An overall review of the tube hydroforming (THF) technology

An overall review of the tube hydroforming (THF) technology

Muammer KocËa,*, Taylan Altanb

aTower Automotive, Technical Center Ð Advanced Technology Division, 3533N. 27th Street, Milwaukee, WI 53216, USAbERC for Net Shape Manufacturing, The Ohio State University, Columbus, OH, USA

Accepted 12 September 2000

Abstract

Increasing use of hydroforming in automotive applications requires intensive research and development on all aspects of this relatively

new technology to satisfy an ever-increasing demand by the industry. This paper summarizes a technological review of hydroforming

process from its early years to very recent dates on various topics such as material, tribology, equipment, tooling, etc., so that other

researcher at different parts of the world can use it for further investigations in this area. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Hydroforming; Tube; Lubrication; Friction; Formability; Pre-forming

1. Introduction

Tube Hydroforming (THF) has been called with many

other names depending on the time and country it was used

and investigated. Bulge forming of tubes (BFTs) and liquid

bulge forming (LBF) were two earlier terms, for instance.

Hydraulic (or hydrostatic) pressure forming (HPF) was

another form of name used for a while by some investigators.

Internal high pressure forming (IHPF) has been mostly used

within German manufacturers and researchers. In some

periods, it was even called as `̀ Unconventional Tee Form-

ing''. Throughout this paper, THF will be used to describe

the metal forming process whereby tubes are formed into

complex shapes with a die cavity using internal pressure,

which is usually obtained by various means such as hydrau-

lic, viscous medium, elastomers, polyurethane, etc., and

axial compressive forces simultaneously, Fig. 1.

Even though THF process has been in practical industrial

use only more than a decade, development of the techniques

and establishment of the theoretical background goes back

to 1940s. Manufacturing of seamless copper ®ttings with T

branches was investigated using internal pressure and axial

load by Grey et al. [1].

Davis tested tubes of medium carbon steel under internal

pressure and tensile axial load in order to determine their

yield and fracture characteristics [2]. Experimental and

numerical studies were conducted to ®nd the bursting pres-

sure of thick-walled cylinders by Faupel, Crossland and

Dietmann during 1950s and 1960s [3±5]. In 1960s, experi-

mental and theoretical investigations on instability of thin-

walled cylinders were performed by many researchers at

different countries [6±8]. Fundamental investigations on

thin- and thick-walled cylinders helped theoretical improve-

ments in LBF operations. Use of hydrostatic pressure in

metal forming processes, in particular, for bulging of tubular

parts was ®rst reported by Fuchs [9]. In this paper, he

reported experimental studies on expansion and ¯anging

of copper tubes using hydraulic pressure.

Ogura and Ueda [10] presented their experimental results

on LBF of Tee shapes from low and medium carbon steel.

Different con®gurations and number of Tee protrusions were

formed using internal pressure and axial compressive load-

ing. Proper forming zones were de®ned for Tee protrusions

using experimental results. Experimental results for forming

of `̀ differential cases'' were also disclosed in this paper. In

the same period, Al-Qureshi and his team [11] performed

bulging and piercing experiments of different materials

including copper, steel and aluminum using polyurethane

to provide internal pressure. They did not report use of axial

loading in their experiments.

In 1970s, research on different aspects of bulge forming

continued both experimentally and theoretically by various

authors. New shapes, materials, different tooling con®gura-

tions and new machine concepts were introduced, whereas

the fundamentals remained the same. For instance, instead

of polyurethane, rubber and elastomer were used to provide

internal pressure [12]. He presented that greater circumfer-

ential expansion of thin-walled tubes was obtained using

Journal of Materials Processing Technology 108 (2001) 384±393

* Corresponding author. Tel.: �1-414-447-4504; fax: �1-414-447-4870.

E-mail address: [email protected] (M. KocË).

