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Source: George E. DieterPresentation by HP
Objectives:
Introduction
Classification of extrusion processes
Extrusion equipment (Presses, dies and tools)
Hot extrusion
Deformation, lubrication, and defects in extrusion
Analysis of the extrusion process
Cold extrusion and cold-forming
Hydrostatic extrusion
Extrusion of tubing
Production of seamless pipe and tubing
What is extrusion?• It is the process by which a block of metal isreduced in cross-section by forcing it to flowthrough a die orifice under high pressure
The products of extrusion are generally called "extrudates".
Extruded partBilletRam
Die
ContainerDummy plate
(extrusion pad)
Process• A heated billet cut from continuous casting is located in a
heated container.
• By applying pressure by means of a ram to one end of thebillet the metal flows through the die, located at the other endof the container to produce a section, the cross sectionalshape of which is defined by the shape of the die
Extruded partBilletRam
Die
ContainerDummy plate
(extrusion pad)
• In general, extrusion is used to produce cylindrical barsor hollow tubes or for the starting stock for drawing rod,cold extrusion or forged products.
• Most metals are hot extruded due to large amount offorces required in extrusion.
• Complex shape can be extruded from the more readilyextrudable metals such as aluminium.
• If better properties are required then it may be heattreated or cold worked
Extruded partBilletRam
Die
• The reaction of the extrusion billet with the container and dieresults in high compressive stresses which are effective inreducing cracking of materials during primary breakdownfrom the ingot.
Extruded partBillet
Die
Pressure
Compressive stresses will help in increasing the utilisation ofextrusion in the working of metals that are difficult to form likestainless steels, nickel-based alloys, and other high-temperaturematerials.
Cold ExtrusionCold extrusion is the process done at room temperature or
slightly elevated temperatures.
This process can be used for most materials-subject to
designing robust enough tooling that can withstand the
stresses created by extrusion.
Examples of the metals that can be extruded are lead, tin,aluminium alloys, copper, titanium, molybdenum, vanadium,steel. Examples of parts that are cold extruded are collapsibletubes, aluminium cans, cylinders, gear blanks.
Advantages:
No oxidation takes place. Good mechanical properties due to severe cold working as long as
the temperatures created are below the recrystallizationtemperature.
Good surface finish with the use of proper lubricants.
Hot extrusion
Hot extrusion is done at fairly high temperaturesapproximately 50 to 75 % of the melting point of the metal.• The pressures can range from 35-700 MPa• The most commonly used extrusion process is the hot direct
process.• The cross-sectional shape of the extrusion is defined by the
shape of the die.
• Due to the high temperatures and pressures and itsdetrimental effect on the die life as well as othercomponents, good lubrication is necessary.
• Oil and graphite work at lower temperatures, whereas athigher temperatures glass powder is used.
Cold extrusion
• It is done at room temp ornear room temp
• The advantages of thisover hot extrusion are thelack of oxidation, higherstrength due to coldworking, closer tolerances,better surface finish, andfast extrusion
Hot extrusion
• It is fairly done at high temp(0.5-0.7 Tm) (aboverecrystallization temp)
• Good lubrication is neededsince it works at elevatedtemp (glass powders areused)
• Due to the high temperaturesand pressures and itsdetrimental effect on the dielife as well as othercomponents.
• So, good lubrication isnecessary.
Extrusion of lead sheath on electrical cable
• Typical parts produced by extrusion are trim parts usedin automotive and construction applications, windowframe members, railings, aircraft structural parts.
Example: Aluminium extrusions are used in commercial and
domestic buildings for window and door frame systems,
prefabricated houses/building structures, roofing and
exterior cladding, curtain walling, shop fronts, etc.
Furthermore, extrusions are also used in transport for
airframes, road and rail vehicles and in marine applications.
Classification of extrusion processes
There are several ways to classify metal extrusion processes
By Direction
• Direct / Indirect extrusion• Forward / backward extrusion
By temperature Hot / cold extrusion
By equipment Horizontal and vertical extrusion
Methods of extrusion
Direct/forward
Indirect/backward
Lateral
Extrusion forging
Impact Extrusion
Hooker extrusion
Hydrostatic extrusion
Direct/forward extrusionThe metal billet is placed in a container and driven through the die
by the ram.
