Chapter 18-Fundamentals Metal Forming

8/2/2019 Chapter 18-Fundamentals Metal Forming

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Chapter 18:

Fundamentals of Metal Forming


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FUNDAMENTALS OF METAL FORMING

Overview of Metal Forming

Material Behavior in Metal Forming

Temperature in Metal Forming Strain Rate Sensitivity

Friction and Lubrication in Metal Forming


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Metal Forming

Metal Forming includes a large group of manufacturing

processes in which plastic deformation is used to

change the shape of metal workpieces

The tool, usually called a die, applies stresses that

exceed yield strength of metal The metal takes ashape determined by the geometry of the die

Stresses to plastically deform the metal are usually

compressive

Examples: rolling, forging, extrusion

However, some forming processes

Stretch the metal (tensile stresses)

Others bend the metal (tensile and compressive)

Still others apply shearstresses


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Material Properties Required in Metal

Forming

Desirable material properties:

Low yield strength and high ductility

These properties are affected by temperature: Ductility increases and yield strength decreases

when work temperature is raised

Effect of temperature gives rise to distinctions

between cold, warm and hot working Other factors:

Strain rate and friction


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Bulk Deformation Processes

Metal Forming processes can be classified as (1)

bulk deformation processes and (2) sheet

metalworking processes Bulk deformation processes are characterized by

significant deformations and massive shape changes

"Bulk" refers to workparts with relatively low surface

area-to-volume ratios

Starting work shapes include cylindrical billets and

rectangular bars


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BASIC BULK DEFORMATION PROCESSES

(a) Rolling (b) Forging

(c) Extrusion (d) Drawing


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Sheet Metalworking

Sheet metalworking processes are forming and

related operations performed on metal sheets, strips,

and coils High surface area-to-volume ratio of starting metal,

which distinguishes these from bulk deformation

Often called pressworkingbecause presses perform

these operations

Parts are called stampings

Usual tooling: punch and die


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BASIC SHEET METALWORKING OPERATIONS

(a) Bending (b) Deep Drawing

(c) Shearing


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Typical stress-strain curve for most metals is dividedinto an elastic region and a plastic region

Plastic region of stress-strain curve is primary interestbecause material is plastically or permanentlydeformed

In plastic region, metal's behavior is expressed by theflow curve:

nKIW !

where K= strength coefficient; and n = strain

hardening exponent

Stress and strain in flow curve are true stress and

true strain

Flow curve is a valid relationship for cold working


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Flow Stress

For most metals at room temperature, strengthincreases when deformed due to strain hardening

Flow stress = instantaneous value of stress requiredto continue deforming the material-to keep metalflowing

where Yf = flow stress, that is, the yield strength as a

function of strain The flow stress of the material is the strength

property that determines forces and power requiredto accomplish a particular forming operation

nf KY I!


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Average Flow Stress

In some processes, like extrusion, average flowstress ( the average value of stress overthe stress-straincurve fromthe beginningof strain tothe finalmaximum

value thatoccurs during deformation) is more useful

than the instantaneous flow stress Determined by integrating the flow curve equation

between zero and the final strain value defining the

range of interest

where = average flow stress; and I = maximum

strain during deformation process

n

KY

n

f

!

1

I_

_

fY


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n

f KY I!

n

KY

n

f

!

1

I

Yf Flow Stress

I Maximum strain

for forming process

K Strength coefficient

Average flow stressfY


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Temperature in Metal Forming

For any metal, Kand n in the flow curve depend on

temperature

Both strength and strain hardening are reduced at

higher temperatures

In addition, ductility is increased at highertemperatures

Any deformation operation can be accomplished with

lower forces and power at elevated temperature

Three temperature ranges in metal forming:

Cold working

Warm working

Hot working


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Cold Working

Performed at room temperature or slightly above

Many cold forming processes are important mass

production operations Minimum or no machining usually required

These operations are nearnetshape ornetshape

processes

Due to strain hardening the strength of the metalincreases and ductility decreases during cold working

Advantages Vs. Disadvantages


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Advantages of Cold Forming vs.

