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外文资料翻译资料来源：书籍文章名：Chapter 4Casting Dies书刊名：English for Die & Mould Design and Manufacturing作 者：刘建雄 王家惠廖丕博主编出版社：北京大学出版社，2002章 节：Chapter 4Casting Dies页 码：P61P96文章译名： 铸造模具 1Chapter 4Forging Die4.1 IntroductionForging is a process in which the workpiece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations, dating back at least to 4000 B.C.perhaps as far back as 8000 B.C. Forging was first used to make jewelry, coins, and various implements by hammering metal with tools made of stone.Simple forging operations can be performed with a heavy hand hammer and an anvil, as was traditionally done by blacksmiths. Most forgings, however, require a set of dies and such equipment as a press or a forging hammer.Typical forged products are bolts and rivets, connecting rods, shafts for turbines, gears, hand tools, and structural components for machinery, aircraft, railroads, and a variety of other transportation equipment.Metal flow and grain structure can be controlled, so forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications (Fig. 4-1). Forg- ing may be done at room temperature (cold forging) or at elevated temperatures (warm or hot forging, depending on the temperature).(a) (b)(c)Fig. 4-1 A part made by three different processes, showing grain flow(a) casting (b) machining (c) forgingBecause of the higher strength of the material, cold forging requires greater forces, and the workpiece materials must have sufficient ductility at room temperature. Cold-forged parts have good surface finish and dimensional accuracy. Hot forging requires smaller forces, butproducesdimensionalaccuracyandsurfacefinishthatarenotasgood.Forgingsgenerallyrequireadditionalfinishingoperations,suchasheattreating,tomodifyproperties,andthenmachiningtoobtainaccuratefinisheddimensions.Theseoperationscanbeminimizedbyprecisionforging,whichisanimportantexampleofthe3 4.2 Open-Die ForgingOpen-die forging is the simplest forging process. Although most open-die forging generally weighs 15 kg500 kg, forging as heavy as 300 tons have been made. Sizes may range from very small parts up to shafts some 23 m long (in the case of ship propellers).The open-die forging process can be depicted by a solid workpiece placed between two flat dies and reduced in height by compressing it (Fig. 4-2). This process is also called upsetting or flat-die forging. The die surfaces in open-die forging may have simple cavities, to produce relatively simple forgings. The deformation of the workpiece under ideal conditions is shown in Fig. 4-2 (b). Because constancy of volume is maintained, any reduction in height increases the diameter of the forged part.Note that, in Fig. 4-2 (b), the workpiece is deformed uniformly. In actual operations, the part develops a barrel shape (Fig. 4-2 (c); this deformation is also known as pancaking. Barreling is caused primarily by frictional forces at the die-workpiece interfaces that oppose theoutward flow of the materials at these interfaces. Barreling can be minimized if an effective lubricant is used.4WorkpieceDie(a)Die(b)(c)Friction forceBarrelingFig. 4-2 (a) Solid cylindrical billet upset between two flat dies (b) Uniform deformation of the billet without friction (c) Deformation with frictionBarreling can also occur in upsetting hot workpieces between cold dies. The material at and near the interfaces cools rapidly, while the rest of the workpiece remains relatively hot. Thus, the material at the ends of the workpiece has higher resistance to deformation than the material at its center. Consequently, the central portion of the workpiece expands laterally to a greater extent than do its ends. Barreling from thermal effects can be reduced or eliminated by using heated dies; thermal barriers such as glass cloth at the die-workpiece interfaces are also used.17Cogging, also called drawing out, is basically an open-die forging operation in which the thickness of a bar is reduced by successive forging steps at specific intervals (Fig. 4-3). Because the contact area per stroke is small, a long section of a bar can be reduced in thickness without requiring large forces or machinery. Blacksmiths perform such operations with a hammer and an anvil using hot pieces of metal; iron fences of various designs are often made by this process.4.3 Impression-Die and Closed-Die ForgingIn impression-die forging, the workpiece acquires the shape of the die cavities (impressions) while being forged between two shaped dies (Fig. 4-4). Note that some of the material flows outward and forms a flash. The flash has a significant role in flow of material in impression-die forging: The thin flash cools rapidly, and, because of its frictional resistance, it subjects the material in the die cavity to high pressures, thereby encouraging the filling of the die cavity.DieBlank DieFlashFig. 