0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 8 3 0 - X

rubber forming methods than using hydraulic forming tech-

nique. Effect of friction between rubber and inner side of the

tubes was also mentioned. Limb and his team [13] per-

formed BFTs of different materials with changing wall

thickness. They reported that increasing the internal pressure

gradually during the application of axial load gives the best

results on thinning and complete ®lling. Thickening of

tube wall at feeding zone was also mentioned due to the

friction between tube and die surface. In addition, experi-

mentation of different lubricants such as PTFE ®lm, colloi-

dal graphite and Rocol RTD spray were carried out. In case

of insuf®cient lubrication, low Tee protrusion heights were

obtained as well as a bulged protrusion area resulted instead

of a fully formed and ¯at area. With proper lubrication, it

was reported that a ¯atter bulging of the Tee protrusion was

obtained.

Woo [14] reported experimental and analytical results for

tubes bulged under internal pressure and axial compressive

loading. He carried out a numerical study assuming that the

entire length of the bulged tube was in tension, and thus, free

bulging took place. Comparison of experimental and theo-

retical results indicated good agreement when stress±strain

properties of tubes obtained from bi-axial tests were used in

calculations. Use of upper-bound technique to calculate

internal pressure as function of material properties and

geometry was presented by Powel and Avitzur [15]. They

tried forming of 90o elbows with sharp radius from straight

tubes using hydraulic pressure and a special tooling devel-

oped for this purpose.

Limb et al. [16] used oil as pressurizing medium in their

experiments to investigate the forming of copper, aluminum,

low carbon steel and brass Tee-shaped tubular parts. Results

of lubricant and material evaluations were reported in terms

of protrusion height attainable. Sauer et al. [17] presented

their theoretical and experimental work on necking criterion

of bulged tubes. Assuming a constant ratio of hoop and

longitudinal stresses in tube wall during expansion, numer-

ical and experimental results were found to be in agreement.

Effective strain at necking was also explained in terms of

pre-strain, strain-hardening exponent and stress ratio. Woo

and Lua [18,19] described their experimental tooling for

BFTs, and presented a theoretical analysis of stresses and

strains taking into account the anisotropy effect of the sheet

metals in two separate papers. They utilized Hill's theory of

plastic anisotropy in their work.

Starting from 1980s, researchers in Japan concentrated on

determining the material properties and their effects on tube

bulging operations. Manabe and Nishimura [20] investi-

gated in¯uence of the strain-hardening exponent and aniso-

tropy on forming of tubes in hydraulic bulging and nosing

processes. They brie¯y presented the maximum internal

pressure as a function of tube radius, thickness, strain-

hardening exponent, and strength coef®cient assuming that

there was no axial loading. Manabe et al. [21] published their

work on examination of deformation behavior and limits of

forming for aluminum tubes under both internal pressure

and axial force. Axial cylinders and internal pressure were

controlled by a computer-control-system to obtain pre-

de®ned stress ratio during their experiments. They utilized

fundamental analysis of thin-walled cylinders in their pre-

dictions for internal pressure and axial force.

Fuchizawa [22] analyzed bulge forming of ®nite-length,

thin-walled cylinders under internal pressure using incre-

mental plasticity theory. He presented the in¯uence of

strain-hardening exponent on limits of bulge height. Similar

to Manabe et al., he utilized the fundamental plasticity and

membrane theories in his predictions. Internal pressure and

maximum expansion radius were expressed in terms of

length, diameter, strength coef®cient (K) and strain-hard-

ening exponent (n). Later, Fuchizawa [23] extended his

studies to explore the in¯uence of plastic anisotropy on

deformation behavior of thin-walled tubes under only inter-

nal pressure. He based his analysis on deformation theory

and Hill's theory of plastic anisotropy. Longitudinal aniso-

tropy was found to be effective on the critical expansion

limit while anisotropy in hoop direction was affecting the

Fig. 1. Elements of a typical THF process. (Fa): Axial force, (Fq): counter force, (Pi): internal pressure, (Rc): corner radius, (Re): ®llet radius, (Do): initial

tube diameter, (Dp): protrusion diameter or bulge width, (Hp): protrusion or bulge height, (Lp): distance between tube edge and protrusion.

