1. Lecture 9: Metal Forming
Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces
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

2. 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 and friction

3. Basic Types of Deformation Processes
Bulk deformation
Rolling
Forging
Extrusion
Wire and bar drawing
Sheet metalworking (Lecture 10)
Bending
Deep drawing
Cutting
Miscellaneous processes

4. 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

5. 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 s = stress (MPa,psi), K = strength coefficient (MPa,psi), e = true strain, and n = strain hardening exponent
Flow curve based on true stress and true strain

6. 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 deforming the material
where Yf = flow stress (MPa,psi), that is, the yield strength as a function of strain. But to estimate the force and power in rolling the average flow stress is used
Where = average flow stress (MPa,psi)

7. Example Problem 1
(SI units) Austenitic stainless steel has a flow curve with strength coefficient = 1200 MPa and strain-hardening exponent = A tensile test specimen with gage length = 100 mm is stretched to a length = 145 mm. Determine the flow stress and average flow stress that the metal experienced at this strain.
Solution:
 = ln (145/100) = ln 1.45 = 0.372
Flow stress Yf = 1200(0.372)0.40 = 808 MPa
Average flow stress = 1200(0.372)0.40/1.40 = 577 MPa

8. 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

9. 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  Room Temp.<T<0.3Tm, where Tm = melting point (absolute temperature) for metal
If it is lower than room Temp., use room Temperature
Warm working  0.3Tm<T<0.5Tm
Hot working  0.5Tm<T
Note that Tm must be in degree Rankine or Kelvin.
oK = oC oR = oF + 460

10. 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

11. 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

12. 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

13. What is Strain Rate?
Strain rate in forming is directly related to speed of deformation v
Deformation speed v = velocity of the ram or other movement of the equipment (m/s, ft/s)
Strain rate is defined:
h = instantaneous height of workpiece being deformed (m, ft)

14. 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

15. 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.

16. Figure 19.1 The rolling process (specifically, flat rolling).
Deformation process in which work thickness is reduced by compressive forces exerted by two opposing rolls
Figure The rolling process (specifically, flat rolling).

17. Diagram of Flat Rolling
Figure Side view of flat rolling, indicating before and after thicknesses, work velocities, angle of contact with rolls, and other features.

18. Types of Rolling Based on workpiece geometry :
Flat rolling - used to reduce thickness of a rectangular cross section
Shape rolling - square cross section is formed into a shape such as an I‑beam
Based on work temperature :
Hot Rolling – most common due to the large amount of deformation required
Cold rolling – produces finished sheet and plate stock

19. Rolled Products Made of Steel
Figure Some of the steel products made in a rolling mill.

20. Flat Rolling Terminology
Draft = amount of thickness reduction
where d = draft; to = starting thickness (in, or mm); and tf = final thickness (in, or mm)
Reduction = draft expressed as a fraction of starting stock thickness:
r = d/to
where r = reduction

21. Conservation of matter
Volume of metal exiting the rolls equal the volume entering.
towovo=tf wf vf
to and tf are the before and after work thickness.
wo and wf are the before and after work widths.
vo and vf are the entering and exiting velocities.

22. Contact length Contact length can be approximated by
Where to and tf are the before and after work thickness.
and R = roll radius (inch or mm)

23. Rolling force F
Can be calculated based on the average flow stress experienced by the work material in the roll gap. That is,
F = w L
where L is contact length, w is the width of work material.
and = average flow stress (psi, or MPa);
Force unit is pound or (N in metric system)

24. Required Power
Power is calculated based on the fact that rolling mill consists of two powered rolls (in.lb/min or Watt).
T = Torque (lb.in or N.m)
N = rotational speed (rev/min)
F = rolling force ( lb, or N)
L = contact length (inch or mm)
Watt = N.m/s = N. mm/min × 1/60,000
HP = 396,000 in.lb/min

25. Example problem 2
(USCS units) A 4-in-thick aluminum slab is 10 in wide and 100 in long. It is reduced in one pass in a two‑high rolling mill to a thickness = 3.5 in. The roll rotates at a speed = 12 rev/min and has a radius = 20 in. The aluminum has a strength coefficient = 25,000 lb/in2 and a strain hardening exponent = 0.20 (Table 3.4). Determine (a) roll force, (b) roll torque, and (c) power required to accomplish this operation.
Solution:
(a) Draft d = 4.00 – 3.5 = 0.5 in,
Contact length L = (20 x 0.5)0.5 = 3.16 in
True strain  = ln(4.0/3.5) = ln =
= 25,000(0.1335)0.20/1.20 = 13,928 lb/in2
Rolling force F = wL = 13,928(10)(3.16) = 440,125 lb
(b) Torque T = 0.5FL = 0.5(440,125)(3.16) = 789,700 lb-in.
(c) N = 12 rev/min
Power P = 2(12)(440,125)(3.16) = 104,863,473 in-lb/min
HP = (104,863,473 in-lb/min)/(396,000) = 265 hp

