5 Hydroforming
5.1 General
On e of th e aim s of th e sh eet-m etal p rocessin g in d u stry is th e m in im iza- tion of costs an d th e op tim ization of its p rod u cts con cern in g weigh t,
stren gth ch aracteristics an d rigid ity. In search for altern ative p rod u ction p rocesses, h yd roform in g – th e m an u factu re of h ollow bod ies with com -
p lex geom etries by m ean s of flu id p ressu re – h as been sh own to offer an in terestin g tech n ical an d econ om ic p oten tial to sh eet m etal m an u fac-
tu rers. Th e ach ievem en t of ben eficial com p on en t ch aracteristics u sin g th is p rocess is on ly p ossible wh ere com p on en t an d p rocess con figu ra-
tion are selected by con sid erin g th e overall system d esign .
5.2 Process technology and example applications 5.2.1 Process technology
Th e h yd roform in g tech n iq u e is based on th e in flation of, for in stan ce, a tu be, cou p led with axial or rad ial com p ression an d by su bseq u en t exp an -
sion an d sizin g again st th e d ie wall. Th is p rocess com p rises in gen eral of th e followin g step s: exp an sion , com p ression , calibration .
Hyd ro fo rm in g is cat ego rized as a co ld fo rm in g p ro cess. It is u sed fo r t h e m an u fact u re o f geo m et rically h igh ly co m p lex h o llo w bo d ies fro m
t u bu lar o r p refo rm s. An u n d erlyin g d ifferen ce is d rawn bet ween free h yd ro fo rm in g an d h yd ro fo rm in g u sin g d ies. Th e lat t er is su bd ivid ed
again acco rd in g t o p ro cesses in vo lvin g lo n git u d in ally an d t ran sversely d ivid ed d ies
Fig. 5.2.1 .
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e exam p le u sed h ere t o exp lain t h e h yd ro fo rm in g p ro cess is t h e p ro d u ct io n o f a T-fit t in g
Fig. 5.2.2 . Th is t yp e o f co m p o n en t can be
eit h er a co p p er fit t in g o r a jo in t elem en t in a lo ad -bearin g st ru ct u re. A sp ecial h yd rau lic p ress is eq u ip p ed wit h a t wo -p art m u lt ip le p u rp o se
d ie. Dep en d in g o n t h e wo rkp iece, t h e d ies h ave t wo seal p u n ch es h o r- izo n t al cylin d ers p o sit io n ed axially relat ive t o t h e t u be en d s an d a
co u n t erp ressu re p u n ch . Th e t u bu lar p refo rm is in t h e bo t t o m d ie an d t h e d ie is clo sed . Th e en d s o f t h e t u be are sealed by t h e axial p u n ch es,
an d t h e t u be is filled wit h p ressu re m ed iu m . In t h e act u al fo rm in g p ro cess, t h e p u n ch es co m p ress t h e t u be, wh ile t h e p ressu re m ed iu m is
fed t o in flat e t h e p art u n t il t h e p art wall rest s again st t h e d ie co n t o u r. Th e co u n t erp ressu re p u n ch co n t ro ls ad d it io n ally t h e m at erial flo w.
Th e calibrat io n p ressu re fo rm s t h e wo rkp iece in su ch a way t h at it s co n t o u r co rresp o n d s t o t h at o f t h e d ie accu rat ely an d rep ro d u cibly. Th e
d ie is fin ally o p en ed an d t h e fo rm ed co m p o n en t is eject ed .
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Hydroforming
Fig. 5.2.1 Process variants of hydroforming: a free hydroforming; b tool-dependent hydro-
forming, transversely divided die; c tool-dependent hydroforming, longitudinally divided die
axial force pi = internal pressure
closing force
pi pi
pi
a b
c
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e com p on en t an d m aterial-sp ecific p rocess p aram eter cu rves Fig. 5.2.3
are u sed to con trol th e form in g p rocess. If an existin g h yd ro- form in g lin e is in ten d ed for th e m an u factu re of a d ifferen t workp iece,
th e d ie is ch an ged an d a d ifferen t p rogram accessed in th e m ach in e’s con trol system .
Exp an sio n t akes p lace u n d er axial m at erial flo w, wh ich is acco m - p lish ed by co m p ressio n in t h e axial o r rad ial d irect io n
Fig. 5.2.4 . Su b-
seq u en t ly, t h e co m p o n en t wh ich h as alread y been largely fin ish fo rm ed is calibrat ed again st t h e wall o f t h e d ie.
Th e act ive fo rm in g fo rces are ap p lied bo t h by t h e m o vin g d ie ele- m en t s an d also by t h e p ressu rized m ed iu m . In t h e d ie-relat ed m et h o d ,
t h e fo rm ed p art is p ressed again st t h e wall o f a co n t o u red d ie wh ich t h u s d et erm in es t h e fin al sh ap e o f t h e p art .
Aft er h yd ro fo rm in g, t h e co m p o n en t s p o ssess a h igh d egree o f d im en - sio n al st abilit y an d h igh st ren gt h valu es. As a resu lt o f t h e t h ree-d im en -
407
Process technology and example applications
Fig. 5.2.2
Production of a T-piece tube
counterpressure punch
contoured die hal es filling with
pressure medium secondary contour
press closure
application of hori ontal cylinders, water pressure
control, control of counterpressure punch
press opening, e ection of part
T-fitting seal punch
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
sio n al st at e o f st ress – wit h a relat ively h igh h yd ro st at ic st ress co m p o - n en t co m p ared t o d eep -d rawin g an d st ret ch d rawin g p ro cesses – rela-
t ively h igh d egrees o f d efo rm at io n can be ach ieved cf. Sect s. 4.2.1
, 4.2.4
an d 4.2.5
.
