Ch 4 Metal Forming

1

Chapter 4: Metal Forming

Learning objectives

Understand the basic metal forming processes, including

forging and sheet metal stamping

Understand how the metal forming process changes the

shape and the material properties of the metal

Recognize different metal forming machines (presses),

including mechanical press and hydraulic press

Recognize the dies for sheet metal forming, their design

and constructions

2

The aspects of metal forming:

The material (metal)

The machine

The tool (dies)

The operation (process)

Process

Material

Press

Die

Product

Heat and Beat

3

A list of different metal forming processes:

Metal Forming

4

Types of metal forming processes

Rolling (Chapter 13)

Bulk deforming (Chapter 14)

Extrusion and Drawing (Chapter 15)

Sheet metal forming (Chapter 16)

5

Rolling

Different types of rolling process

6

Rolling

Rolling machines

7

Rolling

Rolling process

The forces act on the workpiece

The force and torque act on the roll

8

Rolling

The rolling force

The total power (for two rolls)

avgLwYF L = roll-strip contact length

w = width of the strip

Yavg = average true stress of the strip

000,60

2kW)(in Power

FLN N = rolling speed (RPM)

9

Rolling

An example: An annealed copper strip 228 mm wide and

25 mm thick is rolled to a thickness of 20 mm in one pass.

The roll radius is 300 mm, and the rolls rotate at 100 rpm.

Calculate the roll force and the power required in this

operation

10

Rolling

Solution

Roll-strip contact length is calculated through geometry,

Absolute true strain of the strip is

Average true stress is

The roll force is

Total power is

223.020

25ln

mm 7.3820253000 fo hhRL

MN 4.171801000250

10007.38 avgLwYF

MPa 1802/28080

W705000,66

100

1000

7.381074.12

000,66

2 6 FLN

Power

11

Rolling

Dimensional Tolerances

Thickness tolerances for cold-rolled sheets range from

0.1~0.35 mm

Flatness tolerances are within 15 mm/m for cold rolling

and 55 mm/m for hot rolling

Surface Roughness

Cold rolling can produce a very fine surface finish

Cold-rolled sheets products may not require additional

finishing operations

12

Rolling

Straight and long structural shapes can be formed by shape

rolling

13

Rolling

Roll forging: Cross section of a round bar is shaped by

passing it through a pair of rolls with profiled grooves

14

Rolling

Skew rolling: Similar to roll forging and used for making

ball bearings

15

Rolling

Ring rolling:

A thick ring is expanded into a large-diameter thinner one

Thickness is reduced by bringing the rolls closer together as they

rotate

Short production times, material savings and close dimensional

tolerances

Video: https://www.youtube.com/watch?v=wSbywBfXlHg

https://www.youtube.com/watch?v=wSbywBfXlHg

16

Rolling

Thread rolling:

Thread rolling is a cold-forming process by which straight or

tapered threads are formed on round rods or wire

Threads are formed with rotary dies at high production rates

17

Rolling

Rotary tube piercing:

Also known as the Mannesmann process

It is a hot-working operation for making long, thick-walled

seamless pipe and tubing

The round bar is subjected to radial compressive forces while

tensile stresses develop at the center of the bar

18

Rolling

Tube rolling:

Diameter and thickness of pipes and tubing can be reduced by

tube rolling, which utilizes shaped rolls

19

Rolling

Computer simulation of rolling:

https://www.youtube.com/watch?v=k6iODHla6qY

Concluding remarks

Rolling is highly efficient

Rolling is reasonably accurate

Rolling can generate good surface

Rolling results in residual stress

https://www.youtube.com/watch?v=k6iODHla6qY

20

Forging

workpiece is shaped by compressive forces applied

through dies and tools

produce discrete parts

Forged parts have good strength and toughness, and are

reliable for highly stressed and critical applications

Types of forging

Open die forging and closed die forging

Hot forging and cold forging

21

Characteristics of forging

22

Forging

The machine

The process: https://www.youtube.com/watch?v=XTU0Z-

FkhtU

https://www.youtube.com/watch?v=XTU0Z-FkhtU

23

Open-die forging

Workpiece is deformed uniformly under frictionless

conditions

Barreling occurs because of the friction force. It can be

reduced by adding lubrications

24

Open-die forging

Different types of open-die forging

In open die forging, the stress varies continuously

25

Open-die forging (continue)

