IV. Phenomena

  • PRESSURE-INDUCED DUCTILITY REVERSIBLE FLOW, AND METALWORKING UNDER PRESSURE
  • The most significant factor controlling the application of metal forming as a manufacturing process is the ductility of the workpiece. Metallurgical aspects determine the ductility of the workpiece at standard room temperature conditions. The most popular experimental procedure to determine ductility is the tensile test. One traditional method to improve the ductility of metals is heating, which causes most metals to soften and become more ductile. Thus, traditionally, heating was employed both to reduce the required forming forces and to increase the amount of deformation possible.

    The indication that ductility, or the lack of it, is not an inherent and solely metallurgical property, but a property that can be controlled by mechanical means (namely, environmental pressure), was suggested by Bridgman (1949). He showed that the ductility of metals as manifested by the stress-strain curve increases with the mean superimposed hydrostatic pressure. The terms mean stress, average stress, hydrostatic stress or pressure, and environmental pressure are used interchangeably. Not only the metallurgical parameters of the workpiece but also the processing parameter pressure dictate the formability. This phenomenon, the increase in ductility with the environmental pressure, is called pressure-induced ductility (PID). As suggested by Bridgman, the mechanism of PID is the restraining effect of environmental pressure in inhibiting void initiation and growth. Since the growth and coalescence of voids are prerequisites to ductile failure, their arrest extends formability, thus increasing ductility. For the mathematical treatment of the effect of pressure on the deformation and strength of a tensile bar and on void formation and prevention (Talbert and Avitzur, 1977). This increase in ductility by superimposed hydrostatic environmental pressure was confirmed by Bobrowsky et al., (1964), Pugh and Green (1956), Alexander (1964-1965), and many others.

    With the renewed research into hydrostatic extrusion, a convenient tool was developed for the investigation of PID and its implementation in metal forming, namely, metalworking under pressure (MUP). For example, by pressure-to-pressure hydrostatic extrusion, the environmental pressure can be controlled as an independent process parameter, through the control of the receiver pressure, separately from the reduction, die angle, friction, or temperature. Bridgman was able to demonstrate the PID phenomenon by extruding marble, a brittle material by all counts, into a high-receiver pressure. The extrudate came out as a sound product. A prevailing theory today in geology is that rocks deep underground are capable of undergoing plastic deformations in a ductile manner because they are constrained by high environmental hydrostatic pressure. MUP for many hard to deform metals or shapes is demonstrated through pressure-to-pressure extrusion and implemented in industry. While MUP had been sporadically employed earlier, Bridgman’s pioneering work gave the phenomenon an identity, and since then it has been applied deliberately in many processes. A wide range of processes to which PID can be utilized in MUP are included in these five categories: (1) forming, (2) cropping and shearing, (3) bonding, (4) powder compaction, and (5) reversible flow from smaller to larger cross sections (Avitzur, 1983).

  • HIGH ENERGY RATE FORMING
  • Up to this point, the processes described were achieved through static loading, as in the forging of a disk between two platens of a press (Fig. 1). We now examine how the same result (upsetting) can be achieved by a high-energy rate-forming (HERF) process. (The process is also called high-velocity or HVF).

    If, hypothetically, the disk is thrown with a high speed at the bottom platen, the entire kinetic energy of the rushing disk will be absorbed at the moment of impact with the platen. If the projectile achieves bullet speeds, it may, on impact, either penetrate the platen, like an armor-piercing bullet, weld to the platen, deform, or undergo two of the above simultaneously.

    Figure 21 represents the most common design for the use of explosives in a HERF process. A blank made of a plate or sheet metal is placed over a die cavity of the desired shape. A vacuum must be formed in the cavity below the blank by evacuating the air. The tank above the bank is filled with water. An explosive charge is placed just below the surface of the water, directly above the center of the blank.

    When the explosive charge is detonated, a shock wave moves through the water. Water is a very effective shock-wave-transmitting medium through which the impact of the explosion is transmitted from the source to the workpiece target.

