dr.r.narayanasamy - metal forming part - i

05/01/2023 1

Metal Forming Part - I

Dr.R.Narayanasamy B.E.,M.Tech.,M.Engg.,Ph.D.,(D.Sc.)Professor, Department of Production Engineering,

National Institute of Technology, Tiruchirappalli – 620 015, Tamil Nadu, India.

05/01/2023 2

Strain Hardening

• Strain hardening occurs when the metals are deformed at lower temperature.

• Flow stress increases with increasing strain.• This applies to forming temperatures that are

low, that the thermally activated process plays no significant role.

• Strain hardening results in higher forming force and forming work, thus enhancing the load stresses acting on the tool.

05/01/2023 3

Strain Hardening cont…

• As a result of strain hardening (in many cases) heat treatment is required.

• After every forming operation to improve formability for obtaining the required deformation heat treatment is required.

Alternative way: • Metal forming at high (elevated) temperatures.• Dimensional accuracy and surface quality is poor

in hot forming operation.

05/01/2023 4

Strain Hardening cont…

• Strain hardening can be used in all metals and alloys to increase strength and hardness.

• The increase in flow stress and tensile strength permits the use of materials with lower initial strength compared to machined components.

• In many cases heat treatment becomes unnecessary because of strain hardening.

05/01/2023 5

Work hardening

• Atoms are arrested in a certain geometric fashion (called as crystal structure).

• Impossible to have perfect arrangement of atoms (certain type of defects will be in crystal structure).

• The important type of defect is dislocation – line defect.

• To cause plastic deformation – external forces are applied. Dislocations move till they are arrested by barriers (grain boundaries, inclusions etc.).

05/01/2023 6

Work hardening cont…

• During the process, the density of the dislocation increases by Frank Reed sources.

• Dislocation density for annealed metal is .• Dislocation density for cold worked metal is .• As the dislocation density increases, the movement of

dislocation becomes more difficult. (because mean free path of dislocation reduces and this is called back stress).

• This process increases the strength of metal called work hardening.

05/01/2023 7

Annealing

Annealing: when the metal is subjected to cold working.• Strength increases.• Hardness increases.• Percentage elongation decreases.• Percentage area reduction in cross section

decreases.

05/01/2023 8

Annealing

𝝉𝒆𝒓 ≅ (𝑻𝒎

𝟐 )

05/01/2023 9

Annealing cont…

• If the cold work is severe/excessive, the metal will fracture before reaching the desired shape or size.

• So, the cold working is carried out with several steps with intermediate annealing.

• Intermediate annealing- softens the metal and restores ductility

• This repeated sequence (cold work + annealing) is called cold work – anneal cycle.

05/01/2023 10

Annealing cont…

05/01/2023 11

Annealing cont…

• Cold worked metals are annealed in inert gas atmosphere. Otherwise, the metal will react with atmospheric oxygen and get oxidized.

• Manufacturing cost is increased.• After annealing metals have: a. Low yield stress. b. Low hardness. c. High elongation. d. Low strain hardening exponent.

05/01/2023 12

Annealing of cold worked metal

• Annealing cycles/techniques used for cold working metals:

a.• Full annealing

b.• Sub critical annealing

c.• Spheroidize annealing

05/01/2023 13

Annealing of cold worked metal cont…

• Cold worked metal has higher internal energy than un deformed metal.

• When the metal is heated, the cold worked state becomes more unstable.

• The metal softens to a strain - free condition.• The overall process by which this occurs is

known as Annealing. • Annealing restores the ductility of the metal.

05/01/2023 14


Three stages of Annealing process

a.• Recovery

b.• Recrystallization

c.• Grain growth

05/01/2023 15


05/01/2023 16


Recovery:• Restores physical properties. (Ex. Electrical

conductivity). • Reduces dislocation density.• No change in microstructure and Mechanical

properties.

05/01/2023 17


Recrystallization: Replacement of cold worked microstructure with new set of strain- free grains.• It reduces strength and hardness.• Increases the ductility.The stored energy of cold worked metal is the driving force for both recovery and recrystallization.

05/01/2023 18


05/01/2023 19


Main variables of recrystallization are:1. Amount of prior deformation.2. Temperature.3. Time.4. Initial grain size. 5. Composition.6. Amount of recovery.

𝑹𝒆𝒄𝒓𝒚𝒔𝒕𝒂𝒍𝒍𝒊𝒛𝒂𝒕𝒊𝒐𝒏 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆=(𝑴𝒆𝒕𝒂𝒍𝒎𝒆𝒍𝒕𝒊𝒏𝒈 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆𝟐 )

05/01/2023 20


Relationship of variables:• Smaller the degree of deformation, higher temperature is

required for recrystallization.• A minimum amount of deformation is required to cause

recrystallization.• Increasing the annealing time, decreases the

recrystallization temperature.• Greater the degree of deformation and lower the annealing

temperature (recrystallized grain size will be small).• Crystallization temperature decreases with increasing purity

of metals. Solid solution alloying additions raise the recrystallization temperature.

