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Chapter 6. Bulk Deformation

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M2794.001800 M A T E R I A L A N D M A N U F A C T U R I N G P R O C E S S E S

Chapter 6. Bulk Deformation Processes

Prof. Ahn, Sung-Hoon ( )

School of Mechanical and Aerospace Engineering Seoul National University

(2)

© Prof. Ahn, Sung-Hoon

 Temperature

Hot working(열간가공)

Warm working(온간가공)

Cold working(냉간가공)

 Types of operation

Primary working(1차가공) : solid piece of metal(금속괴) → slabs(슬래브), plates(후판), billets(빌랫).

Secondary working(2차가공) : products from primary working(1차가공품) → bolts(볼트), sheet metal parts(금속판재품), wire(선재) etc.

 Size and shape of the workpiece

Bulk deformation(부피성형가공) : thickness or cross section of the work- piece changes (forging(단조), rolling(압연), extrusion(압출), drawing(인발)).

Sheet forming(판재성형가공) : generally shape changes, thickness changes are undesirable.

Classification of Deformation Processes

Tm

T

0 . 6

Tm

T

0 . 3 ~ 0 . 5

Tm

T

0 . 3

(3)

Forging (단조) (1)

Bellows / Blower(풀무)

Anvil(모루)

(4)

© Prof. Ahn, Sung-Hoon

Forging (단조) (2)

 Plastic deformation of the workpiece is carried out by compressive forces.

 Crankshafts, connecting rods, turbine disks, gears, wheels, bolt heads, hand tools, etc.

 The recrystallization temperature(재결정온도) for metals :

 Basic categories of forging

Open die forging(자유단조)

Impression die forging(형단조)

Closed die forging(폐쇄단조)

5 . 0

~

/

Tm T

(5)

Open-die forging (자유단조)

 Reducing the height of workpiece between two flat dies by

compressing it.

(6)

© Prof. Ahn, Sung-Hoon

Forces and work of deformation

 

1

) ( ) (

,

,

, ,

ln

,

% 100

1 1

0

1 1

0

1 0 0 1

1

1 1

0 1

1 0 1

0 1 0 1

0 1 0

1 1

volume Work

volume Work

height in

Reduction

 



 

 

 

 

n d K

K Y

Y A

Y F

K d

h h A A

YA F

h v h

e v

h h h

h e h

h h h

n n f

n

Relative velocity

between the plates

(Yf : Flow stress, Y : Average flow stress)

(7)

Methods of analysis - Slab method (1)

 

2

2 0

0 2

) (

h dx d

h dx

h d

y x

x y

x x

 



Horizontal forces :

Using distortion-energy criterion for plane strain

(8)

© Prof. Ahn, Sung-Hoon h

x y

y y

y y

Ce h dx

d

h dx d

/ 2 O.D.E

2

2 0

 

 

Methods of analysis - Slab method (2)

specimen the

of edges at the

'

0

,

: conditions

Boundary

xa

x  

yY

) )(

2 )(

(

1 '

] 1 [

' '

' '

/ ) ( 2

/ ) ( 2

/ 2

width a

p F

h Y a

p

e Y Y

e Y p

e Y C

av av

h x a y

x

h x a y

h a



 

 

pressure

Average

:

force Forging

:

ⓐ & ⓑ give

(9)

Methods of analysis - Slab method (3)

Die pressure Rectangular Specimen

(10)

© Prof. Ahn, Sung-Hoon

Methods of analysis - others

 Slip-line analysis(미끄럼선 해석)

 Upper-bound technique(상계해법)

 Finite element method(유한요소법)

(11)

Impression-die/closed-die forging(형단조)(1)

 Impression-die forging : the workpiece acquires the shape of the die cavities (impressions) while it is being upset between the closing dies.

Flash(플래쉬) : significant role in the flow of material.

Forces in impression-die forging : depending on its position, F=KpYfA.

Land(랜드부) : controlling the forging load not to be excessive.

) )(

)(

( K Y A

F

Plane strain ratio

L h

(12)

© Prof. Ahn, Sung-Hoon

Impression-die/closed-die forging(형단조)(2)

 Closed-die forging(폐쇄단조) : no flash is formed and the workpiece is completely surrounded by the dies.

Precision forging(정밀단조), flashless forging(밀폐단조) : the part formed is close to the final dimensions, high forging load & capacity equipment needed.

Isothermal forging(등온단조), hot-die forging(가열금형단조) : the dies are heated to the same temperature as the hot blank.