The dummy block or pressure plate, is placed at the end of the ramin contact with the billet.
Friction is at the die and container wall requires higher pressurethan indirect extrusion.
Extruded partBilletRam
Die
ContainerDummy plate
(extrusion pad)
1 Extrusion
2 Die backer
3 Die
4 Billet
5 Dummy block
6 Pressing ram
7 Container liner
8 Container body
Process:Hot metal billet is placed in the container
Compressive forces are applied on the metal billet by ram(hydraulically driven)
Compressive forces will make the metal advance in thecontainer, and then through the die opening
Hot metal is placed in the container
Metal is pressed by Ram
Metal will advance in container, and then
through die
Metal will flow through the die opening
Extruded metal will further be processed
Extruded partBillet
Die
ContainerClosure plate
Ram
Indirect extrusion
The hollow ram containing the die is kept stationary and the containerwith the billet is caused to move.
Friction at the die only (no relative movement at the container wall)requires roughly constant pressure.
Hollow ram limits the applied load.
Typical curves of extrusion Vs. ram travel for direct andindirect extrusion
Lateral extrusion
• Container is in verticalposition and the die islocated in the side
• The metal is kept in thecontainer such that thevertical ram applies force onthe metal
• The extruded part comesout from the bottom die
• This is suitable for very lightalloys that have low meltingpoints
Equal channel angular extrusion• Equal channel angular extrusion (ECAE) is an extrusion process is
developed to refine the microstructure of metals and alloys,thereby improving their strength according to the Hall-Petchrelationship.
• It is also known as “ECAP (Equal Channel Angular Pressing)”
• ECAE is unique because significant cold work can beaccomplished without reduction in the cross sectional area of thedeformed workpiece.
• In conventional deformation processes like rolling, forging,extrusion, and drawing, strain is introduced by reduction in thecross sectional area.
• ECAE produces significant deformation strain without reducingthe cross sectional area.
• This is accomplished by extruding the work piece around acorner.
• For example, a square cross section bar of metal is forced through achannel with a 90o angle.
• The cross section of the channel is equal on entry and exit.
• The complex deformation of the metal as it flows around thecorner produces very high strain.
• Because the cross section remains the same, a work piece can beextruded multiple times with each pass introducing additionalstrain.
Extrusion-forging• It’s a combination of
extrusion and forging
• Metal is extrudedthrough die whilebeing forged
• Used for poppet valvesin IC engines
Punch
Hot metal
Die holder
Die holder
Punch
Completion of extrusion forging process
Impact Extrusion• Impact extrusion is a cold manufacturing process similar
to extrusion and drawing by which products are made with a metal slug.
• The slug is pressed at a high velocity with extreme force into a die/mould by apunch.
• Process is restricted to soft metals like lead, aluminum, and copper
It uses heavy duty mechanical
presses
Metal collapsible tubes,
disposable tubes, ointment
tubes, deodorant bottles are
produced by this process
Impact Extrusion
Advantages
• Simple and veryeconomical
• Suitable for collapsibletubes
• Production cost very low
• Excellent surface finish
• Fast production rates
Limitations
• Limited to soft metals likePb, Al, and Cu
• More wear
• Feeding lubricant is bitdifficult since it splashesout
Hooker Extrusion
Hydrostatic Extrusion
• In this process the billet is completely surrounded by apressurized liquid, except where the billet contacts the die.
• This process can be done hot, warm, or cold, however thetemperature is limited by the stability of the fluid used.
• The process must be carried out in a sealed cylinder tocontain the hydrostatic medium.
• The fluid can be pressurized in two ways:• Constant-rate extrusion: A ram or plunger is used to pressurize the
fluid inside the container.
• Constant-pressure extrusion: A pump is used, possibly with a pressureintensifier, to pressurize the fluid, which is then pumped to thecontainer.