5.2.2 Types of hydroformed components
A basic d ifferen ce is d rawn bet ween h yd ro fo rm ed co m p o n en t s wh ich p erm it t h e fo llo win g p ro cess co n d it io n s, in p art icu lar based o n p art
geo m et ry an d frict io n : – co n t in u o u s exp an sio n an d co m p ressio n o r
– o n ly p art ial exp an sio n an d co m p ressio n o r – o n ly calibrat io n
Th e d ifferen ces are exp lain ed belo w: Th ese com ponents
Fig. 5.2.5 are ch aract erized by a h igh d egree o f
d efo rm at io n d u e t o t h e axial m at erial flo w wh ich t akes p lace o ver t h e en t ire co n t o u r o f t h e wo rkp iece. Th ey are exp an d ed an d co m p ressed
408
Hydroforming
Fig. 5.2.3
Hydroforming process sequence closing
force F
Z
internal pressure p
i
horizontal path s
filling forming
calibration release
time
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
409
Process technology and example applications
Fig. 5.2.4
Forces exerted on a tube section w ith a branch T-fitting compression path
Fr Fa
Ft expansion
Ft compression path
Fa Fr
Fig. 5.2.5 Pure expansioncom-
pression operation
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
d u rin g t h e p ro cess. As a resu lt o f t h e axial m at erial flo w, h igh er d egrees o f d efo rm at io n are p o ssible t h an wh en fo rm in g o n ly by ch an gin g t h e
wall t h ickn ess. In so m e cases, lo cal m at erial co m p ressio n resu lt s. Th ese com ponents are ch aract erized by a n u m ber o f lo cally d ist ribu t ed
bu lges o ver t h e co m p o n en t len gt h wit h o r wit h o u t in t erm ed iat e ben d s Fig. 5.2.6
. Axial m at erial flo w as a resu lt o f co m p ressio n h as an effect o n ly at t h e co m p o n en t en d s. In t h e case o f sh ap ed elem en t s fo llo win g
ben d s o r a m arked bu lge, p o sit io n ed clo se t o t h e h o rizo n t al cylin d ers, o n ly exp an sio n an d calibrat io n t ake p lace, bu t h ard ly an y axial co m -
p ressio n . In t h e case o f su ch com ponents, axial co m p ressio n is n o t p o ssible fo r
vario u s reaso n s Fig. 5.2.7
. A calibrat io n p ro cess is ap p lied h ere, wh ere- by fo rm in g o n ly t akes p lace m ain ly by ch an gin g t h e wall t h ickn ess.
5.2.3 Fields of application
Th e d esign an d st ru ct u ral p o ssibilit ies o ffered by h yd ro fo rm in g are u sed su ccessfu lly in a n u m ber o f ap p licat io n s fo r t h e m an u fact u re o f h igh -
st ren gt h co m p o n en t s an d assem blies wit h o p t im ized service life an d weigh t ch aract erist ics.
410
Hydroforming
Fig. 5.2.6 Expansion
compression and local calibrating
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
411
Process technology and example applications
Fig. 5.2.7 Pure calibration
Fig. 5.2.8
Blank shapes single-w alled tubes
extruded sections w elded or unw elded
sheet metal blanks double-w alled tubes
sheet metal blanks conically rolled
and w elded
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
O n t h e basis o f t h e vario u s sem i-fin ish ed p ro d u ct sh ap es Fig. 5.2.8
, h yd ro fo rm in g can be ap p lied t o a wid e variet y o f field s. Th ree m ain
u ser gro u p s h ave em erged as t h e m o st sign ifican t o ver recen t years Table 5.2.1
: – t h e au t o m o t ive en gin eerin g an d su p p ly in d u st ry,
– san it ary an d d o m est ic in st allat io n in d u st ry, – p ip e an d p ip e co m p o n en t m an u fact u rers.
412
Hydroforming
Sector
automotive industry vehicles
– road – w ater
– air – rail
chemical, gas, oil industry, pow er 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, framesbodyw ork,
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, ABC pillars,
roof frame profiles, steering column w ith 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 p ract ical in d u st rial ap p licat io n , it is in creasin gly im p o rt an t t o d esign a co m p o n en t n o t o n ly fo r fu n ct io n alit y bu t also fo r p ro d u cibilit y.