Hot Working

1. Better accuracy closer tolerances

2. Better surface finish

3. Strain hardening increases strength and hardness4. Grain flow during deformation can cause desirable

directional properties in product

5. No heating of work required lower fuel cost and

higher production rates


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Disadvantages of Cold Forming

1. Higher forces and power required

2. Surfaces of starting workpiece must be free of scale

and dirt3. Ductility and strain hardening limit the amount of

forming that can be done

In some operations, metal must be annealed to

allow further deformation In other cases, metal is simply not ductile enough

to be cold worked


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Warm Working

Performed at temperatures above room temperature

but below recrystallization temperature

Dividing line between cold working and warm workingoften expressed in terms of melting point:

0.3Tm, where Tm = melting point (absolute

temperature) for metal

Lower strength and strain hardening while higherductility


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Advantages of Warm Working

1. Lower forces and power than in cold working

2. More intricate work geometries possible

3. Need for annealing may be reduced or eliminated


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Hot Working

Deformation at temperatures above recrystallizationtemperature

Recrystallization temperature = about one-half of

melting point on absolute scale

In practice, hot working usually performed somewhat

above 0.5Tm

Metal continues to soften as temperature increases

above 0.5Tm, enhancing advantage of hot working

above this level

0.75 Tmis generally considered as upper limit asheat generated due to deformation can lead to

melting above that temperature

Recrystallization may take place during or after the

deformation


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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

Advantages Vs. Disadvantages


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Advantages of Hot Working vs. Cold Working

1. Workpart shape can be significantly altered

2. Lower forces and power required

3. Metals that usually fracture in cold working can behot formed

4. Strength properties of product are generally isotropic

(no grain orientation)

5. No strengthening of part occurs from work hardening Advantageous in cases when part is to be

subsequently processed by cold forming


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Disadvantages of Hot Working

1. Lower dimensional accuracy

2. Higher total energy required (due to the thermal

energy to heat the workpiece)3. Work surface oxidation (scale), poorer surface finish

4. Shorter tool life


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Strain Rate Sensitivity

Theoretically, a metal in hot working behaves like a

perfectly plastic material, with strain hardening

exponent n = 0

The metal should continue to flow at the same

flow stress, once that stress is reached

However, an additional phenomenon occurs

during deformation, especially at elevated

temperatures: Strain rate sensitivity


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What is Strain Rate?

Strain rate in forming 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:

where =true strain rate; and h = instantaneous height

of workpiece being deformed

h

v!

.

I.

I

In most practical operations, valuation of strain rate is

complicated by

Workpart geometry

Variations in strain rate in different regions of the part

Strain rate can reach 1000 s-1 or more for some metal

forming operations


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Effect of Strain Rate on Flow Stress

Flow stress is a strong function of temperature However, at hot working temperatures, flow stress

also depends on strain rate

As strain rate increases, resistance to deformation

increases This effect is known as strain-rate sensitivity

As strain rate is increased, resistance to deformation

increases. This usually plots as a straight line on a

log-log graph:

Strain Rate Sensitivity Equation: mf CY I!

where C= strength constant (similar but not equal to

strength coefficient in flow curve equation), and m =

strain-rate sensitivity exponent


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Figure 18.5 - (a) Effect of strain rate on flow stress at an

elevated work temperature. (b) Same relationship

plotted on log-log coordinates


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Figure 18.6 - Effect of

temperature on flow stress

for a typical metal. Theconstant Cin Eq. (18.4),

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


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Friction in Metal Forming

Friction in metal forming arises because of the closecontact between the tool and work surfaces and the high

pressures that drive the surfaces together in these

operations

In most metal forming processes, friction is undesirable:

Metal flow is retarded

Forces and power are increased

Wears tooling faster

Friction and tool wear are more severe in hot working (see

table)

Friction in metal forming is more severe than machine

parts (gears, cams, bearings etc) where there is low

contact pressures, low to moderate temperatures, and

ample lubrication to avoid metal to metal contact


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Lubrication in Metal Forming

Metalworking lubricants are applied to tool-workinterface in many forming operations to reduce the

harmful effects of friction

Benefits:

Reduced sticking, forces, power, tool wear Better surface finish

Removes heat from the tooling

Lubricants used for cold working operations include:

mineral oils, fats and fatty oils, water basedemulsions, soaps, and other coatings

Hot working is sometimes performed dry; the used

lubricants include minerals oils, graphite and glass


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Considerations in Choosing an

Appropriate Metalworking Lubricant

Type of forming process (rolling, forging, sheet metal

drawing, etc.)

Hot working or cold working Work material

Chemical reactivity with tool and work metals

Ease of application

Toxicity, flammability and cost

Documents

Chapter 18-Fundamentals Metal Forming