Week9

WEEK-9

O C T 7 T H 2 0 1 4

2 0 1 4 - 2 0 1 5 S E M E S T E R - I

TA201 Manufacturing Processes

F U N D A M E N T A L S O F M E T A L F O R M I N G

Classification Material behavior Strain-rate sensitivity Temperature

B U L K D E F O R M A T I O N P R O C E S S E S

Rolling Forging Extrusion

S H E E T M E T A L W O R K I N G

Cutting Bending Drawing

Dr. Shashank Shekhar TA 201

2

Metal Forming [Chap-18, 19 & 20:Groover]

2014-15 Semester-I

Metal Forming Processes

Forging

3

TA 201 Dr. Shashank Shekhar

2014-15 Semester-I

Classification of Metal Working Processes

Plastic deformation based processes

[Metal forming]

Metal removal processes

[Metal Machining]

Direct compression

Indirect compression

Bending

Tension type

Shearing

Cold working

Warm working

Hot working

On

th

e b

as

is o

f fo

rce

Dir

ec

tio

n

On

the b

asis

of w

ork

ing

tem

pera

ture

TA 201

4

Dr. Shashank Shekhar

2014-15 Semester-I

5 Classification of deformation based Metal Forming

Processes

(Thickness: 0.4mm to 6mm)

(Thickness> 6mm)

(on

the b

asis

of t

hickne

ss o

f th

e

wor

kpiece

to

be f

ormed

)


2014-15 Semester-I

Metal Forming Large group of manufacturing processes in which

plastic deformation is used to change the shape of metal work pieces

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

TA 201

6


2014-15 Semester-I

Bulk Deformation Processes

Characterized by significant deformations and massive shape changes

"Bulk" refers to work parts with relatively low surface area to volume ratios

Starting work shapes include cylindrical billets and rectangular bars

Stresses to plastically deform the metal are usually compressive - Examples: rolling, forging, extrusion

TA 201

7


2014-15 Semester-I

8

Strain calculation In metalworking, compressive stress and strain are predominant. If a block of initial height ho is compressed to h1, the axial compressive strain will be:

True strain

Engineering strain

Note: the calculated strain is negative for compressive strains. However the convention is reversed in metalworking problems so that compressive stresses and strains are defined as positive.


h1

2014-15 Semester-I

9

And the fractional reduction (reduction of area) in

metal working deformation is given by

Again from constancy in volume before and after deformation


Strain calculation (rolling)

A0 A1

Compressive stresses

L0 L1

W W

2014-15 Semester-I

Sheet Metalworking

Characterized by localized deformation and configuration changes

Includes processes like cutting, bending and drawing

Stresses required are combination of tensile, compressive and shear

- Stretch the metal (tensile stresses)

- Bend the metal (tensile and compressive)

- Still others apply shear stresses

TA 201

10


2014-15 Semester-I

11

Direct Compression

Indirect Compression

Tension type Bending Shearing

Bulk deformation

Sheet Metalworking TA 201 Dr. Shashank Shekhar

2014-15 Semester-I

Not always Uniaxial!


Process State of Stress in Main Part During

Forming

Rolling Bi-axial compression

Forging Tri-axial compression

Extrusion Tri-axial compression

swaging Bi-axial compression

Deep drawing

In flange of blank, bi-axial tension and

compression. In wall of cup, simple uni-

axial tension.

Wire and tube drawing

Bi-axial compression, tension.

Straight bending At bend, bi-axial

compression and bi-axial tension

12

2014-15 Semester-I

Plasticity


Engineering or true?

• Elastic region • Plastic region • Yielding • Necking • Elastic Strain • Plastic strain

13

2014-15 Semester-I

True Stress (σt) & Strain (ε)

TA 201

14


a

2014-15 Semester-I

Flow Curve


A true stress-strain curve is frequently called a flow curve, because it gives the stress required to cause the metal to flow plastically to given strain

Loading-Unloading causes ‘Hardening’, which is obvious with increasing flow (yield) stress

Plastic stress-strain is a complex behavior (unlike elastic) and many mathematical equations have been used to fit this behavior

Most common is ‘Power Law’ =Kn

Flow curve is strongly dependent

on strain rate and temperature

15

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 (true

stress), that is, the yield

strength as a function of

strain

n

f KY


16

Average Flow Stress

Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest

Where = maximum strain during deformation process

n

KY

n

f

1

_

True strain

0

1)( dYmeanY ff

Given that 2 = and 1 (initial) = 0


17

nKn

1

1

2014-15 Semester-I

Plasticity


In elastic regime, Hooke’s law relates stress and strain, where any stress results in strain

In plastic regime, a minimum stress (yield stress) must be reached before deformation can be attained

The relation between plastic stress-strain is mere approximate, compared to elastic-stress-strain relations which are more “exact”

Plastic stress-strain is a complex behavior (unlike elastic) and many mathematical equations have been used to fit this behavior. Most common is ‘Power Law’ =Kn

Flow stress, the stress required to keep the material deforming, is usually given by and average flow stress is given by

18

n

f KY

n

KY

n

f

1

_

2014-15 Semester-I TA 201 Dr. Shashank Shekhar

19

Flow curve is strongly dependent on

• strain rate

• temperature

2014-15 Semester-I

Effect of Strain Rate on Flow Stress

A flow stress simply gives the stress required to impose certain strain and should be valid at whatever rate you reach that strain. However this is not correct

Theoretically, a metal in hot working, behaves like a perfectly plastic material, with strain hardening exponent n = 0 and the metal should continue to flow at the same flow stress, once that stress is reached. However this is also not the case

Flow stress also depends on strain rate (particularly at hot working temperatures) - As strain rate increases, resistance to deformation increases - This effect is known as strain rate sensitivity

TA 201

20


2014-15 Semester-I

What is Strain Rate?


Rate of straining 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:

Strain-rate is complicated by geometry of workpart

Values can range from 10-3 to 100 s-1

h

v

.

where = true strain rate; and h = instantaneous height of workpiece being

deformed

.

TA 201

21

2014-15 Semester-I

(a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log-log coordinates

Log( )

Log(

)


22

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

m

f CY


23

TA 201

2014-15 Semester-I

Effect of temperature on flow stress for a typical metal. The constant C in the above Eq. indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature

m

f CY


24

2014-15 Semester-I

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 behavior

- As temperature increases, strain rate becomes increasingly important in determining flow stress

TA 201

25


2014-15 Semester-I

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

TA 201

26


2014-15 Semester-I

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 (<0.3Tm)

- Warm working (0.3Tm to 0.5Tm)

- Hot working (>0.5Tm)

TA 201

27


2014-15 Semester-I

Influence of Annealing Temperature on the Tensile Strength and Ductility

TA 201

28


2014-15 Semester-I

Cold Working

Performed at room temperature or slightly below (≤0.3Tm )

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

TA 201

29


2014-15 Semester-I

Hot Working

Deformation at temperatures above recrystallization temperature

Recrystallization temperature = about one-half of melting point on absolute scale 1. In practice, hot working usually performed somewhat above

0.5Tm

2. Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level

TA 201

30


2014-15 Semester-I

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

TA 201

31


2014-15 Semester-I

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 – 0.5 Tm, where Tm = melting point (absolute

temperature) for metal

TA 201

32


Week9

Documents

exactplastic stressstrain

axial compressive strain

minimum stress yield

stress results

hardening flow stress

shashank shekharengineering

shashank shekharta

shashank shekhar16