METAL FORMING

Betzalel Avitzur Lehigh University
 
 

  • Introduction
  • Basic Concepts
  • Typical Processes
  • Phenomena
  • Replacing Brute Force
  • Approaches to Understanding Metal Forming
  • Summary

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    GLOSSARY

    Drawing: Metal forming process whereby the workpiece is a shaped longitudinal prism that undergoes a reduction and change in its cross section area and shape while being pulled through a shaped converging die.

    Extrusion: Metal forming process whereby the workpiece is placed in a chamber with an opening and is forced to escape through the opening, usually being pushed out by a mandrel.

    Forging: Metal forming process whereby the workpiece is placed between an anvil and a hammer and subjected to compressive force between them.

    Friction: Resistance to sliding motion along the interface between two solids.

    Lubrication: Supply of substance on the inter-face between two sliding solids aimed at re-ducing the friction and/or the wear on the interface.

    Metal-forming processes: Processes that cause changes in the shape of solid metal articles via plastic (permanent) deformations.

    Modeling: Procedure to present a physical re-ality by other means.

    Perfectly plastic materials: Idealization of the characteristics of metals undergoing plastic deformations.

    Pressure-induced ductility: Increase in the ability of metals to undergo plastic deformations without fracturing, this ability being enhanced by high environmental pressure.

    Rolling: Metal forming process whereby the workpiece is a longitudinal prism, which is placed between two opposing circular rolls that rotate in opposite directions, drag the workpiece along, and force it to reduce in cross section.

    Simulation procedures: Procedures that provide representation of a physical reality through a mathematical or physical model.
     
     

    Metallic components can be shaped in a manner similar to the molding of pottery. The raw material of a fundamental simpler shape is provided by a primary process like casting, powder consolidation, earlier forming processes, or even by electric deposition. Metals deform very much like soft clay or wax. Even in the solid state, permanent changes in shape can be forced upon them by displacement of relative positions between neighboring material points. To enforce these changes, external forces are applied. While soft plasticine can be molded by tiny toddler’s fingers, for metal forming, specially constructed tooling, usually of hard materials, are manipulated, sometimes by colossal machinery.

    A variety of processes, the equipment and tooling, and the concepts involved will be discussed in this article. This will provide an understanding of the state of the art in metal forming, typical processes (not all), and basic phenomena and concepts involved. 

    I. Introduction

  • PRIMARY AND SECONDARY FORMING < FONT SIZE=2>PROCESSES

  • 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 them is described by Fig. 2. 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 (see Fig. 3). A variety of shapes can be produced between flat platens by manipulation of the ingot while the platens squeeze and release the
     
     


     
     

    FIG. 1. Extrusion (a) and an assortment of extrudates (b).





    FIG. 2. Rolling. Friction forces: F1, driving force; F2, opposing force. Net driving force = F1- F2; v0 < ? < v f and v f / v0 = t0 / t f.


     
     
     
     

    workpiece repeatedly. Alternately, the platens may be shaped with a cavity that imparts its shape on the product.
     
     

    FIG. 3. Forging: open die (a) and closed die (b).

  • 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
     
     

    FIG. 4. Schematic of a hydraulic "C" clamp press.

    closed loop feedback capabilities for automatic corrective measures.

    Almost all of the disciplines of engineering at their most advanced stage interact in providing the present-day metal-forming system. Starting with the workpiece, knowledge of metallurgy and mechanics combine to provide an insight to its behavior. Sliding occurs on the interface between the workpiece and the tool, friction is manifested, and lubrication is exercised. Thus, tribology (i.e., the study of friction and wear) is necessary. The tools are made from hard metals and nonmetals as well, and therefore the latest advances in material science are immediately applied. Furthermore, to optimize tool wear, the latest in surface treatments, by coating, ion implantation, and laser beam surface hardening, are all practiced. In the design of the machine tool itself, all disciplines combine. To mention a few, the frame is made of any material from cast iron to plastic, which will reduce weight and noise. Hydraulics and electronics with robotics combine to provide motion inspection and vibration control. [See MANUFACTURING PROCESSES TECHNOLOGY; METALLURGY, MECHANICAL; PLASTICITY (ENGINEERING); TRIBOLOGY.]

    1. THE STATUS OF THE METAL-FORMING PROCESSES AS TECHNOLOGY AMONG THE INDUSTRIAL PROCESSES
    Metal forming is normally performed after the primary processes of extraction, casting, and powder compaction and before the finishing processes of metal cutting, grinding, polishing, painting, and assembly. With few exceptions, the bulk of the products of the metal fabrication industry are shaped by forming or a combination of forming and other processes like metal cutting or joining. Forming operations are classified as those processes where the desired shape is achieved by imparting plastic deformation to the workpiece in the solid state. Classification by (1) product, (2) material, (3) forming temperature, and (4) nature of deformation (sheet metal versus bulk deformation) can also be helpful. However, the boundaries between categories are not perfectly defined. For example, impact extrusion can be classified as a forging process or as an extrusion. It is needless to say that any specific product can be made from a number of materials, by a variety of processes, and at a range of temperatures.
     

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