Basic classification of

manufacturing processes
➔ On the basis of physical state of the starting material

- Liquid state (casting processes)

- Solid state (forming and machining processes)
- Powders (powder metallurgy, Additive Manufacturing)

➔ On the basis of the way the component is manufactured

- Forming processes (bulk and sheet)

- Subtractive processes (machining)
- Additive processes (Additive Manufacturing)
Forming processes
o Bulk

o Sheet
Bulk Forming Processes

Chapter 6
 5
Rolling process
F

Width of Strip is w0 Width of Strip is wf

Vo Vf
ho hf
Vf = Vo (ho/hf)

Rotating rolls reduce the thickness of the incoming strip.

Friction is essential, otherwise the strip slips

 6

Rolling
schemes
 7
Sheet and plate rolling

• Continuous cast slab

• Scalp (not always)
• Reheat and homogenize
• Hot roll in reversing mill to ~4-5 mm thick
• Cut off ends (alligators)
• Warm roll to 0.2-4 mm thick
• Heat treat to recrystallize (sometimes)
• Cool
• Cold roll to finish thickness
• Heat treat (where appropriate)
• Results in material which must be further worked
 Process variables 8

Independent variables
• Roll speed and radius
• Draft (amount of thickness reduction) or roll gap
• Strip initial thickness
• Strip initial width
• Strip material
F
• Strip initial temperature
• Lubricant N

Width of Strip is w0 Width of Strip is wf

Dependent variables
Vo Vf
ho hf
• Roll force Vf = Vo (ho/hf)
• Power
• Speed of exiting strip
• Final strip temperature F

• Maximum draft
• Roll strip contact length
• Sheet or plate shape
 9
Hot rolling
Advantages
– Stresses lower
– Forces smaller
– Power requirements less
– No work hardening
– Large deformations possible
– Breaks up the cast structure into preferable forms
– Closes porosity
– Sometimes the only way to create sheets

Disadvantages
– Higher friction
– Rolls need to be cooled
– Material handling difficult
– Personnel must be protected from heat
 10

Microstructure in hot rolling

 11
Cold rolling
Advantages
– Deformations of ~ 50 to 80%
– Work hardening increases strength
– Excellent surface finish
– Excellent tolerances on the thickness and shape

Disadvantages
– High forces
– Small reductions give rise to surface stresses and non-
uniform stress distributions
Mechanics of flat rolling – plane strain 12

To keep constant the volume

rate of the material, the velocity
of the strip must increase as it
moves through the roll gap

 h0 
Vf = V0  
 hf 

NEUTRAL POINT:
point in the arc of
contact where the roll
velocity and the strip
velocity are the same
 13

Rolling force and power

R=roll radius

L = Rh h=difference between the original

and final thickness of the strip (draft)

æ mL ö Yf’=average flow stress in plane

F = LwYf ' ç1+ ÷ strain of the material in the roll gap
è 2hav ø
FL
Assume that the roll force acts in
the middle of the arc of contact T = Torque per roll
2
Power = T Power required per roll

FLN F (N)
Power = (kW) L (m)
60000 N (rpm)
Power required per roll
 14

Hot rolling force

• Proper estimation of the friction coefficient at

elevated temperatures
 = 0 .2 → 0 .7
• Strain-rate sensitivity of metals at high
temperatures
Vr  h0 
• Calculation of the average flow stress
considering the dependency from the strain rate
 = ln 
L  hf 
 15

Friction coefficient influence

hmax =  2R Maximum draft

 16
Rolling mills

• Most commercial mills

are 4 high, reversible
– Small rolls in contact
– F a L a R0.5
– Large back up rolls
provide stiffness

Schematic illustration of various roll

arrangements: (a) two high; (b) three
high; (c) four high; (d) cluster; (e) tandem
rolling with three stands; (f) planetary.
 17
Sendzimir rolling mill

