Physical and Mechanical Fundamentals of Metal Forming

The necessary condition for metal forming process is the proper plasticity possessed by material, i.e. ability of material to be deformed under an external load without failure. During forming process elastic and plastic deformations take place.

Elastic strain is a deformation, whose influence on shape, structure and properties of a body completely disappears when external forces causing the deformation are removed. The applied load causes only slight relative and completely reversible displacement of the atoms.

Plastic strain takes place when the shear stresses exceed a certain definite value (elastic limit) and the deformation becomes irreversible (Fig. 4.1). After removement of the load only the elastic component of the deformation is eliminated. The part of the deformation that is called plastic or residua, strain remains.

During plastic deformation the distances between atoms do not change (practically) and, hence, volume of a deforming body remains constant. This main law of metal forming is used for the definition of a deformation degree. The deformation degree represents relative change in cross-sectional area, or dimensions of a half-finished article.

Fig. 4.1. Dislocation mechanism of plastic deformation

 

Let us assume that we have a prism with 90-degree angles and dimensions before forming A, B, C and after it, correspondingly, a, b, c.

The condition (law) of volume constancy may be expressed as follows:

                                                                 (4.1)

 

Hence, deformation coefficients may be calculated. Coefficient of increase in height (length), or extension coefficient:

                                                    (4.2)

 

Coefficient of cross section reduction:

                                 (4.3)

 

Coefficient of decrease in thickness (in breadth):

                                                                      (4.4)

 

Changes in the structure of metal in plastic deformation also take place. The shape of metal is changed in plastic working as a result of plastic deformation of each grain. Since the grains are differently oriented, plastic deformation can’t occur simultaneously and in the same way throughout the whole volume of the polycrystal.

Owing to slip processes, the grains (crystallites) are changed in shape as a result of considerable deformation (Fig. 4.2). The grains before the deformation are of more or less equiaxed shape (a). As a result of displacement along the slip planes the grains are stretched by deformation in the direction of the acting forces to form a fibrous or banded structure (b).

Fig. 4.2. Effect of plastic deformation on microstructure of a metal: a – a grains and

structure before deformation; b – the same after deformation

 

Deformation texture. A high degree of deformation results in preferable crystallographic orientation of the grains. Ordered orientation of the crystallites with respect to the external deforming forces is called texture (deformation texture). The formation of the texture promotes anisotropy of the mechanical and physical properties. Deformation texture is revealed by etching with special reagents. Orientation of texture must be taken into consideration when parts of machines are produced.

Strain Hardening. With an increase in the degree of deformation (at room temperature), properties, which characterize the resistance of steel to deformation (s_u, s_y, HB, etc.), are increased and, on the other hand, the capacity for plastic deformation (percent elongation d and reduction in area j) is reduced (Fig. 4.3).

Hardening of metal in the process of plastic deformation takes place due to the increase in the number of defects of crystal structure (dislocations, vacancies and interstitial atoms). All the defects impede motion of the dislocations, thereby increasing resistance to deformation and reducing ductility. Of prime importance is the increase in dislocation density, which at high degree of deformation may reach extremely high values: 10¹¹ to 10¹² cm^-2 instead of 10⁶ to 10⁸ cm^-2 before the deformation.

Fig. 4.3. Effect of plastic deformation (cold working) on the mechanical properties of low-carbon steel