Metal forming 2

Dr. Amr Shehata Fayed

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

Materials Engineering DepartmentFaculty of Engineering

Zagazig [email protected]

2Dr. Amr Shehata Fayed

Material Properties in Metal Forming

To be successfully formed, a metal must possess certain properties.

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.

Strain rate and friction are additional factors.


Independent Variables in Metal FormingIndependent variables are those aspects of the process over 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 forming process:-

Starting material

Starting geometry of the workpiece

Tool or die geometry

Lubrication

Starting temperature

Speed of operation

Amount of deformation


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 dimensional precision

Nature of the material flow


Material Behavior in Metal Forming• The typical stress strain curve for most metals is divided

into an elastic region and a plastic region

• Plastic region of stress-strain curve is primary interest because material is plastically deformed

Necking starts at maximum engineering stress or, equivalently, at maximum tensile load.


K = strength coefficient (MPa); and n = strain hardening exponent. Stress and strain in flow curve are true stress and true strain.

Material Behavior in Metal Forming

• In plastic region, metal's behavior is expressed by the flow curve:


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

Flow Stress

For most metals at room temperature, strength increases when deformed due to strain hardening. The stress required to continue deformation must be increased to match this increase in strength.

Flow stress is defined as the instantaneous value of stress required to continue deforming the material – to keep the metal “flowing”.


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-rate states should yield the same flow curve for same strain-rate value and same temperature.


Ranges of Equivalent Strain & StrainRates in Metal Forming Processes

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


Some Ugly Facts about theDetermination of Flow Curves

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

It is extremely difficult to have tests with homogeneous deformation.

The flow curves obtained by different tests do not coincide for the same material.


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),


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

For cold flow curves the flow stress increases only 3 to 10 % for an increase of one order in the strain-rates.


Average Flow StressThe average flow stress (or mean flow stress) is the average value of stress over the stress-strain curve from the beginning of strain to the final (maximum) value that occurs during deformation.

Where: Yf = average flow stress; and= maximum strain during deformation


Typical Values of K and n


Temperature in Metal Forming

For any metal, the values of K and n in the flow curve depend on temperature.

Both strength and strain hardening are reduced at higher temperatures.

In addition, ductility is increased at higher temperatures.

These property changes are important because;

Any deformation operation can be accomplished with lower forces and power at elevated temperature


Temperature Ranges Metal Forming

There are three temperature ranges in metal forming processes:

where Tm is the melting point of the metal


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 near net shape or net shape processes


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


Disadvantages of Cold WorkingThere are certain disadvantages or limitations associated with cold working

Higher forces are required to initiate and complete the deformation.

Heavier and more powerful equipment and stronger tooling are required.

Surfaces of starting workpiece must be free of scale and dirt.

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

In some operations, metal must be annealed to allow further deformation. While, in other cases, metal is simply not ductile enough to be cold worked.


Warm Working

Performed at temperatures above room temperature but below recrystallization temperature.

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

0.3Tm, where Tm = melting point for metal


Advantages of Warm Working

The lower strength and strain hardening as well as higher ductility of the metal at the intermediate temperatures provide warm working the following advantages 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.


Hot WorkingDeformation at temperatures above recrystallization temperature

Recrystallization temperature = about one-half of melting point. In practice, hot working usually performed somewhat above 0.5Tm.

Capability for substantial plastic deformation of the metal - far more than possible with cold working or warm working.


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 hot formed.

Strength properties of product are generally isotropic

No strengthening of part occurs from work hardening.

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


Disadvantages of Hot Working

Lower dimensional accuracy.

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

Work surface oxidation (scale), poorer surface finish.

Shorter tool life


Friction in Metal Forming

Friction in metal forming arises because of the close contact 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 for the following reasons:

Metal flow in the work is retarded.

The forces and power to perform the operation are increased.

Rapid wear of the tool occurs.

Friction and tool wear are more severe in hot working


Friction in Metal Forming

If the coefficient of friction becomes large enough, a condition known as sticking occurs.

Sticking in metal working is the tendency for the two surfaces in relative motion to adhere to each other rather than slide.


Lubrication in Metal Forming

Metalworking lubricants are applied to tool-work interface in many forming operations to reduce harmful effects of friction.

Benefits obtained from the application of lubricants are:

Reduced sticking, forces, power, tool wear

Better surface finish

Removes heat from the tooling


Homework (2)

1. If K = 600 MPa and n = 0.2 for certain metal. During a forming operation, the final true strain that the metal experienced = 0.73. Determine the flow stress at this strain and average flow stress that metal experienced during the operation.

2. A particular metal has a flow curve with parameters; strength coefficient = 35000 lb/in2 and strain hardening exponent = 0.26. A tensile specimen of the metal with gage length = 2 in is stretched to a length = 3.3 in. Determine the flow stress at this new length and the average flow stress that the metal was subjected to during deformation.


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 relative to warm and hot working.

5. Why is friction generally undesirable in metal forming operations?

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

Metal forming 2

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