4.6. Extrusion of Metals

Extrusion is a comparatively new and foremost process among the industrial methods by which metal is wrought into useful forms. Essentially it is a process of converting a block of solid metal into a continuous length of uniform cross-section by forcing it to flow under high pressure through a die orifice, which is shaped to impart the required form to the product.

In the majority of cases pressing is a hot working operation, so a billet is heated to provide a suitable degree of softness and plasticity, but sometimes (e.g. for plastic metal) the cold process may be used.

The sketches in figure 4.14 serve to illustrate the essential principle of the process. At the same time, it also explains the distinction between two methods of working, known as direct and inverted extrusion.

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Fig. 4.14 Direct (a, b) and inverted (c) extrusion: 1, 2- ram; 3-billet; 4-container;

5-die; 6-needle (mandrel); 7 - article

The needle is used when a hollow product must be produced.

This method is used for manufacturing of steel and non-ferrous alloys:

- bars from 3 to 250 mm in diameter (thickness);

- pipes from 20 to 400 mm in diameter;

- different shapes which can't be produced by rolling.

Degree of deformation (extension) is the highest, with respect to other methods, from 10 to 80, because metal is compressed in three directions. Dimensional accuracy is higher compared to rolling. The extrusion is carried out by means of powerful hydraulic presses.

The advantages of the method are

- intricate shapes may be produced;

- brittle materials may be deformed, because metal is compressed in three directions and plasticity is high;

- high dimensional accuracy of product.

The disadvantages are the following:

- rapid wear of tools;

- large pressing rest of metal in container.

4.7. Drawing

This process is used for reducing the cross section of shape rolled stock, wire and pipes by drawing half finished article through special tool, named reducing die, or draw hole.

Figure 4.15 represents schemes of drawing the bar and pipe. Procedure is usually executed without heating.

Degree of deformation (drawing coefficient) is the smallest, because tensile stresses act within a billet: =1/L=1.25…1.45

Cold deformation promotes strain hardening of metal. To prevent an article's fracture the recrystallization annealing is carried out after a few drawing operations.

The drawing is used for production wires, which have diameter from 0.002 to 10 mm, pipes with diameter from 0.2 to 500 mm, bars with diameter from 3 to 150 mm and intricate shapes.

Fig. 4.15. Schemes of drawing of a bar (a) and a pipe (b): 1-reducing die; 2-billet (half-finished article)

The drawing provides a very high dimensional accuracy and surface quality, because the drawing die is polished and deformation is mainly cold.

The drawing die is made of tool steel (large sizes), hard-facing alloys (middle sizes) and technical diamond (small sizes for wire less than 0.2 mm in diameter).

Two types of drawing mills are used (Fig. 4.16):

Fig. 4.16. Chain (a) and drum (b) mills; for a: 1 -die-support; 2-billet; 3-reducing die; 4-6-capture device and carriage; 8, 10-sprockets; 9-chain; for b: 1-wire; 2, 4, 6, 8, 10-drums; 3, 5, 7, 9-reducing dies; 1 1-motor; 12-gears

- drum mill for wire production;

- chain mill for rods and pipes production.