    The effectiveness of the energy transfer is demonstrated by observing uses in other fields. For example, sonar under water is most efficient and sensitive. The destructive force of the shock wave has been used for centuries (now illegally) by fishermen to destroy (or to stun) all life in a vast sea or pond space. Submarine warfare demonstrates the sharpness by which the shock wave from a bomb hits the submarine, as if it had been hit directly by a hammer.

    On reaching the blank, the shock wave hits it so hard that the blank rushes downward and conforms to the cavity. Once the shock wave has hit the blank and set it in motion, the rest of the operation is performed by the inertia of the

     

    FIG. 21. Schematic of explosive forming.

    FIG. 22. Intermediate shape.

     

    moving blank. The blank moves downward as a plane during forming. Halfway through the operation the part would look like a flat-bottomed bowl with sides conforming to the cavity. This intermediate shape is shown in Fig. 22. This shape would also result if the explosive charge were insufficient to complete the operation. For smaller parts the surge of energy can be provided through other chemicals or by an abrupt electrical discharge of energy from a battery of capacitors.

  • FRICTION AND LUBRICATION
  • One of the last frontiers in the understanding of metal forming is the friction phenomena between the tool and the workpiece. No matter how much care is taken to form a smooth tool surface, the surfaces of both tool and workpiece are irregular surfaces with peaks and valleys. Opposite peaks clash with each other, resulting in damage to both surfaces. Temperature rises due to the rubbing action. A thin layer under both surfaces undergoes severe plastic deformation.

    FIG. 24. Disciplines affecting friction and wear.

     

    Models of the typical behavior of the asperities of the surfaces of two solids interfacing one another under pressure, and sliding with respect to each other, are described in Fig. 23. Many more possible outcomes of the clashing of the asperities may occur. One specific behavior, described in part (b) of Fig. 23, is the steady state flow of the asperity, identified as the wave motion. In the model of this motion, the "wave model" (Fig. 24), wedges of the harder surface indent into the softer surface because of the applied pressure, thus producing opposing ridges on the surface of the softer component (Leslie, 1804). According to Leslie, the ridges are supressed down under the sliding wedges, only to rise again in front of the moving wedges. This perpetual supression and uprising of the ridges are motions similar to the motion of ocean waves.

     

     

    FIG. 23. Several patterns of distortion of asperities.

    The wedges and the ridges are the asperities. The gap between the opposing asperities is filled with lubricating liquid, establishing boundary lubrication. As sliding is maintained, the ridges are mobilized and an eddy flow is established in the trapped lubricant (Avitzur, 1990). The eddy flow creates high shear within the lubricant. This shear generates power losses, heating, and liquid pressure. The shear, the power losses, and the pressure, all increase with increasing speed and viscosity of the liquid. The power required to mobilize the ridges and to establish eddy flow in the lubricant is calculated, and thus the friction resistance to sliding is determined. Simulta-neously, the pressure generated in the liquid as a result of shear is also evaluated. It becomes clear that the height of the ridge, due to indentation, is inversely proportional to the speed of sliding. The higher the sliding speed, the higher the liquid pressure that is countering the loading pressure and the smaller the indentation. At high enough speeds the entire load is supported by the pressure generated in the liquid, indentation is eliminated. And hydrodynamic lubrication commences.

    A classic presentation of the resistance to sliding as a function of interface load is shown in Fig. 25. When the load (p) is low or intermediate, the resistance to sliding is proportional to the load and, as suggested by Coulomb (1785) and Amonton (1699), t = m p where m is the coefficient of friction (Bowden and Tabor, 1954, 1964). With increasing load the resistance levels to reach a plateau, t = ms 0/Ö 3, where m is a constant friction factor. Both the proportionality factor and the plateau are functions of the irregularities of the surface and of the effectiveness of the lubrication (Wanheim, 1973; Avitzur, 1984). In Fig. 25 m0 represents the inverse effectiveness of the lubrication and a represents the steepness of the irregularities on the surface of the die. During the metal forming the interface pressures required to impose plastic deformations on the workpiece are high, and the constant friction resistance indicated by the flat portion of Fig. 25 is realized, unless film lubrication comes into effect as described next.