Schematic illustration of a cluster (Sendzimir) mill. These mills are very rigid and are
used in cold rolling of thin sheets of high-strength metals, with good control of
dimensions. Rolled products are as wide as 5000 mm and as thin as 0.0025 mm.
 18

Shape rolling
• Non-flat shapes can be produced by using shaped rolls
– I-Beams
– H-sections
– Rails (trains)
– Angle iron

• Design of shaped rolls to provide the right amount of non-

uniform deformation is critical
 19
Sequence for shape rolling

Stages in the shape rolling

of an H-section part.
Various other structural
sections, such as channels
and I-beams, are also
rolled by this kind of
process.
 20

Ring rolling

(a) Schematic illustration of a ring-rolling operation. Thickness reduction results in an increase

in the part diameter. (b-d) Examples of cross-sections that can be formed by ring-rolling.
 21
Thread rolling

Thread-rolling processes: (a) and (b) reciprocating flat dies; (c) two-roller dies. Threaded
fasteners, such as bolts, are made economically by these processes at high rates of production.
 22

Rolled vs. machined threads

(a) Schematic illustration of machined or rolled threads. (b) Grain-flow lines in

machined and rolled threads. Unlike machined threads, which are cut through the
grains of the metal, rolled threads follow the grains and are stronger, because of the
cold working involved.
Application case – prosthetic screw
Cut profile Rolled profile
 24
Mannesmann process

The rod is driven by a pair of cone-shape rolls that have skewed axis and rotate in the same
direction. The frictional load between the rolls and the rod causes the rod to rotate and forces
it to advance longitudinally over the piercer point, where the internal fracture starts. The round
bar rotates and undergoes a cyclic progressive compression which results in high tensile
stresses in the centre of the rod itself.
 Forging procedure

• Prepare raw material including cleaning

• Heat workpiece (for hot forging)
• Descale if necessary
• Preheat and lubricate dies (for hot forging)
• Forge in appropriate dies and in correct sequence
• Remove excess material (for hot forging)
• Clean
• Check dimensions
• Straighten if necessary
• Machine to final dimensions
• Heat treat if necessary
• Inspect
 Characteristics
• Tolerances of 0.5% to 1% can be achieved
• Material properties can be tailored by appropriate die design
– Directed material flow

Schematic illustration of a part made by three different processes showing grain flow. (a)
Casting, (b) machining from a blank, and (c) forging.
• Advantages
– Metal flow and grain structure can be controlled
– Results in good strength and toughness
– Near net shape (low buy to fly ratio)
– Parts of reasonable complexity can be created
• Landing gear
• Connecting rods
• Complex shafts

• Disadvantages
– Dies are expensive, particularly for hot forging
– Highly skilled labor required
 Open die forging
• Simplest and cheapest
• Also called upsetting or flat-die forging
• Advantages
– Cheap
– Can form a wide variety of simple shapes with the same
dies
• Squares, cylindrical
– Useful for preparing material for other forms of forging
or machining
– Can handle large items (35 tons)
• Disadvantages
– Barreling of shape due to high friction
Plane strain open die forging

Distribution of die pressure, in terms of p/Y´, in plane-strain

f compression with sliding friction. Note that the pressure at the
left and right boundaries is equal to the yield stress in plane
strain, Y´. Sliding friction means that the frictional stress is
directly proportional to the normal stress.

æ ma ö
pav @ Yf ' ç1+ ÷
è hø
F = (pav )(2a )(width )
 Axi-symmetric open die forging

Stresses on an element in forging of a solid cylindrical workpiece between flat dies.

