Extrusion

General concepts

One of the very widely used metal-forming processes is that of extrusion. In its basic form, the operation involves preparation
of a, generally, cylindrical billet (sometimes referred to as the ‘slug’), insertion of it into a container (which holds a suitably
profiled die), and applying pressure (by means of a punch or a ram) to tlhe trailing end of the billet. The billet is thus pushed,
or extrudled, through the die.

Since extrusion normally constitutes a more or less direct step from the billet to the finished product, tool design
(includin,g its material) and lubrication are of primary importance. The term ‘extrusion’ is sometimes applied to forging operations
that invo!ve elongation of the product, but it is used here only for ithe case of ‘pure’ extrusion described above.
The extrusion process is used in the manufacture of round or profiled bars and tubes. profiled sections, simple and
shaped containers, finned or ribbed components and, more recently, spur gears. Actual extrusion operations are carried out using either mechanical or hydraulic presses; the former being of the vertical type and the latter, as a rule, of the hoxizontal type.
These machines provide extrusion ratios (initial to final crosssection ratios) varying from 10 to 100, with punch speeds of up
to 500 mm s-’. Slower speeds are used for most of the lighter non-ferrous alloys of aluminium, magnesium and copper, and
higher velocities of deformation are needed when processing ferrous alloys, refractory metals and titanium.
Mechanical presses show a slight advantage over hydraulic machines in terms of speed. The speed of operation precludes
almost completely the possibility of using glass as a lubricant which phys an important role in extrusion of stainless steels.
Consequently. the process becomes more severe and the billet to hollow ouput can be slightly lower than that for hydraulic
presses. A further disadvantage of mechanical presses lies in the fact that the punch velocity varies during the working
stroke, and so very considerable difficulties can be created when processing alloy5 svceptible to strain-rate variations.
In addition, the maximum length of the product is limited by the length of the crankshaft. Hydraulic presses remove these
difficulties because the speed of extrusion is controllable and can be maintained almost constant while the length of the
extrudate depends on the load capacity of the press and not on its mechanical characteristics. A distiinct, additional and practica.1 advantage of this type of press is the ease of removal of long tubing, or other extrudate, as opposed to the vertical, mechanical presses where a provision for, say, a pit must be made. It is, however, advisable to remember that, in general,
alignment is better on vertical presses than on horizontal ones, and that loading conditions are less severe on the fcrmer.
With the great advantages of glass as a lubricant, the number of horizontal presses has inevitably increased, being at
present abour three times that of vertical presses. Because of its very wide range of application, the extrusion
process is looked upon in a vauiety of ways that reflect the particu1a.r use to which it is put in any specific circumstance.
As a consequence of this, the classiEication of extrusion operations follows diverse courses.

The most basic classification recognizes four extruding techniques:

1. forward or direct extrusion,
2. backward or inverse extrusion,
3. side extrusion. and
4. continuous extrusion.

However, depending on the shape of the product, a further subdivision is often used:

1.
2. hollow (tubular) extrusion, and
3. can extrusion.
In view of the importance of die design and, therefore, the die profile, the process is sometimes classified with reference
to the tooling:
1.profiled die extrusion
2.Naturally, extrusion processes can be carried out at elevated
temperatures (hot extrusion)
3. . or below the recrystallization temperature of the alloy (warm or cold extrusion). Square-die processing is used normally in the hot extrusion or profiled light metal (aluminium or copper alloys) components. Continuous profile dies are mainly used for lubricated cold or warm extrusion to enhance material properties and improve surface finish. Streamlined dies are employed
because of the higher degree of homogeneity of flow that can be obtained. Depending on the properties of the starting material and the complexity of the shape of the extrudate, cold extrusion processes (which clearly require high extrusion pressures)
involve three distinct techniques which, under specific conditions, ameliorate to some degree their severity:

1. conventional extrusion,
2. impact extrusion, and
3. hydrostatic extrusion.

As in other cold-forming operations, the mechanical properties of the metal are considerably improved if, of course,
the increase in temperature associated with the operation does not exceed the temperature of recrystallization. Dimensional
tolerances are of a very high order and, as already mentioned, efficient lubrication gives good surface finish, which is also
made possible by the absence of oxidizing effects that are present in hot working conditions.

Basic extrusion operations

The three most basic extrusion operations of forward, backward and side flow are shown in Figure 16.43. Although, as
shown in the figure, these operations refer to axisymmetric solid or hollow components of circular cross-section, profiled
or asymmetric shapes can also be obtained in this way. Again, the techniques are equally applicable to hot, warm or cold
conventional processing.

