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Metal Forming Fundamentals: Processes, Behavior, and Temperature Effects

This lecture covers an overview of metal forming processes, material behavior, temperature effects, and strain rate sensitivity in metal forming. Understand the basics of metal deformation processes such as rolling, forging, extrusion, and sheet metalworking. Learn about the importance of temperature and strain rate in metal forming operations and the advantages and disadvantages of cold, warm, and hot working temperatures. Gain insights into the impact of temperature on flow stress and the effects of strain rate sensitivity on deformation.

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Metal Forming Fundamentals: Processes, Behavior, and Temperature Effects

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  1. ME 350 – Lecture 20 – Chapter 18 FUNDAMENTALS OF METAL FORMING • Overview of Metal Forming • Material Behavior in Metal Forming • Temperature in Metal Forming • Strain Rate Sensitivity

  2. Basic Types of Deformation Processes • Bulk deformation – starting material has low surface area to volume (e.g. billets & bars) • Rolling • Forging • Extrusion • Wire and bar drawing • Sheet metalworking – starting material has high surface area to volume (e.g. sheet & coils) • Bending • Deep drawing • Cutting

  3. Metal Forming • Plastic deformation that changes the shape of a metal workpiece using a tool, called a die, by applying a stress that exceeds the metal’s: • Stresses that are experienced by the workpart can be • Compressive • Tensile • Both compressive and tensile • Shear

  4. Sheet Metalworking - Forming Often called pressworking • Bend Brakes, Turret Presses, Stamping dies etc • Usual tooling: punch and die

  5. Tube Metalworking – Mandrel Bending

  6. Material Behavior in Metal Forming • Plastic region of stress-strain curve is of 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 (typical material values listed in Table 3.4) • Flow curve based on true stress and true strain

  7. Temperature in Metal Forming • K and n in the flow curve depend on temperature • At higher temperatures ductility is: • At higher temperatures both ‘K’ and ‘n’ are: • Thus, the force and power required to perform deformation operations at elevated temperatures are: • Three temperature ranges in metal forming: • Cold working • Warm working • Hot working

  8. Cold Working • Temperature performed at: • Advantages: • Better accuracy, closer tolerances • Due to strain hardening, strength and hardness are: • Directional properties in workpart due to grain flow • No heating of work required • Disadvantages • Metal may not be ductile enough for large deformations • Deformation forces and power are: • Surfaces must be clean - free of scale and dirt

  9. Warm Working • Temperature: • Advantages: • Lower forces and power than cold working • More intricate work geometries possible • Need for annealing may be reduced or eliminated

  10. Hot Working • Temperature: • Advantages: • Large deformations possible (fracture and cracking possibility eliminated or greatly reduced) • Lower forces and power required • Strength properties of product are generally: isotropic • Disadvantages: • Lower dimensional accuracy • Higher total energy required (due to heating) • Work surface oxidation (scale), poorer surface finish • Shorter tool life

  11. 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, where larger stress is needed as deformation velocity increases.

  12. What is Strain Rate? • Strain rate in forming is directly related to speed of deformation, v (i.e. ram velocity): where h = instantaneous workpiece height • As strain rate increases, resistance to deformation:

  13. Strain Rate Sensitivity where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent

  14. Effect of Temperature on Flow Stress where C, is the intersection of each plot with the vertical dashed line at strain rate = 1.0, and m is the slope of each plot. • Observations: • Increasing temp. C: • Increasing temp. m: • Effect of strain rate at room temperature is:

  15. Next Lecture: Design to Cost

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