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Forging

1 Dr. B Gharaibeh Production Processes

Deformation Processes

• Operations that induce shape changes on the workpiece by plastic deformation under forces applied by various tools and dies

- Primary working processes

- Secondary working processes

2 Dr. B Gharaibeh Production

Processes

Deformation Processes • Primary working processes Operations are those that take a solid piece of

metal (ingot) and break it down into shapes like slabs, plates, billets – Forging - Rolling – Extrusion

• Secondary working processes The operation which involves further processing of

the product from the primary working into final products such as bolts, metal parts, wires, cans.

- Deep drawing - blanking


Processes

Deformation Processes

Classified according to shape and size of workpeice

A- Bulk deformation operations

1. The processing of workpeice having a relatively small surface area to volume or (surface area to thickness) ratio

2. In all bulk deformation processing, the thickness or cross-section of the workpiece is changed.

B- Sheet forming operations

1. The surface area to the thickness ratio is relatively large

2. Thickness changes are usually undesirable.


Processes

Dr. B Gharaibeh Production

Processes

5

Forging process

• Plastic deformation by compressive forces, can be done at cold or warm and hot temperatures (homologous temperature)

– Open-die forging

– Impression-die forging

– Closed-die forging


Processes

Open Die forging - Upsetting

Ideal deformation of a solid cylindrical specimen compressed between

flat frictionless dies (platens), an operation known as upsetting. (b)

Deformation in upsetting with friction at the die-workpiece interfaces.

Note barrelling of the billet caused by friction.


Heading

Forming the heads of fasteners, such as bolts and rivets, by the heading process.


Open-Die Forging

(a) Cogging operation on a rectangular bar. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times along the length of the part. (b) Reducing the diameter of a bar by open-die forging; note the movements of the die and the workpiece. (c) The thickness of a ring being reduced by open-die forging.


Processes

Grain Flow

Schematic illustration of grid

deformation in upsetting: (a) original

grid pattern; (b) after deformation,

without friction; (c) after

deformation, with friction. Such

deformation patterns can be used to

calculate the strains within a

deforming body.

Grain flow lines in upsetting a solid,

steel cylindrical specimen at elevated

temperatures between two flat cool

dies. Note the highly inhomogeneous

deformation and barreling, and the

difference in shape of the bottom and

top sections of the specimen. The latter

results from the hot specimen resting

on the lower die before deformation

proceeds. The lower portion of the

specimen began to cool, thus exhibiting

higher strength and hence deforming

less than the top surface. Source: After

J.A. Schey.


Minimize barreling by applying effective lubricants or ultrasonically vibrate the part

Upsetting

Under ideal conditions

% Reduction in height=(h0-h)/h0*100

• e1=(h0-h1)/h0

• True strain=Ln(h0/h1)

• If v is the velocity between platens (dies):

True strain rate increases rapidly as height of the specimen approaches zero.


1

0

h

v

h

ve

• For perfectly plastic material

F=Y A1 ,A1=A0. h0/h1

• For strain hardening material

Upsetting under ideal conditions

is average flow stress

F=Yf A1

Yf = average flow stress ×

(n+1)

= kε1n

Yf is flow stress

1

0

ε

d ε u s

n k ε s

1

0

ε

n dε ε k u

1

1

1 1

ε Yf n

k ε u

n

+

+

Yf

Work =Volume × u = Volume × 1 ε Yf


Slab Analysis of Forging

From equilibrium:

Resulting die pressure prediction:


Processes

Plain stress compression, between flat dies with friction, horizontal stress is uniform across height (h)

02

02

+

++

dxh

d

hdxhd

y

x

xyxx

ss

ssss

Second equation from the yield criterion

Die Pressure

Distribution of die pressure, in dimensionless form of

p/Y’, in plane-strain compression with sliding friction.

Note that the pressure at the left and right boundaries is

equal to the yield stress of the material in plane strain,

Y’. Sliding friction means that the frictional stress is

directly proportional to the normal stress.

