of the forming force falls as dl/d 0 increases. The experimentally determined failure cases show good agreement with the calculated values. The finite element method can thus suitably be used for establishing this process limit in other wall thickness to diameter ratio ranges, so~do, as well. 600.

The circumference of the starting tube can be reduced to a certain extent through the radial movement of the segments towards the centre of the tube. If the tube wall comes into contact with appropriately-shaped segment surfaces, it is possible to achieve round, stepped or conical cross-sections, as well as oval, square, polygonal and nonconcentric ones. By combining the three process variants, it is possible to generate workpiece shapes that involve individual sections of the tube being expanded, reduced or displaced.

~

KN

PROCESS

LIMITS

There are restrictions on the shapes that can be achieved through operations performed on the flexible die system, due to failure through plastic instabilities /1,2,5,6/. The instabilities that can be caused by an excessive forming force F u include collapsing, buckling and wrinkling of the tube in the feed section and also the tube being pushed inwards at the transition in cross-section with high levels of expansion. The type of instability that occurs with an excessive internal pressure pi includes necking or the bursting of the workpiece wall between the supporting die segments. The process limits are a function of the: -

starting parameters of the tube

-

process parameters

-

die geometry.

The tube is pushed inwards (Fig. 4) if the forming force F u exceeds a critical value. Figure 4 shows the results of calculations performed with the

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_ CALCULATION 0 dl/d0 =1,7 [] dl/d0 =1,8 A dl/do =1,9

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INTERNAL P R E S S U R E 4.

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EXPERIMENT • dl/C~0 =1,76 dl/d0 =1,84 ~ dl/do =1,94

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Figure 4. Tube pushed inwards, as a limit on the process Calculating the necking and bursting with the finite element method, by contrast, involves a considerably higher outlay. According to /8/, the effective strain ev that is necessary to bring about necking in a tube under internal pressure and axial load can be calculated in the following manner using the stress ratio ~ of the axial stress o z to the tangential stress a t and the strain-hardening exponent n: cv = n 2 ~ - ~ + l

(1)

If the experimental failure cases are set against the values from Eqn. (1), then it can be seen, as in Fig. 5, that these lie above the calculated curve. This is due to the simplification introduced in/8/, which involves the assumption of a constant stress ratio ~ during the forming operation. This constant stress ratio is not guaranteed in the experiment.

380

0,5

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This second type of process control system is employed if borderline values have to be achieved with the process control, such as the maximum wall thickness. This is illustrated in Fig. 6 with the example of step-by-step rotationally-symmetric expansion.

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Figure 5. Bursting as a limit on the process Wrinkling and buckling can be prevented by increasing the internal pressure which supports the tube from the inside. The level of internal pressure required to do this, Pimin, can be roughly calculated as follows/9/: Pi rain = 2k f (s 1 / f ) 2

2C o

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200 N~, ERNA[

400 PRES ~t It;

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(2) Figure 6, Process control for step-by-step expansion

with the yield stress k/; the wall thickness s 1 and the wrinkle height, f The wrinkle height can be established from the work conducted by /10/, for instance. 5. PROCESS

CONTROL

The functions of process control are avoidance of failure cases attainment of the forming result set out in the design specifications The process control system takes in the quantitative correlations that exist between the forming forces and the internal pressure, as well as any die forces that may prevail. Process control can be performed by adjusting the process parameters as a function of the forming displacement (displacement-controlled system) or by adjusting the force and the action of the pressure on the basis of the process limits (force/pressure-controlled system).

The aim here is to determine the highest level of forming that is possible, taking as the basis the maximum internal pressure that can be applied, with the aid of the degree of forming according to Eqn. (1). Failure through collapsing is prevented in this case, since the tube wall comes to rest on the segments after just a small forming displacement. The fact of the tube coming to rest on the segments makes it possible for higher axial forces to be applied than in conventional expansion processes. Providing that the maximum degree of forming is observed, a good forming result is obtained. This is illustrated by the cross-section shown in Fig. 7. A further example is the process control t"oi displacement. The tube is subjected to an axial force and to internal pressure, which satisfy the flow condition. The displacing operation is then performed by means of a segment being moved in the radial direction At the same time, the opposite side of the tube is supported by a second segment in