In a forward extruding operation involving the manufacture of a solid rod or hollow tube (Figure 16.43(a),(b)) the
previously prepared billet is placed in the container and is either directly extruded to form the rod or, if prepierced, it is
threaded onto a cylindrical mandrel which supports its bore and is then extruded. The deformation of the tube results in
the thinning of its wall and the consequent elongation of the extrudate. If there is no prepiercing, a piercing punch is
positioned in the container to provide, on the one hand, the initial hole and, on the other, to act as a mandrel.
In the extrusion of a can (Figure ¶6.43(a)), a counterpunch in the container acts as a die by causing the material to flow
between the container walls and its own outer surface. At the conclusion of the operation and retraction of the punch an
ejector removes the formed can from the container.

In the backward extrusion of a rod (Figure 16.43(d)), the billet is placed in the bottom of the container and a hollow
punch, of bore diameter corresponding to the outer diameter of the rod to be extruded, is forced into the material, causing it
to flow upwards.
A similar arrangement is used when extruding tubular components (Figure 16.43(e)), but here either a prepierced
hollow is used or piercing in the container forms the first stage which is followed by extrusion. Again, an ejector is necessary
to remove the product.
When extruding a can (Figure 16.43(f)), a reverse of the forward process is effected. The billet is placed in the die and a solid punch is moved axially into the metal. An ejector is necessary to remove the can.

Side extrusion, either one- or two-sided, is limited to rod (Figure 16.43(g)) or tube (Figure 16.43(h)) manufacture. In
the first case, the die is situated in the side of the container with the billet positioned at right angles to it, but supported at
its lower end. The punch moves axially downwards forcing the metal to flow through the die. When making a tube, the billet
must be pierced first, inserted into the container and the mandrel threaded through it. It is only then that the punch can
be actuated and cause plastic flow to commence.

As in other forming processes, extrusion, in any form, produces a number of material defects which are associated
either with the characteristics of the process itself, or with the selected geometry of the forming pass. In more general terms,
these can be summarized in the form given in Figure 16.44. Defective items in extrusion may reach a proportion as high as
10-15% of the total volume of the product and, although inspection of extrudates can prevent the use of defective
components, rejection of parts increases production costs since, in addition to the expense of full processing, the
inspection itself is expensive.

Defects that may occur are usually due to any, or a number, of the folllowing:

1. defective billets,
2.defective QP unsuitable tooling, and/or
3. processing technique.
Irrespective of their origin, all these defects can be reduced or even eliminated by correct design of the extrusion tooling.
A very characteristic feature of extrusion, especially when using flat dies, is the phenomenon of the ‘dead-metal zone’.
It can be seen from Figure 16.45 that in flat dies (90’ die semi-angle) part of the billet material becomes trapped in the
corner of the diekontainer space and does not participate in extrusion. The bulk of the material moving through the die
shears past the trapped annular ring of stationary metal which thus effectively forms a new, curved die surface that merges
with the proper die.

Depending on the point of view adopted, the formation of a dead-metal zone can be regarded either as a defect or as a
desirable phenomenon which may enable the material to adopt the optimal flow path. defective QP unsuitable tooling, and/or

Conventional tube extrusion

One of the most important applications of the extrusion process is the manufacture of seamless tubing and, in particular,
the rnanufacture of the stainless-steel variety. This is because rotary, longitudinal rolling processes often give unsatisfactory
results, and operations such as Assel elongating fail altogether. The importance of extrusion becomes obvious in
these circumstances, and its applicability to the processing of ferrous alloys cannot be stressed too strongly.

Although extrusion has been used for non-ferrous metals since about the middle of the last century, serious interest in
its application to steels was not shown until the mid-1920s when the first experiments carried out in France, the USA and
Germany showed the distinct feasibility of the operation. The problem encountered was the high rate of tool wear, in
particular of the dies. The slow development of tool materials and manufacturing techniques prohibited any extensive use of
extrusion until almost the outbreak of World War 11, but rapid progress was then made in Germany and the USA, whereas in
France, Sejournet continued the development of his glass lubricating technique, which became widely accepted in the 1950s.
Since the extrusion of stainless steel offers wide scope for discussion, these materials are used in the following as the
basis for a review of techniques and practices. The current practice of extruding stainless-steel as tubing and sections, either by direct or reverse methods, is based on the following sequence of operations:

1. preparation of billet,
2. heating,
3. lubricating,
4. providing a pilot hole,
5. reheating and relubricating,
6. extruding,
7. removing the lubricant, and
8. straightening whenever required.
Reheating and relubricating (5) are not necessary in some
processes.
In general, four main variants of the process are in operation. These involve the following techniques:
1. prepiercing, extruding;
2. drilling the billet, expanding, extruding;
3. drilling the billet, extruding; and
4. staving (dumping) the billet, piercing, extruding.
The choice and application of a given technique are examined in the following detailed discussion of the general
sequence of operations.
Preparation of billet Irrespective of the process involved, all billets are faced at least on one end. Furthermore, they are
normally machined, or sometimes ground, since the surface finish of the billet determines to a great extent the surface
quality of the hollow. A surface finish of about 7.5 pn is often required, thus increasing considerably the cost of production.
In the case of direct extrusion (using drilling as the means of providing the pilot hole), billets are drilled centrally at this
stage. Holes of up to 50 mm in diameter are machined out on twist drilling and vertical boring machines. Larger holes are
made by trepanning. Trepanning is also used on smaller size holes in an effort to increase the initial length of billets.
Invariably, billets are either radiused or chamfered externally at the leading ends to offset the tendency to cracking
in this region during extrusion.

Heating Heating of billets is of great importance, not only from the point of view of the time involved, but also in terms
of the economics of the possible descaling operation.The general trend appears to be towards the low frequency
inert gas atmosphere heating of billets. Theoretically, the economic use of a low frequency induction furnace puts severe
limitations on the use of smaller diameter billets below, say, 100 mm diameter; the range above 150 mm being considered
economical. The advantage of obtaining a scale-free billet may sometimes outweigh the disadvantage of higher heating costs
and, consequently, the intermediate range between 100 and 150 mm is occasionally used. When using induction furnaces, the general practice is towards preheating billets to about 1020°C followed by further induction heating to about 1200°C for staving and prepiercing, and expanding processes. Other techniques are also in use, e.g. preheating to about 820°C in a gas-fired furnace to avoid heavy scaling, followed by heating in a salt bath to extrusion temperature. A practice widely adopted in the USA consists of preheating in a Selas radiant heat slot furnace to about 12OO0C, followed by heating in a barium or sodium chloride bath (to dissolve scale) to extrusion temperature.

A new heating system that makes use of dual fuel furnaces is also in use. In this system, steel is preheated to 900°C in a
gas-fired furnace and is brought rapidly to 1250°C in an induction furnace. Scaling appears to be negligible.
In the case of billet drilled for direct extrusion, heating is usually carried out in a salt bath, with the exception of the
revolving Balestra type furnace. The latter combines the heating and lubricating operations, being a drum type furnace in which the refractory lining is coated with a thick layer of molten glass. Prior to heating, glass wool is inserted into each
end of the hole drilled in the billet in order to prevent oxidation. Equiverse type furnaces, which have proven very
successful in the case of low carbon steels, are not often used for stainless steels. Essentially, the final heating operation is carried out either in an induction- or salt-type furnace. Each of these processes has distinct advantages and disadvantages.
Lubrication The inherent difficulty in extruding steels, and particularly stainless steels, requires more efficient and, to a
certain extent, more sophisticated lubricating methods than those used for the processing of ordinary engineering alloys.
Originally, graphite type lubricants were widely used mainly because of the possibility of lubricating the mandrel when
extruding small-bore hollows. The Ugine-Sejournet process, using glass as lubricant, has changed this situation significantly.
The innovation consists not only in using glass, but also in introducing the novel idea (in hot working) of lubricating only
the work piece and not the tools.

However, the possibility of increasing the range of extrudable materials and the amount of deformation in a given
operation when using glass can be predicted from a theoretical analysis of the process. Glass is applied by means of pads of fibre, cloth or pressed powder. The only disadvantage of glass is the slight difficulty in removing it from the extrudate. In a properly controlled process, however, the layer of glass is very thin and on cooling there is a tendency for iron oxides to form. These dissolve glass slightly and facilitate its removal. Providing a pilot hole An initial hole is made either by drilling or machining, or by hot piercing. Drilling and machining,