M. KocË, T. Altan / Journal of Materials Processing Technology 108 (2001) 384±393 385

maximum internal pressure required. With increasing ani-

sotropy in longitudinal axis, thinning is reduced while

obtainable expansion gets larger with less internal pressure

requirement. Experimental results were eventually com-

pared with theoretical ®ndings [24]. Different materials

including aluminum, brass and copper were tested in their

tooling, which only utilized internal pressure in a closed

cavity. Assuming that the tube materials obey power law of

strain hardening, experimental and calculated results were

found to be in good agreement. Studies of Manabe and

Fuchizawa on anisotropy effects were mostly found useful in

THF applications involving aluminum products.

Hydraulic bulging of tubes was later used in determining

the stress±strain characteristics of tubular materials by

Fuchizawa et al. [25]. Annealed aluminum, copper, brass

and titanium tubes were tested under only internal pressure.

With the instrumentation and control systems available, tube

thickness, radius of curvature in both longitudinal and hoop

directions, and internal pressure measured and recorded

during formation of the bulge. Using analytical methods

by membrane and plasticity theories, stress±strain relations

were derived. These ®ndings were also compared with those

obtained from tensile tests. Stress±strain relations for alu-

minum, copper and brass were found to be similar by two

tests, whereas that for titanium were different. Since they did

not use axial compressive load during bulging, stress±strain

relation obtained was limited to low strain values up to 0.7.

Thiruvarudchelvan [26] and his team have worked on

experimental and theoretical aspects of tube bulging process

using both polyurethane and liquid as pressurizing medium.

They used computers to control the process parameters and

for data acquisition in their experimental systems. Optimum

values for axial forces were de®ned to obtain large bulge

heights of tubes without any fracture [27,28]. Ueda [29,30]

presented forming of differential gear casings with hydro-

forming techniques after a series of experimentations in

1980s. Hashimi and his team [31±33] investigated the bulge

forming of axisymmetric and asymmetric components via

experiments, analytical techniques and FEA. Tonghai et al.

[34] presented their experimental and analytical work for

forming of Tee protrusions using polyurethane. Upper-

bound technique was used to predict total forming load.

As a major contribution, use of counter force and its effect

on attainable Tee protrusion height were investigated and

discussed. Use of upper-bound technique in calculation of

maximum internal pressure and axial force was also pre-

sented in another work [35]. Free bulging of aluminum tubes

was conducted, and results were compared with theoretical

solutions. Stress ratios between ÿ1 and 0 were found to be

the optimum range for bi-axial forming of tubes. However,

practical dif®culty to maintain the stress ratio in this zone

was also reported.

Finally, Dohmann and many of his students have been

working on various issues related to THF technology since

early 1980s [36]. Their work was mostly based on the

previous theoretical studies along with real and new indus-

trial applications of this technology [37]. They also utilized

the capabilities of continuously developing FEA and com-

puter controls in their experimental and analytical works

[38,39]. They have established formability diagrams for

different materials under certain circumstances in order to

speed up the practical use of the technology [40].

Controlling of process parameters and investigation of

possible part types were investigated by Schmoeckel and his

students in Darmstadt, Germany [41]. Forming of crank-

shaft-like parts using thick-walled tubes were performed

under experimental conditions [42,43]. They have also used

principle theories for prediction of process parameters, and

applied them into practical industrial use. Their work has

been heavily based on experiments for different aspects of

the technology including formability and producibility of

certain automotive parts, and tribological issues like lubri-

cation, die surface ®nish and wear [44]. THF parts were ®rst

classi®ed by Engel and Dick [43] as part of his dissertation

on development of fuzzy control systems for hydroforming

process. He grouped parts with respect to: (a) their variation

along longitudinal axis, (b) variation of the feature position

relative to the longitudinal axis and (c) variation of the cross-

section. Categorization of parts depending on their shape

complexity was also conducted. However, this classi®cation

was limited with only parts in exhaust systems excluding

structural frame parts, whereas KocË and Altan [45] consid-

ered structural parts such as frame rails, axles, cradles, etc. in

their classi®cation along with exhaust components. Their

classi®cation was based on: (a) spline geometry, (b) com-

mon feature types, (c) cross-section of tubes and (d) ratio of

length to diameter (L/D).

