1. Introduction to
Metal Forming Process

2. Metal Forming Metal Forming
Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces (V1 = V2, S1 > S2)
The tool, usually called a die, applies stresses that exceed the yield
strength of the metal
The metal takes a shape determined by the geometry of the die

3. Comparison of Crystal Structures
Metal Forming
Comparison of Crystal Structures
(a) 절삭가공
섬유조직
(a) 소성가공

4. Stresses in Metal Forming
Stresses to plastically deform the metal are usually compressive
Examples: rolling, forging, extrusion
However, some forming processes
Stretch the metal (tensile stresses)
Others bend the metal (tensile and compressive)
Still others apply shear stresses

5. Metal Forming
Stresses in Metal Forming

6. Material Properties in Metal Forming
Desirable material properties:
Low yield strength
High ductility
These properties are affected by temperature:
Ductility increases and yield strength decreases when work temperature is raised
Other factors:
Strain rate, friction, and lubrication

7. Basic Types of Deforming Processes
Metal Forming
Basic Types of Deforming Processes
Bulk deformation
Rolling
Forging
Extrusion
Wire and bar drawing
Sheet metalworking
Bending
Deep drawing
Cutting
Miscellaneous processes

8. Bulk Deformation Processes
Metal Forming
Bulk Deformation Processes
Characterized by significant deformations and massive shape changes
"Bulk" refers to workparts with relatively low surface area‑to‑volume ratios
Starting work shapes include cylindrical billets and rectangular bars

9. Figure. Basic bulk deformation processes: (a) rolling
Metal Forming
Rolling
Figure. Basic bulk deformation processes: (a) rolling

10. Metal Forming

11. Figure. Basic bulk deformation processes: (b) forging
Metal Forming
Forging
Figure. Basic bulk deformation processes: (b) forging

12. Metal Forming
Forging
FIGURE Schematic illustrations of stages in impression-die forging. Note the formation of a flash, or excess material that subsequently has to be trimmed off.

13. Metal Forming
Forging
FIGURE Typical load-stroke curve for impression-die forging. Note the sharp increase in load when the flash begins to form. Source: After T. Altan.

14. Metal Forming Open Die Forging
FIGURE (a) Schematic illustration of a cogging operation on a rectangular bar. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times along the length of the part. (b) Reducing the diameter of a bar by open-die forging; note the movements of the die and the workpiece. (c) The thickness of a ring being reduced by open-die forging.

15. Metal Forming Roll Forging
FIGURE Two illustrations of roll forging (cross-rolling) operations. Tapered leaf springs and knives can be made by this process using specially designed rolls. Source: After J. Holub.

16. Metal Forming Grain Flow ???
Grain flow lines in upsetting a solid, steel cylindrical specimen at elevated temperatures between two flat cool dies. Note the highly inhomogeneous deformation and barreling, and the difference in shape of the bottom and top sections of the specimen. The latter results from the hot specimen resting on the lower die before deformation proceeds. The lower portion of the specimen began to cool, thus exhibiting higher strength and hence deforming less than the top surface. Source: After J.A. Schey.

17. Metal Forming
Forged Wrenches

18. Metal Forming

19. Figure. Basic bulk deformation processes: (c) extrusion
Metal Forming
Extrusion
Figure. Basic bulk deformation processes: (c) extrusion

20. Metal Forming

21. Metal Forming
Extrusion Parts

22. Figure. Basic bulk deformation processes: (d) drawing
Metal Forming
Wire and Bar Drawing
Figure. Basic bulk deformation processes: (d) drawing

23. Metal Forming Sheet Metal Working
Forming and related operations performed on metal sheets, strips, and coils
High surface area‑to‑volume ratio of starting metal, which distinguishes these from bulk deformation
Often called pressworking because presses perform these operations
Parts are called stampings
Usual tooling: punch and die

24. Metal Forming

25. Metal Forming
Stamping Parts

26. Figure. Basic sheet metalworking operations: (a) bending
Metal Forming
Sheet Metal Bending
Figure. Basic sheet metalworking operations: (a) bending