æ 2m r ö
pav @ Yf ç1+
è
÷
3h ø
F = (pav ) r ( ) 2
 Impression die forging
• Uses dies with the approximate end shape
• Usually requires more than one die to complete the process
• Fullering and Edging dies prepare material to take up die shape
– Fullering moves material away from center
– Edging moves material away from edges
• Flashing produced from excess material in case of hot forging

• Advantages
– Produces near-net-shape products
– Material properties tailored to application
• Disadvantages
– High die costs
– Highly skilled labor required
Connecting rod forging
Forging load
 Forging load
F = k Yf A
where Yf is the flow stress
A is the area and
k is a factor given below

Shapes k
Simple, no flashing 3-5
simple, with flashing 5-8
Complex, with flashing 8-12
 Forgeability

• Defined as the capability of a metal to undergo deformation

without cracking
• Common test is the upset test
– Upset a cylindrical specimen to fixed, large deformation
– Examine barrel surfaces for cracks
• Another is the hot torsion test
– Twist long cylindrical specimen around its axis
– Number of twists to failure is material forgeability
– Also used for rolling and extrusion deformation
capabilities
 Further forging processes

Heading
– Used mostly for bolts

Orbital forging
– Forms the part incrementally
– Small forging forces
because the die contact is
concentrated on a small part
of the workpiece at time
– Applicable to mostly
cylindrical shapes
 Further forging processes
Isothermal forging
– Dies at same
temperature of workpiece
– No workpiece cooling
– Low flow stresses
– Better material flow
– More close tolerances
and finer details can be
achieved

Cogging/swaging
– Cylindrical parts
subjected to radial
impact forces by
reciprocating dies
– Used to reduce tube
diameter and introduce
rifling into gun barrels
 Application case – titanium alloy hip stem

Material:
Ti-15Zr-4Nb

https://www.youtube.com/watch?v=Ssdb9hkS8rU
Forging costs
 Application – Hot forging of a cylindrical billet /1
Data:
Tforging=1120°C
r=7850kg/m3
Component diameter at the parting line =
66mm
Weight of the component after forging and
flash trimming = 1.124kg
Yf=120MPa

Calculate the dimensions of

the billet before forging and
the needed forging force

1) Weight of the initial billet: M = (ma + mb )A

Flash weight = 5-15% forged component weight
Weight loss due to thermal oxidation = 3-5% initial billet weight
 Application – Hot forging of a cylindrical billet /2

2) Volume of the initial billet

V =M
r
3) Dimensions of the initial billet
 - Upsetting design
V= Db2H b
4 - Extrusion design

4) Dimensions of the flash land

1 s = height of the flash land
b 62.5
s = 0.017D + = b = length of the flash land
D+5 s D
D = component diameter at the parting line

5) Forging force  (D + 2b )2
F = K pYf A = K pYf
4
Extrusion process

• Push an ingot through an orifice with the

desired shape
• Both hot and cold extrusion are possible
– Depends on the material
 Direct Extrusion
• Direct or forward extrusion
– Billet pushed through die from behind
 Indirect extrusion
– Die pushed into billet
– No relative motion at the billet-container interface
 Hydrostatic extrusion
Ram compresses fluid surrounding billet which is pushed out
through the die; the fluid transmits the pressure to the billet

• Very recent development

(1950's)
• Advantages
– No friction at walls
– Can successful extrude
brittle materials
– Yield stress reduced by
hydrostatic pressure
– Room temperature
operation
• Disadvantages
– Limited industrial
applications
– Complex tooling
– High pressure experience
required
 Lateral extrusion
Like direct, but material comes out of
the die at right angles to the direction
in which the billet is pushed

→ Chance to nanostructure metals, like pure titanium

 Process scheme

Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed,
billet temperature, and lubrication all affect the extrusion pressure.
 Indipendent variables
• Die angle a
• Extrusion ratio (ratio of input to output cross-sectional
areas) A0/Af
• Circumscribing circle diameter
• Shape factor
– Ratio of perimeter of shape to cross-sectional area
– A circular shape has the minimum
– A measure of the complexity of the extrusion
• Billet temperature
• Ram speed
• Lubricant
 Material flow

Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction,
or in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces.
(c) Pattern obtained at high friction, or with cooling of the outer regions of the billet in the
chamber. This type of pattern, observed in metals whose strength increases rapidly with
decreasing temperature, leads to a defect known as pipe, or extrusion defect.

