Fig. 1. Compression between
narrow dies.
Open Die Forging Open die forging with hammers and presses is a modern-day extension of the pre-industrial metalsmith working with a hammer at his anvil.
In open die forging, the workpiece is not completely confined as it is being shaped by the dies. The open die process is commonly associated with large parts such as shafts, sleeves and disks, but part weights can range from 5 to 500,000 lb.
Most open die forgings are produced on flat dies. Round swaging dies and V dies also are used in pairs or with a flat die. Operations performed on open die presses include:
1. Drawing out or reducing the cross-section of an ingot or billet to lengthen it.
2. Upsetting or reducing the length of an ingot or billet to a larger diameter.
3. Upsetting, drawing out, and piercing--processes sometimes combined with forging over a mandrel for forging rough-contoured rings.
Fig. 2. Roll forging.
As the forging workpiece is hammered or pressed, it is repeatedly manipulated between the dies until it reaches final forged dimensions. Because the process is inexact and requires considerable skill of the forging master, substantial workpiece stock allowances are retained to accommodate forging irregularities. The forged part is rough machined and then finish machined to final dimensions. The increasing use of press and hammer controls is making open die forging, and all forging processes for that matter, more automated.
Compression between flat dies, or upsetting, is an open die forging process whereby an oblong workpiece is placed on end on a lower die and its height reduced by the downward movement of the top die. Friction between end faces of the workpiece and dies prevents the free lateral spread of the metal, resulting in a typical barrel shape. Contact with the cool die surface chills the end faces of the metal, increasing its resistance to deformation and enhancing barreling.
Upsetting between parallel flat dies is limited to deformation symmetrical around a vertical axis. If preferential elongation is desired, compression between narrow dies is ideal. Frictional forces in the ax ial direction of the bar are smaller than in the perpendicular direction, and material flow is mostly axial.
A narrower die elongates better, but a too-narrow die will cut metal instead of elongate. The direction of material flow can also be influenced by using dies with specially shaped surfaces.
Compression between narrow dies is discontinuous since many strokes must be executed while the workpiece is moved in an axial direction. This task can be made continuous by roll forging. Note the resemblance between and. The width of the die is now represented by the length of the arc of contact. The elongation achieved depends on the length of this contact arc.
Fig. 4. Impression die forging
Larger rolls cause greater lateral spread and less elongation because of the greater frictional difference in the arc of contact, whereas smaller rolls elongate more. Lateral spread can be reduced and elongation promoted by using specially shaped rolls.
The properties of roll-forged components are very satisfactory. In most cases, there is no flash and the fiber structure is very favorable and continuous in all sections. The rolls perform a certain amount of descaling, making the surface of the product smooth and free of scale pockets.