ME 134 Metal Forming

metal forming

▪ Large group of processes ▪Uses plastic deformation ▪ Tool applies stress exceeding yield strength ▪ Takes shape of die

compressive stresses

Rolling, forging, extrusion

tensile stresses

stretch the metal

tensile and compressive

bend the metal

shear stresses

cutting and blanking

desirable material properties

• Low yield strength • High ductility • Increasing work temperature -> Increases ductility; Decreases yield strength

bulk deformation

• Rolling • Forging • Extrusion • Wire and bar drawing

sheet metalworking

• Bending • Deep drawing • Cutting • Miscellaneous processes

bulk deformation processes

▪ Characterized by significant deformations and massive shape changes ▪ "Bulk" refers to workparts with relatively low surface area-to-volume ratios ▪ Starting work shapes include cylindrical billets and rectangular bars

rolling

◼ reduce thickness or change the cross section ◼ compressive forces

starting stock

◼ blooms ◼ billets ◼ slabs

blooms

square or rectangular cross section (>6inches)

billets

smaller than a bloom; square or circular cross section

slabs

rectangular solid; width greater than twice the thickness

basic rolling process

1. Passed between two rolls rotating in opposite directions 2. Friction acts to propel the material forward 3. Metal is squeezed and elongates to compensate for the decrease in cross-sectional area

hot rolling

◼ Control temperature for successful form ◼ Uniform Temperature = Uniform deformation ◼ Terminated when the temperature reaches to about 50° to 100 ° C above the recrystallization temperature of the metal.

cold rolling

◼ Produces sheet, strip, bar and rod products ▪ smooth surfaces ▪ accurate dimensions

smaller diameter rolls

produce less length of contact require

less force to produce a given change in shape

reduced stiffness

continuous (tandem) rolling mills

◼ Billets, blooms, and slabs are heated and fed through an integrated series of non reversing rolling mills ◼ Synchronization of rollers may pose issues

ring rolling

◼ One roll is placed through the hole of a thick-walled ring and a second roll presses on the outside ◼ Produces seamless rings ◼ Circumferential grain orientation ◼ Used in rockets, turbines, airplanes, pressure vessels, and pipelines

hot-rolled products

◼ have little directionality in their properties ◼ Uniform grain size and have dependable quality ◼ Surfaces may be rough or may have a surface oxide known as mill scale

cold-rolled products

exhibit superior surface finish and dimensional precision

flatness control and rolling defects

◼ Rollers must be evenly spaced throughout for perfectly flat pieces to be produced ◼ Sometimes this variation in roller “flatness” may be desired

thermomechanical processing and controlled rolling

combines the deformation and thermal processing into a single shape with the desired properties ◼ Requires computer-controlled facilities ◼ Substantial energy savings

forging

Induce plastic deformation through localized compressive forces applied through dies ◼ Oldest known metalworking process ◼ Parts can range in size

open-die hammer forging

Forging done by a blacksmith but mechanical equipment performs the operation ◼ Material needs to be heated to proper temperature

gravity drop machine

simplest industrial hammer

computer-controlled hammers

provide varying blows

impression-die hammer forging

◼ The dies are shaped to control the flow of metal ◼ Upper piece attaches to the hammer and the lower piece to the anvil ◼ Metal flows and completely fills the die

first impression

edging, fullering, or bending

intermediate impressions

blocking

final shape

final forging operation

flash forging

excess metal

flashless forging

total confinement

press forging

◼ Used for large or thick products ◼ Slow squeezing action penetrates completely through the metal ◼ Produces a more uniform deformation and flow ◼ Longer time of contact between the die and workpiece ◼ Dies may be heated (isothermal forging - near net shape and uniform microstructure and mechanical properties) ◼ Presses are either mechanical or hydraulic

upset forging

▪ Increases the diameter of a material by compressing its length ▪ Both cold and hot upsetting

automatic hot forging

Slabs, billets, and blooms can be slid into one end at room temperature and hot-forged products can emerge at the other end, with every process automated

roll forging

◼ Round or flat bar stock is reduced in thickness and increased in length ◼ Produces products such as axles, tapered levers, and leaf springs ◼ Little or no flash is produced

swaging

◼ Also known as rotary swaging and radial forging ◼ Uses external hammering to reduce the diameter or produce tapers or points on round bars or tubes

net-shape and near-net-shape forging

◼ 80% of the cost of a forged-part can be due to post-forging operations ◼ To minimize expense and waste, parts should be forged as close the final shape as possible

extrusion

◼ Metal is compressed and forced to flow through a shaped die to form a product with a constant cross section ◼ May be performed hot or cold ◼ A ram advances from one end of the die and causes the metal to flow plastically through the die

advantages of extrusion

◼ Many shapes can be produced that are not possible with rolling ◼ Dies are relatively inexpensive ◼ Small quantities of a desired shape can be produced economically

direct extrusion

Solid ram drives the entire billet to and through a stationary die ◼ Must provide power to overcome friction

