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Unit III Metal forming process

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Page 1: Metal Forming Process

Unit III

Metal forming process

Page 2: Metal Forming Process

Three Basic Bulk Deformation Processes

1. Rolling – slab or plate is squeezed between opposing rolls

2. Forging – work is squeezed and shaped between opposing dies

3. Extrusion – work is squeezed through a die opening, thereby taking the shape of the opening

Page 3: Metal Forming Process

Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls

The rolling process (specifically, flat rolling).

Rolling

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The Rolls Rotating rolls perform two main functions:

Pull the work into the gap between them by friction between workpart and rolls

Simultaneously squeeze the work to reduce its cross section

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Types of RollingBased on workpiece geometry :

Flat rolling - used to reduce thickness of a rectangular cross section

Shape rolling - square cross section is formed into a shape such as an I‑beam

Based on work temperature :Hot Rolling – most common due to the large

amount of deformation requiredCold rolling – produces finished sheet and

plate stock

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Some of the steel products made in a rolling mill.

Rolled Products Made of Steel

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Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features.

Diagram of Flat Rolling

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Rolling MillsEquipment is massive and expensiveRolling mill configurations:

Two-high – two opposing rollsThree-high – work passes through rolls in

both directionsFour-high – backing rolls support smaller

work rollsCluster mill – multiple backing rolls on

smaller rolls Tandem rolling mill – sequence of two-high

mills

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Various configurations of rolling mills: (a) 2‑high rolling mill.

Two-High Rolling Mill

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Various configurations of rolling mills: (b) 3‑high rolling mill.

Three-High Rolling Mill

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Various configurations of rolling mills: (c) four‑high rolling mill.

Four-High Rolling Mill

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Multiple backing rolls allow even smaller roll diameters

Various configurations of rolling mills: (d) cluster mill

Cluster Mill

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A series of rolling stands in sequence

Various configurations of rolling mills: (e) tandem rolling mill.

Tandem Rolling Mill

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Thread RollingBulk deformation process used to form threads

on cylindrical parts by rolling them between two dies

Important commercial process for mass producing bolts and screws

Performed by cold working in thread rolling machines

Advantages over thread cutting (machining):Higher production ratesBetter material utilizationStronger threads and better fatigue resistance

due to work hardening

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Figure 19.6 Thread rolling with flat dies:

Thread Rolling

(1) start of cycle (2) end of cycle

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Work Rest

Rolls

Work piece

Machined thread Rolled thread

Thread Rolling

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Forging Deformation process in which work is compressed

between two dies Oldest of the metal forming operations, dating from about 5000 B C

Components: engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts

Also, basic metals industries use forging to establish basic form of large parts that are subsequently machined to final shape and size . Forging today is done either with presses or with hammers powered by compressed air, electricity, hydraulics, or steam.

Forged parts can now range in weight from less than a kilogram to 170 metric tons

Forging can be broken down into two main types; cold and hot forging

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Hot Forging Hot forging is defined as working a metal above its

recrystallization temperature.Advantages Disadvantages

Decrease in yield strength, which means that it is easier to work.

Undesirable reactions between the metal and the surrounding atmosphere

Elevated temperatures increase diffusion between the metals

Less precise tolerances due to thermal contraction and warping from uneven cooling

Pores may reduce in size or close completely during deformation

Grain structure may vary throughout the metal

Increase in ductility

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Cold Forging Cold forging is defined as working a metal below its

recrystallization temperature, but usually around room temperature.

Advantages Disadvantages

Better surface finish Higher forces are required

Better reproducibility and interchangeability

Heavier and more powerful equipment and stronger tooling are required

No Heating required Metal is less ductile and Undesirable residual stress may be produced

Directional properties can be imparted into the metal

Metal surfaces must be clean and scale-free

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Forging operationDrawing out: In this operation in which the metal get

elongated with a reduction in the cross sectional area. For this purpose, the force is applied in the direction perpendicular to the length of the axis.

Upsetting: In this operation, cross sectional area of the work piece increases by decreasing length. For this purpose is force is applied in the direction parallel to the length of the axis

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Upsetting

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Types of Forging DiesSmith forging: Traditional operation, done by

manual hammering in open dies.Drop Forging or Impression : done by serious

of blows by means drop hammers -die contains cavity or impression that is imparted to work during this process flash is created.

Machine forging: in case of drop and press forging the material would be continuously drawn out where as in this case only upsetting take place.

Press forging : similar to the drop forging expect force is applied continuously (squeezing type).