27. Metal Forming
Sheet Metal Bending

28. Figure. Basic sheet metalworking operations: (b) drawing
Metal Forming
Deep Drawing
Figure. Basic sheet metalworking operations: (b) drawing

29. Metal Forming
Deep Drawing Parts

30. Figure. Basic sheet metalworking operations: (c) shearing
Metal Forming
Shearing of Sheet Metal
Figure. Basic sheet metalworking operations: (c) shearing

31. Metal Forming
Shearing of Sheet Metal > Clearance

32. Material Behavior in Metal Forming
Plastic region of stress-strain curve is primary interest because material is plastically deformed
In plastic region, metal's behavior is expressed by the flow curve:
where K = strength coefficient; and n = strain hardening exponent
 Flow curve based on true stress and true strain

33. Metal Forming
Material Behavior in Metal Forming (기계재료 자료참조)

34. Metal Forming Flow Stress
For most metals at room temperature, strength increases when deformed due to strain hardening
Flow stress = instantaneous value of stress required to continue plastic deforming the material
where Yf = flow stress, that is, the yield strength as a function of plastic strain

35. Metal Forming Average Flow Stress
Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest
For calculation of forming pressure using the average flow stress
where = average flow stress; and  = maximum strain during deformation process

36. Metal Forming
Flow Stress & Average Flow Stress

37. Temperature in Metal Forming
For any metal, K and n in the flow curve depend on temperature
Both strength (K) and strain hardening (n) are reduced at higher temperatures
In addition, ductility is increased at higher temperatures

38. Temperature in Metal Forming
Any deformation operation can be accomplished with lower forces and power at elevated temperature
Three temperature ranges in metal forming:
Cold working
Warm working
Hot working

39. Metal Forming Cold Working
Performed at room temperature or slightly above
Many cold forming processes are important mass production operations
Minimum or no machining usually required
These operations are near net shape or net shape processes

40. Advantages of Cold Forming
Metal Forming
Advantages of Cold Forming
Better accuracy, closer tolerances
Better surface finish
Strain hardening increases strength and hardness
Grain flow during deformation can cause desirable directional properties in product
No heating of work required

41. Disadvantages of Cold Forming
Metal Forming
Disadvantages of Cold Forming
Higher forces and power required in the deformation operation
Surfaces of starting workpiece must be free of scale and dirt
Ductility and strain hardening limit the amount of forming that can be done
In some cases, metal must be annealed to allow further deformation
In other cases, metal is simply not ductile enough to be cold worked

42. Metal Forming
Multi-step stamping

43. Metal Forming Warm Working
Performed at temperatures above room temperature but below recrystallization temperature
Dividing line between cold working and warm working often expressed in terms of melting point:
0.3Tm, where Tm = melting point (absolute temperature) for metal

44. Advantages of Warm Working
Metal Forming
Advantages of Warm Working
Lower forces and power than in cold working
More intricate work geometries possible
Need for annealing may be reduced or eliminated

45. Metal Forming Hot Working
Deformation at temperatures above the recrystallization temperature
Recrystallization temperature = about one‑half of melting point on absolute scale
In practice, hot working usually performed somewhat above 0.5Tm
Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level

46. Metal Forming
Hot Working

47. Metal Forming Why Hot Working ?
Capability for substantial plastic deformation of the metal ‑ far more than possible with cold working or warm working
Why?
Strength coefficient (K) is substantially less than at room temperature
Strain hardening exponent (n) is zero (theoretically)
Ductility is significantly increased

48. Heat treatment conditions
Metal Forming
Effect of annealing
(In case of sheet metal)
Original material
Heat treatment conditions
900 ℃, holding time 3min
800 ℃, holding time 3min
700 ℃, holding time 3min
Material
Y.S
(MPa)
T.S El
(%)
Hardness
(Hv)
STS 304-1/2H
Original material
651.4
851.8
38.4
270.07
700℃
618.5
845.7
52.5
258.76
800℃
552.5
827.3
57.2
237.32
900℃
272.1
691.3
60.9
155.08

