18-02-2011, 09:53 AM
presented by
Dr. Arindam Kumar Chanda
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Metal Forming
Elastic and plastic deformation
• Elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed.
• This type of deformation involves stretching of the bonds, but the atoms do not slip past each other
• When the stress is sufficient to permanently deform the metal, it is called plastic deformation
• Plastic deformation involves the breaking of a limited number of atomic bonds in the crystal by the movement of dislocations
Hot Working
• Done at temperature above recrystallization temperature but below melting point.
• Work hardening is eliminated.
• Improvement of mechanical properties like elongation, reduction of area and impact value
• Refinement of crystal occurs
• Cracks and blow holes are welded up.
• Promotes uniformity of material
• Internal and residual stresses are not developed in metal
Cold Working
• Done at temperature below re-crystallization temperature
• Work hardening is not eliminated.
• Decrease of mechanical properties like elongation, reduction of area and impact value
• Crystallization does not occur. Grains are elongated and distorted.
• Possibility of crack formation and propagation
• Uniformity of material is lost
• Internal and residual stresses are developed in the metal.
Rolling
• Bars, plates, sheets, rails and other structural sections are made through plain and grooved rolls.
Rolling Mill
Extrusion
Wire Drawing
Forging
Forging Press
Deep drawing
Deep drawing is defined as the process of making of cup shaped parts from flat metal sheet. This process is also called cupping.
Piercing or Seamless tubing
Here a hot rolled billet is passed between two conical shaped rolls and over a mandrel which assist in the piercing and control the size of the hole as the billet is forced over it.
Tresca’s Yield Criteria
• Since the plastic flow depends on slip which essentially is a shearing process, Tresca suggested that “The plastic flow initiates when the maximum shear stress reaches a limiting value”. This limiting value is defined as shear yield stress K.
• If the principal stresses at a point in the material are σ1 , σ2 , and σ3 , then the maximum shear stress τmax , is given by
τmax = (σ1 - σ3 ) / 2
Hence Tresca’s criteria becomes
(σ1 - σ3 ) / 2 = K
Von Mises’ Yield Criteria
• Von Mises’ proposed that the plastic flow occurs when the shear strain energy per unit volume reaches a critical value(say A). The shear strain energy per unit volume is given by
1/6G[(σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 ] , where G is the shear modulus of the material. Hence according to this criteria
1/6G[(σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 ] = A
or, (σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 = 6GA
i.e. (σ1 - σ2 )2 + (σ2 – σ3 )2 + (σ3 – σ1 )2 = C(Constant)