    In processes like wire drawing or rolling, the deforming workpiece continually passes through the tools. These processes (unlike other bulk processing, i.e., forging), classified as "flow through" processes, are executed at high speeds and thus are most efficient. Being continuous, they minimize manual handling and lend themselves easily to mechanization and automation. Rolling on a finishing mill may be

    (a)

    (b)

    FIG. 25. Friction versus load; (a) with wedge angle (a 1) as a parameter, and (b) with the friction factor (m0) as a parameter.

     

    performed at 10,000 ft/min, and even higher speeds are reached in wire drawing. At high speeds, an entry (or inlet) zone develops whereby fluid from the entrance squeezes as a wedge between the workpiece and the die. In Fig. 26a for wire drawing, this wedge extends partway through to the point defined by Ri, where R0 > Ri > Rf. The faster the drawing is, the smaller are Ri and the contact zone between the workpiece and the die. As long as Ri > Rf, the liquid dragged by the workpiece (and the rolls, in the case of rolling) into the wedge cannot escape through the exit and must return to the entrance. The profile of the lubricant flow through any cross section is described in Fig. 26b, showing that at the surface of the workpiece flow, speed

     

    FIG. 26. Lubricant film: (a) entry zone, (b) velocity profile of lubricant, (c) eddy flow in entry zone, and (d) hydrodynamic lubrication.

     

    is equal to the speed of the workpiece, and at the surface of the tool, speed is equal to the speed of tool. The total volume rate of the liquid passing through any section is zero. Thus, in the outer annulus, closer to the surface of the die, liquid flows in the general backward direction. At some intermediate point, where a reversal of the direction of flow occurs, the velocity component is zero (Fig. 26b). Liquid at that point does not flow in or out. It does however flow into the wire or into the die as shown in Fig. 26c.

    The loops in Fig. 26c show flow lines in the inlet zone. The eddy current flowing in a closed loop retains practically the same particles of liquid and its contaminants. This circular motion is associated with high-speed gradients and shear strain rates within the liquid. The temperature of this trapped liquid in motion may rise appreciably. A very thin layer of lubricant at the surface of the wire maintains a sort of laminar flow with the wire. This layer, through the labyrinth of voids between the workpiece and the die escapes with the wire through the die exit. Being extremely fine, this layer does not constitute hydrodynamic film separation. Since metal to metal contact decreases due to an increasing liquid wedge as a result of increasing workpiece speed, it follows that friction drops too with increasing speeds, as shown in Fig. 27.

    The range of the resulting changes in the power consumed through the mobility of the ridge, and through shear losses in the trapped lubricant due to the eddy flow, is wide, as demonstrated by Avitzur (1990). The complexity of the characteristics of friction is evident from the calculated value of the global friction factor m, as presented in Fig. 27. The abscissa is the Sommerfeld number (S), the ordinate is the global friction factor (m), and the parameter is the normal load (p) on the interface between the two sliding bodies. The local friction factor is m0 = 0.6 while the asperity’s angle is a = 1° . For the lower load values (p = 2) the characteristic behavior of the Stribeck curve (1902) is observed. The static friction factor value of m is highest when no sliding occurs. With increasing speed or Sommerfeld number values, resistance to sliding drops because the ridge size reduces sharply. Higher-pressure values produce higher resistance to sliding. Note also that for higher pressures the height of the ridge is higher, and therefore the thickness of the film of the trapped lubricant is thinner. Furthermore, increases in Sommerfeld number values are not as effective in reducing the height of the ridge, and thus for high pressures, the lubricant film remains thin even with increasing values of Sommerfeld number.