26. Two-High Rolling Mill
Figure Various configurations of rolling mills: (a) 2‑high rolling mill.

27. Figure 19.5 Various configurations of rolling mills: (d) cluster mill
Multiple backing rolls allow even smaller roll diameters
Figure Various configurations of rolling mills: (d) cluster mill

28. Thread Rolling
Bulk deformation process used to form threads on cylindrical parts by rolling them between two dies
Important commercial process for mass producing bolts and screws
Performed by cold working in thread rolling machines
Advantages over thread cutting (machining):
Higher production rates
Better material utilization
Stronger threads and better fatigue resistance due to work hardening

29. Thread Rolling
Figure Thread rolling with flat dies: (1) start of cycle, and (2) end of cycle.

30. Ring Rolling
Deformation process in which a thick‑walled ring of smaller diameter is rolled into a thin‑walled ring of larger diameter
As thick‑walled ring is compressed, deformed metal elongates, causing diameter of ring to be enlarged
Hot working process for large rings and cold working process for smaller rings
Applications: ball and roller bearing races, steel tires for railroad wheels, and rings for pipes, pressure vessels, and rotating machinery
Advantages: material savings, ideal grain orientation, strengthening through cold working

31. Ring Rolling
Figure 19.7 Ring rolling used to reduce the wall thickness and increase the diameter of a ring: (1) start, and (2) completion of process.

32. Forging
Deformation process in which work is compressed between two dies
Oldest of the metal forming operations, dating from about 5000 B C
Components: engine crankshafts, connecting rods, gears, aircraft structural components, jet engine turbine parts
Also, basic metals industries use forging to establish basic form of large parts that are subsequently machined to final shape and size

33. Classification of Forging Operations
Cold vs. hot forging:
Hot or warm forging – most common, due to the significant deformation and the need to reduce strength and increase ductility of work metal
Cold forging – advantage: increased strength that results from strain hardening
Impact vs. press forging:
Forge hammer - applies an impact load
Forge press - applies gradual pressure

34. Types of Forging Dies
Open‑die forging - work is compressed between two flat dies, allowing metal to flow laterally with minimum constraint
Impression‑die forging - die contains cavity or impression that is imparted to workpart
Metal flow is constrained so that flash is created
Flashless forging - workpart is completely constrained in die
No excess flash is created

35. Open-Die Forging
Figure Homogeneous deformation of a cylindrical workpart under ideal conditions in an open‑die forging operation: (1) start of process with workpiece at its original length and diameter, (2) partial compression, and (3) final size.

36. Required Force for Open-Die Forging

37. Example problem 3 Solution:
(USCS units) A cylindrical work part has a diameter = 2.5 in and a height = 4.0 in. It is upset forged to a height = 2.75 in. Coefficient of friction at the die‑work interface = The work material has a flow curve with strength coefficient = 25,000 lb/in2 and strain hardening exponent = Determine the force needed to perform this operation.
Solution:
Volume of cylinder V = D2h/4 = (2.5)2(4.0)/4 = in3
At h =2.75,  = ln(4.0/2.75) = ln =
Yf = 25,000(0.3747)0.22 = 20,144 lb/in2
V = in3 calculated above.
At h = 2.75 in, A = V/h = /2.75 = in2
Corresponding D = in (from A = D2/4)
Kf = (0.1)(3.015)/2.75 = 1.044
F = 1.044(20,144)(7.140) = 150,136 lb

38. Impression‑Die Forging
Compression of workpart by dies with inverse of desired part shape
Flash is formed by metal that flows beyond die cavity into small gap between die plates
Flash must be later trimmed, but it serves an important function during compression:
As flash forms, friction resists continued metal flow into gap, constraining material to fill die cavity
In hot forging, metal flow is further restricted by cooling against die plates

39. Impression-Die Forging
Figure Sequence in impression‑die forging: (1) just prior to initial contact with raw workpiece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates.

40. Advantages and Limitations
Advantages of impression-die forging compared to machining from solid stock:
Higher production rates
Less waste of metal
Greater strength
Favorable grain orientation in the metal
Limitations:
Not capable of close tolerances
Machining often required to achieve accuracies and features needed

41. Flashless Forging
Compression of work in punch and die tooling whose cavity does not allow for flash
Starting workpart volume must equal die cavity volume within very close tolerance
Process control more demanding than impression‑die forging
Best suited to part geometries that are simple and symmetrical
Often classified as a precision forging process

42. Flashless Forging
Figure Flashless forging: (1) just before initial contact with workpiece, (2) partial compression, and (3) final punch and die closure.

