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METAL FOAMING
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Dr Juri Metal Forming
Metal Forming
•Cold work
•Warm work
•Hot Work
1
Dr Juri
Overview of processes
2
Dr Juri
Research Interests
Metal Forming Research
Sheet Metal Forming
Micro-Forming Bulk Metal Forming
Deep
Drawing Stamping Sheet Bulk Forging Extrusion
Dr Juri
Importance of Metal Forming in
Manufacturing Engineering
• Net Shape or Close to Net Shape
• High Production Rate
• High Profit Margin
• Low Scrap Rate
• Improving Material Properties
• Etc.
Dr Juri
Applications and Products of
Metal forming in Macro Scale
• Automotive
• Aerospace
• Appliance
• Cookware
• Etc.
Dr Juri
Current Issues of Metal Forming
Industry
• Lack of Experienced Metal Forming
Engineer
• Short Product Life Cycle
• New Metallic Materials
• Developing New Hybrid Process
• High Accuracy and Small Feature Products
• Etc.
Dr Juri
OVERVIEW OF METAL FORMING
• Bulk Deformation Processes:
- Compressive deformation force
- Significant deformations
- Massive shape changes
- Starting work shapes include billets and rectangular bars
• Sheet metal working:
- Also called ―Pressworking‖
- Cold working processes
- Use set of punch and die
- Performed on metal sheets, strips and coils
Surface Area /
Volume
is small
Surface Area /
Volume
is large
Dr Juri
Rolling Forging
Extrusion Wire Drawing
Bending Cup Drawing
Shearing
Bulk Deformation Processes Sheet Metal Working
Metal Forming Processes
Dr Juri
Examples of Precision
Cold Forged Products
Dr Juri
Precision Hot Forging of
Complex Shapes
Dr Juri
Precision Hot Forging of
Complex Shapes
Dr Juri
Temperature in Metal Forming
• For any metal, K and n in the flow curve
depend on temperature
– Both strength (K) and strain hardening (n) are
reduced at higher temperatures
– In addition, ductility is increased at higher
temperatures
Dr Juri
Temperature in Metal Forming
• Any deformation operation can be
accomplished with lower forces and power
at elevated temperature
• Three temperature ranges in metal forming:
– Cold working
– Warm working
– Hot working
Dr Juri
Dr Juri
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
bellow the recrystallization one.
Advantages: lower forces and power, more complex part shapes, no annealing is
required.
Disadvantages: some investment in furnaces is needed.
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.
Dr Juri
Cold Working
• Performed at room temperature or slightly
above
• Many cold forming processes are important
mass production operations
• Minimum or no machining usually required
– These operations are near net shape or net shape
processes
Dr Juri
Advantages of Cold Forming
• Better accuracy, closer tolerances
• Better surface finish
• Strain hardening increases strength and
hardness
• Grain flow during deformation can cause
desirable directional properties in product
• No heating of work required
Dr Juri
Disadvantages of Cold Forming
• Higher forces and power required for
deformation
• Surfaces of starting work must be free of
scale and dirt
• Ductility and strain hardening limit the
amount of forming that can be done
– In some cases, metal must be annealed before further
deformation can be accomplished
– In other cases, metal is simply not ductile enough to
be cold worked
Dr Juri
Warm Working
• Performed at temperatures above room
temperature but below recrystallization
temperature
• Dividing line between cold working and
warm working often expressed in terms of
melting point:
– 0.3Tm, where Tm = melting point (absolute
temperature) for metal
Dr Juri
Advantages and Disadvantages of
Warm Working
• Advantages
– Lower forces and power than in cold working
– More intricate work geometries possible
– Need for annealing may be reduced or
eliminated
• Disadvantage
– Workpiece must be heated
Dr Juri
Hot Working
• Deformation at temperatures above the
recrystallization temperature
– Recrystallization temperature = about one-half
of melting point on absolute scale
• In practice, hot working usually performed
somewhat above 0.5Tm
• Metal continues to soften as temperature increases
above 0.5Tm, enhancing advantage of hot working
above this level
Dr Juri
Why Hot Working?
