Ch5 metalworkproc Erdi Karaçal Mechanical Engineer University of Gaziantep

02/11/14 CHAPTER 5 METAL WORKING
PROCESSES
1
CHAPTER 5
METAL WORKING PROCESSES
5.1 INTRODUCTION
ME 333 PRODUCTION PROCESSES II
Metal forming includes a large group of manufacturing processes in which plastic
deformation is used to change the shape of metal workpieces.
Deformation results from the use of a tool, usually a die in metal forming, which
applies stresses that exceeds the yield strength of the metal.
The metal therefore deforms to take a shape determined by the geometry of the
die.
Stresses applied to plastically deform the metal are usually compressive.
However, some forming processes stretch the metal, while others bend the
metal, still others shear the metal.
To be successfully formed, a metal must posses certain properties.
Desirable properties for forming usually include low yield strength and high
ductility.
These properties are affected by temperature.
Ductility is increased and yield strength is reduced when work temperature is
raised.

5.1.1 DEFINITIONS
PROCESSES
2
Plastic Deformation Processes
Operations that induce shape changes on the workpiece by plastic deformation
under forces applied by various tools and dies.
Bulk Deformation Processes
These processes involve large amount of plastic deformation. The cross-section
of workpiece changes without volume change.
The ratio cross-section area/volume is small. For most operations, hot or warm
working conditions are preferred although some operations are carried out at room
temperature.
Sheet-Forming Processes
In sheet metalworking operations, the cross-section of workpiece does not change
—the material is only subjected to shape changes. The ratio cross-section
area/volume is very high.
Sheet metalworking operations are performed on thin (less than 6 mm) sheets,
strips or coils of metal by means of a set of tools called punch and die on machine
tools called stamping presses. They are always performed as cold working
operations.

5.1.2 Material considerations
PROCESSES
3
Material Behavior
In the plastic region, the metal behavior is expressed by the flow curve:
σ = Κεn
Where;
K is the strength coefficient and
n is the strain-hardening (or work- hardening) exponent.
K and n are given in the tables of material properties or are calculated from the
material testing curves.
Flow stress
For some metalworking calculations, the flow stress σf of the work material (the
instantaneous value of stress required to continue deforming the metal) must be
known:
σf = Κεn

PROCESSES
4
Average (mean) flow stress
In some cases, analysis is based not on the instantaneous flow stress, but on an
average value over the strain-stress curve from the beginning of strain to the final
(maximum) value that occurs during deformation:
σf = Κεn
Fig. 5.1 Stress-strain curve indicating location of average flow stress sf
in relation to
yield strength sy and final flow stress sf
.

s
=
1 0
PROCESSES
5
The mean flow stress is defined as:
K nf
e
+
n
here εf is the maximum strain value during deformation.
Work-hardening
It is an important material characteristic since it determines both the properties of
the workpiece and process power. It could be removed by annealing.
Work hardening, also known as strain hardening or cold working, is the
strengthening of a metal by plastic deformation. This strengthening occurs
because of dislocation movements within the crystal structure of the material.
Any material with a reasonably high melting point such as metals and alloys can
be strengthened in this fashion.

5.1.3 Temperature in metal forming
PROCESSES
6
The flow curve is valid for an ambient work temperature. For any material, K and n
depend on temperature, and therefore material properties are changed with the
work temperature:
Fig. 5.2 True stress-strain curve showing decrease in strength coefficient K and strain-hardening
exponent n with work temperature

PROCESSES
7
There are three temperature ranges-cold, warm & hot working:
Fig. 5.3 Temperature range for different metal forming operations. TA is the ambient (room)
temperature, and Tm is the work metal melting temperature