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Open-Die Forging

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Drop forging or Impression-Die Forging

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Drop forging or Impression-Die Forging

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Drop forging It uses closed impression dies to get the desire shape of the component. The shaping is done by serious of hammering given to the material to

form a desired shape. The equipment used for this : Drop hammers.

Apply impact load against work.

Two types:Gravity drop hammers - impact energy from falling weight of a heavy

ram Power drop hammers - accelerate the ram by pressurized air or steam

Disadvantage: impact energy transmitted through anvil into floor of building

Commonly used for impression-die forging Die consists of two halves, lower half is fixed where as upper half is

movable. The heated stock is placed between the die.

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Diagram showing details of a drop hammer for impression‑die forging.

Drop Hammer Details

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Drop forging Cont…..The required impression are machined in the die.During hammering the between the stock is

compressed, because of continuous blows the metal will completely spread over the die

Complex shape can be possible.Depending upon the requirement of grain shape

and complex shape number of pass will be decided.Application: crank, connecting rod, crank shaft and

crane hook.Disadvantages: in a single stretch it is not possible

to get the final product and huge force will be transmitted to the building.

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Drop forging hammer, fed by conveyor and heating units at the right of the scene (photo courtesy of Chambersburg Engineering Company).

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Advantages and LimitationsAdvantages of impression-die forging

compared to machining from solid stock:Higher production ratesLess waste of metalGreater strengthFavorable grain orientation in the metal

Limitations:Not capable of close tolerancesMachining often required to achieve

accuracies and features needed

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Press forging Similar to the drop forging, it uses closed dies.

Incase of drop forging the force will be supplied as a serious of blows where as in this case the force will be applied as a squeezing force with the help of hydraulic operation.

Because of the continuous action of the hydraulic press the material gets uniformly deformed through out its entire depth.

Press forging Drop forgingThe entire force is

transmitted to the stock or work

The partial forces is transmitted to the building

The finish obtained in the product is fine

the finish of the product is not fine because of its

serious blow

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Machine forging Some times it is called upset forgingEven though the drop and press forge are

done by machine, historically upsetting process is called machine forging.

Normally used for making bolt heads.

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Figure 19.23 Examples of heading (upset forging) operations: (a) heading a nail using open dies, (b) round head formed by punch, (c) and (d) two common head styles for screws formed by die, (e) carriage bolt head formed by punch and die.

Heading (Upset Forging)

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Figure 19.22 An upset forging operation to form a head on a bolt or similar hardware item The cycle consists of: (1) wire stock is fed to the stop, (2) gripping dies close on the stock and the stop is retracted, (3) punch moves forward, (4) bottoms to form the head.

Upset Forging

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Other application of upset forging isMaking of gear, shaft, axles.Upsetting machines are in horizontal in

nature.In this process it is not possible to get the

final shape of the. product in a single pass. It just like drop forging process.

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Upsetting and HeadingForging process used to form heads on nails,

bolts, and similar hardware products More parts produced by upsetting than any

other forging operation Performed cold, warm, or hot on machines

called headers or formers Wire or bar stock is fed into machine, end is

headed, then piece is cut to length For bolts and screws, thread rolling is then

used to form threads

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ExtrusionCompression forming process in which work

metal is forced to flow through a die opening to produce a desired cross‑sectional shape

Process is similar to squeezing toothpaste out of a toothpaste tube

In general, extrusion is used to produce long parts of uniform cross sections

Two basic types:Direct extrusionIndirect extrusion

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Figure 19.30 Direct extrusion.

Direct Extrusion

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Comments on Direct Extrusion Also called forward extrusion As ram approaches die opening, a small

portion of billet remains that cannot be forced through die opening

This extra portion, called the butt, must be separated from extrudate by cutting it just beyond the die exit

Starting billet cross section usually roundFinal shape of extrudate is determined by

die opening

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Figure 19.31 (a) Direct extrusion to produce a hollow or semi‑hollow cross sections; (b) hollow and (c) semi‑hollow cross sections.

Hollow and Semi-Hollow Shapes

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Figure 19.32 Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section.