Capability for substantial plastic deformation
- far more than is possible with cold
working or warm working
• Why?
– Strength coefficient (K) is substantially less than at
room temperature
– Strain hardening exponent (n) is zero (theoretically)
– Ductility is significantly increased
Dr Juri
Advantages of Hot Working • Workpart shape can be significantly altered
• Lower forces and power required
• Metals that usually fracture in cold working
can be hot formed
• Strength properties of product are generally
isotropic
• No strengthening of part occurs from work
hardening
– Advantageous in cases when part is to be
subsequently processed by cold forming
Dr Juri
Disadvantages of Hot Working
• Lower dimensional accuracy
• Higher total energy required, which is the
sum of
– The thermal energy needed to heat the
workpiece
– Energy to deform the metal
• Work surface oxidation (scale)
– Thus, poorer surface finish
• Shorter tool life
– Dies and rolls in bulk deformation
Dr Juri 25
Metal
forming
Principle of the process
Structure
Process modeling
Defects
Design For Manufacturing (DFM)
Process variation
Dr Juri
Principle of Metal Forming
26
Dr Juri Module 8
27
Metal Forming
• Metal forming includes a large group of manufacturing
processes in which plastic deformation is used to change
the shape of metal work pieces
• Plastic deformation: a permanent change of shape, i.e.,
the stress in materials is larger than its yield strength
• Usually a die is needed to force deformed metal into the
shape of the die
Dr Juri 28
• Metal with low yield strength and high ductility is in
favor of metal forming
• One difference between plastic forming and metal
forming is
Plastic: solids are heated up to be polymer melt
Metal: solid state remains in the whole process
- (temperature can be either cold, warm or hot)
Metal Forming
Dr Juri Module 8
29
Metal forming is divided into: (1) bulk and (2) sheet
Metal Forming
Bulk: (1) significant deformation
(2) massive shape change
(3) surface area to volume of the work is small
Sheet: Surface area to volume of the work is large
Dr Juri 30
Bulk deformation processes
Rolling
Forging
Extrusion Drawing
Traditionally
Hot
Dr Juri 31
Sheet deformation processes (Press working/ Stamping)
Bending Drawing
Shearing
Actually
Cutting
Dr Juri
Definitions Plastic Deformation Processes
Operations that induce shape changes on the work piece 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 work piece 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.
Dr Juri
Bulk Deformation Processes
Rolling: Compressive deformation process in which the thickness of a plate is reduced
by squeezing it through two rotating cylindrical rolls.
Forging: The workpiece is compressed between two opposing dies so that the die
shapes are imparted to the work.
Extrusion: The work material is forced to flow through a die opening taking its shape
Drawing: The diameter of a wire or bar is reduced by pulling it through a die opening
(bar drawing) or a series of die openings (wire drawing)
Dr Juri
Rolling
Definition
Rolling is a Bulk Deformation
Process in which the thickness of
the work is reduced by
compressive forces exerted by
two opposing rolls
Dr Juri
35
Dr Juri
Progressive Hot Rolling:
smaller, uniform grains
Kalpakjian
Dr Juri
Rolling
Dr Juri
Rolling
Important Applications:
Steel Plants,
Raw stock production (sheets, tubes, Rods, etc.)
Screw manufacture
Dr Juri
Rolling Basics
Sheets are rolled in multiple stages (why ?)