Metal forming processes can also be classified according to the working
temperature.
The effect of temperature gives the rise to distinctions between cold working, warm
working, and hot working.
Hot and cold working of metals is of great importance in engineering production.
Processes such as forging, rolling, drawing and extrusion predominate in the
primary stages of production and have been perfected through developments.
Hot Working is the initial step in the mechanical working of most metals and alloys.
Hot working reduces the energy required to deform the metal. It also increases
ability of metals to flow without cracking. However, due to high temperature, surface
oxidation and decarburisation can not be prevented.
Cold Working of a metal results in an increase in strength or hardness and a
decrease in ductility. But, when cold working is excessive, the metal will fracture
before final size has been reached (<0.3Tm).
Hot Working of metals takes place above the recrystallization temperature.
Cold Working must be done below the recrystallization range (0.5Tm to 0.75Tm).
PROCESSES
8

PROCESSES
9
Cold working is metal forming performed at room temperature.
Advantages: better accuracy, better surface finish, high
strength and hardness of the part, no heating is required.
Disadvantages: higher forces and power, limitations to the amount of
forming, additional annealing for some material is
required, and some material are not capable of cold
working.
Warm working is metal forming at temperatures above the room temperature but
below the recrystallization one.
Advantages: lower forces and power, more complex part shapes, no
annealing is required.
Disadvantages: some investment in furnaces is needed.

PROCESSES
10
Hot working involves deformation of preheated material at temperatures
above the re-crystallization temperature.
Advantages: big amount of forming is possible, lower forces and
power are required, forming of materials with low
ductility, no work hardening and therefore, no
additional annealing is required.
Disadvantages: lower accuracy and surface finish, higher production
cost, and shorter tool life.

5.1.4 Friction effects
PROCESSES
11
Homogeneous Deformation
If a solid cylindrical workpiece is placed between two flat platens and an applied load
P is increased until the stress reaches the flow stress of the material then its height
will be reduced from initial value of ho to h1.
Under ideal homogeneous condition in absence of friction between platens and work,
any height reduction causes a uniform increase in diameter and area from original
area of Ao to final area Af.
P = As 0
Fig. 5.4 Homogeneous deformation
The load required, i.e. the press
capacity, is defined by;

P =s V + m
d
PROCESSES
12
Inhomogeneous deformation
In practice, the friction between platens and workpiece cannot be avoided and the
latter develops a “barrel” shape.
This is called inhomogeneous deformation and changes the load estimation as
follows:
)
3
0 (1
h
h
Fig. 5.5 Inhomogeneous deformation with barreling of the workpiece
where μ is the frictional
coefficient between
workpiece and platen.

PROCESSES
13
Metal forming processes can be classified as:
1. Bulk deformation processes: Forging, Rolling, Extrusion and Drawing (wire);
2. Sheet metalworking processes: Bending, Drawing (cup, deep), Shearing
Fig. 5.6 Typical metal-working operations

PROCESSES
14
5.2 FORGING
Forging is the working of metal into a useful shape by hammering or pressing. It is
the oldest of the metal working processes.
Most forging operations are carried out hot, although certain metals may be cold
forged.
The two broad categories of forging processes are open-die forging and closed-die
forging.
Closed-die forging uses carefully machined matching die blocks to produce forging to
close dimensional tolerances.
h0 hf
Flash Gutter
(çapak
haznesi)
Flash
A (çapak) 0h0 = Afhf
Open die forging Closed die forging
Fig. 5.7 Forging
processes

PROCESSES
15
According to the degree to which the flow of the metal is constrained by the dies
there are three types of forging:
1. Open-die forging
2. Impression-die forging
3. Flashless forging
Fig. 5.8 Three types of forging: (a) open-die forging, (b) impression die forging, and (c) flashless
forging

PROCESSES
16
Open-die forging
Known as upsetting, it involves compression of a work between two flat dies, or
platens. Force calculations were discussed earlier.
Fig.5.9 Sequence in open-die forging
illustrating the unrestrained flow of material.
Note the barrel shape that forms due to
friction and inhomogeneous deformation in
the work
Fig. 5.10 Open-die forging of a multi
diameter shaft

PROCESSES
17
Impression-die forging
In impression-die forging, some of the material
flows radially outward to form a flash:
Fig. 5.11 Schematics of the impression-die
forging process showing partial die filling at
the beginning of flash formation in the
center sketch, and the final shape with flash
in the right-hand sketch
Fig. 5.12 Stages (from bottom to top) in
the formation of a crankshaft by hot
impression-die forging

PROCESSES
18
Flashless forging
The work material is completely surrounded by the die cavity during compression and
no flash is formed:
Fig. 13 Flashless forging: (1) just before initial contact with the workpiece, (2) partial compression,
and (3) final push and die closure. Symbol v indicates motion, and F - applied force.
Most important requirement in flashless forging is that the work volume must equal
the space in the die cavity to a very close tolerance.

