Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools
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12.11 Example 121
Small to medium-sized presses are mainly run on oil hydraulics. For larger plants with high
extrusion speeds, a water accumulator drive is commonly used. Here, water mixed with oil is
used as a hydraulic fluid.
The following table shows the dimensions of forward extrusion presses.
Table 12.6 Dimensions of forward extrusion presses
Billet dimensions Extrusion force in
kN
Diameter D
0
in
mm
Length L
max
in
mm
5 000 80 – 140 375
8 000 100 – 180 475
16 000 140 – 250 670
25 000 180 – 315 850
40 000 224 – 400 1060
63 000 280 – 500 1330
100 000 355 – 630 1680
125 000 400 – 710 1870
(10 kN = 1 t)
12.11 Example
The aim is to manufacture round rods with a diameter d = 15mm, out of AlMgSil
where:
Extrusion punch speed: 2 mm/s =
X
p
Density of AlMgSil:
U
= 2.7 kg/dm
3
Billet dimensions: D
0
= 180 mm , L = 475 mm long
Forming temperature: 450 °C (Table 12.4).
Find: Extrusion force.
Solution:
1. Principal strain
M
p
= ln
0
1
A
A
= ln
22
22
180 ʌ /4mm
4.96
15 ʌ/4mm
M
p
perm
= 5.5 Table 12.3
122 12 Extrusion
as
M
p
<
M
p
perm
the extrusion dimensions can be produced in one extrusion operation.
2. Exit speed of the extrusion
22
p0
ext
3322
1
ext
60
2mm 60s 1m 180 ʌ /4mm
10 s min 10 mm 15 ʌ /4mm
17.28 m/ min .
A
A
X
X
X
3. Strain rate
M
pp
0
6
~.DD
D
XM
M
1
6 2mm 4.96
0.33s .
s 180mm
M
4. Yield strength k
str
k
str
1
= 48 N/mm
2
for T = 450° C and
M
= 1 s
–1
, Table 12.4
k
str
= k
str
1
·
m
M
, m = 0.108 from Table 12.4
k
str
= 48 · 0.33
0.108
= 48 · 0.887 = 42.57 N/mm
2
.
5. Extrusion force for solid forward extrusion
str p
0
0Wstr
F
ʌ
Ak
F
Dl k
M
P
K
K
F
= 0.5;
P
W
= 0.15 selected.
22
2
2
180 ʌ / 4mm 42.6N 4.96
180mm ʌ 475mm 0.15 42.6 N / mm
0.5 mm
10753655 N 1716393N 12470 kN .
F
12.12 Exercise on Chapter 12
1. What criteria are used to differentiate between the extrusion processes?
2. Name the most important extrusion processes and the typical fields of application.
3. What are the advantages and disadvantages of the extrusion process?
4. What values does k
str
mainly depend upon?
5. How are extrusion presses categorised?
13 Impression-die forging (closed-die forging)
13.1 Definition
Impression-die forging is a hot bulk forming process. It is a pressure forming process where
shaped dies are moved towards one another so that the material is forced in a certain direction,
taking on the shape of the cavity in the die (see Table 13.2, page 124).
13.2 Starting stock
13.2.1 Type of material
Table 13.1 Feedstock associated with the forging processes
Process Type of material
Rod forging Rod stock up to approx. 40
Billet forging Sections of billets with round or square cross-sections
Forging sheared sheet blanks Rolled sheets
13.2.2 Mass of material required m
A
req f fla sc
mmmm
m
req
in kg mass of material required
m
f
in kg mass of the finished forging
m
fla
in kg mass of flash
m
sc
in kg mass of scale.
The mass of material required depends upon the shape of the workpiece and its mass.
However, standard values have been put together so that it is not necessary to determine the
mass of scale and flash for every new workpiece. In these standard values, the mass ratio fac-
tor W indicates how many times greater the mass of the required material m
req
has to be than
the mass of the finished product. As the shape of the flash can be very different depending on
the shape of the forging, however, the W factor takes into account the shape of the forging as
well as the final mass. Certain characteristic shapes which give rise to the same problems dur-
ing manufacture are sorted into groups. Table 13.3 below shows a small extract from this
shape classification table.