Vo
Vfto tf
Vo
Vfto tfVo
Vfto tf
Vo
Vfto tf
thread rolling machine
stationary die
rolling diethread rolling machine
stationary die
rolling die
Reciprocating flat thread-rolling diesReciprocating flat thread-rolling dies
Screw manufacture:
Dr Juri
Forging
Definition
Forging is a Bulk Deformation Process in
which the work is compressed between
two dies. According to the degree to which
the flow of the metal is constrained by the
dies there are three types of forging:
ΠOpen-die forging
• Impression-die forging
Ž Flash less forging
Dr Juri
Forging
Dr Juri
Stages in Open-Die Forging
(a) forge hot billet to max diameter
(b) ―fuller: tool to mark step-locations
(c) forge right side
(d) reverse part, forge left side
(e) finish (dimension control)
[source:www.scotforge.com]
Dr Juri
Stages in Impression-die (Closed-Die) Forging
[source:Kalpakjian & Schmid]
Dr Juri
Stages in Impression-die (Closed-Die) Forging
Dr Juri
Flash less forging
Dr Juri
Forging grain flow
Dr Juri
Quality of forged parts
Stronger/tougher than cast/machined parts of same material
Surface finish/Dimensional control:
Better than casting (typically)
[source:www.scotforge.com]
Dr Juri
A material is pushed or drawn through a die of the
desired cross-section. Any solid or hollow cross-section
may be produced by extrusion, which can create
essentially semi-finished parts. The metal can forcing
through a die in the same direction or opposite direction.
Dr Juri
Extrusion Typical use: ductile metals (Cu, Steel, Al, Mg),
Plastics, Rubbers
Common products:
Al frames of white-boards, doors, windows, …
Dr Juri
hydraulic
piston
chamber
chamber
stock
die
extruded shape
hydraulic
piston
chamber
chamber
stock
die
extruded shape
hydraulic
piston
chamber
chamber
stock
die
extruded shape
Extrusion: Schematic, Dies
Exercise: how can we get hollow parts?
Dr Juri
• The cross-sections that can be produced vary from solid round, rectangular, to L shapes, T shapes.
• Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). Extrusions can be done with the material hot or cold.
• Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.
Dr Juri
Extruded products
• Typical products made by extrusion are railings for
sliding doors, tubing having carious cross-sections,
structural and architectural shapes, and door and
windows frames.
Extruded products
Dr Juri
• Direct extrusion: A metal billet is located into a container, and a ram compresses the material, forcing it to flow through one or more openings in a die at the opposite end of the container.
• Indirect extrusion: The die is mounted to the ram rather than at the opposite end of the container. One advantage of the indirect extrusion process is that there is no friction, during the process, between the billet and the container liner.
Dr Juri
Drawing
Commonly used to make wires from round bars
stock (bar)
F (pulling force)
wirediestock (bar)
F (pulling force)
wiredie
Similar to extrusion, except: pulling force is applied
Dr Juri
WHAT is DRAWING?
Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die.
The principle of this procedure consist of reducing the thickness of a pointed ,tapered wire by drawing it through a conical opening in a tool made of a hard material.The wire will take shape of the hole.
Dr Juri
• Drawing improves strength and hardness when these
properties are to be developed by cold work and not by
subsequent heat treatment
• Where is it used?
This process is widely used for the production of
thicker walled seamless tubes and cylinders
therefore; shafts, spindles, and small pistons and
as the raw material for fasteners such as rivets,
bolts, screws.
Dr Juri
DRAWING TOOLS
• The most important tool in the drawing process is without doubt the
drawplate.This consist of a plate of high grade steel into which
similar shaped holes have been placed whose size in evenly reduced
from one hole to another.
• The most common drawplate have round holes and are used to
reduce the size of round wire.
Drawing wire with the
draw tongs
drawbench
Dr Juri
Deep Drawing (cold, for sheetmetal)
- Punch draws blank into die
- Metal is supported on both sides to avoid wrinkling
- Hold-down pressure (blankholder force)
is primary process variable
if too high: tearing
if too low: wrinkling
Kalpakjian
www.endo-mfg.co.jp
Dr Juri
How such a drawplate hole is made
Dr Juri
www.dissco.co.nz
Spinning
Ideal for
• Lower production volumes
• Large parts
• Inexpensive tooling
www.traditional-building.com
www.ashfordmetalspinning.co.uk
Dr Juri Module 8
61
In the following series of lecture, we discuss:
1. General mechanics principle
2. Individual processes:
- mechanics principles
- design for manufacturing (DFM) rules
- equipment
Dr Juri Module 8
62
1. General mechanics principle
• The underlying mechanics principle for metal forming is
the stress-strain relationship; see Figure 1.