49. Advantages of Hot Working
Metal Forming
Advantages of Hot Working
Workpart shape can be significantly altered
Lower forces and power required
Metals that usually fracture in cold working can be hot formed
Strength properties of product are generally isotropic
No strengthening of part occurs from work hardening
Advantageous in cases when part is to be subsequently processed by cold forming

50. Disadvantages of Hot Working
Metal Forming
Disadvantages of Hot Working
Lower dimensional accuracy
Higher total energy required (due to the thermal energy to heat the workpiece)
Work surface oxidation (scale), poorer surface finish
Shorter tool life

51. Strain Rate Sensitivity
Metal Forming
Strain Rate Sensitivity
Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0
The metal should continue to flow at the same flow stress, once that stress is reached
However, an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivity

52. Metal Forming What is Strain Rate ?
Strain rate in forming is directly related to speed of deformation v (m/s)
Deformation speed v = velocity of the ram or other movement of the equipment
Strain rate is defined:
(m/s/m or simply s-1)
where = true strain rate; and h = instantaneous height of workpiece being deformed (m)

53. Evaluation of Strain Rate
Metal Forming
Evaluation of Strain Rate
If deformation speed v is constant during the operation, strain rate will change as h changes.
In most practical operations, valuation of strain rate is complicated by
Workpart geometry
Variations in strain rate in different regions of the part
Strain rate can reach 1000 s-1 or more for some metal forming operations

54. Effect of Strain Rate on Flow Stress
Metal Forming
Effect of Strain Rate on Flow Stress
Flow stress is a function of temperature
At hot working temperatures, flow stress also depends on strain rate
As strain rate increases, resistance to deformation increases
This effect is known as strain‑rate sensitivity

55. Strain Rate Sensitivity
Metal Forming
Strain Rate Sensitivity
C: strength coefficient, similar to K
m: strain-rate sensitivity exponent
cold working <m<0.05
hot working <m<0.3
superplasticity 0.3<m<0.7
Newtonian fluid m=1
Figure. (a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log‑log coordinates.

56. Strain Rate Sensitivity Equation
Metal Forming
Strain Rate Sensitivity Equation
where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent
The value of C is determined at a strain rate of 1.0 and m is the slope of the curve in the previous figure.
Increasing temperature decreases the value of C and increases the value of m.

57. Effect of Temperature on Flow Stress ???
Metal Forming
Effect of Temperature on Flow Stress ???
Figure. Effect of temperature on flow stress for a typical metal. The constant C, as indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature.
 This is important in hot working because
deformation resistance of the material
increases so dramatically as strain rate
is increased.

58. Observations about Strain Rate Sensitivity
Metal Forming
Observations about Strain Rate Sensitivity
Increasing temperature decreases C and increases m
At room temperature, effect of strain rate is almost negligible
 Flow curve is a good representation of material behavior
As temperature increases, strain rate becomes increasingly important in determining flow stress

59. Friction in Metal Forming
In most metal forming processes, friction is undesirable:
Metal flow is retarded
Forces and power are increased
Tooling wears faster
Friction and tool wear are more severe in hot working

60. Friction in Metal Forming
입력에너지의 50%이상이 마찰 극복에 사용
제품의 표면조도와 치수정밀도는 마찰에 직접 관련
윤활의 변화 ⇒ 마찰변화, 재료유동의 형태를 바꿈 ⇒ 최종제품의 물성을 변화
※ 압연의 경우는 충분한 마찰 필요

61. Lubrication in Metal Forming
Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction
Benefits:
Reduced sticking, forces, power, tool wear
Better surface finish
Removes heat from the tooling

62. Considerations in Choosing a Lubricant
Metal Forming
Considerations in Choosing a Lubricant
Type of forming process (rolling, forging, sheet metal drawing, etc.)
Hot working or cold working
Work material
Chemical reactivity with tool and work metals
Ease of application
Cost