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411

Process technology and example applications

Fig. 5.2.7

Pure calibration

Fig. 5.2.8 Blank shapes

single-walled tubes

extruded sections welded or unwelded

sheet metal blanks

double-walled tubes sheet metal blanks

conically rolled

and welded

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

On the basis of the various semi-finished product shapes (Fig. 5.2.8),

hydroforming can be applied to a wide variety of fields. Three main

user groups have emerged as the most significant over recent years

(Table 5.2.1):

–the automotive engineering and supply industry,

–sanitary and domestic installation industry,

–pipe and pipe component manufacturers.

412

Hydroforming

Sector

automotive industry

vehicles

– road

– water

– air

– rail

chemical, gas, oil industry,

power station construction

domestic appliance industry

bicycle industry

processing of sections

pumps and fittings

heating, ventilation,

air conditioning

furniture industry

lighting industry

optics

Assembly

chassis, exhaust and intake

systems, add-on parts,

drive system, seats,

frames/bodywork,

steering

piping and tank components,

pipe fittings

fittings, machines

pipe fittings

frame construction, semi-

finished pipes, rail-bound

vehicles, utility vehicles

housings

fittings

frames, moulded elements

street lighting

telescopes, torches

Component (examples)

cross members, side members,

manifolds, roof rails, spoilers,

gear shafts, seat frame com-

ponents, A/B/C pillars,

roof frame profiles, steering

column with compensation

T-fittings, reducers,

housings, panelling

tube bends, T-fittings,

reducers, cross pieces,

bends

pedal bearings, joints, frames

joints, girders,

calibrated tubes, sections,

roof arches, frames,

structural members

intake pipes

tube bends, reducers,

T-fittings, manifolds

legs, structural members,

joints, shells, shelves

light masts, lamp shells

housings

Table 5.2.1: Fields of application for the hydroforming technique

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

5 Hydroforming

5.3 Component development

5.3.1 User-oriented project management

In practical industrial application, it is increasingly important to design

a component not only for functionality but also for producibility.

“Simultaneous engineering” and “early involvement” have become

important watchwords in the field of product design, with customer

and process developer cooperating from an early stage in determining

component configuration. Only in this way is it possible to optimize

functional and process engineering related factors within a short time-

frame. A component development activity can be, for example, carried

out as follows:

1. preliminary meeting (customer and developer)

2. technical and economic feasibility study (customer and developer)

3. component configuration suitable for hydroforming (customer and

developer) and definition of a production sequence

4. prototype development: CAD surface data generation (customer) and

CNC milling data generation (developer), manufacture of a forming

die and production of prototypes using different materials and wall

thicknesses (developer)

5. try-out of prototypes (customer)

6. definition of quality-relevant (QA) data (customer)

7. series production (customer or supplier)

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

5.3.2 Feasibility studies

When developing hydroformed components, the precise analysis of

boundary conditions is essential. Optimum design of components tak-

ing into consideration special process-specific factors enhances safety

and also the cost-effectiveness of series production.

The feasibility study, the component configuration and definition of

a production sequence are closely interlinked. Once these processes

have produced a positive result, it is possible to start with prototype

development of the components. When working through the individ-

ual steps, different boundary conditions must be taken into account.

These are briefly outlined below.

Review of feasibility

When reviewing feasibility, a difference is drawn between two groups of

components: new developments with a geometry that has been config-

ured specifically with hydroforming in mind, and the production of

existing components using hydroform technology.

Significant factors here include the geometry, wall thicknesses, mate-

rials and procurement of the semi-finished parts or preforms. Initially,

the geometry data is evaluated with the aid of a CAD/CAM system.

Circumferences, radii, cross sections and transitions are all determined.

A decision must also be taken regarding the characteristics of the pre-

form. In the case of a tube, for instance, these factors would include the

material to be used, the diameter, the wall thickness and also any pos-

sible need for pre-forming.

The expected circumferential expansion is then determined. Taking

into account the material data, the wall thickness, pre-forming of the

tubular blank and other boundary conditions, it is possible to assess the

manufacturing feasibility. Subsequently, the internal pressure, closing

force, forces and movements of the horizontal cylinders have to be cal-

culated. Since the correlations between all the different influencing

variables are highly complex, specific expertise is an essential compo-

nent of successful part configuration.

Part configuration

If it is possible to manufacture the part, the next stage is to design the

details, the position in the die and the slide channels. In the case, when

414

Hydroforming

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

a component does not appear to be producible using hydroforming

technology, an attempt is made to modify the existing conditions in

such a way that the part can be produced without modifying its func-

tional characteristics.

The part geometry must be optimized taking into account the process

limits. Where assemblies are produced, it may under certain circum-

stances be possible to group several parts into a single one or to split the

assemblies up differently. A different material can also be selected. When

making this type of modification, the question of reliability in series pro-

duction must not be neglected, as this is of decisive importance concern-

ing the cost effectiveness and process engineering related issues.

Configuration for series production

This stage includes the definition of a production sequence and config-

uration of the necessary production lines. Consistent implementation

of the prototyping results is of particular importance here. An optimum

product is only achieved through effective interaction of theoretical

analysis and practice-oriented testing. Another aspect is the definition

of quality-relevant QA data.

Forming simulation

The Finite Element Method (FEM) has become an established feature of

metal forming technology. The objective of FEM is to replace costly and

elaborate experimental testing by fast, low-cost computer simulation.