05/01/2023 21

Cold Working

Cold working is the process of deforming the metal under conditions of temperature and strain rate, such that the strain hardening produced by working is not relieved.

• Cold working is carried out below the recrystallization temperature ()of metals.

05/01/2023 22

Cold Working cont…

Structural changes due to cold working

Before working After working

05/01/2023 23

Cold Working cont…

Advantages• No scaling and oxidation occur,

because of low deformation temperature.

• Good surface finish.• Close dimensional tolerance can

be obtained.• Automation is possible because of

low working temperature.• Thin gauge sheets can be

manufactured.• Strength can be controlled by

controlling formability parameters.

Dis advantages• Higher capacity machines are

required because the strength of metal increases at low temperature.

• The formability of metals is poor at low temperatures. Hence, costly annealing operations are required.

• Cold working introduces residual stresses. Hence, stress relieving annealing operation is required.

• Costly tools are required to take care of high forming stresses.

05/01/2023 24

Hot Working• Hot working is a process of plastically deforming a metal

under conditions of temperature and strain rate. • Recrystallization occurs during or immediately after

deformation.• No work hardening occurs in metals.• Microstructure elongates in the direction of the applied

load.• Strain – free grains are formed because of high temperature.• Size of grains depends on following parameters:

a. Temperature.b. Percentage amount of work.

05/01/2023 25

Hot Working cont…

Property changes due to cold working

(a) (b)

05/01/2023 26

Hot Working cont…

05/01/2023 27

Hot Working cont…

Advantages• Lower capacity machines are

sufficient because of lower forming stresses.

• Number of forming stages required to obtain particular shape is less (because of good formability at higher temperatures).

• No work hardening occurs – higher deformation is possible.

Dis advantages• Heating facilities are

required which increases the investment cost.

• Scaling and oxidation occurs during hot working.

• Material losses due to scaling.

• Poor dimensional tolerances.

05/01/2023 28

Hot Working cont…

Advantages• Stress relieving is not

necessary because of higher temperature.

• Reduces chemical in homogeneities because of rapid diffusion at high temperature.

• Blow holes and porosities can be eliminated at high temperature.

Dis advantages• Thin gauge material cannot be

manufactured.• Automation is difficult.• Surface decarburization

reduces the strength of surface.

• Due to non uniform deformation, structure and properties are not same over the cross section. (Normalizing is needed).

05/01/2023 29

Warm Working

• Warm working is a process of plastically deforming a metal under conditions of temperature and strain rate.

• The drawbacks of both cold working and hot working are eliminated with their combined advantages.

05/01/2023 30

Warm Working cont…

Selection of warm working temperature is based on the following factors:1. Yield or flow strength of material.2. Ductility or Formability of the material.3. Dimensional tolerances.4. Scaling and oxidation losses.

05/01/2023 31

Effect of strain rate on Formability

Strain rate/ deformation velocity has three principal effects in metal working.a. Flow stress of the metal increases with strain

rate.b. The temperature of work piece increases

because of adiabatic heating.c. Lubrication at the tool-metal interface.

05/01/2023 32

The strain rate for a cylindrical upset in compression is:

Where h- instantaneous height (varies with axial distance – converging dies)V- the deformation velocity.

It is convenient to define mean strain rate by: Where L- length of contact b/w tool & work piece.

ἐ 𝒕=𝒅𝜺𝒅𝒕 =

𝟏𝒉𝒅𝒉𝒅𝒕 =

𝒗𝒉

Effect of strain rate on Formability cont…

05/01/2023 33

It is more usual to evaluate the mean strain rate in terms of the time for an element to travel through the die

For hot extrusion root mean power strain rate is (for strain rate sensitive material at large deformation)


05/01/2023 34

• Forming velocity of equipment is faster than the cross head velocity of UTM m/c.

• The values of flow stress obtained using tension/compression test are not directly applicable for forming load calculation.

In practical cold work – increase in flow stress due to increase in speed is not considered.• For flow stress determination

camplastometer can be used.• When the forming zone is very small, it is

possible to produce very high local strain rates.


Camplastometer

05/01/2023 35

Examples:• Fine-wire drawing at a speed of 120ft/s results in

a strain rate in the order of • In rolling of thin sheet, the strain r is in the order

of • When the deformation zone is narrow, it

produces a very high particle velocity.• In high energy rate forming, the deformation is

delivered at a much higher rate.


05/01/2023 36

Typical Values of Velocity and Forming Operation

Operation Velocity (m/s)

Tension test 2 x 10^-6 to 2x 10^-2

Hydraulic Extrusion Press 0.01 to 10

Mechanical Press 0.5 to 5

Forging Hammer 10 to 20

Explosive Forming 100 to 400

05/01/2023 37

Effect of Strain Rate cont…• For many materials the elongation to fracture

increases with strain rate until a critical strain rate is reached .

• Once the strain rate reaches a critical value, the ductility falls off sharply.

• Explosive forming hardens the metal to a quite high value with no grain distortion.

• Other extreme forming is super plastic forming .

05/01/2023 38

Effect of Strain Rate cont…• Material having high strain rate sensitivity

(0.3 ≤ m ≤ 1.0) exhibit pronounced resistance to necking.