4-4 Stages in impression-die forging of a solid round billetThe blank to be forged is prepared by such means as (a) cutting or cropping from an extruded or drawn bar stock, (b) a preform in operations such as powder metallurgy, (c) casting, or (d) a preform blank in a prior forging operation. The blank is placed on the lower die and, as the upper die begins to descend, the blanks shape gradually changes, as is shown for the forging of a connecting rod in Fig. 4-5 (a).DieBlank (bar stock)EdgingBlockingFinishingTrimming (a)(b)(c) Workpiece DieFig. 4-5 (a) Stages in forging a connecting rod for an internal combustion engine (b) Fullering(b) Edging operations to distribute the material when preshaping the blank for forgingPreforming processes, such as fullering and edging (Figs. 4-5 (b) and (c), are used to distribute the material into various regions of the blank, much as they are in shaping dough to make pastry. In fullering, material is distributed away from an area; in edging, it is gathered into a localized area. The part is then formed into the rough shape of a connecting rod by a process called blocking, using blocker dies. The final operation is the finishing of the forging in impression dies that give the forging its final shape. The flash is removed usually by a trimming operation (Fig. 4-6).The examples shown in Figs. 4-4 and 4-5 (a) are also referred to as closed-die forgings. However, in true closed-die or flashless forging, flash does not form and the workpiece com-pletely fills the die cavity (right side of Fig. 4-7 (b). Accurate control of the volume of material and proper die design are essential in order to obtain a closed-die forging of the desired dimensions and tolerances. Undersize blanks prevent the complete filling of the die cavity; conversely, oversize blanks generate excessive pressures and may cause dies to fail prematurely or to jam.BeforeAfterPunchFlashTrimming dieSlugStationary punchFig. 4-6 Trimming flash from a forged partForging with flashFlashless forgingUpper dieUpper punchForgingBilletFlashLower dieDieLower punch(a) Start of stroke(b) End of strokeFig. 4-7 Comparison of closed-die forging to precision or flashless forging of a cylindrical billet4.3.1 Precision ForgingFor economic reasons the trend in forging operations today is toward greater precision, which reduces the number of additional finishing operations. Operations in which the part formed are close to the final dimensions of the desired component are known as near-net-shape or net-shape forging. In such a process, there is little excess material on the forged part, and it is subsequently removed (generally by trimming or grinding).In precision forging, special dies produce parts having greater accuracies than those from impression-die forging and requiring much less machining. The process requires higher-capacity equipment, because of the greater forces required to obtain fine details on the parts. Because of the relatively low forging loads and temperatures that they require, aluminum and magnesium alloys are particularly suitable for precision forging; also, little die wear takes place and the surface finish is good. Steels and titanium can also be precision-forged. Typical precision-forged products are gears, connecting rods, housings, and turbine blades.Precision forging requires special and more complex dies, precise control of the billets volume and shape, accurate positioning of the billet in the die cavity, and hence higher investment. However, less material is wasted, and much less subsequent machining is required, because the part is closer to the final desired shape. Thus, the choice between conventional forging and precision forging requires an economic analysis, particularly in regard to the production volume.4.3.2 CoiningCoining essentially is a closed-die forging process typically used in minting coins, medallions, and jewelry (Fig. 4-8 (a), (b). The slug is coined in a completely closed die cavity. In order to produce fine details the pressures required can be as high as five or six times the strength of the material, note, for example, the detail on newly minted coins. On some parts, several coining operations may be required. Lubricants cannot be applied in coining, because they can become entrapped in the die cavities and, being incompressible, prevent the full reproduction of die-surface details.The coining process is also used with forgings and with other products, to improve surface finish and to impart the desired dimensional accuracy. This process, called sizing, involves high pressures, with little change in part shape