(13)

Closed-die forging (폐쇄단조)

 Orbital forging(궤도단조)

(14)

© Prof. Ahn, Sung-Hoon

Miscellaneous forging operations (1)

 Coining(코이닝)

an example of closed-die forging

minting of coins.

 Heading(헤딩) – Fig 6.17

 Piercing(천공) – Fig 6.18

 Hubbing(허빙)

 Cogging(코깅)

 Fullering and edging(풀러링, 에징)

 Roll forging(압연단조)

 Skew-rolling(강구전조작업)

(15)

Miscellaneous forging operations (2)

Cogging Roll forging(Cross rolling)

(16)

© Prof. Ahn, Sung-Hoon

Miscellaneous forging operations (3)

Steel balls by upsetting

Steel balls by skew-rolling

(17)

Defects in forging (1)

 Caused by the material flow patterns in the die cavity.

 Cold shut.

 Grain flow pattern, end grain.

Buckling

Over sized billet

(18)

© Prof. Ahn, Sung-Hoon

Defects in forging (2)

Grain flow pattern (단류선 영향)

(19)

Die design (1)

 Considerations

Strength and ductility of the workpiece material.

Strain rate and temperature.

Frictional characteristics.

Die distortion under high forging loads.

 Die design parameters

Flash clearance between the dies : 3% of the max. thickness.

Length of the land : 5 times of the flash clearance.

Proper radii for corners and fillets.

 The most important rule in die design : the workpiece material flow in the direction of least resistance.

(20)

© Prof. Ahn, Sung-Hoon

Die design (2)

 Draft angles : Internal angles(7~10°) > External angles(3~5°)

(21)

Forging equipment

 Hydraulic presses(유압프레스)

 Mechanical presses(기계프레스)

 Screw presses(나사프레스) : for parts requiring precision(turbine blades) and control of ram speed.

 Hammers(해머) : potential energy of the ram is converted to kinetic energy.

 Counterblow hammers(카운터블로해머)

(22)

© Prof. Ahn, Sung-Hoon

Rolling (압연)

 The process of reducing the thickness or changing the cross-section of a long workpiece by compressive forces applied through a set of rolls.

 Plate(후판) : t ≥6mm / sheet (박판) : t ≤ 6mm

(23)

Changes in the grain structure

Wrought structure

(단련 구조)

(24)

© Prof. Ahn, Sung-Hoon

Mechanics of flat rolling

 The velocity of strip must increase as it moves through the roll gap.

 Sliding occurs between the roll and the strip due to the surface speed of the roll is constant.

 Neutral point or no slip point(중립점)

Left of neutral point : workpiece velocity < roll velocity

Right of neutral point : workpiece velocity > roll velocity

 The frictional forces oppose each other at the neutral point. (Ffriction_left > Ffriction_right)

→ yielding a net frictional force to the right.

r r f

V V V

slip

Forward

(25)

Roll pressure distribution (1)

(26)

© Prof. Ahn, Sung-Hoon

Roll pressure distribution (2)

) 2 (

/

) (

2 1

) (

2 1

) (

] 2 ) [(

3 2

) 1 cos , sin

( , ) (

) 2 (

) cos (sin

) 2 (

0 cos 2

sin 2

) )(

(

h R Y

p Y

p d

d

pR h

d Y d Y

p Y

p d h d Y

pR Y h

Y p d

d

d pR h Y p d

Y Y p

d pR h d

d pR h d

pRd h

pRd dh

h d

f f

f f

f f

f f

f

f f x

x

x

x x

x

Hi

f f f

H f

f f

f f f

h e C R

h R h

H R

Re Y h C p

h C R h

R R

h Y

p

R h h

R h

h

: zone entry In the

tan 2

ln tan

2 ln

ln

) cos 1 ( 2

1

1 2

) (H H o

f

e o

h Y h

p  

hf

C R : zone exit In the

H f

f e

h Y h

p  

(27)

Neutral point / front and back tension

tan 2 1 ln 2

1

) 2 ( 2

f n f

n

f o o

n

H H H

H

f o

H R h R

h

h H h

H e e

e h

h o n

n o

)

) (

( :

zone

Entry H H

o b f

e o

h Y h

p

H f f

f e

h Y h

p ( ) :

zone

Exit

( )

slip roll cause to

required on

Back tensi 6.3

Exmple

0

Ho

o f b

f e

h Y h

p

(28)

© Prof. Ahn, Sung-Hoon

Defects in rolling

 Surface defects

 Structural defects

 Residual stresses

In the case of small-diameter rolls or small reductions, plastic deformation occurs on the surfaces.

→ compressive stresses(surfaces), tensile stresses(bulk).