PROCESSES
19
Coining
Special application of flashless forging in which fine detail in the die are
impressed into the top and bottom surfaces of the workpiece.
There is a little flow of metal in coining.
Fig.14 Coining operation: (1) start of cycle, (2) compression stroke, and (3) ejection of
finished part

5.2.1 FORGING EQUIPMENT
PE = m g h = W h
W
PROCESSES
20
Forging equipment may be classified with respect to the principle of operation:
a)Forging hammers (Şahmerdan):
The force is supplied by a falling weight. These are energy-restricted machines
since the deformation results from dissipating the kinetic energy of the ram. Their
capacity is expressed with energy units.
Fig. 5.15 Drop forging hammer
Raising Rollers
Upper Die
Lower Die

b) Mechanical forging presses are stroke-restricted machines since the length of
stroke and available load at positions of stroke represent their capacity. Their
capacity is expressed with load.
c) Hydraulic presses are load restricted due to pressure in oil. Their capacity is
PROCESSES
21
expressed with load.
WW
Pressurized oil
Fig. 5.16 Hydraulic press
PRESS SPEED RANGE(m/s)
Hydraulic 0.06-0.30
Mechanical 0.06-1.5
Gravity Drop Hammer 3.6-4.8
Power Drop Hammer 3.6-9.0
HERF Machine (High Energy
Rate Forming) 6.0-24.0

PROCESSES
22
Fig. 5.17 Drop forging hammer, fed by conveyor and heating unit at the right of the scene.

5.2.2 FORGING LOAD
s
PROCESSES
23
In determination forging load for free upsetting (open die), two assumptions can
be made for simplicity:
1. Material is perfectly plastic
2. No friction between material and die surfaces.
W Stroke
so
e
Load (P)

where
P: forging load (press force)
A: area and
so: flow strength
Volume A h Ah Const A V 0
then P V
h
0 0 = = = .Þ = .. =s
0
W Pdh s V h
PROCESSES
24
0 0 0 P = As 0 and P = As
The work done
h
= ò =
0
0 ln
h
h f f h

P =s V + m
d
T P c A 1 0 = s
PROCESSES
25
If there is friction: Schey Equation
For Closed die forging
)
3
0 (1
h
h
AT : total cross sectional area and
c1 = 1.2-2.5 for open die forging
c1 = 3.0-8.0 for simple shape closed die forging
c1 = 8.0-12.0 for complex shape closed die forging

Rolling is a metal deformation process where the thickness of the metal is reduced
by successive passes from rolls.
PROCESSES
26
5.3. ROLLING
Fig.18 The process of flat rolling

Reduction ratio = T - t ´ 100
PROCESSES
27
The metal emerges from the rolls traveling at the higher speed than it enters
100
T
A
f -
Reduction in Area o x
A
A
o
or =

PROCESSES
28
Fig. 19 Side view of flat rolling and the velocity
diagram indicating work and roll velocities
along the contact length L
Flat Rolling

PROCESSES
29
Fig. 5.20-a Various configurations of rolling mills

PROCESSES
30
Fig. 5.20-b Various configurations of rolling mills

 Rolling into intermediate shapes-blooms, billets, slabs.
• Processing blooms, billets, slabs into plates, sheets, bar stock, foils.
• Steel is cast into ingots. Theare storedy in that shape. When milling is
necessary, ingots are heated in soaking pits, up to 1200oC. Ingots are rolled into
intermediate shapes: A bloom has a square section above 150x150 mm. A billet
is smaller than a bloom with square section from 40x40 mm up to bloom. Slabs
have a rectangular section with min. width 250 mm and min. thickness 40 mm.
The strips are rolled from slabs.
PROCESSES
31
5.3.1 STEEL ROLLING
Steps in rolling
The preheated at 1200oC cast ingot (the process is known as soaking) is rolled
into one of the three intermediate shapes called blooms, slabs, or billets.
* Bloom has a square cross section of 150/150 mm or more
* Slab (40/250 mm or more) is rolled from an ingot or a bloom
* Billet (40/40 mm or more) is rolled from a bloom

