001 3-D Finite-element Simulation of the Cold Forging

8/11/2019 001 3-D Finite-element Simulation of the Cold Forging

http://slidepdf.com/reader/full/001-3-d-finite-element-simulation-of-the-cold-forging 1/13

LS VI R

J. M ater. Process. Technol. 43 (1994) 279-291

Journal o f

M a t e r i a l s

P r o c e s s i n g

T e c h n o l o g y

3 D f i n i t e e l e m e n t s i m u l a t i on o f t h e c o l d f or g i n g

of he l ical gears

V o l k e r S z e n t m i h a l i , K u r t L a n g e a , Y v e s T r o n e l b , J e a n - L o u p C h e n o t * 'b ,

R i c h a r d D u c l o u x c

aUniversi ti it Stut tga rt Inst i tut f iJr Um formtechnik 700 0 Stuttgart Germany

bEcole nationale sup~rieure des M ine s de Paris Centre de M ise en Fo rm e des Matbriaux Sophia Antipolis

06560 Valbonne France

CTransvalor Les Espaces Delta B P 037 Sophia Antipolis Cedex France

(Received M arch 18, 1993; accepted N ovem ber 24, 1993)

Industrial Sum m ary

Th is ar t icle i llustrates the use of the f inite-element m eth od to simu late the forging of

3 -d imens iona l complex i ndust r ia l pa r t s . The F O RG E3 code , wh ich can s imu la te t he co ld and

hot forg ing of indus t r ia l par t s i s presented toge th er wi th a thermo-m echan ica l form ula t ion an d

i t s num er ica l reso lu tion . I t is kno wn tha t in an up dated lagrangian app roa ch us ing a convec t ive

mesh , degene ra t i on o f the m esh occu r s ve ry r ap id ly . An au tom a t i c mesh gene ra t ion p rocedu re

for 3-D com plex geome tr ies has been d eve loped which enab les the c rea t ion of the in i t ia l mesh o f

the b i llet as well as remeshing . This technique a l lows the s imu la t ion of the w hole process of

forg ing complex indus t r ia l par t s us ing quadra t ic te t rahedra l e lements . In order to show the

effec tiveness of the m ethod , the la te ra l co ld ex t rus ion o f a he lica l g ear has b een com puted . T he

re su l ts show tha t t he com pu ta t i on can be ca r r i ed ou t u s ing the de sc r ibed r emesh ing p rocedu re

and tha t i t can be appl ied successfu l ly to even more complex geometr ies .

1 Introduction

H e l i c a l g ea r s u s e d b y t h e a u t o m o b i l e i n d u s t r y w e r e t h e s u b j e c t o f a r e s e a r c h p r o j e c t

o f t h e F o r s c h u n g s g e s e l ls c h a f t U m f o r m t e c h n i k ( F G U ) , S t u t tg a r t , w i th t h e a i m o f

s u b s t i tu t i n g t h e m e t a l - c u t t i n g p r o d u c t i o n w h i c h h a s b e e n u s e d u n t i l n o w b y a n e w

m e t h o d o f c o l d f o r m i n g [ 1 ] . I t w a s p o s s i b le t o p r o v e t h e a p p r o p r i a t e n e s s o f t h e

s e l e c t e d l a t e r a l e x t r u s i o n p r o c e s s a n d t h e d e v e l o p e d t o o l s . H o w e v e r , t h e h i g h i n t e r n a l

p r e s s u r e o f t h e p r o c e s s w h i c h i s n e c e s s a r y f o r t h e f i ll in g u p o f t h e t o o t h s p a c e ,

* Corresponding author

0924-0136/94/$07.00 © 1994 Else vier Science B.V . All rights reserve d

S S D I

0 9 2 4 - 0 1 3 6 ( 9 3 ) E 0 1 3 5 - 4



28 Volker Szentm ihal i e t a l . /Journal o f Ma ter ia ls Process ing Technology 43 1994, ) 27 9- 291

i n fl u e n ce s t h e d i m e n s i o n a l t o l e r a n c e o f t h e t o o t h e d g e a r . C o m p e n s a t i o n f o r t h e

o c c u r r e n c e o f e l as t ic d e f o r m a t i o n o f t h e t o o l b y m e a s u r i n g t h e d i m e n s i o n a l d e v i a t i o n s

o f t h e t o o t h s y s t e m is o n l y p a r t l y p o s s i b l e , s i n c e t h is d e v i a t i o n a l s o d e p e n d s o n t h e

m a n u f a c t u r e o f t h e t o o l b y E D M a n d p o l is h in g : o n l y a t h e o r e t i c a l t r e a t m e n t o f t h e

l a te r a l e x t r u s i o n p r o c e s s c o u l d p r o v i d e a n a b s o l u t e l y f a u lt le s s to o l p r o f il e . A n u m e r -

i ca l s t u d y o f t h is e x t r u s i o n p r o c e s s is a ls o n e c e s s a r y b e c a u s e m e a s u r e m e n t o f t h e f o r c es

o r s t r e s s e s a c t i n g o n t h e t o o t h s u r f a c e i s d i f f i c u l t o r i m p o s s i b l e .