indirect extrusion

A hollow ram pushes the die back through a stationary, confined billet

mandrels

may be used to produce hollow shapes or shapes with multiple longitudinal cavities

hydrostatic extrusion

◼ High-pressure fluid surrounds the workpiece and applies the force to execute extrusion ◼ Billet-chamber friction is eliminated ◼ High efficiency process ◼ Temperatures are limited because the fluid acts as a heat sink ◼ Seals must be designed to keep the fluid from leaking

continuous extrusion

◼ Pushing force is applied at the periphery of the feedstock resulting to continuous extrusion. • Feedstock can be solid, metal powder, punchouts, or chips • Metallic and nonmetallic powders can be intimately mixed

wire, rod, and tube drawing

◼ Reduce the cross section of a material by pulling it through a die ◼ Similar to extrusion, but the force is tensile

tube drawing

used for long-length product

wire drawing

rotating draw block

cold heading

a form of upset forging ◼ Used to make the enlarged sections on the ends of rod or wire (i.e. heads of nails, bolts, etc.)

impact extrusion

◼ A metal slug is positioned in a die cavity where it is struck by a single blow ◼ Metal may flow forward, backward or some combination ◼ The punch controls the inside shape while the die controls the exterior shape

piercing

Thick-walled seamless tubing can be made by rotary piercing ◼ Heated billet is fed into the gap between two large, convex-tapered rolls ◼ Forces the billet to deform into a rotating ellipse

roll extrusion

thin walled cylinders are produced from thicker-wall cylinders

sizing

involves squeezing all or select regions of products to achieve a thickness or enhance dimensional precision

riveting

permanently joins sheets or plates of material by forming an expanded head on the shank end of a fastener

staking

permanently joins parts together when a segment of one part protrudes through a hole in the other

coining

cold squeezing of metal while all of the surfaces are confined within a set of dies

hubbing

plastically forms recessed cavities in a workpiece

peening

mechanical working of surfaces by repeated blows of impelled shot or a round-nose tool

burnishing

rubbing a smooth, hard object under pressure over the minute surface irregularities

roller burnishing

used to improve the size and finish of internal and external cylindrical and conical surfaces

shearing

mechanical cutting of material without the formation of chips or the use of burning or melting ▪ Both cutting blades are straight

phases of shearing

◼ Elastic deformation ◼ Plastic deformation (between yield and UTS) ◼ Material separation, crack propagation

punching

◼ Punching force ◼ F=0.7tL(UTS)

simple shearing

sheets of metal are sheared along a straight line

slitting

lengthwise shearing process that is used to cut coils of sheet metal into several rolls of narrower width

blanking

the piece being punched out becomes the workpiece

piercing

the punchout is the scrap and the remaining strip is the workpiece

lancing

piercing operation that forms either a line cut or hole

perforating

piercing a large number of closely spaced holes

notching

removes segments from along the edge of an existing product

nibbling

a contour is progressively cut by producing a series of overlapping slits or notches

shaving

finishing operation in which a small amount of metal is sheared away from the edge of an already blanked part

cutoff

a punch and a die are used to separate a stamping or other product from a strip of stock

dinking

used to blank shapes from low-strength materials such as rubber, fiber, or cloth

progressive die sets

two or more sets of punches and dies mounted in tandem

transfer dies

move individual parts from operation to operation within a single press

compound dies

combine processes sequentially during a single stroke of the ram

bending

the plastic deformation of metals about a linear axis with little or no change in the surface area

forming

multiple bends are made with a single die

bar folders

make angle bends up to 150 degrees in sheet metal under 1.5mm thick

press brakes

make bends in heavier sheets or more complex bends in thin material

bottoming dies

contact and compress the full area within the tooling

air bend dies

produce the desired geometry by simple three-point bending

coining dies

If bottoming dies go beyond the full-contact position

roll bending

a continuous form of three-point bending

roll forming

process by which a metal strip is progressively bent as it passes through a series of forming rolls

seaming

a bending operation that can be used to join the ends of sheet metal in some form of mechanical interlock

flanges

can be rolled on sheet metal in a similar manner as seams

straightening

Done before subsequent forming to ensure the use of flat or straight material

stretcher leveling

material is mechanically gripped and stretch until it reaches the desired flatness

drawing

refers to the family of operations where plastic flow occurs over a curved axis and the flat sheet is formed into a three-dimensional part

deep drawing

to form solid-bottom cylindrical or rectangular containers from sheet metal

depth is greater than diameter

shallow drawing

to form solid-bottom cylindrical or rectangular containers from sheet metal

depth is less than diameter

spinning

▪ Sheet metal is rotated and shaped over a male form, or mandrel ▪ Produces rotationally symmetrical shapes ▪ Spheres, hemispheres, cylinders, bells, and parabolas

shear forming

modified version of spinning

stretch forming

a sheet of metal is gripped and a form block shapes the parts

guerin process

◼ Also known as rubber-die forming ◼ Form block replaces male die ◼ Developed by the aircraft industry to produce small quantity parts

bulging

Operation that use fluid or rubber to transmit the pressure required to expand a metal blank