    An interesting point can be observed here regarding the die wear, called the "ring" at the entrance. The eddy current of the trapped liquid in the wedge causes an excessive liquid temperature rise and liquid contamination; together with the pressure rise due to the reversal of flow, it may erode the die in the same manner as the water flow in the river bend erodes its bank.

     

    FIG. 27. Global friction vs. Sommerfeld number, at high pressures.

     

    With increasing speed, a critical value is reached at which Ri = Rf and the wedge extends to the entire conical surface of the die. At that point and beyond, a thin film of laminar flow commences at the surface of the wire, This flow will proceed through, from the entrance to the exit of the die. The film will separate the wire entirely from the die. This separation will exist along the bearing (land) of the die (Fig. 26d). The wedge of eddy flow may or may not disappear while the liquid escapes from the entrance side of the die to the exit of the laminar flow. Full separation between the workpiece and the die and hydrodynamic lubrication commence. When hydrodynamic lubrication prevails, the friction is represented by the shear within the liquid in the following form:

    t = h (D v/e ) (13)

    Where h is the viscosity of the lubricant, D v is the sliding velocity between the workpiece and the tool, and e is the thickness of the lubricant’s film.

    The Sommerfeld (1904) number can be defined as a function of viscosity, velocity, wire size, and strength in the following manner: S = h vf/(Rfs 0). When the Sommerfeld number reaches a critical value (Scr) and above, hydrodynamic lubrication prevails. The higher the Sommerfeld number, above the critical value, the thicker the film becomes, separating the workpiece from the tools.

    For processes where high speeds cannot be attained (forging, deep drawing, etc.), a film of lubricant can be introduced between the tool and the workpiece by externally pressurized liquid. Hydrostatic lubrication then prevails.

  • HOT VERSUS COLD AND WARM FORMING AND IN BETWEEN
  • Up to World War II, only soft metals had been extruded on a large scale. In normal operations, lead was extruded at room temperature, aluminum either cold or hot, and copper hot. The extrusion of steel was severely limited by lubrication problems. Excessive friction along the die wore it out so quickly that a satisfactory extrusion was impossible. On the other hand, even moderate friction along the chamber wall entailed a considerable increase in the required force so that direct extrusion had to be limited to very short billets. Of course, indirect extrusion, where the die is inserted in the movable ram and not in the opposite end of the chamber, could have been quite beneficial by offsetting this increase, but in production it was limited to special cases due to design difficulties. Its main use was in laboratory studies, where it is desirable to eliminate varying friction, the better to observe the process variables.

    The Ugine-Sejournet process (Sejournet, 1955; 1966) is based both on the use of a lubricant in a viscous condition at extrusion temperature and on a separation between the lubrication of the chamber wall and that of the die. A steel billet is heated to the extrusion temperature and then rolled in a powder of glass. The glass melts and forms a thin film, 0.5 to 0.75 mm (20 to 30 mil) thick, of viscous material coating the lateral surface of the billet and separating it from the chamber wall. The relevant coefficient of friction is thus so reduced that the force required for the extrusion is practically constant throughout the extrusion, whatever the length of the billet and with the exception of a starting point.

    On the other hand, a thick solid glass pad, 6 to 18 mm (0.25 to 0.75 in) thick, rests on the entry face of the die, which, for this purpose, is at least partly flat. The front face of the billet (Fig. 28) shapes this glass pad into a longitudinal contour corresponding to the metal flow and, at the same time, melts a thin layer of glass, which will drift along with the outflowing metal and will lubricate its contact with the die. This melting will continue during the whole extrusion and ensure a continuous supply of viscous lubricant between die and extruded product. There is no metal dead zone, and a shear effect occurs in the viscous lubricant. Note that the actual film thickness of the lubricant is exaggerated in Fig. 28.

