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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Hydroforming
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
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.
419
Component development
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
5 Hydroforming
5.4 Die engineering 5.4.1 Die layout