“Sim u lt an eo u s en gin eerin g” an d “early in vo lvem en t ” h ave beco m e im p o rt an t wat ch wo rd s in t h e field o f p ro d u ct d esign , wit h cu st o m er
an d p ro cess d evelo p er co o p erat in g fro m an early st age in d et erm in in g co m p o n en t co n figu rat io n . O n ly in t h is way is it p o ssible t o o p t im ize
fu n ct io n al an d p ro cess en gin eerin g relat ed fact o rs wit h in a sh o rt t im e- fram e. A co m p o n en t d evelo p m en t act ivit y can be, fo r exam p le, carried
o u t as fo llo ws: 1. p relim in ary m eet in g cu st o m er an d d evelo p er
2. t ech n ical an d eco n o m ic feasibilit y st u d y cu st o m er an d d evelo p er 3. co m p o n en t co n figu rat io n su it able fo r h yd ro fo rm in g cu st o m er an d
d evelo p er an d d efin it io n o f a p ro d u ct io n seq u en ce 4. p ro t o t yp e d evelo p m en t : CAD su rface d at a gen erat io n cu st o m er an d
CNC m illin g d at a gen erat io n d evelo p er, m an u fact u re o f a fo rm in g d ie an d p ro d u ct io n o f p ro t o t yp es u sin g d ifferen t m at erials an d wall
t h ickn esses d evelo p er 5. t ry-o u t o f p ro t o t yp es cu st o m er
6. d efin it io n o f q u alit y-relevan t Q A d at a cu st o m er 7. series p ro d u ct io n cu st o m er o r su p p lier
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
5.3.2 Feasibility studies
W h en d evelo p in g h yd ro fo rm ed co m p o n en t s, t h e p recise an alysis o f bo u n d ary co n d it io n s is essen t ial. O p t im u m d esign o f co m p o n en t s t ak-
in g in t o co n sid erat io n sp ecial p ro cess-sp ecific fact o rs en h an ces safet y an d also t h e co st -effect iven ess o f series p ro d u ct io n .
Th e feasibilit y st u d y, t h e co m p o n en t co n figu rat io n an d d efin it io n o f a p ro d u ct io n seq u en ce are clo sely in t erlin ked . O n ce t h ese p ro cesses
h ave p ro d u ced a p o sit ive resu lt , it is p o ssible t o st art wit h p ro t o t yp e d evelo p m en t o f t h e co m p o n en t s. W h en wo rkin g t h ro u gh t h e in d ivid -
u al st ep s, d ifferen t bo u n d ary co n d it io n s m u st be t aken in t o acco u n t . Th ese are briefly o u t lin ed belo w.
Review of feasibility W h en reviewin g feasibilit y, a d ifferen ce is d rawn bet ween t wo gro u p s o f
co m p o n en t s: n ew d evelo p m en t s wit h a geo m et ry t h at h as been co n fig- u red sp ecifically wit h h yd ro fo rm in g in m in d , an d t h e p ro d u ct io n o f
exist in g co m p o n en t s u sin g h yd ro fo rm t ech n o lo gy. Sign ifican t fact o rs h ere in clu d e t h e geo m et ry, wall t h ickn esses, m at e-
rials an d p ro cu rem en t o f t h e sem i-fin ish ed p art s o r p refo rm s. In it ially, t h e geo m et ry d at a is evalu at ed wit h t h e aid o f a CAD CAM syst em .
Circu m feren ces, rad ii, cro ss sect io n s an d t ran sit io n s are all d et erm in ed . A d ecisio n m u st also be t aken regard in g t h e ch aract erist ics o f t h e p re-
fo rm . In t h e case o f a t u be, fo r in st an ce, t h ese fact o rs wo u ld in clu d e t h e m at erial t o be u sed , t h e d iam et er, t h e wall t h ickn ess an d also an y p o s-
sible n eed fo r p re-fo rm in g. Th e exp ect ed circu m feren t ial exp an sio n is t h en d et erm in ed . Takin g
in t o acco u n t t h e m at erial d at a, t h e wall t h ickn ess, p re-fo rm in g o f t h e t u bu lar blan k an d o t h er bo u n d ary co n d it io n s, it is p o ssible t o assess t h e
m an u fact u rin g feasibilit y. Su bseq u en t ly, t h e in t ern al p ressu re, clo sin g fo rce, fo rces an d m o vem en t s o f t h e h o rizo n t al cylin d ers h ave t o be cal-
cu lat ed . Sin ce t h e co rrelat io n s bet ween all t h e d ifferen t in flu en cin g variables are h igh ly co m p lex, sp ecific exp ert ise is an essen t ial co m p o -
n en t o f su ccessfu l p art co n figu rat io n . Part configuration
If it is p o ssible t o m an u fact u re t h e p art , t h e n ext st age is t o d esign t h e d et ails, t h e p o sit io n in t h e d ie an d t h e slid e ch an n els. In t h e case, wh en
414
Hydroforming
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
a co m p o n en t d o es n o t ap p ear t o be p ro d u cible u sin g h yd ro fo rm in g t ech n o lo gy, an at t em p t is m ad e t o m o d ify t h e exist in g co n d it io n s in
su ch a way t h at t h e p art can be p ro d u ced wit h o u t m o d ifyin g it s fu n c- t io n al ch aract erist ics.
Th e p art geom etry m u st be op tim ized takin g in to accou n t th e p rocess lim its. W h ere assem blies are p rod u ced , it m ay u n d er certain circu m -
stan ces be p ossible to grou p several p arts in to a sin gle on e or to sp lit th e assem blies u p d ifferen tly. A d ifferen t m aterial can also be selected . W h en
m akin g th is typ e of m od ification , th e q u estion of reliability in series p ro- d u ction m u st n ot be n eglected , as th is is of d ecisive im p ortan ce con cern -
in g th e cost effectiven ess an d p rocess en gin eerin g related issu es. Configuration for series production
Th is st age in clu d es t h e d efin it io n o f a p ro d u ct io n seq u en ce an d co n fig- u rat io n o f t h e n ecessary p ro d u ct io n lin es. Co n sist en t im p lem en t at io n
o f t h e p ro t o t yp in g resu lt s is o f p art icu lar im p o rt an ce h ere. An o p t im u m p ro d u ct is o n ly ach ieved t h ro u gh effect ive in t eract io n o f t h eo ret ical
an alysis an d p ract ice-o rien t ed t est in g. An o t h er asp ect is t h e d efin it io n o f q u alit y-relevan t Q A d at a.
