Fundamentals of Metal Forming

What is metal forming?

Using plastic deformation to change the shape of metal workpieces using dies to form the desired geometry

What is bulk deformation?

Significant deformations and massive shape changes w.r.t. workparts with low surface area-to-volume ratios

What are the types of bulk deformation processes?

Rolling

Forging

Extrusion

Wire & Bar Drawing

What is rolling?

Slab is heated in a furnace & rolled between powered rollers until at desired thickness

What is forging?

Forming metal by impacting and/or squeezing a preheated part between two halves of a die

Multiple dies may be needed to achieve final shape

What is extrusion?

Billets are preheated and forced by a ram through one or more dies to achieve the desired cross-section

What is wire / bar drawing?

Cross-section of solid rod, wire, tubing is reduced or changed by pulling it through a die

What is sheet metalworking?

Forming operations done on metal sheets, strips, and coils

High surface area-to-volume ratio of starting metal

Also called pressworking as operations use presses creating stampings using punches & dies

What is sheet metal bending?

Straining of metal sheet or plate to create a specific angle

What is deep drawing?

Forming of flat metal sheet into hollow or concave shape by stretching the metal

What is shearing?

Cuts the workpiece using a punch and die

What is the flow curve?

Plastic region of stress-strain curve expressed as a curve using true stress & strain

σ = Kεⁿ

K - strength coefficient
n - strain hardening exponent

What is flow stress?

Instantaneous value of stress required to continue deforming the material

Strength increases due to strain hardening

Y_f = Kεⁿ

Y_f - flow tress (yield strength as a function of strain)

What is the average flow stress?

How does temperature affect the flow curve?

As temperature increases, strength (K) & strain hardening (n) exponents are reduced

Ductility is increased at higher temperatures

Any deformation operation can occur with lower forces & power at higher temperatures

What are the temperature ranges in metal forming?

Cold working

Warm working

Hot working

What is cold working?

Plastic deformation that typically occurs at room temperature

What are some advantages & disadvantages of cold working?

Advantages:

Better accuracy, surface finish & closer tolerances
Strain hardening increases strength & hardness
Grain flow during deformation can improve directional properties
No heating required

Disadvantages:

Higher forces & power required for deformation
Surfaces must be smooth & free of scale (black coating of oxide when heated) and dirt
Ductility & strain hardening can limit amount of forming possible
May require annealing to allow further deformation
Some metals might not be ductile enough to do cold forming

What is warm working?

Plastic deformation carried out at above room but below recrystallization temperatures (~30% of melting)

Compromise between cold & hot working

What are the advantages of warm working?

Advantages:

Lower forces than cold working
More complex geometries possible
Reduced need for annealing (can even be eliminated)

What is hot working?

Plastic deformation done above recrystallization temperature (above ~50% of melting)

Why would you use hot working?

Higher ability for plastic deformation of metal than cold & warm working

Lower strength coefficient (K) and strain hardening exponent (n) is almost zero improves ductility

What are the advantages & disadvantages of hot working?

Advantages:

Significant alteration possible
Lower force & power required
Metals that fracture in cold working can be hot formed
Strength properties are generally isotropic (not directionally dependent)
No strain hardening / strengthening

Disadvantages:

Lower dimensional accuracy
Higher total energy required from needing to heat the workpiece
Work surface oxidation (scale) results in poor surface finish
Shorter tool life