Any practical implementation of preliminary considerations must

take into account the special features of hydroforming:

– consideration of partial strain hardening on pre-formed tubes,

– correlation between process parameters: motions and forces of the

horizontal cylinders, variation of internal pressure in time, motion of

additional die components such as hole punches and counterpres-

sure punches, closing movement of the die,

– functional coordination of process parameters: internal pressure, dif-

ferent additional die elements such as hole punches and counter-

pressure punches and axial cylinders,

– springback,

– failure criteria: buckling and bursting,

– material behavior: anisotropy and flow curve and

– description of the contact (friction) conditions.

415

Component development

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

The following information can be computed using a suitably config-

ured FEM program:

–equivalent plastic strain,

–effective stress,

–thickness distribution,

–distribution of plastic strain rate,

–principal strains and

–distance between outer wall of workpiece and the die wall.

5.3.3Component design

In addition to the use of pre-bent tubes, the process technologies specif-

ic to hydroforming include the generation of cross-sectional changes,

flanges, breakthroughs, simple and multiple branches or the creation of

surfaces for centering welding operations. The key data indicated in

Fig. 5.3.1describes the current state of the art for using tubular-shaped

blanks. The maximum achievable height of a bulge at a straight leg is

markedly higher than one on a bend, as the material is prevented from

further displacement by the geometry of the bend (Fig. 5.3.2).A larger

branched tube height can be achieved if the branch is located near a

horizontal cylinder. In principle, sharp corners and edges should be

416

Hydroforming

part length L

max. 12,000 mm

wall thickness s

0.6 to 50 mm

diameter D

max. 660 mm

Fig. 5.3.1

Present state of the art in hydro-

forming using tubes as blanks

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

avoided when designing a component. Radii must be adjusted to the

wall thickness(Fig. 5.3.3).It is beneficial to provide gentle round tran-

sitions between different cross sections when this is allowable by the

functional requirements.

Some of the fundamental guidelines to be observed when designing

hydroformed components are outlined below.

417

Component development

Fig. 5.3.2 Examples of achievable branched tube heights: the achievable height decreases

with an increasing degree of difficulty

100% D

75% D

15% D

Fig. 5.3.3 Internal pressure as a function of the inside radius

inside radius/wall thickness[–]

internal

pressure

[bar]

5,000

3,000

1,000

1/1

3/1

2/1

4/1

pfrks

ifmax min

min

(,,)=

required internal pressure

hydroforming die

min

=smallest inside radius

of the component

=wall thickness

=stability factor

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Straight component axis

The inflation limit is restricted by the risk of bursting and buckling. The

likelihood of buckling increases with the length of the expanded sec-

tion. With a buckling length # 2 · the diameter of the tubular blank

(starting diameter), and if a rotationally symmetrical cross section is

being processed, the achievable deformation may be assumed to lie well

beyond the ultimate elongation of the material.

Axial curvature

In the case of components with a curved axis in the inflation contour,

axial displacement of material is possible without problems, provided

the curvature radius is R $ 5

the diameter of the initial preform. Partial

inflation takes place here at the inner shell of the bend.

The free buckling length should not exceed the starting diameter,

extend over no more than half the circumference of the tubular blank

and at the same time it should be continuously decreasing.

In free forming without support, the component buckles if axial pres-

sure is applied. The inflation capacity is limited by the ultimate elonga-

tion, as forming takes place purely as a result of the internal pressure.

Cross sections

During free expansion, the maximum deformation is reached when the

free buckling length amounts to a maximum of 2 · starting diameter, and

the cross section is rotationally symmetrical and located in the tube axis.

An asymmetrical cross section with a deep, narrow contour area can

only be formed in cases where material is able to flow axially into the

contour. Sharp contour transitions lead to partial and localized stretch-

ing in the wall and thus to premature bursting. Asymmetrical cross sec-

tions outside the tube axis can be formed in cases where at least half the

circumference of the tubular blank is supported by the die contour.

Longitudinal sections

Contour transitions are best designed with large radii. Suddenly chang-

ing transitions in the direction of material flow lead to the formation of

cross-wrinkles at the component (buckling) and act as a brake to mate-

rial flow in the axial direction. Beyond these underlying guidelines, the

following points should be observed in the configuration of hydro-

formed components:

418

Hydroforming

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Pure expansion/compression

The optimum ratio between the pipe diameter D and pipe wall thickness s is

D/s = 20 … 45

Here too, the free buckling length should be # 2 · the starting diameter D.

If D/s > 45, there is a danger of bursting or buckling. This can be pre-

vented by reducing the free buckling length.

If D/s < 20, the buckling length can be greater with increasing wall

thickness. Thinning of the wall thickness is prevented by axial dis-

placement of the material. However, the same displacement can also

lead to compression of the wall. Only a minimal danger of buckling

exists.

Expansion/compression and local calibration

The maximum inflation capacity is limited by the ultimate elongation

of the material. Half the ultimate elongation represents the limit of

achievable deformation where no axial material flow is possible. In this

case, the buckling length is of no significance.

Pure calibration

The limit of achievable deformation corresponds to half of the ultimate

elongation of the material.

419

Component development

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

5Hydroforming

5.4Die engineering

5.4.1Die layout

Figure 5.4.1illustrates the layout of a hydroforming die. The die set

which comprises an upper and a lower die is used to accommodate the

functional elements. The die holder plates permit adjustment of the

assembly height prescribed by the press stroke or height of the cylinder

holder brackets. The seal punches which are actuated by the displace-

ment or pressure-dependent and controlled horizontal cylinders, they

seal and push the tube ends during the compression process. The piston

Fig. 5.4.1 Schematic view of a die layout

seal punch

slide plate

centering

die holder plate

horizontal cylinder

cylinder holder

bracket with

swivel mounting

die holder plate

hydroformed partbed plate

top die

bottom die

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998