Form ing sim ulation Th e Fin it e Elem en t Met h o d FEM h as beco m e an est ablish ed feat u re o f
m et al fo rm in g t ech n o lo gy. Th e o bject ive o f FEM is t o rep lace co st ly an d elabo rat e exp erim en t al t est in g by fast , lo w-co st co m p u t er sim u lat io n .
An y p ract ical im p lem en t at io n o f p relim in ary co n sid erat io n s m u st t ake in t o acco u n t t h e sp ecial feat u res o f h yd ro fo rm in g:
– co n sid erat io n o f p art ial st rain h ard en in g o n p re-fo rm ed t u bes, – co rrelat io n bet ween p ro cess p aram et ers: m o t io n s an d fo rces o f t h e
h o rizo n t al cylin d ers, variat io n o f in t ern al p ressu re in t im e, m o t io n o f ad d it io n al d ie co m p o n en t s su ch as h o le p u n ch es an d co u n t erp res-
su re p u n ch es, clo sin g m o vem en t o f t h e d ie, – fu n ct io n al co o rd in at io n o f p ro cess p aram et ers: in t ern al p ressu re, d if-
feren t ad d it io n al d ie elem en t s su ch as h o le p u n ch es an d co u n t er- p ressu re p u n ch es an d axial cylin d ers,
– sp rin gback, – failu re crit eria: bu cklin g an d bu rst in g,
– m at erial beh avio r: an iso t ro p y an d flo w cu rve an d – d escrip t io n o f t h e co n t act frict io n co n d it io n s.
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Component development
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Th e fo llo win g in fo rm at io n can be co m p u t ed u sin g a su it ably co n fig- u red FEM p ro gram :
– eq u ivalen t p last ic st rain , – effect ive st ress,
– t h ickn ess d ist ribu t io n , – d ist ribu t io n o f p last ic st rain rat e,
– p rin cip al st rain s an d – d ist an ce bet ween o u t er wall o f wo rkp iece an d t h e d ie wall.
5.3.3 Component design
In ad d ition to th e u se of p re-ben t tu bes, th e p rocess tech n ologies sp ecif- ic to h yd roform in g in clu d e th e gen eration of cross-section al ch an ges,
flan ges, breakth rou gh s, sim p le an d m u ltip le bran ch es or th e creation of su rfaces for cen terin g weld in g op eration s. Th e key d ata in d icated in
Fig. 5.3.1 d escribes th e cu rren t state of th e art for u sin g tu bu lar-sh ap ed
blan ks. Th e m axim u m ach ievable h eigh t of a bu lge at a straigh t leg is m arked ly h igh er th an on e on a ben d , as th e m aterial is p reven ted from
fu rth er d isp lacem en t by th e geom etry of th e ben d Fig. 5.3.2
. A larger bran ch ed tu be h eigh t can be ach ieved if th e bran ch is located n ear a
h orizon tal cylin d er. In p rin cip le, sh arp corn ers an d ed ges sh ou ld be
416
Hydroforming
part lengt
h L max.
12,00 0 mm
w al
l t hi
ck ne
ss s
0. 6
to 5
m m
di am
et er
D m
ax . 6
60 m
m
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
avo id ed wh en d esign in g a co m p o n en t . Rad ii m u st be ad ju st ed t o t h e wall t h ickn ess
Fig. 5.3.3 . It is ben eficial t o p ro vid e gen t le ro u n d t ran -
sit io n s bet ween d ifferen t cro ss sect io n s wh en t h is is allo wable by t h e fu n ct io n al req u irem en t s.
So m e o f t h e fu n d am en t al gu id elin es t o be o bserved wh en d esign in g h yd ro fo rm ed co m p o n en t s are o u t lin ed belo w.
417
Component development
Fig. 5.3.2
Examples of achievable branched tube heights: the achievable height decreases w ith an increasing degree of difficulty
D 100 D
75 D 15 D
Fig. 5.3.3
Internal pressure as a function of the inside radius
inside radiusw all thickness [–]
internal pressure
[bar] 5,000
3,000 1,000
11 31 21 41
p f r
k s
s
i f
m ax m in
r
m in
, ,
=
required internal pressure hydroforming die
r
m i n
s pi
= smallest inside radius of the component
= w all thickness = stability factor
k
f
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Straight com ponent axis Th e in flat io n lim it is rest rict ed by t h e risk o f bu rst in g an d bu cklin g. Th e
likelih o o d o f bu cklin g in creases wit h t h e len gt h o f t h e exp an d ed sec- t io n . Wit h a bu cklin g len gt h 2 · t h e d iam et er o f t h e t u bu lar blan k
st art in g d iam et er, an d if a ro t at io n ally sym m et rical cro ss sect io n is bein g p ro cessed , t h e ach ievable d efo rm at io n m ay be assu m ed t o lie well
beyo n d t h e u lt im at e elo n gat io n o f t h e m at erial. Axial curvature
In t h e case o f co m p o n en t s wit h a cu rved axis in t h e in flat io n co n t o u r, axial d isp lacem en t o f m at erial is p o ssible wit h o u t p ro blem s, p ro vid ed
t h e cu rvat u re rad iu s is R 5 · t h e d iam et er o f t h e in it ial p refo rm . Part ial in flat io n t akes p lace h ere at t h e in n er sh ell o f t h e ben d .
