Comprehensive Notes on Metal Forming Operations

Extrusion

• Extrusion is a process where metal is forced to flow through a shape-forming die.
• The metal deforms due to compressive and shear forces applied at the inlet end.
• Advantage: High compressive and shear forces without tensile forces prevent cracking.
• Extrusion is a hot working process, utilizing the material's deformability.
• Difference between extrusion and drawing: Extrusion uses compressive forces at the entry side, while drawing applies forces to pull the material through the die.

Principle of Extrusion

• Primary shaping process: Metal in a cylinder is forced through a die opening.
• The metal undergoes plastic deformation, resulting in reduction and elongation.
• Types:
  • Hot Extrusion
  • Cold Extrusion

Hot and Cold Extrusion

• Hot extrusion: Carried out at high temperatures, approximately 50-70% of the metal's melting point.
• Cold extrusion: Carried out at room temperature or slightly above.
• Materials like lead, tin, and aluminum are easily extruded.

Types of Hot Extrusion

• Forward Extrusion/Direct Extrusion
• Backward Extrusion/Indirect Extrusion

Forward Extrusion (Direct Extrusion)

• Most common extrusion process.
• The billet is placed in a heavy-walled container and pushed through the die by a ram or screw.
• A reusable dummy block separates the ram and billet.
• Disadvantage: Higher force is required due to friction between the billet and the container.
• Greatest force is needed at the beginning, decreasing as the billet is used up.
• Force increases at the end as material flows radially to exit the die; the butt end is typically not used.

Backward Extrusion (Indirect Extrusion)

• The billet and container move together, while the die is stationary.
• The die is held by a stem that must be longer than the container.
• Maximum extrusion length is limited by the stem's column strength.
• Frictional forces are eliminated because the billet moves with the container.

Cold Extrusion: Impact Extrusion

• Used for small workpieces from ductile materials.
• Material is placed in a blind die, and a ram with clearance forces the metal to flow plastically around the punch.
• Applications: Collapsible medicine tubes and toothpaste tubes.
• Restricted to soft metals like lead, aluminum, copper, tin, etc.

Hydrostatic Extrusion

• The billet is completely surrounded by pressurized liquid, except where it contacts the die.
• Can be done hot, warm, or cold, but temperature is limited by the fluid's stability.
• Requires a sealed cylinder to contain the hydrostatic medium.

Force vs. Ram Displacement

• Comparison of force requirements for direct, indirect, and hydrostatic extrusion.

Hydraulic Presses

• Utilizes Pascal's principle: constant pressure throughout a closed system.
• A piston acts as a pump, applying force on a small area; another piston with a larger area generates a proportionally large mechanical force.
• Small-diameter tubing is sufficient if the pump is separate from the press cylinder.
• Fluid pressure closes the mold and forms the part.

Pneumatic Power Hammer

• Consists of a frame, anvil, and reciprocating ram with a hammer head or die.
• The workpiece is placed on the anvil, and the hammer strikes the work piece.
• Stores potential energy in mechanical linkages, springs, or compressed air, accelerating the ram on the downward stroke.
• Provides more force than simply allowing the weight to fall.

Ironing

• Smoothing and thinning the wall of a shell or cup (cold or hot) by forcing it through a die with a punch.
• Equipment: Mechanical and hydraulic presses.
• Materials: Carbon and alloy steels, aluminum and aluminum alloys, titanium alloys.
• Applications: Shells and cups for various machines.

Swaging

• Process that reduces/increases the diameter, tapers rods, or points round bars or tubes by external hammering.

Cold-Working Operations

• Bending
• Shearing
• Drawing
• Presses
• Rolling
• Cold Forging
• Thread Rolling
• Coining

Rolling

• Used for sheets, strips, bars, and rods to obtain smooth surfaces and accurate dimensions.
• Metal stock is passed through one or more pairs of rolls to reduce and uniform the thickness.
• Subjects the metal to high compressive stresses due to friction between the rolls and the metal surface.