T o d a y s e v e ra l e ff ic i en t a n d u s e r - fr i e n d ly c o d e s a r e a v a i la b l e f o r t h e n u m e r i c a l

t r e a t m e n t o f f o r m i n g o p e r a t i o n s , ev e n t h o s e o f h i g h c o m p l e x i ty [ 2 - 4 ] . H o w e v e r ,

i n d u s t r i a l a p p l i c a t io n s h a v e b e e n l i m i te d u p t o n o w t o s i m u l a t i o n o f a x i s y m m e t r ic a l

o r 2 - D p r o c e s s e s o f b u l k f o r m i n g [ 5 - 8 ] . B e c a u s e th e l a te r a l e x t r u s i o n p r o c e s s u n d e r

c o n s i d e r a t i o n c o u l d n o t b e t r e a t e d i n 2 - D d u e t o i ts c o m p l e x fl o w o f m a t e r i a l a n d

b e c a u s e i n a 2 - D s i m u l a t i o n i t is n o t p o s s i b le t o d r a w c o n c l u s i o n s a b o u t t h e i n f lu e n c e

o f t h e h e l i x a n g l e o f t h e g e a r , o n l y a 3 - D a n a l y s i s is u s e fu l in t h i s c a se . F u r t h e r m o r e , a n

e f fi ci en t r e m e s h i n g - m o d u l e is n e e d e d b e c a u s e o f th e l a rg e p l a s t ic d e f o r m a t i o n i n -

v o l v e d : w i t h o u t t h i s t h e s i m u l a t i o n w o u l d b e p o s s ib l e o n l y f o r sm a l l s tr a in s o r w o u l d

r e q u i r e u n f e a si b le m a n u a l r e m e s h in g .

F o r t h i s p u r p o s e a m o d u l a r 3 - D r e m e s h i n g c o d e is a v a i la b l e w h i c h h a s a l re a d y b e e n

u s e d s u c c e s s f u l ly f o r th e s i m u l a t i n g m e t a l f o r m i n g o f s p u r g e a r s [ 9 ] . H o w e v e r , th i s

m e t h o d is n o t f u l ly a u t o m a t i c a n d d o e s n o t a p p e a r v e r y u s e r - fr i e n d ly f o r c o m p l e x

i n d u s t r i a l g e o m e t r i e s [ 1 0 ] .

F o r 2 - D s i m u l a t i o n s , m o s t l y t r i a n g u l a r o r f o u r - n o d e l i n e a r e l e m e n t s ar e u s e d a n d

f o r 3 - D c a s es c u b i c e i g h t - n o d e l i n e a r e le m e n t s a r e a p p l i e d i n r e m e s h i n g p r o g r a m s . I n

t h e a c t u a l c a s e , f o r t h e s o l u t i o n o f a d if fi c ul t 3 - D p r o b l e m , t e n - n o d e t e t r a h e d r a l

e l e m e n t s a r e u s ed . In t h is p a p e r t h e p r o g r a m s f o r a u t o m a t i c r e m e s h i n g a n d m e s h

g e n e r a t i o n , t h e s i m u l a t i o n c o d e F O R G E 3 , a s w e ll a s t h e e x a m p l e o f t h e c o l d f o r m i n g

o f a t h i c k - w a l l e d h o l l o w c y l i n d e r i n t o a h e l i c a l g e a r , a r e p r e s e n t e d .

2 Bas ic equations

T h e a i m o f t h is s e c t io n i s t o i n t r o d u c e t h e e q u a t i o n s g o v e r n i n g t h e t h e r m o -

d e p e n d a n t m o d e l o f f o rg in g . I t s h o u l d b e n o t e d t h a t t h e c o m p u t e r c o d e F O R G E 3 is

s p e c i f i e d f o r h o t f o r g i n g b u t c o l d f o r g i n g c o u l d b e f o r e s e e n a s a l i m i t c a s e .

2 1 The mechanical equations

S i n c e i n e r t i a a n d g r a v i t y f o r c e s a r e n e g l e c t e d , t h e e q u i l i b r i u m e q u a t i o n i s w r i t t e n :

d iv a ) = 0 , 1)

w h e r e a i s t h e u s u a l C a u c h y s t r e s s t e n s o r .

A s t h e m a t e r i a l i s a s s u m e d t o b e i s o t r o p i c a n d i n c o m p r e s s i b l e , t h e f o l l o w i n g

c o n d i t i o n o n t h e v e l o c i t y f ie l d c a n b e w r i t t e n :

di v v) = tr ~) = O, 2)

w h e r e ~ is t h e s t r a i n - r a t e t e n s o r .



8 Volker Szentmihali et al./Journal o f Materials Processing Technology 43 19 94 ) 279 -2 91

A p o w e r l a w i s u s e d f o r t h e f r i c t i o n p o t e n t i a l :

a K

• f A v )

~

[Imvllp + 1

p - e l

11)

s o t h a t t h e s h e a r s t r e s s m a y b e w r i t t e n :

r = - e K I la v ll p - l a y , 1 2 )

wh ere c t an d p a re re spec t ive ly the f r i c t ion coe f f i c i en t an d th e f r i c t io n - ra te s ens i t iv i ty

p a r a m e t e r . I t i s a s s u m e d t h a t t h e r e i s n o o t h e r m e c h a n i c a l i n t e r a c t i o n o n t h e

b o u n d a r y , w h i c h i m p l i e s t h e r e i s n o f o r c e o n t h e f r e e s u r f a c e s .