381

order to prevent the tube from bursting at that point. It is important to ensure that the movement of the two segments is synchronised by the machine control system.

tubes in St 30 AI with a ratio of so/d O = 0,05 and also AI Mg Si 0.5 tubes that have been displaced, with a ratio of sold 0 = 0,08. E I

MEASURING POINTS r

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MEDIUM WALL THICKNESS

As far as rotationally-symmetric expansion is concerned, the experiments have shown that, for the tube in question, the form of process control described permits a higher level of expansion to be achieved, at dl/d 0 = 1,9, than is possible with the single-stage expansion of conventional processes, where a ratio of only dl/d 0 = 1,6 is attained. Oval expansion was studied with ratios of up to 1,6 for the large cross-section to the diameter and with ratios of up to 1,45 for the large cross-section to the small one. For the displacing operation, tests were conducted for a displacement of up to 0,5.d0.

Figure 7. Expanded cross-section The development of a process control system can be verified by the finite element method. Figure 8 shows the example of one of the forming stages. 6. SELECTED STUDY RESULTS The investigations carried out with the flexible die system were first performed on simple shapes. A number of the results of these are set out in Fig. 9 a-c. What is shown are the rotationallysymmetric expansion and the oval expansion of

Figure9.a Experimental results, expanding rotationally cross section

of

382

Dohmann, F.; Bieling, P.: Werkzeugparameter und ProzeMaten beim aufweitenden Innenlaochdrucloamformen. Umformtechnik 26 (1992) 1~ S.23-31. Dohmann, F.; Dudziak, K.-U.; Bau yon Werkzeugen und Maschinen zum Innenhochdruckumformen. B~nder Bleche Rohre 8 (199 ] ), S. 19-29. Figure 9.b Experimental results, expanding of oval cross section

Klaas, F, Kaehler, K.: Herstellung innovativer Hohlteilformen mit den InnenhochdruckVeffahren. In: HFF-Berichte, Nr.12, Umfonntechnisches Kolloquium Hannover, 17 / 18. Marz 1993, S. 9/1-9/10. Dohmann, F.; BOhm, A.; Dudziak, K.-U,: The shaping of hollow shaft-shaped workpieces by liquid bulge forming. In: Advanced Technolog) of Plasticity 1993, Proc. of the fourth Int. Con£ on Technology of Plasticity, Beijing, China 5./9 Septl 1993, S.447-452.

Figure 9.c Experimental results, displacement 7. CONCLUSION AND PROSPECTS Employed in conjunction with a flexible die system, the principle of internal high-pressure forming permits flexible and cost-efficient production of a large number of different-shaped hollow workpieces and lightweight components for industrial applications. This paper has presented a concept for a flexible die system and has set out the requirements that are placed on the die system, the forming machine and the control facilities. The sample industrial applications referred to serve to underline the importance of this particular technology. REFERENCES Dohmann, F.: Innenhochdruckumformen. In: Umformtechnik, K. Lange (Hrsg.), Bd.4, 2.Aufl., Springer Verlag, Berlin u.a., 1993, S. 253-270.

Dohmann, F.; Hartl, Ch.: M6glichkeiten der Innenhochdruckumformung unter besonderer Beachtung des Formens yon Strangprel3profilen. Tagungsband: Neuere Entwicklungen in der Massivumformung, 8./9..Iuni 1993, Fellbach bei Stuttg., 1993, S. 255-279 Dohmann, F.; BOhm, A : Bedeutung der ProzeBsimulation ftir das Innenhochdruckumformen yon Leichtbauwerkstiicken. B~indcr Blcche Rohre 33 (1992) 1, S.26-34 Mellor, P.B. :Tensile Instability in Thin-Walled Tubes. Journal Mechanical Science Vol 4. No.3, 1962, S.251-256 Dohmann, F.; Hartl, Ch.: Prozefifiihrungen zum flexiblen Innenhochdruckumformen forthcoming in B~ader Bleche Rohre. 10 Geckeler, J.W.: Plastisches Knicken tier Wandung von Hotdzylindem und einige anderc Falterscheinungen an Schalen und Blechen. Z. f. angew. Math. u; Mech. 8 (1928) 5, S.341-35l~