2.1 Introduction

Plastic forming operations arethose in which the shape of a metal piece is changed by plastic deformation; forexample, forging, rolling, extrusion, and drawing are common forming techniques (Fig.1.1). Of course, the deformation must be induced by anexternal force or stress, the magnitude of which must exceed the yield strengthof the material. Most metallic materials are especially amenable to theseprocedures, being at least moderately ductile and capable of some permanentdeformation without cracking or fracturing.

     

FIGURE.1-1  Some plastic forming operations: (a)forging; (b) rolling; (c) extrusion; (d) drawing.

When deformation is achieved at a temperature above that at which recrystallization occurs, the process is termed hot working; otherwise, it iscold working. With most of the forming techniques, both hot- and cold-working procedures are possible. For hot-working operations,largedeformations are possible, which may be successivelyrepeated because the metal remains soft and ductile. Also, deformation energyrequirements are less than for cold working. However, most metals experiencesome surface oxidation, which results in material loss and a poor final surfacefinish. Cold working produces an increase in strength with the attendantdecrease in ductility, since the metal strain hardens; advantagesover hot working include a higher quality surface finish, better mechanical properties and a greater variety of them, and closer dimensional control of the finished piece. On occasion,the total deformation is accomplished in a seriesof steps in which the piece is successively cold worked a small amount and thenprocess annealed; however, this is anexpensive and inconvenient procedure.