followed directly by extrusion, are used where the finished bore is small (usually up to about 30 mm). The cost of
the waste material and labour involved is considerable, but the lack of eccentricity in the finished tube outweighs to a certain
extent this distinct disadvantage. This technique is also used whenever the formed metal is difficult to hot pierce.
To obtain bore sizes of 32-115 mm, small pilot holes are drilled and then expanded. Pilot holes for this operation are
20-25 mm in diameter. For bore sizes larger than 115 mm in diameter, hot piercing followed by extrusion is employed.
Extrusion The techiques of extrusion are discussed in Section 16.2.6.2 but, in the case of stainless steel and the
Ugine-Sejournet process, tube sizes and the corresponding required press capacities need careful assessment.
The inherent weakness in the extrusion process lies in the production of discard associated with the formation of the
dead-metal zone which, naturally, represents the total loss of material. With the high cost of stainless steels, the problem of
discard is more serious than for other materials. The weight of discard is approximately proportional to the cube of the
mandrel diameter. The weight is not affected by the introduced variations in the diameter of the die. Although a
reduction in the die angle gives some improvement, this is small in comparison with the improvement achieved by completing
the piercing-extruding operation in two stages. In this operation, the billet is first pierced against a solid plate which
temporarily replaces the die. On completion of piercing, the plate is removed and the extruding die is introduced into its
place.

The advantage of reducing the weight of the discard in this way must be set against the cost of the labour involved and the
provision of back-plates which, obviously, can be damaged easily.

Removal of lubrccant Graphite-based lubricants are easily removabie by conventional methods. When glass is used as
lubricant, techniques vary slightly but, essentially, they all depend on the application of some type of pickling bath. Thus,
glass is riemoved by pickling in a mixture of 4% hydrofluoric acid and 14% nitric acid. or in a mixture of hydrofluoric acid
and sodium sulphate.

Small amounts of the mixture of partly decomposed glass and scale: are sometimes removed by sand blasting.
The increase in the use of the extrusion process in the last 25 years has been due partly to economical and partly to technical
considerations. Extrusion becomes economical and competitive with rolling processes for a range of sizes of up to 150 mm
outside diameter, and for comparatively short lengths and runs. ThNe cost of plant required for the production of bigger
hollows, both in terms of presses and ancillary equipment, increases rapidly when this limit is exceeded without there
being, ai the same time, any possibility of increasing the length. Pit present, the intermediate range (say, 150-250 mm
outer diameter) is manufactured using extrusion presses but the proci:ss becomes rather expensive owing to considerable
tool wear and increased rate of scrap. The additional, inherent disadvantage of the process is its comparative slowness with,
on average, 50-60 extrusions per hour.

A more important reason for the use of extrusion is the fact that, from the point of view of formability, the process is
capable of dealing with very difficult materials. This is possible due to the lower incidence of redundant strains than in rotary
processe;s, and the consequent reduction in the severity of the operatioin.

Figure 16.46 Different hydrostatic extrusion systems

Cold extrusion processes
Conventional cold extrusion operations are based on the same three basic systems of forward, backward and side operations
(see Figure 16.43). The range of materials usefully employable in engineering applications is limited to steels and aluminium
and copper alloys. Although easily extrudable, materials like tin, lead and magnesium show no benefit from strain hardening
and, in any case, are of no great industrial importance. Although hollow sections are produced by extrusion in
non-ferrous alloys, the bulk of the products consists of profiled sections (particularly in aluminium) used extensively in the car
and aircraft industries, as well as in domestic situations. Very high extrusion pressures, combined with extensive
frictional effects that occur in the container and in the die,make conventional extrusion unsuitable for processing less
ductile materials or composites. Although the individual demand for either is not yet very high, developments in modern
technology rriake it imperative that processes capable of coping with such materials be developed. Two processes
fulfilling the demand are hydrostatic and impact extrusion. Both of these rely on high pressure. but reduced friction.
forming. The process of hydrostatic extrusion differs from conventional extrusion in that it employs a pressurized liquid instead
of an extrusion ram. A diagram of the type of hydrostatic extrusion systems available is shown in Figure 16.46. The
extrusion die has, in this case, a relatively small cone angle, but the high bursting stresses associated with this are offset by
the fact that the fluid pressure acts as a containing element around the die circumference. In pra'ctice, this operation has a number of advantages over conventional extrusion, most of which are related to the fact that bhere is no contact between th,e billet and the extrusion container wall. This results in:

 1. lower extrusion pressure,

2. improved lubrication from the pressurizing liquid,

3. a reduction in redundant work since smaller die angles can
be used, and
4. the extrusion of longer billets-the limit being the length
of the container.

.A further advantage of hydrostatic extrusion is associated with the improvement in ductility which most materials undergo
when deformed under hydrostatic pressure. In practice, this means that the extrudability of materials is improved
when the hydrostatic pressure component is present and under such conditions many nominally brittle materials have been
satisfactorily cold extruded.

As in other processes, it is found that for a given amount of deformation and specific frictional conditions, there is an
optimum die angle which gives the best balance between friction and inhomogeneous st;ain and, hence, there is a
minimum extrusion pressure.

