Several processes representing a diverse spectrum of secondary and primary metal forming operations are covered in this section. Only forging and wire drawing are dealt with in some depth, covering also the press system. The other processes are covered only briefly.
Forging is a most popular production process because it lends itself to mass production as well as to the production of individual sample parts. The origins of forging may be traced to the ancient process of hammering of gold foil, between a rock, the anvil, and a stone, the hammer. In hammering, the inertia of the fast moving hammer provides the required deformation energy and force, while in pressing the force is static. Usually the final shape is imparted on the workpiece by manipulating the workpiece between the flat anvil and the flat hammer as the hammer hits the workpiece repeatedly. Complex shapes can be hammered by skilled blacksmiths. A conical protrusion from the anvil, holes in the anvil, a variety of pegs with different cross sections, and auxiliary tools, including a large selection of shaped hand hammers, may assist the blacksmiths and their helpers (see Fig. 5).
Today, hand-held hammers are replaced by mechanical and hydraulic presses. When a large number of identical components are manufactured, the open dies are replaced by closed dies (Fig. 3), each with a shaped cavity to impart its shape to the product. The workpiece does not have to be manipulated, and the operator therefore does not have to be skilled; completion of the product can be achieved in one stroke and processing efficiency is high. Feeding the blank and ejection of the product are automated. Mass production of a technological age emerges out of the ancient art for ornament and artistic values.
Generally speaking, hydraulic presses (see
FIG. 5.The tools of the blacksmith.
Fig. 4) are slower than mechanical presses or hammers (see Fig. 6). Furthermore, the larger presses, carrying higher loads or longer strokes are hydraulic. Thus, the mechanical press is more suitable for mass production. Contact time between the tool and the workpiece is shorter on a mechanical press, protecting the dies better against heating during hot forging. Mechanical presses are recommended for open-die forging, and when used for closed-die forging, a flash is usually incorporated. The role of the flash and when it can be eliminated are to be discussed soon. For closed-die forging without a flash, a hydraulic press is recommended. Mechanical presses for closed-die, precision flashless forging
as economic alternatives have recently been introduced in the market and proved successful. The largest forgings, for example, airplane wing frames, are forged on the largest presses (with up to and over 50,000 tons forging force), which are hydraulic presses.
The schematic of the forging of a flat component with flash with closed dies is presented in Fig. 7. The cavity between the top and bottom dies dictates the shape of the component. The original shape of the blank may be a predetermined length of a rod with a square or round cross section. It is also quite common to design dies with several cavities for the production of several components in each stroke of the press. The blank passes, in several steps, through a succession of pairs of dies in which it gradually approaches the final nominal desired shape of the product. The cavity between the pair of final dies is designed to be as close as possible to the nominal size of the product, but not precisely to the product's dimensions. The cavityis oversized for a number of reasons; the most important ones are the following.
FIG. 6.Schematic of mechanical presses: knuckle joint press (a) and crank press (b).
FIG. 7.Schematic of forging of a flat component with flash.
prolong tool life when compared to sharp corners that crack easily. The peak forging pressure is achieved when the top die reaches its lowest position; this position can be adjusted and thus the thickness or height of the flash can be controlled. The thinner the flash, is, the higher the forging force is. With the need or desire to fill sharper and sharper corners, a need arises for higher forging pressures, which are obtainable through thinner and longer flashes.
So far, it has been established that the fill of the cavity by the workpiece is promoted through the flow of excess material into the flash, since the blank is cut to a size larger than that of the final product. Variations in the volume of the blank are minimized to a practical tolerance. The smallest permissible volume of the blank is larger than the product size to assure a minimum flash and thus a minimum value of the peak forging pressure, which is dictated by the required corner radii.
Note that excess volume of the blank does not by itself assure a fill of the cavity. If the spacing between the dies at the bottom of the stroke leaves a flash height that is too large (and thus a forging pressure too low), the cavity may not fill, in spite of the fact that a flash has been created. In this case, the flash will flow outward too easily, leaving empty spaces in the corners of the cavity.