    This process has been developed to such an extent that appropriate powdered glass can be

     

    FIG. 28. Steel extrusion by Ugine-Sejournet process.

    found for any specific temperature range. The thickness of the glass layer on the finished product is of the order of 1 mil, and after cooling it is easily removed. Initially devised for steel, the Ugine-Sejournet process has been extended to practically all metals and alloys that have a deformation temperature either above that of steel or limited to a narrow range.

    Present-day trends in metal forming tend toward the replacement of hot forming and other manufacturing processes by cold forming. Some of the advantages are stronger products, better dimensional precision, surface finish, and savings in material waste. In the extrusion (and other forming operations) of steel, cold forming became feasible with the introduction of phosphate coating, a development that complements (and competes with) the Ugine-Sejournet process, When the steel surface is coated, the spongy phosphate coat absorbs the lubricating liquid, which thus becomes highly effective in reducing friction and wear.

    One may say that both developments, the Ugine-Sejournet process and phosphate lubrication, are breakthroughs that solved friction and wear problems. Without these solutions, the metal forming of steel would not be where it is today.

    When forming is conducted at temperatures above room temperature but below the recrystallization temperature, it is called warm forming. Today, many forming processes are performed warm to achieve a proper balance between required forces, ductility during processing, and final product properties. During warm forming of most steels, a specific range of temperatures (where the steel hardens by precipitation hardening) should be avoided. With today’s sophisticated equipment for the control of temperature and its distribution, the choice of the working temperature may be more precisely followed to ensure optimal production, as typically demonstrated by precision closed-die flashless forming.

    Hot forging is usually done with high-alloy tools that can withstand elevated temperature. Tool-life considerations require as short a time of contact as possible between the tool and the workpiece; thus mechanical presses that do not dwell at the bottom of the stroke are recommended for hot forging. Hydraulic presses are commonly used for cold forging, especially of large components; recently they have been replaced for smaller parts by the faster mechanical presses. So today, the choice of mechanical versus hydraulic press may be

    Table I . Process Comparisons

     

    Criteria

    Mode

    Hot

    Cold

    Warm

    Isothermal

    Ductility

    Good

    Poor to Good

    Moderate

    Ideal

    Forming Loads

    Moderate

    High

    Moderate

    Low

    Forming Rate

    Fast

    Fast

    Fast

    Low

    Dimensional Precision

    Poor

    Good

    Moderate to Good

    Good

    Surface Finish

    Poor

    Good

    Moderate

    Good

    Material Conservation

    Poor

    Moderate

    Good

    Good

    Die Cost

    Moderate

    Moderate

    High

    High

    Die Life

    Poor

    Good

    Moderate

    Poor

     

    decided, not only by the temperature of forging, but also by part size and production volume.

    The choice of one process over another, for any product, depends on many factors, including the material of the workpiece, the size, quality, and quantity required, and the producers’ likes and dislikes. Some of the criteria for this choice are covered briefly by Avitzur (1983), and a condensed summary is given in Table I.

    Forming in the mashy state is an emerging technology. Observing the tensile properties of metals, especially those of metal alloys, it is noted that the gradual drop in strength with increase in temperature undergoes a discontinuity in slope at the solidus line (see Fig. 29). The solidus line represents the temperature at which a solid metal alloy starts the transformation into the liquid state. This transformation proceeds gradually on heating, so that larger and larger portions of the specimen become liquid with increasing temperature until the liquidus temperature is reached, at which point the entire specimen melts. The way alloys liquefy is unique. First, drops of liquid nucleate

     

    FIG. 29. Strength characteristics in the mashy state.

    at the grain boundaries. When the temperature rises only slightly above the solidus temperature and only a small percentage of the workpiece is liquid, the entire network of grain boundaries is already liquid and each individual grain floats in liquid. This condition accounts for the drastic change in strength at this temperature and explains the different behaviors of metals during forming in the mashy state (Kiuchi et al., 1979a; 1979b). [See PHASE TRANSFORMATIONS, CRYSTALLOGRAPHIC ASPECTS.]