' / ) ( 2 h x a

y e Y p s

Consider the friction - rectangular cross-section

plane strain (no flow perpendicular to the page)

' Y 1.15Y


Processes

Method of Analysis

02)( ++ hdxhd xyxx ssss

02

+ dxh

dy

x

ss

We know from distortion energy criterion for

plane strain:

'3

2YYxy ss

11/11/2012 15 Dr. B Gharaibeh Production

Processes

Solving two equations:

]1[''

'

',0,

'

/)(2

/)(2

/2

/2

hxa

yx

hxa

y

yx

ha

hx

y

eYY

eYp

Yax

eYC

Ce

ss

s

ss

s

at


Processes

• Paverage can be obtained using:

• For strain hardening material is replaced by

• Yf=kεn = average flow stress × (n+1)

• Upsetting force: • F=Pav*cross-section area

• F=Pav*2a*width

• This is true for any instantaneous h

Upsetting Consider the friction - rectangular cross-section

Assume that the deformation is in plane strain (no flow

perpendicular to the page)

) 1 ( ' h

a Y P av

+

' Yf ' Y


Processes

Cylindrical Workpiece

Average pressure:

Forging force:

r is the final radius after upsetting and h is the final value


Processes

Example 1

• A cylindrical workpeice made of annealed steel (K = 147,000 psi, n=0.17) is 6 in. diameter and 4 in hight. It is upset by open die forging with flat die to a height of 2 in. in plane strain. Assuming that the coefficient of friction is 0.2 . Calculate

A. the force required at the end of stroke. B. The work consumed to decrease the height to 2 in. • Diameter=6 inch • Height=4 inch

– After upsetting=2 inch

• Coeff. Of friction =0.2 • Material is Annealed Steel


Processes

For annealed steel

(r2 from volume conservation)

. 10 ) 24 . 4 )( ( 000 , 177

000 , 177 ) 2 )( 3 (

24 . 4 ) 2 . 0 ( 2 1 [ 000 , 138

. 24 . 4

000 , 138 ) 693 . 0 ( 000 , 147

693 . 0 ) 2

4 ln(

17 . 0

) 000 , 147 ( 1015

) 3

2 1 (

7 2

2

17 . 0

1

1

lb F

psi P

in r

psi y

ε

kε y

n

psi MPa k

h

r Y P

av

f

n

f

f av

] +

+

p

20

Example 2

• A rectangular workpice has the following original dimensions: 2a = 100 mm, h = 30 mm and width = 20 mm. The metal has a strength coefficient of 400 Mpa and a strain hardening exponent of 0.3. It is being forged in plane strain with µ = 0.2, Calculate

A. the force required at the end of stroke.

B. The work consumed to decrease the height to 2 in.


Processes

Impression Die Forging

Stages in impression-die forging. Note the formation of a flash, or

excess material that subsequently has to be trimmed off.

Range of Kp values for impression-die forging.

Forging force:


Processes

Impression Die Forging

• Workpiece takes the shape of the die cavity

• Flashes are created in the radial direction and moving outward

• High Friction in the Flash Flash: the length to height ratio is large Flash cools faster than the bulk and hence resists deformation - This helps the billet to fill the cavity


Processes

Closed Die Forging

• No flash is formed and the workpiece is completely surrounded by the dies

• Workpiece takes the shape of the die cavity

• Proper control of the volume of the material is essential to obtain a forging of desired dimensions.


Processes

Forging Defects

Stages in lap formation in a part during forging, due to buckling of the web. Web

thickness should be increased to avoid this problem.

Stages in internal defect formation in a forging because of an oversized billet. The die cavities are filled prematurely, and the material at the center of the part flows radially outward and past the filled regions as deformation continues.


Processes

Forging Dies

Standard terminology for various features of a typical forging die.

Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash developed, which is important in properly filling die cavities.

11/11/2012 26 Dr. B Gharaibeh Production Processes

Forging Temperatures

Forging temperature ranges for various metals.


Metal Working Equipment

Various types of presses used in metalworking. The choice of a press is an important consideration in the overall operation and productivity.


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