6 Solid forming Forging
6.1 General
Processes and parts Th e t erm s sh eet m et al fo rm in g an d so lid fo rm in g are based o n p ract ical
in d u st rial ap p licat io n , an d su bd ivid e t h e field o f fo rm in g t ech n o lo gy in t o t wo d ist in ct areas. Th is d ist in ct io n is m ad e in ad d it io n t o t h e d efi-
n it io n s est ablish ed by DIN 8582 “Fo rm in g” cf. Fig. 2.1.3
. So lid fo rm - in g en t ails t h e t h ree-d im en sio n al fo rm in g o f “co m p act ” slu gs fo r
exam p le sh eared billet s, wh ile t h e sh eet m et al fo rm in g m et h o d gen er- ally p ro cesses “flat ” sh eet m et al blan ks t o creat e t h ree-d im en sio n al h o l-
lo w st ru ct u res wit h an ap p ro xim at ely co n st an t sh eet m et al t h ickn ess. Th e fo rces exert ed d u rin g so lid fo rm in g are su bst an t ially h igh er t h an
t h o se n ecessary fo r sh eet m et al fo rm in g. As a resu lt , it is n ecessary t o u se relat ively rigid co m p act -d esign m ach in es an d d ies. So lid fo rm in g
in vo lves n o t o n ly t h e p ro cesses o f ext ru sio n , in d en t at io n an d d rawin g wh ich are d ealt wit h in m o st d et ail h ere, bu t also fo r exam p le ro llin g,
o p en d ie fo rgin g cf. Fig. 2.1.4
o r clo sed d ie fo rgin g cf. Fig. 2.1.5
. Th e p ro cesses cold extrusion an d drawing in vo lve fo rm in g a slu g o r bil-
let p laced in a d ie, m o st co m m o n ly fo r t h e production of discrete parts. Ho t ext ru sio n u ses a basically sim ilar fo rm in g m ech an ism t o creat e
sem i-finished products such as rods, tubes and sections. Th e basic p ro cesses in vo lved in co ld ext ru sio n
Fig. 6.1.1 are classified d ep en d in g o n t h eir
fo rm in g d irect io n as fo rward , backward an d lat eral ext ru sio n an d also acco rd in g t o t h e fin al fo rm o f t h e wo rkp iece as ro d so lid , t u be an d cu p
ext ru sio n . W h en p ro d u cin g so lid p art s, t h e classical o p erat io n s o f co ld ext ru sio n t ech n o lo gy are su p p lem en t ed by p ro cesses like u p set t in g an d
iro n in g Fig. 6.1.2
. Th e sh ap e variet y o f p art s fo rm ed fro m slu gs o r bil-
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
434
Solid forming Forging
Fig. 6.1.1
Schematic representation of the cold extrusion cold forging processes: I prior to forging;
II after forging BDC; a punch; b container;
c w orkpiece; d ejector; e counterpunch; f mandrel
I II
I II
I II
I II
I II
I II
I II
a b
c
I II
I II
a b
c
d,e
b
c
e a
a
b c
f
a c
b f
d, e a
c
b
d,e
a b
e c
d
b c
e a
b
c
e a
rod e trusion
tube e trusion cup e trusion
forw ard
e trusion
backward e
trusion
lateral e
trusion
s
d f
f
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
let s t h at can be p ro d u ced t o d ay is in creased su bst an t ially by t h e u se o f piercing, flow piercing, trim m ing and bending processes.
