Lecture Set IV – Fundamentals of Metal

Forming
 Learning Objectives

By the end of this lesson, you will be able to:

• Classify metal forming operations on the basis of working

temperature and work piece modulus.

• Describe metal behaviour during metal forming.

• Perform metal forming calculations involving flow stress,

average flow stress, strain rate etc.

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 Introduction

• Metal forming is the backbone of the modern manufacturing

industry besides being a major industry in itself.
• Throughout the world hundreds of million tons of metals go
through metal forming processes every year.
• As much as 15–20% of GDP of industrialized nations comes from
the metal forming industry.
• Besides, it fulfills a social cause by providing job opportunities to
millions of workers.

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• Metal forming industry, in general, is a bulk producer of semi-
finished and finished goods and this is one reason that it is viable
to undertake large scale research and development projects
because even a small saving per ton adds up to huge sums.
• In metal forming processes, the product shapes are produced by
plastic deformation.
• Hence it is important to know the plastic flow properties of
metals and alloys for optimizing the processes.
• Also the resulting component properties depend upon the
intensity and the conditions of plastic deformation during
forming.
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• Many forming processes produce raw materials for other
processes which in turn produce finished or semi-finished
products.
• For example, steel plants produce sheet metal which is used by
the automobile industry to manufacture components of
automobiles and their bodies.
• In fact sheet metal is used by a number of manufacturers for
producing a large variety of household and industrial products.
• Similarly billets produced by steel plants are used by re-rolling
mills for rolling into products like angles, channels, bars etc.
• Bars may be further used for manufacturing forgings, wires, bright
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bars and machined products.
• Similarly the manufacturers of rivets, screws, bolts and nuts buy
wire from wire manufacturers and process them further.
• Therefore, the producers of semi-finished materials such as sheet
metal, bar stock and wires, etc. have to consider that they
produce such properties in their products which are required by
downstream industry engaged in further processing of these
products.
• For example, deep drawability of sheet metal increases with
increase in anisotropy ratio therefore, rolling parameters such as
finishing temperature, cold reduction etc., are adjusted to
produce higher anisotropy ratio in the sheet metal which is to be
used for deep drawing.
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 Topics
• Forming properties of metals and alloys
• Elastic and plastic behaviour of metals
• Flow stress, average flow stress, and strain rate
sensitivity
• Hot, warm and cold working
• Bulk deformation processes
• Sheet forming processes

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 Metal Forming
• Large group of manufacturing processes in which plastic
deformation is used to change the shape of metal workpieces.
• The tool, usually called a die, applies stresses that exceed yield
strength of metal.
• The metal takes a shape determined by the geometry of the die.
• Stresses to plastically deform the metal are usually compressive.
- Examples: rolling, forging, extrusion

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• However, some forming processes

- stretch the metal (tensile stresses).

- Others bend the metal (tensile and compressive).

-Still others apply shear stresses.

• Desirable material properties:

- Low yield strength and high ductility

• These properties are affected by temperature:

- Ductility increases and yield strength decreases when work

temperature is raised.

• Other factors:

-Strain rate, strain hardening, and friction

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 Testing for yield strength / flow stress

• For determination of yield strength or flow

stress, one or more of the following three
basic tests are conducted.
1. Tension test.

2. Compression test.

3. Torsion test.

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 Material Behaviour in Metal Forming

•Plastic region of stress-strain curve is primary interest

because material is plastically deformed.
• In plastic region, metal's behavior is expressed by the
flow curve:
where K = strength coefficient; and n = strain
hardening exponent
•Stress and strain in flow curve are true stress and
true strain
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 Flow Stress
• For most metals at room temperature, strength
increases when deformed due to strain hardening.
• Flow stress = instantaneous value of stress required to
continue deforming the material.

where Yf = flow stress, that is, the yield strength as a

function of strain.
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 Average Flow Stress

• Determined by integrating the flow curve equation

between zero and the final strain value defining the
range of interest 𝐾𝜀 𝑛
𝑌ത𝑓 =
1+𝑛

Where = average flow stress; and =

maximum strain during deformation process.

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 Temperature in Metal Forming
• For any metal, K and n in the flow curve depend on temperature:
- Both strength and strain hardening are reduced at higher
temperatures.
- In addition, ductility is increased at higher temperatures.
• 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
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 Cold Working
• Performed at room temperature or slightly above. i.e.
below recrystallisation temperature of the workpiece
metal.
• 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.
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 Advantages of Cold Forming vs.
Hot Working
• 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.

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 Disadvantages of Cold Forming

• Higher forces and power required.

• Surfaces of starting workpiece must be free of scale
and dirt.
• Ductility and strain hardening limit the amount of forming
that can be done.
- In some operations, metal must be annealed to
allow further deformation.
- In other cases, metal is simply not ductile enough
to be cold worked.
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 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.