Figure 1
Dr Juri Module 8
63
• True strain: Instantaneous elongation per unit length of
the material
0ln
0 L
L
L
dLL
L
L0: the initial length of a specimen
L: the length of the specimen at time t
the true strain at time t
• True Stress: Applied load divided by instantaneous
value of cross-section area
AF /
Dr Juri Module 8
64
• In the forming process we are more interested in the
plastic deformation region (Figure 1)
Plastic
deformation
region
Dr Juri Module 8
65
• The stress-strain relationship in the plastic deformation
region is described by
nK
Where
K= the strength coefficient, (MPa)
= the true strain, σ=the true stress
n= the strain hardening exponent,
The flow stress (Yf) is used for the above stress
(which is the stress beyond yield)
Called
FLOW
CURVE
Dr Juri Module 8
66
• As deformation occurs, increasing STRESS is required
to continue deformation (shown in curve)
• Flow Stress: Instantaneous value of stress required to
continue deforming the material (to keep metal
―flowing‖)
FLOW STRESS
nKfY
Dr Juri Module 8
67
• For many bulk deforming processes, rather than
instantaneous stress, average stress is used (extrusion)
• The average flow stress can be obtained by integrating
the flow stress along the trajectory of straining, from
zero to the final strain value defining the range of
interest
n
kY
n
f
1
AVERAGE FLOW STRESS
Average flow stress Max. strain during
deformation
Strength Coefficient
Strain hardening exponent
Dr Juri Module 8
68
Example 1:
Determine the value of the strain-hardening exponent for a
metal that will cause the average flow stress to be three-
quarters of the final flow stress after deformation.
According to the statement of the problem, we have
4/3fY of fY
333.0
75.0)1/(1
75.0)1/(
75.0
n
n
KnK
YY
nn
ff
Dr Juri Module 8
69
• The above analysis is generally applicable to the cold
working, where the temperature factor is not considered.
• The metal forming process has three kinds in terms of
temperature: (1) cold, (2) warm, (3) hot
• In the case of warm and hot forming, the temperature
factor needs to be considered, in particular
Temperature up The (yield) strength down and
ductility up
Dr Juri Module 8
70
• Strain rate (related to elevated temperatures)
- Rate at which metal is strained in a forming process
- In the hot forming or warm forming, the strain rate
can affect the flow stress
hv /h
Speed of
deformation (could
be equal to velocity
of ram)
Instantaneous
height of
work-piece
being
deformed h
m
f CY Flow stress
Strain Rate
Dr Juri Module 8
71
m
f CY
where
C strength constant
m strain-rate sensitivity exponent
C and m are determined by the following figure
which is generated from the experiment
nKfY
Strength
coefficient but not
the same as K
Dr Juri Module 8
72
Dr Juri Module 8
73
C and m are affected by temperature
Temperature Up
C Down
m Up
Dr Juri Module 8
74
mn
f AY
Even in the cold work, the strain rate could affect the
flow stress. A more general expression of the flow stress
with consideration of the strain rate and strain is
presented as follows:
A is a strength coefficient, a combined effect of K, C
All these coefficients, A, n, m, are functions of
temperature
Dr Juri Module 8
75
Example 2:
A tensile test is carried out to determine the strength
constant C and strain-rate sensitivity exponent m for a
certain metal at 1000oF. At a strain rate = 10/sec, the
stress is measured at 23,000 lb/in2; and at a strain rate =
300/sec, the stress=45,000 lb/in2. Determine C and m
23000=C(10)^m
45000=C(300)^m
From these two equations, one can find m=0.1973
Solution:
Dr Juri
Finite Element Simulations Predict and minimize tearing and wrinkling locations
nsmwww.eng.ohio-state.edu
www.esi-group.com
tmku209.ctw.utwente.nl