Pressure is applied through a fluid surrounding the billetFluid pressure forces the billet into dieDie compresses the metal with very less friction
Hydrostatic ExtrusionAdvantages
• No friction between thecontainer and the billet reducesforce requirements.
• This ultimately allows for fasterspeeds, higher reduction ratios,and lower billet temperatures.
• Usually the ductility of thematerial increases when highpressures are applied.
• An even flow of material.
• Large billets and large cross-sections can be extruded.
• No billet residue is left on thecontainer walls.
Limitations
• The billets have to beprepared by tapering oneend so that it matches thedie entry angle.
• Only cold extrusion ispossible
• It can be difficult to containthe fluid, under the effectsof high pressures (up to 2GPa).
Extrusion of seamless tubes(With a fixed mandrel)
BilletRam
Die
ContainerDummy plate
(extrusion pad)
Mandrel
Extrusion of seamless tubes(With a floating mandrel)
BilletRam
Die
ContainerDummy plate
(extrusion pad)
Mandrel
• Both the operations can be carried out with hallowbillet or solid one
• Solid billets are always preferable since creating ahallow billet consumes time and wastes morematerial
BilletRam
Die
ContainerDummy plate
(extrusion pad)
Mandrel
Imp extrusion variables1) Yield strength of the material (temp increases
yield strength decreases thereby extrusion forcedecreases )
2) The type of extrusion
3) Extrusion Ratio
4) The working temp
5) The speed of deformation
6) Frictional conditions
• Extrusion Ratio: it is the ratioof the cross-sectional area ofthe billet (Ao) to final cross-sectional area after extrusion(Af)
𝑅 =𝐴𝑜
𝐴𝑓
• Extrusion ratios reach about40:1 for hot extrusion of steeland may be as high as 400:1 forAl.
• Extrusion ratios will increasewith increase in temp
Extrusion Circumscribing circle
CCD = Circumscribing
Circle Diameter is the
diameter of the smallest circle
into which the extruded part can
fit
• CCD will increase when
components are big irregular
• CCD of the square or
rectangular section is just
connecting the diagonals
Ram Speed:• Increasing ram speed produces an increase in theextrusion pressure
• A tenfold increase in the speed results in about a 50%increase in pressure
• Greater cooling of the billet occurs at low extrusionspeeds
• When the ram moves slowly, billet cools fast and thepressure required for direct extrusion will increase
Extrusion pressure
is the extrusion
force divided by
the cross-sectional
area of the billet.
Rapid rise in ‘P’
during initial ram
travel is due to the
initial compression
of the billet to fill
the container
For direct
extrusion the metal
begins to flow
through the die at
the maximum
value of pressure
(breakthrough
pressure)
Effect of temp:
• Decreased flow stress or deformation resistance due toincreasing extrusion temperature.
• Use minimum temperature to provide metal withsuitable plasticity.
• The top working temperature should be safely belowthe melting point or hot-shortness range.
• Oxidation of billet and extrusion tools.• Softening of dies and tools.• Difficult to provide adequate lubrication.
The temperature of the workpiece inmetal working depends on:
1. The initial temperature of the tools and thematerials
2. Heat generated due to plastic deformation
3. Heat generated by friction at the die/materialinterface (highest)
4. Heat transfer between the deforming materialand the dies and surrounding environment
Relationships between extrusion ratio, temperature and pressure
• For a given extrusion pressure, extrusion ratio Rincreases with increasing Extrusion temperature.
• For a given extrusion temperature, a largerextrusion ratio R can be obtained with a higherextrusion pressure.
Extrusion temperature
Extrusion pressure
Extrusion Ratio (R)
Die design
• Die design is at the heart of efficient extrusion production.
• Dies must withstand considerable amount of stresses,thermal shock, and oxidation.
Die Design considerations:
• Wall thickness: different wall thicknesses in one sectionshould be avoided.
• Simple shapes: the more simple shape the more costeffective.
• Symmetrical:more accurate.
• Sharp or rounded corners: sharp corners should be avoided.