Use of FEA for THF process simulations is now a

standard development tool after investigations and valida-

tions conducted by many researchers since early 1990s.

Application of current commercial FEA software, such as

LS-DYNA, PAM-STAMP, ABAQUS, MARC, AUTO-

FORM, DEFORM, etc., for stamping and forging processes

into THF was performed and presented successfully [45±

47]. Consequent and seamless simulation of bending, pre-

forming and hydroforming, and sometimes annealing,

results in accurate predictions in terms of producibility,

formability and thinning of the desired part as well as points

out necessary changes in tool design. In order to shorten the

development time and efforts for THF process, supplemental

codes and techniques are being developed. Adaptive simu-

lation technique, for instance, iterates between appropriate

internal pressure and axial feeding inputs to ensure a part

without any fracture and wrinkles [48,49].

Various authors have presented their application and

practical oriented studies in numerous occasions since the

beginning of 1990s. The following issues were common

topics of these presentations [50±57]:

� industrial applications of THF technology,

� production of structural frame components,

� product development and design procedures,

386 M. KocË, T. Altan / Journal of Materials Processing Technology 108 (2001) 384±393

� evaluation of hydroformed parts in comparison to

stampings,

� incorporation of piercing into hydroforming tooling,

� assembly and welding issues,

� weight savings, etc.

2. THF parts, technology, presses, hydraulic andcontrol systems

Various parts for automotive, appliance and plumbing are

produced by THF technology as listed below:

� Exhaust system parts; usually made of stainless steel for

obtaining required structural, thermal and corrosion pro-

perties: Exhaust parts, engine tubes, catalytic converters,

pressure tubes, tail pipes, connectors and manifolds.

� Chassis parts; common material is low to medium carbon

steels and aluminum for structural and cost related rea-

sons: Frame rails, engine sub-frames (cradles), roof rails

and bows, instrument panels, rear axle frames and radiator

frames.

� Engine and power train components: Suspension cross

members, hollow camshafts, drive shafts and gear shafts.

� Body and safety parts: Windshield headers, A/B/C pillars,

space frame components, seat frames and shock absorber

housings.

Design of the THF system is of special importance since

high hydraulic pressures and complex shaped parts involved.

The system needed for THF consists of the followings:

� presses or clamping devices for closing the dies,

� tooling,

� pressure system; intensifier,

� hydraulic cylinders and punches; for sealing the tube and

move the material,

� process control systems; computers, data acquisition,

transducers, etc.

Fig. 2 illustrates examples of THF parts for automotive

applications. There are also a number of candidate parts in

development, such as camshaft, crankshafts, differential

casings and space frames [58].

2.1. Presses or clamping devices

In contrast to other forming operations, in THF process,

presses are used to open and close the die and to provide

enough clamping load during forming period to prevent

elastic de¯ections and die separation. Necessary tonnage of

the press (or clamping device) is dependent on the required

closing force. It is, in turn, a function of the maximum

internal pressure takes place during forming, part size (i.e.

diameter, length and thickness), and material. Large com-

ponents with thick walls (i.e. chassis components) and

intricate regions (i.e. small corner radii) need high closing

forces up to 7000±8000 t [60]. At present, presses up to

10 000 t capacity are in operation at several plants in the

world. Existing hydraulic presses with appropriate closing

forces and bed sizes can be utilized for THF process [55,60±

64] with some necessary additions and changes in the

system.

Clamping devices, other than regular hydraulic press

systems, are being designed and tested for hydroforming

purposes [65]. The purpose of developing special clamping

devices is to increase capabilities on process control, obtain

better dimensional accuracy via high clamping load, access

larger bed size, reduce cycle time, increase ¯exibility for

different parts and reduce investments, etc. In such a design,

the ram with the upper die half is actuated up and down

through a small cylinder, which would provide rapid motion

and cost less. As the ram closes the dies at its bottom

position, two opposite and horizontally positioned cylinders

are actuated to lock the ram at its required location. More-

over, several other small and short-stroke cylinders at the

bottom of the press bed are moved up to further increase the

clamping load capability. Such a design would not only be

cost effective in terms of initial capital investment, but also

would provide rapid stroking, which consequently contri-

bute reducing the production cost. In principal, a THF press

or machine must have the following features:

� appropriate die closing force;

� appropriate bed size to hold the dies;

� adjustable/movable axial punches with computer con-

trolled positioning;

Fig. 2. Examples of structural frame parts for automobile applications. In (a) roof headers (A), instrument panels (B), radiator frame (C), engine cradle and

rear axle (D), roof rails (E) and lower rail frames (F) can be manufactured by THF [56], (b) exhaust part [59], (c) space frame.


� adjustable/movable rams for counter forces with free and

position control;

� optional: automatic work-piece handling;

� high pressure (2000±5000 bar) and fluid pumping cap-

ability with tight control.

2.2. Tooling

Hydroforming tooling consists of die holders, dies,

inserts, punches, sealing systems and sometimes counter

punches or movable inserts. Due to the high-pressure values

involved in THF process, strong tooling systems are required

to minimize die de¯ection and part tolerance deviations.

Hence, tool steel such as D2 are used for inserts, whereas

1045 steel is used for the dies. Inserts are usually hardened

and polished to achieve smooth surface ®nish to reduce

friction and die wear. Design of part positioning and parting

lines requires full attention since through which not only

necessary closing force can be reduced but also formability

of the part can be guaranteed. For structural parts, diagonal

positioning is one way of balancing the die de¯ection

between vertical and horizontal directions of the part.

Because of con®dentiality issues in this high demanding

technology, limited information regarding tooling design is

released to the public as it goes with other aspects of the

technology. Hence, common guidelines known for forging

and stamping technologies are applied in combination after

necessary improvements and trials.

In general, the followings are main requirements for THF

tooling [55,66±68]:

� high strength against stresses due to large internal pres-

sure and axial loading;

� good surface finish to minimize friction and increase

formability;

� flexibility by interchangeable inserts;

� good guiding systems;

� balanced design to minimize the closing force require-

ments.

2.3. Pressure system

The pressure system (pump, intensi®er and control

valves) should be designed and selected, so as to provide

the required pressure levels for a wide range of parts to

obtain ¯exibility in the system invested. The applied pres-

sure should have a range from 2000 bar (30 ksi) up to

10 000 bar (150 ksi) depending on the parts in consideration

[69]. In many current industrial applications using pressures

up to 3000 bar (45 ksi) are suf®cient. The ¯ow rate can reach

up to 50 l/min in order to allow short cycle times. In order to

increase the production rate, multiple intensi®ers are used to

shorten the pressurizing period and compensate time losses

in case of rapid pressure increases when required by any part

and process design.

2.4. Hydraulic cylinders and punches

The axial punches are necessary to: (a) seal the end of the

tube to avoid pressure losses and (b) feed material into

expansion regions. They should feed the material into the

deformation zone in a controlled way, and in synchroniza-

tion with internal pressure, i.e. pressure versus time and axial

force versus time should be controlled and coordinated.

Counter punches are sometimes used on bulged or protru-

sion sections to avoid premature fracture by providing a

controlled material ¯ow. Axial cylinders are expected to

generate forces of up to 7000 kN (700 t) while counter

cylinder limits extend up to 2000 kN (200 t). The smaller

size also allows close control of the punch position. Various

punch tip designs for effective sealing during hydroforming

have been developed. More information can be found in [67].

3. Materials and formability in THF

The overall success of hydroforming product heavily

depends on the incoming tubular material properties. Mate-

rial properties such as composition, weld type, yield and

tensile strength, ductility, anisotropy must be determined for

tubes. Monitoring and controlling of tube rolling, welding

and annealing processes should be conducted carefully to

produce tubes with desired properties. Followings are the

required characteristics of tubular materials for quality THF

applications:

� high and uniform elongation;

� high strain-hardening exponent;

� low anisotropy;

� close mechanical and surface properties of weld line to

the base material;

� good surface quality, free of scratches;

� close dimensional tolerances (thickness, diameter and

shape);

� burr free ends; should be brushed;

� tube edges perpendicular to the longitudinal axis.

