Chapter 14

Processes Used to Form Metallic Materials

Objectives

• Metal forming by forging• Metal forming by cutting away metal• Metal casting

Introduction

• Choosing a forming process depends on the type of metal, the number of pieces that need to be made, and the cost.

• Another requirement is that the process must impart the desired mechanical properties.

Forging

• Hand forging was initially used to shape simple metal tools. It was later refined to make fine jewelry and coins.

• Today, the most extensive use of hand forging is by craftsmen working with ornamental wrought iron.

• In forging, the shape of the raw material is changed by repositioning material rather than removing it.

• Today forging is used to produce bolts, rivets, connecting rods, gears, and structural members for equipment.

Open Die Forging• In Open die forging (Smith forging) the work piece is formed

between flat dies that compact but do not completely enclose the heated metal part.

• The work piece is heated to improve its plastic flow characteristics and to reduce the force required to work on the metals.

• The process can be used from a range of a few pounds up to around 300 tons.

• Open die forging creates flashing and generally cannot shape the part completely. The excess material has to be removed by subsequent machining operations.

Closed Die Forging

• Closed die forging is usually done with horizontally opposed precision die sets.

• These opposing dies simultaneously hammer against the billet until the forming process is complete.

• This process accounts for a vast majority of all commercial forging production.

• The die parts need to be made from hardened steel because of the tremendous abuse from hammering.

• Softer alloys such as aluminum, magnesium and copper can be easily forged.

Coining

• A movable punch is located within the die.• The movement of this punch cold works the

material and can form intricate features with very fine details and finish.

• Pressures as high as 200,000 psi are required to generate very fine features.

Roller Forming• Roller forming is also called cold roll forming. • No dies are needed. Rollers progressively squeeze the

continuous strips of metal into the desired shape.• Some products are truck frame members, metal building

components, and metal studs.• Stock is usually purchased in large coils.• Roller forming machines consume enormous amounts of floor

space.• Many firms turn flat stock into tubes using roller forming. The

edges are then automatically joined in a continuous welding operation.

Roll Bending

• Roll bending is used to bend circular, curved, and cylindrical shapes from bar, rod, tube, angle and channel stock.

• There are various ending roll arrangements used on a machine called a roll bender.

• Parts can be found in appliances, aircraft, and automobiles.

Thread Rolling

• Creating threads to make fasteners is one of the most common metal forming processes.

• External threading on the outside of a rod creates a bolt or screw. An internal thread forms a nut.

• Thread rolling is a chip less cold forming process that can be used to produce either straight or tapered threads.

Extrusion• Extrusion is a continuous pressure forming process much like squeezing

toothpaste out of a tube.• Metals such as aluminum, copper, magnesium, and stainless steel are

easily extruded.• Stationary mandrel method is used to make hollow tubing.• Impact extrusion is used to make collapsible tubes (used to dispense

glues and grease) and cans.• Hot extrusion is used to apply a plastic insulation coating to copper

electrical wire.• Metal products that are normally extruded include ladders, window

frames, lipstick cases, soft drink cans, and fire extinguishers.• Extrusion is gaining popularity with larger products such as refrigerators,

washing machines, and air conditioners.

Upsetting (cold forming)

• Cold forming is a type of forging that thickens or bulges the work piece while also shortening it by compression – it is a combination of forging and extrusion.

• It is used for high volume production of nails, bolts, and rivets.

• The equipment can take preforms, coiled wire, or bar stock and produce parts requiring no surface finishing (fig 14-16, page 194).

Stamping• Stamping is a cold forming process that uses a set of matched

molds in a stamping press to compact stock under pressure.• One of the largest products for stamped products is the

automotive industry.• The punches and die block assembly form the die set.• One of the major areas being worked on is reducing the time

it takes to change die sets.• Conventional die changes used to take several hours.• Acronym for improving die change time: SMED (single minute

exchange of dies).

Embossing

• Embossing is a process that produces raised areas on a flat sheet of metal.

• A common product application would be placing names and numbers on military dog tags or identification plates.

• Embossing can be done on a stamping press or punch press or rotary dies to continuously emboss patterns on thin metal foil or sheet stock.

Drawing and Expanding• Drawing involves both stretching and compressing.• It is used to make kitchen sinks, automobile fenders, and pots and pans.• Expanding is a process that can be used to increase the diameter of

tubular parts.• Before the work piece is expanded it should be softened or annealed.• If the part is to be expanded 30 percent or less of the diameter of the

work piece, the process can be conducted with one stroke. If the expansion is more than 30 percent several strokes are required with the part being annealed before each stroke.

Bulging and Necking• Bulging is another process that can be used to expand tubular

shapes such as corrugated tubes, bellows, or musical instruments.

• Necking is used to reduce the diameter of the end of a tubular part.

• Necking is a die reduction process that stretches the relatively soft and ductile metal part at the same time it reduces its cross sectional area.

• The necking process must be used with care as there is a point where increasing strain on the part will result in breaking it (breaking point or fracture strength).

Nosing, Forming, Contouring• Nosing is used to partially close the end of a tube.• This is a process that is particularly useful in making rifle and pistol

cartridges.• Electromagnetic Forming (magnetic pulse forming) is a process that forms

a work piece by using intense pulsating magnetic forces.• The process can also be used for internal forming by placing a coil inside

the tube causing the tube to expand. • The process is used extensively as an assembly technique to join tubular

parts to other components.• The process is used to assemble steering gear, coaxial cable and various

parts that would be difficult to assemble by other methods.• Contouring: Magnetic pulse forming can also be used for shaping flat

parts.

Peen Forming, Explosive Forming, HERF

• Peen Forming: Cold working process that is accomplished by bombarding the surface of a part with small spheres called shot.

• Used on parts such as oil well drilling equipment, turbine compressor blades, shafts, gears, and coil springs.

• Explosive Forming: When the charge is detonated, a shock wave forces the sheet stock against the forming die. The limit to the size of the work piece is restricted only by the size of the water tank.

• HERF (high energy rate forming): Use a spark generated shock wave rather than a chemical process.

• Spinning: It is a process that involves stretching the sheet stock over a rotating male or female mold.

• Parts such as flood lamp reflectors, bowls, and bells are normally spun using CNC equipment.

Casting• Cast iron: Gray iron has the lowest melting temperature and best

castability. It is used for machine beds and frames. • Ductile or nodular iron is used to cast axles, brackets, and crank shafts.• Casting Process: Powder metallurgy is a process that involves compacting

metallic powders in a permanent (reusable) mold.• There are 3 steps involved in manufacturing a powdered metal part:

powder mixing, compacting, and sintering.• Impregnation is the process of immersing porous powder metallurgy

parts in heated oil or resin after they are removed from the sintering oven. If the part is a bearing, impregnation provides a reservoir of lubricating oil.

Casting

• Die Casting: The molten metal is forced under high pressure into a cavity in the die.

• The dies are expensive, often costing from $10,000 to $100,000 or more to design and machine.

• Hand tools are often die cast.• Permanent Mold Casting: also known as gravity

casting. • This process is often used to make ornamental parts

and decorative items.

Casting• Sand Casting: Typical products include engine blocks and cylinder heads.• The mold that is created can be used only once since it is destroyed when

the part is removed after casting.• Investment Casting: It is used to produce intricate jewelry, artwork, and

ornate metal products.• A pattern is typically carved from wax or expanded polystyrene. The

pattern is dipped in a ceramic refractory slurry.• Centrifugal Casting: Used to mold cylindrical parts from plastics and

metals.• Process is used for pipes, tanks, and bearing rings.

Summary• Choosing a forming process depends on the type of metal, the number of pieces that need to

be made, and the cost. Another requirement is that the process must impart the desired mechanical properties.

• In forging, the shape of the raw material is changed by repositioning material rather than removing it. Today forging is used to produce bolts, rivets, connecting rods, gears, and structural members for equipment.

• Extrusion is a continuous pressure forming process much like squeezing toothpaste out of a tube. Metals such as aluminum, copper, magnesium, and stainless steel are easily extruded.

• Stamping is a cold forming process that uses a set of matched molds in a stamping press to compact stock under pressure. One of the largest products for stamped products is the automotive industry.

• The punches and die block assembly form the die set.• Electromagnetic Forming (magnetic pulse forming) is a process that forms a work piece by

using intense pulsating magnetic forces.• Centrifugal Casting: Used to mold cylindrical parts from plastics and metals. Process is used

for pipes, tanks, and bearing rings.

Home Work

• 1. What is stamping?• 2. What is a die set?• 3. What is electromagnetic forming? Give

examples of applications.