Th e ran ge o f p art s in clu d es n o t o n ly fastening elem ents screws, bo lt s, n u t s, rivet s, et c., bu t p art s fo r t h e construction industry L an d T-fit t in gs
fo r h eat in g syst em s an d co m p o n en t s u sed in m echanical engineering ap p licat io n s ball bearin g races, balls, ro llers an d cages. Mo st so lid
fo rm ed p art s are u sed in t h e autom otive and m otor cycle industry. En gin e com p on en ts valves, cam sh afts, cam s, con n ectin g rod s, gear p arts gear
sh afts, gear wh eels, syn ch ron ou s rin gs, d ifferen tial bevel gears, ch assis co m p o n en t s st u d axles, ball h u bs, st ep p ed h o u sin g elem en t s, sh o ck
abso rbers, st eerin g co m p o n en t s st eerin g co lu m n jo in t s, t ie ro d en d s, brake com p on en ts brake p iston s an d com p on en ts for electrical m otors
starter m otor p in ion s, d rivers, p ole rotors, can be econ om ically p ro- d u ced by solid form in g. In ad d ition , solid form ed p arts are u sed in
h ou seh old ap p lian ces, office eq u ip m en t, clocks or m ed ical eq u ip m en t. Figure 6.1.3
offers an overview of d ifferen t sh ap es wh ich can be p ro- d u ced . Th e form in g tech n iq u es d escribed h ere are cu rren tly also u sed to
p rod u ce gears th at can be p rod u ced very econ om ically. In m an y cases, n o m ach in in g is n ecessary or at m ost on e fin al grin d in g or h ard en in g
p rocess m ay be req u ired at th e teeth . Com p on en ts with straigh t teeth sp u r gears – su ch as h u b sleeves, bevel wh eels an d gears –, h elical teeth
e. g. p lan etary gears, sm all gears an d sp ecial teeth form s e. g. rin g gears can be p rod u ced . Th is typ e of gears can be form ed with an y
d esired tooth th at is often in volu te in sh ap e. Co n t o u rs are fo rm ed o n t h e su rface u sin g t h e in d en t at io n p ro cesses
DIN 8583 sh eet 5 co in in g an d ro ll em bo ssin g. Here, t h e co m p let e
435
General
Fig. 6.1.2
Upsetting, open die extrusion, ironing upsetting extrusion
ironing α
α punch
punch ironing die
container w orkpiece
w orkpiece upsetting die
2
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
workp iece or th e area to be em bossed is en closed in trap p ed closed d ies. Du rin g em bossin g, th e p art th ickn ess is red u ced , in som e cases by a con -
sid erable am ou n t. Coin in g is a typ ical exam p le of an em bossin g p rocess cf.
Fig. 2.1.6 . Bim etal coin s are a good exam p le h ere, as th ey in volve a
join in g p rocess DIN 8593 com bin ed with coin in g an d roll em bossin g. Em bossin g tech n iq u es are also u sed for exam p le in th e p rod u ction of
road sign s, an d in th e cu tlery or h an d tool in d u stry Fig. 6.1.4
. Cold coining of hot forged parts sizing DIN 8583 sh eet 4 is u sed t o
im p ro ve d im en sio n al, sh ap e an d p o sit io n in g accu racy an d o r t o im p ro ve su rface q u alit y. Th e o bject ive is t o p ress d im en sio n s an d sh ap e
t o t h e req u ired t o leran ces, an d t h u s avo id t h e n eed fo r su bseq u en t m et - al cu t t in g o p erat io n s. In co n t rast t o in d en t at io n , wh en u sin g co ld co in -
in g, t h e co m p o n en t alread y h as alm o st it s co m p let e geo m et ry, m ean in g t h at t h e h eigh t red u ct io n d u rin g co ld co in in g is relat ively sm all. Sizin g
o f t h e bo sses in co n n ect in g ro d s wh ich are req u ired t o h ave a t igh t er t h ickn ess t o leran ce co m p ared t o cast in gs, an d sizin g o f p lier h alves
436
Solid forming Forging
Fig. 6.1.3
Range of cold, w arm and hot forged parts
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
wh o se fu n ct io n al su rfaces req u ire p art icu lar p o sit io n in g accu racy are t yp ical exam p les o f t h is t yp e o f ap p licat io n .
Hot, warm and cold form ing and process com binations It is gen erally t ru e t h at alm o st all so lid fo rm in g p ro cesses can t h eo ret i-
cally be ap p lied at d ifferen t t em p erat u res. It is o n ly wh en d ifferen t fac- t o rs co m e t o get h er t o creat e in t errelat ed co m p lexit ies t h at eit h er h o t ,
co ld o r warm fo rm in g em erges as t h e id eal p ro d u ct io n m et h o d in an y p art icu lar case cf.