According to the requirements above, all alloys that are

used in deep drawing or extrusion are suitable for THF.

Table 1 tabulates some of the tubular materials used in THF

process. In addition, available tube types can be listed as

follows:

� seamless drawn circular tubes;

� seamless drawn tubular profiles;

� longitudinally seam welded circular tubes;

� longitudinally seam welded tubular profiles;

� tailored tubes; round seam welded or longitudinally seam

welded.

Different testing methods have been used to determine the

quality of tubing for purposes other than THF process [70].

These tests can be listed as follows: (a) tensile test, (b)

expansion test (c) cone test and (d) bulge test.


Investigation of formability limits, failure or necking

criterion and ¯ow stress characteristics of tubular materials

started with establishment of instability points in sheet metal

forming processes [6,7].

Fuchizawa et al. [25] conducted experimental and theo-

retical studies to determine the stress±strain relations of

tubular materials. They developed a bi-axial testing method

for tubular materials. This test uses internal hydraulic pres-

sure to bulge tubing, which is supported between two dies.

The ends of the tube are restrained by a set of dies, which are

separated by a predetermined length of tubing. One of the

supporting dies is restricted in movement, while the other is

free to move in the axial direction, thus reducing axial

stretching during the test. The internal pressure, thickness,

diameter and meridonial curvatures are measured continu-

ously, as the test is executed. From the recorded data, a

stress±strain relationship is analytically determined.

This bulge testing was used for analysis of aluminum,

copper, brass and titanium alloys. Results were compared

with those of tensile testing. While the values for the

aluminum, copper and brass showed little distinction

between the two testing methods, the titanium showed great

difference between the properties determined by the tensile

and bulge testing methods. For this reason, it was concluded

that a bi-axial hydrostatic testing method should be used for

testing materials to be used with THF processes.

The effects of the strain-hardening exponent and plastic

anisotropy were thoroughly discussed through theoretical

analyses in his other presentations [22] and [23], respec-

tively. The strain-hardening exponent (n-value) study

showed that as the n-value increased, the internal pressure

required to form a certain bulge height is decreased, thick-

ness distribution became more uniform, and greater expan-

sion was realized. Similar results were also presented in a

paper by Manabe and Nishimura [20]. Results of the plastic

anisotropy study showed that the r-value in hoop direction

affected the internal pressure requirement, while r-value in

longitudinal direction affected the maximum expansion of

the tube.

Sokolowski et al. utilized an approach similar to that of

Fuchizawa in order to determine the ¯ow stress curves of

low carbon and stainless steel tubes, Fig. 3. They introduced

the use of FEA as additional tool to the analytical and

experimental techniques [70,71]. Both studies were limited

with bulging with only internal pressure. Thus, working

strains were in the range 0.1±0.7.

4. Friction in THF process and evaluation of lubricants

Structural frame parts with particularly long and with

varying cross-sections require substantial axial feeding in

order to form into die cavities without much expense of

excessive thinning. Substantial cross-sectional changes from

round-like to rectangular shapes demand minimum resis-

tance against corner forming and material movement. Fric-

tion issues for such cases become very critical. Selection of

an appropriate lubricant and die coating is essential to

overcome sliding friction, prevent sticking and galling to

reduce tool wear, axial forces and excessive thinning.

Until recent years, there was not any reported testing

methods or equipment development to measure or evaluate

friction in THF process. However, effect of friction and

different lubricants on formability and extend of protrusion

Table 1

Common materials for THF

Material US designation German designation Material No. DIN

Steels AISI 1015 C 15 DIN 17007, 1.0401

AISI 1020 C 22 DIN 17007, 1.0402

AISI 1035 C 35 DIN 17007, 1.0501

AISI 1045 C 45 DIN 17007, 1.0503

AISI 1015 St 37 DIN 17007, 1.0100

AISI 1020 St 42 DIN 17007, 1.0130

ASTM A572-575 St 50 DIN 17007, 1.0530

Alloyed steels AISI 5120 21 MnCr 5 DIN 17007, 1.2162

AISI 420 X 20 Cr 13 DIN 17007, 1.4021

AISI 304

AISI 409

Aluminum alloys AA 1050A Al 99.5 DIN 1712 (part 3)