Indirect Extrusion

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Comments on Indirect Extrusion Also called backward extrusion and reverse

extrusion Limitations of indirect extrusion are

imposed by Lower rigidity of hollow ram Difficulty in supporting extruded product as it

exits die

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Advantages of ExtrusionVariety of shapes possible, especially

in hot extrusion Limitation: part cross section must be

uniform throughout length Grain structure and strength enhanced

in cold and warm extrusion Close tolerances possible, especially in

cold extrusion In some operations, little or no waste

of material

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Hot vs. Cold ExtrusionHot extrusion - prior heating of billet to

above its recrystallization temperature Reduces strength and increases ductility of

the metal, permitting more size reductions and more complex shapes

Cold extrusion - generally used to produce discrete parts The term impact extrusion is used to indicate

high speed cold extrusion

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Extrusion RatioAlso called the reduction ratio, it is defined

as

where rx = extrusion ratio; Ao = cross-sectional area of the starting billet; and Af = final cross-sectional area of the extruded section

Applies to both direct and indirect extrusion

f

ox A

Ar

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Figure 19.35 (a) Definition of die angle in direct extrusion; (b) effect of die angle on ram force.

Extrusion Die Features

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Comments on Die Angle Low die angle - surface area is large, which

increases friction at die‑billet interface Higher friction results in larger ram force

Large die angle - more turbulence in metal flow during reductionTurbulence increases ram force required

Optimum angle depends on work material, billet temperature, and lubrication

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Orifice Shape of Extrusion Die Simplest cross section shape is circular die

orifice Shape of die orifice affects ram pressure As cross section becomes more complex,

higher pressure and greater force are required

Effect of cross-sectional shape on pressure can be assessed by means the die shape factor Kx

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Figure 19.36 A complex extruded cross section for a heat sink (photo courtesy of Aluminum Company

of America)

Complex Cross Section

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Extrusion Cross Sections

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IntroductionThe Extrusion ProcessExtrusion PracticeHot ExtrusionCold ExtrusionImpact ExtrusionHydrostatic Extrusion

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Direct Extrusion

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Indirect extrusion and hydrostatic Extrusion

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Important process variables

Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.

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Extrusion Hot extrusion Cold

extrusion1. Forward 1. Forward Hydro static2. backward 2. Backward (a) Cold Extrusion

(b) Impact extrusion

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Comparison of forward and backwardForward hot extrusion Back ward hot extrusion

Flow of metal in the forward direction same as that of ram or plunger

Flow of metal in the forward direction opposite to the movement of ram or plunger

Mostly preferred because of its simple in construction

Not preferred that much because of complicated construction

The friction force in prevalent in case of direct extrusion because of relative movement between billet and cylinder

In this model it is avoided because die is connected with ram, no movement of billet.This is the advantage of indirect extrusion.

Extrusion pressure is not uniform Lubricant is necessary because while extruding steel material temperature are high.

Extrusion pressure is uniform because no movement of billet material. Lubricant is necessary because while extruding steel material temperature are high.

Oil and graphite can be used, if temp is high molten glass can be used.

The surface defect is the major problem, every material would coming out from the extrusion container.

Extrusion should be cooled quickly.

Extrusion should be cooled quickly.

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Forward cold extrusion Similar to the forward extrusion process Extrusion ratio are higher in hot forward

extrusion process where as extrusion ratio are very lower in forward cold extrusion process.

Since materials are heated at higher temperature higher extrusion ratio is possible.

Examples for forward cold extrusion is Aluminum brackets, cans, shock

absorber cylinder.

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Forward extrusion(Hydro static)

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Hydro static extrusion For special application this kind of process

will be employed.Metal is compressed in all direction by a

liquid rather than ram.Since liquid is present in the container no

need to have lubricant to cool the process.Material will compressed uniformly.Gray cast iron material can be

extruded(Brittle material) SAE 30 or castor oil with 10 % alcohol,

glycerin, and ethyl alcohol.

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Hydro static extrusion Cont……The hydro static pressure range from 1110

to 3150 MPa.Application:Making wires for less ductile materialMaking reactor fuel rods.

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Impact extrusion (Backward)

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Impact extrusionMost common process for extruding soft

materials.Also called backward extrusion process.The slug for making the component is kept

on the die and punch strikes the slug.Because of the impact force, side wall go

straight along the punch.The height depends upon the height of the

slug.Commonly used for making collapsible tubes

and containers.

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Cold extrusion Forging (Backward extrusion )

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Cold extrusion ForgingSimilar to the impact extrusion process.The height of the slug will be higher incase

of impact extrusion where as in this case height will be lower and thickness will be higher.

In all conventional extrusion process the area of cross section of the extruded part will be the same.

In this case different area cross section can be possible to made.

Suitable for non ferrous alloys.

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Extruding tubes

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Extruding tubesThis is also one type of hot extrusion

process, for making tubes. Double action ram can be used for making

tubes .In this model the solid ram moves towards

the billet material and than plunger ram will be moved to form a complete required shape.

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Wire drawing die

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Wire drawing dieWire means small diameter and uniform

cross section.Rod means

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