Other means to assure filling of the thin webs and sharp corners with moderate loads call for isothermal forging of super plastic materials (Section IV, D) and forging in the mashy state (Section IV, D). Recently, forming to "near net Shape" was introduced by flashless forging. The matching of the top and bottom dies to the form of a cavity may be designed without provision for a flash. Such design changes require that, rather than facing each other at contact, one die enters the other.
By eliminating the flash, the following advantages are gained.
On the other hand, by eliminating the flash, much stricter tolerances are imposed on the blank. Too small a blank and the cavity will not be filled. Too large a blank and the press load will be excessive to the point of causing die breakage. Better choices of tool material and a higher degree of expertise in die design are required. Flashless forging is gaining popularity and every day new components not produced hitherto by the process are being added.
Figure 8 represents a billet and die. A variety of cross-sectional profiles can be produced; however, in the following simplification, the billet is a cylindrical rod of radius R0; the rod is
FIG. 8.Flow through conical converging dies; s xf = f(R0/Rf, a , and m).
reduced to radius Rf by forcing it to pass through
the conical converging die. Reduction is measured from the cross sectional area of the billet at the entrance to the die (A0) to that at the exit (Af).
Besides the choice of the material itself, three variables (the independent process parameters) involved in the reduction process are noted at once. They are, the reduction, the semicone angle (a ) of the die, and the severity of the friction between the workpiece and the die.
These three process variables-reduction, cone angle, and friction-are independent in that the process planner may exercise a degree of freedom in choosing their values. The severity of friction, for instance, is controlled, within limits, by choices of lubricant, die material and finish, speed, etc.
The above three parameters are the primary factors affecting the process and their effect on the first dependent parameter the drawing or extrusion force will be analyzed first. Other independent parameters play a role during processing. For example, we will find that the drawing or extrusion force is linearly proportional to the flow strength of the material, but when inertia forces are neglected, it is independent of the speed (when a Mises' material is considered). The power, on the other hand, is linearly proportional to speed. Furthermore, one may consider isothermal processing, where temperature is not a factor, and then extend the treatment to handle adiabatic processing and temperature effects. Thus, at first, only the effect of the three independent parameters (r%,a , and friction) is considered.
The force required for drawing or extrusion can now be characterized. In Fig. 8 the drawing force F (or drawing stress,s xf = F/Af) is obviously a function of reduction (larger reduction required higher force), cone angle, and friction, and similarly for extrusion force F (or extrusion stress, s xb = F/A0). In short, the motivation force or stress causing the drawing or extrusion is a dependent variable, which is a function of reduction, cone angle, and friction. Description of the drawing force, for example, as a function of these three independent variables, may be undertaken by either an experimental approach or an analytical approach.
Figure. 9 illustrates the characteristics of the relative drawing stress (or extrusion pressure or force) as a function of the semicone angle of the die (abscissa) and of reduction (parameter). The relative drawing (or extrusion) stress is the motivation force divided by the cross-sectional
FIG. 9. (a) The effect ofa and reduction on the relative drawing stress. (b) Relative extrusion pressure versus semicone angle and constant shear factor.
area on which it acts and by the flow strengths 0 of the workpiece. With too small a cone angle, the length of contact between the die and the workpiece is excessive and thus friction is predominant and makes the force excessive. As the cone angle increases, friction drops very drastically and so does the drawing force. An optimal angle is reached for the power. A further increase in the cone angle causes large distortions and excessive resistance to this distortion to offset what has been gained on friction, and thereafter redundant work (caused by distortion) is a predominant factor, not friction, A further increase in the die angle produces a further increase in the total power. For larger reductions, as well as for higher friction values (t ), the drawing force and the optimal angle that minimize it are increased. For the definition of the friction factor (m) see IV, C.
Flow through conical converging dies can be imposed by drawing on the emerging product (in
FIG. 10.A drawbench (a) and a bull block (b) for wire drawing.
a process called wire drawing), by pushing-in processes called extrusion, or by a combination of the two. Only limited reduction is achieved in drawing in a single pass because the tension permitted on the emerging wire should not exceed the strength of the product or the wire will tear. Wire drawing can be achieved in straight, short products or by pulling while coiling over a drum of very long wire (see Fig. 10). Occasionally a long, straight product can be produced by the equivalent of two, hand-over-hand pulls. A tandem arrangement of many blocks, one after each other, may be used to affect large total reductions.