T h e c o n t a c t i s s u p p o s e d t o b e u n i l a t e r a l , i.e . a n o d e o f t h e p a r t c a n c o m e i n t o

c o n t a c t w i t h o r l ea v e t h e d ie . T h e r e is a ls o a c o n d i t i o n o f n o n - p e n e t r a b i l i ty w h i c h d o e s

n o t a l l o w a p o i n t t o p e n e t r a t e i n t o t h e d i e :

A v .n <~ O, 13)

w h e r e n i s t h e e x t e r n a l u n i t v e c t o r n o r m a l t o t h e p a r t .

2 . 3 . V a r i a t io n a l f o r m u l a t i o n

T h e p a r t i s a d o m a i n I2 a n d t3 t2 f i s t h e b o u n d a r y o f I2 w h i c h i s i n c o n t a c t w i t h t h e

d i e. B y a p p l i c a t i o n o f t h e v a r i a t i o n a l p r in c i p l e , t h e s o l u t i o n o f t h e p r e s e n t p r o b l e m

m i n i m i z e s t h e f u n c t i o n a l J , w h i c h i s l a t e r d e f i n e d b y :

J v ) = I P v ) d ~ + f 4 ~ f A v ) d ~ .

14)

T h i s s e a r c h o f t h e m i n i m u m is p e r f o r m e d u n d e r t h e i n c o m p r e s s i b il i ty c o n s t r a i n t

d i v v ) = 0 a n d t h e c o n d i t i o n o f n o n - p e n e t r a b i l i t y o n t h e i n t e r f a c e 0f2 f b e t w e e n t h e

p a r t a n d t h e t o o ls .

A c c o r d i n g t o th e e x p r e ss i o n o f • a n d ~ f , J v ) c a n b e w r i t t e n :

I K / \ m + l f a K

v ) = ~ - ~ - - ~ x / / 3 ~ ) d ~ + p - -- ~ ql A v ll P + ld 6a,

aOf

15)

a n d d i v v ) = 0. T h e c o n s t r a i n t o f i n c o m p r e s s i b il i ty is e n f o r ce d a p p r o x i m a t e l y b y

a p e n a l t y m e t h o d a n d f i n al l y a m o d i f i e d f u n c t i o n a l Jp v) i s o b t a i n e d :

Jp v) = J v ) + ½p ~ K d i v v ) ) 2 d r e , 1 6 )

Q

w h e r e p i s a l a r g e p o s i t i v e n u m b e r .



Volk e r S ze n tm iha l i e t a l ./ J ourna l o f M a te r ia ls Proc e s s ing T e c hno logy 43 1994 ) 279 - 291

28

E l a s t i c e f fe c ts a r e s u p p o s e d t o h a v e a s m a l l c o n t r i b u t i o n a n d a r e t h u s n e g l e c te d . A s

a c o n s e q u e n c e t h e m a t e r i a l i s a s s u m e d t o o b e y a v i s c o p l a s t i c l a w d e r i v e d f r o m

a v is c o p l a s t i c p o t e n t i a l ~ . T h e n , t h e r e l a t i o n s h i p b e t w e e n t h e d e v i a t o r i c p a r t o f t h e

s t r e s s t e n s o r a n d t h e s t r a i n - r a t e t e n s o r i s g i v e n b y :

~ v )

s - 3 )

T h e c l a s si ca l N o r t o n - H o f f p o w e r l a w h a s b e e n c h o s en , fo r w h i c h t h e a s s o c ia t e d

p o t e n t i a l i s :

K / \m+ 1

~ v ) = m + l / x / ~ ) , 4 )

w h e r e ~ i s t h e e q u i v a l e n t s t r a i n r a t e :

e = 2 i . 5 )

T h u s , f o r t h e d e v i a t o r i c s t r e s s t e n s o r :

s = 2 K i , 6 )

w h e r e K i s t h e s t r e n g t h o r th e c o n s i s t e n c y ) o f t h e m a t e r i a l a n d m i s t h e s t r a i n - r a t e

s e n s i ti v i ty , th e v a l u e o f m b e i n g e q u a l t o u n i t y f o r a N e w t o n i a n f l u id a n d t o 0 f o r

v o n - M i s e s r ig i d - p l a s t i c b e h a v i o u r . I n th i s m o d e l b o t h t e m p e r a t u r e s o f t e n in g a n d

s t ra i n h a r d e n i n g a r e t a k e n i n t o a c c o u n t t h r o u g h t h e c o ns i st e nc y :

K = Ko ~o + ~)n ex p /3 /T ) , 7 )

w h e r e t h e e q u i v a l e n t s t r a i n i s d e f i n e d b y :

= f ~ d r . 8 )

2 2 The boundary conditions

I n f o r g in g , fr i c ti o n b e t w e e n t h e t o o l s a n d t h e p a r t i s v e r y i m p o r t a n t , s o t h a t i t i s

n e c e s s a r y t o i n t r o d u c e a f ri c t io n l a w i n t o t h e m o d e l , A f r i c ti o n l a w c o n s i s t e n t w i t h th e

m a t e r i a l b e h a v i o r is c h o s e n a n d a f ri c ti o n p o t e n t i a l ~ f A v ) d e p e n d i n g o n t h e t a n g e n -

t ia l v e l o c i t y d i f fe r e n c e b e t w e e n t h e p a r t a n d t h e t o o l i s c o n s t r u c t e d .