• Size to weight ratio
• Tolerances: tolerances are added to allow some distortions
Die materialsDies are made from highly alloy tools steels or ceramics
(zirconia, Si3N4 ). (for cold extrusion offering longer tool lifeand reduced lubricant used, good wear resistance).
Wall thickness as small as 0.5 mm (on flat dies) or 0.7 mm(on hollow dies) can be made for aluminium extrusion.
Heat treatments such as nitriding are required (severaltimes) to increase hardness (1000-1100 Hv or 65-70 HRC).
This improves die life. Avoiding unscheduled pressshutdown.
• Metal entering the die willform a dead zone andshears internally to form itsown die angle.
• A parallel land on the exitside of the die helpsstrengthen the die andallow for reworking of theflat face on the entranceside of the die withoutincreasing the exit diameter.
• Requires good lubricants.• Decreasing die angle increasing
homogeneity, lower extrusionpressure (but beyond a point thefriction in the die surfacesbecomes too great.
• For most operation, 45o < α < 60o
Flat-faced dies Dies with conical entrance angle
• The die stack consists of the die, which is supported by a die holderand a bolster, all of which are held in a die head.
• The entire assembly is sealed against the container on a conicalseating surface by pressure applied by a wedge.
• A liner is shrunk in a more massive container to withstand highpressures.
• The follower pad is placed between the hot billet and the ram forprotection purpose. Follower pads are therefore replacedperiodically since they are subject to many cycles of thermal shock.
(a) Low container friction and a well-lubricated billet – nearly homogeneous
deformation.
b) Increased container wall friction, producing a dead zone of stagnant metal at corners
which undergoes little deformation. Essentially pure elongation in the centre and
extensive shear along the sides of the billet. The latter leads to redundant work
c) For high friction at the container-billet interface, metal flow is concentrated toward the
centre and an internal shear plane develops – due to cold container. In the sticky friction,
the metal will separate internally along the shear zone. A thin skin will be left in a
container and a new metal surface is obtained.
d) Low container friction and a well lubricated billet in indirect extrusion.
Extrusion Defects• Defects in extruded products occur predominantly
due to friction and non-homogeneous materialflow.
• Further, temperature variations across the billetduring hot extrusion can also lead toinhomogeneous deformation.
• Three types of defects are prominent in extrusion.
• They are:
1. Inhomogeneous deformation
2. Surface cracks
3. Internal cracks
• Inhomogeneous deformation in direct extrusionprovide the dead zone along the outer surface of the billetdue to the movement of the metal in the centre beinghigher than the periphery.
• After 2/3 of the billet is extruded, the outer surface of thebillet (normally with oxidised skin) moves toward thecentre and extrudes to the through the die, resulting ininternal oxide stringers transverse section can be seen as anannular ring of oxide.
Container wall
friction
Container wall
temp
Extrusion Defects
Extrusion Defects
• Surface cracking, ranging from a badly roughened surfaceto repetitive transverse cracking called fir-tree cracking, seeFig.
• This is due to longitudinal tensile stresses generated as theextrusion passes through the die.
• In hot extrusion, this form of cracking usually isintergranular and is associated with hot shortness.
• The most common case is too high ram speed for theextrusion temperature.
• At lower temperature, sticking in the die land and thesudden building up of pressure and then breakaway willcause transverse cracking.
Surface cracks from heavy die
friction in extrusion
Internal cracks or centre burst:
• It occurs because the stresses within the workpiecebreak the material causing cracks to form along thecentral axis of the extruded region
• High die angles will favour centre cracking
• Metal must be free from inclusions to avoid centreburst
• High extrusion ratio also will favour centre burst
Analysis of extrusion process
• As we have seen the slab analysis for wiredrawing, the analysis of extrusion process ismuch simpler
• Extrusion stress (𝜎𝑥 ) or die pressure Pd iscalculated by
𝐏𝐝 = 𝛔𝐱 =𝛔𝐨 𝟏 + 𝐁
𝐁𝟏 − 𝐑𝐁
𝐵 = 𝜇 𝑐𝑜𝑡𝛼; 𝑤ℎ𝑒𝑟𝑒 𝜇 = 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛
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