Th e free bu cklin g len gt h sh o u ld n o t exceed t h e st art in g d iam et er, ext en d o ver n o m o re t h an h alf t h e circu m feren ce o f t h e t u bu lar blan k
an d at t h e sam e t im e it sh o u ld be co n t in u o u sly d ecreasin g. In free fo rm in g wit h o u t su p p o rt , t h e co m p o n en t bu ckles if axial p res-
su re is ap p lied . Th e in flat io n cap acit y is lim it ed by t h e u lt im at e elo n ga- t io n , as fo rm in g t akes p lace p u rely as a resu lt o f t h e in t ern al p ressu re.
Cross sections Du rin g free exp an sion , th e m axim u m d eform ation is reach ed wh en th e
free bu cklin g len gth am ou n ts to a m axim u m of 2 · startin g d iam eter, an d th e cross section is rotation ally sym m etrical an d located in th e tu be axis.
An asym m et rical cro ss sect io n wit h a d eep , n arro w co n t o u r area can o n ly be fo rm ed in cases wh ere m at erial is able t o flo w axially in t o t h e
co n t o u r. Sh arp co n t o u r t ran sit io n s lead t o p art ial an d lo calized st ret ch - in g in t h e wall an d t h u s t o p rem at u re bu rst in g. Asym m et rical cro ss sec-
t io n s o u t sid e t h e t u be axis can be fo rm ed in cases wh ere at least h alf t h e circu m feren ce o f t h e t u bu lar blan k is su p p o rt ed by t h e d ie co n t o u r.
Longitudinal sections Co n t o u r t ran sit io n s are best d esign ed wit h large rad ii. Su d d en ly ch an g-
in g t ran sit io n s in t h e d irect io n o f m at erial flo w lead t o t h e fo rm at io n o f cro ss-wrin kles at t h e co m p o n en t bu cklin g an d act as a brake t o m at e-
rial flo w in t h e axial d irect io n . Beyo n d t h ese u n d erlyin g gu id elin es, t h e fo llo win g p o in t s sh o u ld be o bserved in t h e co n figu rat io n o f h yd ro -
fo rm ed co m p o n en t s:
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Pure expansioncom pression Th e optim um ratio between the pipe diam eter D and pipe wall thickness s is
D s = 20 … 45 Here too, th e free buckling length sh ou ld be 2 · th e startin g d iam eter D.
If Ds 45, t h ere is a d an ger o f bu rst in g o r bu cklin g. Th is can be p re- ven t ed by red u cin g t h e free bu cklin g len gt h .
If Ds 20, t h e bu cklin g len gt h can be great er wit h in creasin g wall t h ickn ess. Th in n in g o f t h e wall t h ickn ess is p reven t ed by axial d is-
p lacem en t o f t h e m at erial. Ho wever, t h e sam e d isp lacem en t can also lead t o co m p ressio n o f t h e wall. O n ly a m in im al d an ger o f bu cklin g
exist s. Expansioncom pression and local calibration
Th e m axim u m in flat io n cap acit y is lim it ed by t h e u lt im at e elo n gat io n o f t h e m at erial. Half t h e u lt im at e elo n gat io n rep resen t s t h e lim it o f
ach ievable d efo rm at io n wh ere n o axial m at erial flo w is p o ssible. In t h is case, t h e bu cklin g len gt h is o f n o sign ifican ce.
Pure calibration Th e lim it o f ach ievable d efo rm at io n co rresp o n d s t o h alf o f t h e u lt im at e
elo n gat io n o f t h e m at erial.
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5 Hydroforming
5.4 Die engineering 5.4.1 Die layout
Figure 5.4.1 illu st rat es t h e layo u t o f a h yd ro fo rm in g d ie. Th e d ie set
wh ich co m p rises an u p p er an d a lo wer d ie is u sed t o acco m m o d at e t h e fu n ct io n al elem en t s. Th e d ie h o ld er p lat es p erm it ad ju st m en t o f t h e
assem bly h eigh t p rescribed by t h e p ress st ro ke o r h eigh t o f t h e cylin d er h o ld er bracket s. Th e seal p u n ch es wh ich are act u at ed by t h e d isp lace-
m en t o r p ressu re-d ep en d en t an d co n t ro lled h o rizo n t al cylin d ers, t h ey seal an d p u sh t h e t u be en d s d u rin g t h e co m p ressio n p ro cess. Th e p ist o n
Fig. 5.4.1
Schematic view of a die layout
seal punch slide plate
centering die holder plate
horizontal cylinder cylinder holder
bracket w ith sw ivel mounting
die holder plate hydroformed part
bed plate top die
bottom die
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
d iam et er an d st ro ke are ad ju st ed t o t h e sp ecific req u irem en t s o f t h e co m p o n en t . Th e cylin d er h o ld er bracket s abso rb t h e axial fo rces an d
align t h e h o rizo n t al cylin d ers co n cen t rically wit h t h e en d gu id es. Th ese can be m o ved an d p o sit io n ed o n t h e bed o r slid e p lat e.