A v = v - v di~. 9)

T h e f u n c t i o n s h e a r s t r e s s i s d e r i v e d f r o m ~ f b y :

~ ~ f )

r - 10)

c~Av



Volker Szentmihali et al./Journal o f Materials Processing Technology 43 1994 ) 27 9-2 91

83

2 . 4 . T h e t h e r m a l e f f e c t

2 . 4 . 1 . T h e h e a t t r a n s f e r e q u a t i o n

I f a n i s o t r o p ic F o u r i e r l a w i s u s e d f o r t h e h e a t f l ux , t h e h e a t e q u a t i o n m a y b e

w r i t t e n :

d T

p c - ~ = d i v k g r a d T ) + I ~ , 1 7)

w h e r e l/d is th e h e a t p r o d u c t i o n d u e t o m e c h a n i c a l v i sc o p l a st ic d e f o r m a t i o n .

T h e g e n e r a l e x p r e s s i o n o f l ~ i s

I ~ = r a : ~ ,

w it h 0.9 ~< r ~< 1. 0. 18)

I n t h e p a r t i c u l a r c a s e o f t h e N o r t o n - H o f f la w , th i s le a d s to :

= r K

19)

2 . 4 .2 . T h e b o u n d a r y c o n d i t i o n s

O n t h e f r e e s u r f a c e t h e c o n d i t i o n s m a y b e w r i t t e n a s f o l l o w s :

k g r a d T . n = h ( T - Text) , 20)

w h e r e

ex

i s t h e e x t e r n a l t e m p e r a t u r e .

T h e c o e ff ic i en t h is c o m p l e x , ta k i n g i n t o a c c o u n t c o n v e c t i o n a n d r a d i a t i o n p h e -

n o m e n a :

h = h e + e , a r ( T +

Text) T 2 + T2xt), 21)

wh e re he i s the con vec t ion coe f f i c i en t , er i s the m e ta l em is s iv i ty an d tr , i s the S te ph an

c o n s t a n t .

O n t h e s u r fa c e in c o n t a c t w i t h t h e d i e , i n a d d i t i o n t o a c o n v e c ti v e c o n d u c t i o n t e r m ,

t h e r e is s u r fa c e e n e r g y d i s s i p a t i o n d u e t o f r i c t io n .

T h e r e s u l t i n g h e a t f l u x c a n b e a p p r o x i m a t e d b y :

b

bd

ctK 11Av Ip + x 22)

k g r a d

T n = h c d ( T -

Td) b +

wh ere hcd i s the h ea t t r ans fe r coe f f i c i en t, Td i s the d ie t em per a tu re , a nd b an d bd a re the

e f f u si v i ty o f t h e p a r t a n d o f t h e d i e r e s p e ct iv e l y .

T h e r e i s a l so a c o n d i t i o n o f i m p o s e d t e m p e r a t u r e T = T i.

3 . D i s c r e t i z a t i o n a n d r e s o l u t i o n

A s a n u p d a t e d l a g r a n g i a n m e t h o d i s u s e d , t h e p r o c e s s i s d i v i d e d i n t o s m a l l t i m e

s t e p s . A t e a c h t i m e s t e p t h e m e c h a n i c a l a n d t h e r m a l p r o b l e m s a r e s o l v e d s e p a r a t e l y ,

s o t h a t t h e r e is n o t a f u ll t h e r m o - m e c h a n i c a l c o u p l in g . H o w e v e r , t h e t e m p e r a t u r e f ie ld



284

Volk e r S ze n tm iha l i e t aL /J ourna l o f M a te r ia l s Proc e s sing T e chno logy 43 f199 4 279 - 291

is n e e d e d in o r d e r t o s o lv e t h e m e c h a n i c a l p r o b l e m t h r o u g h a t h e r m a l d e p e n d e n c e o f

t h e c o n s i s t e n c y . O n t h e o t h e r h a n d , t h e h e a t e q u a t i o n c o n t a i n s m e c h a n i c a l v a r i a b l e s

t h r o u g h t h e v o l u m e t r i c h e a t p r o d u c t i o n a n d t h e s u rf a c ic h e a t d i s s ip a t i o n d u e t o

f ri c ti o n . T h e r e f o r e t h e t h e r m a l c o u p l i n g i s p e r f o r m e d f r o m o n e s t ep t o t h e n e x t o n e .

3 . 1 . T h e m e c h a n i c a l p r o b l e m

T h e n u m e r i c a l m e t h o d u s e d t o s o l v e t h e m e c h a n i c a l p r o b l e m i s t h e f i n i t e - e l e m e n t

m e t h o d . A t e a c h t i m e s t e p t h e d o m a i n f2 is d i sc r e ti z e d i n t o e l e m e n t s f2 e a n d t h e

v e l o c i t y is th e n a p p r o x i m a t e d b y :

q

v = ~ N i V , (23)

i =

w h e r e q is th e t o t a l n u m b e r o f n o d e s , a n d V a n d N i a r e r e s p e c t iv e l y t h e n o d a l v e l o c i ty

v e c t o r a n d t h e s h a p e f u n c t i o n r e l a t e d t o n o d e i . T h e n a d i s c re t iz e d f u n c t io n a l w h i c h

d e p e n d s o n t h e q n o d a l v e l o c i ty v e c t o r s c a n b e w r it te n , it s m i n i m i z a t i o n l e a d i n g t o

a s et o f 3 q n o n - l i n e a r e q u a t i o n s w h i c h i s s o l v e d b y a c la s si ca l N e w t o n - R a p h s o n

m e t h o d .

T h e n o d a l v e l o c i ty is t h e n u s e d t o c a l c u l a te t h e o t h e r v a r i a b l e s s u c h a s t h e s tr a i n

r a t e a n d t h e s tr e ss t e n s o r . T h e i n t e g r a t i o n s c h e m e i s E u l e r i a n a n d e x p l ic i t a n d is u s e d

t o u p d a t e t h e d o m a i n a n d h i s t o r y v a r i a b l e s s u c h a s t h e e q u i v a l e n t s t r a i n , t h e m i c r o -

s t r u c t u r a l p a r a m e t e r s a n d t h e t o o l w e a r .

F o r a n y v a r i a b l e v :

v(t + A t ) = v( t ) + ~At . (24)

F o r i n s t a n c e , t h e d o m a i n i s u p d a t e d b y

g2(t + A t) = g2(t) + ~ t ) A t , (25)

a n d e q u i v a l e n t s t r a i n b y :

~( t + A t ) = ~( t ) + ~( t )A t . (26)

3 . 2 . T h e t h e r m a l p r o b l e m

T h e t h e r m a l p r o b l e m i s s o l v e d w i t h t h e s a m e d i s c r e t i z a t i o n a n d t h e s a m e s h a p e

f u n c ti o n s a s th e m e c h a n i c a l p r o b le m . T h u s t h e t e m p e r a t u r e f ie ld c a n b e a p p r o x i m a t e d

by :

q

T = ~ N i T i, (27)

i = l

w h e r e T i is th e n o d a l t e m p e r a t u r e o f n o d e i. T h e G a l e r k i n m e t h o d is e m p l o y e d a n d

f i n a ll y a s e t o f q n o n - l i n e a r e q u a t i o n s h a s t o b e s o l v e d . I f t h e s e a r e w r i t t e n i n m a t r i x

f o r m , t h e n :

dr

d t + K T + Q = 0. (28)



Volker Szentmihali et al./Journal of M aterials Processing Technology 43 1994 ) 2 79-29 1

285

F o r t i m e i n t e g r a t i o n a t h r e e -l e v e l i n t e g r a t i o n s c h e m e w h i c h is c o n s i s t e n t t o t h e s e c o n d

o r d e r i s c h o s e n [ 1 3 ]. U n d e r t h e s e c o n d i t io n s , t e m p e r a t u r e a n d i ts d e r iv a t i v e a r e w r i t te n :

lc = a T t - a , , + 3 - 2 a - g ) T t + a - ½ + g ) T t + a , ~ ,

- T , - A , , T , a , 2 - T ,

d T = l _ g ) T t

A t x + g A t z

29)

N o t e t h a t i n t h i s s c h e m e a l l t h e n o n - l i n e a r t e r m s a r e l i n e a r i z e d s o t h a t C i s w r i t t e n :

c * = ½ - g ) c , _ ~ , , + ½ + g ) c , ,

3 0 )

w i t h s i m i l a r f o r m s f o r K a n d Q .

F i n a l l y a li n e a r m a t r i x s y s t e m w i t h t h e n o d a l t e m p e r a t u r e s a t t i m e t + A t 2 a s th e

u n k n o w n s h a s t o b e s o l v e d .

3 . 3 . T h e g l o b a l t h e r m o - m e c h a n i c a l r e so l u t io n

A t e a c h t i m e s t e p , th e f o l l o w i n g s t ag e s a r e p e r f o r m e d :

i) T h e d o m a i n a t t i m e t, t e m p e r a t u r e a t ti m e t a n d t - A t1 a re k n o w n , so t h a t t h e

c o n s i s t e n c y K a t t i m e t is k n o w n . T h e m e c h a n i c a l r e s o l u t i o n i s a c h i e v e d a n d g i v es

the ve loc i ty f i eld .

ii) W i t h t h i s v e l o c i t y f i el d a r e t h e n c o m p u t e d t h e s t r es s t e n s o r , t h e s t r a i n - r a t e t e n s o r

a n d t h e e q u i v a l e n t s t r a i n - r a t e a n d t h e c o n f i g u r a t i o n i s u p d a t e d s o t h a t t h e

d o m a i n , t h e e q u i v a l e n t s t ra i n , t h e h i s t o r y v a r i a b l e s , t h e f r i c ti o n h e a t f lu x a n d t h e

p l a s t i c w o r k r a t e a r e k n o w n a t t i m e t + A t 2 .

iii) F i n a l l y , t h e t h e r m a l c a l c u l a t i o n i s p e r f o r m e d s o t h a t t e m p e r a t u r e a t t i m e t + A t2

i s k n o w n a n d s t e p i) c a n b e r e t u r n e d t o b y m a k i n g t ~ t + A t 2 .