In spite of considerable research and development work on hydrostatic extrusion, the process has not been as widely
accepted as an important industrial process as was originally expected. Away from the conventional operations, including hydrostatic extrusion, it is the high-energy-rate operations that are steadily finding application in specialized industrial areas,
and extrusion is no exception to this development.Impact extrusion can be employed in the case of either bar
or tube forming. The forming energy is supplied through the medium of a ram which, in turn, is actuated by a sudden
expansion of gas produced by, say, an explosive charge or a pneumatic-mechanical system. In the forward extrusion of bar (Figure 16.47). immediately after impact the ram and billet travel together at high speed with the extrusion of the billet taking place in the die.
Relatively high strain rates can be attained and in some alloys these will lead to a substantial increase in the value of the yield
stress Y, combined with a reduction in the strain to fracture. This is particularly noticeable towards the end of the operation
when a high degree of deceleration is reached. It is also at this stage that high tensile stresses are reached at the base of the
extruded bar and result in either the necking or breaking off of the product. The maximum possible deformation depends mainly on the ability of the tool materials to withstand shock conditions. Impact extrusion of tubular components is more conveniently
carried out in a reverse system (Figure 16.48). This employs a ram of either flat or contoured face and a means of
applying additional carefully controlled external pressure up to the walls of the extruded product. Pressure p is required to
minimize the effect of two major faults which characterize the operation, i.e. cavitation and fish skin. Fish skin occurs
throughout the operation and is equivalent to the appearance of circumferential tearing of the surface. Cavitation is associated
with the flow of the material upwards, leaving the corners of the container unfilled. Rounded instead of sharp edges are thus produced. The practical usefulness of this particular process depends, to a considerable degree, on whether the incidence of faults such as break-offs in a bar or cavitation and fish skin in, say, tubular bottle manufacture, can be avoided.

Continuous extrusion
Continuous extrusion processes have been explored in some depth, but it appears that, at present, the Conform process is
the only one that is used extensively.

The process relies on frictional forces generated between the billet and container. These are sufficiently high to effect

extrusion through the die. The operational system is illustrated in Figure 16.49. The product can be delivered either
axially or radially (as in side extrusion) as shown in Figure 16.50(a). The tooling consists of a rotating wheel with a circumferential
groove, a shoe which overlaps a portion of the wheel surface and includes a grip segment and an abutment containing
the die.

Solid and tubular sections can be extruded in aluminium and copper through single- or twin-port dies (Figure 16.50(b)).
Both solid and particulate matter can be used as starting material, but very high extrusion ratios of up to 200 produce
temperatures of up to 500°C and die pressures of up to 1000 MPa. Consequently, good tool materials must be used
and the system must be cooled efficiently. With the groove of the wheel1 undergoing cyclic thermal stressing, the possibility
of fatigue must be considered when designing this part.

Section and sheet extrusion

Although interest often centres on the extrusion of rod and wire, a Parge proportion of extrudates in light-metal alloys is
manufactured as profiled sections ranging from simple curtain rails to very complex structural shapes a selection of which is
shown in Figure 16.51.

In such cases the flow of metal during the operation is far from uniform and the incidence and level of redundancy can
be high unless die profiles are designed correctly. The requirements of modern industry demand more and
more composite bi- or tri-metallic components which possess sufficiently good mechanical and physical properties to be
suitable for forming to specific shapes. Examples of such applications involve the use of bimetallic sheets in the chemical
industry where, say, a thin layer of stainless steel on a low carbon steel base will provide both structural strength and
anticorrosive properties, and can be used in the fabrication of containers and pressure vessels, or in the electrical industry for
the manufacture of bimetallic strip conductors.

Bimetallic strip is often produced by rolling but, more recently, an extrusion method has been developed for small
width and thickness strip in which billets of different metals are extruded simultaneously from two or more containers to
form a composite strip.

This manufacturing method provides a means of obtaining good dimensional tolerances and results locally in sufficiently
high stresses to produce pressure welding. High deformations exceeding 50% are needed for this to occur.
Extrusion pressures, experienced by the metals, are only slightly higher than the corresponding pressures occurring
with similar extrusion ratios in circular sections.

Die materials

For the hot extrusion of aluminium and similar light alloys. H13 steel dies are normally used. alp brass extrusions require
HlOA and H21 steel dies or stellites and cobalt-based alloys. Copper components are extruded through Nimonic 90, or
other nickel based alloy dies, whereas steels are usually processed in H13 and H21 steel dies, or in TZM molybdenum
alloy. Nickel alloys are invariably extruded through Nimonic 90 dies, and titanium alloys through hot-work refractory oxide
coated steel tools