AA 5005A Al Mg 1 DIN 1725 (part 1)

AA 5056A Al Mg 5 DIN 1725 (part 1)

AA 5086 Al Mg 4 Mn DIN 1725 (part 1)

AA 7075 Al Zn Mg Cu 1.5 DIN 1725 (part 1)

AA 5052

AA 5754

AA 6260 T4

AA 6061 T4

AA 6063 T4


height was mentioned at many occasions starting 1970s [13].

In the same source, thickening of tube wall at feeding zone

was reported due to the friction between tube and die

surface. In addition, experimentation of different lubricant

such as PTFE ®lm, colloidal graphite and Rocol RTD spray

were carried out. In case of insuf®cient lubrication, bulging

effect of the dome of Tee protrusion was found to be more

pronounced. With proper lubrication, it was reported that a

¯atter bulging of the Tee protrusion was obtained. As

reported by Ahmed and Hashimi [72], Hutchinson carried

out experimental studies to investigate the effect of different

lubricants on bulging of tubes. The in¯uence of the follow-

ing parameters on tribological conditions in hydroforming

should be examined in detail to improve forming of a

complex part:

� lubricants;

� die coatings;

� surface pressure;

� sliding velocity;

� work piece and die materials;

� their surface conditions of work piece and inserts;

� effect of the parting line (in transversal and longitudinal

direction) on the forming process.

Schmoeckel et al. [73] identi®ed different friction zones

on a typical THF process depending on the effects of axial

force, feeding and geometrical aspects. The surface pres-

sure, sliding velocity and state of stress and strain were

identi®ed to be different in these zones as follows (Fig. 4):

(a) guide zone, (b) transition zone and (c) expansion zone. In

these three zones, the following conditions prevail:

� Guide zone. Medium surface pressure, high sliding velo-

city, high axial pressure, little expansion of the surface.

� Transition zone. Surface expansion or reduction, sliding

velocity smaller than that of the guide zone, but still

appreciable, stresses are somewhere between axial pres-

sure and tensile hoop stress, tensile stresses in the tube are

in hoop direction.

� Expansion zone. Tensile stresses are prevalent (axial and

hoop direction), sliding velocity is small, surface enlarge-

ment is large.

Fig. 3. Flow stress for 304 stainless steel, determined with analytical and simulation means [70].

Fig. 4. Schematic of a basic tooling design for friction testing, and various friction zones during a typical hydroforming process.


In order to investigate the in¯uence of the above para-

meters in different zones of friction, Schmoeckel et al.

[73,74] used an experimental setup where a straight tube

is expanded under internal pressure and pushed to investi-

gate the friction conditions in only guide zone. Simulta-

neously, Dohmann [75] developed another tooling, which

would permit investigation of friction in all zones. Other

researchers conducted pin-on-disk or twist tests to rank the

performance of different lubricants suggested for hydro-

forming applications [76]. As a result, all parameters affect-

ing friction conditions should be improved for an overall

success in hydroforming. For instance, a good hydroforming

lubricant should be selected based on the following criteria:

� lubricity to reduce sliding friction between tooling and

tube surface;

� durability under high pressure values up to 6±15 ksi at the

tube-to-tooling interface to prevent sticking and galling;

� minimum abrasivity to reduce tool wear;

� compatibility with pressurizing medium and environmen-

tal requirements;

� ease of application and removal (washable);

� cost.

Investigations on hydroforming lubricants have been

conducted at all levels not only to determine friction coef®-

cients but also to rank possible lubricants for speci®c

hydroforming applications. Depending on the composition

of the lubricant, they can be listed as follows: (a) dry

lubricants (solid lubricants), (b) wet lubricants (solutions

and emulsions as well as synthetics) and (c) pastes, soaps

and waxes. Each group has their own advantages and

disadvantages in terms of performance, application,

removal, compatibility and cost.

