Fundamental of Materials Forming -Forging of Metals 金属锻造

Introduction A metal is shaped by compressive forces Oldest metal working process – 4000BC Can be performed with a hammer and anvil Typical forged products: Bolts Rivets Connecting rods Gears

Forging锻造 (a) (b) Figure 14.1 (a) Schematic illustration of the steps involved in forging a bevel gear with a shaft. Source: Forging Industry Association. (b) Landing-gear components for the C5A and C5B transport aircraft, made by forging. Source: Wyman-Gordon Company.

(c) Figure 14.1 (c) general view of a 445 MN (50,000 ton) hydraulic press. Source: Wyman-Gordon Company.

Outline of Forging and Related Operations Figure 14.2

Forging may be done at room temperature (cold forging) or at elevated temperatures (warm or hot forging, depending on the temperature, Section 1.7). Because 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, but it produces dimensional accuracy and surface finish that are not as good.

A component that can be forged successfully may also be manufactured economically by other methods, such as by casting, by powder metallurgy or by machining. However, as you might expect, each process will produce a part having different characteristics and limitations, particularly with regard to strength, toughness, dimensional accuracy, surface finish, and internal or external defects.

Grain Flow Comparison Figure 14.3 A part made by three different processes, showing grain flow. (a) casting, (b) machining, (c) forging. Source: Forging Industry Association.

Characteristics of Forging Processes

Upsetting Up setting or Upset Forging is the simplest case of open-die forging involving compression of a workpiece between two flat dies. Upset forging reduces the height of the workpiece but increases its cross-sectional area. We will consider upsetting of a round billet. Under ideal conditions where there is no friction between the work piece and the dies, the billet deforms homogeneously (the cylindrical shape of the billet remains cylindrical throughout the process). But in practical conditions the billet tends to barrel since there is some friction. 10

Upsetting镦粗 Figure 14.4 (a) Solid cylindrical billet upset between two flat dies. (b) Uniform deformation of the billet without friction. (c) Deformation with friction. Note barreling of the billet caused by friction forces at the billet-die interfaces.

Homogeneous upsetting of a cylindrical billet (without friction) Practical upsetting of a cylindrical billet (with friction & barreling) Groover V1 = upper die velocity Do, D, D1 = average billet diameters before, during and at the end of deformation Figure 19.11, Groover ho, h, h1 = billet heights before, during and at the end of deformation 12

Terminology (1) (2) In homogeneous upsetting / no friction: ho = starting height of workpiece (before deformation) h = instantaneous height of the work piece (at an intermediate press stroke) F = instantaneous upsetting force A = instantaneous cross sectional area of the workpiece (1) (2) 13

Terminology (3) (4) In homogeneous upsetting: K = strength coefficient, n = strain hardening coefficient F increases with deformation (press stroke) since Yf and A both increase with deformation and strain (Eqs. (1), (3) & (4)) (3) (4) 14

Terminology Practical Upsetting of a cylindrical workpiece (with friction & barreling): Where  = coefficient of friction (0.05 – 0.3) D = instantaneous workpiece diameter, mm (in), h = instantaneous workpiece height, mm (in) (5) (6) 15

Open-Die Forging Upsetting or flat-die forging – a solid workpiece is placed between flat dies and is compressed Barreling caused by frictional forces at the die-workpiece interfaces Can be minimized if a lubricant is used Thermal effects caused by barreling can be minimized by using heated dies

open-die forging开式模锻 The open-die forging process can be depicted(描述） by a solid workpiece placed between two flat dies and reduced in height by compressing it (Fig. 14.4a). This process is also called upsetting or flat-die forging自由锻造. 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. 14.4b. Because constancy of volume is maintained, any reduction in height increases the diameter of the forged part.

Open-Die Forging Is the simplest forging process Sizes can very from very small parts to very large parts

Cogging拔长 Cogging, 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. 14.5). Because the contact area per stroke is small, a long section of a bar can be reduced in thick- ness 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 design ,are often made by this process.

Figure 14. 5 Two views of a cogging operation on a rectangular bar Figure 14.5 Two views of a cogging operation on a rectangular bar. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil. Note the barreling of the workpiece.

