1. Fundamentals of Metal Forming Chapter 18
Manufacturing Processes, MET1311
Dr Simin Nasseri
Southern Polytechnic State University

2. FUNDAMENTALS OF METAL FORMING
Overview of Metal Forming
Material Behavior in Metal Forming
Temperature in Metal Forming

3. 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

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

6. Basic Types of Deformation Processes
Bulk deformation
Rolling
Forging
Extrusion
Wire and bar drawing
Sheet metalworking
Bending
Deep drawing
Cutting
Miscellaneous processes

7. Bulk Deformation

8. 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 18.2 Basic bulk deformation processes: (a) rolling
The slab is heated in a furnace and rolled between powered rollers until the plate is made with desirable thickness.
Figure Basic bulk deformation processes: (a) rolling

10. Figure 18.2 Basic bulk deformation processes: (b) forging
Forging is the process of forming the metal by impacting and/or squeezing a preheated part between two halves of a die. A succession of dies may be needed to achieve the final shape.
Figure Basic bulk deformation processes: (b) forging

11. Figure 18.2 Basic bulk deformation processes: (c) extrusion
Billets are preheated and forced by a ram through one or more dies to achieve the desired cross-section. The product is long in relation to its cross-sectional dimensions and has a cross section other than that of rod and bar and pipe and tube.
Aluminum part
Figure Basic bulk deformation processes: (c) extrusion

12. Wire and Bar Drawing
Drawing is an operation in which the cross-section of solid rod, wire or tubing is reduced or changed in shape by pulling it through a die.
Figure Basic bulk deformation processes: (d) drawing

13. Sheet Metal Working

14. Sheet Metalworking
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

15. Straining of a metal sheet or plate to take an angle.
Sheet Metal Bending
Straining of a metal sheet or plate to take an angle.
Figure Basic sheet metalworking operations: (a) bending

16. Figure 18.3 Basic sheet metalworking operations: (b) drawing
Deep Drawing
Forming of a flat metal sheet into a hollow or concave shape, by stretching the metal.
Figure Basic sheet metalworking operations: (b) drawing

17. Shearing of Sheet Metal
Shearing operation which cuts the work by using a punch and die.
Figure Basic sheet metalworking operations: (c) shearing

18. Test yourself! sheet metalworking
Name the metal forming process used for each object (bulk deformation or sheet metalworking).
sheet metalworking

19. Test yourself! Name the metal forming process used for each object.
sheet metalworking
sheet metalworking

20. Test yourself! Bulk deformation
Name the metal forming process used for each object.
Bulk deformation

21. Test yourself! sheet metalworking
Name the metal forming process used for each object.
sheet metalworking

22. Forging (Bulk deformation)
Test yourself!
Name the metal forming process used for each object.
Forging (Bulk deformation)

23. Extrusion (bulk deformation)
Test yourself!
Extrusion (bulk deformation)

24. Test yourself!
Gutter
sheet metalworking
Bending
Deep drawing

25. Materials behavior

26. 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

27. 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, that is, the yield strength as a function of strain

28. Average Flow Stress
Determined by integrating the flow curve equation between zero and the final strain value
where = average flow stress; and
 = maximum strain during deformation process

29. Temperature in Metal Forming

30. 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
T K n

31. 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

32. Temperature in Metal Forming
Cold working refers to plastic deformation that occurs usually, but not necessarily, at room temperature.
Warm working: as the name implies, is carried out at intermediate temperatures. It is a compromise between cold and hot working.
Hot working refers to plastic deformation carried out above the recrystallization temperature.

33. Temperature in Metal Forming
Increasing temperature
Melting temperature
Room temperature Tm 0.5 Tm Tm Above Tm
Casting
Hot working
Cold working
Warm Working
Recrystallization

34. Cold Working
Performed at room temperature or slightly above
Cold Forming is the primary manufacturing operation of the fastener industry.
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

35. 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
Cold forming can make tiny, complex precision parts in one machining step. Tolerances of in. are possible.

36. Disadvantages of Cold Forming
Higher forces and power required in the deformation operation
Surfaces of starting workpiece must be free of scale and dirt (Scale=a usually black scaly coating of oxide forming on the surface of a metal (as iron) when it is heated for processing)
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
Annealing: a heat treatment that alters the microstructure of a material causing changes in properties such as strength and hardness

37. 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

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

39. 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

40. 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

41. 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

42. 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

43. Useful site