The directional flow of the material in forging

Folding

26

Open-die forging

The forging force, F, in an open-die forging operation on

a solid cylindrical workpiece can be estimated from

h

rrYF f

3

212

Yf = flow stress of the material

= coefficient of friction between the workpiece and die

r = the instantaneous radius

h = height of the workpiece

27

Open-die forging

An example: A solid cylindrical slug made of 304

stainless steel is 150 mm in diameter and 100 mm high.

It is reduced in height by 50% at room temperature by

open-die forging with flat dies. Assuming that the

coefficient of friction is 0.2, calculate the forging force

at the end of the stroke.

28

Open-die forging

Solution

The final radius is

Absolute value of the true strain is

From Table 2.3, 304 stainless steel has K = 1275 MPa and n =

0.45. Thus for a true strain of 0.69, the flow stress is 1100 MPa.

The forging force is

mm 106100752

10022 rr

69.050

100ln

MN 4505.03

106.02.021106.0101000

26 F

29

Analysis of open die forging

This is the simplest case in metal forming. Consider the

deformation of a square workpiece:

h0 h1d0 d1

0

101

h

hhe

1

01 ln

h

h

F, v

0

1h

ve

1

1h

v

h0d0

F, v

30

The reaction force

The required work for deformation

h0

h1

d0

d1

1YAF

1

001

h

hAA

F, v

10

0

Volume

Volume

VolumeWork

1

1

Y

dK

d

n

1

1

1

0

1

n

KdK

Yn

n

31

Note:

We are primary interested in the plastic deformation and hence,

always use true strain and stress

How to compute the average yield stress and average work

= F/A

5

0

0

555, lnln

A

AY

l

lYYWs

A

A

l

l 0

0

lnln

Average yield stress

32

In order to study the stress distribution, in general, we

need to understand the 3D strain and stress

In general, strain and stress are three dimensional

y

tyzx

txyz

tzx

z

zyy

zxyxx

ij

t

tt

z

zyy

zxyxx

ij

x

v

y

uyx

Shear stressShear strain

h0d0

F, v

33

In open die forging, it can be simplified as a 2D problem. In the

horizontal direction:

Using the distortion-energy criterion:

Therefore,

02 hdxhd xzxx

xz dd

dxh

d

x

x

2

x

dx

x x + dx

zz

z

za

x(a) = 0

z(a) = Y

Square

specimenh

34

Solving the differential equation results in

Similarly, we can find the solution for y The stress distribution is shown below

hxaz eY

'

1' 2 hxax eY

35

Open die forging (continue)

The facture

Under excessive force, the barreling will become fracturing

Improved lubrication can reduce fracture

36

Polycrystalline deformation

Crystal elongation

Cold forming and hot forming

Cold forming is used for near shape manufacturing

The micro-process of cold forming

Reorientation (deformation ratio = 3)

Single crystal slip

Polycrystalline deformation

Crystal elongation (deformation ratio = 8 ~ 10)

Crystallographic fibrous structure

Slip line

37

Cold forming and hot forming

Hot forging is used to form large bulk steels

The temperature effect is significant

Finding the right temperature range using phase diagram

Overheat, steel may

burn or oxidize

Under-heat, much

large force is required

Right temperatureoC

38

The forming process affects the grain size

Finer grain have higher yield strength and toughness and lower

internal strains and stresses

The process of forging and grain size

Grain size increase

because of heating

Grain size after one

forging

Grain size after many

forgings

Heating

punching

39

Closed-die forging

Application example

40

Closed-die forging (continue)

Application example

41

Closed die forging

making an aircraft landing gear

42

Closed-die forging

The workpiece resemble the shape of the die

Anatomy of a die set: gutter and flash, parting line

43

Closed-die forging

Forging Force

The forging force, F, required to carry out an impression-die

forging operation is

AkYF f

k = multiplying factor obtained

Yf = flow stress of the material at the forging temperature

44

Extrusion and drawing

fA

AkAF 00 ln