subject: Aluminum extrusion [print this page] Aluminum extrusion Aluminum extrusion
The process of extrusion is particularly adapted for the production of a variety of metal shapes. An extrusion press is capable of producing as many shapes as there are differently shaped dies to fit it. Intricate shapes can be produced by this process which cannot be made in any other manner, and many shapes which can be produced otherwise are often made by the extrusion process until it is possible to justify single-purpose equipment.
It is common usage to designate the capacity of extrusion presses by the number of tons pressure the press can put upon the metal to be extruded: in aluminum extrusion practice, presses vary from about 200 tons to as high as 5,000 tons. For convenience, the majority of small presses are built to operate in a vertical position. For structural reasons and because of other practical considerations, the larger presses are built to operate in a horizontal position. The commercial designs vary greatly, but in general metal is either extruded at the end of the ingot container farthest from the ram, or the metal is forced through the ram, which, in this case, is hollow. Solid and hollow shapes may be produced by extrusion. Hollow shapes, such as tubes, require a mandrel suitably held in the die opening so that metal flows through the die, around the mandrel. A press may be provided with a number of container of different sizes in which the ingot is held during extrusion. Some of the larger presses will take ingots with a diameter of 14 inches and a length of 35 inches. The dimensions of extruded shapes are limited by the diameter of the ingot container and its length, and these two dimensions are in turn dependent upon the press capacity and the physical properties of the metal or alloy to be extruded, and, to a less extent, upon the kind of section desired.
For extrusion the ingots are usually preheated and placed in the press. Depending upon the particular design of press, the die may be placed in position either before or after the press is charged. Pressure is then applied to advance the ram against the ingot and force the metal through the die opening. In some cases, more especially in smaller presses, the ram is stationary so that the container with the ingot to be extruded moves against the ram. During extrusion the pressure applied is controlled so that the metal flows through the die at a predetermined rate. With pure metal the rate of metal flow is not particularly important but with certain alloy compositions the rate of metal flow has a pronounced effect upon the surface of the extruded product, and also affects the physical properties. This is particularly true when the extruded material has a cross-section of irregular out-line. After the extrusion operation there remains in the container a certain portion of the charged ingot. In most instances this butt portion is scrapped, especially when the physical properties of the product are of prime importance.
The ingots used are, for the most part, cast in suitable molds. In some cases, however, the billet used may be a product of a working operation, as for example, rolled rounds. For the production of solid extruded sections a solid ingot is used, and for hollow sections a hollow ingot is used. Hollow ingots may be cast as such, but they may also be produced by a piercing operation on a solid ingot or billet.
Both pure metal and aluminum alloys are extruded. Pure metal is used for the production of molding and such articles as do not require the physical properties obtained in strong alloys. Aluminum alloys are employed in the production of many structural shapes such as I-beams, channels, H-beams, angles, tees and special and intricate shapes which cannot be produced in any other manner, such as specially designed sections for aircraft construction. This class of product is usually heat treated after the extrusion operation. The extrusion process is also employed for the production of tubing blooms and to provide forging stock.