during sizing. Marking of parts with letters and numbers can be done rapidly by a process similar to coining.Retaining ringDie holder(a)Upper dieDie holder Workpiece Lower dieWorkpieceDie with depressions(b)Flat dieFig.4-8(a)Schematicillustrationofthecoiningprocess(b)AnexampleofacoiningoperationtoproduceanimpressionoftheletterEonablockofmetal4.4Forging-Die DesignThe design of forging dies requires knowledge of the strength and ductility of the workpiece material, its sensitivity to deformation rate and temperature, its frictional characteristics, and the shape and complexity of the workpiece. Die distortion under high forging loads is an importantconsideration, particularly if close tolerances are required.The most important rule in die design is the fact that the part will flow in the direction of least resistance. Thus the workpiece (intermediate shape) should be shaped so that it properly fills the die cavities. An example of the intermediate shapes for a connecting rod is shown in Fig. 4-9. The importance of preforming can be appreciated by noting how a piece of dough is preshaped to make a pie crust or how ground meat is preshaped to make a hamburger.DieTubeFig. 4-9 Swaging of tubes without a mandrel4.4.1 PreshapingIn a properly preshaped workpiece, the material should not flow easily into the flash, the grain flow pattern should be favorable, and excessive sliding at the workpiece-die interlaces should be minimized in order to reduce wear. Selection of shapes requires considerable experience and involves calculations of cross-sectional areas at each location in the forging.Computer-aided design techniques have been developed to expedite these calculations, as well as to predict the material-flow pattern in the die cavity and to predict the formation of defects. Because the material undergoes different degrees of deformation (and at different rates) in various regions in the die cavity, the mechanical properties depend on the particular location in the forging.4.4.2 Die Design FeaturesThe terminology for forging dies is shown in Fig. 4-10, and the significance of various features is described below. Some of these considerations are similar to those for casting. For most forgings, the parting line is usually at the largest cross-section of the part. For simple symmetrical shapes, the parting line is normally a straight line at the center of the forging, but for more complex shapes the line may not lie in a single plane. The dies are then designed in such a way that they lock during engagement, in order to avoid side thrust, balance forces, and maintain die alignment during forging.External and internal draft anglesFlash GutterRibParting line Web Fillet CornerLand Trim lineFig. 4-10 Standard terminology for various features of a typical impressionAfter sufficiently constraining lateral flow to ensure proper die filling, the flash material is allowed to flow into a gutter, so that the extra flash does not increase the forging load unnecessarily. A general guideline for flash clearance between dies is 3% of the maximum thickness of the forging. The length of the land is usually two to five times the flash thickness. Several gutter designs have been developed throughout the years.Draft angles are necessary in almost all forging dies, in order to facilitate the removal of the part from the die. Upon cooling, the forging shrinks both radially and longitudinally, so internaldraft angles are made larger than external ones. Internal angles are about 7to 10, externalangles about 3to 5.Selection of the proper radii for corners and fillets is important, in order to ensure smoothflow of the metal into the die cavity and to improve die life. Small radii are generally undesirable, because of their adverse effect on metal flow and their tendency to wear rapidly (as a result of stress concentration and thermal cycling). Small fillet radii also can cause fatigue cracking of the dies. As a general rule, these radii should be as large as can be permitted by the design of the forging.Instead of being made as one piece, dies may be assembled with die insets (Fig. 4-11), particularly for complex shapes; this alternative reduces the cost of making several similar dies. The inserts can be made of stronger and harder materials, and they can be changed easily in the case of wear or failure in a particular section of the die.Upper die blockInsertInsertWorkpiece Fig.4-11DieinsertsusedindiesforforginganautomotiveaxlehousingAswiththepatternsusedincasting,allowancesareprovidedinforging-diedesignbecausemachiningtheforgingmaybenecessarytoobtainfinaldesireddimensionsandsurfacefinish.Machiningallowanceshouldbeprovidedatflanges,atholes,andatmatingsurfaces.4.5Forging MachinesA variety of forging