Cold, warm and hot forging The forging operation (and metal forming operations in general) can be performed at various temperatures ranges: Where Tm is the melting temperature of the metal Note: for most metals, recrystallization occurs between 30% and 50% of The melting temperature Cold forging T < 0.3 Tm Warm forging 0.3 Tm < T < 0.5 Tm Hot forging T > 0.5 Tm and usually less than 0.75 Tm 21

Cold, warm and hot forging Cold forging vs. Hot forging: You have to think about the reasons behind each of the above mentioned points Cold Hot Strength/ hardness of the forged billet Higher Lower Ductility/ ability to produce intricate shapes Force/ energy/ machine capacity required Load on the tools (dies) Tool wear Less More Dimensional accuracy Better Worse Surface finish The need for heating equipment No Yes Barreling (uniformity of deformation) 22

Original steel specimen Comparison of cold and hot forging: Steel specimen after cold forging (5 steps/hits) Original steel specimen Steel specimen after hot forging 23

closed-Die Forging 闭式模锻 Figure 14.6 Stages in impression-die forging of a solid round billet. Note the formation of flash, which is excess metal that is subsequently trimmed off (see Fig. 14.8).

In impression-die压印 forging, the workpiece acquires the shape of the die cavities impressions) while being forged between two shaped dies (Fig, 14.6). Note that some of the material flows outward and forms a flash. The flash has a significant role in the 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 file filling of the die cavity.

Open-Die Forging

Forging [Heated] metal is beaten with a heavy hammer to give it the required shape Hot forging, open-die

Slider-crank Mechanism TDC BDC Connecting Rod Shut Height Adjustment Force BDC = Bottom Dead Center Bottom position of the slide TDC = Top Dead Center Top position of the slide The length of connecting rod determines the TDC and BDC or shut height. Its length can be modified. The total slide stroke S = 2r is unchanged 28

29 Flywheel Load reading system Connecting rod Shut height adjustment Power box to start and stop the press Yellow pedal to cycle the press Connecting rod Press slide 29

Forging Shims and shut height adjustment: 30 One of the 5 Shims Screw for shut height adjustment Handle to adjust the stroke One of the 5 Shims Slot to place handle and rotate slide screw Shims and shut height adjustment: 30

The Loadgard and the power box of the press Load Reading Reset Button Start Button Stop Button 31

Hardness testing machine Hardness Reading Indenter Handle to load the specimen Plate on which to place the specimen 32

Figure 14.7 (a) Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash required to ensure proper filling of the die cavities. (b) Fullering, and (c) edging operations to distribute the material when preshaping the blank for forging.

CLOSED-DIE FORGING闭式模锻

Impression-Die & Closed-Die Forging The workpiece acquires the shape of the die cavities while being forged between the two shaped dies

Impression-Die & Closed-Die Forging The blank to be forged is prepared by: Cutting from a bar stock Preformed blank Casting Preformed blank from prior forging

Impression-Die & Closed-Die Forging Fullering & edging are used to distribute the material Fullering – material is distributed away from an area Edging – material is gathered into an area Blocking – rough shaping of the part Impression dies – give the part its final shape

Precision Forging Used for economic reasons The part formed is close to the final dimensions Less machining is needed Higher capacity equipment is needed Aluminum and Magnesium alloys work well in the process

Trimming Flash from a Forged Part Figure 14.8 Trimming flash from a forged part. Note that the thin material at the center is removed by punching.

Comparison of Forging With and Without Flash Figure 14.9 Comparison of closed-die forging to precision or flashless forging of a cylindrical billet. Source: H. Takemasu, V. Vazquez, B. Painter, and T. Altan.

Precision Forging精锻 For 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 is close to the final dimensions of the desired component are known as near-net-shape or net-shape forging. The advantages of net-shape forming were described in the General Introduction. 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 part. 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 billet's 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.

Coining精压 Figure 14.10 (a) Schematic illustration of the coining process. the earliest coins were made by open-die forging and lacked sharp details. (b) An example of a coining operation to produce an impression of the letter E on a block of metal. Used for minting coins, medallions, & jewelryLubricants can not be used in coining Can be used to improve surface finish

Range of k Values for Equation F=kYfA The forging force, F, required to carry out an impression-die forging operation can be estimated from the formula F=kYfA where k is a multiplying factor (obtained from Table 14.2), Yf is the flow stress of the material at the forging temperature, and A is the projected area of the forging, including the flash.In hot-forging operations, the actual forging pressure for most metals ranges from 550 MPa to 1000 MPa

Heading镦头 Heading is essentially an upsetting operation, us)Jally performed at the end of a round rod or wire in order to produce a larger cross-section. Typical examples are the heads of bolts螺栓, screws螺丝钉, rivets,铆钉, nails钉子, and other fasteners紧固件 (Fig. 14.1 la). Heading processes can be carried out cold, warm, or hot; they are performed on machines called headers, which are usually highly automated. Production rates are hundreds of pieces per minute for small parts. These machines tend to be noisy; soundproof enclosure or the use of ear protectors is required. Heading operations can be combined with cold-extrusion processes (Section 15.5) to make various parts.