Th e m o d u lar basic st ru ct u re o f t h e d ies p erm it s flexible im p lem en t a- t io n o f t h e req u ired co m p o n en t geo m et ries. Th e o bject ive is t o m in i-
m ize p ro d u ct io n co st s, d ie ch an gin g t im es an d t h e t im e req u ired t o exch an ge wo rn p art s. In ad d it io n , d u rin g t h e d evelo p m en t p h ase it is
p o ssible t o t ry o u t d ifferen t t u be m at erials an d wall t h ickn esses wit h o u t ch an gin g t h e d ies. Th u s, it is p o ssible t o t est vario u s p refo rm geo m et ries
wit h a m in im u m o f t ro u ble an d exp en se. Sim p le ch an geo ver fro m p ro t o t yp e d ies t o p ro d u ct io n d ies is p o ssible
if o n ly cavit y o r slid in g in sert s h ave t o be exch an ged Fig. 5.4.2
. W h en d esign in g a p ro t o t yp e d ie, it s service life is o n ly o f m in o r im p o rt an ce.
Excep t fo r t h e slid in g in sert s, it is n o t n ecessary t o h ave t im e-co n su m - in g co at in gs o r o t h er su rface t reat m en t s o f t h e d ie in sert s. Pro t o t yp in g
– an d t h u s st ep -by-st ep p ro gress t o ward s a d ie su it able fo r series p ro - d u ct io n – t akes p lace in several st ages:
421
Die engineering
Fig. 5.4.2
Die systems for prototyping and production
prototype die production die exchange of w earing parts
modification of individual contour sections w orkpiece basic die horizontal cylinder die cavity insert
counterpressure punch
sliding insert
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
– d evelo p m en t o f t h e basic co m p o n en t geo m et ry, – in t egrat io n o f su p p lem en t ary o p erat io n s su ch as h o le p iercin g,
– p ro d u ct io n o f m o d ified in sert s. Here, t h e fo llo win g p aram et ers in flu en ce t h e select io n o f t o o l m at erials
an d t h e n ecessary co at in g o r t reat m en t : – co m p o n en t geo m et ry,
– co m p o n en t m at erial, – m axim u m in t ern al p ressu re,
– sealin g syst em , – ap p licat io n p ro t o t yp in g, series o p erat io n .
5.4.2 Lubricants
As is t h e case in o t h er co ld fo rm in g o p erat io n s, lu brican t s are am o n g t h e m o st im p o rt an t aid s in t h e h yd ro fo rm in g p ro cess, as t h ey m in im ize
frict io n at lo cat io n s wit h relat ive m o t io n bet ween d ie an d wo rkp iece cf.
Sect . 4.2.3 . Man y co m p o n en t s can o n ly be m an u fact u red in t h e
req u ired sh ap e o n ly t h ro u gh t h e u se o f ap p ro p riat e lu brican t s. Lu brican t s p erm it t o o bt ain m o re even wall t h ickn ess variat io n , sim -
p lify t h e fo rm in g o f t u bu lar bran ch es, red u ce p art ial st ret ch in g p ro cess- es an d im p ro ve fo rm in g in areas t h at d o n o t allo w an y m at erial flo w by
axial feed . A n u m ber o f d ifferen t lu brican t s can be co n sid ered fo r u se in h yd ro -
fo rm in g. Dep en d in g o n t h eir co m p o sit io n , t h ese can be assign ed t o t h e fo llo win g gro u p s:
– so lid lu brican t s, gen erally o n a grap h it e o r Mo S
2
basis, – p o lym er d isp ersio n -based lu brican t s,
– waxes, o ils an d – em u lsio n s.
Lu brican t is gen erally ap p lied by m ean s o f sp rayin g o r im m ersio n . Excep t in t h e case o f o ils, t h e ap p lied co at in g m u st gen erally h ave d ried
an d h ard en ed p rio r t o t h e fo rm in g p ro cess. O n p rin cip le, at t en t io n m u st be p aid t o en su re an even co at in g t h ickn ess. Th e o p t im u m t h ick-
n ess o f t h e lu brican t layer m u st be d et erm in ed by t rial an d erro r.
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5 Hydroforming
5.5 M aterials and preforms for producing hydroformed components
5.5.1 M aterials and heat treatment
Basically th e sam e m aterials wh ich can be u sed for oth er cold form - in g tech n iq u es su ch as d eep -d rawin g cf.
Table 4.2.3 are su itable for
h yd roform in g ap p lication s. Th e m aterial sh ou ld h ave a fin e-grain ed m icrostru ctu re an d be soft-an n ealed .
Th e best fo rm in g resu lt s are ach ieved u sin g m at erials wit h a h igh d egree o f u lt im at e elo n gat io n , wh ereby t h e u lt im at e elo n gat io n at righ t
an gles t o t h e lo n git u d in al d irect io n o f t h e t u be – i. e. gen erally at righ t an gles t o t h e ro llin g d irect io n o f t h e sh eet m et al – is im p o rt an t fo r t h e
co n figu rat io n o f h yd ro fo rm co m p o n en t s. A h igh d egree o f st rain h ard - en in g is ben eficial.
Th e t u be m an u fact u rin g p ro cess an d p re-fo rm in g brin g abo u t a st rain h ard en in g effect wh ich m ay h ave t o be elim in at ed by an n ealin g
fo r cert ain co m p o n en t s p rio r t o t h e h yd ro fo rm in g p ro cess. If t h e fo rm a- bilit y o f t h e m at erial is exceed ed d u rin g a p ro d u ct io n st ep in t h e m an -
u fact u rin g p ro cess, fo rm in g m u st be p erfo rm ed u sin g several st ages p ro gressive fo rm in g wit h in t erm ed iat e an n ealin g. Ho wever, d u e t o
t h e ad d it io n al co st s in vo lved , h eat t reat m en t sh o u ld o n ly be carried o u t wh en all o t h er p o ssibilit ies, fo r exam p le d esign o r m at erial m o d ifi-
cat io n s, h ave been exh au st ed . Th e p referable m et h o d o f h eat t reat m en t fo r austenitic st eels is so lu -
t io n izin g – id eally u sin g in ert gas. Fo r sm all p art s, a co n t in u o u s fu rn ace can be u sed .
