Extrusion of metals is the process, in which a billet, usually round, is pressed by a stem at high pressure through a tool of the desired shape, the die, to one or more lengths (Figure 1). The process is used mainly for the production of bar, wire, tubes, and profiles in
- aluminum alloys and
- copper alloys.
However, tubes, profiles, and semifinished products in other metals also are produced in small and medium quantities by extrusion. These metals and their alloys are :
- stainless steels
Figure 1 – The principle of extrusion 
Сompressive stress state
During extrusion, a compressive stress state is developed throughout the billet (Figure 1). It enables large deformations to be achieved with a low risk of cracking.
The ratio of the billet cross-sectional area to that of the extruded section is known as the extrusion ratio. It usually falls in the range 10 to 100 and more in special cases.
- the material’s being extruded to have a low flow stress
- a high specific press pressure of up to 1000 N/mm2.
Extrusion is therefore normally carried out at a high temperature:
- aluminum alloys usually in the range 420 to 525 ºC
- copper alloys between 600 and 900 ºC
- stainless steels and special materials up to 1250 ºC.
Two main types of extrusion
Figure 2 illustrates the two most important types of extrusion :
- (a) direct
- (b) indirect.
Figure 2 – Variation of friction components and longitudinal speed components
of metal flow in the reduction zone across the billet cross section in
the case of (a) direct, and (b) indirect extrusion processes.
(VD, VC, and VR are speed of die, container, and ram, respectively) 
Direct extrusion (forward extrusion)
- A stem, with a pressure pad in front, pushes the billet in a stationary container through a tool of the desired shape, the die.
- Relative movement takes place between the billet and the container (Figure 2a).
Indirect extrusion (backward extrusion):
- The die is located in front of a hollow stem and pushed against the billet by the forward movement of the container closed at the back.
- There is no relative movement between the billet and the container (Figure 2b).
- There is no relative displacement between the billet and the container. As a result, there is no frictional slress at the billet/container interface.
- Therefore, the extrusion load and the temperature generated by deformation and friction are reduced, as shown in Figure 3 .
Figure 3 – Typical load versus ram displacement curves
for aluminium extrusion processes .
(a) Load versus ram displacement curves
for direct (forward) (1) and indirect (backward) (2) extrusion.
(b) Division of the work of deformation.
A, work of upsetting billet;
B, work needed to initiate deformation;
C, work of deformation;
D, work needed to overcome friction and shearing in direct extrusion
Types of aluminium extrusion presses
A schematic of a direct aluminium extrusion press is shown in Fig. 4. Direct presses are used to make solid bars, rods, strips, and integrated profiles. The presses can also be used to extrude tubes and hollow profiles from softer grade aluminum using a solid billet through a porthole or bridge dies .
The press for indirect aluminium extrusion (Fig. 5) consists mainly of the same elements as the presses used for direct extrusion. Generally, for hard alloy extrusion, especially for use in the aerospace industry, the metal flow properties obtained with indirect extrusion are much more favorable than those obtained with the direct method .
Figure 4 – Schematic of a direct extrusion press.
1, counter platen; 2, die slide or rotary die head; 3, shear; 4, billet container;
5, moving crosshead; 6, stem; 7, cylinder crosshead; and 8, oil tank.
Source: Schloemann-Siemag 
Figure 5 – Schematic of an indirect extrusion press.
1, counter platen; 2, die slide; 3, shear; 4, billet container; 5, moving crosshead;
6, the star 7, sealing element; 8, cylinder crosshead; and 9, oil tank.
Source: Schloemann-Siemag 
Direct extrusion versus indirect extrusion
The indirect method affords the following advantages:
- Longer initial billets
- Higher extrusion speed for many materials
- Thinner butt ends
- More uniform structure over the extruded length
- Thinner sections
- Closer tolerances over the entire length of the product
- More uniform container and billet temperatures during extrusion
- Longer service life of the container and liner
But, there are some important disadvantages of indirect presses:
- Machining of billet skin is required to prevent surface imperfections.
- Size of extrusion is limited with the bore of hollow stem.
- Die handling is difficult.
- The difficulties in ram construction.
- The extruded product must travel the length of the stem before it is possible to quench. Therefore, it is not possible to do press quenching.
Because of these limiting factors indirect extrusion has not found the same extensive application as the direct process.
