Metal Forming

FUNDAMENTALS OF METAL FORMING

Overview of Metal FormingMaterial Behavior in Metal FormingTemperature in Metal FormingStrain Rate SensitivityFriction and Lubrication in Metal Forming

Metal FormingLarge group of manufacturing

processes in which plastic deformation is used to change the shape of metal workpiecesThe tool, usually called a die, applies stresses that exceed yield strength of metal The metal takes a shape determined by the geometry of the dieLoad applied are compressive, tensile, shear, bending and combination

Material Properties in Metal FormingDesirable material properties:

Low yield strength and high ductilityThese properties are affected by temperature:

Ductility increases and yield strength decreases when work temperature is raised

Other factors: Strain rate and friction

rolling

Bulk Deformation Processes

forging

extrusion

drawing

"Bulk" refers to workparts with relatively low surface area-to-volume ratios

Sheet Metalworking

High surface area-to-volume ratio of starting metal, which distinguishes these from bulk deformation

bending

drawing

shearing

Stress-strain curve

0 0

ln ( )l

l

dl lel l

= =∫

True strain

True stressσ = P/A

To determine point of instability

0dP dA dAdl dl dl

σσ= + =

dA Adl l

= −

d dl del

σσ

= =

1 1ddeσ

σ=

0[ ]ce

pW deσ= ∫Total plastic work

Using volume constancy condition, A0l0=Al

----(1)

after substituting in (1)

de is incremental strain

Now draw a tangent at the point of instability I, the length DF will be 1 and strain de is evaluated

This leads to flow curve equation

From true stress equation

Material Behavior in Metal FormingPlastic region of stress-strain curve is primary interest because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve:

nKεσ =where K = strength coefficient; and n = strain hardening exponent

• Stress and strain in flow curve are true stress and true strain

Flow stress = instantaneous value of stress required to continue deforming the material

nf KY ε=

nKY

n

f +=

1ε_

Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest

Stress-strain curve

Spring back effect Temperature and strain rate response

Extend of Plastic deformation

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 reachedHowever, an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivityAs strain rate increases, resistance to deformation increases

What is Strain Rate?

Strain rate in forming is directly related to speed of deformation vDeformation 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

hv

=.ε

.ε

(a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log-log coordinates

Strain rate sensitivity equation m

f CY ε=

Effect of temperature on flow stress for a typical metal.

n mfY A Cε= ε

Observations about Strain Rate Sensitivity

Increasing temperature decreases C, increases m

At room temperature, effect of strain rate is almost negligible

Flow curve is a good representation of material behavior

As temperature increases, strain rate becomes increasingly important in determining flow stress

Recrystallization

Effect of recovery, recrystallization and grain growth

Recovery: stresses in the highly deformed regions are relieved. Sub grain boundaries are formedNo change in mechanical properties

Recrystallization:New equiaxed and stress free grains are formed

Grain growth :Grain grows and affects mechanical properties

Temperature in Metal Forming

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

Both strength and strain hardening are reduced at higher temperaturesIn addition, ductility is increased at higher temperatures

Three temperature ranges in metal forming: Cold working (less than .3 Tm)Warm working (.3 to .5 Tm)Hot working ( above 0.5 Tm)

HotworkingAdvantages:• Very high reduction without fracture• Deformation energy is low• Process is faster• Metal is made tougher because pores get closed• No change in hardness of material Disadvantages• Higher temperature promotes undesirable reaction• Metallurgical structure may not be uniform because of cooling history

after deformation• Heat-resistant tools are required • Close tolerance cannot be achieved• Surface finish is poor• Handling is difficult

Cold Working Advantages

Good surface finish and better dimensional accuracyStrength, fatigue and wear properties are improvedMinimum contamination in workpiece surfaceEnergy saving Handling is easy

DisadvantagesDeformation energy required is largeStrain hardening limits deformationDuctility is reduced

Friction in Metal Forming

In most metal forming processes, friction is undesirable:

Metal flow is retarded Forces and power are increasedWears tooling faster

Friction and tool wear are more severe in hot workingSticking friction instead of slip phenomenon

Lubrication in Metal Forming

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

Reduced sticking, forces, power, tool wearBetter surface finishRemoves heat from the toolingGraphite, hot glass is used for Hot working