This document provides an overview of metal forming processes, including definitions, classifications, and comparisons of hot and cold working. It discusses key topics such as plastic deformation, strain hardening, yield criteria, temperature effects, lubrication, and considerations in process selection. The objectives are to understand elastic and plastic deformation, strain hardening concepts, yield criteria, and differences between hot and cold working processes.

1. Manufacturing Technology II
(ME-202)
Overview of
Metal Forming
Processes
Dr. Chaitanya Sharma
PhD. IIT Roorkee

2. Title of slide
Lesson Objectives
In this chapter we shall discuss the following:
1. Elastic and plastic deformation;
2. Concept of strain hardening;
3. Yield criterions
4. Hot and cold working processes
Learning Activities
1. Look up
Keywords
2. View Slides;
3. Read Notes,
4. Listen to
lecture
Keywords:

3. 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.

4. Stresses in Metal Forming
• Stresses to plastically deform the metal are
usually compressive
Examples: rolling, forging, extrusion
• However, some forming processes stretch the
metal (tensile stresses)
• Others bend the metal (tensile and
compressive)
• Still others apply shear stresses

5. Classification of Metal Forming
Processes
• Based on the type of force applied on to
the work piece
oDirect-compression-type processes
oIndirect-compression processes
o Tension type processes
o Bending processes
o Shearing processes

18. Material Properties in
Metal Forming
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:

19. Bulk Deformation
Processes
• Characterized by significant deformations and
massive shape changes
• "Bulk" refers to workparts with relatively low
surface area-to-volume ratios
• Starting work shapes include cylindrical billets
and rectangular bars

20. Material Behavior 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

21. 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

22. Average Flow Stress
• Determined by integrating the flow
curve equation between zero and the
final strain value defining the range of
interest
where
= average flow stress; and
= maximum strain during deformation

24. 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.

25. 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

26. 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

27. 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
ε= true strain rate and
h = instantaneous height of workpiece being deformed

28. 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

30. Strain Rate Sensitivity
Equation
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
• Increasing temperature decreases C, increases m
 At room temperature, effect of strain rate is almost
negligible
Flow curve is a good representation of material
behavior
 As temperature increases, strain rate becomes

32. 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

33. 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

35. 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

36. Manufacturing Technology
Hot Working: T>0.5Tm
• Mechanical working of a metal above the recrystallization
temperature but below the melting point is known as hot working.
• The temperature at which the complete recrystallization of a metal
take place with in a specified time
• The recrystallization temperature of metal will be about 30 to 40% of
its melting temperature.
Types
• Forging
• Rolling
• Extrusion
• Drawing

37. Manufacturing Technology
Hot Working
• Advantages
– Force requirement is less
– Refined grain structure
– No stress formation
– Quick and Economical
– Suitable for all metals
• Disadvantages
– Poor surface finish
– Less accuracy
– Very high tooling and handling cost
– Sheets and wires cannot be produced

38. Manufacturing Technology
Cold Working :T<0.3Tm
 Mechanical working of a metal below the recrystallization
temperature (Room Temperature) is known as cold working.
 Reduces the amount of plastic deformation that a material can
undergo in subsequent processing and requires more power for
further working
Types
 Drawing
 Squeezing
 Bending

39. Manufacturing Technology
Cold Working
• Advantages
– Better surface finish
– High dimensional accuracy
– Sheets and wires can be produced
– Suitable for Mass production
• Disadvantages
– Stress formation in metal very high
– Close tolerances cannot be achieved
– No Refined grain structure

40. Manufacturing Technology
Comparison of Hot and Cold Working
S.No Hot Working Cold Working
1 Working above
recrystallization temperature
Working below recrystallization
temperature
2 Formation of new crystals No crystal formation
3 Surface finish not good Good surface finish
4 No stress formation Internal Stress formation
5 No size limit Limited size