When a billet is pushed through the die in the process of open-die extrusion (Fig. 11), the reduction is limited, just as in wire drawing, because here the allowable driving force is limited or the feedstock will be upset between the die and the driving force. For larger reductions, the process of extrusion through a closed chamber as described by Fig. 1 is used. In the process of hydrostatic extrusion (Fig. 12), the billet is pushed through the die by a pressurized
FIG. 11.Open die extrusion.
liquid. Occasionally liquid under pressure may be introduced at the exit to affect a process of pressure-to-pressure extrusion.
In the process of rolling, long products of a variety of cross-sectional shapes can be produced
FIG. 12.Hydrostatic extrusion.
by forcing the feedstock to pass through the gap between rotating rolls. The rolls transfer energy to the workpiece through friction (Fig. 2). In flat products (strip), the strip is dragged by the rolls into the gap between them. It decreases in thickness while passing from the entrance to the exit. Meanwhile its speed gradually increases from v0 at the entrance to vf at the exit. Under regular rolling conditions, the strip moves slower than the rolls at the entrance to the gap between the rolls (v0 < Ůi) and faster than the rolls (vf > Ůi) at the exit, with a neutral point in between at which the speeds of strip and the rolls (Ůi) are equal (vn = Ůi). This neutral point is also called the no-slip point. It can be successfully argued that a no-slip region exists about the neutral point. The friction force acting along the surfaces of the rolls between the entrance and the neutral point (F1) advances the strip between the rolls, while the friction force acting between the neutral point and the exits (F2) opposes the rolling action. The difference between the friction on the entrance side and the friction on the exit (F1 – F2 in Fig. 2) provides the necessary power for rolling. The position of the neutral point is automatically determined by the power required to deform the strip and to overcome friction losses. In the conventional range of reductions practiced, the larger the reduction attempted, the farther the neutral point moves toward the exit, so that F1 increases, F2 decreases, and the net friction drag force increases to supply the higher power demand. Larger reductions can be achieved until the neutral point reaches the exit (vf = Ůi). Then the maximum reduction possible is achieved and the process becomes unstable. If larger reductions are attempted, the rolls will skid over the strip and the strip will stop altogether.
Larger reductions also require higher pressures on the rolls. Large pressures on the rolls cause more and more flattening and bending of the rolls. A limit on the amount of reduction that can be taken is set by one of two causes. When excessive pressure is limiting the maximum reduction, it is said that limiting reduction or limiting thickness is reached. If the neutral point reaches the exit and the rolls start to skid over the strip, it is said that maximum reduction is reached. The process of rolling is effectively controlled by the application of front (s xf) and back (s xb) tension to the strip on both ends of the rolls. However, the process of rolling is affected by the friction drag. An increasing number of metal forming processes were introduced recently, whereby friction provided the motivation force. These processes are classified as friction aided processes (Avitzur, 1982, 1983), and because the motivation source is applied directly to the deformation region, these processes are typified by their ability to impose excessive or unlimited reductions in a single pass.
Tubes and tubular products are made essentially from all metal and by all metal-forming processes available. In Fig. 13 a tube of larger diameter is reduced to a smaller one by the process of tube drawing, similar to wire drawing, which is called (free) tube sinking, without specific control of the inner diameter of the product and by the processes of tube drawing with a floating plug, whereby the inner diameter is controlled by a plug, In tube drawing with a floating plug, the plug is free to move axially, and its position at the throat of the die is
FIG. 13. Tube sinking (a) and tube drawing with a floating plug (b).
maintained automatically by the balance of the friction force and interface pressure with the tube. Special care in the design of the plug and die geometry must be taken so that the plug will stay in position, effectively control the inside diameter of the tube, and prevent tube tearing.
