Casting is formed by melting the metal and pouring it into a mold of the desired shape. Since plastic deformation of the metal does not occur, no alteration of the grain shape or orientation is possible. The gain size of the metal can be controlled by the cooling rate, the alloys of the metal, and the thermal treatment. Castings are normally lower in strength and are more brittle than a wrought product of the same material. For intricate shapes or items with internal passages, such as turbine blades, casting may be the most economical process. Except for engine parts, most metal components found on an aircraft are wrought instead of cast. All metal products start in the form of casting. Wrought metals are converted from cast ingots by plastic deformation. For high strength aluminum alloys, an 80 to 90 percent reduction (dimensional change in thickness) of the material is required to obtain the high mechanical properties of a fully wrought structure. Both iron and aluminum alloys are cast for aircraft uses. Cast iron contains 6 to 8 percent carbon and silicon.
Cast iron is a hard unmalleable pig iron made by casting or pouring into a mold. Cast aluminum alloy has been heated to its molten state and poured into a mold to give it the desired shape.
The extrusion process involves the forcing of metal through an opening in a die, thus causing the metal to take the shape of the die opening. The shape of the die will be the cross section of an angle, channel, tube, or some other shape. Some metals such as lead, tin, and aluminum may be extruded cold; however, most metals are heated before extrusion. The main advantage of the extrusion process is its flexibility. For example, because of its workability, aluminum can be economically extruded to more intricate shapes and larger sizes than is practical with other metals.
Extruded shapes are produced in very simple as well as extremely complex sections. In this process a cylinder of aluminum, for instance, is heated to 750– 850 °F and is then forced through the opening of a die by a hydraulic ram. The opening is the shape desired for the cross section of the finished extrusion.
Many structural parts, such as channels, angles, T‑sections, and Z-sections, are formed by the extrusion process. Aluminum is the most extruded metal used in aircraft.
Aluminum is extruded at a temperature of 700–900 °F (371– 482 °C) and requires pressure of up to 80,000 psi (552 MPa). After extrusion, the product frequently will be subjected to both thermal and mechanical processes to obtain the desired properties. Extrusion processes are limited to the more ductile materials.