43. Forging Hammers (Drop Hammers)
Apply impact load against workpart
Two types:
Gravity drop hammers - impact energy from falling weight of a heavy ram
Power drop hammers - accelerate the ram by pressurized air or steam
Disadvantage: impact energy transmitted through anvil into floor of building
Commonly used for impression-die forging

44. Forging Presses
Apply gradual pressure to accomplish compression operation
Types:
Mechanical press - converts rotation of drive motor into linear motion of ram
Hydraulic press - hydraulic piston actuates ram
Screw press - screw mechanism drives ram

45. Upsetting and Heading
Forging process used to form heads on nails, bolts, and similar hardware products
More parts produced by upsetting than any other forging operation
Performed cold, warm, or hot on machines called headers or formers
Wire or bar stock is fed into machine, end is headed, then piece is cut to length
For bolts and screws, thread rolling is then used to form threads

46. Upset Forging
Figure An upset forging operation to form a head on a bolt or similar hardware item The cycle consists of: (1) wire stock is fed to the stop, (2) gripping dies close on the stock and the stop is retracted, (3) punch moves forward, (4) bottoms to form the head.

47. Swaging
Accomplished by rotating dies that hammer a workpiece radially inward to taper it as the piece is fed into the dies
Used to reduce diameter of tube or solid rod stock
Mandrel sometimes required to control shape and size of internal diameter of tubular parts

48. Swaging
Figure Swaging process to reduce solid rod stock; the dies rotate as they hammer the work In radial forging, the workpiece rotates while the dies remain in a fixed orientation as they hammer the work.

49. Trimming After Impression-Die Forging
Figure Trimming operation (shearing process) to remove the flash after impression‑die forging.

50. Extrusion
Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross‑sectional shape
Process is similar to squeezing toothpaste out of a toothpaste tube
In general, extrusion is used to produce long parts of uniform cross sections
Two basic types:
Direct extrusion
Indirect extrusion

51. Figure 19.30 Direct extrusion.

52. Hollow and Semi-Hollow Shapes
Figure (a) Direct extrusion to produce a hollow or semi‑hollow cross sections; (b) hollow and (c) semi‑hollow cross sections.

53. Indirect Extrusion
Figure Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross section.

54. Advantages of Extrusion
Variety of shapes possible, especially in hot extrusion
Limitation: part cross section must be uniform throughout length
Grain structure and strength enhanced in cold and warm extrusion
Close tolerances possible, especially in cold extrusion
In some operations, little or no waste of material

55. Wire and Bar Drawing
Cross‑section of a bar, rod, or wire is reduced by pulling it through a die opening
Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion)
Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening

56. Figure 19.40 Drawing of bar, rod, or wire.
Wire and Bar Drawing
Figure Drawing of bar, rod, or wire.

57. Figure 19.43 Draw die for drawing of round rod or wire.
Draw Die Details
Figure Draw die for drawing of round rod or wire.

58. Preparation of Work for Drawing
Annealing – to increase ductility of stock
Cleaning - to prevent damage to work surface and draw die
Pointing – to reduce diameter of starting end to allow insertion through draw die

59. 1- Reading Assignment 2- Written Assignment Week1 – Rolling
Chapters 17, and 18
2- Written Assignment
Week1 – Rolling
What are the differences between bulk deformation processes and sheet metal processes?
How does increasing temperature affect the parameters in the flow curve equation?
Name the four basic bulk deformation processes.
What is rolling in the context of the bulk deformation processes?
Besides flat rolling and shape rolling, identify some additional bulk forming processes that use rolls to effect the deformation.
(USCS units) Annealed low-carbon steel has a flow curve with strength coefficient = 75,000 lb/in2 and strain-hardening exponent = A tensile test specimen with gage length = 2.0 in is stretched to a length = 3.3 in. Determine the flow stress and average flow stress that the metal experienced during this deformation.
(SI units) A low-carbon steel plate is 300 mm wide and 25 mm thick. It is reduced in one pass in a two‑high rolling mill to a thickness of 20 mm. Roll radius = 600 mm, and roll speed = 8 rev/min. Strength coefficient = 500 MPa, and strain hardening exponent = 0.25 (Table 3.4). Determine (a) roll force, (b) roll torque, and (c) power required to perform the operation.

60. 1- Reading Assignment 2- Written Assignment Week2 – Forging
Chapters 17, and 18
2- Written Assignment
Week2 – Forging
What is forging?
Why is flash desirable in impression die forging?
What are the two basic types of forging equipment?
What is extrusion?
What are wire drawing and bar drawing?
(USCS units) A cylindrical work part with diameter = 2.5 in, and height = 2.5 in is upset forged in an open die to a height = 1.5 in. Coefficient of friction at the die work interface = The work metal has a strength coefficient = 40,000 lb/in2 and strain hardening exponent = Yield strength = 15,750 lb/in2. Determine the instantaneous force in the operation just as the height is reached to h = 2.3 in.