Dry lubricants are usually found to be more effective in

terms of performance to reduce friction and increase tool

life. Their application is easy and consistent with proper

instrumentation. Their compatibility with pressurizing ¯uid

is very good when they are dried appropriately. However,

their removal requires special washing ¯uids. They are

found to be more expensive than wet lubricants when drying

time, application and removal process and their original

costs are added. On the other hand, wet lubricants are cost

effective, easy to remove, most of the time are compatible

with pressure ¯uids, but do not perform well as dry lubricant

do. Hence, a compromise must be made depending on the

part complexity and quality requirements.

5. Pre-forming of tubes for hydroforming process

Many THF operations require a pre-formed tube in order

to: (a) ®t the tube into the hydroforming die cavity and (b)

reach the desired shape at the end of the process. Pre-

forming of tubes usually includes bending and crushing

operations. Furthermore, annealing may be necessary after

bending or crushing to remove residual stresses.

In order to take the effects of pre-forming into account and

to design parts, tooling and process parameters properly,

investigation of bending and crushing of tubes is essential. In

literature, investigations in this area have been dated to very

recent years, and in a limited manner [58,77]. These inves-

tigations are mainly experimental [51,52] or based on FEA

[46,78,79] of complex shaped parts. In addition, there is

enough background and experience in tube bending for other

purposes [80,81]. Thinning and thickening of tubes during

particularly in bending operation may greatly affect the

success of hydroforming process as thinned sections may

not be able to withstand internal pressure during expansion,

and consequently burst, whereas excessive thickening may

lead to wrinkles on the bent tube, and these may require high

pressure for straightening.

In order to analyze the entire THF process, it is necessary

to carry the results of bending and crushing analysis into

hydroforming stage. Use of FEA is so far the only way of

achieving this. Appropriately selected FEA software would

carry the strain history gained during pre-forming directly

into the hydroforming stage just as in actual forming of

complex parts. Along with FEA, theoretical analyses can be

also performed for simple cases [82] or two-dimensional

conditions like cross-section of a part.

6. Innovations and trend in hydroforming technology

Recent innovations are aimed to improve competitiveness

of hydroforming technology by reducing initial investment

cost, increasing production rate, and material utilization,

consolidating more parts into single parts, and ®nding ways

to eliminate draw backs such as excessive thinning.

As mentioned before, new press or clamping device con-

cepts are under development and trial to reduce the amount

of initial capital investment as well as increase the produc-

tivity by having rapid strokes [64,65]. Even some hydro-

forming systems without a press or clamping device are

discussed and seem feasible only for low production rates.

In order to increase the material utilization and avoid

excessive thinning, following innovations are being tested

and used nowadays: (a) tapered (conical) tubes for long

structural parts having substantial expansion degrees

between two ends, (b) tailor-welded tubes for minimizing

thinning at high expansion zones which are usually at the

middle sections of a long part for which other innovations

cannot be utilized practically [83], (c) double tubing is used

to increase the strength of the ®nal part while minimizing the

weight. Particularly used for front rails where extra care has

to be taken for excellent crash properties, (d) multiple tubing

seems to be an innovative way of producing whole assem-

blies at once, which is an excellent way of consolidating

more parts into one. Tubes of different pre-formed shapes

are connected to each other, and placed into a hydroforming

die altogether. Upon completion of hydroforming, all parts

of an assembly are manufactured and assembled [84]. Use of


aluminum alloys and high strength steel is seen as another

way of achieving lighter parts.

Companies and institutes are looking into every chance

and opportunity to make cost effective production with

lighter and stronger products. For instance, consolidation

of lubrication into tube making is considered one way of

increasing production rate. Application of various welding

types, such as gas metal arc welding, laser welding, electron

beam welding, is investigated to search better material

properties. Tube making (forming) cells are in consideration

instead of conventional tube rolling mills in some justi®able

cases.

As a result, almost all aspects of hydroforming technology

require full attention of researchers for better understanding

of its details. Generally, trend in this technology seem to

follow the same development path of other metal forming

processes.

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An overall review of the tube hydroforming (THF) technology

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