Heading/Upset Forging Figure 14.11 (a) Heading operation, to form heads on fasteners such as nails and rivets. (b) Sequence of operations to produce a bolt head by heading.

Piercing穿孔 Piercing is a process of indenting成穴(but not breaking through) the surface of a workpiece with a punch in order to produce a cavity or an impression (Fig, 14,12). The workpiece may be confined in a die cavity, or it may be unconstrained. Piercing may be followed by punching, to produce a hole in the part. Piercing is also performed to produce hollow regions in forgings, using side-acting auxiliary equipment. The piercing force depends on the cross-sectional area and the tip geometry of the punch, on the strength of the material, and on the magnitude of friction at the sliding inter faces. The pressure may range from three to five times the strength of the material-roughly the same level of stress required to make an indentation in hardness testing (Section 2.6.2).

Grain Flow Pattern of Pierced Round Billet Figure 14.12 A pierced round billet, showing grain flow pattern. Source: Courtesy of Ladish Co., Inc.

Roll-Forging滚锻 Figure 14.13 Two examples of the roll-forging operation, also known as cross-rolling(楔横轧）. Tapered leaf springs and knives can be made by this process. Source: (a) J. Holub; (b) reprinted with permission of General Motors Corporation.

Production of Bearing Blanks Figure 14.14 (a) Production of steel balls by the skew-rolling斜轧 process. (b) Production of steel balls by upsetting a cylindrical blank. Note the formation of flash. The balls made by these processes are subsequently ground and polished for use in ball bearings (see Sections 25.6 and 25.10).

Orbital forging （摆碾） Orbital forging is a process in which the upper die moves along an orbital path (Fig. 14.15a) and forms the part incrementally. The operation is similar to the action of a mortar and pestle. Typical components forged by this process are disk-shaped and conical parts (Fig. 14.15b), such as bevel gears. The forging force is relatively small because, at any instant, the die contact is concentrated onto a small area of the workpiece. The operation is relatively quiet, and parts can be formed within 10-20 cycles of the orbiting die.

Orbital Forging Figure 14.15 (a) Various movements of the upper die in orbital forging (also called rotary, swing, or rocking-die forging); the process is similar to the action of a mortar and pestle. (b) An example of orbital forging. Bevel gears, wheels, and rings for bearings can be made by this process.

Swaging环锻 Figure 14.16 (a) Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a tubular workpiece by swaging. (c) A die-closing type swaging machine, showing forming of a stepped shaft. (d) Typical parts made by swaging.

Swaging of Tubes With and Without a Mandrel Figure 14.17 (a) Swaging of tubes without a mandrel; not the increase in wall thickness in the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the tube depends on the mandrel diameter. (c) Examples of cross-sections of tubes produced by swaging on shaped mandrels. Rifling (spiral grooves) in small gun barrels can be made by this process.

FORGING-DIE DESIGN锻模设计 Tile design of forging dies requires a 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 undcr high lorging loads is an important consideration, 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 properl fills the die cavities. An example of the intermediate shapes for a connecting rod were shown in Fig. 14.7a. The importance of prefoming can be appreciated by noting bow a piece of dough is preshaped to make a pic crust or how ground meat is preshaped to make a hamburger.

Preshaping预成形 In 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 interfaces should be minimized in order to reduce wear. Selection of shapes reqnires considerable experience and involves calculations of cross-sectienal 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 ill 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.

Impression-Forging Die and Die Inserts Figure 14.18 Standard terminology for various features of a typical impression-forging die. Figure 14.19 Die inserts used in dies for forging an automotive axle housing. (See Tables 5.5 to 5.7 for die materials.) Source: Metals Handbook, Desk Edition. ASM International, Metals Park, Ohio, 1985. Used with permission.

Draft angles 表2-8 各种金属锻件常用的模锻斜度 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 internal draft angles are made larger than cxternal ones. Internal angles are about 7oto 10o external angles about 3o to 5o. 为便于从模膛中取出锻件，模锻件上平行于锤击方向的表面必须具有斜度，称为模锻斜度，一般为5°～15°之间。模锻斜度与模膛深度和宽度有关，通常模膛深度与宽度的比值（h／b）较大时，模锻斜度取较大值。此外，模锻斜度还分为外壁斜度α与内壁斜度β，如图2-28所示。外壁指锻件冷却时锻件与模壁离开的表面；内壁指当锻件冷却时锻件与模壁夹紧的表面。内壁斜度值一般比外壁斜度大2°～5°。生产中常用金属材料的模锻斜度范围见表2-8。 表2-8 各种金属锻件常用的模锻斜度 锻件材料 外壁斜度 内壁斜度 铝、镁合金 钢、钛、耐热合金 3°～5° 5°～7° 7°10°12°