    Plastic deformations in the mashy state occur mainly through solid grains sliding along the liquid grain boundaries. During compacting, the grains themselves may simply be rearranged relative to one another like sand or powder. The viscosity of the liquid metal and the thickness of the liquid layer determine the strength of the workpiece. Furthermore, the rate of shear within the liquid layer [see Eq. (13) and also Avitzur, 1979] determines the resistance to flow; the higher the rate, the higher is the resistance. The dashed lines in Fig. 29 indicate higher strengths for higher shear rates. In actual forming, these higher shear rates are caused by several factors. For example, they can be brought about by higher forming speeds, as with higher ram speeds in extrusion and forging. In the forging of disks, for a constant ram speed, the thinner the disk, the higher are the shear rate in the liquid and the resistance to flow.

    In the application of forming in the mashy state, precautions must be taken to prevent squeezing of the liquid outward through the surface. For example, in extrusion, the billet is usually heated to the mashy state and placed in a preheated chamber. When the extrusion takes place, the extrudate may heat up due to deformation and friction, and thicker layers of the grain boundaries may then melt. Preventing the liquid grain boundaries from squeezing out is of utmost importance and can be achieved by cooling the product at the exit. Closed-die forging is advantageous for forming in the mashy state because the confinement of the workpiece in the die automatically safeguards against liquid escape.

    In general it is observed that forming in the mashy state at a higher temperature and liquid phase results in lower required pressures but with products of inferior properties (i.e., lower tensile strength combined with cast dendritic structure of lower ductility). Comparisons between forging from the melt and forging in the mashy state are made in Table II.

    Forging from the melt is a process that competes with forming in the mashy state. Forging from the melt is accomplished by pouring molten metal into a mold, which serves as a die, and applying ram pressure while freezing progresses, (Ramati et al. 1978). As the molten metal gradually solidifies, its volume decreases and the ram force must be applied continuously. The surface of the workpiece freezes first. A drastic volume drop is associated with the phase transformation on freezing from the liquid to solid phase. This volume change due to phase transformation is vastly greater than the shrinking that occurs with the temperature drop in the solid state. Thus, as the interior freezes and shrinks, the ram advances into the workpiece and (depending on the skin temperature) causes the skin, which is already

     

     

    TABLE II. Comparison between Forging from the Melt and Forging in the Mashy State

     

    Forging from the melt

    Forging in the Mashy state

    On solidification, the workpiece conforms to the die cavity

     

    Solidification starts at the interface with the die, while the interior is still liquid. When the interior freezes, the volume is reduced, the punch (or top die) goes deeper, and the previously frozen skin wrinkles.

    Upsetting to any degree can be incorporated. The billet’s shape differs from the cavity’s

    The liquid phase is uniformly spread through the workpiece on the gain boundaries, permit-ting easy shifts of grains throughout the workpiece.

    solidified, to fold over, wrinkle, and even crack. Skin defects are a major obstacle in the application of forging from the melt. Nevertheless, aluminum automobile wheels are mass produced by this process, as well as by forging in the mashy state. The two processes are competing for the same products.

    Presently, workers in the field on production problems prefer the process of forging from the melt, also called "forge squeeze," "molten squeeze," or other similar terms, over forging in the mashy state. However, progress in the work on forging in the mashy state suggests some potential advantages, especially in extrusion of composites containing filamentary hard whiskers. A solution is still being sought for the best mode of mixing the matrix alloy with the whiskers.

  • SOFT TOOLING
  • In a typical metal-forming operation, the shape of the product is imposed by the tools. Thus, the tool is required to have a higher strength than that of the workpiece. For our purposes, tools include dies or rolls in drawing, extrusion, forging, and rolling; they also include the chamber and ram or punch in extrusion or deep drawing. However, there are techniques where the tool is softer than the workpiece.

