In this article we will discuss about:- 1. Definition of Extrusion 2. Ways to Extrude Metals 3. Characteristics 4. Forces 5. Design Considerations 6. Extrusion Speeds, Temperatures and Pressures.
Definition of Extrusion:
Extrusion is defined as the process of pushing the heated billet or slug of metal through an orifice provided into a die, thus forming an elongated part of uniform cross-section corresponding to the shape of die orifice.
The pressure is applied either hydraulically or mechanically. Aluminium, nickel and their alloys are the metals used for extrusion directly at the elevated temperatures. Aluminium and its alloys are greatly used for this process. The extrusion of steel and its alloys is also possible.
The extrusion dies may be made either from heat resistant steel or tungsten carbide, the latter having much longer die life. Rods, tubes, moulding trims, structural shapes, cable sheathing, hose, casing, brass cartridge, aircraft parts, gear profiles and hardware items are the typical products of metal extrusion. Intricate shapes in long lengths can be produced by hot extrusions.
Fig. 5.41 shows some of the typical shapes produced by hot extrusion, which might be economically impracticable by any other method. The extrusion of such shapes requires dies of intricate design so that the flow of metal is properly proportioned and fills out any deep recesses in the profiles and does not flow preferentially through the larger cross- sectioned openings.
Horizontal hydraulic presses of capacities between 250 to 5500 tonnes are generally used for conventional extrusion. The pressure acting on the metal varies from 5625 to 7025 kg/cm2. The temperatures of the metals in the die are as follows: 350 to 425°C for magnesium, 425 to 480°C for aluminium, 650 to 900°C for copper alloys, and 1100 to 1250°C for steel.
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It is very important that the metal before extrusion be heated in controlled atmosphere furnaces to avoid formation of oxides which adversely affect the quality of extrusion. Correct operating speed, depending upon the temperature and material, very from few metres to 300 metres per minute.
Vegetable and petroleum oils are generally used for lubrication of extrusion chamber, die and ram.
Ways to Extrude Metals:
The metal can be extruded in three different ways which are as follows:
(a) Direct and Forward Extrusion:
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In this case the hot metal is pushed by the press, operating a ram in the cylinder, also called container, into the die through a small restricted opening called orifice. The process is shown in Fig. 5.42.
With the application of ram pressure, the metal first plastically fills the cylindrical shape and it is then forced out through the die opening until a small amount remains in the container. It is then sawed off next to the die. Direct extrusion is the more popular method, the extrusion press being mechanically simpler. Solder wire is also made by extrusion cylinder.
(b) Sleeve Method of Direct Extrusion:
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These result in a high amount of discard or butt scrap, particularly with zinc, copper-base alloys. As the billet is shortened, rough and oxidised peripheral material is pulled into the centre of the billet, forming a serious longitudinal defect known as ‘pipe’ or ‘core’.
This can be avoided by the sleeve method in which a dummy block about 6 mm smaller in diameter than the container bore and with a central projection on one surface fitting a recess in the end of the ram as shown in Fig. 5.45 is used.
In this way the dummy block is maintained centrally in the container. This small-diameter block shears through the billet leaving a shell about 3 mm thick around its circumference, which contains the oxidised outer surface of the billet. As a result, when inversion begins the inverted metal is all clean and homogeneous and does not appear as defect in extrusion.
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(c) Indirect or Backward Extrusion:
It is similar to the direct extrusion with the difference that extruded metal is forced through the hollow ram as shown in Fig. 5.45. The force required to compress the metal is less, but the equipment used is more complicated in case of indirect extrusion.
The limitations of this process are weakening of ram and impossibility of providing adequate support for the extruded part. This process provides a better quality product, as it minimises the amount of case from the outside of the billet flowing into the extrusion.
The process of direct and indirect extrusion may be done hot or cold, depending upon the material. Copper and its alloys, aluminium, and magnesium are extruded hot; lead solder and zinc are extruded either hot or cold.
(d) Impact Extrusion:
This is a cold extrusion process used for making short tubes of soft alloys, such as tooth-paste containers. It is carried out on a mechanical crank press and a punch is forced into a blind die containing a small slug of metal, with clearance between the punch and die so arranged that the metal flows up and around the punch forms a deep, then-walled cup.
(e) Tube Extrusion:
It is a form of direct extrusion, but uses a mandrel to shape the inside of the tube. The heated metal is put into the cylinder, the die containing the mandrel is pushed through the heated metal. The ram is pushed from other side forming the tube as shown in Fig. 5.46.
(f) Stepped Extrusion:
Stepped extrusion is an intermittent process and can be used to advantage for stepped diameter components by using two-piece die consisting of major and minor dies as shown in Fig. 5.47. When a sufficient length of the smaller cross-section has been extruded, the process is halted, the minor die is removed and the extrusion is completed by forcing the metal through major die to obtain stepped diameter component.
(g) Combined Forging and Extrusion:
A combination of forging and extrusion is used to manufacture stepped parts like poppet valves as shown in Fig. 5.48. The heated steel slug is forged by a forging piston to produce forged head and the extruded stem through the die.
Characteristics of Extrusion:
(1) It is a faster process.
(2) The products have good tolerances.
(3) Mechanical properties are slightly superior than rolling.
(4) The products have good surface finish.
(5) Complex shapes can be easily extruded.
