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  • Publicado : 16 de noviembre de 2010
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METAL FORMING

Fundamentals of metal forming g
Overview of Metal Forming Material Behavior in Metal Forming Temperature in Metal Forming Strain Rate Sensitivity Friction d Lubrication i M l F F i i and L b i i in Metal Forming i

Classification of metal forming process gp

Sheet metal forming – surface to volume ratio - more l ti

Stress in metal forming g
Stresses to plasticallydeform the metal are usually compressive
Examples: rolling, forging, extrusion p g, g g,

However, some forming processes
Stretch the metal (tensile stresses) Others bend the metal (tensile and compressive) Still others apply shear stresses pp y

Metal properties of Metal forming p p g
Desirable material properties:
Low yield strength and high ductility

These properties are affected bytemperature:
Ductility increases and yield strength decreases when work temperature is raised p

Other factors:
Strain rate and friction

Material Behavior in Metal Forming g
Plastic region of stress-strain curve is primary interest stress strain because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: n

σ = Kε

where K = strengthcoefficient; and n = strain hardening exponent • Stress and strain in flow curve are true stress and true strain

Flow Stress
For most metals at room temperature, strength p , g increases when deformed due to strain hardening Flow stress = instantaneous value of stress required to continue deforming the material

Average Flow St A Fl Stress

where Yf = flow stress, that is, the yield strengthas a function of strain

Yf = Kε n

Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest _ n
Kε Yf = 1+ n

Temperature in Metal Forming p g
For any metal, K and n in the flow curve depend on temperature
Both strength and strain hardening are reduced at g g higher temperatures In addition, ductility is increased athigher temperatures

Three temperature ranges in metal forming: Cold working – 0.3 Tm Warm working – 0.5 Tm Hot H t working – 0 7 Tm ki 0.7 T

Advantages and Disadvantages of Cold Working C ld W ki
Good Surface finish and better dimensional accuracy y Strength, fatigue and wear properties are improved Minimum contamination of low temperature Energy saving no heating Handling easy DisadvantagesDeformation energy required is high Strain hardening limits deformation Ductility is reduced

Advantages & Disadvantages of Hot Working – recrystallisation temperature
Very high reduction is possible without fracture Deformation energy required is low Process is fast Metal is made tough Process does not change hardness Structure can be altered to improved final properties Disadvantages Hightemperature promotes reactions Metallurgical str ct re Metall rgical structure are not uniform niform Heat resistance tools are required Surface finish is poor Handling is difficult

Why Hot Working? y g
Capability for substantial plastic deformation of the metal - far more than possible with cold working or warm working g Why?
Strength coefficient is substantially less than at room g ytemperature Strain hardening exponent is zero (theoretically) Ductility is significantly increased

Strain Rate Sensitivity y
Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0 p
The metal should continue to flow at the same flow stress, once that stress is reached However, an additional phenomenon occurs during deformation, especiallyat elevated temperatures: Strain rate sensitivity

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

Strain Rate Sensitivity Equation y q
Yf = Cε &
m

where C = strength...
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