    For example, during the process of conventional ram extrusion, the ram or punch that pushes the billet through the die does not control the shape of the extrudate (product). The shape of the product is controlled solely by the shape of the opening in the die. The hard ram of conventional ram extrusion is replaced by a soft tool, the liquid, in the process of hydrostatic extrusion.

    Components, mainly those produced from sheet metal or from tubing, can be shaped by a hard tool on one surface only. The contour of that surface will control the contour on the opposite side without the use of a hard, shaped tool on that side. Such components may be candidates for soft tooling.

    Brake-press bending of a strip into an angle forming a channel is shown in Fig. 30. In the top picture rigid tooling is shown, while in the bottom picture, the female die is replaced by an elastomer. During rigid tool forming, the region of deformation in the vicinity of the bent corner is in contact with the ram only and is free on the other side. When bent with a female die of a soft yielding material, the bending corner is confined by the hydrostatic pressure imposed by the soft

    FIG. 30. Brake press: (a) hard tool and (b) elastomer pad.

     

    tool, with beneficial effects on ductility in the deformation region. The hydrostatic pressure can easily be controlled through the strength of the elastomer material, the geometry of the elastomer, and its support.

    The examples shown in Fig. 30 pertain basically to sheet-metal bending processes and to thin-wall tube forming. Even in this restricted area, only a small selection of shapes has a potential for soft tooling.

    The main incentives for the use of soft tooling are the low cost of tooling and the ductility of the workpiece affected by the hydrostatic component of pressure that the soft tool exerts.

    Usually the male component of the die is the hard tool, and the female component is the soft tool. There is no shaping of the soft tool, and the die cost is less than half the cost of hard male-female die set.

    The soft tool may have to be replaced more often than the hard tool, but the elastomer pad is easier and cheaper to replace than a machined, heat-treated, and polished tool-steel die. The elastomer pad of Fig. 30b can be placed in four alternative positions before it is replaced. The most usual elastomer material is rubber or urethane of controlled hardness. Avitzur (1983) presents alternative designs in which liquid under pressure replaces the elastomer. Thus reasonable and sometimes unlimited lifetimes can be expected of the soft tool.

    A major problem in sheet-metal forming, especially in a brake press, is springback caused by the residual (elastic) stresses. For example, if a bend with an included angle of 90° is desired for the channel, the ram might have to be designed with an 85° included angle. Any reasonable change in the design of the ram is compatible with the basic design of the soft counterpart.

    Brake-press bending with the hard tools may leave tool marks, which are eliminated when soft tools are employed. On the other hand, when hard tools are used, a smooth product surface with duplication of intricate designs can be achieved, while the surface finish achieved by soft tooling is rough and intricate designs can be duplicated only on the side facing the hard tool.

    Earlier in this section, the beneficial effect of the environmental hydrostatic pressure of soft tooling on the ductile behavior of the workpiece was mentioned. For example, in Fig. 31a, the deep drawing of a cup with soft tooling is described. When a cup is drawn with a spherical-nosed mandrel without the soft tool opposite to the mandrel, the depth of draw is limited by stretching, thinning, and consequent tearing of the workpiece over the mandrel. When a soft tool presses the workpiece against the mandrel, sliding between the workpiece and the tool is arrested and stretching and thinning are minimized, deterring cup tearing and extending the possible depth of draw. Furthermore, soft tooling can be adapted to a standard press.

    Replacement of the elastomer in Fig. 31a with a pressurized fluid is described in Figs. 31b and c. While the design of the liquid pressure chamber can be adapted for a standard press design, hydroforming and hydromechanical forming are performed on specially designed presses with the pressure chamber and pressure supply mechanisms as integral parts of the press. The processes of hydroforming are also called "rubber-diaphragm-forming."