Direct billet-to-billet extrusion
- The most important and common method used in aluminum extrusion is the direct process .
Figures 1 and 2a show the principle of direct extrusion where the billet is placed in the container and pushed through the die by the ram pressure. Direct extrusion finds application in the manufacture of solid rods, bars, hollow tubes, and hollow and solid sections according to the design and shape of the die. In direct extrusion, the direction of metal flow will be in the same direction as ram travel. During this process, the billet slides relative to the walls of the container [2, 3].
Billet-on-billet extrusion is a special method for aluminum alloys that are easily welded together at the extrusion temperature and pressure. Using this process, continuous lengths of a given geometry (shape) can be produced by different methods. Billet-on-billet extrusion is also a viable process in the production of coiled semifinished products for further processing, such as rod and tube drawing production. Perfect welding of the billet in the container with the following billet must take place as the joint passes through the deformation zone.
Two methods of billet-on-billet extrusion have been developed, with removing discard and without removing discard.
Billet-to-billet extrusion with discard (butt)
The most commonly used method of billet-on-billet extrusion is the method, when the discard is removed, and the following billet is welded to the one remaining in the welding or feeder plate (Figure 6) .
Figure 6 – The method of direct extrusion with removing the discard (butt).
The following billet is welded to the one remaining
in the welding or feeder plate 
Billet-to-billet extrusion without discard (butt)
The second method does not need a discard; the subsequent billet is pressed directly onto the billet still in the container as shown in Fig. 7. The dummy block attached with the stem shears an aluminum ring from the container during each return stroke, and this has to be removed from the stem . This method is used much less often.
Figure 7 – The second method does not need a discard (butt).
The subsequent billet is pressed directly onto the billet still in the container.
The dummy block attached with the stem shears an aluminum ring from
the container during each return stroke, and this has to be removed from the stem 
Extrudability of aluminium alloys
Extrudability, which can be measured by the maximum extrusion speed, is one of the most significant factors influencing cost and efficiency of the extrusion process (Figure 8). Temperature and speed parameters, together with the state of stress in the deformation zone, mainly in the die region, play a significant role in improving extrudability of a given alloy.
Alloys easy to extrude – Soft alloys:
- AlMgSi0.5, and
Alloys moderately difficult to extrude – Medium-strength alloys:
Alloys difficult to extrude – Hard alloys
- AlMg > 3%Mg
Extrusion toolings for various aluminium alloys
In hot extrusion of aluminum alloys, flat-face dies are generally used. There are fundamental differences between the configurations of dies and tooling used for extrusion of :
- soft and medium-strength alloys (Figure 9:
– fixed dummy
– billet-on-billet process
- hard alloys (Figure 10):
– no feeder
– floating (loose) dummy
– each billet is extruded separately.
Figure 9 – Tooling configuration in direct extrusion process
with feeder plate die for soft and medium-strength alloys.
1, feeder plate; 2, die; 3, backer; 4, die ring; 5, bolster;
6, pressure pad; and 7, fixed dummy 
Flow stress of aluminium alloys
Figure 11 shows the range of exit speeds encountered in the extrusion of various aluminum alloys.
- The extrusion rate depends greatly on the flow stress of the alloy under the process conditions
- Exit speeds are relatively high for soft (6063 and 3003 alloys), but are quite low for hard alloys such as 7075 and 2024 .
Force and pressure capacities of extrusion presses
Presses for hot extrusion are usually rated in terms of force capacity, that is, the total force the press is capable of exerting upon the billet. However, press operation depends on the actual unit pressure exerted on the metal. For a press with a given force capacity, higher unit pressures can be obtained if the billet container is smaller in diameter. As the container increases in diameter, the unit pressure capability decreases, with a resultant decrease in extrusion capability .
The typical unit pressure, or the specific pressure, is listed in the table of Figure 12 for different press capacities and container sizes. The unit or specific pressure of the press has to be greater than the required pressure for a particular extrusion under certain conditions. The required pressure for extrusion could vary with the alloy and its condition, the extrusion ratio, length and billet temperature, extrusion speed, and circumscribed circle diameter .
- The typical maximum unit pressure that is used on most extrusion presses is about 1035 MPa.
- This pressure is near the upper limit of the mechanical strengths of most tool steels used for extrusion .