Cold Forging

• Deforms metal below its recrystallization point, generally preferred for soft metals like aluminum.
• Less expensive than hot forging, requires little finishing work.
• Slugs of material are squeezed into shaped die cavities to produce finished parts of precise shape and size.
• Example: Cost savings in material for cold-formed parts compared to machined parts.

Thread Rolling

• Metal rolling process used extensively in manufacturing to produce screws, bolts, and other fasteners.
• Threads are formed into the metal of a blank by pressing and rolling action between two dies.
• Die surfaces hold the shape, and the force forms the threads into the material.

Bending

• Manufacturing process that produces a V-shape, U-shape, or channel shape along a straight axis in ductile materials, most commonly sheet metal.
• Equipment: Box and pan brakes, brake presses, and other specialized machine presses.

Shearing

• Cuts stock without forming chips or using burning or melting.
• Straight blades: Shearing; curved blades: Shearing-type operations.
• Materials: Sheet metal, plates, and rods.

Coining

• Closed die forging process where pressure is applied to obtain closer tolerances and smoother surfaces.
• May be done in hot or cold working conditions, predominantly a cold work process.
• Used to form indentations and raised sections, intentionally thinning or thickening metal.

Advantages of Coining

• Produces finer and detailed surface finish.
• Provides accurate bends consistently.
• Does not require expensive machinery.
• Work-hardened surface resists impact and abrasion.
• Eliminates complex finishing processes.
• Plastic flow reduces surface grain size and work hardens the surface.

Industrial Applications of Coining

• Minting of coins and medallions
• Making jewelry
• Making badges, buttons, precision-energy springs
• Making complex electronic parts
• Making precision parts with finer polished surface finishes.

Drawing

• Metalworking process using tensile forces to stretch metal into a desired shape and thickness.
• Two types:
  • Sheet metal drawing
  • Wire, bar, and tube drawing.

Wire Drawing

• Produces rods/bars from metallic or polymeric material.

Deep Drawing

• Process involving a punch, blank holder, die, and flange to form a bottomed part.

Press Forging

• Applies continuous pressure or force, unlike the instantaneous impact of drop-hammer forging.
• Dies are in contact with the work piece for seconds, compared to milliseconds in drop-hammer forges.
• Can be done either cold or hot.

Comparison: Press Forging vs. Drop Forging

• Press forging: Entire force is transmitted to the stock, resulting in a finer finish.
• Drop forging: Partial forces transmitted to the building, resulting in a less fine finish.

Defects in Forging

• Cold Shut or Lap
• Cracks
• Scale Pits
• Unfilled Sections
• Mismatch Sections
• Ruptured Fibers Flow lines
• Fins and Rags
• Burnt and Overheated Metal

Grain Structure

• Forging provides true grain flow, unlike casting (no grain flow) or machining (grain flow broken).

Rolling

• Process used in sheets, strips, bars, and rods to obtain smooth surfaces and accurate dimensions.
• Metal is subjected to high compressive stresses due to friction between rolls and metal surface.

Rolling Process Use

• Most common industrial process for making large length cross sections like sheets and plates of steel and aluminum.
• Rolling mills are used, available in different shapes and sizes.
• Every rolling mill consists of a minimum of two rolls.

Types of Rolling Mills

• Two High Rolling Mill
• Three High Rolling Mill
• Four High Rolling Mill
• Tandem Rolling Mill
• Cluster Rolling Mill
• Planetary Rolling Mill

Two High Rolling Mill

• Consists of two rollers arranged to rotate in opposite directions.
• The workpiece is fed between the rollers, which apply a compressive force.
• Two types:
  • Non-Reversible: Rollers rotate in one direction only.
  • Reversible: Rollers can rotate in both directions.

Three High Rolling Mill

• Consists of three rollers arranged parallel to each other.
• The middle roller rotates in the opposite direction of the other two.
• Machine is used to reduce thickness two times in one pass.

Four High Rolling Mill

• Consists of four rollers (two small and two big).
• Small rollers are in direct contact with the workpiece.
• Big rollers act as backup rolls.
• Used for cold rolling where high rigidity is required.

