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ME 350 – Lecture 20 – Chapter 18

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ME 350 – Lecture 20 – Chapter 18

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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. 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 are typically compressive: • Examples: rolling, forging, extrusion • However, some forming processes • Stretch the metal (tensile stresses): wire drawing • Others bend the metal (tensile and compressive) • Still others apply shear

  3. 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

  4. Sheet Metalworking Often called pressworking • Parts are often called stampings • Usual tooling: punch and die Bending Cutting

  5. 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

  6. Flow Stress & Average Flow Stress • Flow stress, Yf, - the instantaneous (or maximum) stress required to continue deforming the material. (used in upset or impression forging) • Average flow stress, Yf, - integrating area under the stress curve . ( = maximum strain during deformation, used in open forging, rolling or drawing)

  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: • 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. Quotes: • The more a person knows, the more they forgive. - Confucius • Living well and beautifully and justly are all one thing.- Socrates • Adversity introduces a person to themselves. • And in the end, it's not the years in your life that count. It's the life in your years.- Abraham Lincoln • Beware the barrenness of a busy life. - Socrates