DIE MATERIALS AND LUBRICATION Die Materials. Most forging operations, particularly for large parts, are carried out at elevated temperatures. General requirements for die materials therefore are (a) strength and toughness at elevated temperatures, 高温下的强度和刚度 (b) hardenability and ability to harden uniformly, 淬透性 (c) resistance to mechanical and thermal shock, 抗机械和热冲击性能 (d) wear resistance, particularly resistance to abrasive wear, 耐磨性

lubrication润滑 Lubricants greatly influence friction and wear; consequently, they affect the forces required and the flow of the metal in die cavities. A wide variety of lubricants can be used in forging. For hot forging, graphite（石磨）, molybdenum disulfide（二硫化钼）, and sometimes glass are used. For cold forging, mineral oils and soaps are common lubricants. In hot forging, the lubricant is usually applied directly to the dies; in cold forging, it is applied to the workpiece. The method of application and the uniformity of the lubricant's thickness on the blank are important to product quality.

Forgeability 可锻性 Forgeability is generally defined as the capability of a material to undergo deformation without cracking. A number of tests have been developed to quantify forgeability, although none is accepted universally. A commonly used test is to upset a solid cylindrical specimen and observe any cracking on the barreled surfaces (see Fig. 2.19d); the greater the deformation prior to cracking, the greater the forgeability of the metal.

Classification of Metals in Decreasing Order of Forgeablilty

Forging Defects In addition to surface cracking during forging, other defects can develop as a result of the material Bow pattern in the die. If there is an insufficient volume of material to fill the die cavity, the web may buckle皱折 during forging and develop laps重迭 (Fig. 14.20a). On the other hand, if the web金属簿条 is thick, the excess material flows past the already formed portions of the forging and develops interual cracks (Fig. 14.20b).

Defects in Forged Parts Figure 14.20 Examples of defects in forged parts. (a) Labs formed by web buckling during forging; web thickness should be increased to avoid this problem. (b) Internal defects caused by oversized billet; die cavities are filled prematurely, and the material at the center flows past the filled regions as the dies close.

FORGING MACHINES A variety of forging machines are in use.These machines are generally classified as presses or hammers. power drop hammer 蒸汽锤 锻锤, 对击锤

Principles of Various Forging Machines Figure 14.21 Schematic illustration of the principles of various forging machines. (a) Hydraulic press液压机. (b) Mechanical press机械压力机 with an eccentric drive; the eccentric shaft can be replaced by a crankshaft to give the up-and-down motion to the ram. (continued) eccentric 偏心轮 crankshaft n.曲轴 ram n.公羊, 撞锤

Principles of Various Forging Machines (cont.) Figure 14.21 (continued) Schematic illustration of the principles of various forging machines. (c) Knuckle-joint press关节压机. (d) Screw press旋压机. (e) Gravity drop hammer.重力锤

Finite Element (FE) Simulations The next several slides illustrate the simulation of the cylinder, ring, and hot cylinder compression tests, generated by FEA. The following slides include: - Cold upsetting of Al 1100 cylinders (σ = 25.2 ε0.304 Ksi) - Comparison of Al 1100 and Steel AISI 1010 (σ = 103.8 ε0.22 Ksi) upsetting with respect to forging load - Illustration of the effect of μ on the internal diameter in ring compression test of Steel AISI 1010 - Comparison between the upsetting of Steel AISI 1015 at room temperature (68 oF: σ=117.5 ε0.15 Ksi) and at elevated temperature (1112 oF: σ=54.7 ε0.072 Ksi) 71

Simple upsetting simulation Cold upsetting of Aluminum 1100 σ = 25.2 ε0.304 Ksi μ = 0.12 Stage A Stage B Stage C Note the barreling 72

Simple upsetting simulation Load-Stroke curves for Al 1100 (previous simulation) and Steel AISI 1010 σAl = 25.2 ε0.304 Ksi σsteel = 103.8 ε0.22 Ksi μ = 0.12 Note how material properties affect the upsetting force 73

Ring compression simulation Ring compression of Steel AISI 1010 (σ = 103.8 ε 0.22 Ksi) with Two different Friction coefficients: note how increasing μ reduces the final internal diameter. This idea will be used in the lab to determine μ Before After (μ = 0.12) After (μ = 0.3) Note the change in internal diameter 74

Hot vs. Cold upsetting A comparison between cold and hot upsetting of steel: note the following: 1- The load in cold forging > hot forging 2- Barreling in cold forging < hot forging Cold Hot 75