Cans can be made by deep drawing or impact extrusion and then wall thinning can be affected by the process of ironing. The process of deep drawing uses a rolled sheet, from which a properly contoured blank is stamped for the production of cans and other products. Bathtubs, kitchen sinks, and autobody components are typical deep-drawn articles. However, only cylindrical cans (also called "cups"), such as cartridges, aerosol cans, and beverage cans, will be discussed here. Blanks for cylindrical cups are circular disks.
Here, the process of deep drawing is covered and compared with impact extrusion and ironing. In the deep drawing of a cylindrical cup, a planar disk is transformed into a cup with a flat bottom, cylindrical walls, and an open top. As shown in Fig. 14, the disk is placed over the opening in the die and forced to deform by a moving ram (also called the punch). As the ram moves downward, it pulls the flange toward the center. The flange is held between the die and the blank holder, with the purpose of preventing the flange from folding upward. The blank holder is also called the "blankholder," "pressure pad" or "hold-down ring." The flange moves inward radially while its inner side bends over the rounded corner of the die and transforms from a flat disk to a circular tube. The bottom is not deformed, while the cylinder is already deformed but is not undergoing further deformation also, the toroidal
FIG. 14.Deep drawing.
FIG. 15.The pattern of deformations in deep drawing.
section between the cylinder and the flange is bending, and the flange is undergoing plastic deformation (see Fig. 15).
The process of impact extrusion (also called inverse extrusion, backward extrusion, or piercing) is utilized to produce hollow shells from solid rods or disks. Schematically (Fig. 16), a disk (slug) is placed in the cavity of a female die and then the ram (mandrel, punch, or tool) is pushed into the raw material. While the ram moves downward, the wall of the produced can moves upward, escaping through the annular gap between the ram and the die. Because the wall of the product moves upward in the direction opposite to that of the downward motion of the tool, the process is sometimes called inverse or backward extrusion. In most of the manufacturing practices involved, the product is made on a fast mechanical press; and the name "impact extrusion" has resulted. A hexagonal
FIG. 16.Schematic of impact extrusion.
FIG. 17.Hexagonal cavity produced in bolt making.
cavity produced in the head of a steel bolt is shown in Fig. 17 and an assortment of aluminum containers is shown in Fig. 18. The well-known white metal toothpaste tube needs no illustration.
Ironing is usually performed after either deep drawing or impact extrusion when a thin-wall cup is required. This same process is often applied in the thinning of tubes. This presentation is concerned with the ironing of thin-wall cups, of which the beer can is a classic example. The deep-drawing operation is more suited for heavy or medium gauge cups of relatively restricted depths. For the production of longer cups of thinner walls, ironing can be used.
FIG. 18.Variety of shapes possible by impact extrusion.
A cup (Fig. 19) of inner radius Ri, wall thickness t0, and fairly small height H, is first produced by deep drawing. The thickness t0 is usually much less than the radius Ri. Then, during ironing the cup is forced to flow into a conical die of semicone angle a and inner radius Rf and is pushed downward at a velocity vf by a punch of radius Ri over which the cup is mounted. The gap between the die and the punch (Rf -Ri) is the thickness tf: this is the final thickness of the cup, and tf < t0. As the punch advances, the wall of the cup extrudes through the gap and its thickness decreases from t0 to tf while the length H increases. The outer radius of the cup decreases from R0 to Rf while the inner radius remains constant at Ri.
The punch force P is transmitted to the deformation zone (Fig. 20) partly through the pressure on the bottom of the cup, further by tension on the wall, and partly through friction. As the friction between the punch and the inner surface of the cup increases, less tension is exerted on the wall, thus enabling ironing with larger reduction. By differential friction (i.e., by having the ram friction higher than the die friction) and proper choice of die angle, unlimited amounts of reduction can in principle be achieved through a single die (Avitzur, 1983).
As of recently, processing of polymers in the solid state is performed by the same metal-forming process described in the preceding paragraphs. The molecular orientations imposed by this process enhance the strength properties of the product. The product is made into its final shape with no machining (Austen et al., 1982).
FIG. 19.Deep drawn cup; t0 << Ri.
FIG. 20. Transmitting the punch force to the deformation region.