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 Advantages of Warm Working

•Lower forces and power than in cold working

•More intricate work geometries possible
•Need for annealing may be reduced or eliminated

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 Hot Working
• Deformation at temperatures above 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.
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 Why Hot Working?
Capability for substantial plastic deformation of the
metal - far more than possible with cold working or
warm working.
•Why?
- Strength coefficient is substantially less than at room
temperature.
- Strain hardening exponent is zero (theoretically).
- Ductility is significantly increased.
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 Advantages of Hot Working vs. Cold
Working
• Workpiece 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.

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 Disadvantages of Hot Working

• Lower dimensional accuracy.

• Higher total energy required (due to the thermal
energy to heat the workpiece).
• Work surface oxidation (scale), poorer surface
finish.
• Shorter tool life.

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 Strain Rate Sensitivity
• Theoretically, a metal in hot working behaves like a
perfectly plastic material, with strain hardening
exponent n = 0.
• The metal should continue to flow at the same flow
stress, once that stress is reached.
• However, an additional phenomenon occurs during
deformation, especially at elevated temperatures:
Strain rate sensitivity.
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 What is Strain Rate?
• Strain rate in forming is directly related to speed of
deformation v.
• Deformation speed v = velocity of the ram or other
movement of the equipment.
Strain rate is defined:

where = true strain rate; and h = instantaneous height

of workpiece being deformed.
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 Evaluation of Strain Rate
• In most practical operations, valuation of strain
rate is complicated by :
- Workpiece geometry.
- Variations in strain rate in different regions of the
part.
• Strain rate can reach 1000 s-1 or more for some
metal forming operations.

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 Effect of Strain Rate on Flow Stress
• Flow stress is a function of temperature
• At hot working temperatures, flow stress also
depends on strain rate:
- As strain rate increases, resistance to
deformation increases.
- This effect is known as strain-rate sensitivity.

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 Strain Rate Sensitivity Equation

where C = strength constant (similar but not equal

to strength coefficient in flow curve equation), and
m = strain-rate sensitivity exponent

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 Observations about Strain Rate
Sensitivity
• Increasing temperature decreases C, increases m.
• At room temperature, effect of strain rate is almost
negligible.
• Flow curve is a good representation of material
behaviour.
• As temperature increases, strain rate becomes
increasingly important in determining flow stress.

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 Friction in Metal Forming
• In most metal forming processes, friction is
undesirable:
- Metal flow is retarded
- Forces and power are increased
- Wears tooling faster
• Friction and tool wear are more severe in hot
working.

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 Lubrication in Metal Forming

• Metalworking lubricants are applied to tool-work

interface in many forming operations to reduce harmful
effects of friction.
• Benefits:
- Reduced sticking, forces, power, tool wear
- Better surface finish
- Removes heat from the tooling
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 Considerations in Choosing a Lubricant

• Type of forming process (rolling, forging, sheet metal

drawing, etc.)
• Hot working or cold working
• Work material
• Chemical reactivity with tool and work metals
• Ease of application
• Cost

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 Classification of Metal Forming
Processes
• Bulk Deformation Processes:
- Compressive deformation force
- Significant deformations
- Massive shape changes
Volume /
- Starting work shapes include billets and
Surface Area rectangular bars
is large
• Sheet metal working:
- Also called “Pressworking”
- Cold working processes
- Use set of punch and die
- Performed on metal sheets, strips and
Volume /
coils
Surface Area
is small

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 Bulk Deformation Processes

• Characterized by significant deformations and

massive shape changes.

• "Bulk" refers to workpieces with relatively high

volume-to-surface area ratios.

• Starting work shapes include cylindrical billets

and rectangular bars.

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 Rolling

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 Forging

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 Extrusion

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 Drawing

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 Sheet Metalworking
• Forming and related operations performed on
metal sheets, strips, and coils.
• Low volume-to-surface area ratio of starting metal,
which distinguishes these from bulk deformation.
• Often called pressworking because presses perform
these operations.
-Parts are called stampings
-Usual tooling: punch and die
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 Bending

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 Deep Drawing

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 Shearing

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 Importance of Metal Forming in
Manufacturing Engineering
1. Net Shape or Close to Net Shape

2. High Production Rate

3. High Profit Margin

4. Low Scrap Rate

5. Improving Material Properties

6. Etc.

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 Applications and Products of Metal
forming in Macro Scale
• Automotive

• Aerospace

• Appliance

• Cookware

• Etc.

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 o fim

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 YouTube Videos
• https://www.youtube.com/watch?v=RP4Y_aSBoNw

• https://www.youtube.com/watch?v=5EeuYai8Ax8

• https://www.youtube.com/watch?v=32fGcqmvlow

• https://www.youtube.com/watch?v=aEatTMQsGtg

• https://www.youtube.com/watch?v=GOxT9tFGC50

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