Published online by Cambridge University Press: 05 June 2012
The shapes of most metallic products are achieved by mechanical working. The exceptions are those produced by casting and by powder processing. Mechanical shaping processes are conveniently divided into two groups, bulk forming and sheet forming. Bulk forming processes include rolling, extrusion, rod and wire drawing, and forging. In these processes, the stresses that deform the material are largely compressive. One engineering concern is to ensure that the forming forces are not excessive. Another is ensuring that the deformation is as uniform as possible so as to minimize internal and residual stresses. Forming limits of the material are set by the ductility of the work piece and by the imposed stress state.
Products as diverse as cartridge cases, beverage cans, automobile bodies, and canoe hulls are formed from flat sheets by drawing or stamping. In sheet forming, the stresses are usually tensile and the forming limits usually correspond to local necking of the material. If the stresses become compressive, buckling or wrinkling will limit the process.
Bulk Forming Energy Balance
An energy balance is a simple way of estimating the forces required in many bulk-forming processes. As a rod or wire is drawn through a die, the total work, Wt, equals the drawing force, Fd, multiplied by the length of wire drawn, ΔL, Wt = Fd ΔL. Expressing the drawing force as Fd = σd A, where A is the area of the drawn wire and σd is the stress on the drawn wire, Wt = σd AΔL (Figure 14.1).