9312 Extrusions

Extrusions are products formed by pushing heated metal, in a log-shaped form called a billet, through an opening called a die, the outline of which defines the cross-sectional shape of the extrusion. Some examples are shown in Fig. 9.3. Thousands of pounds of pressure are exerted by the extrusion press as it forces the aluminum through the die and onto a runoff table, where the extrusion is straightened by stretching and cut to length. Artificial aging heat treatment may then be applied to extrusions made of heat-treatable alloys.

Extruding was developed in the 1920s and replaced rolling around 1950 for producing standard shapes such as I beams and angles. Today it is used for virtually all aluminum shapes. Shapes, called profiles in the aluminum industry, are all products that are long relative to their cross-sectional dimensions and are not sheet, plate, tube, wire, rod, or bar. Commonly available shapes include I beams, tees, zees, channels, and angles; a great variety of custom shapes can be produced with modest die costs and short lead times. In the late 1960s, the

FIGURE 9.3 Extruded shapes.

Aluminum Association developed standard I beam and channel sizes that are structurally efficient and have flat (as opposed to tapered) flanges for convenient connections (ASTM B308 6061-T6 Standard Structural Shapes).

Profiles are sized by the smallest diameter circle that encloses their cross section, called the circumscribing circle size. The size of an extruded shape is limited by the capacity and size of the extrusion press used to produce the shape. Presses that are used for common applications are usually limited to those profiles that fit within about a 15-in. (375 mm) diameter circle; presses for military and aerospace applications can handle shapes as large as 31 in. (790 mm) in circle size. Standard I beams range up to 12 in. (305 mm) in depth because the entire shape can fit in a 15-in.-diameter circle. Standard channels are as large as 15 in. deep.

Some alloys are more difficult to extrude than others; generally, the stronger the alloy, the more difficult it is to extrude. The relative extrudability of some alloys is given in Table 9.20. Alloy 6063 is the benchmark for extrudability because it's easy to extrude and widely used; about 75% of all extrusions are 6xxx series alloys. The 1xxx, 3xxx, and 6xxx series alloys are called soft alloys, while the 2xxx, 5xxx, and 7xxx are called hard alloys, based on the relative difficulty of extruding.

Hollow shapes are extruded with hollow billets or with solid billets through porthole or bridge dies. The billet metal must divide and flow around the support for the die outlining the inside surface of the hollow extrusion and then weld itself back together as it exits the die. Such dies are more expensive than those for solid shapes. Hollow shapes produced with porthole or bridge dies are not considered seamless and are not used for parts designed to hold internal pressure. (See Section 9.3.1.4 for information on tubes used to contain internal pressure.)

A profile's shape factor (the ratio of the perimeter of the profile to its area) is an approximate indication of its extrudability. The higher the ratio, the more difficult the profile is to extrude. In spite of this, profiles that are thin and wide are extrudable, but the trade-off between the higher cost of extruding and the additional cost of using a thicker section should be considered.

Sometimes a thicker part is less expensive in spite of its greater weight because it is more readily extruded. Parts as thin as 0.04 in. (1 mm) can be extruded, but as the circle size increases, the minimum extrudable thickness also increases.

Other factors also affect cost. Unsymmetric shapes are more difficult to extrude, as are shapes with sharp corners. Providing a generous fillet or rounded corner decreases cost. Profiles with large differences in thickness across the cross section or with fine details are also more difficult to extrude. Consulting extruders when designing a shape can help reduce costs [14].

Metal flows fastest at the center of the die, so as shapes become larger, it becomes more difficult to design and construct dies that keep the metal flowing at a uniform rate across the cross section. For this reason, larger shapes have larger dimensional tolerances. Tolerances on cross-section dimensions, length, straight-ness, twist, flatness, and other parameters are given in Aluminum Standards and Data [5]. Tolerances on cross-sectional dimensions are also given here in Table 9.21.

Cold extrusions (also called impact extrusions or impacts) are also produced, an effective method for making tubes or cuplike pieces that are hollow with one end partially or totally closed [22]. The metal is formed at room temperature, and any heating of the metal is a consequence of the conversion of deformation energy to heat. The slug or preform is struck by a punch and deformed into the shape of a die, resulting in a wrought product with tight tolerances and zero draft and no parting lines. The five most commonly cold-extruded alloys and their relative cold extrudability are shown in Table 9.22. An example of a cold extrusion is irrigation tubing, which can be produced up to 6 in. in diameter, 0.058-in. wall thickness, and 40 ft in length.

Wire, rod, and bar are defined as products that have much greater lengths than cross-section dimensions (ASTM B211 Bar, Rod, and Wire). Wire is rectangular (with or without rounded corners), round, or a regular hexagon or octagon in cross section and with one perpendicular distance between parallel faces less than 0.375 in. (10 mm or less). Material that has such a dimension of 0.375 in. or greater (greater than 10 mm) is bar, if the section is rectangular or a regular hexagon or octagon, and rod, if the section is round (see Table 9.23).

Rod and bar can be produced by hot rolling long, square ingot or hot extruding; the product may also be subsequently cold finished by drawing through a die. Wire can be hot extruded or drawn (pulled through a die or series of dies that

TABLE 9.23 Wire, Rod, and Bar
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