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Dr. Amr Shehata Fayed

Amr Shehata Fayed, Ph.D.Assistant Professor of Mechanical Engineering

Materials Engineering Department

Faculty of Engineering

Zagazig University

[email protected]


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2Dr. Amr Shehata Fayed

Material Properties in Metal Forming

To be successfully formed, a metal must possesscertain properties.

Desirable material properties:

Low yield strengthand high ductility

These properties are affected by temperature:

Ductility increases and yield strengthdecreases when work temperature israised.

Strain rate and friction are additional factors.


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Independent Variables in Metal Forming

Independent variables are those aspects of the processover which the engineer has direct control, and they are

generally selected or specified when setting up a process.

Some of independent variables in a typical formingprocess:-

Starting materialStarting geometry of the workpiece

Tool or die geometry

Lubrication

Starting temperatureSpeed of operation

Amount of deformation


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Dependent Variables in Metal Forming

Dependent variables are the consequences of the

independent variable selection.

Example of dependent variables include:

Force or power requirements

Material properties of the product

Exit (or final) temperature

Surface finish and dimensionalprecision

Nature of the material flow


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Material Behavior in Metal Forming

The 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 deformed

Necking starts atmaximum engineeringstress or, equivalently,at maximum tensileload.


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K = strength coefficient (MPa); and n = strain hardeningexponent. Stress and strain in flow curve are true stressand true strain.

Material Behavior in Metal Forming

In plastic region,metal's behavior isexpressed by the flowcurve:


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Where: Yf = flow stress, that is, the yield strength as afunction of strain

Flow Stress

For most metals at room temperature, strength increases

when deformed due to strain hardening. The stressrequired to continue deformation must be increased tomatch this increase in strength.

Flow stress is defined as the instantaneous value of

stress required to continue deforming the material tokeep the metal flowing.


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Relevance of the Flow Curve

The flow curve is used to determine the new yield strength

after a plastic deformation process.

The flow curve is used to judge the formability of metals.

The flow curve describes the hardening behavior of

metals during plastic deformation in terms of equivalent

strain, equivalent strain rate and temperature.The flow curve is a property of each individual metal.

various experiments with different stress and strain-ratestates should yield the same flow curve for same strain-

rate value and same temperature.


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Ranges of Equivalent Strain & StrainRates in Metal Forming Processes

Therefore, the flow curves should be up to these strains andstrain rates..


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Some Ugly Facts about theDetermination of Flow Curves

It is not possible to obtain flow curves up to the requiredplastic strains and strain rates practically.

It is extremely difficult to have tests with homogeneousdeformation.

The flow curves obtained by different tests do notcoincide for the same material.


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Reasons for Deviations in the FlowCurves Obtained by Different Tests

Effect of stress state,

Effect of equivalent stress equation,

Effect of anisotropy (Bauschinger effect),

Effect of experimental inaccuracies (e. g. friction),

Effect of temperature (heating of the specimens),


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Flow Curve: Mathematical Representation (1)(Cold Flow Curves)

For cold flow curves the flow stress increases only 3 to10 % for an increase of one order in the strain-rates.


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Average Flow StressThe average flow stress(ormean flow stress) is the

average value of stressover the stress-straincurve from the beginningof strain to the final(maximum) value that

occurs during deformation.

Where: Yf= average flow stress; and

= maximum strain during deformation


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Typical Values of K and n


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Temperature in Metal Forming

For any metal, the values of Kand nin the flow curve

depend on temperature.

Both strength and strain hardening are reduced athigher temperatures.

In addition, ductility is increased at higher

temperatures.

These property changes are important because;

Any deformation operation can be accomplishedwith lower forces and power at elevatedtemperature


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Temperature Ranges Metal Forming

There are three temperature ranges in metal forming

processes:

whereTm

is the melting point of the metal


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Cold Working

Performed at room temperature or slightly above.

Many cold forming processes are important massproduction operations.

Minimum or no machining usually required.

These operations are near net shape or net shapeprocesses


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Advantages of Cold Working

Significant advantages of cold forming compared to

hot working

Better accuracy, meaning closer tolerances

Better surface finish

Strain hardening increases strength and hardness

Contamination problems are minimized

No heating of work required


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Disadvantages of Cold Working

There are certain disadvantages or limitationsassociated with cold working

Higher forces are required to initiate and completethe deformation.

Heavier and more powerful equipment and strongertooling are required.

Surfaces of starting workpiece must be free of scaleand dirt.

Ductility and strain hardening limit the amount offorming that can be done.

In some operations, metal must be annealed to allowfurther deformation. While, in other cases, metal issimply not ductile enough to be cold worked.


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Warm Working

Performed at temperatures above room temperaturebut below recrystallization temperature.

Dividing line between cold working and warmworking often expressed in terms of melting point:

0.3Tm, where Tm= melting point for metal


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Advantages of Warm Working

The lower strength and strain hardening as well ashigher ductility of the metal at the intermediatetemperatures provide warm working the followingadvantages over cold working:

Lower forces and power than in cold working.

More intricate work geometries possible.

Need for annealing may be reduced or eliminated.

Finishing machining is reduced.

Less scaling and steel decarburization compared to

hot working.


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Hot Working

Deformation at temperatures above recrystallizationtemperature

Recrystallization temperature = about one-half ofmelting point. In practice, hot working usuallyperformed somewhat above 0.5Tm.

Capability for substantial plastic deformation of themetal - far more than possible with cold working orwarm working.


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Advantages of Hot Working

Workpart shape can be significantly altered.

Lower forces and power than in cold working.

Metals that usually fracture in cold working can be hotformed.

Strength properties of product are generally isotropic

No strengthening of part occurs from work hardening.

Advantageous in cases when part is to besubsequently processed by cold forming.


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Disadvantages of Hot Working

Lower dimensional accuracy.

Higher total energy required (due to the thermalenergy to heat the workpiece).

Work surface oxidation (scale), poorer surface finish.

Shorter tool life


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Friction in Metal Forming

Friction in metal forming arises because of the close

contact between the tool and work surfaces and thehigh pressures that drive the surfaces together inthese operations.

In most metal forming processes, friction isundesirable for the following reasons:

Metal flow in the work is retarded.

The forces and power to perform the operationare increased.

Rapid wear of the tool occurs.

Friction and tool wear are more severe in hot working


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Friction in Metal Forming

If the coefficient of friction becomes large enough, acondition known as stickingoccurs.

Sticking in metal working is the tendency for the twosurfaces in relative motion to adhere to each otherrather than slide.


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Lubrication in Metal Forming

Metalworking lubricants are applied to tool-work interface inmany forming operations to reduce harmful effects offriction.

Benefits obtained from the application of lubricants are:

Reduced sticking, forces, power, tool wearBetter surface finish

Removes heat from the tooling


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Homework (2)

1. If K = 600 MPa and n = 0.2 for certain metal. During a

forming operation, the final true strain that the metalexperienced = 0.73. Determine the flow stress at thisstrain and average flow stress that metal experiencedduring the operation.

2. A particular metal has a flow curve with parameters;

strength coefficient = 35000 lb/in2 and strain hardeningexponent = 0.26. A tensile specimen of the metal withgage length = 2 in is stretched to a length = 3.3 in.Determine the flow stress at this new length and theaverage flow stress that the metal was subjected to during

deformation.


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Homework (2), cont.

3. Why is the term press working often used for sheet-

metalworking processes?

4. Mention some of the advantages of cold working relativeto warm and hot working.

5. Why is friction generally undesirable in metal forming

operations?

6. What are the main differences between bulk deformationand sheet metalworking processes?

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