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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. Basic Types of Deformation Processes.
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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
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
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
Sheet Metalworking - Forming Often called pressworking • Bend Brakes, Turret Presses, Stamping dies etc • Usual tooling: punch and die
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
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
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
Warm Working • Temperature: • Advantages: • Lower forces and power than cold working • More intricate work geometries possible • Need for annealing may be reduced or eliminated
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
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.
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:
Strain Rate Sensitivity where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent
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:
