CHAPTER 14. Forging of Metals

Transcription

1 CHAPTER 14 Forging of Metals

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7 Forging (a) (b) (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. 7

8 (c) Figure 14.1 (c) general view of a 445 MN (50,000 ton) hydraulic press. Source: Wyman-Gordon Company. 8

9 Grain Flow Comparison Figure 14.2 A part made by three different processes, showing grain flow. (a) casting, (b) machining, (c) forging. Source: Forging Industry Association. 9

10 Grain Flow Pattern in Forging Figure A pierced round billet, showing grain flow pattern. Source: Courtesy of Ladish Co., Inc. 10

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12 Open die Closed die Upper die Workpiece Lower die 簡單 精度較差 複雜 精度較高 12

13 Upsetting Forging force, F Y f r r 3h Figure 14.3 (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. 13

14 Cogging Figure 14.4 (a) Schematic illustration 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. Reduction in thickness is accompanied by barreling, as in Fig. 14.3c. (b) Reducing the diameter of a bar by open-die forging; note the movements of the dies and the workpiece. (c) The thickness of a ring being reduced by open-die forging. 14

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16 Why need to know forging force Selection of machine Control of dimensions and shapes of product F Y f r 2 r 2 1 3h h r X; F 16

17 X; F h r F Y f r h Stroke (X) 17

18 Example Given:strainless steel 304, d 0 =150mm, h 0 =100mm, h f = 0.5 h 0 = 50mm, μ= 0.2 Find: F (at the end of forging stroke) Solution: r f Y F f d ln f 2 h h 0 f 1000MPa Y r d N 0 ln h h 0 f r 3h mm lb 0.69 ( Figure 2.6) f ton

19 True Stress-True Strain Curves Figure 2.6 True stress-true strain curves in tension at room temperature for various metals. The curves start at a finite level of stress: The elastic regions have too steep a slope to be shown in this figure, and so each curve starts at the yield stress, Y, of the material. 19

20 Impression-Die Forging Figure 14.5 (a) through (c) 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. 14.7). (d) Standard terminology for various features of a forging die. 20

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22 Forging a Connecting Rod 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 properly distribute the material when preshaping the blank for forging. 22

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24 20% higher strength than die casting 24

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26 Stepped Pin (a) The stepped pin used in Case Study (b) Illustration of the manufacturing steps used to produce the stepped pin. Source: Courtesy of National Machinery, LLC. 26

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29 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. 29

30 Comparison of Forging With and Without Flash Figure 14.9 Comparison of closed-die forging with flash (left side of each illustration) and precision or flashless forging (right side) of a round billet. Source After H. Takemasu, V. Vazquez, B. Painter, and T. Altan. 30

31 Impression die (with flash) Closed die (without flash, more precise control of material volume) Use of thin flash Strength (Cools rapidly) Flow resistance (more friction) enhance cavity filling Flash Thickness : 3 % of workpiece thickness Length : 2~5 times of thickness 31

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33 Forging Force in Impression/Closed-Die F = ky f A 33

34 Heading/Upset Forging Figure (a) Heading operation, to form heads on fasteners such as nails and rivets. (b) Sequence of operations to produce a bolt head by heading. 34

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38 Swaging with and without a Mandrel Figure (a) Swaging of tubes without a mandrel; note 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 crosssections of tubes produced by swaging on shaped mandrels. Rifling (internal spiral grooves) in small gun barrels can be made by this process. 38

39 Temperature Ranges for Various Processes TABLE 1.2 Process Cold working Warm working Hot working T/T m < to 0.5 >

40 Annealing Cold working T<0.3T m Hot working T>0.6T m Warm working in between Figure 1.14 Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: G. Sachs. 40

41 Metals in Decreasing Order of Forgeability 41

42 Characteristics of Forging Processes TABLE 14.1 Process Advantages Limitations Open die Simple, inexpensive dies; useful for small quantities; wide range of sizes available; good strength characteristics Limited to simple shapes; difficult to hold close tolerances; machining to final shape necessary; low production rate; relatively poor utilization of Closed die Relatively good utilization of material; generally better properties than open-die forgings; good dimensional accuracy; high production rates; good reproducibility material; high degree of skill required High die cost for small quantities; machining often necessary Blocker type Low die costs; high production rates Machining to final shape necessary; thick webs and large fillets necessary Conventional type Requires much less machining than blocker type; high production rates; good utilization of material Somewhat higher die cost than blocker type Precision type Close tolerances; machining often unnecessary; very good material utilization; very thin webs and flanges possible Requires high forces, intricate dies, and provision for removing forging from dies 42

43 Ch 14.6 Die design (core technology in forming, compared to mask in IC manufacturing) Parting line ( 分模線 ): avoid side thrust Flash: higher success rate for production Draft: 外部 3 ~4 ; 內部 7 ~10 Enough radii of corner/fillet: smooth flow of material and longer die life 43

44 Die Material Strength and Toughness (at high temperature) Hardenability Shock Resistance Wear Resistance 44

45 Lubrication friction wear of die cooling rate of product Parting agent ( 幫助脫模 ) 45

46 Defects in Forged Parts Figure 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. 46

47 Principles of Various Forging Machines Crank Figure Schematic illustration of the principles of various forging machines. (a) Hydraulic press. (b) Mechanical press with an eccentric drive; the eccentric shaft can be replaced by a crankshaft to give the up-and-down motion to the ram. (continued) 47

48 Principles of Various Forging Machines Knucklejoint Figure Schematic illustration of the principles of various forging machines. (c) Knuckle-joint press. (d) Screw press. (e) Gravity drop hammer. 48

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52 Speed Ranges of Forging Equipment 52

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55 Unit Cost in Forging Figure Typical unit cost (cost per piece) in forging; note how the setup and the tooling costs per piece decrease as the number of pieces forged increases, if all pieces use the same die. 55

56 Relative Unit Costs of a Small Connecting Rod Figure Relative unit costs of a small connecting rod made by various forging and casting processes. Note that, for large quantities, forging is more economical. Sand casting is the more economical process for fewer than about 20,000 pieces. 56

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58 Manufacture of a spark plug body: (left) by machining from hexagonal bar stock; (right) by cold forming. Note the reduction in waste. (Courtesy of National Machinery Co.) 58

59 Ch 14 精讀 :14.2, 14.6, 14.9 (pay attention to Figure &14.19) 略讀 : 14.1, 14.3, 14.5,