PROCESSES
32
These intermediate shapes are then rolled into different products as illustrated in the figure:
Fig. 5.21 Production steps in rolling

PROCESSES
33
Shape rolling
The work is deformed by a gradual reduction into a contoured cross section (I-beams,
L-beams, U-channels, rails, round, squire bars and rods, etc.).
Ring rolling
Thick-walled ring of small diameter is rolled into a thin-walled ring of larger
diameter:
Fig. 5.22 Ring rolling used to reduce the wall thickness and increase the diameter of a ring

PROCESSES
34
Thread rolling
Threads are formed on cylindrical parts by rolling them between two thread dies:
Fig. 5.23 Thread rolling with flat dies

PROCESSES
35
Gear rolling
Gear rolling is similar to thread rolling with three gears (tools) that form the gear
profile on the work
Fig. 5.24 Gear rolling between three gear roll tools

PROCESSES
36
5.3.2 LIMITING ROLLING CONDITION
Plain strain conditions are valid for rolling (i.e. no change in width of plate) and
speed of neutral plane (N) is equal to tangential speed of rolls:
b ho vo = b hf vf = b h v b: width

FCos P Sin F
q q q Tan
r ³ Þ ³ ³
Cos
F
r
F
r
Þ = ³ limiting condition for rolling.
PROCESSES
37
The angle a between the entrance plane and the centerline of the rolls is called angle of
contact or angle of bite. For the workpiece to enter into the gap between the rolls, horizontal
component of the normal force Pr and the frictional force F should be equal or frictional force
should be bigger.
q
q
Sin
P
r
Tanq
P
³ where r F = mP
m Tana
P

é
P 2
1 ( Q 1)
= e - b RDh
L
m
= L R h p and = D
p
h
Q
PROCESSES
38
5.3.3 ROLLING FORCE
The following parameters should be considered in rolling force
calculations:
• The roll diameter
• Flow strength of material which is affected by strain rate and temperature
• Friction between rolls and the work piece
• The presence of front and/or back tension
úû ù
êë
Q
3
0 s m
R: radius of rolls,
hm: mean thickness between entry and exit, and
D h: reduction in thickness.

PROCESSES
39
5.3.4 ROLLING CALCULATIONS
f f f Volume t w l = t w l 0 0 0
f f f t w v = t w v 0 0 0
r f v < v < v 0
Volume rate

æ - = - =
r T t f d = t - t 0
PROCESSES
v v
f r
v
40
There is a point where v = vr which is called no-slip point or neutral point.
Before and after this point slipping and friction occur between roll and workpiece.
The amount of the slip can be measured by means of the forward slip:
r
S
-
=
The reduction ratio sometimes used as draft
ö
÷ ÷ø
ç çè
t t
0
0
t
T
In practice max. draft: d R 2
max = m (i.e. μ=0 no draft)
μ : coefficient of friction
R : roll radius
μ ~ 0.1 0.2
0.4
cold warm
hot

PROCESSES
41
Length of contact:
L = Rq
θ: angle of contact (rad.)
Force:
F wL 0 =s
0 s : average flow stress
w : width
Torque:
T = 0.5FL
Power:
P = NT on each roll
N: rotational speed of roll
two rolls P = 2N(0.5FL)
P = NFL

A 300x25 mm strip is fed through a rolling mill with two powered rolls each of 250
mm radius. The work thickness is to be reduced to 22 mm in one pass at a roll
speed of 50 rpm. The flow stress of work material is 180 MPa and the coefficient
of friction between roll and work is about 0.12. Determine if the friction is
sufficient to permit rolling operation to be accomplished. If so calculate the roll
force, torque and horsepower.
PROCESSES
42
Example:
RCosq = 250 - (12.5 -11)Þq = 6.28
for limiting rolling conditions m ³ TanqÞ0.12 > Tan(6.28 )
0.12 > 0.11 feasible

