The ingots of a relatively large volume, coming as cast billets through solidification of molten metal, are usually shaped through plastic deformations into intermediate shapes. This primary shaping provides profiles that are closer to the profile of the final product and also causes a refinement of the crystal structure of the cast ingot. This refining of the structure, called recrystallization, occurs at elevated temperatures. Furthermore, metals are softer and more ductile at elevated temperatures. Thus, primary forming is done at elevated temperatures.
In the process of extrusion (Fig. 1), a billet is placed into a chamber with a shaped opening (called a die) on one end and a ram on the other. As the ram is forced into the chamber, the workpiece is forced out through the die. The extrudate, a long product (i.e., a rod), emerges through the die duplicating its cross sectional shape. The flow lines indicate that a dead metal zone forms in the corner on the exit side of the chamber where the separated ring of a triangular cross section remains stagnant.
The process of rolling, whereby the ingot is gripped by two rolls and squeezed between The rolls are identical and they are rotating in opposite directions so that they grab the ingot and drag it by friction into the narrowing gap between them. The product may become thinner while passing through the rolls. Flat products are produced by cylindrical rolls, while profiles are provided by grooved rolls.
The process of forging is performed on a press or a hammer. Basically, the ingot is placed between two platens that are forced one against each other, squeezing the ingot between them . A variety of shapes can be produced between flat platens by manipulation of the ingot while the platens squeeze and release the
Interaction between the Machine, the Tool, and the Workpiece
A typical system for a metal-forming process is presented here through forging (see Fig. 4). The platens are manipulated by a hydraulic cylinder. The force applied to the workpiece through the piston and platens is contained by the frame. The resultant force on the system is zero. However, the frame must be strong enough to contain the forming forces. While the largest forging press during World War II was a 5,000 ton press available only in Germany in limited numbers, there are throughout the world today a few production presses of 50,000 to 80,000 tons. These presses are huge and expensive. The power supply needed for a press this size is an impressive system by itself. Not so long ago, the control and manipulation of the workpiece and tools were manual. Today’s modern presses are automated. The following description is the state of the art in several of the most advanced designs (Lange, 1985).
The shape of the product, together with other information about the feed stock is given as the input to an online computer that activates the press and its accessories. The entire workpiece, tooling, and press manipulations schedules are calculated by the computer. Workpiece after workpiece is automatically fed to the press from its storage. An assortment of tools is stored on a rack at the press, and automatic selection of the desired tools at the proper portion of the cycle is affected. The tools and workpiece are manipulated in synchronization to shape the workpiece to the proper design by repeated forging actions. When forming of one workpiece is completed, the workpiece is removed to make room for the next one. On-the-spot automatic inspection is, on occasion, affected with possible.