125
Numerical tables for the W factor have been put together from the shape classification table
and the final mass of the impression-die forgings. The mass of material required, m
req
, can be
easily determined using these tables (13.4).
req f
mWm
Table 13.3 Shape classification table (excerpt from Billigmann & Feldmann, Stauchen und Pressen)
Examples of application Shape
group
Explanation
1.1
Roughly spherical and cube-shaped parts, solid hubs with small
flanges, cylinders and parts with no secondary elements.
1.2
Roughly spherical and cube-shaped parts, cylinders with sec-
ondary elements on one side.
2.1
Hubs with small flanges; formed partly by the vertical flow of
material in the upper die, partly by vertical flow in the lower
die.
2.2
Rotationally symmetric forgings with hole in the hub and an
outside rim. Hub with hole and outside rim are connected by
thin webs.
3.1
Two-armed lever with solid cross-section, thicker at the centre
and at both ends; parts must be pre-shaped, e.g. foot pedals and
clutch levers.
3.2
Very long forgings with several large changes in cross-section
where the material has to flow a great deal vertically; crank-
shafts with forged counterweights and more than six throws.
Very high amount of flash due to repeated intermediate trim-
ming
Table 13.4 Mass ratio factor W as f (m
f
and shape group)
m
f
in kg
1.0 2.5 4.0 6.3 20 100
1 1.1 1.08 1.07 1.06 1.05 1.03
2 1.25 1.19 1.17 1.15 1.08 1.06
W
with shape group
3 1.5 1.46 1.41 1.35 1.20 –
13.3 Types and application of the process
126 13 Impression-die forging (closed-die forging)
13.4 Processes in the forging die
The processes in the forging die can be split into three phases (Figure 3.1):
1. Upsetting
2. Lateral flow
3. Vertical flow (rise)
Upsetting
During upsetting, the height of the work-
p
iece is reduced with no appreciable flow
paths along the walls of the die.
Lateral flow
Lateral flow is when the material flow is
mainly transverse to the movement of the
die. The flow paths where the material
touches the walls of the die are relatively
long. Because of this, a lot of friction
occurs and high deformation forces are
required.
Figure 13.1
Processes in the forging die, a) upsetting, b) lateral
flow, c) vertical flow
Vertical flow
The vertical flow of the material is the last forging phase in the die. Here, the flow of the mate-
rial is in the opposite direction to the forging motion. The initial height of the workpiece is
raised in places.
However, in order for vertical flow to occur in the die to begin with, the resistance to flow in
the flash gap must be higher than that required for vertical flow in the die. The material must
not flow into the flash gap until the die cavity is completely filled. This resistance to flow in
the flash gap depends upon the ratio of flash land width to flash land height (w/s).
13.4.1 Calculation of the resistance to flow in the flash gap
According to Siebel, the resistance to flow p
fl
for upsetting between parallel plates is:
fl f
2
b
pk
s
P
p
fl
in N/mm
2
resistance to flow
w in mm flash land width
s in mm flash land height
P
– coefficient of friction
k
str
in N/mm
2
yield strength.
Figure 13.2 Shape of the flash land
13.5 Calculation of force and mechanical work 127
By changing the flash land ratio w/s the internal pressure in the die can be varied to meet the
requirements of the workpiece. Where the material in the die has to flow a long way vertically,
workpieces need high internal pressure and thus a high flash land ratio (e.g. w/s = 5 to 10).
13.4.2 Calculation of the flash gap
The flash gap height can be calculated approximately:
s
0.015
s
A
s in mm flash land height
A
s
in mm
2
projected area of the workpiece without flash.
Table 13.5 flash land ratio w/s = f (A
s
and the type of deformation)
A
s
in mm
2
w/s mainly for
Upsetting Lateral flow Vertical flow
up to 2000
8 10 13
2 001 – 5 000 7 8 10
5 001 – 10 000 5.5 6 7
10 001 – 25 000 4 4.5 5.5
26 000 – 70 000 3 3.5 4.5
71 000 –150 000 2 2.5 3.5
13.5 Calculation of force and mechanical work
It is not possible to calculate force and mechanical work more precisely for impression-die forg-
ing, as here, the deformation process is affected by too many influencing factors, such as the
forging temperature, intergranular processes in the material, strain rate, the shape of the work-
piece, the kind of material and the kind of machine used for forging.