Sect . 2.2.1 . Abo ve t h e recryst allizat io n t em p erat u re,
hot form ing forging t akes p lace. Fo rgin g is t h e o ld est kn o wn m et a fo rm - in g t ech n iq u e. If t h e billet o r wo rkp iece is n o t h eat ed p rio r t o p ressin g,
t h e m et h o d u sed is referred t o as cold form ing cold forging. Th is m et h o d d id n o t beco m e feasible fo r u se wit h st eel u n t il t h e t h irt ies, wh en a su it -
able sep arat in g layer bet ween t h e d ie an d t h e wo rkp iece co u ld be d evelo p ed . W arm form ing or forging, t h e last o f t h e t h ree t ech n iq u es,
d evelo p ed fo r t h e in d u st rial p ract ice, was in t ro d u ced in t h e eigh t ies. Fo r st eels, fo r exam p le, t h is m et h o d wo rks at a t em p erat u re ran ge o f
bet ween 750 an d 950 °C. Fo r m an y p art s an d sp ecial wo rkp iece m at eri- als wit h a h igh p ro p o rt io n o f carbo n , t h is m et h o d co m bin es t h e t ech -
n ical ad van t ages o f h o t fo rgin g wit h t h o se o f co ld fo rgin g fo r o p t im u m eco n o m ical co n d it io n s.
Th e flo w st ress Fig. 6.1.5
d ecreases wit h in creasin g t em p erat u re. Dep en d in g o n t h e m at erial bein g p ro cessed , wh en u sin g t h e warm fo rg-
in g m et h o d , t h e flo w st ress reach es o n ly aro u n d o n e h alf t o a t h ird o f
437
General
Fig. 6.1.4
Examples of coined parts, such as coins, medals, cutlery and various hand tools, as w ell as cold-coined hot forged parts
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
t h e co rresp o n d in g valu es fo r co ld fo rgin g. W h en h o t fo rgin g, even lo w- er flo w st ress valu es can be ach ieved , co rresp o n d in g t o as lit t le as o n e
fift h o f t h e flo w st ress level d u rin g co ld fo rgin g. Again d ep en d in g o n t h e m at erial u sed , in warm fo rgin g an d h o t fo rgin g resp ect ively fo rge-
abilit y is t wo o r six t im es h igh er t h an t h at in co ld fo rgin g. In t h e h igh - er t em p erat u re ran ge fro m aro u n d 700 °C u p ward s, t h e in crease in scale
fo rm at io n wh ich h as a d et rim en t al effect o n d im en sio n al accu racy an d su rface q u alit y o f t h e p art s m u st be t aken in t o acco u n t . Th e t em p era-
t u re req u ired fo r warm fo rgin g m u st be carefu lly select ed as in it s “lo w- er ran ge” it is lim it ed by t h e size o f t h e p ress fo rce an d t h e ach ievable
d egree o f d efo rm at io n an d in it s “u p p er ran ge” by scale fo rm at io n . Table 6.1.1
p ro vid es a co m p ariso n o f t h e ad van t ages o f all t h ree m et h o d s o n t h e basis o f p ro cessin g en gin eerin g crit eria. Th e weigh t
o f t h e co m p o n en t s p ro d u ced u sin g h o t fo rgin g ran ges bet ween 0.05 an d 1,500 kg, in warm an d co ld fo rgin g bet ween 0.001 an d 50 kg.
Dim en sio n al accu racy an d su rface q u alit y d ecrease wit h in creasin g
438
Solid forming Forging
Fig. 6.1.5
Flow stress, scale formation and achievable degree of deformation forgeability in function of temperature
500 1000
1500
200 400 600 800 1000 1200 [
° C]
forming temperature flo
w s
tr es
s k
f
1 2
3 4
5
fr ac
tu re
s tr
ai n
ϕ
B
N mm
2
sc al
e la
ye r
20 40
60 80
µ m
X8 CrN
iTi1 810
C60 X10C
r13
ϕ =70s
. ϕ
=0,7 .
C15 16M nCr5
Ck45 15
5 min C60
42 M n V7
Ck 15
-1
ϕ = 90s
.
-1
min
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
t em p erat u re. Th is is d u e t o t h erm al exp an sio n , wh ich can n o t be p re- d ict ed wit h accu racy, an d also t o in creased d ie wear.