= q = 250´6.28´ p =
F wL 180 N 300 27.4 1.4797 0 2 =s = ´ ´ =
T FL N mm m 20.272
0.5 0.5 (1.4797 10 ) 27.4 13
= = ´ ´ 6 ´ =
P NFL rev 3
N mm m
= = æ p
501 ´ ´ ´ ÷ø
(1.4797 10 ) 27.4 1
2
1min
ö çè
PROCESSES
43
OR draft d = 25 - 22 = 3mm
d 2R (0.12)2250 3.6mm
max = m = =
max d < d feasible
L R 27.4mm
180
mm mm MN
mm
kNm
mm
10
mm
rev
6
10
1
60sec
min
P = 212.287kw
P = 212.287´1.34
P = 284.46hp

Extrusion is a Bulk Deformation Process in which the work is forced to flow through a
die opening to produce a desired cross-sectional shape.
Extrusion is the process by which a block of metal is reduced in cross-section by
forging it to flow through a die under pressure. In general, extrusion is used to produce
cylindrical bars or hollow tubes, but irregular cross-sections may also be produced.
Lead, tin, aluminum alloys can be cold extruded. Horizontal type presses are used.
Speed is depends on temperature and type of material used.
PROCESSES
44
5.4 EXTRUSION
Fig. 5.25 Typical shapes produced by extrusion

Extrusion is performed in different ways therefore different classifications are
available:
• Direct and indirect extrusion
• Hot and cold extrusion
• Continuous and discrete extrusion
PROCESSES
45
5.4.1 EXTRUSION TYPES
Fig. 5.26 Direct extrusion to produce hollow or semihollow cross section

Direct extrusion
PROCESSES
46
Fig. 5.27 Direct extrusion to produce solid cross section. Schematic shows the various
equipment components.

PROCESSES
47
Indirect extrusion
Fig. 5.28 In indirect extrusion (backward, inverse extrusion) the material flows in the
direction opposite to the motion of the ram to produce a solid (left) or a hollow cross
section (right)

PROCESSES
48
5.4.2 EXTRUSION FORCE AND ENERGY
Fig. 5.29 Ram pressure vs ram stroke

It can be calculated similar to forging. Here, ram power and ram force are:
h
W =s V f
P A A0
PROCESSES
49
Df
ho hf
and
0
0 ln
h
f A
0 0 =s ln

5.5 WIRE DRAWING
Drawing operation involves pulling a metal through a die by means of a tensile
force applied to the exit side. The end is grasped by tongs on a draw bench and
pulled through. There may be few successive drawing dies on continuous drawing.
PROCESSES
50
Wire and Bar Drawing is a Bulk Deformation Process in which the cross-section of
a bar, rod or wire is reduced by pulling it through a die opening, as in the next
figure:
Fig. 5.30 Drawing of a rod, bar, or wire

P A A0
0 =s ln
PROCESSES
51
5.5.1 DRAWING LOAD
If friction is zero, then;
f
f A
Fig. 5.31 Wire drawing processes
The number of dies varies between 4 to12. The maximum possible reduction per
pas is 0.63. In practice, draw reductions per pass are well below the theoretical
limit. Reductions of 0.5 for single-draft bar drawing and 0.3 for multiple-draft wire
drawing seem to be the upper limits in industrial practice.

PROCESSES
52
THE END

Ch5 metalworkproc Erdi Karaçal Mechanical Engineer University of Gaziantep

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (13)

Similar to Ch5 metalworkproc Erdi Karaçal Mechanical Engineer University of Gaziantep

Similar to Ch5 metalworkproc Erdi Karaçal Mechanical Engineer University of Gaziantep (20)

More from Erdi Karaçal

More from Erdi Karaçal (20)

Recently uploaded

Recently uploaded (20)

Ch5 metalworkproc Erdi Karaçal Mechanical Engineer University of Gaziantep