The force and work can be calculated approximately as follows:
Calculation process:
1. Strain rate
0
h
X
M
M
in s
–1
strain rate
X
in m/s ram or punch impact velocity
h
0
in m initial height of the blank
Table 13.6 shows
X
values for the presses generally used in this case.
128 13 Impression-die forging (closed-die forging)
Table 13.6
Strain rate
ij =
f (r and initial height
h
0
of the blank)
13.5 Calculation of force and mechanical work 129
2. Yield strength
1
m
str
str
kk
M
m – material exponent
k
str
in N/mm
2
yield strength for the strain rate
M
and the forging temperature T
k
str1
in N/mm
2
yield strength for
M
= 1 (s
–1
) at forging temperature T (k
str
1
from Table
12.4, page 117)
M
in s
–1
strain rate (for
M
from 1 to 300 s
–1
, k
str
can also be taken from Table
13.7)
3. Yield strength at the end of forging
re str
kyk
k
re
in N/mm
2
yield strength at the end of forging Take shape factor y (takes the shape
of the workpiece into account) from Table 13.8.
4. Maximum forging force
dre
F
Ak
F in N max. forging force
A
d
in mm
2
projected area of the forging including flash land.
5. Mean principal strain
As the height h
1
of the forging can not be precisely defined, here the mean final height h
1m
is
worked with, determined from the volume and the projected area of the forging (Figure 13.3).
1
p
0
ln ,
h
h
M
m
1
d
V
h
A
m
p
d0
ln
V
A
h
M
Figure 13.3 Definition of the mean final
height
h
1
in mm height after forging
h
0
in mm blank height
V in mm
3
volume of the forging
M
p
m
– mean principal strain
130 13 Impression-die forging (closed-die forging)
Table 13.7 yield strength depending on the forging speed for the forging temperature T = constant
Material T
(°C)
str
kf
M
for T = const. k
str
in N/mm
2
M
=1
(s
–1
)
M
=2
(s
–1
)
M
=4
(s
–1
)
M
=6
(s
–1
)
M
=10
(s
–1
)
M
=20
(s
–1
)
M
=30
(s
–1
)
M
=4
0
(s
–1
)
M
= 50
(s
–1
)
C 15
1200 84 93 104 110 120 133 141 145 153
C 35
1200 72 80 88 93 100 111 118 122 126
C 45
1200 70 78 88 94 102 114 122 128 132
C 60
1200 68 76 86 92 100 112 120 126 131
X 10 Cr l3
1250 88 94 100 104 109 116 120 123 126
X5CrNi l8 9 1250 116 124 132 137 144 154 160 164 168
X 10 CrNiTi l8 9
1250 74 84 94 101 111 125 135 142 147
E-Cu
800 56 61 67 70 75 82 86 89 92
CuZn 28
800 51 59 68 75 83 96 105 111 117
CuZn 37
750 44 51 58 63 70 80 87 92 97
CuZn 40 Pb 2
650 35 41 47 51 58 67 73 78 82
CuZn 20 Al
800 70 79 90 97 106 120 129 136 142
CuZn 28 Sn
800 68 76 85 91 99 110 118 124 128
CuAl 5
800 102 114 128 137 148 166 178 186 193
Al 99.5
450 24 27 30 32 35 39 41 43 45
AlMn
480 36 40 44 46 49 54 57 59 61
AlCuMg 1
450 72 78 85 90 95 104 109 113 116
AlCuMg 2
450 77 84 92 97 104 114 120 125 129
AlMgSi 1
450 48 52 56 58 62 66 69 71 73
AlMgMn
480 70 80 92 99 109 125 135 143 150
AlMg 3
450 80 85 91 94 99 105 109 112 114
AlMg 5
450 102 110 119 124 131 142 148 153 157
AlZnMgCu l.5
450 81 89 98 103 110 121 128 133 137