Frictional conditions at billet-container-die

interfaces and thermal gradients in the billet
 Process mechanics
Case of ideal deformation

A0
Extrusion Ratio R=
Af

 A0   Lf 
 1 = ln  = ln  = ln R
 Af   L0 
Work = (A0 )(L0 )(u ) Work = FL0 = pA0L0 F: ram force

1
p = u =  d Extrusion Pressure at the Ram
0
 Process mechanics
Case of ideal deformation and friction (slab method)

æ tan a ö m cot a : friction at the

p = Yfç1+ ÷éë R -1ùû die-billet interface
è m ø a: die angle

æ 2L ö Hp: the frictional stress is equal to

p = Yfç1.7ln R+ ÷ the shear yield stress; because of
the dead metal zone, the material
è D0 ø flows along a 45° “die angle”

Actual forces (also non-homogenous deformation)

p = Yf ( a+ bln R) where a and b are experimentally determined

constants a=0.8; b=1.2-1.5
Optimal die angle

Schematic illustration of the extrusion force

as a function of the die angle: (a) total
force; (b) ideal force; (c) force required for
redundant deformation (inhomogeneous
deformation); (d) force required to overcome
friction (the length of contact at the billet-die
interface decreases).

Note that there is an optimum die angle

where the total extrusion force is a
minimum.
 Cold extrusion

• More recent development (1940's)

• Includes some operations similar to forging

Advantages
– Improved mechanical properties
– Work hardening
– Good control of tolerances
– Improved surface finish
– Elimination of heating costs
– High production rates
Disadvantages
– Higher stresses require more expensive tooling and die material
– Difficult die design essential to success
– Lubrication is critical
Examples of cold extrusion
 Hot extrusion
Advantages
– Lower stresses
– Sometimes the only way to extrude certain metals
• e.g. titanium, refractory alloys, high strength alloys
Disadvantages
– Higher die wear
– Oxide film buildup results in non-uniform flow, inclusions and
poor surface finish
– Cooling of the billet in the container results in non-uniform flow
– Distorted parts

Process temperature ranges

Material Temp
Range (oC)
Lead 200-250
Aluminum and alloys 375-475
Copper and alloys 650-975
Steels 875-1300
Refractory alloys 975-2200
 Drawing process
• Like extrusion but the material is pulled out the orifice of the
required shape instead of being pushed in.
• The resulting shape variety is more limited than in extrusion
– Rod (larger cross sections)
– Wire (smaller cross sections)
Advantages
– Very long product with non-flat cross-section than in extrusion - wire,
rod (total length can exceed miles)
– Non circular cross sections can be produced, but uncommon

Disadvantages
– Higher forces required
– If exceed tensile strength further deformation can be non-uniform
– Limited size reduction per pass (Maximum 63%)
– Need multiple dies and take up rolls
 Mechanics of the drawing process
Case of ideal deformation
æA ö æA ö
s d = u = Yf ln çç 0 ÷÷ F = YfAf ln çç 0 ÷÷
è Af ø è Af ø

Case of ideal
deformation with friction
æ tan a öé æ Af ö
m cot a ù

s d = Yf ç1+ ÷ê1- ç ÷ ú
è m øêë è A0 ø úû
Variation in strain and flow stress in the deformation zone in drawing. Note that the strain
increases rapidly toward the exit. The reason is that when the exit diameter is zero, the
true strain reaches infinity. The point Ywire represents the yield stress of the wire.
 Die pressure
d: the tensile stress in the deformation
zone at any diameter

p = Yf - s
Yf: the material flow stress at any diameter
d p: die pressure at any diameter along the
die contact

Variation in the drawing stresses and die contact pressure along the
deformation zone. Note that as the drawing stress increases, the die
pressure decreases. Note the effect of back tension.
 Maximum reduction per pass
Perfectly plastic material

Strain hardening material