machines are in use, with a range of capacities, speeds, and speed- stroke characteristics. These machines are generally classified as presses or hammers.4.5.1 Presses1. Hydraulic PressesThese presses operate at constant speeds and are load limited, or load restricted. In other words, a press stops if the load required exceeds its capacity. Large amounts of energy can be transmitted to a workpiece by a constant load throughout a stroke, the speed of which can be controlled. Because forging in a hydraulic press takes longer than in other types of forging machines, the workpiece may cool rapidly unless the dies are heated. Compared to mechanical presses, hydraulic presses are slower and involve higher initial cost, but they require less maintenance.A hydraulic press typically consists of a frame with two or four columns, pistons, cylinders (Fig. 4-12 (a), rams, and hydraulic pumps driven by electric motors. The main landing-gear support beam for the Boeing 747 aircraft is forged in a 450-MN (50,000-ton) hydraulic press, shown in Fig. 4-12 (c) (with the part in the forefront). This part is made of a titanium alloy and weighs approximately 1350 kg (1.35 tons).2. Mechanical PressesThese presses are basically of either the crank or the eccentric type (Fig. 4-12 (b). The speed varies from a maximum at the center of the stroke to zero at the bottom of the stroke, so they are stroke limited. The energy in a mechanical press is generated by a large flywheel powered by an electric motor. A clutch engages the flywheel to an eccentric shaft. A connecting rod translates the rotary motion into a reciprocating linear motion. A knuckle-joint mechanical press is shown in Fig. 4-12 (c). Because of the linkage design, very high forces can be applied in this type of press (see also Fig. 4-12 (a).The force available in a mechanical press depends on the stroke position; it becomes extremely high at the bottom dead center. Thus proper setup is essential to avoid breaking the dies or equipment components. Mechanical presses have high production rates; they are easier to automate and require less operator skill than do other types of forging machines. Press capacitie operation is repeated until the forging is completed.Screw presses are used for various open-die and closed-die forging operations; they are particularly suitable for small production quantities and precision parts, such as turbine blades. Capacities range from 1.4 MN to 280 MN (160 tons to 31,500 tons).4.5.2 HammersHammers derive their energy from the potential energy of the ram, which is converted into kinetic energy (Fig. 4-12 (e); thus they are energy limited. Unlike hydraulic presses, they operate at high speeds, and the resulting low forming time minimizes the cooling of a hot forging. Low cooling rates allow the forging of complex shapes, particularly those with thin and deep recesses. To complete the forging, several successive blows are usually made in the same die. Hammers are available in a variety of designs; they are the most versatile and the least expensive type of forging equipment.1. Gravity Drop HammersIn the operation of this hammer, a process called drop forging, the energy is derived from the free-falling ram (the hammer shown in Fig. 4-12 (e) is known as a board hammer). The available energy of the hammer is the product of the rams weight and the height of its drop. Ram weighs range from 180 kg to 4500 kg, with energy capacities ranging up to 120 kJ.2. Power Drop HammersIn this hammer, the rams downstroke is accelerated by steam, air, or hydraulic pressure at about 750 kPa. Ram weighs range from 225 kg to as much as 22,500 kg, with energy capacities ranging up to 1150 kJ.3. Counterblow HammersThis hammer has two rams that simultaneously approach each other horizontally or vertically to forge the part. As in open-die forging operations, the part may be rotated between blows for proper shaping of the workpiece during forging. Counterblow hammers operate at high speeds and transmit less vibration to their bases. Capacities range up to 1200 kJ.4. High-Energy-Rate MachinesIn a high-energy-rate machine, the ram is accelerated by inert gas at high pressure, and the part is forged in one blow at a very high speed. Although there are several types of these machines, various problems associated with their operation and maintenance, with die breakage, and with safety considerations have greatly limited their actual use in forging plants.4.5.3 Selection of Forging MachinesSeveral considerations are important in the selection of forging machines: force or energy requirements; the size, shape and complexity of the forging; the strength of the workpiece material;
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