In the case of large-diameter rolls or high reduction, plastic deformation of the bulk is greater than the surfaces.

tensile stresses(surfaces), compressive stresses(bulk)

(29)

Flat-rolling practice (평판 압연 작업)

 Hot rolling : converts the coarse-grained, brittle and porous cast structure to wrought structure(finer grains and enhanced ductility).

 Products of the first hot-rolling operation

Bloom : square cross-section(정사각형), at least 150mm side, structural shapes by shape rolling(e.g. I-beam and railroad rails).

Slab : rectangular in cross-section(직사각형), plates and sheet.

Billet : square cross-section(정사각형) smaller than bloom, various shapes (e.g. round rods(환봉강) and bars)

 Rolling equipment

Small-diameter rolls are preferable, because the smaller the roll radius the lower the roll force. However, small rolls deflect under roll forces and have to

(30)

© Prof. Ahn, Sung-Hoon

Rolling equipment

(31)

Miscellaneous rolling operations (1)

 Shape rolling(형상압연) : H-section part, I-Beam and channels.

 Ring rolling(링압연) : rings for rocket, turbines and gearwheel rims.

 Thread and gear rolling(나사 및 기어전조) : high production rates, great strength, improved fatigue life.

 Rotary tube piercing(회전천공) : hot-working process to make long, thick-walled seamless tubing.

(32)

© Prof. Ahn, Sung-Hoon

Miscellaneous rolling operations (2)

 Ring rolling

(33)

Miscellaneous rolling operations (3)

 Thread and gear rolling

(34)

© Prof. Ahn, Sung-Hoon

Miscellaneous rolling operations (4)

 Rotary tube piercing

Thick-walled seamless tube.

(35)

Extrusion (압출)

Direct extrusion(직접압출) : high friction between billet and wall

Indirect extrusion(간접압출) : no friction between billet and wall, die moves to the billet.

(36)

© Prof. Ahn, Sung-Hoon

Mechanics of extrusion (1)

) )(ln (

ln Work

) )(

)(

( Work

ln ln

ln

ratio, Extrusion

1 1

R A Y

Y A u p

L pA FL

u L A

Y u

L R L A

A

A R A

f o

o o o

o o

o f f

o

f o

 

 

 

 

 

 

 

 

 

Energy dissipated in plastic deformation per unit volume

(37)

Mechanics of extrusion (2)









o o

friction

o friction

f o f

o

f o o

friction

f o o

plastic

o total

friction plastic

total

D Y L D k L p

kL D D

p

R Y p

D D D

D Y

p

D D Y

D V

P

D D Y

D V P

D pV P

P P

P

R Y

p

2 4

) 4 (

ln 7 . 1

] ) / [ln(

7 . 1 ) / ln(

41 . 3 /

) / ln(

) 2 / )(

2 / )(

(

] ) / )[ln(

)(

4 / (

) 4 / (

] 1 )

tan [(

1

2 0

2 2

0

2 2

0

2 0

cot

 

When friction is included

(38)

© Prof. Ahn, Sung-Hoon

Defects in extrusion

 Surface cracking(표면균열)

Too high extrusion temperature, friction, or extrusion speed.

Periodic sticking of the extruded product along the die land.

 Extrusion defect(파이프결함)

Surface oxides and impurities drawn toward the center of the billet.

Reduced by modifying the flow pattern, machining the surface of the billet or using a dummy block.

 Internal cracking(내부균열) : due to hydrostatic tensile stress at the center line.

(39)

Drawing (인발)

 The cross-sectional area of a bar or tube is reduced by pulling it through a converging die.

 Rod and wire drawing are usually finishing process.

 

 

 

 

 

 

 

f o f

n n

f o d

A A A

Y F

n Y K

K A Y A

ln 1 ln

1

(40)

© Prof. Ahn, Sung-Hoon

Defects in drawing

 Similar to defects in extrusion.

 Seams(솔기결함) : longitudinal scratches or folds in the material.

(41)

Swaging (스웨이징) (1)

 The metal flows in to the dies which apply compressive force or impact to the metal at room temperature.

 Rotary swaging(회전 스웨이징) : a solid rod or a tube is reduced in diameter by reciprocating radial movement of dies.

 The Internal diameter and thickness or shape of the tube can be controlled with or without mandrels(맨드릴).

(42)

© Prof. Ahn, Sung-Hoon

Swaging (스웨이징) (2)

(43)

Case study (Space shuttle)

(44)

© Prof. Ahn, Sung-Hoon

Solid rocket casing

(45)

Forging processes for solid rocket casing

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