Manufacturing Processes - 3 Metal Forming

3.1.1 Introduction
About 90% of all metal production starts off as cast, however a very large proportion of this is then processed by a bulk plastic deformation process, either to improve the structure and properties and / or to give the desired final shape (or close to that) required.

Plastic deformation involves change in shape without change in volume and without melting. For many metals this means processing at a temperature a bit above half the melting point in oK (0.5 Tm - termed 'hot working'). In this region most metals have low strength and high ductility.

The material must not fracture while being deformed which is not a problem with many metals, but some metals can fracture easily (molybdenum) and for these deformation must be carried out under hydrostatic pressure to prevent cracking or fracture.

The deformation occurs in the solid state so die filling is much more difficult than with casting, sharp corners and thin webs are particularly difficult to fill.

There are two types of processes, steady state or homogeneous, where all parts of the workpiece undergo the same deformation and non steady state (or inhomogeneous) where the geometry of the part changes continually and the analysis must be repeated as the process proceeds.

3.1.2 Key Considerations When Specifying a Process to facilitate determination of tool pressures, forces and required machine power.

It should be kept in mind that owing to variability of friction and to some extent variability in material properties, there is little point in using an elaborate theory to try and obtain an accuracy of a few percent. It is more realistic and useful to use a simple method to obtain a result that is within + or - 20%.

3.1.3 Stress States
For most metal forming calculations, some simplifications are usually possible:

  • Plane stress - where one of the three principal stresses, often sigma2 = 0. This may occur in operations on sheet metal where for example the metal is being stretched - as in a tensile test.
  • Plane strain - where one of the three principal strains, usually epsilon2 = 0. This occurs in components where the width is much greater than the length of the deformation zone - where sheet is being rolled. Here sideways spread of material in the deformation zone is prevented by material adjacent to the deformation zone.
  • Axisymmetric Stress - where the state of stress and strain is symmetrical in any radial direction about an axis, this usually occurs in wire drawing and tube forming.

References:
1. 'Introduction to Manufacturing Processes', J A Schey, McGraw-Hill International, 1987.

David J Grieve, 19th November 2012.