• Generally this occurs with a very fine grain size, of the order of 1 µm and at deformation temperature above 0.4 Tm .

• For any super plastic material, there is a limit strain rate above which it is no longer super plastic.

05/01/2023 39

Effect of Microstructure on Forming

05/01/2023 40

05/01/2023 41

05/01/2023 42

05/01/2023 43

05/01/2023 44

05/01/2023 45

Effect of Temperature on Metal Working

• Forming processes are commonly classified into hot-working and cold-working operations.

• Hot working is defined as deformation under conditions of temperature and strain rate such that recovery and recrystallization of processes takes place simultaneously with the deformation.

• On the other hand, cold working is deformation carried out under condition where strain hardening is dominant.

05/01/2023 46

05/01/2023 47

05/01/2023 48

05/01/2023 49

Hot Working

1. No strain hardening.2. Distorted grains are replaced

by new strain free grains.3. Recovery, recrystallization

and grain growth take place.4. Very large deformation is

possible.5. At constant flow stress hot

working occurs because flow stress decreases with temperature.

6. Energy required is less.

Cold Working

1. Strain hardening exists.2. Grains get distorted.

3. No such reaction.

4. No large deformation is possible.

5. Flow stress increases with deformation.

6. Energy required is more.

05/01/2023 50

Hot Working

• It is an initial step in mechanical working of most metal and alloys.

• The rapid diffusion at hot working temperature aids in decreasing the chemical inhomogenities.

• Blow holes and porosity are eliminated by the welding together of these cavities.

05/01/2023 51

Hot Working cont…

• Coarse columnar grains of the casting are broken and refined into smaller equiaxed recrystallized grains.

• These changes in structure result in increase in ductility and toughness over the cast state.

05/01/2023 52

Hot Working cont… Disadvantages :-i. Surface oxidization.ii. Scale formation takes place.iii. Embrittlement of metal by O2 is possible.iv. Surface decarburization.v. Poor surface finish.

05/01/2023 53

Hot Working cont…

Disadvantages :-vi. More allowance must be made for thermal

expansion and contraction in design.vii. Structure and properties are not uniform

because the deformation is always greater in the surface than core. This results in fine grains at the surface and coarse grain at the core.

05/01/2023 54

Hot Working cont…• The lower temperature limit for the hot-working

of metals is the lowest temperature at which the rate of recrystallization is rapid enough to eliminate strain hardening in the time when the metal is at temperature of recrystallization.

05/01/2023 55

Hot Working cont… Hot working temperature depends on amount of deformation and the time that the

metal is at temperature. the amount of deformation Tre

• Metal which is rapidly deformed and cooled rapidly from temperature will require a higher hot working temperature for the same degree of deformation than will metal slowly deformed and slowly cooled.

05/01/2023 56

Hot Working cont…

• The upper limit for working is determined by the temperature at which melting or excessive oxidization takes place.

• Maximum temperature = Tmelting – 100° F• Whenever the metal contains low melting

constituents along the grain boundaries, they melt and make the material crumble into pieces during hot-working.

• Such condition is known as hot shortness or grain boundary burning.

05/01/2023 57

Hot Working cont…

• Finishing temperature is always just above Tre

(in order to get fine recrystallized grains) compared to initial and intermediate forming stages.

Defo

rmati

on (%

) Cold short

05/01/2023 58

Hot Working cont…

• Hot shortness is a limitation.• As the strain rate of deformation increases,

more heat is retained in the workpiece.• The temperature is to be reduced to keep its

final temperature from reaching the hot shortness temperature.

05/01/2023 59

Mechanics of Metal Forming• The primary objective is to express Stress and

Strain involved in metal forming operation in some mathematical form.

• These are required to produce desired shape.• Siebel and other German worker proposed

that the maximum shear stress law was the proper criterion for describing stresses during forming.

05/01/2023 60

Mechanics of Metal Forming cont…• Subsequent work showed that Von-Mises flow

criterion provided better agreement with experimental data.

• Von-Mises criterion is preferred and most widely used in the analysis of forming operations.

• These two criteria differ by only 15%.

05/01/2023 61

Mechanics of Metal Forming cont…

• Maximum shear stress law will be used wherever it needs simplification to the analysis.

• In metal forming, the volume is assumed to be constant .

05/01/2023 62

Work of Plastic Deformation

• WT = Total Work• Wτ = Redundant Work• Wd = work of deformation (or) work required for homogenous deformation• Wf = Work done against friction

05/01/2023 63

Work of Plastic Deformation cont…• Redundant work is energy expended on

deforming the body which is not involved in a pure change in shape.

05/01/2023 64

Formability Test• It is necessary to predict the success of forming

operation on the basis of simple laboratory tests.• Tension test : stretch forming• Compression test : Forging, Rolling• Torsion test : Extrusion, Rolling• Shift cup test : Deep Drawing • Erichson test : Stretch forming • Bent test : Bending

Formability of metal can be studied using the above tests for forming behaviour.

05/01/2023 65

Thank You