4 Th e automat ic remeshing procedure

P r e s e n t e d h e r e t h e m a i n s ta g e s o f t h e a u t o m a t i c p r o c e d u r e , w h i c h l a te r is c o n c e r n e d

w i t h t h e c r e a ti o n o r t h e r e m e s h i n g o f a s p a t i a l d o m a i n w i t h t e t r a h e d r o n b r ic k s. F o r

t h e f i n it e - el e m e n t c a l c u l a t io n a n d i n p a r t i c u l a r t o t a k e i n t o a c c o u n t t h e c o n d i t i o n o f

i n c o m p r e s s i b i l i t y , t e n - n o d e s q u a d r a t i c e l e m e n t s , a r e u s e d .

4 . 1 . Th e i n i t ia l d a t a

A f u l ly a u t o m a t i c p r o c e d u r e o f m e s h c r e a t i o n n e e d s a m i n i m u m o f d a t a , i n th e

p r e s e n t c as e, t h is b e i n g a d e s c r i p t io n o f t h e b o u n d a r y . T w o c a se s m a y o c c u r, b u t h a v e

t h e s a m e i n i t i a l d a t a .

1) T h e d a t a i s g i v en i n it i al l y b y a C A D c o d e t h r o u g h a d i s c r e t iz a t i o n in t o l i n e a r

t r ia n g l e s o f t h e b o u n d a r y .



86 Volk e r S ze n tm iha l i e t aL /J ourna l ~ / M a te r ia l s Proce s s ing T e c hno logy 43 ( 199 4 ) 279 291

2 ) A r e m e s h i n g is re q u i r e d : t h e s a m e d a t a i s o b t a i n e d b y e x p l o i t i n g t h e c u r r e n t m e s h .

T w o s u b - s t a g e s a r e m a d e :

i) t h e b o u n d a r y o f t h e p a r t w h i c h i s m a d e u p o f q u a d r a t i c t r i a n g l e s is o v e r-

d i s c r e ti z e d i n t o l i n e a r t r i a n g l e s b y c u t t i n g e a c h f a c e i n t o s m a l l e r t r ia n g l e s .

ii) A s n e w n o d e s h a v e b e e n c r e a t e d , a c o n t a c t a n a l y s i s is m a d e i n o r d e r t o k e e p t h e

o v e r - d i s c r e t i z e d s u r f a c e a s c l o s e a s p o s s i b l e t o t h e p r e v i o u s c o n t a c t c o n f i g u r a t i o n :

p r o j e c t i o n o f t h e i n s i d e n o d e s , p r o j e c t i o n o f n e w n o d e s w h i c h a r e f o u n d t o b e i n

c o n t a c t w i t h i n a g iv e n m a r g i n ) a r e n e c e s s a r y t o s a t is f y t h i s g e o m e t r i c a l c o n s t r a i n t .

4 1 1 The remeshing procedure

A s m e n t i o n e d a b o v e , th e b o u n d a r y is o v e r -d i s cr e ti z ed , so t h a t t h e n u m b e r o f s u r fa c e

n o d e s i s v er y i m p o r t a n t . T h e p r e v i o u s d e s c r i p t i o n o f t h e b o u n d a r y i s th e n m o d i f ie d in

o r d e r t o o b t a i n t h e s a m e s t r u c t u r a l d e s c r i p t i o n w i t h a m o r e t r a c t a b l e n u m b e r o f

n o d e s , w h i l st a g o o d p r e c is i o n o f t h e g e o m e t r y is m a i n t a i n e d .

T h e n a q u a d r a t i c s u r f a c e is c r e a t e d b y c u r v i n g t h e l i n e a r t r i a n g l e s o f t h e p r e v i o u s

s u r f a c e . T h e w a y t h i s c u r v a t u r e i s d o n e m a t c h e s a s p r e c i s e l y a s p o s s i b l e t h e r e a l

s u r f a ce o f th e p a r t w h i c h is r e p r e s e n t e d b y t h e o v e r - d i s c r e t iz e d s u r f ac e .

A v o l u m e t r i c l i n e a r t e t r a h e d r a l m e s h i s c r e a t e d f i r s t w h i c h f i t s t h e s u r f a c e . I t i s

r e fi n e d b y a d d i n g i n t e r n a l n o d e s , t h e b o u n d a r y f a ce s a r e c u r v e d w h e n n e c e s s a r y a n d

t e n n o d e e l e m e n t s a r e c r e a t e d in t h e w h o l e d is c r e ti z e d d o m a i n . I t s h o u l d b e n o t e d t h a t

a l l i n t e r n a l e l e m e n t s h a v e s t r a i g h t f a c e s : o n l y t h e s u r f a c e c a n p r e s e n t c u r v a t u r e .

4 1 2 The whole simulation

U p t o n o w t h e w h o l e s i m u l a t i o n is n o t f u l l y a u t o m a t i c d u e t o d i f f e re n t s t ag e s o f

m e s h c r e a t i o n . T h e p r o c e d u r e d e s c r i b e d a b o v e n e e d s t h e in t e r v e n t i o n o f t h e u s er ,

m a i n l y t o c o n t r o l i n t e ra c t iv e l y th e p r e c is i o n , th e q u a l i t y o f th e m e s h a n d t h e t o t a l

n u m b e r o f n o d e s i n o r d e r to c o n t r o l t h e c o m p u t a t i o n t i m e a n id e n t ic a l p r o c e d u r e i n

2 - D i s f u l l y a u t o m a t i c b e c a u s e t h e p r o c e d u r e is n o t s o l i m i t e d b y t h e n u m b e r o f n o d e s ) .

T h e s i m u l a t i o n o f a c o m p l e t e p a s s o f f o r g i n g c a n b e d i v i d e d i n t o 4 s t ag e s .

1 ) U s i n g a C A D d e s c r i p t i o n o f t h e i n it i a l b o u n d a r y , a n i n i t i a l m e s h i s c r e a t e d .

2 ) T h e f o r g i n g s i m u l a t i o n i s p e r f o r m e d s t e p b y s t e p u n t i l r e m e s h i n g i s n e c e s s a r y :

e i t h e r b y d e g e n e r a c y o f a n e l e m e n t o r b y t h e c h o i c e o f t h e u s e r .

3 ) A n e w m e s h is c r e a t e d w i t h t h e s u r f a c e d e s c r i p t i o n o f t h e d e f o r m e d m e s h w h e n

t h e c o m p u t a t i o n i s i n t e r r u p t e d .

4 ) S t a t e v a r ia b l es a r e t r a n s p o r t e d f r o m t h e o l d m e s h t o th e n e w m e s h t e m p e r a t u r e ,

s t r a i n . . . . ). O t h e r v a r i a b l e s c o n c e r n i n g t h e t o o l c a n b e t r a n s p o r t e d : f o r i n s t a n c e

t h e w e a r , w h i c h i s a h i s t o r y v a r i a b l e .

S t e p s 2 ) - 4 ) a r e r e p e a t e d u n t i l th e e n d o f t h e p r o c e s s is re a c h e d .

5 R e m e s h i n g o f a h e l ic a l g e a r

T h e h e li ca l g e a r i s c o m p o s e d o f 30 t ee t h: a n a p p r o x i m a t i o n w a s d e v e l o p e d t o t a k e

i n t o a c c o u n t o n e t o o t h o n l y . T h e c u t e d g e s p r es e n t n o p l a n e o f s y m m e t r y , b u t t h e

p e r i o d i c a l s t r u c t u r e o f t h e e n t i r e w o r k p i e c e p e r m i t s a m e a n i n g f u l s u b - d i v i s io n .



Volker Szentrnihali et al./Journal of Materials Processing Technology 43 1994) 279-291 2 8 7

A s s u m i n g t h a t t h e r e i s n o f l o w o f ma t e r i a l p e r p en d i cu l a r t o t h e cu t ed ges t h e s e

d eg r ees o f f r eed o m w er e s u p p r e s s ed b y t w o ad d i t i o n a l f ic t it io u s t o o l s w i t h n o f r i c ti o n

w h i ch a r e a d j acen t t o t h e t w o cu t ed g es F i g . 1 .

The fo rm ing fo rce is t ransm i tt ed t o t he w orkp iece by two pu nches w hich m ove tow ards

each other so that the m ater ial f low perpend icular to the mo ving direct ion of the punches

a

b

z

F i g . 1 . a ) W o r k p i e c e ; b ) a n d c ) a d d i t i o n a l l i m i t a t i o n s u r f a c e s f i c t i ti o u s t o o l s ).

c

z

F i g . 2 . a ) W o r k p i e c e ; b ) t o o t h e d d i e w i t h c l o s i n g d i e s a t t h e u p p e r a n d l o w e r si d e ; c ) u p p e r p u n c h ;

d ) m a n d r e l ; e ) l o w e r p u n c h .



288 Volk e r S ze n tm iha l i e t a l ./ J ourna l o f M a te r ia l s Proc e s sing T ec hno logy 43 1994 ) 279 291

o c c u rs , m a i n l y i n th e r a d i a l d i r e c ti o n i n t o t h e t o o t h s p a c e o f t h e d ie . T h e m o v e m e n t o f

t h e m a n d r e l i n t h e c e n tr a l b o r e i s c o u p l e d w i t h th e m o v e m e n t o f t he u p p e r p u n c h .

T h e g e o m e t r i c a l m o d e l l i n g f o r t h e s i m u l a t i o n w a s d o n e b y t h e p r e - a n d p o s t -

p r o c e s s o r P A T R A N , w h e r e b y t h e t h r e e - n o d e t r i a n g u l a r e l e m e n t s o f t h e s u rf a c e o f t h e

Fig. 3. (a) initial mesh; (b) 6th mesh at 60 reduction; (c) 12th me sh at 75 reduction; (d) final mesh at the

end of the simulation.



Volker Szentmihali et al./Journal of Materials Processing Technology 43 1994) 279-291

289

Fig. 3. continued).

F ig . 4. Dis t r ibu t ion of equiva lent s t ra in in the work piece a t the end of the forming process .

raw piece only served as a geometric basis for the mesh generator of FORGE3

whereas the tool meshes Fig. 2 could be taken directly for the tools which latter were

assumed to be rigid. The workpiece meshes had between 180 and 205 boundary linear

triangles out of which 622 quadratic tetrahedral elements with 1260 nodes were

formed at the beginning of the process and finally 781 quadrat ic tet rahedral elements

with 1523 nodes were generated. The forming process was achieved after a punch

displacement of 5.15 mm for which 192 load increments were needed. For this

purpose 20 remeshing processes were required the first of which was conducted after

40 steps because separate tool nodes penetrated too deeply into the tools. All further



290

Vo lke r S zen tmih a l i e t a / . / Jo u r n a [ o [ M a ter ia l s P ro cess in g Tech n o/og ) 4 3 f 1 9 94 J 2 7 9 2 9 1

r e m e s h i n g p r o c e s s e s h a d t o b e c o n d u c t e d b e c a u s e o f m e s h d e g e n e r a c y . T h e c r it ic a l

p o s i t i o n w a s , a l w a y s t h e r a d i u s a t t h e f il le t f r o m t h e v e r t i c a l i n t o t h e h o r i z o n t a l p a r t o f

t h e c l o s i n g d i e . T h i s r a d i u s w a s c h o s e n a s 0 . 2 m m , w h i c h i s r a t h e r l a r g e a s c o m p a r e d

t o t h e s h a r p e d g e i n t h e e x p e r i m e n t , b u t s t i l l r a t h e r s m a l l f o r a s u d d e n c h a n g e i n t h e

m o v i n g d i r e c t io n o f t h e e le m e n t n o d e s .

F i g . 3 (a ) s h o w s t h e f i rs t w o r k p i e c e m e s h f o r t h e g i v e n r a w p i e c e g e o m e t r y . F i g s . 3(b )

a n d 3 (c ) s h o w th e 6 t h a n d t h e 12 t h m e s h a f te r a b o u t 6 0 r es p . 7 5 o f f o r m i n g w a y

a n d i n F i g . 3(d ) t h e s t a t e a t t h e e n d o f t h e s i m u l a t i o n is sh o w n .

A p a r t f r o m t h e f lo w o f th e m a t e r ia l , t h e d i s t r i b u t i o n o f t h e e q u i v a l e n t s t r a in i n t h e

w o r k p i e c e a n d t h e d i s t ri b u t i o n o f c o n t a c t n o r m a l s t re s se s o n t h e w o r k p i e c e s u rf a c e

a r e i n c l u d e d ( F ig . 4 ). T h e s e a r e u s e d f o r t h e d e t e r m i n a t i o n o f e l as t ic t o o l d e f o r m a t i o n

b y m e a n s o f th e b o u n d a r y - e l e m e n t m e t h o d , w h i c h d e f o r m a t i o n s a r e f in a ll y t o b e

c o m p e n s a t e d f o r .

6 Conclusions

T h e F O R G E 3 c o d e t o g e t h e r w i th i ts ne w p r o c e d u r e f o r g e n e r a t io n o f F E - m e s h e s is

p r e s e n t e d . T h e s t e p - w is e g e n e r a t i o n o f t h e v o l u m e m e s h o f q u a d r a t i c t e t r a h e d r a l

e l e m e n t s r e s u l ti n g f r o m a w o r k p i e c e s u r f a c e m e s h e d w i t h l i n e a r t r ia n g u l a r e l e m e n t s i s

d e s c r ib e d . A s a n e x a m p l e , t h e s i m u l a t i o n o f t h e c o l d l a t e ra l e x t r u s i o n o f a h e l i ca l g e a r

o u t o f a t h i c k - w a l l e d h o l l o w c y l i n d e r i s d e s c r i b e d a n d t h e r e s u l ts a r e p r e s e n t e d . I t h a s

b e e n s h o w n t h a t b y u s i n g q u a d r a t i c t e t r a h e d r a l e l e m e n t s , i t b e c o m e s p o s si b le t o

d e s c r i b e a c c u r a t e l y c o m p l i c a t e d i n d u s t r i a l c o m p o n e n t s w i t h s u r f a c e s w i t h s t r o n g

c u r v a tu r e s , d es p it e th e r e l a ti v e ly s m a ll n u m b e r o f e le m e n ts . T h e p r o g r a m F O R G E 3 is

t h e r e f o r e a n i m p o r t a n t t o o l f o r th e a n a l y s i s o f f o r m i n g p r o c e s s e s f o r t h e r a p i d l y a d -

v a n c i n g t e h n o l o g i e s o f b u l k f o r m i n g .

7 Acknowledgement

T h e a u t h o r s w i s h to t h a n k t h e M i n i s t e r i u m f fi r W i r ts c h a f t , M i t t e ls t a n d a n d T e c h -

n o l o g i e , B a d e n - W i i r t te m b e r g , f o r fi n a n c ia l s u p p o r t o f th i s p r o j e c t.

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001 3-D Finite-element Simulation of the Cold Forging

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