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
In t h e case o f ferritic st eel, t h e t yp e an d p ract icalit y o f h eat t reat m en t d ep en d h eavily o n t h e resp ect ive m at erial an d it s fo rm in g h ist o ry. O n
p rin cip le, eit h er o f t wo p o ssible p ro cesses can be u sed : recryst allizat io n an d n o rm al an n ealin g.
Gen erally, Mn o r Si allo ys are u sed fo r alum inium . Th e m o st favo rable m et h o d o f h eat t reat m en t m u st be d et erm in ed in each in d ivid u al case,
wh ereby p art icu lar at t en t io n m u st be p aid t o t h e lo n g-t erm beh avio r o f t h e m at erial, an d in t h e case o f st rain h ard en in g allo ys, t o rap id fu rt h er
p ro cessin g.
5.5.2 Preforms and preparation
Fo r h yd ro fo rm in g, p art s m ad e o f t h e fo llo win g sem i-fin ish ed p ro d u ct s are u sed
Fig. 5.2.8 :
– d rawn o r weld ed t u bes, – d o u ble-walled t u bes,
– ext ru d ed p ro files, – weld ed o r p re-fo rm ed t u bu lar blan ks.
In gen eral, t u bu lar p refo rm s wit h circu lar cro ss sect io n s are u sed . Weld ed an d ext ru d ed n o n -ro u n d sect io n s are u sed less freq u en t ly.
Th e p ro d u ct io n st ep s req u ired fo r t h e p rep arat io n o f p refo rm s fo r t ech n ically an d eco n o m ically o p t im u m p ro d u ct io n are d escribed in t h e
fo llo win g wit h referen ce t o t u bu lar blan ks. Th e t u be cutting p ro cess m u st be ext rem ely p recise in o rd er t o avo id
u n d esirable effect s su ch as leaks at t h e begin n in g o f t h e h yd ro fo rm in g seq u en ce. Th ese wo u ld lead t o failu re o f t h e co m p o n en t as a resu lt o f
bu cklin g. Flu ct u at io n s in t h e wall t h ickn ess alo n g t h e p erip h ery resu lt in a h igh scrap level, an d in ext rem e cases even t o failu re o f t h e co m -
p o n en t . Th e fo llo win g t o leran ces are gen erally ad m issible: – len gt h : + – 0.5 m m
– an gle o f t h e cro ss sect io n relat ive t o t h e lo n git u d in al axis: + – 0.5° Co n cen t ricit y at t h e t u be en d s is im p o rt an t in o rd er t o avo id u n wan t ed
sh earin g wh en t h e d ie is clo sed . Th e t u bes m u st be clean ed o f an y ch ip s, as t h ese lead t o t o o l d am age, fo rm in g erro rs o r m arks o n t h e su r-
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face o f t h e co m p o n en t . Tu be en d s m u st be d ebu rred fo r t h e sam e rea- so n , alt h o u gh ch am fers m u st be avo id ed u n d er an y circu m st an ces.
Th e id eal p refo rm fo r t h e h yd ro fo rm in g p ro cess is a st raigh t cylin d ri- cal t u be. Ho wever, d eviat io n s fro m t h is sh ap e are co m m o n , as t h e t u be
m u st be laid ad eq u at ely in t h e d ie in o rd er t o en su re p art q u alit y. Th is n ecessitates a pre-form ing p rocess. On p rin cip le, an y p re-form in g p rocess
red u ces su bseq u en t form ability an d lead s to d ifferen ces in th e wall th ickn ess an d fau lts in th e form in g beh avior of th e p arts as a resu lt of
local strain h ard en in g. However, su itable con figu ration of th e com p o- n en t an d op tim u m selection of th e p re-form in g p rocess can cou n teract
th ese n egative effects. Bending is p erfo rm ed o n a m an d rel, p ro file, cam o r in d u ct io n ben d -
in g m ach in e d ep en d in g o n t h e ben d in g rad iu s, t u be d iam et er an d wall t h ickn ess cf.
Sect . 4.8 . Becau se o f t h e n o t ch in g effect s t h at m ay o c-
cu r, su rface d am age, in p art icu lar sh arp -ed ged im p ressio n s creat ed by clam p in g jaws, m ay lead t o p rem at u re bu rst in g o f t h e co m p o n en t . Th e
t o leran ce fo r t h e st raigh t en d s, as fo r cu t t in g, is + – 0.5 m m . Ben d in g erro rs, su ch as wrin kles o n in sid e ben d s, im p ressio n s left by clam p in g
jaws o r d rawin g gro o ves m u st be avo id ed , p art icu larly in t h e case o f t h in -walled t u bu lar blan ks.