Tandem Rolling Mill

• Also known as continuous rolling.
• Two or more sets of four high rolling mills are arranged in a series.
• Stands are arranged so they can roll the workpiece in decreasing cross section.
• Used for rolling thick workpieces in a single pass.

Cluster Rolling Mill

• Each working roller is backed up by two or more larger backup rollers.
• Used for rolling hard material.

Planetary Rolling Mill

• A large backup roller is surrounded by planetary working rollers.
• Each planetary roll gives constant reduction.
• Used to reduce large thickness in a single pass of steel strip.

Hot Rolling

• Occurs above the recrystallization temperature of the material.
• Used mainly to produce sheet metal or simple cross sections.
• Reduces the average grain size of metal, improving the strength of material.

Cold Rolling

• Used to produce sheet & strip with superior surface finish and dimensional tolerance.
• Strain hardening results from the cold reduction, giving increased strength.
• Done usually at room temperature.
• Four-high or cluster mills are used (Al, Cu alloys).
• Increases strength via strain hardening.

Maximum Thickness Reduction in One Pass

• $H_{max} = μ^2R$
• Where:
  • $H_{max}$ = Maximum thickness reduction in one pass
  • $μ$ = Static friction coefficient between rolls and metal
  • $R$ = Radius of rolls

Comparison Between Hot and Cold Rolling

• Hot Rolling
  • Blowholes and porosity eliminates by welding together.
  • Results into better ductility and toughness.
  • Surface decarburization of steels.
  • Not good surface finish.
  • Dimensional tolerance expansion/due to contraction of metal.
• Cold Rolling
  • Results in increased strength or hardness.
  • Better dimensional control.
  • Good surface finish.
  • Ductility decreases.
  • Yield point phenomenon (results in inhomogeneous deformation), occur in annealed steel sheet.

Types of Rolling Processes

• Continuous Rolling
• Transverse Rolling
• Section or Shape Rolling
• Ring Rolling
• Thread Rolling

Continuous Rolling

• Objective: Decrease the thickness of the metal with an increase in length and with little increase in width.

Transverse Rolling

• Using circular wedge rolls.
• Heated bar is cropped to length and fed in transversely between rolls.
• Rolls are revolved in one direction.

Section or Shape Rolling

• Flat slap is progressively bent into complex shapes.
• Suitable for producing molded sections such as irregular shaped channels and trim.

Ring Rolling

• Specialized type of hot rolling that increases the diameter of a ring.
• The resulting grain structure is circumferential, giving better mechanical properties.

Thread Rolling

• Rolled threads are produced in a single pass at speeds far in excess of those used to cut threads.
• Dies are pressed against the surface of cylindrical blank.
• As the blank rolls against the in-feeding die faces, the material is displaced to form the roots of the thread, and the displaced material flows radially outward to form the thread's crest.

Terminology

• Bloom: Product of first breakdown of ingot (cross sectional area > $230 cm^2$).
• Billet: Product obtained from further reduction by hot rolling (cross sectional area > $40x40 mm^2$).
• Slab: Hot rolled ingot (cross sectional area > $100 cm^2$ and width ≥ 2 x thickness).

Plate, Sheet, Strip

• Plate: Product with a thickness > 6 mm.
• Sheet: Product with a thickness < 6 mm and width > 600 mm.
• Strip: Product with a thickness < 6 mm and width < 600 mm.

Shape Rolling Operation

• Structural shape rolling is the rolling and roll forming of structural shapes by passing them through a rolling mill while maintaining a constant cross-section.
• Structural shapes: I-beams, H-beams, T-beams, U-beams, angle iron, channels, bar stock, and railroad rails.

Principle of Wire Drawing

• Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling it through a single or series of drawing die(s).
• Applications: Electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for wheels, and stringed musical instruments.
• Different from extrusion: Wire is pulled, rather than pushed, through the die.

Tube Drawing

• Metalworking process to size a tube by shrinking a large diameter tube into a smaller one by drawing it through a die.
• Produces high-quality tubing with precise dimensions, good surface finish, and the added strength of cold working.
• Suitable for both large- and small-scale production.

