ME 215 – Section 6: Metal Forming

Why does forming often need high production volume?

Specialized presses, dies and tooling can be expensive; a large run spreads fixed tooling cost across many parts.

What material data matter in forming?

Flow stress/stress-strain behavior, strain hardening, ductility, anisotropy, temperature and strain-rate sensitivity are key inputs for forces and safe deformation.

How does higher temperature ease forming?

Yield/flow stress and strain-hardening resistance usually decrease; ductility and metal flow often improve, especially with recovery/recrystallization.

Why heat the dies in hot working?

To prevent a hot workpiece from chilling at the die surface, giving more uniform flow/temperature and reducing surface cracks or excess loads.

Why analyze forging and rolling differently?

Forging commonly creates complex 3-D, nonuniform flow and pressure; rolling typically has more constrained, continuous deformation suited to simpler analysis.

Typical upset-forging products?

Bolt and screw heads, rivets, nails, valves and similar parts formed by compressing short lengths of rod or wire.

Sheet forming vs. bulk forming?

Sheet forming acts on thin stock where thickness is small relative to width/length; bulk forming substantially deforms the cross-section of bars, billets or slabs.

Progressive dies vs. transfer dies?

Progressive dies perform stages on a strip carried between stations; transfer dies move separated workpieces from one station to the next.

Benefit and cost of coining during bottom bending?

Coining reduces springback and gives a highly repeatable bend angle, but needs much higher forces and creates greater tool loads/wear.

What two processes are called drawing?

Drawing of wire/rod/tube through a die to reduce cross-section, and sheet-metal deep drawing of a flat blank into a cup or shell.

Plasticity

Ability of a solid metal to undergo permanent deformation without fracture.

Elastic deformation

Reversible shape change recovered when load is removed.

Plastic deformation

Permanent shape change after the material passes its elastic limit.

Formability

Ability of a material to undergo a particular forming operation without unacceptable failure.

Bulk forming

Large plastic flow of thick stock, as in rolling, forging, extrusion and drawing of bars/wires.

Sheet forming

Deformation of thin sheet/strip into desired shapes using shearing, bending or stretching/drawing.

Independent variable

Input controlled in a process: workpiece material, geometry, temperature, lubrication, die shape or speed.

Dependent variable

Result of processing, such as forming load, final shape, flow pattern, defects or properties.

Flow stress

Stress needed to sustain plastic deformation under given strain, strain rate and temperature conditions.

Strain hardening

Increasing strength from plastic deformation due mainly to dislocation interactions.

Anisotropy

Direction dependence of material properties, often from preferred grain orientation/processing.

Friction in forming

Resistance to relative motion at workpiece/tool interfaces; affects loads, metal flow, and surface quality.

Lubrication

Reducing friction and wear at tool/workpiece contact, sometimes also helping cooling or protecting surfaces.

Tribology

Study of friction, lubrication and wear between contacting surfaces.

Hot working

Plastic deformation above the material's recrystallization temperature so strain hardening is relieved as it occurs.

Cold working

Plastic deformation below the effective recrystallization temperature; generally increases strength and reduces ductility.

Warm working

Forming at temperatures between conventional cold and hot working to balance force, finish and ductility.

Isothermal forming

Holding die and workpiece near the same temperature to provide uniform flow and prevent unwanted chilling.

Recrystallization

Formation of new, relatively strain-free grains during appropriate heating or hot deformation.

Recovery

Rearrangement/reduction of lattice defects during heating, with partial removal of work hardening.

Springback

Elastic recovery after unloading a formed part, changing its final bend or dimensions.

Residual stress

Locked-in stress caused by nonuniform plastic flow, temperature changes or other processing differences.

Rolling

Reducing workpiece thickness or changing cross-section by passing metal between rotating rolls.

Hot rolling

Rolling above recrystallization temperature to obtain large deformation loads with less strain hardening.

Cold rolling

Rolling at lower temperatures to improve thickness control, finish and strength by work hardening.

Four-high rolling mill

Two small work rolls supported by two large backup rolls to limit deflection.

Cluster mill

Rolling mill using multiple backup rolls around small work rolls; suitable for thin strip and foil.

Roll crown

Deliberate roll diameter profile designed to compensate for bending and thickness variation.

Mill scale

Oxide layer that forms on steel during elevated-temperature processing.

Forging

Shaping metal by localized or bulk compressive force using hammers, presses and dies.

Open-die forging

Compressive shaping between simple dies without fully enclosing the workpiece.

Impression-die forging

Shaping within contoured die cavities; displaced excess metal may form flash.

Flash

Thin excess metal squeezed out of a forging die, which can help build cavity pressure and later is trimmed.

Upset forging

Shortening a rod/wire segment by compression to increase its local cross-section, as in bolt heads.

Extrusion

Forcing metal through a shaped die opening to form a long product with a constant profile.

Direct extrusion

Ram pushes billet toward stationary die; billet slides against the container.

Indirect extrusion

Die moves toward billet, reducing billet/container sliding friction.

Wire drawing

Pulling wire or rod through a smaller die opening to reduce its diameter.

Tube drawing

Pulling tubing through a die, optionally with an internal mandrel, to control size and wall thickness.

Swaging

Incremental radial hammering or squeezing used to reduce or shape rods, wires, or tubes.

Shearing

Separating sheet material by applying opposing cutting edges.

Blanking

Shearing operation in which the punched-out piece is the desired product.

Punching/piercing

Shearing a hole where the removed slug is typically scrap.

Bending

Deforming sheet around a straight or curved axis, causing tension outside and compression inside the bend.

Air bending

Bend controlled by punch depth with less full contact between workpiece and V-die.

Bottoming

Sheet forced firmly against die faces to set the bend geometry.

Coining (bending)

High-pressure localized compression at a bend that strongly reduces springback but increases required load.

Progressive die

Tooling with multiple stations acting successively on a moving strip of sheet metal.

Transfer die

Multiple-station die where individual parts are mechanically transferred between operations.

Deep drawing

Pulling a sheet blank into a die cavity with a punch to make cups, boxes, or shell shapes.

Blank holder

Device that controls sheet flange flow and reduces wrinkling during drawing.

Wrinkling

Buckling/folding of sheet caused by compressive stresses during forming.

Earing

Uneven cup rim formed in drawing because sheet properties depend on orientation.