As an exam p le, let u s co n sid er warm fo rgin g as t h e m o st eco n o m ical p ro cess fo r select ed p art s, su bject t o t ech n ical feasibilit y. If t h is p ro cess
is t h en assign ed a co st fact o r o f 100, t h e relat ive co st s o f h o t fo rgin g can be u p t o 13 an d o f co ld fo rgin g u p t o 47 h igh er. Th erefo re, in warm
an d esp ecially h o t fo rgin g t h e co st o f rem o vin g ad d it io n al m at erial d u e t o larger m ach in in g allo wan ces m u st be co n sid ered . In t h e case o f co ld
fo rgin g, in co n t rast , it is freq u en t ly p o ssible t o co m p en sat e co m p let ely fo r t h e “h igh est fo rm in g co st s” by ext rem ely lo w o r even en t irely elim -
in at ed fin ish in g co st s. In ext ru sio n , t h erefo re, co ld an d warm fo rgin g m u st be co m p ared as t h e m o st eco n o m ical p ro cess alt ern at ives. Wit h
an in creasin g d egree o f geo m et rical co m p lexit y h igh er d egree o f d efo r- m at io n , fo r exam p le fo r st eerin g kn u ckles, h o wever, h o t fo rgin g is st ill
an eco n o m ical p ro cess. In p ractice, th e “id eal p rocesses” of solid form in g d efin ed h ere are also
u sed as p rocess com bin ation s in th ree d ifferen t resp ects. First, alm ost all com p lex workp ieces are m an u factu red tod ay by u sin g m u ltip le-stage
com bin ation s p rocess or p rocessin g step seq u en ce of in d ivid u al op era- tion s. To p rod u ce a bu sh in g, for exam p le, a com bin ation of th e p rocess-
es u p settin g, cen terin g, backward cu p extru sion , p iercin g an d iron in g are ap p lied in seq u en ce. Secon d , it is p ossible for two p rocesses to be car-
ried ou t sim u ltan eou sly in a form in g op eration . Usin g com bin ed back-
439
General Table 6.1.1: Comparison betw een hot, w arm and cold forging
Forming Hot
W arm Cold
w orkpiece w eight 0.05 – 1,500 kg
0.001 – 50 kg 0.001 – 30 kg
precision IT 13 – 16
IT 11 – 14 IT 8 – 11
surface quality Rz .
50 – 100 mm flow stress
~ 20 – 30 ~ 30 – 50
100 formability
f T, material w
6 w
4 w
1.6 “ Forming costs“
VDW study 1991, Darmstadt
up to 113 100
up to 147 machining required
high low
very low f T, material
. 30 µ
m .
10 µ
m
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
ward an d forward cu p extru sion , for exam p le, tu be-sh ap ed workp ieces with a cen tral web
Fig. 6.1.1 can be p rod u ced . Th ese an d sim ilar
p rocess com bin ation s red u ce th e n ecessary n u m ber of p rocessin g step s an d gen erally in volve lower p ress forces th an th e su m of p ress forces
u sed for in d ivid u al op eration s. Su ch p rocess com bin ation s are m ore eco- n om ical, bu t are on ly p ossible u sin g m u ltip le-action p resses an d d ies.
Th ird , in fo rm in g p ro cesses wh ere wo rkp ieces are in it ially h o t o r warm fo rged an d t h en co ld fo rged , “p ro cess co m bin at io n ” h as also
beco m e an est ablish ed t erm . Usin g t h is t ech n o lo gy, it is p o ssible t o ben efit sim u lt an eo u sly, fo r exam p le, fro m a t h e eco n o m ic ben efit s o f
warm fo rm in g t o creat e “n ear-n et sh ap e” co m p o n en t s an d b t h e t ech - n ical ben efit s o f co ld ext ru sio n o r co ld fo rgin g, in p art icu lar it s d im en -
sio n al accu racy an d su rface q u alit y. In t h is p ro cess, t h e co ld fo rm in g st age is n o t by an y m ean s rest rict ed sim p ly t o a sizin g o p erat io n , bu t
can in clu d e a m u lt ip le-st age fo rgin g p ro cess.
440
Solid forming Forging
Metal Forming Handbook Schuler c Springer-Verlag Berlin Heidelberg 1998
6 Solid forming Forging
6.2 Benefits of solid forming 6.2.1 Economic aspects
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