Types of Tube Drawing

• Tube sinking
• Mandrel drawing/ Rod drawing
• Stationary mandrel/ Fixed plug drawing
• Floating mandrel/ Floating plug drawing

Tube sinking

• Also known as free tube drawing, reduces the diameter of the tube without a mandrel inside.
• Especially on thick-walled tubes and tubes smaller than 12 mm.

Rod drawing/Mandrel drawing

• Draws the tube with a mandrel inside; the mandrel is drawn with the tube.
• Advantage: Mandrel defines the ID and surface finish has a quick setup time for short runs.
• Disadvantages: Lengths are limited and a second operation is required to remove the mandrel.

Fixed plug drawing/ Stationary mandrel

• Uses a mandrel at the end of the die to shape the ID of the tube.
• Slow and has limited area reductions, but gives the best inner surface finish.

Floating plug drawing/ Floating mandrel

• Uses a mandrel not anchored to shape the ID of the tube.
• Greatest advantage: Can be used on extremely long lengths, sometimes up to 1,000 feet.
• Used for oil-well tubing

Metal Forming

• Fashioning metal parts and objects through mechanical deformation. Workpiece is reshaped without adding or removing material, and its mass remains unchanged.
• Operates on the principle of plastic deformation, where the physical shape of a material is permanently deformed.

Metal Forming Processes

• Forging
• Rolling
• Extrusion
• Drawing
• Pressing
• Can be done hot, warm, or cold working.

Advantages of Metal Forming

• Quick and economical.
• Deformation occurs with less force.
• Mechanical properties such as ductility, elongation, and toughness can be improved.
• Suitable for nearly all commercial metals.
• Blow holes and cracks are eliminated.

Hot Working

• Plastic deformation of metal above its recrystallization temperature but below the melting point.
• New crystals start forming when atoms reach a certain higher energy level under heat and force (Recrystallization).
• Keeps materials from strain hardening, maintaining yield strength, hardness, and high ductility.

Warm Working

• Deformation carried between hot and cold working.
• Deforming metal heated to a temperature that maximizes the material's malleability without recrystallization, grain growth, or metallurgical fracture.
• Temperatures can range from 200-850 degrees C.

Advantages of Warm Working

• Equipment works under lighter loads.
• Metal ductility is more.
• Less strain hardening
• Less heat energy is required.
• Good precision is achieved.
• Good surface finish.

Cold Working

• Plastic deformation of metal below its recrystallization temperature at room temperature (or above room temperature).

Advantages of Cold Working

• No heating is required.
• Strength, fatigue, and wear properties are improved.
• Better surface finish is obtained.

Disadvantages of Cold Working

• Higher forces are required.
• Less ductility is available.
• More powerful equipment is required.
• Metal surfaces must be clean and scale-free.

Recrystallization

• Temperature at which the first new grains appear.
• Requires realignment or diffusion in materials.
• Process by which deformed grains are replaced by new, defect-free grains.
• Usually accompanied by reduced strength and hardness, and a simultaneous increase in ductility.

Strain Hardening

• Also known as work hardening, strengthening metal by plastic deformation.
• This occurs because of dislocation movements and dislocation generation.

Hot vs. Cold Working Processes

• Hot Working
  • Processing done at the temperature above recrystallization
  • Forces required are lower
  • New crystals are formed due to recrystallization
  • Blow holes, porosity, and cracks are eliminated due to the high working pressure.
  • Difficulty in handling; require heat resistant tools
  • Dimensional accuracy is poor
  • Oxidation and scaling result in a poor appearance.
  • Thin gauge sheet and fine wires cannot be produced.
  • Cold Working
  • Processing done at the temperature below recrystallization
  • Forces required are higher
  • Recrystallization does not occur
  • Excessive cold working can lead to production and propagation of cracks
  • No such requirement
  • Dimensional accuracy is much better
  • Finish obtained is much better
  • Even very thin sheets and fine wires can be produced.