Forces in Extrusion:
The die for extrusion is commonly flat-face because the equivalent half-cone angle in extrusion die as compared to drawing die is very large. Due to flat face die and high friction between the material and the container wall, a dead zone develops in which no flow of material is able to take place.
The dead zone can be approximated by a half cone angle of 45°. The material between ram face and section AA can be considered as a rigid body motion and the flow of material between sections AA and BB is analogous to that in a drawing operation.
Problem:
Determine the maximum force required for extruding a cylindrical aluminium billet of 25 mm diameter and 50 mm length to a final diameter of 5 mm. σy for aluminium = 170 N/mm2. Also calculate the power loss in friction.
Solution:
Ram force is maximum at the start of extrusion process when l is maximum.
Value of µ can be determined by hit and trial from equation
Design Considerations for Extrusion:
The various types of extruded shapes can be:
i. Rod,
ii. Tubing,
iii. Semi-hollow shape,
iv. Hollow shape.
A great economic advantage can be derived by production of complicated cross-section by extrusion process. The sections designed must be smaller than the billet diameter used; also the ratio of billet area to section area (extrusion ratio) will limit the maximum weight per unit length of the shape as well as the length.
The die material must support the extrusion pressure needed to manufacture a given shape. For this reason an extrusion ratio greater than 45 to 1 may take too high extrusion pressures and break the die, while a low ratio smaller than 10 to 1 will use so little pressure that the cast structure of the ingot will not be hot worked enough to give guaranteed mechanical properties.
On a high extrusion ratio it is possible to use more than one die opening, thus reducing the ratio so that several sections may be extruded simultaneously with a favourable extrusion ratio. Billets should be uniformly preheated upto a suitable temperature depending upon the material (1200°C for steel) before extrusion.
Since temperature rise of the order of 50 to 60°C is experienced during extrusion, the extruded product is liable to be cracked. In order to a avoid it, the material near the ram end is heated less and this type of temperature gradient in heating of metal can be best controlled by low-frequency induction heating.
It has been observed that pressure in direct extrusion initially rises rapidly when billet is expanded to fill the cylindrical shape completely before stating of extrusion. The pressure then starts decreasing as the ram moves further because the force between the cylinder wall and billet is reduced, which initially is high.
In the case of indirect extrusion, the pressure is more or less constant throughout ram travel, corresponding to pressure exerted at the end of ram travel in the case of direct extrusion. At the end of travel, it is very difficult force the thin plate of metal left out in the cylinder to flow out of die and as such pressure again starts rising.
In practice, the whole of the billet is never extruded; the end portion (stub end or discard) is left out to avoid oxide inclusions on extruded surface. In order to leave behind the oxide layer and surface imperfections at the outside diameter of the billet (which if extruded would adversely affect the quality of extrusion) in the case of brass, it is usual to employ a ram smaller than cylinder diameter so that a thin tube of metal (skull) is left behind in the cylinder.
In the case of aluminium alloys, the billet before extrusion is turned to remove surface imperfections.
As the speed of extrusion is increased, the metal gets on time for heat transfer and the billet temperature rises, which ultimately may lead to melting of an alloy constituent and hence appearance of crack in the extruded part (hot shortness). On the other hand, if extrusion speed is very low then billet may become stiffer requiring high extrusion pressure and in extreme cases it may not be extruded even.
For this reason many extrusion presses are fitted with means of heating of the cylinder to prevent cooling of billet during slow extrusion. This is particularly essential for extrusion of high strength aluminium or magnesium alloys which are extruded at low speeds.
Fig 5.51 shows how the extrusion ratio, billet inlet temperature are affected by the extrusion speed and pressure. It would be noted that as the speed of extrusion increases, billet initial temperature should be low for same extrusion ratio.
Similarly for high extrusion pressure, low billet temperature can be used for same extrusion ratio. However, it may be appreciated that both extrusion pressure and velocity of extrusion impose limit because beyond certain extrusion pressure, billet becomes too stiff to be extruded and beyond certain high velocity, metal starts to melt. Intersection of these curves decides the limit of maximum extrusion ratio.
Extrusion pressure required to extrude a billet of length L and diameter D can be shown as:
= 4 µ L/D
where µ = coefficient of friction between billet and cylinder wall which can be determined by extruding two billets of similar material but different lengths L1 and L2 under same conditions and measuring the ram pressures p1 and p2, then-
Extrusion Speeds, Temperatures and Pressures:
If billet is too hot or if metal is heated up due to extrusion at excessive speed, then lateral cross cracks called ‘checks’ are formed. Correct extrusion speed depends on the material, e.g. for light metals extrusion speed is 1.8 to 2.4 m/min, and for copper-base alloy it is as high as 250 m/min.
Temperature for aluminium alloys is 427°C, for 60% copper, and 40% zinc leaded alloys it is 650°C and for 30% cupronickel alloys it is 1105°C. Pressure is of the order of 2400 kg/cm2 for the low- copper mixtures and 9000 kg/cm2 for silicon bronze and phosphor bronzes. Still higher pressures are obtained for light alloys.
The extrusion pressure required for a given material depends upon extrusion temperature, reduction in area from billet to extruded product, and speed of extrusion. Pressure in beginning is high, gradually diminishes as extrusion proceeds and becomes higher than original pressure when the billet approaches a thickness of about 15—25 mm. Extrusion pressure is proportional to the log or ratio of reduction in area of cross-section.