Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces. Deformation results from the use of a tool usually called a die in metal forming, which applies stresses that exceed the yield strength of the metal.
The metal, therefore, deforms to take a shape determined by the geometry of the die. Stresses applied to plastically deform the metal are usually compressive. However, some forming processes stretch the metal, while others bend the metal, and still, others apply shear stresses to the metal. To be successfully formed, a metal must possess certain properties. Desirable properties include low yield strength and high ductility. These properties are affected by temperature. Ductility is increased and yield strength is reduced when work temperature is raised. The effect of temperature gives rise to distinctions between cold working, warm working, and hot working. Strain rate and friction are additional factors that affect performance in metal forming.
Classification of Metal Forming Operations
Metal forming processes can be classified into two basic categories: bulk deformation processes and sheet metal working processes. Each category includes several major classes of shaping operations.
Bulk Deformation Processes – Bulk deformation processes are generally characterized by significant deformations and massive shape changes, and the surface area-to-volume of the work is relatively small. The term bulk describes the work parts that have this low area-to-volume ratio. Starting work shapes for these processes include cylindrical billets and rectangular bars.
Rolling – This is a compressive deformation process in which the thickness of a slab or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotate so as to draw the work into the gap between them and squeeze it.
Forging – In forging, a workpiece is compressed between two opposing dies, so that the die shapes are imparted to the work. Forging is traditionally a hot working process, but many types of forging are performed cold.
Extrusion – This is a compression process in which the work metal is forced to flow through a die opening, thereby taking the shape of the opening as its own cross-section.
Drawing – In this forming process, the diameter of a round wire or bar is reduced by pulling it through a die opening.