Overview of Mechanical Working Processes: Part Two

Deformation Processes and Classification

Deformation is only one of several processes which may be used to obtain intermediate or final shapes in metal. Liquid metal may be cast to shape in moulds, sprayed to form intermediate or final shapes or made into powder which is pressed into shape and sintered to produce strong components. While each of these has a field of application the overwhelming bulk of metal is shaped from the simple cast ingot by a series of deformation processes. The applicability and development of these processes is completely dependent on the plasticity of the solid metal.

The study of plasticity is concerned with the relationship between metal flow and applied stress. If this can be determined, then the required shapes can be achieved by the application of calculated forces in specified directions at controlled rates.

In practice the external load is applied by a tool and its shape controls the direction of application necessary to achieve the desired flow. The type of tool can be used to classify the different categories of deformation processes.

Common industrial processes fall into six categories:

• deep drawing or pressing,
• rolling,
• forging,
• stretching,
• extrusion and
• wire drawing.

There are other working processes, e.g. roll forging, spray forming, etc., but these are not yet of great industrial significance. An outline of each of the important processes is given in the following.

Deep Drawing and Pressing

Deep drawing is an extension of pressing in that the metal blank is given a substantial third dimension after flowing through a die. Simple pressing is carried out by loading a blank between a punch and a die so as to indent the blank and give the product a measure of rigidity. Can ends in food and beverage containers are the most widespread examples.

This process can only be carried out cold. Any attempt at hot drawing results in the metal necking and failing.

Rolling

This is a process which reduces the thickness of the material passed between a pair of revolving rolls. The rolls are generally cylindrical producing a flat product such as sheets or strip. They can also be grooved or textured on the surface in order to change profile as well as emboss patterns.

This deformation process can be carried out either hot or cold. Hot working is very widely used because it is possible to achieve rapid and cheap change of shape. Cold rolling is carried out for special reasons such as the production of good surface finish or special mechanical properties. More metal is rolled than the total treated by all other processes.

Forging

In the simplest case, the metal is compressed between a hammer and an anvil and the final shape is obtained by turning and moving the work piece between blows. For bulk production and the shaping of large sections, the hammer is replaced by a tup or die sliding in a frame and impelled by mechanical, hydraulic or steam power. While forging can be carried out on either hot or cold metal, the high expenditure of power and wear on the dies, as well as the relatively small extent of deformation possible, limit cold forging applications.

Stretch Forming

This is essentially a process for the production of shapes in sheet metal. The sheets are drawn over shaped formers to the extent that they deform plastically and assume the required profiles. It is a cold-working process and is currently the least used of all the working processes.

Extrusion

In this process a cylinder or billet of metal is forced through an orifice by means of a ram to such effect that the elongated and extruded metal has a transverse shape which is that of the die orifice.

There are two kinds of extrusion, direct and indirect or inverted. In the former case the ram and die are at opposite ends of the billet and the metal is pushed up to and through the die. With indirect extrusion the die is held at the end of a hollow ram and is forced into the billet so that metal is extruded backwards through the die.

Wire Drawing

Metal rod is pointed at one end and then drawn through the tapered orifice of a die. The rod entering the die has a large diameter and leaves with a smaller diameter. In the early examples of this process, short lengths were drawn by hand through a series of holes of diminishing size in a cast-iron or forged-steel "draw plate". Modern installations, in which long lengths are drawn continuously through a series of dies by the use of a number of mechanically driven blocks, can produce very large quantities of wire in long lengths at high speed, using very little manpower. By using the appropriately shaped orifice it is possible to draw a variety of shapes such as ovals, squares, hexagons, etc., by this process.