Reduction and expansion p ro cesses are u sed wh ere large p erip h eral d if- feren ces are req u ired o n lo n g elo n gat ed co m p o n en t s o r in case o f p ar-
t ial o versize o f t h e u sed t u be in t h e cen t ral sect io n . Here, t o o , p ro cess- sp ecific d efect s su ch as gro o ves, im p ressio n s et c. co u ld h ave a d et ri-
m en t al effect . Pre-form ing in the hydroform die rep resen t s an id eal so lu t io n if it can
elim in at e an y o t h er p re-fo rm in g st ep s req u ired o n t h e st raigh t t u be. Pre-fo rm in g is also carried o u t o n p art s wh ich h ave been p revio u sly
ben t , exp an d ed o r red u ced . Pre-form ing in the die is u sed in t h e case o f co m p lex geo m et ries, wh ere
t h e abo ve p ro cesses fail t o p ro d u ce sat isfact o ry resu lt s. Sp ecial p ro cesses also exist fo r t ap ered , weld ed t u bes, weld ed t u bes
wit h n o n -ro u n d sect io n s, d eep -d rawn h alf sh ells, ext ru d ed sect io n s an d jo in ed t u bes. In t h e case o f co m p lex co m p o n en t s, several p re-fo rm in g
st ep s can be n ecessary.
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M aterials and preforms for producing hydroformed components
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
5 Hydroforming
5.6 Presses for hydroforming
Hyd ro fo rm in g p resses are sp ecially d esign ed p resses t h at h ave: – t h e elect ro n ic syst em ,
– t h e o il h yd rau lics, – t h e p ressu re m ed iu m h yd rau lics an d co n t ro l syst em ,
– t h e bed an d slid e ad ap ted to th e sp ecial req u irem en ts of h yd roform in g. Th e p ress typ es u sed
are eith er fou r-u p righ t p resses Fig. 5.6.1
or fram e p resses Fig. 5.6.2
. Th e fou r-u p righ t con stru ction gu aran tees op tim u m accessibility of d ies for
load in g, p rocess con trol an d d ie com p on en t ch an geover. Th e d ie can be m ou n ted ou tsid e th e p ress an d fed in to th e p ress u sin g a d ie ch an gin g sys-
tem . In sid e th e p ress, th e d ie is locked in p osition by h yd rau lic clam p s: All th at is n ecessary to create th e h yd rau lic an d electrical con n ection s. Die
ch an ge can be execu ted d ep en d in g on req u irem en ts at th e p ress location on on e of th e fou r sid es cf.
Sect. 3.4 . Th e p osition of th e h orizon tal cylin -
d er is flexible. Fram e p resses are u sed as sin gle-p u rp o se p resses fo r t h e p ro d u ct io n o f
fit t in gs. Th e h o rizo n t al cylin d ers o f t h is t yp e o f p ress are p erm an en t ly in st alled .
Th e clo sin g fo rce is ap p lied by m ean s o f vert ical p lu n ger o r d ifferen - t ial cylin d ers cf.
Sect . 3.3 . User-sp ecific fact o rs are t aken as a basis fo r
– co n figu rat io n o f t h e p lu n gers an d d ifferen t ial cylin d ers, – t h e st ro ke an d d iam et er o f t h e h o rizo n t al an d co u n t erp ressu re cylin -
d ers, – d im en sio n in g t h e p ressu re in t en sifier an d
– d et erm in in g t h e size o f t h e bed .
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
Co n t ro llable p u m p s fill t h e wo rkp ieces u p t o a d efin ed in it ial p res- su re wit h p ressu re m ed iu m . Pressu re levels exceed in g t h is are gen erat ed
by t h e p ressu re in t en sifier. Th e lin e co n t ro l syst em is co n figu red fo r at least fo u r co n t ro lled h yd rau lic axes – o n e p ressu re in t en sifier, t wo h o r-
izo n t al cylin d ers an d o n e co u n t erp ressu re cylin d er. All t h ese axes can be co n t ro lled t o be d isp lacem en t o r p ressu re-d ep en d en t . Th e clo sin g
p ro cess fo r co in ed ben d s is p ro vid ed as t h e fift h axis. Th e n u m ber o f axes can be in creased , fo r exam p le if d ifferen t co m p o n en t s o r co m p o -
n en t s wit h su bseq u en t st ro ke h ave t o be fo rm ed in a sin gle p ress st ro ke. All t h e n ecessary p ro cess d at a is en t ered at t h e h o st co m p u t er cf.
Sect . 3.5 . Th e seq u en ce o f t h e fo rm in g p ro cess is execu t ed in t h e fo rm
o f a series o f st ep s co n t ain in g t h e d at a o n vario u s axes an d an y ad d i- t io n al co n t ro l co m m an d s. Th e p ressu re an d p o sit io n o f all axes can be
m o n it o red d u rin g execu t io n o f t h e p ro cess o n -lin e o n t h e screen . Th ey can also be reco rd ed . Th e p ro gram m ed d at a fro m t h e st ep ch ain can be
viewed at an y t im e in t h e fo rm o f a d iagram . It is p o ssible t o d raw u p a
427
Presses for hydroforming
Fig. 5.6.1 Four-upright hydroforming press
w ith a closing force of 25,000 kN
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
resp o n se p lan wh ich co rrect s cert ain erro rs au t o m at ically u sin g t h e p re- scribed m easu res wh ere t h ese o ccu r.
Takin g in t o acco u n t t h e n ecessary safet y m easu res, t h e lin e can be lo ad ed eit h er fu lly au t o m at ically o r m an u ally.
428
Hydroforming
Fig. 5.6.2
Frame-type hydroforming press
slide cylinder
slide counterpressure
punch
horizontal cylinder pressure beam w ith
support roll filling valve
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
5 Hydroforming
5.7 General considerations 5.7.1 Production technology issues
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