Classification of Mechanical Working of Metals

• Hot working
  • Forging
  • Rolling
  • Extrusion
  • Spinning
  • Drawing
  • Tube Piercing
• Cold working
  • Drawing
  • Coining
  • Sizing
  • Extrusion
  • Squeezing
  • Bending
  • Shearing

Hot-Working Operations

• Forging
  • Open die forging
  • Closed die forging
  • Impression die forging
• Rolling
• Extrusion
• Spinning
• Drawing
• Tube Piercing

Forging

• A manufacturing process involving the shaping of metal using localized compressive forces.
• Classified according to temperature:
  • Cold forging
  • Warm forging
  • Hot forging.

Forging Temperatures

• Hot forging: Temperatures above recrystallization temperatures (0.6 Tm or above).
• Warm forging: Temperature range of 0.3 Tm to 0.5 Tm.
• Cold forging: Advantages include good surface finish, high strength, and greater accuracy.

Forging Applications and Advantages

• Applications: Bolts, disks, gears, turbine disks, crankshafts, connecting rods, valve bodies, small components for hydraulic circuits. etc.
• Advantages: Closer dimensional accuracies, material saving, higher strength, greater productivity, favorable grain orientation, high degree of surface finish.

Forging Process

• Open die forging
• Closed die forging
• Impression die forging

Forging Tools

• Tongs
• Hot chisel
• Round punch
• Cold chisel
• Anvil
• Hammer

Open Die Forging

• Deforming a piece of metal between multiple dies that do not completely enclose the material.
• Metal is altered as the dies "hammer" or "stamp" through a series of movements until the desired shape is achieved.
• Equipment: Hydraulic presses, hammers.
• Materials: Carbon and alloy steels, aluminum alloys, copper alloys, titanium alloys, all forgeable materials.

Closed Die Forging

• A billet is formed in dies (usually with two halves) such that the flow of metal from the die cavity is restricted.
• Excess material is extruded through a restrictive narrow gap and appears as flash around the forging at the die parting line.
• Materials: Carbon and alloy steels, aluminum alloys, copper alloys, magnesium alloys, beryllium, stainless steels, nickel alloys, titanium and titanium alloys, iron, nickel, and cobalt superalloys.

Impression Die-Forging

• Heated workpiece is placed between two shaped dies and is pressed or hammered.
• During hammering or pressuring, it takes the shape of the die.
• A small amount of materials is forced outside the die impression, which is then trimmed.

Comparison of Open-Die and Closed-Die Forging

• Open Die Forging
  *Hot forging process in which metal is shaped by hammering or pressing between flat or simple contoured dies
  *Small mass production
  *low production efficiency
  *high labour intensity
  *Hydraulic presses and Hammers Equipment
  *Using Carbon and alloy steels and aluminium alloys Material
  *Forging ingots, large and bulky forgings application
• Closed Die Forging
  *a billet is formed (hot)in dies (usually with two halves)
  *heavy mass production
  *High production efficiency
  *Low labour intensity
  *Carbon and alloy steels Equipment
  *Copper alloys and many more Materials
  *automibiles and more applications appilcation

Upset Forging

• Increases the diameter of the workpiece by compressing its length.
• Examples: Engine valves, couplings, bolts, screws, and other fasteners.

Roll Die Forging

• Reduces the thickness and increases the length of round or flat bar stock.
• Uses two cylindrical or semi-cylindrical rolls with shaped grooves.

Drop Forging

• A forging process where a hammer is raised and then "dropped" onto the workpiece to deform it according to the shape of the die.

Hot Spinning

• Metalworking process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part.
• Performed by hand or by a CNC lathe.

Piercing

• Used to create a cavity or hole in the workpiece.
• A shearing process where a punch and die are used to create a hole in sheet metal or plate.

Ring Rolling

• Metal rolling in which a ring of smaller diameter is rolled into a precise ring of larger diameter and reduced cross section using two rollers acting on either side of the ring's cross section.

Skew Rolling

• Metal forging process that used two designed opposing rolls.
• Round stock is fed into rotating rolls;Forged by grooves;emerges as metal ball.