    In Figs. 31b and c (hydroforming), the pressure chamber is sealed by a diaphragm, separating the workpiece from the fluid. During hydromechanical forming the workpiece is in direct contact with the liquid, with the seal at the blank holder to prevent liquid leakage. As the punch advances, the pressure in the chamber rises, causing the workpiece to wrap around the punch. Raising the pressure too early may lead

    FIG. 31. Deep drawing with an elastomer die. (a) Deep drawing with an elastomer pad; (b) & (c) deep drawing with pressurized fluid; (b) before, (c) after.

     

    to tearing, while too little pressure at the early stages leads to wrinkling. When a very thin blank is to be hydroformed, it is customary to back the blank with a dummy blank of iron or copper. The sandwich material is formed in unison, so that wrinkling is avoided. The dummy blank is called a "water sheet."

    One fluid pressure chamber may serve for the production of a variety of components in a range of sizes. When a changeover to another product takes place, only the punch need be changed. The liquid pressure can be programmed independently of the position of the punch. Thus, changes in the pressure versus time can be effected to maximize the drawability of the component. In contrast with the use of elastomers, these pressure changes can be effected without tool modification. The advantage of controlling the pressure sequence independently is in higher drawability.

    The use of soft tooling and high temperature combine together in the implementation of metal forming with super plastic flow. The phenomena of superplasticity have been observed in the characteristics of many metal alloys. With certain small-grain structures and in certain ranges of elevated temperature, these alloys may flow practically without limit under very small loads but at very low rates of straining. A tensile specimen made of a superplastic metal when tested at the proper temperature at a low enough rate will undergo over 1000% elongation and will stretch like viscous glass into a fiber before separation. The superplastic material in the state of superplasticity is highly sensitive to the rate of straining. Any section that temporarily remains slightly larger in cross section will instantly proceed to stretch at higher rates than the smaller sections because the strain rate is inversely proportional to the area. Thus, the stretched portion of the specimen is automatically maintained at a uniform diameter. When the same tensile rod made of superplastic alloy is tested at room temperature. The superplastic behavior vanishes. Such a test will exhibit higher strength and a stress-strain curve (Avitzur, 1983).

    Thus, a superplastic alloy may be formed with simple tooling and very low loads when it is brought up to its superplastic temperature. When the desired shape is obtained and the temperature is lowered to room temperature, the part possesses high strength and can serve as a load-carrying structural component. However, some superplastic materials will creep slowly and get out of shape if loaded for a long time.

    Titanium alloys can be forged in the superplastic range into a rotor of a turbine engine with the blades intact. Another typical superplastic material is the alloy of zinc with aluminum and minute additions of copper, magnesium, and other elements. A popular superplastic zinc alloy is 78% zinc by weight with 22% aluminum and is called 78-22 alloy. At room temperature, the strongest zinc alloys exhibit ultimate strength of about 4000 kg/cm2 (60,000 psi) and a reasonable ductility. At 260° C this material is in its best superplastic state. The range over which this material exhibits its high formability is between 250° C (480° F) and 275° C (527° F), which is a narrow range. Objects such as the carrying case of a typewriter are made of this alloy or from a polymer.

    In Fig. 32 a mold made of two halves split

    FIG. 32. Pressure-aided deep drawing.

     

    parallel to the axis of symmetry is delicately decorated on its interior. A thin-wall, deep-drawn cup is heated and placed in the die assembly. The opening is sealed and air or argon gas at low pressure (above atmospheric) is pumped into the cup. Very slowly the cup expands and fills the mold perfectly to reproduce every detail of curved design.

    Alternately, a flat sheet may be placed over the cavity of the mold. Vacuum may be introduced in the interior, sucking the sheet into the cavity. If desired, the operation may be initiated by vacuum and completed by pressure.

    While the above process is slow, the equipment is simple and inexpensive and so are the dies. Very low loads are exerted, and the die life is unlimited. Even when a battery of dies is engaged to increase production rates, the overall economy of the process is retained because of the low cost of tooling and equipment. Because the processing is conducted at a constant temperature, it is also termed isothermal processing.

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